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Vibration wave downhole communication technique
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 44, Issue 2, April 2017 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2017, 44(2): 321–327.
RESEARCH PAPER
Vibration wave downhole communication technique ZHENG Lichen*, YU Jiaqing, YANG Qinghai, GAO Yang, SUN Fuchao PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China
Abstract: To overcome the disadvantages of traditional downhole communication methods, a vibration wave downhole communication technique is proposed, and a vibration wave downhole communication system is developed. This technique has been verified by field test and is applied to separated layer water injection. It is shown by theoretical and test research that transmission of the vibration wave through tubing and casing appears as the alternate distribution of pass-band and stop-band. According to that, a multi-baseband transmission strategy is formulated. The on-off keying modulation and Manchester encoding scheme are used to load the control information into the vibration wave. A generation system of vibration signals is developed to realize the controllable conversion from electric energy into vibration wave energy. A receiving and decoding system of vibration waves, which uses a micro-vibration acceleration sensor as the signal pickup element, is developed too. A test system for vibration wave downhole remote transmission is designed and applied to field test. The feasibility of the technique and the accuracy and reliability of communication system are verified and the attenuation characteristics of casing vibration wave signals are obtained. This technique has been applied to separated layer water injection successfully with wide application prospect in wellbore control field. Key words: vibration wave; downhole communication; on-off-keying modulation; Manchester encoding; magne-tostrictive material; micro-vibration acceleration sensor
Introduction With the progress of reservoir development technology, separated layer injection-production and reservoir stimulation techniques have presented more strict demand for wellbore control. Downhole communication technique is the key to realizing remote downhole control from the ground and data transmission between the ground and downhole, but due to complicated downhole conditions, it is impossible to apply directly the traditional communication techniques such as wireless communication to downhole communication process. At present, cable communication technique, pressure wave communication technique and intelligent well technique are commonly used means of downhole communication. Cable communication technique, employs carrier or load modulation method to realize the long-distance wire communication via steel armored cable[1–2], and can be divided into two types based on its working mode, i.e. inside-tubing cable communication and through-annulus cable communication. Insidetubing cable communication generally adopts the mode of relay communication to make near-distance wireless communication between downhole instruments. In this case, accurate positioning is required, and site execution is complicated due to the use of a test vehicle for communication. For application
of through-annulus cable communication technique, cable shall be run in together with the string upon well completion, which is difficult to execute. With poor adaptability and low reliability, this technique currently has limited application. Pressure wave communication technique, which uses a ground fracturing truck as the remote control tool, controls the water injection and drainage time interval to generate pressure wave communication signals by injecting water into the wellbore to cause pressure change inside the wellbore. The downhole pressure transmitter picks up the pressure change signals, and then identifies and processes them further[3–4]. This method features long control distance and simple downhole tools. However, requiring the use of large equipment such as a fracturing truck, it is costly and complicated in operation. In recent years, intelligent well technique has made rapid progress overseas, and been applied to some high productivity wells. In intelligent well technique, hydraulic control tubing and cable are preset downhole, the signals of downhole pressure, temperature and flow rate are transmitted to the ground via cable, and downhole valves are control remotely with hydraulic equipment via hydraulic tubing based on ground judgment[5–6]. With this technique, the real time monitoring, signal transmission and real time control of downhole status can be realized. Nevertheless, due to big initial investment
Received date: 19 Apr. 2016; Revised date: 16 Jan. 2017. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National High Technology Research and Development Program (863 Program), China (2012AA061300). Copyright © 2017, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
ZHENG Lichen et al. / Petroleum Exploration and Development, 2017, 44(2): 321–327
and high operation risk, this technique is applicable to high productivity wells only. In order to overcome the disadvantages of traditional downhole communication methods and realize remote control of wellbores, a vibration wave downhole communication technique is proposed in this paper, which uses tubing or casing as the transmission medium and vibration wave as the carrier to realize downhole information transmission by digital modulation. A vibration wave downhole communication system has been developed and tested on site.
