Coalbed Methane Extraction Research Articles

The effect of lamination on elastic anisotropy of primary coals under confining pressure: Experiment and theoretical modelling

AbstractThe deep coalbed methane development is in its initial stage, with limited research on elastic anisotropy of coals in deep coalbed methane reservoirs. To study the elastic anisotropy of coals in deep coalbed methane reservoirs, three sets of cylindrical primary coals are collected from the relatively shallow area of the Taiyuan 8# coal formation, a key site for deep coalbed methane extraction. The microscopic observation, basic physical property test, ultrasonic velocity measurement and theoretical modelling are constructed to study the pressure sensitivity of elastic properties and the factors affecting the elastic anisotropy of coal samples. The velocity increases rapidly with confining pressure increasing at confining pressures below 13 MPa but relatively stabilizes at higher pressures. The velocity perpendicular to the bedding plane is more sensitive to pressure than that parallel to the bedding plane. As confining pressure increases, the velocity anisotropy decreases but remains noticeable at the highest pressure. Based on the microstructure and ultrasonic experiment results, an anisotropic rock physics model for the deep coalbed methane reservoir is constructed to quantitatively analyse the effects of the clay and organic matter and pore structure on the elastic anisotropy of coals. The modelling analysis indicates that P‐ and S‐wave velocity anisotropies decrease as the degree of preferred orientation of organic matter and clay decreases, which increases with the increase of organic matter and clay content. The equivalent pore aspect ratio and lamination index of studied coals are obtained from a practical inversion scheme based on the proposed model. The results can support the anisotropic rock physics inversion of deep coalbed methane reservoirs and the accurate prediction of engineering sweet spot parameters.

Impact of controllable parameters in hydraulic slotting on stress relief effects around coal seam

Hydraulic slotting technology has broad application prospects in coal seam gas management. Using the FLAC3D finite difference software and an evolution model considering plastic damage, the unloading damage characteristics under unequal pressure conditions during the hydraulic slotting process are simulated. The results show that factors such as borehole diameter, drilling depth, slot radius, and borehole spacing significantly impact the unloading damage effect. Specifically, increasing the borehole diameter, drilling depth, and slot radius expands the unloading damage area, forming “butterfly-shaped” or “elliptical” plastic zones. Reducing the spacing between parallel slots causes overlapping unloading damage in the coal body between adjacent slots, further enhancing the stress reduction effect. The “high–low–high” pattern of parallel slots effectively supplements and strengthens the weak stress unloading zones, better fracturing the coal seam. Field testing confirms that hydraulic slotting significantly increases borehole radius and gas extraction. These results validate simulations and enhance coalbed methane extraction's safety and efficiency, providing robust guidance for optimizing parameters.

Study of Key Factors for Efficient Coalbed Methane Extraction: Pore Structure, Desorption Rate and Seepage Characteristics.

Effective coalbed methane extraction is a key strategy for boosting natural gas production and storage in addition to raising the safety of the production standard of coal mines. In order to examine the effects of important variables such as pore parameters, desorption rate under temperature and pressure conditions, and seepage capacity on coalbed methane production during the coalbed methane extraction process. The low-temperature nitrogen adsorption tests (LTN2A), methane adsorption/desorption experiments at various temperatures, and the changes in the desorption rate constants, the initial desorption rate, and the desorption rate decay index with temperature and pressure were quantitatively analyzed. And COMSOL software was used to simulate the seepage characteristics of coalbed methane under various reservoir pressures. The results demonstrated that the large interior pore volumes of the samples are beneficial for gas adsorption. Because of its better pore connectivity, the QL sample had a desorption amount that was 55.65% and 48.01% higher at 313.15 and 333.15 K than the WJB sample. The three parameters of k, V1 and kt are at a high level with increasing temperature; hence, the desorption rate peaks at 333.15 K and pressures between 1 and 4 MPa. Furthermore, the WJB sample is more sensitive to the temperature than the QL sample. The COMSOL simulation shows that the methane pressure inside the reservoir can be released better when the reservoir pressure is below 10 MPa. The Darcy seepage rate is fast and stable, which is favorable for the seepage of coalbed methane. The findings show a relationship between the desorption and seepage properties of CBM and its pore structure, which may offer empirical and theoretical support for the successful development of the CBM.

