Fluid Flow Mechanisms in Shale Organic Nanopores

From a fluid dynamics perspective, production of hydrocarbons from unconventional reservoirs represents a challenge because (1) the fluids are located in porous organic matter in shale rocks where the pore sizes range from a few to a couple hundred nanometers, (2) the permeability of shale rocks is on the order of nanodarcy (1 darcy ≈ 10⁻¹² m²), and (3) the behavior of the fluids and their interactions with the pore surface are poorly understood. In this work we use molecular dynamics simulations to create molecular models of organic matter (kerogen) that host a hydrocarbon mixture that is liquid at normal conditions of pressure and temperature. Our models account for the chemical functionality and pore geometry of kerogen and serve as rigid frameworks to study the mechanisms by which the different species of the mixture flow from organic pores to a microfracture.

Our simulations indicate that the two phases free- and adsorbed-fluid coexist in the pores, and the mechanisms by which the molecules of each phase flow are different. While the free phase can expand and diffuse, the strong interaction between adsorbed molecules and the pore surface gives rise to surface diffusion and retards or impedes the arrival of the longer, heavier and aromatic molecules to the microfracture.