A Numerical Study of Benefits of Adding Polymer to WAG Processes for a Pilot Case

CO₂ flood and polymer flood are two proven and commercially practiced technologies for several decades. Traditional gas flood methods suffer from inadequate sweep efficiency and incomplete recovery of oil. Several methods to improve the volumetric sweep efficiency are practiced in field operations such as water alternating gas (WAG), polymer-gel, and foam-WAG. Polymer is traditionally added to the water flood projects to reduce the mobility of water in an effort to improve the water sweep efficiency and increase oil production especially from heterogeneous reservoirs. Both polymer and gas floods are mature technologies for improved oil recovery. However, few studies have been done on combined application of the two methods.

In this paper, we conducted simulation study of the potential benefit of adding polymer to the water in the CO₂ WAG process, taking the advantage of CO₂ miscibility with oil and polymer conformance control during water cycle. A commercial reservoir simulator CMG-STARS is used in this study, with its PVT module CMG-WinProp. A comprehensive polymer property module is used to calculate the shear thinning rheology and non-linear mixing of polymer solution in addition to the polymer adsorption and mobility reduction factor. Field scale simulations are performed based on a real field geological model and light oil fluid properties, taking into account the heterogeneity and EOR design. The performance of each EOR method on oil recovery is also evaluated, including the water flood, CO₂ flood, water alternating gas (WAG), and polymer alternating gas (PAG). Based on this pilot study, polymer alternating gas flood has the highest oil recovery factor of 74%, while WAG has 68% and water flood has 59% oil recovery factor. This field scale simulation study demonstrated positive response of PAG compared to WAG, water, or CO₂ floods in oil recovery and total injection.