Integrated CH4 joint production and CO2 sequestration: insights from various depressurization schemes and post-production partially depleted hydrate and shallow gas reservoirs

Joint methane hydrate and shallow gas extraction can improve the economic viability of marine hydrate production. However, the gas production rate typically declines in the later stages, making it uneconomical to fully recover the remaining hydrates. Interestingly, the reservoir conditions during these later stages may favor CO 2 sequestration. This study proposes an integrated approach for the co-production of marine hydrates and shallow gas coupled with CO 2 storage. Various depressurization schemes were employed to extract hydrates and shallow gas, while monitoring temperature and pressure responses, as well as variations in gas and water production rates. Following joint production, CO 2 was injected into the in-situ depleted reservoir for storage. Based on experimental research, further numerical simulations were conducted to reveal the characteristics of joint production and the mechanism of depleted reservoir environment in CO 2 sequestration. The results indicate that residual hydrate nuclei, combined with CO 2 displacement, drive the hydrate layer into a rapid CO 2 storage stage. In the hydrate layer, the CO 2 sequestration rate is jointly controlled by residual methane hydrates, remaining hydrate crystals, and CO 2 partial pressure. In contrast, the rapid sequestration stage in the shallow gas layer occurs later and is predominantly governed by CO 2 partial pressure. Stepwise and deep depressurization enhances CO 2 partial pressure and sequestration efficiency by reducing residual methane. Combining stepwise and deep depressurization during dual-gas production represents an optimal strategy for improving both gas recovery and CO 2 storage efficiency. This study provides new insights into enhanced exploitation of marine hydrates and shallow gas, as well as effective CO 2 sequestration in depleted reservoirs.