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Introducing oxy-coal burning flue gas into coal reservoirs has the advantages of mitigating emissions of CO 2 , NO x , and SO 2 , and producing in-situ coalbed methane (CBM). Given the characteristics of the geologic time scale for fluid sequestration, the long-duration NO exposure was conducted for various ranks of coal matrices to access the potential impacts of NO exposure on CO 2 storage. Afterwards, the changes in critical physico-chemical properties and adsorbability of various ranks of coal matrices to CO 2 because of long-duration NO exposure were revealed. Finally, the implications to implement oxy-coal burning flue gas sequestration in unmineable coal reservoirs with CBM production were indicated. The results demonstrate that the long-duration interaction with NO reduces the number of micro- and meso-pores of various ranks of coals, especially those with diametres below 4.00 nm. Moreover, the long-duration NO exposure reduces the oxygen-containing functional groups while significantly increasing the amine/amide-N for all the coals. The aforementioned alterations in the surface chemistry property imply that the coal matrix is capable of chemically adsorbing NO, thereby achieving its stable storage in target coal reservoirs. Given the electron donor-acceptor interactions between amine/amide-N and CO 2 , the NO exposure can elevate the CO 2 storage capability of various ranks of coals under typical reservoir temperature and pressure. In summary, introducing oxy-coal burning flue gas into coal reservoirs is capable of stably storing critical gaseous pollutants and simultaneously enhancing the CO 2 storage potential of coal reservoirs, thereby updating the existing oxy-coal burning technology and CO 2 sequestration in unmineable coal reservoirs with the enhanced CBM recovery technology.

Trace elements are associated with the microbial degradation of organic matter and methanogenesis, as enzymes in metabolic pathways often employ trace elements as essential cofactors. However, only a few studies investigated the effects of trace elements on the metabolic activity of microbial communities associated with biogenic coalbed methane production. We aimed to determine the effects of strategically selected trace elements on structure and function of active bacterial and methanogenic communities to stimulate methane production in subsurface coalbeds. Microcosms were established with produced water and coal from coalbed methane wells located in the Powder River Basin, Wyoming, USA. In initial pilot experiments with eight different trace elements, individual amendments of Co, Cu, and Mo lead to significantly higher methane production. Transcript levels of mcrA , the key marker gene for methanogenesis, positively correlated with increased methane production. Phylogenetic analysis of the mcrA cDNA library demonstrated compositional shifts of the active methanogenic community and increase of their diversity, particularly of hydrogenotrophic methanogens. High-throughput sequencing of cDNA obtained from 16S rRNA demonstrated active and abundant bacterial groups in response to trace element amendments. Active Acetobacterium members increased in response to Co, Cu, and Mo additions. The findings of this study yield new insights into the importance of essential trace elements on the metabolic activity of microbial communities involved in subsurface coalbed methane and provide a better understanding of how microbial community composition is shaped by trace elements.IMPORTANCEMicrobial life in the deep subsurface of coal beds is limited by nutrient replenishment. While coal bed microbial communities are surrounded by carbon sources, we hypothesized that other nutrients such as trace elements needed as cofactors for enzymes are missing. Amendment of selected trace elements resulted in compositional shifts of the active methanogenic and bacterial communities and correlated with higher transcript levels of mcrA . The findings of this study yield new insights to not only identify possible limitations of microbes by replenishment of trace elements within their specific hydrological placement but also into the importance of essential trace elements for the metabolic activity of microbial communities involved in subsurface coalbed methane production and provides a better understanding of how microbial community composition is shaped by trace elements. Furthermore, this finding might help to revive already spent coal bed methane well systems with the ultimate goal to stimulate methane production., Competing Interests: The authors declare no conflict of interest.

An in-depth understanding of gas diffusion characteristics in coal is of great value to coalbed methane (CBM) production planning and coal mine safety management. However, the mechanism and model of gas diffusion is still unclear, and some methods for determining diffusion coefficients are not accurate enough. Accordingly, a free gas density gradient (FGDG)-driven coal particle gas desorption and diffusion model was established in this work, and numerical solutions were performed via finite difference method (FDM) and dimensionless method. The variation rules of dimensionless gas pressure, gas content, desorption capacity, and desorption rate were obtained. Finally, the application of the dimensionless method in diffusion modeling and diffusion coefficient inversion was discussed. The results show that the dimensionless method can simplify mathematical equation processing and analyze the common phenomena of desorption and diffusion under different parameters. The gas desorption diffusion in coal particles is from the surface to the inside, and there is obvious desorption hysteresis effect. The larger the dimensionless radius or dimensionless time, the smaller the dimensionless gas pressure, gas content, and dimensionless desorption rate. The dimensionless cumulative gas desorption amount increased rapidly first and then tended to flat with dimensionless time. The simulated curve can be easily converted into the variation curves of several different measured parameters, and the diffusion coefficient can be calculated accurately. The prediction curve of the FGDG diffusion model is in good agreement with the experimentally measured data, which verifies its reasonableness. The research content aims to provide some ideas for modeling gas desorption and diffusion behavior., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Coalbed methane (CBM) is a clean energy source, but its utilization is inefficient due to the complexity and low accuracy of its emission prediction model. In this research, we constructed a mathematical model of gas emission from the excavation workface, and combined the experimental results to propose a new model for accurate and concise prediction. The new model was validated in the field workface and compared with the traditional prediction model. Moreover, the sensitivity of gas emission parameters and the participation ratio of gas emission sources were analyzed. The study results show that the new model has higher calculation accuracy than the old model prediction, with an average error reduction of 4.693%. In the excavation workface, the coal fall gas emission conforms to the negative power function equation, and the coal wall gas emission conforms to the negative exponential function equation. In the early stage of excavation, the proportion of coal fall gas emission is higher than that of coal wall gas emission, and the peak proportion reaches 58.5%. In the later stage, the proportion of coal fall gas emission gradually decreases to below 30%. The order of the sensitivity of gas emission parameters is coal wall gas initial velocity > coal fall gas decay coefficient > coal fall gas initial velocity > coal wall gas decay coefficient. The new model is successfully applied in engineering, which helps to improve the efficiency of coal mine gas disaster control and utilization., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

The global trend of transiting to more renewable energy sources requires transition fuels, such as coal seam gas, to supplement and secure energy needs. In order to optimise strategies and technologies for enhancing gas production, an understanding of the fundamental microbial processes and interactions would be advantageous. Models have recently begun mapping the microbial roles and interactions in coal seam environments, from direct coal degradation to methanogenesis. This study seeks to expand those models by observing community compositional shifts in the presence of differing organic matter by conducting 16S rRNA microbial surveys using formation water from the Surat and Sydney Basins grown on varying types of organic matter (black and brown coal, oil shale, humic acid, and lignin). A total of 135 microbes were observed to become enriched in the presence of added organic matter in comparison to carbon-free treatments. These surveys allowed detailed analysis of microbial compositions in order to extrapolate which taxa favour growth in the presence of differing organic matter. This study has experimentally demonstrated shifts in the microbial community composition due to differing carbon sources and, for the first time, generated a conceptual model to map putative degradation pathways regarding subsurface microbial consortia., (© 2021. Crown.)

