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Coalbed methane (CBM) is an increasingly important unconventional natural gas. Production logging can provide important information about the production status of each layer in a CBM well, which is crucial for developing and adjusting development plans. However, currently, only open-hole logging is done for CBM wells, and there is no mature technology for production testing of wells that produce low amounts of gas. To address this issue, a new method has been proposed in this paper for measuring the production profile of CBM wells. This method is based on the pressure difference method and measures the gas–liquid two-phase flow in a 125 mm vertical rising circular tube. The researchers established a simulation model of the CBM wellbore pressure difference method and obtained four flow patterns: bubble flow, slug flow, churn flow, and annular flow. We studied the relationship between the pressure difference and gas and water flow rates at different positions and spacing between measuring points in the wellbore. A differential pressure flowmeter without a throttling device was developed, and gas–liquid dynamic experiments were carried out through a simulation experiment platform to verify the feasibility of the flowmeter. Two well field tests were conducted in Shanxi CBM fields using differential pressure flowmeters, which accurately and quantitatively measured the stratified gas production of CBM wells. This technology can help improve the productivity and development efficiency of CBM wells. [ABSTRACT FROM AUTHOR]

Spontaneous combustion of coal is a major problem in coal industry. In order to improve the environmental performance and effectiveness of existing inhibitors used to inhibit coal spontaneous combustion, this study utilized physical inhibitors such as chitosan (CS), acrylic acid (AA), and acrylamide (AM) and copolymerized them with the chemical inhibitor procyanidin (OPC). The resulting composite inhibitor was then in situ polymerized with modified nano-TiO2 to create an environmentally friendly gel-based composite inhibitor, referred to as TiO2/OPC/CPAAM. Scanning electron microscopy analysis revealed a significant increase in the number of laminar folds and pores within the inhibited coal samples. Additionally, thermogravimetric analysis showed that all characteristic temperature points of the inhibited coal samples were raised, with an average increase of 21.34%. Differential scanning calorimetry also indicated a reduction in the total heat release and heat release intervals of the inhibited coal samples. Gas infrared spectroscopy provided evidence that the inhibitory treatment significantly slowed the growth of CO and CO2 during the heating process of the coal samples, leading to an improvement in environmental performance. Furthermore, diffuse reflection infrared spectroscopy testing demonstrated that the inhibitory treatment resulted in the passivation of active groups' free hydroxyls and an increase in the content of stable group ether bonds. Overall, the results of this study indicate that the composite inhibitor TiO2/OPC/CPAAM offers superior inhibition effects on coal spontaneous combustion. [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]

Fractures influence the mechanical strength of coal roof and floor, constraining the design of hydraulic fracturing for coalbed methane production. Currently, the predominant approach involves the integration of petrophysical logging with machine learning for fracture prediction. Nevertheless, challenges exist regarding the model's accuracy. In this study, we present a novel approach to predict fracture density. Our method optimises a back‐propagation (BP) neural network and utilises principal component analysis for feature extraction. We employ logging parameters (density, compensated neutron and acoustic time difference) obtained from Shouyang Block well SY‐1 and fracture density data from electrical imaging logging to construct the FVDC model's dataset. The BP neural network model is optimised using the Sparrow Search algorithm and Tent Chaotic Mapping. The results demonstrate a substantial enhancement over the BP neural network model, with reductions of 80.102% in mean absolute error, 94.182% in mean square error, 75.879% in root mean square error and 79.764% in mean absolute percentage error. When considering accuracy, the optimised model (97.098%) surpasses the support vector regression model (96.478%), the random forest model (94.404%) and the BP neural network model (85.657%). Scalability testing for the optimised model was conducted using data from well SY‐2, yielding a remarkable prediction accuracy of 96.775%. This performance exceeds that of the BP neural network (with an accuracy of 85.102%), as well as the random forest and support vector regression models (with accuracies of 91.234% and 90.384%, respectively). These results underscore the potential of well logging and machine learning in FVDC prediction. [ABSTRACT FROM AUTHOR]

ABSTRACT The low‐permeability oil‐bearing tuffaceous sandstones of the second member of the lower Jurassic Sangonghe Formation (J1s2) in the Well Pen‐1 west sag of the central Junggar Basin occur in a deep burial setting (> 4000 m). They contain abundant oil and gas shows and hold promising exploration prospects. However, the reservoir heterogeneity is strong due to complex lithofacies and diagenesis, leading to significant productivity differences between adjacent wells. Therefore, identifying the lithofacies and diagenesis of the J1s2 sandstones and elucidating their influence on the evolution of relatively high‐quality reservoirs is of utmost importance for oil and gas exploration and development in this area. Samples from the Well Pen‐1 west sag, representing the J1s2 sandstones, were investigated utilising core and thin section observations, scanning electron microscopy, X‐ray diffraction, fluid inclusions and carbon and oxygen isotope analyses. The J1s2 sandstones are mainly medium‐ to fine‐grained and moderate‐ to‐well‐sorted feldspathic litharenites and litharenites. The tuffaceous contents range from 2.6% to 25% and the authigenic clay contents, produced by diagenesis, range from 0.6% to 12%, although carbonate cements are not abundant (av. 3.1%). Four sandstone lithofacies have been identified based on mineral compositions, leading to variations in diagenetic evolution and reservoir quality. Early diagenetic events included compaction, alteration of tuffaceous matrix and feldspar, and development of smectite, chlorite, kaolinite and early calcite. Mesogenic alteration included feldspar and tuffaceous matrix dissolution, alteration of kaolinite, chlorite and illite, and precipitation of quartz, anhydrite, late calcite and ferrocalcite. The alteration of the tuffaceous matrix resulted in a complex pore‐throat structure in the J1s2 sandstones. The pebbly sandstone and conglomerate (SC) and fine‐grained sandstone (Sm) lithofacies are generally characterised by high compaction resistance, low tuffaceous matrix and cement contents, and abundant secondary dissolution pores, and they exhibit better reservoir quality and great potential for oil and gas enrichment. [ABSTRACT FROM AUTHOR]

