Dynamic Evolution of Gas–Water Displacement and Microscopic Fluid Occurrence in Deep Coalbed Methane

Deep coalbed methane (CBM) has become an important contributor to natural gas production worldwide. Its fluid occurrence characterized by high free gas content and low water saturation suggests substantial gas-driven displacement caused by hydrocarbon generation overpressure. However, the microscopic evolution of this process and the corresponding occurrence remain poorly understood. To address these issues, we combined centrifugation experiments, nuclear magnetic resonance (NMR) monitoring, and theoretical modeling to systematically investigate pore-scale displacement dynamics and the associated fluid distribution. A dynamic evolution model for gas–water displacement in nanopores is developed by incorporating the capillary pressure and disjoining pressure, and validated against the centrifugation experimental data. At the pore scale, gas–water displacement is governed by critical displacement pressure and water film thickness. Water saturation declines sharply once the displacement pressure exceeds a critical threshold, after which it decreases slowly as the water film progressively thins. At the porous media scale, water saturation continuously decreases with increasing displacement pressure. For the high-rank coal samples in this study, the overall water saturation decreases to 49.15% as the displacement pressure increases to 10 MPa. The water film is negligible for pores larger than 20 nm, but significant for pores smaller than 20 nm. This critical pore size is not fixed, but is a dynamic threshold controlled by the disjoining pressure parameter. The occurrence of free gas in deep CBM is governed by the relative matching between hydrocarbon generation overpressure and reservoir pore structure. These findings provide a theoretical basis for resource assessment and efficient development of deep CBM.