ABSTRACT

Acoustic logging-while-drilling (LWD) is a technology that is used to measure formation elastic properties during drilling. When the formation shear slowness is smaller than the borehole fluid slowness (i.e., fast formation), monopole logging can be used to obtain formation compressional and shear slownesses by measuring the corresponding refracted waves. In a slow formation in which the shear slowness is larger than the borehole fluid slowness, other logging methods, such as quadrupole LWD, are used instead for shear slowness measurement due to the lack of a fully refracted S-wave. Through modeling analysis, we find that the transmitted S-wave generated by a monopole LWD tool in a slow formation can be detected and used to measure the formation shear slowness. This phenomenon can be explained by Huygens’ principle, which states that every point on a wavefront can be considered as a secondary source that induces particle motion. It is hard to discern the transmitted S-wave in monopole wireline data because it strongly interferes with the Stoneley mode in wireline logging. However, the transmitted S-wave decouples from the Stoneley in the LWD environment because the drill collar slows down the low-frequency part of the Stoneley mode. The nondispersive nature of the transmitted S-wave makes it suitable for shear slowness extraction using time semblance analysis, but sophisticated signal preprocessing might be needed because this wave is generally weak compared with the Stoneley wave. Moreover, our study helps us better understand how the Stoneley mode behaves and interferes with other modes in a slow formation under LWD conditions.

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