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In Western Guizhou, China, multi-layer development is a successful way for CBM development, with drainage rate control being the essential technology. In this article, a coupled hydraulic-mechanical numerical model considering permeability velocity-sensitive damage was established to analyze the impact of drainage rate on the gas production and reservoir parameters of multi-layer CBM development. The CBM development in the study area can be divided into four stages. The permeability velocity-sensitive damage and Jamin effect mainly occurred in the first two stages. Increasing the drainage rate during the first two stages will cause more serious permeability velocity-sensitive damage. Reducing the drainage rate in the first two stages could alleviate the permeability velocity-sensitive damage and Jamin effect. During the stable production stage II, the gas seepage being dominant in the c409 coal seam, the gas production would be significantly reduced under the permeability stress-sensitive damage as the drainage rate increasing. Based on the simulation results, three recommendations concerning the drainage rate optimization of multi-layer CBM development were advanced, and gas production was successfully improved. This study has important theoretical and practical significance for guiding the multi-layer CBM development in Western Guizhou and Southern China. [ABSTRACT FROM AUTHOR]

Coal is a typical dual-porosity structural material. The injection of CO2 into coal seams has been shown to be an effective method for storing greenhouse gasses and extracting coal bed methane. In light of the theory of dual-porosity media, we investigate the impact of non-homogeneity on seepage anisotropy and examine the influence of CO2 gas injection on the anisotropy of coal and the permeability of fractures. The results demonstrate that under constant pressure conditions, coal rock has the greatest permeability variation in the direction of face cleats and the smallest changes in the direction of vertical bedding. The more pronounced the heterogeneity, the more evident the change in permeability and the less pronounced the decreasing stage of permeability. Additionally, the larger the diffusion coefficient is, the less pronounced the permeability change. The change in permeability is inversely proportional to the size of the adsorption constant and directly proportional to the size of the fracture. As the matrix block size increases, the permeability also increases, whereas the decrease in permeability becomes less pronounced. The findings of this study offer a theoretical basis for further research into methods for enhancing the CO2 sequestration rate. [ABSTRACT FROM AUTHOR]

In the process of deep coal seam mining, the problem of coal–gas compound disasters is increasingly prominent, with the safe and efficient extraction of gas serving as the key to disaster reduction. A deep coal seam gas extraction project is a complex coupled system involving multiple physical fields, such as stress fields, gas flow fields, and energy. Constructing a systematic theoretical framework of multiphysics field coupling is crucial for improving the safety and efficiency of gas extraction. This paper examines all existing multiphysics field coupling theories. It then suggests a theoretical modeling framework that is based on three important scientific issues: the coal deformation law, the gas flow law, and the coal porosity and permeability spatiotemporal distribution law. We further analyze the application and development of the model in typical coal seam gas extraction engineering on this basis. Finally, this paper points out the shortcomings of the current research and looks forward to the future research directions for the coupled coal and gas multiphysics field model, aiming to provide a theoretical basis and guidance for the model's construction and application in gas extraction engineering. [ABSTRACT FROM AUTHOR]

Frequent coal mine gas disasters pose significant threats to the safety of miners and the continuity of coal mining operations. Understanding and mastering the patterns of gas occurrence is the foundation for controlling gas outbursts. This study, drawing on previous theories, research, and practical coal mine production data, analyzes the structural characteristics of the Kaiping syncline, with particular emphasis on the structural differentiation at its northeastern uplifted end. The study examines how gas generation and storage are influenced by progressively layered structures and their effect on coal mine gas management. The results indicate that the Kaiping syncline has a NE-SW axial orientation, which gradually shifts to an asymmetric syncline with a nearly EW trend, rising towards the northeastern end. At the turning end, the strata on the northwest limb are steep—locally vertical or overturned—gradually transitioning into the gentler southeast limb with dips of 10° to 30°, further complicated by a series of sub-parallel secondary folds. The gas formation process in coal seams has undergone multiple stages, regulated by structural burial and thermal evolution. The current gas storage characteristics result from the combined effects of these structural factors. The Kaiping syncline can be divided into two gas zones: a high-gas zone in the northwest limb and a shallow low-gas zone paired with a deep high-gas zone in the southeast limb. At the turning end, structural differentiation results in significant variations and gradations in the gas storage conditions of the coal seam. This differentiation directly causes a transition from coal and gas outburst mines in the northwest limb to low-gas mines in the southeast limb, highlighting the significant influence of structural factors on gas generation, preservation, and mine gas emissions. This study integrates theoretical analysis with measured data to enhance the understanding of structural evolution and its influence on gas storage. It offers guidance for preventing coal seam gas disasters and ensuring the safe production of coal mines in the Kaiping coalfield. [ABSTRACT FROM AUTHOR]

The mineral composition in coalbed methane (CBM) reservoirs significantly influences fracture morphology, making the description of reservoir heterogeneity challenging. This study develops a fracture propagation model for CBM reservoirs that incorporates the varying mineral properties within the reservoir's lithology. Dynamic logging data are considered to characterize rock mechanical properties, which form the basis for in situ stress estimation. Using an adjusted critical circumferential stress calculation for coal rock, the model considers the impact of complex lithology on fracture propagation. A comprehensive fractal index is introduced to capture the influence of different minerals on fracture morphology and propagation randomness. Models representing clay, quartz, and pyrite with varied compositions were constructed to explore the effects of each mineral on fracture characteristics. In single-component models, clay-rich reservoirs exhibited the highest induced fracture density, with quartz and pyrite showing approximately 65% and 20% of the fracture density observed in clay, respectively. Fractures primarily propagated toward quartz-rich regions, while pyrite significantly inhibited fracture growth. In mixed-mineral models, increasing the quartz proportion by 40% resulted in a 20 m increase in fracture length and a 30% reduction in fracture density. Fractures predominantly propagated around pyrite boundaries, demonstrating pyrite's resistance to fracture penetration. Clay and quartz promote fracture development, whereas pyrite hinders fracture formation. The fracture inversion model presented here effectively captures the influence of complex mineral distributions on fracture morphology, offering valuable insights for optimizing fracturing production strategies. [ABSTRACT FROM AUTHOR]

