Diffusion of fluids confined in carbonate minerals: A molecular dynamics simulation study for carbon dioxide and methane-ethane mixture within calcite

The ability to calculate how different compounds diffuse within mesoporous structures is paramount for a number of applications in the oil & gas sector, from oil exploration to separation, and even for the design of Carbon Capture, Utilization, and Storage (CCUS) processes. Molecular Dynamics simulations entail an excellent alternative to cases for which an experimental determination is unfeasible or extremely difficult, as it happens for fluids in mesopores. Nonetheless, being confined within a mineral mesopore makes the fluid spatial distribution inhomogeneous, requiring appropriate methods to compute the diffusion coefficients. Recently, some of us presented a new method for this purpose (Franco et al. 2016). In this work, we present a detailed study on how to apply such a method exploring fluids confined within calcite walls, which is a mineral representative of carbonate rocks found in several geological formations. From our results, we were able to map the evolution of the self-diffusion tensor components throughout the pore, showing the anisotropy among the components at different directions. We also show the influence of confinement and observe a significant effect at the center of the pore for small mesopores (< 7.5 nm), where the density distribution is constant. This is an unexpected result that shows how the confinement effect is manifested even at the so-called "bulk-like"region at the center of the pore.