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Experimental verification of the fracture density and shear-wave splitting relationship using synthetic silica cemented sandstones with a controlled fracture geometry

Tillotson, Philip; Sothcott, Jeremy; Best, Angus Ian; Chapman, Mark; Li, Xiang-Yang. 2012 Experimental verification of the fracture density and shear-wave splitting relationship using synthetic silica cemented sandstones with a controlled fracture geometry. Geophysical Prospecting, 60 (3). 516-525. 10.1111/j.1365-2478.2011.01021.x

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Abstract/Summary

We present laboratory ultrasonic measurements of shear-wave splitting from two synthetic silica cemented sandstones. The manufacturing process, which enabled silica cementation of quartz sand grains, was found to produce realistic sandstones of average porosity 29.7 ± 0.5% and average permeability 29.4 ± 11.3 mD. One sample was made with a regular distribution of aligned, penny-shaped voids to simulate meso-scale fractures in reservoir rocks, while the other was left blank. Ultrasonic shear waves were measured with a propagation direction of 90° to the coincident bedding plane and fracture normal. In the water saturated blank sample, shear-wave splitting, the percentage velocity difference between the fast and slow shear waves, of <0.5% was measured due to the bedding planes (or layering) introduced during sample preparation. In the fractured sample, shear-wave splitting (corrected for layering anisotropy) of 2.72 ± 0.58% for water, 2.80 ± 0.58% for air and 3.21 ± 0.58% for glycerin saturation at a net pressure of 40 MPa was measured. Analysis of X-ray CT scan images was used to determine a fracture density of 0.0298 ± 0.077 in the fractured sample. This supports theoretical predictions that shear-wave splitting (SWS) can be used as a good estimate for fracture density in porous rocks (i.e., SWS = 100εf, where εf is fracture density) regardless of pore fluid type, for wave propagation at 90° to the fracture normal.

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.1111/j.1365-2478.2011.01021.x
Programmes: BGS Programmes 2010 > Earth hazards and systems
NOC Programmes
ISSN: 0016-8025
Date made live: 31 Jul 2012 15:12
URI: http://nora.nerc.ac.uk/id/eprint/18982

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