Pore-scale simulation of gas-water two-phase flow in volcanic gas reservoir based on Volume of Fluid method

Understanding the transport process of gas-water flow in volcanic gas reservoir is of key importance for natural gas production. However, there is still limited evidence on the precise influence of volcanic reservoir type, capillary number and wettability. We thus performed gas-water flow simulations (using Volume of Fluid method) at different capillary numbers under different wettability conditions directly on microcomputed tomography (μ-CT) images. The simulation results demonstrated that the eddy current in dead-end corners is the main mechanism for the formation of residual gas. The gas phase near the wall of fractured porous medium was mainly dominated by drag force, resulting in lower residual gas saturation. Moreover, it is generally believed that a low capillary number facilitates the displacement of residual gas in dead-end corners. However, we found that under high temperature (>100 °C) and high pressure (>100 MPa), less residual gas distributed in dead-end corners at higher capillary number. This showed that the conventional percolation law was unlikely to provide reliable predictions in fluid distribution under high temperature and high pressure. The wettability of rock affected the shape of displacement front. The water-gas flow dynamics under water-wet condition was piston like. However, fingering flow occurred under non-hydrophilic condition, and snap-off trapping was more likely to occur, resulting in higher residual gas saturation. This work provides fundamental data on the influence of pore structure, capillary number and wettability on gas-water flow and aids in the further advancements of improved nature gas recovery in volcanic reservoirs.