1. The vibration wave transmitting through tubing or casing As the basic component of a wellbore, tubing or casing possesses the conditions to serve as the transmission medium of vibration waves. To enable the use of downhole tubing or casing string as the transmission medium of vibration signals, it is necessary to analyze the transmission characteristics of vibration waves through tubing and casing string. The transmission rules of vibration waves through tubing or casing have been analyzed theoretically in references [7–12]. It has been found that due to their periodic structure, tubing and casing show the characteristics of a comb filter in signal transmission, i.e. the alternate presence of passband and stopband. Based on theoretical analysis, a transmission characteristic test of a 100 m-long tubing string was carried out to obtain more accurate transmission characteristics of vibration wave in a system composed of tubings and couplings. During the test, a signal hammer was used to strike the end face of tubing string to generate vibration waves, acceleration sensors were placed at the end face of tubing string and every tubing coupling, and a vibration signal analysis system was then utilized to analyze the hammering signals and the acceleration sensor signals. The test data is shown in Figs. 1-3. Fig. 1 shows the hammering signals input. Fig. 2 shows the frequency spectrum of hammering signals input. Fig. 3 shows the signals received by acceleration sensors placed at different locations. In this figure, signal 1 is the signal received by a sensor placed at the end face of tubing string, while signal 2 is the signal received by another sensor 100m away. It is found by analysis that the output signals and the input signals have very good correlation with a correlation coefficient of 1, that is to say, apart from the change of amplitude, other characteristics of the vibration waves remain unchanged after transmitted through the tubing string, which is the basis of signal transmission by utilizing vibration wave. Fig. 4 shows the transmission characteristics of vibration signals through tubing string. It can be seen that the transmission of vibration wave through tubing string shows the characteristics of a comb filter, that is to say, some frequencies have stopbands where signals attenuate acutely, and cannot be used for signal transmission. A vibration wave transmission test was conducted on a casing string, and similar conclusions were reached. The
Fig. 1.
Fig. 2.
Hammering signals.
Frequency spectrum of hammering signals.
Fig. 3. Signals received by acceleration sensors at different locations.
Fig. 4. Transmission characteristics of vibration signals through tubing string.
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theoretical and test analysis have verified the feasibility of vibration wave remote transmission. Meanwhile, it is found that the transmission of vibration wave appears as alternate distribution of passband and stopband.
2. Key technology for vibration wave downhole communication The vibration wave downhole communication system consists of a vibration signal generation system and a vibration signal receiving and decoding system. To make it work, a high power vibration signal generator has to be fixed onto the wellhead, which can generate vibration signals containing control information as requested, the signals then will be transmitted downhole through the casing or tubing. A vibration signal receiving and decoding system equipped inside the downhole actuator receives and decodes the vibration signals transmitted through tubing or casing, and then converts them into control instructions to realize the control of downhole instruments. 2.1.
Transmission strategy of vibration wave signals
Transmission of vibration wave signals through tubing or casing string follows certain pattern, which appears as alternate distribution of passband and stopband. Therefore, the rational selection of fundamental-frequency is of great importance for successful downhole transmission of vibration waves. The stopbands of a tubing or casing string can be avoided by utilizing the distribution rule. A multi-fundamental-frequency information transmission strategy is formulated in this paper in which multiple fundamental-frequencies at certain interval are used as carriers, and then modulated by digital control signals separately. Due to the resonant frequency limitation of vibration wave generation system, the upper frequency limit of vibration waves generated is 350 Hz. It is found by theoretical analysis and modeling test that there is only one stopband existing with the size series of tubings and casings used in oilfields at the frequency range of 0350 Hz, and so the stopband of tubing or casing system can be avoided to ensure satisfactory downhole transmission of information so long as more than two fundamental-frequencies are selected for transmission. In this paper, three fundamental-frequencies of 100 Hz, 200 Hz and 300 Hz are used to transmit vibration signals.
Fig. 5.
OOK modulation.
In the case of transmitting vibration waves through tubing or casing, there will be greater reflection during transmission and fast attenuation of signals because it is difficult to have continuous impedance. In order to stabilize the system and prevent code errors, Manchester encoding is selected as the encoding scheme. See Fig. 6 for its principles. For every bit, the middle falling edge is 0, and the rising edge is 1. 16-bit data (4-bit layer information + 8-bit control information + 4-bit parity check) are used to encode the vibration signals. The first 4-bit layer information can control any downhole layer separately. According to the normal conditions of wellbores, the encoding of vibration signals currently employs the 4-bit layer control, that is to say, altogether 16 layers can be controlled. The middle 8-bit control information is the control instruction sending from the ground to the target layer. To ensure the validity of communication data, four parity check bits are added to the ending of data bits. Take a set of communication data “1010101000000110” for example. See Fig. 7 for its encoding. 2.3.
Generation system of vibration wave signals
The generation system of vibration wave signals consists of a vibration signal generator and a power supply system (Fig. 8). The power supply system controls the drive coils to produce alternating magnetic field, which drives the magnetostrictive material to vibrate, in this way, the electric energy is converted into the mechanical vibration energy. The power supply system can control the vibration signal generator effectively to output encoding waveform.
2.2. Vibration signal modulation and encoding Technology In order to load the control information into the carrier, it is necessary to modulate and encode the vibration waves. Learning from the modulation mode of electric signals, the on-off keying (OOK) mode is used for the modulation of vibration wave signals to facilitate transmitting system. When the digital signal modulated is “1”, the carrier is transmitted; when the digital signal modulated is “0”, no carrier is transmitted. See Fig. 5 for OOK modulation. 323
Fig. 6.
Manchester encoding principles.
Fig. 7.
Vibration signal encoding.
ZHENG Lichen et al. / Petroleum Exploration and Development, 2017, 44(2): 321–327
Fig. 8.
