The relationship between the pore structure characteristics and methane adsorption behavior evolution during coalification is vital for elucidating coalbed methane (CBM) storage and the impact on gas production. The middle-high rank coals collected from the Western Guizhou were analyzed by the full-scale pore structure characterization and methane adsorption isotherms. The evolution of pore, including pore type, structure, and fractal dimension, and gas adsorption behavior were established. Based on the quantitative characterization of coal samples' desorption, diffusion, and permeability capabilities, the impact of the gas storage mechanism on the gas production at the microscale and the geology-adapted technologies for gas recovery was elucidated. The results show that as the coal rank increases, the methane adsorption spaces and sites within coal undergo a substantial expansion primarily due to the enhanced development of micropores. During this process, the quantity of mesopores remains low, but their proportion increases while macropores gradually diminish. Coal petrographic and quality parameters related to the pore structure parameter exhibit a strong correlation with saturated adsorption capacity (SAC), with micropores playing a dominant role in controlling methane molecule adsorption. Coalification, on the one hand, increases the methane adsorption site, coupled with an increase in gas-solid interaction due to the condensation of macromolecular structures, leading to an increase in SAC. On the other hand, it results in a reduction in the micropore diameter and an intensification of monolayer molecular adsorption, causing a significant decrease in average adsorbed molecular layers (AAML). Therefore, the increase in SAC accompanies a decrease in AAML. Although high-rank coals exhibit higher methane desorption volume, desorption efficiency, and diffusion capacity, their low permeability characteristics hinder fluid seepage. To facilitate efficient development of high-rank CBM, it is imperative to implement geological compatibility techniques aimed at reducing solid-gas interactions within coal reservoirs and enhancing the connectivity of the pore network.

Optimizing the start-extraction time for coalbed methane (CBM) wells in mining areas remains challenging. This is due to the limited understanding of mining-induced mechanical changes and fluid migration in protected seams, which restricts the development of clean fossil energy. To address this, a geological-engineering model is constructed to investigate the mining-induced zonal evolution of stress, strain, permeability, and gas migration in protected seams, with the goal of optimizing the start-extraction time. The results show that gas production is controlled by the mechanical properties and gas pressure of protected seams near the well. Initially, these seams experience prolonged elastic strain. Plastic compressive strain develops at close-distance protected seams only when the coalface advances to within 5 m of them. Subsequently, rapid stress relief and complex stress directions lead to continuous plastic shear and expansion strains. As the distance from the mining seam increases, the plastic strains delay and diminish, reverting to elastic strain. These transitions collectively characterize the dynamic development of five distinct permeability regimes. Within permeability-reduced zones, an enhanced gas pressure gradient mitigates production declines. As the start-extraction time is progressively delayed, post-initiation gas production manifests in four phases: gradual decline, slow rebound, rapid increase, and surge. The optimal start-extraction time aligns with the rapid increase phase, when the coalface reaches the well, shortening extraction by at least 5.75 days and reducing electricity consumption by more than 2.07 × 10⁴ kWh in the study area. This research provides practical solutions for methane emission reduction and sustainable CBM development in mining areas.

Efficient stimulation of coal reservoirs is crucial for enhancing coalbed methane (CBM) well productivity. As an emerging unconventional stimulation technology, low-frequency pulsating water injection (LF-PWI) has attracted widespread attention for its ability to transform the pore-fracture system of coal reservoirs and its potential for engineering application. In this study, LF-PWI experiments are conducted using NMR, CT, and a triaxial apparatus to investigate the influence of LF-PWI parameters on pore-fracture properties of coal samples. The dynamic gray relational analysis (DGRA) method is employed to quantitatively evaluate the correlations of pore-fracture properties with porosity and permeability, respectively. Finally, the stimulation mechanism of productivity is elucidated and validated through a case study. The results demonstrate that LF-PWI can reduce the coal elastic modulus by 35.15%. The volume and connectivity of micropores and mesopores fluctuate, while macropores with diameters of 200–300 μm increase significantly, forming dominant flow pathways (>500 μm). Permeability continuously increases due to macropore development (k/k0 = 4.07). Porosity initially shows a rapid rise driven by macropore expansion (φ/φ0 peaks near 1.05); it then declines (by up to 9.95%) and gradually recovers toward unity as the micropores vary. Increasing the pressure amplitude simultaneously enhances the deterioration of mechanical properties, the fluctuation of porosity, and the improvement of permeability. A segmented control strategy employing high–medium–high pressure amplitude can help balance the pore-fracture network, preserve the gas source, and expand gas seepage pathways. Field application in the DFZ-1 well confirmed that LF-PWI mitigates coal fines flowback and increases cumulative gas production by 17.07%. This study provides a theoretical basis for employing LF-PWI to improve CBM well productivity.

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