A device and method for restoring the volume of coal core

Abstract

The present invention discloses a device and method for restoring the volume of a coal core, comprising a water injection mechanism, an oil injection mechanism, a constant speed and constant pressure pump, a piston container and a coal sample tank, wherein the water injection mechanism and the oil injection mechanism are both connected to the liquid inlet end of the constant speed and constant pressure pump, the piston container has the same structure as the coal sample tank, and both the piston container and the coal sample tank comprise a tank body, a tank cover, a piston, an upper liquid inlet and a lower liquid inlet, the tank body and the tank cover are flange-connected to form a closed cavity, the piston is tightly matched with the inner wall of the tank body, a displacement sensor is arranged on the piston of the piston container, the liquid outlet end of the constant speed and constant pressure pump is connected to the upper liquid inlet and the lower liquid inlet of the piston container through a pipeline, and the liquid outlet end of the constant speed and constant pressure pump is connected to the upper liquid inlet and the lower liquid inlet of the coal sample tank through a pipeline; the present invention adopts a piston container with a piston and a coal sample tank to compress and fill the free space of the coal core, and can accurately restore the volume of the coal core to its original volume before the coal core is collected.

Description

Technical Field

[0001] The invention relates to the technical field of coal mine safety, and in particular to a device and method for restoring the volume of a coal core. Background Art

[0002] Gas pressure plays a very important role in the occurrence and development of coal and gas outbursts. It is the preferred indicator for predicting and evaluating the danger of coal seam outbursts and testing the effectiveness of regional outburst prevention measures. When using gas pressure to test the regional outburst prevention effect, the measured pressure is required to be the residual gas pressure. However, after the pre-extraction of gas from the coal seam, a network of interconnected fractures has been formed inside the coal seam. The pressure measured in the regional effect test is not the residual gas pressure corresponding to the residual gas content, but the gas flow resistance inside the coal seam or even the negative pressure of extraction. The existing gas pressure measurement method is to measure the gas pressure by drilling and sealing the hole. This method is not suitable for the measurement of coal seam gas pressure after extraction measures have been taken. Therefore, a method for measuring coal seam gas pressure without sealing the hole is proposed. The test method is based on the three changes that occur in coal samples during gas pressure testing from underground coal seam storage conditions to being loaded into the experimental coal sample tank: First, during the sampling process from the coal sample being exposed to being loaded into the coal sample tank, coal gas desorption causes gas loss; second, the coal body breaks and expands from a dense block form under the confining pressure conditions of the underground coal seam into a loose particle state after being loaded into the coal sample tank, and the volume of the dead space occupied by the coal sample before and after sampling has changed; third, the ambient temperature of the coal sample before and after the two states changes from the underground coal seam temperature to the laboratory ambient temperature. The unsealed coal seam gas pressure test technology compensates for the three different changes that occur before and after the coal sample, restores the coal core volume, and artificially simulates the coal reservoir temperature to restore the gas pressure under the coal seam storage conditions. Among these three links, the compensation for gas loss and the simulation of coal reservoir temperature are easy to achieve, so how to restore the coal core volume has become the top priority. Summary of the invention

[0003] In order to overcome the above-mentioned shortcomings, the present invention provides a device and method for restoring the volume of coal cores, the purpose of which is to achieve accurate restoration of the volume of coal cores and restore them to their original volume before coal core collection.

[0004] To achieve the above object, the present invention is implemented according to the following technical solutions:

[0005] A device for restoring the volume of a coal core, comprising a water injection mechanism, an oil injection mechanism, a constant speed and constant pressure pump, a piston container and a coal sample tank, wherein the water injection mechanism and the oil injection mechanism are both connected to the liquid inlet end of the constant speed and constant pressure pump, the piston container has the same structure as the coal sample tank, and both the piston container and the coal sample tank comprise a tank body, a tank cover, a piston, an upper liquid inlet and a lower liquid inlet, the tank body and the tank cover are flange-connected to form a closed cavity, the piston is tightly matched with the inner wall of the tank body, the upper liquid inlet is located at the top of the tank cover, the lower liquid inlet is located at the bottom of the tank body, a displacement sensor is provided on the piston of the piston container, the liquid outlet end of the constant speed and constant pressure pump is connected to the upper liquid inlet and the lower liquid inlet of the piston container through a pipeline to inject liquid into the piston container, and the liquid outlet end of the constant speed and constant pressure pump is connected to the upper liquid inlet and the lower liquid inlet of the coal sample tank through a pipeline to inject liquid into the coal sample tank;

