Coal sample fracturing structure, carbon dioxide pulsating fracturing experiment system and experiment method

Abstract

The present application discloses a kind of coal sample fracturing structure, carbon dioxide pulsating pressure fracturing experimental system and experimental method, coal sample is drilled in advance for the coal sample inner hole for passing into pulsating pressure flow, coal sample inner hole is spiral and is used to reduce the pressure offset effect of pulsating pressure flow to reflected pressure flow.Experimental system includes carbon dioxide filling mechanism, true triaxial loading device for providing true triaxial stress loading for coal sample, gas tightness inspection mechanism, vacuum pumping mechanism, constant temperature jacket for providing constant temperature environment for true triaxial loading device and metering detection mechanism;The present application makes the energy contained in pulsating pressure flow act on coal sample more, substantially reduces energy offset phenomenon, produces better fracturing permeability effect to coal sample under the same energy consumption condition of pulsating pump, has important significance to optimization supercritical carbon dioxide fracturing technology, improvement coal seam permeability, strengthening gas extraction effect and reduction " greenhouse effect " caused by gas emission, energy saving and consumption reduction etc..

Description

Technical Field

[0001] This invention relates to the field of safe coal mining technology, specifically to a deep coal and rock supercritical carbon dioxide pulsating fracturing experimental system and method. Background Technology

[0002] Currently, coal remains one of my country's main energy sources. Coal seams often contain large amounts of methane gas, which can easily cause significant hazards. To prevent these methane disasters, gas drainage is necessary. However, with increasingly deeper coal mining, the permeability of coal seams is decreasing, and the effectiveness of gas drainage is also declining.

[0003] To improve coal seam permeability and gas extraction efficiency, permeability enhancement technologies such as hydraulic fracturing and supercritical carbon dioxide fracturing have emerged. However, hydraulic fracturing has drawbacks such as short effective extraction time, the formation of simple fractures, and damage to the coal reservoir. Supercritical carbon dioxide fracturing, on the other hand, can create a complex fracture network in the coal seam while avoiding damage to the coal reservoir. Furthermore, in today's dual-carbon environment, it can improve coal seam permeability while simultaneously sequestering carbon dioxide into the reservoir, achieving a geological sequestration effect. Therefore, it is receiving increasing attention in the coal industry.

[0004] my country's deep coal seam supercritical carbon dioxide injection technology is still in the exploratory stage and currently faces the following problems: (1) No experimental research has been conducted on supercritical carbon dioxide pulsed fracturing of deep coal and rock; (2) ... Figure 1 As shown, the existing pulse fracturing technology ignores the fluid backflow loss factor; the coal sample 55 has a coal sample inner hole for communicating with the injection pipe. The incident flow entering the coal sample inner hole impacts the bottom of the coal sample inner hole vertically. The pulse pressure flow 57 (wave) transmitted to the coal sample 55 is largely canceled out by the reflected pressure flow (wave) reflected back, resulting in poor fracturing and permeability enhancement effect. (3) Existing supercritical carbon dioxide fracturing experiments all collect the seepage gas first and then test the gas composition, but cannot detect the seepage gas composition in real time.

[0005] In supercritical carbon dioxide immersion fracturing, the higher the pressure, the more difficult it is for fractures in the coal seam to extend. However, if the pressure is too low, the coal seam cannot be fracturing effectively. How to achieve both fracturing the coal seam and extending the fractures as far as possible with lower pressure is a difficult problem to solve. Summary of the Invention

[0006] The purpose of this invention is to provide a coal sample fracturing structure that reduces energy cancellation phenomena, provides conditions for fracturing coal bodies under lower pressure conditions, facilitates the extension of fractures further, and improves the permeability of coal seams.

[0007] To achieve the above objectives, the present invention provides a coal sample fracturing structure, wherein the coal sample has a pre-drilled inner hole for introducing a pulsating pressure flow, and also includes a fracturing injection pipe, the upstream end of which is connected to a pulsating pump, and the downstream end of which is connected to the inner hole of the coal sample. The pulsating pressure flow impacts the pore wall of the coal sample, forming a reflected pressure flow. The inner pores of the coal sample are spiral-shaped and designed to reduce the pressure offset effect of reflected pressure flow on pulsating pressure flow.

[0008] The present invention also provides a carbon dioxide pulse fracturing experimental system using the above-mentioned coal sample fracturing structure, including a carbon dioxide injection mechanism, a true triaxial loading device for providing true triaxial stress loading to the coal sample, an airtightness inspection mechanism, a vacuuming mechanism, a constant temperature jacket for providing a constant temperature environment for the true triaxial loading device and converting liquid carbon dioxide into a supercritical state, and a metering and detection mechanism. The carbon dioxide injection mechanism is used to supply supercritical carbon dioxide to the coal sample; the vacuuming mechanism is used to evacuate the system before testing the permeability of the coal sample; the metering and detection mechanism is used to detect the composition of the emitted gas, the amount of emitted gas, and the weight of the emitted liquid. A true triaxial loading device is used to generate confining pressure on coal samples to simulate the pressure environment of deep coal and rock. The true triaxial loading device includes a loading container, inside which is a coal sample holder for holding the coal sample from the fracturing structure; vertical loading hydraulic pistons are connected to the upper and lower ends of the loading container, respectively. The left and right ends of the loading container are respectively connected to left and right loading hydraulic pistons. The front and rear ends of the loading container are respectively connected to forward and backward loading hydraulic pistons; The vertical loading hydraulic piston, the left-right loading hydraulic piston, and the front-back loading hydraulic piston are collectively referred to as the loading hydraulic piston. Each loading hydraulic piston is equipped with a loading injection port. The vertical loading hydraulic piston at the top of the loading container is connected to the coal sample air inlet and the loading container air outlet. Each loading hydraulic piston's loading injection port is connected to the outlet of the booster water pump via a loading pipeline, and the booster water pump is connected to a booster water pump controller. With the flow direction of the fluid as the downstream direction; the upstream end of the coal sample inlet is connected to the downstream end of the carbon dioxide injection mechanism and the downstream end of the air tightness inspection mechanism; the downstream end of the coal sample inlet is connected to the fracturing injection pipe of the coal sample fracturing structure. The upstream end of the air outlet of the loading container is used to communicate with the loading container, and the downstream end of the air outlet of the loading container is connected to the upstream end of the metrology and testing institution.

