Is Smart Water Flooding Smarter Than Seawater Flooding in a Fractured Chalk Reservoir?

Abstract

Abstract It is well-known that seawater flooding has been a huge success for hydrocarbon recovery from the fractured chalk reservoir, Ekofisk, on the Norwegian Continental Shelf. Extensive laboratory studies the last decades have shown that Smart Water flooding has potential of greatly improving oil recovery beyond that obtained by standard waterflooding, due to wettability alteration, which improves reservoir sweep efficiency. However, to be economically viable compared to seawater injection, the Smart Water must be cheap, easily available, and must substantially improve oil production. Thus, the objective of this work was to investigate if a tailor-made, but cheap Smart Water could enhance oil production compared to seawater injection in an offshore chalk reservoir. Is seawater the smarter choice in offshore chalk reservoirs? Two reservoir chalk cores were used in this study and initial reservoir core wettability was estimated from optimized, in-house laboratory core restoration procedures. The potential for wettability alteration and resulting oil recovery by seawater (∼33000 ppm salinity) and Smart Water (<5000 ppm salinity, containing 20 mM SO42-, Ca2+ and Mg2+) were compared in spontaneous imbibition tests at reservoir temperature (>100 °C). Waterflooding at various rates was also performed to evaluate the displacement performance, with regards to water breakthrough and ultimate recovery, of the two injection brines studied. Reproducible initial wettability was confirmed in both reservoir cores, making a comparison of brine performance easier in spontaneous imbibition tests. The restored cores behaved initially mixed to oil-wet, imbibing limited amount of water. Both seawater and Smart Water showed potential for wettability alteration, although oil recovery by spontaneous imbibition by Smart Water was not improved compared to that by seawater. By low-rate waterflooding the Smart Water was more efficient than seawater due to the water being forced into the interior of the cores causing faster and more pronounced wettability alteration at microscopic scale, hence generating stronger positive capillary forces than in the spontaneous imbibition process. It was concluded that Smart Water flooding can potentially improve recovery beyond that obtained by seawater flooding in fractured chalk reservoirs. This high-temperature offshore chalk reservoir case study demonstrates that seawater is able to alter wettability of mixed to oil-wet reservoir chalk in a similar way as previously reported for outcrop chalk. Additionally, although seawater injection seems to be a good choice offshore, there is still potential of tailoring a Smart Water composition to both accelerate oil production, delay water breaktrough, increase ultimate oil recovery, thus lowering the field residual oil saturation if its injection is timely implemented.