Optimized dual-gas co-production from hydrate reservoirs with underlying gas through depressurization & well closure cycling: An experimental evaluation

The regular depressurization method for the exploitation of natural gas hydrate, while effective in gas production, carries a high risk of hydrate regeneration and significant reservoir shrinkage. In this study, the hydrate reservoirs containing underlying gas were synthesized by controlling the initial water distribution, and confining pressure was applied to replicate sub-seafloor conditions. An innovative cyclic depressurization (DP) + well closure (WC) cycling scheme was implemented to facilitate the joint production of hydrate and underlying gas. The heat transfer, gas production, and sediment deformation characteristics during dual-gas co-production were compared and examined considering different DP and WC duration. The findings clearly indicate that the DP + WC cycling method yielded significantly superior results compared to the regular DP approach. Specifically, it achieved effective CH4 recovery rates 1.83–2.33 times higher, temperature recovery rates 1.2–1.3 times greater, and a reduced the duration of reservoir contraction. More importantly, the extent of influence on these factors could be controlled by adjusting DP or WC duration. A discovery like this offers diverse gas production approaches tailored to various hydrate reservoir requirements. Overall, this study provides preliminary evidence that the DP + WC cycling method allows for the simultaneous attainment of efficient production and reservoir security, highlighting its potential significance for gas production from hydrate reservoirs containing underlying gas.