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.