Discussion on the Application of Ultrasonic Flowmeter in Oilfield Wastewater Measurement

1. Current status of sewage metering in Shengli Oilfield

The metering of sewage reinjection in oilfields is a key link in the reasonable allocation of oil wells. A large amount of industrial sewage is generated every day in oilfield production. These sewage are reinjected by various joint stations through metering instruments. If the metering data is inaccurate, it will cause the phenomenon that the geological water production, sewage treatment volume and water injection volume are inconsistent, affecting the implementation of the injection and production plan. At present, Shengli Oilfield has about 500 large-caliber water injection flowmeters in use. These flowmeters play a very important role in sewage metering. However, since Shengli Oilfield does not have the ability to calibrate large-caliber flowmeters, many users can only disassemble the instruments and send them outside the oilfield for calibration. This method is time-consuming, labor-intensive and sometimes delays production. Some users cannot disassemble the instruments due to process flow, resulting in the situation that the instruments cannot be calibrated after many years of use. For this reason, after on-site experimental demonstration, we proposed a method of using ultrasonic flowmeters to conduct online testing of large-caliber sewage metering instruments in oilfields to solve the calibration problem of large-caliber sewage flowmeters.

2. Working Principle and Characteristics of Ultrasonic Flowmeter

2.1 Working Principle Ultrasonic flowmeters are classified into two measurement principles: propagation time method, Doppler effect method, etc. The propagation time method is currently widely used. Its basic principle is shown in the figure.

The time it takes for a high-frequency sound pulse to be transmitted from the upstream transducer A to the downstream receiving transducer B is

The time from the downstream transducer B to the upstream receiving transducer A is

In the formula, L is the distance of the sound channel; C is the speed of sound in the liquid; —— the angle between the sound channel and the axis of the pipe; V is the average axial velocity in the pipe.

From the above formula, we can deduce

It can be seen that the fluid flow rate V is related to the time difference. The flow rate V can be obtained by measuring tBA-tAB.

2.2 Features

(1) The flow measurement accuracy of the ultrasonic flowmeter is almost unaffected by the temperature, pressure, density, conductivity and other parameters of the measured fluid, and can be made into a non-contact and portable measuring instrument.

(2) The ultrasonic flowmeter can adapt to the measurement of various pipe diameters and various flow ranges. Compared with flowmeters based on other principles, the ultrasonic flowmeter generally has a range ratio of up to 300:1, does not produce pressure loss, has a full linear measurement range and low maintenance.

(3) The ultrasonic flowmeter is operated by a built-in battery and is suitable for use in occasions without external power supply.

(4) The ultrasonic flowmeter has low requirements on the pipe material. The pipe material can be made of metal, plastic, glass and other materials. [page]

3. Methods for using ultrasonic flowmeters to achieve accurate measurement

3.1 Selection of probe position. The straight pipe section length upstream of the flowmeter should be more than 10D, and the straight pipe section length downstream should not be less than 5D. If there is an elbow upstream of the installation point, a straight pipe section length of 30D should be guaranteed. The probe should be installed horizontally as much as possible and the two probes should be on the same axis. At the same time, the special coupling agent provided by the manufacturer should be used as much as possible.

3.2 A 1-meter stainless steel straight pipe section is installed in front of the flow meter. Due to poor water quality, the inner wall of ordinary carbon steel pipes is easily scaled, which affects the accurate reception of acoustic signals. Therefore, according to the characteristics of oilfield sewage measurement, a 1-meter stainless steel pipe section can be added to the probe installation point. This can not only ensure the measurement accuracy of the wall thickness, but also maximize the signal reception strength, thereby accurately measuring.

3.3 Reasonable selection of ultrasonic flowmeter. At present, there are many types of ultrasonic flowmeters. If the selection is not appropriate, it will result in low measurement accuracy or even the inability to measure flow.

(1) The time difference ultrasonic flowmeter is the most widely used ultrasonic flowmeter. It is mainly used to measure the flow of clean liquids and is widely used in the fields of tap water and industrial water. In addition, it can also measure the flow of homogeneous fluids with low impurity content, such as sewage, with an accuracy of up to ±1.5%. Practical applications show that the use of time-difference ultrasonic flowmeters can obtain satisfactory results for the measurement of corresponding fluids.

(2) Doppler ultrasonic flowmeters can only be used to measure fluids containing appropriate amounts of particles or bubbles. It should be noted that it has strict requirements on the measured medium. It cannot be clean water. At the same time, the impurity content must be relatively stable to be measured normally. Otherwise, large measurement errors will occur in actual use.

4. Several issues that should be noted in the field application

of ultrasonic flowmeters 4.1 Accurate measurement of parameters. According to the measurement principle of ultrasonic waves, the measured values ​​of the outer diameter and wall thickness of the pipeline are related to the flow rate, so the outer diameter and wall thickness should be accurate during measurement. When measuring the outer diameter, a vernier caliper should be used for measurement. When measuring the wall thickness, several measurement points should be selected for measurement, and then the average value is taken as the wall thickness parameter input.

4.2 Correct installation of the probe.

(1) The installation position of the probe should meet the requirements of the straight pipe section, 10D in the front and 5D in the back.

(2) Determine the correct installation method of the probe. There are two most commonly used probe installation methods for ultrasonic flowmeters, namely the z method and the V method. The z method is generally used for large-diameter pipeline measurement. The advantage of this method is that the signal loss is small, but the disadvantage is that the sound path is short and it is not suitable for small-diameter pipeline measurement; the V method is generally suitable for small-diameter pipeline measurement. The advantage of this method is that the sound path is long, which can obtain better time resolution. On a horizontal pipeline, the sensor is best installed at the "9 o'clock or 3 o'clock" position of the pipeline cross-sectional area. This can avoid the influence of bubbles gathering at the top and particles sinking to the bottom flow on the measurement when other installation methods are used.

(3) The pipeline surface where the probe is installed should be smooth and coated with an appropriate amount of ultrasonic special coupling agent to ensure that there is no gap between the probe and the pipe wall. Too little coupling agent or too thin coupling agent will cause inaccurate measurement data.

(4) The probe should be firmly tied to make it in close contact with the outer wall of the pipeline and ensure that it will not move, otherwise it will cause the attenuation of sound waves and lead to inaccurate data.

4.3 Regularly remove the scale on the inner wall of the measuring pipe section. Since the scale formed on the inner wall of the pipe for a long time in sewage measurement can easily affect the accuracy of ultrasonic measurement, users who have installed stainless steel measuring pipe sections can regularly dismantle the stainless steel measuring pipe sections for cleaning. This can ensure the accuracy of the ultrasonic flowmeter when measuring the pipeline flow and minimize the impact caused by the scale on the inner wall of the pipe.

5. Conclusion

5.1 The use of ultrasonic flowmeters can realize online measurement of sewage flowmeters without affecting the normal production of users.

5.2 Portable Doppler ultrasonic flowmeters have the characteristics of small size, easy to carry, and easy to install. Compared with the previous offline calibration, they save the demolition and transportation of the sewage flowmeter to be tested. Some large-caliber sewage flowmeters did not consider the future calibration issues during design and installation, so offline calibration is impossible. The use of ultrasonic flowmeters for online detection can save users a lot of costs.

References
[1] Cai Wuchang, Sun Huaiqing, Ji Gang. Flow measurement methods and instrument selection [M]. Beijing: Chemical Industry Press, 2001.
[2] Luo Shounan, Liu Yan, Feng Guanping. Continuous wave ultrasound and sensor, 2OO4 (12): 44-46 (end)