Experimental Investigation of the Role of Rock Components in Enhanced Oil Recovery Using Low Salinity Water Flooding in Shadegan Oil Field

Due to the complex nature of carbonate reservoirs, mechanisms behind low salinity water injection (LSWI) in these reservoirs are not well understood compared to sandstone ones. Using a limestone core and crude oil from one of the wells of the Shadegan oil field, a core injection test was performed with a sequence of high salinity-low salinity water. 10-time diluted seawater, as low salinity water, was able to recover 14.5% of initial oil in place (IOIP) following the secondary injection of seawater. In contrast, the tertiary injection of the low salinity bine into artificial pure calcite rock (with petrophysical properties similar to real rock) recovered only 3.4% of IOIP. Using the results of the contact angle test, the improved oil recovery by LSWI is attributed to wettability alteration towards a more water-wet state. However, the amount of wettability alteration in Shadegan oil field limestone (42°) is significantly higher than that of pure calcite rock (16°). Based on the results of zeta potential, the wettability alteration by low salinity water is attributed to the more negative electric charge of the crude oil/brine and rock/brine interfaces. The increasing trend of pH with decreasing the salinity showed that more CO32- adsorption at low salinity reduces the electric charge on the calcite surface. The electric charge of Shadegan oil field limestone, by reducing the salinity of injected water from seawater to 10-time diluted seawater, experienced a greater decrease (7.7 mV) than pure calcite (4.1 mV). XRF results attributed this behaviour to the presence of impurities such as silica, sulphate, and phosphate in the Shadegan oil field limestone.