In most coal mines in China, the target coalbed methane (CBM) and its adjacent CBM generally require joint extraction before the target coalbed can be mined. But only the mixed CBM from multi-seams can be measured at the mouth of boreholes, and it is not possible to individually measure the CBM from a target coalbed. In view of this issue, the application of isotope geochemistry is proposed for the first time in the industry. Based on the law of mass conservation for the chemical composition of mixed CBM, according to the gas components and isotope difference characteristics of each coalbed in multi-seams, a calculation model for the extraction ratio from a target coalbed is established. Field experiments were selected in the Xiaotun Mine in Guizhou Province. The extraction ratio of the target coalbed 6m determined by isotope geochemistry ranged from 57.81% to 69.58%, and the deviation from the extraction ratio determined by the measured flow method was less than 7%. The residual gas content of the No. 14 mining face was calculated by combining the results of the above two methods, and the deviation from the measured residual gas content was less than 6%, indicating that the results obtained scientifically by isotope geochemistry were reliable. The results of the study can provide a new research tool for the judgment of target CBM extraction standards and, ultimately, ensure the safety of coal mine operation. [ABSTRACT FROM AUTHOR]

With the increase of mining depth, the gas management problem in coal and gas outburst mine is more prominent. At the same time, gas is used as a clean resource to mine together with coal which is to realize the simultaneous extraction of coal and gas more practical significance. The relation between coal mining and gas extraction is mutually reinforcing and restricting. It is of great theoretical significance and application value to study simultaneous extraction of coal and gas technique optimization. The Qidong Coal Mine of the Wanbei Coal and Electricity Group is a coal and gas outburst mine. The spacing of the coal seam is small, the occurrence conditions are complex, and the mining is difficult. This study used theoretical analysis, numerical simulation, and engineering analogy to investigate the coordinated mining technology of coal and gas in outburst coal seams. [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]

The freezing method compensates for the defect of sacrificing coal integrity to reduce gas content, which is the case with traditional methods, achieving the improvement of coal body strength while reducing coal seam gas energy storage, improving the safety of coal and gas outburst accidents in deep coal seams during the process of rock cross-cut coal uncovering. This study conducted water injection and low-temperature freezing experiments on coal/rock samples under the gas atmosphere, analyzing the effects of water and temperature on sample temperature, deformation, and gas adsorption and desorption characteristics. The results indicate that water can displace adsorbed gas in coal/rock samples, and the relationship between the gas displacement and the water content of the sample satisfies an improved exponential function. The center temperature Tm of low water content coal/rock samples decreases with time and gradually tends to stabilize, while the Tm of high water content samples experiences a short-term deceleration or stagnation due to the phase transition heat release of water when it drops to around 0 °C. The cooling rate of samples with low water content and no gas is higher and that of rocks is higher than that of coal samples. Coal/rock samples with high water content experience frost heave during the freezing process, but the overall deformation is still dominated by cold shrinkage, and the amount of deformation is negatively correlated with temperature and water. The gas adsorption capacity of coal decreases linearly with the temperature. At the same time, an increase in water content and a decrease in freezing temperature will significantly reduce the gas desorption capacity of coal samples, effectively reducing the gas expansion energy of coal samples, especially the desorption gas expansion energy. In engineering implementation of this method, the ice phase network can fill the coal pores and cracks and improve the mechanical properties of the coal/rock mass, and the gas pressure in the coal seam and stress concentration near the coal rock interface can be reduced by low temperature and cold shrinkage, thereby achieving safe exposure of the coal seam and preventing accidents from occurring. [ABSTRACT FROM AUTHOR]

Deep coal seams in China have the characteristics of low permeability, low porosity, and poor gas permeability, which makes the extraction of coalbed methane very difficult. To study the modification effect of acidic solutions on coal pore structure and mineral components, this study used three different concentrations (5%, 10%, and 15%) of organic acids (HCOOH and CH3COOH) to treat coal. Through industrial/elemental analysis, low-temperature nitrogen adsorption experiments, and x-ray fluorescence spectroscopy detection experiments, the pore size distribution, mineral elements, and oxides of the acidified coal were analyzed. The strength of the acid solution's modification effect on coal was analyzed, as well as the relationship between different mineral elements and changes in pore structure. The results show that the two acids have a pore expanding effect on coal, with an increase in average pore size; the pore size distribution is between 2 and 50 nm, belonging to mesopores; acetic acid has a stronger overall dissolution efficiency on inorganic minerals, while formic acid has a better effect on the removal of Ca and P elements, but none of them can effectively remove silicoaluminates; the content of Ca element in coal is positively correlated with the specific surface area provided by mesopores; and iron dolomite is not sensitive to the reaction with acetic acid, resulting in an opposite trend in Fe content and mesoporous specific surface area. Organic acid solution changes the pore and fracture distribution structure of coal through chemical dissolution by dissolving the mineral components inside the coal, thereby affecting its internal structural characteristics. [ABSTRACT FROM AUTHOR]

