Research On Methods Of Signal Transmission And Detection In Periodic Drill Strings Acoustic Channel

With the rapid development of the social economy,the fossil energy such as the petroleum and natural gas has become an indispensable resource across all industries.In order to meet the increased demands of energy sources and solve the complex problems of the oil and gas explorations,automation and intellectualization logging technologies have become research priorities.As the core technology of the intellectualization logging,logging while drilling(LWD)can measure the down-hole formation rock properties and upload the log data quickly in real time,and then the formation evaluation results can guide the drill track and achieve the intelligent geosteering.The real-time and efficient transmission of down-hole data is a technological difficulty of LWD,and its research is significant in the development of LWD.At present,the data transmission rate for LWD is very low,which can only transmit a few information of drill bit working condition.However,a large amount of log data is stored in the down-hole circuit,and the log data can only be obtained after the completion of the drilling operation,which cannot meet the real-time demand for LWD.The acoustic transmission for LWD log data utilizes the acoustic wave as carrier,and the drill strings with periodic drill pipes and joints connected as channel,which can transmit the log data in real time.The acoustic transmission has advantages of high data rate,good applicability,low development cost,simple operation and so on.However,the multipath time delay of drill strings and the strong noise interferences from the down-hole bit and ground environment affect the transmission and detection of the acoustic wave seriously and limit the research and development of the LWD acoustic transmission technology.Based on this,this dissertation primarily solves the problems of the acoustic wave transmission and detection in the drill strings channel to realize the efficient transmission of acoustic wave.The main works and contributions of the dissertation are as follows:(1)On the basis of analyzing the wave principle and types of the elastic wave propagation in the infinite solid medium,three propagation characteristics which are longitudinal wave,torsional wave and the flexural wave,and the attenuation characteristics of the acoustic wave in pipe are researched,respectively,then based on the advantages of convenient receiving and analysis of the longitudinal wave,the longitudinal wave is chosen as acoustic transmission carrier.For the drill strings with periodic structure,its dispersion characteristics have been researched,and the channel models based on the finite difference method,the transfer matrix method,and the equivalent acoustic transmission method are built,respectively.In order to acquire the acoustic transmission performance of drill strings completely,the effects of the drill strings structure parameters such as the unit cascade number,the pipe length,the joint cross-sectional area and the non-uniform unit size,and the viscous coupling of drilling fluid are mainly analyzed.Meanwhile,the design requirements of LWD acoustic transmission system in oil field are analyzed,the test device of the acoustic wave transmission is designed and built,and the design schemes are given,which can provide the experimental platform for the subsequent theoretical technical validation.(2)Based on the theory of the signal transmission in wireless channel,the system model of the acoustic transmission for LWD log data is built.The uplink and downlink channel capacities of the drill strings are theoretically analyzed and the noise interferences along the acoustic transmission in drill strings and the corresponding suppression methods are researched,respectively.Based on the acoustic characteristics of the periodic drill strings,a simple acoustic insulator design scheme is proposed,which can suppresses the in-band noise interferences effectively.Meanwhile,aiming for the high data rate of the uplink transmission,the design scheme based on the double receiving sensor array is researched according to the single input multiple output(SIMO)system with space diversity gain,which can suppress the ground environment noise interferences in the uplink transmission and improve the uplink transmission efficiency.(3)Aiming for the comb filter characteristics of periodic drill strings channel,an acoustic transmission method for LWD based on non-contiguous orthogonal frequency division multiplexing(NC-OFDM)is proposed,which can utilize the multiple noncontiguous transmission sub-passbands effectively to improve the transmission rate.On the basis of the transmission requirements and the drill strings transmission characteristics,the transmission subcarriers are allocated adaptively.In the design of the transmitter,the peak-to-average power ratio(PAPR)reduction methods are studied and a low complexity PAPR reduction method is applied based on miniaturization and low power consumption requirements of the instruments in the down hole,which can reduce the PAPR at the transmitter effectively.Considering the continuity of each sub-passband of the drill strings,an adaptive pilot design scheme is designed,which can meet the requirements of channel estimation in real time.In the design of the receiver,the effects of two kinds of symbol timing errors and carrier frequency offset on the received data are studied theoretically,and an adaptive timing synchronization technology is proposed,which can achieve symbol timing synchronization and estimation of carrier frequency offset.The least squares(LS)and discrete Fourier transform-based least squares(DFT-LS)channel estimation are studied,and the zero-forcing(ZF)and minimum mean square error(MMSE)equalization detection methods are also studied,respectively.Meanwhile,aiming for the possible deep fade in the transmission bands,a simple precode NC-OFDM transmission scheme based on the Haar wavelet transform is designed,and the design principle of the precoding matrix at the transmitter and the detection method at the receiver are studied,respectively.This transmission scheme,which meets the requirements of miniaturization and low power consumption,can reduce the PAPR and improve the transmission performance under the deep fade.(4)Aiming for the periodic drill strings channel with non-uniform unit size under the complex drilling conditions,an acoustic transmission method for LWD based on vector orthogonal frequency division multiplexing(V-OFDM)is proposed.It can improve the transmission performance under the condition of poor channel,and transfer the complexity between the transmitter and receiver,which are very suitable for the requirements of uplink transmission.The principles of modulation and demodulation of V-OFDM,the relation between V-OFDM and conventional OFDM,the complexity of the transmitter and receiver,and the transmission performance are studied,respectively.In the design of the receiver,the effects of two kinds of symbol timing errors on the received data are studied theoretically,the pilot design scheme and channel estimation are also studied.In the signal detection of receiver,the performances of the maximum likelihood(ML),ZF,and MMSE equalization detection methods are analyzed theoretically.In order to achieve the optimal rotation performance at the V-OFDM transmitter,a precoding matrix scheme is applied,and the design principle of the precoding matrix at the transmitter and the detection method at the receiver are given,respectively.This technology can further improve the transmission performance of V-OFDM on the premise of that the design complexity is not significant increased.