A system and method for utilizing extracted gas to fracture coal and displace gas

Abstract

A method and system for utilizing extracted gas to fracture coal and displace gas. The method partly utilizes the cooperation of a group of holes to pressurize the gas extracted by the group of holes, directly injects high-concentration gas in the early stage of extraction, and injects the concentrated high-concentration gas into a borehole in the later stage of extraction, and combines with gas extraction to realize gas fracture, positive and negative pressure alternating stress fatigue fracture, and gas displacement of the coal body around the borehole. The system adopts a combination of an impurity removal device, a bypass pipe, a concentration pipe, a centrifugal pump, a gas booster pump, an air injection pipe, and a gas extraction branch pipe. Through the action of the centrifugal pump and the gas booster pump on the booster pipe, gas with a concentration higher than 30% is directly injected into the borehole during the entire extraction cycle. By combining borehole transformation with gas extraction, high-concentration gas high-pressure fracture of the coal body, positive and negative pressure alternating stress fatigue fracture of the coal body, and gas displacement are realized. The present invention can significantly improve the gas extraction efficiency and gas extraction concentration.

Description

Technical Field

[0001] The invention relates to a method for increasing coal seam gas production, in particular to a method system and method for utilizing extracted gas to fracture coal bodies and displace gas, belonging to the technical field of gas extraction. Background Art

[0002] The structure of coal seams in my country is complex, and the permeability is generally low. Many high-gas coal seams are low-permeability coal seams. At the same time, with the gradual increase in coal mining depth, the increase in stress further reduces the permeability of coal seams. The effect of pre-extraction of gas from undepressurized coal seams in mines with poor permeability is often not ideal.

[0003] At present, the technical measures for efficient extraction of gas from coal mines in my country include hydraulic fracturing and gas injection. When hydraulic fracturing is used, the coal body is easily muddied after being soaked in water, blocking the gas flow channel. The permeability enhancement range is limited by the surface tension of water. When hydraulic permeability enhancement is used, the coal body fracture pressure is required to be 25MPa. Under the action of high-pressure water, it is very easy to induce coal and gas outburst, which brings serious safety hazards to the efficient production of coal mines. Gas injection mainly injects nitrogen, carbon dioxide, air, etc. into the coal seam to displace and extract coal seam gas. Although these methods greatly improve the gas extraction rate and extraction efficiency, they dilute the gas concentration of the later extraction, resulting in a decrease in gas utilization rate, or even inability to be used. Therefore, how to improve the efficiency of gas extraction without affecting the utilization of gas has become an urgent problem to be solved. Summary of the invention

[0004] The purpose of the present invention is to provide a method and system for utilizing extracted gas to fracture coal and displace gas, which does not require an external gas source, has a simple process, is easy to operate, and has a low cost. It integrates gas fracturing, alternating stress fatigue fracturing, and gas displacement to solve the problem of short high-concentration gas extraction cycle, realizes gas fracturing, alternating stress fracturing, and gas displacement of the coal body, and significantly improves the gas extraction efficiency and gas extraction concentration.

[0005] In order to achieve the above object, the present invention provides a method for utilizing extracted gas to fracture coal and displace gas, comprising the following steps:

[0006] ① According to the inclination of the coal seam, several boreholes for gas extraction are constructed. After the construction of several boreholes is completed, the gas extraction boreholes are sealed by high-pressure cement slurry injection;

[0007] ② Every five boreholes form a group, the first borehole, the second borehole, the third borehole, the fourth borehole, and the fifth borehole are connected to the gas collection pipe, the control valves on the sealing pipes from the second borehole to the fifth borehole are opened, and the gas injection joint of the first borehole is connected to the gas injection pipe;

[0008] ③ Open the fifth control valve, the fourth control valve, the third control valve and the first control valve in sequence. This is the initial stage of gas extraction, and the gas concentration is close to 100%. The second gas concentration meter transmits the gas concentration signal to the controller. When the gas concentration of the second gas concentration meter received by the controller is higher than 30%, the controller transmits a signal through the data line to open the first intelligent ball valve. After a delay of 30 seconds, the controller transmits a signal through the data line to open the gas booster pump and the centrifugal pump in sequence. Under the negative pressure of the extraction of the centrifugal pump, the gas from the second borehole, the third borehole, the fourth borehole and the fifth borehole is removed by the impurity removal device to remove impurities such as water and coal powder carried by the gas, and then injected into the first borehole through the sealing pipe after passing through the bypass pipe, the booster pipe, the gas injection pipe and the gas injection joint. The coal body near the first borehole is subjected to the same direction of the negative pressure from the gas injection pressure as the extraction of the second borehole, and both diffuse outward to form a combined force to fracture the coal body, and can also displace the gas in the coal body. The gas injection time is 3-5 hours;

