As a critical reservoir property, permeability governs both fluid transport dynamics in coal seams and consequent coalbed methane recovery rates. The pore-fracture structure evolution of soft, low-permeability coal under stress disturbance is complex, making permeability prediction challenging. There is an urgent need to establish a permeability prediction model for such coal. Based on nuclear magnetic resonance characterization of pore-fracture structures and permeability testing of variable-pressure saturated coal rock, fractal permeability prediction models (Sw-FPPM) K(Sw)T-C and K(Sw)SDR incorporating variable water saturation (Sw) were established to reveal the evolution characteristics of pore-fracture structures and permeability under variable-pressure saturated water conditions. The results indicate that variable-pressure water saturation significantly enhances the permeability of coal seams. At the maximum pressure of 9 MPa, the permeability increased by 7.2–51.38 times compared to the initial coal samples. Softer coal samples with lower firmness coefficients exhibited higher initial permeability but showed smaller permeability increments under higher water pressures. The fractal dimension (Ds) of seepage pores approached 3, indicating strong heterogeneity in the coal's pore-fracture structure. Ds slightly decreased with increasing water saturation pressure, while permeability significantly increased as Ds decreased. Additionally, pore shrinkage effects inhibit seepage, and the control exerted by small-aperture pores on permeability is commonly underestimated. The improved models K(Sw)T-C and K(Sw)SDR, incorporating Sw and Ds, demonstrated significantly enhanced accuracy by 61.09% and 53.84%, respectively. This provides an effective method for predicting permeability in soft, low-permeability coal seams.

Experiments on corrosion reactions of pulverized coal with monomeric and polymeric (mixed) acid solutions reveal that monomeric acids are listed in a descending order as HF, HCl, and CH₃COOH according to their corrosion effects on tectonic coal collected in Faer Coal Mine (Liupanshui City, Guizhou Province, China). In addition, the optimal mixing ratio of mixed acids is 6% HCl + 6% HF + 3% CH₃COOH + 2% KCl. The mineral grains filled in pores in coal samples treated with mixed acid solutions are dissolved, so the porosity increases. The volumes of transition pores and mesopores are obviously affected by acidization, and some transition pores are transformed into mesopores and macropores to form dissolved pores. At the same time, inkbottle-shaped pores reduce, while slit pores or open pores increase. The coal samples after acidization show a higher aromatization degree and an increased relative content of oxygen-containing functional groups, with a generally lower hydroxyl content, so the methane (CH₄) adsorption capacity of coal declines, which promotes CH₄ desorption. The control effect of pore structures after acidization reactions on CH₄ desorption was revealed from perspectives of the diffusion coefficient (Kn), adsorption volume (ω), average pore-throat ratio (P_T), and average sinuosity (τ_av). That is, CH₄ molecules in tectonic coal after acidization turn from Knudsen diffusion to transitional diffusion, the adsorption volume of CH₄ molecules shrinks, the average pore-throat ratio decreases, and the average sinuosity reduces, which promotes CH₄ desorption from tectonic coal.

Currently, the utilization of coalbed methane resources in the Guizhou region faces challenges such as complex reservoir structure, high gas content, and microporous development. Based on these, the pore structure and adsorption capacity of Guizhou tectonic deformed coals (TDCs) were evaluated using a suite of integrated diagnostic techniques including low-temperature nitrogen adsorption (LT-N₂A), mercury intrusion porosimetry (MIP), methane isothermal adsorption. Through the above methods, the pore structure and adsorption characteristics of the samples were characterized; The samples were divided into the range of joint pores by combining the results of MIP and LT-N₂A; Using the molecular simulation software, the 2 nm, 4 nm, 10 nm pores affecting the methane endowment state were investigated respectively, and from the perspective of the heat of adsorption and energy, the concept of the three-phase transition of methane was proposed, and explore the change of the pore spacing affecting the endowment state of methane from the solid state pore to the gas state pore. The results provide new ideas for the in-depth study of gas storage in tectonic coal reservoirs in Guizhou Province.

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.