The self-diffusivity of natural gas in the organic nanopores of source rocks

Abstract

Natural gas stored in source rocks has become a significant contributor to supply the energy demand. Source rocks are a special subclass of sedimentary rocks where the matrix serves as both the source and the reservoir at the same time. Attributed to their complex mineralogy and multi-scale pore systems, source rocks exhibit transport and storage processes that are not within the continuum framework. Significant portion of source rocks pores is of few nanometers in size. These nanopores offer large surface area to host hydrocarbons in the free and sorbed forms. Our ability to model the mechanisms by which hydrocarbons are stored and transported is, however, at infancy stages. In this paper, representative organic nanopores were formed from kerogen at different thermal maturation states. Free molecular diffusion was found to be the dominant mechanisms based on the calculated Knudsen number. Furthermore, diffusivity analysis was performed using molecular dynamics for some range of pressure that is typically encountered during the production span. The results revealed some deviation of the diffusivity coefficient from the value calculated theoretically. The deviation was even more pronounced for the post-mature case. The gap between the theoretically calculated and molecularly simulated diffusivity coefficients was found to reduce with increasing the pressure and the pore size. The sorption and diffusion data were coupled to redefine the mean free path for gas transport in organic nanopores. The reported values can serve as input for better description of the hydrocarbons transport in source rocks.