Gas outbursts in coal seams represent a severe and formidable hazard, posing a significant threat to the safety of coal mining operations. The advanced early warning is a crucial preventive measure against outbursts. Acoustic emission (AE) and electromagnetic radiation (EMR) are advanced monitoring and early warning techniques for gas outbursts. However, during the mining operations, interference signals from AE and EMR may arise. Due to the impact of these interference signals, the use of statistical indicators and time-frequency feature analysis may lead to false alarms and missed detections in outburst warnings. The advancement of deep learning offers new methods for intelligent identification of gas outburst risks. This article proposes an outburst warning method for detecting outburst precursor signals and conducting comprehensive index analysis based on deep learning techniques for AE and EMR. First, reconstruct the signal using wavelet packet decomposition and then process the resulting signal with the diffusion-semi-supervised classification algorithm, employing partially labeled signals to train the model for intelligent identification of outburst precursor risk indicators of AE and EMR. By analyzing the prominent risk precursor signals of AE and EMR, establish a gas outburst risk analysis method based on Bayesian networks, thereby achieving early warning of gas outbursts. The findings suggest that the method in question, which employs a training dataset comprising 60% manually annotated data, is proficient in precisely identifying to outburst precursor signals of AE and EMR, and is adept at identifying a range of precursor signals. It provides a basis for distinguished multi-level early warning. The research outcomes significantly enhance the reliability of AE and EMR monitoring signals, offering effective monitoring and early warning for gas outbursts in coal seams, gas power manifestations, and abnormal gas. [ABSTRACT FROM AUTHOR]

With the continuous mining of shallow coal resources, deep mining has increasingly become the norm. However, the migration mechanism of coalbed methane (CBM) in coal seams becomes exceptionally complex due to the combined influence of multiple factors in deep mining, posing considerable challenges to coal and gas co-mining. Therefore, studying the coal's mechanical behavior and seepage evolution mechanisms during deep mining is necessary. This study established a coal permeability model based on elastoplastic mechanics, considering the impacts of coal matrix destruction on the average fracture aperture. It assumed that the fracture aperture follows an exponential distribution and further introduced plastic strain to characterize the damage process in coal. The proposed permeability model was validated using the indoor experimental data. Subsequently, the control mechanisms of force-heat coordination effects on coal permeability were discussed, and the sensitivity of model parameters was analyzed. The results demonstrated that the established permeability model effectively described the evolution of coal permeability under the combined impacts of temperature and effective stress. Moreover, the fracture number ratio (η) and the influence coefficient of plastic strain increment on the average fracture aperture (β) not only connected the dilation of microfractures and plastic deformation in coal but also effectively reflected the relationship between permeability and plastic deformation during the failure process of coal. The results presented in this paper contributed to understanding the evolution of permeability during coal and gas co-mining, which should be of great significance for reducing coal and gas outburst hazards. [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]

With the gradual increase in China's coal mining depth, coal seams exhibit high gas content, high gas pressure, and low permeability, exacerbating the difficulty of gas drainage. Hydraulic punching has become an effective means to improve the efficiency of coal seam gas drainage by relieving pressure and increasing permeability in coal seams. To solve the problem of low permeability and difficult gas drainage in the No. 3 coal seam of Changping Coal Mine, hydraulic punching was implemented in the 5302 floor drainage tunnel and 53 023 tunnel, which were used as the engineering background. First, based on the elastic mechanics strain softening model and the diffusion seepage coal seam gas flow theory model, the evolution equation of coal seam permeability and gas flow equation after hydraulic punching was established, and numerical simulation software was used to analyze the pressure relief and permeability enhancement mechanism and permeability distribution law of hydraulic punching. Then, the field tests of ordinary and hydraulic punching boreholes were conducted to compare the gas drainage effect. The results showed that the effective influence radius of hydraulic punching was 4–5 m, and the influence radius was 5–8 m. The reasonable punching water pressure was 17.51 MPa, and the reasonable punching time was 60 min. The gas drainage concentration of cross-measure borehole hydraulic punching and inseam borehole hydraulic punching was 1.77 and 1.49 times higher than that of ordinary borehole, respectively. From the gas drainage concentration index perspective, the drainage effect of cross-measure borehole hydraulic punching was the best. However, the gas drainage purity was 2.88 and 3.08 times, respectively. From the gas drainage purity index perspective, the drainage effect of inseam borehole hydraulic punching was the best. The coal seam driving efficiency in the area affected by punching increased by 37%. Hydraulic punching can effectively eliminate coal seam outburst hazards, improve coal seam permeability, and ensure the safe and efficient production of mining faces. [ABSTRACT FROM AUTHOR]

Deep coalbed methane (CBM) flow exhibits various transportation mechanisms within the multiscale pore structures of reservoirs, including continuous flow, Knudsen diffusion, and surface diffusion. Current research predominantly emphasizes the effects of individual or partial flow mechanisms and single-factor influences on the multiscale migration of CBM. We proposed a new apparent permeability model that integrates multiple flow mechanisms to enhance our understanding of the factors governing CBM flow in complex fractured networks. This model accounted for stress sensitivity, adsorbed gas desorption, and matrix shrinkage. By assigning appropriate weights to different flow mechanisms, the model yielded a more accurate representation of the deep CBM apparent permeability, avoiding the overestimation resulting from the linear superposition of diverse migration mechanisms. Our findings indicated that the apparent permeability was positively correlated with compressibility and negatively correlated with the tortuosity and Poisson's ratio. In the presence of the adsorbed gas, the apparent permeability of organic matter showed heightened sensitivity to formation pressure, rock compressibility, and tortuosity. However, the impacts of these factors became less pronounced when the pressure differential was small. The proposed model was applied to the flow simulations for a multi-fractured horizontal well within a deep coal reservoir characterized by a complex fracture network. The simulation results agreed well with the production data. We found that continuous flow was the dominant contributor to the apparent permeability of organic and inorganic matter within the coal rock, followed by Knudsen diffusion and surface diffusion. This study provided insights into the evolution of apparent permeability of CBM during development and offered valuable guidance for the analysis of CBM production dynamics, productivity forecasting, and production system design. [ABSTRACT FROM AUTHOR]

