Carbonated Water Injection and In Situ CO2 Exsolution in Oil-Wet Carbonate: A Micro-Scale Experimental Investigation
journal contributionposted on 05.04.2021, 13:35 by Ziqiang Qin, Maziar Arshadi, Mohammad Piri
Recent advances in X-ray microtomography (micro-CT) technology have enabled investigations of multiphase flow through natural porous media at the pore scale. In this study, we probed the displacement physics that govern the oil mobilization during tertiary carbonated water injection (CWI) and the subsequent depressurization process in an oil-wet carbonate. To this end, a series of miniature core-flooding experiments were performed at elevated pressure and temperature conditions, utilizing a customized core-flooding setup integrated with a high-resolution micro-CT scanner. The results showed that CWI and the in situ CO2 exsolution substantially reduce the remaining oil saturation left stranded after blank (i.e., unadulterated) waterflooding. Furthermore, in situ contact angle measurements yielded direct evidence of wettability reversal toward reduced oil wetness due to the CWI. The wettability reversal occurred nonuniformly mainly due to the interactions between the rock surface and carbonated water. We found that pore-scale mechanisms governing oil mobilization during the tertiary CWI included a decrease in the threshold brine pressure of displacements, swelling and coalescence of oil ganglia due to the CO2 diffusion from brine to the oil phase, and brine flow diversion. During the in situ CO2 exsolution process, gas bubbles preferably formed, grew, and resided in larger pore elements. The growth of the gas globules induced gas-to-oil-to-brine double displacements. Further, oil formed spreading layers sandwiched between the gas and brine phases. The synergistic effects of spreading oil layers and oil displacements prompted the isolated oil globules to coalesce, enhanced the local connectivity of the oil phase, and facilitated the oil mobilization. The oil mobilization realized by in situ degassing was mainly achieved in medium-sized pores.