Evaluation of CO2 Substitution for CH4 as a Mechanism for Concurrent Gas Production and CO2 Sequestration in Hydrate-Bearing Geologic Media

Natural hydrates in geologic media contain CH4 in overwhelming abundance. In addition to the conventional methods of CH4-hydrate dissociation (depressurization, thermal stimulation, the use of inhibitors, and combinations thereof), CO2 substitution for CH4 has been proposed as a potential method that could simultaneously achieve two goals: the release of CH4 from the CH4-hydrates for gas production, and the replacement of CH4 by CO2 in the clathrates, thus forming either binary CH4+CO2-hydrates or pure CO2-hydrates and enabling CO2 sequestration. The reaction of CO2 substitution for CH4 in the clathrates is thermodynamically favorable, resulting in binary or CO2-hydrates that are more thermodynamically stable than CH4-hydrates. In this numerical study we first evaluate the performance of a new equation-of-state (EOS) module developed as a code unit for the TOUGH+ general simulator. The EOS describes the thermodynamic and flow behavior of binary CH4+CO2-hydrates in geologic media, and covers the entire composition spectrum in both the gas and the hydrate phase. The EOS includes fast parametric relationships that describe the 3-dimensional P-T-X phase diagram of the CH4+CO2+H2O system, which (a) were developed from 3D regression of thermodynamic data obtained from the CSMGem code (a statistical thermodynamics simulator that is based on the minimization of Gibbs energy of hydrate systems) and (b) were validated using laboratory measurements. The TOUGH+ simulator with the CH4+CO2-hydrate EOS is then validated using results from laboratory studies that involve replacement of CH4 by CO2 in hydrate-bearing cores. Finally, we investigate the technical feasibility of such replacement at the reservoir scale, and the conditions under which it may be successful, in realistic settings involving systems of vertical and horizontal wells.