Vibration signal generation system.
The vibration signal generator employs an energy conversion element made of magnetostrictive material. Length of the magnetostrictive material changes under the action of an alternating magnetic field to produce mechanical vibration with the same frequency[1314]. Featuring big magnetostrictive strain, high pressure resistance, high energy conversion efficiency, high energy density and fast response, this material is an ideal energy conversion material for vibration wave generation. The working principle of a vibration signal generator is: When the length of magnetostrictive material rod changes under the action of drive coils, the induction hammer is driven to move up and down, but remains its pressure on the magnetostrictive rod under the action of gravity and spring restoring force and keeps on contact. The reactive force of magnetostrictive rod on the base is transmitted outwards via the base connecting flange. For communication, the vibration signal generator is installed onto the wellhead through the base connecting flange to generate vibration waves, which will be transmitted through the tubing or casing for remote communication. See Fig. 9 for the signal waveform actually output by the vibration signal generator. 2.4.
Receiving and decoding system of vibration signals
Consisting of a microvibration acceleration sensor and a decoding control panel, the receiving and decoding system of vibration signals is an important component of downhole actuating device, which receives and decodes the vibration signals transmitted through tubing or casing, and then converts
them into control instructions for downhole actuator. Because of the greater reflection and fast signal attenuation associated with the transmission process of vibration waves, the receiving sensor has to be highly sensitive, and the receiving and decoding system of vibration signals must have good signal processing capacity. In this study, special microvibration acceleration sensors with the resolution of 4×10-4 m/s2 are used to pick up signals. In the case of multi-fundamental-frequency transmission strategy, the system can receive and decode vibration wave signals of different frequencies. With small signals and the impact of extraneous noises taken into account, there are two options of magnification factors (1 000 and 100) available for signal acquisition. According to the test, the system resolution has reached 10-3 m/s2, which is capable of meeting the requirements for receiving and decoding vibration signals.
3. Remote transmission field test of vibration wave signals In order to verify the feasibility of vibration signal transmission in wellbores, find the relationship between transmission distance and signal intensity, and test the accuracy and reliability of the signal processing system, a remote transmission field test of vibration wave signals was conducted in Well Zhong 20-310 (with a well depth of 940 m) of Daqing Oilfield. A remote transmission test system of vibration signals was designed for this test (Fig. 10). When the system works, the motor rotates, driving the eccentric shaft to turn and the cam-controlled support arm to stretch out and retract. When the support arm is stretched out, the system can be pushed to rest against the casing wall at any depth under the action of disc spring force to receive and decode the vibration signals transmitted through the casing. When the support arm is retracted, the system can move freely up and down inside the casing. The system, which is connected with a ground-based computer via steel armored cable, transmits the decoded downhole signals and other relevant information in real time to the ground computer. In the field test, the vibration signal generator was fixed onto the wellhead to serve as the signal source, and the casing was used as the transmission medium of vibration signals to realize the data transmission to the downhole. The steel armored cable loaded the device and served as the transmission medium of electric signals. The remote transmission test system of vibration signals received and decoded the vibration
Fig. 9. Waveform actually output by the vibration signal generator.
324
Fig. 10.
Remote transmission test system of vibration waves.
ZHENG Lichen et al. / Petroleum Exploration and Development, 2017, 44(2): 321–327
signals, and transmitted the vibration signals received and the signals decoded to the ground computer via steel armored cable. The test system was sent into wellbore via the steel armored cable. In this process, the test system was pushed to rest against the casing wall every 100 m to acquire vibration signals sent out by the vibration signal generator at the same interval. See Table 1 for the test data. It can be seen from Table 1 that vibration wave signals with the fundamental frequency of 100 Hz can be transmitted at any location between the wellhead and the well depth of 900 m. Due to the bottomhole vibration wave reflection, vibration wave signals with the fundamental frequency of 200 Hz and 300 Hz cannot be transmitted at the well depth of 900 m. The effective transmission and correct decoding of signals in the range of 900 m from the wellhead downhole have verified the feasibility of transmitting vibration wave digital signals through casing, the reliability of vibration wave signal encoding system and the correctness of signal decoding algorithm. The results that vibration signals with the fundamental frequency of 200 Hz and 300 Hz cannot be transmitted correctly, but vibration signals with the fundamental frequency of 100 Hz can be transmitted normally at the well depth of 900 m have verified the rationality of the multi-fundamental-frequency transmission strategy. The intensity of vibration signals received downhole follows a linear attenuation rule after treated mathematically, but the intensity of signals near the wellhead and bottomhole behaves abnormally. This is because the intensity of signals received near the wellhead is slightly less than that received downhole due to the impact of ground noises, while the signal intensity is amplified or reduced suddenly near the bottomhole due to the interference of vibration waves reflected by the bottomhole with the vibration waves normally transmitting downwards. See Figs. 11 and 12 for the attenuation trend of vibration signals with the wellhead and bottomhole factors eliminated. Under existing technical conditions, downhole Table 1.
