Direct Flue Gas Injection into Ocean for Simultaneous Energy Recovery and CO<sub>2</sub> Sequestration in Solid Hydrate Reservoirs
Siddhant Kumar Prasad, Yogendra Kumar, Dnyaneshwar R. Bhawangirkar, Namrata Gaikwad, Jitendra S. Sangwai
Abstract
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 CO 2 or a N 2 /CO 2 mixture stands out as a promising method, as it provides hydrate structural stability due to CH 4 replacement with CO 2, leading to simultaneous methane production and carbon dioxide sequestration. Insights into the kinetics of methane replacement using a mixture of N 2 and CO 2 (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 CO 2 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 N 2 and CO 2 with three different N 2 /CO 2 molar ratios (3:1, 1:1, and 1:3) and pure CO 2 by maintaining ∼4 MPa driving force based on the corresponding hydrate equilibrium pressure. It has been observed that N 2 assisted in CH 4 replacement from hydrate cages, improving CH 4 recovery to a certain extent depending on the N 2 /CO 2 molar ratio. The maximum CH 4 recovery is observed for the injected gas mixture with a N 2 /CO 2 molar ratio of 1:3. Although pure CO 2 in the fluid state led to some improvement in CH 4 recovery (comparable to N 2 /CO 2 = 1:1), it is not advisable owing to cost and operability concerns. We anticipate that DFI can provide energy security, enhanced geothermal stability, and CO 2 sequestration potential in marine basins.