Water Based EOR by Wettability Alteration in Dolomite

Water based oil recovery from carbonates is a great challenge due to unfavorable wetting properties. Especially in naturally fractured formations, when spontaneous imbibition is an important drive mechanism, the oil recovery is low. In the past decade, much scientific work has been published focusing on the chemical understanding of wetting properties in chalk and limestone. Very little systematic work has been addressed to dolomite, which is also an important reservoir rock in the carbonate family. Recent work has shown that seawater acts as a Smart Water wettability modifier in calcite at higher temperatures due to symbiotic interaction between Ca2+, Mg2+, and SO42- and the rock surface. In the present work, the affinity of these active components toward the dolomite surface is discussed and compared to previous experimental work in calcite. The affinity of sulfate toward the carbonate surface, which is the catalyst for the wettability alteration process, was very low toward dolomite. Spontaneous imbibition studies confirmed that seawater was not a good wettability modifier in dolomite at 70 degrees C. Using 10 times diluted seawater as imbibing brine increased oil recovery due to wettability alteration by 15% of OOIP compared to ordinary seawater. No extra oil was recovered by using 100 times diluted formation water without sulfate as imbibing fluid, confirming that the low salinity brine must contain some sulfate as catalyst to achieve wettability alteration.