Well placement and control optimization for WAG/SAG processes using ensemble-based method

Compared to the CO2 flooding, the alternative injection of water/surfactant solution and CO2 gas (WAG/SAG) can provide good control over the mobility ratio, stabilize the displacement front and improve the macroscopic sweep efficiency of the flooding. We show that the performance of a WAG/SAG flood can be significantly improved by optimizing the locations and optimizing rates or pressure in order to avoid gas/water channelling and improve sweep efficiency. To maximize the life-cycle net present value (NPV) for a WAG/SAG process, we implemented a steepest ascent algorithm combined with a simplex stochastic gradient to find the optimal well trajectories and controls. Both simultaneous and sequential approaches for the joint optimization of well locations and controls are investigated for a synthetic channelized reservoir model. Well spacing constraints are also included in the optimization process using the penalty method in order to keep the distance between wells greater than a specified minimum value. Four synthetic reservoir problems are studied and optimized to illustrate the viability of the methodology.