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The production of ultra-light weight coke proppants for fracturing coal bed methane deposits is presented. The raw material is blast-furnace coke, foundry coke and a coke breeze. The method used obtains coke proppants that meet all the requirements of the ISO 13503-2:2006/Amd.1:2009 standard for proppants used in hydraulic fracturing, with the exception of crush resistance. They have porosity up to 40 % and they are mainly macroporous materials, containing predominantly cylindrical, less frequently bottle-shaped or wedge-shaped pores. The proppants with the lowest crush rate were obtained from the blast furnace coke (CSR > 65 %). • A coke modification to obtain proppant for hydraulic fracturing. • Proppants with the best properties were obtained by modifying blast furnace coke. • Coke proppant is suitable for fracturing shallow CBM deposits. [ABSTRACT FROM AUTHOR]

The pressure regimes of the No. 2 coalbed methane (CBM) reservoir in the Yanchuannan field located in the southeastern Ordos Basin are highly variable and divided into overpressured (pressure gradient >9.80 kPa/m), slightly underpressured (pressure gradient of 8–9.80 kPa/m), and moderately underpressured (pressure gradient of 5–8 kPa/m). The controlling factors for the variable pressure regimes were investigated through the analysis of geological and hydrological characteristics. The pressure regimes are controlled by different mechanisms in different hydrodynamic environments. In the closed hydrodynamic environment characterized by TDS > 10,000 mg/L and NaCl type of groundwater, the pressure regime is dominated by overpressured to slightly underpressured and is controlled by CBM migration. Overpressure was developed by thermogenic CBM generation during the coalification process and is maintained by thermogenic CBM migration from the extended northwestward and deeply buried CBM reservoir during tectonic uplift. The transition from overpressure to slight underpressure and then to moderate underpressure towards the southeast is the result of the progressively weakened migrated thermogenic CBM with increasing migration distance. In the open hydrodynamic environment characterized by TDS < 10,000 mg/L and NaHCO 3 type of groundwater, the pressure regime is dominated by slightly to moderately underpressured and is governed by hydrodynamics. Groundwater is fed by meteoric recharge along the structurally upturned basin margin and creates the hydrodynamic framework during tectonic uplift. The transition from moderate to slight underpressure towards the southwest is associated with the minor decrease range in ground elevation from basin margin to basin interior and the gradually weakened runoff intensity of groundwater with increasing distance to meteoric recharge. The idealized models for the pressure regimes are established, which can provide guidance to deep CBM sweet spot identification in CBM fields in the eastern Ordos Basin and elsewhere. [Display omitted] • There are two different hydrodynamic environments because of impermeable faults. • The pressure regimes are controlled by the two different mechanisms. • CBM migration governs the pressure regime in the closed hydrodynamic environment. • Hydrodynamics controls the pressure regime in the open hydrodynamic environment. • The idealized models for the pressure regimes are created in eastern Ordos Basin. [ABSTRACT FROM AUTHOR]

This research presents a new method for studying gas-water two-phase flow in fractured coal, integrating cutting-edge imaging techniques. We combine dynamic positron emission tomography (PET), high-resolution X-ray micro-computed tomography (micro-CT), and unsteady-state fluid flow experiments. First, micro-CT under reservoir pressure conditions maps the sample's fracture structure at high-resolution. Then, helium injection into a water-saturated sample simulates gas flow in a coal seam during production. Real-time PET monitoring captures the dynamic displacement process within the fractures. This approach yields crucial data on gas injection volume, pressure variations, and water production, enabling relative permeability curve prediction. Finally, multi-scale image analysis merges high-resolution micro-CT with dynamic PET images, overlaying the flow path onto the fracture network. This innovative method leverages the strengths of both PET and micro-CT, offering unprecedented visualization of gas-water flow behaviour in fractured coal. PET images play a crucial role in providing both spatial and temporal water saturation profiles since the activity mapping directly correlates with water volume distribution in the fractures. The consistency between the initial activity profile along the sample from PET and the fracture volume distribution calculated from micro-CT images confirms the reliability of PET data. The workflow proposed in this paper can be used to monitor two phase flow displacement in unconventional rocks such as coal and be applied for determination of relative permeability curves. • Multi-instrumental imaging is employed to understand coal relative permeability. • PET imaging offers real-time visualization, revealing the dynamic of fluid flow in fractures. • Multi-scale analysis of PET and micro-CT offers a novel method for flow in complex coal fractures. [ABSTRACT FROM AUTHOR]

The Orzesze-1 exploratory well with a depth of 3708 m (TVD) was drilled in 2019–2020 in the depocentre of the Upper Silesian Coal Basin (USCB). The methane content in the coal seams has been tested to a depth of 2840 m and the sorption capacity of the coal to a depth of 2576 m. These are the deepest measurements in the USCB so far. The vertical distribution of methane content in the borehole shows two depth zones of interest, the first at a depth 883 m to about 1300 m (maximum methane content about 12 m3/t coaldaf) and another in the range of 1500–2840 m, that is, to the maximum measurement depth, so the actual lower boundary depth of this zone is unknown. The maximum methane content here exceeds 18 m3/t coaldaf at a depth of >2800 m. Both zones are separated by an interval of reduced methane content of about 5 m3/t coaldaf at a depth of approximately 1400 m. The gas composition is dominated by methane (∼90%), and the content of carbon dioxide increases to approximately 15% at a depth of >2300 m. The methane-bearing zone at ∼900–1300 m corresponds to the zone of high- and medium-volatile bituminous coal (second coalification jump), while the highest methane content at a depth of >2800 m was determined in anthracite. The methane sorption capacity of the coal seams oscillates between 16 and 40 m3/t coaldaf with a maximum in anthracite at a depth of >2800 m, where the temperature of the rock approaches 100 °C and the deposit pressure exceeds 28 MPa. The highest sorption capacity in anthracite results from its inner structure characterised by the predominance of ordered aromatic lamellas and the dominance of vitrinite macerals (>70%), which contain coal micropores accumulating adsorbed methane. The comparison of the sorption capacity of the tested coal and the measured methane content displays undersaturation of 11–59%, however, due to significant gas content in the deep zone (depth > 1500 m), the drilling area can be considered as a prospect for further exploration and development of coalbed methane (CBM). • A vertically extensive zone of maximum gas contents was found in the USCB depocentre. • Anthracite has the highest methane content and sorption capacity. • Methane is the main gas component, but the CO 2 content reaches >15% at a depth of >2500 m. • Coal is characterised by significant methane undersaturation and low permeability. • Work on deep CBM mining technology should be continued due to the promising gas-bearing potential of the area. [ABSTRACT FROM AUTHOR]

The chemical characteristics of coal reservoir water are important for studying the formation and enrichment of biogenic coalbed methane (BCM). Based on geological and sampling test data, this paper studied the geochemical characteristics and formation mechanisms of high-salinity coal reservoir water (CRW) in the Jurassic Yan'an Formation of the Binchang area in the southern Ordos Basin. The results show that the TDS contents of the CRW in the Binchang area are between 7577.38 and 15,138.61 mg/L (av. 13,268.95 mg/L), which is high-salinity brackish water. The ion types of CRW are mainly Na+, Cl− and HCO 3 −, and the correlations between TDS and Na+ and Cl− are close to 1. The Piper trilinear diagram indicates that the evolution direction of the CRW is deep concentrated brine, and the hydrochemical type is the Na Cl type. The 127I concentrations of CRW are between 285 and 484 μg/L, which are much higher than the values of 55.88 μg/L for seawater. The results of 129I dating show that the minimum age of the CRW in the study area is between 6.7 Ma and 39.97 Ma, which is much younger than the actual geological age of the Yan'an Formation. The hydrogen and oxygen isotope results show that the CRW in the study area experiences an apparent oxygen drift, indicating that the coal reservoir of the Yan'an Formation has good sealing and a long retention time for the CRW. The hydrodynamic factors show that the hydrodynamic conditions of the coal reservoir are weak, and the primary ions in the CRW originated from the dissolution of salt rocks. The main ion differentiation indices show that high-salinity coal seam water is mainly formed by evaporation, and the ion exchange between CRW and the surrounding rock and the alternating adsorption of cations in water are very weak. Evaporation and diagenesis lead to an increase in the contents of Na+, Cl− and I+ in coalbed water, which in turn leads to an increase in the total dissolved solids contents of CRW and its evolution toward concentrated brine. The genesis and evolution of the CRW in the study area are affected by the combination of the relationships among the paleoclimate, aquifers and aquifuges, and tectonic evolution processes. The CRW in the study area has experienced five evolution stages, i.e., sedimentary water and diagenetic water, high-salinity infiltration water, primary mixed water, paleoatmospheric precipitation recharge water, and secondary mixed water. The above understanding can provide a basis for studying the formation period and accumulation mechanism of BCM and provide a hydrogeological basis for water resource utilisation and pollution prevention and the control of high-salinity water. • The high-salinity coal reservoir water in the Yan'an Formation in the southern Ordos Basinis is caused by evaporation. • δD and δ18O isotopes indicate that coal reservoir water is early retained water, and the alternating effect of water is weak. • The minimum age of the coal reservoir water calculated by 129I dating is between 6.7 Ma and 39.97 Ma. • The coal reservoir water in the Binchang area has experienced five evolution stages. [ABSTRACT FROM AUTHOR]

Understanding inter-aquifer connectivity or leakage of greenhouse gases and groundwater to aquifers overlying gas reservoirs is important for environmental protection and social licence to operate. Australia's Great Artesian Basin (GAB) is the largest artesian groundwater system in the world with groundwater extracted for agriculture, livestock, mines, energy, private or town water supply. Microbial coal seam gas (CSG) and production water are also extracted from the GAB. Here a range of groundwater tracers is used to investigate the potential for gas and groundwater connectivity between the CSG reservoir and aquifers. The GAB aquifer and alluvium contained a range of methane concentrations (0.001 to 2100 mg/L) that exhibit an increase with depth and δ13C-CH 4. Aquifer and alluvium groundwater 87Sr/86Sr were in the range 0.7042 to 0.7082. CSG production waters however had non-radiogenic, distinctive 87Sr/86Sr signatures <0.7036, indicating a lack of significant groundwater leakage. One gassy aquifer bore with 160 mg/L methane conversely has 87Sr/86Sr, δ13C-CH 4 , δ2H-CH 4 and δ13C-DIC values overlapping the CSG waters. In several aquifers δ34S-SO 4 and δ18O-SO 4 are sourced from windblown surface salts of inland Australian playa lakes in recharge waters. Bacterial sulphate reduction is additionally occurring in a regional aquifer. Cosmogenic isotopes and tritium show recent recharge and mixing with older groundwaters in several shallow aquifers. Groundwater and gas signatures indicate that leakage of groundwater and methane from the CSG reservoir was not occurring in the majority of areas investigated here. Methane was consistent with in situ generation in shallow GAB aquifers by primary microbial CO 2 reduction or acetate fermentation. Connectivity of one alluvial bore and the underlying GAB aquifer could not be completely ruled out. Separately, one gassy Springbok GAB aquifer bore is either connected to the underlying CSG gas reservoir, or has in situ secondary microbial CO 2 reduction producing methane from interbedded coal within the aquifer. This study is relevant to other basins in Australia and internationally where gas is observed in aquifers that overly conventional, unconventional or coal seam gas reservoirs. [Display omitted] • Coal seam gas region multi-isotope GAB aquifer groundwater gas connectivity study. • CSG reservoir non-radiogenic distinctive 87Sr/86Sr and gas isotope vs GAB aquifers, alluvium. • δ34S, δ18O-SO 4 source: surface salts in recharge waters, bacterial SO 4 reduction in one aquifer. • Possible gas leakage into one gassy bore. Mixing of modern waters in shallow aquifers. • Signatures support dis-connectivity of the CSG reservoir with majority aquifer bores. [ABSTRACT FROM AUTHOR]

