Numerical Analysis of Immiscible WAG Recovery Mechanisms in a Large North Sea Field

A large North Sea oil field with excellent reservoir properties in which water injection with full voidage replacement has been the primary drainage mechanism since production start-up is considered. Full-field immiscible WAG injection is planned to further enhance the oil recovery by improving microscopic and macroscopic displacement efficiency. The objective of this work is to use numerical simulations to gain a detailed understanding of the WAG recovery mechanisms and quantify the stake.

This paper presents a numerical analysis of the main physical mechanisms contributing to the incremental WAG recovery compared to a standard waterflooding approach. Three-phase relative permeabilities are studied in detail to evaluate the reduction in residual oil saturation generated by successive water and gas sweeps. Using a zero-dimensional compositional simulation, the gas-oil interactions taking place during gas injection before saturation (strong condensing drive) and after saturation (weak condensing drive followed by stripping) are analyzed. Three-dimensional generalized Black-Oil simulation results are employed to quantify the magnitude of the deviscosification and oil swelling processes over time and the potential recovery increment induced by these mechanisms. Oil saturation maps are used to compute the recovery increment achieved thanks to the displacement of attic oil by the gas plume in areas not swept by water at the top of the reservoir.

Despite the simple Black-Oil model considered in the 3D study, the methodology summarized above provides a detailed insight into the key mechanisms yielding the excellent overall WAG incremental recovery observed in the full-field simulations. The study demonstrates that attic oil displacement is critical for the success of WAG in this field and is complemented by strong gas-oil interactions boosted by the large initial undersaturation of the oil. The results also illustrate the long-lasting beneficial impact of WAG on recovery years after the end of the cycles. A numerical sensitivity study is proposed combining grid refinement and advanced relative permeability hysteresis modeling to gain more confidence in the findings. The study concludes with a review of the limitations of the numerical modeling approach (no 3D compositional modeling, need for a more accurate time-dependent modeling of gas-oil interactions on coarse grids).