Abstract

In this paper the effects of borehole stress concentrations on both dipole flexural waves and azimuthally resolved, axially propagating compressional waves (i.e., microsonic) are studied. Dipole sonic logging tools provide the capability to measure acoustic anisotropy in the borehole. However, one needs to distinguish between two forms of anisotropy: intrinsic or stress-induced. A method is presented, based on an analysis of dipole dispersion curves, which can make this distinction. Once stress-induced anisotropy is determined, then the fast shear direction is the maximum stress direction. In addition, an azimuthal microsonic measurement technique is used to measure the azimuthally resolved, axially propagating, refracted compressional wave which can identify both stress direction and detect mechanical damage (i.e., zone of plasticity). Laboratory borehole experiments are presented for both dipole and compressional measurements. The experiments compare favorably with the theory of acoustoelasticity. The maximum stress direction is important in hydraulic fracturing while detection of mechanical damage can be important for wellbore stability.

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