Coal seam gas enhanced mining method based on methane in-situ burning explosion fracturing

Abstract

The invention discloses a coal seam gas enhanced mining method based on methane in-situ burning explosion fracturing. A top plate horizontal well and a bottom plate horizontal well are drilled in a top plate and a bottom plate correspondingly; the sand-carrying fluid forms high-speed and high-pressure jet flow through a perforating device and impacts a bottom plate, holes are formed in the coal seam, and meanwhile a plurality of cracks are generated in the surfaces of the holes; a plurality of fishbone wells are drilled downwards in the horizontal well section of each roof horizontal well; fracturing operation is conducted on the holes through liquid CO2; coal ash and oxygen are injected into the horizontal well section of the bottom plate horizontal well together, and a mixture is formed; detonating the mixture, and generating impact on the coal seam by utilizing instantaneous high pressure generated by burning explosion, so that the crack continues to expand; performing underground methane burning and explosion operation for multiple times, and forming a highly-fractured impact crack network near the horizontal well section; meanwhile, part of coal is mixed with oxygen to be combusted, and a coal seam loosening ring with weak bearing capacity is formed near the bottom plate; closing the well for mineralization reaction; and gas extraction operation. According to the method, the gas extraction efficiency can be remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of coalbed methane resource extraction, and particularly relates to a coalbed gas enhanced extraction method based on methane in-situ combustion fracturing. BACKGROUND

[0002] Coalbed methane is a kind of efficient and clean fossil energy, and coalbed methane accounts for an important position in the energy structure in China, with a total resource amount of about 43 trillion m 3 , which is equivalent to the natural gas reserves, and the coalbed methane geological resource amount existing in the coal seams with a depth of 2000 m or less is as high as 30.05 trillion m 3 , of which the extractable resource amount is about 12.50 trillion m 3 . The development of coalbed methane can greatly make up for the shortage in the development and utilization of conventional energy. However, the coal seams in China not only have the characteristics of high adsorption, low saturation, low permeability and low pressure, but also the gas content in the deep coal seams is continuously increasing as the coal development is increasingly directed to the deep part. The former leads to a low yield of the developed coalbed methane well, and the latter seriously threatens the safety of the mine. Coal seam hydraulic fracturing is one of the consistent technologies for improving the gas extraction efficiency. If the traditional hydraulic fracturing development technology is selected, the purpose of increasing the coal seam fracture volume cannot be achieved, the fracture direction is single, the fracture scale is small, the fracturing scale cannot be expanded, a large amount of water resources are wasted, the invasion and retention of the water phase cause potential damage and serious water lock effect of the reservoir, underground water is polluted, and the backflow liquid is difficult to handle, and many other problems, so the conventional hydraulic fracturing technology has a serious blind area problem in the technical innovation, and cannot realize the large-area gas desorption and release. Therefore, exploring a new fracturing extraction technology has become one of the research hotspots in the field of energy mining development technology and oil and gas exploitation.

[0003] The methane in-situ combustion fracturing technology is a technology for fracturing the reservoir rock by using the instantaneous high-pressure impact of the methane in-situ combustion. The pressure generated by the methane in-situ combustion in the wellbore and the fracture can reach more than 60 MPa within tens of milliseconds, and the impact fracture can overcome the limitation of the ground stress on the fracture expansion direction, and finally form multiple radial fractures along different directions of the wellbore. This not only solves the limitation of the easy expansion of the fracture along the maximum horizontal stress direction in the traditional hydraulic fracturing, but also effectively increases the complexity of the fracture and improves the reservoir stimulation effect. However, the existing methane in-situ combustion fracturing technology cannot form large-area three-dimensional fractures in the coal seam, and the permeability of the coal seam is not ideal, which limits the efficiency of the gas extraction. In order to effectively solve this technical problem, it is urgent to provide a coalbed gas enhanced extraction method based on methane in-situ combustion fracturing. SUMMARY

[0004] In view of the problems in the prior art, the present application provides a coal seam gas enhanced mining method based on methane in-situ combustion fracturing, which has simple implementation process and low implementation cost, can significantly improve the permeability of the coal seam, greatly increase the content of the gas, and greatly improve the gas extraction efficiency.

