CN106761641B - Coal body electric pulse fracturing and permeability increasing experimental system and method - Google Patents

Abstract

An experimental system and method for increasing the permeability of coal by electric pulse cracking is suitable for improving the gas extraction rate of low permeability and high adsorption coal seams. The system includes a high-voltage charging power supply, a high-voltage energy storage capacitor, a discharge switch, and a high-voltage breakdown generator. The high-voltage charging power supply rectifies and boosts the 220V alternating current to an adjustable voltage in the range of 0-300kV; the high-voltage energy storage capacitor can store high-voltage electricity; the discharge switch can instantly release the high-voltage The high-voltage electricity stored in the energy storage capacitor; the high-voltage breakdown generator can apply pressure in the three-axis direction of the coal sample through the hydraulic control system. The system uses high-voltage pulse discharge technology to achieve coal seam fracturing, which can better dredge the channels in the process of gas seepage, which is of great significance for improving the gas drainage rate of low-permeability and high-adsorption coal seams.

Description

An experimental system and method for fracturing and increasing permeability of coal body by electric pulse

technical field

The invention relates to an experimental system and method for fracturing and permeation enhancement, in particular to a coal body electric pulse fracturing and permeation-increasing experimental system and method for breaking down coal samples by using high-voltage electric pulse discharge technology.

Background technique

In recent years, as my country's coal mines have entered the stage of deep mining, most mines in my country are mainly mining low-permeability coal seams. These mines are faced with the difficulty of gas extraction in the process of mining. The key to solving this problem is It is to improve the permeability of coal seam. There are two traditional methods to improve the effect of gas extraction: one is to promote gas desorption, and the other is to clear the channel of gas seepage. The main method currently used to improve gas extraction is to dredge gas seepage channels, that is, a series of fracturing technologies are used to form a fracture network inside the coal seam to provide a path for gas migration in the coal seam. According to the traditional coal seam fracturing and permeability enhancement methods: mining liberated layers, hydraulic fracturing, intensive drilling and deep hole blasting have also achieved certain results, but these methods themselves also have some defects, for example: mining liberated layers will There is a lot of gangue and it is not suitable for single coal seam mining; the direction of hydraulic fracturing is difficult to control and will waste a lot of water resources; intensive drilling technology is prone to string drilling, and the increase in the number of drilling holes will greatly increase. Increase the cost of coal mining; deep hole blasting technology is difficult to deliver explosives to preset locations in soft coal seams. Based on the shortcomings of the existing coal seam permeability enhancement technology, it is particularly important to develop a simpler and more effective coal seam permeability enhancement method to solve the problem of difficult gas drainage in low permeability coal seams.

Since the United States and other countries successfully applied the high-voltage electric pulse crushing technology to oil blockage removal in the 1970s, some scholars proposed to apply the high-voltage pulse discharge technology to the cracking and anti-reflection of coal seams. There are two main ways to destroy materials by high-voltage electric pulse discharge technology: one is the hydroelectric effect, and the other is electric crushing. The hydroelectric effect means that the solid material and the electrode are immersed in the liquid, and the electrode and the solid material are in a non-contact state. The high-voltage electricity first breaks down the liquid dielectric, thereby generating a strong shock wave. The shock wave acts on the solid material to break the solid material. , in essence, the hydroelectric effect uses the pressure generated by the shock wave to break the solid material. Electro-fragmentation means that high-voltage electricity directly acts on the surface of solid materials, causing a large amount of plasma to be formed inside the solid material. With the accumulation of the plasma, the plasma channel expands rapidly, resulting in a large tensile stress, forcing the solid material to be broken.

At present, most scholars apply high-voltage pulse discharge technology to the field of crushing solids mainly by hydroelectric effect crushing. However, the energy utilization rate of hydroelectric effect crushing solid materials is very low, so it is more direct to study high-voltage pulse discharge. The technique of crushing solids is of great significance. In the present invention, by directly loading high-voltage electric pulses to both ends of the coal sample, a large amount of plasma will be generated during the discharge process. These plasmas carry huge energy and accumulate in an instant to form a plasma channel. The accumulated plasma is in the form of stress waves. The coal sample does work, forcing the coal sample to rupture. The method directly loads high-voltage electric pulses to both ends of the coal sample and crushes the coal sample, which can more effectively improve the energy utilization rate, and at the same time dredge the gas migration channel, thereby realizing the improvement of gas extraction in the coal seam with low permeability and high adsorption. the purpose of the extraction rate.

