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• A novel approach for modelling subsidence induced by fluid extraction from coal seams. • Transient groundwater and subsidence models are linked. • Coal shrinkage induced by gas desorption can significantly impact subsidence. Several coal seams are contained within the formations of Great Artesian Basin (GAB), the largest natural underground water reservoir in Australia and the world. The extraction of coal seam gas (CSG) and its associated water from thousands of wells within the coal measures of the GAB has led to tens of millimetres of subsidence in CSG development areas. Since highly developed farming systems located in these areas rely on very low slope landforms, even this scale of subsidence has caused significant community concern about the potential for CSG extraction to impair farming operations and productivity through changes in land slope and drainage. Coal seam compaction associated with dewatering and gas extraction has two key components: poromechanical compaction and desorption-induced bulk shrinkage. The former results from pore pressure depletion (due to dewatering and gas extraction) and an increase of vertical effective stress, while the latter is induced by gas desorption from the coal matrix, leading to further deformation. While poromechanical compaction of fluid-bearing formations has been extensively addressed in the literature, relatively little research has been conducted on the role of coal shrinkage in CSG-induced subsidence. This paper introduces an innovative practical modelling approach for assessing CSG-induced subsidence at a subregional/regional scale. The approach utilises an analytical model for CSG-induced subsidence derived from constitutive stress–strain relations for poroelastic-sorptive media. This is a novel approach in the context of CSG-induced subsidence which considers both poromechanical compaction and desorption-induced shrinkage. A further distinction to previous work is the integration of the geomechanical-sorptive subsidence model with a numerical groundwater model. Based on this approach, this paper examines the effect of coal shrinkage on subsidence, and its proportions with respect to total compaction for one of the major coal measures in the Surat Basin. Input data are derived from three-dimensional geological, geomechanical, and groundwater models, as well as methane adsorption and desorption reports. Results show that (a) the impact of coal shrinkage on CSG-induced subsidence is likely to be significant in the study area and (b) the contribution of coal shrinkage to CSG-induced subsidence depends on gas content, Langmuir isotherm, shrinkage strain parameters, and the saturation state of coal. This study provides important insights into CSG-induced subsidence and lays the foundation for the development of robust, efficient, and localised predictive models to support environmental impact assessment and management. [ABSTRACT FROM AUTHOR]

In Australia, coal seam gas (CSG) could help to meet growing demands for cleaner alternatives to coal, thus making it important for the energy transition. By developing such an industry, not only does an energy resource become available, but large water volumes are also extracted, especially in the first stages of production. The purpose of this paper is to analyze the impacts of a large-scale coal seam gas industry on the landscape. Specifically, the research intends to explore how coal seam gas has changed the landscape of the agricultural production area in Western Downs, Queensland. In this Australian State, the water associated with coal seam gas production is considered an asset and the legislation in force encourages beneficial use of such resources. Comparison of aerial pictures highlights how, in a short timeframe, the local rural landscape has changed considerably due the extensive development of unconventional gas resources. The coal seam gas industry has provided the host region with opportunities for local development, but the issue of the sustainability of these new mining and non-mining activities still needs to be properly addressed. • Coal seam gas-associated water is potentially a valuable commodity. • In Queensland, the coal seam gas development footprint is quite extensive. • Landscape changes are related both to gas and CSG-associated water extraction. • Unsteady amounts of CSG-associated water pose challenges to the agricultural sector. [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]

This paper reviews recent developments on the cleat-scale characterisation of coal. Novel micro-CT imaging and image calibration methods are described. The application of micro-CT imaging for studying diffusion processes in ultralow permeability media is shown. The extraction of statistical information from micro-CT images to reconstruct cleats are demonstrated. The developments of microfluidic methods for understanding complex displacement mechanisms in coal seams and variation of coal contact angles are described. We explain numerical methods for prediction of petrophysical properties from micro-CT images and discuss limitations when dealing with coal. The paper concludes by addressing the challenges for microscale coal characterisation. [ABSTRACT FROM AUTHOR]

There is an ever growing demand for energy worldwide and the demand for gas alone is predicted to double between 2010 and 2035. This demand together with concurrent advances in drilling technologies caused the production of unconventional natural gas such as shale gas and coal seam gas (CSG), which is in the focus of this paper, to grow rapidly in the last decades. With the gas bearing coal seams extending across vast areas within their respective basins and with CSG production having to follow these seams through a network of production wells, pipelines and access roads, CSG activity affects large areas and therefore interferes with existing land uses, predominantly agriculture. For the eastern Australian Surat Basin and the southern Bowen Basin alone there are projected well numbers in excess of 15,000 to 20,000 between the years 2020 and 2030. The interference of CSG with agriculture on a large scale has raised concerns about the impact of CSG on farmland, food security, water resources and the socio-economic environment within the affected regions and beyond. This paper presents a newly developed spatial model which provides order of magnitude figures of the impact of CSG activity on gross economic returns of current agricultural land uses in a given region over the time of CSG production. The estimated gross figures do not account for any compensation payments received by farmers. The model is capable of accounting for a variation in a variety of parameters including impact frequency of distinct infrastructure elements, differences in soil types and associated varying responses of soil productivity, varying length of the CSG production phase and more. The model is flexible in that it can be transferred and applied in other regions as well. Based upon a literature review and given that CSG is an industry that started operating at larger scales relatively recently, we claim that the presented model is the first of its kind to provide these important agro-economic indicators. [ABSTRACT FROM AUTHOR]

