Incorporating electromagnetic measurements into drilling systems with a relay station

Journal Pre-proof Incorporating electromagnetic measurements into drilling systems with a relay station

Chunhua Lu, Tao Zhang PII:

S0926-9851(19)30532-4

DOI:

https://doi.org/10.1016/j.jappgeo.2020.103946

Reference:

APPGEO 103946

To appear in:

Journal of Applied Geophysics

Received date:

9 June 2019

Revised date:

9 January 2020

Accepted date:

9 January 2020

Please cite this article as: C. Lu and T. Zhang, Incorporating electromagnetic measurements into drilling systems with a relay station, Journal of Applied Geophysics(2020), https://doi.org/10.1016/j.jappgeo.2020.103946

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© 2020 Published by Elsevier.

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Incorporating electromagnetic measurements into drilling systems with a relay station Chunhua Lu, Tao Zhang

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Faculty of Engineering, China University of Geosciences, Wuhan 430074, Hubei, People’s Republic of China E-mail:[email protected] Received… Accepted… Published…

Abstract：Electromagnetic measurement while drilling (EM-MWD) systems transmit downhole

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data by emitting electromagnetic waves into the formation, thereby carrying the emitted data to a

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surface antenna; however, these electromagnetic waves are attenuated when transmitted into the

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formation. To further extend depth capabilities, Extended-Range EM-MWD systems have been developed. Expanding on this approach, this paper introduces an EM-MWD system that

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incorporates a relay station within ultra-deep wells; our proposed system consists of a downhole instrument assembly, a relay station and a ground receiver assembly. After decoding a received

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signal, the relay station amplifies, filters and recodes the signal; then, the relay station emits the augmented signal. Through numerical simulations, as presented in this paper, propagation

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characteristics of electromagnetic waves in heterogeneous and stratified strata show that when the resistivity of the upper formation and the lower formation is kept constant, the voltage of the ground receiving signal first increases and then decreases with the increased resistivity of the interlayer. Similarly, when the resistivity of the upper stratum and interlayer is kept constant, the voltage of the ground receiving signal first increases and then decreases with the increased resistivity of the lower stratum. This paper also presents the design schemes, the technology used to process the insulation gap, and the power-saving technology to increase the lifetime of the downhole instruments. Furthermore, the working principle of the relay station is described. Moreover, by analysing the electric and magnetic excitation methods, under the same conditions, signal attenuation is fast and the signal strength obtained on the ground is low when magnetic excitation is used. Therefore, an electric excitation method is adopted for implementing the relay station. To conclude, suggestions as to reasonable locations of the relay station and reception and

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Keywords：EM-MWD system, relay station, electromagnetic wave, insulated gap, power-saving technology

1.Introduction Modern directional drilling systems with primarily oil, gas and geothermal applications require

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accurate real-time information on the location and attitude of their corresponding drill bits with

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respect to gravity, magnetic north and true north(Timothy and Yu 2009). Measurement while

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drilling (MWD) is a real-time information interaction technology that involves downhole

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measurement instruments and ground-monitoring equipment; MWD implementations can monitor a variety of real-time downhole geological and drilling parameters, thereby supporting the field

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analysis, processing and explanation of obtained downhole information and contributing to timely

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and effective formation evaluation(Wait and Hill 1979).

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There are two types of MWD systems typically used in production. The first is known as Mud-MWD it uses pressure signals communicated through a column of fluid (i.e. mud) present in the given well. The second type of MWD system is known as electromagnetic MWD (EM-MWD), and this type transmits data via low-frequency electromagnetic waves. In EM-MWD systems, the electromagnetic wave attenuates as it propagates through the stratum; its propagation depth is called the skin depth and can be expressed as

  [2f

 2

[ 1 (

1 2 )  1 ]]1 , 2fpe

(1)

where δ represents skin depth (m), μ represents magnetic permeability (with μ=4π×10−7Ω·s/m), ε represents a dielectric constant (F/m), f represents frequency (Hz) and pe represents formation

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resistivity (Ω·m). At very low frequencies, i.e. 1/2πfεpe＞＞1, Equation (1) can be simplified as



1 f

.