Field test data of vibration wave remote transmission Signal intensity, dB
Fundamental freWell quency, 100 Hz depth/ m Amplification factor
Fundamental frequency, 200 Hz Amplification factor
Fundamental frequency, 300 Hz Amplification factor
100
1,000
100
1,000
100
1,000
100
30
44
43
44
25
32
200
45
44
43
45
30
30
300
41
44
38
44
30
36
400
39
43
35
41
24
35
500
36
41
33
41
25
37
600
33
38
35
43
25
36
700
33
38
33
42
18
31
800
39
43
25
34
11
23
900
17
21
Fig. 11. Vibration signal attenuation trend (with 100 amplification factor selected).
Fig. 12. Vibration signal attenuation trend (with 1000 amplification factor selected)
communication within the depth range of 3 000 m can be realized.
4. Application of vibration wave downhole communication technique The vibration wave downhole communication technique has been tested in separated layer water injection in Well +5-10.2 of Jilin Oilfield. See Fig. 13 for the design of test string. A vibration wave controlled water distributor, which consists of batteries, a gear motor, water nozzles and a vibration signal receiving & decoding system, was set at Layer 3 of the well at the depth of 416 m. The wellhead vibration signal generator generates vibration signals containing control information as requested, the vibration signals are then transmitted downhole through the tubing. The vibration wave controlled water distributor receives and decodes the vibration signals transmitted from the wellhead, and then converts them into control instructions to control the rotation of gear motor, which in turn controls the nozzle opening of water distributor. The water distributor is equipped with a nozzle opening sensor to provide 0100% full range valve opening control. The procedures of field test are given below: a. Ran in operation string. Pressurized and actuated the packer with a ground pump truck. b. Pressurized the tubing to 15 MPa, and the tubing was able to maintain the pressure of 10 MPa for more than 5
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ZHENG Lichen et al. / Petroleum Exploration and Development, 2017, 44(2): 321–327
The field test shows that it is feasible to transmit digital signals to do remote control with tubing as the transmission medium and vibration waves as the carrier, and the vibration wave downhole communication technique is applicable to separated layer water injection.
5.
Fig. 13.
Test string design.
minutes, which indicated that all downhole water nozzles were closed. c. The ground vibration signal generator sent out a control instruction “0000011001000111”. The first four bits denoted the public address of water distributor, the middle 8 bits denoted the nozzle opening, which meant open it to 100% in this case, and the last 4 bits were the parity check bits. To prevent the stopband from interrupting signals, three frequencies of 100 Hz, 200 Hz and 300 Hz were used to transmit signals at the interval of 5 minutes. d. Pressurized the tubing to 15 MPa with the ground pump truck, and the tubing pressure dropped quickly to 2 MPa in one minute (the pressure drop time was influenced by the low permeability oil layer), which indicated that the downhole water nozzle in the vibration wave controlled water distributor was open, and the vibration waves could transmit the control information normally. e. The ground vibration signal generator sent out a control instruction “0000000000000000” at the carrier frequency of 100 Hz, 200 Hz and 300 Hz respectively. The first 4 bits denoted the public address of water distributor. The middle 8 bits denoted the nozzle opening, which meant close it to 0% in this case. The last 4 bits were the parity check bits. f. Pressurized the tubing to 15 MPa, and the tubing was able to maintain the pressure of 10 MPa for more than 5 minutes, which indicated that the downhole water nozzles were closed, and the vibration waves could control the closing of water nozzles normally.
Conclusions
In order to overcome the disadvantages of traditional downhole communication methods and realize the remote control of wellbores, a vibration wave downhole communication technique is proposed in this paper. Firstly, the transmission characteristics of vibration waves through tubings and casings were analyzed, based on which a multi-fundamental-frequency transmission strategy was formulated. The OOK modulation and Manchester encoding scheme were used to modulate and encode the vibration signals, and then the control information was loaded into the carrier. A vibration signal generation system with an energy conversion element made of magnetostrictive material is developed to realize the controllable conversion of electric energy into vibration wave energy. A test system for vibration wave downhole remote transmission is developed and tested in field, which has verified the feasibility of realizing vibration wave communication with casing as the transmission medium and the accuracy and reliability of the encoding scheme and decoding algorithm of vibration wave communication system. With this test system, the attenuation characteristics of vibration wave signals transmitted through casings are obtained. Under existing technical conditions, downhole communication within the depth range of 3000 m can be realized. As a fundamental common technique for wellbore control, the vibration wave downhole communication technique provides a completely new technical means for downhole signal transmission. It has the advantages of simple operation, low cost, fast speed and long transmission distance. This technique has a wide application prospect in the areas of separated layer water injection, separated layer oil production, fracturing, formation test, and drilling.
