Experimental and Numerical Study on Gas-Displacing-Water in Coal Reservoirs under Different Confining Pressures Using LF-NMR.

With the depletion of shallow coal resources, deep coalbed methane (CBM) extraction has become critical for the global energy supply, but it is severely constrained by the low permeability, strong heterogeneity of deep coal reservoirs, and the coupled effects of in situ stress and geothermal conditions. Existing studies rarely consider dynamic stress paths and the synergistic control of the temperature field, limiting their guiding significance for field CBM development practices. To address this gap, this study systematically investigates the confining pressure-dominated gas-displacing-water behavior in deep coal reservoirs by integrating low-field nuclear magnetic resonance (LF-NMR) real-time monitoring and thermo-hydro-mechanical (THM) multiphysics coupled numerical simulation. Laboratory experiments were conducted at a constant temperature of 25 °C using the control variable method to isolate the independent effect of confining pressure, eliminating temperature interference and providing high-reliability benchmark data for model validation. A complete THM coupled numerical model was established and validated by experimental data (cumulative water production error <5%), successfully bridging the laboratory core scale and field engineering scale. Results show that increasing confining pressure compacts macropores/fractures and promotes micropore-to-mesopore transformation, leading to permeability reduction following a power-law relationship. Gas preferentially displaces free water from macropores/fractures at ΔP < 2 MPa, while bound water in micropores requires ΔP > 4 MPa for effective displacement. Field-scale simulations incorporating actual reservoir temperature (348 K) indicate that a 150 m borehole spacing maximizes CBM productivity and ensures compliance with the safety pressure threshold, outperforming 200 m spacing by 8.3% in daily gas yield. This study reveals the intrinsic laws of gas-water two-phase seepage dominated by confining pressure in deep coal reservoirs, and provides a quantitative basis for optimizing deep CBM extraction technologies.