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Kerogen is the dispersed organic matter in sedimentary rocks from which natural gas and oil are generated over time by thermal maturation. There has been widespread interest in developing atomistic models of kerogen for numerical investigations of adsorption and diffusion behavior. Currently, the most popular kerogen models for use in molecular simulations are "molecular models," which consist in packing and annealing small macromolecules in order to create a 3D kerogen model. This method neglects the cross-linking that occurs as maturity increases, which can strongly control both the amount of pore space and the mechanical properties of kerogen, and is crucial for studying the transport properties of adsorbed fluids. Whereas, Leyssale and coworkers have pursued a different approach to kerogen modeling by using statistical mechanics-based methods to simulate the formation process of kerogen from organic precursors [1], [2], [3], [4]. This new generation of kerogen models, called "mimetic" models, capture the evolution of the cross-linking and chemistry with the maturity [5].
Here, we report on an exhaustive investigation of the self-diffusion coefficient of CH$_4$ in kerogen using eleven different mimetic models of kerogen derived from fatty acid precursors, spanning the range of maturity from immature to post-mature. Kerogen swelling and matrix flexibility must be considered in order to accurately estimate the self-diffusion coefficient for soft matrices [6]. It is well-established now that the collective effects on CH$_4$ (or CO$_2$) transport in kerogen are negligible even when flexibility matters [7], [8]. So, the self-diffusion coefficient can capture the impact of the adsorption and mechanical properties of kerogen on transport. Furthermore, CH$_4$ and CO$_2$ transport in flexible kerogen are known to be quite similar, as both can be modeled by the same free volume theory [8]. Therefore, gas loading was calculated at pressures between 0.1–50 MPa by using a hybrid method that alternates between hybrid grand canonical Monte Carlo and isothermal–isobaric ensemble molecular dynamics simulation in order to explicitly allow for adsorption-induced deformation of the kerogen matrix due to the presence of adsorbed fluid. Thermomechanical and chemical equilibrium are thus simultaneously maintained during adsorption. Molecular dynamics simulation are then performed at a constant temperature of 45 °C in the canonical ensemble starting from the fluid-loaded matrix.
A free volume model inspired by Fujita–Kishimoto theory can fit the observed trends in the self-diffusion coefficient of CH$_4$, with regard to both gas loading and kerogen maturity, in the kerogen models that display significant adsorption-induced swelling. Maturity influences transport in kerogen by both static and dynamic effects. On the one hand—consistent with the experimentally observed gradual stiffening of kerogen during maturation—the flexibility of kerogen matrices decreases with increasing maturity, which reduces the enhancement of diffusive transport due to the fluctuating microstructure. However, more mature kerogen is also more porous, which naturally allows for more efficient diffusion as mean free paths are lengthened due to greater pore connectivity. With regard to gas loading, the fluid content of kerogen mainly influences transport through swelling effects, which again depend on the maturity [9].
| References | [1] Atmani, L.; Bichara, C.; Pellenq, R. J.-M.; Damme, H. V.; van Duin, A. C. T.; Raza, Z.; Truflandier, L. A.; Obliger, A.; Kralert, P. G.; Ulm, F. J. et al. From cellulose to kerogen: molecular simulation of a geological process. Chem. Sci. 2017, 8, 8325–8335. [2] Atmani, L.; Valdenaire, P.-L.; Pellenq, R. J.-M.; Bichara, C.; Damme, H. V.; van Duin, A. C. T.; Ulm, F. J.; Leyssale, J.-M. Simulating the geological fate of terrestrial organic matter: Lignin vs cellulose. Energy Fuels 2020, 34, 1537–1547. [3] Leyssale, J.-M.; Valdenaire, P.-L.; Potier, K.; Pellenq, R. J.-M. Replica Exchange Molecular Dynamics Simulation of Organic Matter Evolution: From Lignin to Overmature Type III Kerogen. Energy Fuels 2022, 36, 14723–14733. [4] Leyssale, J.-M.; Valdenaire, P.-L.; Potier, K.; Pellenq, R. J.-M. Replica-Exchange Molecular Dynamics Simulation of the Natural Evolution of a Model Type I Kerogen. Energy Fuels 2023, 37, 14811–14823. [5] Obliger, A.; Bousige, C.; Coasne, B.; Leyssale, J.-M. Development of Atomistic Kerogen Models and Their Applications for Gas Adsorption and Diffusion: A Mini-Review. Energy Fuels 2023, 37, 1678–1698. [6] Potier, K.; Ariskina, K.; Obliger, A.; Leyssale, J.-M. Molecular simulation of argon adsorption and diffusion in a mature kerogen with poroelastic couplings. Langmuir 2023, 41, 6364–6375. [7] Obliger, A.; Valdenaire, P.-L.; Ulm, F.-J.; Pellenq, R. J.-M.; Leyssale, J.-M. Methane diffusion in a flexible kerogen matrix. J. Phys. Chem. B 2019, 123, 5635–5640. [8] Ariskina, K.; Galliéro, G.; Obliger, A. Adsorption-induced swelling impact on CO2 transport in kerogen microporosity described by free volume theory. Fuel 2024, 359, 130475. [9] Obliger, A.; Valdenaire, P.-L.; Capit, N.; Ulm, F. J.; Pellenq, R. J.-M.; Leyssale, J.-M. Poroelasticity of methane-loaded mature and immature kerogen from molecular simulations. Langmuir 2018, 34, 13766–13780. |
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| Country | France |
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