The Effect of Core Wettability on Oil Mobilization, Capillary Forces and Relative Permeability in Chalk

Abstract

Abstract Rock wettability is of utmost importance when assessing reservoir recovery processes, because it controls key transport properties of fluid flow in porous media. The effects of wettability on capillary forces, fluid distribution, and oil mobilization are of great interest for understanding waterflooding and water-based EOR processes such as Smart Water injection. Two strongly water-wet and three reduced water-wet chalk cores containing Swi = 20% and 80 % non-wetting mineral oil were used in this study. Spontaneous imbibition (SI) experiments were used to assess the wettability of restored core material and forced imbibition (FI) tests were carried out to capture fluid flow behavior under a viscous force dominated environment. Oil recovery and pressure drop profiles, start and endpoint core saturations and pressure drops were collected in front of and during FI tests with formation water (FW) as injection fluid to avoid any chemical induced wettability alteration. The SI oil recovery results showed that the cores exposed to crude oil possessed reduced water wetness compared to the strongly water-wet reference cores. The FI oil recovery results showed only small differences in oil production profiles and ultimate recoveries. The oil recovery profiles displayed a piston-like displacement indicating that oil recovery was controlled by capillary forces at the injection rate used. SENDRA was used to simulate the effect of wettability on relative permeability and capillary pressure curves for the strongly to reduced water-wet cores from FI processes. On average, higher oil relative permeability end points and lower water relative permeability end points were measured for the strongly water-wet cores compared to the cores reduced in water-wetness. The core scale simulation with SENDRA indicates continuous production of water and oil taking infinite time to reach residual oil saturation, however, the end of production was reached at a finite time in the experiments. A history matching approach based only on single rate injection did not yield reliable results, partly, because the capillary and viscous forces cannot easily be separated in the history matching process. This affects estimates of residual oil saturation and water end points of relative permeability.