[0005] In order to achieve the above-mentioned purpose, the present application provides a coal seam gas enhanced mining method based on methane in-situ combustion fracturing, comprising the following steps: Step one: drilling operation; A plurality of roof horizontal wells are drilled in the roof above the coal seam, and a plurality of floor horizontal wells are drilled in the floor below the coal seam, and meanwhile, the plurality of roof horizontal wells and the plurality of floor horizontal wells are ensured to be distributed one by one in correspondence with each other in up and down directions; Step two: jet cavitation and permeability increasing operation of the floor horizontal well; S21: lower the perforating device into the horizontal well section of the floor horizontal well, and make the perforating direction of the perforating device face the coal seam above; meanwhile, use the mixture of blast furnace slag, carbide slag and fly ash in the coal-based solid waste to make abrasive; S22: first, use the ground stirring tool to fully mix and stir the supercritical CO2 and the abrasive to form a sand-carrying liquid, then start the plunger pump, and pump the sand-carrying liquid into the perforating device through the pipeline, so that the sand-carrying liquid forms a high-speed high-pressure jet through the perforating device and impacts the floor upwards, and the sand-carrying liquid continues to impact the coal seam after penetrating the floor, forming a hole in the coal seam; meanwhile, use the low-temperature expansion characteristics of supercritical CO2 to act on the coal seam around the hole, so that the surrounding coal seam produces tensile brittle failure, and a plurality of cracks are formed around the hole; S23: change the position of the perforating device in the horizontal well section, form a plurality of holes in the horizontal well section of each floor horizontal well, and form a plurality of cracks around each hole; use the plurality of holes and the plurality of cracks to connect the floor horizontal well and the intermediate coal seam; Step three: drilling fishbone wells in the roof horizontal well; In the horizontal well section of each roof horizontal well, a plurality of fishbone wells extending into the intermediate coal seam are drilled towards the coal seam below; Step four: fracturing and permeability increasing operation of the floor horizontal well; Use liquid CO2 filled in the sand-carrying liquid as fracturing fluid to perform fracturing operation on the hole, guide the cracks on the surface of the hole to be oriented upward and expand, and form complex cracks; Step five: downhole methane combustion operation; S51: inject fly ash and oxygen into the horizontal well section of the floor horizontal well through the pipeline, and enter into the cracks through the hole to form a mixture of methane, oxygen and fly ash in the cracks; S52: Ignite and explode the mixture, use the instantaneous high pressure generated by the explosion to impact the horizontal well section of the floor horizontal well and the coal seam around the hole, causing the cracks to continue to expand and form a plurality of radial impact cracks in the coal seam; Step six: cycle the combustion and explosion; S61: Perform multiple underground methane combustion and explosion operations in the coal seam to form a highly fractured impact crack network near the horizontal well section of the floor horizontal well; at the same time, use the high-temperature environment generated during the multiple methane combustion and explosion processes to cause part of the coal around the impact crack network to burn with oxygen, and break the coal seam around the impact crack network through the combined action of methane combustion and coal seam combustion, forming a coal seam loose circle with weak bearing capacity near the coal seam near the floor; S62: Use the gravity of the entire coal seam to overcome the weak bearing force of the coal seam loose circle, causing the entire coal seam to collapse and move downward, while forming a large number of fissures that connect the fishbone wells; Step seven: shut in and perform mineralization reaction; Close the roof horizontal well and the floor horizontal well, and make the in-well blast furnace slag, carbide slag, and fly ash in the well react with CO2; at the same time, use the CO2 generated by methane combustion and coal seam combustion to displace the methane in the coal seam; Step eight: gas extraction operation; Lower the extraction pipe into the roof horizontal well and use the extraction device to perform gas extraction operation.

[0006] As a preferred, in step one, before drilling construction, coal seam resource positioning and well site layout design are performed first, wherein the process of coal seam resource positioning and well site layout design is as follows: First, determine the development area geological conditions according to the underground rock stratum geological parameters and exploration data, accurately position the resource location and well layout depth, and then design the well site layout according to the geological conditions.

[0007] Further, in order to avoid heat loss during transportation, in S21 of step two, the pipe is subjected to heat insulation treatment; in step eight, the extraction pipe is subjected to heat insulation treatment.

[0008] Further, in order to ensure the connection effect, in S22 of step two, the diameter of the hole ranges from 0.6 to 0.8 m.

[0009] Further, in order to ensure the connection effect, in step three, the height of the fishbone well ranges from 5 to 8 m.