[0006] Furthermore, the piston container and the upper liquid inlet and the lower liquid inlet of the coal sample tank are both provided with a test liquid valve, and the test liquid valve comprises a valve body, a test liquid cavity, a first liquid inlet connected to the test liquid cavity, a test liquid port, a first liquid discharge port, a first tightening valve adapted to the test liquid port, and a second tightening valve adapted to the first liquid discharge port;

[0007] Furthermore, the water injection mechanism includes a water storage cup and a water delivery pipe, and the water storage cup is connected to the water delivery pipe;

[0008] Furthermore, the oil filling mechanism includes an oil storage cup and an oil delivery pipe, and the oil storage cup and the oil delivery pipe are connected;

[0009] Furthermore, the liquid inlet end of the constant speed and constant pressure pump is connected to a liquid inlet pipe, and the liquid outlet end is connected to a liquid discharge pipe, and the water delivery pipe and the oil delivery pipe are connected to the liquid inlet pipe of the constant speed and constant pressure pump through a three-way joint;

[0010] Further, the liquid discharge pipe is respectively connected to a first liquid inlet pipe and a second liquid inlet pipe through a three-way joint, the first liquid inlet pipe is connected to a lower liquid inlet of the piston container, the second liquid inlet pipe is connected to a third liquid inlet pipe and a fourth liquid inlet pipe through a three-way joint, the third liquid inlet pipe is connected to an upper liquid inlet of the coal sample tank, and the fourth liquid inlet pipe is connected to a lower liquid inlet of the coal sample tank;

[0011] Furthermore, the fourth liquid inlet pipe is connected to a fifth liquid inlet pipe via a joint, and the fifth liquid inlet pipe is communicated with the upper liquid inlet of the piston container;

[0012] Furthermore, valves are provided on the water pipe, the oil pipe, the first liquid inlet pipe, the second liquid inlet pipe, the third liquid inlet pipe, the fourth liquid inlet pipe, and the fifth liquid inlet pipe.

[0013] A method for using a device for restoring the volume of a coal core comprises the following steps:

[0014] Step 1, calculate the dead volume increment of the coal sample using the formula ΔV=V0-m / ARD, where ΔV is the dead volume increment of the coal sample, V0 is the volume of the coal sample tank; m is the mass of the coal sample; ARD is the density of the coal sample;

[0015] Step 2, compression of free space: first open the suction valve of the constant speed and constant pressure pump and the valve of the water pipe, and close the suction valve after the constant speed and constant pressure pump is full of water, and then open the valve on the second liquid inlet pipe, the valve on the third liquid inlet pipe, the test liquid valve on the liquid inlet of the coal sample tank and the discharge valve of the constant speed and constant pressure pump, first fill the pipeline from the valve on the second liquid inlet pipe to the valve on the third liquid inlet pipe, and from the valve on the third liquid inlet pipe to the second tightening valve section of the liquid inlet of the coal sample tank with water, and when liquid flows out of the test liquid port of the test liquid valve at the upper end of the coal sample tank, quickly close the first tightening valve and the second tightening valve of the test liquid valve at the upper end of the coal sample tank, and open the first tightening valve of the test liquid valve at the lower end of the piston container and the second tightening valve of the test liquid valve at the upper end of the piston container, continue to inject liquid into the upper end of the piston container to push the piston downward, and then close the valve of the second liquid inlet pipe, the piston container The first tightening valve of the test liquid valve at the lower end of the device and the discharge valve of the constant speed and constant pressure pump; set the constant speed and constant pressure pump to the constant pressure injection mode, and open the discharge valve of the constant speed and constant pressure pump, the second tightening valve of the test liquid valve at the lower end of the piston container and the second tightening valve of the test liquid valve at the upper end of the coal sample tank, and inject liquid into the lower end of the piston container to push the piston upward to achieve quantitative injection into the hydraulic chamber at the upper end of the coal sample tank and push the piston of the coal sample tank downward to compress the dead volume increment of the coal sample. When the current pressure of the constant speed and constant pressure pump reaches the initial setting value and the injection flow rate is stable at zero, stop the injection, close the second tightening valve of the test liquid valve at the upper end of the coal sample tank, the second tightening valve of the test liquid valve at the lower end of the piston container, the discharge valve of the constant speed and constant pressure pump and the water pipe valve; the dead volume compensation amount can be obtained according to the upper cavity bottom area S of the piston container and the displacement sensor moving distance ΔH1 by the formula V1=S×ΔH1;

