A comparison of logging while drilling (LWD) and wireline acoustic measurements

Abstract

The instruments used to measure borehole acoustic data can be classified as either wireline or logging while drilling (LWD). The wireline tool measures formation speeds after the borehole is drilled, and the LWD tool measures formation speeds while the borehole is drilled. This thesis focuses on comparing the data collected by these tools and how formation properties affect their measurements. LWD and wireline measurements taken from the same borehole are compared. Discrepancies in estimated shear and compressional velocities, as calculated by time semblance methods, were found between the two data sets. We modeled radially layered formations with increasing or decreasing radial velocity profile to estimate the acoustic measurement penetration for each tool. We reprocessed sections of the data using frequency semblance methods and compared with layered model results. We found that a frequency-domain analysis is feasible and reduces the overall difference between the LWD and wireline shear and compressional velocity estimates. The remaining discrepancy can be explained by the different radial depths of penetration of these two tools, which naturally leads to a difference in the velocity estimates when there is a radial gradient in the velocity profile.

(cont.) We model axisymmetric propagation of waves in a borehole with a transversely isotropic (TI) formation. An algorithm is developed for an arbitrarily radially layered medium that can be used to approximate the steel LWD tool inside the fluid-filled borehole. We present a full description of modal arrivals, as a function of frequency and phase velocity, for the LWD tool and compare with the wireline case, both for isotropic and TI formation. The tool modes were found to be largely unaffected by the presence of a TI medium while the modes associated with the borehole fluid and formation, i.e. Stoneley, pseudo-Rayleigh and borehole flexural modes, displayed sensitivity to the TI formation parameters, specifically to C44 in the elastic stiffness matrix. Our analysis demonstrates that at a lower frequency of operation, the LWD tool can potentially measure the effect of a TI medium in the fundamental shear modes, if the modes are well-coupled to the formation (i.e., the formation is soft). This thesis makes two new contributions to the field of borehole geophysics. Firstly, we make an independent comparison of LWD and wireline measurements, and our work suggests that frequency-domain semblance processing may be a better method of analysis, particularly in radially varying formations.

(cont.) As industry moves toward faster and more cost efficient LWD measurements, it is essential to understand the implications of the LWD tool geometry in relation to its wireline counterpart and how traditional velocity processing methods are affected. Secondly, we present a modal analysis of the LWD tool in a transversely isotopic formation, which suggests that the newer generation of LWD tools, operating in a lower range of frequency may be able to measure TI formations. The TI medium is of major importance to exploration geophysics as it represents the anisotropy found in thinly layered media, i.e. sedimentary strata. This is the predominant form of anisotropy seen in many sedimentary basins, so that its effect on LWD measurements is of great interest to exploration geophysics.