Supercritical CO2 and Ionic Strength Effects on Wettability of Silica Surfaces: Equilibrium Contact Angle Measurements

Wettability of reservoir mineral surfaces is a critical factor controlling CO2 mobility, trapping, and safe-storage in geological carbon sequestration. Although recent studies have begun to show that wettability of some minerals can change in the presence of supercritical CO2 (scCO(2)), different laboratories have reported significantly different wetting behavior. We studied wettability alteration of silica in CO2 brine systems through measuring equilibrium water contact angles under wide ranges of pressures (0.1 to 25 MPa) and ionic strengths (0 to 5.0 M NaCl), at 45 degrees C. Using two independent approaches for each of the experiments, we found the following: (1) Equilibrium water contact angles on silica increased up to 17.6 degrees +/- 2.0 degrees as a result of reactions with scCO(2). This increase occurred primarily within the pressure range 7-10 MPa, and the contact angles remain nearly constant at pressure greater than 10 MPa. (2) The contact angle increased with ionic strength nearly linearly, with a net increase of 19.6 degrees +/- 2.1 degrees at 5.0 M NaCl. These changes in contact angle induced by changes in scCO(2) pressure and aqueous solution ionic strength are approximately additive over the range of tested conditions. These findings can be used to estimate the wetting behavior of silica surfaces in reservoirs containing supercritical CO2.