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

Hydrate-based CO 2 sequestration is a novel effective method for sequestrating large amounts of CO 2 in subsea sediments. CO 2 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 CO 2 injected are critically important as they determine the fate of CO 2 in the long term. In this study, we develop a numerical model to analyze the long-term CO 2 sequestration behavior in response to various CO 2 injection modes. We systematically design simulation cases to investigate the effects of CO 2 injection position in relation to CHSZ, amount of CO 2 injection, and the interval of CO 2 injection on both short-term and long-term CO 2 sequestration performance. The spatiotemporal evolution of CO 2 hydrate, liquid CO 2, and dissolved CO 2 is examined. CO 2 injection within the CHSZ yields a relatively thicker CO 2 hydrate cap with hydrate saturation gradually decreasing downward, covering liquid CO 2 underneath for continuous CO 2 hydrate formation. However, CO 2 injection across and below CHSZ both results in a much thinner CO 2 hydrate cap above the base of CHSZ with inferior CO 2 conversion to hydrate, yet liquid CO 2 underneath remains stable over 100 years without significant upward migration. Increasing CO 2 injection amount practically yields an extended area of CO 2 hydrate cap; however, CO 2 conversion to hydrate is reduced. Reducing CO 2 injection interval further increases near-well overpressure and results in enhanced upward migration of CO 2 with risk of potential leakage. Based on the long-term CO 2 sequestration behavior, we further propose a method for estimating CO 2 storage capacity accounting for stable storage as both liquid CO 2 and CO 2 hydrate. The findings provide practical guidance for designing an optimal CO 2 injection strategy for future offshore CO 2 sequestration projects in South China Sea.