Coalbed Methane Research Articles

Modelling and analysis of coalbed methane drainage quality in penetrating-strata boreholes considering air leakage

The quality and efficiency of coalbed methane (CBM) drainage are critical for the safe production of coal mining enterprises in China. Air leakage should be prevented during CBM drainage because it can reduce drainage concentration and induce produce accidents such as coal seam spontaneous combustion. However, existing theoretical models for CBM drainage do not consider the air leakage effect. This study sets up a gas–solid coupling theoretical model in which the air leakage effect is taken into account as well as gas diffusion and gas seepage. The influencing factors on the drainage quality of CBM were analyzed using numerical simulation. The results show that the CBM drainage quality linearly improves with the increase of drainage negative pressure, while increasing initial CBM pressure can linearly enhance drainage quality at the early drainage stage. The CBM drainage quality exponentially increases with Langmuir content and exponentially decreases with Langmuir pressure. With the increase of coal’s initial permeability, CBM drainage quality exponentially increases at the early drainage stage but exponentially decreases at the middle and late drainage stages. The new theoretical model effectively depicts the drop in CBM drainage quality and the increase of air leakage during the entire drainage process.

Research on Coal and Gas Outburst Prediction and Sensitivity Analysis Based on an Interpretable Ali Baba and the Forty Thieves–Transformer–Support Vector Machine Model

Coal and gas outbursts pose significant threats to underground personnel, making the development of accurate prediction models crucial for reducing casualties. By addressing the challenges of highly nonlinear relationships among predictive parameters, poor interpretability of models, and limited sample data in existing studies, this paper proposes an interpretable Ali Baba and the Forty Thieves–Transformer–Support Vector Machine (AFT-Transformer-SVM) model with high predictive accuracy. The Ali Baba and the Forty Thieves (AFT) algorithm is employed to optimise a Transformer-based feature extraction, thereby reducing the degree of nonlinearity among sample data. A Transformer-SVM model is constructed, wherein the Support Vector Machine (SVM) model provides negative feedback to refine the Transformer feature extraction, enhancing the prediction accuracy of coal and gas outbursts. Various classification assessment methods, such as TP, TN, FP, FN tables, and SHAP analysis, are utilised to improve the interpretability of the model. Additionally, the permutation feature importance (PFI) method is applied to conduct a sensitivity analysis, elucidating the relationship between the sample data and outburst risks. Through a comparative analysis with algorithms such as eXtreme gradient boosting (XGBoost), k-nearest neighbour (KNN), radial basis function networks (RBFNs), and Bayesian classifiers, the proposed method demonstrates superior accuracy and effectively predicts coal and gas outburst risks, achieving 100% accuracy in the sample dataset. The influence of parameters on the model is analysed, highlighting that the coal seam gas content is the primary factor driving the outburst risks. The proposed approach provides technical support for coal and gas outburst predictions across different mines, enhancing emergency response and prevention capabilities for underground mining operations.

Advanced Physical Simulation Technique for Investigating Coalbed Methane Coproduction in Multicoal Seams

Advanced Physical Simulation Technique for Investigating Coalbed Methane Coproduction in Multicoal Seams

Influences of Different Factors and Sensitivity Analysis of Permeability of Gassy Coal

Influencing factors and sensitivity analysis of coal permeability are significant for reasonably setting coalbed methane (CBM) extraction parameters and increasing CBM output. Seepage tests were conducted on gassy coal using a seepage test system for damaged coal and rock mass under various conditions of axial pressure, confining pressure, and gas pressure. Moreover, the influences of different factors on the permeability of gassy coal and the sensitivity of permeability to these factors were analyzed. Research results show that under the same confining pressure, the relationship between permeability and axial pressure of gassy coal meets the quadratic polynomial function; under the same axial pressure, the permeability changes with the confining pressure as a power function. The permeability of gassy coal is far more sensitive to confining pressure than to axial pressure during axial seepage. Under the same axial pressure and confining pressure (same stress), the permeability of gassy coal reduces at first and then increases in a V-shaped trend with growing gas pressure. There is a turning point in the seepage tests, that is, the critical gas pressure. When the gas pressure is lower than the critical value, the slippage effect plays the leading role in the variation of permeability of the coal; on the contrary, effective stress plays the dominant role. In the non-isobaric deviatoric stress state, the permeability of gassy coal is most sensitive to the confining pressure, followed successively by gas pressure and axial pressure. The research results provide a theoretical basis for precise gas extraction and control in coal seams.

