Research on test method of multi-channel ultrasonic wave detection for gas well injection volume

In the early stage of natural gas extraction, the fracturing and blowout process is often used to obtain stable production. However, during the fracturing process, natural gas combustion is frequently directly released, which causes significant pollution to the environment and a waste of resources. In order to measure the flow rate of the gas-liquid mixture in the process of gas well blowout, a multi-channel velocity measurement model is established. The multi-channel sensor structure is designed based on an analysis of the gas well blowout process and fluid configuration simulations. The influence of pipe diameter on the flow characteristics of gas-liquid two-phase flow was analyzed by FLUENT software. The simulation results show that the distribution of flow velocity in a gas-liquid two-phase flow tube is not uniform, the flow velocity of different flow layers is different, and the stratification phenomenon is more obvious with the increase of pipe diameter, highlighting the necessity of multi-channel measurements. Therefore, we developed a gas-liquid two-phase flow velocity measuring device. The experimental results demonstrate that accurately describing velocity in the tube can be achieved by measuring velocities in different channels using the gas-liquid two-phase flow velocity measurement device.


Introduction
Gas well blowout is a controlled process in oil and gas exploration, involving the intentional opening of the wellhead for testing.This allows oil and gas to be released from the well in a regulated manner.The blowout process is divided into three stages: the initial stage, where the fluid is mostly liquid; the middle stage, dominated by mixed fluids; and the late stage, where the fluid is mostly gas.Throughout these stages, the overall gas phase quantity is significantly greater than the liquid phase quantity.In the event of a significant blowout, it is crucial to prevent the release of natural gas into the atmosphere near oil and gas fields to avoid explosive concentrations.Therefore, it is essential to measure the total gas and liquid amounts during the gas well blowout process [1].Due to the fluid temperature being close to atmospheric temperature, on-site construction to open the pipeline and install instruments is inconvenient, leading to limited use of contact flow measurement methods.Current non-contact measurement methods for detecting gas-liquid two-phase flow include optical and X-ray technologies.Optical methods require transparent pipe materials, making them impractical for non-transparent pipes.Although X-ray can measure phase distribution with high time resolution, its cost is prohibitive [2][3].
Liu et al. proposed a fusion method using Kalman filtering to integrate multiple flow results from gas ultrasonic flow meter channels [4].Kuang et al. focused on measuring gas-liquid two-phase flow in offshore oil and gas production, using pressure fluctuation signals for gas phase error prediction through a BP neural network [5].However, implementing the complex Kalman filtering algorithm and BP network structure in a single-chip microcomputer poses challenges.In scenarios where the pipeline cannot be disturbed, a non-contact multi-channel ultrasonic method is applied to measure blowout amounts during gas well blowouts [6][7][8].By analyzing the impact of various pipe diameters on gas-liquid phase distribution and fluid velocity, a gas well blow-off quantity detector based on a single-chip microcomputer is developed, facilitating quantitative blow-off detection.

Detection model of blowout volume of muti-channel gas wells
The multi-channel parallel structure involves placing sensor pairs at various chord lengths in the pipeline section.Each channel runs parallel to the others, measuring flow rate information for different layers of the pipeline.This design is suitable for pipelines with layered flow rate distributions, where each layer has a distinct flow rate.The three channels are arranged on different flow layers of the pipeline, the different channels cross each other in the y direction, the z direction is parallel to each other, and the Angle between each channel and the axis direction is θ.The three-channel structure is shown in Figure 1.The working frequency of the ultrasonic sensor is selected at 200~250 kHz.

Modeling of ultrasonic sensors in chord direction
First of all, according to the mathematical model of monophonic measurement, the average flow velocity along the axis of any flow layer channel in the pipeline can be obtained, as shown in Formula (1): Where ( ) =2√ − / sin is the chordwise sound path, is the radius of the measuring pipe section, is the distance of the chordwise soundtrack from the axis, is the measured value of the ultrasonic countercurrent time, is the measured value of the ultrasonic downstream time, and is the angle between the channel and the axis direction.We make − =∆ , and then Formula (2) can be obtained: At this time, the formula for multi-channel flow velocity measurement can be expressed as Formula (3): where is the surface average velocity of the channel , and is the weight coefficient of the channel , which depends on the numerical integration method used in the calculation.Therefore, the fluid flow flowing through the pipe with diameter D in unit time T is Formula (4):

