A system and method for controlling pressure reduction and gas extraction after coal seam fracturing in a mine

Abstract

This invention relates to a system and method for controlling and reducing gas pressure after fracturing in an underground coal seam and extracting gas. The system includes: a wellhead device for controlling pressure reduction, fracturing equipment, fracturing pipelines, gas pipelines, drainage pipelines, a gas-water separator, a gas flow meter, and / or negative pressure extraction equipment. The wellhead device has a four-way valve, four connection ports, a pressure gauge, four valves, and a pressure relief nozzle. The wellhead device is installed underground and connected to the coal seam borehole opening via fracturing pipelines connected to two of the four connection ports. The pressure gauge and pressure relief nozzle are connected to the other two connection ports of the wellhead device. The method for controlling and reducing pressure during gas extraction involves installing the wellhead device of this invention in the underground roadway of the coal mine, and using the pressure relief nozzle of the wellhead device to control the pressure reduction, thereby ensuring the stable outflow of water and gas from the coal seam. This prevents the sand supporting the coal seam fractures from being washed away by the water flow due to excessive water velocity, which would cause the fractures to close again. It also prevents gas from mixing with air and surging out in large quantities under high pressure, which would reduce the concentration of gas during subsequent gas extraction.

Description

Technical Field

[0001] This invention relates to underground gas extraction in coal mines, and more particularly to a control and pressure reduction system and method for underground gas extraction. Background Technology

[0002] To prevent gas outbursts and methane emissions, surface coalbed methane extraction technology was once used as a gas control method. Theoretically, deploying a sufficient number of extraction wells on the surface could achieve the goal of regional outburst mitigation. However, the complex terrain makes it unsuitable to deploy surface extraction wells in many areas.

[0003] However, the permeability of coal seams in my country is generally low, typically averaging between 0.01 and 0.5 × 10⁻⁶. -3 µm 2 For coal seams that are difficult or impossible to drain, my country has introduced coalbed methane fracturing technology to increase or decrease their permeability. Coalbed methane fracturing involves drilling into the coal seam, injecting water and pressurizing it to fracture the seam, creating fissures and increasing its permeability, thus effectively releasing coalbed methane. To prevent the fractures from reclosing, sand is typically injected after fracturing to support them. Coalbed methane fracturing technology effectively increases coalbed methane permeability, improving both emissions and production.

[0004] However, after coal seam fracturing and before gas extraction, the water injected during fracturing needs to be drained. Drainage from surface coalbed methane wells requires a "stable," "gradual," and "gradual" process to prevent excessive pressure drop. This would cause sand particles supporting the coal seam fractures to flow out with the water, thus re-blocking the fractures that serve as channels for coalbed methane migration. Because there is a distance of thousands of meters between surface coalbed methane wells and underground boreholes, by the time sand and coal dust are observed in the drainage at the wellhead, a considerable amount of sand has already flowed out of the coal seam fractures. Since it is difficult to observe the presence of sand and coal dust in the drainage in a timely manner, most surface extraction wells must release pressure very cautiously, always ensuring that the pressure in the borehole does not drop too quickly. This results in a long drainage time, generally requiring more than a year to completely drain the injected water and enter the stable production period of coalbed methane. The excessively long drainage time from surface coalbed methane wells prevents coal mines from quickly starting production.

[0005] Furthermore, surface extraction is negative pressure extraction, requiring the extraction equipment to operate continuously. During the extraction process, a continuous power supply is needed to ensure the equipment operates 24 hours a day. The extraction equipment (mostly electric motors) consumes a staggering amount of electricity, significantly increasing energy costs. Summary of the Invention

[0006] (a) Technical problems to be solved

[0007] To address the issue of long drainage times in surface drilling for coalbed methane extraction, this invention provides a pressure-controlled extraction system and method for downhole drainage and gas extraction.

[0008] Due to limitations imposed by underground construction conditions and coal seam fracturing equipment, existing underground coalbed methane extraction technologies do not involve coal seam fracturing. Instead, they involve precisely drilling boreholes in methane-accumulating areas to extract localized methane gas. While localized extraction can quickly reduce methane concentration within the coal seam, its coverage area is limited, and the extraction is not thorough, resulting in low methane production. To address these shortcomings of existing technologies, this invention provides an underground pressure-controlled extraction system and method that improves coal seam permeability, enables thorough gas extraction, and achieves high gas production.

