Assessing CO2 storage mechanisms in marine shallow sediments to Mitigate leakage risks from sub-seabed carbon storage: A Numerical Simulation Study

Gang Hu¹Xiangyi Yi²Xuanhua Tian^1,3*Pengchun Li⁴Linxiong Peng¹Xi Liang⁵

• ¹School of Petroleum Engineering (Engineering Technology Research Center of Guangdong Province for Unconventional Energy), Guangdong University of Petrochemical Technology, Maoming, China, Maoming, China
• ²Energy Resource School (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation), Chengdu University of Technology, Chengdu, China
• ³Guangdong University of Petrochemical Technology, Maoming, China
• ⁴CAS Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences,, Guangzhou, China
• ⁵University College London, London, England, United Kingdom

The geological storage of CO₂ in offshore saline aquifers has been implemented at various sites worldwide. While subsea sediments can serve as a critical barrier against potential leakage from deep storage formations, the mechanisms and storage capacity for CO₂ trapping within these sediments remain inadequately understood. This study developed a two-dimensional conceptual model of shallow sediments based on the Enping15-1 Carbon Capture and Storage (CCS) project site in the South China Sea. Furthermore, the CO2-water-rock reaction and storage mechanism were simulated using CMG-GEM, and the process of CO2 leakage into the sediments was investigated. The results indicate that CO₂ leakage into the shallow seabed sediments is primarily sequestered through dissolution in pore water, accounting for 70% of the total sequestration, while residual and mineral trapping contribute 10-20%. The dissolution of CO₂ leads to pore water acidification, which triggers the dissolution of anorthite and K-feldspar under the prevailing initial geochemical conditions. Dynamic reaction behavior is mainly observed at the leading edge of the acidified plume. However, if the leakage rate exceeds a critical threshold, the advancing acidified plume front causes partial dissolution of previously precipitated carbonate minerals. The critical leakage rate is determined to be 0.2 m³/day for single-point leakage and 0.3 m³/day for multi-point leakage. Notably, multi-point, low-velocity leakage enhances secondary storage within the sediment, thereby reducing the risk of CO₂ release into the overlying seawater.