Long-term safety of continental slope carbon storage in the South China Sea: 600,000-year simulation of the effect of hydrate cap and density pump coupling mechanism

Hydrate-based CO2 Sequestration (HBCS) targets deep-ocean settings (>3,000 m) to exploit negative buoyancy to sink CO2 as a primary fail-safe. On the other hand, the vast potential of shallower continental slopes is often overlooked due to risks associated with positive buoyancy. In this study, a 600,000-year simulation of CO2 storage in sediments at a water depth of ∼1,300 m in the Shenhu area of South China Sea is conducted with an extended TOUGH+Hydrate framework with improved salinity-dependent thermodynamics. Contrary to concerns over CO2 leakage, the results obtained reveal a robust hydrate cap-density pump coupling mechanism, which ensures long-term sequestration. A critical “temporal transition” process is identified: a self-sealing hydrate cap forms rapidly with liquid CO2 injection, serving as a transient barrier that effectively contains the buoyant plume for approximately 1000 years; such a containment time window secures the initiation of the density pump mechanism, which is driven by the gravitation-associated downward convective mixing of CO2-saturated brine. The density pump mechanism is composed of two stages: liquid CO2 dissolution via intense brine fingering, followed by hydrate dissolution. As a result of the above process, all free-phase CO2 is sequestered with less leakage risk. Based on the results of this research, the continental slope is a potential place for safe, large-scale carbon neutrality.