Change of Longitudinal Velocity in Rock around the Hydraulic Fracture Area

Abstract

We measured changes of longitudinal wave velocity caused by hydraulic fracturing in rock specimens. Three fracturing experiments were performed in cubical granite samples (coarse grained lnada granite) of 296 mm edge length under triaxial stress state generated by means of a twin-oil-jack sytem. The direction of fracturing plane was controlled by the stress state as well as natural weak planes in granite. The measurements of longitudinal velocity were made along several paths in the planes perpendicular to the bore hole before and after fracturing experiments, and along three paths perpendicular both to the maximum stress axis and the bore hole during an injection of pressurized water.
In these experiments, heavy differences of velocity that reach more than 20 % were observed between fractured area, which was created by injection of pressurized water, and unfractured area. The velocity differences also depend on the direction of fracture planes. When wave propagates parallel to the fracture plane, very high velocities were observed only in the vicinity of the fracture plane. However, high velocities were observed in the wide area when wave propagates perpendicular to the fracture plane. These results imply strong anisotorpies of velocity in fractured rock, and suggest that these anisotropies must be taken into account when we try to estimate extents of fractured area by the measurements of microseismic hypocenters.