[0009] (II) Technical Solution

[0010] This invention places the downhole pressure-controlled extraction system at a distance from the coal seam borehole. In its gas-water separator, it is possible to observe in time whether there are sand particles and / or coal dust flowing out of the drainage. If not, the pressure relief nozzle can be opened wider to accelerate drainage; if so, the pressure relief nozzle should be closed until there are no more sand particles and / or coal dust in the discharged water.

[0011] This invention uses fracturing pipelines to connect a bulky coal seam fracturing device placed on the ground to a borehole underground, in order to fracture the coal seam.

[0012] This invention combines surface coalbed methane well fracturing and permeability enhancement technology with downhole precision extraction technology, giving it the advantages of short extraction cycle, high gas concentration, and large gas extraction volume.

[0013] One of the features of this invention is that, apart from the surface fracturing equipment, all other equipment can be installed underground; in this invention, apart from surface fracturing construction, all other engineering work is completed in underground roadways, such as drilling coal seams, laying fracturing pipelines, depressurization, and extraction.

[0014] The system for controlling and reducing gas extraction pressure according to the present invention includes: a coal seam borehole opening, a wellhead device for controlling and reducing gas extraction pressure, coal seam fracturing equipment, fracturing pipeline, gas pipeline, drainage pipeline, gas-water separator, etc.

[0015] In a preferred embodiment of the present invention, the system for controlling the depressurization and extraction of gas of the present invention further comprises: a gas flow meter for measuring the amount of gas produced.

[0016] In another preferred embodiment of the present invention, the system for controlling and reducing gas extraction pressure of the present invention also includes a negative pressure extraction device when necessary.

[0017] The wellhead device for controlling and reducing gas extraction has a four-way valve, a pressure relief nozzle, a pressure gauge, and four valves.

[0018] The fracturing pipeline is divided into a first fracturing pipeline and a second fracturing pipeline.

[0019] The gas pipeline is divided into a first section and a second section.

[0020] The wellhead device for controlling and reducing gas extraction pressure has four connection ports. The first connection port is connected to one end of the first section of fracturing pipeline, and the other end of the first section of fracturing pipeline is connected to the borehole opening of the coal seam. The second connection port is connected to one end of the second section of fracturing pipeline, and the other end of the second section of fracturing pipeline is connected to fracturing equipment placed on the ground. The third connection port is connected to the pressure gauge for observing pressure changes at the coal seam borehole opening. The fourth connection port is connected to the pressure relief nozzle. A valve is installed between each connection port and its respective connected pipeline or equipment. The pressure relief nozzle is connected to the gas-water separator through the first section of gas pipeline. The gas-water separator has two outlets: the first outlet is connected to the drainage pipeline, and the second outlet is connected to the second section of gas pipeline.

[0021] The gas flow meter can be used selectively and can be installed on the second section of the gas pipeline.

[0022] When the coal seam pressure is 0 MPa, a negative pressure extraction device can be used, which can be connected to the end of the second gas pipeline.

[0023] The negative pressure extraction equipment includes a negative pressure extraction motor and a negative pressure extraction pipeline, with the negative pressure extraction motor installed at the outlet end of the negative pressure extraction pipeline.

[0024] The negative pressure extraction equipment may further include a negative pressure regulating valve for adjusting the intensity of the negative pressure. The negative pressure extraction equipment can be installed underground or on the surface. The negative pressure regulating valve is installed between the negative pressure extraction motor and the negative pressure extraction pipeline.

[0025] During construction, the coal seam fracturing equipment installed on the surface injects high-pressure water into the fracturing pipeline. The water flows through the fracturing pipeline into the coal seam borehole underground. The impact force of the high-pressure water fractures the coal seam. Then, sand is injected through the fracturing pipeline to support the fracturing coal seam fractures, completing the fracturing operation. Before coal seam fracturing, the fourth valve between the fourth connection port of the wellhead device and the pressure relief nozzle is closed. After fracturing, the second valve between the second connection port of the wellhead device and the second section of the fracturing pipeline is closed, and the fourth valve between the fourth connection port of the wellhead device and the pressure relief nozzle is opened. The wellhead pressure gauge is observed. The wellhead device controls the pressure of water and gas in the coal seam through the pressure relief nozzle to perform pressure relief and extraction operations.

