In offshore petroleum exploration, due to increasing demands of logging in highly deviated and horizontal drilling wells, acoustic logging while drilling has wide applications. Bidirectional sandwich piezoelectric transducer is a good option for the acoustic logging tool with advantages of simple structure, anti-vibration, easy implementation and high electro-acoustic power.
Recently, researchers from the State Key Laboratory of Acoustics of the Institute of Acoustics of the Chinese Academy of Sciences have optimized the bidirectional sandwich piezoelectric transducer, by taking account of drilling environments with high temperature and pressure, as well as strong collar drilling vibration during the drilling process.
In the research, the optimized structural scheme of bidirectional sandwich piezoelectric transducer is shown in Fig. 1. The transducer consists of a metal mass in the middle, two sets of piezoelectric stacks and two radiation masses. Each piezoelectric stack is formed by 𝑛 pieces of piezoelectric wafers with thickness of 𝑙p, and the total length of each piezoelectric stack is 𝐿p. Here 𝑅1 and 𝑆1 represent the radius and area of middle mass and piezoelectric ceramics, 𝑅2 and 𝑆2 represent the radius and area of radiation mass, and 𝑙h and 𝑙0 represent the length of radiation and middle mass, respectively.
Aimed to improve the transmitting performance, numerical and experimental researches for the transducer optimization are conducted. The impact of location and length of the piezoelectric stack on resonance characteristics and effective electromechanical coupling coefficient are calculated and analyzed. Admittance and transmitting performance of the proposed transducer are measured in laboratory experiments, and the results are compared with simulated ones.
Research shows that the newly proposed transducer has higher transmitting performance with lower resonance frequencies. By numerical analyses of influencing factors on transmitting performance, it is pointed out that higher performance of the transducer is obtained with suitable length of piezoelectric stack located near the center of the bar. And the optimized transducer (Model 2) behaves at three times the amplitude of the original transducer (Model 1) given by (CONG J S et al 2011 China Patent ZL201120157165.1), which is shown in Fig. 2.
This research provides theoretical and experimental bases for transducer designing and acoustic wave measurements in acoustic logging, especially in acoustic logging while drilling.
Fig. 1 Structural schematic of the bidirectional sandwich piezoelectric transducer (Image by WEI Qian et al.)
Fig. 2 Model borehole experimental results obtained for two transducers: (a) received waveforms and (b) frequency responses (Image by WEI Qian et al.)
Reference:
WEI Qian, WANG Xiuming,CHE Chengxuan, CONG Jiansheng. Optimization Design for the Sandwich Piezoelectric Transmitters in Acoustic Logging. Chinese Physics Letters (Vol. 33, No.7, July 2016). DOI: 10.1088/0256-307X/33/7/074301
Contact:
WEI Qian
Institute of Acoustics, Chinese Academy of Sciences, 100190 Beijing, China
Email: weiqian@mail.ioa.ac.cn
