Mineralogical and transport controls on the evolution of porous media texture using direct numerical simulation

Abstract

The evolution of porous media due to mineral dissolution and precipitation can change the bulk properties of subsurface materials. The pore‐scale structure of the media, including its physical and mineralogical heterogeneity, exerts controls on porous media evolution via transport limitations to reactive surfaces and mineral accessibility. Here we explore how these controls affect the evolution of the texture in porous media at the pore scale. For this purpose, a pore‐scale flow and reactive transport model is developed that explicitly tracks mineral surfaces as they evolve using a direct numerical simulation approach. Simulations of dissolution in single‐mineral domains provide insights into the transport controls at the pore scale, while the simulation of a fracture surface composed of bands of faster‐dissolving calcite and slower‐dissolving dolomite provides insights into the mineralogical controls on evolution. 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. Due to increasing velocities, the evolution in these regions is like that observed under conditions closer to strict surface control at the pore scale. That is, grains evolve to have oblong shapes with their long dimensions aligning with the local flow directions. Another example of an evolving reactive transport regime that affects local rates is seen in the evolution of the fracture surface. As calcite dissolves, the diffusive length between the fracture flow path and the receding calcite surfaces increases. Thus, the calcite dissolution reaction becomes increasingly limited by diffusion.

Multi-scale Model of Reactive Transport in Fractured Media: Diffusion Limitations on Rates

“…The simulations in this manuscript did not consider evolution of the media as a result of the reactions, and therefore, the pore-scale and Darcy-scale domains did not evolve over time. An embedded-boundary method for time-evolving domains developed by Miller and Trebotich (2012) and implemented in Molins et al (2017) for pore-scale geometry evolution could however be applied in the multi-scale model. In this case, the model could capture both the evolution of the porous matrix and the displacement of the pore-scale/Darcy-scale interface as a result of dissolution and other relevant processes such as altered layer erosion, e.g., Deng et al (2017b).…”

“…4b). Effluent concentrations are obtained by flux-averaging concentrations at the fracture outlet (Li et al 2008;Molins et al 2012Molins et al , 2014Molins et al , 2017.…”

“…In pore-scale models, the interfaces between the different fluid and solid phases that make up porous media are resolved and it is therefore possible to solve for flow and reactive transport within the pore space. Pore-scale models can capture the diffusion boundary layers that develop around reactive surfaces and contribute to the formation of effective reaction rates (Li et al 2008;Molins et al 2014Molins et al , 2017. They can also simulate the channelization of the flow path due to dissolution in transport-limited conditions (Szymczak and Ladd 2009).…”

Simulation of mineral dissolution at the pore scale with evolving fluid-solid interfaces: review of approaches and benchmark problem set

“…Chombo-Crunch is a code suite for the solution of flow, reactive transport and geometry evolution at the pore scale developed since 2010 by Trebotich and co-workers [64][65][66]. Flow, transport, and geometry evolution processes are implemented using the Chombo software package [3,25], while geochemical reactions are implemented in the CrunchFlow code [101] which is coupled to Chombo via a custom interface.…”

Reactive Transport in Evolving Porous Media

“…Imaging and pore scale modeling techniques have already provided outstanding results and insights. Molins et al (2017) reproduced dissolution processes in a multi-mineral fracture zone by incorporating the heterogeneity of the spatial distribution of minerals. In another study, it was possible to reproduce the wormholing processes by direct simulation at the pore scale (Molins 2015).…”