Direct Flue Gas Injection into Ocean for Simultaneous Energy Recovery and CO₂ Sequestration in Solid Hydrate Reservoirs

Natural gas hydrate deposits in subsea sediments and permafrost have garnered significant interest as an unconventional energy resource and could potentially help in a sustainable energy transition. Among various methane production strategies, gas exchange by CO2 or a N2/CO2 mixture stands out as a promising method, as it provides hydrate structural stability due to CH4 replacement with CO2, leading to simultaneous methane production and carbon dioxide sequestration. Insights into the kinetics of methane replacement using a mixture of N2 and CO2 (simulated flue gas) are not studied enough in the open literature. Direct flue gas injection (DFI) in hydrate reservoirs can circumvent the costly carbon capture step, thereby reducing overall operational expenses. In this direction, we have investigated the effect of the injected flue gas composition on methane production and CO2 sequestration from lab-simulated hydrate-bearing sediments. Initially, methane hydrate formation experiments were carried out at ∼275.15 K in porous media having 35% porosity and 75% water saturation. After the initial methane hydrate saturation phase, the gas exchange reaction (methane production) was carried out by injecting a gas mixture containing N2 and CO2 with three different N2/CO2 molar ratios (3:1, 1:1, and 1:3) and pure CO2 by maintaining ∼4 MPa driving force based on the corresponding hydrate equilibrium pressure. It has been observed that N2 assisted in CH4 replacement from hydrate cages, improving CH4 recovery to a certain extent depending on the N2/CO2 molar ratio. The maximum CH4 recovery is observed for the injected gas mixture with a N2/CO2 molar ratio of 1:3. Although pure CO2 in the fluid state led to some improvement in CH4 recovery (comparable to N2/CO2 = 1:1), it is not advisable owing to cost and operability concerns. We anticipate that DFI can provide energy security, enhanced geothermal stability, and CO2 sequestration potential in marine basins.