The Surat Basin Coal Seam Gas (CSG) play requires a vast number of wells to develop, and reservoir productivity changes quickly over small distances. Well testing provides a means to evaluate these changes. The high cost of conventional testing precludes sufficient tests to ensure areal and statistical coverage across a large well stock. This paper discusses a new approach, with a reduced cost, that allows five times more wells to be tested than conventional methods. By testing more wells, there is more data for subsurface model calibration and productivity fairways can be better mapped and developed. In the Surat Basin CSG play there is an industry-wide completion technique that uses air to lift every well to size the initial downhole pump for dewatering. This paper describes an enhancement to this standard pump sizing test which facilitates the provision of key reservoir information: full well permeability, skin and productivity. This novel well test is termed an Air Assisted Flow Test (AAFT). During the AAFT the well is produced at a predictable drawdown for up to an hour whilst accurate rate measurements are made at surface. A downhole shut-in is then performed and a pressure gauge measures the build-up pressure response. The additional equipment and rig time have been designed to add a minimal cost to the standard completion operation and test cost is 20% of a traditional Drill Stem Test (DST). Field tests using the AAFT and multiple dual packer DST's in the same well clearly demonstrate that both testing methodologies deliver the same results. It is therefore valid to incorporate the AAFT into the extensive permeability database for the Surat Basin. Additionally drawdown dependent skin has been observed in data from the new AAFT. This phenomenon is interpreted to be the result of stress dependent coal fracture permeability in the near well bore region. This new testing method is only directly relevant to CSG developments. However the awareness to gather additional data, from enhancing a standard operation, is applicable to all unconventional and conventional reservoirs. [ABSTRACT FROM AUTHOR]

• Demonstrated capability of three-phase foam flow in dewatering unconventional wells. • Examined flow regimes of foam flows in vertical tubing and annulus. • Examined the effect of pressure on dynamic and static foam flow characteristics. • Tested the effect of salt and particles on foam stability. This paper aims to investigate and demonstrate, for the first time, the feasibility of using an artificial lift system, referred to as foam assisted gas lift (FAGL) for dewatering unconventional gas wells to depressurise coal and allow gas to be produced. More specifically, this paper investigates the capability of a three-phase foam flow to lift gas, liquid and solid particulates to the surface to optimise gas production by minimising the flowing bottom-hole pressure (FBHP) and minimising downtime, operational costs and production losses caused by downhole pump failures. A laboratory set-up of a 9 m vertical tubing with 80 mm (3.1 in) ID and a 7 m annulus with a casing inner diameter of 170 mm (6.69 in) and tubing outer diameter of 70 mm (2.76 in) are used to investigate the foam flow behaviour in both geometries for a wide range of gas and water flowrates, 40–6500 L/min and 10–80 L/min, respectively. The 80 mm tubing is pressurised up to 6 bar, allowing examining the foam flow characteristics and foam stability at different depths of a gas well, by changing the operating pressure. Foam flow regimes in the tubing and the annulus, which are key in determining the FBHP are characterised by analysing differential pressure signals, collected at 100 Hz frequency. The addition of foamer (sodium dodecylbenzene sulfonate) significantly advanced the transition from churn to the annular flow regime, resulting in a lower required gas flow rate to obtain the least attainable FBHP. A model is developed to evaluate the FBHP of the FAGL, compared against the measured FBHP in 36 gas wells in Australia, equipped with down-hole pumps for dewatering. Our results indicate that FAGL is capable of maintaining a low FBHP for a wide range of gas and water flow rates. The effective particle lifting capacity of foam flow is also demonstrated. Further, the positive impact of water salinity (0.1 M NaCl) in the presence of particles (15–25 g/l) on foam integrity is presented. [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 The rate of gas production from coal seam gas reservoirs is highly dependent on coal lithotype, cleat attributes and multiphase flow in cleats. In this paper, we explore and categorize the wettability, roughness data, sorption capacity, and the flow in bright and dull coal lithotypes. The roughness measured on the wall surface of coal fractures indicates different values for each lithotype. Multiphase flow through rough-walled fractures is measured and the pressure variation and relative permeability data are compared for samples from bright and dull bands. Finally, microfluidic experiments are conducted to visualize and analyze the flow in the bright and dull bands. A flow scenario is investigated in which gas is transported through coal regions that have parallel and adjacent lithotypes. It is visualized that the production from these regions can be 53% less than the production from dull band regions. Herein, the impact of coal lithotypes properties on displacement efficiency in coal seams is discussed. Highlights • Microscale properties of coal lithotypes are measured and compared. • Roughness values observed on wall surface of coal fractures are analyzed. • Relative permeability data for dull and bright band coals are estimated. • Microfluidics are utilized to investigate displacement in coal different bands. [ABSTRACT FROM AUTHOR]

Coal seam gas (CSG) is an unconventional resource drawing attention globally and is considered to play a crucial role in the future natural gas market. In CSG reservoirs, the characterisation of fracture aperture sizes is critical since coal permeability and wellbore performance are highly dependent on this parameter. To optimise gas recovery and predict gas production rate, it is important to analyse reservoir properties at in situ conditions. By using a high-pressure flow cell and X-ray microcomputed tomography (micro-CT) imaging, we visualise how the internal structure of coal changes under prevailing reservoir conditions. Important differences in the size of fractures can be seen from images of the coal sample obtained at different pressure. As anticipated, aperture size was found to decrease with increasing confining stress from atmospheric to reservoir pressures and it was more sensitive to the change of initial pressure. Similar conclusions were drawn for both gas and Klinkenberg-corrected permeability using helium and methane gases. In the permeability experiments, methane gas is used to mimick in situ conditions whereas helium is used for comparison as a non-adsorbing gas. There was less change in fracture aperture size if the ends of a coal cleat are sealed by minerals. We successfully applied micro-CT imaging on coal samples under high confining stress. The coal sample studied has a relatively large diameter compared to the common core sizes used for high-pressure imaging. Overall, this paper demonstrates a quantitative analysis of coal permeability at reservoir conditions, which provides a better understanding of deformation and further insights into enhanced CSG exploration and development techniques. [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]

