Pore-scale controls on mineral dissolution and porous media evolution
The use of the subsurface in applications related to energy storage and security often involves the injection of fluids that are far from equilibrium with respect to the constituents of the native formation. Geologic sequestration of CO2, an approach for mitigating greenhouse gas emissions, is one of these subsurface applications. Heterogeneous reactions driven by fluid-rock disequilibrium, such as mineral dissolution and precipitation, may change the properties of the porous medium, including porosity, permeability, and reactivity. In these applications, one is typically concerned with the evolution of these properties at large scales. However, the changes take place at the scale of individual pores and grains, where small, localized effects may translate into significant impacts at larger scales via nonlinear emergent processes.
In this colloquium, I will present recent developments in reactive transport modeling that have made it possible to simulate processes at the pore scale. At this scale, individual grains and fluid interfaces are resolved and thus must be considered explicitly in the models, which in their most general form entail the solution of the coupled processes of fluid flow, multicomponent reactive transport, and the evolution of geometry. In our approach, an embedded boundary finite volume discretization is used to capture the complex geometry of natural porous media within a high-performance computational framework.
Pore scale modeling has proven particularly useful in quantifying the effect of transport limitations to reactive surfaces. Using the case of calcite dissolution as an example, I show that the nonuniformity in the flow field at the pore scale has the effect of decreasing the overall reactivity of the porous medium. Transport-limited conditions at the grain-pack scale may result in unstable evolution, a situation in which dissolution is focused in a fast-flowing, fast-dissolving path. Further, simulation of dissolution in mineralogically heterogeneous media shows that non-uniform dissolution owing to heterogeneous reactivity leads to increasing transport limitations to reactive surface, affecting overall rates.
The pore-scale approach has the potential to provide mechanistic explanations for some long-standing questions in reactive geochemistry in porous media, including the so-called discrepancy between laboratory and field rates. It also offers the chance to understand reaction-induced porosity, permeability, and reactivity change.
Sergi Molins is a Research Scientist at the Lawrence Berkeley National Laboratory with extensive experience in the field of reactive transport modeling in porous media. His research focuses on elucidating processes affecting the formation of effective reaction rates at different spatial scales in subsurface applications relevant to energy and the environment. Code development is an integral part of his work having contributed among others to MIN3P and CrunchFlow. He holds a PhD from the University of British Columbia and a Civil Engineering degree from the Technical University of Catalonia. He currently serves as Associate Editor for Water Resources Research.