Deep coalbed methane (CBM) resources are enormous and have become a hot topic in the unconventional exploration and development of natural gas. The fractures in CBM reservoirs are important channels for the storage and migration of CBM and control the high production and enrichment of CBM. Therefore, fracture prediction in deep CBM reservoirs is of great significance for the exploration and development of CBM. First, the basic principles of calculating texture attributes by gray-level cooccurrence matrix (GLCM) and gray-level run-length matrix (GLRLM) were introduced. A geological model of the deep CBM reservoirs with fractures was then constructed and subjected to seismic forward simulation. The seismic texture attributes were extracted using the GLCM and GLRLM. The research results indicate that the texture attributes calculated by both methods are responsive to fractures, with the 45° and 135° gray level inhomogeneity texture attributes based on the GLRLM showing better identification effects for fractures. Fracture prediction of a deep CBM reservoir in the Ordos Basin was carried out based on the GLRLM texture attributes, providing an important basis for the efficient development and utilization of deep CBM. [ABSTRACT FROM AUTHOR]

The utilization of CO2‐Enhanced Coal Bed Methane (CO2‐ECBM) technology is pivotal in realizing the environmentally responsible and efficient exploitation of Coalbed Methane (CBM) energy resources. The optimization of carbon capture, utilization, and storage (CCUS) for carbon reduction mandates a nuanced understanding of the diverse geological attributes present in CBM reserves globally. Traditional estimations of CO2‐ECBM's carbon sequestration potential have predominantly relied on rudimentary empirical models, notably those proposed by the United States Department of Energy (DOE), which overlook the intrinsic geological conditions and the physicochemical properties of subsurface fluids. Addressing these limitations, our study implements the advanced DR/Henry mixed adsorption model in tandem with the Peng‐Robinson equation of state (PR‐EOS). This approach meticulously identifies the critical parameters governing the mass exchange ratios between CO2 and CH4, pertinent to in‐situ geological environments. Subsequently, we have formulated a comprehensive carbon sequestration potential assessment framework. This innovative model adheres to the mass conservation principles for individual CO2 and CH4 components, taking into account the specific surface and stratigraphic conditions prevalent. Employing this refined methodology, we evaluated the CO2‐ECBM carbon sequestration potential of the 40 evaluation units of extensional, compressive, and cratonic continental coal bearing basins in China's three major temperature‐pressure systems across different depth domains and coal ranks within 2000 m. Our findings reveal that the theoretical carbon sequestration capacity of China's continental coal‐bearing basins is approximately 59.893 billion tons. Concurrently, the potential ECBM output stands at an estimated 4.92 trillion cubic meters, underscoring the substantial environmental and energy benefits inherent in harnessing CO2‐ECBM technology effectively. The regional analysis revealed that North and Northwest China hold the highest sequestration and recovery potential, followed by the Northeast and Southern regions, respectively. Specific areas, including the eastern edge of the Ordos Basin and southern Junggar Basin, Qinshui, Huoxi, Xishan, and other areas in Shanxi, present promising future prospects for geological carbon storage in unrecoverable coal seams. [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]

The expansion of hydraulic fractures in coalbed methane (CBM) reservoirs is key to effective stimulation, making it essential to understand fracture propagation and its influencing factors for efficient resource development. Using petrological characteristics, logging data, microseismic monitoring, and fracturing reports from the Baode Block on the eastern Ordos Basin, this study systematically investigates the geological and engineering factors influencing hydraulic fracture propagation. The real-time monitoring of fracture propagation in 12 fractured wells was conducted using microseismic monitoring techniques. The results indicated that the fracture orientations in the study area ranged from NE30° to NE60°, with fracture lengths varying between 136 and 226 m and fracture heights ranging from 8.5 to 25.3 m. Additionally, the fracturing curves in the study area can be classified into four types: stable, descending, fluctuating, and falling. Among these, the stable and descending types exhibit the most effective fracture propagation and are more likely to generate longer fractures. In undeformed–cataclastic coals and bright and semi-bright coals, long fractures are likely to form. When the Geological Strength Index (GSI) of the coal rock ranges between 60 and 70, fracture lengths generally exceed 200 m. When the coal macrolithotype index (Sm) is below 2, fracture lengths typically exceed 200 m. When the difference between the maximum and minimum horizontal principal stresses exceeds 5 MPa, fractures with length >180 m are formed, while fracture heights generally remain below 15 m. From an engineering perspective, for the study area, hydraulic fracturing measures with a preflush ratio of 20–30%, an average sand ratio of 13–15%, and a construction pressure between 15 MPa and 25 MPa are most favorable for coalbed methane production. [ABSTRACT FROM AUTHOR]

The exploration of coal bed methane (CBM) plays a pivotal role in mitigating the greenhouse effect and promoting clean energy supply on a global scale. The Radial Jet Drilling (RJD) technology can create several drainage holes to improve the recovery of CBM by creating fracture network in coal seam. However, the damage pattern and fracture mechanism of coal between radial branch boreholes (RBBs) remain unclear. In this study, we performed RJD experiments under triaxial stress using raw coal specimens. Our findings reveal that the triaxial stress state significantly inhibits coal damage induced by RJD. Specifically, as the stress differential increases, the damage effect becomes more pronounced, accompanied by crack coalescence between RBBs. The fundamental principle underlying RJD's ability to enhance coal seam permeability lies in the initial deformation of RBBs under stress difference, followed by the interconnection of cracks induced by this deformation. Notably, the deformation behavior of RBBs in coal differs markedly from that observed in other geological materials. While boreholes in sandstone, shale, and similar materials exhibit deformation tips pointing toward the σ3 direction, RBB deformation in coal aligns with the σ1–σ2 plane. This observation underscores the crack-inducing capacity of RBBs, allowing them to overcome the constraints imposed by in-situ stress within coal seams. The results presented herein serve as an experimental reference for designing RJD applications in coal seams. Highlights: Radial Jet Drilling (RJD) experiments under triaxial stress using raw coal specimens are conducted. The damage pattern and fracture mechanism between RBBs under various stress difference is revealed. The mechanism of RJD to enhance coal seam permeability by inducing fracture between RBBs is investigated. The unique occurrence of borehole breakout in coal rock RBBs is outlined and contrasted with deformation patterns observed in boreholes across diverse geological materials. [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]

