(China Petrochemical Engineering Technology Research Institute, Beijing 100 10 1)

Aiming at the phenomenon that electromagnetic wave while drilling (MWD) system is easily affected by well site noise, a remote weak low-frequency electromagnetic wave array receiving and processing method based on ZigBee protocol is proposed, which is used in the field of remote receiving of electromagnetic wave signals of MWD system and real-time measurement of downhole information, so as to improve the detection performance of ground receivers of MWD system. The experimental results show that compared with the traditional receiving method, this method can improve the processing gain by about 10dB, and has a good application prospect in the field of measurement while drilling.

Electromagnetic wave measurement while drilling; Array signal; ZigBee protocol of electromagnetic wave

Research on Remote Electromagnetic Signal Receiving and Processing Based on ZigBee Technology

Liu, Liu Xiushan, Yang,

(China Institute of Petroleum Engineering, Beijing1001KLOC-0/,China)

A remote electromagnetic signal receiving and processing algorithm of electromagnetic measurement system while drilling based on ZigBee technology is proposed. The proposed method measures electromagnetic signals by using an electrode array far away from the well site, which can enhance the ability of the communication receiver to extract very weak signals from a large amount of ambient noise. The field experimental results show that this method can improve the processing gain of 10dB, and has a good application prospect in electromagnetic measurement while drilling system.

Keywords electromagnetic measurement while drilling; Array signal processing; Electromagnetic wave; ZigBee protocol

Electromagnetic measurement while drilling (EM -MWD), as the main technical means to solve the problem of measurement while drilling in gas drilling and various gas-filled drilling, has been highly valued by oil service companies at home and abroad. However, due to the particularity of the working environment of electromagnetic MWD, the propagation of low-frequency electromagnetic waves in the stratum medium is inevitably influenced by the channel medium, especially in the non-uniform stratum transmission channel medium, the attenuation and distortion of electromagnetic wave propagation are more serious, which leads to the rapid degradation of the transmission performance of electromagnetic MWD system and the greatly reduced transmission depth [1 ~ 4]. Therefore, under the interference of well site noise and channel noise, the research on receiving and processing technology of low-frequency electromagnetic wave signals has always been the focus and difficulty of electromagnetic MWD system research.

At present, the ground receiver of electromagnetic MWD system mainly uses the electromagnetic signal with downhole information between the ground electrode and the derrick to obtain downhole information, and the distance between the electrode and the derrick is about1100mm ... Its working principle is: coupling transformer induces weak electromagnetic signal with downhole information, then processes the data through preamplifier and low-pass filter, and finally decodes the electromagnetic signal with downhole measurement information through digital signal processing technology to obtain downhole information. Patent application No.2008101407 invented an electromagnetic MWD system, which uses two antennas to receive electromagnetic signals sent underground and noise signals from the well site respectively, and its ground receiver has the function of processing electromagnetic signals and measurement data information sent underground [5]. The patent application number is 2004 10005527. X, invented a measurement while drilling telemetry system [6], which can process electromagnetic wave signals in the medium. The patent application number is 20 102098570.0, which invented a method to solve the problem that it is difficult for oil well measurement system to pick up weak electromagnetic signals. These patents all deal with weak electromagnetic signals in the electromagnetic interference environment of well sites. However, these methods are affected by electromagnetic noise in the well site to varying degrees, especially the electromagnetic noise generated by various electrical equipment in the well site, such as drilling rig, diesel engine, generator, mud pump, transmission chain, vibrating screen, etc., which seriously reduces the performance of the ground receiver of electromagnetic MWD system in processing low-frequency electromagnetic signals and greatly reduces the receiving sensitivity.

Fund Project: National Major Project "Monitoring Technology for Wellbore Environment of Offshore Oil and Gas Wells" (201kloc-0/zx05005-006).

Aiming at the phenomenon that electromagnetic wave measurement system while drilling is easily affected by well site noise, a remote weak low-frequency electromagnetic wave array receiving and processing method based on ZigBee protocol is proposed. This method takes array signal processing as the core, uses sensor array to receive and process weak electromagnetic wave signals with underground measurement information, and uses ZigBee protocol chip to realize remote transmission of signals. This method can effectively reduce the influence of well site noise on electromagnetic MWD and improve the processing gain and sensitivity of the ground receiver of electromagnetic MWD system.

Design of 1 electromagnetic MWD remote receiving system

1. 1 working principle and main functions of electromagnetic MWD remote control receiving system

Electromagnetic MWD remote receiving system consists of well site receiver and remote receiver; The remote receiver includes sensor array, coupling transformer, preamplifier, bandpass filter, DSP signal processor and wireless transceiver. The remote wireless receiving system for electromagnetic wave measurement while drilling is shown in figure 1.