Coalbed methane not only is a new clean energy source, but also has potential damage to ecological environment. Water and methane coexist in coal reservoir; understanding the adsorption of water on coal and its impact on pore structure and methane adsorption of coal is vital to evaluate the reserves and productivity of coalbed methane. In the paper, water adsorption characteristics of various rank coals are firstly investigated by ten mathematical models. The modified Dent model provides a best fit, followed by GAB and Dent models. For GAB model, the primary site adsorption is more difficult to reach saturation, and the contribution rate of the secondary site adsorption is surprisingly high at P/P 0 approaching 0, which can be attributed to the possible overestimation of GAB monolayer adsorption capacity and secondary site adsorption. Besides, the low-rank coal sample YZG2 exhibits more prominent hysteresis than middle- to high-rank coals. The low-pressure hysteresis can be attributed to the water-water interactions over the primary site and the strengthened binding forces of water molecules in the water desorption process. In contrast, the high-pressure hysteresis largely depends on pore structure of coal such as ink-bottle pores, especially for the studied sample YZG2. Besides, pore analyses by low-temperature nitrogen adsorption method show that the pre-adsorbed water has remarkable influence on micropores smaller than 10 nm, and the micropores smaller than 4 nm almost disappear for water-equilibrated coals, which is closely related to the formed water clusters and capillary water in pore throats. This finding reveals that more methane gas can only be adsorbed in the larger pores of moist coal, and provides an explanation for water weakening methane adsorption capacity., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Subsurface microbial (biogenic) methane production is an important part of the global carbon cycle that has resulted in natural gas accumulations in many coal beds worldwide. Laboratory studies suggest that complex carbon-containing nutrients (e.g., yeast or algae extract) can stimulate methane production, yet the effectiveness of these nutrients within coal beds is unknown. Here, we use downhole monitoring methods in combination with deuterated water (D 2 O) and a 200-liter injection of 0.1% yeast extract (YE) to stimulate and isotopically label newly generated methane. A total dissolved gas pressure sensor enabled real-time gas measurements (641 days preinjection and for 478 days postinjection). Downhole samples, collected with subsurface environmental samplers, indicate that methane increased 132% above preinjection levels based on isotopic labeling from D 2 O, 108% based on pressure readings, and 183% based on methane measurements 266 days postinjection. Demonstrating that YE enhances biogenic coalbed methane production in situ using multiple novel measurement methods has immediate implications for other field-scale biogenic methane investigations, including in situ methods to detect and track microbial activities related to the methanogenic turnover of recalcitrant carbon in the subsurface.

An in-depth understanding of the hydrogeochemical characteristics of coal mines is helpful in establishing an effective and successful exploration program of coalbed methane (CBM). This study provides a comprehensive analysis of hydrogeological characteristics, characteristics of coalbed water, and characteristics of the coal sample from a coal seam located in the Red River Basin (RRB). These physicochemical characteristics along with the microbial composition of coalbed water were critically analyzed. A high concentration of chloride and sodium was found in the coalbed water, presumably due to the coal mine's stratigraphic association with marine or marine-transitional beds. A correlation between the occurrence of microbes and the chemical components in the coalbed water was established. The characteristics of the coal were systematically analyzed, including proximate, ultimate, and petrographic analyses. Based on the coal macerals, coal rank is classified as low-rank (sub-bituminous) with a vitrinite reflectance (R o, max ) of 0.36%, suggesting that this type of low-rank coal is favorable for biogenic methane generation. Pore structures and pore types were characterized using different methods, including low-temperature nitrogen adsorption/desorption (LTNA), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM). Coal from the study area has microporous and macroporous features. Pore types of the coal were also characterized using SEM. The primary genetic pore types of the Red River coal include plant tissue holes and blowholes., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Methane is an important energy resource internationally, and a large proportion of this methane is produced by microbial communities living in coal seams. Despite the value of this resource for human energy security, our understanding of the metabolic roles played by specific taxa during the biodegradation of coal to methane in situ is quite limited. In order to develop a greater understanding of microbial catabolism on coal, a community from a coal seam in the Surat Basin, Australia, was incubated on 10 different aromatic organic compounds: coronene, benzo[a]pyrene, pyrene, phenanthrene, naphthalene, ethylbenzene, phenol, benzoate, vanillate and syringate. Each of these aromatic compounds either occurs in coal or is a possible product of the coal biodegradation process. 16S rRNA sequencing revealed substantial changes to each community in response to each aromatic carbon substrate provided. Abundant taxa from these substrate-specific communities were identified and their probable catabolic roles proposed based on literature searches of related taxa. This study is the first to link specific coal seam taxa to aromatic substrates available in coal seam environments. Two conceptual models of the putative degradation pathways and key taxa responsible are proposed., (© The Author(s) 2021. Published by Oxford University Press on behalf of FEMS.)

Microbial communities in subsurface coal seams are responsible for the conversion of coal organic matter to methane. This process has important implications for both energy production and our understanding of global carbon cycling. Despite the environmental and economic importance of this process, little is known about which components of the heterogeneous coal organic matter are biodegradable under methanogenic conditions. Similarly, little is known about which taxa in coal seams carry out the initial stages of coal organics degradation. To identify the biodegradable components of coal and the microorganisms responsible for their breakdown, a subbituminous coal was fractionated into a number of chemical compound classes which were used as the sole carbon source for growth by a coal seam microbial community. This study identifies 65 microbial taxa able to proliferate on specific coal fractions and demonstrates a surprising level of substrate specificity among members of this coal-degrading microbial consortia. Additionally, coal kerogen, the solvent-insoluble organic component of coal often considered recalcitrant to microbial degradation, appeared to be readily converted to methane by microbial degradation. These findings challenge our understanding of coal organic matter catabolism and provide insights into the catabolic roles of individual coal seam bacteria., (© FEMS 2019.)

It is now clear that active methanogens are present in the deep-subsurface. This paper reviews microbial population structures and the biodegradation of organic compounds to methane in situ within oil reservoirs and coal deposits. It summarizes our current knowledge of methanogenes and methanogenesis, fermenters, synthrophs and microbial metabolism of complex organic compounds in these two widely occurring organic-rich subsurface environments. This review is not intended to be an exhaustive report of microbial diversity. Rather, it illustrates the similarities and differences between the two environments with specific examples, from the nature of the organic molecules to the methanogenic metabolic pathways and the structure of the microbial populations to demonstrate that widely diverging microbial populations show surprisingly similar metabolic capabilities., (Copyright © 2013 Institut Pasteur. Published by Elsevier Masson SAS. All rights reserved.)

To enhance biogenic coalbed methane production, it is imperative to comprehend the processes by which methane is generated when microbes decompose coal in perched water zones situated in steeply inclined fire zones. Low-rank coal from Fukang mining was used to simulate biogenic gas production and investigate methane generation in perched water zones in steeply inclined coal fire zones. This study integrated characterization techniques, multifractal theory, and field data. Microbial activity increased oxygen-containing functional groups, reduced aromatic rings, and disrupted aliphatic clusters in samples. Aromatic nucleus interlayer spacing expanded; microcrystalline stacking, extensibility, and aromatic layers decreased. Coal structure degraded as microbes targeted aromatic then aliphatic carbon. Increased pore and fracture density enhanced pore connectivity (H) and reduced pore heterogeneity (△α), while pore types remained unchanged. Methane production via microbial degradation follows acetate and hydrogenotrophic pathways. Methane rates were affected by perched water zone distance, coal seam salinity, and pH. • Identified methane production mechanisms in steep coal seams under water recharge. • Microbial degradation improved pore connectivity and reduced heterogeneity in coal. • Biogenic methane generation via acetate and hydrogenotrophic pathways was confirmed. • Findings provide guidance for biogenic gas recovery in steeply dipping coal seams. [ABSTRACT FROM AUTHOR]

