Characterization and Alteration of Wettability States of Alaskan Reserviors to Improve Oil Recovery Efficiency (including the within-scope expansion based on Cyclic Water Injection - a pulsed waterflood for Enhanced Oil Recovery)

Abstract

Numerous early reports on experimental works relating to the role of wettability in various aspects of oil recovery have been published. Early examples of laboratory waterfloods show oil recovery increasing with increasing water-wetness. This result is consistent with the intuitive notion that strong wetting preference of the rock for water and associated strong capillary-imbibition forces gives the most efficient oil displacement. This report examines the effect of wettability on waterflooding and gasflooding processes respectively. Waterflood oil recoveries were examined for the dual cases of uniform and non-uniform wetting conditions. Based on the results of the literature review on effect of wettability and oil recovery, coreflooding experiments were designed to examine the effect of changing water chemistry (salinity) on residual oil saturation. Numerous corefloods were conducted on reservoir rock material from representative formations on the Alaska North Slope (ANS). The corefloods consisted of injecting water (reservoir water and ultra low-salinity ANS lake water) of different salinities in secondary as well as tertiary mode. Additionally, complete reservoir condition corefloods were also conducted using live oil. In all the tests, wettability indices, residual oil saturation, and oil recovery were measured. All results consistently lead to one conclusion; that is, a decrease in injection water salinity causes a reduction in residual oil saturation and a slight increase in water-wetness, both of which are comparable with literature observations. These observations have an intuitive appeal in that water easily imbibes into the core and displaces oil. Therefore, low-salinity waterfloods have the potential for improved oil recovery in the secondary recovery process, and ultra low-salinity ANS lake water is an attractive source of injection water or a source for diluting the high-salinity reservoir water. As part of the within-scope expansion of this project, cyclic water injection tests using high as well as low salinity were also conducted on several representative ANS core samples. These results indicate that less pore volume of water is required to recover the same amount of oil as compared with continuous water injection. Additionally, in cyclic water injection, oil is produced even during the idle time of water injection. It is understood that the injected brine front spreads/smears through the pores and displaces oil out uniformly rather than viscous fingering. The overall benefits of this project include increased oil production from existing Alaskan reservoirs. This conclusion is based on the performed experiments and results obtained on low-salinity water injection (including ANS lake water), vis-a-vis slightly altering the wetting conditions. Similarly, encouraging cyclic water-injection test results indicate that this method can help achieve residual oil saturation earlier than continuous water injection. If proved in field, this would be of great use, as more oil can be recovered through cyclic water injection for the same amount of water injected.