| The technology of electromagnetic measurement while drilling is that the sensor modulate the measured data,and then use the signal transmitter to transmit signals to the ground in the form of electromagnetic waves,the electromagnetic wave passes through the stratum to the ground along the drill pipe,the received signal is detected by the ground receiving system,measurement data is obtained by decoding.Electromagnetic wave transmission has the advantages of fast transmission speed,simple implementation and low cost.This kind of transmission method makes up for many shortcomings of drilling mud pulse transmission mode and becomes an indispensable key technology for natural gas drilling and underbalanced drilling.In recent years,this technology has gradually been applied in oil and gas exploration.The advanced electromagnetic wave while drilling measurement technology has been blocked by foreign countries,and Chi na's EM-MWD technology is still in the experimental stage,which requires us to overcome difficulties and develop our own electromagnetic measurement technology while drilling.Therefore,the electromagnetic wave relay transmission technology based on drilling is proposed.Based on the existing results of the electromagnetic wave relay transmission technology while drilling,this paper analyzes some problems that the technology is currently facing.On this basis,the analog circuit system of the electromagnetic wave relay transmission while drilling is studied.Based on the actual engineering needs,this paper proposes the overall design of the electromagnetic wave relay transmission system while drilling.Aiming at the problem of coordinated transmission and interference between multiple relays,two relay forwarding schemes are designed to realize the function of electromagnetic wave underground relay transmission,then design specifically the analog circuit of underground launching system,relay forwarding system and ground receiving system.In order to ensure that the system can achieve long-distance stable transmission performance at high speed,compare the bit error rate and power spectral density curve of several modulation methods in the EM-MWD electromagnetic communication channel,finally choose MSK modulation mode.Since the downhole space of the drilling system is limited and completely sealed,the input impedance of the transmitting antenna is constantly changing,and the combination of the class D power amplifier and the emission current detecting circuit is adopted to ensure the stable power output of the transmitting system.The electromagnetic signal transmitted from the downhole is very weak and is interfered by noise.This paper designs a low-noise receiving system circuit from the perspective of receiver design,and uses the noise analysis software LTspice to analyze and optimize the noise of the receiving system circuit to obtain good reception of weak signals.Due to the large variation of the electromagnetic signal intensity in the well,a 60 dB dynamic range automatic gain control circuit based on PID algorithm is designed to ensure the stable output of the received signal.Finally,according to a series of field tests of the system circuit,the electromagnetic wave relay transmission system can be operated stably and reliably for a long time,and can achieve a symbol transmission rate of 100 bit/s,and a bit error rate of less than 0.1%,which satisfies the design requirements.The results of this paper provide a good basis for the further research. |
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