Low permeability renders a significant fraction of coal seam gas (CSG) resources sub-economic. An effective permeability enhancement strategy is thereby crucial in monetising a large proportion of low permeability CSG resources. This paper introduces the concept of using oxidants for permeability enhancement, describes a practical screening method to evaluate potential oxidants and provides knowledge about the coal behaviour in oxidants. A test based on time-lapse photography and image analysis of coal particles immersed in liquid oxidants was used to assess the extent and rate of change of coal particle size. Complementary leaching tests determined the extent of coal solubilisation by quantifying the change in coal mass and leachate organic content. The swelling profiles of coal particles hand-picked from a low permeability CSG coal core (Bowen Basin, Australia) were first examined in solutions of potassium chloride, and then pyridine for the purpose of method development and validation. Finally, the swelling ratio, S R , and rate of swelling S r (% area change per 6 h), of coal particles immersed in oxidising solutions of sodium hypochlorite (0.1%, 1% and 10% NaClO), potassium permanganate (0.015%, 0.03%, 0.1%, 1%, 3% and 5% KMnO 4 ), hydrogen peroxide (1%, 3%, 10% and 30% H 2 O 2 ) and potassium persulfate (1% and 3% K 2 S 2 O 8 ) were examined. Results provide evidence for coal solubilisation (maximum mass loss = 15%) and the propensity to swell (maximum particle size increase = 15%) in all the candidate oxidant stimulants as well as coal breakage in specific oxidants and at specific concentrations (1% NaClO and 3%, 5% KMnO 4 ). The swelling and solubilisation of the coals used in this study tends to increase with higher oxidant concentrations. Anisotropic swelling was also clearly observed in 1% NaClO. Coal reacted vigorously with NaClO and KMnO 4 , but only slightly with K 2 S 2 O 8 and H 2 O 2 . Massive coal solubilisation occurs in NaClO and KMnO 4 , but negligible in K 2 S 2 O 8 and H 2 O 2 . In terms of coal oxidation to enhance permeability, NaClO and KMnO 4 seem to be more promising than K 2 S 2 O 8 and H 2 O 2 . For the situation of in situ application, it remains unclear if the net effect of coal swelling and coal solubilisation will increase or decrease permeability. In addition, coal breakage may lead to void space or new cracks in the coal matrix, which could have the potential to increase the coal permeability. Confined core-flooding tests that simulate in situ conditions are required to elucidate this behaviour. [ABSTRACT FROM AUTHOR]

Seam gas pre-drainage, is widely used as an effective method to control gas and coal outburst in underground coal mines. However, in CO 2 abundant low permeable seams, this technology seems to be less efficient due to the CO 2 sorption characteristics and the lower safe mining threshold limit for CO 2 applied in many outburst risk management plans. This paper presents the experimental investigations of the applicability of nitrogen (N 2 ) injection to improve gas drainage in CO 2 rich seams. Core specimens were obtained from Bulli coal seam, Sydney Basin and N 2 flood tests were conducted under different permeability conditions (0.3 mD and 0.06 mD). A triaxial permeability test rig equipped with a back pressure regulator was used to conduct the test. The variation of gas composition, gas outlet flow rate, N 2 flushing efficiency, and permeability variation were analyzed. In addition, a comparative study between N 2 flushing coal seam CO 2 and N 2 -ECBM was conducted and it was observed that CO 2 was much more difficult to drain out and with longer time compared with CH 4 . Based on the test results, a two-stage flushing mechanism was obtained. In the first stage, the original free phase coal seam gas accounted for the large percentage and this stage last for a shorter time. In the second stage, desorption time governed the flushing efficiency and desorbed gas was the primary gas. It took much longer time than the first stage. The final recovery rate in 0.3 mD and 0.06 mD scenarios were 90.5% and 87.7%, and the N 2 consumption/CO 2 production ratios were 15 and 11.2, respectively. Through these laboratory experiments, we concluded that N 2 injection can significantly decrease coal seam CO 2 content level and increase gas drainage efficiency. [ABSTRACT FROM AUTHOR]

Coal is highly heterogeneous at multiple scales, such that the characterisation of coal lags behind that of conventional reservoir rocks. This paper demonstrates the capability of a digital coal model that can be used to characterise the multiscale structure of coal as well as its petrophysical properties at the core scale (cm). X-ray micro-computed tomography (micro-CT) is applied to obtain the internal cleat structure of a heterogeneous fractured coal sample, based on which statistics of coal lithotype distributions and cleat geometrical properties are extracted. Digital coal models are constructed stochastically according to the measured statistics. Resulting models preserve the core scale (cm) heterogeneity and pore scale (µm to mm) cleat properties of the imaged coal sample. Furthermore, petrophysical evaluation is performed based on the stochastic digital coal models. We find that the generated digital coal models can provide permeability estimation with errors of 26.7% in comparison to that of the original micro-CT data. Since the presented model is able to preserve the multiscale heterogeneities as well as petrophysical properties of coal, our work provides an alternative to segmented micro-CT images. Therefore, we can avoid the challenges that are inherent to micro-CT images, such as segmentation errors, size and resolution limitations. The digital coal model can also be a tool of linking the rock microstructure to petrophysical properties at the core scale, such that we can predict the permeability of coal based on its geometrical measures rather than expensive laboratory experiments. [ABSTRACT FROM AUTHOR]

The disposal of CO 2 is a key issue for greenhouse gas control. We apply a reactive transport model based on lattice Boltzmann and finite volume methods to simulate the injection of CO 2 -saturated brine into a fractured coal. Fluid flow, transport of solute and dissolution are modelled at the pore scale. Fracture closure under overburden pressure is considered in the numerical model by performing mineral detachment identification and aperture adjustment. Variations of permeability and porosity are investigated. Mechanical properties are calculated to study the effect of mineral dissolution on formation stability. The results show that injection of CO 2 causes fracture demineralisation. However, the removal of minerals may result in closure of existing fractures increasing resistance to fluid flow. This paper helps improve understanding of CO 2 injection into coal seams and discusses its potential effect on coal properties. [ABSTRACT FROM AUTHOR]

