The effect of CO2 solubility on the long term fate of CO2 stored in a water saturated subsurface storage location

CO2 storage in aquifers is an attractive solution to reduce CO2 emissions into the atmosphere. The successful example of CO 2 storage in the Sleipner injection project proves that large volumes of CO2 can be stored in a subsurface water-bearing formation. In general, we know that the most of the injected CO2 will displace water and will accumulate at the top of the aquifer in a free gas phase. Following the Sleipner success, European research studies and pilots such as the K12-B injection site in the Netherlands focus on CO2 storage in trap structures, by preference in depleted fields where the seal integrity is well proven. However depleted (gas) fields of Europe lack storage capacity in the order of magnitude required for a significant reduction of emitted CO 2. Typical power stations produce of the order of 10 million tons/year of CO2; over a life-time of 40 years this means that space for about 400 million tons is required, far more than can be accommodated in even the largest depleted oil or gas fields in Europe. On the other hand, we also know that CO2 is soluble in water, therefore, the aquifer or depleted reservoir will eventually be saturated with CO2. In this paper we investigate various aspects of the solution process, through numerical simulation studies for the Sleipner field and other field structures. The case studies indicate that solubility only in the long term (> 1000 yrs) provide a mechanism for solubility of most of stored CO2 in depleted fields. This is a consequence of the strict containment of the gas phase CO2 with limited exposure to the water phase. However if injection strategies are focused towards exposing the injected CO2 to as much "fresh water" as possible (i.e. through extended and dispersed migration paths) the level of CO2 free gas phase can reduce significantly and on a shorter timescale. Consequently aquifer and depleted field storage, targeted towards extended migration pathways can provide a mechanism for more sustainable and saver storage than direct injection in depleted fields. On the other hand the actual dispersion process through extended migration pathways is subject to large geological and physical uncertainties, which should be carefully studied.