Keywords: Multi-morph bender bar; Fundamental frequency; Conductance; FEM

In the well logging industry, measuring formation shear velocity is desirable since it provides important information for exploration and production of oil and gas. Using a monopole acoustic toolthe shear velocity can be measured from the shear wave refracted along the borehole wall, if the formation shear wave velocity is greater than the borehole fluid acoustic velocity. However, in a slow formation where the shear velocity is less than borehole fluid velocity, it cannot be directly measured by monopole logging.

To solve this problem, dipole acoustic logging tools had been developed. At low frequency, the flexural wave travels at the formation shear velocity. The trilaminar bender (Fig. 1) bar is widely used as a dipole source in sonic well logging tool, but current bender bars have difficulties in producing high quality logs for large holes and soft formation applications.

Recently, the multi-morph bar (Fig. 2), a new type of flexural transducer, is proposed by researchers from the Institute of Acoustics of the Chinese Academy of Sciences for well logging. The multi-morph bar comprises an inert substrate having two pairs of piezoelectric elements attached to its two sides.

A comparison between the multi-morph bender bar and the trilaminar bender bar has been conducted, showing that the multi-morph bender bar has a lower fundamental frequency than the trilaminar bender bar while the radiation efficiency remains high (Fig. 3).

Numerous numerical simulations find out how the multi-morph bender bar behaves as its geometry size changes. With the first pair of piezoelectric elements increasing, the fundamental frequency will increase. Meanwhile, the conductance will decrease after reaching the peak when the first pair of piezoelectric elements reaches 90 percentages of the inert substrate. As the second pair of piezoelectric elements increases, or as the gap increases, both the fundamental frequency and conductance will decrease.

Further study will be focused on wide band transducer which is of great importance to the acoustic well logging. And the simulation model should include the fixing skeleton as close to the logging surroundings as possible.

Fig.1 Schematic diagram of the trilaminar bender bar (Image by WU)

Fig. 2 Schematic diagram of the multi-morph bender bar (Image by WU)

Fig.3 The input electrical admittance of bender bars (in air) (Image by WU)

References:

WU Delin, WANG Dong. Simulation of the Multi-morph Bender Transducer. Piezoelectricity, Acoustic Waves, and Device Applications (SPAWDA), 2014 Symposium on, ISBN 978-1-4799-6424-6, IEEE (pp. 162-165, Oct. 30 2014-Nov. 2, 2014, Beijing, China). DOI: 10.1109/SPAWDA.2014.6998551 

Contact：

WU Delin

Institute of Acoustics, Chinese Academy of Sciences, 100190 Beijing, China

Email:wdl19911218@126.com