Oilfield wastewater reinjection is an effective way to achieve reasonable equipment and utilization of water resources, as well as energy conservation and environmental protection. The appropriate amount of sewage reinjection is related to the optimization of formation structure, the restoration and maintenance of reservoir pressure, and directly affects the oil recovery rate of the oilfield. Therefore, the accuracy of sewage flow meters is important. It is difficult to disassemble sewage flow meters on oilfield sites, and there are often no backup flow meters. Therefore, ultrasonic flow meters are usually calibrated online on site. In recent years, with the increasing emphasis on sewage measurement in oil fields, the on-site calibration method of ultrasonic flow meters has been widely used, but it has also exposed the problem of low accuracy of this flow meter. This article combines the principle of ultrasonic flow meters and on-site application experience, and discusses the adaptability of ultrasonic flow meters in the calibration of sewage flow meters through on-site experiments and data analysis

1 Principle of Ultrasonic Flowmeter Discussion on the Adaptability of Ultrasonic Flowmeters in the Calibration of Sewage Flowmeters

There are two commonly used principles of ultrasonic flowmeters: time difference method and Doppler method.

1.1 Principle of Time Difference Method

When ultrasonic signals are in a fluid, the signal propagation speed is appropriate and the propagation time is short when flowing downstream.

; When in reverse flow, the signal propagation speed is slow and the propagation time is long. When using ultrasonic signals in the fluid, there is a difference in the propagation time of acoustic signals in the direction of forward and backward flow, which can be used to calculate the fluid flow rate. The time difference method is more suitable for measuring pure liquids. Liquids or suspended solids with more bubbles can hinder the normal propagation of sound pulses, leading to abnormal measurement

1.2 Principle of Doppler Method

Ultrasonic signals are scattered when they encounter small particles (suspended substances or small bubbles) on the propagation path, and the ultrasonic receiver can receive the scattered ultrasonic signal.

. The frequency of the transmitted signal is affected by the Doppler effect and changes to different frequencies of the received signal. The difference between the two frequencies has a definite quantitative relationship with the fluid flow rate, from which the fluid flow rate can be calculated. Doppler flow meters cannot measure clean fluids and can only measure fluids with a turbidity of about 50mg/L or higher. At the same time, the measurement accuracy of Doppler method is lower than that of time difference method, generally at level 2.0, which is 2% full scale error (2% F.S.)