Impact of carbonate surface mineralogy on wettability alteration using stearic acid

Wettability alteration of the reservoir rock toward a favorable wetting state is one of the main recovery mechanisms during enhanced oil recovery (EOR) processes. A clear understanding of the initial wettability of the reservoir rock can be considered the first step toward a successful wettability alteration during any applied EOR technique. The present study investigates the impact of stearic acid on the wetting properties of several carbonate surfaces (calcite, limestone, and dolomite). A systematic experimental approach was followed in which contact angle measurements, Fourier transform infrared (FTIR) spectroscopy, energy dispersive X-ray spectroscopy (EDXS), scanning electron microscopy (SEM), thermo-gravimetric analysis (TGA), and zeta potential measurements were conducted on fresh carbonate samples as well as treated samples with stearic acid. Contact angle results revealed that all fresh carbonate surfaces were strongly water-wet. Aging in the model oil altered the wettability of the carbonates toward oil-wetness with an average contact angle of 159 degrees, 151 degrees, and 137 degrees for calcite, limestone, and dolomite surfaces, respectively. FTIR and EDXS results demonstrated the adsorption of stearic acid on the carbonate surfaces. Moreover, TGA results showed that the amount of adsorbed stearic acid on carbonate surfaces decreases in the following order: calcite (158.3 mu mol/g) > limestone (116.1 mu mol/g) > dolomite (46.4 mu mol/g). Zeta potential measurements revealed a shift toward more negative values after aging the carbonate surfaces in the model oil which confirms the adsorption of the stearic acid on surfaces. Despite the similarities between carbonate surfaces, the experimental observations clearly highlighted the significant impact of the mineralogical composition of the carbonate surface on the electrical and wetting properties of the surfaces. More positively charged surface of calcite resulted in more electrostatic attraction between the calcite and stearate ions which led to superior affinity of stearic acid toward calcite as compared with limestone and dolomite. The presence of other minerals with calcite on the carbonate surfaces affected the adsorption of stearic acid and the electrical properties of the surface.