[0009] ④ Remove the gas injection pipe connected to the sealing pipe of the first borehole, and connect the sealing pipe of the first borehole to the gas collection pipe, then separate the second borehole from the gas collection pipe, connect the sealing pipe of the second borehole to the gas injection pipe through the gas injection joint thereon, and repeat step ③ until the gas injection coal fracture and gas displacement work of the first borehole to the fifth borehole is completed;

[0010] ⑤ Connect the first to fifth boreholes to the gas collection pipe, open the fourth intelligent ball valve through the controller, and extract gas for 4-6 days;

[0011] ⑥ Repeat steps ②, ③, ④, and ⑤ for 5-7 times. During this process, the coal bodies around the first borehole to the fifth borehole undergo gas positive pressure fracturing, gas dynamic fatigue fracturing with positive and negative pressure conversion, and gas displacement, ensuring efficient and high-concentration extraction of coal seam gas. Then, the fourth intelligent ball valve is opened through the controller to perform normal gas extraction, and the gas injection pipe is kept connected to the gas injection joint of the first borehole;

[0012] ⑦ During the gas extraction process, when the second gas concentration meter measures a gas concentration lower than 30%, the gas concentration signal measured by the second gas concentration meter is transmitted to the controller through the data line. The controller closes the fourth intelligent ball valve and opens the third intelligent ball valve, and then opens the fourth control valve and the first control valve in sequence. After a delay of 30 seconds, the controller transmits a signal through the data line to open the gas booster pump and the centrifugal pump in sequence. The gas passes through the deoxygenation device and the denitrification device to increase the gas concentration. When the first gas concentration meter measures a gas concentration lower than 30%, the controller transmits a signal through the data line to open the second intelligent ball valve, and the gas flows back to the gas collection pipe until the first gas concentration meter measures a gas concentration higher than 30%. The controller transmits a signal through the data line to open the first intelligent ball valve, and the gas with a concentration higher than 30% is injected into the first borehole. The injected gas pressure is 1-3MPa, and the gas injection time is 2-3h.

[0013] ⑧ Remove the gas injection pipe connected to the sealing pipe of the first borehole, and connect the first borehole to the gas collection pipe, then separate the second borehole from the gas collection pipe, connect the gas injection joint of the second borehole to the gas injection pipe, repeat step ⑦, cycle in sequence, until the gas extraction from the first borehole to the fifth borehole is used to fracture the coal body and displace the gas;

[0014] ⑨ Connect the first to fifth boreholes to the gas collection pipe, open the fourth intelligent ball valve through the controller, and extract gas for 2-3 days;

[0015] ⑩ Repeat steps ⑦, ⑧ and ⑨ until the gas content in the coal seam reaches the mining requirements. During this process, ensure that the extracted gas concentration does not affect gas utilization.

[0016] In step ③, after being pressurized by the gas booster pump, the pressure of the gas injected into the first borehole is 5-8 MPa.

[0017] A system for utilizing extracted gas to fracture a coal body and displace the gas, comprising a first borehole, a second borehole, a third borehole, a fourth borehole, a fifth borehole and a controller, wherein the first borehole, the second borehole, the third borehole, the fourth borehole and the fifth borehole extend into a coal seam, and a sealing pipe is arranged in each of the first borehole, the second borehole, the third borehole, the fourth borehole and the fifth borehole, wherein one end of the sealing pipe extends into the coal seam, and the other end of the sealing pipe extends out of the coal seam and is connected to a branch interface of a gas collecting pipe after sealing the borehole, and a sealing pipe is arranged in the sealing hole. A gas injection joint is installed on each pipe, a control valve is arranged on the sealing pipes in the second borehole, the third borehole, the fourth borehole and the fifth borehole, a second control valve is arranged on the first borehole, and the control valve and the second control valve are arranged between the gas injection joint and the gas collecting pipe, and a first control valve is arranged on the pipeline of the gas injection joint; the gas outlet of the gas collecting pipe is connected with the gas inlet of the impurity removal device through the impurity removal connecting pipe, and a fifth control valve and a second gas concentration measuring instrument are arranged in sequence on the impurity removal connecting pipe, the gas outlet on one side of the impurity removal device is connected with one end of the first branch pipe, the other end of the first branch pipe is connected to one of the ports of the third three-way valve, and the other two ports of the third three-way valve are respectively connected to one end of the concentration pipe and one end of the bypass pipe, a fourth control valve is arranged on the first branch pipe, the other end of the concentration pipe is connected to one of the ports of the second three-way valve, and the other two ports of the second three-way valve are respectively connected to the other end of the bypass pipe and one end of the boosting pipe, a third control valve and a second check valve are arranged in sequence on the bypass pipe, and a third intelligent ball valve and a deoxygenating valve are arranged in sequence on the concentration pipe. Device, denitrification device, first gas concentration measuring instrument, third check valve; the other end of the boosting pipe is connected to one of the ports of the first three-way valve, the other two ports of the first three-way valve are respectively connected to the air inlet of the gas collection pipe and the air inlet of the gas injection pipe, a centrifugal pump and a gas boosting pump are sequentially arranged on the boosting pipe, a second intelligent ball valve and a first check valve are sequentially arranged on one end of the gas collection pipe close to the first three-way valve, the air outlet of the gas injection pipe is connected to the air inlet of the gas injection joint, and a first intelligent ball valve is arranged on the gas injection pipe;

