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

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).

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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