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327
RESEARCH PAPER
Vibration wave downhole communication technique ZHENG Lichen*, YU Jiaqing, YANG Qinghai, GAO Yang, SUN Fuchao PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China
Abstract: To overcome the disadvantages of traditional downhole communication methods, a vibration wave downhole communication technique is proposed, and a vibration wave downhole communication system is developed. This technique has been verified by field test and is applied to separated layer water injection. It is shown by theoretical and test research that transmission of the vibration wave through tubing and casing appears as the alternate distribution of pass-band and stop-band. According to that, a multi-baseband transmission strategy is formulated. The on-off keying modulation and Manchester encoding scheme are used to load the control information into the vibration wave. A generation system of vibration signals is developed to realize the controllable conversion from electric energy into vibration wave energy. A receiving and decoding system of vibration waves, which uses a micro-vibration acceleration sensor as the signal pickup element, is developed too. A test system for vibration wave downhole remote transmission is designed and applied to field test. The feasibility of the technique and the accuracy and reliability of communication system are verified and the attenuation characteristics of casing vibration wave signals are obtained. This technique has been applied to separated layer water injection successfully with wide application prospect in wellbore control field. Key words: vibration wave; downhole communication; on-off-keying modulation; Manchester encoding; magne-tostrictive material; micro-vibration acceleration sensor
Introduction With the progress of reservoir development technology, separated layer injection-production and reservoir stimulation techniques have presented more strict demand for wellbore control. Downhole communication technique is the key to realizing remote downhole control from the ground and data transmission between the ground and downhole, but due to complicated downhole conditions, it is impossible to apply directly the traditional communication techniques such as wireless communication to downhole communication process. At present, cable communication technique, pressure wave communication technique and intelligent well technique are commonly used means of downhole communication. Cable communication technique, employs carrier or load modulation method to realize the long-distance wire communication via steel armored cable[1–2], and can be divided into two types based on its working mode, i.e. inside-tubing cable communication and through-annulus cable communication. Insidetubing cable communication generally adopts the mode of relay communication to make near-distance wireless communication between downhole instruments. In this case, accurate positioning is required, and site execution is complicated due to the use of a test vehicle for communication. For application
of through-annulus cable communication technique, cable shall be run in together with the string upon well completion, which is difficult to execute. With poor adaptability and low reliability, this technique currently has limited application. Pressure wave communication technique, which uses a ground fracturing truck as the remote control tool, controls the water injection and drainage time interval to generate pressure wave communication signals by injecting water into the wellbore to cause pressure change inside the wellbore. The downhole pressure transmitter picks up the pressure change signals, and then identifies and processes them further[3–4]. This method features long control distance and simple downhole tools. However, requiring the use of large equipment such as a fracturing truck, it is costly and complicated in operation. In recent years, intelligent well technique has made rapid progress overseas, and been applied to some high productivity wells. In intelligent well technique, hydraulic control tubing and cable are preset downhole, the signals of downhole pressure, temperature and flow rate are transmitted to the ground via cable, and downhole valves are control remotely with hydraulic equipment via hydraulic tubing based on ground judgment[5–6]. With this technique, the real time monitoring, signal transmission and real time control of downhole status can be realized. Nevertheless, due to big initial investment
Received date: 19 Apr. 2016; Revised date: 16 Jan. 2017. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National High Technology Research and Development Program (863 Program), China (2012AA061300). Copyright © 2017, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
ZHENG Lichen et al. / Petroleum Exploration and Development, 2017, 44(2): 321–327
and high operation risk, this technique is applicable to high productivity wells only. In order to overcome the disadvantages of traditional downhole communication methods and realize remote control of wellbores, a vibration wave downhole communication technique is proposed in this paper, which uses tubing or casing as the transmission medium and vibration wave as the carrier to realize downhole information transmission by digital modulation. A vibration wave downhole communication system has been developed and tested on site.
1. The vibration wave transmitting through tubing or casing As the basic component of a wellbore, tubing or casing possesses the conditions to serve as the transmission medium of vibration waves. To enable the use of downhole tubing or casing string as the transmission medium of vibration signals, it is necessary to analyze the transmission characteristics of vibration waves through tubing and casing string. The transmission rules of vibration waves through tubing or casing have been analyzed theoretically in references [7–12]. It has been found that due to their periodic structure, tubing and casing show the characteristics of a comb filter in signal transmission, i.e. the alternate presence of passband and stopband. Based on theoretical analysis, a transmission characteristic test of a 100 m-long tubing string was carried out to obtain more accurate transmission characteristics of vibration wave in a system composed of tubings and couplings. During the test, a signal hammer was used to strike the end face of tubing string to generate vibration waves, acceleration sensors were placed at the end face of tubing string and every tubing coupling, and a vibration signal analysis system was then utilized to analyze the hammering signals and the acceleration sensor signals. The test data is shown in Figs. 1-3. Fig. 1 shows the hammering signals input. Fig. 2 shows the frequency spectrum of hammering signals input. Fig. 3 shows the signals received by acceleration sensors placed at different locations. In this figure, signal 1 is the signal received by a sensor placed at the end face of tubing string, while signal 2 is the signal received by another sensor 100m away. It is found by analysis that the output signals and the input signals have very good correlation with a correlation coefficient of 1, that is to say, apart from the change of amplitude, other characteristics of the vibration waves remain unchanged after transmitted through the tubing string, which is the basis of signal transmission by utilizing vibration wave. Fig. 4 shows the transmission characteristics of vibration signals through tubing string. It can be seen that the transmission of vibration wave through tubing string shows the characteristics of a comb filter, that is to say, some frequencies have stopbands where signals attenuate acutely, and cannot be used for signal transmission. A vibration wave transmission test was conducted on a casing string, and similar conclusions were reached. The
Fig. 1.
