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 and 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. (doi:10.1111/j.1365-2478.2011.01021.x).

Abstract

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 = 100?f, where ?f is fracture density) regardless of pore fluid type, for wave propagation at 90° to the fracture normal.

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