[0010] Further, in order to ensure the subsequent connection effect, in step three, ensure that the plurality of fishbone wells are staggered with the plurality of holes below.

[0011] Further, in order to realize sufficient mineralization reaction, and in order to realize efficient replacement of methane, in step seven, the length of time for closing the top plate horizontal well and the bottom plate horizontal well is not less than 5 hours.

[0012] The application provides a deep coal seam gas enhanced mining method based on a methane in-situ combustion fracturing technology. First, multiple top plate horizontal wells and multiple bottom plate horizontal wells are drilled in the upside top plate and the downside floor of the coal seam, and the multiple top plate horizontal wells and the multiple bottom plate horizontal wells are distributed in a one-to-one correspondence. The method can provide a good foundation condition for subsequent large-scale communication between the top plate horizontal well and the underlying coal seam. Second, a jet sandblasting operation is performed on the upward coal seam by using a jet formed by supercritical CO2 and abrasive in the horizontal well section of the bottom plate horizontal well, multiple holes connecting the bottom plate horizontal well and the coal seam are formed, the coal seam around the holes is subjected to tensile brittle failure by using supercritical CO2, and multiple cracks are formed around the holes, so that the effective communication between the bottom plate horizontal well and the intermediate coal seam is initially realized by using the holes and the cracks. Third, multiple fishbones extending into the intermediate coal seam are drilled in the downward coal seam by using the top plate horizontal well, so that the effective communication between the top plate horizontal well and the intermediate coal seam is realized. Fourth, a crack forming operation is performed in the bottom plate horizontal well by using liquid CO2 as a fracturing fluid, and cracks extending upward and having better complexity are formed, so that the communication effect between the bottom plate horizontal well and the intermediate coal seam is further improved. Subsequently, fly ash and oxygen are injected into the cracks, a mixture of methane, oxygen and fly ash is formed in the cracks, ignition is performed to make the methane and the oxygen combust and explode, and the combustion pressure is effectively increased by using the fly ash, and the combustion time is prolonged, so that impact cracks having a more ideal communication effect are formed in the coal seam. On this basis, multiple in-situ methane combustions are performed in the coal seam by cyclic injection of oxygen and fly ash, and a highly cracked impact crack network is formed near the bottom plate horizontal well. At the same time, high temperature is generated in the methane combustion process, the coal seam in contact with the oxygen is subjected to a large-scale deflagration, the coal seam near the bottom plate horizontal well is effectively crushed, and the whole coal seam can move downward under the action of gravity. In the whole coal seam movement process, a large number of cracks are generated in the coal seam, so that the whole coal seam permeability is significantly increased. Then, the blast furnace slag, the calcium carbide slag and the fly ash are subjected to a mineralization reaction with CO2 for a long time, and the methane is replaced by using the CO2 generated in the methane combustion and the coal seam combustion, so that the content of free methane gas is effectively increased, and the subsequent gas extraction efficiency is ensured. Finally, the gas can be efficiently extracted by using the top plate horizontal well.

[0013] The application effectively combines the methane in-situ combustion fracturing technology and the supercritical CO2 abrasive jet technology, and realizes the efficient gas extraction operation through multiple fracturing processes, wherein the supercritical CO2 fluid has special properties, the density of which is close to and controllable, and the viscosity is close to gas. The use of supercritical CO2 jet to break rock can reduce the threshold pressure and improve the breaking speed, the fluid has no pollution to the reservoir, and can be miscible with crude oil to enhance the flow capacity and increase the oil flow channel, especially for unconventional oil and gas resources such as shale gas and coalbed methane with low porosity and low permeability, which will further help to improve the oil and gas single well production and recovery. The supercritical CO2 enters the floor horizontal well in a low-temperature state, which can cause the temperature of the coal seam to change suddenly and generate thermal stress, induce micro cracks, reduce the difficulty of fracturing, and thus increase the fracturing effect; the abrasive particles are added to the supercritical CO2 jet to form a supercritical CO2 abrasive jet, which can fully utilize the low-temperature characteristics and gasification expansion characteristics of supercritical CO2 to further increase the permeability, and effectively increase the effect of subsequent combustion and explosion fracturing. The abrasive of the mixture of blast furnace slag, calcium carbide slag and fly ash in coal-based solid waste can be used as the material for mineralization reaction in the downhole after the well is closed, which effectively improves the problems of water resource consumption, water phase invasion and retention causing potential damage to the reservoir in the prior art.

