Research On Mud Pulse Signal Processing Of Measurement While Drilling System

Presently, the difficulty in the process of drilling the oil wells is growing. MWD(Measurement While Drilling) technology has been widely applied with the purpose of obtaining the formation parameters accurately and improving the borehole data acquisition rate. In MWD system, down-hole parameters can be measured closing to the drill bit while drilling and transferred in real time to the surface through the telemetry system. Recently, the mud pulse signal transmission is the most mature and most widely applied wireless transmission technology, whose data is transmitted to the surface via a mud column channel, and a pressure pulse modulated with measured information. Mud channel is so harsh that the received signal would be attenuated sharply due to the mechanical noise of the drilling platforms, signal attenuation, and the nonlinear distortion. Furthermore, the bandwidth of the mud channel is also quite narrow, only about 20 Hz. Therefore, for MWD telemetry system, the research on mud pulse signal processing is particularly important on improving the transmission rate and anti-noise performance.Aiming at solving the problem of narrow channel bandwidth, complex noise composition and low transmission efficiency in MWD system, the research and algorithm innovations in this paper mainly analyze mud pulse channel, pump noise cancellation and signal encoding are as follows:1. The mud pulse signal has been studied in transmission speed and attenuation model. By setting up the time frequency domain model, it is found that the channel amplitude frequency response has a characteristic, that resonant frequency point is infinite. And the fluid channel frequency response range and time delay characters is obtained through a method combined with correlation estimation and MMSE, which is used to process the actual logging data.2. In order to remove pump noise from acquired signals on the ground effectively, six algorithms is discussed, including wavelet, EEMD, FFT, null space pursuit, an adaptive denoising algorithm based on the fundamental frequency of the pump stroke interference signal and image decomposition via the texture and natural parts. Simulation results show that these algorithms could eliminate noise effectively. With obvious pulse and low pulse interval error, FFT and the pump noise cancellation method processing based on image decomposition via the texture and natural not only improve the pulse signal detection rate, but reduce the pulse interval decoding error number.3. In order to improve the transmission speed and obtain better error performance, a trellis coded pulse interval modulation(TC-PIM) is proposed aiming at issues like less transmitted pulse information and low transmission rate. The joint foreword error correlation coding(FEC) and pulse interval modulation(PIM), maximize the minimal time distance between the modulation output sequences with convolution code redundancy to reduce the Viterbi decoding bit error rate and obtain the significant coding gain, then extend the transmission distance of the mud pulse telemetry system. Experimental results also show the algorithm can get significant coding gain without transmission rate loss.