CO2 water-alternating-gas injection for enhanced oil recovery: Optimal well controls and half-cycle lengths

•An optimization framework based on augmented Lagrangian method and StoSAG algorithm is proposed to optimize both the well controls and injection half-cycle lengths.•Both deterministic optimization and robust optimization are performed to investigate the influence of adding WAG injection half-cycle lengths as optimization variables.•The conditions under which optimizing the injection time intervals of WAG significantly enhances the NPV of production and the conditions under which optimization of each injection periods has a very small effect on the NPV are established. CO2 water-alternating-gas (WAG) injection is an enhanced oil recovery method designed to improve sweep efficiency during CO2 injection with the injected water to control the mobility of CO2 and to stabilize the gas front. Optimization of CO2 -WAG injection is widely regarded as a viable technique for controlling the CO2 and oil miscible process. Poor recovery from CO2 -WAG injection can be caused by inappropriately designed WAG parameters. In previous study (Chen and Reynolds, 2016), we proposed an algorithm to optimize the well controls which maximize the life-cycle net-present-value (NPV). However, the effect of injection half-cycle lengths for each injector on oil recovery or NPV has not been well investigated. In this paper, an optimization framework based on augmented Lagrangian method and the newly developed stochastic-simplex-approximate-gradient (StoSAG) algorithm is proposed to explore the possibility of simultaneous optimization of the WAG half-cycle lengths together with the well controls. The proposed framework is demonstrated with three reservoir examples.