Shear-Velocity Measurement In The Logging-While- Drilling Environment: Modeling And Field Evaluations

In the logging while drilling (LWD) environment, the presence of the drill collar substantially influences the acoustic wave propagation characteristics. Measurement principles that are proven in wireline logging may suffer drawbacks or become invalid. Therefore, new LWD measurement approaches need to be investigated. This paper evaluates the LWD shear-wave velocity measurement using low-frequency monopole, dipole, and quadrupole acoustic sources. Theoretical analyses show that the LWD monopole and dipole acoustic waves contain strong tool waves traveling along the collar. Further, the LWD dipole-flexural wave, unlike its wireline counterpart, has a velocity that is significantly lower than the shear velocity of a slow formation. In comparison, the quadrupole wave is a better measurement candidate for two reasons. First, the tool quadrupole-wave contamination can be avoided by operating at low frequencies. Second, the formation quadrupole wave velocity is close to shear-wave velocity at low frequencies and can thus be used to measure it. A multipole LWD array acoustic tool has been constructed for the shear measurement. Several field measurements have been made in various fast and slow formations. In a frequency range of 2-10 kHz, the monopole acoustic data consist primarily of a collar (extensional) wave and Stoneley waves. The dipole data consist of both collar and formation flexural waves. The quadrupole data are least contaminated by the collar wave and comprise mostly the formation quadrupole wave. The characteristics of the measured multipole-wave data are found to be in good agreement with the modeling. Theoretical analysis and field evaluations demonstrate that the quadrupole acoustic measurement is a promising technology for determining an LWD shear-wave velocity.