The daily gas production of a single well (coalbed methane (CBM) vertical well) in the western Fukang mining area in the southern margin of the Junggar Basin, Xinjiang, China is relatively high. However, there are significant differences between gas and water production of CBM wells at different locations in the area, and the reason has not been adequately explained. To explore the distribution characteristic of coalbed methane and water in the Fukang mining area, the three-phase CBM gas (adsorption gas, free gas, and water-soluble gas) content was determined based on theoretical analysis and simulation. Combining the calculation results and the basis of geological data, the CBM-water differential distribution in the study area was discussed. The results show that the average daily water production and average daily gas production of CBM wells show a negative correlation in the study area. The CBM wells with high daily gas production are mainly located in the high areas of the structure, and these wells commonly begin to produce gas within a short period. The calculation of three-phase CBM gas content and the test results of gas composition show that the gas content (especially the free gas) is relatively high in the high areas of the structure, while the concentration of C₂H₆ is relatively low. Meanwhile, the concentration of C₂H₆ shows a positive correlation with buried depth, which indicates that CBM migrates from the deep areas to the higher areas. The calculation of equivalent water level elevation and hydraulic head shows that the groundwater mainly flows from the central part to the east and the west within the CBM well areas, and the groundwater flows downward along the coal seam controlled by gravity, which results in the CBM-water differential distribution. The CBM-water differential distribution in the western Fukang mining area is the result of coupling control of tectonic and hydrological geology factors. Multistage tectonic movements formed large-scale folds and faults in the area, which controlled the migration direction of CBM and groundwater. The dip angle of the stratum in the study area is commonly greater than 45°, and the gravity effect is greater in the process of groundwater flow, which promotes the CBM-water differential distribution. Free gas migrates to the high area of the structure, and groundwater accumulates in the axial part of the syncline. The results in this study provide a basis for the large-dip angle CBM exploration and development in the Fukang mining area.

The heterogeneity of the pore structure of coal reservoirs affects the desorption and diffusion characteristics of coalbed methane, and determining its distribution law is conducive to improving the theory of coalbed methane development. The central and eastern parts of the Qinshui Basin are rich in coalbed methane resources, but the heterogeneity characteristics of the pore structure of coal reservoirs are not clear. NMR has the advantages of being fast, non-destructive and full-scale, and multifractal can describe the self-similarity of NMR T2 curve at different scales so as to analyze the complexity of pore distribution. Based on this, 15 samples with different coal ranks were collected from the central and eastern Qinshui Basin (R_o,max between 1.54 and 2.78%), and quantitative pore characterization experiments such as low-field nuclear magnetic resonance (LF-NMR) and low-temperature liquid nitrogen adsorption (LTN₂A) were conducted. Based on multifractal theory, the heterogeneity law of pore structure was quantitatively evaluated, and its influencing factors were elucidated. The results showed that the BJH pore volume of coal samples in the study area ranged from 0.0005–0.0028 cm³/g, with an average of 0.0014 cm³/g, and the BET specific surface area was 0.07–2.52 m²/g, with an average of 0.41 m²/g. The NMR T₂ spectrum peaked at 0.1–1, 10–100 and 100–1000 ms, and the spectrum was mostly bimodal or trimodal, indicating that pores of different pore sizes were developed. There were great differences in the pore structure of different coal ranks; high-rank coal was dominated by micropores, and the proportion of mesopores and macropores of medium-rank coal was higher. The pore structure of coal samples showed obvious multifractal characteristics, and the fractal characteristics of the sparse region (low-value information) were more significant; they dominated the pore distribution and had a stronger influence on the distribution of pore space. Pore structure heterogeneity is closely related to the degree of coalification, and with the increase in coalification, it is closely related to coal lithotype and quality, and high mineral and inertinite contents lead to the enhancement of pore structure heterogeneity in coal reservoirs, while R_o,max, M_ad and vitrinite group contents have opposite effects. The research results provide theoretical guidance for the subsequent exploration and development of coalbed methane in the region.

