Azimuthally Focused LWD Sonic Logging for Shear Wave Anisotropy Measurement and Borehole Imaging

Abstract

Abstract An azimuthally focused LWD sonic tool has been developed to resolve shear wave anisotropy in fast formations, and provide borehole images of compressional and shear acoustic slowness. As the drillstring rotates, waveform measurements are recorded in 16 fixed-orientation azimuthal bins. The waveforms are processed to yield azimuthally oriented compressional and refracted shear slowness values. Most shale reservoirs exhibit significant intrinsic anisotropy. Knowledge of the acoustic velocities and anisotropy ratio is vital for properly computing formation mechanical properties and modeling hydraulic fracturing performance, as well as for seismic processing and interpretation. Yet VTI shear wave anisotropy cannot be resolved from dipole measurements in vertical wells, and such data are rarely available from horizontal wellbores. The LWD azimuthal sonic sensor has been successfully tested in horizontal shale reservoir wells in the northeastern U.S., where VTI shear wave anisotropy of 5 to 50 percent was clearly measured in the horizontal sections. Borehole images constructed from azimuthal shear slowness data provided a clear visual indication of the orientation and magnitude of the shear wave anisotropy, while compressional slowness borehole images compared well with conventional images from LWD azimuthal gamma ray and density tools, and can provide information on formation dip and the stratigraphic position of the horizontal wellbore. Azimuthally focused LWD sonic measurements of compressional and anisotropic shear wave slowness provide new and valuable information from horizontal shale wells, supplying geomechanical data necessary for more informed hydraulic stimulation programs, as well as providing geophysicists with key anisotropy parameters and complementing data available from vertical wells to provide a complete three-dimensional compressional and shear velocity model.