Reservoir wettability representing the competitive adsorption for different fluids on the surface, helps to predict the capture capacity and risk assessment for CO2 geological sequestration. Numerous simulated models have been applied to reveal rock wettability with various pressures. However, most papers investigated the wettability alteration without considering CO2 flow in the pores. There required an accurate model to describe the change in wettability of the reservoir during CO2 injection. In this paper, the molecular simulation was conducted to investigate the wettability alteration of reservoirs during the CO2 injection process. Considering the continuous CO2 injection, we employed a model referring to quartz-CO2-solution. In this model, CO2 flow is regarded as a stationary layer. After that, we studied the wetting behavior of reservoirs with various pressures ranging from 0 MPa to 62.3 MPa. The results show that the contact angle first dramatically increases until 12.2 MPa from 67 °to 102.9 °and after that enters a ramp region and ultimately reaches a finial value 120.7 °, which shows the CO2 injection pressure weakens the water-wet property of reservoirs. Water clusters predicting the wettability are hard to move through the CO2 atmosphere with the increase of pressure. Thus, the water cluster exhibits a hysteresis at a high pressure, resulting in the water cluster being hard to change and expend a long time to be equilibrated. Moreover, it is noted that the interaction of rock-CO2 gradually increases with the increase of pressure, indicating that more CO2 can be captured in tight sandstones. This paper proposed a model considering CO2 flow in the CO2 injection process, which can deepen the understanding of the wettability alteration in different CO2 densities during CO2 injection for CO2 geological sequestration, which further guides the operation of CO2 in Carbon Capture, Utilization and Sequestration project.
Keywords Tight sandstones, CO2 sequestration, wettability, CO2 injection, molecular dynamics simulation