Water alternating gas (WAG) processes have been successfully performed as an oenhanced oil recovery (EOR) method. The main purpose is to increase the sweep efficiency of miscible/immiscible gas flooding by reducing the mobility ratio between injected gas and the formation oil which is the major problem in solvent EOR floods with very poor sweep and early gas breakthrough. Forecasting the WAG performance using fast and robust analytical techniques are of great importance to study and better design the EOR projects as an alternative to very costly and time consuming numerical simulation. In this study we present a novel approach to predict the performance of WAG processes based on a general isothermal EOR forecasting model.

We have developed a general isothermal EOR forecasting tool using fundamental principles of material balance, segregated flow and fractional flux. The effects of mobility ratio, miscibility conditions, heterogeneity and WAG ratio are all incorporated using Koval fractional flux model with its general applicability to even extremely heterogeneous formations. The validity of our WAG forecasting tool was confirmed by matching numerous fields and pilots WAG projects and also numerical simulation results.

For forecasting purposes, we have developed proxy models for both Koval factor (called effective mobility ratio) and final average oil saturation as a function of reservoir and process variables such as reservoir pressure, heterogeneity, and WAG ratio. Numerous WAG simulation scenarios were designed and history matched by the forecasting model. The strength of the correlations between the forecasting model parameters (Koval factor and final average oil saturation) and reservoir/process variables confirms the ability of the model for forecasting in addition to history matching.

The forecasting tool provides time evolutions of key flood performances such as average oil saturation, cumulative oil recovery, oil cut, oil rate, recovery efficiency and volumetric sweep for miscible/immiscible WAG EOR processes. The volumetric sweep efficiency predictions enable us to study the sweep efficiency of the miscible flood that has a major impact on the success of gas flooding projects. In addition it is able to predict the oil bank creation, existence and durability of the miscible slug that greatly affects the WAG results.

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