Figure 1 Remote wireless receiving system for electromagnetic wave measurement while drilling

The working principle of the remote wireless receiving system for electromagnetic wave measurement while drilling is that the directional probe measures the downhole information and transmits the measurement signal to the downhole transmitter according to the specified protocol. After the underground transmitter encodes and modulates the measurement information, it sends out electromagnetic waves with underground measurement information, which are transmitted to the ground through drill pipes, exposed well walls and strata, and the remote sensor array installed on the ground receives the electromagnetic signals. The received electromagnetic wave signal is filtered, amplified, demodulated, encoded and encrypted, and then transmitted through the wireless transceiver based on ZigBee technology. The well site receiver receives wireless signals, saves and manages the received data, and displays them on the driller's display.

The main functions of remote receiving are: (1) using sensor array to receive signals transmitted by underground electromagnetic MWD system far away from the well site; (2) signal processing, noise reduction, superposition and other processing are carried out on the received weak electromagnetic wave signal; (3) The processed signals are packaged and encrypted, and transmitted through the ZigBee protocol module.

The difference between the remote wireless receiving system and the existing electromagnetic wave measurement while drilling system lies in the addition of (1) ZigBee technology, which can effectively reduce the noise interference of the well site even if the ground electrode is far away from the well site; (2) Adopt array receiving mode, that is, adopt multi-channel information acquisition technology and array signal processing technology to process signals, so as to reduce noise interference.

1.2 sensor array design and array signal processing technology

The existing electromagnetic MWD system uses single-channel differential reception to receive the signal between the derrick and the grounding electrode. In order to effectively reduce the interference of well site noise, the ground electrode array is used to receive electromagnetic signals with downhole information.

Assume that the sensor group of the receiving device is an antenna array with n elements, as shown in fig. 2. The signal processing block diagram of multi-channel remote control receiving device is shown in Figure 3.

Fig. 3 signal processing block diagram of remote receiving device

The number of array elements is 1 # and 2 #...n #, and the spacing of equally spaced array elements is d (Figure 4). The carrier frequency of the transmitter is ω, the wavelength is λ, the propagation speed is V, the propagation time of the signal reaching the 2 # array element is τ, and the delay distance is u, then the delay between adjacent array elements is.

Figure 4 Remote Ground Receiving Linear Array

Oil and gas accumulation theory and exploration and development technology: Proceedings of Postdoctoral Academic Forum of China Petrochemical Petroleum Exploration and Development Research Institute 20 1 1. 4

Assuming that the received signal is X(t), the useful electromagnetic signal is s(t) and the noise is n(t), there are

Oil and gas accumulation theory and exploration and development technology: Proceedings of Postdoctoral Academic Forum of China Petrochemical Petroleum Exploration and Development Research Institute 20 1 1. 4

Then the total response of the observation signal is

Oil and gas accumulation theory and exploration and development technology: Proceedings of Postdoctoral Academic Forum of China Petrochemical Petroleum Exploration and Development Research Institute 20 1 1. 4

Oil and gas accumulation theory and exploration and development technology: Proceedings of Postdoctoral Academic Forum of China Petrochemical Petroleum Exploration and Development Research Institute 20 1 1. 4

λ0 is the wavelength corresponding to the central frequency of X(t). According to transmitter frequency, array spacing, delay and other parameters, an appropriate sensor array can be arranged. From the point of view of signal analysis, the received electromagnetic signal containing downhole information is different from the interference wave as follows: (1) The spectrum of carrier signal is different from that of noise; (2) The statistical laws are different. Therefore, digital signal processing techniques such as signal superposition method and time-frequency filtering method can be used to effectively reduce noise interference and improve processing gain and receiving sensitivity. The purpose of array design in electromagnetic MWD remote receiving system is to maximize the signal-to-noise ratio of received data and reduce the influence of well site noise. In the actual implementation process, it is necessary to arrange and select the receiving points and their mutual positions reasonably. Using linear array instead of area array avoids the complexity of debugging and reduces the cost.