SUMMARY OF THE INVENTION

Technical problem: The purpose of the present invention is to solve the deficiencies in the existing low-permeability coal seam gas drainage technology, and to provide an experimental system and method for coal body electric pulse cracking and permeability enhancement, which utilizes the process of instantaneous high-voltage discharge to carry plasma The huge energy of the coal does work on the coal sample in the form of stress waves, thereby dredging the primary fractures in the coal seam or generating new fracture channels to promote gas drainage.

Technical scheme: The coal body electric pulse fracturing and infiltration experiment system of the present invention includes: high-voltage charging power supply, high-voltage energy storage capacitor, discharge switch, voltage divider, high-voltage breakdown generator, current detector, oscilloscope, discharge switch control Table, computer, three-axis loading device, charge discharge device and hydraulic control system; the output end of the high-voltage charging power supply is connected to the positive electrode of the high-voltage energy storage capacitor, and the output end of the high-voltage energy storage capacitor is connected to the input end of the discharge switch connected, the output end of the discharge switch is connected with the input end of the voltage divider, the output end of the voltage divider is connected with the input end of the high-voltage breakdown generator, and the output end of the high-voltage breakdown generator is connected with the input end of the high-voltage breakdown generator. The input end of the current detector is connected with the hydraulic control system through the hydraulic pipeline; the output end of the current detector is connected with the negative electrode of the high-voltage energy storage capacitor; the output end of the discharge switch is controlled by the discharge switch The input end of the oscilloscope is connected with the input end of the stage; the output end of the voltage divider and the current detector are respectively connected with the input end of the oscilloscope, and the output end of the oscilloscope is connected with the input end of the computer through a data line; the output end of the oscilloscope The terminal is connected with the input terminal of the computer through a data line; the high-voltage energy storage capacitor is connected with the charge discharge device.

The high-voltage breakdown generator includes a box with a cubic coal sample in the middle, and an electrode sleeve and an electrode sleeve shoulder for fixing the electrode sleeve are symmetrically arranged on the left and right sides of the box. The electrode sleeve is respectively provided with "needle-needle" positive and negative electrodes for the opposite sides of the cubic coal sample, and the positive electrode pressure plate and the negative electrode pressure plate are respectively provided at one end of the electrode sleeve opposite to the cubic coal sample. Hydraulic cylinders are respectively arranged on the front, rear, upper and lower sides of the box body, and pressure plates on the front, rear, upper and lower sides of the cubic coal sample are respectively arranged on the piston rods of the four hydraulic cylinders.

The high-voltage breakdown generator includes a box with a cubic coal sample in the middle, and an electrode sleeve and an electrode sleeve shoulder for fixing the electrode sleeve are symmetrically arranged on the left and right sides of the box. The electrode sleeve is respectively provided with "needle-plate" positive and negative electrodes for the opposite sides of the cubic coal sample, and a positive electrode pressure plate and a negative electrode pressure plate are respectively provided at one end of the electrode sleeve opposite to the cubic coal sample. There are hydraulic control rods on the front, rear, upper and lower sides of the box, respectively, and pressure plates on the front, back, up and down sides of the cubic coal sample on the four hydraulic control rods; the four hydraulic control rods are connected to the connected to the control system.

The "needle-needle" positive and negative electrodes include a needle-shaped positive electrode that passes through the positive electrode pressure plate and touches the left end face of the cube coal sample, and a needle-shaped negative electrode that passes through the negative electrode pressure plate and touches the right end face of the cube coal sample. constitute.

The "needle-plate" positive and negative electrodes are composed of a needle-shaped positive electrode that passes through the positive electrode pressure plate and touches the left end face of the cube coal sample, and a plate-shaped negative electrode that is attached to the right end face of the cube coal sample.

The output voltage range of the high-voltage charging power supply is 0-300kV.

