TY - JOUR
AU - Wang,, Chenli
AB - Abstract An electromagnetic measurement while drilling system (EM-MWD) can transfer well track state parameters to the ground in real time, which makes it an indispensable technology for deep-hole drilling. This paper introduces the development of and experiments on an EM-MWD system used for deep exploration in the People’s Republic of China. The designed EM-MWD system is composed of a downhole instrument and a ground instrument, and we elaborate on the structural design of the downhole instrument, the design of the transmission and control circuits and the signal modulation. This work also covers the software and hardware design of the ground instrument and signal demodulation technologies. Finally, some indoor signal decoding experiments and some in-hole signal transmission experiments are performed. This study indicates that the designed EM-MWD system can measure information for downhole drilling parameters and send it to the ground effectively, while the ground receiver can decode the signal accurately and reliably, and the desired signal can be obtained. Furthermore, the strength of the received signal is not affected by the polar distance within a certain polar distance. deep exploration, EM-MWD, signal transmission, development and experiment 1. Introduction The People’s Republic of China launched a major deep exploration project on the mainland (called SinoProbe5) in 2008. The primary mission of the project is to make key technical preparations for the future engineering of Earth crust exploration; solve key technical difficulties for detection and establish core technology integration for the 3D detection of solid earth layers; conduct experiments in some key areas such as different natural settings, complex ore concentration areas, deep oil and gas basins and geological disaster areas, etc; and form a number of experimental bases for deep exploration. In order to reach the goal of the special major project, many deep holes need to be drilled. Measurement while drilling (MWD) is a kind of real-time information interaction technology that links downhole measurement instruments and ground monitoring equipment. It can monitor downhole geological parameters, drilling parameters, etc in real time, provide guarantees for field analysis, process and explain downhole information, and contribute to timely and effective formation evaluation. Consequently, it can improve drilling efficiency and safety, and reduce drilling costs. Furthermore, MWD technology is also an important technical support for drilling automation and is an indispensable technology for deep drilling (Vong and Kheong 2005). At present, two kinds of MWD systems are usually used in production, the mud pulse MWD (MUD-MWD) and the electromagnetic MWD (EM-MWD). Compared with MUD-MWD, EM-MWD has many advantages, such as the fact that the signal transmission is not affected by drilling fluid, its structure is simple and has no moving parts, it has a high data transfer rate (the data transfer rate of MUD-MWD is about 8 bits s-1, while that for EM-MWD is about 100 bits s-1), it can realize two-way communication between the ground and downhole, and it is low cost, etc. Moreover, MUD-MWD cannot be used in unbalanced or air drilling, and EM-MWD plays an irreplaceable role in these situations. In recent years, EM-MWD has become a hot issue in the international drilling field. Famous drilling companies, such as Schlumberger, Halliburton and Weatherford from the USA, СaМaрские and гориэонты from Russia, Ryan and Phoenix from Canada, and Cryoton and Geolink from the UK, have launched their own EM-MWD products and applied them all over the world (Whitacre and Yu 2009, Shi and Peng 2011). This work belongs to SinoProbe-09-05 (equipment research used for deep exploration in mainland China), and this paper introduces a kind of EM-MWD that has been developed for deep exploration, and describes the overall frame design, the downhole instrument design, the ground receiver design, signal denoising processing, signal decoding experiments, ground signal transmission experiments and downhole signal transmission experiments. 2. Overall scheme of electromagnetic MWD An EM-MWD system launches electromagnetic wave signals via an emitting dipole and receives electromagnetic signals via a receiving dipole; its working principle is shown in figure 1. One end of the emission dipole is the downhole measurement and emission short section and the other end is the drilling pipe, which is connected to the drilling rig. Electromagnetic waves radiate into the surrounding infinite space from the emission source and the signal gradually decays in the course of transmission (Liu et al2014). Figure 1. Open in new tabDownload slide Working principle of a typical EM-MWD system. Figure 1. Open in new tabDownload slide Working principle of a typical EM-MWD system. The designed EM-MWD system is composed of a downhole instrument and a ground instrument and its overall scheme is shown in figure 2. The downhole instrument is powered by a battery pack and sensors installed in the downhole instrument are used to measure parameters such as pressure, temperature, hole deviation angle and azimuth angle, etc, which are translated into a simulation electrical signal which can