Among the three types of geological CO2 sequestration (mature oil and gas fields, unminable coal beds and deep saline formations), depleted gas condensate reservoirs are particularly interesting. Firstly, because of the high compressibility of gas, these reservoirs have larger storage capacity than oil reservoirs or aquifers. Secondly, the condensate that has dropped out from the gas phase during natural depletion will re-vaporize due to re-pressurization of the reservoir and by miscibility with the injected CO2. This condensate can be recovered from producing wells and leaves more pore volume for available for storage of CO2.
We analyzed the injection of a CO2-based stream into a depleted gas condensate reservoir and into a saline aquifer using a compositional reservoir simulation model.
The dynamics of the reservoir impose a minimum period of injection that is required in order for the storage scheme to benefit from 100% of the reservoir storage capacity. Hence, over and above a certain CO2 injection rate, it becomes meaningless to invest in bigger compressors to increase this rate to reduce the time of injection.
When the CO2 stream contains impurities such as N2 or methane, the storage capacity of the reservoir decreases proportionally to the impure stream's compressibility factor and its concentration of impurities. This finding suggests that an economic optimum between the costs of separation, compression and injection can be determined.
Finally, the mass of CO2 sequestrated per pore volume in the equivalent aquifer model is about 13 times lower that of the depleted gas condensate reservoir model. This confirms that, due to their low overall compressibility, aquifers offer a far lower ratio of CO2 stored per pore volume than depleted gas condensate reservoirs. However, aquifers tend to have a far larger extent, which often compensates somewhat for this lower ratio and so provides storage for significant volumes of CO2.
In 1992, international concern about climate change led to the United Nations Framework Convention on Climate Change (UNFCCC). The ultimate objective of that Convention is the " stabilization of greenhouse gas concentrations in the atmosphere at a level that prevents dangerous anthropogenic interference with the climate system". Depending on the scenario considered, cumulative emissions of hundreds or even thousands of gigatonnes of CO2 would need to be prevented during this century to stabilize the CO2 concentration at 450 to 750 ppmv1. CO2 Capture and Storage (CCS) could facilitate the achievement of stabilization goals. CCS would be an option for countries that have significant sources of CO2 suitable for capture, that have access to storage sites and experience with oil or gas operations, and that need to satisfy their development aspirations in a carbon-constrained environment. Among the three main types of geological storage (mature oil and gas fields, unminable coal beds and deep saline formations), the injection of CO2 into depleted gas condensate reservoirs is particularly interesting for two main reasons:
Depleted gas condensate and/or natural gas reservoirs have larger storage capacity than oil reservoirs or aquifers.