Coal measure stratum hard coal rock composite structure modifying and cracking system and method

Abstract

The invention discloses a coal measure stratum hard coal rock composite structure modifying and cracking system and method. The system comprises a cracking device arranged in a directional drill hole extending into a cracking area, a high-pressure pump and a hydraulic pump, wherein the high-pressure pump and the hydraulic pump are arranged in a coal seam roadway and inject a high-pressure cracking agent into the cracking device. The method comprises the following steps: 1, determining the position of a modified cracking area of the overlying strata of the coal seam; 2, determining drilling parameters; 3, obtaining a directional drill hole; fourthly, a fracturing device is installed, and the hole is sealed; 5, calculating the fracture pressure of the rock stratum and carrying out primary water fracturing; 6, liquid nitrogen reinforced cracking of the rock mass; and 7, judging whether the coal measure stratum hard coal rock composite structure modification cracking effect is achieved or not. On the basis of hydraulic fracturing, liquid nitrogen fracturing and mechanical auxiliary fracturing are introduced and ingeniously combined, meanwhile, the fracturing means can be autonomously adjusted according to different working conditions, the hard roof fracturing modes are enriched, the method has the advantages of being small in engineering amount, low in pollution, high in safety and high in working efficiency, and the cracking effect is enhanced.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal-rock composite structure modification and fracturing, and in particular relates to a system and method for modifying and fracturing hard coal-rock composite structure in coal-bearing strata. Background Art

[0002] The hard roof of a coal mine refers to the hard rock layer that exists directly above the coal seam or above the thin direct roof. It has the characteristics of high hardness, good integrity, and large layer thickness. With the mining of coal seams, the hard roof is difficult to collapse in time, and it is very easy to form a large area of ​​hanging roof in the goaf, such as Figure 1 As shown in the figure. Large-scale suspended roof and its sudden breakage are prone to impact loads and large-area roof pressure. The stress disturbance caused by the bending and breaking of the hard roof is easy to induce impact ground pressure, coal and gas outbursts and other mining dynamic disasters, which seriously affect the safety of underground coal mine production. Therefore, it is particularly important to deal with the hard roof in time during coal seam mining.

[0003] At present, the commonly used fracturing methods for hard rock strata in mines include roof blasting forced release method, hydraulic fracturing, directional long drilling collaborative fracturing, microwave fracturing, carbon dioxide fracturing, liquid nitrogen fracturing, etc.

[0004] The blasting roof caving method refers to the use of blasting to induce pre-cracking of the hard roof, thereby improving mining efficiency. This method has a good cracking effect, but it requires in-depth investigation and prediction based on the physical and mechanical properties of the mining area and rock, and the rational selection of the drilling method and charging method. It has high technical requirements, and there are certain safety hazards in controlling the roof through drilling and blasting in high-gas mines. The safety is poor and it is easy to pollute the underground air.

[0005] Hydraulic fracturing refers to the process of injecting pressurized water into a sealed open hole to cause tensile fractures in the rock. This process is actually the process in which microcracks in the rock initiate, expand, and penetrate under the action of water pressure until finally macro cracks are produced, leading to unstable fractures. This method has low cost and little impact on the underground production environment. At the same time, it can avoid the dangers brought by blasting operations. However, traditional hydraulic fracturing methods have certain limitations when used. The effect of hydraulic fracturing is affected by the rock properties of the roof. When encountering hard rock formations, the working efficiency is low and the fracturing effect is not ideal, which increases the cost of use. The high-pressure water injection method has a limited range of roof weakening when the water injection volume is limited, and this method takes a certain amount of time to be effective, which can easily affect the production progress.

[0006] The directional drilling collaborative fracturing method is to construct dense directional holes in the roof rock strata. A large number of dense holes are used to destroy the integrity of the hard roof. Under the action of mine pressure, the roof rock strata collapse along the direction of the drilling arrangement, thereby avoiding the formation of large-scale hanging roof. This method can pre-crack the hard roof to a certain extent and promote the collapse of the roof. However, due to the large number of drilling holes, the project volume is large, the construction cost is increased, and noise and dust will be generated during construction, and the labor efficiency is low.

