Prevention of seabed subsidence of class-1 gas hydrate deposits via CO₂-EGR: A numerical study with coupled geomechanics-hydrate reaction-multiphase fluid flow model

The geomechanics effects and seabed subsidence are critical issues that should be considered in the development of a hydrate reservoir. The purpose of this study is to couple the geomechanics, hydrate reaction, and multiphase fluid flow modules to investigate the feasibility of CO₂ enhanced gas recovery (CO₂-EGR) of a Class-1 hydrate deposit by observing the formation deformation, and the seabed subsidence. The production methods of depressurization and CO₂-EGR are modeled, respectively. The production behaviors and seabed subsidence of different production methods are compared. The positive influence on the gas recovery for a Class-1 hydrate deposit via CO₂-EGR is observed. The calculations of seabed subsidence showed a significant improvement can be achieved when CO₂-EGR was used. The subsidence is only 6.8% of that from the pure depressurization in the case of a pressure drop of 30%. The effects of production pressure drop and production gas rate are investigated. The association between the gas production and the pressure drop of the well is different from the cases of pure depressurization and the CO₂-EGR. The appropriate initial time for the CO₂ injection is tested. Slighter seabed subsidence is observed when the CO₂ injection is initiated earlier. The case of different injection pressure control showed that a lower injection pressure leads to a heavier seabed subsidence. A higher CO₂ fraction allowed in the produced gas stream results in a higher cumulative gas production, but there is no significant impact on the seabed subsidence.