Acoustic characterization of fracture permeability at Chalk River, Ontario

Abstract

This study was undertaken to test recently formulated acoustic-analysis methods for fracture interpretation. The study area was selected because surface outcrops of igneous and metamorphic rocks have numerous, interconnected fractures and major lithology changes. In-situ acoustic-refraction data were obtained by digitally recording the entire pressure signal received by a conventional acoustic borehole logging system. The acoustic energy source had a centerband frequency of 34 kHz, and data were obtained at 60 and 90 cm source/receiver spacing. Borehole geometry produces waveforms with strong shear arrivals and high amplitudes associated with the fundamental guided fluid mode known as the tube wave. Waveforms refracted across fractures that are open on the borehole have shear mode excitation and tube-wave attenuation effects similar to previously described effects for isolated fractures in very uniform lithologies. Independent permeability data for the Chalk River boreholes are available in the form of effective-fracture apertures determined by straddle packer isolation and injection tests. The best correlation between the permeabilities measured by packer tests and acoustic data is obtained by integrating the difference between local tube-wave amplitude and an average amplitude from many adjacent stations. This synthetic amplitude-deficit log shows close correlation with zones of large measured permeabilities; however, there are some quantitative differences. These are attributed to: (1) differing radii of investigation, (2) effects of fracture interconnectivity, and (3) drilling damage in highly weathered and fractured zones. The tube-wave-amplitude method also does not seem applicable to depths less than about 50 m where the tube-wave mode is relatively unexcited. Keywords: fracture permeability, borehole acoustics, fracture hydrology.