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

Authors

  • Philip Tillotson,

    Corresponding author
    1. National Oceanography Centre, University of Southampton, Waterfront Campus, European Way, Southampton SO14 3ZH, UK
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  • Mark Chapman,

    1. Edinburgh Anisotropy Project, British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK
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    • Now at: School of Geosciences, The University of Edinburgh, Grant Institute, The King's Buildings, West Mains Road, Edinburgh, EH9 3JW, UK

  • Angus Ian Best,

    1. National Oceanography Centre, University of Southampton, Waterfront Campus, European Way, Southampton SO14 3ZH, UK
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  • Jeremy Sothcott,

    1. National Oceanography Centre, University of Southampton, Waterfront Campus, European Way, Southampton SO14 3ZH, UK
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  • Clive McCann,

    1. National Oceanography Centre, University of Southampton, Waterfront Campus, European Way, Southampton SO14 3ZH, UK
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  • Wang Shangxu,

    1. Key Laboratory of Geophysical Prospecting, CNPC, University of Petroleum, 102249 Beijing, China
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  • Xiang-Yang Li

    1. Edinburgh Anisotropy Project, British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK
    2. School of Geosciences, The University of Edinburgh, Grant Institute, The King's Buildings, West Mains Road, Edinburgh, EH9 3JW, UK
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  • This paper is based on extended abstract Z020 presented at the 71st EAGE Conference & Exhibition Incorporating SPE EUROPEC 2009, 8–11 June 2009 in Amsterdam, the Netherlands.

E-mail: philip.tillotson@noc.soton.ac.uk

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. Shear-wave 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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