Composition Routes for Three-Phase Partially Miscible Flow in Ternary Systems

Abstract

SummaryThree-phase flow often occurs in reservoirs, particularly for secondary or tertiary oil recovery methods such as miscible gas or chemical flooding. In these cases, there is often significant mutual solubility of components in the phases. Unfortunately, there is insufficient understanding of how three partially miscible phases can affect flow. Furthermore, there are currently no benchmark analytical solutions available to validate numerical simulations for this complex flow regime.In this research, compositional solution routes are developed by the method of characteristics (MOC) for 1D, dispersion-free flow wherein up to three partially miscible flowing phases may be present. The method is applied to a water/alcohol/oil system that exhibits a large three-phase region in laboratory experiments. Unique solutions are found based on continuity arguments, shock-jump conditions, entropy constraints, and velocity constraints. The analytical solutions are compared to fine-grid finite-difference simulations to verify that they converge to the same dispersionfree limit.The results show that composition routes within the three-phase region often exhibit one phase below its residual saturation so that only two phases are flowing. As miscibility is approached, cumulative oil recovery initially declines because of the development of constant states in the solution, which cause the leading shock to speed up. We show that multicontact miscibility is developed at the critical point of one two-phase region and along the boundary of the three-phase region where all shocks and waves flow at a dimensionless velocity of one. Last, we show that injectivity changes by a factor of two for the specific relative permeabilities and injection compositions used.