Determining the porosity of mudrocks using methodological pluralism

Abstract

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 Rock 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, and low pressure N2 sorption, as well as small-angle to ultra-small-angle neutron scattering (SANS–USANS). 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 approximately 23% for the neutron-scattering method to approximately 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: this 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.

Predicting Effective Diffusion Coefficients in Mudrocks Using a Fractal Model and Small‐Angle Neutron Scattering Measurements

“…All VSANS measurements were conducted at the KWS‐3 instrument at JCNS (Pipich & Zhendong, ). Results for Opalinus Shale have been reported earlier (Busch et al, ). With the exception of one Boom Clay sample (K4), all samples were cut parallel to bedding, fixed on quartz glass carriers, and polished to a thickness of about 200 m. Samples were dried at room temperature, and measurements were performed under ambient pressure and temperature conditions.…”

“…Pores can be spherical, slit‐shaped, or ellipsoidal or have more complex geometries, as has been shown in previous studies (Desbois et al, ; Leu et al, ; Loucks et al, ). We have used the spherical model in a previous study on Opalinus Shale and demonstrated that porosity calculations originating from this approach match porosity determinations using Helium pycnometry or water porosimetry (Busch et al, ). Furthermore, using a different approach (e.g., ellipsoid pores) would not have an impact on the fractal dimension or the slope of the scattering intensity versus pore radius and we are therefore confident that the approach used in this study is suitable.…”

“…The method provides information on fluid accessible and fluid nonaccessible pores; these can be differentiated by filling the pore space with deuterated water or deuterated methane and comparing with the air‐filled measurement. Neutron scattering data provide important information on porosity, specific surface area, and pore size distribution, and depending on the instrument and instrument setting used, the pore size range can vary from 5 Å to 100 μm (Busch et al, ; Melnichenko, ). Despite the valuable data and information that can be gathered from such measurements, access to nuclear research facilities is limited to a few places worldwide only.…”

“…Furthermore, using a different approach (e.g., ellipsoid pores) would not have an impact on the fractal dimension or the slope of the scattering intensity versus pore radius and we are therefore confident that the approach used in this study is suitable. Details on the calculation steps to obtain pore volume, specific surface area, and pore size distribution have been described previously (Busch et al, ; Kampman et al, ).…”

“…From an ~2‐m core section, ~1‐cm disks were cut in 10‐cm intervals. From these disks, samples for neutron scattering measurements have been prepared for this study and other petrophysical properties and mineralogy have been published previously (Busch et al, ).…”