Determining the porosity of mudrocks using methodological pluralism

Porosity of shales is an important parameter that impacts rock strength for seal or wellbore integrity, gas-in-place calculations for unconventional resources or the diffusional solute and gas transport in these microporous materials. From a well section obtained from the Mont Terri underground laboratory in St. Ursanne, Switzerland we determined porosity, pore size distribution and specific surface areas on a set of 13 Opalinus Clay samples. The porosity methods employed are Helium pycnometry, water and mercury injection porosimetry, liquid saturation and immersion, low pressure N2 sorption as well as small/very small angle neutron scattering. These were used in addition to mineralogical and geochemical methods for sample analysis that comprise X-ray diffraction, X-ray fluorescence, total organic carbon content and cation exchange capacity. We find large variations in total porosity, ranging from ~23% for the neutron scattering method to ~10% for mercury injection porosimetry. These differences can partly be related to differences in pore accessibility while no or negligible inaccessible porosity was found. Pore volume distributions between neutron scattering and low pressure sorption compare very well but differ significantly from those obtained from mercury porosimetry which is realistic since the latter provides information on pore throats only and the two former methods on pore throats and pore bodies. Finally we find that specific surface areas determined using low pressure sorption and neutron scattering match well.