Effect of pH on the Dominant Mechanisms of Oil Recovery by Low Salinity Water in Fractured Carbonates

The spontaneous imbibition process involves the fluid flow driven by gravity and capillary forces between the matrix and fractures, which is a critical mechanism in oil production in naturally fractured reservoirs (NFRs). Previous studies have explored how various rock and fluid parameters, such as temperature, permeability, connate water saturation, and initial wettability, impact the performance of low salinity water in NFRs using spontaneous imbibition tests. In this particular study, we aimed to investigate the influence of pH on wettability alteration and oil recovery in carbonate-fractured porous media through imbibition at high temperatures. To accomplish this, we utilized a combination of imbibition tests, ion chromatography analysis, contact angle study, and ξ-potential and pH value measurements to verify changes in fluid-rock interactions and evaluate the driving mechanisms of incremental oil recovery by low salinity water at different pH conditions. Our test results have demonstrated that the ultimate recovery factor for each brine remains consistent regardless of the pH levels of the brine solution. However, we observed a significant variation in the oil recovery rate by imbibition. This suggests that the pH of the contacted brine has an impact on the dominant recovery mechanism. Our analysis of ion chromatography and data on the contact angle, ξ-potential, and pH variation has indicated that calcite dissolution and the alkali effect are the primary mechanisms at different pH values. At high pH conditions, the production rates are initially low due to the alkali effect, but they increase as calcite dissolution becomes active. In contrast, under low pH conditions, the recovery rate decreases during production due to the activation of the alkali effect.