Numerical Study on Gas Recovery and Escape during Gas Hydrate Exploitation Enhanced by Continuous CO2/H2 Injection

The combined technology integrating CO₂/H₂ injection and CH₄ production with in situ steam reformation of CH₄ has provided a promising way to address green energy production and CO₂ sequestration. As both the injectant and product in the circular process, the behaviors of H₂ recovery and escape are crucial to the exploitation performance. In this work, gas production from water-rich unconfined hydrate deposits enhanced with a CO₂/H₂ injection was numerically simulated in a continuous injection-production mode. A series of field-scale simulations were conducted mainly to investigate the influence of injection/production pressure, the injected gas composition, and the permeability of overburden and underburden on gas recovery, gas escape, and CO₂ sequestration. The results indicated that for unconfined water-rich hydrate reservoirs, the injection pressure was crucial for CH₄ release, gas escape, and CO₂ sequestration, while the production pressure was a key factor in achieving favorable recovery of CH₄ and H₂. The H₂ recovery ratio varied little with the composition of the injected gas, while H₂ loss was positively correlated with its content in the injection gas. The net escaped amount of released CH₄ and injected H₂ decreased as the distance from the injection well increased, and the H₂ recovery ratios are 48%, 82%, and 86% in open, semiclosed (with only an impermeable overburden), and closed reservoirs, respectively. The semiclosed reservoir shows the best performance in CH₄ release and extraction, CO₂ sequestration, as well as the recovery and net production of H₂. Targeted reservoir reformation, aimed at artificially creating a low-permeability overburden, can effectively increase the net production of H₂.