Pore Size of Shale Based on Acyclic Pore Model

Abstract

The transport properties inside a nanosize conduit, such as viscosity and density, deviate from those reported at unconfined conditions with an identical pressure and temperature. The deviation from the nominal value is well studied for the simple topology in nanofluidics. A straight circular tube is an example of a simple topology. The pore space inside the matrix of a shale formation is also a nanofluidic system, because its characteristic size is smaller than or equal to 100 nm, but it includes a complex structure. With this in mind, we determine the pore-throat and pore-body size distributions of different shales whose data are available in the literature. The main objective is to quantitatively distinguish the two sizes, whose importance is overlooked in the study of shale formations. The pore-throat size distribution is determined from mercury injection capillary pressure measurements, and the pore-body size distribution is derived from nitrogen adsorption–desorption. The acyclic pore model, which is physically representative of the pore space at the core scale, allows us to interpret the petrophysical measurements. Our study of different shales shows that the average pore-body size is usually larger than 20 nm; thus, there is no need to account for the pore proximity (confinement). The pore-throat size distributions of different shales usually fall below 20 nm, which entails the modification of a transport property that is relevant to the formation resistance against the flow.