In-situ stress plays a pivotal role in influencing the desorption, adsorption, and transportation of coalbed methane. The reservoir gas content represents a pivotal physical parameter, encapsulating both the coalbed methane enrichment capacity and the underlying enrichment law of the reservoir. This investigation collates, computes, and consolidates data concerning pore pressure, breakdown pressure, closure pressure, triaxial principal stress, gas content, lateral pressure coefficient, and other pertinent variables from coal reservoirs within several coal-bearing synclines in the Liupanshui coalfield, China. This study elucidates the characteristics of longitudinal stress development in the study area, the gas content of the longitudinal reservoirs and their interrelationships. The study area is situated within the middle-high stress zone, exhibiting discernible evolution patterns from reverse fault mechanism to strike-slip fault mechanism to normal fault mechanism, progressing from shallow to deep. In the deeper stratigraphy, a strike-slip-normal fault mechanism emerges. The relationship between burial depth and triaxial principal stress is subjected to linear regression, resulting in the proposal of a simplified model for vertical in-situ stress. The hyperbolic regression algorithm was employed in order to derive both the envelope and median formulas for lateral pressure coefficient (k values). The k value displays discrete behavior along the vertical axis in shallow depths, gradually converging in deeper strata and ultimately stabilising at approximately 0.65 with increasing depth. A comprehensive examination of the k value substantiates the efficacy of the simplified in-situ stress model along the vertical axis, offering profound insights into the vertical interrelationships and evolving patterns of the triaxial principal stresses. The mean gas content in the study area was found to be 11.89 m³/t, exhibiting a general increase in depth, followed by a subsequent decrease. The pore pressure (Pp) displays a discernible positive correlation with gas content. This study comprehensively elucidates the developmental patterns of the stress field, the simplified model of vertical in-situ stress, the attributes of the stress ratio (KH, kh, lateral pressure coefficient k), the characteristics of reservoir gas content, and the corresponding and transformative relationships between coupled geostress field parameters and gas content. The lateral pressure coefficient conversion depth, in-situ stress conversion depth, and gas inversion depth are delineated, accompanied by a detailed exposition of their definition process, physical significance, and interrelations. Within the study area, the lateral pressure coefficient conversion depth is estimated to range between 450 and 500 m, while the critical depth for in-situ stress conversion is approximately 670 m. Moreover, the critical depth for gas content conversion falls within the range of 700–800 m. It is noteworthy that the critical depth for deep coalbed methane within the Liupanshui coalfield has been identified as approximately 800 m. Subsequently, a vertical "in-situ stress-gas content mode" relationship model for coalbed methane development was formulated, thereby providing a structured framework for understanding the dynamic interactions between vertical in-situ stress and gas content. [ABSTRACT FROM AUTHOR]

ABSTRACT In response to the severe problem of coal powder production in the Panhe block coalbed methane wells in the southern Qinshui Basin, the characteristics of coal powder production in the study area were identified through sample testing, indoor experiments, and theoretical calculations. The settling velocity of coal powder with different mesh sizes was clarified, and a correction factor α was proposed for the experimental and theoretical results of settling velocity. The research results indicate that the coal powder concentration in the Panhe block ranges from 0.03 to 7.14 g/L, with an average of 1.26 g/L. The particle size of the coal powder produced was 4.10–237.64 μm, with an average of 35.82 μm. The settling velocity of coal powder particles with a mesh size of 40–400 is between 0.0041 and 0.029 m/s. The larger the particle size of coal powder particles, the higher the settling velocity of coal powder; the correction coefficient ranges from 2.3 to 13.67. A corrected settling velocity calculation model was obtained by fitting the coal powder particle size data to the correction coefficient. The research results provide a theoretical basis for developing production conditions, coal powder prevention, and control measures for coalbed methane wells in the Panhe area. [ABSTRACT FROM AUTHOR]

Deep coalbed methane reservoirs generally exhibit characteristics such as extremely low permeability, significant heterogeneity, high in situ stress, and dense geological discontinuities. Notably, these geological discontinuities cleats, bedding planes, and natural fractures, as mechanically weak planes, significantly contribute to the creation of extremely complex and tortuous hydraulic fracture (HF) networks near the wellbore, but impede the propagation of HFs to the far-field region. This will lead to insufficient stimulated reservoir volume, thereby limiting the CBM production. Under this background, a series of physical simulation experiments of temporary plugging and diverting fracturing (TPDF) were carried out on large-size coal blocks under true triaxial stress conditions. Combining high-energy industrial computed tomography scanning technology, first, the morphology of fracture propagation of the sample before TPDF is divided into two fracture propagation modes. Then, TPDF experiments were conducted to analyze the behavior of fracture propagation under different modes. Finally, a mode of TPDF tailored for adjusting the HF network geometry in deep CBM reservoirs was explored innovatively. The effects of concentration and particle size of the temporary plugging agent (TPA) on pressure increment, plugging location and fracture diversion behavior during TPDF were examined in particular. Experimental results indicate that optimizing the concentration and particle-size of TPA based on the resulting fracture geometry is crucial for adjusting the fracture network geometry (simplifying the growth behavior of HF near the wellbore while increasing fracture complexity in the far-field region) during conventional fracturing (before using the TPA). When a complex fracture network is created under the condition of formation with high-dense natural fractures (NFs) near the wellbore region, using small-particle-size TPA (e.g., 70/140 mesh) is optimal for adjusting the fracture geometry, as it can effectively plug the NFs and allow them to continue extending toward the far-field region. Meanwhile, a higher concentration of TPA is beneficial for plugging the interval of HF closer to the wellbore, and then causing the creation of complex fracture networks. When a long single HF is generated under the condition of a formation with low-dense NFs, using the medium-particle-size TPA (e.g., 40/70 mesh) is optimal for enhancing the fracture complexity near the wellbore region. Using TPA of excessively large particle sizes (e.g., 20/40 mesh) tends to plug the HFs at their entrances, causing HFs to be reinitiated from the unstimulated segment of the wellbore. This study can provide crucial theoretical guidance for optimizing the scheme design of TPDF in deep CBM reservoirs. [ABSTRACT FROM AUTHOR]

Coal structures are widely regarded as a critical influencing factor for the dynamic behaviors of CH4 migration in coalbed methane (CBM) reservoir. In this paper, geophysical logging data were analyzed to explore the logging response characteristics of coal structures, and their application on identification of coal structures by using the machine learning methods. Meanwhile, the correlations between coal structures and gas-bearing properties were revealed. The results show that with the increase in coal deformation intensities, acoustic transit time, caliper logging, compensated neutron, and natural gamma values positively increase and that for density logging and lateral resistivity show a negative correlation. The multi-logging parameter identification models of coal structures were constructed by using random forest algorithm, radial basis function neural network, and long short-term memory neural network, with their accuracy reaching to 96.67%, 93.33%, and 91.67%, respectively. Based on the identification results of RFA model, the highest distribution percentages of cataclastic coal are 50.2%, which is controlled by tectonic activities and buried depth. The origins of gases are mainly thermogenic gases whose average value of δ13C(CH4) is −37.51‰. The gas content in granulated coal is smaller than 12 cm3/g, but it is higher than 15 cm3/g in cataclastic coal, resulting the higher gas saturation of cataclastic coal. The average extension length of artificial fractures in cataclastic coals is nearly two times as long as in granulated coals. It is suggested that cataclastic coal zone is the favorable area for CBM development. [ABSTRACT FROM AUTHOR]

Due to tectonics, it is impossible in some areas to accurately assess the water abundance of strata based solely on the current groundwater level. This study focused on an aquifer located ≈ 10 m above the no. 15 coal seam in the Shouyang Block of the Qinshui Basin, Shanxi Province, China. Well-logging data was used to determine the aquifer water abundance. After classifying and analyzing the water-rich aquifer and its effect on the water output of the discharge wells, development measures were proposed. The strongest hydrodynamic force was observed in the superposition area of the syncline core + syncline core. In contrast, the least hydrodynamic force was observed in the superposition area of the anticline core + anticline core. The southeastern and northeastern regions of the study area exhibited the greatest water abundance. Strongly hydrodynamic coalbed methane (CBM) wells have high daily water production, which makes drainage and pressure reduction challenging. Therefore, CBM development should be approached cautiously. CBM wells with strong water abundance have high water production, and efforts should be made to avoid aquifer communication during development. Wells with weak hydrodynamics and weak water abundance have low water production, making drainage and pressure reduction easier. Reducing reservoir damage can further enhance the gas production potential. The findings of this study provide a methodology and reference for calculating aquifer water abundance and forecasting daily water production. [ABSTRACT FROM AUTHOR]

