The role of phase saturation in depressurization and CO2 storage in natural gas hydrate reservoirs: Insights from a pilot-scale study

The natural gas recovery and CO2 storage in natural gas hydrate (NGH) reservoirs is a promising integrated strategy for implementing clean energy production and CCUS. However, the fundamental characteristics (phase saturation) of NGH reservoirs on the feasibility and efficiency of the above-integrated processes remain unclear. In this study, we synthesized methane hydrate-bearing sediments (MHBS) with varying hydrate saturation (S-H= 18-66 %) and aqueous saturation (S-A = 7-92 %) at marine NGH conditions (T = 283.8 K, P = 11.0 MPa). The effect of phase saturation on depressurization-induced hydrate dissociation, and subsequent CO2 storage and hydrate restoration by CO2/N-2 injection was investigated. The water-saturated MHBS with high S-A can promote faster MH dissociation during the depressurization stage and prevent secondary hydrate formation, but a slower MH dissociation during the later stage due to slow heat transfer. A relatively high S-H (> 63 %) results in sustained low temperatures due to insufficient heat supply slowing down MH dissociation. The CH4/CO2/N-2 mixed hydrates (Mix-H) formation and CO2 storage in depleted MHBS after CO2/N-2 injection initially increase and then decrease with post-mining S-A (peak at S-A = 54.5 %). The rate of Mix-H formation decreases as the post-mining S-A increases. Higher pre-mining S-H and CO2/N-2 injection rate facilitates rapid gas phase mixing within the sediments, thereby improving the kinetics and homogeneity of Mix-H formation and hydrate-based CO2 storage efficiency. MHBS with relatively low S-H and S-A exhibit enhanced safety for the integrated process, offering reduced depressurization-induced sediment subsidence and increased hydrate restoration ratios (similar to 85 %) by Mix-H formation.