The coal matrix exhibits significant heterogeneity at the micro-scale. In this study, a heterogeneous mechanical model of coal matrix from different coal ranks was constructed by using nanoindentation. Additionally, a fluid-solid coupling model was developed to consider matrix shrinkage and stress sensitivity, enabling the incorporation of porosity changes during methane diffusion. The methane concentration and matrix deformation characteristics in the simulation area at different times was obtained. The effects of matrix shrinkage and stress sensitivity on porosity changes and matrix deformation were compared. And the deformation of the matrix under different micromechanical properties was compared. The findings indicate that the nanoindentation method effectively characterizes the micromechanical heterogeneity of coal matrices. Furthermore, during the diffusion process, we observed an initial increase followed by a decrease in matrix deformation, with matrix shrinkage leading to more pronounced deformation. Notably, harder coal matrices exhibited reduced deformation. This study enhances the understanding of the dynamic changes in the matrix during coalbed methane extraction. [ABSTRACT FROM AUTHOR]

Coal seams are typically self-sourced reservoirs. However, in certain special cases, the coal seams also contain large amounts of external gas from other source rocks. Successful development strategies in relation to Jurassic coalbed methane (CBM) in recent years have confirmed the presence of large amounts of external gas within the coal seams of the Baijiahai Uplift, Junggar Basin, China, which has important exploration and development prospects. However, the genesis and source of Jurassic CBM in the area remain unclear, limiting our understanding of its formation and accumulation as well as the planning of further development strategies. Based on a comprehensive analysis of the geochemical characteristics of Jurassic CBM, combined with research on the burial and thermal evolution history of the Jurassic coal seams, this study discusses the production curves of typical wells as well as regional tectonic styles, genesis, source, and accumulation of Jurassic CBM in the Baijiahai Uplift. Depending on the genesis, Jurassic CBM in the study area is divided mainly into coal-type (humic-type) gas, oil-type (sapropelic-type) gas, and mixtures of them, all of which are thermogenic gases of medium to high maturity. There are two main sources of Jurassic CBM in the Baijiahai Uplift. On the one hand, it comes from the vertical migration of gas generated by Permian and Carboniferous source rocks in the Baijiahai Uplift directly. On the other hand, it comes from the regional lateral migration from the Dongdaohaizi and Fukang Sags. Gas generated by source rocks in the Dongdaohaizi and Fukang Sags is driven by pressure, migrates towards the Baijiahai Uplift through sandstone channels, and accumulates in the Jurassic coal seams through faults. Compared with common self-sourced coal seams, enrichment of the coal seams with large amounts of external gases requires two necessary conditions: (1) Other layers of gas-generating source rocks are present at levels deeper than those of the coal seams, constituting additional gas sources for the coal seams, and (2) an open fault system is developed regionally, providing pathways for gas from other layers to enter the coal seams. • The genesis and source of external gas in deep coal seams are explained. • Identification methods of the gas originates from multiple source rocks. • Reveal the migration and accumulation of external gas in deep coal seams. • The necessary conditions for the formation of external gas are determined. [ABSTRACT FROM AUTHOR]

This study represents the first assessment of CO 2 storage potential in Austrian coal seams. Coal samples were taken from Fohnsdorf and Leoben abandoned coal mines, with particular emphasis on the Fohnsdorf coal since Leoben coal reserves were largely mined during previous coal production. Several methods were used to compare coal characteristics, including Rock-Eval pyrolysis (RE), organic petrography, and low-pressure N 2 and CO 2 sorption measurements. Both Fohnsdorf and Leoben coal samples show low sulfur and ash yields, as well as correspondingly high total organic carbon (TOC) contents. The pyrolysis T max and vitrinite reflectance values agree with a low coal rank for both sites. According to the N 2 adsorption measurements at 77 K, low-lying mire coals from Fohnsdorf show a higher BET-specific surface area (BET-SSA) and BJH pore volume compared to raised-mire coals from Leoben. However, sapropelic shales and high-ash coals from Leoben show the highest BET-SSA and BJH pore volumes of all investigated samples and considerably exceed the N 2 adsorption volumes of pure coals from both locations (N 2 uptake up to 16 cm3/g; avg. for all samples 5.4 cm3/g). In contrast, the mean adsorbed CO 2 uptake measured at 273 K and ∼ 1 bar followed the order of Fohnsdorf low-lying mire coals > Leoben raised-mire coals > Leoben sapropelic coals and shales, ranging at ∼0.8 mmol/g, ∼0.7 mmol/g, and ∼ 0.2 mmol/g, respectively. This shows that BET-SSA and BJH equations did not allow for adequate estimation of CO 2 adsorption capacity trends in the investigated sample set. Furthermore, based on the existence of a hysteresis loop between CO 2 adsorption and desorption branches for all investigated samples, the occurrence of weak chemisorption phenomena during CO 2 adsorption is indicated. This effect helps to increase CO 2 uptake and storage safety since the chemisorption process is not fully reversible upon pressure decrease. Ultimately, the theoretical CO 2 sequestration potential of the remaining unmined Fohnsdorf coal reserves was estimated at 4.65 million tons, with an additional potential for enhanced coal bed methane production due to the gas-rich nature of Fohnsdorf coals with an estimated 1.2 billion m3 of CH 4 in place. [ABSTRACT FROM AUTHOR]

Accurate quantification of the coexistence of adsorbed and free gas content holds the utmost significance for estimating gas-in-place resources and predicting gas production dynamics. In this study, we conducted real-time isothermal adsorption experiments and NMR fluid monitoring on stress-confining core samples, from the Zhengzhuang Block's No.3 coal seam in the southern Qinshui Basin. Our focus was on assessing multi-phase methane gas contents within coal under various pressure and temperature (P/T) conditions. By integrating experimental findings with adsorption potential theory and the SDR adsorption model, we developed comprehensive models for adsorbed, free, and total gas contents as functions of P/T and water/gas volume saturation. Utilizing these models, we predicted vertical variations in adsorbed and free gas contents within the coal seam. Our results revealed that the interplay between positive reservoir pressure effects and adverse reservoir temperature effects influenced both adsorbed and free methane gases. With increasing burial depth, the influence of pressure on adsorbed gas diminished, while temperature effects became more pronounced. Conversely, free gas content responded noticeably to reservoir pressure, with temperature exerting a marginal influence. Additionally, we performed a numerical simulation to reconstruct the thermal history, burial trajectory, and evolution of reservoir pressure for the No.3 coal seam. The simulation results served as foundational data for understanding the evolution of free and adsorbed gas contents across different geological epochs within the in-situ reservoir. Our findings unveiled a four-stage evolutionary progression in both adsorbed and free gas contents, correlating with the uplift and subsidence of the coal seam. In conclusion, our study provides a conceptual model elucidating the intricate, deep-time evolution process and mechanisms governing the occurrence of multiphase gases across distinct geological epochs. The implications of this research are crucial for accurately evaluating gas-in-place resources and guiding the exploration and development of deep coalbed methane resources. [ABSTRACT FROM AUTHOR]

Permeability plays a significant role in the economic viability of coalbed methane (CBM) development. In China, in situ permeability obtained from injection fall-off tests range from 0.0001 to 41 mD at depths of 130–1728 m, with the majority (78%) falling between 0.01 and 1 mD. Coal reservoirs with high rank, common cleat mineralization, and depth-dependent high stress may result in extremely low permeability. Low-rank coal and cataclastic coal exhibit more favorable permeability conditions at shallow depths. Heterogeneity in coal rank, coal texture, and stress magnitude results in a discrete distribution of permeability both regionally and vertically. Most deep seams are highly compressed with extremely low permeability (<0.1 mD) as high stress levels. Multilateral horizontal wells work best with high permeability seams, while vertical wells with conventional hydraulic fracturing demonstrate limited adaptability to deep and low-permeability seams due to insufficient proppant-supported fractures. With increasing depth, both vertical and horizontal wells should employ larger fracturing scales to create effectively supported zones with higher fracture conductivity and reduced stress sensitivity to enhance CBM productivity. Horizontal wells with large-scale volume fracturing demonstrate superior performance compared to vertical wells in extracting CBM from deep, low-permeability, and high gas saturation reservoirs. [ABSTRACT FROM AUTHOR]

In conjunction with implementing the Indonesian Act on coal downstream research and development, the geochemical characterization of the Barito Basin coals and an evaluation of the critical elements in coal and coal ashes compared with those found in conventional deposits must be fulfilled. This study documented the signature characteristics of the coal geochemistry in the Barito Basin. Coals from Tanjung Formation mainly comprise huminite macerals and inorganic constituents are mainly dominated by kaolinite, pyrite, quartz, carbonate minerals (i.e. , calcite, Sr-bearing carbonate), Ti-oxide minerals (anatase or rutile), chlorite, and apatite. Meanwhile, the Warukin Fm is characterized by high contents of inertinite and inorganic materials comprising kaolinite, pyrite, and quartz. Moreover, the B3 seam from the Tanjung Fm shows normal slight enrichments for Hf, Zr, and V, demonstrating the highest REY content of up to 194.56 ppm. Meanwhile, coal samples from the Warukin Fm show depletion in all the trace elements and REY. The provenance source of the Barito Basin coals is derived from epiclastic mafic–intermediate volcaniclastic rocks, including the Early Cretaceous Pitap and Haruyan Fm, which are mainly derived from low-Ti mafic magmas. Subsequently, marine water infiltration during peat accumulation is subjected to the influence of the geochemical characteristics of coals from the Eocene Tanjung Fm, showing the transgressive settings. On the other hand, the Miocene Warukin Fm. represents a non-marine regressive setting during peat accumulation. Furthermore, local geological controls in the Eocene Tanjung Fm. may lead some roof, floor, parting and coals from B3 seam to critical elements enrichment (such as REY, Zr, Hf, and V), and their affinity is related to Ti-oxide minerals, aluminosilicates, and organic matter. • First report of coal geochemistry from the Barito Basin. • Sediment source is from the low-Ti mafic volcaniclastic rocks of Pitap and Haruyan Fm. • Marine water infiltration subjected to influencing peat accumulation in the Tanjung Fm. • Some layers of roof, floor, parting, and B3 seam from the Barito Basin are elevated in certain critical elements. • Zr, Hf, V, and REY are related to the affinity of Ti-oxide minerals, aluminosilicates, and organic matter. [ABSTRACT FROM AUTHOR]

The Syncline of Umbita (SU) in Boyaca, Eastern Cordillera of Colombia contains reserves of coalbed methane (CBM) estimated in 122 Bcf of gas, the major CBM play in the Cundi-Boyacense plateau. Despite its potential, the role of groundwater on gas desorption is yet to be addressed. The main constraint is water yield, the more sustainable, lower the potential for gas desorption since sustainable yield or additional water influx into reservoir is expected to inhibit the dewatering of the coal seams and thereby limit the coalbed methane recovery. Such flow attributes are known as "infinite acting aquifer" (IAA) type-of-flow, a condition with sustainable groundwater yield without imminent flow boundaries, also known in CBM as "ineffective-type of water". Presence of IAA at the SU is primarily inferred by sustainable water yield of the coal reservoir interjected from prolific aquifers above and below by means of vertical hydraulic communication or cross-formational flow. In addition, conditions of sustainable water yield are reinforced by converging linear type-of-flow, which develops within the U-Tube-like hydraulic setting with vented ends conformed by the complete, quasi-symetric syncline in the Umbita region. Assessing conditions of IAA is achieved by weighing drawdown vs. dewatering under multiple production scenarios, which is best accomplished with the use of numerical methods. Therefore, a series of flow simulations were undertook forwarded to determining whether IAA would be a limiting factor for commercial development of methane in the SU. The results suggest that "infinite acting aquifer" type-of-flow likely drives groundwater flow in the syncline which may impact gas desorption. Main evidence provided by simulation is the numerical feasibility of having a coal reservoir with conditions of sustainable water yield under extreme dewatering regimes, conditions that are supported by induced recharge from aquifers above and below via cross-formational flow and development of converging linear type-of flow. This work provides a preliminary assessment regarding the hydrodynamics and reservoir depressurization style of what constitutes a unique field analog of a hydraulically-complex CBM reservoir. The main goal is to assist in future hydrogeological investigations and reservoir characterization that set the basis for CBM degassing of the coal reservoir to economically produce methane in the Syncline of Umbita. • Infinite acting aquifer (IAA) conditions in CBM reservoirs trigger sustainable water yield limiting gas desorption • Prolific aquifer systems, flow paths and structural controls prompt IAA conditions in the Synclinal of Umbita • Reservoir depressurization under IAA conditions demand high dewatering schemes challenging production of CBM [ABSTRACT FROM AUTHOR]

