Ensemble-Based Optimization of the WAG Injection Process

For a low-permeability reservoir or a reservoir with fractures, CO₂-water-alternating-gas (CO₂-WAG) flooding generally leads to higher recovery than either continuous CO₂ flooding or waterflooding. Although CO₂ injection increases displacement efficiency, unless complete miscibility is achieved, poor sweep efficiency is generally obtained due to gravity segregation and the channeling of CO₂ through high permeability zones or viscous fingering. Alternating water injection with CO₂ injection results in better mobility control and increases sweep efficiency. Water injection also increases pressure which promotes miscibility. However, poorly designed WAG parameters can result in sub-optimal WAG performance. In this work, given the number of WAG cycles and the duration of each WAG cycle, we apply a slight modification of a standard ensemble-based optimization technique to estimate the optimal well controls which 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 synthetic, channelized-reservoir examples. The performances of the optimized WAG flooding, optimized waterflooding and optimized continuous CO₂ flooding are compared. Due to 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 by partially using WAG optimization as a proxy.