Effects of Reservoir Pressure and Temperature on CO2–Brine Wettability in Low-Permeability Sandstone: Implications for CO2-Enhanced Oil Recovery and Geological Storage

Wettability is a key interfacial property affecting CO2-enhanced oil recovery and geological CO2 storage in reservoirs. However, experimental data on the wettability of CO2–brine–rock systems remain limited for low-permeability sandstone, a major lithology in unconventional oil reservoirs. In this study, low-permeability sandstone samples (∼1 mD) from the Chang 7 Member of the Ordos Basin, China, were used to measure the contact angle (CA) and interfacial tension (IFT) of the CO2–brine–sandstone system at 25–125 °C and 0.1–20 MPa. Results show that the measured CA increases with pressure, although its pressure dependence becomes weaker above approximately 7–8 MPa. Pressure and temperature exerted statistically significant but opposing effects on CA. At 85 °C, CO2–brine IFT decreases from 49.85 to 29.65 mN·m–1 as pressure increases from 3 to 20 MPa, with a gradual-transition region near 8–11 MPa, while its temperature response exhibits a clear pressure dependence. For the sandstone samples studied, an empirical power-law relationship between surface roughness and permeability was established, while the roughness-induced apparent CA differences progressively diminished at 17–20 MPa. To support practical interpretation, an interfacial capillary term was introduced to combine CA and IFT, thus providing a first-order interfacial reference for capillary behavior relevant to CO2-enhanced oil recovery and geological CO2 storage in low-permeability sandstone systems.