WAG Injection to Reduce Capillary Entrapment in Small-Scale Heterogeneities
Authors: van Lingen P.P. , Barzanji O.H.M. , van Kruijsdijk C.P.J.W. 
Published: December 2007

Abstract
We present an experimental study of water-alternate-gas (WAG) injection, or more accurately simultaneous water and gas injection (SWAG) as a means to reduce the capillary entrapment of oil in small (centimeter) scale reservoir heterogeneities. We describe the physical principle of the displacement method, which we illustrate with simulation results. In the experimental study, we compare the displacement efficiency of water injection and SWAG injection in scaled models with heterogeneity in wettability and grain size distribution. In these experiments, the average WAG recovery was twice as high as the water flood recovery.

Introduction
Small-scale heterogeneities in grain-size distribution are common in oil reservoirs. The dominant heterogeneity is often in the form of laminated structures such as cross-bed sets.
The importance of capillary entrapment in reservoir heterogeneities during immiscible flooding is generally recognized. The impact of capillary trapping can be estimated with simple analytical models. The important reservoir parameters are the length-scale of the heterogeneity, the contrast in grain-size (which affects the permeability and the capillary pressure curve), the volume-fraction of the porous medium occupied by coarse- and line-grained material, and wettability (which affects capillary pressure curves and relative permeability). Equally important are the interfacial tension, the injection rate, and the viscosity of the injected phase. These last three parameters can be manipulated to a considerable extent to reduce the amount of trapped oil.
Previous Work on WAG Injection. The WAG process, patented by Parrish in 1966, was initially proposed as a method to increase the sweep efficiency during gas injection. Since that time, numerous studies have been performed on front stability during WAG processes, mainly for miscible flooding.
In addition to improving sweep efficiency, WAG injection was found to improve the displacement efficiency in heavy oil reservoirs by oil phase swelling and viscosity reduction. An adverse effect caused by oil trapping was reported. This type of trapping is not exactly the same as the capillary entrapment discussed here; when the saturation of a continuous gas phase is low, its network may not branch to all the pores containing residual oil, with a negative effect on the swelling process.
WAG injection was reported to improve oil and condensate recovery on pore-scale level due to the reduction of the residual oil saturation by film flow.
Surguchev et al. performed a screening study of WAG injection strategies in heterogeneous reservoirs. They concluded that WAG injection increases the microscopic displacement efficiency, and that the sweep efficiency of the waterdrive was increased due to gas-blocking of the high permeable zones.
In This Work. We focus on WAG injection to reduce the capillary entrapment of oil in heterogeneous reservoirs. We describe the physical principle, which we illustrate with simulation results. Experimental verification is conducted in a scaled laboratory setup. We model the porous medium with unconsolidated grain packs with a lamina-type heterogeneity. Wettability effects are modeled using either glass beads (water-wet) or resin coated sand (oil-wet). In different experiments, we study the effect of flooding both perpendicular, and parallel to lamina-type heterogeneities. In all experiments, we compare the injection of only water, with the simultaneous injection of both water and air. To justify this comparison, the total injection rate was kept constant.
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