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

[Display omitted] • Depressurization followed CO 2 -N 2 injection for CH 4 recovery and CO 2 sequestration. • Effect of MH dissociation ratio on subsequently mixed hydrate formation. • CH 4 hydrate re-formation and dissociation occurred depending on CH 4 hydrate left. • CO 2 hydrate formation depended on the CH 4 left in both gas phase and hydrate phase. • Increasing CH 4 production decreased the CO 2 sequestration and hydrate restoration. 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 CO 2 -N 2 injection processes for NGHs recovery has been identified as an environmental method to harvest energy, CO 2 sequestration, and to maintain the mechanical stability of sediment. In this regime, partial CH 4 was produced by depressurization followed by CO 2 -N 2 injection to form CH 4 -CO 2 -N 2 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 CH 4 production ratio) on the subsequent processes of Mix-H formation, CO 2 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 CO 2 /N 2 hydrate in Mix-H is positively correlated with the amount of remaining CH 4 in the combined phases of gas and hydrate after depressurization. The formation of CH 4 hydrate in Mix-H during the reformation process occurred in five cases when MH dissociation ratio (D MH) reaches above 45.1 % during depressurization. MH was only observed to dissociate continuously after CO 2 /N 2 injection when D MH is low at 30.8 %, which indicates possible "CH 4 -CO 2 replacement reaction". Overall, increasing CH 4 production at the same BHP decreases the CO 2 sequestration and hydrate restoration. We first verified the feasibility of the combination method for synergistic CH 4 recovery and CO 2 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 CH 4 recovery and CO 2 sequestration that need to be wholistically balanced. [ABSTRACT FROM AUTHOR]