Pore-Scale Insights into the Contribution of Clays during Low-Salinity Waterflooding in Sandstones

The presence of clay minerals is generally accepted to be a necessary condition for low-salinity flooding to improve oil recovery in sandstones. However, there have been no in situ pore-scale observations that low-salinity brine releases more oil from pores with higher proportions of clay minerals present. In fact, there is a growing body of evidence that significant oil release can occur from quartz and feldspar mineral surfaces during low-salinity flooding. In this work, we use X-ray Micro-CT imaging technology to image unsteady-state tertiary low-salinity waterflooding experiments in Berea, Bunter, and Castlegate sandstone samples. We make the first pore-scale in situ observations of oil release from clay-rich and clay-poor pores during low-salinity flooding in oil-wetting sandstone samples. We find that clay minerals are initially significantly more oil-wetting than other mineral groups in all samples. In the Bunter and Berea samples, we observe a significant wettability alteration during low-salinity flooding for clay minerals and nonclay minerals; however, no significant wettability alteration is observed in either mineral group in the Castlegate sample. In all samples, clay-poor pores contribute significantly more to total oil production during low-salinity waterflooding than clay-rich pores. Clay-rich pores are found to account for only 15, 5, and 3% of total additional recovery in the Berea, Bunter, and Castlegate samples, respectively. We see no evidence that clays dominate the response to low-salinity flooding in any of the samples. This work adds to the growing body of evidence that questions the presence of clays as a necessary condition for a favorable response to low-salinity flooding in sandstones.