Wettability and Its Effect on Oil Recovery

Reservoir wettability is determined by complex interface boundary conditions acting within the pore space of sedimentary rocks. These conditions have a dominant effect on interface movement and associated oil displacement. Wettability is a significant issue in multiphase flow problems ranging from oil migration from source rocks to such enhanced recovery processes as alkaline flooding or alternate injection of CO2 and water. In this paper, wettability will be discussed mainly in the context of recovery of light Oow-viscosity) oils by waterflooding. Waterflooding has been widely applied for more than half a century; secondary recovery by waterflooding presently accounts for more than one-half of current U.S. oil production. Many research papers have addressed the effect of wettability on waterflood recovery during this period. For much of the past 50 years, however, a large body of reservoir engineering practice has been based on the assumption that most reservoirs are very strongly water-wet (VSWW); i.e., the reservoir-rock surface always maintains a strong affmity for water in the presence of oil. The rationale for assuming VSWW conditions was that water originally occupied the reservoir trap; as oil accumulated, water was retained by capillary forces in the fmer pore spaces and as films on pore surfaces overlain by oil. Wettability behavior other than VSWW was observed for reservoir core samples, but was often ascribed to artifacts related to core recovery and testing procedures. The majority of reservoir engineering measurements have been made on cleaned core with refined oil or air as the nonwetting phase to give results for, or equivalent to, VSWW conditions. Examples of such measurements are laboratory waterfloods, determination of electrical resistivity vs. water saturation relationships, and capillary pressure measurements for determination of reservoir connate water saturation. Mounting evidence on the effects of crude oil on wetting behavior l has now led to