Multi-objective optimization of operation conditions by smart proxy model for geological CO2 storage design in an over-pressured aquifer in the Ulleung Basin

Abstract

This study aims to optimize the geological CO2 storage (GCS) design in the D structure in the East Sea using a Smart Proxy Model (SPM). Analyzing the geomechanical stability of the overpressured aquifer, it was found that the pore pressure in the fault plane must be maintained below the current pressure value. Assuming that two existing wells were repurposed as an injection well and a relief well, Feature Engineering was adopted to derive new parameters for training the SPM with a reduced number of simulation runs. The optimization was conducted to maximize cumulative CO2 injection and minimize brine production, considering several design parameters, including pre-relief period, the selection of pre-existing wells, and perforation intervals. The model exhibited high predictive accuracy in blind testing, with higher R2 than 90%. 14 optimal solutions and the Pareto front were obtained by employing the SPM in the NSGA-II algorithm. As a result, the lowest cumulative CO2 injection and brine production are 3.89 Mt and 5.12 Mt, respectively, while the largest CO2 injection and brine production are 33.9 Mt and 59.7 Mt, respectively. In addition, it was found that the optimal perforation intervals strongly depend on both the operation conditions and reservoir characteristics. Therefore, the perforation intervals of the injection and relief wells should be optimized when a relief well design is incorporated into a GCS process. Although this study is specific to the D structure, the proposed ML-based optimization framework incorporating geomechanical risks can be adapted to other geological formations for GCS designs.