ABSTRACT The accurate identification of fracture extension patterns in hydraulic fracturing can provide important guidance for the optimisation of fracturing parameters. In this paper, factors such as effective hole friction and wellbore flow friction during fracturing are fully considered, and a calculation model of net bottom‐hole pressure of fracturing is constructed. By introducing the change rate of net bottom‐hole pressure and the changing characteristics of the fracturing curve, seven fracture extension modes during hydraulic fracturing in coalbed methane vertical wells are established. The accuracy of the identification method is verified by the fracture monitoring and production results in Shizhuang South Block. The results show that fracture elongation is mainly controlled by in situ stress difference, angle between natural fracture and maximum principal stress, coal tensile strength, fracturing time, proppant and angle between other factors. When the fracture construction parameters are fixed, the smaller the difference between maximum and minimum horizontal principal stresses and the smaller the natural fractures and maximum horizontal principal stresses. When the reservoir potential is similar, the effective extension index is positively correlated with the gas production effect, and the effective extension index can effectively judge the fracturing effect. The higher the proportion of effective extension mode, the longer the extension time and the higher the stable daily gas production. The research results provide a method and reference for clearly identifying the fracture extension and the occurrence conditions of different extension modes in the hydraulic fracturing process. [ABSTRACT FROM AUTHOR]

Coal-and-gas outbursts represent a significant hazard in coal mining, with gas expansion energy (GEE) in coal seams being a primary energy source. Accurate GEE assessment is vital for outburst prediction and mitigation, thereby enhancing mining safety. Traditional calculation models have struggled with limited understanding of outburst mechanisms and experimental constraints, leading to broad GEE estimates with considerable discrepancies. Addressing this gap, this study introduces an experiment-driven, highly practical calculation model, along with innovative experimental methods to measure accurately key determinants of GEE: fracture porosity, CH4 desorption amount, and gas pressure in coal seams. For the first time, this study employed remade and raw coal columns as media to simulate accurately the real conditions of tectonic and raw coal seams for exploring the coupling effects of stress and gas pressure on GEE. This study calculated the GEE as stress increases from 5 to 50 MPa and gas pressure decreases from 2 to 0.5 MPa. The results indicate that, for two remade coal columns, the GEE decreased from 1870 to 62 kJ/t and from 2039 to 356 kJ/t while for the raw coal column, the GEE dropped from 130 to 6 kJ/t. [ABSTRACT FROM AUTHOR]

The deep coal seams in the southern Sichuan region contain abundant coalbed methane resources. Determining the characteristics and distribution patterns of coal structures in this study area, and analyzing their impact on pore and fracture structures within coal reservoirs, holds substantial theoretical and practical significance for advancing coal structure characterization methods and the efficient development of deep coalbed methane resources. This paper quantitatively characterizes coal structures through coal core observations utilizing the Geological Strength Index (GSI) and integrates logging responses from different coal structures to develop a quantitative coal structure characterization model based on logging curves. This model predicts the spatial distribution of coal structures, while nitrogen adsorption data are used to analyze the development of pores and fractures in different coal structures, providing a quantitative theoretical basis for accurately characterizing deep coal seam features. Results indicate that density, gamma, acoustic, and caliper logging are particularly sensitive to coal structure variations and that performing multiple linear regression on logging data significantly enhances the accuracy of coal structure identification. According to the model proposed in this paper, primary-fragmented structures dominate the main coal seams in the study area, followed by fragmented structures. Micropores and small pores predominantly contribute to the volume and specific surface area of the coal samples, with both pore volume and specific surface area increasing alongside the degree of coal fragmentation. Additionally, the fragmentation of coal structures generates more micropores, enhancing pore volume and suggesting that tectonic coal has a greater adsorption capacity. This study combines theoretical analysis with experimental findings to construct a coal structure characterization model for deep coal seams, refining the limitations of logging techniques in accurately representing deep coal structures. This research provides theoretical and practical value for coal seam drilling, fracturing, and reservoir evaluation in the southern Sichuan region. [ABSTRACT FROM AUTHOR]

Introduction: Coal reservoir fractures serve as critical storage spaces and migration pathways for coalbed methane (CBM), significantly influencing CBM enrichment. The characteristics of coal reservoir fracture development can be obtained using traditional simulation methods, but these still have shortcomings. This work presents an optimization approach for the traditional method. Methods: This study introduces an optimization approach for traditional methods with two novel contributions. This study integrates the simulation of tectonic stress fields with fracture prediction, using surface sandstone fractures as constraints to reconstruct the paleostress field of the coal seam, while also accounting for the influence of coal thickness on fracture development to calculate fracture density. Results: The predicted fracture density results were validated against measured values from the Bailongshan mine and Xiongdong coal mine with a relative error of approximately 12%, suggesting a reasonable degree of reliability. Discussion: Based on the results of the fracture simulation predictions, it is believed that the coal seam fracture density in the study area is mostly 10–20 lines/m and that the sweet spot for CBM development is located in the Yuwang block. [ABSTRACT FROM AUTHOR]

Injecting CO2 into coal seams to enhance coal bed methane (ECBM) recovery has been identified as a viable method for increasing methane extraction. This process also has significant potential for sequestering large volumes of CO2, thereby reducing the concentration of greenhouse gases in the atmosphere. However, for deep coal seams where formation pressure is relatively high, there is limited research on CO2 injection into systems with higher methane adsorption equilibrium pressure. Existing studies, mostly confined to the low-pressure stage, fail to effectively reveal the impact of factors such as temperature, high-pressure CO2 injection, and coal types on enhancing the recovery and sequestration of CO2-displaced methane. Thus, this study aims to investigate the influence of temperature, pressure, and coal types on ECBM recovery and CO2 sequestration in deep coal seams. A series of CO2 core flooding tests were conducted on various coal cores, with CO2 injection pressures ranging from 8 to 18 MPa. The CO2 and methane adsorption rates, as well as methane displacement efficiency, were calculated and recorded to facilitate result interpretation. Based on the results of these physical experiments, numerical simulation was conducted to study multi-component competitive adsorption, desorption, and seepage flow under high temperature and high pressure in a deep coal seam's horizontal well. Finally, the optimization of the total injection amount (0.7 PV) and injection pressure (approximately 15.0 MPa) was carried out for the plan of CO2 displacement of methane in a single well in the later stage. [ABSTRACT FROM AUTHOR]

Biogenic methane (BM) production from coal is quite limited due to its complex chemical nature, which makes the responsible microbes sensitive to its denaturation. In the current investigation, wheat straw (WS) at different concentrations as a cosubstrate were used with coal to augment methane generation from coal. Results revealed that the codigestion approach significantly enhanced the cumulative methane generation in CS1 (17.25 mmol) (coal:straw, 75:25), which was 59.52%–256.47% higher than the CS2 (coal:straw, 85:15), and WS single digestion, respectively. Moreover, the lowest methane yield (0.295 mmol/g) was achieved from single coal digestion. Particularly, the highest methane content, 68.37% in the collected biogas, was observed in CS1, followed by CS2 (49.53%), WS (44.92%), and coal (3.53%). Microbial community analysis illustrates that Methanobacteriaceae (51.33%) and Methanosarcinaceae (48.66%) were the leading archaeal communities at the peak methanogenic stage in CS1. While the abundant bacteria at this stage were Hungateiclostridiaceae (40.18%), Rhodobacteriaceae (21.40%), and Lentimicrobiaceae (19.20%) in CS1. According to scanning electron microscopy (SEM) analysis, several microbes were seen to be attached to the coal surface in CS1 and CS2. Moreover, the nontargeted metabolomic results showed that aromatics, aliphatic, long‐chain fatty acids, and alkane (C19–C36) compounds were found to be highly expressed in only coal, CS1, and CS2 reactors compared to WS. In addition, volatile fatty acids (VFAs) analysis showed that acetic acids were abundantly present in CS1 (2,190,175 ng/ml), followed by WS (1,543,492.88 ng/ml), CS2 (1,159,050 ng/ml), and coal (50,998.31 ng/ml). These results promote the significance of using WS as a cosubstrate with coal to enhance methane production, which can be used as a fuel in power plants. Further studies are required to investigate the potential cosubstrate with coal to enhance methane production and identify the specific metabolic pathways involved. [ABSTRACT FROM AUTHOR]