[0009] The carbon dioxide refueling mechanism includes a carbon dioxide cylinder and a liquid carbon dioxide storage tank; the carbon dioxide cylinder is connected to a sixth pressure gauge, and the liquid carbon dioxide storage tank is connected to a seventh pressure gauge. The outlet of the carbon dioxide cylinder is connected to a gas booster pump via a first pipeline. A first pressure reducing valve and a first valve are connected in series on the first pipeline. A first pressure gauge is connected to the first pipeline to display the pressure at the inlet of the gas booster pump in real time. The gas booster pump is connected to a first controller. The outlet of the gas booster pump is connected to a second pipeline, and the downstream end of the second pipeline is connected to the inlet of the liquid carbon dioxide storage tank. A second valve and a condenser are connected in series on the second pipeline; The outlet of the liquid carbon dioxide storage tank is connected to the inlet of the supercritical carbon dioxide storage tank via a third pipeline, and the outlet of the supercritical carbon dioxide storage tank is connected to the coal sample inlet via a fourth pipeline; the supercritical carbon dioxide storage tank is connected to a fifth pressure gauge, and the downstream end of the fourth pipeline serves as the downstream end of the carbon dioxide refueling mechanism. The third pipeline has a third valve connected in series, and the fourth pipeline has a pulsating pump, a first swing check valve and a fourth valve connected in series from upstream to downstream. The pulsating pump is connected to a pulsating pump controller.

[0010] The thermostatic jacket is connected to a temperature controller; the supercritical carbon dioxide storage tank, the pulse pump, the first swing check valve, the fourth valve, the fourth pipeline, and the loading container of the true triaxial loading device are all located inside the thermostatic jacket.

[0011] The airtightness inspection device includes a helium cylinder. The outlet of the helium cylinder is connected to the coal sample inlet via a fifth pipeline. The fifth pipeline is equipped with a second pressure reducing valve, a second pressure gauge, and a fifth valve. The helium cylinder is connected to a third pressure gauge.

[0012] The vacuum pumping mechanism includes a vacuum pump, the inlet of which is connected to the main exhaust pipe via a sixth pipeline, and the outlet of the vacuum pump is open to the atmosphere; the upstream end of the main exhaust pipe is connected to the exhaust port of the loading container; a sixth valve is provided on the sixth pipeline; and a fourth pressure gauge is provided on the main exhaust pipe.

[0013] The metrology and testing facility includes a metering manifold, the upstream end of which is connected to the downstream end of the gas outlet manifold, and the downstream end of the metering manifold is connected to an analysis pipeline and a flow pipeline. The metering main is connected in series from upstream to downstream with a seventh valve, a second swing check valve, and a gas-liquid separator; a weighing device is located below the gas-liquid separator, and a weighing container is located on the weighing device. The bottom of the gas-liquid separator is connected to the weighing container through a weighing pipeline. The analysis pipeline is connected in series from upstream to downstream with the eighth valve and the gas chromatograph. The outlet of the gas chromatograph is open to the atmosphere. The flow pipeline is connected in series from upstream to downstream, consisting of a ninth valve, a dryer, and a gas flow meter.

[0014] The present invention also provides an experimental method using the above-described carbon dioxide pulsating fracturing experimental system, which is carried out according to the following steps: The first step is to prepare coal samples; Samples of the required shape and size are cut from the simulated deep coal seam. A spiral-shaped inner hole is drilled in the sample using a auger drill. If the drilling is successful, the coal sample required for the experiment is obtained, and the coal sample preparation is completed. If the coal sample cracks or breaks during drilling, the coal sampled from the simulated deep coal seam is pressed into powder or broken into powder. The powdered coal is then bonded together to form the shape and size required for the experiment. Then, a spiral drilling machine is used to drill a spiral-shaped inner hole in the sample. After drilling, the coal sample required for the experiment is obtained, and the coal sample preparation is completed. If the coal sample cracks or breaks during drilling, a spiral tube coated with a release agent can be placed into the coal sample mold after the coal from the simulated deep coal seam has been pressed or crushed into powder, and before the powdered coal is bound into the required shape and size using a coal sample mold. This creates a coal sample with spiral holes. After the coal sample is prepared, it is dried and demolded to complete the coal sample production.

[0015] The second step is experimental preparation; Connect the carbon dioxide pulse fracturing experimental system, place the coal sample into the coal sample holder, and close all valves; The third step is an airtightness check; First, open the second pressure reducing valve to introduce helium from the helium tank into the fifth pipeline; then close the second pressure reducing valve and open the fifth valve to allow the helium in the fifth pipeline to be injected into the coal sample through the coal sample inlet; the helium then seeps from the coal sample into the loading container. After the reading of the fourth pressure gauge has stabilized for more than 2 hours, indicating that the system is well sealed, close the fifth valve, end the third step, and proceed to the fourth step. If the reading on the fourth pressure gauge continues to drop, perform leak detection and sealing of the system; after leak detection and sealing are completed, close all valves and repeat step three. The fourth step is loading; The working pressure of the booster pump is set according to the pressure of the simulated deep coal seam through the booster pump controller; the booster pump is turned on, and six hydraulic pistons apply six-sided confining pressure to the coal sample from six directions to simulate the pressure conditions of the deep coal seam. The fifth step is to create a vacuum; Open the sixth valve and vacuum pump to evacuate the loading container and the main exhaust pipe; after evacuating for 2 hours, close the sixth valve and vacuum pump and proceed to the sixth step. The sixth step is to test the initial permeability of the coal sample; First, the pressurization process is performed. The pressurization process involves: opening the second pressure-reducing valve, allowing helium gas from the helium tank to enter the fifth pipeline; then closing the second pressure-reducing valve and opening the fifth valve, allowing helium gas in the fifth pipeline to be injected into the coal sample through the coal sample inlet; helium gas then seeps from the coal sample into the loading container. If the pressure shown by the fourth pressure gauge remains lower than the predetermined pressure for the experiment, the pressurization action is repeated until the pressure shown by the fourth pressure gauge reaches the predetermined pressure for the experiment. Then, the fifth valve is closed and the seventh and ninth valves are opened. The gas in the loading container enters the gas flow meter through the main gas outlet pipe, the seventh valve, the second swing check valve, the gas-liquid separator, the ninth valve, and the dryer. The permeability of the coal sample is calculated based on the data measured by the gas flow meter. Repeat step six N times to obtain N sets of permeability. Take the average of the N sets of permeability as the initial permeability of the coal sample; N is a natural number and N≥3; After obtaining the initial permeability of the coal sample, close all valves and proceed to step seven; The seventh step is to adjust the pressure and temperature of the carbon dioxide fluid; The action of adjusting the pressure of carbon dioxide fluid is: Open the first pressure-reducing valve to introduce gaseous carbon dioxide into the first pipeline, then close the first pressure-reducing valve and open the first valve, the second valve, and the gas booster pump. The working pressure of the gas booster pump is controlled by the first controller, and the pressure of carbon dioxide in the second pipeline is adjusted to the predetermined pressure for the experiment, thus becoming high-pressure carbon dioxide. The action of adjusting the temperature of carbon dioxide fluid is: The temperature inside the thermostatic jacket is adjusted to the predetermined temperature for the experiment using a temperature controller. During the process of adjusting the pressure of the carbon dioxide fluid, the high-pressure carbon dioxide is condensed into liquid carbon dioxide in the condenser and enters the liquid carbon dioxide storage tank; when the pressure value displayed by the seventh pressure gauge reaches the predetermined value of the experiment, the first valve, the second valve and the gas booster pump are closed, and the third valve is opened; At this point, liquid carbon dioxide enters the supercritical carbon dioxide storage tank through the third pipeline. In the constant temperature jacket, the temperature gradually changes to the temperature maintained by the constant temperature jacket, and the liquid carbon dioxide enters the supercritical state. When the pressure shown by the fifth pressure gauge reaches the upper limit of the pressure of the supercritical carbon dioxide storage tank predetermined for the experiment, the third valve is closed. The eighth step is supercritical carbon dioxide pulsating fracturing of coal and rock; Open the pulsating pump and the fourth valve, control the pulsation frequency and pressure of the pulsating pump to the predetermined values ​​of the experiment through the pulsating pump controller, and record the pressure data during pulsating pressurization; After pulsating pressurization for M minutes, the pulsating pump and the fourth valve are closed to end the pulsating fracturing and proceed to the ninth step; M is a natural number and M≥3; During the eighth step, the pressure inside the supercritical carbon dioxide storage tank is observed in real time through the fifth pressure gauge. When the pressure inside the supercritical carbon dioxide storage tank drops to the lower limit of the pressure inside the supercritical carbon dioxide storage tank predetermined by the experiment, the seventh step is executed so that the pressure displayed by the fifth pressure gauge reaches the upper limit of the pressure inside the supercritical carbon dioxide storage tank predetermined by the experiment. The ninth step is exhaust gas and component analysis; Open the seventh valve, weighing device, eighth valve, and gas chromatograph; weigh the liquid discharged from the gas-liquid separator using the weighing device, and use the gas chromatograph to detect the gas composition; after the detection is completed, close the seventh valve, weighing device, eighth valve, and gas chromatograph. The tenth step is to test the permeability of the sample after fracturing.

