Migration and Trapping of Injected CO2: Insights From Numerical Modeling of China's First Offshore Storage in the Enping Saline Aquifer

Abstract

Located in the Pearl River Mouth Basin, the Enping 15-1 project marks China's pioneering offshore CO₂ storage endeavor. This study conducts a coupled thermohydrogeochemical simulation to investigate the mechanisms governing CO₂ migration, trapping, and reservoir–caprock interactions within the target saline aquifer of the Yuehai Formation. Simulation results reveal that CO₂ will be confined within the anticline of the target reservoir. Over a 10-year period, about 55% of the injected CO₂ dissolves into formation water, increasing to more than 80% after 500 years, indicating that solubility and structural trapping are the dominant long-term trapping mechanisms. A moderate increase in CO₂ injection rate can enhance the dissolution rate. The pressure buildup near the injection well remains below 0.5 MPa—well within the caprock integrity threshold—while CO₂ dissolution lowers formation pH, driving transient dissolution of carbonates and feldspars, followed by limited secondary mineral precipitation. Overall, net mineral trapping is negligible. These findings highlight that in offshore formations such as Yuehai, which are characterized by high permeability, low salinity, and limited potential for mineral trapping, the long-term security of CO₂ storage primarily depends on structural trapping and solubility trapping. The results provide an empirical and modeling framework to support the monitoring design and risk assessment of future offshore CO₂ storage projects in China and similar continental margin basins.