[0007] Liquid nitrogen fracturing is an emerging, efficient and green fracturing technology. The chemical properties of nitrogen are stable and will not react with minerals inside the formation. However, actual experiments have found that although directly injecting high-pressure liquid nitrogen into the borehole can play a certain role in increasing permeability, its fracturing effect is limited. The low water content of the rock will cause the fracturing effect to be unsatisfactory, and when encountering hard and intact rock formations, its fracturing efficiency will be reduced, which has certain limitations. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a composite structure modified fracturing system for hard coal rock in coal-bearing strata in view of the deficiencies in the above-mentioned prior art. The system has a novel and reasonable design, introduces liquid nitrogen fracturing and mechanically assisted fracturing on the basis of hydraulic fracturing, and cleverly combines them. At the same time, the fracturing means can be independently adjusted according to different working conditions, enriching the fracturing methods of hard roof. The system has the advantages of small engineering workload, low pollution, high safety and high work efficiency, strengthens the cracking effect, improves underground work efficiency, provides guarantee for safe and efficient underground mining, and is easy to promote and use.

[0009] In order to solve the above technical problems, the technical solution adopted by the present invention is: a coal-bearing stratum hard coal rock composite structure modification fracturing system, characterized in that: it includes a fracturing device arranged in a directional drill hole extending into the fracturing area, and a high-pressure pump and a hydraulic pump arranged in the coal seam tunnel and injecting high-pressure fracturing agent into the fracturing device, the fracturing device includes a mounting seat, an outer tube and a fracturing nozzle connected in sequence, a T-shaped partition is arranged in the outer tube to divide the inner cavity of the tube into a water pressure cavity and a liquid nitrogen cavity, a water injection pipe for connecting the water pressure cavity and the high-pressure pump, and a nitrogen injection pipe for connecting the liquid nitrogen cavity and the hydraulic pump are connected to the mounting seat, a water pressure fracturing nozzle, a liquid nitrogen fracturing nozzle and a mixed fracturing nozzle located between the water pressure fracturing nozzle and the liquid nitrogen fracturing nozzle are arranged on the fracturing nozzle, a transverse partition of the T-shaped partition extends into the fracturing nozzle, and the top of the transverse partition is connected to the liquid nitrogen fracturing nozzle and the liquid nitrogen fracturing nozzle. A first directional valve is arranged at the intersection of the mixed fracturing nozzle, a second directional valve is arranged at the intersection of the side wall of the transverse partition and the liquid nitrogen fracturing nozzle, a third directional valve is arranged at the intersection of the top of the transverse partition and the hydraulic fracturing nozzle and the mixed fracturing nozzle, a fourth directional valve is arranged at the intersection of the side wall of the transverse partition and the hydraulic fracturing nozzle, a fifth directional valve is arranged at the connection position between the nitrogen injection pipe and the liquid nitrogen chamber, a sixth directional valve is arranged at the connection position between the water injection pipe and the hydraulic chamber, a vibrator is installed on the outer wall at the connection position between the outer tube and the fracturing nozzle, a first packer and a second packer are arranged on the outer side of the outer tube, a hydraulic fracturing channel is opened on the outer tube between the first packer and the second packer corresponding to the hydraulic chamber, and a liquid nitrogen fracturing channel is opened on the outer tube between the first packer and the second packer corresponding to the liquid nitrogen chamber.

[0010] The above-mentioned coal-bearing stratum hard coal rock composite structure modification and fracturing system is characterized in that: there are multiple hydraulic fracturing channels, and multiple hydraulic fracturing channels are opened on the outer tube between the first packer and the second packer corresponding to the hydraulic chamber along the length direction of the outer tube; there are multiple liquid nitrogen fracturing channels, and multiple liquid nitrogen fracturing channels are opened on the outer tube between the first packer and the second packer corresponding to the liquid nitrogen chamber along the length direction of the outer tube.

[0011] The above-mentioned hard coal rock composite structure modification and fracturing system in coal-bearing strata is characterized by: it also includes an industrial computer, a borehole peep instrument, and microseismic equipment and an acoustic emission instrument installed in the overburden layer, and the high-pressure pump and the hydraulic pump are controlled by the industrial computer.

[0012] The above-mentioned hard coal rock composite structure modification and fracturing system in coal-bearing strata is characterized in that there are slits at the locations of the fracturing nozzles, hydraulic fracturing channels and liquid nitrogen fracturing channels in the directional drilling hole.