[0026] The drainage and air extraction process of this invention is divided into four stages: 1) pressure release and liquid drainage stage, 2) critical desorption stage, 3) positive pressure extraction stage and 4) negative pressure extraction stage. The extraction in the first three stages can be controlled by adjusting the pressure release nozzle, and the extraction in the fourth stage can be started by activating the negative pressure extraction equipment.

[0027] (III) Beneficial Effects

[0028] The above-described technical solution of the present invention has the following beneficial effects:

[0029] 1. There is no extraction equipment on the ground, so there is no power consumption issue for the equipment, which provides a guarantee for energy conservation and emission reduction;

[0030] 2. The amount of gas extraction can be more than doubled, which will correspondingly reduce methane (gas) emissions, reducing methane emissions in coal mining by more than half;

[0031] 3. Gas extraction efficiency is increased by more than 3 times, and the extraction time to meet standards is shortened by more than 100%.

[0032] 4. The probability of gas outbursts and exceeding limits is significantly reduced, achieving safe, continuous, and stable production in coal mines;

[0033] 5. Gas extraction is not limited by surface terrain conditions and has wide applicability. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the connection of various devices and the extraction method of a pressure-controlled extraction system according to a certain embodiment of the present invention.

[0035] Figure 2 This is a schematic diagram of the wellhead device structure of the present invention.

[0036] Figure 3 This is a graph showing the actual gas extraction efficiency of this invention. Detailed Implementation

[0037] The embodiments of the present invention will be described in further detail below with reference to the accompanying drawings. The following examples are for illustrative purposes only and should not be construed as limiting the scope of the invention.

[0038] Example 1

[0039] like Figure 1 and Figure 2 As shown, in one embodiment of the present invention, the system for controlling and reducing pressure to extract gas underground after coal seam fracturing includes: a system for controlling and reducing pressure to extract gas underground after coal seam fracturing, the system comprising:

[0040] Coal seam borehole opening (1);

[0041] A wellhead device (2) having the function of controlling pressure reduction, the wellhead device (2) includes a four-way valve (10), four valves, namely a first valve (11), a second valve (12), a third valve (13), a fourth valve (14), a pressure gauge (15), and a pressure relief nozzle (16);

[0042] Fracturing equipment (3);

[0043] The fracturing pipeline (4) is divided into a first fracturing pipeline (4.1) and a second fracturing pipeline (4.2). One end of the first fracturing pipeline (4.1) is connected to the four-way valve (10), and the other end is connected to the coal seam borehole opening (1).

[0044] Drainage pipeline (5);

[0045] Gas-water separator (6);

[0046] The gas pipeline (7) is divided into a first gas pipeline (7.1) and a second gas pipeline (7.2). One end of the first gas pipeline (7.1) is connected to the pressure relief nozzle (16), and the other end of the first gas pipeline (7.1) is connected to the inlet end of the gas-water separator (6). One end of the second gas pipeline (7.2) is connected to the gas outlet port of the gas-water separator (6), and the drain pipeline (5) is connected to the water outlet port of the gas-water separator (6).

[0047] The four-way valve (10) of the wellhead device (2) has four connection ports: the first connection port (17) is connected to the first section of fracturing pipeline (4.1), the second connection port (18) is connected to the second section of fracturing pipeline (4.2), the third connection port (19) is connected to the pressure gauge (15), and the fourth connection port (20) is connected to the pressure relief nozzle (16).

[0048] The first valve (11) of the wellhead device (2) is located between the first connection port (17) and the first section of fracturing pipeline (4.1); the second valve (12) is located between the second connection port (18) and the second section of fracturing pipeline (4.2); the third valve (13) is located between the third connection port (19) and the pressure gauge (15); and the fourth valve (14) is located between the fourth connection port (20) and the pressure relief nozzle (16).

[0049] Except for the fracturing equipment (3), all other systems for controlling and reducing gas extraction can be installed underground in coal mines.

[0050] like Figure 1 and Figure 2As shown, in another embodiment of the present invention, the system for controlling and reducing pressure to extract gas in the mine after coal seam fracturing of the present invention includes a gas flow meter (8) installed on the second gas pipeline (7.2) to measure the gas production.

[0051] like Figure 1 As shown, in another embodiment of the present invention, when the underground gas pressure drops to 0 MPa, a negative pressure extraction device (9) may be selectively used. The negative pressure extraction device (9) includes a negative pressure extraction motor (9.1) and a negative pressure extraction pipeline (9.2).