then be sent to a single chip microcomputer. After frequency shift keying (FSK) modulation and power amplification, the signal is emitted via the emitting dipole. Electromagnetic waves propagate through the earth and are detected by antenna (receiving dipole), and after decoding and filtering they are displayed and saved in the form of charts on a personal computer. Figure 2. Open in new tabDownload slide Overall scheme of the designed EM-MWD system. Figure 2. Open in new tabDownload slide Overall scheme of the designed EM-MWD system. The technical indicators of the designed EM-MWD system are as follows: Working depth: 1000 m–4500 m. Highest working temperature:  >150 °C. Constant temperature crystal oscillator. GPS synchronization module error:  <20 ns. Downhole instrument supply voltage: 12 V (powered by battery pack). Signal emission frequency: 2 Hz–20 Hz. Consumption:  ⩽100 W. Electric current:  ⩽10 A. 3. Downhole instrument 3.1. Structure The downhole instrument includes an inner tube and an outer tube, and support rings are installed between the inner and outer tubes. Drilling fluid flows through the gap between the inner and outer tubes. The inner tube is not subjected to torque, which is fixed on the outer tube with support rings. Both ends of the outer tube are connected to the drilling pipe, the outer tube needs to transfer the torque of the drilling pipe. In order to form the dipole to emit electromagnetic waves, an insulated gap must be set in the middle of the inner and outer tubes. The inner tube is mainly used to install batteries, circuit boards and sensors, and its structure is shown in figure 3. It is composed of a top connection, a bottom connection, an insulated gap, a battery pack, an emitting circuit board, a control circuit board and a sensor holder. The top connection, bottom connection and insulated gap form a seal chamber, and the battery pack, emission circuit board, control circuit board and sensors are installed in the seal chamber. The top and bottom connections are made from non-magnetic steel and the circuit board holder and sensor holder are made from copper. A pressure sensor, temperature sensor, borehole deviation angle sensor, azimuth angle sensor and tool face angle sensor are installed on the sensor bracket. Figure 3. Open in new tabDownload slide Structure of the inner pipe of the downhole instrument. Figure 3. Open in new tabDownload slide Structure of the inner pipe of the downhole instrument. In order to emit electromagnetic waves, an insulated gap should be installed on the outer tube, and the outer tube needs to bear the torque, pressure and pull of the drilling pipe, so the strength of the insulated gap is very important for the design of the outer tube. High-strength engineering plastic polyetheretherketone (PEEK) is used to produce the insulated gap, which can then meet the strength requirements. The structure of the outer tube is shown in figure 4. The outer tube is mainly composed of the top connection, the bottom connection, an insulated locating ring, epoxy glue, a key and glass fiber reinforced plastic. The top connection and bottom connections are made of non-magnetic steel and the insulated locating ring and the key are made of PEEK. In the structure of the outer tube, the top and bottom connections are separated by the insulated locating ring and the epoxy glue, while the epoxy glue binds the top and bottom connections together strongly, and this plays a certain role in transmitting torque, pressure and pull. In addition, six high-strength PEEK keys play the same role in transmitting torque, pressure and pull. Glass fiber reinforced plastic is used to protect the PEEK keys and enhance the overall strength of the outer tube (Wait and Hill 1978). Figure 4. Open in new tabDownload slide Structure of the outer tube of the downhole instrument. Figure 4. Open in new tabDownload slide Structure of the outer tube of the downhole instrument. 3.2. Circuit of the downhole instrument The circuit of the downhole instrument includes the control and transmission circuits. The control part is composed of a single chip computer and sensors, etc, with the sensors including pressure, temperature, accelerated speed, flux-gate, pressure and temperature sensors. The last two are used to record the downhole pressure and temperature, respectively, while the accelerated speed and flux-gate sensors are used to measure and calculate the hole deviation angle, azimuth angle and tool face angle (Liu et al2006). The analog signal collected by the sensors is sent to the single chip computer, which converts the analog to digital and then carries out FSK modulation of the collected signal. After that, the signal is transmitted at 40 bits per frame (12 bits is used for the azimuth angle and hole deviation angle signals, respectively, and eight bits is used for the temperature and pressure signals respectively; together, this is 40 bits). The digital signal output from the single chip computer is driven by a V-groove metal oxide semiconductor (VMOS) bridge and is finally sent to the emitting dipole. The overall design diagram for the downhole instrument is shown in figure 5. Figure 5. Open in new tabDownload slide Overall design diagram for the downhole instrument. Figure 5. Open in new tabDownload slide Overall design diagram for the downhole instrument. The transmission circuit uses an H-bridge to amplify the power. The H-bridge is composed of the metal oxide semiconductor (MOS) IRF2807, which is driven by the special drive chip IR2184. Meanwhile, the electricity and voltage monitoring circuits are designed. After voltage dividing resistance, the transmission voltage is input into the single chip computer for analog to digital conversion, and we can monitor the transmission voltage by digital signal. In order to monitor the transmission electricity, a 0.1 Ω sample resistance is series connected in the H-bridge. 