Research and discussion on online calibration method of electromagnetic flowmeter in water injection station

Overview of this article
The water injection station of the oil field is a very important production link in the oil field operation. At present, the water injection flowmeters used in major water injection stations in China are mostly electromagnetic flowmeters, with diameters ranging from DN100 to DN400. According to our summary data, most of them are DN300 electromagnetic flowmeters. Electromagnetic flowmeters are calibrated and qualified before leaving the factory. After the oil field is put into production, they are often not calibrated after the flowmeter is installed due to the lack of online calibration methods. With the accumulation of measurement errors, the error of water injection volume is too large, which has an adverse effect on the precise control of production. Therefore, how to realize the online calibration method in the oil field development and production process is of great significance to provide accurate and reliable first-hand information for the production management department, so that they can grasp the dynamics of crude oil production in a timely manner.
 
According to the on-site visits and surveys of the technical personnel of Runzhong Instrument Technology, the data collected is that the water injection error of major oil field water injection stations is basically 10%-20%. Because most of the small-diameter water injection meters in the water distribution room are calibrated regularly, the water volume at the wellhead of the water distribution room is used as the basis for calculating the error of water injection volume. The reasons why flowmeters are not calibrated regularly are:
first, many oil fields currently do not have devices for calibrating large-caliber flowmeters;
second, the comprehensive cost of external calibration is high;
third, flowmeters are troublesome to disassemble and assemble.
The flowmeter cannot be calibrated regularly, so the amount of water pumped out by the water injection station cannot be accurately measured, resulting in over-injection or under-injection of oil wells. Excessive water injection will cause formation disorder and a significant increase in the water content of the produced fluid; insufficient water injection will cause formation pressure to drop and no liquid can be produced. It can be seen that the accurate measurement of water injection directly affects the overall production of the oil field and plays a very important role in the stable and high production of the oil field. At the same time, the oil field conducts water injection system efficiency tests on each oil production unit every year. The water injection volume parameters are required for the test. Due to the large error of the water injection instrument, the inaccurate measured water injection volume, and the low system efficiency, the management department cannot grasp the accurate production conditions in time.
In addition, as oil fields fully implement digital management in the field of exploration and development, accurate and reliable basic data are also needed. In order to provide accurate and reliable first-hand information to the oilfield development and production management departments, so that they can grasp the dynamics of crude oil production in a timely manner, it is very important to use an online calibration method suitable for large-diameter water injection flowmeters in oilfields.
　　
2. Technical solutions
　　Through technical consultation and on-site investigation, three technical solutions were proposed to solve the problem of large water injection flow errors in water injection stations:
　　Solution 1: Establish a calibration device for large-diameter flowmeters and calibrate water injection flowmeters offline; Solution 2: Use ultrasonic flowmeters to calibrate flowmeters online; Solution 3: Use water injection tanks at water injection stations to calibrate flowmeters online.
　　If a large-diameter flowmeter calibration device is established according to Solution 1, there are problems such as large device investment (each device costs about 2 million yuan) and long recovery period. In addition, non-real liquid calibration will bring certain calibration errors. In addition, a flowmeter must be disassembled and unloaded twice to calibrate it, and the disassembly and assembly of large-diameter flowmeters is quite difficult. Therefore, Solution 1 is not easy to implement. If Solution 2 is implemented, the on-site technical conditions are difficult to meet the requirements. First, the existing pipelines have different degrees of corrosion, rust or scaling problems, the thickness of the pipe wall is uneven, and the front and rear straight pipe sections are difficult to meet the installation conditions of the ultrasonic flowmeter. Second, the sewage composition of each water injection station is different, and the propagation speed of ultrasonic waves in sewage of different components is also different, which brings certain difficulties to accurate measurement. Third, the on-site transformation investment of the ultrasonic detection method is large. Therefore, the second plan is also difficult to implement. If the third plan is implemented, the water injection tank is used to calibrate the water injection flowmeter online, and the water injection tank in the water injection station is calibrated in accordance with the national calibration regulations, the accuracy level of the water injection tank above 700m3 can be guaranteed to reach 0.1. In this way, the existing water injection tank of the water injection station is fully utilized as a standard for testing the flowmeter, which saves investment and does not require process modification. Therefore, the third plan is recommended. The specific method is as follows:
　　1. Perform volume calibration on the water injection tank in the station to obtain the water injection tank volume table. The specific steps are operated in accordance with the calibration regulations of JJG168-2005 "Vertical Metal Tank Capacity".
　　2. Use the water injection tank to calibrate the water injection flowmeter online and obtain the flowmeter measurement error. The specific steps are operated according to the calibration procedures of JJG198-1994 "Velocity Flowmeter".
　　
III. Field Experiment
　　1. Experimental Principle
　　At present, the accuracy level of water injection flowmeters installed in oil field water injection stations is mostly level 1, and the highest level is level 0.5. A water injection station is selected for field experiment. The water injection flowmeter of the station is a qualified level 0.5 flowmeter. The water injection flowmeter is calibrated online with a standard device composed of a water injection tank. If the test result is consistent with the calibration certificate of the flowmeter (the accuracy level of the flowmeter is level 0.5), it proves that the water injection tank can be used as a standard container for online calibration of water injection flowmeters, and the standard device composed of water injection tanks can be used to calibrate water injection flowmeters with an accuracy level of 0.5 online. Otherwise, the water injection tank cannot be used as a standard device for online calibration of the field water injection flowmeter.
　　2. Experimental method
　　The experiment combines the actual working conditions on site, and uses the water injection tank in the station to perform volume calibration to obtain the water injection tank volume table. Then, the calibration device composed of the water injection tank is used to calibrate the 0.5-level electromagnetic flowmeter that has been calibrated by the national metrology and calibration department online, and the relevant technical indicators of the electromagnetic flowmeter are obtained and compared with the data on the calibration certificate.
　　3. Experimental content and results A 0.5-level electromagnetic flowmeter
　　that has been calibrated by the national metrology and calibration department is installed on the water injection pipeline of a water injection station . The flowmeter is calibrated online by the standard device composed of the water injection tank. After half a month of field tests, the test results are: the basic error of the electromagnetic flowmeter is 1.65%, which does not meet the requirement of ≤0.5%. Through the analysis and research of the experimental data, it is found that the error of the first set of data in each test is relatively large. The reason is that after the test personnel have completed the process and adjusted the inlet and outlet valves of the water injection tank, they immediately go to the top of the tank to detect the first set of numbers. At this time, the water inlet gate of the water injection tank has just been closed, and the liquid level in the tank has not yet stabilized, resulting in a large error in the first set of gauge data. In view of the above problems, the test process was improved. After adjusting the process, the test personnel stabilized for 10 minutes before testing the first set of data. In this way, the repeatability of the test data is better. After the test method was improved, a one-month field test was carried out at the water injection station, and the measured data error reached the requirement of ≤0.5%. IV. Prospects for promotion and application 　　Through the field experiment at the water injection station for nearly 3 months, a relatively ideal experimental effect was achieved. The promotion and application of this test method in oil fields has the following advantages: 　　1. It solves the problem of online detection of water injection flowmeters at water injection stations and reduces the inspection costs. 　　2. There is no need to modify the existing process of the water injection station, which basically will not affect the actual production on site. It has strong operability and saves the cost of field process modification. 　　3. The oil field tests the efficiency of the water injection system of each oil production plant every year, in which the water injection flow rate is used. The accuracy of the water injection amount directly affects the accuracy of the water injection system efficiency. The developed test system can accurately measure the water injection amount of the flowmeter, and can provide reliable flow data for system efficiency testing. 　　4. In this test system, only the volume of the water injection tank above 6m is actually used, and the volume at the bottom has no effect on the test. Therefore, the volume calibration can be carried out without cleaning the water injection tank on site (only the basic outer diameter of the tank needs to be measured with the "external measuring ruler method", the radial deviation of each ring plate from the basic outer diameter needs to be measured with the "guide rail optical radial measuring instrument", the thickness of each ring plate tank wall needs to be measured with an ultrasonic thickness gauge, and the height of each ring plate needs to be measured with a standard steel tape measure). It can ensure that the accuracy level of the volume of the segmented tank reaches 0.1%. V. Conclusion and understanding
　　