(2)

pe Equation (2) shows that the transmitting frequency of the electromagnetic wave and the formation resistivity jointly determine the transmission depth of the electromagnetic wave in the formation. When the resistivity of the formation is certain, the transmission depth decreases with increased

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electromagnetic wave emission frequency(Mugoya R et al 2011).

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At present, EM-MWD generally uses a transmitting frequency of 5–20 Hz. At certain

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frequencies of the electromagnetic waves, transmission depth increases with formation resistivity.

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Assuming a transmission frequency of 5 Hz and a formation resistivity of 2 Ω • m, the propagation

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depth of an electromagnetic wave in the formation can be calculated as 2 km using Equation (2). However, due to well site noise and other environmental factors, in practice, the actual

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transmission distance is less than 2 km. At present, the application depth range of conventional

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EM-MWD systems is in the range 2000–2500 m (Hu Y F et al 2015). Note that Equation (2) does not consider the influence of transmission power on transmission depth. In fact, as transmission power increases, the transmission depth also increases; however, increasing transmission power increases the size of the transmission system, and given the limitation in wellbore space, only one suitable transmission power can be selected. In addition, increasing transmission power reduces the lifetime of EM-MWD systems with the same battery capacity. The prior analysis shows that the depth of electromagnetic wave propagation in the formation is limited; furthermore, transmission depth is the primary bottleneck currently restricting the development and increased application of EM-MWD systems. Therefore, the present research

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focuses on improving the transmission depth of EM-MWD systems through the three following aspects(Meng X F et al 2010). First, the receiving capacity of the ground device is increased, and the weak signals submerged in the noise are extracted. Second, the extended EM-MWD of the transmitting antenna are incorporated. Here when the downhole emitter reaches a certain depth close to its launch limit, a cable is dropped from the ground through the drill string channel; the

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lower end of the cable docks with the transmitting antenna of the downhole emitter to become an

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extended transmitting antenna, thereby increasing transmission depth. While effective, this method is disadvantageous in that during the drilling process, the cables often get tangled with the

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drill string and are not in stable contact with the emitter. Third, the relay transmission technology

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is extended by installing a relay station in the middle of the drill string; this relay station is

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designed to receive, process and transmit electromagnetic signals. Directly incorporating relay

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transmission technology improves the transmission depth of electromagnetic signals and requires simple handling and low cost.

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2. Working mechanism of a relay station-infused EM-MWD system As shown in figure 1, the relay station-infused EM-MWD system consists of a downhole instrument assembly, a relay station and a ground receiver assembly. The downhole instrument assembly includes a sensor module, a battery pack, an insulated gap, a signal processing module and a signal transmitting module. The battery pack powers the sensor, signal processing module and signal transmission module. Next, the sensor module measures the bottom hole drilling parameters and converts them into electrical signals. Furthermore, the signal processing module is responsible for analogue-to-digital (A/D) conversion, pre-amplification and filtering of the electrical signals measured by the sensor, and the monitoring and management of the battery

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voltage and temperature. The signal transmitter modulates the signal, amplifies it, power and transform impedance. Note that the insulated gap is also called an emission antenna, dividing the drill string into two poles, i.e. the upper and lower insulating poles; the modulated signals are transmitted through the formation with the electromagnetic wave as the carrier. The relay station includes a signal receiving module, a battery pack, an insulated gap, a signal

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processing module and a signal transmitting module. The battery pack powers the receiver module,

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signal processor module and signal transmitter module. After the signal receiving module receives a weak electromagnetic wave signal from the formation, the signal processing module

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demodulates and decodes the received weak signal. Next, the signal transmitting module recodes

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and modulates the processed signal and then transmits it through the insulated gap (i.e.

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transmitting antenna). The ground receiving assembly primarily includes an antenna, a receiver, a

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computer and a cable. Its role is to receive weak electromagnetic signals from the formation; perform signal amplification, filtering, trapping, decoding, A/D conversion and other processing

computer.