4. Ground receiver Due to the greater transmission depth, the smaller received signal and the lower signal to noise ratio, a low noise amplifier filtering circuit is for the extraction of useful signal and decoding. The ground receiver first amplifies the received signal with a programmable amplifier and then filters the high-frequency noise and power frequency interference with a low pass filter and a 50 Hz band trap; after that, the signal is amplified again. Finally, the signal is shaped to a square wave with a comparator and input into the single chip computer, and the data are displayed on an LCD or personal computer after decoding by the single chip computer. The overall design diagram for the EM-MWD system’s ground receiver is shown in figure 6. Figure 6. Open in new tabDownload slide Overall design diagram for the ground receiver of the EM-MWD system. Figure 6. Open in new tabDownload slide Overall design diagram for the ground receiver of the EM-MWD system. 4.1. Hardware circuit design The ground receiver’s hardware circuit design includes a pre-amplifier and zeroing circuit, a filter circuit, an analog digital converter (ADC) and a power circuit, etc. The majority of the noise that the ground receiver must deal with comes from earth, the self-potential can reach hundreds of mV, and the signal to noise ratio is very low. Large first level magnification may cause amplifier saturation, so the design of the pre-amplifier is very important. An instrumentation amplifier with high common mode rejection ratio characteristics is selected in this work (Williamson 2000, Timothy and Yu 2009). The main noise the ground receiver must deal with is high-frequency noise that comes from earth and power line interference, therefore, filter circuit design focuses on low pass filter and power frequency band trap. In this work, the low-pass filter design uses the six step Sallen–Key active power filter; the actual measurement amplitude and frequency curve for the Sallen–Key low-pass filter is shown in figure 7. Figure 7. Open in new tabDownload slide Amplitude and frequency curve for the Sallen–Key low-pass filter. Figure 7. Open in new tabDownload slide Amplitude and frequency curve for the Sallen–Key low-pass filter. The power frequency band trap adopts the classical active double T structural circuit, the frequency is 50 Hz, and its transfer function is shown in equation (1): H( jω)=ω2−ω02ω2−ω02⁢ −j4ω0(1−k)ω,1 where j is the reactance, ω is the frequency, ω0  =  1/RC, R is the resistance, C is the capacitance and K is the reaction coefficient of the circuit. From equation (1) it can be obtained that when ω  =  ω0, H( jω)  =  0 some special frequency can be filtered, but for another frequency H( jω) is close to 1 and the signal can be passed well. When K is less than and close to 1, we can obtain the narrow-band filtering effect and make the band trap achieve optimum performance (Liu and Feng 2012, Lu et al2015a, 2015b). When the amplification factor is too large, the direct current signal coupled mode is not convenient for instrument zero processing. In order to identify small signal at a low amplification factor, a 24 bit analog–digital conversion chip is used in the ADC; the supply voltage of the instrument is  +5 V. 4.2. Software design The software design includes natural potential and offset voltage compensation, signal decoding, communication with the ground receiver and other supporting functions. An FIR digital filter can eliminate noise interference further, and the parameters of the designed FIR digital filter are as follows: the sampling frequency is 250 Hz, the pass band cut-off frequency is 20 Hz, the attenuation band frequency is 49 Hz, the pass band damping is 0 dB and the attenuation band damping is 40 dB. When we test the designed FIR digital filter the sine wave signal produced by the forcing function generator is put in the analog channel of the receiver and the amplitude–frequency response after different circuit and module sampling conversions is shown in figure 8. It is clear that noise interference is further removed after FIR digital filtering, as can be seen in figure 8. Figure 8. Open in new tabDownload slide Amplitude and frequency response of FIR digital filter. Figure 8. Open in new tabDownload slide Amplitude and frequency response of FIR digital filter. The self-potential will change in the course of drilling, with the influence of the offset voltage in the circuit, DC bias will occur in the measured signal, so the original signal amplitude will be enlarged or diminished, and this may cause the amplifier to become saturated after the signal is amplified. This influence can be eliminated