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.

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.

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

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In this work, phase behavior was analyzed for recombined mixtures of a Brazilian pre-salt crude oil with carbon dioxide and methane. This oil was sampled at stock tank conditions during a well-test, and mixtures with gas were prepared with a gas molar composition ranging from 20.0 to 86.0 mol %. Their phase behavior was investigated from 293.15 to 378.15 K and up to 100.0 MPa. Direct phase transition detections were achieved by using a full visibility PVT cell, coupled with a short-wave infrared camera. At this wavelength, crude oil has a lower absorbance, and it has been noted that macroscopic phase transitions can be easily detected by visual inspection. For the pseudo-binary system crude oil + carbon dioxide systems, liquid−liquid phase transitions were observed when the gas content is higher than 70.0 mol %. In addition, at the higher carbon dioxide composition, an asphaltic phase was formed at high pressures together with liquid−liquid phases. Similarly, crude oil and methane systems presented a liquid−liquid immiscibility region at methane composition above 60.0 mol % in all temperature domain studied. The Peng−Robinson equation of state was used for modeling purposes, and liquid−liquid and liquid−liquid−vapor transitions were qualitatively described. Additionally, it was observed that liquid−liquid equilibria behavior was highly dependent on the crude oil heavy fraction immiscibility because of the increasing system asymmetry by increasing mixture gas content.

Silicon photomultipliers (SiPM) are the photon detectors chosen for the tracking readout in NEXT, a neutrinoless β β decay experiment which uses a high pressure gaseous xenon time projection chamber (TPC). The reconstruction of event track and topology in this gaseous detector is a key handle for background rejection. Among the commercially available sensors that can be used for tracking, SiPMs offer important advantages, mainly high gain, ruggedness, cost-effectiveness and radio-purity. Their main drawback, however, is their non sensitivity in the emission spectrum of the xenon scintillation (peak at 175 nm). This is overcome by coating these sensors with the organic wavelength shifter tetraphenyl butadiene (TPB). In this paper we describe the protocol developed for coating the SiPMs with TPB and the measurements performed for characterizing the coatings as well as the performance of the coated sensors in the UV-VUV range.

The multi-component diffusive mass transport is generally quantified by means of the Maxwell-Stefan diffusion coefficients when using molecular simulations. These coefficients can be related to the Fick diffusion coefficients using the thermodynamic correction factor matrix, which requires to run several simulations to estimate all the elements of the matrix. In a recent work, Schnell et al. ["Thermodynamics of small systems embedded in a reservoir: A detailed analysis of finite size effects," Mol. Phys. 110, 1069-1079 (2012)] developed an approach to determine the full matrix of thermodynamic factors from a single simulation in bulk. This approach relies on finite size effects of small systems on the density fluctuations. We present here an extension of their work for inhomogeneous Lennard Jones fluids confined in slit pores. We first verified this extension by cross validating the results obtained from this approach with the results obtained from the simulated adsorption isotherms, which allows to determine the thermodynamic factor in porous medium. We then studied the effects of the pore width (from 1 to 15 molecular sizes), of the solid-fluid interaction potential (Lennard Jones 9-3, hard wall potential) and of the reduced fluid density (from 0.1 to 0.7 at a reduced temperature T* = 2) on the thermodynamic factor. The deviation of the thermodynamic factor compared to its equivalent bulk value decreases when increasing the pore width and becomes insignificant for reduced pore width above 15. We also found that the thermodynamic factor is sensitive to the magnitude of the fluid-fluid and solid-fluid interactions, which softens or exacerbates the density fluctuations.

Shale gas is a rising energy supply that is set to change the global energy market in coming decades. Even though thousands of shale gas wells are still in production throughout the world, the physics underlying the nano-scale of shale gas remains poorly understood, especially when complex fluid mixtures are involved. Effectively, compositional changes will arise in the well gas stream, resulting not only from phase changes but also from selective desorption and molecular sieving in the nano pores. The aim of this paper is twofold:  Highlight the potential use of molecular simulation, to determine key reservoir parameters in term of storage and kinetics.  Suggest an integrated workflow for implementing the parameters obtained in a commercial simulator and provide guidelines for tailoring the physics in the simulator to the needs of shale gas modeling.Molecular simulation is an emerging technique which can be put to relevant use for shale gas. It offers insights into nanoscale properties such as sorption or transport coefficients. The technique is based on numerical simulations at the molecular scale for a given set of pressure and temperature conditions. Our work comprised the following actions:  We started building an organic molecule compound representative of a mature kerogen.  Molecular simulation was used to produce adsorption isotherms for multi-component gas. For a C1-C2 mixture, we have established that the standard Extended Langmuir isotherm model available in ECLIPSE is poorly suited to matching with actual data. We propose an enhanced model in which partial pressures are replaced by fugacity.  Because of the nano-scale size of the pores in the kerogen, the gas flows not to a Darcy pattern but in a slip or transitional process instead. This kind of flow displays different transport properties, depending on molecule sizes. We used molecular simulation to check the relevance of analytical flow models such as Beskok-Karniadakis's one and investigated how individual component flow properties change when a gas mixture is involved. We then made a comprehensive review of commercial simulators to check their suitability for shale gas modeling. A triple porosity model was built to capture the actual nature of a shale gas formation which features organic and inorganic pores along with fractures. We show how alpha factors can successfully be used to model the molecular sieving ofgas flow in the organic pores (depending on size). We also elaborate on how the extended Langmuir curve was distorted to match the results of molecular simulation. At the end of this paper, we offer insight into simulation forecasting of gas stream composition changes over time.