The experimental method for fracturing and increasing permeability of coal body using the above system includes the following steps:

a. Open the box cover of the high-voltage breakdown generator, use the hydraulic control system to preset the pressure, and fix the cubic coal sample through the positive electrode pressure plate, the negative electrode pressure plate and the 4 pressure plates;

b. Rotate the shoulder of the electrode sleeve so that the discharge electrode is close to the cubic coal sample;

c. Cover the cover of the box and fix the cover of the box with screws;

d. Turn on the high-voltage charging power supply to charge the high-voltage energy storage capacitor, stop charging when the voltage reaches the preset value, and turn off the high-voltage charging power supply to avoid damaging the high-voltage charging power supply during discharge;

e. Trigger the discharge button on the discharge switch console, and directly load the preset voltage in the high-voltage energy storage capacitor to both ends of the cubic coal sample to break down the coal sample; at the same time of discharging, the voltage divider and current detection are carried out. The device measures the voltage and current signal of the coal sample at the moment of discharge, and records it through the computer;

f. Unload the remaining voltage in the high-voltage energy storage capacitor through the charge discharge device;

g. Open the box cover of the box, take out the broken coal sample, and complete the experiment.

Beneficial effects: the method of directly loading high-voltage electricity on both ends of the coal sample breaks down the coal sample to form new cracks or connect the original cracks, thereby promoting the development of coal seam cracks, realizing coal seam permeability enhancement, improving low permeability, High-adsorption coal seam gas extraction rate; by directly loading high-voltage electricity on both ends of the coal sample, it can effectively overcome the energy loss in the traditional crushing process using the hydroelectric effect; the mechanism of electric pulse cracking and permeation enhancement can be mastered, It plays an important role in studying the influencing factors of high-voltage electric pulse anti-reflection technology and the application of high-voltage electric pulse anti-reflection technology in downhole. The use of high-voltage breakdown generator and hydraulic control system can apply pressure in the three-axis direction of the coal sample, and use high-voltage pulse discharge to achieve coal seam fracturing, which can better dredge the channel in the process of gas seepage. The extraction rate of coal seam gas is of great significance. It can solve the problem of difficulty in gas extraction in low-permeability coal seams. The huge energy carried by ions in the process of instantaneous high-voltage discharge is used to perform work on the coal sample in the form of stress waves, thereby dredging the original cracks in the coal seam or generating new crack channels. to facilitate gas extraction. The utility model has the advantages of simple structure, convenient operation and good effect, and has wide practicability in the technical field.

Description of drawings

FIG. 1 is a schematic structural diagram of the present invention.

FIG. 2 is a schematic structural diagram of the needle-needle electrode high voltage breakdown generator of the present invention.

FIG. 3 is an I-I sectional view of FIG. 2 .

4 is a schematic structural diagram of the needle-plate electrode high-voltage breakdown generator of the present invention.

In the picture: 1- high voltage charging power supply, 2- high voltage energy storage capacitor, 3- discharge switch, 4- voltage divider, 5- high voltage breakdown generator, 6- current detector, 7- oscilloscope, 8- discharge switch control Table, 9-Computer, 10-Three-axis loading device, 11-Charge discharge device, 12-Hydraulic control system, 13-Box, 14-Pressure plate, 15-Needle positive electrode, 16-Needle negative electrode , 17-electrode sleeve, 18-electrode sleeve shoulder, 19-hydraulic cylinder, 20-plate negative electrode, 21-cube coal sample, 22-hydraulic pipeline, 23-positive electrode pressure plate, 24-negative Electrode pressure plate.

Detailed ways

Embodiments of the present invention are further described below in conjunction with the accompanying drawings:

Example 1. The coal body electric pulse fracturing and permeation-increasing experimental system of the present invention, as shown in Figure 1, the system includes: a high-voltage charging power supply 1, a high-voltage energy storage capacitor 2, a discharge switch 3, a voltage divider 4, a high-voltage shock Transmitter 5, current detector 6, oscilloscope 7, discharge switch console 8, computer 9, triaxial loading device 10, charge discharge device 11 and hydraulic control system 12; The positive pole of the energy storage capacitor 2 is connected, the output end of the high voltage energy storage capacitor 2 is connected to the input end of the discharge switch 3, the output end of the discharge switch 3 is connected to the input end of the voltage divider 4, and the voltage divider The output end of the device 4 is connected with the input end of the high-voltage breakdown generator 5, the output end of the high-voltage breakdown generator 5 is connected with the input end of the current detector 6, and is connected with the hydraulic control system through the hydraulic pipeline 22. 12 is connected; the output end of the current detector 6 is connected with the negative pole of the high-voltage energy storage capacitor 2; the output end of the discharge switch 3 is connected with the input end of the discharge switch console 8; the voltage divider 4 The output end of the current detector 6 is respectively connected with the input end of the oscilloscope 7, and the output end of the oscilloscope 7 is connected with the input end of the computer 9 through the data line; the output end of the oscilloscope 7 is connected with the input end of the computer 9. Connected through a data line; the high-voltage energy storage capacitor 2 is connected to the charge discharge device 11 . The output voltage range of the high-voltage charging power supply 1 is 0-300kV.

