Coal mine gas extraction system and method based on carbon dioxide phase change induced cracking

Abstract

The application discloses a novel coal mine gas extraction system and method based on carbon dioxide phase change fracturing, which comprises drilling construction, screen hole casing assembly and delivery, carbon dioxide fracturing device assembly and delivery, carbon dioxide phase change fracturing temporary blocking, carbon dioxide phase change fracturing permeability improvement, carbon dioxide fracturing device removal and recovery, and drilling hole fixing, sealing and gas extraction. When the carbon dioxide phase change fracturing is used for permeability improvement of soft coal seams, the carbon dioxide fracturing device can be sent to the deep drilling hole even if the coal quality is soft, the roadway excavation and the drilling disturbance effect are superimposed, and the coal body is prone to cross-hole and hole collapse after the hole forming. Meanwhile, the carbon dioxide fracturing device can be normally removed under the protection of the screen hole casing, and the coal body collapse and hole collapse do not block the gas extraction channel, so that the extraction effect of the soft coal seam is improved.

Description

Technical Field

[0001] This invention relates to the field of coalbed methane development technology, and in particular to a coal mine gas extraction system and method based on carbon dioxide phase change fracturing. Background Technology

[0002] For soft, low-permeability coal seams in underground coal mines, the low permeability coefficient means that gas extraction alone cannot efficiently meet extraction standards, necessitating coal seam permeability enhancement. Common permeability enhancement measures include mechanical vibration enhancement, hydraulic enhancement, and gas-phase fracturing enhancement. Mechanical vibration enhancement, exemplified by deep-hole pre-fracturing blasting, is limited in practicality due to the high risks and strict regulations associated with the underground application of explosives, and the tendency for "duds" to occur. Hydraulic enhancement measures, such as hydraulic fracturing and hydraulic slotting, require the introduction of large amounts of water, which can easily cause collapse in soft coal seams, leading to difficulties in drainage and slag removal, and poor extraction efficiency. Gas-phase fracturing enhancement, represented by carbon dioxide phase change fracturing, is well-suited to soft coal seams, but the soft coal quality combined with the disturbance effects of roadway excavation and drilling can cause the coal body to easily cross the borehole or collapse after drilling, making it difficult to lower fracturing devices to the deepest parts of the borehole. Even if the fracturing device is managed to be lowered to a depth in the borehole, it cannot be properly removed after fracturing due to cross-hole or collapse of the coal seam. In some cases, removal may even block the gas flow channel, causing the borehole to be scrapped, thus affecting the subsequent extraction effect.

[0003] Therefore, how to improve the extraction efficiency of soft coal seams has become a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0004] This invention proposes a coal mine gas extraction system and method based on carbon dioxide phase transformation fracturing to improve the extraction effect of soft coal seams.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] This invention provides a coal mine gas extraction method based on carbon dioxide phase change fracturing, characterized by comprising the following steps:

[0007] Drilling operation: Using a complete set of drilling equipment equipped with hollow drill rods and open-end drill bits, drill holes along the coal seam in the coal roadway and record the drilling conditions; when the drilling reaches the designed depth, stop drilling, retract the drill and disassemble several drill rods;

[0008] Screen casing assembly and placement: Install the bottom fixing device on the top of the screen casing, manually insert the screen casing with the bottom fixing device into the drill rod, connecting the screen casing as it is inserted, and insert it to the bottom of the drill rod. Open the crossbeam of the opening drill bit and continue to insert it to the bottom of the borehole, ensuring that the umbrella claw of the bottom fixing device is open, and pull it back until it is under force, ensuring that the umbrella claw of the bottom fixing device is hanging upside down on the coal wall; withdraw all drill rods and drill bits;

[0009] Carbon dioxide fracturing device assembly and delivery: Check the safety performance of the carbon dioxide fracturing device. After ensuring safety, assemble the carbon dioxide fracturing device. Place the carbon dioxide fracturing device at the fracturing point and use the top rod to connect at the non-fracturing point. Push while connecting. Use the power system of the drilling equipment to push the carbon dioxide fracturing device through the inside of the screen hole sleeve to the bottom of the drill rod, so that the explosion vent of the carbon dioxide fracturing device is consistent with the passage of the screen hole sleeve.