This paper evaluates the potential methane storage capacity of six clay-rich interburden rock samples from coal seam gas (CSG) wells in the Surat Basin, Australia. Clay minerals identified in all samples included kaolinite, illite, smectite, and illite-smectite mixed-layers. The total organic carbon concentrations in these interburden rocks ranged from 0.66–1.19 wt%, and thus these rocks can be classified as fair to good hydrocarbon source rocks. The effective porosity of the rocks determined from mercury intrusion porosimetry and helium pycnometry varied from 6.8% to 12.5%, and included volumes of micropores and mesopores. The adsorption isotherm results indicated that the average adsorption capacity of six interburden was 3.64 cm3/g, a value corresponding to approximately 20% that of Surat Basin coal. Based on the clay compositions and porosity of the samples, the permeability of these Surat interburden rocks is estimated to be <5 nano Darcy using Yang and Aplin's empirical correlation, which was too low for reliable measurement in our laboratory core flooding apparatus even with a differential pressure of 10 bar applied over a shortened 20 mm length core. However, after stimulation by electrohydraulic discharge (EHD) shockwaves the permeability of one of the interburden samples (S2) increased to 0.6 ± 0.11 mD due to development of fractures and new pores by the EHD stimulation. We characterised the development of the fractures after EHD shockwaves using x-ray computer tomography. The findings of this study suggest that dynamic shockwaves such as those generated by EHD have potential to increase permeability of soft and clay-rich interburden layers in CSG reservoirs and other layered reservoirs. This potentially opens these ultra-tight gas resources to exploitation and recovery. • A suite of coal seam interburden samples has been characterised and investigated for gas potential appraisal. • After EHD stimulation, the confining-constrained interburden discs were successfully cracked with fractures/voidages. • The shock pressure at both radial and axial orientations was recorded to reveal the philosophy of interburden breakage. • As a pioneering trial to stimulate interburden under stress-state, EHD was employed on stimulating clay-rich interburden. [ABSTRACT FROM AUTHOR]

Abstract Unconventional natural gas, including coalbed methane and shale gas, has become important natural gas resources. Coal and shale reservoirs are characterised by low porosity and low permeability and difficult for gas production. These reservoirs are also considered as fractured reservoirs, i.e. the natural fracture/cleat system in coals and bedding direction microfractures in shales. Permeabilities of these reservoirs are sensitive to stress change. During gas production, the pressure drawdown significantly increases effective stress, and thus decreases the absolute permeability. The relationship between permeability and stress is characterised by fracture compressibility, which is difficult and costly to be obtained from the field, but can be acquired easily from laboratory measurement. In this review article, the laboratory methods to obtain fracture compressibility were reviewed. Literature data on fracture compressibility for coals and shales were collated and the relationships between fracture compressibility and pressure, stress and rock properties were discussed. It is found that fracture compressibility is higher for coals than for shales, and the fracture compressibility for proppant supported fracture is even lower than that for the same shale or coal. Moreover, fracture compressibility is variable depending on gas type, gas pressure, and stress. Fracture compressibility has no correlation with absolute permeability in general, but has a weak positive correlation for the same sample. Highlights • Fracture compressibility is found higher for coals than for shales in general. • Proppant supported fracture compressibility is typically smaller than that for natural fractures or original samples. • Fracture compressibility is variable and dependent on gas type, gas pressure, and stress. • Fracture compressibility has no correlation with absolute permeability. [ABSTRACT FROM AUTHOR]

Abstract Accurate determination of permeability is critical for the efficient development of coalbed methane (CBM) reservoirs. Permeability determination is mainly based on the feature straight-line analysis method and the plate fitting method requiring pressure build-up data. Although few analytical methods only require dynamic production data, they usually have limitations in application due to complicated solution processes or insufficient consideration of CBM reservoir characteristics. This study improves the previous method for determining the permeability of undersaturated CBM reservoirs. Firstly, mathematical models are established for calculating permeability of undersaturated CBM reservoirs with constant permeability and variable permeability due to pulverized coal output/blockage, matrix shrinkage, and stress dependence during the production process. Secondly, the proposed models are validated by numerical simulation, analytical, and pressure build-up test methods. Finally, the models are applied to the Muai CBM reservoir in Sichuan Basin, China. High-, middle-, and low-permeability zones of the Muai reservoir are classified. The results show that when bottom-hole pressure and water production data of CBM wells during single-phase water flow period are processed by the proposed method and plotted in a rectangular coordinate system, a strong linear relationship can be obtained for CBM reservoirs with constant permeability and variable permeability. The slope of the fitted equations can be used to determine permeability efficiently and accurately. Compared with previous methods, the new method provides better interpretation results for CBM wells with strongly fluctuating production data since the use of the integration of fluctuating data and cumulative water production reduce the effect of production fluctuation on the results. Due to its conciseness, this method can rapidly calculate reservoir permeability around a large number of production wells and obtain permeability distribution in the reservoir, providing guidance for CBM productivity forecasting and development policy adjustment. Highlights • An accurate method for evaluating permeability of undersaturated coalbed methane reservoirs is developed. • The calculation steps are simplified and the calculation accuracy is improved. • The proposed method is suitable for coalbed methane reservoirs with constant or variable permeability. • Only production data during the early dewatering stage is needed. • The method can also be applied to wells with large fluctuation in water production. [ABSTRACT FROM AUTHOR]

Abstract This study investigates the interaction between supercritical CO 2 (scCO 2) and minerals in coal that may occur during CO 2 sequestration into deep coal seams. Two low-rank coals with low-ash content, one low-rank coal with high-ash content and one high-rank coal with low-ash content were used to study the interactions. A coal-CO 2 geochemical interaction experiment with a typical high-ash coal was conducted to investigate change of the water-soluble elements after scCO 2 interaction. X-ray powder diffraction (XRD) was used to determine the effects of scCO 2 –H 2 O on the minerals in the four coals. In addition, the influence of scCO 2 on pore structure of four coals was studied based on liquid nitrogen adsorption experiment. The results suggest that scCO 2 can effectively enhance the solubility of all elements from coal in water because of scCO 2 interaction. ScCO 2 can significantly improve the solubility of trace elements than that of major elements. Because of the relatively low mineral content in coals and the short experimental period, secondary minerals formed by scCO 2 -mineral-water interaction were not determined with XRD. However, the content of primary minerals changed; the apparent content of quartz increased and the content of kaolinite, carbonate minerals and pyrite decreased. For the experimental coals in this study, scCO 2 can enlarge the pore size of the micropores for low-ash coal and mesopores for low-rank coal, but shrinks that of micropores for high-ash coal and mesopores for high-rank coal. Highlights • ScCO 2 can effectively enhance the solubility of all elements in coal. • Enhancement degree of the solubility for trace elements is significantly higher than that of major elements. • ScCO 2 can increase the apparent content of quartz. • ScCO 2 can decrease the content of kaolinite, carbonate minerals and pyrite. • ScCO 2 can change the pore size of the micropores and the mesopores, depending on coal rank and ash content. [ABSTRACT FROM AUTHOR]

Abstract Carbon dioxide geosequestration into deep unmineable coal seams is a technique which can mitigate anthropogenic greenhouse gas emissions. However, coal composition is always complex, and some minerals such as calcite chemically react when exposed to the acidic environment (which is created by scCO 2 mixing with formation water). These reactive transport processes are still poorly understood. We thus imaged a water-bearing heterogeneous coal (calcite rich) core before and after scCO 2 injection in-situ at high resolution (3.43 μm) in 3D via X-ray micro-tomography. Indeed, the calcite- fusinite mix phase was partially dissolved, and absolute porosity and connectivity significantly increased. We thus suggest that such a process could be used as an acidizing method for enhanced coal bed methane (ECBM) production, thus significantly improving the permeability performance, CO 2 injectivity and the associated methane permeability. Highlights • The calcite-fusinite mix phase partially dissolved was observed in 3D as scCO 2 injected into water-bearing bituminous coal. • Morphological results showed the dissolved volume significantly increased the porosity and pore connectivity. • An environmentally friendly and relatively harm free acidizing method in ECBM was proposed. [ABSTRACT FROM AUTHOR]

Abstract Sorption studies of gaseous mixtures of ethane, ethylene, propane, and propylene allowed us to obtain a better understanding of flammable gas behavior during flow through a coal seam. The dynamic sorption method was used to characterize the behavior of the multi-component gas mixtures. In these experiments, a gas mixture was released during a simulated heating process for coal and was then flown through a fixed bed of coal with particle sizes of 0.5–0.7 mm. The composition of gaseous effluent flow at the inlet and outlet of the sorption column was analyzed by gas chromatography (GC). The mixture composition influenced the amounts of sorbed gas, and variations occurred as a result of differences in the coal permeability. Both the high content of ethane in the mixture and its mechanism of sorption resulted in faster retention times for this gas. Propylene, which was characterized by low concentrations, was sorbed in the largest amounts and had the least steep adsorption curves compared to the other gases. The coal adsorbent with the largest surface area as determined by micropores and mesopores sorbed the largest amount of gases. These findings suggest that sorption measurements in which only a single gas is considered are not reflective of the real conditions in coal mines. Therefore, studies on flammable gas mixtures under dynamic states have great significance in terms of fire hazard assessments. Highlights • The order of coal permeability for gases from high to low was: C 2 H 6 , C 2 H 4 , C 3 H 6 , and C 3 H 8. • Variations in sorbed flammable gas amounts were indicative of competitive sorption. • The strongest affinity was observed for propylene on the coal samples. • The gas mixture composition influenced the flow and sorption behavior of gases. [ABSTRACT FROM AUTHOR]

Abstract Coalbed methane (CBM) drilling technology is critical in achieving efficient CBM development. The geological conditions for CBM development in China are complex, including special terrain, high-rank coal areas, deep coal seams, multiple superposed coal seams and coal measure gas symbiosis areas, steeply inclined coal seam areas, and tectonic coal seam areas. Therefore, it is critical to apply the optimum drilling technology in each different area. This paper first reviews the current CBM drilling technologies and gas production behaviors in China. It is found that although vertical wells and cluster well groups are common well types for CBM development in China, U-, V- and L- shaped horizontal wells and multilateral horizontal wells have been favored for CBM development in recent years. The paper then discusses the applicability of different well types to geological conditions and proposes a modification method for well design in various geologic conditions. This method uses the coal structure, R o , in situ stress, and ratio of critical desorption pressure to the reservoir pressure as the main inputs. Highlights • The current CBM well technologies and gas production in China were reviewed. • The applicability of different well types to geological conditions was discussed. • An optimization method for well type selection was proposed. [ABSTRACT FROM AUTHOR]

