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

Compositional grading within a mixture has a strong impact on the evaluation of the pre-exploitation distribution of hydrocarbons in underground layers and sediments. Thermodiffusion, which leads to a partial diffusive separation of species in a mixture due to the geothermal gradient, is thought to play an important role in determining the distribution of species in a reservoir. However, despite recent progress, thermodiffusion is still difficult to measure and model in multicomponent mixtures. In this work, we report on experimental investigations of the thermodiffusion of multicomponent n-alkane mixtures at pressure above 30 MPa. The experiments have been conducted in space onboard the Shi Jian 10 spacecraft so as to isolate the studied phenomena from convection. For the two exploitable cells, containing a ternary liquid mixture and a condensate gas, measurements have shown that the lightest and heaviest species had a tendency to migrate, relatively to the rest of the species, to the hot and cold region, respectively. These trends have been confirmed by molecular dynamics simulations. The measured condensate gas data have been used to quantify the influence of thermodiffusion on the initial fluid distribution of an idealised one dimension reservoir. The results obtained indicate that thermodiffusion tends to noticeably counteract the influence of gravitational segregation on the vertical distribution of species, which could result in an unstable fluid column. This confirms that, in oil and gas reservoirs, the availability of thermodiffusion data for multicomponent mixtures is crucial for a correct evaluation of the initial state fluid distribution.

Fluid transport through microporous carbon-based materials is inherent in numerous applications, ranging from gas separation by carbon molecular sieves to natural gas production from coal seams and gas shales. The present study investigates the steady-state permeation of supercritical methane in response to a constant cross-membrane pressure drop. We performed dual control volume grand canonical molecular dynamics (DCV-GCMD) simulations to mimic the conditions of actual permeation experiments. To overcome arbitrary assumptions regarding the investigated porous structures, the membranes were modeled after the CS1000a and CS1000 molecular models, which are representative of real microporous carbon materials. When adsorption-induced molecular trapping (AIMT) mechanisms are negligible, we show that the permeability of the microporous material, although not significantly sensitive to the pressure gradient, monotonically decreases with temperature and reservoir pressures, consistent with diffusion theory. However, when AIMT occurs, the permeability increases with temperature in agreement with experimental data found in the literature.

When an initially stretched rubber band is suddenly released at one end, an axial-stress front propagating at the celerity of sound separates a free and a stretched domain of the elastic material. As soon as it reaches the clamped end, the front rebounds and a compression front propagates backward. When the length of the compressed area exceeds Euler critical length, a dynamic buckling instability develops. The rebound is analysed using Saint-Venant's theory of impacts and we use a dynamical extension of the Euler–Bernoulli beam equation to obtain a relation between the buckled wavelength, the initial stretching and the rubber band thickness. The influence of an external fluid medium is also considered: owing to added mass and viscosity, the instability growth rate decreases. With a high viscosity, the axial-stress front spreads owing to viscous frictional forces during the release phase. As a result, the selected wavelength increases significantly.