[0010] Temporary sealing for carbon dioxide phase change fracturing: Connect the temporary sealing device to the carbon dioxide fracturing device and send it to the predetermined position. Lead the water inlet pipe of the temporary sealing device and the busbar of the fracturing device out of the borehole to avoid it getting stuck in the borehole after sealing. Connect the water inlet pipe to the water tank and pressurize the rubber bag of the temporary sealing device until it expands and gets stuck on the borehole wall to form a temporary seal. At the same time, use a push rod outside the borehole to hold the temporary sealing device with one end and the other end against the roadway wall opposite the borehole to ensure that the bag sealer will not be forced out by high pressure during carbon dioxide phase change fracturing, thus improving the quality of temporary sealing.

[0011] Carbon dioxide phase change fracturing and permeability enhancement: After checking that the carbon dioxide fracturing device has no explosion failure, start the detonator. The carbon dioxide fracturing device releases high-pressure carbon dioxide gas through the explosion relief port to fracture the coal seam. After fracturing, maintain pressure for a preset holding time.

[0012] Carbon dioxide fracturing device removal: The ventilation dispatcher checks the first gas concentration in the roadway. When the first gas concentration in the roadway is less than the first threshold, the temporary sealing device is depressurized, and gas and carbon dioxide gas gush out from the borehole. The second gas concentration and carbon dioxide concentration within a preset distance near the borehole are checked. When the second gas concentration is less than the second threshold and the carbon dioxide concentration is less than the third threshold, the removal work is carried out.

[0013] Drainage sealing and connection: After the carbon dioxide fracturing device is removed, a sealing device with a drainage pipe and a bag is installed. The sealing device has two bags at both ends. The grouting pump is used to pressurize and grout the bags at both ends of the sealing device. After the bags expand and support the coal wall, the expansion cement is injected into the middle blank area through the grouting pipe to seal it under pressure, so as to form a fixed sealing section. After the cement solidifies, the drainage pipe is connected to the drainage system for drainage.

[0014] Preferably, in the extraction method of the present invention, the drilling equipment is a fully hydraulic tunnel drilling equipment, the drill rod is a shallow-bladed hollow spiral drill rod, and the drill bit of the drilling equipment is a large-hole open-closed PDC drill bit.

[0015] Record borehole conditions, including coal, rock, gas, and pressure.

[0016] Medium-pressure air is used for slag removal, and the air pressure is supplied by an explosion-proof mobile air compressor underground, with a medium-pressure of 1.0-1.2MPa.

[0017] Preferably, in the extraction method of the present invention, the hole bottom fixing device and the screen hole sleeve are connected by threads or plug-in connection.

[0018] The distance between the rear end of the sieve sleeve and the orifice is greater than 15m, and the length of a single sieve sleeve is 1m or 1.5m.

[0019] Preferably, in the extraction method of the present invention, the carbon dioxide fracturing devices are connected to each other or to the connecting rod via steel pipe threads.

[0020] Preferably, in the extraction method of the present invention, checking the safety performance of the carbon dioxide fracturing device includes: detecting the circuit of the carbon dioxide fracturing device; and checking whether there is any gas leakage in the carbon dioxide fracturing device.

[0021] Preferably, in the extraction method of the present invention, the sealing depth is not less than 15m; and the preset pressure holding time is not less than 60min.

[0022] Preferably, in the extraction method of the present invention, the first gas concentration, the second gas concentration, and the carbon dioxide concentration are volume concentrations, the first threshold, the second threshold, and the third threshold are 0.5%, and the preset distance is 50m.

[0023] Preferably, in the extraction method of the present invention, the dismantling work includes depressurizing the sealing device; after the sealing device is depressurized, the drilling equipment is started to remove the sealing device and the carbon dioxide fracturing device together, leaving the bottom fixing device in the borehole.

[0024] Preferably, in the extraction method of the present invention, the length of the extraction pipe is ≥15m; the length of the fixed sealing section is ≥15m.

[0025] This invention also discloses a coal mine gas extraction system based on carbon dioxide phase change fracturing, comprising a complete set of drilling equipment with hollow drill rods and open-end drill bits, a bottom hole fixing device, a screen casing, a carbon dioxide fracturing device, a temporary sealing device, a top rod, a first gas concentration detector, a carbon dioxide concentration detector, a second gas concentration detector, a bag sealing device, and an extraction pipe, which are executed according to the corresponding steps in the above-mentioned coal mine gas extraction method based on carbon dioxide phase change fracturing.