This study assessed the impacts of wheat straw as a cosubstrate on coal biocoverion into methane and the associated mechanism within methane metabolic pathways. Co-digestion of coal with varying wheat straw concentrations resulted in a remarkable (1246.05%) increase in methane yield compared to that of the control (CK). Moreover, microbial analysis revealed a uniform distribution of Methanosarcinaceae (51.14%) and Methanobacteriaceae (39.90%) in the co-digestion of coal and wheat straw (CWS1) at a ratio of 3:1 (w/w) compared to other treatments such as coal and wheat straw (CWS2) at a ratio of 3:0.5. In addition, Hungatieclostridiaceae and Rhodobacteriaceae were abundant in both co-digesters, whereas the bacterial communities in the CK group were significantly different and more abundant than those in the Peptostreptococcaceae and Enterobacteriaceae groups. The key enzymes related to methanogenic metabolic pathways, including EC: 1.2.99.5 and EC: 2.1.1.86 (facilitating the conversion of CO2 into methane), and EC:1.12.98.1 exhibited significant abundance within CWS1. Aromatic compounds such as 4-(2-chloroanilino)-4-oxobutanoic acid and phthalic acid were substantially more abundant in CWS1 and CWS2 than in CK, indicating the increased bioavailability of coal to microbial activities. This novel approach demonstrates that wheat straw co-digestion with coal during anaerobic digestion modulates microbial communities and their metabolic pathways to enhance methane production from complex substrates such as coal. [ABSTRACT FROM AUTHOR]

In order to determine the reasonable parameters of high-gas and extra-thick coal seam drainage, considering the factors of the coal seam metamorphic degree, stress condition, gas occurrence state, and permeability dynamic change, the gas desorption, diffusion, and transport process of coal seam gas are analyzed. A secondary distribution model of coal around the borehole, a porosity variation model of coal around the borehole, a stress–seepage coupling model, a pore flow model of the pressure-driven transition flow zone, and a free molecular flow zone are established. Taking the gas drainage of Zhangcun Coal Mine of Lu'an Group as the research object, the influence of drilling hole diameter, coal seam permeability, gas original pressure, and other factors on the control range of coal seam drainage drilling is simulated by ANSYS Fluent 6.3.26. The results show that secondary stress distribution occurs in the coal seam drill hole under the action of lead stress, which leads to the change in porosity; the seepage zone, transition zone, molecular flow zone, and original rock stress zone are presented around the drill hole; and the range of influence of the drill hole is mainly based on the seepage zone and the transition zone, supplemented by the molecular flow zone. The control range of the drill hole is in a positive proportional relationship to the diameter of the drill hole, the porosity of the coal seam, and the original pressure of the gas. [ABSTRACT FROM AUTHOR]

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This document introduces a novel equivalent algorithm for forecasting gas content within deep coal seams, which is subject to constraints stemming from the advancements and precision achieved in well and roadway engineering endeavors. This algorithm meticulously acknowledges that coal seam gas content comprises three fundamental components: the inherent gas emission rate of the equivalent stratum, the residual gas content retained within the coal seam itself, and the influence imparted by the gas content within the coal seam. Furthermore, the approach thoroughly considers variations in the level of porosity development within the coal seam and its surrounding rock formations, as well as the occurrence of gas within these structures. The equivalent layer is classified into two distinct groups: the sandstone zone and the clay zone. The sandstone zone utilizes pertinent parameters pertaining to fine sandstone, whereas the clay zone distinguishes between clay rock and thick mudstone. The influencing factor considerations solely encompass natural elements, such as the coal seam's occurrence and geological structure. The residual gas content employs either existing measured parameters or acknowledged experimental parameters specific to the coal seam. Based on this predictive approach, an intelligent auxiliary software (V1.0) for mine gas forecasting was devised. The software calculates the gas content of deep coal seams within the mine at intervals of 100 m × 100 m, subsequently fitting the contour lines of gas content across the entire area. The gas content predictions derived from this equivalent algorithm demonstrate robust adaptability to variations in gas content caused by construction activities, and the prediction results exhibit an acceptable level of error on-site. Notably, the prediction process is not constrained by the progress of tunnel engineering, ensuring that the prediction outcomes can accurately represent the distribution characteristics of deep coal seam gas content. After a year of application, the prediction results have consistently met on-site requirements, providing a scientific foundation for the implementation of effective gas prevention and control measures in the mining area. Furthermore, this approach can effectively guide the formulation of medium- and long-term gas prevention and control plans for mines. [ABSTRACT FROM AUTHOR]

To explore the influence of surfactant concentration on the pore structure and permeability of coal samples during the chemical enhancement of coalbed methane production, different kinds and different concentrations of surfactants were added to the chemical solution, and the coal samples were soaked. Methods such as low-field nuclear magnetic resonance testing (NMR), fractal theory, permeability testing, surface tension testing, and contact angle testing were employed to analyze the variation patterns of coal sample pore structure, fractal characteristics, and permeability, and to explore the correlation between surface tension, contact angle, and the degree of pore structure development. The results show that the increase in total porosity of coal samples, the increase in the seepage pore porosity, the decrease in Dt, and the growth rate of permeability increase with the increase in surfactant concentration, and are negatively correlated with the surface tension of the solution and the contact angle of the coal-solution interface, while the decrease in Ds is not significantly correlated with surfactant concentration, surface tension, or contact angle. In terms of the erosion effect of a chemical solution on coal samples, the influence of contact angle is greater than that of surface tension, while surface tension has the greatest impact on the development of adsorption pores. By adding different surfactants, the surface tension of the chemical solution and the contact angle of the coal-solution interface can be controlled, further promoting the erosion of coal samples, which is of positive significance for the chemical enhancement of coalbed methane production. [ABSTRACT FROM AUTHOR]

To reveal the enrichment conditions and resource potential of coal-rock gas in the Ordos Basin, this paper presents a systematic research on the sedimentary environment, distribution, physical properties, reservoir characteristics, gas-bearing characteristics and gas accumulation play of deep coals. The results show that thick coals are widely distributed in the Carboniferous-Permian of the Ordos Basin. The main coal seams Carboniferous 5# and Permian 8# in the Carboniferous-Permian have strong hydrocarbon generation capacity and high thermal evolution degree, which provide abundant materials for the formation of coal-rock gas. Deep coal reservoirs have good physical properties, especially porosity and permeability. Coal seams Carboniferous 5# and Permian 8# exhibit the average porosity of 4.1% and 6.4%, and the average permeability of 8.7×10-3 µm2 and 15.7×10-3 µm2, respectively. Cleats and fissures are developed in the coals, and together with the micropores, constitute the main storage space. With the increase of evolution degree, the micropore volume tends to increase. The development degree of cleats and fissures has a great impact on permeability. The coal reservoirs and their industrial compositions exhibit significantly heterogeneous distribution in the vertical direction. The bright coal seam, which is in the middle and upper section, less affected by ash filling compared with the lower section, and contains well-developed pores and fissures, is a high-quality reservoir interval. The deep coals present good gas-bearing characteristics in Ordos Basin, with the gas content of 7.5-20.0 m3/t, and the proportion of free gas (greater than 10%, mostly 11.0%-55.1%) in coal-rock gas significantly higher than that in shallow coals. The enrichment degree of free gas in deep coals is controlled by the number of macropores and microfractures. The coal rock pressure testing shows that the coal-limestone and coal-mudstone combinations for gas accumulation have good sealing capacity, and the mudstone/limestone (roof)-coal-mudstone (floor) combination generally indicates high coal-rock gas values. The coal-rock gas resources in the Ordos Basin were preliminarily estimated by the volume method to be 22.38×1012 m3, and the main coal-rock gas prospects in the Ordos Basin were defined. In the central-east of the Ordos Basin, Wushenqi, Hengshan-Suide, Yan'an, Zichang, and Yichuan are coal-rock gas prospects for the coal seam #8 of the Benxi Formation, and Linxian West, Mizhi, Yichuan-Huangling, Yulin, and Wushenqi-Hengshan are coal-rock gas prospects for the coal seam #5 of the Shanxi Formation, which are expected to become new areas for increased gas reserves and production. [ABSTRACT FROM AUTHOR]

