An experimental investigation of long-distance gas–water two-phase flow behavior in unconsolidated sandstone gas reservoirs

Gas–water two-phase flow in porous media is vital in groundwater management and hydrocarbon development, yet most experiments use small cores (5–10 cm) or etched micro-models. These studies often overlook the quantitative characterization of residual gas, long-distance gas–water flow behavior, and effects of gas–water flow on pore structure. This study presents a series of 3 m-long artificial unconsolidated sandstone models with permeabilities of 5, 10, 30, 50, and 100 mD, fabricated via rock–electric testing techniques to simulate edge-water invasion in gas reservoirs. The results indicate that (1) by adjusting clay content, cementing agents, grain size, and sand mix, artificial cores achieve permeability, porosity, cementation strength, sensitivity, and pore structure similar to natural cores; this approach addresses the sampling challenge from unconsolidated sandstone. (2) During long-distance gas–water flow, pressure drops rapidly in the gas–water transition zone. As permeability increases, the zone shifts downstream and becomes narrower. (3) The flow of gas–water causes a large number of particles to gather near the gas–water interface and block the throat, and effective stress on unconsolidated sandstone intensifies this blockage effect. (4) Residual gas exists in the forms of dead-end trapped gas, bypass trapped gas, and snap-off trapped gas. The residual gas volume is mainly controlled by gas saturation and pressure, but the largest amount of residual gas accumulates near the gas–water interface. This study addresses the research gap in understanding long-distance gas–water flow and presents a novel experimental method for unconsolidated porous media.