Pore Size Distribution and Morphology in the Horn River Shale, Middle and Upper Devonian, Northeastern British Columbia, Canada

Abstract

Abstract Porosity and pore size distributions are key attributes for characterizing shale gas storage capacity and flow in shale. In this study of shale samples from the Horn River Formation, analysis of field emission scanning electron microscopic (SEM) images of argon-ion-milled samples, nitrogen adsorption and desorption experiments, and mercury injection capillary pressure were applied to characterize pore volumes, pore morphology, and pore sizes. Helium porosity measurements range from 2.3 to 6.6%. Porosity estimated from analysis of SEM images was significantly lower, suggesting that this approach fails to resolve a significant pore volume represented by small pores. Based on SEM image analysis, most observed pores were intraparticle pores within organic matter and less commonly within inorganic matrix. Interparticle pores were observed in pyrite framboids, between grains, and surrounding grain rims. Microfractures noted in some images may influence both gas storage and transport. Estimates of pore size distributions also depend on the analytical method. Image analysis of secondary electron SEM images indicated the dominant pore size range is 10 to 40 nm, whereas nitrogen adsorption/desorption isotherm (BET) analysis indicated a much smaller dominant pore size range of 3 to 5 nm. Mercury injection capillary pressure results demonstrated a dominant pore throat size ranging from 3 to 8 nm, consistent with the nitrogen adsorption/desorption isotherm analysis. The discrepancy between SEM and nitrogen adsorption/desorption results also suggests that SEM images obtained with field emission SEM failed to resolve an important class of pores.