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

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