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

Abstract

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.