Numerical simulation and field examples of critically refracted shear arrivals in a borehole in soft formations

Abstract

Summary Nowadays, dipole or cross-dipole tools are widely used t o obtain formation shear velocity or anisotropy, especially in soft formations (whose shear velocities are less than t he borehole fluid velocity.) The design of a dipole tool and its applications were initiated by the traditional inference th at no critically refracted shear arrivals (called shear head wave hereafter) existed in a borehole embedded in soft formation. However, in this paper we would demonstrate by numerical simulations and field examples that a low-frequency and localized acoustic source regardless of its order or monopole, dipole, etc. can excite shear head wave in soft formation. In general, for a monopole case, shear head wave in soft formation is of the same order as compressional head wave in amplitude. Nevertheless, shear head wave is usually interfered with the subsequent arriv als such as strong Stoneley arrival so that it is difficult to be observed visually. For a dipole case, the amplitude of shea r head wave in soft formation is about one-fifth of the flexural mode amplitude. It is particularly interested th at in a cased hole, the cutoff frequency of flexural mode moves toward to high frequency up to 8kHz when the formation shear wave velocity is near the borehole fluid wave velocity so that for a conventional dipole logging tool (low frequency band 0.5~5kHz), only shear head wave can be