Reservoir Modeling of Production of CH4 from Natural Gas Hydrates by Injection of a CO2+N2 Gas Mixture

Garapati, Nagasree; McGuire, Patrick; Anderson, Brian J.

Natural gas hydrate reservoirs are likely to contain more carbon than in all other fossil fuel reserves combined. Different techniques currently being proposed for production of CH4 from hydrate deposits include depressurization, thermal stimulation and inhibitor injection. These processes involve the dissociation of hydrate and the release of significant volumes of water, which may cause geomechanical stress on the reservoir leading to subsidence. A series of experimental studies over the decade reviewed the feasibility of using CO2/CO2+N2 gas mixtures to recover CH4 gas from the hydrates, which serves dual purpose of CO2 sequestration and production of CH4 while maintaining the geomechanical stability of the reservoir.

The current state-of-the-art in reservoir simulators only accounts for pure CH4 hydrates or CH4+CO2 hydrates. In order to analyze a CH4 production process by injection of a CO2+N2 gas mixture in gas hydrate reservoirs, a new simulation tool was developed to account for multi-component hydrates. In this work, HydrateResSim, an open-source code available for public through the National Energy Technology Laboratory has been modified to handle ternary hydrates. The modified new software is called Mix3HydrateResSim (Mix3HRS), which can allow distribution of heat and up to 5 components between four possible phases. Mix3HRS is used in this work to predict the CH4 production from a methane hydrate reservoir at 7 MPa and 5.5°C by injecting N2+CO2 (77%+23%) at 9.65 MPa and 5.5°C for 14 days, followed by depressurization of the domain by maintaining the boundary at 5.5 MPa and 5.5°C over a 30-day production. During the injection phase, there is initially an increase in the saturation of hydrate indicating the formation of secondary hydrate from injected gas and available free water. There is also a slight increase in the temperature due to the exothermic nature of hydrate formation. As the hydrate becomes saturated with the injected gases it releases CH4, and there is hydrate dissociation and gas production. After the initial 14 days of injection, a mixture of the three gases was produced through depressurization. The amount of CH4 released from the hydrate phase during the injection and production phases and the amount of CO2 and N2 gases sequestered in reservoir have been examined in this study. A model-based history-matching of the gas flow rates from the ConocoPhillips field test and lab scale exchange experiments is done using Mix3HRS.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013