电遥测钻柱信号传输的建模分析与信道特性改善

电遥测钻柱信息遥测系统由多节布线钻杆级联形成,用于井下随钻测量数据的高速率上传。由于信道传输的高频电压信号衰减严重,目前每隔约300米需要中继器进行信号续传,因此如何延长中继距离成为系统性能改善的关键。布线钻杆由布有同轴电缆的杆体及电磁耦合器构成,高频下布线杆体的同轴电缆可看做传输线,电磁耦合器可等效为高频变压器,将二者结合建立了布线钻杆的等效电路模型及电遥测钻柱信道的电压传输函数数学模型；通过外置补偿元件使传输线达到阻抗匹配以确立布线钻杆的电磁参数；通过改变补偿元件参数在传输线终端产生反射电压来加强传输线上的信号幅度,使信道的信号传输能力得到有效改善。研究表明,适当增大补偿元件电阻值可以大幅度提高信道的电压传输函数值,在目前信道带宽及信号检测灵敏度条件下中继距离可提高1倍,达到大幅度延长中继距离的目的。本文的研究可以为布线钻杆的电磁设计及电遥测钻柱信息传输系统的性能改进提供理论指导。

Modeling Analysis and Channel Characteristics Improvement of the Telemetry Drill String Signal Transmission

The telemetry drill string is formed by a string of wired drill pipes, it is used to upload downhole MWD data at high speed. Due to the serious attenuation of high-frequency voltage signal in the channel, repeaters are needed for continuous transmission at every 300 meters. Thus, how to extend the relay distance is the key to improve the channel characteristics. The wired drill pipe consists of a rod body with coaxial cable and an electromagnetic coupler. The coaxial cable embedded in the rod body can be regarded as a transmission line at high frequency condition, and the electromagnetic coupler can be equivalent to high frequency transformer. Combining the transmission line and high frequency transformer, the equivalent circuit model of the wired drill pipe and the mathematical model of the voltage transfer function of the telemetry drill string channel are established. The impedance matching of transmission line is achieved by placing external compensation element to determine the electromagnetic parameters of the wired drill pipe. By changing the parameters of the compensation element, the reflected voltage is generated at the transmission line terminal to enhance the signal amplitude of the transmission line, so that the signal transmission capability of the channel is effectively improved. The research shows that the voltage transfer function of the channel can be greatly improved by appropriately increasing the resistance of the compensation element, and the relay distance can be doubled under the current channel bandwidth and signal detection sensitivity, so as to greatly extend the relay distance. This paper can provide theoretical guidance for the electromagnetic design of wired drill pipe and improve the performance of the telemetry drill string information transmission system.