Sustainable and clean production of natural gas is of paramount importance in transitioning to a low-emission economy. Gas production in an unconventional well such as a coal seam gas (CSG) relies on continuous dewatering, commonly performed through a progressive cavity pump (PCP). PCPs are susceptible to failure for different reasons, primarily the interference of solid particles. This results in interruptions, pump replacement, well cleaning and recommissioning which pose long downtimes (i.e., reduced well deliverability), work-over at substantial costs, along with environmental impacts such as increased methane emissions to the atmosphere. This paper aims to demonstrate and assess the performance and suitability of a sustainable dewatering technique, referred to as foam-assisted gas lift (FAGL) which offers uninterrupted dewatering, minimal downtime and minimised environmental impacts. FAGL is capable of maintaining the gas well deliverability by replacing the downhole pump with a gas compressor at the surface and an engineered gas-liquid-solid flow dynamics by injecting foamers and a small amount of surplus gas into the wellbore. The performance of FAGL is evaluated in 45 vertical CSG wells in Australia under different operating conditions and depths ranging between 400 and 1050 m and is compared with well data using PCPs. The flowing bottomhole pressure (FBHP) is calculated to assess the suitability of FAGL under different operating conditions. The results from this study show that FAGL outperforms PCPs in more than 50% of the cases under this study, offering on average 34% lower FBHP. The FAGL's capability in lifting solid particles is demonstrated using solids taken from gas wells in Queensland, showing a reliable and continuous lift of particles. In addition, chemical defoaming of the foam at the surface is demonstrated to avoid re-foaming in the downstream and mitigate the risk of contamination of the surrounding area. • Evaluated the FAGL performance compared with 45 gas wells equipped with PCPs. • FAGL outperformed PCPs in 50% of the case studies offering 11%–77% reduction in FBHP. • FAGL is capable of lifting of solids with more than 20 wt%. • A defoamer can rapidly break down the foam and mitigate re-foaming at the surface. [ABSTRACT FROM AUTHOR]

Coal seam gas (CSG), an unconventional resource of energy, is gaining global interests due to its natural abundance and environmental benefits in comparison to more traditional energy sources. Its production is mainly controlled by the underlying fracture network of coal, called “cleats”. The discrete fracture network (DFN) model is widely applied for characterising fracture networks due to its ability of accounting for physical geological parameters of cleats. However, common discrete fracture network (DFN) models fail to preserve the local heterogeneity by assuming planar and smooth cleat surfaces, while real cleats have rough walls and variable opening apertures. This paper aims to characterise the roughness of cleat surfaces at the core scale by developing a novel framework: rough-walled discrete fracture network (RW-DFN) model. The model integrates the pore-scale roughness obtained from micro-computed tomography (micro-CT) imaging of coals into the discrete representation of fracture networks. Analysis of the fracture surfaces obtained from micro-CT imaging demonstrates random, isotropic surfaces following a Gaussian distribution. RW-DFN gives lower permeability than that of the traditional DFN by up to 30%, and its permeability estimation is more accurate with significantly fewer errors (6.5%) than traditional ones (25.1%). This indicates that, to be able to characterise these reservoirs, traditional DFN models may over-estimate the production while our proposed RW-DFNs provide more deterministic results. Overall, the method applies micro-CT imaging to obtain the internal surfaces of coal fractures in a non-destructive manner and reconstruct representative RW-DFN models. The developed RW-DFN models are not restricted by the imaging resolution, so that they are favourable for direct numerical simulation of permeability. In addition, RW-DFN models can be constructed with extended domain size, so they can be incorporated into existing reservoir characterisation frameworks for the prediction of coal properties. [ABSTRACT FROM AUTHOR]

Coal seam gas (CSG) is gaining global interests due to its natural abundance and environmental benefits in comparison to more traditional energy sources. However, due to its significant heterogeneity and complex porous structure, it is challenging to characterise and thus predict petrophysical properties. Moreover, the fracture network of coal poses a major challenge for direct numerical simulations on segmented images collected from X-ray micro-computed tomography (μCT). The segmentation of coal images is problematic and often results in misclassification of coal features that subsequently causes numerical instabilities. This paper aims to develop an advanced image analysis method and a novel discrete fracture network model to circumvent these issues. Coal μCT data are utilised for the acquisition of structural parameters and then discrete fracture networks are built to reconstruct representative coal images. The modelling method mimics the cleat formation process and reproduces particular cleat network patterns. The reconstructed network preserves the key attributes of coal, i.e. connectivity and cleat structure, while not being limited in terms of size and/or resolution. Furthermore, direct numerical simulations based on lattice Boltzmann method are performed on the cleat network realisations to evaluate coal permeability. We find that directional permeabilities result in different system scaling effects because of the dependence on the underlying structure of the cleat network. The developed method facilitates the evaluation of the relationship between coal cleat structure and resulting flow properties, which are steps forward in the evaluation of coal petrophysical properties at the core scale. [ABSTRACT FROM AUTHOR]

