CH4 hydrate production coupled with CO2 sequestration and hydrate restoration employing depressurization assisted by CO2-N2 injection at marine conditions

Natural gas hydrates (NGHs) buried below the seafloor are globally abundant, energy-dense, and essential to the world's future energy mix. The combination of depressurization and CO2-N2 injection processes for NGHs re-covery has been identified as an environmental method to harvest energy, CO2 sequestration, and to maintain the mechanical stability of sediment. In this regime, partial CH4 was produced by depressurization followed by CO2- N2 injection to form CH4-CO2-N2 mixed hydrate (Mix-H). In this study, a series experiments were designed to examine the key factors in the depressurization process (i.e. bottom hole pressure BHP and CH4 production ratio) on the subsequent processes of Mix-H formation, CO2 sequestration and hydrate restoration. It was observed that Mix-H formation occurred in all cases, but the final amount of Mix-H were all lower than the initial amount of MH before depressurization. Particularly, the formation of CO2/N2 hydrate in Mix-H is positively correlated with the amount of remaining CH4 in the combined phases of gas and hydrate after depressurization. The formation of CH4 hydrate in Mix-H during the reformation process occurred in five cases when MH dissociation ratio (DMH) reaches above 45.1 % during depressurization. MH was only observed to dissociate continuously after CO2/N2 injection when DMH is low at 30.8 %, which indicates possible "CH4-CO2 replacement reaction". Overall, increasing CH4 production at the same BHP decreases the CO2 sequestration and hydrate restoration. We first verified the feasibility of the combination method for synergistic CH4 recovery and CO2 sequestration in marine conditions. The experimental results provide possible guidance on the optimal design of the coupled processes and shed light on the relationship between CH4 recovery and CO2 sequestration that need to be wholistically balanced.