　　

　　1. Through experiments, it can be known that the 0.5-level electromagnetic flowmeter is calibrated online through the standard device composed of water injection tanks, and the error of the electromagnetic flowmeter is <0.5%. Therefore, it is feasible to use the standard device composed of water injection tanks to calibrate the water
　　injection flowmeter online to meet the value transfer requirements. 2. The online calibration time of large-caliber water injection flowmeters using the standard device composed of water injection tanks should be between May and October each year, otherwise the weather is too cold and the thickness of the floating oil in the water injection tank is uneven, which will cause a large error in the gauge.
　　3. During the on-site test, since there is no automatic control device, the stopwatch can only be used to measure the gauge of the water injection tank and read the flowmeter indication at the same time. This requires the inspection personnel to have a high sense of responsibility. The gauge inspection personnel at the top of the water injection tank and the flowmeter reading personnel must operate at the same time at the agreed time.
　　4. During the test, since it is a dynamic gauge inspection, the test personnel must have good operating skills. Once the measurement time is reached, the gauge inspection should be carried out quickly. It is necessary to be proficient in operation and try to keep the speed of the gauge inspection consistent each time. During the operation, ensure that the speed of lowering the gauge at the starting point and the end point is consistent to reduce system errors.
　　5. It is necessary to ensure the sealing of the inlet and outlet valves of the water injection tank to prevent water from entering the water injection tank during the test and affecting the accuracy of the test.