WAG Injection Experiments With In-Situ Saturation Measurements at Reservoir Conditions and Simulations

Abstract

Abstract This paper presents Water-Alternating-Gas (WAG) injection experiments with x-ray CT in-situ saturation measurements of the gas, oil and water phases in a normal size single core plug, and simulation of the experiments with hysteresis in wetting and non-wetting phase rock functions. The experiments were simulated and history matched mainly by adjusting the rock functions in a reservoir simulator to analyze the physical behavior and to obtain relative permeability and capillary pressure functions. The history matching includes the fluid saturation profile match for all mobile fluids in the core under dynamic conditions (CT-scans were taken at pre-selected locations during the experiments), and the fluid production history match. This combined history matching procedure gives reliable drainage and imbibition relative permeability and capillary pressure functions, including hysteresis functions related to the different processes. The resulting relative permeability and capillary pressure functions for relevant WAG injection sequences were then applied in simulation of equilibrium gas- and water-injection in an eight-plug composite core comprising the mentioned single core plug as one of the similar eight core plugs. Very good history match was obtained for the composite core flood experiments. The WAG injection sequences are;water injection (WI) at the bottom of the core at very low rate (capillary imbibition),equilibrium gas injection (GI) at the top of the core at very low rate (gravity stable GI),WI at the bottom of the core at moderately high rate to mobilize residual oil,GI at high rate to displace residual oil, andWI at very high rate to obtain "true" end-point saturations. The two experiments performed on the composite core are;equilibrium gas injection at the top, andwater injection at the bottom. Both the WAG and composite core experiments were performed on restored North Sea reservoir cores with equilibrium gas/oil/water systems at reservoir conditions. Introduction Water-Alternating-Gas (WAG) and combined gas-water injection schemes have received noticeable interest as improved oil recovery (IOR) methods in the North Sea quite from the beginning of implementing IOR strategies for the province1. These injection schemes are designed to improve sweep efficiency and mobilize oil in zones of the reservoir that are by-passed in gas and water flood alone. Pilot projects have been performed in the North Sea to evaluate these IOR methods on field scale2. A successful WAG pilot test was initiated on the Gulfaks field in 19913 and later on field tests were implemented in Snorre4 and Brage5. A significant contribution from WAG to increase and accelerate oil production was reported for all tests. Simulation of laboratory SCAL experiments for the purpose of design and analysis is utilized to assess the potential of implementing these techniques in selected field developments. For multiphase flow situations, this implies that relative permeability and capillary pressure functions are to be specified at all locations throughout the reservoir. Consequently, accurate determination of multiphase flow functions is an issue of great concern to the oil industry. As there is typically insufficient information to make reliable estimates of these functions from data gathered from field tests or production history, they are generally determined through analyses of data gathered from laboratory experiments on small core samples2 recovered from the reservoir.