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steps; and then present results ultimately as graphics and tables displayed and stored on the

3. Influence of stratum stratification on electromagnetic signal transmission Hu S L et al (2011) analysed the influence that changes in formation resistivity had on the ground receiving signal in a homogeneous formation based on the equivalent transmission line model, with the effect of formation stratification on the ground received signal not being considered. Electromagnetic waves emitted by an EM-MWD system propagate in heterogeneous stratified formations in ultra-deep well environments. Therefore, in this paper, a numerical simulation was used to analyse the influence formation stratification has on EM-MWD signal

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transmission. As shown in figure 2, the assumed calculation model is summarised as follows. First, the formation is divided into three layers, i.e. an upper formation, an interlayer, and a lower formation. Second, the drill string consists of drill rods, all with the same specifications. Here, the inner and outer diameters of the insulated gap and drill pipe are equal. Third, the borehole is a vertical hole

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with a circular cross-section; the borehole axis coincides with the drill string axis. The radius of

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the cross-section of the formation is r0 = 1410 m, the depth of the well is h = 1410 m, the thickness of the upper formation is h1, the thickness of the interlayer is h2 and the thickness of the

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lower formation is h3. Furthermore, the upper drill string length is h4 = 1399.4 m, the insulation

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gap length is h5 = 0.6 m, and the lower drill string length is h6 = 10 m. The inner diameter of the

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of the borehole is 2r3 = 0.216 m.

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drill pipe is 2r1 = 0.109 m, the outer diameter of the drill pipe is 2r2 = 0.127 m, and the diameter

The analysis assumes that the emission power of the EM-MWD is 6 W, the resistivity of the

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drill string is 10-7 Ω • m, the resistivity of the insulated gap is 107 Ω • m, and the resistivity of the drilling fluid is 1 Ω • m. To improve the quality of the meshing, the following approach was used to divide the mesh. First, the drill string, insulated gap and the drilling fluid grid unit are all confirmed to be quadrilaterals; furthermore, the ratio of the length to the width of the unit is not more than eight. Second, within 60 m of the drill string, the mesh from the drill string begins to change from dense to sparse. Third, beyond the drill string, i.e. beyond 60 m, the grid is thinner, and the unit shape is a triangle. The relationship between the received signal voltage and the interlayer resistivity simulated by ANSYS software is shown in figure 3, while the relationship between the received signal voltage

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and the resistivity of the lower formation is shown in figure 4. Figure 3 shows that when the resistivity of the upper formation and the lower formation is constant, the voltage of the ground receiving signal first increases and then decreases as the resistivity in the interlayer increases. The thicker the interlayer, the more obvious the influence that resistivity changes have on the transmission of electromagnetic signals; likewise, lower the interlayer resistivity correspond to

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greater influence resistivity on signal transmission.

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Figure 4 shows that when the resistivity of the upper formation and interlayer is constant, the voltage of the ground receiving signal first increases and then decreases with the increased

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resistivity in the lower formation. Here, as the lower formation resistivity decreases, the influence

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on the received signal increases. When the lower formation resistivity exceeds 20 Ω • m, the

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change of thickness of the lower formation has little influence on the received signal.

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4. Downhole instrument assembly

The function of the downhole instrument assembly is to measure the drilling parameters

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through the sensors and then process and send these parameters to the relay station in the form of electromagnetic waves. As noted, the relay station-infused EM-MWD system can delve deeper than conventional EM-MWD systems. Here, as well depth increases, the temperature and pressure in the well also increase, thus the inner tube of the downhole instrument must have better sealing; furthermore, the electronic components that make up the sensors and circuit boards must withstand higher temperatures. Therefore, the inner tube is sealed with an annealed copper face seal that can withstand pressure greater than 54 Mpa; furthermore, a high temperature-resistance electron device was used. The experiments showed that these electronic devices can work for a long time at 175°C.

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4.1 Insulated gap (transmission antenna) As the depth of the well increases, the environment of the insulated gap worsens, increasing the requirements to ensure its proper operation. Insulating materials are generally non-metallic materials, and their strength is limited; therefore, it is necessary to improve the overall strength of the insulated gap in the structural design and through processing technology (Xue Q L et al 2016).

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Figure 5 provides a structure diagram and a photo of the designed insulated gap. As suggested in

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the figure, the insulated gap is primarily composed of an upper joint, a lower joint, a high-strength insulating layer and glass fibre reinforced plastic. The upper joint is designed as a male buckle

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with a large pitch rectangular thread. The lower joint is designed as a female buckle that

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cooperates with the male buckle of the upper joint. Here, the gap between the male and female

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buckles is approximately 1 mm.

