Optimized design of water-alternating-gas eor projects in heterogeneous reservoirs

Abstract

To overcome reduced oil recovery resulting from poor mobility ratio and sweep efficiency by the low viscosity of CO2, water-alternating-gas (WAG) process has been commonly used as an advantageous alternative. This study examines how to optimize WAG processes in heterogeneous reservoirs by rigorous simulation of WAG injection. Compositional simulations of areally heterogeneous reservoirs were used to quantify the effects of process parameters such as WAG ratio and CO2 slug size, well parameters including well length and type, and reservoir heterogeneity characteristics such as Dykstra-Parsons' coefficient and correlation lengths on the oil recovery and economic value. Optimization was carried out with net present value (NPV) as an objective function. The main conclusions of this study show that more WAG cycles are required for maximum NPV at a given operation period. Higher water fraction in WAG injection is needed to improve sweep efficiency in more heterogeneous reservoirs. The longer horizontal wells maximize NPV as their orientation and permeability correlation direction is parallel. The proposed methodology has significant improvement on the oil recovery up to 43% and hence yields higher NPV up to 47%. The importance of using the reservoir simulation as a tool for optimum design before implementing WAG injection in practice is evidenced. It also demonstrated that the approach is viable to give the optimistic scenario of oil recovery prediction under WAG injection into heterogeneous reservoirs.