Temperature-driven enhancement of methane-water displacement in low-rank coal: Experimental insights for deep CBM recovery

Gas-water two-phase seepage is a key factor governing recovery efficiency during deep coalbed methane (CBM) extraction. Gas-driven water displacement experiments in coal provide essential insights into flow behavior, seepage dynamics, and competitive gas-water distribution. In this study, methane (CH 4 )-driven water displacement experiments were conducted on a low-rank deep-coal sample at ambient (20 °C) and reservoir (70 °C) temperatures. Outlet gas and water flow rates were continuously monitored to calculate key parameters, including displacement efficiency and gas-phase relative permeability K rg . The dynamic process of CH 4 displacing pore water in coal was compared across temperatures to assess the influence of temperature on displacement behavior. Results suggest that elevated temperature may enhance gas-driven water displacement efficiency and accelerate the displacement process to some extent. At 70 °C, water production, gas saturation ( S g ), displacement efficiency, and the dimensionless parameters V w ( t ) and V ( t ) were all significantly higher than those at 20 °C. Under the studied conditions, gas saturation in coal increased logarithmically with cumulative injection time. As gas saturation rose, K rg tended to increase, whereas water relative permeability ( K rw ) declined, which may reduce gains in displacement efficiency and water production during later stages. The gas-driven water displacement process could be tentatively divided into four stages: initial water displacement startup, early gas breakthrough, mid-term gas-dominated flow, and late gas-driven stabilization. This study improves understanding of gas-injection behavior and competitive gas-water seepage in deep coal reservoirs and provides preliminary experimental reference for optimizing temperature parameters during CBM development.