Gas hydrates are a significant resource of natural gas existing both on-shore buried under the permafrost and off-shore buried under oceanic and deep lake sediments. Recent investigations consider the possibility of sequestering carbon dioxide (CO2), a greenhouse gas (GHG), in gas hydrate reservoirs and at the same time recovering the methane (CH4) from the hydrates. Numerical studies can provide an integrated understanding of the process mechanisms in predicting the potential and economic viability of CH4 gas production and CO2 gas sequestration in a geological reservoir. This study numerically investigates possible sequestration of CO2 as a stable gas hydrate in reservoir geological formations. A unified gas hydrate model coupled with a thermal reservoir simulator (CMG STARS) was applied to simulate CO2 hydrate formation in three reservoir geological formations. These reservoirs can be described as follows. The first reservoir (reservoir I) is similar to tight gas reservoir with mean porosity 0.25 and mean absolute permeability 10 mD. The second reservoir (reservoir II) is similar to a conventional sandstone reservoir with mean porosity 0.25 and mean permeability 20 mD. The third reservoir (reservoir III) is similar to hydrate-free Mallik silt with mean porosity 0.30 and mean permeability 100 mD. The fourth reservoir (reservoir IV) is similar to hydrate-free Mallik sand with mean porosity 0.35 and mean permeability 1000 mD. The Mallik gas hydrate bearing formation can be described as several layers of variable thickness with permeability varied from 1 mD to 1000 mD. This paper describes numerical methodology, model input data selection, and reservoir simulation results, including an enhancement to model the effects of ice formation and decay. The numerical investigation shows that the gas hydrate model effectively captures the spatial and temporal dynamics of CO2 hydrate formation in geological reservoirs by injection of CO2 gas. Practical limitations to CO2 hydrate formation by gas injection are identified and potential improvements to the process are suggested.
Background – Gas hydrates are ice-like solids composed of gas molecules and water. Gas hydrates form when relatively small guest molecules (such as carbon dioxide (CO2) and methane (CH4)) come into contact with water under low temperature, high-pressure conditions, both above and below the freezing point of water. Depending on the types of gas present, several crystal structures of gas hydrate are known to occur (such as Structure I, Structure II and Structure H), each with different physical and stability properties 1,2. In natural environments, the pressure-temperature conditions favouring gas hydrate formation could occur offshore in shallow depths below the ocean floor and onshore beneath the permafrost. Areas offshore of Canada's west coast and a number of onshore Arctic locations are know to contain some of the most concentrated CH4 hydrate deposits in the world. One large CH4 hydrate reserve is located in the Mallik field, Mackenzie Delta on the coast of the Beaufort Sea, in Canada's Northwest Territories. In addition, geological reservoirs with favorable pressure – temperature conditions for CO2 hydrate formation exist offshore of Canada's east and west coasts widespread in num