Investigation of the high-frequency wavefield of an off-center monopole acoustic logging-while-drilling tool

Abstract

During logging-while-drilling (LWD) operations, complex drill string movements and the weight of the drill pipe often lead to a measurement tool that is not centralized. Therefore, studies of the response of an off-center acoustic LWD tool are essential to facilitate better interpretation of measurements made in an actual drilling environment. Such studies will be helpful for tool design and data processing. We used a finite-difference method to simulate the response of a noncentralized monopole acoustic LWD tool at high frequency (10 kHz). We analyzed the effects on the waveforms for receivers at different azimuths caused by an off-center tool with differing amounts of offset. We used velocity-time semblance and dispersion analysis methods to help us to understand the modes in the waveforms at different azimuth receivers for different tool offsets. We have found that the waveforms in the direction of the tool offset, that is, where the fluid column is smallest, were affected the most. Waveforms in the orthogonal direction were less affected by tool offset. Collar flexural and collar quadrupole modes appear when the tool is off center. In addition, the formation flexural and quadrupole modes contaminate the Stoneley wave. Waveforms in a fast formation are more strongly affected by the offset of the tool than those in a slow formation. In a fast formation, the new collar modes make it difficult to determine the P-wave velocity in the direction of tool offset whereas it is easier in the orthogonal direction. However, P-waves are less contaminated by new modes in a slow formation. Due to the significant changes in waveforms with azimuth when the tool is off center, the simple addition of all waveforms from an azimuthal distribution of receivers will not result in a clean waveform that is sensitive to only the surrounding formation.

A method to determine the strike of interface outside of borehole by monopole borehole acoustic reflections

“…The staggered grid will be used in the code with second-order accuracy in both space and time domain (Wang et al, 2015). More details about the 3DFD method can be found in Cheng (1994) and Tao et al (2008).…”

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

“…For the simulation, multiple point sources will be set on the outer collar to approximate a ring source (Chen et al, 2010). Wang and Tao (2011) and Wang et al (2015) show how to get the dispersion curves for different modes in the borehole from the wavenumber integration formulation by using a root-finding Newton-Raphson mode-search routine (Tang and Cheng, 2004).…”

“…Drill collars occupy most of the space in a fluid-filled borehole and divide the borehole fluid into two thin-fluid columns (Byun and Toksöz, 2003;Wang et al, 2009b;Wang and Tao, 2011), which makes the wavefield in acoustic LWD (ALWD) different from that in acoustic wireline logging. The collar makes it more difficult to identify the modes that are sensitive to formation (Wang et al, 2013a(Wang et al, , 2015. It is difficult to identify the P-wave in a fast formation because of the interference between it and the collar wave.…”

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

“…Previous studies of eccentered ALWD tools have mainly focused on the responses of dipole and quadrupole LWD tools (Huang 2003;Tang et al, 2009). Huang (2003) and Wang et al (2013aWang et al ( , 2015 have studied the response of an off-center monopole tool. Pardo et al (2013) use a finite-element method to study the responses of eccentered monopole tools in the fast and slow formations for a few ALWD scenarios.…”

“…In our previous study (Wang et al, 2015), we investigate the response of an eccentered monopole tool in a fast formation. However, the effect of the tool eccentering on velocity measurement has not yet been quantified.…”