Fundamental insights into multistep depressurization of CH4/CO2 hydrates in the presence of N2 or air

Exploitation of natural gas hydrates provides an alternative way to address energy crisis. Dilute CO2 gas (13–30mol%CO2 with remaining N2/Air) injection into CH4 hydrates for hydrate swapping (SW) allows cheaper and more practical CH4 recovery and in-situ CO2 sequestration. However, the roles of N2/Air in dilute CO2 gas during exploitation remain unknown. It is unclear whether depressurization should be coupled after SW and continued below CH4 hydrate stability pressure. This work employed multistep depressurization (MD) to dissociate the mixed hydrates formed after SW from 86.5 to 97.9 bar at 0.7–1.2 °C in bulk-water and sandpack with CH4 hydrate saturation of 4.1–25.3 %. Effects of N2/Air on exploitation were investigated by examining hydrate morphologies and gas compositions. Morphological results in bulk-water indicated higher N2 fraction in 20mol%CO2/N2 triggered more CO2-rich hydrate reformation and CH4-rich hydrate dissociation. Exploitation results in sandpack indicated 13mol%CO2/N2 produced the highest CH4 swapping percent (46.6 %) and CO2 hydrate sequestration percent (29.1 %). Air exerted weaker promoting effects on exploitation compared with equivalent N2. The promotion of N2/Air on exploitation was dominated by dilute CO2 gas injection altering mixed hydrate equilibrium which varied with time-dependent gaseous compositions during MD. l-methionine of 3000 ppm had stronger promoting effects on CO2 sequestration in sandpack than bulk-water depending on mass transfer and water availability. Ceasing points (13.9–31.4 bar) suggested MD could be continued below CH4/above CO2 hydrate stability pressures and before water production. For the first time, this study provided insights into the roles of N2/Air to determine injection gas types and depressurization schemes for efficient and safe hydrate exploitation in gas-rich hydrate-bearing sediment.