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

The spontaneous imbibition process involves the fluidflow drivenby gravity and capillary forces between the matrix and fractures,which is a critical mechanism in oil production in naturally fracturedreservoirs (NFRs). Previous studies have explored how various rockand fluid parameters, such as temperature, permeability, connate watersaturation, and initial wettability, impact the performance of lowsalinity water in NFRs using spontaneous imbibition tests. In thisparticular study, we aimed to investigate the influence of pH on wettabilityalteration and oil recovery in carbonate-fractured porous media throughimbibition at high temperatures. To accomplish this, we utilized acombination of imbibition tests, ion chromatography analysis, contactangle study, and & xi;-potential and pH value measurements to verifychanges in fluid-rock interactions and evaluate the drivingmechanisms of incremental oil recovery by low salinity water at differentpH conditions. Our test results have demonstrated that the ultimaterecovery factor for each brine remains consistent regardless of thepH levels of the brine solution. However, we observed a significantvariation in the oil recovery rate by imbibition. This suggests thatthe pH of the contacted brine has an impact on the dominant recoverymechanism. Our analysis of ion chromatography and data on the contactangle, & xi;-potential, and pH variation has indicated that calcitedissolution and the alkali effect are the primary mechanisms at differentpH values. At high pH conditions, the production rates are initiallylow due to the alkali effect, but they increase as calcite dissolutionbecomes active. In contrast, under low pH conditions, the recoveryrate decreases during production due to the activation of the alkalieffect.