[0026] As can be seen from the above technical solution, in the coal mine gas extraction method based on carbon dioxide phase change fracturing provided by this invention, when carbon dioxide phase change fracturing is used to enhance permeability in soft coal seams, the carbon dioxide fracturing device is pushed to the bottom of the borehole through the inside of the screen casing. Even if the soft coal quality, combined with the disturbance effect of roadway excavation and borehole drilling, leads to situations where the coal body is prone to cross-hole or collapse after borehole formation, the carbon dioxide fracturing device can still be delivered to the depth of the borehole. Simultaneously, since the phase change fracturing and permeability enhancement of the carbon dioxide fracturing device is carried out inside the screen casing, under the protection of the screen casing, the carbon dioxide fracturing device can be normally removed when it is dismantled, thereby improving the extraction effect of soft coal seams. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some examples or embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort, and the present invention can be applied to other similar scenarios based on the provided drawings. Unless obvious from the linguistic context or otherwise specified, the same reference numerals in the drawings represent the same structures or operations.

[0028] Figure 1 This is a schematic diagram of the process of a coal mine gas extraction method based on carbon dioxide phase change fracturing provided by the present invention.

[0029] Figure 2 This is a schematic diagram of the drilling construction provided by the present invention;

[0030] Figure 3 and Figure 4 This is a schematic diagram of the assembly and feeding of the screen sleeve provided by the present invention;

[0031] Figure 5 This is a schematic diagram of the assembly and delivery of the carbon dioxide fracturing device provided by the present invention;

[0032] Figure 6 This is a cross-sectional view of the carbon dioxide fracturing device and the sieve sleeve provided by the present invention.

[0033] Figure 7 This is a schematic diagram of the carbon dioxide phase change-induced crack sealing method provided by the present invention;

[0034] Figure 8 This is a schematic diagram of the dismantling of the carbon dioxide fracturing device provided by the present invention;

[0035] Among them: 1 is drilling equipment, 2 is screen casing, 3 is bottom hole fixing device, 4 is carbon dioxide fracturing device, 5 is temporary sealing device, 6 is top rod, 11 is drill rod, 12 is drill bit, 13 is crossbeam, 10 is coal seam, and 20 is roadway. Detailed Implementation

[0036] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. The described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0037] Definitions:

[0038] 1. In-seam borehole: Gas extraction boreholes arranged along the coal seam in underground coal mines.

[0039] 2. Carbon Dioxide Phase Change Fracturing of Coal Seams to Displace Methane: Carbon dioxide exists in liquid form below 31℃ and at a pressure of 7.2MPa. When the temperature exceeds 31℃, 1kg of liquid carbon dioxide absorbs 60kJ of heat and vaporizes within 40ms. Liquid carbon dioxide is filled into a carbon dioxide fracturing device and placed in a coal seam borehole. Utilizing the phase change characteristic of liquid carbon dioxide expanding 600 times in volume when heated into a gas, the coal seam is fractured, forming a highly permeable area with well-developed fractures around the borehole. Simultaneously, taking advantage of the coal-affinity property of gaseous carbon dioxide (8 times higher adsorption capacity for coal than for methane), a large amount of adsorbed methane is displaced by the gaseous carbon dioxide and converted into free methane, thus improving both coal seam permeability and methane freeness, achieving the goals of safe and rapid extraction and outburst suppression.

[0040] See Figure 1 This invention provides a coal mine gas extraction method based on carbon dioxide phase change fracturing, comprising the following steps: step S100 drilling construction, step S200 screen casing assembly and placement, step S300 carbon dioxide fracturing device assembly and placement, step S400 temporary sealing of carbon dioxide phase change fracturing, step S500 carbon dioxide phase change fracturing permeability enhancement, step S600 carbon dioxide fracturing device dismantling and recovery, and step S700 drilling, fixing, sealing, and extraction.

[0041] In the coal mine gas extraction method based on carbon dioxide phase change fracturing provided by this invention, when carbon dioxide phase change fracturing is used to enhance permeability in soft coal seams 10, the carbon dioxide fracturing device 4 is pushed to the bottom of the borehole through the inside of the screen casing 2. Even if the soft coal quality, combined with the disturbance effect of roadway 20 excavation and borehole drilling, leads to situations where the coal body is prone to cross-hole or collapse after borehole formation, the carbon dioxide fracturing device 4 can still be delivered to the depth of the borehole. At the same time, since the phase change fracturing and permeability enhancement of the carbon dioxide fracturing device 4 is carried out inside the screen casing 2, under the protection of the screen casing 2, the carbon dioxide fracturing device 4 can be removed normally when it is removed, and a good gas extraction channel can be maintained, thereby improving the extraction effect of soft coal seams 10.

