Gas injection is the second largest enhanced oil recovery process practiced in the world. To increase the macroscopic sweep efficiency, the gas is generally injected intermittently with water either simultaneously or as slugs. This mode of injection, called water alternating gas (WAG), is being widely used in the oil fields. This experimental and numerical study is aimed at understanding mechanisms of oil recovery following slug first-contact miscible WAG (FCMWAG) injection in the presence of discontinuous shales. A particular objective is to quantify the accuracy of numerical simulation when modeling slug FCMWAG displacements in these heterogeneous systems. A series of well-characterized laboratory experiments in two-dimensional heterogeneous bead packs were carried out. Slug FCMWAG displacement experiments were performed in the porous media containing discontinuous shale. Slug injection was carried out at a WAG ratio of 1:1 with slug sizes of 5 and 50% PV. All experiments were then modeled using IMPES finite difference simulator without using history matching (all simulations used directly measured porous media properties as inputs) to quantify the accuracy of the simulation when modeling the slug FCMWAG displacements in a porous media with a discontinuous shale. The large water slug (50% PV) displacement outperformed the 5% PV slug process because the large water slug injected ahead of solvent significantly reduced the extent of fingering and the bypassed oil around the shale barrier. Simulation of the slug WAG experiments showed good agreement for the oil recovery. However, a discrepancy in the physics of the fluid flow during the displacements, especially downstream and upstream of the shale barrier, was observed in the fluid cut curves. Simulation predicted less fingering and fewer breakdowns of the injected slugs.
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