[0016] Step 3, filling the free space V2: First, open the suction valve and oil pipe valve of the constant speed and constant pressure pump, fill the pump body with incompressible liquid materials that are not absorbed by the coal body (for example: silicone oil, organic oil, inorganic oil, etc.), then close the suction valve, and then open the second liquid inlet pipe valve, the fourth liquid inlet pipe valve, the test liquid valve at the lower end of the coal sample tank, and the constant speed and constant pressure pump discharge valve. First, fill the pipeline section from the second liquid inlet pipe valve to the fourth liquid inlet pipe valve and the fourth liquid inlet pipe valve to the test liquid valve at the lower end of the coal sample tank with liquid materials that are incompressible and not absorbed by the coal body. When liquid flows out from the test liquid port of the test liquid valve at the lower end of the coal sample tank, quickly close the test liquid valve at the lower end of the coal sample tank, and open the first tightening valve of the test liquid valve at the lower end of the piston container and the second tightening valve of the test liquid valve at the upper end of the piston container. Tighten the valve, inject liquid into the upper end of the piston container to push the piston down to the corresponding scale H2, and when H2=V2 / S, immediately close the second liquid inlet pipe valve, the first tightening valve of the test liquid valve at the lower end of the piston container and the discharge valve of the constant speed and constant pressure pump; then open the second tightening valve of the test liquid valve at the lower end of the piston container, the second tightening valve of the test liquid valve at the lower end of the coal sample tank and the discharge valve of the constant speed and constant pressure pump, inject liquid into the lower end of the piston container to push the piston upward to quantitatively inject liquid into the coal sample chamber in the lower cavity of the coal sample tank, and continue to inject liquid until the piston of the coal sample tank moves to the topmost position, close the second tightening valve of the test liquid valve at the lower end of the piston container, the fourth liquid inlet pipe valve, the second tightening valve of the test liquid valve at the lower end of the coal sample tank and the discharge valve of the constant speed and constant pressure pump, complete the dead volume increment difference compensation V2 of the coal sample of the gas pressure test, and also realize the recovery of the volume state of the coal core.

[0017] Compared with the prior art, the device and method for restoring the volume of coal core of the present invention have the following beneficial effects:

[0018] The present invention adopts a piston container with a piston and a coal sample tank to compress the free space of the coal core and fill the free space, and can accurately restore the volume of the coal core to its original volume. By restoring the volume of the coal core, an important guarantee is provided for the gas pressure measurement technology of the coal seam without sealing. Through this pressure measurement technology, a fast and simple new way can be provided for accurately obtaining the residual gas pressure when testing the gas pressure of the coal seam in the mine and the effect of the regional sudden elimination measures. When measuring the gas content under high pressure, the restoration of the volume of the coal core can provide an accurate numerical value for the calculation of the adsorption constant ab value.

[0019] The present invention designs a test liquid valve. When injecting liquid into a piston container or a coal sample tank, the test liquid port of the test liquid valve can not only play the role of exhaust, but also monitor whether the pipeline is full of liquid, providing necessary conditions for accurate filling of the coal core. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0021] Figure 1 It is a structural schematic diagram of the present invention;

[0022] Figure 2 It is a schematic diagram of the structure of the test liquid valve of the present invention. DETAILED DESCRIPTION

[0023] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. The illustrative embodiments and descriptions of the present invention are used to explain the present invention but are not intended to limit the present invention.

