Ultrasonic tomography and acoustic emission in hydraulically fractured Lac du Bonnet Grey granite

Abstract

Ultrasonic tomography and acoustic emission (AE) data were obtained during laboratory hydraulic fracturing tests on two large, unconfined cylinders of Lac du Bonnet grey granite. The cylinders were internally pressurized over four to five cycles prior to final failure. Compressional velocities were measured before and after each pressurization cycle with an array of 16 evenly spaced transducers around the central, cross‐sectional plane of each sample. Sixteen channels of whole waveform AE data were recorded during most pressurization cycles and for a period of about 1 hour after final failure in one sample. Compressional velocities were found to be strongly anisotropic, with the in situ vertical direction being the most rapid direction in both samples. The velocity anisotropy is related to the rock's preexisting microcrack fabric. Owing to radial penetration of fluid into the rock, compressional velocities rose over the course of the experiment. A regression analysis showed that the velocity changes can be explained by variations in crack density, inferred from initial velocities, and radial distance from the borehole. Saturation levels consistent with the observed velocity changes were calculated on the basis of the O'Connell and Budiansky theory. Acoustic emissions reoccurred in a few distinct zones over several pressurization cycles. The AE locations allowed two distinct fracture planes to be sharply delineated in one sample. The fracture plane orientations were controlled by the preexisting microcrack fabric in both samples. AE occurred too rapidly during peak pressure failure to permit us to isolate distinct events. Source mechanism analysis of the AE which occurred prior to peak pressure failure in both samples, and during postfailure monitoring in one sample, showed a predominance of double‐couple sources. Compressive sources, thought to be related to crushing of asperities during crack closure, and tensile sources, related to mode I crack growth, were also recorded, as well as more complex sources that could not be modeled by the simple source types listed above.