TY - JOUR T1 - Impact of Nanoporosity on Hydrocarbon Transport in Shales’ Organic Matter JF - Nano Letters Y1 - 2018 A1 - Amaël Obliger A1 - Franz-Josef Ulm A1 - Roland Jean-Marc Pellenq KW - Kerogen; microporosity; diffusion; hydrocarbons; shales AB -

In a context of growing attention for shale gas, the precise impact of organic matter (kerogen) on hydrocarbon recovery from unconventional reservoirs still has to be assessed. Kerogen's microstructure is characterized by a very disordered pore network that greatly affects hydrocarbon transport. The specific structure and texture of this organic matter at the nanoscale is highly dependent on its origin. In this study, by the use of statistical physics and molecular dynamics, we shed some new lights on hydrocarbon transport through realistic molecular models of kerogen at different level of maturity [Bousige et al. Nat. Mater. 2016, 15, 576]. Despite the apparent complexity, severe confinement effects controlled by the porosity of the various kerogens allow linear alkanes (from methane to dodecane) transport to be studied only via the self-diffusion coefficients of the species. The decrease of the transport coefficients with the amount of adsorbed fluid can be described by a free volume theory. Ultimately, the transport coefficients of hydrocarbons can be expressed simply as a function of the porosity (volume fraction of void) of the microstructure, thus paving the way for shale gas recovery predictions.

https://pubs-acs-org.libproxy.mit.edu/appl/literatum/publisher/achs/journals/content/nalefd/2018/nalefd.2018.18.issue-2/acs.nanolett.7b04079/20180208/images/medium/nl-2017-04079p_0004.gif

VL - 18 UR - http://pubs.acs.org/doi/10.1021/acs.nanolett.7b04079 IS - 2 JO - Nano Lett. ER - TY - JOUR T1 - Mesoscale structure, mechanics, and transport properties of source rocks’ organic pore networks JF - Proceedings of the National Academy of Sciences Y1 - 2018 A1 - Berthonneau, Jeremie A1 - Amaël Obliger A1 - Valdenaire, Pierre-Louis A1 - Grauby, Olivier A1 - Ferry, Daniel A1 - Chaudanson, Damien A1 - Pierre E. Levitz A1 - Kim, Jae Jin A1 - Franz-Josef Ulm A1 - Roland Jean-Marc Pellenq KW - porous media; electron tomography; mechanics; fluid transport; mesoscale AB -

Organic matter is responsible for the generation of hydrocarbons during the thermal maturation of source rock formation. This geochemical process engenders a network of organic hosted pores that governs the flow of hydrocarbons from the organic matter to fractures created during the stimulation of production wells. Therefore, it can be reasonably assumed that predictions of potentially recoverable confined hydrocarbons depend on the geometry of this pore network. Here, we analyze mesoscale structures of three organic porous networks at different thermal maturities. We use electron tomography with subnanometric resolution to characterize their morphology and topology. Our 3D reconstructions confirm the formation of nanopores and reveal increasingly tortuous and connected pore networks in the process of thermal maturation. We then turn the binarized reconstructions into lattice models including information from atomistic simulations to derive mechanical and confined fluid transport properties. Specifically, we highlight the influence of adsorbed fluids on the elastic response. The resulting elastic energy concentrations are localized at the vicinity of macropores at low maturity whereas these concentrations present more homogeneous distributions at higher thermal maturities, due to pores' topology. The lattice models finally allow us to capture the effect of sorption on diffusion mechanisms with a sole input of network geometry. Eventually, we corroborate the dominant impact of diffusion occurring within the connected nanopores, which constitute the limiting factor of confined hydrocarbon transport in source rocks.

VL - 115 UR - http://www.pnas.org/lookup/doi/10.1073/pnas.1808402115 IS - 49 JO - Proc Natl Acad Sci USA ER - TY - JOUR T1 - From cellulose to kerogen: molecular simulation of a geological process JF - Chemical Science Y1 - 2017 A1 - Atmani, Lea A1 - Christophe Bichara A1 - Roland Jean-Marc Pellenq A1 - Henri Van Damme A1 - Adri CT Van Duin A1 - Raza, Zamaan A1 - Truflandier, Lionel A. A1 - Amaël Obliger A1 - Kralert, Paul G. A1 - Franz-Josef Ulm A1 - Jean-Marc Leyssale AB -

The process by which organic matter decomposes deep underground to form petroleum and its underlying kerogen matrix has so far remained a no man's land to theoreticians, largely because of the geological (Myears) timescale associated with the process. Using reactive molecular dynamics and an accelerated simulation framework, the replica exchange molecular dynamics method, we simulate the full transformation of cellulose into kerogen and its associated fluid phase under prevailing geological conditions. We observe in sequence the fragmentation of the cellulose crystal and production of water, the development of an unsaturated aliphatic macromolecular phase and its aromatization. The composition of the solid residue along the maturation pathway strictly follows what is observed for natural type III kerogen and for artificially matured samples under confined conditions. After expulsion of the fluid phase, the obtained microporous kerogen possesses the structure, texture, density, porosity and stiffness observed for mature type III kerogen and a microporous carbon obtained by saccharose pyrolysis at low temperature. As expected for this variety of precursor, the main resulting hydrocarbon is methane. The present work thus demonstrates that molecular simulations can now be used to assess, almost quantitatively, such complex chemical processes as petrogenesis in fossil reservoirs and, more generally, the possible conversion of any natural product into bio-sourced materials and/or fuel.

VL - 8 UR - http://xlink.rsc.org/?DOI=C7SC03466K IS - 12 JO - Chem. Sci. ER - TY - JOUR T1 - Free Volume Theory of Hydrocarbon Mixture Transport in Nanoporous Materials JF - The Journal of Physical Chemistry Letters Y1 - 2016 A1 - Amaël Obliger A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm A1 - Benoit A. Coasne AB -
Despite recent focus on shale gas, hydrocarbon recovery from the ultraconfining and disordered porosity of organic matter in shales (kerogen) remains poorly understood. Key aspects such as the breakdown of hydrodynamics at the nanoscale and strong adsorption effects lead to unexplained non-Darcy behaviors. Here, molecular dynamics and statistical mechanics are used to elucidate hydrocarbon mixture transport through a realistic molecular model of kerogen [Bousige, C.; et al. Nat. Mater. 2016, 15, 576]. Owing to strong adsorption effects, velocity cross-correlations between the mixture components and between molecules of the same species are shown to be negligible. This allows estimation of each component permeance from its self-diffusivity, which can be obtained from single-component data. These permeances are found to scale with the reciprocal of the alkane length and decrease with the number of adsorbed molecules following a simple free volume theory, therefore allowing mixture transport prediction as a function of the amount of trapped fluid.
 
 
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VL - 7 IS - 19 JO - J. Phys. Chem. Lett. ER -