The accurate estimation of methane reserves in closed coal mines is crucial for supporting clean energy recovery and reducing greenhouse gas emissions. This study addresses the technical challenges associated with complex geological conditions and limited post-closure data in China’s closed mines. Three mainstream estimation methods—the material balance, resource composition, and decline curve—are systematically reviewed and applied to a case study in the Huoxi Coalfield. Results indicate that the material balance method provides upper-bound estimates but is highly sensitive to incomplete historical data, whereas the resource composition method yields more conservative and geologically realistic values. Although the decline curve method is not applied in this case, it offers potential for forecasting when long-term monitoring data are available. A multi-method integration approach, supported by enhanced data archiving and uncertainty assessments, is recommended to improve the accuracy and reliability of methane reserve evaluations in post-mining environments.

Methane accumulation in the upper corners of coal mine working faces poses significant safety risks. Y-type ventilation combined with gob-side entry retaining has been applied to improve gas dispersion; however, airflow convergence can still result in high methane concentrations near the return airway. To address this, a computational fluid dynamics (CFD) model was developed to simulate methane migration in the goaf under different drainage strategies. Results indicate that methane concentration increases with elevation and distance from the working face, and is consistently higher along the gob-side entry than on the intake side. Buried pipelines effectively reduced methane levels near the floor and along the gob-side, while high-level boreholes were more effective in the caved zone but less so in the overlying fractured zone. A combined drainage approach was found to yield the most comprehensive reduction across all regions of the goaf. These findings provide valuable insights for optimizing methane control in Y-type ventilated mining operations.

Hot flue gas displacement of coalbed methane has significant potential in enhancing coal mine safety and CO₂ emission reduction. However, the influence mechanisms of injection engineering parameters on permeability and displacement efficiency remain unclear, which restricts technological optimization. To address this, this study constructed and validated a fully coupled THMC (Thermo-Hydro-Mechanical-Chemical) multi-field model for hot flue gas displacement of coalbed methane. It systematically analyzed the spatiotemporal evolution of adsorption, mineral dissolution, and thermal effects under different injection pressures, quantified their respective contributions to fracture porosity, and clarified the response patterns of permeability, recovery rate, CO₂ storage, and displacement efficiency. The results show that adsorption strain increases with higher injection pressure, with greater magnitude and range adjacent to the injection well than adjacent to the production well. Mineralization is dominated by calcite dissolution; as pressure increases, dissolution near the injection well intensifies and expands, and the pH of the reservoir solution shifts from alkaline to acidic. The contributions of adsorption and dissolution to porosity exhibit opposite trends: in the near‐well region, adsorption dominates in the beginning while dissolution dominates in the later period, whereas the thermal effect is negligible. Permeability and porosity follow the same spatiotemporal evolution. Increasing injection pressure can significantly improve displacement efficiency, increase CH₄ recovery and CO₂ and N₂ storage, with the most prominent enhancement for CH₄ production. However, it also accelerates N₂ breakthrough, increasing the difficulty of CH₄ purification. It is recommended to adopt measures such as using pure CO₂ or high‐concentration CO₂ pre‐slug to block preferential channels, optimizing the CO₂/N₂ ratio, adjusting injection‐production well spacing, and implementing stratified gas injection, so as to balance production, storage, and economy, achieving the synergistic goal of ”increasing production, storing carbon, and reducing costs.”

Based on mechanical experiments conducted on bulk raw coal and coal of different types in order to explore the correlations between the fractal dimension and the grain size gradation and strength parameters of coal samples, the fractal statistics method was used to statistically analyze the grain size distribution characteristics of tectonic soft coal, while fractal theory was applied to study the grain size fractal characteristics of tectonic soft coals of categories III–V. The results of this study show that coal types III–V have increasing fractal dimension numbers, and the content of coarse particles decreases with an increasing fractal dimension number. Within this sampling range, the Class V coal is better graded, and the fractal dimension number decreases as the distance of the sampling point from the fault zone increases. In the direct shear experiments, the internal friction angle of the bulk raw coal decreased linearly with an increasing fractal dimension number, and the regularity of the cohesive force and the fractal dimension number was not strong, but the adhesion cohesion of the types of coal exhibited a positive exponential relationship with the fractal dimension, and the relationship between the internal friction angle and the fractal dimension was not strong. There was a positive exponential relationship, and the internal friction angle was relatively stable. The uniaxial compressive strength of the types of coal exhibited a good correlation with the coefficient of firmness of the coal samples and the fractal dimension, and the coefficient of firmness of the coal samples was the main factor influencing the uniaxial compressive strength of the types of coal compared with the particle size gradation.

The efficiency of gas extraction in coalbed methane extraction is governed by coal seam permeability and borehole structural integrity. Over prolonged extraction, some boreholes become ineffective owing to degraded performance, necessitating precise identification criteria. In this study, a mathematical model for cumulative gas production under borehole failure was developed based on a gas flow decay function. Numerical analysis and data visualization were conducted via MATLAB, with mean values for collapsed/blocked boreholes adopted to eliminate the interference of unstable intervals. The analysis produced an equation that correlates the critical borehole flow rate with the time of loss of stability, providing a quantitative criterion for identifying ineffective boreholes in coal mining operations.