[0042] See Figure 2 In the drilling process, step S100 includes: using a complete set of drilling equipment 1 equipped with hollow drill rods 11 and open-end drill bits 12 to drill along the coal seam 10 in the roadway 20, and recording the drilling conditions; when the drilling reaches the designed depth, stop drilling, retract the drill and disassemble 3-5 drill rods 11.

[0043] It should be noted that the drilling equipment 1 includes a drill rod 11, a drill bit 12, and a power head. The power head drives the drill bit 12 to rotate at high speed via the drill rod 11. Preferably, the drilling equipment 1 is a fully hydraulic tunnel drilling equipment 1. The drill rod 11 is equipped with a shallow-bladed hollow spiral drill rod 11 on its outer periphery, and the drill bit 12 of the drilling equipment 1 is a large-hole open-closed PDC (polycrystalline diamond composite) drill bit. The drill rod 11 is a hollow drill rod 11. When the screen casing 2 passes through the drill bit 12, the crossbeam 13 at the drill bit 12 is lifted, and the screen casing 2 is drilled out. Drill rod 11: Integral wide-blade auger drill rod 11 (specifications: outer diameter Φ108mm, inner diameter Φ50.8mm, length 1.0m or 1.5m or customized); Drill bit 12: Large through-hole open-closed PDC drill bit (specifications: outer diameter Φ133mm, inner diameter Φ50mm), for medium wind pressure (wind pressure ≥1.2MPa) drilling operations.

[0044] Medium-pressure air is used for slag removal, with air supplied by an explosion-proof mobile air compressor underground at a pressure of 1.0-1.2 MPa. Construction must strictly adhere to design parameters, employing a slow-speed advancement method to ensure a straight borehole. Detailed records of all drilling conditions must be kept, including coal, rock, gas, and pressure encounters. When the borehole reaches the designed depth, drilling should be stopped, the drill string retracted, and 3-5 drill rods 11 removed. During this retraction and removal, the power head of the hydraulic drilling equipment 1 should be moved to the bottom of the frame to avoid interfering with the lowering of the carbon dioxide fracturing device 4.

[0045] See Figure 3 and Figure 4Step S200: Assemble and deploy the screen casing 2: Install the bottom fixing device 3 on the top of the screen casing 2. Manually insert the screen casing 2 with the bottom fixing device 3 into the drill rod 11, connecting the screen casing 2 as it is inserted. Insert it to the bottom of the drill rod 11, opening the crossbeam 13 of the opening drill bit 12. Continue inserting it to the bottom of the borehole, ensuring the umbrella claw of the bottom fixing device 3 is open, and pull it backward until it is under force, ensuring the umbrella claw of the bottom fixing device 3 hangs upside down on the coal wall. Withdraw all drill rods 11 and drill bits 12. The rear end of the screen casing 2 is more than 15m from the borehole opening. Each screen casing 2 is 1m or 1.5m long and uses a plug-in or spiral connection.

[0046] See Figure 5 Step S300, the assembly and delivery of the carbon dioxide fracturing device 4, includes: checking the safety performance of the carbon dioxide fracturing device 4, ensuring safety, assembling the carbon dioxide fracturing device 4, placing the carbon dioxide fracturing device 4 at the fracturing point, and connecting it to the non-fracturing point using the push rod 6, pushing it while connecting, and using the power system of the drilling equipment 1 to push the carbon dioxide fracturing device 4 through the inside of the screen hole sleeve 2 to the bottom of the drill rod 11, so that the explosion vent 41 of the carbon dioxide fracturing device 4 is aligned with the passage 21 of the screen hole sleeve 2.

[0047] The carbon dioxide fracturing devices 4 are connected to each other or to the connecting rod via steel pipe threads.