The presence of water in coal and its interaction plays pivotal roles in the storage and migration of coalbed methane (CBM), making it imperative to understand the water-occurrence across different coal ranks to guide CBM exploitation effectively. Here, a novel method for categorizing water into condensed and adsorbed forms based on their dehydration dynamics is proposed using differential thermogravimetric curve and the Arrhenius equation, offering a straightforward process and enabling the assessment of the interaction strength between water and coal. The result indicates that the total water capacity decreases initially before subsequently increasing as coal rank increases from 0.28 to 2.33% Ro,max, with the ratio of condensed water exhibiting an S-shaped curve. Remarkably, the condensed water capacity is correlated linearly with the total pore volume. The adsorbed water in low-rank coal is controlled primarily by the level of oxygen functional groups, whereas in medium-high rank coal it is controlled primarily by the specific surface area. Based on this, the controlling equations of water capacity and coal–water structure models were established. Additionally, coal–water interaction strength decreases significantly after the first coalification jump, with the strength of low-rank coal being approximately 2.54 times higher than that of medium-high rank coal. This discrepancy arises from the combined influence of multiple oxygen functional groups in low-rank coal on adsorbed water. This paper enhances the understanding of drainage process in coal reservoirs of varying ranks, which can facilitate the efficient extraction of CBM. [ABSTRACT FROM AUTHOR]

Mercury intrusion porosimetry (MIP) is widely used for coal pore structure characterization, however, the matrix compressibility (MC) can lead to overestimated measurement results. Determination of MC is crucial for revealing the influence of pore structure on coalbed methane (CBM) flow behavior. In this study, MIP and low temperature N2 adsorption (LT-N2A) were conducted on 15 coal samples from major coal-producing regions in Northern China. The MIP data were corrected using MC theory, and the effects of coal rank and pore structure on coal MC were analyzed. The influence of MC on fractal dimension was elucidated, and the sensitivity of three fractal models to MC was effectively evaluated. Finally, the impact of MC on the coalbed methane (CBM) exploitation was discussed. The results show that low-rank coals have higher MC than medium/high-rank coals, and the MC coefficient follows a cubic polynomial relationship with coal rank, with two inflection points located at 1.4–2.5%, respectively. Micropores and transition pores are the main contributors to MC, for corrected data, the pore volume of both types of pores decreases significantly. The corrected pore size distribution exhibits better agreement with the LT-N2A measurement results, particularly in peak position and size for pores between 5 and 50 nm. This suggests the potential of corrected MIP data to supersede the combined use of MIP and LT-N2A data. MC can lead to overestimation of the fractal dimension, with the thermodynamic model showing the lowest sensitivity to MC. After the microfractures in medium/high-rank coal are greatly compressed, the compressional deformation of micropores and transition pores begins to have a significant impact on the CBM transport. The research results are of great significance for deeply understanding the mechanism of CBM transport. [ABSTRACT FROM AUTHOR]

Coal is an important fossil energy source in the world, which provides important support for the development of industry. However, the chemical composition of coal is complex, and it may cause harm to the human body and environment during the process of mining and utilization, especially some aromatic hydrocarbons in coal that are strongly carcinogenic to human beings; thus, it is necessary to analyze the organic chemical compositions of coal so as to realize the clean and harmless utilization of coal. In this article, three different coal samples were extracted by seven solvent-graded extractions, and then the extracts were tested by gas chromatography–mass spectrometry (GC-MS). According to the results of the GC-MS test, it was found that CS2 could dissolve a large amount of aromatic hydrocarbons in the coal, n-hexane could dissolve a larger amount of aliphatic hydrocarbons, methanol could dissolve a larger amount of ketones, benzene could extract phenolic compounds in the coal, acetone could dissolve alcoholic compounds, and the mixed solvent methanol/THF could dissolve coal esters. Then, by analyzing these extracts, researchers can clearly understand the microscopic organic components of coal, which have a significant role in the development of the coal chemical industry and ecological environment protection. [ABSTRACT FROM AUTHOR]

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Coalbed methane thermodynamic extraction, as an emerging ECBM recovery method, can effectively improve gas recovery rates. And clarifying methane diffusion and migration law in coal under thermal stimulation is crucial for the selection of its process parameters. Based on laboratory methane adsorption-release experiments, the evolution law of methane diffusion characteristics with temperature and pressure was studied, and the control mechanism of heat-dependent methane diffusion behavior was explored. The results show that both thermal stimulation and high adsorption pressure accelerated the methane diffusion rate in coal. Adsorption pressure had little effect on methane diffusion percentage, but thermal stimulation promoted a significant increase in diffusion percentage and improved the net methane yield. The influence mechanisms of adsorption pressure and thermal stimulation on methane diffusion characteristics are elucidated in relation to the amount and proportion of methane-activated molecules in the diffusion process. The constant diffusion coefficient of methane is heat-dependent, based on which a diffusion model is derived to accurately predict the methane release process in coal. Additionally, temperature has a more important effect on transient diffusion coefficient than pressure. Thermal stimulation leads to a net increase rather than a decrease in diffusion coefficient in the early diffusion stages and can also accelerate the attenuation of diffusion coefficient, with this intensifying effect becoming more pronounced at higher temperatures. The research results can provide some reference for the determination of coal seam gas content and the selection of heat injection process parameters. [ABSTRACT FROM AUTHOR]

Nowadays, some fracking fluids can enable resourceful extraction of coalbed methane and reduce greenhouse gas emissions. However, their toxicity or corrosiveness will cause harm to downhole workers and pollute groundwater resources. Thus, five kinds of clean composite fracturing fluids were developed in this paper by using starch solution as the matrix and adding various preparations. The change rule of methane adsorption capacity by microstructure changes of coal samples was investigated systematically, and the optimal composite fracturing fluid was determined. The results showed that the new fracturing fluid increased the degree of aromatic ring condensation by 43.3% and the average pore size by 52.1%. Also, the adsorption constants of a value decreased by 11.6% and b value decreased by 23.9%, which can remarkably reduce the methane adsorption. The experimental results provide theoretical support for the clean production of coalbed methane. [ABSTRACT FROM AUTHOR]

