Investigation of collar properties on data-acquisition scheme for acoustic logging-while-drilling

Abstract

We have used the wavenumber integration, velocity-time semblance, and dispersion methods to investigate the influence of collar properties including velocities, density, and attenuation on acoustic logging-while-drilling wavefields. We have found that when the velocities of the collar wave and the P-wave of the formation are similar, they interfere. However, the interference disappears when the velocity difference increases. Having a collar with large velocities (especial large shear velocity) and density makes the direct P-velocity determination possible in a fast formation even without isolators. For a slow formation, the interference of the collar flexural wave with the formation flexural and leaky P-waves is slight for a dipole tool when collar velocities are large. For this case, the S velocity can be determined by the flexural formation wave at low frequency (approximately 2 kHz). Based on these observations, we propose that the measurement of the P-and S-velocities can be easier if the collar is made of an advanced composite material that has high compressional and shear velocities as well as density. This is a direct and easy change to implement and a new idea for an acoustic logging-while-drilling tool design.

Reliability of velocity measurements made by monopole acoustic logging-while-drilling tools in fast formations

“…Part of the ALWD wavefield propagates along the collar and has a strong influence on the formation wave measurements. Different collar waves are excited with different ALWD tools according to the characteristics of the collar wave in different cases (Wang et al, 2016). The collar extensional wave, which is excited when the monopole tool is used, has a weak dispersion with the velocity being 0.8-0.9 times the compressional velocity of the collar (see Figure 2c in Wang et al, 2016).…”

“…The collar flexural wave is a shear motion of the collar that is excited by a dipole source. The collar flexural wave has a strong dispersive behavior with velocity from 0 m∕s at 0 Hz to approximately 0.8 times the shear velocity of the collar material at high frequency (more than 10 kHz) (see Figure 12 in Wang et al, 2016).…”

“…However, the P-wave in Figure 1d is separated from the collar extensional wave in the borehole because of the large velocity gap between the collar extensional and P-wave. This suggests a possible approach for eliminating the interference of the collar extensional wave on the P-wave velocity measurement when using monopole tools in fast formations: using a collar made of a material with large velocity rather than adding an isolator on the collar (Wang et al, 2016). Our choice of formation F2 was made to allow us to investigate the effects of tool eccentering on P-wave measurements when an isolator is not present in the ALWD tool.…”

Understanding acoustic methods for cement bond logging

“…The S 0 , S 1 , S 2 correspond to the first, second, third orders of the collar extensional modes in a monopole ALWD model as Fig. 4 in Wang et al (2015). The antisymmetric (A) modes exhibit the feature as different collar shear modes in a multi-pole ALWD model with the dipole or quadrupole tools.…”

“…We compare the dispersion curves of different modes of Figs. 5(g) and 5(h) with collar modes in an acoustic logging-while-drilling (ALWD) model because the thickness of the plate is almost similar to the drill collar (Wang et al, 2015). The symmetric (S) modes are the extensional modes according to the definition and the description in Fig.…”