Microstructural controls on the pressure-dependent permeability of Whitby mudstone
Published:January 01, 2017
Rosanne McKernan, Julian Mecklenburgh, Ernie Rutter, Kevin Taylor, 2017. "Microstructural controls on the pressure-dependent permeability of Whitby mudstone", Geomechanical and Petrophysical Properties of Mudrocks, E. H. Rutter, J. Mecklenburgh, K. Taylor
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A combination of permeability and ultrasonic velocity measurements allied with image analysis is used to distinguish the primary microstructural controls on effective-pressure-dependent permeability. Permeabilities of cylindrical samples of Whitby mudstone were measured using the oscillating pore-pressure method at confining pressures ranging between 30 and 95 MPa, and at pore pressures ranging between 1 and 80 MPa. The permeability–effective pressure relationship is empirically described using a modified effective pressure law in terms of confining pressure, pore pressure and a Klinkenberg effect. Measured permeability ranges between 3 × 10−21 and 2 × 10−19 m2 (3 and 200 nd), and decreases by approximately one order of magnitude across the applied effective pressure range. Permeability is shown to be less sensitive to changes in pore pressure than changes in confining pressure, yielding permeability effective pressure coefficients (χ) between 0.42 and 0.97. Based on a pore-conductivity model, which considers the measured changes in acoustic wave velocity and pore volume with pressure, the observed loss of permeability with increasing effective pressure is attributed dominantly to the progressive closure of bedding-parallel, crack-like pores associated with grain boundaries. Despite only constituting a fraction of the total porosity, these pores form an interconnected network that significantly enhances permeability at low effective pressures.
Supplementary material: A CSV file containing all experimental conditions and a tabulation of results is available at https://doi.org/10.6084/m9.figshare.c.3785741
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Geomechanical and Petrophysical Properties of Mudrocks
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A surge of interest in the geomechanical and petrophysical properties of mudrocks (shales) has taken place in recent years following the development of a shale gas industry in the United States and elsewhere, and with the prospect of similar developments in the UK. Also, these rocks are of particular importance in excavation and construction geotechnics and other rock engineering applications, such as underground natural gas storage, carbon dioxide disposal and radioactive waste storage. They may greatly influence the stability of natural and engineered slopes. Mudrocks, which make up almost three-quarters of all the sedimentary rocks on Earth, therefore impact on many areas of applied geoscience.
This volume focuses on the mechanical behaviour and various physical properties of mudrocks. The 15 chapters are grouped into three themes: (i) physical properties such as porosity, permeability, fluid flow through cracks, strength and geotechnical behaviour; (ii) mineralogy and microstructure, which control geomechanical behaviour; and (iii) fracture, both in laboratory studies and in the field.