Kinetic Mechanism and Experimental Study of Initial Temperature and Blended Gas Addition on Combustion Characteristics of Coal Bed Methane-Air

ABSTRACT Coal bed methane (CBM) is a clean energy resource, CH4 and various blends of C2H6, C2H4, CO, and H2 in different proportions were chosen to formulate a surrogate fuel for coal bed methane. The objective was to study the explosion characteristics of coal bed methane while alleviating the computational complexity associated with intricate reaction mechanisms. The explosion properties (peak explosion pressure (P max) and the maximum pressure rise rate (dp/dt) max) of the mixtures were tested. The unstretched laminar burning velocity (U l ) of the surrogate components was determined under a range of initial temperatures (T u ) (298–373 K) and blended fuel concentrations (0.4–2.0%). Two simplified kinetics models for the surrogate fuel were developed using the GRI Mech 3.0 and USC Mech II mechanisms. These models were constructed through the application of a direct relation graph with error propagation (DRGEP) and full species sensitivity analysis (FSSA). To assess the accuracy of the models, simulations based on these simplified mechanisms were compared with both the detailed mechanism and experimental data. Additionally, sensitivity analyses, utilizing the simplified mechanism, were conducted in order to identify precisely the reactions that govern the interaction dynamics between blended fuel and methane. Within the range of the experimental conditions, the results indicated that P max decreased linearly and with increasing initial temperature. (dp/dt) max was only slightly sensitive to the variation in the initial temperature. However, U l increased with increasing initial temperature, as expected. When blended fuel was added to a 7% CH4-air mixture, P max, (dp/dt)max, and unstretched U l exhibited increasing trends. An equation was derived using the experimental data, to forecast changes in U l across elevated temperatures and blended gas concentrations. The simplified mechanism model successfully replicated the results of the detailed mechanism model and demonstrated good agreement with experimental data concerning species concentrations, ignition delay times, and the U l of the coal bed methane surrogate fuel. Compared to sample 2, the chemistry of sample 1 exhibits greater sensitivity toward intermediate temperature reactions, particularly at higher temperatures.

Pore Chemical Modification of Bimetallic Coordination Networks for Coal-Bed Methane Purification under Humid Conditions.

The recycling of low-concentration coal-bed methane (CBM) is environmentally beneficial and plays a crucial role in optimizing the energy mix. In this work, we present a strategy involving pore chemical modification to synthesize a series of bimetallic diamond coordination networks, namely CuIn(ina)4, CuIn(3-ain)4, and CuIn(3-Fina)4 (where ina = isonicotinic acid, 3-ain = 3-amino-isonicotinic acid, and 3-Fina = 3-fluoroisonicotinic acid). Among these, the amino-functionalized CuIn(3-ain)4 exhibits excellent CH4 adsorption capacity (1.71 mmol g-1) and CH4/N2 selectivity (7.5) due to its optimal pore size and chemical environment, establishing it as a new benchmark material for CBM separation. Dynamic breakthrough experiments confirm the exceptional CH4/N2 separation performance of CuIn(3-ain)4. Notably, CuIn(3-ain)4 demonstrates excellent stability under wet conditions and maintains outstanding separation performance even in high-humidity environments. Additionally, theoretical simulations provide valuable insights into how selective adsorption performance can be fine-tuned by manipulating the pore size and geometry. Regeneration tests and cycling evaluations further underscore the remarkable potential of CuIn(3-ain)4 as a highly efficient adsorbent for the separation of CBM.