The demand for natural gas in Queensland, Australia has historically been supplied from conventional reservoirs. However, depletion in conventional sources has led producers to turn to extensive supplies in Queensland's coal resources. These coal seam gas (CSG) developments not only represent new supplies for the domestic market in eastern Australia, they are also the first time that CSG (aka coal bed methane or CBM) has been liquefied to serve the expanding world LNG market. In order to make this development occur, considerable infrastructure had to be installed, with field developments still on-going. This AUD$60 billion investment precipitated a major overhaul of state regulations to provide not only a safe and clean operating environment, but also to allay the concerns of certain stakeholders. The gas is primarily produced from thin high permeability coals in the Jurassic-age Walloon Coal Measures in the Surat Basin and from several relatively thick Permian-age coal seams in the Bowen Basin, of which the Baralaba Coal Measures and the Bandanna formation are the most important. There are numerous technical challenges with this production, such as fines production from the inter-burden clays, which can form a thick paste that is difficult to pump. Salt extraction by reverse osmosis, from associated water produced to depressurise the coal seams and enable the flow of gas, allows for the beneficial use of the water. Technical challenges also include mathematical modelling of the counter-current two-phase flow (gas and water) in the well annuli because conventional models in simulators only handle co-current two-phase flow in the well-bores. Also, the subject of on-going investigations is decommissioning of the large number of shallow wells over the next few decades in a safe and cost effective manner, with compressed bentonite being a promising option for well plugging. As with any major commercial development, in addition to the technical challenges there have been social challenges as well. These include interaction and coexistence of extensive surface operations with an established agricultural sector, interactions between gas production and ground water aquifers in water-stressed areas, and the cumulative social and economic impacts of 3 large projects on a rural area. Ultimately, the State of Queensland expects to produce more than 1800 BCF/annum, of which about 1400 BCF/annum will be exported as LNG. Depending on the demand and well productivity, up to 1000 CSG wells may be drilled per year for the next thirty years. A review of CSG resources, development, and challenges is presented in this paper to provide context for a stream of research findings that are emerging on the Queensland CSG experience. [ABSTRACT FROM AUTHOR]

The ability of coal to absorb gas is one of the most important factors affecting the prevention and control of mine gas-related disasters and safe exploitation of coal seam gas. In this paper, high gas content coal samples from the Pingdingshan mining area are selected as the research object. The micro molecular structure of coal is altered by pyrolysis, combined with experimental methods such as infrared spectroscopy, isothermal adsorption and liquid nitrogen adsorption. The evolution characteristics of functional groups of coal during high temperature pyrolysis and the variation of gas adsorption capacity of coal after pyrolysis at different temperatures were studied from a micro level. The results show that when the pyrolysis temperature increased from 20 °C to 750 °C, the methane adsorption capacity of coal samples decreased with the increase of pyrolysis temperature. C C, –OH, C O, –CH 2 in coal molecules gradually decreased. In terms of the effect of functional groups and the pore structure on methane adsorption, the methane adsorption capacity in descending order is: C C > –OH > C O > –CH 2. The results of this study provide theoretical guidance for the development of new technologies for the development of coalbed methane thermal injection mining technology. • Discover the evolution of functional groups and pore structure in coal under pyrolysis. • Understand the variation of methane adsorption capacity by modifying the molecular structure of coal under pyrolysis. • Establish the correlation between coal microstructure and methane adsorption capacity via macro and microscopic tests. [ABSTRACT FROM AUTHOR]

Coal seam gas or coal bed methane production is becoming a significant industry in Australia. The area around the Broke township (located in the north-west of the Sydney Basin) has had coal seam gas exploration activities over the last decade. One methodology of well stimulation, hydraulic fracturing, has the potential to cause some environmental problems. The Broke region includes 4 aquifers (3 confined and semi-confined), more than 3 major coal seams (3 gas bearing) and has been covered by variety of different sedimentary rocks of Permian age. The groundwater system, with the gas bearing deposits is the function of conductivity and storativity (K and S) of the coal seam and geometry of properties of the fractures. The required data to investigate the hydraulic properties in the fractured zone includes; corehole data to assessment the geology and underground system, micro-seismic data to infer the fracture properties, pumping tests results, and monitoring wells data to evaluate the hydraulic properties of the coal seams. This paper investigates the shape of drawdown curves, resulting from the hydraulic pumping tests at Broke performed to determine the hydraulic and fracture properties, such as length, width, conductivity of fractures, and the proportion of the water and gas contained in the targeted coal seam. In order to reduce data uncertainty and increase the reliability of the hydraulic property estimation the Generalized Likelihood Uncertainty Estimation (GLUE) will be applied. Finally, with investigation of the shape (length and width) and conductivity of the fractures, the best monitoring method to ensure public safety of provide assurances that if problems occur the monitoring system will provide early warning for this study area will be determined. [ABSTRACT FROM AUTHOR]

Rapid global expansion of unconventional natural gas development (UNGD) raises environmental health concerns. Many studies present information on these concerns, yet the strength of epidemiological evidence remains tenuous. This paper is a review of the strength of evidence in scientific reporting of environmental hazards from UNGD activities associated with adverse human health outcomes. Studies were drawn from peer-reviewed and grey literature following a systematic search. Five databases were searched for studies published from January 1995 through March 2014 using key search terms relevant to environmental health. Studies were screened, ranked and then reviewed according to the strength of the evidence presented on adverse environmental health outcomes associated with UNGD. The initial searches yielded > 1000 studies, but this was reduced to 109 relevant studies after the ranking process. Only seven studies were considered highly relevant based on strength of evidence. Articles spanned several relevant topics, but most focussed on impacts on typical environmental media, such as water and air, with much of the health impacts inferred rather than evidenced. Additionally, the majority of studies focussed on short-term, rather than long-term, health impacts, which is expected considering the timeframe of UNGD; therefore, very few studies examined health outcomes with longer latencies such as cancer or developmental outcomes. Current scientific evidence for UNGD that demonstrates associations between adverse health outcomes directly with environmental health hazards resulting from UNGD activities generally lacks methodological rigour. Importantly, however, there is also no evidence to rule out such health impacts. While the current evidence in the scientific research reporting leaves questions unanswered about the actual environmental health impacts, public health concerns remain intense. This is a clear gap in the scientific knowledge that requires urgent attention. [ABSTRACT FROM AUTHOR]

