Molecular dynamics of aqueous salt solutions in clay nanopores under the thermodynamic conditions of hydraulic fracturing: Interplay between solution structure and molecular diffusion

B Plankova and M Lisal, FLUID PHASE EQUILIBRIA, 505, 112355 (2020).

DOI: 10.1016/j.fluid.2019.112355

Shale gas has become an important unconventional energy resource and is extracted by hydraulic fracturing of shale rocks. In shale rocks, kerogen and clays are present, the former providing storage for hydrocarbons, the latter preventing hydrocarbon transport. Clays are hydrophilic and have a layered structure. They can adsorb aqueous salt solutions in the interlayer space, and the interlayer ions contribute to a very high salinity of the flow-back water. We used montmorillonite (MMT) as a proxy of clays and study the interplay between the interlayer structure and the molecular diffusion of the aqueous salt solutions confined in the clay nanopores. We considered water with monovalent Na and divalent Ca ions in the MMT slit pores under a typical shale gas reservoir condition of a temperature of 365 K and a pressure of 275 bar. The confined systems were electrostatically balanced by Cl ions. We varied the amount of water to cover one-, two-, three-, and four-layer hydrate states. We quantified the solution structure in terms of the interlayer atomic density profiles, complemented by the charge density and water orientation profiles. We further evaluated the in-plane self- diffusivity of water and ions to provide insight into the diffusion behaviour of the concentrated water-NaCl and water-CaCl2 solutions in the interlayer galleries of the Na- and Ca-MMT pores. We found that the interlayer water structure displays an attraction of water hydrogens to the clay surfaces as a result of the strong H-bond interactions of water molecules with the surface oxygens and formation of a diffusive layer inside wide clay pores. The presence of divalent Ca ions has more pronounced effects on the interlayer water structures than the monovalent Na ions. Divalent Ca ions exhibit a preference for inner- sphere complexes over outer-sphere complexes due to the strong adsorption on the clay surfaces while Na ions show the opposite trend. The diffusion of water and ions in the clay pores is reduced in comparison with the diffusion in the bulk, especially for the one-layer hydrate states. The presence of the salts in the clay pores further decreases the water diffusion, with a stronger decrease for the divalent CaCl2 than the monovalent NaCl. The diffusion of small Na+ and Ca2+ is lower than large Cl- due to the low mobility of the adsorbed cations and the distribution of Cl ions inside the pores where they can diffuse faster. (C) 2019 Elsevier B.V. All rights reserved.

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