[0052] like Figure 1 As shown, the negative pressure extraction equipment may also have a negative pressure regulating valve (9.3). The negative pressure regulating valve (9.3) can adjust the intensity of the negative pressure during negative pressure extraction to prevent the fracturing sand and coal powder in the hole from being extracted due to excessive negative pressure, thereby causing the coal seam cracks to close again and reducing the gas permeability.

[0053] like Figure 1 As shown, the negative pressure extraction pipeline (9.2) of the present invention is connected to the second gas pipeline (7.2).

[0054] like Figure 1 and Figure 2As shown, the method for controlling and reducing gas extraction underground after coal seam fracturing according to the present invention involves arranging the gas extraction system with the function of controlling and reducing pressure of the present invention underground. Except for the fracturing equipment, all other equipment of the system can be installed in the underground roadway of the coal mine and operated underground. The wellhead device (2) of the gas extraction control system of the present invention has four connection ports (10), namely the first connection port (17), the second connection port (18), the third connection port (19) and the fourth connection port (20). The first connection port (17) is connected to one end of the first section of fracturing pipeline (4.1), and the other end of the first section of fracturing pipeline (4.1) is connected to the coal seam borehole (1). The second connection port (18) is connected to one end of the second section of fracturing pipeline (4.2), and the other end of the second section of fracturing pipeline (4.2) is connected to the fracturing equipment (3) set on the ground. The third connection port (19) is connected to the pressure gauge (15). The fourth connection port (20) is connected to the pressure relief nozzle (16). A valve is set between each connection port and its respective connected pipeline or equipment, namely the first valve (11), the second valve (12), the third valve (13) and the fourth valve (14). The first valve (11) is located between the first connection port (17) and the first section of fracturing pipeline (4.1); the second valve (12) is located between the second connection port (18) and the second section of fracturing pipeline (4.2); the third valve (13) is located between the third connection port (19) and the pressure gauge (15); and the fourth valve (14) is located between the fourth connection port (20) and the pressure relief nozzle (16).

[0055] The method further includes a fracturing step, which is to inject water into the coal seam borehole by pressurizing the second section of the fracturing pipeline (4.2) of the ground fracturing equipment (3) of the present invention. Under the impact of the water, the coal seam is fracturing. Then, sand supporting the coal seam fracture is injected. Before the coal seam fracturing operation, the fourth valve (14) between the fourth connection port (20) of the wellhead device (2) and the pressure relief nozzle (16) is closed to prevent the fracturing water from entering the gas pipeline (7). After the fracturing operation, the second valve (12) between the second connection port (18) of the wellhead device (2) and the second section of the fracturing pipeline (4.2) is closed, and the fourth valve (14) between the fourth connection port (20) of the wellhead device (2) and the pressure relief nozzle (16) is opened to allow the water injected into the coal seam and the gas released from the coal seam to be discharged from the gas pipeline (7).

[0056] Observe the wellhead pressure gauge (15) to determine the steps of the extraction stage;

[0057] The gas-water separator (6) is used to observe whether there are sand particles and / or coal dust in the drainage. The pressure relief intensity of the pressure relief nozzle (16) is adjusted accordingly to avoid sand and / or coal dust flowing out with the water due to excessive pressure relief.

[0058] The step of separating gas and water using the gas-water separator (6);

[0059] The steps involved in extracting coalbed methane via gas pipelines.

[0060] like Figure 1 As shown, in the initial stage of gas extraction, water used for fracturing the coal seam and the produced gas flow out from the first gas pipeline. Therefore, in a preferred embodiment of the present invention, the method for controlling and reducing pressure to extract gas underground after coal seam fracturing also includes a gas-water separation step. This step utilizes a gas-water separator (6), and by operating the gas-water separator (6), the extracted liquid and gas are separated. The gas-water separator (6) has two outlets. The first outlet is connected to the drainage pipeline (5), and the second outlet is connected to the second gas pipeline (7.2), so that water flows out from the drainage pipeline (5) and gas flows out from the second gas pipeline (7.2).

[0061] like Figure 1 As shown, in another embodiment of the present invention, the method for controlling and reducing the pressure of underground gas extraction after coal seam fracturing further includes a gas metering step, which uses a gas flow meter (8) to record the gas production. The gas flow meter (8) is installed on the second gas pipeline (7.2).