Fig. 2.
Hammering signals.
Frequency spectrum of hammering signals.
Fig. 3. Signals received by acceleration sensors at different locations.
Fig. 4. Transmission characteristics of vibration signals through tubing string.
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ZHENG Lichen et al. / Petroleum Exploration and Development, 2017, 44(2): 321–327
theoretical and test analysis have verified the feasibility of vibration wave remote transmission. Meanwhile, it is found that the transmission of vibration wave appears as alternate distribution of passband and stopband.
2. Key technology for vibration wave downhole communication The vibration wave downhole communication system consists of a vibration signal generation system and a vibration signal receiving and decoding system. To make it work, a high power vibration signal generator has to be fixed onto the wellhead, which can generate vibration signals containing control information as requested, the signals then will be transmitted downhole through the casing or tubing. A vibration signal receiving and decoding system equipped inside the downhole actuator receives and decodes the vibration signals transmitted through tubing or casing, and then converts them into control instructions to realize the control of downhole instruments. 2.1.
Transmission strategy of vibration wave signals
Transmission of vibration wave signals through tubing or casing string follows certain pattern, which appears as alternate distribution of passband and stopband. Therefore, the rational selection of fundamental-frequency is of great importance for successful downhole transmission of vibration waves. The stopbands of a tubing or casing string can be avoided by utilizing the distribution rule. A multi-fundamental-frequency information transmission strategy is formulated in this paper in which multiple fundamental-frequencies at certain interval are used as carriers, and then modulated by digital control signals separately. Due to the resonant frequency limitation of vibration wave generation system, the upper frequency limit of vibration waves generated is 350 Hz. It is found by theoretical analysis and modeling test that there is only one stopband existing with the size series of tubings and casings used in oilfields at the frequency range of 0350 Hz, and so the stopband of tubing or casing system can be avoided to ensure satisfactory downhole transmission of information so long as more than two fundamental-frequencies are selected for transmission. In this paper, three fundamental-frequencies of 100 Hz, 200 Hz and 300 Hz are used to transmit vibration signals.
Fig. 5.
OOK modulation.
In the case of transmitting vibration waves through tubing or casing, there will be greater reflection during transmission and fast attenuation of signals because it is difficult to have continuous impedance. In order to stabilize the system and prevent code errors, Manchester encoding is selected as the encoding scheme. See Fig. 6 for its principles. For every bit, the middle falling edge is 0, and the rising edge is 1. 16-bit data (4-bit layer information + 8-bit control information + 4-bit parity check) are used to encode the vibration signals. The first 4-bit layer information can control any downhole layer separately. According to the normal conditions of wellbores, the encoding of vibration signals currently employs the 4-bit layer control, that is to say, altogether 16 layers can be controlled. The middle 8-bit control information is the control instruction sending from the ground to the target layer. To ensure the validity of communication data, four parity check bits are added to the ending of data bits. Take a set of communication data “1010101000000110” for example. See Fig. 7 for its encoding. 2.3.
Generation system of vibration wave signals
The generation system of vibration wave signals consists of a vibration signal generator and a power supply system (Fig. 8). The power supply system controls the drive coils to produce alternating magnetic field, which drives the magnetostrictive material to vibrate, in this way, the electric energy is converted into the mechanical vibration energy. The power supply system can control the vibration signal generator effectively to output encoding waveform.
2.2. Vibration signal modulation and encoding Technology In order to load the control information into the carrier, it is necessary to modulate and encode the vibration waves. Learning from the modulation mode of electric signals, the on-off keying (OOK) mode is used for the modulation of vibration wave signals to facilitate transmitting system. When the digital signal modulated is “1”, the carrier is transmitted; when the digital signal modulated is “0”, no carrier is transmitted. See Fig. 5 for OOK modulation. 323
Fig. 6.
Manchester encoding principles.
Fig. 7.
Vibration signal encoding.
ZHENG Lichen et al. / Petroleum Exploration and Development, 2017, 44(2): 321–327
Fig. 8.
