Modeling the Interaction between Low Salinity Water and Sandstone Rock by Coupling Fluid Flow Equations with the PHREEQC Geochemical Model

One of the enhanced oil recovery methods that has been recently received more attention is low salinity water injection (LSW). Among the proposed mechanisms to describe LSW, geochemical reactions seem to mainly affect rock wettability alteration and thus incremental oil recovery. The goal of this paper is to investigate the effect of geochemical reactions of calcite dissolution and ion exchange on the performance of LSW in a sandstone reservoir by using the concept of threshold salinity. Fluid flow equations according to Buckley-Leverett theory were numerically coupled with the PHREEQC software. Results showed that as LSW was injected, initial equilibrium between reservoir rock surface and aqueous phase was disturbed leading to calcite dissolution and also cation exchange between aqueous phase and rock surface. Analysis of geochemical reaction near the injection well showed that cation exchange may cause to separation of organic compound from rock surface leading to wettability alteration to more water- wet. Also, pH profile through the reservoir showed that rate of calcite dissolution was very high near injection well, but only partial dissolution of calcite induced by cation exchange occurs at farther distance. Fractional flow analyses in line with total salinity profile showed that two distinct saturation shock fronts were established during LSW injection: first one represents high salinity condition with a water saturation of 0.43, and the second one represents low salinity condition with a water saturation as much as 0.58. Under the conditions of this study and selection of a salinity threshold of 3000 ppm, such increase of water saturation reveals EOR potential of LSW by which an incremental oil recovery of 10% of the oil initially in place was obtained as compared to high salinity water injection.