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

Abstract

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.

Study of Interfacial Phenomena of High Asphaltene Crude Oil with Different Salinity Water and ZrO2 Nanoparticles

“…16−18 Additionally, IFT reduction tends to correlate with an increase in pH resulting from mineral dissolution in carbonate rocks, thereby leading to the formation of in situ surfactants from carboxyl compounds in crude oil. 19 Changes in wettability are closely related to the presence of divalent ions in the brine. A small amount of divalent cations exhibits a "bridging effect" when combined with the polar components in crude oil, adsorbing on the surface of the rock, and altering wettability.…”

“…The study of low-salinity waterflooding primarily focuses on interfacial tension (IFT), wettability, and zeta potential. − IFT reduction predominantly occurs due to the interaction of natural surfactants such as asphaltene and colloids in crude oil with trace amounts of Ca 2+ and Mg 2+ in water. These ions are adsorbed at the oil/water interface, changing IFT. , However, studies have shown that IFT is not the primary mechanism for low-salinity waterflooding, with reductions typically limited to 3–4 mN/m. − Additionally, IFT reduction tends to correlate with an increase in pH resulting from mineral dissolution in carbonate rocks, thereby leading to the formation of in situ surfactants from carboxyl compounds in crude oil . Changes in wettability are closely related to the presence of divalent ions in the brine.…”