Wettability Alteration and Enhanced Oil Recovery in Carbonate Porous Media by Tuning Waterflood Chemistry

Waterflooding for enhanced oil recovery (EOR) is described by a multiphase system consisting of oil, the hosting rock reservoir matrix, and an invading fluid (IF) that is injected into the reservoir to displace the residual oil. Wettability is one of the unique physicochemical properties of this multiphase system that is known to influence oil displacement by the IF and affect oil recovery. Tuning IF chemistry, such as through low salinity waterflooding, has been shown to alter the wettability of the reservoir media for increased oil recovery. For carbonate rock reservoirs, specifically, reports of wettability alteration with changes in IF chemistry have been inconsistent, and the role of divalent potential-determining ions in altering carbonate reservoir wettability is debated. The lack of understanding of how IF chemistry alters wettability in carbonate reservoirs limits field-scale predictions of EOR effectiveness. This research systematically evaluated the interactions of different IF compositions with model oil-wet carbonate media and correlated these interactions to fluid displacement patterns observed during waterflooding of carbonate granular media in a Hele-Shaw cell. Fluid displacement stability was quantified by the fractal dimension of the displaced fluid area, which was compared across time and length scales using the effective capillary number. Results show that dissolved calcium ions specifically interact with oil-wet carbonate surfaces to reduce the magnitude of the negative surface charge and alter wettability, resulting in more stable fluid displacement patterns during waterflooding. The findings improve fundamental understanding of wettability alteration and have implications for the design of IFs for EOR in carbonate reservoirs.