A linear stability analysis for immiscible displacements

A linear stability analysis has been performed for an immiscible displacement of a nonwetting phase by a wetting phase in a semi-infinite system of finite width and thickness. It is found that instabilities become a more severe problem as the capillary number based upon the characteristic thickness of the system increases. A displacement can always be stabilized, if the capillary number is sufficiently small. As the aspect ratio (ratio of thickness to width) decreases for a fixed capillary number, the potential for unstable behavior increases. All of this means that a displacement in the laboratory is likely to be more stable than a similar displacement in the field. Since the solution to the base state assumes the analytical solution of Yortsos and Fokas (1983), the effect of the mobility ratio could not be examined.