[0018] The gas outlet on the other side of the impurity removal device is connected to one end of the second branch pipe, the other end of the second branch pipe is connected to the gas extraction branch pipe, and a fourth intelligent ball valve is arranged on the second branch pipe;

[0019] The controller is respectively connected to the first intelligent ball valve, the second intelligent ball valve, the third intelligent ball valve, the gas booster pump, the centrifugal pump, the fourth intelligent ball valve, the first gas concentration meter and the second gas concentration meter.

[0020] The controller is a programmable logic controller.

[0021] The deoxygenation device is provided with a commercial nitrogen-making carbon molecular sieve; the denitrification device is provided with an activated carbon adsorbent with a nitrogen absorbing function.

[0022] The boosting ratio of the gas booster pump is greater than or equal to 100:1; the extraction negative pressure provided by the centrifugal pump is greater than or equal to 0.05MPa.

[0023] Compared with the prior art, the method of the present invention makes full use of the cooperation of the borehole group, pressurizes the gas extracted from the borehole group, directly injects high-concentration gas in the early stage of extraction, and combines it with gas extraction to achieve high-pressure gas fracturing, positive and negative pressure alternating stress fatigue fracturing, and gas displacement of the coal body around the borehole, thereby significantly improving the gas extraction efficiency and gas extraction concentration; in the later stage of extraction, purifies the low-concentration gas to ensure that the gas concentration is higher than 30%, and achieves low-pressure fracturing, positive and negative pressure alternating stress fatigue fracturing, and gas displacement of the coal body around the borehole; uses the extracted gas to fracture the coal body and displace the gas, so that there is no need to add other gases. The gas extraction method ensures the quality of gas utilization, and also makes the compressed air flow and the extracted air flow appear alternately in the gas extraction borehole, so that the coal body around the borehole is repeatedly subjected to the effects of expansion and contraction. Under the action of this alternating stress, the coal body is easy to reach the fatigue limit and rupture, so that the cracks continue to expand and gradually develop to the deep, increasing the gas extraction radius of the borehole, increasing the total amount of gas extraction from the borehole, and improving the gas extraction effect; In summary, this method does not require an external gas source, has a simple process, is easy to operate, and has a low cost. It integrates gas fracturing, alternating stress fatigue fracturing and gas displacement, solves the problem of short extraction cycle of high-concentration gas, and has wide practicability;

[0024] The extraction system of the present invention adopts a method of combining an impurity removal device, a bypass pipe, a concentration pipe, a centrifugal pump, a gas booster pump, an air injection pipe, and a gas extraction branch pipe. In the initial stage of extraction, high-concentration gas is directly injected into the borehole through the action of the centrifugal pump and the gas booster pump on the bypass pipe and the booster pipe. By combining borehole transformation with gas extraction, high-concentration gas high-pressure fracturing of the coal body, positive and negative pressure alternating stress fatigue fracturing of the coal body and gas displacement are achieved, thereby realizing efficient and high-concentration gas extraction; in the later stage of gas extraction, the low-concentration gas is concentrated (concentration is greater than 30%) and then injected into the gas extraction borehole, and the gas that does not meet the concentration requirement flows back to the gas collection pipe, thereby realizing low-pressure fracturing of the coal body around the borehole, positive and negative pressure alternating stress fatigue fracturing and gas displacement, ensuring high-concentration and efficient extraction of the extracted gas without affecting gas utilization. The gas extracted by the collaborative extraction of the borehole group is used to fracture the coal body and displace the gas. This not only ensures the quality of gas utilization without adding other gases, but also makes the compressed air flow and the extracted air flow appear alternately in the gas extraction borehole, so that the coal body around the borehole repeatedly bears the effects of expansion and contraction. Under the action of this alternating stress, the coal body is likely to reach the fatigue limit and rupture, so that the cracks continue to expand and gradually develop toward the deep, increasing the gas extraction radius of the borehole, increasing the total amount of gas extracted from the borehole, and improving the gas extraction effect. In summary, this system does not require an external gas source, has a simple process, is easy to operate, and has a low cost. It integrates gas fracturing, alternating stress fracturing and gas displacement, solves the problem of short extraction cycle of high-concentration gas, and has wide practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of the structure of the present invention after assembly.