[0014] The application innovatively uses the combined action of methane combustion and in-situ combustion of coal seam to greatly reduce the density and strength of the bottom coal seam, form a loose zone at the bottom of the coal seam, cause the collapse of the bottom coal seam, and thus cause the overall coal seam to greatly sink downward, so that a large range of three-dimensional cracks can be formed in the coal seam between the roof and the floor, the gas permeability of the coal seam is significantly improved, the content of gas is increased through the action of combustion and explosion, the difficulty of gas extraction is significantly reduced, and the problems of single crack direction, small crack size and low gas extraction efficiency in the prior art are effectively improved.

[0015] The method has simple implementation process and low implementation cost, fully combines the methane in-situ combustion fracturing and supercritical CO2 abrasive jet technology, overcomes the problems of single crack formed by the traditional fracturing technology, large water resource demand, and easy pollution of the working face, and uses multiple fracturing, mineralization reaction and CO2 displacement method to significantly improve the permeability of the coal seam and greatly increase the content of gas, which can greatly improve the gas extraction efficiency. The method can effectively overcome the technical bottleneck that the large-area gas desorption and release cannot be realized in the prior art, and can overcome the problems of low recovery efficiency of coal seams with high adsorption, low saturation, low permeability and low pressure, small crack range of coal seam fracturing, and the like, and can provide technical support for the effective exploitation of deep coal seam resources. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1is a distribution diagram of the roof horizontal well and the floor horizontal well in the present application; Figure 2 is a state diagram of perforating in the direction of the coal seam by the floor horizontal well in the present application; Figure 3 is a state diagram of drilling the fishbone well in the direction of the coal seam by the roof horizontal well in the present application; Figure 4 is a state diagram of fracturing operation by using liquid CO2 in the present application; Figure 5 is a state diagram of filling oxygen and fly ash to reach the combustion condition in the well in the present application; Figure 6 is a state diagram of the cracks generated after the combustion in the well in the present application; Figure 7 is a state diagram of the loose circle formed after the combustion in the well in the present application; Figure 8 is a state diagram of the shut-in mineralization process in the present application; Figure 9 is a state diagram of the gas extraction operation in the present application.

[0017] In the figure, 1 is the coal seam, 2 is the roof, 3 is the floor, 4 is the roof horizontal well, 5 is the floor horizontal well, 6 is the ground mixing tool, 7 is the supercritical CO2, 8 is the abrasive, 9 is the sand-carrying liquid, 10 is the plunger pump, 11 is the pipeline, 12 is the perforating device, 13 is the hole, 14 is the fishbone well, 15 is the liquid CO2, 16 is the crack, 17 is the fly ash, 18 is the oxygen, 19 is the impact crack, 20 is the coal seam loose circle, 21 is the fracture, 22 is the extraction device, and 23 is the extraction pipeline. DETAILED DESCRIPTION

[0018] The present application will be further described below in combination with the drawings.