[0062] like Figure 1 As shown, in another embodiment of the present invention, the method for controlling and reducing the pressure of underground gas extraction after coal seam fracturing further includes a negative pressure extraction step. This step utilizes a negative pressure extraction device (9) to continue extracting gas when the well pressure drops to 0 MPa. Negative pressure extraction can be achieved using the negative pressure extraction device (9). The negative pressure extraction device (9) includes a negative pressure extraction motor (9.1) and a negative pressure extraction pipeline (9.2). The negative pressure extraction device may also optionally include a negative pressure regulating valve (9.3) to control the negative pressure intensity during extraction. The negative pressure extraction pipeline (9.2) is connected to the second section of the gas pipeline (7.2).

[0063] like Figure 3 As shown, the drainage and air extraction process of this invention is divided into four stages: 1) pressure release and liquid drainage stage, 2) critical desorption stage, 3) positive pressure extraction stage and 4) negative pressure extraction stage.

[0064] Phase 1: Pressure Release and Drainage Phase

[0065] Coal seam fracturing involves injecting large amounts of water and fracturing sand into the coal seam under pressure from the surface. The high water pressure causes the coal seam to fracture, creating fissures. Fracturing sand is then injected to fill and support the fractures. After fracturing, a large amount of water remains in the coal seam, increasing its pressure. At this stage, the water must be drained to reduce the pressure and allow gas production.

[0066] Since the coal seam is located above the rock tunnel, water will flow to lower places. In addition, water and gas in the coal seam will automatically overflow under the heavy pressure of the ore body. Therefore, underground gas extraction is positive pressure extraction, which does not require negative pressure extraction equipment such as electric motors. At least in this stage and the positive pressure extraction stage, the participation of negative pressure extraction power equipment is not required.

[0067] Based on the coal seam fracturing mechanism and the damage of fracturing fluid to the coal seam, the coal seam condition is determined. After fracturing, pressure release and drainage measures should be taken promptly. The principle of pressure release and drainage is to ensure that no coal dust is ejected or only a small amount of coal dust is ejected. In order to achieve the goal of removing as much fracturing fluid as possible, the optimal depressurization rate and drainage volume at this stage are calculated based on the mathematical model of unidirectional water flow.

[0068] The drainage volume and pressure reduction rate are controlled by adjusting the pressure relief nozzle (16) of the wellhead device (2). If the rate is too fast, the sand in the fracture will be discharged with the water, causing the fracture to close again. If the pressure reduction is too slow, it will delay the gas extraction time and make it impossible to meet the gas extraction requirements in a short time. The optimal drainage rate to pressure ratio can be calculated by the following formula.

[0069]

[0070] m w Expressed as the mass flow rate of water; K represents the coal seam permeability; K rw μ represents the relative permeability of the aqueous phase. w B is expressed as the viscosity of water. w P is expressed as the volume coefficient of water; w It is expressed as water phase pressure.

[0071] The drainage volume during the pressure release and drainage stage can be calculated based on the formula and data.

[0072] Pressure release speed control table

[0073]

[0074] Key management points: During depressurization, it is important to frequently and carefully observe the quality of the discharged water and whether any solid substances are discharged.

[0075] Second stage of gas extraction: Critical gas production stage

[0076] Once the wellhead pressure reaches the critical desorption pressure, the coal seam gas begins to desorb and be produced, the gas phase permeability begins to increase, and the water phase permeability begins to decrease. At this point, the drainage rate decreases significantly. To prevent the wellhead pressure from dropping too quickly, excessive coal seam gas desorption leading to a steep gas pressure funnel and reduced coal seam permeability at the wellhead, the primary focus should be on stabilizing the bottom hole pressure and controlling the gas desorption rate. (The critical desorption pressure can be calculated based on isothermal adsorption and desorption experiments.)

[0077] Key management points: Strictly control the orifice pressure, maintain a stable pressure decrease, observe changes in water quality, and determine the actual coal seam desorption pressure. The critical desorption point can be calculated using the following formula:

[0078] Pcd = V·PL / (VL-V)

[0079] Pcd represents the critical desorption pressure (MPa); V represents the measured gas content (m³). 3 / t); PL is the Langmuir pressure (MPa); VL is the Langmuir volume (m³). 3 / t)

[0080] The critical desorption pressure value was obtained through calculation.