Over the past decade, the exploration and development of deep coalbed methane (CBM) resources in China have attracted significant attention for their immense potential. Typically found in geological conditions marked by high formation temperature, reservoir pressure, and in-situ stress, exploring and developing these resources present a multitude of challenges. As the coal seams extend to greater depths, adsorption behavior, pore-fracture structure, and mechanical properties of deep coal seams become increasingly intricate, impacting properties such as gas content, permeability, and fracability. Despite the presence of significant free gas in deep CBM reservoirs, production remains challenging due to the extremely low permeability of coal seams. Additionally, high in-situ stress and complex mechanical parameters of deep coal seams can significantly reduce hydraulic fracturing performance, further complicating the development of deep CBM. This comprehensive review provides a detailed analysis of the geological conditions and formation property variations at different depths for CBM reservoirs, illuminating the distinctions between deep and shallow CBM reservoirs. Furthermore, this review summarizes the pilot tests and research perspectives in China to address the challenges associated with exploring and developing deep CBM resources. By raising awareness of the particularities of deep CBM and offering practical and accessible references, this review aims to contribute to the better exploration and development of these valuable resources. • Deep CBM reservoirs present complex geological features influenced by high temperature, pressure, and stress conditions. • Accurate depth definition of deep CBM resources requires thorough understanding of geological and engineering attributes. • Deep CBM wells display distinct production patterns: minimal water, initial gas peak, and stable adsorbed gas production. • "Large-scale, large-displacement, multiple-rounds" fracturing boosts gas in deep CBM wells. • Pilot tests in China confirm substantial development potential for deep CBM resources. [ABSTRACT FROM AUTHOR]

A substantial portion of global coal seam methane reserves are biogenically produced. To enhance methane production, recent research has focussed on genetic surveying of the microbial life inhabiting these coal seams. These surveys have revealed that the most abundant organisms in these microbial communities are taxa specialised in metabolism of various simple substrates, and are typically methanogens, their (presumably) syntrophic acetate-oxidising bacterial partners, and other bacterial taxa associated with biomass recycling. Although abundant, these microbes are incapable of directly accessing the vast majority of recalcitrant carbon contained within the coal geopolymer. Instead, these more abundant taxa likely rely on the low abundance but diverse taxa, known as the 'rare taxa', to transform the carbon within coal into simpler, more labile substrates. Since these rare taxa typically have low taxonomic resolution, their probable activities within the coal seam environment are difficult to infer using the standard 16S rDNA community surveys. Therefore, the present study aimed to develop a method for identifying and characterising these rare taxa using a metagenomic approach. Using this method, a near-complete genome from a Deltaproteobacteria sp. (CSMB_808; designation taken from the Coal Seam Microbiome reference set) was recovered from a metagenome sampling the Surat Basin, Australia, and comprised only 0.5% of the total 16S rDNA sequences recovered from this metagenome. Genetically, CSMB_808 appears to be a motile fatty acid and possibly alkane-degrader within the coal seam environment, and may play an important role in the coal-to-methane degradative pathways. Despite the near-completeness of the recovered genome, the taxonomic resolution of this deltaproteobacterium was unable to be clarified due to insufficient described close relatives. Together, the low taxonomic resolution, relatively low 16S rDNA abundance, and ability to degrade putative upstream metabolites that may facilitate methanogenesis in the coal seam environment, indicates that this method of analysis can assist in understanding other important but uncharacterised rare taxa of the coal seam microbiome. A clearer understanding of the function of these taxa would allow targeted strategies to enhance methane production, as well as contributing valuable knowledge of carbon cycling in the terrestrial subsurface. [ABSTRACT FROM AUTHOR]

Abstract Both fines migration and creep can damage coal permeability during coal seam gas production. This study investigates the individual and coupled effect of fines migration and creep on coal permeability. We perform three types of flow experiments on coal samples: (1) water flow with both fines migration and creep, (2) gas flow with creep and negligible fines migration, and (3) water flow with only fines migration. During the experiments involving creep, both radial and axial strains are measured. Analytical models for fines migration and creep are used to explain the permeability decrease during experiments. After stress was applied, the coal samples creep after elastic deformation. Fines migration or creep alone causes permeability reduction by about 25%, whereas their combined effect causes about a 60% reduction. Additional fines were generated due to stress loading causing additional pore blockage, which explains the greater permeability damage when both fines migration and creep are present. An expansive radial creep was observed, which is strongly correlated with permeability reduction. Existing creep model could not match the observed expansive radial creep. [ABSTRACT FROM AUTHOR]

Abstract Chemical oxidation is proposed as an effective means to react and dissolve small regions of coal in the near wellbore region, thereby raising permeability for gas flow. In this study, we investigated the effect of sodium hypochlorite (NaClO) treatment on the structure of bituminous coal (Coal B) and subbituminous coal (Coal S) separately from the Bowen and Surat basins in Queensland, Australia. Swelling and leaching tests showed that both coals swelled, dissolved and broke in 5%wt. aqueous solutions of NaClO. Coal S reacted more vigorously in 5% NaClO, with 49% mass loss and 3840 mg/L of dissolved organic carbon (DOC) measured in the oxidant filtrate, than Coal B. The Coal B mass loss in 5% NaClO was 4.5% with 430 mg/L DOC measured in the filtrate. After NaClO treatment the total accessible pore volume of Coal S particles increased from 4.6% to 6.1%, and the porosity of Coal B increased from 8.6% to 8.9%. Pore size distributions determined from mercury intrusion porosimetry (MIP) indicated that oxidation enlarged the pores in Coal S more significantly than Coal B. Scanning electron microscopy (SEM) confirmed oxygen generated large pores on the surface of Coal S particles, but there were no significant changes on Coal B. We used a microfluidic cleat flow cell (CFC) to inject NaClO into artificial channels scribed in polished samples of Coal S and Coal B, and measured an increase in the widths of the channels after NaClO treatment. The increase in channel width observed in the CFC indicated that coal solubilisation was a more dominant mechanism than coal swelling. Similarly, the channel aperture of Coal S increased more than Coal B. CFC results also showed that NaClO etched dull coal bands (inertinite-rich) more significantly than bright coal bands (vitrinite-rich), and we proposed this result was due to the greater porosity in semi-fusinite, which allowed greater penetration of NaClO in dull coal bands than in bright coal bands. The low coal rank sample (Coal S) with higher liptinite content and more oxygen content was more susceptible to oxidisation by NaClO than Coal B. [ABSTRACT FROM AUTHOR]

Abstract The permeability of coal seams is pore pressure-dependent. A number of analytical models have been proposed to investigate this problem, but many disregard a crucial factor: the anisotropy of coal. This paper is devoted to investigating the role of anisotropy in modeling the change of horizontal permeability with pressure. Analysis is conducted using a fully anisotropic model that incorporates both the anisotropies of mechanical properties and of the permeability dependence on stresses. Analytical expressions of the pressure-permeability relationship are derived in oedometric and isochoric geomechanical conditions, and validations are conducted against both laboratory and field data. Then, the roles of the anisotropy of stiffness and of permeability dependence on stresses in the permeability-change model are explored. We demonstrate that the mechanical anisotropy can be simplified to an isotropic model without introducing significant errors in prediction of pressure-permeability relationship, while neglecting the anisotropy of permeability dependence on stresses leads to considerable errors. When both anisotropy sources are disregarded, the pressure-permeability curve can be exactly reproduced by a totally isotropic material in both oedometric and isotropic conditions. However, the material properties (e.g., bulk modulus) are skewed; moreover, such an equivalent material might lead to significant errors in other geomechanical conditions. Finally, the permeability change is investigated at the reservoir scale, and the reservoir simulation results confirm the conclusions obtained from the analytical analysis. [ABSTRACT FROM AUTHOR]

Abstract Micropores (<2 nm) play an important role in coalbed methane adsorption, desorption, and diffusion. However, the mechanisms of micropore formation and evolution still need further study. This study primarily focuses on molecular structure controls on micropore evolution during coalification. CO 2 adsorption experiments were employed to characterize micropore structure in coals, and 13C nuclear magnetic resonance (13C NMR) and X-ray diffraction (XRD) were employed to characterize the molecular structures of those coal samples. The results demonstrated that micropore evolution during laboratory simulated coalification was similar to that in geo-time-scale natural maturation. This suggests that laboratory simulated coalification can be an effective method for studying micropore evolution during coalification. Micropore evolution in laboratory simulated coalification and natural maturation both exhibited a parabolic curvature with coal rank, with a minimum at ~1.4%. When vitrinite reflectance (R o) was <1.4%, micropores were mainly controlled by the aliphatic part of the coal samples, and micropore volume decreased with decreasing aliphatic functional groups. When R o varied from 1.4% to 4.0%, micropore structure was mainly controlled by aromatic parts and coal crystallite structure. In this coalification stage, micropore volume showed linear correlation with the fraction of aromatic bridgehead carbon, the fraction of protonated aromatic carbon, and the ratio of aromatic bridge carbon to aromatic peripheral carbon (X BP). Moreover, the increase in lateral sizes (L a) and the decrease in interlayer spacing (d 002) both resulted in the increase in micropore volume. In conclusion, the coal microporosity and its evolution were primarily determined by coal molecular structure. In different coalification stages, microporosity and its evolution were controlled by different sub-portions of the whole molecular structure. These findings can provide mechanistic insights of gas sorption and diffusion, as gas sorption and diffusion behaviors are simultaneously controlled by both pore structure and molecular structure. Highlights • Crystallite structure and chemical composition control micropore evolution • Micropore evolution in natural samples is similar to those matured artificially. • In low rank coals, micropore volume decreases with decreasing aliphatic structures. • In medium-high rank coal, micropores increase with increasing aromatic structures. [ABSTRACT FROM AUTHOR]

Abstract Exploration in the Beetaloo Sub-basin in Australia's Northern Territory has indicated gas-saturated, quartz-rich source rocks that are gas mature and laterally continuous over large areas. In the Sub-basin shale targets have been identified within the Velkerri Formation and the Kyalla Formation. These shales are of Mesoproterozoic age and thus significantly older than any of the North American shale plays and most other shales currently under investigation for their gas production potential. In this work, we characterise two sets of drill cutting samples from the Beetaloo Sub-basin for their composition, pore structure, and CH 4 adsorption capacity. The objective of this study is to assess the adsorption potential of these shales and investigate the properties controlling this. Measurements from other researchers are included in the analysis to determine to what degree commonly characterised properties such as bulk clay and total organic carbon (TOC) content control gas adsorption capacity on a larger scale and thus provide an indication of the range of adsorption behaviours that may be expected. The adsorption measurements are carried out at reservoir conditions - that is high pressures (up to 30 MPa) and high temperatures (up to 110 °C). The results show that the organic-rich (TOC: 3.7–6.2%) middle Velkerri shale samples have a significantly higher adsorption capacity (expressed by the Langmuir volume) than the clay-rich, organic-lean (TOC: 0.85–1.8%) lower Kyalla shale samples: 2.89–3.38 m3/t compared to 1.88–2.81 m3/t. This is in agreement with the higher average micropore volume of the middle Velkerri shale samples. Results of our analysis demonstrate that, depending on a shale's composition, different properties control gas adsorption in shale, though in the organic-rich middle Velkerri B shale samples the controlling paramters cannot be clearly determined. A positive correlation between TOC and CH 4 adsorption capacity is observed, but only up to a TOC of 4.5%. Above this point the adsorption capacity appears to decrease again. The lack of a strictly positive correlation between TOC and adsorption capacity is likely caused by other parameters affecting sorption behaviour, in particular variations in organic matter type and thermal maturity. However, analysis of a global adsorption data set of marine shales of non-differentiated maturity nevertheless indicates that, on a larger scale, the TOC content can provide a reasonable first estimate of a shale's CH 4 adsorption capacity. In the organic-lean lower Kyalla shale samples clay minerals control microporosity and CH 4 adsorption. It is the high illite/muscovite content (30–40%) in particular that is indicated to be the main contributor to microporosity and gas adsorption capacity. Results from the analysis of the global shale adsorption data set are in agreement with these findings, showing that for organic-lean shales with a TOC < 2% clay may be the primary control on CH 4 adsorption. Highlights • Beetaloo Sub-basin drill cuttings were characterised for their composition, pore structure, and CH 4 adsorption capacity • A positive correlation between TOC and CH 4 adsorption is observed for the middle Velkerri B shale up to a TOC of 4.5%. • For a set of marine shales the TOC content provides an initial indication of a shale's expected CH 4 adsorption capacity. • In the lower Kyalla shale samples clay minerals (in particular illite/muscovite) control microporosity and CH 4 adsorption. • The global shale data set indicates that in shales with a TOC < 2% clay may be the primary control on CH 4 adsorption. • Beetaloo Sub-basin drill cuttings were characterised for their composition, pore structure, and CH4 adsorption capacity • A positive correlation between TOC and CH4 adsorption is observed for the middle Velkerri B shale up to a TOC of 4.5%. • For a set of marine shales the TOC content provides an initial indication of a shale's expected CH4 adsorption capacity. • In the lower Kyalla samples clay minerals (in particular illite/muscovite) control microporosity and CH4 adsorption. • The global shale data set indicates that in shales with a TOC < 2% clay may be the primary control on CH4 adsorption. [ABSTRACT FROM AUTHOR]

