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Modeling and Understanding the Influence of Geological Complexity on CO2 Storage

Final Report Summary - MUIGECCOS (Modeling and Understanding the Influence of Geological Complexity on CO2 Storage)

MUIGECCOS has handled the fundamental processes determining the capacities of CO2 geological sequestration. Green house gas (GHG) emissions and other human actions are promoting Climate and Global Change and need to be minimized. Carbon capture and storage in geological formations has been proposed in the last ten years to reduce the emissions of CO2 to the atmosphere by concentrated sources like power plants. Among the key processes conditioning CO2 sequestration are the density driven flows, the solute transport, dissolution and reaction with the host rock. All these phenomena are coupled and strongly depend on the geological structure and hydraulic underground heterogeneity. The main challenge to demonstrate long term storage of CO2 is to understand how simple to complex phenomena develop in complex geological settings.

While most modelling efforts had been undertaken in homogeneous porous media, we have studied more realistic heterogeneous porous and fractured media and determine how hydraulic heterogeneity and connectivity can impact dispersion, mixing and reactivity processes. We show that heterogeneity not only modifies the effective parameters but more likely cause new processes to emerge. Heterogeneity is implicitly the heterogeneity of permeability as permeability is both the most variable and the most important hydraulic parameter of underground hydraulic processes. Objectives were to determine the size of the injected plume, as it determines the spatial extent over which the seal is needed, and the impact of heterogeneity on reactivity.

We have first focused on the transport processes on complex flows, where complexity of flows is fundamentally determined by transient variations of the boundary conditions and hydraulic heterogeneity. We have developed intensive numerical simulations means to determine both the mean behavior and the uncertainty around this global trend. Development has specifically been directed to handle a broad range of flows for solute conservative transport. Chemical reactivity has been derived in a postprocessing step.

The main results have been obtained on the Monte-Carlo derived upscaling laws. Simulations have been taken as “numerical experiments”. Most of our effort have been directed at interpreting the simulation results. From these simulations, we identified the upscaling rules expressed either as equivalent parameters or as “effective” equations when appropriate. We have shown first that solute spreading is not limited to spreading longitudinally to the flow direction but also extends transversally. We have shown secondly that spreading is not enough to predict mixing and reactivity. We detail these two main points in the two following paragraphs.

Dispersion of solutes transverse to the main flow gradient can come several sources. It can come from 3D structures while it is highly limited in 2D structures. In 3D, flow lines can intertwine without intersecting themselves and produce important transverse spreading of solute plumes. It can also come from fluctuations of the boundary conditions. While they do not yield any transverse dispersion in homogeneous media, they enhance it critically in heterogeneous media to a much larger extent than heterogeneity itself. This is an example of the coupled effect of heterogeneity and flow conditions typical of the emergence of new phenomena that would be absent in the sole presence of flow fluctuations or heterogeneity.

Dispersion is an essential driver of mixing and chemical reactivity. It enhances mixing and reactivity but it does not translate all to mixing at once. Dispersion simultaneously enhances the mixing front, develops chemical gradients, but concentrates concentrations in restricted filaments. Effective mixing remains smaller than the dispersion that represents an upper bound potential mixing reached only at very long times when the asymptotic regime for dispersion has been reached. We have quantified the deviation between the potential and effective mixings and shown that this deviation may be a useful measure for characterizing chemical reactivity. In fact, it follows a generic trend with a steep increase of the deviation at short time when advective structures develop and a slow decrease when diffusion progressively homogenizes these structures.

We propose simple measures that can be effective ways to characterized mixing and reactivity in heterogeneous porous structures, and to develop model that do not only comply with dispersion but also with mixing and chemical reactivity. We are following up on these ideas in a close collaboration between the French and Spanish teams. We aim at finding “effective” equations to represent the macro-scale effect of smaller scale processes like fingering and heterogeneity. These effective equations are at the basis of risk-assessment studies.