Analysis of the storage capacity for CO2 sequestration of a depleted gas condensate reservoir and a saline aquifer

Among the three types of geological CO2 sequestration (mature oil and gas fields, unminable coalbeds and deep saline formations), depleted gas condensate reservoirs are particularly interesting. First, because of the high-compressibility of gas, these reservoirs have larger storage capacity than oil reservoirs or aquifers. Second, the condensate that has dropped out from the gas phase during natural depletion will re-vaporize because of 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 storage of CO2. The objective of this study is to investigate the CO2 storage capacity in different formation types, for different levels of CO2 purity and different injection schedules. To this aim, 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 approximately 13 times lower that of the depleted gas condensate reservoir model. This confirms that, because of 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 therefore provides storage for significant volumes of CO2