[0019] As shown in the drawings, the present application provides a coal seam gas enhanced mining method based on methane in-situ combustion fracturing, which comprises the following steps: Figures 1 to 9 Step one: drilling construction, as shown in the drawings; Drill multiple roof horizontal wells 4 in the roof 2 above the coal seam 1 and drill multiple floor horizontal wells 5 in the floor 3 below the coal seam 1, and at the same time, ensure that the multiple roof horizontal wells 4 and the multiple floor horizontal wells 5 are distributed one by one in the up-down direction; Figure 1 Step two: jet flow cavity forming and permeability increasing operation of the floor horizontal well 5, as shown in the drawings; Step three: fishbone well drilling operation of the roof horizontal well 4, as shown in the drawings; Step four: fracturing operation by using liquid CO2, as shown in the drawings; Figure 2 Step five: combustion operation in the well, as shown in the drawings; S21: use the pipeline 11 to lower the perforating device 12 into the horizontal well section of the floor horizontal well 5, and make the perforating direction of the perforating device 12 towards the coal seam 1 above, at the same time, use the plunger pump 10 on the ground to establish the connection between the ground mixing tool 6 and the pipeline 11; at the same time, use the mixture of blast furnace slag, carbide slag and fly ash in coal-based solid waste to make the abrasive 8; S22: first use the ground mixing tool 6 to fully mix and stir the supercritical CO2 7 and the abrasive 8 to form the sand-carrying liquid 9, then start the plunger pump 10, pump the sand-carrying liquid 9 into the perforating device 12 through the pipeline 11, make the sand-carrying liquid 9 form a high-speed high-pressure jet through the perforating device 12 and impact the floor 3 upwards, use the perforating device 12 to carry out the hole-making and permeability-increasing operation, the sand-carrying liquid 9 continues to impact the coal seam 1 after penetrating the floor 3, and forms a hole 13 in the coal seam 1; at the same time, use the low-temperature expansion characteristics of supercritical CO2 7 to act on the coal seam 1 around the hole 13, make the coal seam 1 around the hole 13 produce tensile brittle failure, and form multiple cracks 16 around the hole 13, effectively improve the permeability of the coal seam; S23: change the position of the perforating device 12 in the horizontal well section, form multiple holes 13 in the horizontal well section of each floor horizontal well 5, and form multiple cracks 16 around each hole 13; use the multiple holes 13 and the multiple cracks 16 to connect the floor horizontal well 5 and the intermediate coal seam 1; Step three: drill the fishbone well 14 in the roof horizontal well 4, as shown in Figure 3 In the horizontal well section of each roof horizontal well 4, multiple fishbone wells 14 extending into the intermediate coal seam 1 are drilled at different positions in the length direction towards the coal seam 1 below, in order to ensure the subsequent connection effect, the multiple fishbone wells 14 are distributed staggered with the multiple holes 13 below; in this way, the multiple fishbone wells 14 can be used to effectively connect the roof horizontal well 4 and the intermediate coal seam 1; Step four: the fracturing and permeability-increasing operation of the floor horizontal well 5, as shown in Figure 4 The sand-carrying liquid 9 in the floor horizontal well 5 contains liquid CO2 15, use the liquid CO2 15 filled in the sand-carrying liquid 9 as fracturing fluid to carry out the fracturing operation on the hole 13, the fluid moves upwards in the gasification process of the liquid CO2 15, which can effectively guide the cracks 16 on the surface of the hole 13 to be oriented upwards and initiate and expand, forming more complex and vertically extending cracks 16; Step five: downhole methane combustion and explosion operation, as shown in Figure 5 and Figure 6 S51: inject the fly ash 17 and oxygen 18 into the horizontal well section of the floor horizontal well 5 through the pipeline 11, and into the cracks 16 through the hole 13, form a mixture of methane, oxygen 18 and fly ash 17 in the cracks 16; ​​​S52: Ignite the mixture by using the igniting device, wherein, during the combustion and explosion of the methane and oxygen 18, the fly ash 17 can effectively increase the combustion and explosion pressure and prolong the combustion and explosion time, the instantaneous high pressure generated by the combustion and explosion is used to impact the horizontal well section of the floor horizontal well 5 and the coal seam 1 around the hole 13, so as to cause the crack 16 to continue to expand, and during the combustion and explosion, the expansion direction of the crack 16 can overcome the limitation of the ground stress and expand along different directions, and finally, a plurality of radial impact cracks 19 are formed in the coal seam 1 near the floor horizontal well 5; Step six: cyclic combustion and explosion, as shown in Figure 7 S61: repeatedly perform step five, and perform the underground methane combustion and explosion operation in the coal seam 1 multiple times to form a highly cracked impact crack network near the horizontal well section of the floor horizontal well 5; at the same time, the high-temperature environment generated during the multiple methane combustion and explosion processes is used to cause part of the coal around the impact crack network to burn with oxygen 18, and the coal seam 1 around the impact crack network is broken through the joint action of the methane combustion and explosion and the coal seam 1 burning, so as to form a coal seam loose circle 20 with weak bearing capacity in the coal seam 1 near the floor 3; S62: due to the existence of the coal seam loose circle 20, the coal seam 1 as a whole will move downward, specifically, the gravity of the coal seam 1 as a whole is used to overcome the weak bearing force of the coal seam loose circle 20, so that the coal seam 1 as a whole collapses and moves downward, and at the same time, a large number of cracks 21 connected with the fishbone well 14 are formed in the coal seam 1 during the moving process, so that the permeability of the coal seam 1 as a whole is significantly increased; Step seven: shut in and perform mineralization reaction, as shown in Figure 8 The floor horizontal well 5 and the floor horizontal well 5 are closed, so that the high-temperature blast furnace slag, the carbide slag and the fly ash in the well and the CO2 occur mineralization reaction; at the same time, the CO2 generated by the methane combustion and explosion and the coal seam 1 burning is used to replace the methane in the coal seam 1, so as to increase the content of the free methane gas; finally, the gas in the coal seam 1 can enter the floor horizontal well 4 through the cracks 21; Step eight: gas extraction operation, as shown in Figure 9 The extraction pipe 23 is lowered into the floor horizontal well 4, and the extraction pipe 23 is connected with the extraction device 22 on the ground, and the extraction device 22 is used to perform the gas extraction operation.