Vibration signal generation system.
The vibration signal generator employs an energy conversion element made of magnetostrictive material. Length of the magnetostrictive material changes under the action of an alternating magnetic field to produce mechanical vibration with the same frequency[1314]. Featuring big magnetostrictive strain, high pressure resistance, high energy conversion efficiency, high energy density and fast response, this material is an ideal energy conversion material for vibration wave generation. The working principle of a vibration signal generator is: When the length of magnetostrictive material rod changes under the action of drive coils, the induction hammer is driven to move up and down, but remains its pressure on the magnetostrictive rod under the action of gravity and spring restoring force and keeps on contact. The reactive force of magnetostrictive rod on the base is transmitted outwards via the base connecting flange. For communication, the vibration signal generator is installed onto the wellhead through the base connecting flange to generate vibration waves, which will be transmitted through the tubing or casing for remote communication. See Fig. 9 for the signal waveform actually output by the vibration signal generator. 2.4.
Receiving and decoding system of vibration signals
Consisting of a microvibration acceleration sensor and a decoding control panel, the receiving and decoding system of vibration signals is an important component of downhole actuating device, which receives and decodes the vibration signals transmitted through tubing or casing, and then converts
them into control instructions for downhole actuator. Because of the greater reflection and fast signal attenuation associated with the transmission process of vibration waves, the receiving sensor has to be highly sensitive, and the receiving and decoding system of vibration signals must have good signal processing capacity. In this study, special microvibration acceleration sensors with the resolution of 4×10-4 m/s2 are used to pick up signals. In the case of multi-fundamental-frequency transmission strategy, the system can receive and decode vibration wave signals of different frequencies. With small signals and the impact of extraneous noises taken into account, there are two options of magnification factors (1 000 and 100) available for signal acquisition. According to the test, the system resolution has reached 10-3 m/s2, which is capable of meeting the requirements for receiving and decoding vibration signals.
3. Remote transmission field test of vibration wave signals In order to verify the feasibility of vibration signal transmission in wellbores, find the relationship between transmission distance and signal intensity, and test the accuracy and reliability of the signal processing system, a remote transmission field test of vibration wave signals was conducted in Well Zhong 20-310 (with a well depth of 940 m) of Daqing Oilfield. A remote transmission test system of vibration signals was designed for this test (Fig. 10). When the system works, the motor rotates, driving the eccentric shaft to turn and the cam-controlled support arm to stretch out and retract. When the support arm is stretched out, the system can be pushed to rest against the casing wall at any depth under the action of disc spring force to receive and decode the vibration signals transmitted through the casing. When the support arm is retracted, the system can move freely up and down inside the casing. The system, which is connected with a ground-based computer via steel armored cable, transmits the decoded downhole signals and other relevant information in real time to the ground computer. In the field test, the vibration signal generator was fixed onto the wellhead to serve as the signal source, and the casing was used as the transmission medium of vibration signals to realize the data transmission to the downhole. The steel armored cable loaded the device and served as the transmission medium of electric signals. The remote transmission test system of vibration signals received and decoded the vibration
Fig. 9. Waveform actually output by the vibration signal generator.
324
Fig. 10.
Remote transmission test system of vibration waves.
ZHENG Lichen et al. / Petroleum Exploration and Development, 2017, 44(2): 321–327
signals, and transmitted the vibration signals received and the signals decoded to the ground computer via steel armored cable. The test system was sent into wellbore via the steel armored cable. In this process, the test system was pushed to rest against the casing wall every 100 m to acquire vibration signals sent out by the vibration signal generator at the same interval. See Table 1 for the test data. It can be seen from Table 1 that vibration wave signals with the fundamental frequency of 100 Hz can be transmitted at any location between the wellhead and the well depth of 900 m. Due to the bottomhole vibration wave reflection, vibration wave signals with the fundamental frequency of 200 Hz and 300 Hz cannot be transmitted at the well depth of 900 m. The effective transmission and correct decoding of signals in the range of 900 m from the wellhead downhole have verified the feasibility of transmitting vibration wave digital signals through casing, the reliability of vibration wave signal encoding system and the correctness of signal decoding algorithm. The results that vibration signals with the fundamental frequency of 200 Hz and 300 Hz cannot be transmitted correctly, but vibration signals with the fundamental frequency of 100 Hz can be transmitted normally at the well depth of 900 m have verified the rationality of the multi-fundamental-frequency transmission strategy. The intensity of vibration signals received downhole follows a linear attenuation rule after treated mathematically, but the intensity of signals near the wellhead and bottomhole behaves abnormally. This is because the intensity of signals received near the wellhead is slightly less than that received downhole due to the impact of ground noises, while the signal intensity is amplified or reduced suddenly near the bottomhole due to the interference of vibration waves reflected by the bottomhole with the vibration waves normally transmitting downwards. See Figs. 11 and 12 for the attenuation trend of vibration signals with the wellhead and bottomhole factors eliminated. Under existing technical conditions, downhole Table 1.
