Investigating CO2 sequestration-induced acidification in marine depleted NGH reservoirs with carbonate caprock via HCM coupling

Sequestration of CO2 in marine depleted natural gas hydrate (NGH) reservoirs is proposed as a method to mitigate climate change. In this scenario, it is worth considering that the dissolution of CO2 in the pore fluid induces acidification, which may cause the dissolution of carbonate minerals in the sediments and the consequent increase of the risk of leakage. A Hydraulic-Chemical-Mechanical (HCM) coupled approach is here proposed to reproduce CO2 injection into depleted NGH reservoirs, considering transport, chemical reactions and potential failure processes. In this framework, CO2 is represented as a dissolved species (single phase aqueous transport) and thermal effects are considered parametrically trough a reaction-kinetic dependence, enabling a focused assessment of carbonate dissolution-driven alteration and associated mechanical response. The model is applied to a real case scenario in the South China Sea (SCS). The migration of dissolved CO2 is numerically simulated and carbonate reaction processes over a 100-year scale are evaluated. The corresponding evolution of displacement and instability during injection is systematically evaluated via a simplified mechanical model. In the injection scenario considered, preliminary results indicate that CO2 injection causes acidification of pore fluids within an 80 m radius, leading to severe dissolution of carbonate near the well. A maximum top displacement of 0.63 m and an instability zone length of 18.1 m are observed, which may lead to CO2 leakage in severe cases. Furthermore, some sensitivity analyses have been performed to evaluate the effects of initial carbonate layer permeability, injection velocity, and temperature on the CO2 injection process. The results of this study provide preliminary insights and a conceptual pathway in the context of climate change mitigation and sustainable energy utilization, providing a benchmark regarding CO2 injection into marine sediments.