[0024] like Figure 1 The device for restoring the volume of a coal core shown in the figure comprises a water injection mechanism, an oil injection mechanism, a constant speed and pressure pump 1, a piston container 2 and a coal sample tank 3, wherein the water injection mechanism and the oil injection mechanism are both connected to the liquid inlet end of the constant speed and pressure pump 1, the piston container 2 has the same structure as the coal sample tank 3, and the piston container 2 and the coal sample tank 3 both comprise a tank body 21, a tank cover 22, a piston 23, an upper liquid inlet 24 and a lower liquid inlet 25, the tank body 21 and the tank cover 22 are flange-connected to form a closed cavity, the piston 23 is tightly matched with the inner wall of the tank body 21, and the piston can be A small handle is provided to facilitate the storage of coal samples. The upper liquid inlet 24 is located at the top of the tank cover 22, and the lower liquid inlet 25 is located at the bottom of the tank body 21. A displacement sensor 27 is provided on the piston 23 of the piston container 2. The liquid outlet end of the constant speed and constant pressure pump 1 is connected with the upper liquid inlet 24 and the lower liquid inlet 25 of the piston container 2 through a pipeline to inject liquid into the piston container 2. The liquid outlet end of the constant speed and constant pressure pump 1 is connected with the upper liquid inlet 24 and the lower liquid inlet 25 of the coal sample tank 3 through a pipeline to inject liquid into the coal sample tank 3.

[0025] like Figure 2As shown, in this embodiment, a test liquid valve 26 is provided on the upper liquid inlet 24 and the lower liquid inlet 25 of the piston container 2 and the coal sample tank 3, and the test liquid valve 26 includes a valve body 261, a test liquid cavity 262, a first liquid inlet 263 connected to the test liquid cavity 262, a test liquid port 264, a first liquid discharge port 265, a first tightening valve 266 adapted to the test liquid port 264, and a second tightening valve 267 adapted to the first liquid discharge port 265; when liquid is injected into the piston container 2 or the coal sample tank 3, the test liquid port 264 of the test liquid valve 26 can not only play a role in exhausting air, but also can monitor whether the pipeline is full of liquid, thereby providing necessary conditions for accurate filling of the coal core.

[0026] like Figure 1 As shown, in this embodiment, the water injection mechanism includes a water storage cup 4 and a water delivery pipe 5, and the water storage cup 4 is connected to the water delivery pipe 5; the oil injection mechanism includes an oil storage cup 6 and an oil delivery pipe 7, and the oil storage cup 6 and the oil delivery pipe 7 are connected.

[0027] like Figure 1 As shown, in this embodiment, the liquid inlet end of the constant speed and constant pressure pump 1 is connected to a liquid inlet pipe 8, and the liquid outlet end is connected to a liquid discharge pipe 9, and the water pipe 5 and the oil pipe 7 are connected to the liquid inlet pipe 8 of the constant speed and constant pressure pump 1 through a three-way joint.

[0028] like Figure 1 As shown, in the present embodiment, the discharge pipe 9 is respectively connected to the first liquid inlet pipe 10 and the second liquid inlet pipe 11 through a three-way joint, the first liquid inlet pipe 10 is connected to the lower liquid inlet 25 of the piston container 2, the second liquid inlet pipe 11 is connected to the third liquid inlet pipe 12 and the fourth liquid inlet pipe 13 through a three-way joint, the third liquid inlet pipe 12 is connected to the upper liquid inlet 24 of the coal sample tank 3, the fourth liquid inlet pipe 13 is connected to the lower liquid inlet 25 of the coal sample tank 3; the fourth liquid inlet pipe 13 is connected to the fifth liquid inlet pipe 14 through a joint, the fifth liquid inlet pipe 14 is connected to the upper liquid inlet 24 of the piston container 2.

[0029] like Figure 1 As shown, in this embodiment, valves 15 are provided on the water pipe 5 , the oil pipe 7 , the first liquid inlet pipe 10 , the second liquid inlet pipe 11 , the third liquid inlet pipe 12 , the fourth liquid inlet pipe 13 , and the fifth liquid inlet pipe 14 .

