The Significance of Dipping Bed Effects on CO2 Plume Migration

Recent investigations regarding CO2 sequestration in deep, saline aquifers have focused on characterization of the injected plume, its migration within the aquifer over time, and possible leakage out of the aquifer. As part of our efforts to understand and quantify leakage potential in CO2 storage systems, a semi-analytical solution has been developed that describes the plume shape evolution as well the amount of leakage, with a focus on leakage along abandoned wells. The semi-analytical solutions require a number of simplifying assumptions, including a perfectly horizontal aquifer, negligible capillary pressure, and symmetry of the injection plume. Each of these assumptions can be tested systematically through application of more general numerical simulators. For example, in typical sedimentary basins, it is common to have sloping aquifers with a vertical rise of up to 3-4 km over the total horizontal length of the basin (hundreds of kilometers). Although the slope may only be 1% or less, the effects on the upward migration of the CO2 plume may be significant over the time scales appropriate for carbon sequestration. Similarly, the role of capillarity in these systems may be significant due to capillary diffusion or to capillary exclusion. In this study, we use a general two-phase numerical simulator to assess the limitations of the assumptions required to derive semi-analytical solutions to these systems. For example, we can simulate injection of CO2 into a confined saline aquifer for an extended period (we have used 30 years) and examine the effect of different degrees of slope on the centroid and maximum upslope extent of the plume. These measures of plume asymmetry can then be related to an appropriate dimensionless grouping that takes into account the fluid properties and aquifer characteristics. In this presentation we will present results from these simulations and discuss their implications regarding the extent to which CO2 injection systems can be simplified.