| Acoustic logging while drilling(LWD)is an important method and technology for measuring P and S wave velocities of formation while drilling.It is very important in the process of efficient exploration and development of oil and gas resources.Monopole P-wave LWD acoustic technology has been developed for 30 years,and multipole S-wave LWD technology has been developed for 20 years.These technologies are enough mature abroad,however,there is no relative research or apply about monopole LWD acoustic technology at home,and the development of tool is extremely difficult.Comparing with wireline acoustic logging,the LWD measurement faces tough challenges as follows: low-frequency drilling interferences,drill collar noise contamination,the limited real-time data transmission speed,confined space room for the electronic circuits,and system become more complicate.To get LWD acoustic logging signals of high signal to noise ratio,follow researches should be conducted systematically:acoustic numerical simulation to get the optimal working frequency,frequency-controllable high efficiency excitation source to suppress drill collar wave,active sensor integration for receiving transducer and electronic system for multi-channel parallel data processing,efficiency underground power management,high-speed data processing and electronic system for data storage.Therefore,aiming at problems that LWD is facing and basing on the research situation of LWD acoustic tools,the feasibility study of using dipole source in remote detection of LWD acoustic logging is adopted.Aiming at the problems faced by acoustic logging while drilling(LWD)and the status of domestic LWD tools development,this paper discusses the feasibility of using dipole acoustic source for LWD remote detection,and carries out the key technology research of high-power acoustic source excitation in controlled frequency band,digital active reception of near transducer,fast processing and storage of downhole data,and low power consumption power management.Based on these,a prototype of multi-pole acoustic logging tool while drilling is developed and well group experiments and field drilling are carried out.The results show that the tool has obtained high quality data,and that proves the tool is stable and reliable.The following research results and innovations have been achieved:(1)Researches on dipole sources of acoustic logging while drilling: to certify the applicability of dipole source in acoustic logging while drilling,basing on the displacement farfield asymptotic solution of SH and SV shear waves in infinite formation,the outside well radiated sound fields are contrasted in the situations of have and not have drill collar.Also,the reflection imaging logging technology of dipole source acoustic LWD is discussed,and the advantage exciting wave bands of dipole source are found,which should be adopted in the design of logging tools.Thereafter,the multipole LWD tools should be extended from monopole and quadrupole combination to the possible application of monopole,dipole and quadrupole combination.(2)Frequency conversion and high-power excitation methods and technologies for multipole source: rigorous demands are proposed while multipole source is adopted in acoustic logging while drilling.Focused on the core of the technologies of digital signal processor and full bridge drive,and based on the method of sine pulse width modulation(SPWM),the problems of impulse response based on the condition of high frequency and high voltage,impedance matching and heavy-tailed noise suppressing have been resolved.Therefore,frequency conversion,bandwidth controllable and powerful resources are realized,thus,the multipole excitation mode and drilling interference suppressing are achieved in the process of transducer excitation.(3)Researches of active receiving transducer,whose transducer and data collection circuit are integrated: the analog signal transmission along the long distance between the acoustic receiving transducer and the signal collection circuits caused many problems,such as the lack of anti-crosstalk ability,the huge amount of parallel signal cables and the massive volume.Based on the active transducer design principle,a data collection method is proposed for 32 channel synchronous parallel data collection.The low noise signal processing in small spaces,synchronous parallel control and the suppressing of random noise are realized,thereafter,active transducer collection technology is realized,and signal crosstalk and random noise can be avoided in the process of data collection,and also the design complexity of the system is reduced too.(4)Researches about methods for underground data processing and storage for low power consumption management: the communication rate of LWD telemetry restricts the real time uploading of underground logging data,causing the formation acoustic interval transit time cannot be calculated in real time,and also,the power consumption is restricted by the underground power supply.Focusing on the core of high-speed digital signal processor(DSP)and NAND Flash storage,the time division power supply and fast time-slowness correlation methods are realized to calculate formation wave slowness and realize underground storage management.The mass storage of acoustic wave data and the real time calculation of the formation acoustic interval transit time can be obtained.Thereafter,the LWD acoustic interval data can be acquired in the process of data transmission in real time.(5)The accuracy and reliability are certified by tool testing in indoor wells and outdoor field experimental verification.The exactitude and stability are proved by the spot measurement of indoor soft formation,hard formation and anisotropy formation.Testing by trip and real drilling testing in calibration well and field test wells are induced.Under the interference of drilling noise and collar mode waves,field data of the equivalent quality as the cable acoustic logging are obtained,which verifies the applicability and reliability of the tool. |
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