[0026] In the figure: 1, coal seam; 2, first borehole; 3, sealing pipe; 4, gas injection joint; 5, first control valve; 6, gas injection pipe; 7, first intelligent ball valve; 8, second control valve; 9, first check valve; 10, second intelligent ball valve; 11, first three-way valve; 12, gas booster pump; 13, centrifugal pump; 14, booster pipe; 15, second three-way valve; 16, second check valve; 17, concentration pipe; 18, controller; 19, bypass pipe; 20, third check valve; 21, first gas concentration meter ; 22. Denitrification device; 23. Deoxygenation device; 24. The third intelligent ball valve; 25. The third three-way valve; 26. The third control valve; 27. The first branch pipe; 28. The fourth control valve; 29. ​​The second gas concentration meter; 30. The fifth control valve; 31. The impurity removal connecting pipe; 32. The impurity removal device; 33. The fourth intelligent ball valve; 34. The second branch pipe; 35. The gas extraction branch pipe; 36. The gas collecting pipe; 37. The second borehole; 38. The third borehole; 39. The fourth borehole; 40. The fifth borehole. DETAILED DESCRIPTION

[0027] The present invention will be further described below in conjunction with the accompanying drawings.

[0028] like Figure 1 As shown, a method for utilizing extracted gas to fracture coal and displace gas comprises the following steps:

[0029] ① According to the inclination angle of the coal seam 1, several boreholes for gas extraction are constructed. After the construction of several boreholes is completed, the gas extraction boreholes are sealed by high-pressure cement slurry injection;

[0030] ② Every five boreholes form a group, the first borehole 2, the second borehole 37, the third borehole 38, the fourth borehole 39, and the fifth borehole 40 are connected to the gas collection pipe 36, wherein the control valves on the sealing pipes 3 from the second borehole 37 to the fifth borehole 40 are opened, and the gas injection joint 4 of the first borehole 2 is connected to the gas injection pipe 6;

[0031] ③ Open the fifth control valve 30, the fourth control valve 28, the third control valve 26 and the first control valve 5 in sequence. At this time, it is the initial stage of gas extraction, and the gas concentration is close to 100%. The second gas concentration meter 29 transmits the gas concentration signal to the controller 18. When the gas concentration of the second gas concentration meter 29 received by the controller 18 is higher than 30%, the controller 18 transmits a signal through the data line to open the first intelligent ball valve 7. After a delay of 30 seconds, the controller 18 transmits a signal through the data line to open the gas booster pump 12 and the centrifugal pump 1 in sequence. 3; Under the negative pressure of the centrifugal pump 13, the gas from the second borehole 37, the third borehole 38, the fourth borehole 39 and the fifth borehole 40 is removed of impurities such as water and coal powder carried by the gas through the impurity removal device 32, and then injected into the first borehole 2 through the sealing pipe 3 after passing through the bypass pipe 19, the booster pipe 14, the gas injection pipe 6 and the gas injection joint 4. The coal body near the first borehole 2 is subjected to the same direction of the gas injection pressure as the negative pressure of the second borehole 37, and diffuses outward, forming a combined force to fracture the coal body, and can also displace the gas in the coal body. The gas injection time is 3-5h;

[0032] ④ Remove the gas injection pipe 6 connected to the sealing pipe 3 of the first borehole 2, and connect the sealing pipe 3 of the first borehole 2 to the gas collection pipe 36, then separate the second borehole 37 from the gas collection pipe 36, and connect the sealing pipe 3 of the second borehole 37 to the gas injection pipe 6 through the gas injection joint 4 thereon, and repeat step ③ until the gas injection coal fracture and gas displacement work of the first borehole 2 to the fifth borehole 40 is completed;

[0033] ⑤ Connect the first borehole 2 to the fifth borehole 40 to the gas collection pipe 36, open the fourth intelligent ball valve 33 through the controller 18, and perform gas extraction. The gas time is 4-6 days;