Horizontal well fracturing in the roof of coal seams has been demonstrated to be an effective technique for extracting coalbed methane. However, the mechanism of hydraulic fracturing fracture expansion from roof to coal seam and penetrating the interlayer in multiple stage fracturing is not comprehensive. Mechanical properties of the reservoir, fracturing parameters, and fracturing mode are the mainly factors that influence extraction efficiencies. To explain the relationship between fracturing behaviors and coal, the interlayer and roof along with the fracture propagation and penetration in multiple stages of fracturing; the interaction mechanism of fracture propagation; and penetration with elastic modulus, injection velocity, and fluid viscosity are simulated in single and staged fracturing. Additionally, the effect of the fracturing mode on fracture propagation is considered. The results show that the coal seam strength dominantly influences longitudinal fracture propagation while marginally affecting circumferential stress. Significant modulus differences between coal-rock interlayers and coal seams hinder fracture propagation. In the case of a low injection rate and fluid viscosity, increasing the injection rate and fluid viscosity can help enlarge the fracture area and facilitate penetration. Multistage fracturing and its high injection rates are conducive to maximizing the efficiency of fracturing operations. These results contribute to understanding and optimizing hydraulic fracturing in coal seams for efficient coalbed methane extraction. [ABSTRACT FROM AUTHOR]

The nanometer-sized pores within coal are the primary sites for CH4 adsorption and competitive adsorption with CO2. Reasonable modification of the nanopore structure to enhance CH4 desorption, diffusion rates, and CO2 competitive adsorption effects can enhance significantly coalbed methane (CBM) production. However, ultrasonic synchronous modification of multiple features of nanopores leads to complex and variable gas adsorption behaviors in coal. To reveal the effect of ultrasonic modification of coal nanopores on gas adsorption, pore measurement experiments and molecular simulation studies were conducted. The results showed that the volume ratio of diffusion pores to adsorption pores (V2/V1) decreased significantly after ultrasonic excitation. In the original coal sample, V2/V1 was 3.05, while in the coal sample after ultrasonic treatment, V2/V1 ranged from 0 to 2.54. With decrease in the proportion of the volume of diffusion pores, the proportion of CH4 migration from the pore walls of the adsorption pores increased continuously. The proportion of CH4 migration from the pore walls of the diffusion pores to the pore space of the diffusion pores decreased continuously. The results of gas–solid interaction energy calculation showed that ultrasonic treatment of coal decreases the V2/V1 ratio, leading to 7.1–23.3% increase in CO2 competitive adsorption effect. It also resulted in 4–49% improvement in competitive adsorption efficiency. Additionally, based on gas–solid interaction energy data, an adsorption capacity evaluation model for coal under different gas compositions and pore volume ratios was constructed. The findings can guide ultrasonic-enhanced CBM. [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]

Accurate characterization of coal reservoir micro- and nanopores is crucial in evaluating coalbed methane storage and gas production capacity. In this work, 12 coal-bearing rock samples from the Jurassic Yan'an Formation, Longdong area, Ordos Basin were taken as research objects, and micro- and nanopore structures were characterized via scanning electron microscopy, high-pressure mercury pressure, low-temperature N2 adsorption and low-pressure CO2 adsorption experiments. The main factors controlling coal pore structure development and the influence of pore development on the gas content were studied by combining the reflectivity of specular samples from the research area, the pore microscopic composition and the pore gas content determined through industrial analyses and isothermal absorption experiments. The results show that the coal strata of the Yan'an coal mine are a very important gas source, and that the coal strata of the Yan'an Formation in the study area exhibit remarkable organic and clay mineral pore development accompanied by clear microfractures and clay mineral interlayer joints, which together optimize the coal gas storage conditions and form efficient microseepage pathways for gas. Coalstone, carbonaceous mudstone and mudstone show differential distributions in pore volume and specific surface area. The general trend is that coal rock is the best, carbonaceous mudstone is the second best, and mudstone is the weakest. The coal samples' microporous properties are positively correlated with the coal sample composition for the specular group, whereas there is no clear correlation for the inert group. An increase in the moisture content of the air-dried matrix promotes adsorption pore development, leading to increases in the microporous volume and specific surface area. CH4 adsorption in coal rock increases with increasing pressure, and the average maximum adsorption is approximately 8.13 m3/t. The limit of the amount of methane adsorbed by the coal samples, VL, is positively correlated with the pore volume and specific surface area, indicating that the larger the pore volume is, the greater the amount of gas that can be adsorbed by the coal samples, and the larger the specific surface area is, the greater the amount of methane that can be adsorbed by the coal samples. The PL value, pore volume and specific surface area are not correlated, indicating that there is no direct mathematical relationship between them. [ABSTRACT FROM AUTHOR]

