Molecular modeling is applied to a representative array of kerogens for the purpose of obtaining quantitative predictions of thermodynamic properties from quantum mechanics and volumetric properties from molecular dynamics. The kerogen model units (175−260 carbon atoms) have been built in the MedeA environment from the sole consideration of the elemental analysis and functional group analysis documented in the work of Exxon and IFP-EN scientists [Kelemen, S. R., et al., Energy Fuels, 2007, 21 (3), pp 1548−1561. The density results are in good agreement with the well-documented trends of kerogen density with thermal maturity and organic type. The heat capacity in the ideal gas state is predicted to increase as a function of temperature, as obtained from quantum mechanics at the semiempirical level (MOPAC-PM7). This result is in quantitative agreement with experimental heat capacity data on type I kerogen and on coal. This behavior appears clearly as a nonclassical feature, because of the quantization of energy levels in molecular vibrations. Also, the residual heat capacity estimated from molecular dynamics appears subordinate, compared with the ideal heat capacity evaluated from quantum mechanics. The change from negative to positive standard enthalpy of formation when changing from low-maturity kerogen to high-maturity kerogen is also predicted in agreement with correlative methods based on numerous experimental data from coals and fossil fuels. Kerogen model units are available for download free of charge in .xyz or .sci formats from www.materialsdesign. com/science/structures/kerogens_and_coals.

The parameters of the anisotropic united atoms potential for linear alkanes proposed by Toxvaerd [S. Toxvaerd, J. Chem. Phys. 107, 5197 (1997)] have been optimized on the basis of selected equilibrium properties (vapor pressures, vaporization enthalpies, and liquid densities) of ethane, n-pentane, and n-dodecane. The optimized parameters for the CH2 and CH3 groups form a regular sequence with those of methane and the force centers are found between the carbon and hydrogen atoms, as expected. The resulting potential, called AUA4, has been compared with Toxvaerd’s potential (AUA3) by using several molecular simulation methods (Gibbs ensemble Monte Carlo, thermodynamic integration, and molecular dynamics). An investigation performed at temperatures ranging from 140 to 700 K and with various chain lengths up to 20 carbon atoms has shown AUA4 to provide systematic improvements of vapor pressures, vaporization enthalpies, and liquid densities for pure n-alkanes. Significant improvements have been also noticed on the critical temperatures of n-alkanes, estimated from coexistence density curves, and on the equilibrium properties of CO2–n-alkane binary mixtures. Self-diffusion coefficients of n-hexane, however, are slightly improved by the new potential, but still exceed experimental measurements at low temperature. As we have only optimized the intermolecular potential in the present study, it is suggested that further optimization of the intramolecular potentials of the anisotropic united atoms model could allow simultaneous prediction of thermodynamic properties and of transport coefficients, particularly in very dense liquids.

We report a series of Grand Canonical Monte Carlo simulations of water adsorption in NaY and NaX faujasite, as well as in silicalite-1. Computed adsorption isotherms and heats of adsorption were in good agreement with the available experiments. The existence of cyclic water hexamers in NaX located in the 12-ring windows, recently disclosed by neutron diffraction experiments (Hunger et al., J. Phys. Chem. B, 2006, 110, 342-353) was reproduced in our simulations. Interestingly enough, such cyclic hexamer clusters were also observed in the case of NaY, in which no stabilizing cation is present in the 12-ring window. We also report cation redistribution upon water adsorption for sodium faujasite with varying cation contents (Si ratio Al ratio in the range 1.53-3). A simple and transferable forcefield was used, that enabled to reproduce the different aspects of water physisorption in stable zeolites. The high pressure water condensation in hydrophobic silicalite-1 was reproduced without any parameter readjustment. The method and forcefield used here should be useful for engineering oriented applications such as the prediction of multi-component mixture adsorptive separations in various stable zeolites. It allows to address the issue of the effect of the small amounts of water that are almost inevitably present in zeolite-based separation processes.

In this study, we propose a new global procedure to perform optimization of semiempirical intermolecular potential parameters on the basis of a large reference database. To obtain transferable parameters, we used the original method proposed by Ungerer ͓Ungerer et al., J. Chem. Phys. 112, 5499 ͑2000͔͒, based on the minimization of a dimensionless error criterion. This method allows the simultaneous optimization of several parameters from a large set of reference data. However, the computational cost of such a method limits its application, because it implies the calculation of an important number of partial derivatives, calculated by finite differences between the results of several different simulations. In this work, we propose a new method to evaluate partial derivatives, in order to reduce the computing time and to obtain more consistent derivatives. This method is based on the analysis of statistical fluctuations during a single simulation. To predict equilibrium properties of olefins, we optimize the Lennard-Jones potential parameters of the unsaturated hydrocarbon groups using the anisotropic united atoms description. The resulting parameters are consistent with those previously determined for linear and branched alkanes. Test simulations have been performed at temperatures ranging from 150 to 510 K for several ␣-olefins ͑ethylene, propene, 1-butene, 1-pentene, 1-hexene, 1-octene͒, several ␤-olefins ͑trans-2-butene, cis-2-butene, trans-2-pentene͒, isobutene, and butadiene. Equilibrium properties are well predicted, and critical properties can be evaluated with a good accuracy, despite the fact that most of the results constitute pure predictions. It is concluded that the AUA potential, due to a relevant physical meaning, can be transferred to a large range of olefins with good success.