[0034] ⑥ Repeat steps ②, ③, ④, and ⑤ for 5-7 times. During this process, the coal bodies around the first borehole 2 to the fifth borehole 40 undergo gas positive pressure fracturing, gas dynamic fatigue fracturing with positive and negative pressure conversion, and gas displacement, ensuring efficient and high-concentration extraction of coal seam gas. Then, the fourth intelligent ball valve 33 is opened through the controller 18 to perform normal gas extraction, and the gas injection pipe 6 is kept connected to the gas injection joint 4 of the first borehole 2;

[0035] ⑦ During the gas extraction process, when the second gas concentration meter 29 measures a gas concentration lower than 30%, the gas concentration signal measured by the second gas concentration meter 29 is transmitted to the controller 18 through the data line. The controller 18 closes the fourth intelligent ball valve 33 and opens the third intelligent ball valve 24, and then opens the fourth control valve 28 and the first control valve 5 in sequence. After a delay of 30 seconds, the controller 18 transmits a signal through the data line to open the gas booster pump 12 and the centrifugal pump 13 in sequence. The gas passes through the deoxygenation device 23 and the denitrification device 22 to increase the gas concentration. When the first gas concentration meter 21 measures a gas concentration lower than 30%, the controller 18 transmits a signal through the data line to open the second intelligent ball valve 10, and the gas flows back to the gas collection pipe 36 until the first gas concentration meter 21 measures a gas concentration higher than 30%. The controller 18 transmits a signal through the data line to open the first intelligent ball valve 7, and the gas with a concentration higher than 30% is injected into the first borehole 2. The injected gas pressure is 1-3MPa, and the gas injection time is 2-3h.

[0036] ⑧ Remove the gas injection pipe 6 connected to the sealing pipe 3 of the first borehole 2, and connect the first borehole 2 to the gas collection pipe 36, then separate the second borehole 37 from the gas collection pipe 36, connect the gas injection joint 4 of the second borehole 37 to the gas injection pipe 6, repeat step ⑦, cycle in sequence, until the coal body is fractured and the gas is displaced by the extracted gas from the first borehole 2 to the fifth borehole 40;

[0037] ⑨ Connect the first borehole 2 to the fifth borehole 40 to the gas collection pipe 36, open the fourth smart ball valve 33 through the controller 18, and perform gas extraction. The gas time is 2-3 days;

[0038] ⑩ Repeat steps ⑦, ⑧ and ⑨ until the gas content in the coal seam reaches the mining requirements. During this process, ensure that the extracted gas concentration does not affect gas utilization.

[0039] In step ③ of the present invention, after being pressurized by the gas booster pump 12, the pressure of the gas injected into the first borehole 2 is 5-8 MPa.

[0040] A system for utilizing extracted gas to fracture a coal body and displace the gas, comprising a first borehole 2, a second borehole 37, a third borehole 38, a fourth borehole 39, a fifth borehole 40 and a controller 18, wherein the first borehole 2, the second borehole 37, the third borehole 38, the fourth borehole 39, and the fifth borehole 40 extend into a coal seam 1, and a sealing pipe 3 is disposed in each of the first borehole 2, the second borehole 37, the third borehole 38, the fourth borehole 39, and the fifth borehole 40, wherein one end of the sealing pipe 3 extends into the coal seam 1, and the other end of the sealing pipe 3 extends out of the coal seam and is connected to a branch interface of a gas collecting pipe 36 after sealing. The gas injection joint 4 is installed on each borehole pipe 3, a control valve is installed on the sealing pipe 3 in the second borehole 37, the third borehole 38, the fourth borehole 39, and the fifth borehole 40, and a second control valve 8 is installed on the first borehole 2, and the control valve and the second control valve 8 are arranged between the gas injection joint 4 and the gas collection pipe 36, and the first control valve 5 is arranged on the pipeline of the gas injection joint 4; the gas outlet of the gas collection pipe 36 is connected to the gas inlet of the impurity removal device 32 through the impurity removal connecting pipe 31, and the fifth control valve 30 and the second gas concentration measuring instrument 29 are arranged on the impurity removal connecting pipe 31 in sequence, and the gas outlet on one side of the impurity removal device 32 is connected to the first One end of the branch pipe 27 is connected, the other end of the first branch pipe 27 is connected to one of the ports of the third three-way valve 25, the other two ports of the third three-way valve 25 are respectively connected to one end of the concentration pipe 17 and one end of the bypass pipe 19, a fourth control valve 28 is arranged on the first branch pipe 27, the other end of the concentration pipe 17 is connected to one of the ports of the second three-way valve 15, the other two ports of the second three-way valve 15 are respectively connected to the other end of the bypass pipe 19 and one end of the boost pipe 14, a third control valve 26 and a second check valve 16 are arranged on the bypass pipe 19 in sequence, and a third smart ball valve 24, a check valve 26 and a third smart ball valve 26 are arranged on the concentration pipe 17 in sequence. Oxygen device 23, denitrification device 22, first gas concentration measuring instrument 21, third check valve 20; the other end of the boosting pipe 14 is connected to one of the ports of the first three-way valve 11, and the other two ports of the first three-way valve 11 are respectively connected to the air inlet of the gas collection pipe 36 and the air inlet of the gas injection pipe 6, a centrifugal pump 13 and a gas boosting pump 12 are sequentially arranged on the boosting pipe 14, a second smart ball valve 10 and a first check valve 9 are sequentially arranged on one end of the gas collection pipe 36 close to the first three-way valve 11, the air outlet of the gas injection pipe 6 is connected to the air inlet of the gas injection joint 4, and a first smart ball valve 7 is arranged on the gas injection pipe 6;