[0048] The safety performance check of the carbon dioxide fracturing device 4 includes: testing the circuitry of the carbon dioxide fracturing device 4; and checking for gas leaks in the carbon dioxide fracturing device 4. Testing the circuitry of the carbon dioxide fracturing device 4 involves using a multimeter to test the circuitry to ensure that the resistance of each carbon dioxide fracturing device 4 used does not exceed 500Ω. Checking for gas leaks in the carbon dioxide fracturing device 4 is crucial to avoid using fracturing devices with severe leaks. After connecting the required number of carbon dioxide fracturing devices 4 one by one and measuring the resistance, ensuring it is normal, the cumulative length of the carbon dioxide fracturing devices 4 should cover the coal seam.

[0049] See Figure 6 The vent 41 of the carbon dioxide fracturing device 4 is aligned with the passage 21 of the screen sleeve 2. Thus, when the carbon dioxide fracturing device 4 undergoes phase change fracturing, the carbon dioxide released from the vent 41 can enter the borehole through the passage 21, thereby reducing resistance.

[0050] See Figure 7Step S400: Temporary sealing of carbon dioxide phase change fracturing: Connect the temporary sealing device 5 to the carbon dioxide fracturing device 4, and insert it into the predetermined position. Lead the water inlet pipe of the temporary sealing device 5 and the busbar of the fracturing device out of the borehole to prevent it from getting stuck in the borehole after sealing. Connect the water inlet pipe to the water tank and pressurize the rubber bag of the temporary sealing device 5 until it expands and gets stuck on the borehole wall to form a temporary seal. At the same time, use a push rod 6 outside the borehole to hold the temporary sealing device 5 at one end and press the other end against the wall of the tunnel 20 opposite the borehole to ensure that the bag sealer will not be forced out by high pressure during carbon dioxide phase change fracturing, thus improving the quality of the temporary sealing. Preferably, the depth of the above sealing is not less than 15m.

[0051] See Figure 8 Step S500: Carbon dioxide phase change fracturing and permeability enhancement: After checking that the carbon dioxide fracturing device has no explosion failure, the detonator is activated. The carbon dioxide fracturing device 4 releases high-pressure carbon dioxide gas through the explosion vent to fracture the coal seam 10. After fracturing, pressure is maintained for a preset time. Specifically, to ensure safety during fracturing, other workers must be evacuated to at least 100m away from the work site. When fracturing is initiated, a warning line must be set up, and a dedicated person must be assigned to set up a warning line. Other personnel are strictly prohibited from entering, and the warning personnel must not remove the warning line without the order from the technical personnel. Before fracturing is implemented, a dedicated gas inspector measures the gas and carbon dioxide concentrations near the work site. Work can only begin when the gas concentration does not exceed 0.5% and the carbon dioxide concentration does not exceed 0.5%. A designated person must check the electrical equipment at the working face to ensure that the electrical equipment in the warning zone is intact and has no explosion failure before fracturing can be initiated. During implementation, all non-intrinsically safe power supplies in the drilling roadway 20 must be shut down. After technicians inspect the circuit system for integrity and to ensure there are no potential explosion defects, the carbon dioxide fracturing device 4 is activated to release high-pressure carbon dioxide gas to fracture the coal seam 10. The pressure holding time after fracturing must be no less than the preset holding time. During this period, personnel are not allowed to enter the fracturing area, and no live equipment may be started. Preferably, the preset holding time is 60 minutes.

[0052] See Figure 8Step S600: Removal and Recovery of Carbon Dioxide Fracturing Device 4: The ventilation dispatcher checks the first gas concentration in roadway 20. When the first gas concentration in roadway 20 is less than the first threshold, the temporary sealing device 5 is depressurized, and gas and carbon dioxide gas flow out of the borehole. The second gas concentration and carbon dioxide concentration within a preset distance near the borehole are checked. When the second gas concentration is less than the second threshold and the carbon dioxide concentration is less than the third threshold, removal work is carried out. Specifically, after the pressure holding time ends, the ventilation dispatcher checks the first gas concentration displayed by the first gas concentration detector in roadway 20. When the first gas concentration in roadway 20 is below the first threshold, a dedicated gas inspector can enter the working face to check the second gas concentration and carbon dioxide concentration displayed by the second gas concentration detector and the carbon dioxide concentration detector within a preset distance near the borehole. When the second gas concentration is less than the second threshold and the carbon dioxide concentration is less than the third threshold, relevant technical personnel enter the fracturing location to begin removal work. Preferably, the first, second, and third thresholds are 0.5%; the preset distance is 50m. It should be noted that the above-mentioned first gas concentration, second gas concentration, and carbon dioxide concentration are volume concentration, mass concentration, or molar concentration, with volume concentration being preferred.

