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

Abstract

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.