A Model for Wettability Alteration in Fractured Reservoirs

Many of the world's oil reservoirs are naturally fractured. Injected water will flow into this highly permeable network and not sweep the matrix containing most of the oil. Spontaneous imbibition can drive oil into the fractures by capillary forces. However the wettability of the reservoir is critical for the ultimate potential of oil recovery. Experimental results indicate that exposing chalk cores to seawater or similar brines may alter the wettability towards a more water-wet state. In a fractured reservoir this exposure will happen by molecular diffusion and by water imbibing into the matrix. The injected brine is not in chemical equilibrium with the in-situ rock, water and oil so that chemical reactions occur. Experiments have shown that for chalk rocks the relevant processes are adsorption/desorption of ions on the mineral surface, precipitation/dissolution of minerals or ion exchange. We have developed a model where a reservoir is defined by a fracture surrounded by matrix on both sides. The injected brine contains a component that can adsorb on the matrix surface. These changes of the mineral surface are assumed to alter the wettability towards a more water-wet state. The wettability alteration is described by shifting curves for relative permeability and capillary pressure from curves representing oil-wet conditions towards curves representing more water-wet conditions. The model can make predictions regarding the effect of matrix properties such as wettability and external parameters such as schedule of brine compositions and injection rate. The understanding of the coupling between wettability alteration controlled by water-rock chemistry and fracture-matrix flow is highly relevant for gaining more insight into the observed high recovery in the North Sea chalk field Ekofisk.