As shown in FIG. 2 and FIG. 3 , the high-voltage breakdown generator 5 includes a box body 13 with a cubic coal sample 21 in the middle. The left and right sides of the box body 13 are symmetrically provided with electrode sleeves 17 and fixed electrode sleeves. The electrode sleeve shaft shoulder 18 of the tube 17, the symmetrically arranged electrode sleeve 17 is respectively provided with “needle-needle” positive and negative electrodes for the opposite sides of the cubic coal sample 21, and the electrode sleeve 17 is opposite to the cubic coal sample One end of 21 is respectively provided with a positive electrode pressurizing plate 23 and a negative electrode pressurizing plate 24, and hydraulic cylinders 19 are respectively provided on the front, rear, upper and lower surfaces of the box body 13, and the piston rods of the four hydraulic cylinders 19 are respectively provided with hydraulic cylinders 19. There are pressure plates 14 on the front, back, up and down sides relative to the cubic coal sample 21 . The "needle-needle" positive and negative electrodes include a needle-shaped positive electrode 15 that passes through the positive electrode pressurizing plate 23 and touches the left end face of the cube coal sample 21 and a negative electrode pressurizing plate 24 that touches the right end face of the cubic coal sample 21. The needle-shaped negative electrode 16 is formed. The needle-shaped positive electrode 15 and the needle-shaped negative electrode 16 are composed of solid copper cylinders with a diameter of 1 cm and a length of 25 cm. Conical, the outer surfaces of the needle-shaped positive electrode 15 and the needle-shaped negative electrode 16 are threaded, the electrode sleeve 17 is cylindrical, the center of the electrode sleeve 17 has an internal thread hole, the needle-shaped positive electrode 15 and the needle-shaped negative electrode 16 is connected with the electrode sleeve 17 through threads, the outer surface of the electrode sleeve 17 also has threads, the center of the C and D surfaces of the box body 13 each has a circular hole, and the inside and outside of the box body 13 have an electrode sleeve shaft shoulder. 18. The two electrode sleeve shaft shoulders fix the electrode sleeve 17 through the thread; the input end of the needle-shaped positive electrode 15 is connected with the output end of the voltage divider 4, and the output end of the needle-shaped negative electrode 16 is connected with the current detector. The input end of 6 is connected; the cubic coal sample 21 is fixed by the positive electrode pressure plate 23, the negative electrode pressure plate 24 and the four pressure plates 14.

The positive electrode pressing plate 23 and the negative electrode pressing plate 24 have through holes in the center, the needle-shaped positive electrode 15 passes through the positive electrode pressing plate 24, the needle-shaped negative electrode 16 passes through the negative electrode pressing plate 24, and the discharge is discharged. The end is next to the cube coal sample 21; the positive electrode pressure plate 23, the negative electrode pressure plate 24 and the pressure plate 14 are controlled by the hydraulic cylinder 19, the hydraulic cylinder 19 is connected with the hydraulic pipeline 22, and the hydraulic pipeline 22 is controlled by the hydraulic pressure The control system 12 controls.

