Near‐wellbore characterization using radial profiles of shear slownesses

Near‐wellbore alteration can be caused by several sources such as overpressure drilling, borehole stress concentrations, mechanical damage, shale swelling, and super‐charging of permeable formations. The radial heterogeneity can be expressed in terms of variations in the compressional and shear slownesses as a function of radial position. This paper describes results from a forward model for computing synthetic waveforms at an array of receivers produced by a dipole source placed on the borehole axis. These waveforms are then processed by a modified matrix pencil algorithm for isolating both nondispersive and dispersive arrivals in the wavetrain. The Backus‐Gilbert (B‐G) technique is then used for the inversion of flexural dispersions over bandwidths ranging from about 1 to 8 kHz for estimating radial variations in formation shear slownesses. The radial variation in the inverted shear slowness is essentially uniform in the case of a homogeneous formation and agrees remarkably well with the assumed radial profile of the shear slowness in the case of radially heterogeneous formation. The inversion algorithm can handle both radially increasing and radially decreasing shear slownesses away from the borehole surface. Agreement has been obtained to within 2% between the inverted shear slownesses and assumed shear profiles in generating the synthetic data. This technique has thus been validated against synthetic data and can have a variety of applications for probing materials with varying properties using wideband dispersive signals.