Pore-Fissure Compressibility and Structural Dynamic Evolution of Coal Reservoir under Confining Pressure

Coal reservoirs exhibit a ternary structure comprising pores, microfissures, and macro-fissures, crucial for determining permeability and influencing the adsorption–desorption–diffusion–seepage processes of coalbed methane (CBM). These factors significantly impact the CBM recoverability and production. Through dynamic permeability experiments, nuclear magnetic resonance (NMR) under varied confining pressures, and triaxial compression-CT scanning, the stress sensitivity of coal in different directions under varying confining pressures, pore pressures, and effective stress conditions was investigated. It is obtained that (1) the stress sensitivity of coal fissures is notably high, and they tend to close first under confining pressure. Seepage and adsorption pores exhibit two trends: a gradual decrease or an initial increase followed by a decrease. With deeper coal metamorphism, the stress sensitivity of fissures gradually diminishes, while the stress sensitivity of adsorption pores increases. (2) The fissure compressibility measured by He is the lowest, CO₂ is always the highest, and CH₄ is between the two. The fissure compressibility measured by He decreases exponentially with the increase of pore pressure, while the adsorbed gases CH₄ and CO₂ change complicatedly, decreasing exponentially, or parabolically. (3) The significant stress sensitivity and permeability damage rate occur in the direction of parallel-face cuttings of experimental coals, while the vertical direction exhibits the weakest characteristics. Nonhomogeneity is most pronounced between these two directions. The anisotropy of the coal reservoir diminishes gradually with increasing peripheral pressure.