Study On The Measurement And Wireless Short-distance Communication Near-bit Downhole

With the development of oil and gas exploration, the quality of drilling becomes more important than ever. Information near bit is critical to assure the well s quality. However, the technology of measurement and telecommunication near bit was not mature. Using the longitudinal wave as carrier, the paper analyzes the acoustic properties of channels near bit with theory analysis and experimental methods. Also, a near bit measurement and wireless short-distance communication system is developed. It gives a new way for domestic near bit information s measurement and telecommunication.The downhole drilling tools have the characteristic such as compact, consecutive, consistent and steady. Also they connect the down hole and surface. So they are idea channel for acoustic communication. Through analyze the basic properties of acoustic wave propagated in solid, it finds that reflections and refractions will occur at the edges of tools and fluid, tool and tool, tools and formation. Due to the differences of P-wave and S-wave, the P-wave has better transmitting performance. The tools near bit are more complex than upper tools. So the hypotheses of slim pole and periodic pipe used in analyzing drill string s acoustic properties are not suitable for the near bit tools. A new research method must be developed.The screw tool is important for BHA (Bottom Hole Assembly) in directional drilling. It's acoustic properties should be clear first. According to the real structure and construction, the FEA (Finite Element Analysis) model was build and the pulse response was resolved by FEA method. Using time-domain and frequency-domain analysis , the acoustic property of screw tool was got. The results show that the amplitude of acoustic wave will attenuate and peaks number will increase while the extension of spread. The screw tool has the low pass filter property for the longitudinal wave. The 6KHz carrier frequency is good for communication in the distance about 12m without diameter changing. Also, an experiment was executed. The experimental results show that real screw tool's pass band is narrower than the calculated results and shift to low frequency.According to the real condition of BHA, 5 available acoustic channels and structures are introduced. Their time domain and frequency domain properties are analyzed by FEA method. In analyzed channels, when the longitudinal wave is used as carrier, the reflection and attenuation in straight channel is faint. In by-pass channels, the reflection is strong due to the branch. When the by-pass channel must be used, the 2nd and 3rd channel have more good performance. For better performance, the absorption structure should be added at the terminal. Diameter changing along the tools will make the wave s amplitude smaller and phase lagged. So the reflection will be very strong when the distances between diameter changing and wave length have an appropriate relationship. This point must be given more attention in some special tools. On the other hand this can be used to whittle the reflection.Based on the features of the down hole channels, the OOK with time space modulation, no-coherent demodulation, convolutional coding and Viterbi decoding method was applicated in near bit communication for saving the power and reducing equipments down hole.A measurement and short-distance transmitting system is designed and realized. The system can measure the inclination at 64cm far from the bit, and transmit it to the receiver 12.5m far from the transmitter using acoustic wave. The receiver can transmit the information to MWD and to surface by MWD then. It was tested on surface successfully, also in a well.