Influence of pH on Acidic Oil-Brine-Carbonate Adhesion Using Atomic Force Microscopy

摘要

Wettability alteration seems to be the main physicochemical process during low salinity water injection in carbonate formations. A pH increase due to calcite dissolution during low salinity water injection may affect oil–brine–rock interaction and thereby wettability. However, far too little attention has been paid to quantifying the impact of such an pH increase on acidic oil–carbonate adhesion. Therefore, we measured contact angles between acidic oil (−COOH) and calcite crystal in the presence of 10 000 ppm NaCl solutions at different pHs (6.5, 9.5 and 11) at ambient conditions. Furthermore, we used atomic force microscopy (AFM) to measure the adhesion force between acidic oil groups (−COOH) and calcite substrate at different pHs (9.5 and 11) at similar conditions of the contact angle experiments. Moreover, to confirm the contact angle and AFM results, we measured the zeta potential of acidic oil−brine and calcite−brine interfaces at the same condition and calculated the thermodynamic isotherm disjoining pressure. Our results show that the contact angle reduces from 134 to 95° as the pH increases from 6.5 to 11, increasing hydrophilicity. Adhesion force measurements show that increasing pH reduces the adhesion force between the acidic oil and carbonate in line with contact angle results. Zeta potential results show that increasing the pH increases the negativity of the zeta potential of acidic oil–brine and calcite–brine interfaces, implying the increase of the electrical double layer force. Furthermore, the total disjoining pressure isotherm becomes less negative with increasing pH, implying an increase of hydrophilicity. Taken together, our results confirm that a local pH increase due to calcite dissolution during low salinity water injection would prevail in the wettability alteration process in particular for high acidic oil bearing carbonate reservoirs.