The uncertainty and complexity of the geological parameters of coalbed methane (CBM) reservoirs may result in the assignment of atypical exact values to a whole block. Therefore, the influence of different resources and productivity characterization indices on the development of CBM blocks was analyzed systematically in this research. Furthermore, this research reported an interval-number index evaluation system and a gray relational analysis optimization model for CBM development. The weights of the evaluation indices were determined by combining the entropy method and the best worst method (BWM) with the objective deviation law of the data and the subjective experience of experts. Based on the gray system theory, a multi-index gray relational analysis optimization model was established to evaluate the interval values and optimize the development potential of CBM blocks with complex geological conditions. The results show that the order of importance of the eight resources and reservoir evaluation indices, from highest to lowest, is permeability, reservoir pressure gradient, gas content, reservoir thickness, reservoir depth, critical reservoir ratio, Young’s modulus, and reservoir temperature. The order of the target blocks, from most to least optimal, is the Sanjiao block, Fanzhuang-Zhengzhuang block, Daning-Jixian block, Gujiao block, and Fengrun block. The evaluation results are consistent with actual conditions. Thus, the multi-index gray relation analysis optimization model, on account of the gray system theory, is feasible for the development evaluation of CBM blocks under complex conditions. This model is suitable for the CBM blocks in the range of 547 km²~675 km². When using this model, the index values must be representative and accurate.

A quantitative characterization of the change in coal molecular structures with different cyclical microwave modification parameters and a better understanding of the reaction mechanism of the modification are of great significance for the commercial extraction of coal bed methane (CBM). Therefore, long-flame coal samples obtained from the Ordos Basin, China, were modified by microwave radiation with different times, and the long-flame coal molecular structure parameters were determined by solid-state ¹³C nuclear magnetic resonance (ss¹³C NMR), Fourier transform infrared (FTIR) spectrometry, and X-ray photoelectron spectrometry (XPS). Atomistic representations of the raw long-flame coal molecular model and modified long-flame coal molecular models were established. The temperature rise, pore volume increase, mineral removal, and functional group changes after the modification have a negative effect on methane adsorption. After the modification, the decrease in surface area of the micropores reduced the adsorption site of methane in coal. As a result, the methane adsorption amount decreased linearly with the decreasing surface area. The CH₄ adsorption isotherms of the long-flame models were dynamically simulated and analyzed. The results of this study can prove that after multiple cycles of microwave modifications, the functional groups in long-flame coal were fractured, and the number of micropores was reduced, which effectively decreased the methane adsorption performance in long-flame coal seams, thereby promoting methane extraction. Microwave modification is a promising method for enhancing CBM recovery.

The gas-trapping mechanisms of coalbed methane (CBM) cap rocks in the Shanxi Coalfield remain unclear, and single-lithology assessment methods have inherent limitations. Based on 273 datasets from 125 coal mines, this study classifies cap rock stacking types and quantitatively evaluates their gas-trapping capacity using Markov chain analysis, analytic hierarchy process (AHP), and COMSOL numerical simulation. Cap rocks are classified into 3 major types and 8 sub-types, including composite (28%), thick (58%), and coal cap rocks (14%). The dominant types (I, V, VIII) display distinct gas pressure evolution characteristics, with Type VIII showing a unique “decrease–increase–decrease” trend. A lithology-weighted and distance-attenuated equivalent thickness coefficient model is established, yielding the following sealing capacity ranking: Type VIII (1.1908) > Type V (1.1506) > Type I (0.6664). Gas-trapping capacity is hierarchically controlled: lithology dominates within 0–5 m, while the equivalent thickness coefficient prevails at 5–10 m. The effective sealing zone is within 10 m above the coal seam. These results provide theoretical support for efficient CBM development and CO₂ geological sequestration.

The sandy mudstone within collapse columns is characterized by its density and high mechanical strength, posing significant challenges for direct excavation by coal shearers. Consequently, coal mining operations are impeded by the inability to traverse collapse columns directly. This investigation represents the inaugural elucidation of the chemical weakening mechanism of sandy mudstone subjected to hydrochloric acid (HCl) etching, and it substantiates the practicality of this method within engineering contexts. The employed research methodologies encompass HCl acid etching experiments, rock mechanical property assessments, X-ray diffraction (XRD) analyses, and scanning electron microscopy (SEM) examinations. The findings demonstrate that: The uniaxial compressive strength of sandy mudstone exhibits a negative logarithmic correlation with the volume fraction of the HCl solution; Post-etching, the concentrations of muscovite and kaolinite within the sandy mudstone diminish by 44.74% and 62.96%, respectively, with extensive dissolution of framework particles leading to interconnected pores; The HCl induces a synergistic weakening effect via H ₂ O intrusion and H ⁺ dissolution, substantially degrading cohesion and structural integrity; The engineering applicability indicates that prior to mining, the injection of a 2% HCl solution into the collapse column weakens the rock for over 10 days, enabling the mining face to traverse it directly. A subsequent injection of an optimal amount of Ca(OH) ₂ solution into the goaf then neutralizes any residual HCl and mitigates acid corrosion in the mined-out area. This study furnishes a technical strategy for managing complex geological formations, such as waterless collapse columns, in the context of coal mining engineering.