[0016] First, the pressurization action is performed until the pressure displayed on the fourth pressure gauge reaches the predetermined pressure for the experiment. Then, the fifth valve is closed and the seventh and ninth valves are opened. The gas in the loading container enters the gas flow meter through the main outlet pipe, the seventh valve, the second swing check valve, the gas-liquid separator, the ninth valve, and the dryer. The permeability of the coal sample is calculated based on the measured data from the gas flow meter. Repeat step 10 N times to measure N sets of permeability. Take the average of the N sets of permeability as the permeability of the coal sample after fracturing. After obtaining the permeability of the coal sample after fracturing, close all valves and end the experiment.

[0017] The present invention has the following advantages: This invention employs a spiral-shaped inner hole in the coal sample, causing the reflected pressure flow to deviate significantly from the pulsating pressure flow. This greatly reduces the pressure cancellation effect of the reflected pressure flow on the pulsating pressure flow, allowing more of the energy contained in the pulsating pressure flow to act on the coal sample. This significantly reduces the energy cancellation phenomenon and produces a better fracturing and permeability enhancement effect on the coal sample under the same energy consumption conditions of the pulsating pump.

[0018] The true triaxial loading device is a common technology. When combined with the coal sample fracturing structure, it can apply specific confining pressure to the coal sample to simulate the stress state of deep coal and rock. Then, the coal sample fracturing structure is used to conduct pulsating fracturing experiments, which greatly reduces the energy cancellation phenomenon and produces a better fracturing and permeability enhancement effect on the coal sample.

[0019] The carbon dioxide filling mechanism forms liquid carbon dioxide through pressurization and condensation, and then heats it through a constant temperature jacket to form supercritical carbon dioxide. The overall structure is simple and effective.

[0020] Thermostatic jackets are used to simulate the temperature environment of deep coal seams, making the experimental conditions closer to those in actual strata. This allows the experimental results of the system to provide more accurate predictions and guidance for phenomena occurring in actual strata.

[0021] Metrology and testing institutions can perform functions such as weighing liquids, detecting gas composition and flow rate, and the obtained data is used to calculate the permeability of coal samples.

[0022] During the airtightness test, instead of directly introducing helium from the helium tank into the coal sample inlet, helium is first introduced into the fifth pipeline. After closing the second pressure reducing valve, the fifth valve is opened to inject the helium from the fifth pipeline into the coal sample inlet. The advantage of this is that the helium tank with a higher storage pressure can be used directly without damaging the coal sample due to the high pressure of the helium.

[0023] In supercritical carbon dioxide immersion fracturing, the higher the pressure, the more difficult it is for the fractures in the coal body to extend during supercritical carbon dioxide fracturing. However, if the pressure is too low, the coal body cannot be fracturing effectively.

[0024] This invention is the first to combine supercritical carbon dioxide immersion fracturing with pulsed fracturing. Utilizing the advantage of the periodic tensile stress in pulsed fracturing, the fracturing pressure of this invention is lower than that of existing supercritical carbon dioxide fracturing technology (allowing for fracturing of the coal body at a lower pressure). In other words, the periodic tensile stress in pulsed fracturing enables this invention to fracture the coal body at a lower pressure compared to constant pressure fracturing.

[0025] Meanwhile, by employing a coal sample fracturing structure, this invention further reduces the fracturing pressure requirements of coal fracturing by minimizing energy offsetting phenomena. This allows for a further reduction in fracturing pressure, increasing the fracture extension distance during fracturing and improving the permeability enhancement effect of the coal seam. Furthermore, the reduced fracturing pressure naturally decreases energy consumption, resulting in significant energy conservation and emission reduction.

[0026] When fluid is injected into the coal body, it generates backflow pressure, which can offset part of the injection pressure, thereby reducing the efficiency of pulsating fracturing. The spiral injection method of the fracturing fluid in this invention can hinder the backflow of supercritical carbon dioxide fluid, significantly reducing the energy loss of the system, allowing supercritical carbon dioxide fluid to penetrate the microporous structure of the coal and rock mass more efficiently, and the energy brought by the fluid to act more on the coal body.

[0027] Pulsating fracturing causes supercritical carbon dioxide to form a pressure flow that fluctuates between high and low pressure. Compared with a constant pressure flow, this can enhance the flow capacity of supercritical carbon dioxide in coal and rock masses, forming a more interconnected complex fracture network structure and improving the fracturing and permeability enhancement effect.

[0028] This invention can directly test the change in permeability of coal samples before and after supercritical carbon dioxide pulsating fracturing, and can detect the microscopic components extracted from coal and rock by supercritical carbon dioxide in real time, which greatly reduces the amount of experimentation and improves experimental efficiency.

