| Electromagnetic wave transmission technology while drilling,namely EM-MWD technology,the data acquired by the LWD tool at the bottom of the downhole drilling tool is modulated by communication,and then transmitted to the ground by the transmitter in the form of electromagnetic waves,because it is excited by the quasi-TEM mode The electromagnetic wave will reach the surface along the path of the drill pipe.The surface receiving system receives the electromagnetic signal and decodes the data measured by the downhole sensor.This method of electromagnetic wave transmission while drilling has the advantages of faster transmission speed,lower cost,and no drilling fluid than the mature mud pulse technology.The potential of this transmission method is huge.After mature commercial use,it can be used as a good Alternative solutions,instead of drilling mud pulses,have great advantages especially in scenarios where mud pulses such as natural gas drilling and underbalanced drilling are difficult to apply.With the continuous deepening of the research on electromagnetic wave transmission technology while drilling,the electromagnetic wave transmission technology while drilling has also begun to change from a simple functional experimental stage to a practical application in the oil and gas exploration stage.This topic is based on the domestic and foreign achievements of the electromagnetic wave transmission technology while drilling,and points out the difficulties of this topic.One is that the attenuation of the transmission signal while drilling is large,the signal swing varies greatly with the formation resistivity,and the signal-to-noise ratio is low;Transmission requires high-speed transmission under the extremely limited spectrum resources.In view of the difficulties of large attenuation,large swing of signal swing with formation resistivity,and low signal-to-noise ratio,as well as the limitation of underground battery power supply,select a high-efficiency,large transmission power Class D amplifier in the transmission system.FSK modulation method with good noise performance.At the same time,the relay transmission method is used to extend the transmission distance.Reduce the inherent noise of the circuit in the receiving system,analyze the circuit noise through the theory and simulation tool Pspice,and verify the noise in the order of 10 uV by the actual measurement.For the extremely low frequency power frequency interference,the power frequency notch filter is used to filter out the actual measurement.The maximum notch depth is-40 dB,and a two-stage amplification and filtering scheme is adopted for weak signals with low signal-to-noise ratio.The maximum amplification factor exceeds 120 dB,and the average out-of-band rejection is-45dB@1kHz.In view of the large variation of the signal swing with the formation resistivity,an automatic gain control circuit with a dynamic range of 60 dB is designed to control the gain of the secondary amplifier circuit.Aiming at the difficulties of high-speed transmission under the condition of limited spectrum resources,an OFDM-based transmission while drilling scheme is proposed,which uses OFDM modulation as a backup optimized modulation scheme,and is expected to break through 100 bit/s in further tests in the future.At the same time,due to the actual needs of the project,the design has realized the function of two-way transmission.Finally,based on experimental tests and outdoor tests,the performance indicators of the system are verified.In the Yanjiao system test,the uploading-relay-receive upload scheme was verified.In Yanjiao calibration well,two-way downhole transmission was achieved,and in Xinjiang experimental well,a single-section long-distance transmission of 900 meters and 10 bit/s was achieved. |
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