Mechanistic studies for improved understanding of low salinity waterflooding based enhanced oil recovery and potential application to the Alaskan North Slope reservoirs

Abstract

Improvement in the recovery of oil by low or reduced salinity water has been reported by many researchers. However, a consistent mechanistic explanation behind low salinity waterflood has not yet emerged. A thorough literature review was conducted that pertains to low salinity water based enhanced oil recovery and preliminary screening criteria were proposed which may help in narrowing down the responsible mechanisms and identifying suitable candidates for low salinity waterflood. Altogether nine different variables, such as clays, oil characteristics, salinity ranges etc. were considered in developing the screening criteria. With the exception of some tests on standard Berea sandstone cores, all other experimental studies were carried out on representative Alaska North Slope (ANS) reservoir core samples and oil and brine samples. Experimental studies involved a direct visualization of the release of crude oil from the clay surface with low salinity waterflood as observed through a simple substrate type test. Amott type spontaneous displacement tests were performed to quantitatively determine the effect of low salinity water using core materials containing different types of clays. Two sets of low salinity water coreflooding experiments were conducted in the tertiary recovery mode; first using dead oil and the second using recombined oil at pseudo reservoir conditions to examine the potential in improving oil recovery. Oil recoveries were also compared with continuous injection vs slug-wise injection of low salinity water. Finally, surface level investigation was performed using an optical microscope to visually analyze the impact of low salinity water on core samples. All the experiments performed with low salinity water on Alaska North Slope (ANS) reservoir core samples consistently showed anywhere between a 3-30 % increase in oil production with the use of low salinity brine. The literature review identified wettability alteration, cation exchange capacity, clay type and clay content as some of the dominant mechanisms influencing low salinity waterflooding.