The sustainable in situ regeneration of microbial (biogenic) methane or microbially enhanced coal bed methane (‘MECoM’) is an emerging concept being investigated globally. Promising results and recommendations of a preliminary culture study of Walloon Subgroup co-produced coal seam gas (CSG) waters from the Surat Basin, Queensland established the presence of viable methanogens and suggested that in situ methanogenesis could be stimulated using physical and chemical reservoir treatments. This paper represents the culmination of a stepwise basin analysis project ultimately aimed at siting potential in situ bioreactor locations in the eastern Surat Basin. The integration of stratigraphically located data on the molecular and isotopic composition of desorbed gases, host coal properties and spatially associated waters through the core production zone has allowed the spatial variability and relative influence of hydro-geological factors on methanogenesis to be evaluated in detail for the first time. Higher gas contents and systematically enriched CH 4 and CO 2 carbon isotopic compositions in the stratigraphically central coal seams (oldest-youngest: in the upper Taroom Coal Measures, Tangalooma Sandstone lower Juandah Coal Measures) are best explained by increased rates of microbial CO 2 reduction and substrate depletion. There is a building case to trial MECoM in the central coal seams in a depleted/underperforming well in an area of high permeability. Integrated microbiological, chemical engineering, hydro-chemical and geological studies are ongoing to further enhance understanding of Walloon Subgroup CSG and the bioreactor potential of the Surat Basin. [ABSTRACT FROM AUTHOR]

Coal seam gas zonation in the Sydney Basin, NSW, Australia is related to basin hydrodynamics and hydrochemical facies evolution along the flow path from the subcrop to the basin center. Biogenic methane corresponds with meteoric water under hydrostatic pressure and persists down to the top of the geopressured zone (~ 800 to 1000 m). Thermogenic gases, including wet hydrocarbons, can reach up to relatively shallow horizons of less than 500–600 m depth. In the transition zone between the top of the geopressured and base of the hydrostatic zone, a mixed water and gas regime prevails, comprising brackish waters, and gases of mixed biogenic, thermogenic and inorganic origins, including CO 2 . Mechanisms for and the role of stress in the development of this layered hydrogeological and gas environment are investigated in this paper. The inverse relationship between effective horizontal stress and permeability in coals through regulation of cleat volumes is well documented, and there is evidence of regionally compartmentalized stress regimes with depth within the Sydney and other eastern Australian coal basins. This regional stress regime can be overprinted by the effect of localized geological features. It is hypothesized that the in situ stress regime plays an important role in the regulation of groundwater flow regimes and extents, resulting in the development of the reported gas content and compositional zonation. Analysis of regional gas and stress data obtained from public and private databases, as well as literature, supports this hypothesis. Changes in gas concentration and composition with depth correspond with discernable variations in horizontal stress anisotropy. Gas contents generally increase with depth down to a ‘peak gas’ horizon, below which concentrations decrease. This ‘peak gas’ zone is coincident with a horizontal stress anisotropy change from moderately high to low levels, associated with reverse to strike–slip faulting conditions, respectively. The stress release zone also marks the top of the thermogenic gas zone, identified by the first appearance of ethane in the vertical profile. This zone also hosts gases of mixed origins: biogenic, thermogenic and inorganic (CO 2 ) and represents a mixed (transitional) groundwater flow environment. The base of the mixed gas zone is the top of the ‘geopressured-only’ flow associated with thermogenic gases and is signaled by the return to high stress reverse faulting conditions below 850–900 m depth in the Sydney Basin. [ABSTRACT FROM AUTHOR]

This paper introduces the development of a new predictive model in support of proppant injection in naturally fractured coal seam gas (CSG) reservoirs. In the proposed model, the finite element method (FEM) is used for the prediction of proppant embedment and elastoplastic deformation of the coal. The lattice Boltzmann method (LBM) is applied to the modelling of fluid flow through propped fractures, in which the modified partially saturated method (MPSM) is implemented to characterise the fluid–solid interactions. Permeability diagrams of the fractures are then generated as functions of particle packing ratio and effective stress. Finally, these results are incorporated into a radial Darcy flow analytical solution to predict the productivity index of the CSG wells under various proppant injection pressures and cleat compressibilities. The developed model is applied to selected coal samples from the Bowen and Surat Basins in Queensland, Australia. Modelling results indicate that proppant injection leads to increased fracture permeabilities and enhanced well productivity indexes. The elastoplastic deformation results in smaller permeability increase and less production enhancement when compared to the traditional linear elastic models. Although focused on Australian coals, the developed workflow can be broadly applicable to the assessment of potential stimulation efficacy in other unconventional reservoirs. In addition, a better understanding and implementation of the proppant injection scheme can effectively improve the post-fracturing performance, particularly in low-permeability coal intervals, which benefits the CSG industry. • Elastoplasticity is incorporated into computational models of proppant embedment. • The deformation of idealised coal cleats is assessed for residual permeability. • Optimal monolayer packing is determined for selected Bowen and Surat Basin coals. • Resulting fracture permeaibilities used in well productivity sensitivity analyses. [ABSTRACT FROM AUTHOR]

