Screening criteria for CO2 storage in oil reservoirs

Oil fields are likely to the first category of geologic formation where carbon dioxide (CO2) is injected for sequestration on a large scale, if geologic sequestration proves feasible. About 1.4 BCF per day (69 300 tonnes/day) of CO2 are currently injected for oil recovery in the U.S. Replacing this naturally occurring CO2 with anthropogenic CO2 would have a minor, but measurable, effect on overall CO2 emissions. However, CO2 is injected into only a small fraction of reservoirs and it is estimated that upwards of 80% of oil reservoirs worldwide might be suitable for CO2 injection based upon oil recovery criteria alone. These. facts combined with the generally extensive geologic characterization of oil reservoirs and the maturity of CO2-Oil recovery technology make oil reservoirs attractive first targets as CO2 sinks. This paper lays the groundwork necessary to evaluate whether an oil reservoir might. be suitable for CO2 storage. As such, a series of criteria for injection into currently producing, depleted, or inactive reservoirs are proposed. Aspects considered include the reservoir depth, storage capacity, water and oil volumes in place, formation thickness, and permeability. Importantly, the effect of oil production on reservoir properties, especially fault movement and induced fractures must be gauged and included in assessments. It is demonstrated that CO2 density with depth alone is not a sufficient criterion for choosing candidate sites. It is necessary to consider also porosity and the amount of water and oil that are displaceable. The end result is a criteria table for rapid screening of candidate reservoirs.