Piezoelectric characteristics of coal rock leakage under uniaxial compression

In coal mining, coal rock fracturing damage and leakage pose significant challenges. To study the relationship between piezoelectric signals and seepage characteristics during uniaxial compression, and to achieve leakage monitoring based on piezoelectric signals, a coal rock fracturing damage and leakage monitoring experiment was carried out using coal from the Zhaogu Mine in Henan Province. The wavelet packet energy was introduced to investigate the piezoelectric characteristics. The results indicate the relative variation in wavelet packet energy server as a damage index during the uniaxial compression of coal, correlating closely with permeability changes. The wavelet packet energy at the end of the elastic deformation stage was 0.26 V2, similar to the pre-compression value of 0.29 V2. Therefore, the overall damage in the compression and elastic deformation stages was approximately 0. Permeability values remained stable, fluctuating from 0.26 × 10−16 m2 to 0.11 × 10−16 m2. In the plastic deformation stage, wavelet packet energy decreased while the damage index and permeability rose. Prior to peak stress, wavelet packet energy and damage index plateaued, signaling a precursor to a sudden increase in permeability. Post-peak stress, a marked decline in wavelet packet energy and an increase in the damage index and permeability were observed. This study offers a method for monitoring coal rock leakage, contributing valuable insights for efficient coalbed methane extraction and timely safety alerts.

Experimental and field verification research on the influence of flow velocity and stress on coalbed methane production

The efficacy of coalbed methane extraction significantly impacts coal mine operations and profitability. To investigate the determinants of this extraction efficacy, we utilized standard core flow velocity stress sensitivity apparatus alongside low-field nuclear magnetic resonance technology. We conducted tests on coal specimens of varying thicknesses to assess quantitatively the extent of damage to the samples, the patterns of permeability and porosity changes, and the effects of stress alterations on permeability. Our findings indicate a direct relationship between the permeability of coal specimens and their thickness. At equivalent flow rates, the velocity sensitivity of thicker coal specimens is notably reduced—by 30.49%—compared to thinner ones. Flow velocity exerts a dual impact on porosity, exhibiting a complex, nonlinear correlation. Net stress demonstrates a heightened sensitivity to the permeability of coal seams, with the stress loading and unloading curves failing to align perfectly. Stress-induced adsorption-desorption processes reveal a pronounced lag effect. Furthermore, dynamic surveillance of bottom hole pressure, water output, and daily gas production from coalbed methane at Wenjiaba Coal Mine revealed that daily gas production initially rises with flow velocity but subsequently declines. It also decreases as bottom hole pressure increases. The observed outcomes closely align with our experimental findings, thereby confirming the validity of our tests. This alignment elucidates the regulatory effects of stress and flow velocity on coalbed methane extraction, offering a theoretical basis for enhancing coalbed methane drainage strategies.

Study on the evolution of coal fracture behavior and stress energy consumption model: Experimental study and implications for in situ coal seam cracking

The extent of coal fracturing directly affects the efficacy of coalbed methane extraction. Therefore, we performed an examination of the mechanical properties, energy dynamics, and damage characteristics of coal under varying strain rates (30–120 s−1) using the Split Hopkinson Pressure Bar. The findings reveal that peak strength, strain, average rate of stress increase, energy dissipation, and damage severity in coal specimens exhibits a positive correlation with the strain rate. Conversely, the duration of stress escalation and the average size of fragments demonstrate a negative correlation with strain rate. As the strain rate increases, the cumulative curve of fragments transitions from a concave to a convex configuration, and the degree of coal sample fracturing gradually progresses from type I to II and III. Based on these findings, stress and energy models for coal samples subjected to varied strain rate conditions were developed and validated with experimental data, providing significant insights into the stress and energy dissipation mechanisms critical for coal mining safety. The study introduces an innovative evaluation index, q (0–1, 1–5, >5), for assessing the degree of coal sample fragmentation, which increases with increasing strain rate, reflecting the transition from initial to complete fracturing of coal samples. This discovery holds substantial engineering value for refining new dry ice fracturing techniques in coal mining safety and enhancing the efficiency of gas extraction.

