Effect of core cleaning solvents on wettability restoration and oil recovery by spontaneous imbibition in surface reactive, low permeable limestone reservoir cores

Master's thesis in Petroleum engineeringThe success of oil recovery, by water flooding, in naturally fractured, low permeable limestone reservoirs strictly depends on imbibition of injected water into the reservoir matrix block. Prerequisite of imbibition process is the presence of positive capillary pressure inside the reservoir which is directly related to the wetting state. In previous experimental studies on surface reactive limestone cores performed by the EOR group in University of Stavanger, two different oil recovery ratios were encountered during spontaneous imbibition tests on the same core after cleaning the core with different solvents. After mild cleaning with kerosene & n-heptane (preserved state) around 40% of OOIP was recovered indicating that the core behaved preferentially water-wet. On the other hand, after cleaning the same core with toluene and methanol oil recovery was below 10% indicating that the core was oil-wet. Chromatographic wettability tests were utilized to examine the wetting states inside the cores after mild cleaning with kerosene & n-heptane and after cleaning with toluene and methanol. The results of the tests have shown small changes in water-wet fraction inside the cores indicating infinite small wettability changes due to the adsorbed organic components to the rock surface. It has been concluded that the reason behind this recovery difference is due to successive toluene and methanol injection which removes the water in smaller pores. Polar components dissolved in toluene could adsorb on the initially water-wet smaller pores, change the capillary pressure and affect oil recovery