Coupled Processes in Smectites
Carl Steefel
Overview: The goal of this effort is to understand the origins of swelling and transport in ideal, pure smectite systems at the molecular scale and pore and particle scales, and to encapsulate the new knowledge into a continuum scale model that captures the coupling between hydration/dehydration behavior, ion exchange, and swelling while rigorously considering the molecular processes at the clay-water interfaces, particularly in confined smectite interlayers. Our objective is to be able to predict at the continuum scale the transient changes in clay mineral pore structure, including compaction and grain reorganization, water and solute transport, and swelling pressure, that accompany hydration or ion exchange processes. We will then extend our models to clay-rich sediments and rocks (especially shales) so as to evaluate how these coupled processes at work at the nanopore scale impact rates of diffusion-controlled reactions, osmotic flow, and pore pressure evolution (including both overpressuring and underpressuring) in sedimentary basins.
Background and Open Questions
Coupled Processes in Nanoporous Clay Media
Clay-rich geologic systems are characterized by nanoscale pores, heterogeneous charge distribution, confined water and often exhibit dynamic swelling behavior in response to changes in fluid chemistry. As a consequence, clay-rich materials show a remarkable array of macroscale properties such as very low permeability, partial or complete ion exclusion, and a strong coupling between geochemical, mechanical, and osmotic properties. These properties arise from the interactions of charged clay nanopore surfaces with water and solutes, which leads to highly nonlinear coupling between flux terms. It is a grand challenge to fully elucidate the forms and implications of such coupled processes (Massat et al. 2016).
Based on our recent advances in thermodynamic descriptions of swelling clays we are poised to develop a models for transient coupled processes, studying how ion exchange causes dynamic evolution of swelling pressures that feedback to structure, transport pathways and microscopic chemical-mechanical equilibrium. The implications for geologic transport of ions, including anomalous diffusion behavior, will be established and modeled using an updated version of the powerful reactive transport model, CrunchClay (Tournassat & Steefel 2019). Enhancements planned for CrunchClay are described in the Crosscutting Section on CrunchClay.
We anticipate the completion of predictive model for swelling pressure and solute transport in fully saturated smectites as a function of aqueous solution chemistry that will have great benefit for engineering and geologic modeling. This model will aid in the design of clay-rich engineered barriers used in the geologic storage of radioactive waste.
Impacts of Smectites in Sedimentary Formations
Many of the coupled processes that affect pure clay systems impact the behavior of clay-rich sedimentary systems, including shale. Fractured shale, as in hydrofracking, involves interactions between fracture water (typically bulk water) and the pore water within the clay-rich sediment or sedimentary rock, and this interaction typically involves either molecular diffusion or anomalous flow because of the low permeabilities involved (Neuzil 1994; Neuzil 2015). Building on our mesoscale models for transport in clays and clay rocks, we will investigate the rates of mineral precipitation reactions that occur as a result of interdiffusion between the fracture and clay material (Chagneau et al. 2015). We will also investigate the development of overpressures in sedimentary basins, which has been attributed in part to clay swelling and/or chemical osmosis as the clay-rich zones behave as semi-permeable membranes (e.g., Goncalvès et al. 2010; Tournassat & Steefel 2019).
Goals
To understand ion exchange, hydration/dehydration, and swelling and transport in ideal, pure smectite systems, and to transfer this understanding to the problem of reactive transport and pore pressure evolution in clay-rich sediments and sedimentary rocks.
Proposed Work
References
Chagneau, A., Claret, F., Enzmann, F., Kersten, M., Heck, S., Made, B., & Schäfer, T. (2015). Mineral precipitation-induced porosity reduction and its effect on transport parameters in diffusion-controlled porous media. Geochemical Transactions, 16, 13.
Gonçalvès, J., & Trémosa, J. (2010). Estimating thermo-osmotic coefficients in clay-rocks: I. Theoretical insights. Journal of Colloid and Interface Science, 342(1), 166-174.
Massat, L., Cuisinier, O., Bihannic, I., Claret, F., Pelletier, M., Masrouri, F., & Gaboreau, S. (2016). Swelling pressure development and inter-aggregate porosity evolution upon hydration of a compacted swelling clay. Applied Clay Science, 124-125, 197-210.
Neuzil, C. E. (1994). How permeable are clays and shale? Water Resources Research, 30(2), 145-150.
Neuzil, C. E. (2015). Interpreting fluid pressure anomalies in shallow intraplate argillaceous formations. Geophysical Research Letters, 42(12), 4801-4808.
Tournassat, C., & Steefel, C. I. (2019). Reactive Transport Modeling of Coupled Processes in Nanoporous Media. Reviews in Mineralogy and Geochemistry, 85(1), 75-109.