Field test data of vibration wave remote transmission Signal intensity, dB
Fundamental freWell quency, 100 Hz depth/ m Amplification factor
Fundamental frequency, 200 Hz Amplification factor
Fundamental frequency, 300 Hz Amplification factor
100
1,000
100
1,000
100
1,000
100
30
44
43
44
25
32
200
45
44
43
45
30
30
300
41
44
38
44
30
36
400
39
43
35
41
24
35
500
36
41
33
41
25
37
600
33
38
35
43
25
36
700
33
38
33
42
18
31
800
39
43
25
34
11
23
900
17
21
Fig. 11. Vibration signal attenuation trend (with 100 amplification factor selected).
Fig. 12. Vibration signal attenuation trend (with 1000 amplification factor selected)
communication within the depth range of 3 000 m can be realized.
4. Application of vibration wave downhole communication technique The vibration wave downhole communication technique has been tested in separated layer water injection in Well +5-10.2 of Jilin Oilfield. See Fig. 13 for the design of test string. A vibration wave controlled water distributor, which consists of batteries, a gear motor, water nozzles and a vibration signal receiving & decoding system, was set at Layer 3 of the well at the depth of 416 m. The wellhead vibration signal generator generates vibration signals containing control information as requested, the vibration signals are then transmitted downhole through the tubing. The vibration wave controlled water distributor receives and decodes the vibration signals transmitted from the wellhead, and then converts them into control instructions to control the rotation of gear motor, which in turn controls the nozzle opening of water distributor. The water distributor is equipped with a nozzle opening sensor to provide 0100% full range valve opening control. The procedures of field test are given below: a. Ran in operation string. Pressurized and actuated the packer with a ground pump truck. b. Pressurized the tubing to 15 MPa, and the tubing was able to maintain the pressure of 10 MPa for more than 5
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ZHENG Lichen et al. / Petroleum Exploration and Development, 2017, 44(2): 321–327
The field test shows that it is feasible to transmit digital signals to do remote control with tubing as the transmission medium and vibration waves as the carrier, and the vibration wave downhole communication technique is applicable to separated layer water injection.
5.
Fig. 13.
Test string design.
minutes, which indicated that all downhole water nozzles were closed. c. The ground vibration signal generator sent out a control instruction “0000011001000111”. The first four bits denoted the public address of water distributor, the middle 8 bits denoted the nozzle opening, which meant open it to 100% in this case, and the last 4 bits were the parity check bits. To prevent the stopband from interrupting signals, three frequencies of 100 Hz, 200 Hz and 300 Hz were used to transmit signals at the interval of 5 minutes. d. Pressurized the tubing to 15 MPa with the ground pump truck, and the tubing pressure dropped quickly to 2 MPa in one minute (the pressure drop time was influenced by the low permeability oil layer), which indicated that the downhole water nozzle in the vibration wave controlled water distributor was open, and the vibration waves could transmit the control information normally. e. The ground vibration signal generator sent out a control instruction “0000000000000000” at the carrier frequency of 100 Hz, 200 Hz and 300 Hz respectively. The first 4 bits denoted the public address of water distributor. The middle 8 bits denoted the nozzle opening, which meant close it to 0% in this case. The last 4 bits were the parity check bits. f. Pressurized the tubing to 15 MPa, and the tubing was able to maintain the pressure of 10 MPa for more than 5 minutes, which indicated that the downhole water nozzles were closed, and the vibration waves could control the closing of water nozzles normally.
Conclusions
In order to overcome the disadvantages of traditional downhole communication methods and realize the remote control of wellbores, a vibration wave downhole communication technique is proposed in this paper. Firstly, the transmission characteristics of vibration waves through tubings and casings were analyzed, based on which a multi-fundamental-frequency transmission strategy was formulated. The OOK modulation and Manchester encoding scheme were used to modulate and encode the vibration signals, and then the control information was loaded into the carrier. A vibration signal generation system with an energy conversion element made of magnetostrictive material is developed to realize the controllable conversion of electric energy into vibration wave energy. A test system for vibration wave downhole remote transmission is developed and tested in field, which has verified the feasibility of realizing vibration wave communication with casing as the transmission medium and the accuracy and reliability of the encoding scheme and decoding algorithm of vibration wave communication system. With this test system, the attenuation characteristics of vibration wave signals transmitted through casings are obtained. Under existing technical conditions, downhole communication within the depth range of 3000 m can be realized. As a fundamental common technique for wellbore control, the vibration wave downhole communication technique provides a completely new technical means for downhole signal transmission. It has the advantages of simple operation, low cost, fast speed and long transmission distance. This technique has a wide application prospect in the areas of separated layer water injection, separated layer oil production, fracturing, formation test, and drilling.
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