[0030] A method for using a device for restoring the volume of a coal core comprises the following steps:

[0031] Step 1, calculate the dead volume increment of the coal sample using the formula ΔV=V0-m / ARD, where ΔV is the dead volume increment of the coal sample, V0 is the volume of the coal sample tank; m is the mass of the coal sample; ARD is the density of the coal sample;

[0032] Step 2, compression of free space: first open the suction valve of the constant speed and constant pressure pump and the valve of the water pipe, and close the suction valve after the constant speed and constant pressure pump is full of water, and then open the valve on the second liquid inlet pipe, the valve on the third liquid inlet pipe, the test liquid valve at the liquid inlet of the coal sample tank and the discharge valve of the constant speed and constant pressure pump, and first fill the pipeline from the valve on the second liquid inlet pipe to the valve on the third liquid inlet pipe, and from the valve on the third liquid inlet pipe to the second tightening valve section of the liquid inlet of the coal sample tank with water, and when liquid flows out of the test liquid port of the test liquid valve at the upper end of the coal sample tank, quickly close the first tightening valve and the second tightening valve of the test liquid valve at the upper end of the coal sample tank, and open the first tightening valve of the test liquid valve at the lower end of the piston container and the second tightening valve of the test liquid valve at the upper end of the piston container, continue to inject liquid into the upper end of the piston container to push the piston downward, and then close the valve of the second liquid inlet pipe, the piston container The first tightening valve of the test liquid valve at the lower end of the device and the discharge valve of the constant speed and constant pressure pump; set the constant speed and constant pressure pump to the constant pressure injection mode, and open the discharge valve of the constant speed and constant pressure pump, the second tightening valve of the test liquid valve at the lower end of the piston container and the second tightening valve of the test liquid valve at the upper end of the coal sample tank, and inject liquid into the lower end of the piston container to push the piston upward to achieve quantitative injection into the hydraulic chamber at the upper end of the coal sample tank and push the piston of the coal sample tank downward to compress the dead volume increment of the coal sample. When the current pressure of the constant speed and constant pressure pump reaches the initial setting value and the injection flow rate is stable at zero, stop the injection, close the second tightening valve of the test liquid valve at the upper end of the coal sample tank, the second tightening valve of the test liquid valve at the lower end of the piston container, the discharge valve of the constant speed and constant pressure pump and the water pipe valve; the dead volume compensation amount can be obtained according to the upper cavity bottom area S of the piston container and the displacement sensor moving distance ΔH1 by the formula V1=S×ΔH1;

[0033] Step 3, filling the free space V2: First, open the suction valve and oil pipe valve of the constant speed and constant pressure pump, fill the pump body with incompressible liquid materials that are not absorbed by the coal body (for example: silicone oil, organic oil, inorganic oil, etc.), then close the suction valve, and then open the second liquid inlet pipe valve, the fourth liquid inlet pipe valve, the test liquid valve at the lower end of the coal sample tank, and the constant speed and constant pressure pump discharge valve. First, fill the pipeline section from the second liquid inlet pipe valve to the fourth liquid inlet pipe valve and the fourth liquid inlet pipe valve to the test liquid valve at the lower end of the coal sample tank with liquid materials that are incompressible and not absorbed by the coal body. When liquid flows out from the test liquid port of the test liquid valve at the lower end of the coal sample tank, quickly close the test liquid valve at the lower end of the coal sample tank, and open the first tightening valve of the test liquid valve at the lower end of the piston container and the second tightening valve of the test liquid valve at the upper end of the piston container. Tighten the valve, inject liquid into the upper end of the piston container to push the piston down to the corresponding scale H2, and when H2=V2 / S, immediately close the second liquid inlet pipe valve, the first tightening valve of the test liquid valve at the lower end of the piston container and the discharge valve of the constant speed and constant pressure pump; then open the second tightening valve of the test liquid valve at the lower end of the piston container, the second tightening valve of the test liquid valve at the lower end of the coal sample tank and the discharge valve of the constant speed and constant pressure pump, inject liquid into the lower end of the piston container to push the piston upward to quantitatively inject liquid into the coal sample chamber in the lower cavity of the coal sample tank, and continue to inject liquid until the piston of the coal sample tank moves to the topmost position, close the second tightening valve of the test liquid valve at the lower end of the piston container, the fourth liquid inlet pipe valve, the second tightening valve of the test liquid valve at the lower end of the coal sample tank and the discharge valve of the constant speed and constant pressure pump, complete the dead volume increment difference compensation V2 of the coal sample of the gas pressure test, and also realize the recovery of the volume state of the coal core.