Coal mine methane utilization: environmental friendly catalytic technology for sustainable development

Coal mine methane utilization: environmental friendly catalytic technology for sustainable development

Numerical simulation study of coal seam dual-porosity medium fluid-solid coupling model and directional long borehole gas drainage

To accurately reflect the gas migration law during coal seam gas drainage, this study established a coal seam dual-porosity medium fluid-solid coupling model that takes into account the influence of matrix gas diffusion, fracture gas seepage, Klinkenberg effect and tortuosity. A three-dimensional numerical simulation of directional long boreholes gas drainage in coal seams was then performed using COMSOL simulation software and the accuracy of the model was verified by field test results. The research results showed that the maximum error between the effective drainage radius of directional long boreholes obtained from numerical simulation and the field test results is 6.0%, which validates the accuracy of the constructed dual-porosity medium fluid-solid coupling model in coal seams. In addition, this paper compared and analyzed the effective drainage radius of long boreholes and coal seam permeability obtained by numerical simulation under different influencing factors, and found that the effective drainage radius and permeability without considering Klinkenberg effect and fracture gas seepage are relatively small, while the effective drainage radius and permeability without considering tortuosity and matrix gas diffusion are relatively large. This indicated that Klinkenberg effect and fracture gas seepage promote coal seam gas flow, while tortuosity and matrix gas diffusion inhibit it.

Preparation and performance evaluation of a low-damage thickener suitable for fracturing medium and deep coalbed methane

The middle to deep coalbed methane reservoirs have the characteristics of higher burial depth, developed natural fractures and cleats, and are susceptible to damage from gel residue. To solve those problems, a CBM thickener was synthesized by using 2-acrylamido-2-methylpropanesulfonic acid (AMPS), acrylic acid (AA), methacryl propyl trimethyl ammonium chloride (DMC), and hydroxyethyl methacrylate (HEMA) monomers as raw materials through aqueous solution polymerization. The molecular structure and aqueous solution morphology were characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance hydrogen spectroscopy (1H NMR), and scanning electron microscopy (SEM), respectively. The drag reduction rate and viscosity of different thickener dosages were tested. When the thickener dosage were 0.1 wt%∼1.0 wt%, the drag reduction rate were 81.3%∼61.5%, and the viscosity was 6 mPa·s∼117 mPa·s, and further exploration was conducted by using a rheometer to investigate the temperature and shear resistance at 25–120 °C and the viscoelasticity at 25 °C and 120 °C of 0.6 wt% thickener aqueous solution, indicating its good performance as a fracturing fluid. Finally, the gas permeability changes of the gel breaker with a thickener dosages of 0.4 wt%∼1.0 wt% were investigated to calculate its matrix damage rate, which corresponds to 9.2%∼11.1%, and SEM have been used to characterize the morphological changes of the rock core before and after the gel breaker solution displacement, and verify its damage mechanism. Ultimately, it was proven that the synthesized CBM thickener can be suitable for fracturing operations in middle to deep coalbed methane, with high efficiency in drag reduction and low matrix damage rate.

Dynamics change of coal methane adsorption/desorption and permeability under temperature and stress conditions

