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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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Monte Carlo simulation method for calculating pore size distributions from high-pressure CO2 adsorption in Micro-Mesoporous Carbons
An original methodology is suggested for evaluating the pore size distribution in carbons in the wide range of micro- and mesopores from 0.385 to 10 nm from a single isotherm of high-pressure adsorption of CO2 at 273 K. The proposed method is based on the reference theoretical isotherms calculated by Monte Carlo simulations in model pores of slit and cylindrical geometry. The relationship between the pore size and the pore filling pressure is established. Special attention is given to predicting of the capillary condensation transitions in mesopores by using the meso-canonical ensemble (gauge cell) Monte Carlo simulations. The proposed technique is demonstrated and verified against the conventional N2 and Ar low temperature adsorption methods drawing on the example of micro-mesoporous carbons of the CMK family. Advantages and limitations of CO2 adsorption characterization of nanoporous materials are discussed and further improvements are proposed.
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- Award ID(s):
- 1834339
- PAR ID:
- 10291011
- Date Published:
- Journal Name:
- Carbon
- Volume:
- 173
- ISSN:
- 0008-6223
- Page Range / eLocation ID:
- 842-848
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.carbon.202011.059
