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

Pore fluid viscosity effects on P- and S-wave anisotropy in synthetic silica-cemented sandstone with aligned fractures
Pore fluid viscosity effects on P- and S-wave anisotropy in synthetic silica-cemented sandstone with aligned fractures
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 ?, ? & ?. A theory of frequency-dependent seismic anisotropy in porous, fractured, media was able to predict the observed effect of viscosity and bulk modulus on ? and ? in water- and glycerin-saturated samples, and the higher ? and ? values in air-saturated samples. Theoretical predictions of fluid independent ? 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.
Anisotropy, Fractures, Fluid saturation
0016-8025
1238-1252
Tillotson, Philip
b8c84738-5ad2-4360-a7a1-b83996fa7188
Chapman, Mark
6f13eb72-ad0e-4c7a-b60c-92387a707cf4
Sothcott, Jeremy
71ab4088-7b13-46d6-9e28-67538a02d595
Best, Angus Ian
cad03726-10f8-4f90-a3ba-5031665234c9
Li, Xiang-Yang
b902023b-aa5b-477b-a97b-013173349ce1
Tillotson, Philip
b8c84738-5ad2-4360-a7a1-b83996fa7188
Chapman, Mark
6f13eb72-ad0e-4c7a-b60c-92387a707cf4
Sothcott, Jeremy
71ab4088-7b13-46d6-9e28-67538a02d595
Best, Angus Ian
cad03726-10f8-4f90-a3ba-5031665234c9
Li, Xiang-Yang
b902023b-aa5b-477b-a97b-013173349ce1

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).

Record type: Article

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 ?, ? & ?. A theory of frequency-dependent seismic anisotropy in porous, fractured, media was able to predict the observed effect of viscosity and bulk modulus on ? and ? in water- and glycerin-saturated samples, and the higher ? and ? values in air-saturated samples. Theoretical predictions of fluid independent ? 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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More information

Published date: November 2014
Keywords: Anisotropy, Fractures, Fluid saturation
Organisations: Ocean and Earth Science, Marine Geoscience

Identifiers

Local EPrints ID: 370302
URI: http://eprints.soton.ac.uk/id/eprint/370302
ISSN: 0016-8025
PURE UUID: 78184600-442e-49fe-9d94-ccd18c488b0f

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Date deposited: 21 Oct 2014 13:49
Last modified: 15 Jul 2019 21:41

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