Methane adsorption/desorption and permeability measurements are critical for evaluating reserves and production potential in coalbed methane (CBM) extraction. The varying temperature and stress in CBM wells have an impact on these characteristics. To understand these effects, take the Wenjiaba mining area and the Qinglong mining area in Guizhou, China, as the research objects, which are called WJB and QL for short. Characterizing the coal's surface area and pore structure using low-field nuclear magnetic resonance and low-temperature nitrogen adsorption is essential for methane flow and storage. The coal's adsorptive capacity under in situ conditions was revealed by isothermal methane adsorption tests conducted at pressures ranging from 0 to 18 MPa at different temperatures. Triaxial stress-controlled adsorption experiments simulated the impact of effective stress on methane adsorption. Stress-permeability tests evaluated the stress sensitivity and its effect on the coal's methane transmission ability, a key factor in CBM well producibility. The results showed that increased temperature reduced adsorption capacity for WJB and QL coals by 14.2% and 16.3%, respectively, while desorption rates and diffusion coefficients increased, suggesting that higher temperatures enhance desorption and diffusion. However, higher coal ranks can hinder desorption. Effective stress application led to over a 90% decrease in both adsorption capacity and permeability, emphasizing the need for stress management in CBM extraction. These insights provide a theoretical framework for the interplay between coal's pore structure, adsorption/desorption properties, and permeability under different stress and temperature conditions, guiding the optimization of CBM extraction strategies for efficient and sustainable methane recovery.

The impact of multiscale cleat geometry on coal’s petrophysical properties in the Lorraine basin, NE France: implications for Coalbed Methane (CBM) production and CO2 storage

The impact of multiscale cleat geometry on coal’s petrophysical properties in the Lorraine basin, NE France: implications for Coalbed Methane (CBM) production and CO2 storage

Experimental Investigation of the Effect of Microstructure on Coalbed Methane Adsorption Characteristics Affected by Chemical Solvents.

Coalbed methane (CBM) reservoir modification based on chemical solvent treatment could change the coal microstructure, which further affects the adsorption capacity and flow characteristics of this clean energy. Coal samples were extracted by tetrahydrofuran (THF), carbon disulfide (CS2), and hydrochloric acid (HCl). Low-pressure nitrogen adsorption, carbon dioxide adsorption, Fourier transform infrared spectroscopy, and methane isothermal adsorption test were adopted. The variations of pore structure and chemical composition in coal were evaluated by density functional theory, Barrett-Joyner-Halenda model, and peak fitting method, and their impacts on methane adsorption characteristics were studied. Results show that (1) the micropores less than 2 nm show a bimodal distribution. After acidification, micropore specific surface areas of coal samples I and II increase by 16.79% and 11.72%, respectively, while that of coal sample III-HCl decreases by 4.29% closely related to carbonate and silicate minerals in coal. Microporous specific surface areas of sample II-CS2 and III-CS2 affected by solvent rise by 4.28% and 4.17%, respectively. (2) Among the three original coal samples, the highest level of OH-OH hydrogen bonds content is observed. Solvents have a tendency to interact with -CH2 groups, which shorten the length of fat chains and increase the number of branched chains. THF tends to dissolve C-O-C groups and carbonyl C═O groups. Following CS2 treatment, samples II and III show 38.42% and 47.76% decrease in aromatic C═C area, respectively. (3) The methane adsorption capability in three coal samples is I > III > II. Except for sample I-HCl and sample III-THF, other coal samples have a lower methane adsorption capability than raw coal. Methane adsorption is reduced not just by a decrease in the number of micropores but also by aliphatic groups, aromatic structures, and oxygen-containing structures. Relevant results could deliver guiding significance for understanding methane adsorption and flow characteristics in coal after chemical solvent modification.

The plausibility of claimed induced seismicity

Claims of industrially induced seismicity vary from indisputable to unpersuasive and yet the veracity of industrial induction is vital for regulatory and operational practice. Assessment schemes have been developed in response to this need. We report here an initial assessment of the reliability of all globally known cases of proposed human-induced earthquakes and invite specialists on particular cases to refine these results. 1235 cases were assessed, requiring over 1000 h of work. From the 881 cases for which scorable evidence is available, we class 87% as ‘Confidently Induced’, 10% as ‘Probably Induced’, 2% as ‘Equivocal’ and < 1% as ‘Confidently Natural’. The most seismogenic activities are fracking, research, geothermal, water reservoir impoundment, conventional oil and gas. Least seismogenic activities are construction, deep penetrating bombs, coal bed methane. 354 cases (29%) lack enough information to be assessable. Future work could include applying data mining techniques including natural language processing and AI to uncover new evidence. Future best practice for rapid assessment of cases would ideally involve an independent panel of scientists who rapidly apply a questionnaire scheme, reach consensus, and inform a response.

