Fracture Detection by Circumferential Propagation of Acoustic Energy

Abstract

ABSTRACT An acoustic logging tool that propagates waves circumferentially around the borehole wall is described. This tool, The Circumferential Acoustic Device (CAD), shows excellent promise for the detection of vertically-inclined fractures, for the measurement of shear wave velocity, and for the resolution of thinly interbedded zones. The CAD is a four-arm pad tool. Appended to one pair of opposite arms are cylindrical PZT transducers that act as transmitters. Similarly, the other pair of arms have transducers appended that act as receivers. Acoustic energy travels circumferentially between transmitter and receiver as compressional mud waves and as refractively-coupled formation waves. The formation waves are identified as p-, s-, and pseudo-Rayleigh (pR) waves. Absorbing material is used to preferentially attenuate the mud waves and thereby enhance the formation wave content of the received acoustic wavetrain. We have used the CAD to log open-hole wells in the Williston Basin. The zones logged are primarily tight limestone, have no mudcake, and have a p-wave velocity of around 18,000 ft/sec. The CAD is found to delineate fractured from non-fractured zones through the attenuation of the pR-waves. These results are verified by comparing CAD well-logs to core. The CAD is potentially useful for making s and pR-wave velocity measurements, and for sorting out the elastic moduli of strata describable as transversely isotropic. In a transversely isotropic medium the sand pR-wave velocities as measured from CAD logs depend on the C55 elastic constant. Thus the CAD gives unique elasticity information since the more conventional acoustic tools would measure s-wave velocities that depend on C44.