459789 Thermodynamic and Transport Properties of Natural Gas Fluids Confined By Calcite: A Molecular Dynamics Study
In this context, molecular simulations represent an alternative approach to understand the effects of the confinement on these systems, and at the same time, to provide accurate values for both thermodynamic and transport properties for the confined fluid. Molecular simulations comprise two major techniques of sampling the phase space, so that, for an ergodic sampling, the trajectory average of a certain property will be equal to the macroscopic value. One technique, called Metropolis Monte Carlo simulation, is based on the stochastic sampling of the phase space. Monte Carlo simulations have been extensively used to predict fluid phase equilibria. The other technique, called Molecular Dynamics, is based on the integration of Newton’s equation of motion. Molecular Dynamics is particularly useful for transport coefficient calculations through the fluctuation-dissipation theorem. Provided an initial set of coordinates for the particles and a potential energy function for the interaction between each pair of particles, both techniques can be applied to generate trajectories on the phase space. The most important information for this sort of approach is the potential energy function, which is commonly called the force field. The force field may be derived either by quantum calculations for a single molecule in vacuum or by fitting experimental data. In the past two decades, a lot of effort was devoted to develop accurate force fields for natural gas fluids, such as n-alkanes, carbon dioxide and nitrogen, for water, and, even though not at the same scale, for calcite as well.
In this work, we carried out an extensive study based on Molecular Dynamics simulations for methane, nitrogen, carbon dioxide, and water confined between two calcite crystals (forming a slit pore with dimensions of few nanometers) at temperatures and densities compared to those in the operation of the oil reservoirs. Using this technique, we calculated density profiles, adsorption isotherms, and self-diffusion coefficients for the confined fluid. The density profiles show that all the investigated molecules are adsorbed on calcite surface. Nevertheless for mixtures, a competing process for adsorption was observed. This competing process may be quantitatively evaluated by the calculation of the equilibrium local selectivity near calcite surface. For mixtures of methane and carbon dioxide, the equilibrium local selectivity of carbon dioxide over methane is quite large. In addition, the higher the average density inside the calcite slit pore, the higher the selectivity. Pure methane adsorption isotherms in calcite were also calculated.
The results are in good agreement to the experimental data of methane adsorption in shale rocks that contain ~ 50 wt % of calcite in their mineral composition. For self-diffusion coefficients of fluids in confinement, the tensorial nature of this transport property ought to be considered, for an inhomogeneous density distribution inside the pore causes a significant impact on the value of the self-diffusivity in the perpendicular direction in relation to the crystal surface when compared to the parallel self-diffusivities. Therefore, a variation on the value of this property along the perpendicular axis is found. As a matter of fact, we have observed that near the crystal surface, even the parallel self-diffusion coefficients are different in one parallel direction to the other . We believe that this happens due to the anisotropic arrangement of the atoms in the calcite crystal and the strong interaction between the fluid particles and the crystal.
In conclusion, the effects of confinement are rather relevant for thermodynamic and transport properties of confined fluids. The improvement of the existing models, and the development of new ones to account for such effects is crucial for more accurate predictions. Molecular simulations surely constitute a set of valuable techniques that help chemical engineers to get better understanding on phenomena for which experimental data are scarce or extremely difficult to obtain.
- L.F.M. Franco, M. Castier and I.G. Economou, “Anisotropic Parallel Self-Diffusion Coefficients Near Calcite Surface: A Molecular Dynamics Study”, submitted for publication (2016).