Evolution of methane adsorption characteristics in coal under N-Methylpyrrolidone treatment

The extraction of coalbed methane (CBM) from low-permeability coal beds is a key matter for coal mine safety. The microporous system of coal has a significant influence on its gas adsorption capacity. To reveal the evolution of the microporous system and methane adsorption characteristics of coal under solvent treatment, CO2 and methane isothermal adsorption experiments were carried out. The experimental results show that the N-Methylpyrrolidone (NMP) solvent significantly modifies the micropore structure of coal. After NMP treatment, the maximum reductions in the pore volumes of lignite, bituminous coal, and anthracite are 64.99%, 24.05%, and 11.29%, respectively. Their adsorption constant a values decrease by 55.13%, 27.48%, and 9.32%, respectively. and b values decrease by 52.14%, 21.47%, and 16.25%, respectively. The swelling of macromolecular polymers is the main contributor to the reduction in micropore volume. NMP has a significant impact on lignite and bituminous coal, while its effect on anthracite is slightly weaker. This indicates that NMP can help to solve the challenges of coalbed methane extraction.

Mechanical degradation characteristics and permeability evolution law of coal under liquid nitrogen freeze–thaw cycles

Considering that liquid nitrogen (LN2) refracturing may become a potential coalbed methane extraction technology, this study systematically studied the changes in physical mechanics, permeability, and fracture behavior of coal treated with LN2 freeze–thaw using experimental methods such as uniaxial compression, acoustic emission monitoring, scanning electron microscope (SEM) and permeability testing, and three dimension (3D) cross-sectional scanning. The crack phase field model was used to reveal the thermal cracking mechanism of coal during the freeze–thaw process. Research has found that LN2 freeze–thaw can lead to the deterioration of the physical and mechanical properties of coal, and the deterioration effect increases with the increase in freeze–thaw cycles. In addition, this method effectively improves the degree of fragmentation and cross-sectional roughness of coal after damage, thereby enhancing the permeability of coal. Based on the numerical calculation results of the thermo-mechanical coupling phase-field model, it was found that the initiation and propagation of microcracks are mainly caused by temperature gradients and stress mismatches between mineral particles. In addition, compared to the freezing process, the size and area of microcracks that appear during the thawing process are larger. The overall research results have important guiding significance for the utilization of LN2 fracturing in coalbed methane mining.

Experimental validation and mechanistic elucidation of crack division and two-phase flow-mediated sealing in coalbed methane extraction boreholes

Experimental validation and mechanistic elucidation of crack division and two-phase flow-mediated sealing in coalbed methane extraction boreholes

Evolution Characteristics and Mechanism of Microwave-Induced Fracture Structure in Loaded Coal for Enhancing Coalbed Methane Extraction

Evolution Characteristics and Mechanism of Microwave-Induced Fracture Structure in Loaded Coal for Enhancing Coalbed Methane Extraction

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.

Deformation patterns of casing in coalbed methane extraction wells under conditions of thick alluvial layers and thin bedrock: a numerical simulation study

Coalbed methane wells on the ground in coal mining areas are used to extract a large amount of gas from the working face, but mining activities can cause strata movement, resulting in casing deformation. Numerical simulation was used to study the shear stress loading of casing under thick alluvial conditions. The study found that the casing will have shear stress peak points at the interface between the aquifer, alluvial layer and bedrock, and the mudstone layer in the bedrock, with a stress intensity of 49.9–77.1 MPa. The peak shear stress point of the casing in the aquifer will appear 4 m below the aquifer floor as the working face is mined. The casing stress at the junction of the thick alluvial layer and the bedrock increases linearly with depth, and the stress peak appears just above the junction. The casing stress in the bedrock section fluctuates periodically with lithology changes, and the stress peak mostly appears in weak rock formations such as mudstone. The introduction of non-uniform stress theory explains the reasons for casing deformation in these layers, and proposes suggestions for casing deformation control. This study can provide a reference for the formulation of casing deformation control methods.

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.