[0020] As a preferred, in step one, before the drilling construction, the coal seam resource positioning and well site layout design are performed, wherein, the process of the coal seam resource positioning and well site layout design is as follows: first, according to the underground rock stratum geological parameters and exploration data, the development area geological conditions are determined, the resource position is accurately positioned and the well depth is designed, and then the well site layout design is performed according to the geological conditions.

[0021] ​​​As a kind of preferred, in step two S21, the pipeline 11 is treated by heat insulation, to ensure that heat loss does not occur during fluid transport;In step eight, the extraction pipe 23 is treated by heat insulation, to ensure that heat loss does not occur during fluid transport.

[0022] To ensure the effect of communication, in step two S22, the diameter of the hole 13 is in the range of 0.6 to 0.8 m.

[0023] To ensure the effect of communication, in step three, the height of the fishbone well 14 is in the range of 5 to 8 m.

[0024] As a kind of preferred, in step seven, the length of time for closing the top plate horizontal well 4 and the bottom plate horizontal well 5 is not less than 5 h, and long time shut-in is beneficial to mineralization reaction and efficient replacement of CO2 and coal seam methane.

[0025] The application provides a deep coal seam gas enhanced mining method based on methane in-situ combustion fracturing technology, which comprises the following steps: firstly, drilling multiple roof and floor horizontal wells on the upside roof and downside floor of the coal seam respectively, and making the multiple roof horizontal wells and the multiple floor horizontal wells correspondingly distributed one by one, so as to provide good basic conditions for subsequent roof horizontal well and large-scale communication with the underlying coal seam; secondly, using the jet sandblasting formed by supercritical CO2 and abrasive to perform jet perforation operation on the upward coal seam, so as to form multiple holes connecting the floor horizontal well and the coal seam, using supercritical CO2 to make the coal seam around the holes produce tensile brittle failure, and then forming multiple cracks around the holes, so as to preliminarily realize the effective communication of the floor horizontal well and the middle coal seam by using the holes and cracks; thirdly, drilling multiple fishbones in the downward coal seam from the roof horizontal well into the middle coal seam, so as to realize the effective communication of the roof horizontal well and the middle coal seam; fourthly, using liquid CO2 as fracturing fluid to perform crack forming operation in the floor horizontal well, so as to form cracks with better complexity extending upward, thereby further improving the communication effect of the floor horizontal well and the middle coal seam; fifthly, injecting fly ash and oxygen into the cracks to form a mixture of methane, oxygen and fly ash in the cracks, igniting the mixture to make the methane and oxygen combust and explode, and using the fly ash to effectively improve the combustion and explosion pressure and prolong the combustion and explosion time, so as to form impact cracks with more ideal communication effect in the coal seam; on this basis, repeatedly injecting oxygen and fly ash to perform multiple methane in-situ combustion in the coal seam, so as to form a highly cracked impact crack network near the floor horizontal well; simultaneously, using the high temperature generated in the methane combustion process to make the coal seam in contact with the oxygen produce large-scale deflagration, so as to effectively crush the coal seam near the floor horizontal well, and make the whole coal seam move downward under the action of gravity; in the whole coal seam moving process, a large number of cracks are generated in the coal seam, so as to significantly increase the whole coal seam permeability; then, through long-term shut-in, the blast furnace slag, calcium carbide slag and fly ash are fully mineralized with CO2, and the CO2 generated in the methane combustion and coal seam combustion is used to fully displace the coal seam methane, so as to effectively increase the content of free methane gas, and then ensure the subsequent gas extraction efficiency; finally, the gas can be efficiently extracted through the roof horizontal well.