by AC coupling, but low frequency communication should be used, even lower than 4 Hz (Stephenson and Wilson 1992). The received signal can be expressed as s(k)  =  x(k)  +  n(k)  +  a, where x(k) is the received useful signal, n(k) is the interference signal and a is the self-potential. The received signal is a bipolar signal and therefore when complete alternation sampling to the signal, the amplitude accumulation of the signal and noise in the complete alternation is approximate to zero, that is ∑k=0N−1x(k)=0 (Meng et al2010). The computational process for signal superposition is shown in equation (2): ∑k=0N−1s(k)=∑k=0N−1x(k)+∑k=0N−1n(k)+N⋅a.2 After signal superposition, ∑k=0N−1x(k)=0, ∑k=0N−1n(k)=0.3 Therefore, ∑k=0N−1s(k)=N⋅a.4 Hence, a=1N⋅∑k=0N−1s(k).5 We can obtain a relatively pure signal with s(k) minus the self-potential a. The design idea for decoding is as follows. The ADC collects data points continuously with a certain frequency and the data will be processed all at once after the gathered data points reach a certain number. This involves the continuity of two sets of discrete series, hence, the data need to be merged. Because the sampling frequency of the ADC is definite, the interval time of the data points is certain, and the pulse width and other time information will be reflected in the data points, and at the same time, the data points will include the amplitude information, from which the pulse changing process can be mastered and then decoded (Wait and Hill 1979, Xia and Chen 1993). 5. Experimental study 5.1. Signal decoding experiment in the laboratory In order to verify the capacity of the receiver to dispose of noise and self-potential, some signal transmission and decoding experiments are performed in the laboratory. A sine wave signal of 80 mV is emitted from the transmission terminal. The wave on the oscilloscope is shown figure 9(a), and it can be seen from this that the sine signal is obviously disturbed by the power frequency. The signal wave after transfer through the earth is shown in figure 9(b), and it can be seen from this that the signal is interfered with by the earth entirely, and the peak of the received signal points and noise is up to 800 mV. The signal wave after filtering by the receiver is shown in figure 9(c), and this time the peak of the signal is 11.2 mV; meanwhile, it is found that there is a DC offset of about 20 mV. Finally, the signal wave after analog to digital conversion and digital filtering is shown in figure 9(d); the peak of the signal points is about 7.5 mV. This indicates that the transmission attenuation of the 80 mV signal is approaching   -20 dB, the noise and self-potential are approaching 800 mV and the signal to noise ratio is close to  -40.6 dB. It can be seen from figure 9(d) that the final signal wave after disposal is very clean, with almost no noise, and it is good enough for decoding (Aboelmagd et al2004, Noureldin et al2003). Figure 9. Open in new tabDownload slide Different stages waveform of the 80 mV original signal. (a) Emission signal, (b) electrode signal, (c) signal after analog channel and (d) signal after A/D conversion. Figure 9. Open in new tabDownload slide Different stages waveform of the 80 mV original signal. (a) Emission signal, (b) electrode signal, (c) signal after analog channel and (d) signal after A/D conversion. 5.2. Signal transmission experiments on the ground Some signal transmission experiments are performed on the ground so as to test the designed transmitter and receiver. The transmitter and receiver both have two electrodes and a copper bar is used to simulate electrodes. Four electrodes are inserted a certain depth into earth, one transmitter electrode is connected to one of the receiver electrodes by wire, and an electromagnetic wave is emitted from the transmitter and transfers to the receiver through earth. The experimental parameters are as follows: the measured average formation resistivity is 60 Ω · m, the transmission frequency is 10 Hz, the transmitter and receiver are powered by 12 V batteries and the peak of the transmitting voltage is 24 V. Experiments with different test distances from 100 m to 2500 m are performed. The strength of the received signal reduces with increasing distance, but it all can be decoded correctly. When the test distance is 150 m, the strength of the received signal is about 4.7 V. The noise is mainly power frequency interference. The collected ADC data after amplification and filtering are shown in figure 10. Figure 10. Open in new tabDownload slide ADC date of receiver when distance is 150 m. Figure 10. Open in new tabDownload slide ADC date of receiver when distance is 150 m. 5.3. In-hole signal transmission experiments The site of the in-hole signal transmission experiments is located in Lingbo of Henan province, and the purpose of the drilling is gold ore surveying. The experimental conditions are as follows. The depth of the exploratory hole used for the experiment is 860 m, the finished diameter of the drill-hole is 75 mm, the drilling rig model is XY-4 and the diameter of the drill pipe is Φ50mm. When the experiments are performed, the drilling depth is 616 m, the drilling casing has a length of 439 m and a diameter of 127 mm, and the underground water level is about 18.5 m. The