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In this work, we use molecular simulations to determine the structural and physical properties of the organic matter present in type II shales in the middle of the oil generation window. The construction of molecular models of organic matter constrained by experimental data is discussed. Using a realistic molecular model of organic matter, we generate, by molecular dynamics simulations, structures that mimic bulk organic matter under typical reservoir conditions. Consistent results on density, diffusion, and specific adsorption are found between simulated and experimental data. These structures enable us to provide information on the fluid distribution within the organic matter, the pore size distributions, the isothermal compressibility, and the dynamic of the fluids within the kerogen matrix. This study shows that a consistent description at the molecular level combined with molecular simulations can be useful, in complement of experiments, to investigate the organic matter present in shales.

Résumé -Applications de la simulation moléculaire à la production et au traitement du pétrole et du gaz -La simulation moléculaire est une technique émergente qui consiste à simuler un système de quelques centaines de molécules de façon très détaillée. Par des moyennes statistiques appropriées, on peut déterminer sur la base des résultats de cette simulation des propriétés d'équilibre et de transport qui peuvent être comparées à des résultats expérimentaux. Le but de l'article est de fournir au lecteur des notions de base sur les méthodes de simulation. Ensuite, des exemples d'application sont abordés qui couvrent divers domaines de l'industrie pétrolière. En ingénierie de réservoir, ces exemples ont trait aux propriétés d'équilibre des hydrocarbures lourds, aux propriétés thermiques des gaz naturels, ainsi qu'aux propriétés volumétriques et aux équilibres de phase des mélanges d'hydrocarbures et de gaz acides. Les applications du domaine de la production et du traitement sont illustrées par l'exemple des équilibres de phase impliquant le méthanol (couramment employé comme solvant ou comme inhibiteur d'hydrates) et celui de la solubilité des gaz dans les matériaux polymères à haute pression. En raffinage, c'est à la solubilité de l'hydrogène sulfuré dans les hydrocarbures à haute température que la simulation a été appliquée. Dans chacun de ces problèmes, la simulation a apporté des prédictions très utiles ainsi qu'une compréhension des causes profondes des relations entre la structure chimique et les propriétés des fluides. Elle fournit ainsi une voie intermédiaire à mi-chemin entre les mesures expérimentales et les modèles thermodynamiques classiques. Moins chère et plus rapide que de nouvelles mesures, elle peut améliorer la détermination des paramètres des modèles utilisés en routine. Abstract -Applications of Molecular Simulation in Oil and Gas Production and ProcessingMolecular simulation is an emerging technique which consists in performing a detailed simulation of microscopic systems involving typically a few hundreds of molecules. On the basis of these simulations

International audienceDuring the past decade, gas recovered from shale reservoirs has jumped from 2 to 40% of natural gas production in the United States. However, in response to the drop of gas prices, the oil and gas industry has set its sights on the oil-prone shale plays, potentially more lucrative. This shift from dry to condensate-rich gas has raised the need for a better understanding of the transport of hydrocarbon mixtures through organic-rich shale reservoirs. At the micrometer scale, hydrocarbons in shales are mostly located in amorphous microporous nodules of organic matter, the so-called kerogen, dispersed in an heterogeneous mineral matrix. In such multiscale materials, a wide range of physical mechanisms might affect the composition of the recovered hydrocarbon mixtures. More specifically, kerogen nodules are likely to act as selective barriers due to their amorphous microporous structure. In this work, we study the transport of hydrocarbon mixtures through kerogen by means of molecular simulations. We performed molecular dynamics simulations of hydrocarbons permeating through a molecular model representative of oil-prone type II kerogen. Our results show that the permeation mechanisms through this type of material is purely diffusive. Consequently, we have computed the Onsager's species-specific transport coefficients of a typical condensate-rich gas mixture within kerogen. Interestingly, we have observed that the transport coefficients matrix can be reasonably approximated by its diagonal terms, the so-called Onsager's autocorrelation coefficients. Inspired by the classical Rouse model of polymer dynamics and surface diffusion theory, we propose a simple scaling law to predict the transport coefficient of linear alkanes in the mixture. In good agreement with our simulations results, the Onsager's autocorrelation coefficients scale linearly with the adsorption loading and inversely with the alkane chain length. We believe our results and predictions are applicable to other materials, such as carbon-based synthetic microporous membranes, with structural properties close to that of kerogen