Numerical study of Low Salinity Water Alternating CO2 injection for enhancing oil recovery in a sandstone reservoir: Coupled geochemical and fluid flow modeling

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Abstract The combination of low salinity water with gas in the form of water alternating CO2 injection (CO2-LSWAG) is a promising EOR method that could simultaneously benefit from gas mobility control and also rock wettability alteration to more water-wet as consequence of geochemical interaction between rock and reservoir fluids. This paper aims to investigate the EOR potential of CO2-LSWAG under miscible conditions in a sandstone oil reservoir by coupling fluid flow and geochemical modeling. The wettability alteration of the rock surface was described by a series of relevant geochemical reactions between rock and low salinity water in a compositional simulator. The concept of ion equivalent fraction was used as the wettability index during CO2-LSWAG injection. It consists mainly of the effects of ion exchange and clay properties on oil-water relative permeability functions. Also, the effect of calcite mineral dissolution was studied as result of interaction with low salinity water and CO2 gas using relevant geochemical reactions. Results showed that salinity difference between injected and formation water triggers ion exchange processes between water and rock. Also, the rate of calcite dissolution increased due to the CO2 partitioning into the aqueous phase. The calcite dissolution caused the increase of calcium ions in the reservoir leading to more water-wet condition. However, results indicated that the rate of calcite dissolution decreased far from the injection well most likely due to reduced interaction time between low salinity water with dissolved CO2 and the reservoir rock. The oil recovery results showed that CO2-LSWAG is capable of producing incremental oil as much as 10% of the initial oil in place on top of high salinity CO2-WAG. The results of this numerical study support the potential application of CO2-LSWAG as an efficient EOR method in sandstone reservoirs.