Effects of fines migration on oil displacement by low-salinity water

Enhanced oil recovery by low-salinity waterflooding (LSW) has been shown to result from the combined effect of various physical mechanisms. In this study, laboratory experiments were designed in such a way that only fines migration mechanism remains active. Then, experimental observations and detailed characterizations of core plugs and produced fluids (water and oil) were used to analyze the effect of fines migration-induced formation damage on displacement efficiency. In the first set of experiments, core plugs of Berea and Obernkirchener sandstones were subjected to single-phase injections of water at salinities 40, 30, 20, 10, 5, 2.5, 0.5 and 0 g/L NaCl. An order-of-magnitude decrease in permeability was observed for both core plugs and was associated with an increase in fines concentration in the produced water. SEM-EDS images of the produced fines showed that kaolinite is mobilized as the salinity is reduced. SEM images of the pores before and after injection also showed fines retention and blockage. In the second set of experiments, two-phase LSW was performed on a pair of core plugs comparable to those in the first set of experiments. Kaolinite migration results were consistent with those of single-phase LSW. The two-phase LSW experiments showed 50% reduction in water relative permeability and 3% increase in oil recovery. Because no wettability alteration mechanism was active, the increase in oil recovery is attributed solely to enhanced displacement due to fines migration.