Hydrate-based CO2 sequestration in post-exploitation natural gas hydrate reservoir: A numerical method considering mixed hydrate

Hydrate-based CO2 sequestration (HBCS) in post-exploitation natural gas hydrate (NGH) reservoirs via forming CO2-CH4 mixed hydrate has been proposed to achieve CO2 sequestration and geological restoration simultaneously. However, existing numerical models struggle to accurately simulate the mixed hydrate formation and geomechanical responses. Thus, we proposed a numerical method to solve the limitations of the existing model that can only consider pure CH4 and CO2 hydrates, which was verified by laboratory experiments with an error of ≤3.54 %. Then, we evaluated the HBCS in the post-exploitation NGH reservoir under medium-temperature and high-pressure conditions (10–15 °C, 11–16 MPa) based on geological data at Site W17. Results show: (i) Depressurization-induced NGH dissociation enlarges pore spaces and causes subsidence, while CO2 injection restores pressure and forms mixed hydrates with residual CH4 for structural support. (ii) The injection process may cause hydrate accumulation near the wellbore, blocking the seepage channel and inhibiting the sequestration effect. (iii) Some NGH stimulation measures have a synergistic effect with HBCS. For example, complex structural wells allow CO2 to be injected more dispersed, forming a wider area of mixed hydrates and avoiding local blockage. (iv) The high permeability case (20 mD) exhibits superior synergy effects with an energy efficiency of 448 and a sequestration capacity of 5786 tons, but the higher the exploitation degree, the worse the geological restoration effect. These findings provide a reference for future HBCS in post-exploitation NGH reservoirs, including blockage risk mitigation and well design optimization, obtaining additional benefits from carbon trading to reduce costs.