Velocity anisotropy and attenuation in shale in under and over pressured conditions

Domnesteanu, P.R.; McCann, C.; Sothcott, J.. 1999 Velocity anisotropy and attenuation in shale in under and over pressured conditions. Geophysical Prospecting, 47 (3). 359-392.

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Ultrasonic compressional- and shear-wave attenuation measurements have been made on 40, centimetre-sized samples of water- and oil-saturated oolitic limestones at 50 MPa effective hydrostatic pressure (confining pressure minus pore-fluid pressure) at frequencies of about 0.85 MHz and 0.7 MHz respectively, using the pulse-echo method. The mineralogy, porosity, permeability and the distribution of the pore types of each sample were determined using a combination of optical and scanning electron microscopy, a helium porosimeter and a nitrogen permeameter. The limestones contain a complex porosity system consisting of interparticle macropores (dimensions up to 300 microns) and micropores (dimensions 5–10 microns) within the ooids, the calcite cement and the mud matrix. Ultrasonic attenuation reaches a maximum value in those limestones in which the dual porosity system is most fully developed, indicating that the squirt-flow mechanism, which has previously been shown to occur in shaley sandstones, also operates in the limestones. It is argued that the larger-scale dual porosity systems present in limestones in situ could similarly cause seismic attenuation at the frequencies of field seismic surveys through the operation of the squirt-flow mechanism.

Item Type: Publication - Article
Digital Object Identifier (DOI):
ISSN: 0016-8025
Date made live: 13 Aug 2008 +0 (UTC)

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