[0029] This invention has relatively high experimental efficiency, low experimental pressure, significantly reduced fracturing energy consumption, and better coal fracture extension. Compared with existing supercritical carbon dioxide fracturing technology, it has a better permeability enhancement effect, thereby strengthening the gas extraction effect and improving underground safety.

[0030] Drilling spiral holes in coal samples can easily damage them; this invention, by breaking the coal and then bonding it together to form the shape and size required for the experiment, makes it easier to drill spiral-shaped holes in the coal sample.

[0031] This invention is of great significance for optimizing supercritical carbon dioxide fracturing technology, improving coal seam permeability, enhancing gas extraction efficiency, reducing the greenhouse effect caused by gas emissions, and saving energy and reducing consumption. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the structure of a coal sample fracturing device in the existing technology; Figure 2 This is a schematic diagram of the coal sample fracturing structure in this invention; Figure 3 This is a schematic diagram of the carbon dioxide pulsating fracturing experimental system in this invention; Figure 4 This is a structural schematic diagram of a true triaxial loading device. Detailed Implementation

[0033] like Figures 1 to 4 As shown, the present invention provides a coal sample fracturing structure, wherein a coal sample 55 is pre-drilled with an inner hole 56 for introducing a pulsating pressure flow 57, and also includes a fracturing injection pipe 53, the upstream end of which is connected to a pulsating pump through a pipeline structure, and the downstream end of which is connected to the inner hole 56 of the coal sample. The pulsating pressure flow 57 impacts the pore wall of the inner hole 56 of the coal sample, forming a reflected pressure flow 58. The inner hole 56 of the coal sample is spiral-shaped and is used to reduce the pressure counteracting effect of the reflected pressure flow 58 on the pulsating pressure flow 57.

[0034] This invention employs a spiral-shaped inner hole 56 in the coal sample, which causes the reflected pressure flow 58 to deviate significantly from the pulsating pressure flow 57. This greatly reduces the pressure cancellation effect of the reflected pressure flow 58 on the pulsating pressure flow 57, allowing more of the energy contained in the pulsating pressure flow 57 to act on the coal sample 55. This significantly reduces the energy cancellation phenomenon and produces a better fracturing and permeability enhancement effect on the coal sample 55 under the same energy consumption conditions of the pulsating pump.

[0035] The present invention also discloses a carbon dioxide pulse fracturing experimental system, which uses the above-mentioned coal sample fracturing structure, including a carbon dioxide injection mechanism, a true triaxial loading device for providing true triaxial stress loading to the coal sample 55, an airtightness inspection mechanism, a vacuuming mechanism, a constant temperature jacket 15 for providing a constant temperature environment for the true triaxial loading device, and a measurement and detection mechanism. The carbon dioxide injection mechanism is used to supply supercritical carbon dioxide to coal sample 55; the vacuuming mechanism is used to evacuate the system before testing the permeability of the coal sample; the metering and detection mechanism is used to detect the composition of the emitted gas, the amount of emitted gas (the ultimate goal is to calculate the gas permeability of coal sample 55 from the emission data), and the weight of the emitted liquid. The true triaxial loading device is used to generate confining pressure on coal sample 55 to simulate the pressure environment of deep coal and rock. Thermostatic jacket 15 is also used to convert liquid carbon dioxide into a supercritical state; The true triaxial loading device includes a loading container 23, and a coal sample holder 52 is provided inside the loading container 23. The coal sample holder 52 is used to hold the coal sample 55 in the coal sample fracturing structure. The upper and lower ends of the loading container 23 are respectively connected to vertical loading hydraulic pistons 46. The left and right ends of the loading container 23 are respectively connected to left and right loading hydraulic pistons 48. The front and rear ends of the loading container 23 are respectively connected to forward and backward loading hydraulic pistons 50; The vertical loading hydraulic piston 46, the left-right loading hydraulic piston 48, and the front-back loading hydraulic piston 50 are collectively referred to as loading hydraulic pistons. Each loading hydraulic piston is equipped with a loading injection port 47. The vertical loading hydraulic piston 46 at the upper end of the loading container 23 is connected to the coal sample air inlet 44 and the loading container air outlet 45. Each loading hydraulic piston's loading injection port 47 is connected to the outlet of the booster water pump 24 through the loading pipeline 69. The booster water pump 24 is connected to the booster water pump controller 25. With the flow direction of the fluid as the downstream direction; the upstream end of the coal sample inlet 44 is connected to the downstream end of the carbon dioxide injection mechanism and the downstream end of the airtightness inspection mechanism. The downstream end of the coal sample inlet 44 is connected to the fracturing injection pipe 53 of the coal sample fracturing structure; The upstream end of the loading container outlet 45 is used to communicate with the loading container 23, and the downstream end of the loading container outlet 45 is connected to the upstream end of the metrology and testing institution.

[0036] The true triaxial loading device is a common technology. When combined with the coal sample fracturing structure, it can apply a specific confining pressure to the coal sample 55 to simulate the stress state of deep coal and rock. Then, the coal sample fracturing structure is used to conduct a pulsating fracturing experiment, which greatly reduces the energy cancellation phenomenon and produces a better fracturing and permeability enhancement effect on the coal sample 55.

[0037] The carbon dioxide refueling mechanism includes a carbon dioxide cylinder 1 and a liquid carbon dioxide storage tank 2; the carbon dioxide cylinder 1 is connected to a sixth pressure gauge 3, and the liquid carbon dioxide storage tank 2 is connected to a seventh pressure gauge 12. The outlet of the carbon dioxide cylinder 1 is connected to a gas booster pump 7 via a first pipeline 59. A first pressure reducing valve 4 and a first valve 5 are connected in series on the first pipeline 59. A first pressure gauge 6 is connected to the first pipeline 59. The first pressure gauge 6 is used to display the pressure at the inlet of the gas booster pump 7 in real time. The gas booster pump 7 is connected to a first controller 8. The outlet of the gas booster pump 7 is connected to a second pipeline 60, and the downstream end of the second pipeline 60 is connected to the inlet of the liquid carbon dioxide storage tank 2; the second pipeline 60 is connected to an eighth pressure gauge 9.

[0038] A second valve 10 and a condenser 11 are connected in series on the second pipeline 60; the internal space of the condenser 11 provides space for carbon dioxide condensation, and the condenser 11 is preferably a coil structure, but it can also be a hollow container structure.

[0039] The outlet of the liquid carbon dioxide storage tank 2 is connected to the inlet of the supercritical carbon dioxide storage tank 18 through the third pipeline 61, and the outlet of the supercritical carbon dioxide storage tank 18 is connected to the coal sample inlet 44 through the fourth pipeline 62; the supercritical carbon dioxide storage tank 18 is connected to the fifth pressure gauge 14, and the downstream end of the fourth pipeline 62 serves as the downstream end of the carbon dioxide refueling mechanism. A third valve 13 is connected in series on the third pipeline 61. A pulse pump 19, a first swing check valve 21, and a fourth valve 22 are connected in series from upstream to downstream on the fourth pipeline 62. The pulse pump 19 is connected to a pulse pump controller 20.