Example 2. The experimental system for fracturing and permeation enhancement of coal body by electric pulse of the present invention is the same as that of Example 1, and the same parts are omitted. The difference, as shown in FIG. 4 , the high-voltage breakdown generator 5 includes a box body 13 with a cubic coal sample 21 in the middle, and the left and right sides of the box body 13 are symmetrically provided with electrode sleeves 17 and fixed electrodes. The electrode sleeve shaft shoulder 18 of the electrode sleeve 17, the symmetrically arranged electrode sleeve 17 is respectively provided with "needle-plate" positive and negative electrodes for the opposite sides of the cube coal sample 21, and the electrode sleeve 17 is opposite to the cube One end of the coal sample 21 is respectively provided with a positive electrode pressurizing plate 23 and a negative electrode pressurizing plate 24, and hydraulic cylinders 19 are respectively provided on the front, rear, upper and lower surfaces of the box body 13, and the four hydraulic cylinders 19 are respectively provided with hydraulic cylinders 19. The four hydraulic cylinders 19 are connected to the hydraulic control system 12 through the hydraulic pipeline 22 . The "needle-plate" positive and negative electrodes are composed of a needle-shaped positive electrode 15 that passes through the positive electrode pressurizing plate 23 and touches the left end face of the cube coal sample 21 and a plate-shaped negative electrode 20 that is attached to the right end face of the cube coal sample 21. . The needle-shaped positive electrode 15 is composed of a solid copper cylinder with a diameter of 1 cm and a length of 25 cm. The discharge end of the needle-shaped positive electrode 15 is a cone with a height of 2.5 cm and a bottom diameter of 1 cm; the plate-shaped negative electrode 20 is It consists of a square copper plate with a side length of 10cm and a thickness of 0.5cm. The outer surface of the needle-shaped positive electrode 15 has threads, the electrode sleeve 17 is cylindrical, and the center of the electrode sleeve 17 has an inner thread hole, and the needle-shaped positive electrode 15 is connected to the electrode sleeve 17 through threads. The outer surface of the electrode sleeve 17 is also threaded, the center of the C surface and the D surface of the box body 13 each has a circular hole, the inner side and the outer side of the box body 13 have an electrode sleeve shaft shoulder 18, and the two electrode sleeve shaft shoulders pass through. Threads fix the electrode sleeve 17; the input end of the needle-shaped positive electrode 15 is connected to the output end of the voltage divider 4, the output end of the plate-shaped negative electrode 16 is connected to the input end of the current detector 6; the cubic coal sample 21 It is fixed by the positive electrode pressing plate 23 , the negative electrode pressing plate 24 and the four pressing plates 14 .

The specific steps of the coal body electric pulse cracking and permeation-increasing experimental method of the present invention are as follows:

a. Open the box cover of the box body 13 in the high-voltage breakdown generator 5, use the hydraulic control system 12 to preset the pressure, and press the cube coal through the positive electrode pressure plate 23, the negative electrode pressure plate 24 and the four pressure plates 14. Sample 21 is fixed;

b. Rotate the shaft shoulder of the electrode sleeve 17 so that the discharge electrode is close to the cubic coal sample 21;

c. Cover the cover of the box body 13, and fix the cover of the box body with screws;

d. Turn on the high-voltage charging power supply 1 to charge the high-voltage energy storage capacitor 2, stop charging when the voltage reaches the preset value, and turn off the high-voltage charging power supply 1 to avoid damaging the high-voltage charging power supply 1 during discharging;

e. Trigger the discharge button on the discharge switch console, and directly load the preset voltage in the high-voltage energy storage capacitor 2 to both ends of the cubic coal sample 21 to break down the coal sample; and the current detector 6 to measure the voltage and current signals of the coal sample at the moment of discharge, and record them through the computer 9;

f. Unload the remaining voltage in the high-voltage energy storage capacitor 2 through the charge discharge device 11;

g. Open the box cover of the box body 13, take out the broken coal sample, and the experiment is completed.

Claims (4)