[0041] The gas outlet on the other side of the impurity removal device 32 is connected to one end of the second branch pipe 34, the other end of the second branch pipe 34 is connected to the gas extraction branch pipe 35, and a fourth intelligent ball valve 33 is arranged on the second branch pipe 34;

[0042] The controller 18 is connected to the first smart ball valve 7 , the second smart ball valve 10 , the gas booster pump 12 , the centrifugal pump 13 , the third smart ball valve 24 , the fourth smart ball valve 33 , the first gas concentration meter 21 and the second gas concentration meter 29 , respectively.

[0043] The controller 18 is a programmable logic controller.

[0044] The deoxygenation device 23 is provided with a commercial nitrogen-producing carbon molecular sieve; the denitrification device 22 is provided with an activated carbon adsorbent having a nitrogen-absorbing function.

[0045] The boosting ratio of the gas booster pump 12 is greater than or equal to 100:1; the extraction negative pressure provided by the centrifugal pump 13 is greater than or equal to 0.05 MPa.

[0046] The present invention adopts a method of combining an impurity removal device 32, a bypass pipe 19, a concentration pipe 17, a centrifugal pump 13, a gas booster pump 12, a gas injection pipe 6, and a gas extraction branch pipe 35. In the initial stage of extraction, the high-concentration gas is directly injected into the borehole through the centrifugal pump 13 and the gas booster pump 12 on the bypass pipe 19 and the booster pipe 14. The borehole transformation is combined with gas extraction to achieve high-concentration gas high-pressure fracturing of the coal body, positive and negative pressure alternating stress fatigue fracturing of the coal body, and gas displacement, thereby achieving efficient and high-concentration gas extraction; in the later stage of gas extraction, the low-concentration gas is concentrated (the concentration is greater than 30%) and then injected into the gas extraction borehole, and the gas that does not meet the concentration requirement flows back to the gas collection pipe 36, thereby achieving low-pressure fracturing of the coal body around the borehole, positive and negative pressure alternating stress fatigue fracturing, and gas displacement. Gas displacement is carried out to ensure that the extracted gas is extracted at a high concentration and efficiently without affecting the utilization of gas; the gas extracted by the collaborative extraction of the hole group is used to fracture the coal body and displace the gas. In this way, there is no need to add other gases, which ensures the quality of gas utilization. In addition, the compressed air flow and the extracted air flow alternate in the gas extraction borehole, so that the coal body around the borehole repeatedly bears the effects of expansion and contraction. Under the action of this alternating stress, the coal body is prone to reach the fatigue limit and rupture, thereby causing the cracks to continue to expand and gradually develop into the deep, increasing the gas extraction radius of the borehole, increasing the total amount of gas extracted from the borehole, and improving the gas extraction effect; in summary, this system does not require an external gas source, has a simple process, is easy to operate, and has a low cost. It integrates gas fracturing, alternating stress fracturing and gas displacement, solves the problem of short high-concentration gas extraction cycle, and has wide practicality.