• A novel hybrid Fracture-Micropore Network Model (FM-PNM) is developed for coal. • FM-PNM is able to model multiphysics gas flow in both coal matrix and fractures. • The model can be applied for CSG simulations and predicting GHG emission of coal mines. Coal is not only a combustible black sedimentary rock, but also a source and reservoir rock for natural gas, known as coal seam gas (CSG). A thorough understanding of gas transport through coal matrix micropores as well as coal cleats is of significance in both mining and CSG industries. Multiple physical mechanisms are identified during gas flow in coal. However, there is not a comprehensive modelling framework, where multiscale flow behaviours with multiphysics are coupled. In this paper, we develop a hybrid Fracture-MicroPore Network Model (FM-PNM), coupling viscous gas flow in fractures and gas diffusion in coal matrix. This model includes multiphysics gas flow mechanisms in coal, including gas sorption, diffusion, slip flow, and compressibility. With FM-PNM, methane flowing through fractured coal is simulated to compute apparent permeability and gas flow rates, which are critical for CSG developments. Besides, the process of gas emission from coal over time is simulated, where desorption curves are obtained and analysed at different scenarios. This multiphysics FM-PNM gives a promising framework for studying gas flow in fractured coal, which can be integrated with other works to study different coal internal structures, flow models and stress conditions. This model has a wide range of applications, including prediction of greenhouse gas (GHG) emission of coal mines, ventilation design during mining, CSG development, and CO 2 storage management. [ABSTRACT FROM AUTHOR]

• Coal mining and coal seam gas developments can have important hydrological impacts. • There is large uncertainty associated with the magnitude of these impacts. • There is potential for cumulative impacts from multiple developments in proximity. • A cumulative impact assessment approach is hereby presented and applied. This paper presents an assessment of the hydrological impacts of proposed coal mines and coal seam gas (CSG) developments in the Gloucester, Hunter, Namoi and Galilee subregions of eastern Australia. Surface water and groundwater modelling was carried out to assess the cumulative regional-scale hydrological impacts of multiple coal and coal seam gas developments. The hydrological metrics assessed were groundwater drawdown and changes in low-flow characteristics as measured by the number of low-flow or zero flow days per year. Uncertainty in the modelling was also considered, and the results are presented as the median and plausible range (5th to 95th percentile) from many simulations which attempt to capture a wide range of uncertainty. The results indicate that under the best-case (5th percentile) scenario, the impact of additional/proposed coal resource developments on groundwater drawdown and surface water characteristics can be quite small. However, under the worse-case (95th percentile), groundwater drawdown is very likely for up to 5 km from the coal resource developments, but unlikely beyond 20 km. Altered surface water characteristics can extend for many kilometres downstream, particularly in unregulated systems. The cumulative hydrological impact is enhanced where there are multiple developments in close proximity. The results are being used to inform policy and the approval process for coal resource developments. [ABSTRACT FROM AUTHOR]

• A new method introduced for fast estimation of permeability of fractured coal images. • Fracture Pipe Network Model (FPNM) extracted directly from micro-CT images of coal. • Accuracy and computation time of FPNM is checked against direct simulations. In coal seam gas (CSG) reservoirs, the underlying fracture networks play a critical role in controlling flow pathways of methane and water phases. Thus, study of flow in fracture networks via different numerical modelling methods is of importance. Compared to direct flow simulation, the Fracture Pipe Network Model (FPNM) is an effective means of modelling fluid flow in a fracture network due to its computational efficiency. However, constructing FPNM for authentic samples is challenging due to the limited fracture data at the core scale. In addition, application of FPNM for coal is challenging due to its complex internal fracture network. With X-ray micro-computed tomography (micro-CT) imaging, internal fracture network of coal samples can be visualised non-destructively, providing deterministic input data for FPNM. However, analysing fractures individually and characterising network connectivity based on micro-CT images are difficult, due to complex interactions, irregular fracture geometry, resolution limitation and image noises. In this paper, we develop a framework for constructing image-based FPNM for real fractured coal cores for fast prediction of permeability. Compared with conventional FPNM, each pipe element of our improved FPNM has the identical properties (orientations, length, aperture and roughness) with its corresponding data from the real sample. By comparing permeability values obtained from FPNMs, micro-CT images and voxelised DFNs, we conclude that FPNM can effectively estimate the permeability of original fracture networks while requiring significantly lower computational cost that make it a suitable framework for expensive multi-phase multiphysics flow simulations. [ABSTRACT FROM AUTHOR]

Coal seam gas (CSG) reservoirs typically have low rock strength. During gas production, pressure depletion and matrix shrinkage may cause the differential stress around the wellbore to exceed the coal mechanical strength and result in rock failure. Coal failure has several detrimental consequences including coal fines production, permeability reduction, wellbore filling and damage to pumps and compressors. The matrix shrinkage causes a unique stress path in CSG reservoirs. However, the details of how matrix shrinkage affects coal failure still have remained uncertain. This paper presents a new workflow to evaluate stress distribution around CSG wells and predicts coal failure by coupling the effects of pressure depletion, matrix shrinkage and wellbore simultaneously. The model calculates Maximum Coal Free Drawdown Pressure (MCFDP) by considering the effects of all contributing parameters and Mogi-Coulomb failure criterion. Data from a vertical well in the San Juan Basin in USA were used to evaluate the validity of the developed model. The developed model was applied to evaluate coal failure under three different stress regimes. The results indicate that pressure depletion and matrix shrinkage have a significant effect on coal failure in all stress regimes. In the case of a normal stress regime, it is found that vertical wellbores are the most stable during the life of a reservoir. However, in the case of strike-slip and reverse stress regimes, pressure depletion and matrix shrinkage could cause the change of stress regime and therefore, the optimum wellbore trajectory could change. Additionally, it is found that in the normal stress regime the depletion and matrix shrinkage reduces the MCFDP of horizontal wellbores more than the vertical wells. However, in the reverse stress regime, depletion and shrinkage cause more reduction of MCFDP in vertical wellbores compared to horizontal wells. [ABSTRACT FROM AUTHOR]