[0034] The present invention adopts a piston container with a piston and a coal sample tank to compress the free space of the coal core and fill the free space, and can accurately restore the volume of the coal core to its original volume. By restoring the volume of the coal core, an important guarantee is provided for the gas pressure measurement technology of the coal seam without sealing. The pressure measurement technology can also provide a fast and simple new way to accurately obtain the residual gas pressure when testing the gas pressure of the coal seam in the mine and the effect of the regional sudden relief measures. When the high-pressure capacity method is used to measure the gas adsorption isotherm, the volume of the coal core is restored, which can improve the measurement reliability of the gas adsorption constants a and b.

[0035] The technical solution of the present invention is not limited to the above-mentioned specific embodiments. All technical variations made according to the technical solution of the present invention fall within the protection scope of the present invention.

Claims

1. A device for restoring the volume of a coal core, characterized in that: It includes a water injection mechanism, an oil injection mechanism, a constant speed and constant pressure pump, a piston container and a coal sample tank. The water injection mechanism and the oil injection mechanism are both connected to the liquid inlet end of the constant speed and constant pressure pump. The piston container has the same structure as the coal sample tank. The piston container and the coal sample tank both include a tank body, a tank cover, a piston, an upper liquid inlet and a lower liquid inlet. The tank body and the tank cover are flange-connected to form a closed cavity. The piston is tightly matched with the inner wall of the tank body. The upper liquid inlet is located at the top of the tank cover, and the lower liquid inlet is located at the bottom of the tank body. A displacement sensor is provided on the piston of the piston container. The liquid outlet end of the constant speed and constant pressure pump is connected to the upper liquid inlet and the lower liquid inlet of the piston container through a pipeline to inject liquid into the piston container. The liquid outlet end of the constant speed and constant pressure pump is connected to the upper liquid inlet and the lower liquid inlet of the coal sample tank through a pipeline to inject liquid into the coal sample tank.

2. The device for restoring the volume of coal core according to claim 1, characterized in that: The piston container and the upper liquid inlet and the lower liquid inlet of the coal sample tank are both provided with test liquid valves, and the test liquid valve includes a valve body, a test liquid cavity, a first liquid inlet connected to the test liquid cavity, a test liquid port, a first liquid discharge port, a first tightening valve adapted to the test liquid port, and a second tightening valve adapted to the first liquid discharge port.

3. The device for restoring the volume of coal core according to claim 2, characterized in that: The water injection mechanism comprises a water storage cup and a water delivery pipe, and the water storage cup is connected with the water delivery pipe.

4. The device for restoring the volume of coal core according to claim 3, characterized in that: The oil filling mechanism comprises an oil storage cup and an oil delivery pipe, and the oil storage cup and the oil delivery pipe are connected.

5. The device for restoring the volume of coal core according to claim 4, characterized in that: The liquid inlet end of the constant speed and constant pressure pump is connected with a liquid inlet pipe, and the liquid outlet end is connected with a liquid discharge pipe. The water pipe and the oil pipe are connected with the liquid inlet pipe of the constant speed and constant pressure pump through a three-way joint.

6. The device for restoring the volume of coal core according to claim 5, characterized in that: The discharge pipe is respectively connected to the first liquid inlet pipe and the second liquid inlet pipe through a three-way joint, the first liquid inlet pipe is connected to the lower liquid inlet of the piston container, the second liquid inlet pipe is connected to the third liquid inlet pipe and the fourth liquid inlet pipe through a three-way joint, the third liquid inlet pipe is connected to the upper liquid inlet of the coal sample tank, and the fourth liquid inlet pipe is connected to the lower liquid inlet of the coal sample tank.

7. The device for restoring the volume of coal core according to claim 6, characterized in that: The fourth liquid inlet pipe is connected to a fifth liquid inlet pipe via a joint, and the fifth liquid inlet pipe is communicated with the upper liquid inlet of the piston container.

8. The device for restoring the volume of coal core according to claim 7, characterized in that: Valves are arranged on the water delivery pipe, the oil delivery pipe, the first liquid inlet pipe, the second liquid inlet pipe, the third liquid inlet pipe, the fourth liquid inlet pipe and the fifth liquid inlet pipe.

