Applying Fractional-Flow Theory Under the Loss of Miscibility

Abstract

Summary This paper examines the limits of the Walsh and Lake (WL) method for predicting the displacement performance of solvent flood when miscibility is not achieved. Despite extensive research on the applications of fractional-flow theory, the prediction of flow performance under the loss of miscibility has not been investigated thoroughly. We introduce the idea of an analogous first-contact miscible (FCM) flood to study miscibly degraded simultaneous water and gas (SWAG) displacements using the WL method. Furthermore, numerical simulation is used to validate the WL solution on one oil/solvent pair. In the simulations, the loss of miscibility (degradation) is attributed to either flow-associated dispersion or insufficient pressure to develop the miscibility. 1D SWAG injection simulations suggest that results of the WL method and the simulations are consistent when dispersion is limited. For the 2D displacements, the predicted optimal water-alternating-gas (WAG) ratio is accurate when the permeable medium is fairly homogeneous with a limited crossflow or is heterogeneous with a large lateral correlation length (the same size or greater than the interwell spacing). The results suggest that the accuracy of the WL method improves as crossflow is reduced. In addition, linear growth of the mixing zone with time is observed in cases for which the predicted optimal WAG ratio is consistent with the simulation results. Hence, we conclude that the WL solution is accurate when the mixing zone grows linearly with time.