Coal structures exhibit considerable differences in rock properties and adsorption capacities. The physical properties of coal rocks are fundamental to understanding oil and gas reservoirs, while adsorption capacity directly impacts the gas content in coal seams. The accurate recognition of coal structures is essential for evaluating productivity and guiding coalbed methane well development. This study examines coal rocks of Benxi Formation in Ordos Basin. Using core photographs and logging curves, we classified the coal structures into undeformed coal, cataclastic coal, and granulated-mylonitized coal. AC, DEN, CAL, GR, and CN15 logging curves were selected to build a coal structure recognition model utilizing a long short-term memory (LSTM) neural network. This approach addresses the gradient vanishing and exploding issues often encountered in traditional neural networks, enhancing the model's capacity to handle nonlinear relationships. After numerous iterations of learning and parameter adjustments, the model achieved a recognition accuracy of over 85%, with 32 hidden units, a minimum batch size of 28, and up to 150 iterations. Validation with independent well data not involved in the model building process confirmed the model's effectiveness, meeting the practical needs of the study area. The results suggest that the study area is predominantly characterized by undeformed coal, with cataclastic coal and granulated-mylonitized coal more developed along fault trends. [ABSTRACT FROM AUTHOR]

Deep coalbed methane (CBM) resources are abundant in China, and in the last few years, the country's search for and extraction of CBM have intensified, progressively moving from shallow to deep strata and from high-rank coal to medium- and low-rank coal. On the other hand, little is known about the gas content features of deep coal reservoirs in the eastern Junggar Basin, especially with regard to the gas content and the factors that affect it. Based on data from CBM drilling, logging, and seismic surveys, this study focuses on the gas content of Baijiahai Uplift's primary Jurassic coal seams through experiments on the microscopic components of coal, industrial analysis, isothermal adsorption, low-temperature CO2, low-temperature N2, and high-pressure mercury injection. A systematic investigation of the controlling factors, including the depth, thickness, and quality of the coal seam and pore structure; tectonics; and lithology and thickness of the roof, was conducted. The results indicate that the Xishanyao Formation in the Baijiahai Uplift usually has a larger gas content than that in the Badaowan Formation, with the Xishanyao Formation showing that free gas and adsorbed gas coexist, while the Badaowan Formation primarily consists of adsorbed gas. The coal seams in the Baijiahai Uplift are generally deep and thick, and the coal samples from the Xishanyao and Badawan formations have a high vitrinite content, which contributes to their strong gas generation capacity. Additionally, low moisture and ash contents enhance the adsorption capacity of the coal seams, facilitating the storage of CBM. The pore-specific surface area of the coal samples is primarily provided by micropores, which is beneficial for CBM adsorption. Furthermore, a fault connecting the Carboniferous and Permian systems (C-P) developed in the northeastern part of the Baijiahai Uplift allows gas to migrate into the Xishanyao and Badaowan formations, resulting in a higher gas content in the coal seams. The roof lithology is predominantly mudstone with significant thickness, effectively reducing the dissipation of coalbed methane and promoting its accumulation. [ABSTRACT FROM AUTHOR]

In view of the gas flow problem in the hydraulic fracturing engineering of L-type horizontal well and in the process of gas production in coalbed methane development, the permeability stress test of single fracture coal-rock combination was carried out. The permeability evolution law of fractured coal-rock combination under axial compression, confining compression, and gas pressure is analyzed. The axial compression has little effect on the permeability of coal-rock combination with fractures, and the permeability has no obvious change. Taking 0.25 MPa gas pressure as an example, the permeability of N1 fluctuates between 0.004 and 0.005 mD, and the permeability of N2 and N3 samples fluctuates between 0.002 and 0.0035 mD. With the increase of confining compression, the permeability decreases with the increase of confining compression. Taking the N4 sample as an example, when the gas pressure is 0.50 MPa, the permeability of N4 rapidly decreases from 0.351 to 0.0025 mD. The permeability has decreased by 99.3%. With the increase of gas pressure, the permeability and stress sensitivity of fractured coal-rock combination decrease gradually. In the process of gas pressure loading, the permeability decreases greatly due to the existence of gas slippage effect in the low-pressure stage. When the gas pressure exceeds 1 MPa, the joint action of slip effect and velocity-sensitive effect makes the permeability almost unchanged. Taking the N4 sample as an example, when the axial and confining pressures are 6 MPa, the permeability decays from 1.298 to 0.382 mD, with a decay ratio of 70.6%. Finally, the permeability calculation model of single fracture coal-rock combination under in situ stress and gas pressure can well match the experimental data and clarify the influence of each permeability on the overall permeability. The permeability model shows that the overall permeability depends on the part of the smaller permeability, and the higher permeability only makes the overall permeability increase slightly. [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]

The objective of this work is to investigate the implications of geological influence factors on gas content and geochemical characteristics of deep-buried (> 800 m) coalbed methane (CBM) reservoirs. Results show that bituminous coal accounts for the majority, which exhibits similar maturity but differ in maceral and chemical constituents. CBM reservoirs show low porosity, low permeability and moderate temperature, with thickness of 0.85–4.15 m. In addition, the total gas content is 4.58–12.33 m3/t (average of 8.83 m3/t). CH4 is the main component with concentration of 92.83−99.22% (average of 96.68%), and the δ13CCH4 and δ13DCH4 is − 53.78‰–−44.62‰ (average of − 48.82‰) and − 223.93‰––−4.49‰ (average of − 215.37‰), respectively. All CBM samples are the mixtures of thermogenic gases and secondary biogenic gases with CO2 reduction. In addition, gas content characteristic at the critical point of burial depth is the result by positive and negative geological effects. CH4 concentration shows a wide range with the increases of buried depth, while the numerical values of which for the selected samples display complex variation characteristics. Furthermore, the values of δ13CCH4 and δ13DCH4 become heavier with the increases of buried depth. Besides, the above two geochemical parameters are related to Ro, max and reservoir temperature. [ABSTRACT FROM AUTHOR]