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Furthermore, the high-strength insulating layer is made up of alumina ceramic and high-strength adhesive. More specifically, a layer of alumina ceramic is processed by hot pressing

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on the thread of the upper joint. Next, a thin layer of silicon film is coated on the surface of the alumina ceramic by chemical vapour deposition. Then, a toughening treatment is applied to the alumina ceramic layer at a temperature of 1200–1580°C. The strength of the alumina ceramic layer is then greatly improved after this toughening treatment. Next, a high-strength epoxy resin adhesive is coated on the surface of the alumina ceramic layer, and the upper and lower joints are tightened through the thread. Finally, to protect the alumina ceramic layer and epoxy glue, and to improve the overall strength of the insulated gap, the outside of the threaded connection section is wrapped in glass fibre reinforced plastic.

4.2 Power-saving technology

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Most EM-MWD systems are battery-powered; with the proposed relay station-induced EM-MWD system working far deeper than conventional EM-MWD systems, increasing battery life and operating time is crucial. When battery capacity is fixed, the use of power-saving technology is an effective approach to extending battery lifetime. The proposed EM-MWD system is powered by a high temperature lithium battery and uses several power-saving technologies.

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First, in the underlying circuit design, ultra-low-power consumption electronic components and

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sensors were used. For example, low-power sensors such as MEMS accelerometers and MEMS gyroscopes are applied to control measurement units in the hole. At the same time, signal

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processing, control circuit, high-performance electronic integrated devices, interfaces,

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communications and other modules are integrated in the control unit to reduce power consumption.

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Second, the working mode was optimised by turning off the radio-frequency (RF) emission circuit

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when data transmission is not needed, such as when drilling stops and while running the drilling tool in the well. Third, the transmission power of the signal transmitter was adjusted according to

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the depth of the well and the electromagnetic attenuation characteristics of the formation. Because the attenuation of signals is related to formation parameters, signal transmission frequency and well depth, the intensity of received signals on the ground varies in different working environments. If the power required in the case of maximum signal attenuation is used as the fixed transmission power, a large number of battery power will be wasted. For this reason, we establish multivariate functions of transmitting power and well depth, signal transmitting frequency and formation parameters. Formation resistivity is measured in real time by EM-MWD system and well depth is provided by surface. When the radio frequency rate is determined, the transmitting power is continuously adjusted within a fixed time interval according to the well depth and

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formation parameters, thus prolonging the working time of the battery. Fourth, at the duty period of the pulse signal, differential pulse phase modulation (DPPM) technology is used to turn off components with larger power consumption, such as RF emission circuits and signal transmitters. By using these power-saving methodologies, the lifetime of the downhole instrument assembly was increased from 150 hours to 180 hours.

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5. Relay station

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After receiving a signal from the downhole transmitter, the relay station does not merely amplify and transmit the signal. Instead, the signal is demodulated and decoded, and the measured

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data are restored, recoded, re-modulated and transmitted via the transmitter of the relay station.

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These steps ensure a quality signal and support a multi-level relay configuration(Xia M Y and

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Chen Z Y 1993). The most important function that the relay station realises is to receive and

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transmit downhole data in real time; as such, to prevent data loss caused by slow receiving speeds, the data processing and calculation requirements must be completed as quickly as possible after

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receiving the data. Therefore, it is necessary to use a high-performance receiving processor, and in software development, relevant algorithms are used to speed up these calculations. Overall, as illustrated in figure 6, the relay station is primarily composed of the receiving and transmitting antenna, the receiving processor module, the signal forwarding processor module and the power module (Wait J R and Hill D A 1979).

5.1 Electromagnetic excitation mode of the relay station The electromagnetic excitation of the transmission antenna can be divided into the vertical magnetic field method and the vertical electric field method according to the corresponding electromagnetic characteristics. To determine the electromagnetic excitation method, the

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excitation field generated by the vertical electric dipole and magnetic dipole in the ground surface is calculated. According to classical electromagnetic theory, the field component expression of the vertical electric dipole on the ground under the cylindrical coordinate system can be deduced. Likewise, for the perpendicular magnetic dipole, according to the equations, boundary conditions, and the relationship between field components and the hertz magnetic vector, the field of the