[0053] When the pressure inside the fracturing borehole drops to 0.2 MPa, open the low-pressure test valve until it drops to less than 0.1 MPa, ensuring there is no pressure inside the borehole before starting dismantling. Depressurize the temporary sealing device 5; during depressurization, unauthorized personnel are strictly prohibited from entering within 50 meters of the fracturing borehole opening. After the temporary sealing device 5 is depressurized, start the drilling equipment 1 to remove the temporary sealing device 5 and the fracturing device together, leaving the bottom fixing device 3 inside the borehole. If the fracturing device is stuck in the borehole, measures must be taken to remove it; do not forcibly pull it out. Inventory the used "carbon dioxide fracturing device 4" and recover it to the surface.

[0054] Step S700: Drilling, fixing, sealing, and pumping: After the carbon dioxide fracturing device 4 is removed, a sealing device with a pumping pipe and a bag is installed. The sealing device has two bags at both ends. The grouting pump is used to pressurize and grout the bags at both ends of the sealing device. After the bags expand and support the coal wall, expansive cement is injected into the middle blank area through the grouting pipe to seal under pressure, forming a fixed sealing section. After the cement solidifies, the pumping pipe is connected to the pumping system for pumping.

[0055] After fracturing, the borehole should be sealed and pumping resumed promptly. A pumping pipe, ≥15m in length, should be installed, using a pressurized sealing process with expansive cement ("two plugs, one injection"), with a fixed plugging section length ≥15m. Since single-hole flow rate data is difficult to measure, a manifold is used. Five boreholes are connected to a manifold device via flexible hoses. The manifold device has openings for orifice plate flow meters or V-cone flow meters to facilitate the measurement of pumping concentration and flow rate. The manifold is connected to the pumping system for pumping, with a combined negative pressure of not less than 13 kPa.

[0056] See Figure 8 This invention also discloses a coal mine gas extraction system based on carbon dioxide phase change fracturing, comprising a complete set of drilling equipment 1 equipped with a hollow drill rod 11 and an opening drill bit 12, a screen casing 2, a bottom hole fixing device 3, a carbon dioxide fracturing device 4, a temporary sealing device 5, a top rod 6, a first gas concentration detector, a carbon dioxide concentration detector, a second gas concentration detector, a bag sealing device, and an extraction pipe. Since the above extraction method has the aforementioned beneficial effects, the extraction system executing the above extraction method has corresponding effects, which will not be elaborated further here.

[0057] The drilling equipment 1 includes a drill pipe 11, a drill bit 12, and a power head. The power head drives the drill bit 12 to rotate at high speed via the drill pipe 11. Preferably, the drilling equipment 1 is a fully hydraulic tunnel drilling equipment 1. The drill pipe 11 is equipped with a shallow-blade auger drill pipe 11 around its outer periphery, and the drill bit 12 of the drilling equipment 1 is a large-hole open-closed PDC drill bit 12. The drill pipe 11 is a hollow drill pipe 11. When the carbon dioxide fracturing device 4 passes through the drill bit 12, the crossbeam 13 at the drill bit 12 is lifted, allowing the carbon dioxide fracturing device 4 to be drilled out. Drill rod 11: Integral wide-blade auger drill rod 11 (Specifications: Outer diameter Φ108mm, Inner diameter Φ50.8mm, Length 1.0m or 1.5m or customized); Drill bit 12: Large-diameter open-close PDC drill bit 12 (Specifications: Outer diameter Φ133mm, Inner diameter Φ50mm), for medium-pressure (pressure ≥1.2MPa) drilling. The carbon dioxide fracturing device 4, with a diameter of 40mm or customized, is suitable for the large-diameter drill rod 11, with a length of 1.0m or 1.5m or customized. The bottom hole fixing device 3 is made of PE or PVC material, with a blade unfolded dimension of Φ140mm.

[0058] The screen sleeve 2 is an integral screen tube with an outer diameter of 42mm, an inner diameter of 35mm, and a screen aperture diameter of 3~5mm. The connection method is either plug-in or spiral. Of course, the present invention is not limited to this specification of screen sleeve 2. The selection and specification of screen sleeve 2 can also be adjusted according to the type of coal seam 10 and the size of the borehole.

