The design of the dynamic liquid level depth tester based on the digital signal processor

“The digital signal processor-based dynamic liquid surface depth tester introduced in the article uses digital signal processing technology to overcome the inherent shortcomings of traditional instruments and provides a reliable, accurate, and highly automated method for the dynamic liquid surface depth test of pumping wells. Testing means.

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Author: Pan Zhuojin, Zhu Guoxin

1 Introduction

The depth of the movable liquid level is the distance from the wellhead of the oil well to the surface of the downhole oil layer, and it is an important parameter in the regular test of the oil well. The average sound velocity in the well pipe can also be calculated from the depth of the moving liquid surface. The combination of the depth of the liquid surface, the average sound velocity in the well pipe and the results of other test items can fully reflect the working status and output of the pumping well, and provide a basis for the diagnosis and maintenance of the oil well.

2 Measuring principle of dynamic liquid surface depth

The dynamic liquid surface depth test instrument finds out the location of the wellhead by collecting the nodal wave signal reflected by the well pipe joint and the liquid surface wave signal reflected by the oil layer surface (as shown in Figure 1) from the gun gun installed at the wellhead. Using formula (1) to calculate the depth of the moving liquid surface and the reference nodal hoop wave.

Figure 1 Schematic diagram of the hoop wave and liquid surface wave waveforms

In formula (1), A, B, C, and D represent wellhead position, liquid level position, reference start point of reference nodal hoop wave and end point of reference nodal hoop wave respectively. L is the length of a single-section well pipe, and N is between C and D. The number of reference nodal hoop waves between. Since each hoop corresponds to a section of well pipe, N is the number of well pipes between C and D.

Due to the existence of various noise sources such as the noise of the sensor itself, environmental noise, etc., the collected waveforms are not always easy to find the above-mentioned feature points as shown in Figure 1, especially the reference nodal hoop wave, which is accurate It is difficult to calculate the depth of the dynamic liquid surface, and sometimes it is impossible to calculate it at all. Therefore, the filtering processing of the sensor output signal becomes the key to accurately calculating the dynamic liquid surface depth.

3 The structure of the traditional dynamic liquid level tester

Currently, the commonly used dynamic liquid level testers have an analog circuit-based structure as shown in Figure 2 and a microcontroller-based structure as shown in Figure 3.

Figure 2 Block diagram of the dynamic liquid level depth tester based on analog circuit

Figure 3 The principle block diagram of a micro-controller-based dynamic liquid surface depth tester

In the structure of Figure 2, the microphone picks up the low-frequency sound wave signal that is emitted by the gun and returned to the wellhead after being reflected by the well pipe joint and the oil layer surface. The signal is amplified by the amplifier and enters two narrow-band filters: band-pass filter BPF and low-pass filter LPF. The output of the band-pass filter is the hoop wave signal, and the output of the low-pass filter is the surface wave signal. The data acquisition is completed by the driving circuit to control the drawing pen to draw the hoop wave and liquid surface wave curve on the paper tape. The extraction of characteristic positions such as the position of the wellhead, the position of the liquid level, the starting point of the reference nodal hoop wave, and the end point of the reference nodal hoop wave, as well as the calculation of the depth of the moving liquid surface and the average sound velocity in the well pipe are completely manually completed by the technicians.

In the structure of Figure 3, the picking, amplifying and filtering parts of the analog signal are exactly the same as the structure shown in Figure 2, but the data acquisition part is completed by the microcontroller system. The microcontroller system consists of a microcontroller, A/D converter, memory, Display and printing interface. In this structure, as long as each characteristic position is found, the calculation of the depth of the moving liquid surface and the speed of sound can be automatically completed by the microcontroller. In the case that the collected waveform is ideal, the feature location extraction can be automatically completed by the microcontroller after the data collection is completed, but in most cases, due to the poor filtering effect, the automatically extracted feature location is inaccurate, requiring technical personnel Manual intervention.

Although these two structures are quite different in data acquisition methods, the quality of the acquired waveforms depends on the frequency response characteristics of the two analog filters in the signal processing channel. Because the frequency bands of the two signals are very narrow, this leads to the need for higher orders in the realization of the two filters, and because the higher-order filters are more sensitive to changes in device parameters, they are designed and debugged. It is more difficult to come.

4 Structure of dynamic liquid surface depth tester based on digital signal processor

The structure of the dynamic liquid surface depth tester based on the digital signal processor is shown in Figure 4. The microphone output signal passes through the preamplifier and anti-aliasing filter and then enters the A/D converter. The A/D conversion result is sent to the digital signal processor for digital filtering and sampling frequency conversion, and then stored in the nonvolatile Flexible memory for feature location extraction, dynamic liquid surface depth calculation, sound velocity calculation in well pipe, waveform display and printing, and other post-processing.

Figure 4 The principle block diagram of the dynamic liquid surface depth tester based on the digital signal processor

In this structure, the function of the traditional analog filter is completed by a digital filter realized by a digital signal processor, which uses an oversampling digital signal processing technique. The system samples the input signal at a very high sampling rate, and the sampled data is filtered through two narrowband digital filters implemented by the digital signal processor, namely the section band band pass filter BPF and the liquid surface wave low pass filter LPF. The latter two sampling data enter two sampling rate compressors respectively.

Suppose the output sampling frequency of the system is fs, the input sampling frequency is Mfs, the impulse response of the nodal band wave filter BPF is hB(k), the impulse response of the liquid surface wave filter LPF is hL(k), and the input sampling data is x (N), the sampling data of the output nodal hoop wave is yB(n), and the sampling data of the output liquid surface wave is yL(n), then the data flow diagram of the digital filtering and sampling rate conversion part of the digital signal processor is shown in Figure 5. .

Figure 5 Signal processing flow diagram in DSP

The input-output relationship of the system in the time domain is:

yB(m)=wB(mM)=∑hB(k)x(mM-k)

wB(n)=∑hB(k)x(nk)

yL(m)=wL(mM)=∑hL(k)x(mM-k)

wL(n)=∑hL(k)x(nk)

Since the digital signal processor is a processor designed for digital signal processing algorithms, it has a very high MAC (multiply and accumulate) operation speed, so it is easy to complete the operation of these two high-order narrowband digital filters in real time. Since the output sampling frequency of this system is not high, the over-sampling M value can be very large, so that the anti-aliasing filter can be implemented with a simple second-order active analog low-pass filter. A wide margin can be left in the passband during design. Even if the parameters of the analog components in the anti-aliasing filter drift with time or temperature, it will not affect the performance of the entire signal processing system. As the main process of signal processing adopts digital processing, the performance of the system is stable and reliable, and it overcomes the inherent inoperability of the system caused by the drift or inconsistency of the filter component parameters inherent in the two structures shown in Figure 2 and Figure 3. Stability or difficulty in design and debugging.

This structure uses a digital signal processor to replace the traditional analog filter and microcontroller, and only uses one analog-to-digital converter. Due to the good filtering effect, the success rate of automatically extracting the feature position is greatly improved compared with the structure based on the microcontroller, and the work efficiency is improved.

5 concluding remarks

The digital signal processor-based dynamic liquid surface depth tester introduced in the article uses digital signal processing technology to overcome the inherent shortcomings of traditional instruments and provides a reliable, accurate, and highly automated method for the dynamic liquid surface depth test of pumping wells. Testing means.

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