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vertical magnetic dipole on the ground was determined (Vong P and Kheong R 2005). The ratio of

was determined as

E

z 0



s T , lh

(3)

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m

e 0

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E0m

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the horizontal components of the electric field generated by the two dipole antennas on the ground

where E0 represents the horizontal component of the electric field on the ground generated by e

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magnetic excitation, E0  represents the horizontal component of the electric field on the ground

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generated by electric excitation, s represents the ring area of the magnetic dipole, l represents the length of the electric dipole, h represents the well depth, and T represents a coefficient with T∝1. m

e

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Equation (3) shows that the ratio of E0 to E0  is primarily determined by s/l h. Due to the diameter of the drill collar, the ring area of the magnetic dipole (denoted as s) is small, while the length of electric dipole l can be lengthened further. Therefore, given the same hole depth, the m

e

ratio of E0 to E0  is less than one, i.e. using magnetic excitation, the signal attenuates quickly, and the signal intensity of the ground is low. Hence, a vertical electrically excited antenna structure was used for the transmission antenna of the relay station.

5.2 Installation position of the relay station Since the relay station plays the critical role of relaying the given signal during signal transmission, the maximum distance supported by signal transmission is affected by setting the

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position of the drill string. If it is close to the signal source at the bottom of the well or close to the ground, the relay station may not receive the signal. To maximise the signal transmission distance of any given EM-MWD system, a reasonable position for the relay station in the drill string must be identified. Equation (2) shows that the maximum distance δ of the electromagnetic wave signal transmitted

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by the downhole emitter is closely related to emission frequency f and formation resistivity pe. In

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order to obtain the reasonable position of the relay station, the analog transmission and reception experiments of the relay signal are carried out. The transmitting, relaying and receiving modules

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are shown in Fig. 7. Firstly, the position of the relay station from the transmitter at the bottom of

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the hole is changed until the signal received on the ground can not be decoded due to the high

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error code. At this time, the maximum distance of the measured electromagnetic wave signal

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transmission is 1 500 meters. Next, the strength and decoding rate of the received signal on the ground are detected by changing the position of the relay station without changing the transmitter

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at the bottom of the hole and the receiver on the ground. The experimental results are shown in Table 1. As can be seen from Table 1, when the distance between the relay station and the bottom-hole transmitter exceeds 0.8δ (about 1200 meters), the bit error rate of the signal received by the relay station increases significantly. When the distance between the relay station and the bottom-hole launcher is less than 0.75δ (about 1125 meters), the bit error rate of the received signal is relatively stable and low. In addition, when the relay station is in or near the casing, the received signal strength on the ground was greatly attenuated. Therefore, the relay station must be at a distance of several tens of metres under the casing.

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1 2 3 4 5 6 7 8 9 10

3500 3500 3500 3500 3500 3500 3500 3500 3500 3500

Distance of relay station from bottom of hole /(m) 800 900 1000 1100 1200 1300 1400 1500 1600 1700

Distance of relay station from casing /(m) 900 800 700 600 500 400 300 200 100 0

Bit error rate /(%)

Strength of received signal /(mV)

1.23 1.22 1.23 1.22 2.33 3.35 4.52 25.38 35.24 100

24.32 23.20 22.50 20.35 19.99 18.23 16.42 10.18 1.43 0.00

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Hole depth / (m)

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Point position

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6. Ground signal reception

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The EM-MWD system obtains a useful signal by monitoring the voltage difference between the

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antenna at a distance from the wellhead and the drill rod. Because the potential of leakage current density line transmitted by the drill pipe to the ground is the concentric circle centred on the drill

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pipe, the potential difference of the antenna receiving signal is only related to the distance between

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the antenna and the drill pipe but has nothing to do with direction. The primary function of the ground receiving equipment is to process weak signals received by the antenna and restore them to an acceptable data signal to the greatest extent possible. More specifically, the ground receiving equipment does not need to perform any frequency and waveform conversion processing, i.e. the structure is already relatively simple, but its anti-interference, noise characteristics, and distortion requirements are more difficult. This structure is composed of an input protection circuit, a trap circuit, an active filter circuit, an integrated circuit and a magnifying circuit(Lu C D et al 2015). The primary parameters of the proposed EM-MWD system must measure the well angle, the azimuth, the tool face angle and temperature. The designed system defines the signal start bit and end bit as three cycles of low and high levels. For decoding, an A/D converter (ADC) collects