[0059] The bottom hole fixing device 3 is made of PE (polyethylene) or PVC (polyvinyl chloride), with the following specifications: the unfolded size of the wing is Φ140mm. Of course, the present invention is not limited to this specification of bottom hole fixing device 3, and the selection of the specifications of the bottom hole fixing device 3 can also be adjusted according to the type of coal seam 10 and the size of the borehole.

[0060] The carbon dioxide fracturing device 4 has a diameter of 30mm or is customized to fit the size of the large-diameter drill pipe 11, and a length of 1.5m or is customized. By reducing the diameter and increasing the length of each section, the amount of liquid carbon dioxide filled is maintained, ensuring the required permeability enhancement energy is met. Of course, this invention is not limited to this type of carbon dioxide fracturing device 4; the selection and specifications of the carbon dioxide fracturing device 4 can also be adjusted according to the type of coal seam 10 and the borehole size.

[0061] It should be noted that, for ease of description, only the parts relevant to the invention are shown in the accompanying drawings. Unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0062] It should be understood that the terms "system," "device," "unit," and / or "module" used in this invention are methods for distinguishing different components, elements, parts, sections, or assemblies at different levels. However, if other terms can achieve the same purpose, they may be replaced by other expressions.

[0063] As indicated in this invention and the claims, unless the context clearly indicates otherwise, the words "a," "an," "an," and / or "the" do not specifically refer to the singular and may also include the plural. Generally speaking, the terms "comprising" and "including" only indicate the inclusion of explicitly identified steps and elements, which do not constitute an exclusive list, and the method or apparatus may also include other steps or elements. An element defined by the phrase "comprising an..." does not exclude the presence of other identical elements in the process, method, product, or apparatus that includes the element.

[0064] In the description of the embodiments of the present invention, unless otherwise stated, " / " means "or", for example, A / B can mean A or B; "and / or" in this document is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Furthermore, in the description of the embodiments of the present invention, "multiple" refers to two or more.

[0065] Hereinafter, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature.

[0066] This invention uses flowcharts to illustrate the operations performed by the system according to embodiments of the invention. It should be understood that the preceding or following operations are not necessarily performed precisely in sequence. Instead, the steps can be processed in reverse order or simultaneously. Furthermore, other operations can be added to these processes, or one or more steps can be removed from them.

[0067] The above description is merely a preferred embodiment of the present invention and an explanation of the technical principles employed, and is not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. The scope of the invention is not limited to the technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the above-described inventive concept. For example, technical solutions formed by substituting the above features with (but not limited to) technical features with similar functions disclosed in the present invention.

Claims

1. A method for coal mine gas extraction based on carbon dioxide phase change-induced fracturing, characterized in that, Includes the following steps: Drilling operation: Using a complete set of drilling equipment equipped with hollow drill rods and open-end drill bits, drill holes along the coal seam in the coal roadway and record the drilling conditions; when the drilling reaches the designed depth, stop drilling, retract the drill and disassemble several drill rods; Screen casing assembly and placement: Install the bottom fixing device on the top of the screen casing, manually insert the screen casing with the bottom fixing device into the drill rod, connecting the screen casing as it is inserted, and insert it to the bottom of the drill rod. Open the crossbeam of the opening drill bit and continue to insert it to the bottom of the borehole, ensuring that the umbrella claw of the bottom fixing device is open, and pull it back until it is under force, ensuring that the umbrella claw of the bottom fixing device is hanging upside down on the coal wall; withdraw all drill rods and drill bits; Carbon dioxide fracturing device assembly and delivery: Check the safety performance of the carbon dioxide fracturing device. After ensuring safety, assemble the carbon dioxide fracturing device. Place the carbon dioxide fracturing device at the fracturing point and use the top rod to connect at the non-fracturing point. Push while connecting. Use the power system of the drilling equipment to push the carbon dioxide fracturing device through the inside of the screen hole sleeve to the bottom of the drill rod, so that the explosion vent of the carbon dioxide fracturing device is consistent with the passage of the screen hole sleeve. Temporary sealing for carbon dioxide phase change fracturing: Connect the temporary sealing device to the carbon dioxide fracturing device and send it to the predetermined position. Lead the water inlet pipe of the temporary sealing device and the busbar of the fracturing device out of the borehole to avoid it getting stuck in the borehole after sealing. Connect the water inlet pipe to the water tank and pressurize the rubber bag of the temporary sealing device until it expands and gets stuck on the borehole wall to form a temporary seal. At the same time, use a push rod outside the borehole to hold the temporary sealing device with one end and the other end against the roadway wall opposite the borehole to ensure that the bag sealer will not be forced out by high pressure during carbon dioxide phase change fracturing, thus improving the quality of temporary sealing. Carbon dioxide phase change fracturing and permeability enhancement: After checking that the carbon dioxide fracturing device has no explosion failure, start the detonator. The carbon dioxide fracturing device releases high-pressure carbon dioxide gas through the explosion relief port to fracture the coal seam. After fracturing, maintain pressure for a preset holding time. Carbon dioxide fracturing device dismantling and recovery: The ventilation dispatcher checks the first gas concentration in the roadway. When the first gas concentration in the roadway is less than the first threshold, the temporary sealing device is depressurized, and gas and carbon dioxide gas in the borehole gush out. The second gas concentration and carbon dioxide concentration within a preset distance near the borehole are checked. When the second gas concentration is less than the second threshold and the carbon dioxide concentration is less than the third threshold, the dismantling work is carried out. Drilling, fixing, sealing, and extraction: After the carbon dioxide fracturing device is removed, a sealing device with extraction pipes and bags is installed. The sealing device has two bags at the front and rear ends. The grouting pump is used to pressurize and grout the bags at both ends of the sealing device. After the bags expand and support the coal wall, expansive cement is injected into the middle blank area through the grouting pipe to seal it under pressure, forming a fixed sealing section. After the cement solidifies, the extraction pipe is connected to the extraction system for extraction. Under the protection of the screen sleeve, the carbon dioxide fracturing device can be removed normally when it is removed, and a good gas extraction channel can be maintained.