During the exploration and development of deep coalbed methane (CBM), delineating the thickness of coal seam and lithofacies of the roof and floor is one of the major challenging tasks. In past attempts, the prediction methods of these parameters have been limited to the conventional inversion. However, the effect of coal shielding on adjacent reflecting layers restricts the identification of underlying sand effectively by conventional inversion. Also, the depth at which the deep CBM zone is located (1,500-2000 m) produces a significant overlap of P-wave impedance and Vp/Vs of sands and shale which increases classification uncertainty between these two lithofacies. We proposed a new workflow for high-precision quantitative seismic interpretation of deep CBM reservoir. Not only P-wave impedance but also GR is selected as the optimized attributes for lithofacies classification. To reduce the effect of strong reflection of coal seam and identifying thin coal layers, the seismic waveform indication inversion method is used to obtain high-resolution results of P-wave impedance and GR. It uses horizontal changes in seismic waveforms to reflect lithological assemblage characteristics for facies-controlled constraints. Then, Bayesian classification theory is used to achieve three-dimensional lithofacies classification with multi-source data. To improve the continuity and accuracy of the interpreted results, a Markov chain is applied in the Bayesian rule as the spatial prior constraint. A well-associated synthetic test and field data application in Ordos Basin demonstrates the accuracy of the proposed workflow. Compared with conventional inversion, the results of proposed workflow showed higher resolution and accuracy. By providing a new solution for the identification of roof and floor lithofacies of deep CBM reservoir, this workflow aims to contribute to the better exploration and development of deep CBM. [ABSTRACT FROM AUTHOR]

Purpose. To develop a mathematical model of the “coal-gas” medium heat transfer during underground coal gasification to predict the combustion face advance velocity and the duration of gasification column mining. Methodology. To detect the temperature fields in the coal seam and gas, depending on the displacement length of the phase transition boundary, a boundary-value problem of mathematical physics has been developed. To solve this boundary-value problem, Boltzmann transformations, as well as methods for solving ordinary differential equations, are used. Newton-Raphson method, which has quadratic convergence, is used to find the roots of the transcendental equation. Findings. Tendencies in the formation of mathematical models when studying temperature fields around an underground gasifier have been analyzed, with highlighting their disadvantages. A mathematical model of heat transfer during underground gasification has been developed, taking into account the phase transition boundaries of the “coal-gas” medium. A computational experiment was conducted to determine the temperature of the phase transition boundary based on the length of the gasification column and the duration of the process. Originality. A mathematical model for heat transfer during coal gasification in the form of a boundary-value problem of mathematical physics, which consists of parabolic heat-transfer equations, the Stefan condition at the phase transition boundary, and the Dirichlet boundary conditions, has been constructed. As a result of solving the boundary-value problem, a self-similar solution has been obtained for the distribution of the coal seam and gas temperature fields, as well as the position of the phase transition boundary depending on the gasification duration and on the medium density, thermal conductivity coefficients, specific heat capacity of gas and coal, specific calorific value and temperature of coal combustion, initial coal temperature and constant temperature of the gasification process. The conducted analysis of numerical calculations provides a deeper understanding of the dynamics of underground coal gasification process and makes necessary corrections to achieve the maximum process efficiency. Practical value. A methodology for determining the displacement length of the phase transition boundary of the “coal-gas” medium has been developed, taking into account the change in the combustion face temperature along the gasification zone length depending on the duration of this process. Application of the methodology makes it possible to predict the time of mining the gasified coal column for drawing up a calendar plan for mining operations. [ABSTRACT FROM AUTHOR]

Gas saturation is a critical parameter for the selection and development of coalbed methane, as well as a key indicator reflecting the challenges in coalbed methane development and productivity evaluation of coalbed methane wells. As one of the significant factors influencing gas saturation, gas content plays a vital role in comprehensively investigating coal pore properties to fully comprehend the process and conditions of methane adsorption and desorption. In this study, 3# and 15# coals from Qinshui Basin, China was selected as research subjects. The experimental evaluation encompassed an examination of composition, pore characteristics, permeability characteristics of coal, rock mechanical parameters while discussing the impact of temperature and pressure on coal's adsorption and desorption capacity. The mineral characteristics analysis revealed that vitrinite is the main component with varying percentages and reflectance values in both 3# and 15# coal seams. The gas content and methane concentration in the 15# coal seam are higher than those in the 3# coal seam. The relationship between gas content within a coal seam and burial depth depends on achieving a balance between positive pressure effects caused by overburden stress exertion on gases trapped within pores under high pressures during burial history versus negative temperature effects due to cooling during geological processes over time. Predictions were made regarding deep-coal gas content which holds significant implications for accurately understanding variations in desorption behavior along with optimizing fracturing engineering. [ABSTRACT FROM AUTHOR]

Due to the significant pollution caused by the current extraction technology for deep coalbed methane (CBM), we propose the application of anhydrous cracking technology—liquid nitrogen cyclic freeze–thaw technology—to crack deep coal seams to achieve the release and extraction of coal bed methane. The self-made system for applying confining pressure is employed to simulate the deep coal seam in coal sample testing. Subsequently, the effect of joint structure with different angles of coal surface and bedding on the liquid nitrogen cyclic freeze–thaw process is evaluated. The investigation study identifies the damage patterns of the coal sample against the sole factor and coupling effect of confining pressure and joint angle. The findings reveal a clear impact of the cyclic freeze–thaw of liquid nitrogen on the expansion of deep coal seams. The application of appropriate inclined joints, combined with confining pressure, significantly minimizes the crushing time of coal samples, with the maximum expansion rate occurring at joints with a bedding angle of 45°±5°. This study provides vital theoretical support for the green and efficient exploitation of deep CBM. Practical Applications: The results of this study indicate that the waterless hydraulic fracturing technology—liquid nitrogen cyclic freeze–thaw technology—is highly suitable for coalbed methane extraction in deep, low-permeability coal seams, leading to increased gas recovery. The deeper the coal seam, the more pronounced the effects. Moreover, this technology does not cause environmental pollution. It specifically targets the pores, fractures, and joint structures within the coal, preserving the overall framework of the coal and preventing tunnel instability, thereby enhancing coal mining safety. Additionally, under the influence of liquid nitrogen cyclic freeze–thaw, joints at different angles exhibit varying rates of expansion. Joints extending at a 45° angle to the coal's bedding planes show the most rapid expansion, indicating that this orientation stores the maximum amount of coalbed methane postliquid nitrogen freeze–thaw. This pattern can be scientifically applied in the actual engineering design of extraction wells. The experimental results can provide a theoretical basis for guiding the on-site application of deep coal seam permeability enhancement and green coalbed methane extraction technology, thereby reducing environmental pollution. [ABSTRACT FROM AUTHOR]

The characteristics of coal desorption strain play a crucial role in coal permeability, coalbed methane (CBM) recovery, and the prevention of outbursts. This study developed an improved thermomechanical diffusion–seepage (TMDS) coupling model to investigate the strain evolution during the gas desorption process in coal. The model considers the time-varying diffusion coefficient, the Klinkenberg permeability effect, and the impact of moisture on adsorption, amending the traditional coal deformation equation and coal permeability model. Utilizing this model, the study explored the mechanism, contribution, and spatiotemporal evolution of desorption strain, while analyzing quantitatively the effects of gas types and TMDS parameters on the dynamics of desorption strain. The results demonstrate that desorption strain consists of fracture pressure, matrix pressure, desorption action, and temperature effects, with desorption action being the predominant factor. The impact of gas type, especially CO2, on desorption strain is significant, with CO2 enhancing CH4 desorption strain more than N2. Additionally, the study explored the sensitivity of desorption strain to TMDS parameters, revealing that gas pressure, permeability, and Langmuir pressure significantly impact desorption strain. Desorption strain can serve as an indicator for predicting and evaluating the risk of outbursts, and the injection of low-temperature liquid nitrogen could help reduce this risk. This research provides insights for further understanding the desorption mechanism in gas-bearing coal, improving CBM recovery, and preventing disasters. [ABSTRACT FROM AUTHOR]