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digital points continuously at a certain frequency and then concentrates on processing one set of data after receiving n data points. This approach involves the continuity between two discrete sets of sequences, so data splicing is required. As the ADC sampling rate and the time interval of the digital point are both fixed, the time information of the pulse width is reflected by the number of digital points. In addition, the digital point contains amplitude information that can be determined

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by the rising and descending edges; finally, the process of pulse change can be understood and

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then decoded. Pressure sensor, temperature sensor, borehole deviation angle sensor, azimuth angle sensor and tool face angle sensor are installed on sensor bracket, model and manufacturer of

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sensors are shown in table 2. Table 3 is the ground receiving data measured in an experimental

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hole.

2 3 4

Hole deviation angle Azimuth angle Pressure Temperature

Model

Output Signal

MODEL 750

RS-232

American APS

RS-232 Differential signal Analog voltage

701 research institute American Kulite Taiwan Shenjie

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1

Name

MIS3D-3 IPT-25-750 SJ-300P

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Sequence Number

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Table 2. Sensors used in designed EM-MWD system Manufacture

Table 3. Ground receiving signal in a survey hole

Hole Depth m

Polar Distance m

Received Signal Strength mV

Hole deviation angle°

Azimuth Angle °

Pressure MPa

Temperature ℃

Battery Voltage V

2790 3120 3210 3309 3418

50 50 50 70 70

36 32 29 27 25

4.5 4.6 4.7 4.7 5.0

222.8 221.3 220.4 218.9 218.4

33.2 36.4 37.9 39.7 41.1

60.5 73.4 75.2 77.3 79.4

25.36 25.30 24.38 24.20 24.12

7. Conclusions

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In this paper, the working principle of a relay station-infused EM-MWD system, the main factors affecting the signal transmission, electromagnetic excitation mode and some experiments are studied. The numerical simulation shows that the change of formation resistivity and thickness will have a significant impact on signal transmission. when the resistivities of the upper and lower

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formations remain constant, the voltage of the ground receiving signal first increases and then

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decreases as the resistivity of the interlayer increases. A thicker interlayer corresponds to a more obvious influence resistivity change on the transmission of the electromagnetic signal.

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Furthermore, when the resistivity of the upper stratum and interlayer remains constant, the voltage

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of the ground receiving signal first increases and then decreases as the resistivity of the lower

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signal increases.

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stratum increases. Here, as the lower formation resistivity decreases, the influence on the received

To improve the lifetime of downhole instruments required in EM-MWD systems, methods for

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optimising the working mode, adjusting the emitting frequency and using DPPM coding were adopted. Through a comparative analysis of both electric and magnetic excitation methods, the results show that signal attenuation was faster using magnetic excitation and that the signal strength obtained by the ground was low. Therefore, a vertically electrically excited antenna structure was used for the relay station antenna. The experimental results show that the design scheme of the relay station-infused EM-MWD system is feasible and the optimum installation position of the relay station is about 0.75δ from the bottom of the hole. When the relay station approaches the casing, the received signal intensity on the ground will be severely attenuated.

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Acknowledgment

The work was supported by 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 research fund from China Geological Survey Project: research on coring technology and tools for ultra deep and ultra deep wells. The authors wish to thank Doctor

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Chun Sao, Jiahao Wang and other colleagues who helped with the tests.

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References

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Timothy W and Yu X H 2009 A neural network receiver for EM-MWD baseband communication systems IEEE

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Trans. Neural Netw. 3360–4

IEEE Trans. Geosci. Electron. 17 21–4

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Wait J R and Hill D A 1979 Theory of transmission of electromagnetic waves along a drill rod in conducting rock

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Lu C H, Jiang G S, Wang Z Q and Wang J H 2016 The development of and experiments on electromagnetic

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measurement while a drilling system is used for deep exploration J. Geophys. Eng. 13 824–831

