Is Wettability Alteration the Main Cause for Enhanced Recovery in Low-Salinity Waterflooding?

Abstract

Summary Low-salinity waterflooding is gaining some attention in the recent literature as a means of improving oil recoveries. However, a clear understanding of the key mechanism(s) of low-salinity waterflooding does not seem to have emerged, although wettability alteration is repeatedly cited as the cause for the observed effects, though without much experimental evidence. Moreover, much of the reported low-salinity work appears to be related to sandstone reservoirs (Morrow and Buckley 2011). In this study, we have attempted to investigate the role wettability alteration plays in low-salinity waterflooding by conducting dual-drop/dual-crystal contact-angle measurements to characterize wettability changes and coreflood experiments for oil recovery and oil/water relative permeability measurements by use of a dolomite-reservoir-rock/fluids system. Additional experiments have been conducted to examine the roles of brine chemistry and the temperature in altering wettability in low-salinity waterfloods. The contact-angle results clearly indicate the wettability alteration from an oil-wet state (with a water-advancing contact angle of 158°) to an intermediate state (with an advancing contact angle of 113°) caused by diluting the reservoir brine to 1/50 of its original strength. A similar result was obtained when the sulfate concentration in the reservoir brine was doubled and when the temperature was increased to 250°F from the reservoir temperature of 80°F. These wettability alterations as measured by contact angles were confirmed by secondary-oil-recovery-mode coreflood experiments that yielded significantly higher recoveries (from approximately 46 to approximately 76%) because of low-salinity flooding, alteration of brine composition, and temperature. This experimental study confirms the major influence of wettability in low-salinity waterflooding of a dolomite reservoir.