Pore fluid viscosity effects on P- and S-wave anisotropy in synthetic silica-cemented sandstone with aligned fractures

Tillotson, Philip, Chapman, Mark, Sothcott, Jeremy, Best, Angus Ian and Li, Xiang-Yang (2014) Pore fluid viscosity effects on P- and S-wave anisotropy in synthetic silica-cemented sandstone with aligned fractures. Geophysical Prospecting, 62 (6), 1238-1252. (doi:10.1111/1365-2478.12194).

Abstract

Ultrasonic (500 kHz) P- and S-wave velocity and attenuation anisotropy were measured in the laboratory on synthetic, octagonal-shaped, silica-cemented sandstone samples with aligned penny-shaped voids as a function of pore fluid viscosity. One control (blank) sample was manufactured without fractures, another sample with a known fracture density inline image (measured from X-ray CT images). Velocity and attenuation were measured in four directions relative to the bedding fabric (introduced during packing of successive layers of sand grains during sample construction) and the coincident penny-shaped voids (fractures). Both samples were measured when saturated with air, water (viscosity 1 cP) and glycerin (100 cP) to reveal poro-visco-elastic effects on velocity and attenuation, and their anisotropy. The blank sample was used to estimate the background anisotropy of the host rock in the fractured sample; the bedding fabric was found to show transverse isotropy with shear wave splitting (SWS) of 1.45 ± 1.18% (i.e. for S-wave propagation along the bedding planes). In the fractured rock, maximum velocity and minimum attenuation of P-waves was seen at 90° to the fracture normal. After correction for the background anisotropy, the fractured sample velocity anisotropy was expressed in terms of Thomsen's weak anisotropy parameters ε, γ & δ. A theory of frequency-dependent seismic anisotropy in porous, fractured, media was able to predict the observed effect of viscosity and bulk modulus on ε and δ in water- and glycerin-saturated samples, and the higher ε and δ values in air-saturated samples. Theoretical predictions of fluid independent δ are also in agreement with the laboratory observations. We also observed the predicted polarisation cross-over in shear-wave splitting for wave propagation at 45° to the fracture normal as fluid viscosity and bulk modulus increases.

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