Porosity of Coal and Shale: Insights from Gas Adsorption and SANS/USANS Techniques

TLDR

The Illinois Basin coal and shale samples are early mature, with vitrinite reflectance values around 0.5–0.6%. The study used N₂/CO₂ adsorption and SANS/USANS on two Pennsylvanian coals and two Upper Devonian–Mississippian shales to probe porosity and pore accessibility. Mesopore size distributions differ markedly between coal and shale, while micropore distributions are similar; pore volumes correlate with organic matter, and coal pores show size‑specific accessibility that enhances adsorption, whereas shale pores are largely inaccessible.

Abstract

Two Pennsylvanian coal samples (Spr326 and Spr879-IN1) and two Upper Devonian-Mississippian shale samples (MM1 and MM3) from the Illinois Basin were studied with regard to their porosity and pore accessibility. Shale samples are early mature stage as indicated by vitrinite reflectance (Ro) values of 0.55% for MM1 and 0.62% for MM3. The coal samples studied are of comparable maturity to the shale samples, having vitrinite reflectance of 0.52% (Spr326) and 0.62% (Spr879-IN1). Gas (N2 and CO2) adsorption and small-angle and ultrasmall-angle neutron scattering techniques (SANS/USANS) were used to understand differences in the porosity characteristics of the samples. The results demonstrate that there is a major difference in mesopore (2–50 nm) size distribution between the coal and shale samples, while there was a close similarity in micropore (<2 nm) size distribution. Micropore and mesopore volumes correlate with organic matter content in the samples. Accessibility of pores in coal is pore-size specific and can vary significantly between coal samples; also, higher accessibility corresponds to higher adsorption capacity. Accessibility of pores in shale samples is low.