[0040] The upstream end of the fracturing injection pipe 53 is connected to the pulsating pump 19 through the pipeline structure formed by the coal sample air inlet 44 and the fourth pipeline 62.

[0041] The carbon dioxide filling mechanism forms liquid carbon dioxide through pressurization and condensation, and then heats it through a constant temperature jacket to form supercritical carbon dioxide. The overall structure is simple and effective.

[0042] The thermostatic jacket 15 is connected to a temperature controller 16; the supercritical carbon dioxide storage tank 18, the pulse pump 19, the first swing check valve 21, the fourth valve 22, the fourth pipeline 62, and the loading container 23 of the true triaxial loading device are all located inside the thermostatic jacket 15. The thermostatic jacket 15 is connected to a temperature gauge 17 for displaying the real-time temperature of the thermostatic jacket 15.

[0043] The thermostatic jacket 15 is a conventional structure and will not be described in detail. The thermostatic jacket 15 is used to simulate the temperature environment of deep coal seams, making the experimental conditions closer to those in actual strata, thereby enabling the experimental results of the experimental system to have a more accurate predictive and guiding role for phenomena occurring in actual strata.

[0044] The airtightness inspection mechanism includes a helium tank 38. The outlet of the helium tank 38 is connected to the coal sample inlet 44 through a fifth pipeline 63. The fifth pipeline 63 is equipped with a second pressure reducing valve 40, a second pressure gauge 41 and a fifth valve 42. The helium tank 38 is connected to a third pressure gauge 39.

[0045] The vacuum pumping mechanism includes a vacuum pump 37. The inlet of the vacuum pump 37 is connected to the outlet manifold 43 through the sixth pipe 64. The outlet of the vacuum pump 37 is open to the atmosphere. The upstream end of the outlet manifold 43 is connected to the outlet port 45 of the loading container. A sixth valve 36 is provided on the sixth pipe 64. A fourth pressure gauge 26 is provided on the outlet manifold 43.

[0046] The metrology and testing mechanism includes a metering manifold 65, the upstream end of which is connected to the downstream end of the gas outlet manifold 43, and the downstream end of the metering manifold 65 is connected to an analysis pipeline 66 and a flow pipeline 67. A seventh valve 27, a second swing check valve 28, and a gas-liquid separator 29 are connected in series from upstream to downstream on the metering main pipe 65. A weighing device 30 is located below the gas-liquid separator 29. The weighing device 30 can be a platform scale, electronic balance, or weight sensor, etc., all of which are existing technologies and will not be described in detail. A weighing container 70 is attached to the weighing device 30, and the bottom of the gas-liquid separator 29 is connected to the weighing container 70 via a weighing pipe 68. The analysis pipeline 66 is connected in series from upstream to downstream with the eighth valve 31 and the gas chromatograph 32. The outlet of the gas chromatograph 32 is open to the atmosphere. The flow pipeline 67 is connected in series from upstream to downstream, including the ninth valve 33, the dryer 34, and the gas flow meter 35. The metrology and testing mechanism can perform functions such as weighing liquids, detecting gas composition and flow rate, and the obtained data is used to calculate the permeability of coal samples.

[0047] This invention also discloses an experimental method using the above-mentioned carbon dioxide pulsating fracturing experimental system, which is carried out according to the following steps: The first step is to prepare coal sample 55; Samples of the required shape and size are cut from the simulated deep coal seam. A small auger drill is used to drill a spiral-shaped inner hole 56 in the sample. If the drilling is successful, the coal sample 55 required for the experiment is obtained, and the preparation of coal sample 55 is completed. If the coal sample 55 cracks or breaks during drilling, the coal sampled from the simulated deep coal seam is pressed into powder or broken into powder. The powdered coal is mixed with an appropriate amount of binder to form the shape and size required for the experiment. Then, a small auger is used to drill a spiral coal sample inner hole 56 on the sample. After drilling, the coal sample 55 required for the experiment is obtained, and the preparation of the coal sample 55 is completed. The second step is experimental preparation; Connect the carbon dioxide pulsed fracturing experimental system, place the coal sample 55 into the coal sample holder 52, and close all valves; The third step is an airtightness check; First, open the second pressure reducing valve 40, and helium gas is introduced into the fifth pipeline 63 from the helium tank 38; then close the second pressure reducing valve 40 and open the fifth valve 42, so that the helium gas in the fifth pipeline 63 is injected into the coal sample 55 through the coal sample inlet 44; the helium gas seeps into the loading container 23 from the coal sample 55. After the reading of the fourth pressure gauge 26 has been stable for more than 2 hours, it indicates that the system is well sealed. Then close the fifth valve 42, end the third step, and proceed to the fourth step. If the reading on the fourth pressure gauge 26 continues to drop, perform leak detection and sealing of the system; leak detection and sealing are standard techniques and will not be described in detail here. After leak detection and sealing are completed, close all valves and repeat step three. The fourth step is loading; The working pressure of the booster pump 24 is set by the booster pump controller 25 according to the pressure of the simulated deep coal seam; the booster pump 24 is turned on, and six hydraulic pistons apply six-sided confining pressure to the coal sample 55 from six directions to simulate the pressure conditions of the deep coal seam. The fifth step is to create a vacuum; Open the sixth valve 36 and vacuum pump 37 to evacuate the loading container 23 and the exhaust manifold 43; after evacuating for 2 hours, close the sixth valve 36 and vacuum pump 37 and proceed to the sixth step. The sixth step is to test the initial permeability of coal sample 55; First, the pressurization action is performed; the pressurization action is as follows: open the second pressure reducing valve 40, and helium gas is introduced into the fifth pipeline 63 from the helium tank 38; then close the second pressure reducing valve 40, open the fifth valve 42, and inject the helium gas in the fifth pipeline 63 into the coal sample 55 through the coal sample inlet 44; the helium gas seeps from the coal sample 55 into the loading container 23; If the pressure displayed by the fourth pressure gauge 26 remains below the predetermined experimental pressure, the pressurization process is repeated until the pressure displayed by the fourth pressure gauge 26 reaches the predetermined experimental pressure. At this point, the fifth valve 42 is closed, and the seventh valve 27 and the ninth valve 33 are opened. The gas in the loading container 23 enters the gas flow meter 35 through the main gas outlet 43, the seventh valve 27, the second swing check valve 28, the gas-liquid separator 29, the ninth valve 33, and the dryer 34. The coal sample permeability is calculated based on the data measured by the gas flow meter 35. Calculating the coal sample permeability based on the data from the gas flow meter 35 is a conventional technique and will not be described in detail.