For the remote receiving system of electromagnetic wave measurement while drilling system, the remote receiving system uses single chip microcomputer or DSP system to build a remote master control unit, and uses the powerful signal processing ability of DSP to process the received array signals, and sends the processing results to the well site receiver through ZigBee protocol module. In this design, TMS320LF28 12 and a chip containing ZigBee protocol module are selected to build a remote small receiving device, and the well site receiver is also equipped with the same ZigBee protocol module. In this way, the remote receiving device can transmit the weak electromagnetic signals received by the remote sensor array to the well site receiver, and complete the functions of collecting, receiving and processing remote information. The monitoring software design of remote receiving system of electromagnetic wave measurement while drilling system includes DSP main program, algorithm processing program and monitoring program. Among them, DSP program and algorithm processing program are written in C language, and monitoring program is written in Labview. The software of ground receiver includes DSP program and ground monitoring program. Programming with C language, it mainly completes signal and noise data acquisition, A/D conversion, digital filtering and decoding, and connects with the data transmission module through RS-232 interface, and the data is sent out by the data transmission module. I won't go into details here.

2 Analysis of experimental results

The system was tested in the field in Well D66- 129 in Daniudi gas field in North China. The main working parameters of the remote receiving system of electromagnetic wave measurement system while drilling are as follows: the amplification factor of the preamplifier of the electromagnetic MWD remote receiving system is 1 ~ 100000 times, the band-pass filter has a frequency range of 1 ~ 35hz, and the bandwidth is adjustable; The frequency of the signal transmitted by the downhole transmitter is 3 ~ 25 Hz (adjustable according to the formation characteristics); The grounding electrode is 500 ~1000m away from the derrick, and eight groups of electrodes are used. Sampling 8 channels of data; The observation sensitivity is about-12BV. Figure 5 shows the real-time acquisition of well site noise waveform. Sampling frequency fs = 2000 Hz. As can be seen from Figure 5, the working frequency band of electromagnetic MWD system has two line spectrums, which are 6Hz and 1 1.6Hz respectively, and the 50Hz power frequency of well-site generator set and related drilling equipment is also the main component of well-site noise. However, it is difficult to find the line spectrum with the transmitter signal frequency of 10Hz from the noise spectrum.

Figure 5 Noise waveform of Well D66-129

After eight channels of data are collected, the noise spectrum shown in Figure 6 can be obtained by using Equations (6) and (7). Obviously, the 10Hz line spectrum of the transmitter frequency in Figure 7 is about 10dB higher than that in Figure 6. It should be noted that the start and stop of well site working equipment and instruments will greatly affect the performance of the original EM-MWD system, especially when high-power electrical equipment (such as generator sets and mud pumps) is working.

Figure 6 Noise Spectrum of Well D66-129

Fig. 7 Noise spectrum observed after array signal processing

Through the analysis of noise in D66- 129 and DPS-2 well sites, it is found that the noise in well sites is approximately stationary Gaussian distribution when the rotary table or top drive is closed. However, when the turntable or top drive is turned on, the well site noise has obvious non-stationary and non-Gaussian characteristics. This non-stationary and non-Gaussian noise directly affects the ground decoding performance of EM-MWD systems at home and abroad, especially when the turntable is turned on or the top drive is turned on, the data error rate increases and the data reliability decreases. Although the receiving method far from the well site proposed in this paper reduces the amplitude of the received signal to a certain extent, the noise amplitude is more obvious than the signal. Through array signal processing and multi-channel data superposition, the processing gain of remote receiving system can be effectively improved. The ZigBee remote receiving device is powered by button cell, which saves the inconvenience of long-distance wiring in the field and greatly reduces the cost.

3 Conclusion theory

Aiming at the problem that electromagnetic wave signal of electromagnetic MWD system is easily disturbed by well site noise, a remote weak low-frequency electromagnetic wave array receiving and processing method based on ZigBee protocol is proposed.

The field test shows that:

1) When the grounding electrode is placed at a distance of 100m from the derrick, with the increase of the displacement of the electrode from the well site, the attenuation amplitude of noise is more obvious than that of signal under the same working conditions.

2) When laying a suitable sensor array, it is necessary to consider the underground transmitter and borehole position, especially the transmitter frequency, array element spacing, delay and other parameters. The experimental results show that compared with the traditional receiving method, this method can improve the processing gain by about 10dB.

refer to

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Liu Xiushan, Hou, Tu Yulin. Present situation and development trend of electromagnetic survey while drilling [J]. Petroleum Drilling Technology, 200634 (5): 4 ~ 9.

[3] McDonald W.J. Four different systems for MWD [J]. Petroleum &Gag,1978,76 (14):115 ~124.

[4] Zulier, Louis, Lemaistre, Michel. Present situation and prospect of MWD data transmission [C].SPE/IADC 25686,1993:121~128.

Liu Xiushan, Gao, Yang, et al. Ground signal receiving device of electromagnetic measurement while drilling system and its receiving method [P]. China: 2008101407, 2008.

Su Yinao, Sheng Limin, Li Lin. Electromagnetic telemetry method and system for measurement while drilling. China: 2004 1005527. X, 2004

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