Abstract The Zonguldak Basin contains the main bituminous coal reserves of Turkey (ca 1.4 Gt), and several coal seams are mined in underground coal mines within the basin. This study focuses on coal petrography, mineralogy, Rock-Eval pyrolysis, and the meso-and microporosity characteristics of the coal samples collected from several Late Carboniferous (Pennsylvanian) bituminous coal seams in the basin. Forty channel samples were collected from various coal seams in underground mines at the Armutçuk, Zonguldak, and Amasra coalfields in the Zonguldak Basin. The studied coal samples have low total moisture contents (avg. 2.3%, on as-received basis), low total S contents (avg. 0.5%, on dry basis), and relatively high ash yields (avg. 21.2%, on dry basis). The random vitrinite reflectance values (0.69–1.21%) and gross calorific values (avg. 33.0 MJ/kg, on dry, ash-free basis) indicate that the coal samples are of high-volatile C bituminous to medium-volatile bituminous coal. Minerals identified by XRD in bulk coal samples are clay minerals (mainly kaolinite and rarely illite and chlorite), quartz, and carbonates (calcite, dolomite, and siderite). In a few samples, pyrite and feldspar were also identified. The SEM-EDX analyses performed on the carbon-coated polished blocks imply that the carbonate minerals are mainly epigenetic, whereas the silicate minerals are generally syngenetic in origin. The results of low-pressure N 2 adsorption analysis indicate that the BET surface areas (1.1–5.5 m2/g) and mesopore volumes (0.003–0.011 cm3/g) vary within a wide range. The results of the low-pressure CO 2 adsorption analyses show that micropores are abundant in the coal samples; however, the BET surface areas (10.41–78.11 m2/g) and micropore volumes (0.010–0.049 cm3/g) vary widely among the three coal mines and different coal seams. Our results suggest that variations in the mesopore characteristics are mainly controlled by mineral matter, while microporosity is controlled by a combination of organic matter (maceral compositions) and coal rank of the studied coal seams. Highlights • Bituminous coal seams from mines in the Zonguldak Basin have been studied using different analytical methods. • The variations in the mesopore characteristics are mainly controlled by mineral matter. • Microporosity in the studied coal seams is mainly controlled by maceral composition, coal rank, and tectonic deformation. [ABSTRACT FROM AUTHOR]

Abstract The impact of a 120 cm thick dike on dispersed organic matter in the Triassic Marry Harbour (MH) bituminous marl in the Raanes Peninsula, Ellesmere Island, Arctic Canada, was studied using reflected light microscopy and Rock-Eval pyrolysis. Results were compared to a coal seam of similar maturity that was thermally altered by an igneous intrusion. The MH bituminous marl is mature and within the oil window (Ro = 0.94%), whereas the coal is high-volatile B bituminous rank (Ro = 0.80%). Thermal alteration impacted differently the organic-rich marl than the coal seam. Most of the heat generated by the intrusive emplacement in contact with coal is spent transforming coal to coke in the plastic layer/zone, similar to the process that coal undergoes in industrial coking operations. In contrast, the thermal alteration of sedimentary rocks occurs over longer distances as a result of the absence of a plastic layer/zone. The impact zone of the 120 cm thick igneous intrusion was greater in the MH bituminous marl, which became thermally altered (Ro = 2.34%) at 190 cm from the contact with the dike. On the contrary, the coal became thermally altered (%Ro = 5.0%) by a 240 cm alkali-basalt intrusion (twice the thickness of the MH dike) and its impact zone was only 65 cm. Heat and hot volatile matter produced by the thermal decomposition of organic matter and minerals (carbonate, dehydration of clay minerals) in contact with the igneous intrusion body is transferred through the sedimentary rock matrix by convection through fractures. This transformation is evident by the occurrence of the pyrolytic carbon at various distances in the fractured MH marl from contact. The hydrogen index (HI) shows a similar trend to that of the remaining source potential (S 2). Both parameters decrease systematically toward the dike, and go through a minimum at 20 cm from the dike contact, as the indigenous organic matter is converted to hydrocarbons or devolatilized. At the dike contact, both S 2 and HI have higher values. These trends are similar to those of the volatile elements H and Cl in the intruded coal seam. The free hydrocarbons (S 1) showed a slightly different trend than S 2 and HI. It increased initially from 300 cm and reached a maximum at about 190 cm from the dike contact before decreasing continually and reaching a minimum at 20 cm from the contact. At the dike contact, the S 1 increased slightly, similar to the HI and S 2. The presence of oil droplets trapped in carbonate concretions, in combination with the highest value of oil expelled from the source rock (based on S 1 /TOC) at distance of 190 cm from dike, is an indication of the influence of contact metamorphism on oil generation locally. Furthermore, it indicates that intrusion of igneous dikes into the bituminous MH marl resulted in an increase of organic matter maturation locally and in hydrocarbon generation. Highlights • The thermal alteration of coal and organic-rich marl differs. • Heat generated in contact with coal is consumed in the coal-coke plastic zone. • Thermal alteration in bituminous marl is 3× wider than in coal seam. • Intrusion into marl resulted in combustion and devolatilisation of organic matter. • Fracturing allowed movement of hot gases and pyrolytic carbon formation. • Intrusion into marl generated and expelled hydrocarbons locally (oil droplets, S 1). [ABSTRACT FROM AUTHOR]

Abstract The Mafangquan (MFQ) fault zone, transecting a coal seam in Jiulishan coal mine in Jiaozuo coalfield, Henan Province, China, was investigated in detail in view of coal mining safety, including its architecture, stress and permeability features and implication for coal and gas outburst. 10 boreholes have been drilled for formation testing as well as coal sample retrieval for laboratory analysis. Gas content, Protodyakonov strength, gas emission index and pore volume are tested and analysed at different distances from the MFQ fault. A fault structure that can be separated into three distinct zones, including the fault core, the mylonitized zone and the granulated/cataclastic zone, was identified. In-situ stress tests, numerical simulations and model predictions show a principal stress rotation and fault zone stress regime. It is found that the maximum principal stress has shifted to a lower angle pent to the fault zone extend direction gradually with a destress zone from fault core to the damage zone. The permeability of the fault zone has a correlation to fault zone architecture and stress regimes in the coal seam with a high permeability in the mylonitized coal zone. The coal weakened due to deformation, high pore pressure and stress superimposition have a negative impact on coal and gas outburst. In particular, the stress rotation limits the formation of hydrofractures in the case of high pore pressure and reduces fault zone strength, which may have an orthogonal relation to the mining galleries, favouring coal and gas outburst. Highlights • Coal properties in MFQ fault zone are investigated through field test and laboratory experiment. • MFQ fault zone architecture is divided according to coal properties, stress state and permeability analysis. • The link between coal and gas outburst and fault zone stress rotation, permeability structure is revealed. [ABSTRACT FROM AUTHOR]

Abstract The pressure transient technique, such as pulse decay method (PDM), has been considered an advanced approach to measure the permeability of the coalbed methane(CBM) and shale gas formations as such approach is more efficient, accurate and effective compared to that of the conventional flowrate measurement method. A constant isothermal gas compressibility factor is assumed over the testing pressure range in the experimental studies when using the PDM, which could lead to an inaccurate measured permeability. In this study, a nonlinear parabolic partial differential equation (PDE) model, taking the isothermal gas compressibility factor (gas compressibility) as a pressure dependent variable, was established to examine how pressure responses are affected by the gas compressibility variation. A novel and practical finite difference scheme was then proposed to efficiently and accurately solve the nonlinear PDE model, providing a powerful tool to numerically investigate the gas flow behavior in the CBM and shale rocks. Analytical solution for the PDE model was derived rigorously to investigate the influence of gas compressibility on the calculated permeability. Laboratory experiments were also conducted on coal permeability measurement and characteristics of the pressure pulse decay plots verified the effect of the gas compressibility on the pressure responses, especially under low pressures. Both numerical and experimental results demonstrated that the gas compressibility is a crucial parameter that should be considered for the permeability calculations in CBM and shale reservoirs. It is found that the difference in the pressure response that is predicted between the nonlinear model and the linear model with a constant gas compressibility can reach up to 41.1% at the testing pressure of 0.7 MPa. In addition, the difference in the measured permeability obtained from the two models increases as gas depletion proceeds. And also, it is suggested that both pressure changes in the upstream and downstream reservoirs should be used for permeability calculation to minimize the influence of gas compressibility on calculated permeability. This study extends the applicability of the pressure transient technique and provides an effective way to accurately measure permeability of unconventional tight gas reservoirs. Highlights • A nonlinear model is derived to represent pressure decay in time and space. • A novel finite difference scheme is proposed to solve the nonlinear model. • An in-depth comparison is conducted between the linear and nonlinear model. • Three different analytical solutions are derived for permeability calculation. • Gas compressibility is verified to play a role in measured permeability. [ABSTRACT FROM AUTHOR]

