A Review of Molecular Models for Gas Adsorption in Shale Nanopores and Experimental Characterization of Shale Properties

Abstract

Shale gas, as a promising alternative energy source, has received considerable attention because of its broad resource base and wide distribution. The establishment of shale models that can accurately describe the composition and structure of shale is essential to perform molecular simulations of gas adsorption in shale reservoirs. This Review provides an overview of shale models, which include organic matter models, inorganic mineral models, and composite shale models. Molecular simulations of gas adsorption performed on these models are also reviewed to provide a more comprehensive understanding of the behaviors and mechanisms of gas adsorption on shales. To accurately understand the gas adsorption behaviors in shale reservoirs, it is necessary to be aware of the pore structure characteristics of shale reservoirs. Thus, we also present experimental studies on shale microstructure analysis, including direct imaging methods and indirect measurements. The advantages, disadvantages, and applications of these methods are also well summarized. This Review is useful for understanding molecular models of gas adsorption in shales and provides guidance for selecting experimental characterization of shale structure and composition.

Figure 1

Snapshots of configurations of methane molecules in a multilayer graphene slit under different pore sizes. Reprinted from ref (39), Copyright 2017, with permission from Elsevier, 10.1016/j.fuel.2017.03.083.

Figure 2

Excess adsorption isotherms of methane at 298 K in slit pores with pore sizes ranging from 0.4 to 9 nm. Reprinted from ref (35), Copyright 2013, with permission from Elsevier, 10.1016/j.coal.2013.01.001.

Figure 3

Two-component density profiles of three temperatures at 290, 343, and 423 K. Molecular density profiles for CO₂ (top) and CH₄ (bottom) in mixtures of a total of 600 molecules. The CO₂:CH₄ ratios of mixtures are at 15:85, 50:50, and 85:15 with black, red, and blue lines, respectively. Reprinted from ref (42), Copyright 2018, with permission from Elsevier, 10.1016/j.jngse.2018.02.034.

Figure 4

Adsorption model of MD simulations. The blue and gray balls represent CH₄ molecules and carbon atoms, respectively. Republished with permission of the Royal Society of Chemistry, from ref (55), Copyright 2015; permission conveyed through Copyright Clearance Center, Inc.

Figure 5

(a) Selectivity of CO₂/CH₄ in the organic-rich shale model and pristine pillared shales model at T = 370 K. (b) Selectivity of CO₂/CH₄ at different depths. (c) Selectivity of CO₂/CH₄ at different basal spacings. The bulk CO₂ mole faction of y_CO2 = 0.5. Reprinted from ref (59), Copyright 2017, with permission from Elsevier, 10.1016/j.jngse.2017.01.024.

Figure 6

Van Krevelen diagram for kerogen using hydrogen from elemental analysis and organic oxygen from XPS analysis. Reproduced from ref (63), Copyright 2007, American Chemical Society.

Figure 7

(a) Initial structure of the kerogen II-D unit; (b) structure of the kerogen II-D unit after the geometry optimization and annealing dynamics; (c) initial kerogen model configuration; (d) final kerogen model configuration. Atoms: C in gray, H in white, O in red, N in blue, and S in yellow. Reprinted from ref (69), Copyright 2021, with permission from Elsevier, 10.1016/j.jngse.2021.103903.

Figure 8

Absolute adsorption isotherms of CH₄ and CO₂ in the binary mixtures on dry kerogen models of different organic types at 338 K with y_CO2 = 0.5. (a) CH₄ absolute adsorption isotherms; (b) CO₂ absolute adsorption isotherms. Reprinted from ref (72), Copyright 2017, with permission from Elsevier, 10.1016/j.apenergy.2017.10.122.

Figure 9

(a) Molecular model of a type I-A kerogen molecule with the chemical formula C₂₅₁H₃₈₅O₁₃N₇S₃; (b) bulk kerogen configuration with 10 kerogen molecules; (c) bulk kerogen model with porosity; (d) structure of a realistic slit kerogen nanopore. Reproduced from ref (79), Copyright 2020, Multidisciplinary Digital Publishing Institute.

Figure 10

Visualization of constructed kerogen pore structures and corresponding internal surfaces/isolated pore surfaces. Reproduced from ref (78), Copyright 2020, American Chemical Society.

Figure 11

(a) Loading amount of CH₄ (black line) and sequestration amount of CO₂ (red line) with the variation of bulk pressures in kerogen slit nanopores at 323 K, with the corresponding snapshots of the residual gases in kerogen slit nanopores at the bulk pressure of 6 (b) and 20 (c) MPa. Reproduced from ref (69), Copyright 2017, American Chemical Society.

Figure 12

Schematic diagram showing competitive adsorption of a CH₄/C2H₆ mixture in a 3.0 nm MMT slit. Dark green and orange spheres represent the united-atom models of CH₄ and C₂H₆, respectively. Color scheme: pink, Al; light green, Mg; blue, Ca; red, O; yellow, Si; white, H. Reprinted from ref (92), Copyright 2019, with permission from Elsevier, 10.1016/j.cej.2018.08.067.

Figure 13

Selectivity of C₂H₆ relative to CH₄ versus pore pressure P (y_C2H6 = 0.3). Reprinted from ref (92), Copyright 2019, with permission from Elsevier, 10.1016/j.cej.2018.08.067.

Figure 14

Illustration of a molecular model of illite (a) basal slit pore, (b) A and C chain slit pore, and (c) B chain slit pore with adsorbate CH₄ in the equilibrium state (from an orthographic view) and (d) basal slit pore, (e) A and C chain slit pore, and (f) B chain slit pore (from a perspective view). Color scheme: yellow, silicon; pink, aluminum; green, magnesium; red, oxygen; purple, potassium; cyan, carbon; white, hydrogen. Reproduced from ref (97), Copyright 2018, American Chemical Society.

Figure 15

(a) Pore size dependence (333 K, 10 MPa) and (b) temperature dependence (3 nm pore, 10 MPa) of simulated adsorption capacity in different illite slit pores. Reproduced from ref (97), Copyright 2018, American Chemical Society.

Figure 16

Molecular models: (a) kaolinite, Si₄Al₄O₁₀(OH)₈; (b) slit-shaped supercell kaolinite pore. Color scheme: red, oxygen; white, hydrogen; pink, aluminum; yellow, silicon. Reproduced from ref (99), Copyright 2019, American Chemical Society.

Figure 17

Selectivity for different molar fractions at T = 333.15 K in (a) montmorillonite, (b) illite, and (c) kaolinite nanopores. Reproduced from ref (83), Copyright 2019, American Chemical Society.

Figure 18

Selectivity of CO₂/CH₄, S, at varied y_CO2 in anhydrous quartz nanoslit. Reprinted from ref (87), Copyright 2022, with permission from Elsevier, 10.1016/j.energy.2021.122789.

Figure 19

MMT–kerogen composite simulation models with lattice-related parameters. Reprinted from ref (107), Copyright 2022, with permission from Elsevier, 10.1016/j.molliq.2022.119263.

Figure 20

Composite shale model of kaolinite and kerogen II-D. Reproduced from ref (108), Copyright 2020, Multidisciplinary Digital Publishing Institute.

Figure 21

Comparison of CH₄ excess adsorption isotherms between simulated results and experimental data at 338 K. Reprinted from ref (72), Copyright 2017, with permission from Elsevier, 10.1016/j.apenergy.2017.10.122.

Figure 22

Various analytical methods/techniques used for estimating porosity and pore size distributions in unconventional gas reservoirs. Reproduced from ref (14), Copyright 2018, IOP Science.