Study on multi-cycle gas-water displacing mechanism in underground gas storage of low-permeability reservoir based on PNM

A deep understanding of the pore-scale multi-cycle two-phase seepage mechanism of gas-water systems in low-permeability reservoirs is crucial for enhancing oil and gas recovery and optimizing the operating conditions of underground gas storage. A pore network model was reconstructed using micro-CT images of low-permeability core samples from the Dagang Oilfield, China. The mathematical models of gas-water flow in the pore-scale models were established based on Poiseuille's law and the quasi-static displacement theory, considering the gas compressibility and slip effects. The porosity, pore size, absolute permeability, and relative permeability (K_r) of oil-water and gas-water were calculated using pore network simulations and validated against experimental benchmark data of the same samples. The effects of multi-cycle displacement, rock wettability, and average pore pressure on the gas-water two-phase flow were simulated and analyzed. The results showed that the relative permeability of gas (K_rg) at the same water saturation level significantly increased when the gas compressibility and slip effects were considered. With an increase in the number of gas-water displacement cycles, K_rg at the same water saturation level showed a decreasing trend, indicating a reduction in the gas seepage capability. K_rg decreased with increasing water contact angle. With an increase in the average pore pressure, K_rg decreased and gradually increased when the average pressure exceeded 25 MPa.