[0026] The application effectively combines methane in-situ combustion fracturing technology and supercritical CO2 abrasive jet technology, and realizes efficient gas extraction operation through multiple fracturing processes, wherein the supercritical CO2 fluid has special properties, the density is close to and controllable, and the viscosity is close to gas. The use of supercritical CO2 jet to break rock can reduce the threshold pressure and improve the breaking speed, the fluid has no pollution to the reservoir, and can be miscible with crude oil to enhance its flow capacity and increase the oil flow channel, especially for unconventional oil and gas resources such as shale gas and coalbed methane with low porosity and low permeability, which will help to improve the oil and gas single well production and recovery efficiency. The supercritical CO2 enters the floor horizontal well in a low-temperature state, which can cause a sudden change in the temperature of the coal seam to generate thermal stress, induce micro cracks, reduce the difficulty of fracturing, and thus increase the fracturing effect; the abrasive particles are added to the supercritical CO2 jet to form a supercritical CO2 abrasive jet, which can take full advantage of the low-temperature characteristics and gasification expansion characteristics of supercritical CO2 to further increase the permeability, effectively increase the effect of subsequent combustion and explosion fracturing, and the abrasive of the mixture of blast furnace slag, calcium carbide slag and fly ash in coal-based solid waste can be used as the material for mineralization reaction in the downhole after the well is closed, which effectively improves the problems of water resource consumption, water phase invasion and retention causing potential damage to the reservoir in the prior art.

[0027] The application innovatively uses the combined action of methane combustion and in-situ combustion of coal seam to greatly reduce the density and strength of the bottom coal seam, form a loose zone at the bottom of the coal seam, cause the collapse of the bottom coal seam, and thus cause the overall coal seam to greatly sink downward, form a large range of three-dimensional cracks in the coal seam between the roof and the floor, significantly improve the permeability of the coal seam, increase the content of gas through the action of combustion and explosion, and significantly reduce the difficulty of gas extraction, thereby effectively improving the problems of single crack direction, small crack size and low gas extraction efficiency in the prior art.

[0028] The method has simple implementation process and low implementation cost, fully combines methane in-situ combustion fracturing and supercritical CO2 abrasive jet technology, overcomes the problems of single crack formed by traditional fracturing technology, large water resource demand, and easy pollution of working surface, and uses multiple fracturing, mineralization reaction and CO2 displacement method to significantly improve the permeability of the coal seam and greatly increase the content of gas, thereby greatly improving the gas extraction efficiency, effectively overcoming the technical bottleneck that large-area gas desorption and release cannot be realized in the prior art, and overcoming the problems of low recovery efficiency, small fracturing crack range and the like of coal seams with high adsorption, low saturation, low permeability and low pressure characteristics, and providing technical support for effective exploitation of deep coal seam resources.

