Application of nitrogen gas-adsorption technique for characterization of pore structure of mudrocks

Abstract

Subcritical N2 gas-adsorption techniques are used more frequently now for characterizing the pore structure of mudrocks. This article discusses some application challenges associated with mudrocks: sample preparation, choice of inversion techniques of the raw isotherm data, representation of pore-size distribution (PSD) data, and reproducibility of measurement data. Gas-adsorption analysis on hand-ground < 40-mesh powder samples is recommended over using intact rock samples because of the slow diffusion process. Crushing the sample to <40-mesh powder has no significant effect on any of the measured pore-structure attributes. The dV/d(logD) representation is recommended for graphical PSD results because log-scale pore-size abscissa are used most commonly for geologic materials. Pore-size distribution inversion using the Barett-Joyner-Halenda (BJH) method should be preferred over the density-functional-theory (DFT) method because it yields more consistent and less noisy inverted data. The Harkins-Jura thickness equation is recommended as the choice of thickness equation. The Brunauer–Emmett–Teller (BET) theory specific surface area, total pore-volume, and PSD are reproducible within a variation range of 10% to 20%.