Stress-dependence of porosity and permeability of the Upper Jurassic Bossier shale: an experimental study
R. Fink, B. M. Krooss, A. Amann-Hildenbrand, 2017. "Stress-dependence of porosity and permeability of the Upper Jurassic Bossier shale: an experimental study", Geomechanical and Petrophysical Properties of Mudrocks, E. H. Rutter, J. Mecklenburgh, K. Taylor
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In order to characterize the stress-dependence of porosity and permeability of Bossier shale, a series of measurements was conducted on three dry, horizontally orientated samples using various gases under controlled stress conditions.
The Klinkenberg-corrected permeability and gas slippage factors varied by more than two orders of magnitude (0.21–86 µD) and by one order of magnitude (0.09–0.89 MPa), respectively. Porosity values measured under in situ stress conditions were lower by up to 30% than those measured at ambient conditions. Therefore, disregarding the stress-dependence of porosity may lead to a substantial overestimation of the free gas storage capacity.
The stress sensitivity of Klinkenberg-corrected permeability coefficients (−0.012–−0.063MPa−1) is much larger than the stress sensitivity of porosity (−0.0014–−0.0033 MPa−1). Particularly for pore systems dominated by microfractures or slit-shaped pores, the permeability is highly sensitive to effective stress changes. While conventional pore models use porosity stress-sensitivity exponents (m) ranging between 3 and 5, we measured values of up to 27. Strongly stress-sensitive permeability behaviour is a result of effective stress preferentially reducing the volume and effective cross-section of transport pathways. In contrast, stress-dependent permeability of a less stress-sensitive sample is instead controlled by the redistribution of flow.
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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.