1. A coal body electric pulse fracturing and permeation increasing experimental system, characterized in that: the system comprises: a high-voltage charging power supply (1), a high-voltage energy storage capacitor (2), a discharge switch (3), and a voltage divider (4) , high voltage breakdown generator (5), current detector (6), oscilloscope (7), discharge switch console (8), computer (9), triaxial loading device (10), charge discharge device (11) and a hydraulic control system (12); the output end of the high-voltage charging power supply (1) is connected to the positive pole of the high-voltage energy storage capacitor (2), and the output end of the high-voltage energy storage capacitor (2) is connected to the output end of the discharge switch (3). The input end is connected, the output end of the discharge switch (3) is connected with the input end of the voltage divider (4), and the output end of the voltage divider (4) is connected with the input end of the high voltage breakdown generator (5). connected, the output end of the high-voltage breakdown generator (5) is connected with the input end of the current detector (6), and is connected with the hydraulic control system (12) through the hydraulic pipeline (22); the current The output end of the detector (6) is connected with the negative pole of the high-voltage energy storage capacitor (2); the output end of the discharge switch (3) is connected with the input end of the discharge switch console (8); the voltage divider (4) and the output end of the current detector (6) are respectively connected with the input end of the oscilloscope (7), and the output end of the oscilloscope (7) is connected with the input end of the computer (9) through a data line; the The output end of the oscilloscope (7) is connected with the input end of the computer (9) through a data line; the high-voltage energy storage capacitor (2) is connected with the charge discharge device (11); the output of the high-voltage charging power supply (1) The voltage range is 0-300kV. 2. The coal body electric pulse fracturing and permeation-increasing experimental system according to claim 1, characterized in that: the high-voltage breakdown generator (5) comprises a box (13) with a cubic coal sample (21) in the middle ), the left and right sides of the box (13) are symmetrically provided with an electrode sleeve (17) and an electrode sleeve shoulder (18) for fixing the electrode sleeve (17). ) are respectively provided with "needle-needle" positive and negative electrodes for the opposite sides of the cubic coal sample (21), and a positive electrode pressure plate ( 23), a negative electrode pressurizing plate (24), hydraulic cylinders (19) are respectively provided on the front, rear, upper and lower surfaces of the box body (13), and the piston rods of the four hydraulic cylinders (19) are respectively provided with opposing cylinders (19). The pressure plates (14) on the front, rear, upper and lower sides of the cube coal sample (21); the "needle-needle" positive and negative electrodes include the left end of the cube coal sample (21) being touched by passing through the positive electrode pressure plate (23) The needle-shaped positive electrode (15) on the surface and the needle-shaped negative electrode (16) passing through the negative electrode pressure plate (24) and touching the right end face of the cubic coal sample (21) are constituted. 3. The coal body electric pulse fracturing and permeation-increasing experimental system according to claim 1, characterized in that: the high-voltage breakdown generator (5) comprises a box (13) with a cubic coal sample (21) in the middle ), the left and right sides of the box (13) are symmetrically provided with an electrode sleeve (17) and an electrode sleeve shoulder (18) for fixing the electrode sleeve (17). ) are respectively provided with "needle-plate" positive and negative electrodes for the opposite sides of the cubic coal sample (21), and a positive electrode pressure plate ( 23), a negative electrode pressure plate (24), hydraulic cylinders (19) are respectively provided on the front, rear, upper and lower surfaces of the box body (13), and relatively cubic coal samples are respectively provided on the four hydraulic cylinders (19) (21) Compression plates (14) on the front, rear, upper and lower sides; the four hydraulic cylinders (19) are connected to the hydraulic control system (12) through the hydraulic pipeline (22); the positive and negative electrodes of the “needle-plate” are connected by A needle-shaped positive electrode (15) passing through the positive electrode pressing plate (23) and touching the left end face of the cubic coal sample (21) and a plate-shaped negative electrode (20) attached to the right end face of the cubic coal sample (21) are constituted. 4. A coal body electric pulse fracturing and permeation-increasing experimental method using the system of claim 1, 2 or 3, characterized in that it comprises the steps: a. Open the box cover of the box (13) in the high-voltage breakdown generator (5), use the hydraulic control system (12) to preset the pressure, and pass the positive electrode pressure plate (23) and the negative electrode pressure plate (24) and 4 pressure plates (14) to fix the cube coal sample (21); b. Rotate the shoulder of the electrode sleeve (17) so that the discharge electrode is close to the cubic coal sample (21); c. Cover the cover of the box (13), and fix the cover of the box with screws; d. Turn on the high-voltage charging power supply (1) to charge the high-voltage energy storage capacitor (2), stop charging when the voltage reaches the preset value, and turn off the high-voltage charging power supply (1) to avoid damaging the high-voltage charging power supply (1) during discharge; e. Trigger the discharge button on the discharge switch console, and directly load the preset voltage in the high-voltage energy storage capacitor (2) to both ends of the cubic coal sample (21) to break down the coal sample; The voltage divider (4) and the current detector (6) measure the voltage and current signals of the coal sample at the moment of discharge, and record them through the computer (9); f. Unload the remaining voltage in the high-voltage energy storage capacitor (2) through the charge discharge device (11); g. Open the box cover of the box body (13), take out the broken coal sample, and the experiment is completed.

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