The study aimed to investigate the adsorption properties of methane (CH4) and carbon dioxide (CO2) in subbituminous coals from the Mamu and Nsukka formations, focusing on the CO2-Enhanced Coalbed Methane (ECBM) method. Proximate, ultimate, and FT-IR analyses determined the quality, age, and functional categories of these coals, confirming their subbituminous nature. Using molecular dynamics (MD) simulations, a unique amorphous subbituminous coal model was developed to study adsorption phenomena. Isosteric heat and adsorption isotherms for pure CO2 and CH4 were analyzed, alongside Grand Canonical Monte Carlo (GCMC) simulations to assess CO2 adsorption selectivity in a binary CO2 and CH4 mixture. Results showed that CO2 required more isosteric heat than CH4 in single-component scenarios and demonstrated stronger electrostatic interactions with heteroatom groups in the coal model, explaining its higher adsorption preference. In binary adsorption experiments, CO2 exhibited a higher affinity under specific conditions, particularly influenced by pressure variations. At lower pressures, CO2 selectivity decreased rapidly with increasing temperature, while at higher pressures, the influence of temperature diminished. These findings have established a theoretical and practical basis for optimizing CO2-ECBM extraction in Nigeria, highlighting the preferential adsorption of CO2 over CH4 in subbituminous coals from the Mamu and Nsukka formations under varying pressure and temperature conditions. Implementing CO2-ECBM extraction and storage in Nigeria could boost economic viability and help achieve net-zero goals, using insights from this study to guide policy development. Highlights: • Studied CH4 and CO2 adsorption on subbituminous coal, focusing on CO2-ECBM method. • Proximate, ultimate, and FT-IR analyses confirmed the subbituminous nature of coals from the Mamu and Nsukka formations in the Anambra Basin and developed a unique amorphous subbituminous coal model using molecular dynamics (MD). • Results showed that CO2 required more isosteric heat than CH4 and exhibited stronger electrostatic interactions, leading to higher adsorption preference. • Establish a theoretical and practical basis for optimizing CO2-ECBM extraction in Nigeria. [ABSTRACT FROM AUTHOR]

The development of deep coalbed methane has become an important way to obtain natural gas in China. The development of deep CBM mainly depends on horizontal well technology. The different orientations of horizontal wells will have an important impact on the productivity of coalbed methane wells. The angle grid geological model of coalbed methane reservoirs with different inclination angles is established, and the deployment orientation of horizontal wells is changed to study the optimal deployment orientation of deep-saturated coalbed methane reservoirs. When CBM horizontal wells in deep saturated CBM reservoirs are deployed upward along the dip, well-controlled reserves, peak daily gas production, and cumulative gas production increase as the dip decreases. When deploying down the dip, with the increase in dip angle, the well-controlled reserves increase, and the peak daily gas production and cumulative gas production first increase and then decrease. In the low-dip reservoir, the development effect of horizontal wells deployed in different directions is better than that in the up-dip direction. In the high-dip reservoir, the development effect of horizontal wells deployed along the strike is better than that in the up-dip and down-dip directions. The development effect of horizontal wells is controlled by both well-controlled reserves and reservoir pressure drop. Because this method is targeted at different geological conditions, it can be used to guide the horizontal well optimization of other coalbed methane blocks and has very important significance for the development and optimization of coalbed methane reservoirs. [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]

The assessment of highly mature source rocks linked to hydrocarbon generation remains a challenge in oil and gas exploration. However, substantial terrigenous influences and thermal variations have complicated the formation and evolution of source rocks. This study presents an integrated assessment of highly mature source rocks in the Fuxin Basin, based on sedimentological, geochemical, and organic petrological analyses. Two types of oil- and coal-bearing source rocks were deposited in the semi-deep lake and shore–shallow lake facies during the Jiufotang and Shahai periods. The development of source rocks migrated eastward alongside the lacustrine depocenter, influenced by basin evolution related to extensional detachment tectonism. Furthermore, a gradual increase in thermal records was detected from the western to eastern basins. Consequently, thermal decomposition of source rocks in the Jiufotang formation reduced the organic matter (OM) abundance in the central and eastern basins. Meanwhile, OM types of source rocks range from kerogen type-II1/-I to type-II2/-III, with intense hydrogen generation observed from the western to eastern basins. Consequently, the quality and hydrocarbon accumulation of source rocks are influenced by sedimentation and thermal maturity variation. The spatiotemporal variation in mature source rocks enhances the potential for exploring conventional petroleum, coalbed methane, and shale gas across different strata and locations. Our findings illustrate the significance of the sedimentary and thermal effects in characterizing the evolution of highly mature source rocks, which is relevant to determine oil and gas exploration in similar geological settings. [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]

The high gas pressure, low permeability, and strong gas absorption of coal seams in China complicate gas extraction, severely restricting the efficient development of coalbed gas. Liquid CO2 (LCO2) has a dual effect of cracking and enhancing the permeability of coal rock, thus, enhancing gas recovery. In this study, experimental testing and comparative analyses were performed to analyze the LCO2 acidification of antireflection coal, for which reference and variable experimental groups were designed. The acidification effect was quantitatively analyzed by examining changes in the pH value of the reaction solution, mineral content of coal, and pore structure during the experimental process. The experimental results indicated that a higher pressure resulted in a greater amount of CO2 being dissolved and a stronger acidity of the reaction water sample. As the reaction time increased, the minerals in the coal gradually dissolved and more H+ ions in the solution were consumed. The calcite (carbonate) and illite (clay mineral) contents significantly decreased, which is the main reason for the change of coal pore structure. The minerals on the pore surface of coal react with (CO2–H2O) weak acid, which increases the development of pore branches, improves the complexity of coal pores, and roughens the pore surface. Acidification significantly increases the number of micropores (<10 nm) and small pores (10–100 nm) in coal; furthermore, pore development is simple, the surface is smooth, and the pore connectivity is improved, which makes this part of pores have fractal characteristics. CO2–H2O–coal acidification increases the pore volume corresponding to the CO2/CH4 gas slippage flow in coal and strengthens the mass and energy transfer of CO2/CH4 in coal. CO2 can enter more pores than CH4 and displace the adsorbed and free CH4 in the pores. [ABSTRACT FROM AUTHOR]