[0048] Repeat step six N times to obtain N sets of permeability. Take the average of the N sets of permeability as the initial permeability of coal sample 55; N is a natural number and N≥3; the preferred value of N is 5.

[0049] After obtaining the initial permeability of coal sample 55, close all valves and proceed to step seven; The seventh step is to adjust the pressure and temperature of the carbon dioxide fluid; The action of adjusting the pressure of carbon dioxide fluid is: Open the first pressure reducing valve 4 to introduce gaseous carbon dioxide into the first pipeline 59, then close the first pressure reducing valve 4, and open the first valve 5, the second valve 10, and the gas booster pump 7. The working pressure of the gas booster pump 7 is controlled by the first controller 8, and the carbon dioxide pressure in the second pipeline 60 is adjusted to the predetermined pressure for the experiment, thus becoming high-pressure carbon dioxide. The action of adjusting the temperature of carbon dioxide fluid is: The temperature inside the thermostatic jacket 15 is adjusted to the predetermined temperature for the experiment by the temperature controller 16. During the process of adjusting the pressure of carbon dioxide fluid, high-pressure carbon dioxide is condensed into liquid carbon dioxide in condenser 11 and enters liquid carbon dioxide storage tank 2; when the pressure value displayed by the seventh pressure gauge 12 reaches the predetermined experimental value, the first valve 5, the second valve 10 and the gas booster pump 7 are closed, and the third valve 13 is opened. At this time, liquid carbon dioxide enters the supercritical carbon dioxide storage tank 18 through the third pipeline 61. In the constant temperature jacket 15, the temperature gradually changes to the constant temperature maintained by the constant temperature jacket 15, and the liquid carbon dioxide enters the supercritical state. When the pressure shown by the fifth pressure gauge 14 reaches the upper limit of the pressure of the supercritical carbon dioxide storage tank 18 as predetermined in the experiment, the third valve 13 is closed. The eighth step is supercritical carbon dioxide pulsating fracturing of coal and rock; Open the pulsating pump 19 and the fourth valve 22, control the pulsation frequency and pressure of the pulsating pump 19 to the predetermined values ​​of the experiment through the pulsating pump controller 20, and record the pressure data during pulsating pressurization; After pulsating pressurization for M minutes, the pulsating pump 19 and the fourth valve 22 are closed to end the pulsating fracturing and proceed to the ninth step; M is a natural number and M≥3; During the eighth step, the pressure inside the supercritical carbon dioxide storage tank 18 is observed in real time through the fifth pressure gauge 14. When the pressure inside the supercritical carbon dioxide storage tank 18 drops to the lower limit of the pressure inside the supercritical carbon dioxide storage tank 18 predetermined by the experiment, the seventh step is executed so that the pressure displayed by the fifth pressure gauge 14 reaches the upper limit of the pressure inside the supercritical carbon dioxide storage tank 18 predetermined by the experiment (this process is also the process of replenishing liquid into the supercritical carbon dioxide storage tank 18). The ninth step is exhaust gas and component analysis; Open the seventh valve 27, the weighing device 30, the eighth valve 31, and the gas chromatograph 32; weigh the liquid discharged from the gas-liquid separator 29 using the weighing device 30, and detect the gas composition using the gas chromatograph 32; after the detection is completed, close the seventh valve 27, the weighing device 30, the eighth valve 31, and the gas chromatograph 32. The tenth step is to test the permeability of the sample after fracturing.

[0050] First, the pressurization action is performed until the pressure displayed on the fourth pressure gauge 26 reaches the predetermined pressure for the experiment. Then, the fifth valve 42 is closed, and the seventh valve 27 and the ninth valve 33 are opened. The gas in the loading container 23 enters the gas flow meter 35 through the gas outlet main pipe 43, the seventh valve 27, the second swing check valve 28, the gas-liquid separator 29, the ninth valve 33, and the dryer 34. The permeability of the coal sample is calculated based on the data measured by the gas flow meter 35.

[0051] Repeat step 10 N times to measure N sets of permeability. Take the average of the N sets of permeability as the permeability of coal sample 55 after fracturing. The N value in step 10 is the same as the N value in step 6.

[0052] After obtaining the permeability of coal sample 55 after fracturing, all valves were closed, and the experiment was terminated.

[0053] During the airtightness test, instead of directly introducing helium from helium tank 38 into the coal sample inlet 44, helium is first introduced into the fifth pipeline 63. After closing the second pressure reducing valve 40, the fifth valve 42 is opened to inject the helium that has entered the fifth pipeline 63 into the coal sample inlet 44. The advantage of this is that the helium tank 38 with its higher storage pressure can be used directly without damaging the coal sample 55 due to the high pressure of helium.

[0054] In supercritical carbon dioxide immersion fracturing, the higher the pressure, the more difficult it is for the fractures in the coal body to extend during supercritical carbon dioxide fracturing. However, if the pressure is too low, the coal body cannot be fracturing effectively.

[0055] This invention is the first to combine supercritical carbon dioxide immersion fracturing with pulsed fracturing. Utilizing the advantage of the periodic tensile stress in pulsed fracturing, the fracturing pressure of this invention is lower than that of existing supercritical carbon dioxide fracturing technology (allowing for fracturing of the coal body at a lower pressure). In other words, the periodic tensile stress in pulsed fracturing enables this invention to fracture the coal body at a lower pressure compared to constant pressure fracturing.

[0056] Meanwhile, by employing a coal sample fracturing structure, this invention further reduces the fracturing pressure requirements of coal fracturing by minimizing energy offsetting phenomena. This allows for a further reduction in fracturing pressure, increasing the fracture extension distance during fracturing and improving the permeability enhancement effect of the coal seam. Furthermore, the reduced fracturing pressure naturally decreases energy consumption, resulting in significant energy conservation and emission reduction.

[0057] When fluid is injected into the coal body, it generates backflow pressure, which can offset part of the injection pressure, thereby reducing the efficiency of pulsating fracturing. The spiral injection method of the fracturing fluid in this invention can hinder the backflow of supercritical carbon dioxide fluid, significantly reducing the energy loss of the system, allowing supercritical carbon dioxide fluid to penetrate the microporous structure of the coal and rock mass more efficiently, and the energy brought by the fluid to act more on the coal body.

[0058] Pulsating fracturing causes supercritical carbon dioxide to form a pressure flow that fluctuates between high and low pressure. Compared with a constant pressure flow, this can enhance the flow capacity of supercritical carbon dioxide in coal and rock masses, forming a more interconnected complex fracture network structure and improving the fracturing and permeability enhancement effect.

[0059] This invention can directly test the change in permeability of coal samples before and after supercritical carbon dioxide pulsating fracturing, and can detect the microscopic components extracted from coal and rock by supercritical carbon dioxide in real time, which greatly reduces the amount of experimentation and improves experimental efficiency.