Weisbeck D, Blackwell G, Park D and Cheatham C 2002 Case history of first use of extended-range EM MWD in

off shore, underbalanced drilling IADC/SPE 74461 1-7

Mugoya R, Yao A G, Mupenzi J P 2011 The study of signal propagation in electromagnetic-measurement while

drilling (EM-MWD) telemetry systems, Journal of American Science, 7(3), 153-156

Hu Y F, Yang C G, Gao B T, Wang L S, 2015 Reasearch and application of downhole electromagnetic relay

transmission technology, Sciences & Technology Review, 33(15), 66-71

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Science and Technology, 39(9), 114-117

Lu C D, Dong H B, Wang J H and Zhang C 2015 A design of EM-MWD signal reciever circuit based on STM32,

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Chinese Journal of Engineering Geophysics, 12(4), 423-427

Meng X F, Chen Y J, Zhou J and Meng Y F 2010 Microwave propagation in air drilling Pet. Sci. 7 390–6

Vong P and Kheong R 2005 Modeling and electromagnetic telemetry system for signal transmission in oil fields

IEEE Trans Magn. 41 2008–11

Wait J R and Hill D A 1979 Theory of transmission of electromagnetic waves along a drill rod in conducting rock

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IEEE Trans. Geosci. Electron. 17 21–4

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Xia M Y and Chen Z Y 1993 Attenuation predictions at extremely low frequencies for measurement-while-drilling

electromagnetic telemetry system IEEE Trans. Geosci. Remote Sens. 31 1222–8

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Xue Q L, Henry L, Wang R H, Liu B L and Wu Y S 2016 Continuous real-time measurement of drilling trajectory

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with new state-space models of kalman filter, IEEE Trans.Instrumentation and measurement 65,144-148

Figure 2. Assumed calculation model diagram

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Figure 1. Working mechanism diagram of a relay station-infused EM-MWD system

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1-upper formation；2-interlayer；3-lower formation；4-drilling fluid；5-upper drill string；6-insulated gap；7- lower drill string

Figure 3. Relationship between the voltage of the received signal and the resistivity of the interlayer

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Figure 4. Relationship between the voltage of the received signal and the resistivity of the lower formation

Figure 5. Structure diagram and photo of the designed insulated gap 1-upper joint；2- alumina ceramic；3- high strength adhesive；4- glass fiber reinforced plastic； 5-lower joint; 6-photo of insulated gap

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Figure 6. Working principle diagram of a relay station

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Figure 7. The transmitting, relaying and receiving module for experiments

Journal Pre-proof Author statement: We have completed all modifications according to the reviewer's comments. See response to reviewer. For the marked modification paper, see revised [02] please.

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Chunhua Lu

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This manuscript has not been published elsewhere and is not under consideration by another journal. We have approved the manuscript and agree with submission to Journal of

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Applied Geophysics. There are no conflicts of interest to declare.

Journal Pre-proof Highlight In this paper, an EM-MWD system that incorporates a relay station within ultra-deep wells is developed. The system consists of a downhole instrument assembly, a relay station and a ground receiver assembly. A model was also established to study the propagation characteristics of electromagnetic waves emitted by an EM-MWD system in heterogeneous and stratified strata. The numerical simulation results showed that when the resistivities of the upper and lower formations remain constant, the voltage of the ground receiving signal first increases and then decreases as the resistivity of the interlayer increases. A thicker interlayer corresponds to a more obvious influence resistivity change on the transmission of the electromagnetic signal. Furthermore, when the resistivity of the upper stratum and interlayer remains constant, the voltage of the ground receiving signal first increases and then decreases as the resistivity of the lower stratum increases. Here, as the lower

of

formation resistivity decreases, the influence on the received signal increases.

Based on the stated results, a design scheme for an insulated gap was proposed (i.e. a downhole

ro

transmitting antenna) in which a layer of alumina ceramic was thermally pressed on the surface of a male connector to play an insulating role. To improve the lifetime of downhole instruments required in

-p

EM-MWD systems, methods for optimising the working mode, adjusting the emitting frequency and using DPPM coding were adopted. Through a comparative analysis of both electric and magnetic

re

excitation methods, the results show that signal attenuation was faster using magnetic excitation and that the signal strength obtained by the ground was low. Therefore, a vertically electrically excited antenna structure was used for the relay station antenna. The results showed that the installation

lP

position of the relay station influenced the maximum depth of the electromagnetic signal transmission. Thus, a reasonable position of the relay station was analysed, and the ground receiving system and

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na

decoding method were introduced.