Low salinity waterflooding can improve oil recovery from reservoir rocks by changing the wettability from oil-wet towards more water-wet conditions. To investigate the underlying mechanisms of this process, high-resolution X-ray imaging combined with high-pressure and temperature flow apparatus were used to compare secondary and tertiary low salinity waterflooding in carbonate rock samples. Two different crude oil, brine, and rock (COBR) systems were examined.
In the experiments on Estaillades quarry limestone, low salinity waterflooding caused a decrease in average contact angle to 102°, indicating a wettability change from oil-wet to more water-wet conditions. The shift in mean curvature and capillary pressure to positive values further supported this change. Micro-droplets of water were observed within the oil phase and oil layers detached from the rock surface after tertiary low salinity waterflooding. Similar mechanisms were observed in the secondary low salinity waterflooding experiment, with a faster pace and recovery of 85% of the oil initially in place in the resolved porosity. Pore and throat occupancy analysis revealed a redistribution of fluids as more brine was injected.
In the experiments on a reservoir limestone with another crude oil, emulsification of oil was observed with the injection of reduced salinity brine. An intermediate phase that appeared to be a mixture of oil and brine was imaged when low salinity brine was injected, and more emulsification was observed when only low-salinity brine was injected initially. The formation of this intermediate phase is the mechanism by which the oil is mobilized.
Overall, the analysis captured the in situ mechanisms and processes associated with the low salinity effect and ultimate increase in oil recovery. The study found that improved recovery was associated with a change in contact angle and pore occupancy, as well as the formation of micro-dispersions of water in oil and the emulsification of oil.