downhole instrument of the EM-MWD system is assembled and tested on the ground before being sent into the hole, and the frequency of the electromagnetic wave is set to be 10 Hz. The experimental process is as follows. The assembled downhole EM-MWD instrument is sent into the hole through a drill pipe and after that each part of the ground receiver is connected with wire. Four positions at different depths are selected for the experiment at intervals of 100 m. The arrangement of the positions is shown in figure 11 and the experimental results and received signals are shown in figures 12–14. Figure 11. Open in new tabDownload slide Diagram of test points location. Figure 11. Open in new tabDownload slide Diagram of test points location. Figure 12. Open in new tabDownload slide Waveform when hole depth is 442 m. Figure 12. Open in new tabDownload slide Waveform when hole depth is 442 m. Figure 13. Open in new tabDownload slide Waveform when hole depth is 510 m. Figure 13. Open in new tabDownload slide Waveform when hole depth is 510 m. Figure 14. Open in new tabDownload slide Waveform at different polar distances when the hole depth is 609 m. (a) Waveform when polar distance is 50 m. (b) Waveform when polar distance is 70 m. Figure 14. Open in new tabDownload slide Waveform at different polar distances when the hole depth is 609 m. (a) Waveform when polar distance is 50 m. (b) Waveform when polar distance is 70 m. In order to study the influence of polar distance (the distance between the drilling rig and the antenna) on received signal, two polar distances are used in the experiment, 50 m and 70 m, at a depth of 609 m, and the strength of the received signals is compared. The experimental results indicate that the ground receiver of the developed EM-MWD system can receive strong signal, and the signal strength has a tendency to increase with depth increase within a certain depth. It is may be that the downhole instrument of the designed EM-MWD extends from the drill casing, with the transmission of the electromagnetic wave signal being affected by the shielding action of the drill casing. By analyzing the signal transmission results at the same hole depth but different polar distances, it is found that the strength of the received signal is high and unchanged when the dipolar distance is not more than 70 m, that is to say, the strength of the received signal is not affected by the dipolar distance within a certain dipolar distance. 6. Conclusion This paper introduces the development of and experiments on a type of EM-MWD system that is used for deep exploration in the People’s Republic of China. The designed EM-MWD system consists of a ground instrument and a downhole instrument. The research work on the downhole instrument includes structural design, the selection of model sensors and the design of control and transmission circuits, etc. The research work on the ground instrument includes the overall plan of the signal processing, and the hardware and software design for the receiver. In order to test the designed EM-MWD system, an indoor signal decoding experiment, a signal transmission experiment on the ground and an in-hole signal transmission experiment are performed. The research indicates that the developed EM-MWD system can monitor downhole drilling parameters effectively and send information to the ground via electromagnetic wave signals. After the ground instrument receives the signal, the signal will be amplified, filtered, trapped, AD converted and demodulated, and finally the desired signal will be extracted. In addition, we perform some signal transmission experiments at different polar distances, and it is seen from the experimental results that the strength of the received signal is not affected by the dipolar distance within a certain dipolar distance. Because of the restrictions of drilling depths, the maximum depth of the downhole experiments is 609 m, but we received very strong signal of 200 mV. Acknowledgments The work was supported by a major project of the People’s Republic of China—deep exploration in mainland of China (SinoProbe-09-05), the National Natural Science Foundation of China—the data transmission mechanical and experimental study about electromagnetic measurement while drilling based on relay station (no. 41572355) and the research fund from the Engineering Research Center of Rock-Soil Drilling and Excavation and Protection, Ministry of Education, China University of Geosciences (Wuhan). The paper was also funded by the China Scholarship Council. The authors wish to thank Dr Wang Hongliang, Master Lu Chenda and other colleagues who helped with the tests. References Aboelmagd N , Dave I H , Martin P M . , 2004 Measurement-while-drilling surveying of highly inclined and horizontal well sections utilizing single-axis gyro sensing systems , Meas. Sci. 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TI - The development of and experiments on electromagnetic measurement while a drilling system is used for deep exploration
JF - Journal of Geophysics and Engineering
DO - 10.1088/1742-2132/13/5/824
DA - 2016-10-01
UR - https://www.deepdyve.com/lp/oxford-university-press/the-development-of-and-experiments-on-electromagnetic-measurement-535NYJeEdw
SP - 824
VL - 13
IS - 5
DP - DeepDyve
ER -