Size-Dependent Pore Responses to Stress and Temperature of Anthracite Samples from the Qinshui Basin in North China Using NMR: Implications for Deep Coalbed Methane Exploitation

High-production wells of deep coalbed methane have been widely reported during the last decade. The Qinshui Basin in China is rich in deep coalbed methane resources, but the pore size distribution characteristics of coal under high-temperature and high-stress conditions remain unclear, affecting the formulation of coalbed methane development strategies. In this study, nuclear magnetic resonance simulations were conducted on anthracite samples from the Zhaozhuang and Sihe mines in the Qinshui Basin under varying temperatures and confining pressures, and the difference in stress and temperature sensitivity of pores with varying sizes was determined. The results show that the pores exhibit strong stress sensitivity but weak temperature dependence. Total porosity decreases with increasing confining pressure, with a maximum damage rate of 6.55%. Pore-size heterogeneity governs differential responses: micropores and macropore fractures show reduced porosity, whereas mesopores exhibit minor increases. Temperature-driven porosity changes occur in distinct phases: at lower temperatures (20–35 °C), damage rates escalate with heating, while at elevated temperatures (35–50 °C), sensitivity diverges due to the variations in native fracture structures. Furthermore, stress and temperature responses correlate with the developmental state of pre-existing pores/fractures and mineral infill. These findings provide critical insights for optimizing coalbed methane exploitation in deep anthracite reservoirs.