Sustainable gas production from methane hydrate reservoirs by the cyclic depressurization method

The cyclic depressurization method, which uses alternating depressurization and shut-in periods over decades, has been proposed to achieve sustainable gas production from methane hydrate reservoirs. Numerical simulations were conducted to investigate the dissociation and reformation behaviors of methane hydrate during depressurization and shut-in periods. A high gas production rate was obtained for a few years after primary depressurization; however, the production rate drastically decreased because the sensible heat of the reservoir was exhausted owing to hydrate dissociation. During the shut-in period after 10 years of production, methane hydrate continued to dissociate owing to the geothermal heat flow for a few decades and then started to reform in accordance with pressure recovery. Case studies with shut-in periods of 10-30 years showed that 20 years of shut-in was the most effective period before the next depressurization. A conceptual operation plan for a hypothetical field showed that the production time increased to 120 years from 70 years when the cyclic depressurization method was considered. The recovery factor increased from 42.4% to 71.5%. The number of operating wells was reduced to less than one-third compared with the operation with normal depressurization method only. The results suggest that the cyclic depressurization method is a sustainable heat supply method driven by the geothermal heat flow and is both economically and environmentally sound.