Low-salinity water flooding LoSal™ has been used to improve oil recovery for many decades. Historically, the physical mechanism behind this improvement in oil recovery was attributed to the pressure maintenance and displacement of oil by injected water. Recently, evidence from laboratory and field tests indicated that water flooding also involves chemical processes and that modifying of the injection brine salinity and its ionic composition can significantly impact the oil recovery. Several theories regarding the mechanism of LoSal™ flooding have been discussed in the literature. These include interfacial tension reduction, wettability alteration (cation exchange), change in pH (increase), emulsion formation, and clay migration. There is no agreement, however, among the researchers regarding the mechanism of LoSal™ flooding and although limited work has been done on carbonates, some studies have concluded that LoSal™ have no effect on oil recovery.
This work presents the results of flooding tests on selected carbonate core samples taken from Bu Hasa field in Abu Dhabi using sea water and two field injection waters, Um-Eradhuma (UER) at 197,584 ppm and Simsima (SIM) at 224,987 ppm. These results were used to evaluate the effects of brine salinity and ionic composition on the possible interactions of limestone rock/ brine/ and oil system and to identify the oil recovery mechanism. The field inj ection waters were diluted to salinities of 5000 and 1000 ppm and the optimum salinity was determined and then modified by varying the sulfate and calcium ion concentrations. Wettability alteration was determined by contact angle measurements. Interfacial tension measurements of the studied systems were also performed in an attempt to evaluate the flow mechanism with LoSal™ flooding.
The experimental results revealed that a significant improvement in the oil recovery can be achieved through alteration of the injection water salinity. Reducing the salinity of UER water from 197,362 ppm to 5000 ppm resulted in an improvement of oil recovery from 63% to 84.5% of OOIP and the latter salinity was used to evaluate the impact of changing the sulfate and calcium ion concentrations on oil recovery. Results also indicated that sulfate concentration has a significant effect on the flooding process and that increasing the sulfate concentration beyond some optimum concentration of 47 ppm resulted in a negative effect on the flooding process. Contact angle measurements indicated that lowering the solution salinity could shift the wettability of the system towards intermediate wettability levels and that the UER water exhibits higher shift toward intermediate wettability compared to other waters. Results also indicated that there is no clear correlation between the improvements in oil recovery and interfacial tension and the pH of the studied systems.
Results of composite core flooding experiments with LoSal™ UER water have shown similar trends to the results observed with short plugs.