Stress-Dependent Fractal Evolution and Compressibility of Multiscale Pore-Fracture Systems in Coals with Different Ranks

Abstract

Understanding the stress sensitivity of multiscale pore fracture structures (PFS) in coals with different ranks is critical for evaluating coalbed methane (CBM) reservoir behavior. In this study, low-rank and high-rank coals were subjected to effective confining pressure loading–unloading tests under constant pore pressure, and the dynamic evolution of PFS was investigated using low-field nuclear magnetic resonance (LF-NMR), nuclear magnetic resonance imaging (NMRI), and fractal analysis. For the tested specimens, the Fengjiata low-rank coals exhibited higher proportions of seepage pores (SPs) and generally greater stress sensitivity, whereas the Sijiazhuang high-rank coals were dominated by adsorption pores (APs) and showed comparatively stable PFS. SPs are more sensitive to effective stress than APs, and stress-induced pore deformation shows partial irreversibility after unloading. Furthermore, an NMR-based method was proposed to quantify stress-dependent pore compressibility, revealing that pore compressibility decreases logarithmically with increasing effective stress due to the progressive loss of compressible pore space. These findings provide new insights into the multiscale stress response of coal pore fracture systems and improve the evaluation of stress-sensitive permeability evolution in CBM reservoirs.