Variation of CO2-Brine Contact Angles on Natural Rocks of Different Compositions

Contact angles were measured for CO₂-brine interactions on 20 different rocks that represent the properties of various CO₂ storage depositional environments to characterize the wettability during geologic carbon storage. Three different CO₂ phases (gaseous, liquid, and supercritical) were considered to investigate the effect of pressure and temperature. Bubbles were studied in two groups of larger and smaller than 500 μm, the latter being more relevant to pore scale. Variation was observed among contact angle measurements, even while controlling the sample preparation and cleanliness. The contact angle variations could mainly be attributed to natural sample heterogeneity, as shown by an increased range of measured values for the smallest bubbles studied. Despite these variations, the analysis of 1139 contact angle measurements on 20 samples under 3 different experimental conditions showed that 92.8% of the angles were below 40°, meaning that the samples were primarily strongly-to-moderately water-wet with the average contact angle of 22°. 10% of the angles under supercritical conditions were between 40° and 60°. This range of angles constitutes 5% and 4% of the measurements under liquid and gaseous conditions, respectively. Therefore, supercritical CO₂ was observed to be more wetting than liquid or gaseous CO₂.