Accurate Formation Shear-wave Measurement While Drilling - Theoretical Modeling and Application to Field Data

Logging-while-drilling (LWD) shear-wave velocity measurement is mainly made with low-frequency quadrupole acoustic sources. The LWD quadrupole waves consist of a formation quadrupole wave propagated along borehole and a collar quadrupole wave propagated along collar, respectively. The collar quadrupole wave exists only above a certain frequency called the cut-off frequency. However, tool-wave contamination occurs if the collar-wave cut-off frequency is close to the frequency band of formation quadrupole shear-wave measurement, especially in slow formation. Both numerical simulation and field data analysis results show that the collar-wave cut-off frequency mainly depends on the thickness of the collar section where the receiver array is located. The cut-off frequency is neither sensitive to the formation property nor to the groove-isolator between source and receivers. Based on the sensitivity analysis of the LWD tool model, we produce a cut-off frequency and collar size template. The template can be used to aid the design of the quadrupole tool or to guide the data processing measured by existing tools, so as to obtain accurate formation shear velocity from the LWD quadrupole measurement.