Molecular Modeling and Adsorption Characterization of Micro-Mesoporous Kerogen Nanostructures

S Parashar and PI Ravikovitch and AV Neimark, ENERGY & FUELS (2022).

DOI: 10.1021/acs.energyfuels.2c02876

The aim of this work is to enhance the understanding of the pore structure and adsorption properties of kerogens as applied to organic- rich shales and mudstone rocks. Conventional methods of adsorption characterization from low temperature N2 isotherms rely on the use of the so-called standard isotherms on nonporous substrates (typically silica or amorphous carbons), which may not be accurate for the surfaces of kerogens. In this work, we present a new methodology for pore size characterization of kerogens that relies on a realistic molecular model of kerogen surfaces. Taking advantage of recent advances in modeling the molecular structure of kerogens, we create atomistic three-dimensional (3D) models of amorphous bulk kerogens, rough kerogen surfaces, and mesopores imbedded in the amorphous kerogen matrix. Using grand canonical Monte Carlo (GCMC) simulations, we calculate the reference N2 adsorption isotherms in the micropores of the bulk kerogen matrix, on the kerogen surface, as well as in a series of mesopores confined by rough kerogen walls. Next, we parameterized the quenched solid density functional theory (QSDFT) to reproduce the kerogen surface heterogeneity and GCMC-simulated N2 adsorption isotherms. Furthermore, we approximated the isotherm on the reference kerogen surface by a macroscopic disjoining pressure isotherm, which allows us to use the Derjaguin- Broekhoff-de Boer (DBdB) model to predict adsorption and capillary condensation in meso/macropores. The reference GCMC, QSDFT, and DBdB isotherms are combined into the kernel for calculating the micropore volume, meso-and macropore surfaces, and mesopore size distribution from the experimental adsorption isotherms. The proposed methodology is demonstrated on a typical example of a kerogen II-A sample with a wide mesopore size distribution. The methodology can be extended to other kerogen structures of different maturities to provide a comprehensive characterization of organic porosity in kerogen fractions.

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