Numerical Simulation And Experiments On Isolator Of Logging-While-Drilling (LWD) Acoustic Tool

Without an isolator, the acoustic wave propagating along the tool collar will make it difficult to measure formation acoustic waves in the acoustic LWD environment. The design of LWD acoustic isolator is crucial to LWD tool development. It must satisfy the drilling intensity and should be effective to attenuate the jamming of the direct collar wave.By using the borehole acoustic wavefield theory, the responses of the LWD monopole, dipole, and quadruple waves are simulated by a finite-element waveform modeling software module called ANSYS, and the acoustic interference of the LWD tool-collar wave with the formation waves is analyzed systematically. It is proved theoretically that eliminating or suppressing the collar wave is the only way in measuring the formation P-wave. Furthermore, the inherent rejection band of the tool-collar wave is found by analyzing the wave's propagation characteristics, and the relationship between the center frequency and band width to the collar size is established. The results show that grooving or cutting slots on the dill collar can expand the inherent rejection band.Taking the mechanical strength into consideration, a new type of isolator is designed and its capability in isolating is simulated by ANSYS. The results show that it has more than 20dB sound isolation capability. An experimental isolator model was designed and built. A laboratory acoustic experiment was done to measure and characterize the isolator's tool-wave inherent rejection band by sweeping the acoustic source frequency in the frequency band of interest. The experiment has confirmed that the isolator can obtain more than 20 dB acoustic isolation for the tool wave.