Nanopores, or pores some nanometers in diameter, are a majority of pores in rock types such as shale. Similar to the way a nanoparticle has a differing reactivity from its micro- and macro-scopic counterparts, a nanopore might also have a different reactivity. The major questions for this field of research are thus “how the reactivity in nanopores differs from micropores?” and “over what pore sizes is this significant?” Net reactivity in nanopores can be separated into three overlapping effects: transport, confinement, and mineral-water interfaces. To illustrate the need for fundamental research on this subject, two case studies are shown. First, during replacement reactions in a nanoporous limestone by magnesium carbonate, we find that the rates of reactions may be dominated by nanopores and grain boundary transport. This apparent nanoporous reactive transport leads to apparent rates of reactions faster than solid state diffusion, but slower than in open solution. I then show a case where nanoporosity doesn’t play a role in determining the outcome, during fracture sealing of the Eagle Ford shale after exposure to scCO2 and water for a month. To understand how these widely varying levels of reactivity of nanopores and their participation in the geochemical reactions, I use precipitation reactions in nanoporous amorphous silicas as idealized systems. I first show how a calcium carbonate phase fails to precipitate in a nanoporous silica, except when functionalized with a self-assembled monolayer. Changing the precipitate the precipitate to barite (BaSO4) not only causes precipitation to proceed in native nanoporous silica, but nucleation rates are orders of magnitude higher than on planar substrates. To explain these differences, a surface complexation model was used to quantify the extent of ion exclusion in the nanopores. The model explains some of the observations well, but fails to explain the precipitation trends in all the different nanoporous and planar systems. I argue that this failure is a symptom of a deficiency of our understanding of size of mineral-water interfaces. To improve on our understanding, we need to move beyond classical concepts of surface charge based on mean field approximations, to a more localized charge approach that includes atomic-scale interface structure and reactivity.

Precipitation in nanopores: Sources and magnitudes of anomalous reactivity

ACS Spring 2022

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