[0060] This invention has relatively high experimental efficiency, low experimental pressure, significantly reduced fracturing energy consumption, and better coal fracture extension. Compared with existing supercritical carbon dioxide fracturing technology, it has a better permeability enhancement effect, thereby strengthening the gas extraction effect and improving underground safety.

[0061] This invention is of great significance for optimizing supercritical carbon dioxide fracturing technology, improving coal seam permeability, enhancing gas extraction efficiency, reducing the greenhouse effect caused by gas emissions, and saving energy and reducing consumption.

[0062] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A carbon dioxide pulsed fracturing experimental system, using a coal sample fracturing structure, wherein the coal sample has a pre-drilled inner hole for introducing pulsed pressure flow. The coal sample fracturing structure also includes a fracturing injection pipe, the upstream end of which is connected to a pulsed pump, and the downstream end of which is connected to the inner hole of the coal sample. The pulsed pressure flow impacts the wall of the inner hole of the coal sample, forming a reflected pressure flow. The inner hole of the coal sample is spiral-shaped and is used to reduce the pressure cancellation effect of the reflected pressure flow on the pulsed pressure flow. The carbon dioxide pulsed fracturing experimental system includes a carbon dioxide injection mechanism, a true triaxial loading device for providing true triaxial stress loading to the coal sample, an airtightness inspection mechanism, a vacuuming mechanism, a constant temperature jacket for providing a constant temperature environment for the true triaxial loading device and converting liquid carbon dioxide into a supercritical state, and a metering and detection mechanism. The carbon dioxide injection mechanism is used to supply supercritical carbon dioxide to the coal sample; the vacuuming mechanism is used to evacuate the system before testing the permeability of the coal sample; the metering and detection mechanism is used to detect the composition of the emitted gas, the amount of emitted gas, and the weight of the emitted liquid. A true triaxial loading device is used to generate confining pressure on coal samples to simulate the pressure environment of deep coal and rock. Its features are: The true triaxial loading device includes a loading container, inside which is a coal sample holder for holding the coal sample from the fracturing structure; vertical loading hydraulic pistons are connected to the upper and lower ends of the loading container, respectively. The left and right ends of the loading container are respectively connected to left and right loading hydraulic pistons. The front and rear ends of the loading container are respectively connected to forward and backward loading hydraulic pistons; The vertical loading hydraulic piston, the left-right loading hydraulic piston, and the front-back loading hydraulic piston are collectively referred to as the loading hydraulic piston. Each loading hydraulic piston is equipped with a loading injection port. The vertical loading hydraulic piston at the top of the loading container is connected to the coal sample air inlet and the loading container air outlet. Each loading hydraulic piston's loading injection port is connected to the outlet of the booster water pump via a loading pipeline, and the booster water pump is connected to a booster water pump controller. With the flow direction of the fluid as the downstream direction; the upstream end of the coal sample inlet is connected to the downstream end of the carbon dioxide injection mechanism and the downstream end of the air tightness inspection mechanism; the downstream end of the coal sample inlet is connected to the fracturing injection pipe of the coal sample fracturing structure. The upstream end of the air outlet of the loading container is used to communicate with the loading container, and the downstream end of the air outlet of the loading container is connected to the upstream end of the metrology and testing institution.

2. The carbon dioxide pulsed fracturing experimental system according to claim 1, characterized in that: The carbon dioxide refueling mechanism includes a carbon dioxide cylinder and a liquid carbon dioxide storage tank; the carbon dioxide cylinder is connected to a sixth pressure gauge, and the liquid carbon dioxide storage tank is connected to a seventh pressure gauge. The outlet of the carbon dioxide cylinder is connected to a gas booster pump via a first pipeline. A first pressure reducing valve and a first valve are connected in series on the first pipeline. A first pressure gauge is connected to the first pipeline to display the pressure at the inlet of the gas booster pump in real time. The gas booster pump is connected to a first controller. The outlet of the gas booster pump is connected to a second pipeline, and the downstream end of the second pipeline is connected to the inlet of the liquid carbon dioxide storage tank. A second valve and a condenser are connected in series on the second pipeline; The outlet of the liquid carbon dioxide storage tank is connected to the inlet of the supercritical carbon dioxide storage tank via a third pipeline, and the outlet of the supercritical carbon dioxide storage tank is connected to the coal sample inlet via a fourth pipeline; the supercritical carbon dioxide storage tank is connected to a fifth pressure gauge, and the downstream end of the fourth pipeline serves as the downstream end of the carbon dioxide refueling mechanism. The third pipeline has a third valve connected in series, and the fourth pipeline has a pulsating pump, a first swing check valve and a fourth valve connected in series from upstream to downstream. The pulsating pump is connected to a pulsating pump controller.

3. The carbon dioxide pulsed fracturing experimental system according to claim 2, characterized in that: The thermostatic jacket is connected to a temperature controller; the supercritical carbon dioxide storage tank, the pulse pump, the first swing check valve, the fourth valve, the fourth pipeline, and the loading container of the true triaxial loading device are all located inside the thermostatic jacket.

4. The carbon dioxide pulsating fracturing experimental system according to any one of claims 1-3, characterized in that: The airtightness inspection device includes a helium cylinder. The outlet of the helium cylinder is connected to the coal sample inlet via a fifth pipeline. The fifth pipeline is equipped with a second pressure reducing valve, a second pressure gauge, and a fifth valve. The helium cylinder is connected to a third pressure gauge.

5. The carbon dioxide pulsating fracturing experimental system according to claim 4, characterized in that: The vacuum pumping mechanism includes a vacuum pump, the inlet of which is connected to the main exhaust pipe via a sixth pipeline, and the outlet of the vacuum pump is open to the atmosphere; the upstream end of the main exhaust pipe is connected to the exhaust port of the loading container; a sixth valve is provided on the sixth pipeline; and a fourth pressure gauge is provided on the main exhaust pipe.

6. The carbon dioxide pulsed fracturing experimental system according to claim 5, characterized in that: The metrology and testing facility includes a metering manifold, the upstream end of which is connected to the downstream end of the gas outlet manifold, and the downstream end of the metering manifold is connected to an analysis pipeline and a flow pipeline. The metering main is connected in series from upstream to downstream with a seventh valve, a second swing check valve, and a gas-liquid separator; a weighing device is located below the gas-liquid separator, and a weighing container is located on the weighing device. The bottom of the gas-liquid separator is connected to the weighing container through a weighing pipeline. The analysis pipeline is connected in series from upstream to downstream with the eighth valve and the gas chromatograph. The outlet of the gas chromatograph is open to the atmosphere. The flow pipeline is connected in series from upstream to downstream, consisting of a ninth valve, a dryer, and a gas flow meter.

