Multi-state CO2 distribution patterns for subsea carbon sequestration assisted by large-scale CO2 hydrate caps

Hydrate-based CO2 storage in subsea sediments presents a promising solution for safe carbon sequestration, as CO2 hydrate caps effectively reduce CO2 leakage risk. However, the effectiveness of using large-scale hydrate caps to achieve substantial CO2 sequestration is still uncertain. This study developed a numerical model for CO2 sequestration in sediment environments. The distribution patterns of multi-state CO2 (i.e., free, dissolved, and hydrate states) and the effectiveness of hydrate caps were investigated using single-horizontal-well and dual-horizontal-well systems. The findings indicated that a higher injection rate expedited the formation rate of CO2 hydrate caps but reduced the dissolved CO2 sequestration efficiency within the hydrate formation zone and the free phase zone. At the same CO2 sequestration amount, a low-flow-rate prolonged injection strategy could mitigate the pressure accumulation near the well and broaden the distribution range of the hydrate cap. Smaller well spacing facilitated the formation of a larger hydrate cap during the dual-well CO2 sequestration, with the thickness of the hydrate cap increasing by approximately 12 m over 50 years after CO2 injection cessation. Furthermore, a low-permeability mud cap interfered with the processes of CO2 plume migration and heat transfer, exacerbating the stratum instability near the injection well within the hydrate formation zone. This study provided new insights into forming large-scale CO2 hydrate caps and contributed to developing the CO2 storage technology in subsea sediments.