[0081] Third stage of drainage: Positive pressure extraction stage

[0082] As the orifice pressure decreases, the coal seam desorption area increases, and the gas production gradually rises. At this point, a reasonable pressure drop rate is crucial to ensuring gas extraction. If the pressure drop rate is too fast, excessive gas desorption will cause fracturing sand in the coal seam to be produced rapidly with the gas flow rate. Once too much sand is produced from the coal seam, it will cause the coal seam fractures to close and reduce permeability. Therefore, it is necessary to control the pressure drop at the orifice, slow down the desorption rate of gas in the reservoir, and allow water and gas at the far end to flow and be produced, further increasing the desorption area.

[0083] Management focus: The core of this stage is to control the pressure drop at the orifice, determine a reasonable extraction system, and ensure a steady increase in gas extraction and stable coal powder production.

[0084] Based on data simulations from the gas-water bidirectional flow stage, the pressure drop at the wellhead during the positive pressure extraction stage can be calculated using the following formula.

[0085] ( - )

[0086] ( - )

[0087] Indicates gas production; Indicates aquatic output; Indicates the thickness of the coal seam; Indicates the effective radius; Indicates the radius of the orifice; Indicates the gas phase flow coefficient; Indicates the skin depth coefficient; Indicates the gas phase pressure of the grid; Indicates the flow pressure inside the orifice.

[0088] Substituting the data, we can see the decrease in orifice pressure.

[0089] Fourth stage of drainage: Negative pressure extraction stage

[0090] When the orifice pressure drops to 0 MPa, it indicates that the coal seam desorption area has expanded to its maximum. Under normal extraction conditions, the extraction volume will gradually decrease. At this time, it is necessary to use underground negative pressure pipelines in the coal mine for extraction. By adjusting the negative pressure intensity, the residual gas in the coal seam can be extracted, and the gas content in the coal seam can be reduced to the minimum.

[0091] Management focus: The core of this stage is to adjust the negative pressure intensity to avoid the extraction of fracturing sand and coal powder from the borehole due to excessive negative pressure extraction intensity, which would reduce the permeability of the coal seam.

[0092] Negative pressure extraction valve opening control table

[0093]

