| Abstract | An idealized logging tool is a rigid cylinder centered within a cylindrical borehole. In practice, even when mechanical centralizers are used, the central axis of the borehole may not perfectly coincide with central axis of the tool. In the simplest case, the two axes are parallel, but the tool axis is offset from the borehole axis. The tool is eccentered with respect to the borehole and a single eccentering vector defines the azimuth and magnitude of the eccentering. More generally, the tool might be both tilted and eccentered with respect to the borehole, requiring a description in terms of a pair of eccentering vectors or some equivalent description.
In particular, during Logging-While-Drilling (LWD) operations, complex drill string movements and the weight of drill pipe often lead to a measurements being made by an eccentered tool. Therefore, studies on the response of an eccentered 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. In this study, we use a 3D finite difference method (FDM) to simulate the responses of a non-centralized multi-pole acoustic LWD tool in various borehole environments. For monopole tools at high frequency (10 kHz) in both fast and slow formations, we analyze quantitatively the effects of tool eccentering on the waveforms from receivers at different azimuths with respect to the eccentering azimuth. For dipole and quadrupole tools, we have studied the response of an eccentered tool with different eccentering azimuths and eccentering magnitudes at low frequency (2 kHz) in a slow formation. We have found from these simulations that the waveforms in the direction of tool offset, i.e. where the fluid column is smallest, are affected most. Waveforms in the orthogonal direction are less affected by tool eccentering. Collar flexural and collar quadrupole modes appear when the tool is eccentered. In addition, modes such as formation flexural and quadrupole modes contaminate the Stoneley wave. Waveforms in a fast formation are more strongly affected by the tool eccentering 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 eccentering while there is less distortion in the orthogonal direction. Stoneley mode could appear in dipole measurements; the flexural collar wave could become increasingly stronger with an increase of tool eccentering, and the Stoneley mode may be the later arrival event, especially in the case of a severely eccentered quadrupole tool. Due to the significant changes in waveforms with azimuth when the tool is eccentered, data processing methods based on presumed axisymmetry will not result in a clean waveform that is sensitive to only the surrounding formation.
Based on these studies, we introduce a method to quantify the extent and the angle of the tool eccentering with the phase difference in eccentered dipole measurements. These parameters are very useful for the analysis of the bottom-hole assembly (BHA) performance in geo-steering. A data correction method for data acquired by a multipole acoustic Logging-While-Drilling tool in horizontal and highly deviated wells is anticipated. |
