This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 137313, ’CO2 Storage as Hydrates in Depleted Gas Reservoirs,’ by O.Y. Satsepina, University of Calgary, and M. Pooladi-Darvish, SPE, Fekete Associates, originally prepared for the 2010 Canadian Unconventional Resources and International Petroleum Conference, Calgary, 19-21 October. The paper has not been peer reviewed.
The full-length paper presents a study of the carbon dioxide (CO2) hydrate storage capacity of depleted gas pools. The effect of the in-situ gas in formation of mixed-gas hydrate; the effect of temperature increase as a result of the exothermic reaction of hydrate formation; the effect of initial reservoir pressure, temperature, and porosity; and conditions for avoiding the deleterious formation of hydrates around the well-bore are studied.
Approximately 725 Mt of CO2 was emitted to the atmosphere in Canada in 2000, with more than 80% from combustion of coal, petroleum, and natural gas. CO2 is a greenhouse gas, so its capture and storage to avoid accumulation in the atmosphere are important components of climate-change mitigation. Safety issues are vital in choosing a geological formation in which to store CO2 because its sudden release poses danger not only to the environment but also to human life. Therefore, a low probability of CO2 leakage and an effective/efficient trapping mechanism are of great significance in choosing a storage site.
Sequestration of CO2 in the form of hydrate is quite attractive. Hydrates are solids composed of a framework of water molecules with gas molecules captured within the framework. As a result, storing CO2 in the form of hydrate provides a solid structure and a high density. Gas hydrate forms at conditions of high pressure and low temperature, which easily could be found in the deep ocean or in its sediments. Furthermore, researchers are studying sequestration of CO2 through replacement of methane (CH4) by CO2 in the hydrate structure when CO2 is injected into a hydrate reservoir at the appropriate conditions.
When CO2 is injected into a reservoir where methane is present, mixed-gas hydrates form. The stability conditions of CH4/CO2 hydrates are different from those of either CH4 hydrate or CO2 hydrate. The work presented in the full-length paper used a reservoir simulator that is capable of handling compositional processes and reactions with deviation from equilibrium that are used in modeling mixed hydrate. The simulator has been used previously to predict dissociation/formation of CH4/CO2 hydrate.
In the Results section, hydrate formation in a single block is studied first. The model considered acts as a uniform cell and eliminates variations in pressure, temperature, and composition with space. An isothermal approximation is considered as a first step. This is followed by nonisothermal modeling of a single-block model, before proceeding to reservoir modeling.