Characteristics of the Microfracture and Pore Structure of Middle- and High-Rank Coal and Their Implications for CBM Exploration and Development in Northern Guizhou

The microfracture and pore structure characteristics of coal reservoirs are crucial for coalbed methane (CBM) development. This study examines the evolution of pore and fracture structures at the microscopic level and their fractal characteristics, elucidating their impact on CBM development in the northern Guizhou coal reservoirs. The results indicate that the pores and fractures in the coal reservoirs are relatively well-developed, which facilitates the adsorption of CBM. The density of primary fractures ranges from 5.8 to 14.4 pcs/cm, while the density of secondary fractures ranges from 3.6 to 11.8 pcs/cm. As the metamorphic degree of coal increases, the density of primary fractures initially increases and then decreases, whereas the density of secondary fractures decreases with increasing metamorphic degree. With increasing vitrinite reflectance, the specific surface area and pore volume of the coal samples first decrease and then increase. The fractal dimension ranges from 2.3761 to 2.8361; as the vitrinite reflectance of the coal samples increases, the fractal dimension D1 decreases initially and then increases, while D2 decreases. In the northern Guizhou region, CBM is characterized by an enrichment model of “anticline dominance + fault-hydrogeological dual sealing” along with geological controlling factors of” burial depth controlling gas content and permeability + local fault controlling accumulation”. The research findings provide a theoretical basis for the occurrence and extraction of CBM in northern Guizhou.

Effect of the Heterogeneity of Coal on Its Seepage Anisotropy: A Micro Conceptual Model

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.

Structural Characteristics of the Turning End of the Kaiping Syncline and Its Influence on Coal Mine Gas

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.

Influence of Complex Lithology Distribution on Fracture Propagation Morphology in Coalbed Methane Reservoir

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.

Adsorption and Diffusion of CH4, N2, and Their Mixture in MIL-101(Cr): A Molecular Simulation Study.

A comprehensive quantitative grasp of methane (CH4), nitrogen (N2), and their mixture's adsorption and diffusion in MIL-101(Cr) is crucial for wide and important applications, e.g., natural gas upgrading and coal-mine methane capturing. Previous studies often overlook the impact of gas molecular configuration and MIL-101 topology structure on adsorption, lacking quantitative assessment of primary and secondary adsorption sites. Additionally, understanding gas mixture adsorption mechanisms remains a research gap. To bridge this gap and to provide new knowledge, we utilized Monte Carlo and molecular dynamics simulations for computing essential MIL-101 properties, encompassing adsorption isotherms, density profiles, self-diffusion coefficients, radial distribution function (RDF), and CH4/N2 selectivity. Several novel and distinctive findings are revealed by the atomic-level analysis, including (1) the significance of C=C double bond of the benzene ring within MIL-101 for CH4 and N2 adsorption, with Cr and O atoms also exerting notable effects. (2) Density distribution analysis reveals CH4's preference for large and medium cages, while N2 is evenly distributed along pentagonal and triangular window edges and small tetrahedral cages. (3) Calculations of self-diffusion and diffusion activation energies suggest N2's higher mobility within MIL-101 compared to CH4. (4) In the binary mixture, the existence of CH4 can decrease the diffusion coefficient of N2. In summary, this investigation provides valuable microscopic insights into the adsorption and diffusion phenomena occurring in MIL-101, thereby contributing to a comprehensive understanding of its potential for applications, e.g., natural gas upgrading and selective capture of coal-mine methane.

Application of Isotope Geochemistry in Determining the Extraction Ratio of Target CBM from Multi-Seams

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.

Multi-Field Coupling Models of Coal and Gas and Their Engineering Applications to CBM in Deep Seams: A Review

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.