Influence of Surface Roughness on the Contact Angle due to Calcite Dissolution in an Oil-Brine-Calcite System: A Nanoscale Analysis Using Atomic Force Microscopy and Geochemical Modeling

Low-salinity water flooding appears to be a promising means to improve oil recovery in carbonate reservoirs because of a wettability alteration process. Contact angle measurement is a direct approach to reveal the wettability alteration in an oil-brine-carbonate system. However, questions have been raised about using contact angle measurement to justify the wettability alteration. This is because the contact angle may be significantly affected by surface roughness variation in the presence of low-salinity water because of calcite dissolution during the contact angle measurement. To clarify the cause and effect of wettability alteration during low-salinity water flooding, we measured the contact angle on two calcite substrates with similar surface roughness (7 and 4 nm) in the presence of high-salinity water (1 mol NaCl + 0.01 mol CaCl2) and low-salinity water (100 times diluted high-salinity water). Moreover, we measured the surface roughness of the substrates before and after the contact angle measurements using atomic force microscopy (AFM). Furthermore, we performed a geochemical study to quantify the amount of calcite dissolution in the presence of low- and high-salinity brines and compared it with surface roughness measurements. Our contact angle and AFM results reveal that surface roughness increase due to calcite dissolution in low-salinity water plays a negligible role in the contact angle, rather confirming that oil-brine-rock interactions govern the system wettability. Furthermore, our geochemical study shows that low-salinity water only dissolves 1.16 x 10(-14) mol/mol of calcites in low-salinity water during the contact angle measurement. We, therefore, eradicate the possibility that surface roughness variation due to calcite dissolution in low-salinity water would affect contact angle results. Consequently, we argue that contact angle measurement remains a valid approach to directly examine the wettability alteration process in low-salinity water flooding.