Analysis of the influence of pipe diameter on the flow characteristics of gas-liquid two-phase flow
Using FLUENT software, gas-liquid two-phase flow in horizontal tubes with varying diameters was simulated to analyze its characteristics.Under the condition of 10% gas content and 1 m/s fluid velocity, the distribution law of gas and liquid phases in a pipe with an inner diameter of 80 mm was analyzed, and the simulation results were shown in Figure 2 and Figure 3.Under the same conditions, it can be seen from Figure 2 and Figure 3 that the distribution changes of liquid and gas phases of pipelines with an inner diameter of 80 mm are roughly the same, and the influence of pipe diameter is little.Both of them are distributed stably around the pipe axis and change significantly along the radial direction near the pipe wall.

Design of discharge volume detector
The jet discharge detector is designed by the ultrasonic time difference method, and its hardware block diagram is shown in Figure 4.The single-chip microcomputer, acting as the main control chip, commands the TDC time difference measurement module to generate ultrasonic pulses.It also controls the analog switch on the three-channel flow measurement channel, enabling two sensors on each channel to alternately transmit and receive.This facilitates the measurement of ultrasonic downstream time (t ) and reverse time ( t ).Amplified by the driving circuit, the received pulse signal undergoes acoustoelectric conversion.After passing through the mixed fluid, the attenuated signal is conditioned, with the time difference (∆t) of the primary sound path calculated by the TDC module.According to the above simulation results, due to the uniform distribution of velocity around the pipe axis and obvious stratification of velocity distribution near the pipe wall, the installation method of parallel arrangement of three-channel sensors was selected in consideration of system design errors (He et al. 2011).Three sound channels are arranged on different flow layers of the pipeline, and the flow velocity information of different flow layers of the pipeline is measured respectively.The different channels cross each other in the y direction and are parallel to each other in the z direction, but their lengths are not equal.

Experimental results and analysis of flow rate in tube
According to the Gaussian integral method, the weighted coefficients of the three channels are 0.3, 0.5, and 0.3 respectively.The measurement data in Table 1 is the average of the three measurement results, and the weighted flow velocity of the fluid in the tube is obtained after the weighted calculation of the measurement results of the three sound channels.

Comparative experiment of three-channel and monophonic flow velocity measurement
Without changing the hardware circuit, through the control of the software program, one of the channels is selected to measure the time difference ∆t, and the measurement data of the mono channel can be calculated.Because of the uncertainty of the mono channel installation position, the measurement result of channel A is selected to replace the measurement result of mono channel flow.Table 2 lists the measurement results and relative errors of mono and tri-channel at 10% gas content.As per specs, errors for fluid flow below 1m/s must be within 10%, between 1 and 4m/s capped at 8%, and above 4m/s within 5%.Table 2 indicates lower relative errors for three-channel measurements compared to monophonic ones, decreasing with increasing flow velocity.This emphasizes that higher gas-liquid two-phase fluid velocities lead to more accurate three-channel flow measurements.

Conclusion
(1) A mathematical model for multi-channel gas well injection volume detection is proposed based on literature analysis.(2) Fluent software is used to simulate the impact of pipe diameter on two-phase flow velocity distribution.The results show that 1) Inner diameter minimally affects fluid velocity change, but larger diameters accentuate flow stratification.2) Gas-liquid phase distribution is stable around the pipe axis, with notable variations near the pipe wall.3) Mixed fluid velocity distribution is non-uniform, highest at the center, gradually decreasing radially in a circular pattern.(3) Based on fluid velocity simulation results, it is suggested to use multi-channel sensors to detect characteristic points, enhancing the measurement accuracy of two-phase flow velocity through a reasonable multi-channel measurement algorithm.(4) This paper develops a gas well blowout detector and simulates the blowout process in a laboratory environment.Results demonstrate significantly lower measurement errors with three-channel velocity compared to mono-channel.

Figure 4 .
Figure 4.The principle block diagram of the discharge and spray quantity detector.

Table 1 .
Measurement results of three-channel flow rate with 10% gas content.

Table 2 .
Mono and tri-channel measurement results and measurement errors.