Copyright of Coal Geology & Exploration is the property of Xian Research Institute of China Coal Research Institute and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The application of the hydrodynamic method has enhanced the extraction of coal bed methane (CBM). In this method, fracturing fluid rapidly penetrates the coal reservoir, altering its intrinsic pore structure and microscopic mechanical properties. These changes impact the properties of the coal reservoir and CBM depletion. It is, therefore, crucial to explore how these micro-characteristics evolve following water invasion. In this context, using nanoindentation tests, the microscopic characteristics of three coal samples were measured under dry conditions and at water saturations corresponding to 44% and 75% relative humidity. The influence of water immersion on the pore structure was also assessed using mercury injection experiments. Moreover, cluster analysis was used to categorize the extensive measured data into three sub-components: fractures (large pores), inertinite, and vitrinite, to investigate the impact of water saturation on microscopic properties. The findings indicate that cluster analysis is well-suited to these data, showing excellent agreement with porosity and maceral tests. The relationship between the elastic modulus and hardness of dry and wet coal samples varies across the sub-components. There is a notable dependency in the case of vitrinite, whereas water content tends to reduce this dependency. It is also found that water content negatively affects elastic modulus and hardness and reduces the anisotropy ratio. The mechanical properties of inertinite are highly responsive to water immersion, whereas vitrinite exhibits lesser sensitivity. The softening mechanisms of coal when immersed in water, such as calcite phase dissolution, swelling stress fracturing, and weakening of macerals, are identified. This study offers new perspectives on the impact of moisture on the alteration of micromechanical properties in coal. [ABSTRACT FROM AUTHOR]

The characteristics and heterogeneity of coal pores are crucial for understanding the production mechanism of coalbed methane (CBM). In this study, coal samples with varying degrees of metamorphism (0.58% ≤ RO, max ≤ 3.44%) were collected. The characteristics of pore development and the heterogeneous properties of pores were revealed through low-temperature nitrogen adsorption (LTNA) and low-field nuclear magnetic resonance (NMR) experiments. The results indicate that pores with varying diameters exhibit favorable development in low-rank coals, along with favorable pores connectivity. The micropores composition of middle-rank coals was found to be 73.56%, however, the connectivity among transitional, meso, and macropores was observed to be poor. In high-rank coals, the proportion of micropores was 92.74%, with numerous micropores being closed or semi-closed. This resulted in inferior connectivity between micropores and transitional pores. As coal metamorphism progressed, the DL1 (characterizing the roughness of adsorption pores (AP) surface, ranging from 2.13 to 2.45) and DL2 (characterizing the complexity of AP structure, ranging from 2.56 to 2.77) initially decreased and then increased, whereas the DN (characterizing the heterogeneity of seepage pores (SP), ranging from 2.92 to 2.95) consistently improved. Furthermore, the roughness of pore surface and the complexity of pore structure in AP increased as the specific surface area and volume of pores increased. On the contrary, as the SP content increased, the uniformity of the pore structure improved. When the volume of SP remained constant, the complexity of the pore structure decreased due to increased pore connectivity. [ABSTRACT FROM AUTHOR]

Multi-stage fracturing in Horizontal well increases the permeability of coalbed methane by generating multiple fractures. However, the heterogeneity of coal reservoirs is a crucial factor that cannot be ignored in the study of multiple hydraulic fracture propagation. Therefore, we established a two-dimensional model for multiple hydraulic fracture propagation based on the combined finite- discrete element method (FDEM) and assigned a Weibull distribution function to the heterogeneity of the physical parameters of the cohesive elements in the model. The objective was to simulate and study the fracture propagation law of multi-cluster fracturing in horizontal wells in heterogeneous coal reservoirs. The research results indicated that: 1) as the heterogeneity of the coal reservoir weakened, the deflection angle of the main fracture increased. More secondary fractures were generated in the coal reservoir, leading to significant discontinuity. 2) The fracturing disturbance area was always concentrated at the tip of the main fracture, with a double wing shape. However, the fracturing disturbance areas at the tips of multiple main fractures could easily converge, with a square shape; 3) It is recommended to use a moderate injection rate and increase the perforation spacing appropriately when hydraulic fracturing is carried out in coal reservoirs with a heterogeneity coefficient m=5. [ABSTRACT FROM AUTHOR]

The traditional Langmuir equation displays drawback in accurately characterizing the methane adsorption behavior in coal, due to it assuming the uniform surface of coal pores. Additionally, the decay law of gas adsorption capacity with an increasing coal reservoir temperature remains unknown. In this study, the fractal adsorption model is proposed based on the fractal dimension (Df) of coal pores and the attenuation coefficient (n) of the adsorption capacity. The principles and methods of this fractal adsorption model are deduced and summarized in detail. The results show that the pore structures of the two coal samples exhibit obvious fractal characteristics, with the values of fractal dimensions (Df) being 2.6279 and 2.93. The values of adsorption capacity attenuation coefficients (n) are estimated as −0.006 and −0.004 by the adsorption experiments with different temperatures. The proposed fractal adsorption model presents a greater theoretical significance and higher accuracy than that of the Langmuir equation. The accuracy of the fractal adsorption model with temperature effect dependence is verified, establishing a prediction method for methane adsorption capacity in deep coal reservoirs. This study can serve as a theoretical foundation for coalbed methane exploration and development, as well as provide valuable insights for unconventional natural gas exploitation. [ABSTRACT FROM AUTHOR]

Coalbed methane is a hot spot for gas exploration at present, and fractures are widely developed in the coals. However, despite being essential for a number of geophysical applications such as reservoir prediction and hydraulic fracturing, the influence of fractures on the elastic properties and anisotropy of coals is still poorly understood. Therefore, three groups of cylindrical coals were drilled along the face and butt cleat directions to study the effects of pressure and fracture on the elastic properties and anisotropy of tectonic coals. The velocity along the face cleat direction is less sensitive to the pressure than that along the butt cleat direction due to the difference in compliant pore and fracture content. Coals display a strong directional dependence in petrophysical and elastic properties. The permeabilities and velocities of samples along the butt cleat direction are much lower than those along the face cleat direction. The fracture densities are quantitatively characterized by the theoretical model. The values along the face cleat direction are much smaller than those along the butt cleat direction. The influence of fractures on the elastic properties and anisotropy is comprehensively studied by theoretical and laboratory methods. The mineralogy and fracture effects are important factors to the elastic anisotropy of coals, the latter can be a dominant factor to the coal elastic anisotropy. The results can contribute to the understanding of the cleat system of coalbed methane reservoir and can provide a critical rock physics basis for micro‐seismic fracture monitoring and reservoir prediction. [ABSTRACT FROM AUTHOR]

The development of coalbed methane (CBM) in China is susceptible to the influence of microstructure. Therefore, it is crucial to understand the extension laws of hydraulic fractures and engineering responses in coal reservoirs affected by microstructure development. Utilizing the Dahebian block in western Guizhou as the study area, this investigation examines the coal reservoir characteristics, fracturing, and drainage engineering analysis of the well DC1 group in the region. The aim is to discuss the spatial distribution characteristics of hydraulic fractures, geological controlling factors influenced by microstructure, and their corresponding engineering responses. The results indicate that, for coal reservoirs unaffected by microstructure, the extension laws of the fracture network in both longitudinal and planar directions are influenced by burial depth and the regional stress field. In the microstructural belt, tectonic stress dominates, causing changes in the ground stress field. Consequently, the hydraulic fracture network deviates from the direction of the maximum principal stress during the extension process. When a secondary fracture is nearby, the hydraulic fracture network extends towards the shortest path radial secondary fracture direction, leading to a rapid increase in fracture width per unit length until it intersects with the secondary fracture. Additionally, the presence of secondary joints near the microfault structure decreases fracturing pressure and results in a dense distribution of the fracture network. This promotes the formation of a complex fracture network, favorable for fracturing. The extension of the fracture network in complex structural development areas is influenced by the microfault structure between wells, which is reflected in the fracturing construction pressure and fluid output. This accounts for the significant variations in the early drainage performance of CBM wells. [ABSTRACT FROM AUTHOR]

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