Deep coal reservoirs, as opposed to their shallower counterparts, exhibit characteristics of higher temperatures and pressures. These conditions affect the fracture structure and mechanical properties of coal, which in turn controls permeability. Substantial studies have been conducted to determine the effects of overburden pressure on permeability, but the correlation between the temperature and mechanical parameters/permeability of coal remains unclear. This study focused on low-rank bituminous coal from the southern edge of the Junggar Basin in Xinjiang. Using experiments conducted on seepage and mechanics at different depths (considering effective stress and temperature), the study investigated how temperature affects the mechanical parameters and permeability of coal column samples. A permeability prediction model was established incorporating temperature, mechanical parameters, and effective stress. The results show that from 20 to 80 °C, the elastic modulus of coal column samples decreases by 31.0%, and the Poisson ratio increases by 72.0%. Permeability decreases between 48.37 and 90.12% under different depths. The stress sensitivity coefficient under various temperature conditions decreased exponentially as the effective stress increased, and the temperature sensitivity coefficient under various effective stress conditions decreased with increasing temperature. The permeability was more sensitive to a temperature below 40 °C. In the permeability prediction model, the fracture compressibility coefficient is bifurcated into two coefficients, each controlled by temperature and effective stress. The permeability prediction error of the model was 12.7% under constant effective stress and 17.2% under varying effective stress and temperature conditions. The study could provide guidance for fracturing and coalbed methane production in deep coal reservoirs.

CO₂ sequestration in coal is effective for reducing carbon emissions, but related projects have encountered challenges in sustained CO₂ injection, highlighting the need for a comprehensive understanding of CO₂ sequestration in coal. This study reviews variations in the properties of coal/rock during/after CO₂ injection, demonstrating the potential and stability of CO₂ sequestration in coal. The coal with a high V_L-CO₂/V_L-CH₄ is accompanied by high CO₂ sequestration capacity. The matrix swelling and acid corrosion restrict CO₂ sequestration efficiency, which can be enhanced by employing coatings and increasing permeability. Long-term CO₂–water–rock interactions weaken the integrity of coal/caprocks and decrease the adsorption capacity of coal, leading to the CO₂ leakage risk. Three issues are critical in future studies: (1) Increasing CO₂ adsorption capacity. (2) Establishing optimal approaches to enhance CO₂ injection efficiency. (3) Accurately predicting variations in the adsorption capacity of deep coal and the integrity of coal/caprocks during long-term CO₂–water–rock interactions. This review provides foundations for formulating CO₂ sequestration strategies in coal.

The key to the variation in permeability within coal reservoirs lies in the stress-induced deformation and desorption-induced deformation during the coalbed methane (CBM) production. The differences in sample scale and measurement methods between stress-induced deformation and desorption-induced deformation significantly affect the accuracy of permeability measurements. Therefore, in order to elucidate the relationship between stress-induced deformation and adsorption-induced deformation, as well as the influencing factors, and to assess the accuracy of permeability evolution prediction, this study conducted a series of parallel experiments, including compression deformation experiments under stress loading (stress-induced deformation), methane adsorption-induced deformation experiments (adsorption-induced deformation), mu CT scanning, and overburden permeability measurements.The results of the study indicate that stress-induced deformation and adsorption-induced deformation are negatively correlated but exhibit a relatively weak correlation. Stress-induced deformation encompasses deformation of coal matrix, minerals, and fractures, whereas adsorption-induced deformation primarily reflects coal matrix deformation. While there is some overlap between the two, they are not entirely identical. The main influencing factor of stress-induced deformation is the mechanical strength of coal, with minerals in coal increasing the Young's modulus of coal reservoirs. Among them, minerals that are more dispersed and have smaller particles have a more significant impact on stress-induced deformation. The primary influencing factor of adsorption-induced deformation is the deformation capability of the coal matrix, with minerals and fractures having less significant effects. Permeability changes are controlled by fracture deformation, but stress-induced deformation measurements weakly reflect this aspect, leading to an inability to accurately predict the scale of the impact of effective stress changes on permeability during CBM production and CO2-ECBM processes. In contrast, adsorption-induced deformation relatively accurately reflects the deformation capability of the coal matrix and provides a more accurate prediction of permeability rebound under the condition of almost unchanged effective stress in the late stages of mining. Therefore, deformation parameters under stress loading are challenging to directly apply to the prediction of permeability evolution, while adsorption-induced deformation parameters can be effectively utilized.