In acoustic logging while drilling (ALWD), it is crucial to have an ALWD isolator to reduce collar waves and to meet the requirement of wideband acoustic measurements. In this work, we proposed a new acoustic isolator based on phononic crystal structures for reducing collar waves, and its acoustic insulation performance is numerically studied through the finite difference simulation. For the typical ALWD tool, the optimized acoustic isolator has eight-section graded grooves and each contains 11 small grooves varying from 3 to 5 mm, with an interval of 0.2 m between adjacent sections. Performances of the isolators we designed are verified by numerical results. The attenuation effects of the improved isolator and the traditional one on collar arrivals are compared. The comparison of synthetic waveforms reveals that the newly proposed design with grooves characterized by gradually varying width displays better acoustic isolation performance than the traditional isolator with uniform grooves when operated with various source central frequencies from 13 to 15 kHz. By applying the proposed isolator, the formation longitudinal wave velocities can be separated effectively from the wave group containing the collar waves in the fast formation model. These studies may be useful for the ALWD tool design and data processing.
Acoustic logging while drilling (LWD) can extract P-wave and S-wave information from the formation. However, the transmission of the collar wave propagated directly from the emitter to the receiver may interfere with the P-wave and S-wave and affect the extraction of formation information. Therefore, it is necessary to design a suitable acoustic isolator between the transmitter and the receiver to attenuate the drill waves. The commonly used acoustic LWD isolator is that the outer surface of the drill collar is evenly grooved to attenuate the collar wave. However, there are still disadvantages such as the lack of mechanical strength of the evenly grooved acoustic insulators and the ability to extract clean longitudinal wave under certain circumstances. Therefore, there is an urgent requirement to design a new type of acoustic LWD isolator with sufficient strength and acoustic insulation requirements. In recent years, spoof surface acoustic waves (SSAWs) generated by periodic corrugated surface rigid plates have attracted the attention of many researchers, who can spatially separate the surface waves to attenuate acoustic waves. In this paper, a new type of acoustic LWD insulator based on SAW space separation structure is proposed. The finite element software ANSYS is used for acoustic analysis.
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