Variation characteristics and molecular dynamics simulation of key parameters including reservoir wettability and interfacial tension in the whole petroleum system

Accurately measuring the wetting angles, interfacial tension, and pore-throat radii of reservoir rocks under high temperature and pressure conditions or in ultra-tight reservoir environments remains challenging due to laboratory limitations and instrument constraints. These challenges complicate the determination of capillary pressure during hydrocarbon migration and accumulation. Given this, we calculate the wetting angles of the calcite, montmorillonite, quartz, and feldspar surfaces, as well as oil-water and gas-water interfacial tension, under relatively low temperature and pressure conditions using molecular dynamics simulation. After validating the high consistency between the simulation results and experimental data, we expand this method to high temperature and pressure conditions to simulate the corresponding variations in the above parameters. Furthermore, we calculate the pore-throat radii of reservoirs using the multiple linear regression method. Through systematic simulations based on the regional settings of the Junggar, Ordos, and Songliao basins, we obtain data on burial depth-varying wetting angles, interfacial tension, and pore-throat radii of sandstone, volcanic, and carbonate reservoirs under high-, medium-, and low-temperature geothermal fields in China. Accordingly, the quantitative relationships are established between these parameters and the temperature-pressure conditions of petroliferous basins. The simulation results indicate that the water wetting angles of calcite, montmorillonite, quartz, and feldspar decrease gradually with increasing temperature, suggesting enhanced hydrophilicity. In contrast, the water wetting angles of these minerals increase with pressure, leading to reduced hydrophilicity. Generally, in oil reservoirs, calcite tends to exhibit oil-wet behavior, montmorillonite shows neutrality to water-wet characteristics, while quartz and feldspar primarily display water-wet properties. The oil-water interfacial tension decreases gradually with rising temperature but increases progressively with pressure. In contrast, the gas-water interfacial tension decreases with rising temperature and declines further with increasing pressure. The pore throat radii of rocks show certain correlations with porosity and permeability, suggesting that reservoir physical properties play a significant role in fluid migration. The minerals exhibit significantly different wettability, interfacial tension, and pore-throat structures across varying geothermal fields. Their wetting angles trend downward with increasing geothermal gradient. Meanwhile, their interfacial tension also decreases gradually with increasing geothermal gradient, which reduces molecular interactions at oil-water or gas-water interfaces, thereby enhancing fluid mobility.