With the increasing drilling scale of oil and gas fields and the development of science and technology, especially the rapid development of a LWD technology, it is urgent to make the present advanced science and technology play an important role in the development of the oil and gas fields. An azimuthally acoustic LWD technology is one of the LWD technology. Acoustic LWD enables acoustic logging while drilling, which can effectively detect lithological characters, physical properties and reservoir parameters of a wellbore wall formation with respect to wireline logging, wherein data obtained by the LWD is slightly influenced by invasion of a drilling fluid.
A compressional wave velocity and a shear wave velocity of a formation are obtained by an acoustic LWD instrument, and can be used to establish a pore pressure gradient and a permeability, evaluate a wellbore stability, interpret lithological changes, and detect a flow effect of a fluid in a wellbore, thereby providing important geosteering information for a drilling operation. However, since acoustic LWD is affected by a drilling noise, circulation of a drilling fluid and drill collar waves, and due to the particularity of a drilling working environment, the realization of the acoustic LWD is much more complicated than that of a wireline logging technology.
How to effectively achieve function matching of a transmitting acoustic system and improve transmitted energy, how to improve the receiving sensitivity of receiving transducers and improve a signal-to-noise ratio of signal reception and developing a sound insulation technology for drill collar direct waves, which is effective for monopole, dipole, quadrupole and polarized pole, become the focus of research and development of the acoustic LWD instrument.
In the prior art, the acoustic LWD instrument typically employs a monopole operation mode, a dipole operation mode, and a monopole, dipole and quadrupole operation mode.
However, in the prior art, shear wave information cannot be obtained at a soft bottom layer by adopting monopole logging, and Stoneley wave information of a formation cannot be obtained. In the prior art, when the dipole operation model is adopted, a manner of inner notch grooves and outer notch grooves cannot effectively realize the propagation of bending waves in the drill collar waves, thereby causing overlapping of dipole acoustic waves with the drill collar waves. A signal to noise ratio of the received signal is extremely low, resulting in inaccurate measurements. Moreover, an acoustic dipole instrument is widely applied in the wireline logging. In terms of logging while drilling, the dipole acoustic waves are the bending waves, which easily overlap with the drill collar waves, causing interference to the dipole acoustic waves, so that information about the formation cannot be accurately measured. For the dipole acoustic waves, how to achieve effective sound insulation for the drill collar waves is a key to a design of the instrument. CN 104806234 A relates to an acoustic quadrupole LWD instrument, which employs ceramic tile stacked transmitting transducers and receiving transducers, and the transmitting transducers and the receiving transducers are both cylindrical and are uniformly distributed on a drill collar at intervals of 90 degrees. In terms of a sound insulator, sound insulation on the drill collar is performed by adopting the manner of the inner notch grooves and the outer notch grooves, and the LWD apparatus has monopole, dipole and quadrupole operation modes. However, since monopole acoustic waves are high-frequency signals and quadrupole acoustic waves are low-frequency signals, acoustic signals are excited by employing the same set of transducers. Due to a natural frequency property of the transducers, there is a signal with weak energy inherently. It is difficult for the receiving transducers to receive useful acoustic signals.