Long-Term Hydrate-Based CO ₂ Sequestration in South China Sea Subsea Sediments: Role of CO ₂ Injection Modes

Hydrate-based CO2 sequestration is a novel effective method for sequestrating large amounts of CO2 in subsea sediments. CO2 hydrate stability zone (CHSZ) primarily extends ∼130 m below the seafloor in South China Sea (SCS) Shenhu area. The location and the amount of liquid CO2 injected are critically important as they determine the fate of CO2 in the long term. In this study, we develop a numerical model to analyze the long-term CO2 sequestration behavior in response to various CO2 injection modes. We systematically design simulation cases to investigate the effects of CO2 injection position in relation to CHSZ, amount of CO2 injection, and the interval of CO2 injection on both short-term and long-term CO2 sequestration performance. The spatiotemporal evolution of CO2 hydrate, liquid CO2, and dissolved CO2 is examined. CO2 injection within the CHSZ yields a relatively thicker CO2 hydrate cap with hydrate saturation gradually decreasing downward, covering liquid CO2 underneath for continuous CO2 hydrate formation. However, CO2 injection across and below CHSZ both results in a much thinner CO2 hydrate cap above the base of CHSZ with inferior CO2 conversion to hydrate, yet liquid CO2 underneath remains stable over 100 years without significant upward migration. Increasing CO2 injection amount practically yields an extended area of CO2 hydrate cap; however, CO2 conversion to hydrate is reduced. Reducing CO2 injection interval further increases near-well overpressure and results in enhanced upward migration of CO2 with risk of potential leakage. Based on the long-term CO2 sequestration behavior, we further propose a method for estimating CO2 storage capacity accounting for stable storage as both liquid CO2 and CO2 hydrate. The findings provide practical guidance for designing an optimal CO2 injection strategy for future offshore CO2 sequestration projects in South China Sea.