Towards a Fundamental Understanding of the Evolution of Porosity and Permeability During Mineral Nucleation and Growth

A fundamental understanding of how porosity and permeability evolve in response to geochemical reactions is necessary to accurately predict how the subsurface responds to anthropogenic perturbation. Historically, porosity and permeability occlusion due to mineral precipitation in particular have been treated using empirical expressions based on grain parameters such as Kozeny-Carmen, or a "bundle of tubes" approach that assumes that minerals nucleate and grow homogeneously over all rock surfaces. Recent results have called into question these assumptions by showing that there can be a distinct pore-size preference for precipitation reactions, but there is little agreement over whether a preference for precipitation in small or large pores. Pores with diameters in the nanometers (nanopores) in particular have the potential to display special reactivity and can constitute the majority of pores in fine-grained rocks such as shale. In this talk, I will show our recent results that illustrate the special reactivity of nanopores and discuss the path forward for using what we have learned to start to think about porosity and permeability relationships. In particular, I will show results on using synchrotron-based X-ray scattering (PDF analysis) to examine precipitation of calcium carbonate in nanoporous amorphous silica, as well as X-ray tomography results of barium sulfate precipitation in a glass bead column that show how the pore size in which precipitation occurs depends in part on the interaction between precipitate and substrate. To ground this experimental work in a relevant real-world system, I will lastly show recent results of how the porosity distribution of a nanoporous wellbore cement exposed to CO2 over the course of 30 years evolves during mineralization reactions.