Abstract

A sonic measurements-while-drilling tool consists of acoustic transmitters and receivers mounted on a drill collar that has the form of a thick steel pipe. This mandrel is designed to meet the strength requirements of tough logging conditions encountered in deviated and horizontal drilling. A primary goal of sonic measurement is to estimate formation compressional and shear slownesses that are not affected by the presence of a drill collar. Understanding the basic physics of monopole, flexural, and quadrupole modes in a cylindrically layered structure consisting of a concentric steel pipe in a water-filled borehole can help in estimating formation compressional and shear slownesses from at least one branch, either of the monopole, dipole, or quadrupole family of modes. Results show that coupling between the pipe and formation modes is strongly dependent on formation properties relative to the borehole-fluid compressional slowness. Thepresence of a steel pipe tool in a borehole causes a significant increase in the Stoneley dispersion at low frequencies. However, the Stoneley slownesses at high frequencies remain essentially the same as those in the open hole without a mandrel. The leaky compressional dispersion in slow formations produced by a monopole source also is perturbed at frequencies where the open-hole leaky compressional and Stoneley-wave dispersions are close to each other. Although there is a strong interference between the pipe and formation flexural modes in fast formations, such interference in slow formations occurs at very low frequencies, and the formation flexural dispersion is less affected by the presence of a drill collar at higher frequencies. Interestingly, the presence of a drill collar in a borehole causes rather small changes in the lowest-order quadrupole dispersion from the case of an open hole without a mandrel. Consequently, quadrupole logging is a preferred choice for estimating the formation shear slowness.

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