Coalbed methane has emerged as a viable natural gas resource in India since 2007. The understanding of gas genesis kinetics, storage mechanisms and the geological controls is vital for exploration and successful recovery of gas in a cost-effective manner. In this respect, a multidisciplinary analytical approach including gas content, stable isotopes ( δ 13 C 1 ), hydrocarbons distribution, reconstruction of original organic matter, chemical properties, sorption kinetics and the role of petrographic constituents have been assessed. The volatile matter content varies from 20.2 to 32.1 wt%; indicating medium volatile to high volatile bituminous rank of coal with the maximum vitrinite reflectance (VR o %) ranging from 0.63 to 0.98%. The studied coals are vitrinite rich and contains vitrinite group macerals in the range of 41–65 vol%. Dendritic micro-fractures are confined to vitrite microlithotype, and secondary mineralization helps preserve fracture connectivity. The average gas content of coal seams is 2.06 cc/g. The increase in gas content values per 100 m is about 0.38 cc/g emphasizing escape and migration of hydrocarbons during restructuring and tectonic activities in the basin. The relationship of C3/C1 and C2/C1 ratios is indicating that the hydrocarbons in desorbed gas originated from the thermogenic process. The large concentration of methane in desorbed gas and its stable isotope signatures ( δ 13 C 1  < −40‰) also indicates the thermogenic origin of gases in coal seams. Few desorbed gas samples have been found in the mixed gas region, which may be due to the influx of fresh water carrying bacteria leading to the generation of biogenic gases and subsequent mixing with the sorbed thermogenic gases. The values of S1 (0.19–0.78 mg HC/g) and S2 (10.31–25.63 mg HC/g) indicate excellent source rock potentials. The positive correlation of original hydrogen index (HI O ) with present-day hydrogen index (HI PD ) and original organic content (TOC O ) with present-day organic content (TOC PD ) suggests uniform consistency in organic carbon conversion to hydrocarbons. The gas content compared with sorption capacity reveals the undersaturation of coal seams. The experimental results summarize that the low gas content is a critical issue. Similarly, the low, quality and quantity of CH 4 may affect the exploration and recovery of methane in the long term into this basin. But, the low values of sorption time (0.12–6.65 days) signifies good diffusion characteristics that may support the recovery of gas. Finally, it is concluded that depth of occurrence, maturity and pore/cleats associated with microconstituents control the gas accumulation and transport in coal seam reservoirs of South Karanpura coalfield. [ABSTRACT FROM AUTHOR]

Peat formed in mire settings sensitively records environmental fluctuations during deposition including changes in water table or base level and accommodation. On this basis coal seams, as geologically preserved peats, can provide evidence of high-resolution paleoclimatic fluctuations as well as paleobotanical evolution through periods of peat-formation. The No. 2 and No.1 (in ascending order) thick coal seams from the Early Cretaceous Yimin Formation in the Zhalainuoer Coalfield (Hailaer Basin, NE China) are investigated using sedimentological, sequence stratigraphic and petrographic analyses to understand the evolution of their peat forming environments. These ‘single’ thick coal seams, lacking siliciclasitic partings, are well-developed in the central area of the Zhalainuoer coalfield. Petrographic analyses demonstrate that water-table or base-level fluctuations in ‘single’ seams can be revealed by a number of significant surfaces formed by various events including paludification, give-up transgressive, accommodation reversal, flooding, and exposure surfaces. These surfaces can separate the single coal into a number of “wetting-up” and “drying-up” cycles. The wetting-up cycle is characterized by a gradual upward increasing trend in the huminite:inertinite ratio and in the ash yields. In contrast, the rapid drying-up cycle is characterized by an upward-increasing trend in the inertinite-dominated coal (46% on average) that represents a phase of exposure and oxidation resulting from a falling water table. This drying-up cycle can be correlated with the scouring surface in landward parts of the basin and terrestrialization surface basinward. The No. 2 coal seam occurs in the transgressive systems tract and comprises three high-frequency depositional sequences in which each coal cycle is characterized by a gradual wetting-up cycle and ends with a rapid drying-up cycle. The No. 1 coal seam occurs in the highstand system tract and consists of several high-frequency depositional sequences in which each coal cycle is characterized by a gradual drying-up cycle and ends with a rapid wetting-up cycle. These coals could also superpose to constitute the thick coal seam in which various sequence-stratigraphic surfaces can be recognized including terrestrialization, accommodation reversal and exposure surfaces. Stratigraphic relationships between coal and clastic components in the Yimin Formation enable us to demonstrate that thick coals span the formation of several coal cycles and high-resolution boundaries, allowing us to interpret the effects of accommodation on coal seam composition. Recognition that environmental changes can be recorded by thick coals has significance for studies that incorrectly suppose that peat or coal cycles can offer high-resolution and time-invariant records of paleoclimatic fluctuations and paleobotany evolution. [ABSTRACT FROM AUTHOR]

Nuclear magnetic resonance (NMR) has been widely used to evaluate the pore size distribution (PSD) properties of coals. However, the NMR signal itself only provides a relative distribution of pore size. To calculate an absolute pore size distribution from the NMR data, the T 2 cutoff value needs to be known. Meanwhile, the T 2 cutoff value is an indicator to divide the irreducible fluid and movable fluid in porous rock and a key factor for permeability prediction. Conventionally, the T 2 cutoff value is obtained by centrifugal experiments, the process of which is complicated and time consuming, and some T 2 cutoff value prediction models are not suitable for coals with complex pore structures. Hence, a new method is needed for T 2 cutoff value prediction. In this study, we performed scanning electron microscopy (SEM), low-temperature nitrogen adsorption/desorption (LTNA) and NMR experiments on 12 coal samples. The results of SEM and LTNA reveal the complex pore structures of the coals. According to the results from centrifugal experiments, the T 2 cutoff value is in the range from 0.5–2.8 ms for subbituminous coals, whereas it is 15–32 ms for anthracite coals. We present a fractal theory based method for T 2 cutoff value prediction. Using the estimated T 2 cutoff values, we calculated the movable porosities, PSD and permeability for the selected coals. The results show that the proposed permeability model provides significantly better permeability estimation than classic (Coates and SDR) models. These methods are applicable not only for coals, but also for other unconventional gas reservoir rocks such as gas shales. [ABSTRACT FROM AUTHOR]

Microcrystalline graphite can form from coal naturally as a result of igneous intrusion, but the mechanism of alteration from anthracite to graphite remains elusive. The occurrence of microcrystalline graphite in altered coal seams is not common globally; however, several microcrystalline graphite occurrences that are associated with igneous intrusions are currently being mined commercially in China. In this study, petrographic, geochemical, and X-ray diffraction analyses of a series of Carboniferous coals with different levels of graphitization were used to study transformations in coal structure and chemical composition evolution during natural graphitization. The graphitized coals were collected from mines located at varying distances from a large (~130 km 2 in extent) Indosinian-period granite intrusion; the coals in this region have been highly altered by this intrusion. Mean random reflectance of the samples increases from 4.36% to 8.23% approaching the intrusion, but decreases to 4.58% in the most graphitized samples. Vitrinite and inertinite become difficult to distinguish under white light with increased coal rank, and newly formed components including pyrolytic carbon, needle graphite, and flake graphite are seen in the most graphitized samples. The pyrolytic carbon accumulated from a vapor phase that was likely generated during intrusion. The needle and flake graphite occur as fracture and void fills and probably represent graphitization of a mobile phase that migrated through the coal. Timing of generation and the source of the mobile phase is unclear, but predates graphitization and could have been associated with the intrusion event. Microcrystalline graphite, with a characteristic texture consisting of fine granular particles, is the dominant component in the most graphitized samples. Reflectance of microcrystalline graphite using standard coal procedures does not accurately reflect its rank. Approaching the intrusion, structural parameters show a progressive change, especially in highly graphitized coals. Samples closest to the intrusion have transformed into graphite (as confirmed by X-ray diffraction and geochemical data). A plot of volatile matter (VM) versus R r suggests that the maturation pathway for graphitized coals differs from that of coals that have undergone normal burial maturation. Elemental changes in graphitized coals also differ from coals that have been intruded by smaller-scale sills and dikes. This is probably due to the extreme level of metamorphism associated with the large-scale intrusion that graphitized these coals. [ABSTRACT FROM AUTHOR]

This coupled geomechanical-fluid flow numerical modelling study investigates the geomechanical impact of depressurization of coal beds on fault and coal seam geomechanical stability during coal seam gas production, using a simplified 3D reservoir structure based on the Gloucester Basin, Eastern Australia. The model examines transient stress, pore pressure, and fluid flow patterns during gas production under three different background regional stress scenarios, with the aim of understanding possible geomechanical failure states in a fault and coal beds. The model results show that depressurization leads to shear stress increase in normal-faulting stress regimes, but decreases in reverse-faulting (dominant in eastern Australia) and strike-slip faulting stress regimes. This is in combination with an overall effective stress increase associated with pore pressure reduction. Depressurization-resultant fault failure appears to be possible only under normal-stress regimes. In addition, a critically-stressed fault segment is likely to be a precursor requirement for fault reactivation to occur as a result of depressurization/fluid depletion within normal-faulting stress conditions. Results further show fluid flow is dominated by lateral flow along coal beds, converging towards depressurization locations (“wells”). Fault permeability governs fluid transport along/across the fault and development of pore pressure compartmentalisation across the fault. Under all three background stress scenarios, flow velocities in coal beds are governed by coal permeability. [ABSTRACT FROM AUTHOR]

As one of the important unconventional resources, coalbed methane (CBM) can mitigate energy shortage issue, and its efficient exploitation has received widespread attention globally. The development of CBM is of great significance to coal mine safety and energy supply. CBM reserve evaluation provides a basis for selecting development zones and determining development strategies. However, most existing CBM reserve evaluation methods do not consider the effects of dissolved gas, free gas and the difference between initial reservoir pressure and critical desorption pressure. Thus, the predicted results usually deviate from the actual reserve. In this paper, firstly, the material balance equation (MBE) for early dewatering stage considering the effect of stress sensitivity on porosity is established, and the initial free gas and dissolved gas reserves of undersaturated CBM reservoirs can be obtained. Secondly, the MBE for gas desorption stage is derived, in which the effects of stress sensitivity, matrix shrinkage and dissolved gas are considered. So the original gas in place (OGIP) of CBM reservoirs can be solved. Next, the correctness and rationality of MBEs for early dewatering stage and gas desorption stage are verified against King's MBE method and CBM dynamic analysis software. Finally, this method is applied to actual production wells. The results show that in early dewatering stage, 1 − S wi α c p + α S wi c w p ¯ Z ¯ − p i − p ¯ p ¯ Z ¯ and W p B w − W e α p ¯ Z ¯ + G p p sc T Z sc T sc have a linear shape, and the control area of CBM reservoir can be calculated based on the slope of the straight line. In addition, the ratio of the intercept to the slope of the straight line can be used to calculate the initial free gas and dissolved gas reserves of undersaturated CBM reservoirs. During gas desorption stage, p / Z * and G p have a linear relationship. OGIP of CBM reservoirs can be obtained by the ratio of y -intercept to the slope of the straight line. Using gas and water production data provided by CBM dynamic analysis software, the reserves of undersaturated CBM reservoirs evaluated by the proposed method are in good agreement with those from CBM dynamic analysis software, which proves that the proposed material balance equations and corresponding methods are reasonable and reliable. The material balance equations and methods presented in this paper take into account the effects of various factors such as the difference between initial reservoir pressure and critical desorption pressure, pore compressibility, water compressibility, coal matrix shrinkage, dissolved gas, and free gas. The proposed reserve calculation methods for undersaturated CBM reservoirs can provide an important basis for selecting dominant production area, determining well spacing and guiding development policy. [ABSTRACT FROM AUTHOR]

Apart from implications related to coal seam gas (CSG) reservoir engineering such as stress- and desorption- dependent permeability, 2-phase flow of gas and water adds further complications to rate-transient analysis (RTA) techniques in CSG reservoirs. This paper introduces the concept of area of influence (AI) to calculate water saturation in transient formation linear flow for 2-phase RTA in hydraulically fractured CSG wells. The relevant equations are derived and discussed and the application of the AI is demonstrated using reservoir simulation. The area of influence allows a superior estimation of water saturation compared to the dynamic drainage area (DDA) in under-saturated CSG reservoirs and hence improves the estimation of fracture half-length. A practical workflow is also developed and presented to estimate reservoir and hydraulic fracture properties using 2-phase RTA for hydraulically fractured vertical wells. The presented workflow works in a reverse order by first analysing boundary-dominated flow regime and then proceeds to other existing flow regimes developed at earlier production history of the well. Although analysis of each flow regime provides a standalone method to calculate some of reservoir properties, the full development of all flow regimes allows crosschecking and brings more confidence into calculation. In this workflow, the reservoir properties derived from later flow regimes, feeds earlier flow regimes to extract other reservoir variables such as fracture half-length. A hydraulically fractured CSG well from the Ordos Basin, China, is analysed using AI and the proposed workflow to demonstrate the field application of this study. [ABSTRACT FROM AUTHOR]