9. A method for using the device for restoring the volume of a coal core as claimed in claim 8, characterized in that: The following steps are involved: Step 1, calculate the dead volume increment of the coal sample using the formula ΔV=V0-m / ARD, where ΔV is the dead volume increment of the coal sample, V0 is the volume of the coal sample tank; m is the mass of the coal sample; ARD is the density of the coal sample; Step 2, compression of free space: first open the suction valve of the constant speed and constant pressure pump and the valve of the water pipe, and close the suction valve after the constant speed and constant pressure pump is full of water, and then open the valve on the second liquid inlet pipe, the valve on the third liquid inlet pipe, the test liquid valve on the liquid inlet of the coal sample tank and the discharge valve of the constant speed and constant pressure pump, first fill the pipeline from the valve on the second liquid inlet pipe to the valve on the third liquid inlet pipe, and from the valve on the third liquid inlet pipe to the second tightening valve section of the liquid inlet of the coal sample tank with water, and when liquid flows out of the test liquid port of the test liquid valve at the upper end of the coal sample tank, quickly close the first tightening valve and the second tightening valve of the test liquid valve at the upper end of the coal sample tank, and open the first tightening valve of the test liquid valve at the lower end of the piston container and the second tightening valve of the test liquid valve at the upper end of the piston container, continue to inject liquid into the upper end of the piston container to push the piston downward, and then close the valve of the second liquid inlet pipe, the piston container The first tightening valve of the test liquid valve at the lower end of the device and the discharge valve of the constant speed and constant pressure pump; set the constant speed and constant pressure pump to the constant pressure injection mode, and open the discharge valve of the constant speed and constant pressure pump, the second tightening valve of the test liquid valve at the lower end of the piston container and the second tightening valve of the test liquid valve at the upper end of the coal sample tank, and inject liquid into the lower end of the piston container to push the piston upward to achieve quantitative injection into the hydraulic chamber at the upper end of the coal sample tank and push the piston of the coal sample tank downward to compress the dead volume increment of the coal sample. When the current pressure of the constant speed and constant pressure pump reaches the initial setting value and the injection flow rate is stable at zero, stop the injection, close the second tightening valve of the test liquid valve at the upper end of the coal sample tank, the second tightening valve of the test liquid valve at the lower end of the piston container, the discharge valve of the constant speed and constant pressure pump and the water pipe valve; the dead volume compensation amount can be obtained according to the upper cavity bottom area S of the piston container and the displacement sensor moving distance ΔH1 by the formula V1=S×ΔH1; Step 3, filling of free space V2: First, open the suction valve and oil pipeline valve of the constant speed and constant pressure pump, fill the pump body with incompressible liquid material that is not absorbed by the coal body, then close the suction valve, and then open the second liquid inlet pipe valve, the fourth liquid inlet pipe valve, the test liquid valve at the lower end of the coal sample tank, and the constant speed and constant pressure pump discharge valve, first fill the pipeline from the second liquid inlet pipe valve to the fourth liquid inlet pipe valve and the fourth liquid inlet pipe valve to the test liquid valve at the lower end of the coal sample tank with liquid material that is incompressible and not absorbed by the coal body, and when liquid flows out from the test liquid port of the test liquid valve at the lower end of the coal sample tank, quickly close the test liquid valve at the lower end of the coal sample tank, and open the first tightening valve of the test liquid valve at the lower end of the piston container and the second tightening valve of the test liquid valve at the upper end of the piston container, and pour liquid into the piston container. The liquid injected at the upper end pushes the piston down to the corresponding scale H2. When H2=V2 / S, immediately close the second liquid inlet pipe valve, the first tightening valve of the test liquid valve at the lower end of the piston container, and the discharge valve of the constant speed and constant pressure pump; then open the second tightening valve of the test liquid valve at the lower end of the piston container, the second tightening valve of the test liquid valve at the lower end of the coal sample tank, and the discharge valve of the constant speed and constant pressure pump, inject liquid into the lower end of the piston container, push the piston upward, and quantitatively inject liquid into the coal sample chamber in the lower cavity of the coal sample tank. Continue to inject liquid until the piston of the coal sample tank moves to the topmost position, close the second tightening valve of the test liquid valve at the lower end of the piston container, the fourth liquid inlet pipe valve, the second tightening valve of the test liquid valve at the lower end of the coal sample tank, and the discharge valve of the constant speed and constant pressure pump, and complete the dead volume increment difference compensation V2 of the coal sample in the gas pressure test, and also realize the recovery of the volume state of the coal core.

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