7. The experimental method using the carbon dioxide pulsating fracturing experimental system described in claim 6, characterized in that... Follow these steps: The first step is to prepare coal samples; Samples of the required shape and size are cut from the simulated deep coal seam. A spiral-shaped inner hole is drilled in the sample using a auger drill. If the drilling is successful, the coal sample required for the experiment is obtained, and the coal sample preparation is completed. If the coal sample cracks or breaks during drilling, the coal sampled from the simulated deep coal seam is pressed into powder or broken into powder. The powdered coal is then bonded together to form the shape and size required for the experiment. Then, a spiral drilling machine is used to drill a spiral-shaped inner hole in the sample. After drilling, the coal sample required for the experiment is obtained, and the coal sample preparation is completed. If the coal sample cracks or breaks during drilling, a spiral tube coated with a release agent can be placed into the coal sample mold after the coal from the simulated deep coal seam is pressed into powder or broken into powder, and before the powdered coal is bonded into the required shape and size for the experiment using a coal sample mold, thereby producing a coal sample with spiral holes. After the coal sample is prepared, it is dried and demolded to complete the coal sample preparation. The second step is experimental preparation; Connect the carbon dioxide pulse fracturing experimental system, place the coal sample into the coal sample holder, and close all valves; The third step is an airtightness check; First, open the second pressure reducing valve to introduce helium from the helium tank into the fifth pipeline; then close the second pressure reducing valve and open the fifth valve to allow the helium in the fifth pipeline to be injected into the coal sample through the coal sample inlet; the helium then seeps from the coal sample into the loading container. After the reading of the fourth pressure gauge has stabilized for more than 2 hours, indicating that the system is well sealed, close the fifth valve, end the third step, and proceed to the fourth step. If the reading on the fourth pressure gauge continues to drop, perform leak detection and sealing of the system; after leak detection and sealing are completed, close all valves and repeat step three. The fourth step is loading; The working pressure of the booster pump is set according to the pressure of the simulated deep coal seam through the booster pump controller; the booster pump is turned on, and six hydraulic pistons apply six-sided confining pressure to the coal sample from six directions to simulate the pressure conditions of the deep coal seam. The fifth step is to create a vacuum; Open the sixth valve and vacuum pump to evacuate the loading container and the main exhaust pipe; after evacuating for 2 hours, close the sixth valve and vacuum pump and proceed to the sixth step. The sixth step is to test the initial permeability of the coal sample; First, the pressurization process is performed. The pressurization process involves: opening the second pressure-reducing valve, allowing helium gas from the helium tank to enter the fifth pipeline; then closing the second pressure-reducing valve and opening the fifth valve, allowing helium gas in the fifth pipeline to be injected into the coal sample through the coal sample inlet; helium gas then seeps from the coal sample into the loading container. If the pressure shown by the fourth pressure gauge remains lower than the predetermined pressure for the experiment, the pressurization action is repeated until the pressure shown by the fourth pressure gauge reaches the predetermined pressure for the experiment. Then, the fifth valve is closed and the seventh and ninth valves are opened. The gas in the loading container enters the gas flow meter through the main gas outlet pipe, the seventh valve, the second swing check valve, the gas-liquid separator, the ninth valve, and the dryer. The permeability of the coal sample is calculated based on the data measured by the gas flow meter. Repeat step six N times to obtain N sets of permeability. Take the average of the N sets of permeability as the initial permeability of the coal sample. N is a natural number and N≥3; After obtaining the initial permeability of the coal sample, close all valves and proceed to step seven; The seventh step is to adjust the pressure and temperature of the carbon dioxide fluid; The action of adjusting the pressure of carbon dioxide fluid is: Open the first pressure-reducing valve to introduce gaseous carbon dioxide into the first pipeline, then close the first pressure-reducing valve and open the first valve, the second valve, and the gas booster pump. The working pressure of the gas booster pump is controlled by the first controller, and the pressure of carbon dioxide in the second pipeline is adjusted to the predetermined pressure for the experiment, thus becoming high-pressure carbon dioxide. The action of adjusting the temperature of carbon dioxide fluid is: The temperature inside the thermostatic jacket is adjusted to the predetermined temperature for the experiment using a temperature controller. During the process of adjusting the pressure of the carbon dioxide fluid, the high-pressure carbon dioxide is condensed into liquid carbon dioxide in the condenser and enters the liquid carbon dioxide storage tank; when the pressure value displayed by the seventh pressure gauge reaches the predetermined value of the experiment, the first valve, the second valve and the gas booster pump are closed, and the third valve is opened; At this point, liquid carbon dioxide enters the supercritical carbon dioxide storage tank through the third pipeline. In the constant temperature jacket, the temperature gradually changes to the temperature maintained by the constant temperature jacket, and the liquid carbon dioxide enters the supercritical state. When the pressure shown by the fifth pressure gauge reaches the upper limit of the pressure of the supercritical carbon dioxide storage tank predetermined for the experiment, the third valve is closed. The eighth step is supercritical carbon dioxide pulsating fracturing of coal and rock; Open the pulsating pump and the fourth valve, control the pulsation frequency and pressure of the pulsating pump to the predetermined values ​​of the experiment through the pulsating pump controller, and record the pressure data during pulsating pressurization; After pulsating pressurization for M minutes, the pulsating pump and the fourth valve are closed to end the pulsating fracturing and proceed to the ninth step; M is a natural number and M≥3; During the eighth step, the pressure inside the supercritical carbon dioxide storage tank is observed in real time through the fifth pressure gauge. When the pressure inside the supercritical carbon dioxide storage tank drops to the lower limit of the pressure inside the supercritical carbon dioxide storage tank predetermined by the experiment, the seventh step is executed so that the pressure displayed by the fifth pressure gauge reaches the upper limit of the pressure inside the supercritical carbon dioxide storage tank predetermined by the experiment. The ninth step is exhaust gas and component analysis; Open the seventh valve, weighing device, eighth valve, and gas chromatograph; weigh the liquid discharged from the gas-liquid separator using the weighing device, and use the gas chromatograph to detect the gas composition; after the detection is completed, close the seventh valve, weighing device, eighth valve, and gas chromatograph. The tenth step is to test the permeability of the sample after fracturing; First, the pressurization action is performed until the pressure displayed on the fourth pressure gauge reaches the predetermined pressure for the experiment. Then, the fifth valve is closed and the seventh and ninth valves are opened. The gas in the loading container enters the gas flow meter through the main outlet pipe, the seventh valve, the second swing check valve, the gas-liquid separator, the ninth valve, and the dryer. The permeability of the coal sample is calculated based on the measured data from the gas flow meter. Repeat step 10 N times to measure N sets of permeability. Take the average of the N sets of permeability as the permeability of the coal sample after fracturing. After obtaining the permeability of the coal sample after fracturing, close all valves and end the experiment.

Images