Pore-scale mechanisms and hysteresis effect during multi-cycle injection and production process in underground hydrogen storage reservoir

Injection and production of hydrogen gas storage through multiple cycles often lead to significant variations in the gas water occurrence state and the associated relative permeability curve (RPC). In this study, a visual sandstone etching model and numerical simulation method are used to analyze the alteration of the occurrence state after multiple cycles of gas-water inter drive. The influence of the occurrence state on the variation of the RPC is revealed. The experimental results demonstrate that with an increasing number of cyclic injection and production cycles, the gas saturation in the model shows an upward trend, rising from 64.26% to 73.89%. However, the water block phenomenon also increases, leading to only a small amount of gas bubbles undergoing discrete flow, resulting in a reduction of the porosity utilization from 51.06 % to 40.57 %. In different injection and production cycles, the liquid phase at the corners and blind ends of large pores is difficult to displace. The continuity of gas-liquid distribution enhances with the increase in injection and production cycles. The phenomenon of phase trapping during hydrogen multi-cycle injection is evident, and the relative permeability of the gas phase rapidly declines during the imbibition first stage. The difference in gas saturation between the displacement and imbibition processes gradually decreases, reducing from 0.314 to 0.054. The Carlson model achieves a final simulation accuracy of over 90 % for wettability hysteresis. The numerical simulation results indicate that the modified relative permeability curves can more accurately simulate the changes in hydrogen gas storage during injection and production. The relative error of the hydrogen saturation in the uppermost layer of the reservoir at 1673 days of operation is 8.51 %. And the pressure increase is 13.32% without considering the wetting hysteresis model.