Wettability Impacts on Oil Displacement in Large Fractured Carbonate Blocks

Two-dimensional imaging of water and oil saturations during waterfloods in fractured carbonate rock models was obtained using nuclear tracer imaging and magnetic resonance imaging. Large outcrop chalk and limestone blocks were aged in crude oil to obtain wetting conditions from strongly water-wet to weakly oil-wet. The change in the oil recovery mechanism as the wettability shifted was investigated with and without the presence of fractures. Visualization of local, in situ fluid saturations during waterfloods improved the interpretation of the displacement process and oil recovery mechanisms. Experimental results demonstrate how fractures determine the displacement pattern differently depending on the matrix wettability conditions during waterfloods. At strongly water-wet conditions, the fractures had a minor impact on the ultimate recovery but significantly changed the progression of the water front compared to the unfractured case. At less water-wet or oil-wet conditions, capillary imbibition of water from the fracture to the matrix was reduced and fractures had a major impact on the ultimate recovery and water breakthrough time.