Coal seam gas (CSG) or coal bed methane production has grown exponentially in Australia over the last two decades with currently nearly 7000 productive wells and another 18,750 wells planned over the next five to ten years. To address concerns over environmental contamination by hydraulic fracture fluids (HFF) attributed to (i) incidents involving poor surface handling of HFF and other fluids with potential migration pathways in soil and shallow groundwater, and (ii) leakage pathways arising from HFF injection into coal seams and unintended migration of these chemicals, the National assessment of chemicals associated with coal seam gas extraction in Australia (the Assessment ) was undertaken. The Assessment included a review of international studies on unconventional gas migration pathways to provide prior knowledge and comparison for use in the Australian study. The international literature highlighted that accidents with HFF were mainly due to surface operations and included leaking equipment and spills. Contamination risks of groundwater from hydraulic fracturing itself were found to be very small based on analysis of micro-seismic and groundwater hydrochemistry data; geologic modelling of vertical fracture growth further reveals a low risk of leakage pathways developing in aquifers confined by deep shale formations. This is due to limitations on the fracture growth vertically across aquitards often hundreds of meters thick, retention within the shale of limited amounts of injected fluid, and preferential fracture growth at shallow depths in the horizontal direction. These arguments are broadly applicable to Australian CSG basins. However, important differences exist, such as the reduced thickness in Australia of the aquitards that separate the targeted gas resource from groundwater. Also, hydraulic fracturing in Australian coal seams is practised only when permeabilities are too low for gas extraction to be economical without stimulation. From the nearly 10,000 CSG wells drilled in the Bowen and Surat Basins in Queensland, only 6% have received hydraulic fracturing to date. The reduced risk of leakage pathways developing during CSG production in Australian basins is further corroborated by geologic criteria based on modelling and observations. Criteria include encountering certain favourable conditions during drilling/injection, i.e. (i) interfaces, such as natural fractures, faults, and bedding planes, which promote offsetting and branching, or rock layers with higher fracture toughness causing the cessation or reduction in fracture growth, (ii) narrow fractures due to a relatively higher elastic modulus, leading to higher viscous flow loss, (iii) high permeability layers, and (iv) overlying layer(s) with a higher confining stress. In addition to the leakage pathways considered in North American shale gas basins involving (i) pre-existing fractures and faults, or (ii) flow up the production well due to poor casing or cementing, plausible chemical migration pathways in Australian CSG basins are due to (iii) connectivity via a fracture in or extended out of the coal seam formation between CSG wells and water bores co-located in the same formation, and (iv) poor integrity water bores enhancing inter-aquifer connectivity. These plausible pathways are described and used in subsequent work as the basis of risk assessments for contamination at the local- and regional scale. [ABSTRACT FROM AUTHOR]

In this study, carbon-13 cross-polarization magic-angle-spinning solid-state nuclear magnetic resonance ( 13 C CP-MAS SS NMR) was undertaken on a parent coal sample and its density-fractionated derivatives, an inertinite-rich and a vitrinite-rich sample, obtained from the Witbank Coalfield of South Africa (medium rank C bituminous coal). The formation of inertinite macerals has been extensively researched and the conclusions remain largely controversial. However, most research has been confined to northern hemisphere coals, which are typically dominated by vitrinite. South African coals are widely known for their high inertinite content. Earlier workers have ascribed the inertinite macerals, in their diversity, to aerial oxidation. However, this oxidation-only pathway fails to recognize that inertinite macerals can form through other processes such as charring of plant matter. Microscopically, charred matter possesses anatomical structures that closely resembles those observed in inertinite macerals, with lignin and/or cell walls largely preserved, perhaps mechanically fragmented or compressed in some instances. Based on the NMR structural parameters, the average aromatic cluster size in the inertinite-rich sample (petrographically dominated by fusinite, semifusinite of varying reflectance, and inertodetrinite) corresponds to monocyclic, 6-aromatic carbon rings. In the vitrinite-rich sample of the same coal, the cluster sizes are larger, corresponding to multi-ring aromatic hydrocarbons. The 6-carbon rings in the inertinite-rich sample are interpreted to correspond to guaiacol and syringol, the principal products of low-temperature (below 400 °C) lignin pyrolysis. Since charring depletes cellulose and moisture, fire-affected plant matter is interpreted to have a lower compaction potential than vitrinite-forming material, sustaining 6-aromatic carbon ring isolation in the former. Lignin-derived aromatic rings in compactable plant matter merge to form the multi-ring clusters present in the collotelinite- and collodetrinite-rich sample of the same coal. [ABSTRACT FROM AUTHOR]

Coal horizons in terrestrial sedimentary sections are key to understanding the structural and thermal evolution of sedimentary basins. Precise and accurate age information for coal horizons can define correlations and the temporal evolution of sedimentary successions. While the biostratigraphy for coal horizons yields relative ages, tied to radiometric ages through the continuously refined geological time scale, direct radiometric ages of coal horizons have been derived only from tonsteins, lithified ash beds. The Re-Os geochronometer has been proven to record accurate and precise depositional ages for sedimentary rocks rich in organic matter. However, the reliability of the Re-Os geochronometer has not yet been assessed for coals deposited in strictly terrestrial environments. Here, in order to test the Re-Os geochronometer, we measure the Re, Os concentrations and isotopic composition of terrestrial coals from two distinct environments: the Finnmark Platform, southern Barents Sea and the Central Appalachian Basin (CAB). Samples from the Finnmark platform (Soldogg Formation) also include an adjacent organic-rich shale horizon. The shale and two coal horizons were also analysed for total organic carbon (TOC) and Rock-Eval data. Results show that the Soldogg coals and shale reached the oil generation widow (T max  = 448 ± 2 °C), with Type III or mixed Type II/III organic matter, supporting the model for deposition in overbank deposits in a fluvial setting. In contrast, the Fire Clay coals in the CAB were deposited in a broad expanse of mires. Average Re (0.5 ± 0.4 ng/g) and Os (23 ± 26 pg/g) concentrations of terrestrial Soldogg and Fire Clay coals are similar to those of terrestrial Maghara coal and a few orders of magnitude lower than the marine-influenced Matewan coal. Elevated Re/Os* ratio (Os* = Common Os; Os corrected for ingrowth of 187 Os since deposition of the sample) for the Fire Clay coals suggests highly reducing and/or euxinic conditions prevailing in the water column of the extensive Carboniferous peat-forming mire in which these coals were deposited. In contrast, lower Re/Os* for Soldogg coals highlight less reducing conditions, likely in a short-lived mire in a dynamic braided stream system. Re and Os* concentrations of terrestrial coals are on the order of Re and Os of plant materials and river sediments, suggesting that debris from plant materials and weathered continental crust are significant contributors of Re and Os to terrestrial coals, with some contribution from water column of the coal mire. This study is the first attempt to apply the Re-Os geochronometer on terrestrial coals. In case of the Soldogg coals and shale, the 187 Re/ 188 Os vs. 187 Os/ 188 Os relationship displays scatter with no discernible linear trend. The scatter is ascribed to heterogeneity in the initial 187 Os/ 188 Os ratios (Os i ) and lack of post-depositional isotopic equilibration during subsequent diagenesis. For the Fire Clay coals, 187 Os/ 188 Os and 187 Re/ 188 Os are well correlated; linear regression of the data, however, does not yield a geologically reasonable age. Rather, the linear relationship results from binary mixing between two end-members: The Fire Clay coal and tonstein, suggesting that Re-Os systematics of terrestrial coals depositing very close to tonstein beds can be significantly disturbed. We conclude that the post-depositional equilibration with a large Re-Os reservoir, as with a marine incursion, is necessary to record a meaningful Re-Os radiometric age for the coals. [ABSTRACT FROM AUTHOR]

Microbially enhanced coalbed methane has become an important research topic in the later years. The biological conversion of coal to methane can be conceived as a feasible and environmental friendly approach for improving coalbed methane production. Within the strategies for stimulation of gas production, the addition of a microbial consortium or bioaugmentation can be seen as a promising alternative. However, relatively few studies have been conducted on the strategies for enriching microbial population ex-situ under initial atmospheric exposure for subsequent injection into coal seams to stimulate biodegradation and methanogenesis. The development of methanogenic microbial consortia, especially those that can tolerate moderate and low oxygen concentrations and still retain anaerobic functionality, can be considered as an attractive biological complement for coal biogasification. The performance of promising microbial consortia was evaluated at low concentrations of nutrient amendments (e.g., 22.4% v/v, 3.36 mg/cm 3 TSB) and [NaCl] 6.6 mg/cm3 as a possible scenario and to foresee the elevated costs of nutrient utilization at large-scale operations (i.e., in-situ and/or ex-situ applications). Incubation periods of up to four months were tested at 23 °C. Headspace concentrations of CH 4 and CO 2 were analyzed by gas chromatography. After 61 days of incubation for the most promising microbial samples, generated headspace gas concentrations reached 95,700 ppm (14 sft 3 /ton) for methane and 37,560 ppm (5.5 sft 3 /ton) for carbon dioxide. Microbial diversity in promising consortia was investigated. Both bacteria and archaea were identified. [ABSTRACT FROM AUTHOR]

The study was performed on 4 different coals of varied rank. Pyrolysis was conducted on coals of different ranks in gold capsules, with the yield and stable carbon isotopic values of selected gas components and liquid hydrocarbons separately measured to investigate the hydrocarbon generation chemistry, isotopic compositions, and kinetic parameters in coal. With increasing heating temperature, the methane concentrations and total gaseous hydrocarbons increase consistently with temperature. The liquid hydrocarbon concentrations first increase to maximum values and then decrease with temperature. Coals of higher ranks are more likely to primarily produce methane, while coal with low maturity is more likely to produce wet gases. The compositional differences of the four coals become similar at the late stage of the pyrolysis experiment, with a methane content >95%. All of the experiments show a similar isotope trend δ 13 C 1  < δ 13 C 2  < δ 13 C 3 at the same temperature. The δ 13 C values of ethane and propane became less negative with increasing temperature for all four experiments. The gap between δ 13 C 1 and δ 13 C 2 is greater than that between δ 13 C 2 and δ 13 C 3 . There is a linear relationship between ln(C 1 /C 2 ) versus ln(C 2 /C 3 ). When ln(C 2 /C 3 ) increases sharply as ln(C 1 /C 2 ) increases, the decomposition rates exceed the generation rates for C 4   +5 , C 3 , and C 2 as secondary oil cracking occurs. At high temperatures, both parameters are larger. The activation energy distribution of 48–72 kcal/mol for Yilan coal is slightly higher than 55–66 kcal/mol for Xinjing coal, but significantly higher than the distribution of 46–61 kcal/mol for Qinan coal and 51–67 kcal/mol for Shitai coal. [ABSTRACT FROM AUTHOR]

Carbon geo-sequestration (CGS) and recovery enhancement with carbon dioxide injection (CO 2 -ECBM) have brought increasing focus on the CO 2 and water wettability of coal. The CO 2 and water contact angles measured using existing conventional methods, such as the pendent drop tilting plate technique and the captive-bubble technique, show low reproducibility due to coal heterogeneity and operational complexity. In this study, a novel NMR-based approach was developed to evaluate the CO 2 and water wettability of coal. The experimental results of nine bituminous and anthracite coals show that water wettability can be linearly correlated with the changes of the T 2 spectra peak positions. Based on the measured contact angle from the profile of the water adhering to the coal powder disc and the T 2 from the NMR of coal powder, we proposed a linear formulation to evaluate the contact angle using the change of T 2g of P3. Using this method, we analyzed the CO 2 -water wettability characteristics of coal. The results show that CO 2 reduces the water wettability of coal. Low temperature and/or high CO 2 pressure can enhance the CO 2 wettability of coal. The change of water-coal wetting behavior with injection of CO 2 , is resulted by three factors: change of CO 2 adsorption capacity of coal, change of interfacial tension, and dissolution of CO 2 in water. This study makes it possible to evaluate changes in the water and CO 2 wettability of coal, which is essential for evaluating the fluid-interaction mechanisms during the process of carbon geo-sequestration and enhanced coalbed methane recovery with carbon dioxide injection. [ABSTRACT FROM AUTHOR]

