The classification of and much of the understanding behind miscible flooding is based on fractional flow theory. The purpose of this paper is to extend such understanding to the displacement of oil by a miscible solvent in the presence of an immiscible aqueous phase. We pay special attention to the effects of simultaneous water-solvent injection and of the behavior of chase fluids following a solvent-water slug.
Besides the general enhancement of understanding, practical results of this work are: (1) graphical procedures which are valid for arbitrary water-solvent ratios and arbitrary initial conditions; (2) development of the notion of an optimum water-solvent ratio (Caudle and Dye's definition is valid only for secondary floods); (3) a procedure for sizing solvent slugs; (4) procedures for selecting the best chase fluid; and (5) the concept of and methods to compute a water-solvent ratio in the chase fluid which minimizes solvent usage.
Our results must be interpreted within the limits of the fractional flow assumptions. Nevertheless, the insight provided by the fractional flow solutions yields a strong base for interpreting the behavior of complex solvent floods.
