Modelling of Convective Mixing in CO2 Storage

Abstract Accurate modelling of the fate of injected CO2 is necessary if geological storage is to be used at a large scale. In one form of geological storage, CO2 is injected into an aquifer that has a sealing caprock, forming a CO2 cap beneath the caprock. The diffusion of CO2 into underlying formation waters increases the density of water near the top of the aquifer, bringing the system to a hydro-dynamically unstable state. Instabilities can arise from the combination of an unstable density profile and inherent perturbations within the system, e.g., formation heterogeneity. If created, this instability causes convective mixing and greatly accelerates the dissolution of CO2 into the aquifer. Accurate estimation of the rate of dissolution is important for risk assessments because the timescale for dissolution is the timescale over which the CO2 has a chance to leak through the caprock or any imperfectly sealed wells. A new 2D numerical model which has been developed to study the diffusive and convective mixing in geological storage of CO2 is described. Effects of different formation parameters are investigated in this paper. Results reveal that there are two different timescales involved. The first timescale is the time to onset the instability and the second one is the time to achieve ultimate dissolution. Depending on system Rayleigh number and the formation heterogeneity, convective mixing can greatly accelerate the dissolution of CO2 in an aquifer. Two field scale problems were studied. In the first, based on the Nisku aquifer, more than 60% of the ultimate dissolution was achieved after 800 years, while the computed timescale for dissolution in the same aquifer in the absence of convection was orders of magnitude larger. In the case of the Glauconitic sandstone aquifer, there was no convective instability. Results suggest that the presence and strength of convective instability should play an important role in choosing aquifers for CO2 storage. Introduction The use of technologies to capture and store CO2 is rapidly emerging as a potentially important tool for managing carbon emissions. Geological storage, defined as the process of injecting CO2 into geologic formations for the explicit purpose of avoiding atmospheric emission of CO2, is perhaps the most important nearterm option. Geological storage promises to reduce the cost of achieving deep reductions in CO2 emissions over the next few decades. While the technologies required to inject CO2 deep underground are well established in the upstream oil and gas sector, with such methods as CO2 -EOR(1,2) and Acid Gas disposal(3), methods for assessing and monitoring the long-term fate of CO2, and for assessing the risk of leakage, are in their infancy. Assessments of the risk of leakage of CO2 from a storage formation may need to analyze leakage mechanisms and their likelihood of occurrence during the full-time period over which mobile free-phase CO2 is expected to remain in the reservoir. Once dissolved, risk assessments may well ignore the leakage pathways resulting from the very slow movement of CO2 -saturated brines.

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