Building upon the previous modelling work on coalbed methane reservoirs by the authors, recently reported experimental horizontal stress data for two coal samples tested under uniaxial strain conditions have been analysed. As well as CH 4 , non-adsorbing Helium has also been used in the experiments. Deviation between the model and the experimental CH 4 stress data at low pore pressures (< 3 MPa) are observed, which, as in the previous study, may be attributed to the onset of shear failure experienced by the coal samples. In addition, CO 2 was also used on one of the samples after the CH 4 experiment. The post-failure CO 2 stress data could be matched using a combination of a reduced Young's Modulus and an increased Poisson's ratio. The results of this study have provided further evidence for the validity of the equation for computing the changes in the effective horizontal stress in producing coalbed reservoirs, which forms an integral part of the Shi and Durucan permeability model. [ABSTRACT FROM AUTHOR]

Gas management at longwall faces has always been a challenging issue to mine operators. Along a longwall face of an Australian colliery where CO 2 is the dominant coal seam gas, frequent power trip off occurred due to the high goaf gas (CO 2 ) emission, which significantly affected the normal longwall production and brought about safety threats to the longwall crew. CO 2 fringe fluctuation changes close to tailgate (TG) was identified as a major concern. To better understand the CO 2 fringe behaviour on the longwall face and develop the corresponding effective control measures, Computational Fluid Dynamics (CFD) model was developed and validated based upon field data collected from the colliery. General gas flow characteristics on the longwall face were obtained based on which parametric studies were then carried out to investigate the impact of ventilation system and gas drainage options on goaf gas fringe behaviour. The use of a back-return ventilation system was demonstrated to be an effective approach to control the CO 2 fringe at TG; however, it was not practical due to the restriction of existing panel layout. As a compromise of the back-return system, a new gas drainage option using TG borehole which was more practical and cost effective was proposed and assessed by the CFD models. Model results indicated that goaf gas fringe at TG can be effectively controlled when a suction pressure between − 1500 Pa and − 2000 Pa was applied to the TG borehole. Therefore, it can be concluded from this study that gas drainage conducted through TG borehole can be an effective approach to solve the CO 2 accumulation at TG on a longwall face, especially when the back-return system is not applicable on site. [ABSTRACT FROM AUTHOR]

Lignite from the Konin area (Poland) was used as a substrate for incubation experiments to evaluate its potential for simulation of biogenic methane production. Lignite was incubated with a bacterial inoculum enriched from lake sediments, mineral media, and various supplemental components for microbial life. Additives, such as acetate, methanol, glucose, nutrient broth, and yeast extract, can significantly increase methane production. At the same time, biodegradation of these additional carbon sources leads to overestimation of methane yield. In this paper, selected geochemical properties (total organic carbon content; stable isotopic composition of carbon δ 13 C) were analyzed in order to evaluate changes in the organic matter of fermented lignite. Stable isotope analysis of carbon and hydrogen was applied in order to identify sources of methane and carbon dioxide formation. TOC decreased in range from 1.4 to 9.6% in lignite after fermentation. The δ 13 C value of lignite used in the experiments (−25.2‰) decreased after incubation to values in range from −27.1 to −26.2‰. Methane yield per g of TOC (lignite + organic carbon in nutrients) ranged from 0.47 to 2.60 mM/g. Glucose, acetate, and methanol significantly increased biogas production. Nutrient broth and yeast extract were not a source of organic carbon for methane formation, but their presence enhanced biogas production. Values of δ 13 C(CH 4 ) and δ 2 H(CH 4 ) across incubation conditions ranged from −70.2 to −24.2‰ and from −396.6 to −290.5‰, respectively. Values of δ 13 C(CO 2 ) ranged from –55.2 to 45.0‰. The high level of variation of δ 13 C(CH 4 ) and δ 13 C(CO 2 ) suggests mixing of gases from different carbon sources during incubation, but can also be caused by mixing of metabolic modes by the microbial community. Understanding the δ 2 H(CH 4 ) variation is even more difficult than the δ 13 C(CH 4 ) and δ 13 C(CO 2 ), since many substrates often contain exchangeable hydrogen (e.g. in water, lignite, and elements of nutrients). The combined values of δ 13 C(CH 4 ) and δ 13 C(CO 2 ) suggests that the dominant methanogenesis pathway in our experiments may be acetate fermentation. In biodegraded lignite, high relative concentrations of p -cresol (one of the most abundant in the sample) and n -(2-acetylphenyl)formamide were identified. These compounds are most probably lignin decomposition products, or, in the case of the latter, bacterial by-products or remnants. Organic compounds with low molecular weights, n -alkanes, and biomolecules including ferruginol, sugiol, and 6,7-dehydroferruginol, as well as amyrins and tocopherols, were preferentially degraded. The potential for methane production from lignite spiked with carbon-bearing additives is at least one magnitude lower than that from agricultural wastes. The lignite utilization as the single substrate for methanogenic fermentation is economically unprofitable. Mixing of lignite with the external substrate as biomass may be an alternative for consideration and future research. [ABSTRACT FROM AUTHOR]

Microbial generation of methane from organic matter in coal represents an opportunity to improve environmental and economic outcomes of coal seam gas production. This has prompted research interest in microbial communities responsible for its generation. While advances have been made in understanding the overall structure of these communities and in methods to stimulate methane generation through nutrient additions, many questions still remain as to the metabolic roles played by the majority of the taxa in coal seams. One confounding issue faced by the coal seam microbiology community is an inability to compare OTUs (Operational Taxonomic Units) from 16S rDNA community surveys between studies. The aim of this study is to provide a common language for researchers studying coal seam microbes so that any given OTU from a coal seam community can be readily referenced to OTUs from other studies. To meet this aim a new Coal Seam MicroBiome (CSMB) reference set of OTU sequences was constructed from both new data obtained through community surveys of 28 coal seam formation water samples across three eastern Australian coal basins, along with all compatible, publicly available community surveys. To illustrate the power of the CSMB set for enabling comparisons the set was used to compare the co-occurrence of OTUs from publicly available compatible coal seam community surveys. Additionally, 16S rDNA sequence data from all compatible coal seam isolates and/or reconstructed genomes from published studies were mapped to the CSMB set. Resultant analysis revealed that a large number of published surveys have shared CSMB OTUs and a number of isolate and reconstructed genomes matched CSMB OTUs observed in surveys of the coal seam formation waters across the USA, Canada, China, India, Japan and Australia. This report details these observations of co-occurrence, as well as providing information on downloading, mapping and adding new sequences to the CSMB set. [ABSTRACT FROM AUTHOR]

Igneous intrusions can bring hydrothermal fluids, metals, and ligands to the surrounding host sequences, changing their physical, textural and chemical features to some extent depending on the degree of thermal exposure. However, regarding hydrothermally-altered coals, previous studies have predominately focused on variations in coal quality, chemistry and mineralogy, despite the growing research on stable and radioactive isotopes within coal-bearing sequences. The Pennsylvanian coal from the Daqingshan Coalfield, Inner Mongolia, China, was thermally affected by a granitic intrusion in the eastern part of the coalfield during the Yanshanian Orogeny, between the Late Jurassic and Early Cretaceous. The investigated Hailiushu, Datanhao, and Adaohai mines in present study are distributed in Daqingshan Coalfield from west to east. The Adaohai Mine is located to the southwest of the intrusion point, at a distance of approximately 50 km. The Hailiushu and Adaohai mines are about10 km apart, and the Datanhao Mine is located between them. With decreasing distance to the intrusion, there is an increase in vitrinite reflectance values (from 0.84% through 1.17% to 1.58%) and a decrease in the volatile matter content (from 44.57% through 31.31% to 19.60%) in the studied three coal mines. The present study revealed that the westernmost Hailiushu coals have the most negative δ 13C org values (from −24.8 ‰ to −23.5 ‰; −24.3 ‰ on average). In contrast, with the distance from the intrusion decreases, the δ 13C org values of coals from the Datanhao mine range from −24.8 ‰ to −23.5 ‰ (−24.2 ‰ on average). The δ 13C org values of the easternmost Adaohai coals range from −23.8 ‰ to −22.8 ‰, (−23.3 ‰ on average), about 1 ‰ less negative than those in the Datanhao and Hailiushu coals. It is not just the maceral composition that causes the difference in organic carbon isotopes, but the loss of 12C-enriched volatiles due to the thermal effects associated with the igneous intrusion. This hypothesis was further supported by Rock-Eval analysis on the organic matter: the average T max of the Hailiushu, Datanhao, and Adaohai coals increases but generation of hydrocarbons and pyrolysable carbon decrease as approaching the intrusion. Moreover, fracture/cleat-filling carbonates are more abundant in the Adaohai coals than in the Datanhao and Hailiushu coals. The δ 13C PDB (from −14.2 ‰ to 1.7 ‰) and δ 18O PDB (from −16.9 ‰ to −7.0 ‰) of these vein carbonates all indicate a hydrothermal origin. Hydrothermal fluids derived from igneous intrusions is also responsible for the elevated 87Sr/86Sr ratios (0.711392–0.717643) in the carbonate mineral in the thermally altered coals. • δ 13C org values are less negative in coals closer to the igneous intrusion. • The closer the coal to the intrusion, the less pyrolyzable the organic matter. • C O isotopes show that the coal seam was intensely hydrothermally-altered. • High 87Sr/86Sr values for carbonates indicate an igneous rock origin for the Sr. [ABSTRACT FROM AUTHOR]

Stable carbon isotopic compositions of natural gas have long been employed to determine its origin, accumulation, and migration. A reversal of the natural gas carbon isotope sequence, i.e. δ 13C 1 > δ 13C 2 > δ 13C 3 , was discovered in the tight sandstone gas (TSG) of the Shixi gas field at the eastern margin of the Ordos Basin. This paper presents a study to determine the causes of carbon isotopic reversal hence to better understand the gas accumulation in these tight sandstone reservoirs. The molecular composition and carbon isotopes on 81 wellhead gas samples were measured from the TSG reservoirs of the Pennsylvanian in the Shixi gas field. Results indicate that the molecular compositions of the Shixi gas field comprise methane ranging from 88.9 wt% to 98.9 wt%, ethane ranging from 0.49 wt% to 1.12 wt%, and minor propane with content <0.01 wt%. The non-hydrocarbons are mostly CO 2 which is <1.2 wt% and the rare helium ranges from 0.02 wt% to 0.23 wt% with an average of 0.09 wt%. The CO 2 was probably derived from the decomposition of carbonate sediments under a high temperature owing to the varied δ 13C CO2. Hydrocarbon is considered predominantly from coal-derived humic source rocks and minor oil-type gas. The isotopes of CO 2 , helium and vitrinite reflectance of the source rocks, i.e. coal and carbonaceous mudstone, indicate that the study area is lacking conditions for the formation of biogenic gas. The carbon isotopic reversal with δ 13C 1 > δ 13C 2 > δ 13C 3 in the Shixi gas field was interpreted as a result of secondary cracking under a local high temperature >200 °C. [ABSTRACT FROM AUTHOR]