Claims

1. A coal seam gas enhanced mining method based on methane in-situ combustion fracturing, characterized in that, The method comprises the following steps: Step one: drilling well construction; Drill multiple roof horizontal wells (4) in the roof (2) above the coal seam (1), and drill multiple floor horizontal wells (5) in the floor (3) below the coal seam (1); Step two: jet cavitation and permeability increasing operation of the floor horizontal well (5); S21: Lower the perforating device (12) into the horizontal well section of the floor horizontal well (5), and make the perforating direction of the perforating device (12) face upward to the coal seam (1); at the same time, use the mixture of blast furnace slag, carbide slag and fly ash in coal-based solid waste to make abrasive (8); S22: First, use the ground stirring tool (6) to fully mix and stir the supercritical CO2 (7) and abrasive (8) to form a sand-carrying liquid (9), then start the plunger pump (10), and pump the sand-carrying liquid (9) into the perforating device (12) through the pipeline (11), so that the sand-carrying liquid (9) forms a high-speed high-pressure jet through the perforating device (12) and impacts the floor (3) upward, and the sand-carrying liquid (9) continues to impact the coal seam (1) after penetrating the floor (3), forming a hole (13) in the coal seam (1); at the same time, use the low-temperature expansion characteristics of supercritical CO2 (7) to act on the coal seam (1) around the hole (13), so that the surrounding coal seam (1) produces tensile brittle failure, and multiple cracks (16) are formed around the hole (13); S23: Change the position of the perforating device (12) in the horizontal well section, form multiple holes (13) in the horizontal well section of each floor horizontal well (5), and form multiple cracks (16) around each hole (13); use multiple holes (13) and multiple cracks (16) to connect the floor horizontal well (5) with the intermediate coal seam (1); Step three: drill fishbone wells (14) in the roof horizontal well (4); In the horizontal well section of each roof horizontal well (4), drill multiple fishbone wells (14) that extend into the intermediate coal seam (1) toward the coal seam (1) below; Step four: fracturing and permeability increasing operation of the floor horizontal well (5); Use liquid CO2 (15) filled in the sand-carrying liquid (9) as fracturing fluid to perform fracturing operation on the hole (13), guide the cracks (16) on the surface of the hole (13) to orient upward and initiate and expand, and form complex cracks (16); Step five: downhole methane combustion and explosion operation; S51: Inject fly ash (17) and oxygen (18) into the horizontal well section of the floor horizontal well (5) through the pipeline (11), and into the cracks (16) through the hole (13), to form a mixture of methane, oxygen (18) and fly ash (17) in the cracks (16); S52: Ignite and detonate the mixture, use the instantaneous high pressure generated by combustion and explosion to impact the horizontal well section of the floor horizontal well (5) and the coal seam (1) around the hole (13), so that the cracks (16) continue to expand, and multiple radial impact cracks (19) are formed in the coal seam (1); Step six: cyclic combustion and explosion; S61: multiple underground methane combustion operations are carried out in the coal seam (1) to form a highly fractured impact crack network near the horizontal well section of the floor horizontal well (5); at the same time, the high-temperature environment generated during the multiple methane combustion operations is used to promote the combustion of part of the coal around the impact crack network mixed with oxygen (18), and the coal seam (1) around the impact crack network is broken through the combined action of methane combustion and coal seam (1) combustion, forming a coal seam loose circle (20) with weak bearing capacity in the coal seam (1) near the floor (3); S62: the gravity of the entire coal seam (1) is used to overcome the weak bearing force of the coal seam loose circle (20), so that the entire coal seam (1) collapses and moves downward, and a large number of cracks (21) connected with the fishbone wells (14) are formed during the downward movement; Step seven: shut-in mineralization reaction; The roof horizontal well (4) and the floor horizontal well (5) are closed to make the in-well blast furnace slag, carbide slag and fly ash in the well react with CO2 to form mineralization; at the same time, the CO2 generated by methane combustion and coal seam (1) combustion is used to replace the methane in the coal seam (1); Step eight: gas extraction operation; The extraction pipe (23) is lowered into the roof horizontal well (4), and the gas extraction operation is carried out by using the extraction device (22).

2. The coal seam gas enhanced mining method based on methane in-situ combustion fracturing according to claim 1, characterized in that, In step one, before drilling construction, coal resource positioning and well site layout design are carried out first, wherein the process of coal resource positioning and well site layout design is as follows: First, according to the underground rock stratum geological parameters and exploration data, the development area geological conditions are determined, the resource position is accurately positioned and the well depth is designed, and then the well site layout design is carried out according to the geological conditions.

3. The coal seam gas enhanced mining method based on methane in-situ combustion fracturing according to claim 1, characterized in that, In step one, ensure that the multiple roof horizontal wells (4) and the multiple floor horizontal wells (5) are distributed one by one.

4. The coal seam gas enhanced mining method based on methane in-situ combustion fracturing according to claim 1, characterized in that, In S21 of step two, the pipe (11) is subjected to heat insulation treatment; in step eight, the extraction pipe (23) is subjected to heat insulation treatment.

5. The coal seam gas enhanced mining method based on methane in-situ combustion fracturing according to claim 1, characterized in that, In S22 of step two, the diameter of the hole (13) ranges from 0.6 to 0.8 m.

6. The coal seam gas enhanced mining method based on methane in-situ combustion fracturing according to claim 3, characterized in that, In step three, the height of the fishbone well (14) ranges from 5 to 8 m.

7. The coal seam gas enhanced mining method based on methane in-situ combustion fracturing according to claim 1, characterized in that, In step three, ensure that the multiple fishbone wells (14) and the multiple holes (13) below them are staggered.

8. The coal seam gas enhanced mining method based on methane in-situ combustion fracturing according to claim 3, characterized in that, In step seven, the time length for closing the roof horizontal well (4) and the floor horizontal well (5) is not less than 5 h.

Images