Acoustic Character Of Hydraulic Fractures In Granite

Abstract

Hydraulic fractures in homogeneous granitic rocks were logged with conventional acoustic-transit-time, acoustic-waveform, and acoustic-televiewer logging systems. Fractured intervals ranged in depth from 45 to 570m. and logs were made both before and after the fractures were induced. Unambiguous identification of almost all induced fractures was made on the three types of logs, with depth alignments verified through identification of nearby natural fractures. Most televiewer images of the induced fractures indicate the presence of discontinuous hairline fractures parallel to or splaying from the main fractures. Induced fractures also were similar in orientation to nearby natural fractures. All but the two most steeply dipping hydraulic fractures produced small apparent decreases in compressional velocity. Waveforms that propagated across hydraulic fractures at depths greater than 100 m had negligible shear anomalies and tube-wave attenuation. These results indicate that the hydraulic fractures at these depths were slightly open within a few centimeters or less of the borehole wall, and did not produce a measurable increase in permeability. Two induced fractures at depths of less than 100 m had more pronounced shear anomalies, indicating that these fractures may not have fully reclosed upon pressure release at the end of the fracture experiment.