Dynamic Evolution Mechanism of Pore-Adsorption in Moisture-Bearing Coal Reservoirs and Its Implications for Optimization of Coalbed Methane Development Methods

The adsorption capacity of moisture-bearing coal reservoirs is the key factor influencing the occurrence, desorption, and development efficiency of coalbed methane (CBM). While actual coal reservoirs generally exist in moisture-unsaturated states, most current studies use dried or moisture-equilibrated samples, resulting in evaluation biases for pore characteristics and methane adsorption capacity. This study systematically investigates pore evolution and methane adsorption inhibition under varying moisture conditions using low (S_L), medium (S_M), and high (S_H) rank coal samples with controlled humidity states via the humidity balance method, combined with NMR and methane isothermal adsorption experiments. Results indicate coal pores are predominantly mesopores and macropores, with increasing humidity enlarging the critical disappearing pore size; each 1% water saturation increase reduces Langmuir volume by 1.55% (S_L), 2.48% (S_M), and 0.70% (S_H); moisture mainly adsorbs in small pores, transitioning to capillary condensation at ≥75% relative humidity, significantly reducing methane adsorption through pore occupation and site competition, synergistically controlled by coal rank and pore structure. These findings guide moisture-bearing coal reservoir development: utilizing initial high critical desorption pressure, mid-term pressure adjustment based on Langmuir pressure (P_L) and Langmuir volume (V_L)changes, and late-stage gas injection or reservoir transformation for sustained production.