Claims

1. A system for utilizing extracted gas to fracture coal and displace gas, characterized in that: The invention comprises a first borehole (2), a second borehole (37), a third borehole (38), a fourth borehole (39), a fifth borehole (40) and a controller (18); the first borehole (2), the second borehole (37), the third borehole (38), the fourth borehole (39) and the fifth borehole (40) extend into a coal seam (1); a sealing pipe (3) is arranged in each of the first borehole (2), the second borehole (37), the third borehole (38), the fourth borehole (39) and the fifth borehole (40); one end of the sealing pipe (3) extends into the coal seam (1); the other end of the sealing pipe (3) extends out of the coal seam and is connected to a branch interface of a gas collecting pipe (36) after sealing; and a gas injection joint is installed on each of the sealing pipes (3). (4), control valves are arranged on the sealing pipes (3) in the second borehole (37), the third borehole (38), the fourth borehole (39) and the fifth borehole (40), and a second control valve (8) is arranged on the first borehole (2), and the control valve and the second control valve (8) are arranged between the gas injection joint (4) and the gas collection pipe (36), and a first control valve (5) is arranged on the pipeline of the gas injection joint (4); the gas outlet of the gas collection pipe (36) is connected to the gas inlet of the impurity removal device (32) through the impurity removal connecting pipe (31), and a fifth control valve (30) and a second gas concentration measuring instrument (29) are arranged on the impurity removal connecting pipe (31) in sequence, and the gas outlet on one side of the impurity removal device (32) is connected to the first branch pipe (27) The first branch pipe (27) is connected to one end of the first branch pipe (27), the other end of the first branch pipe (27) is connected to one port of the third three-way valve (25), the other two ports of the third three-way valve (25) are respectively connected to one end of the concentration pipe (17) and one end of the bypass pipe (19), a fourth control valve (28) is provided on the first branch pipe (27), the other end of the concentration pipe (17) is connected to one port of the second three-way valve (15), the other two ports of the second three-way valve (15) are respectively connected to the other end of the bypass pipe (19) and one end of the boost pipe (14), a third control valve (26) and a second check valve (16) are sequentially provided on the bypass pipe (19), and a third intelligent ball valve (24) and a deoxygenation device (28) are sequentially provided on the concentration pipe (17). 3), a denitrification device (22), a first gas concentration measuring instrument (21), and a third check valve (20); the other end of the boosting pipe (14) is connected to one of the ports of the first three-way valve (11), and the other two ports of the first three-way valve (11) are respectively connected to the air inlet of the gas collecting pipe (36) and the air inlet of the gas injection pipe (6); a centrifugal pump (13) and a gas boosting pump (12) are sequentially arranged on the boosting pipe (14); a second intelligent ball valve (10) and a first check valve (9) are sequentially arranged on one end of the gas collecting pipe (36) close to the first three-way valve (11); the air outlet of the gas injection pipe (6) is connected to the air inlet of the gas injection joint (4); and a first intelligent ball valve (7) is arranged on the gas injection pipe (6); The gas outlet on the other side of the impurity removal device (32) is connected to one end of the second branch pipe (34), the other end of the second branch pipe (34) is connected to the gas extraction branch pipe (35), and a fourth intelligent ball valve (33) is arranged on the second branch pipe (34); The controller (18) is respectively connected to the first intelligent ball valve (7), the second intelligent ball valve (10), the gas booster pump (12), the centrifugal pump (13), the third intelligent ball valve (24), the fourth intelligent ball valve (33), the first gas concentration measuring instrument (21) and the second gas concentration measuring instrument (29).

2. A system for fracturing coal bodies and displacing gas using extracted gas according to claim 1, characterized in that: The controller (18) is a programmable logic controller.

3. A system for fracturing coal bodies and displacing gas using extracted gas according to claim 2, characterized in that: The deoxygenation device (23) is provided with a commercial nitrogen-producing carbon molecular sieve; and the denitrification device (22) is provided with an activated carbon adsorbent having a nitrogen-absorbing function.

4. A system for fracturing coal bodies and displacing gas using extracted gas according to claim 3, characterized in that: The boosting ratio of the gas booster pump (12) is greater than or equal to 100:1; and the extraction negative pressure provided by the centrifugal pump (13) is greater than or equal to 0.05 MPa.

