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Observations of fluid-dependent shear-wave splitting in synthetic porous rocks with aligned penny-shaped fractures

Observations of fluid-dependent shear-wave splitting in synthetic porous rocks with aligned penny-shaped fractures
Observations of fluid-dependent shear-wave splitting in synthetic porous rocks with aligned penny-shaped fractures
P- and S-wave velocity and attenuation coefficients (accurate to ±0.3% and ±0.2 dB/cm, respectively) were measured in synthetic porous rocks with aligned, penny-shaped fractures using the laboratory ultrasonic pulse-echo method. Shearwave splitting was observed by rotating the S-wave transducer and noting the maximum and minimum velocities relative to the fracture direction. A block of synthetic porous rock of fracture density 0.0201 ± 0.0068 and fracture size 3.6 ± 0.38 mm (measured from image analysis of X-ray CT scans) was sub-sampled into three 20–30 mm long, 50 mm diameter core plugs oriented at 0?, 45? and 90? to the fracture normal (transversely isotropic symmetry axis). Full waveform data were collected over the frequency range 500–1000 kHz for both water and glycerin saturated cores to observe the effect of pore fluid viscosity at 1 cP and 100 cP, respectively. The shear-wave splitting observed in the 90? core was 2.15 ± 0.02% for water saturated and 2.39 ± 0.02% for glycerin saturated, in agreement with the theory that suggests that the percentage splitting should be 100 times the fracture density and independent of the saturating fluid. In the 45? core, by contrast, splitting was 0.00 ± 0.02% for water saturation and ?0.77 ± 0.02% for glycerin saturation. This dependence on fracture orientation and pore fluid viscosity is consistent with the poro-visco-elastic theory for aligned, meso-scale fractures in porous rocks. The results suggest the possible use of shear- or converted-wave data to discriminate between fluids on the basis of viscosity variations.
0016-8025
111-119
Tillotson, Philip
b8c84738-5ad2-4360-a7a1-b83996fa7188
Chapman, Mark
6f13eb72-ad0e-4c7a-b60c-92387a707cf4
Best, Angus Ian
cad03726-10f8-4f90-a3ba-5031665234c9
Sothcott, Jeremy
71ab4088-7b13-46d6-9e28-67538a02d595
McCann, Clive
afadcf9b-2751-434a-99a9-6fdbb4d56575
Shangxu, Wang
f0d2274a-88d1-46a3-a3e5-15a0482fe7c6
Li, Xiang-Yang
b902023b-aa5b-477b-a97b-013173349ce1
Tillotson, Philip
b8c84738-5ad2-4360-a7a1-b83996fa7188
Chapman, Mark
6f13eb72-ad0e-4c7a-b60c-92387a707cf4
Best, Angus Ian
cad03726-10f8-4f90-a3ba-5031665234c9
Sothcott, Jeremy
71ab4088-7b13-46d6-9e28-67538a02d595
McCann, Clive
afadcf9b-2751-434a-99a9-6fdbb4d56575
Shangxu, Wang
f0d2274a-88d1-46a3-a3e5-15a0482fe7c6
Li, Xiang-Yang
b902023b-aa5b-477b-a97b-013173349ce1

Tillotson, Philip, Chapman, Mark, Best, Angus Ian, Sothcott, Jeremy, McCann, Clive, Shangxu, Wang and Li, Xiang-Yang (2011) Observations of fluid-dependent shear-wave splitting in synthetic porous rocks with aligned penny-shaped fractures. Geophysical Prospecting, 59 (1), 111-119. (doi:10.1111/j.1365-2478.2010.00903.x).

Record type: Article

Abstract

P- and S-wave velocity and attenuation coefficients (accurate to ±0.3% and ±0.2 dB/cm, respectively) were measured in synthetic porous rocks with aligned, penny-shaped fractures using the laboratory ultrasonic pulse-echo method. Shearwave splitting was observed by rotating the S-wave transducer and noting the maximum and minimum velocities relative to the fracture direction. A block of synthetic porous rock of fracture density 0.0201 ± 0.0068 and fracture size 3.6 ± 0.38 mm (measured from image analysis of X-ray CT scans) was sub-sampled into three 20–30 mm long, 50 mm diameter core plugs oriented at 0?, 45? and 90? to the fracture normal (transversely isotropic symmetry axis). Full waveform data were collected over the frequency range 500–1000 kHz for both water and glycerin saturated cores to observe the effect of pore fluid viscosity at 1 cP and 100 cP, respectively. The shear-wave splitting observed in the 90? core was 2.15 ± 0.02% for water saturated and 2.39 ± 0.02% for glycerin saturated, in agreement with the theory that suggests that the percentage splitting should be 100 times the fracture density and independent of the saturating fluid. In the 45? core, by contrast, splitting was 0.00 ± 0.02% for water saturation and ?0.77 ± 0.02% for glycerin saturation. This dependence on fracture orientation and pore fluid viscosity is consistent with the poro-visco-elastic theory for aligned, meso-scale fractures in porous rocks. The results suggest the possible use of shear- or converted-wave data to discriminate between fluids on the basis of viscosity variations.

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Published date: 2011
Organisations: Marine Geoscience

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Local EPrints ID: 170217
URI: http://eprints.soton.ac.uk/id/eprint/170217
ISSN: 0016-8025
PURE UUID: 66d76039-0a87-43bc-a5ee-66a68daa7988

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Date deposited: 04 Jan 2011 14:16
Last modified: 16 Jul 2019 23:48

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