2. The coal mine gas extraction method based on carbon dioxide phase change fracturing as described in claim 1, characterized in that, The drilling equipment is a fully hydraulic tunnel drilling equipment, the drill rod is a shallow-bladed hollow spiral drill rod, and the drill bit of the drilling equipment is a large-hole open-closed PDC drill bit.

3. The coal mine gas extraction method based on carbon dioxide phase change fracturing as described in claim 1, characterized in that, The hole bottom fixing device is connected to the screen hole sleeve by thread or plug-in connection; The distance between the rear end of the sieve sleeve and the orifice is greater than 15m, and the length of a single sieve sleeve is 1m or 1.5m.

4. The coal mine gas extraction method based on carbon dioxide phase change fracturing as described in claim 1, characterized in that, The carbon dioxide fracturing devices are connected to each other or to the connecting rod via steel pipe threads.

5. The coal mine gas extraction method based on carbon dioxide phase change fracturing as described in claim 1, characterized in that, The safety performance inspection of the carbon dioxide fracturing device includes: testing the circuit of the carbon dioxide fracturing device and checking whether there is any gas leakage in the carbon dioxide fracturing device.

6. The coal mine gas extraction method based on carbon dioxide phase change fracturing as described in claim 1, characterized in that, The depth of the sealing hole shall not be less than 15m; the preset pressure holding time shall not be less than 60min.

7. The coal mine gas extraction method based on carbon dioxide phase change fracturing as described in claim 1, characterized in that, The first gas concentration, the second gas concentration, and the carbon dioxide concentration are volume concentrations; the first threshold, the second threshold, and the third threshold are all 0.5%; and the preset distance is 50m.

8. The coal mine gas extraction method based on carbon dioxide phase change fracturing as described in claim 1, characterized in that, The dismantling work includes depressurizing the sealing device; after the sealing device is depressurized, the drilling equipment is started to remove the sealing device and the carbon dioxide fracturing device together, leaving the bottom fixing device in the borehole.

9. The coal mine gas extraction method based on carbon dioxide phase change fracturing as described in claim 1, characterized in that, The length of the extraction pipe is ≥15m; the length of the fixed sealing section is ≥15m.

10. A coal mine gas extraction system based on carbon dioxide phase change fracturing, characterized in that, The method for coal mine gas extraction based on carbon dioxide phase change fracturing, as described in claim 1, includes a complete set of drilling equipment equipped with hollow drill rods, an opening drill bit, a bottom hole fixing device, a screen casing, a carbon dioxide fracturing device, a temporary sealing device, a top rod, a first gas concentration detector, a carbon dioxide concentration detector, a second gas concentration detector, a bag sealing device, and an extraction pipe.

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