[0094] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

[0095] Explanation of reference numerals in the attached figures

[0096] 1. Coal seam borehole opening

[0097] 2. Wellhead Equipment

[0098] 3. Fracturing equipment

[0099] 4. Fracturing pipeline

[0100] 4.1 First-stage fracturing pipeline

[0101] 4.2 Second-stage fracturing pipeline

[0102] 5. Drainage pipes

[0103] 6. Gas-water separator

[0104] 7. Gas lines

[0105] 7.1 First section of gas pipeline

[0106] 7.2 Second section of gas pipeline

[0107] 8. Gas Flow Meter

[0108] 9. Negative pressure extraction equipment

[0109] 9.1 Negative pressure extraction motor

[0110] 9.2 Negative Pressure Extraction Pipeline

[0111] 9.3 Negative pressure regulating valve

[0112] 10 Sitong

[0113] 11 First Valve

[0114] 12 Second valve

[0115] 13 Third Valve

[0116] 14. Fourth valve

[0117] 15. Pressure gauge

[0118] 16 Pressure relief nozzle

[0119] 17 First connection port

[0120] 18 Second connection port

[0121] 19 Third connection port

[0122] 20 Fourth connection port

Claims

1. A system for controlling and reducing gas extraction after fracturing an underground coal seam, the system comprising: Coal seam borehole opening (1); A wellhead device (2) having the function of controlling pressure reduction, the wellhead device (2) including a four-way valve (10), a first valve (11), a second valve (12), a third valve (13), a fourth valve (14), a pressure gauge (15), and a pressure relief nozzle (16); Fracturing equipment (3); The fracturing pipeline (4) is divided into a first fracturing pipeline (4.1) and a second fracturing pipeline (4.2). One end of the first fracturing pipeline (4.1) is connected to the four-way valve (10), and the other end is connected to the coal seam borehole opening (1). Drainage pipeline (5); Gas-water separator (6); The gas pipeline (7) is divided into a first gas pipeline (7.1) and a second gas pipeline (7.2). One end of the first gas pipeline (7.1) is connected to the pressure relief nozzle (16), and the other end of the first gas pipeline (7.1) is connected to the inlet end of the gas-water separator (6). One end of the second gas pipeline (7.2) is connected to the gas outlet port of the gas-water separator (6), and the drain pipeline (5) is connected to the water outlet port of the gas-water separator (6). The four-way valve (10) has four connection ports: the first connection port (17) is connected to the first section of fracturing pipeline (4.1), the second connection port (18) is connected to the second section of fracturing pipeline (4.2), the third connection port (19) is connected to the pressure gauge (15), and the fourth connection port (20) is connected to the pressure relief nozzle (16). The first valve (11) is located between the first connection port (17) and the first section of fracturing pipeline (4.1); the second valve (12) is located between the second connection port (18) and the second section of fracturing pipeline (4.2); the third valve (13) is located between the third connection port (19) and the pressure gauge (15); and the fourth valve (14) is located between the fourth connection port (20) and the pressure relief nozzle (16). Except for the fracturing equipment (3), all other systems for controlling and reducing gas extraction are installed underground in coal mines. The system further includes a negative pressure extraction device (9), which includes a negative pressure extraction motor (9.1) and a negative pressure extraction pipeline (9.2). The negative pressure extraction motor (9.1) is installed downhole or above ground, and the negative pressure extraction pipeline (9.2) is connected to the gas outlet port of the second gas pipeline (7.2). The negative pressure extraction device (9) is used to perform negative pressure extraction after the positive pressure extraction stage when the orifice pressure drops to 0 MPa. After fracturing, water in the injected coal seam and gas released from the coal seam are discharged from the gas pipeline (7); the wellhead pressure gauge (15) is observed to determine the steps of the extraction stage; the gas-water separator (6) is used to observe whether there are sand particles and / or coal dust in the drainage, and the pressure relief oil nozzle (16) is adjusted accordingly to avoid sand and / or coal dust flowing out with the water due to excessive pressure relief.

2. The system for controlling and reducing pressure to extract gas after underground coal seam fracturing according to claim 1, wherein, The system further includes a gas flow meter (8) which is located near the gas outlet of the second gas pipeline (7.2).

3. The system for controlling and reducing pressure to extract gas after underground coal seam fracturing according to claim 1, wherein, The negative pressure extraction device (9) further includes a negative pressure adjustment valve (9.3) for controlling the intensity of the extraction negative pressure.

4. A method for controlling and reducing pressure to extract gas after fracturing an underground coal seam, the method being used in the system for controlling and reducing pressure to extract gas after fracturing an underground coal seam as described in any one of claims 1 to 3, characterized in that, The method includes: The fracturing step involves fracturing the target coal seam using the fracturing equipment (3) and fracturing pipeline (4). Before the coal seam fracturing operation, the fourth valve (14) between the fourth connection port (20) of the wellhead device (2) and the pressure relief nozzle (16) is closed. After the fracturing operation, the second valve (12) between the second connection port (18) of the wellhead device (2) and the second section of the fracturing pipeline (4.2) is closed, and the fourth valve (14) between the wellhead device (2) and the pressure relief nozzle (16) is opened. The pressure at the borehole opening is observed using the pressure gauge (15) to determine the steps of the extraction stage; The step of observing whether there are sand particles and / or coal dust in the drainage through the gas-water separator (6) and adjusting the pressure of the pressure relief nozzle (16) accordingly; The step of separating gas and water using the gas-water separator (6); The steps involved in extracting coalbed methane via gas pipelines; The method further includes a negative pressure extraction step, which uses a negative pressure extraction device (9). The negative pressure extraction device (9) is located underground or on the surface and connected to the end of the second gas pipeline (7.2). After the positive pressure extraction stage, when the coal seam pressure is 0 MPa, the negative pressure extraction device (9) is activated. The negative pressure extraction device (9) includes a negative pressure extraction motor (9.1) and a negative pressure extraction pipeline (9.2). The negative pressure extraction motor (9.1) is installed at the gas outlet of the negative pressure extraction pipeline, and the negative pressure extraction pipeline (9.2) is connected to the outlet of the second gas pipeline (7.2).

5. The method for controlling and reducing gas extraction after fracturing underground coal seams according to claim 4, the method further includes a gas metering step, wherein the gas metering step uses a gas flow meter (8) to measure the amount of gas produced, and the gas flow meter (8) is installed on the second gas pipeline (7.2).

6. The method for controlling and reducing gas extraction after underground coal seam fracturing according to claim 4, characterized in that, The negative pressure extraction device (9) further includes a negative pressure regulating valve (9.3) for controlling the intensity of the negative pressure, the negative pressure regulating valve being installed between the negative pressure extraction motor and the negative pressure extraction pipeline.

Images