A multi-scale experimental study of crude oil-brine-rock interactions and wettability alteration during low-salinity waterflooding

Abstract

In this study we present the results of a multi-scale experimental investigation of the mechanisms responsible for the increased oil recovery by low-salinity waterflooding (LSWF), i.e., low-salinity effect (LSE). Using core- and substrate-flooding experiments as well as imaging techniques at macro, micro, and nanometer scales, we probe the intricate crude oil-brine-rock interactions and the subtle pore-scale displacement events governing recovery enhancement during LSWF. We performed several LSWF and high-salinity waterflooding (HSWF) experiments on macro-scale (1 inch-diameter) reservoir sandstone core plugs while collecting effluent samples for further analysis.LSWF experiments exhibited a prolonged oil recovery response and notably higher oil recovery compared to HSWF. Imaging effluent samples, at sub-nanometer resolutions using transmission electron microscopy, demonstrated significant fines mobilization during LSWF. The occurrence of electric double layer expansion (DLE) as a source of fines mobilization during LSWF was confirmed by examining scanning electron microscopy images of the reservoir rock substrates before and after contacting the low-salinity brine. Moreover, geochemical analysis of the effluent samples showed evidence of proton excahnge and multi-component ion exchange (MIE) during LSWF. The mobilization of fine particles (due to DLE) and the ion exchange processes are believed to result in removal of oil molecules from the rock surfaces leading to an enhancement in oil recovery as well as alteration of wettability toward increased water-wetness. These observations were then linked to our findings from an earlier study during which LSWF and HSWF experiments were conducted on miniature (5 mm-diameter) core samples while the pore space was being imaged at micrometer resolution. In-situ oil/brine contact angles measured during flow tests conducted on the miniature core samples revealed a significant level of wettability alteration from weakly oil-wet toward increased water-wetness during LSWF. Analysis of fluid occupancy maps showed that this change in wettability lowers the threshold water pressure needed for water-displacing-oil events to take place in the pore elements, which in turn enhances displacement efficiency and oil recovery.