Effect of brine salinity and crude-oil properties on oil recovery and residual saturations

Centrifuge experiments were conducted on Berea cores to measure the drainag e and imbibition relative permeabilities in two-and three-phase systems. Th ree crude oils, Prudhoe Bay and Shell Mars A-1 and A-20 crude were used in our tests. No effect on oil recovery was obtained in the drainage experimen ts when the crude oil was displaced by air at connate water. However, in th e imbibition experiments the oil recovery increased significantly with the salinity of the connate brine. The salinity of the displacing brine had no significant influence on the oil recovery. The relative permeability curves obtained during drainage were also found to be insensitive to the salinity of the brine. However, the imbibition relative permeability curves show st rong salinity dependence. A comparison of the two crude oils and a nonpolar mineral oil indicates that the more water-wetting Mars crude oil shows a h igher oil relative permeability at the same bond number compared to the mix ed-wetting Prudhoe Bay crude oil. The salinity dependence of the residual s aturations and the relative permeabilities clearly indicate that the change in wetting properties of the rocks surfaces from water-wet to mixed-wet du ring the drainage process is an important factor controlling the imbibition relative permeability curves. This suggests that the performance of waterf loods will be strongly affected by the composition of the crude oil and its ability to wet the rock surfaces, the salinity of the connate brine in the reservoir, and the height above the oil/water contact.