Ensemble-Based Optimization of the Water-Alternating-Gas-Injection Process

CO2-water-alternating-gas (CO2-WAG) flooding generally leads to higher recovery than either continuous CO2 flooding or waterflooding. Although CO2 injection increases microscopic displacement efficiency, unless complete miscibility is achieved, suboptimal sweep efficiency may be obtained because of gravity segregation and the channeling of CO2 through high-permeability zones or by viscous fingering. Alternating water injection with CO2 injection results in better mobility control and increases sweep efficiency. Water injection also increases pressure that promotes miscibility. However, poorly designed WAG parameters can result in suboptimal WAG performance. In this work, given the number of WAG cycles and the duration of each WAG cycle, we apply a modification of a standard ensemble-based optimization technique to estimate the optimal well controls that maximize life-cycle net present value (NPV). By optimizing the well controls, we implicitly optimize the WAG ratio (volume of water injected divided by the volume of gas injected). We apply the optimization methodology to a synthetic, channelized reservoir. The performances of optimized WAG flooding, optimized waterflooding, and optimized continuous CO2 flooding are compared. Because of the similarity between WAG and surfactant alternating gas (SAG foam), we also optimize the SAG process and provide a more computationally efficient way to optimize the SAG process with the optimal well controls obtained from WAG as the initial guesses for the optimal controls for SAG.