In Situ Investigations Of Wettability And Pore Scale Displacements During Two And Three Phase Flow In Oil Wet Carbonates

Preferentially oil-wet characteristics of carbonate reservoir rocks hinder oil recovery due to conventional waterflooding. The remaining oil in these reservoirs is the target of enhanced oil recovery (EOR) techniques such as low-salinity waterflooding (LSWF), immiscible/near-miscible gas injection, Water-Alternating-Gas (WAG) flooding, and carbonated water injection (CWI). An improved fundamental understanding of wettability and displacement mechanisms governing multiphase flow behavior in oil-wet carbonates is critical to ensure the success of these EOR schemes in the field. However, there is a dearth of literature focused on the displacement physics of such EOR processes at the pore scale. To fill in these research gaps, a series of miniature core-flooding experiments were performed in oil-wet carbonates at elevated temperature and pressure conditions, using a three-phase core-flooding system integrated with a high-resolution x-ray micro-CT scanner. Our observations indicated that higher temperature and lower initial water saturation established greater equilibrium in situ oil-brine contact angles during dynamic aging-induced wettability alteration process. For LSWF, wettability reversal towards neutral-wetness and the consequent reduction in threshold brine pressure required for the fluid to invade medium-sized oil-filled pores led to a higher oil recovery than that of high-salinity waterflooding. Under tertiary immiscible gas injection scheme, gas-to-oil-to-brine double displacements were the main pore-scale events responsible for the enhancement in oil production. During this process, the greater was the degree of oil-wetness of the rock, the larger became the additional oil recovery. Furthermore, WAG flooding significantly increased the displacement efficiency of both gas and brine phases because of the shield effect of the trapped gas ganglia. In the first WAG cycle, oil was produced through a series of direct and double displacements. Multiple displacements started taking place and further contributed to oil recovery as more WAG cycles were implemented. As for the CWI scheme, we found that pore-scale mechanisms governing oil mobilization included decrease in the threshold brine pressure of displacements due to wettability reversal, swelling and coalescence of oil ganglia, and brine flow diversion. After CWI, as the in situ CO2 exsolution progressed due to depressurization, gas bubbles preferably formed, grew, and resided in larger pores. The synergistic effects of spreading oil layers and double displacements prompted the isolated oil globules to coalesce and facilitated the oil mobilization. Finally, in near-miscible supercritical CO2 (scCO2) injection scheme, we observed a distinct wettability state where the wetting preference of the solid to scCO2, oil, and brine phases was similar. Consequently, pore sizes neither dictated any preferential invasion order nor restricted the displacement efficiency. Furthermore, we identified a new type of spreading system where spreading oil layers formed but did not exist globally across the pore space between the scCO2 and brine phases. In this experiment, the frequencies of double and multiple displacements were much higher than those observed during N2 injection in oil-wet systems. The interplay of scCO2-oil miscibility, the distinctive wettability state, favorable fluid connectivity, and frequent double/multiple displacements resulted in an exceptional displacement efficiency.