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

Abstract

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.