5. A method for fracturing coal bodies and displacing gas systems using extracted gas according to any one of claims 1 to 4, characterized in that: The steps include: ① According to the inclination of the coal seam, several boreholes for gas extraction are constructed. After the construction of several boreholes is completed, the gas extraction boreholes are sealed by high-pressure cement slurry injection; ② Every five boreholes form a group, and the first borehole (2), the second borehole (37), the third borehole (38), the fourth borehole (39), and the fifth borehole (40) are connected to the gas collection pipe (36), wherein the control valves on the sealing pipes (3) from the second borehole (37) to the fifth borehole (40) are opened, and the gas injection joint (4) of the first borehole (2) is connected to the gas injection pipe (6); ③ The fifth control valve (30), the fourth control valve (28), the third control valve (26) and the first control valve (5) are opened in sequence. At this time, it is the initial stage of gas extraction, and the gas concentration is close to 100%. The second gas concentration measuring instrument (29) transmits the gas concentration signal to the controller (18). When the gas concentration of the second gas concentration measuring instrument (29) received by the controller (18) is higher than 30%, the controller (18) transmits a signal through the data line to open the first intelligent ball valve (7). After a delay of 30 seconds, the controller (18) transmits a signal through the data line to open the gas valves (7) and the first intelligent ball valves (7). A booster pump (12) and a centrifugal pump (13) are provided; under the action of the negative pressure of extraction by the centrifugal pump (13), the gas from the second borehole (37), the third borehole (38), the fourth borehole (39) and the fifth borehole (40) is injected into the first borehole (2) through the sealing pipe (3) after passing through the impurity removal device (32), the bypass pipe (19), the booster pipe (14), the gas injection pipe (6) and the gas injection joint (4); the coal body near the first borehole (2) is subjected to the gas injection pressure in the same direction as the negative pressure of extraction by the second borehole (37), thereby forming a combined force to fracture the coal body; and the gas injection time is 3-5 hours; ④ Remove the gas injection pipe (6) connected to the sealing pipe (3) of the first borehole (2), and connect the sealing pipe (3) of the first borehole (2) to the gas collection pipe (36), then separate the second borehole (37) from the gas collection pipe (36), and connect the sealing pipe (3) of the second borehole (37) to the gas injection pipe (6) through the gas injection joint (4) thereon, and repeat step ③ until the gas injection coal fracture and gas displacement work of the first borehole (2) to the fifth borehole (40) is completed; ⑤ Connect the first borehole (2) to the fifth borehole (40) to the gas collection pipe (36), open the fourth intelligent ball valve (33) through the controller (18), and perform gas extraction. The gas time is 4-6 days; ⑥ Steps ②, ③, ④, and ⑤ are repeated 5 to 7 times. During this process, the coal bodies around the first borehole (2) to the fifth borehole (40) undergo gas positive pressure fracturing, gas dynamic fatigue fracturing with positive and negative pressure conversion, and gas displacement. Then, the fourth intelligent ball valve (33) is opened through the controller (18) to perform normal gas extraction, and the gas injection pipe (6) is kept connected to the gas injection joint (4) of the first borehole (2); ⑦ During the gas extraction process, when the second gas concentration measuring instrument (29) measures a gas concentration lower than 30%, the gas concentration signal measured by the second gas concentration measuring instrument (29) is transmitted to the controller (18) through the data line, and the controller (18) closes the fourth intelligent ball valve (33) and opens the third intelligent ball valve (24), and then opens the fourth control valve (28) and the first control valve (5) in sequence. After a delay of 30 seconds, the controller (18) transmits a signal through the data line to open the gas booster pump (12) and the centrifugal pump (13) in sequence, and the gas passes through the deoxygenation device. (23) and a denitrification device (22), when the first gas concentration measuring instrument (21) measures a gas concentration lower than 30%, the controller (18) transmits a signal through the data line to open the second intelligent ball valve (10), and the gas flows back to the gas collection pipe (36), until the first gas concentration measuring instrument (21) measures a gas concentration higher than 30%, the controller (18) transmits a signal through the data line to open the first intelligent ball valve (7), and the gas with a concentration higher than 30% is injected into the first borehole (2), the injection gas pressure is 1-3MPa, and the injection time is 2-3h; ⑧ Remove the gas injection pipe (6) connected to the sealing pipe (3) of the first borehole (2), and connect the first borehole (2) to the gas collection pipe (36), then separate the second borehole (37) from the gas collection pipe (36), connect the gas injection joint (4) of the second borehole (37) to the gas injection pipe (6), repeat step ⑦, and cycle in sequence until the coal body is fractured and the gas is displaced by utilizing the extracted gas from the first borehole (2) to the fifth borehole (40); ⑨ Connect the first borehole (2) to the fifth borehole (40) to the gas collection pipe (36), open the fourth intelligent ball valve (33) through the controller (18), and perform gas extraction. The gas time is 2-3 days; ⑩ Repeat steps ⑦, ⑧ and ⑨ until the gas content in the coal seam reaches the mining requirements. During this process, ensure that the extracted gas concentration does not affect gas utilization.

6. A method for utilizing extracted gas to fracture coal and displace gas according to claim 5, characterized in that: In step ③, after being pressurized by the gas booster pump (12), the pressure of the gas injected into the first borehole (2) is 5-8 MPa.

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