Evaluation of Low-Salinity Enhanced Oil Recovery Effects in Sandstone: Effects of the Temperature and pH Gradient

Recently, we proposed a chemical mechanism for the low-salinity enhanced oil recovery (EOR) effect, where change in pH was a key parameter. In this paper, we demonstrate the importance of pH in the low-salinity water-flooding process in sandstones. We also try to relate the rate and size of the pH gradient to the low-salinity EOR potential at various temperatures. Static adsorption studies of basic material quinoline onto illite in the pH range of 3-8 showed that the adsorption was always higher in low-salinity water compared to high-salinity water, confirming that a decrease in salinity itself cannot be responsible for the wettability alteration in a low-salinity EOR process. In fact, previous work has shown that the adsorption and desorption processes of the organic material quinoline are mostly pH-controlled. Two reservoir and one outcrop sandstone cores were water-flooded successively with high-salinity low-salinity high-salinity brine at temperatures ranging from 40 to 130 degrees C, and pH and concentrations of Ca2+ and SO42- were recorded. In all cases, an increase in pH was observed as the injected fluid was switched from high-salinity brine to low-salinity brine and the pH decreased again to its initial value as the fluid was switched back to the high-salinity brine. Both the pH gradient and the rate of the pH gradient can be related to the desorption rate of Ca2+ from the clay surface, which is an exothermic process; i.e., the pH gradient and desorption rate decreased as the temperature increased. The presence of anhydrite in the rock material reduced the size of the pH gradient as well as the rate of desorption of Ca2+. The observed pH gradient in cleaned core material can be linked to the low-salinity EOR effect, and in that sense, the method can be used as a first screening test for possible low-salinity EOR potential.