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Abstract
With the increasing concern about climate change, the public, industry, and government are showing increased interest towards reducing CO2 emissions. Geological storage of CO2 is perceived to be one of the most promising methods that could provide significant reduction in CO2 emissions over the short and medium term. However, one major concern regarding geological storage of CO2 is the possibility of leakage. Carbon dioxide under the pressure and temperature conditions encountered in most geological settings remains more buoyant than water. Processes that could lead to permanent trapping of CO2 include geochemical reactions with the formation of solid minerals. This trapping mechanism is attractive because it converts the CO2 into a solid compound. However, the time-scale of such reactions is perceived to be centuries to millennia. In contrast, the kinetics of CO2-hydrate formation — leading to trapping of CO2 in the solid form — is quite fast, providing the opportunity for long-term storage of CO2. In this paper, geological settings suitable for formation of CO2 hydrates are investigated. We study storage of CO2 in depleted gas pools of Northern Alberta.

Thermodynamic calculations suggest that CO2 hydrate is stable at temperatures that occur in a number of formations in Northern Alberta, in an area where significant CO2 emissions are associated with production of oil sands and bitumen. Simulation results presented in this paper suggest that, upon CO2 injection into such depleted gas reservoirs, pressure would initially rise until conditions are appropriate for hydrate formation, enabling storage of large volumes of CO2 in solid form. Numerical simulation results suggest that, because of tight packing of CO2 molecules in the solid (hydrate), the CO2 storage capacity of these pools is many times greater than their original gas-in-place. This provides a local option for storage of a portion of the CO2 emissions there.

In this paper, we study the storage capacity of such depleted gas pools, and examine the effect of various reservoir properties and operating conditions thereon. In particular, we study the effect of the in-situ gas in formation of mixed gas hydrate; the effect of rise in temperature as a result of the exothermic reaction of hydrate formation; the effect of initial reservoir pressure, temperature, and porosity; as well as conditions for avoiding the deleterious formation of hydrates around the wellbore.

Introduction
Approximately 725 megatons (Mt) of carbon dioxide was emitted to the atmosphere in Canada in 2000, with more than 80% by combustion of coal, petroleum, and natural gas.

Carbon dioxide is a greenhouse gas, so its capture and storage to avoid accumulation in the atmosphere are important components of climate change mitigation. Since CO2 released from biological, igneous, or chemical activities has been stored in the upper crust of the Earth for millions of years, options for its storage may involve geological settings including sedimentary basins or saline aquifers. However, safety issues are vital in choosing a geological formation to store CO2 because its sudden release poses danger not only to health but also to human life. In addition, build up in CO2 underground concentration could be harmful to the ecosystem and could contaminate groundwater. Therefore, a low probability of CO2 leakage and an effective/efficient trapping mechanism are of great significance in choosing a storage facility.