Water Alternating Gas in Stratified Reservoirs - A Sensitivity Study of WAG Parameters
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Water alternating gas (WAG) injection is an EOR method where water and gas is injected in cycles. The process exploits the microscopic displacement advantages of gas while using subsequent water slugs for mobility control. The gas is also effective in displacing attic oil not reached by water, and as such WAG can improve both microscopic and macroscopic sweep.Heterogeneities and high permeability streaks and channels can be an issue for effective WAG application. However, the distribution and location of these heterogeneities are not irrelevant and can in some situations be advantageous for gas injection. This study investigates the importance of the position of the high quality units in immiscible WAG by using a modified simulation model of a Gullfaks satellite. The simulations use a black oil fluid description and are run in Eclipse 100. In addition to selected sensitivities, six different stratification sequences are investigated. The layers are moved within the model to explore stratifications where the highest quality layers are on top of the formation, termed downwards fining , the other way around, termed upwards fining , and other varieties. The study showed WAG provided additional recovery between 1.0 % and 3.0 % in all simulations. WAG was 25-50 % more effective in upwards fining sequences than downwards fining sequences and gas breakthrough is reduced and delayed. Vertical permeability was found to have a large impact on results in both waterflooding and WAG in the base case. Increased vertical permeability reduced the incremental oil of WAG over waterflood in the base case, but it is not given that this is the case in other setups. Further investigation is recommended on the matter.