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.

Improving coalbed methane recovery in fragmented soft coal seams with horizontal cased well cavitation

Effective coalbed methane extraction from soft coal seams is essential for mine safety and energy supply. To enhance the extraction efficiency of coal mine methane (CMM) and reduce the risk of gas outbursts in coal mining areas, we developed an original and innovate horizontal well hydraulic cavitation method. A mathematical model that can quantitatively optimize construction parameters and improve the effectiveness of engineering applications was also constructed to calculate the technological parameters of this construction method. This technology differs from traditional approaches by relying on hydraulic erosion rather than water jets, and it can be implemented in cased horizontal wells. Utilizing the mathematical model grounded in porous media theory and Darcy’s law, numerical simulations with COMSOL Multiphysics were conducted and construction parameters optimized. The proposed technology significantly advances safe and efficient coalbed methane recovery, benefiting coal mine safety and environmental sustainability.

Effects of supercritical CO2 on coal microstructure in VES fracturing fluid environment: Experiments and mechanisms

Effects of supercritical CO2 on coal microstructure in VES fracturing fluid environment: Experiments and mechanisms

Gas Content and Geological Control of Deep Jurassic Coalbed Methane in Baijiahai Uplift, Junggar Basin

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.

A transient source-function-based embedded discrete fracture model for simulation of multiscale-fractured reservoir: Application in coalbed methane extraction

A transient source-function-based embedded discrete fracture model for simulation of multiscale-fractured reservoir: Application in coalbed methane extraction

Influence of Microwave-Assisted Supercritical Carbon Dioxide Treatment on the Pore Structure of Low-Rank Coal.

CO2 injection into coal seams not only enhances coalbed methane (CBM) extraction but also allows for CO2 sequestration. Microwave irradiation is considered to be an effective technology to enhance CBM extraction. In this paper, the effects of microwave irradiation and supercritical CO2 immersion on the pore structure of low-rank coals were investigated by scanning electron microscopy (SEM), mercury-in-pressure (MIP), low-temperature nitrogen adsorption (LTN2GA), and carbon dioxide isothermal adsorption/desorption (CO2IA/D) of coal samples. The results showed that the macropores and micropores of the coal samples were more developed after microwave irradiation. After carbon dioxide immersion, the coal samples showed huge fissures, and the meso- and micropores were reduced. In contrast, microwave-assisted carbon dioxide not only reduced the specific surface area in the meso- and microporous stages and decreased the adsorption sites of methane but also enhanced the pore connectivity in the macroporous stage instead of the appearance of huge fissures. This study illustrates the potential of microwave-assisted supercritical carbon dioxide for enhanced coalbed methane extraction and carbon dioxide sequestration.

Analysis of dominant flow in tectonic coal during coalbed methane transport

Diffusion and seepage are the main flow forms of coal seam gas transport, and are one of the key factors in the selection of gas extraction improvement methods. Changes in the physical structure of tectonic coal make gas transport more complex during coalbed methane extraction. In this paper, we develop a multi-field coupled model of methane transport in coal seams, taking into account the effects of tectonics, and theoretically analyze the dominant flow patterns for methane extraction. Then, the evolution of gas dominated flow is analyzed for different initial pressures, initial permeabilities, and initial diffusion coefficients of tectonic and intact coal seams. The results show that the amount of daily methane seepage in tectonic coal increases with the initial pressure of the coal reservoir, but decreases with the initial diffusion coefficient of the coal reservoir. Methane seepage in tectonic coal has a longer control time than in intact coal at different initial pressures, initial permeabilities, and initial diffusion coefficients of the coal reservoir. For different coal reservoir initial pressures, coal reservoir initial permeabilities, and coal reservoir initial diffusion coefficients, the maximum seepage control time for tectonic coal is 20, 17, and 15 times longer than for intact coal, respectively. Finally, the discrepancies of methane dominant flow in tectonic coal and intact coal during methane extraction were analyzed by using the double bottleneck flow model, and methods for methane enhanced extraction in tectonic coal and intact coal were discussed. The results presented in this paper may provide a theoretical reference for the extraction of differentiated gas in coal seams.