f Application of Shear Wave Anisotropy for Natural Fracture Detection in a Cased Well (Best of AAPG)

Fracture detection in an open borehole has become a routine process through the use of resistivity imaging. However, once the well has been cased, this technique is no longer viable and fracture detection becomes difficult. Certain acoustic measurements can still be utilized, even with the presence of cement and casing. Shear waves transmission is known to become anisotropic when fractures alter the intrinsic nature of the rock, affecting the amplitude and velocity of the waves that cross them. In addition, dispersion analysis, comparing shear slowness to frequency response, shows a unique signature when this intrinsic anisotropy exists. New acquisition hardware, software, and interpretation techniques have been developed to observe this occurrence in cased wells. On a recent cased well, in which openhole data were limited, these techniques were used to identify fractured zones. Waveform, dispersion, and spatial analysis were performed on the oriented shear sonic data and then combined with knowledge of the geologic model to select perforation intervals to maximize production from the fractured zones.