In-situ stress pattern and subsurface fractures control the production of Coal Seam Gas (CSG) reservoirs and, hence, comprehensive analyses are required for better development of these reservoirs. The Surat Basin is a prolific CSG province with major economic impacts in eastern Australia. Recent studies have reported significant stress variations in this basin due to local geological structures. However, a systematic analysis of localised stress perturbations within the basin, and their impacts on reservoir permeability have not been presented thus far in the published literature. In this paper, we combine different sets of data including conventional log data, wellbore image logs, 2D seismic lines and interpreted permeability (from well test data) in order to characterise the role of geological structures in the subsurface fluid flow of the Walloon Coal Measures in eastern Surat Basin. Analysis of 7.3 km of borehole image log across 33 vertical wells revealed a regional ENE-WSW orientation of the maximum horizontal stress (S Hmax) in the study area. However, there are massive variability in the S Hmax orientation, spatially and with depth. Analysis of 2D seismic lines highlight the role of geological structures in stress perturbation, and the interpretation of natural fractures using image logs show that most of the open fractures are oriented sub-parallel to the acute angle of local S Hmax orientation. Interpreted well test data in the study wells demonstrates that most of the high permeability data points are located where in-situ stress rotation is observed. This indicates the influence of local S Hmax orientation responding to geological structures on the subsurface fluid flow in the eastern part of the Surat Basin. • Variability of S Hmax orientation laterally and with depth. • Small scale geological structures control the local in-situ stress pattern. • Open fractures are oriented sub-parallel to the acute angle of local S Hmax orientation. • Most of the high permeability data points are located where in-situ stress rotation is observed. [ABSTRACT FROM AUTHOR]

Many potential coalbeds have sub-economic permeability for coal seam gas (CSG) extraction even though all the other key characteristics (gas content, thickness etc.) can meet the requirement for successful production. To enhance coal seam permeability various CSG stimulation techniques, including hydraulic fracturing, cavity well completions and horizontal wells, have been used, but none is ubiquitously successful. The potential of chemical stimulation including both acid (1% HCl) and oxidant (1% NaClO) stimulation are examined in this paper as an alternative method to enhance coal seam permeability. Acid stimulation targets the removal of certain minerals in coal cleats, while the oxidant dissolves the coal matrix, thereby etching pre-existing coal cleats and perhaps forming new ones. The permeability variation during stimulations are measured with core flooding tests and the corresponding coal structural changes are examined using X-ray microcomputed tomography (μCT) technique. The chemical reaction mechanisms are confirmed by elemental analysis via inductively coupled plasma optical emission spectroscopy (ICP-OES) and dissolved organic carbon (DOC) in the effluent. Acid stimulation was performed horizontally with cube samples cut from coal cores (Bowen Basin, Australia) and it exhibited a positive effect on coal permeability, which is proposed to result from demineralisation as shown by before and after CT scanning observations together with high calcium (Ca) content in the effluent. NaClO oxidation was able to etch pre-existing coal cleat surfaces, widen cleat apertures and generate new horizontal fractures and/or void space. These changes were caused primarily by chemical attack, confirmed by the high DOC concentration in the effluent. Oxidant stimulation caused a decrease in vertical permeability and an increase in horizontal permeability for some cores. The decrease in permeability is proposed to be due to a combination of increased void space weakening the coal associated with movement of the coal into newly created space against confining pressure. Minerals in the cleats appear to play a role keeping void space open and preventing collapse under confining pressure. Furthermore, the results for these samples indicate that NaClO oxidation appears to be lithotype independent and preferentially attacks coal sections that have more initial pores or fractures. • Acid stimulation could enhance horizontal coal permeability through demineralisation. • NaClO oxidation could widen coal cleat apertures and create horizontal fractures. • Oxidant may increase permeability where the mineralogy acts as a support. • NaClO oxidation is lithotype independent and attacks the porous coal section. [ABSTRACT FROM AUTHOR]

Unconventional energy such as coal seam gas is trapped in a low porosity/permeability environment and poses challenges for production. Of particular interest is the mode of microscale gas transport through so-called coal cleats which form a network of micro-channels/fractures and allow for gas transfer through impermeable coal seams. The problem has attracted interest from many fields such as microstructural characterization, permeability-, porosity-, fluid flow-, diffusion-, adsorption/desorption- and adsorption-induced deformation studies. The main mode of gas flow is supported by the cleat network pattern for which the surface roughness is a very significant property of gas transport. Roughness plays an important role because adsorption is very sensitive to the change in the area. Direct studies on the effects of surface roughness for methane migration in coal by experimental methods are difficult to perform because the micro-scale heterogeneous pore structure and methane sorption properties are difficult to observe and therefore robust numerical simulations provide the most comprehensive method of investigation. In this paper, we develop a numerical model of coal seam gas diffusion and adsorption considering surface roughness based on fractal statistics and the lattice Boltzmann method. The rough surface is characterized by a 3D-Laser profiler and the fractal dimension of the cleat surface is quantified. Generic variants of the measured rough profiles are generated by the Weierstrass-Mandelbrot function through altering the fractal dimension D and length-scale parameter G. The fractal-based lattice Boltzmann method is introduced to solve the governing equations for the gas flow and diffusion processes and scaling laws may be a standardized analysis method and are applied for the non-dimensionalization of gas adsorption. By considering D ranging from 1.5 to 1.8 and G ranging from 1 to 20 it is found that the fractal dimension D has a small effect on the flow but a significant effect on the gas diffusion. In addition, the length-scale parameter G significantly affects gas transport through varying characteristic cleat aperture, and gas diffusion and adsorption by changing the contact area. The data rescaled by scaling laws provide a better analysis of gas adsorption through simplifying the assessment of the effects of multiple variables. Results of this work allow direct quantification of the effect of surface roughness on gas diffusion and adsorption in coal cleats and can provide an improved assessment of the effect of microscopic mechanisms of gas transfer/adsorption in coal on the overall large recovery of gas in the reservoir. [ABSTRACT FROM AUTHOR]