[0013] At the same time, the present invention also discloses a method for modifying and fracturing the hard coal-rock composite structure in coal-bearing strata, which is characterized in that the method comprises the following steps:

[0014] Step 1: Determine the location of the coal seam overburden modification and fracturing area: collect mine geological materials and physical and mechanical parameters of the rock formation, analyze the detection data of the on-site microseismic equipment and acoustic emission instrument, and determine the location of the coal seam overburden modification and fracturing area;

[0015] Step 2: Determine drilling parameters: According to the actual working conditions of the mine and the collected data, the drilling parameters are determined in combination with numerical simulation of the actual working conditions. The drilling parameters include the spacing, aperture, angle and depth of the directional drilling.

[0016] Step 3: Obtain directional drilling: Use a drilling rig to drill a directional drilling hole into the roof rock layer, and use the drill rod rotary head to rotate and cut at the designed position to create cracks in the rock mass;

[0017] Step 4: Install the fracturing device and seal the hole: install the fracturing device at the designed position of the directional drilling hole, seal the borehole with a packer, and connect the fracturing device with the high-pressure pump and hydraulic pump in the coal seam tunnel;

[0018] Step 5: Calculate the rock fracture pressure and perform the initial water fracturing: According to the formula P = 1.04 × 10 -6 ρgh+2.34, calculate the rock fracture pressure P, in MPa, where ρ is the average density of the overburden, in 10 3 kg·m -3 , g is the acceleration of gravity, h is the burial depth, the unit is m;

[0019] According to the rock formation fracture pressure, the high-pressure pump is started to supply high-pressure water to the fracturing device, and the fracture is further expanded under the action of directional high water pressure until it is penetrated, thus completing the initial water fracturing;

[0020] At the same time, the vibrator is started to assist in vibration, so as to avoid the formation of rock damage and cracks. The vibration frequency of the vibrator is the natural frequency of the roof rock formation.

[0021] Step 6: Liquid nitrogen strengthens cracking of rock mass: After hydraulic fracturing is completed, the water content of the rock formation increases, which is conducive to liquid nitrogen cracking of the rock mass. The hydraulic pump is started to provide high-pressure liquid nitrogen to the fracturing device. The pressure of the injected liquid nitrogen is not greater than the water pressure. After the liquid nitrogen is injected, cold shock, water-ice phase change frost heave force and liquid nitrogen gasification expansion force are generated inside the coal rock mass, so that a porous structure is gradually formed inside the coal rock mass. The porous spaces are interconnected to form a fracture network and eventually change the internal structure of the coal mass, making the coal mass pores larger and the fractures more numerous, thereby further weakening the hard rock mass and strengthening the cracking of the rock mass;

[0022] Step 7, determine whether the modified fracturing effect of the hard coal-rock composite structure of the coal-bearing stratum is achieved: observe the water seepage of the nearby boreholes and roof, and combine the monitoring results of the microseismic equipment, acoustic emission instrument and borehole peep instrument to evaluate the modified fracturing effect of the hard coal-rock composite structure of the coal-bearing stratum. If the fracturing effect is ideal, the modified fracturing effect of the hard coal-rock composite structure of the coal-bearing stratum is completed; if the fracturing effect is not ideal, repeat steps 5 and 6 until the modified fracturing effect of the hard coal-rock composite structure of the coal-bearing stratum is achieved.

[0023] The above-mentioned method for modifying and fracturing the composite structure of hard coal rock in coal-bearing strata is characterized in that: in step two, the spacing of directional drilling holes is 10m~30m; the aperture of the directional drilling holes is 76mm, 89mm or 114mm; the angle of the directional drilling holes is 30°~60°, and the depth D of the directional drilling holes is calculated according to the formula D=L+S, wherein L is the influence range of the working face mining, and S is the single-wing expansion distance of the hydraulic fracture.

[0024] The above-mentioned method for modifying and fracturing the hard coal-rock composite structure of coal-bearing strata is characterized in that: in step five, the high-pressure pump is started, the sixth directional valve and the fourth directional valve are opened, the third directional valve and the hydraulic fracturing channel are closed, and water is injected into the hydraulic chamber through the high-pressure pump. At this time, high-pressure water is output through the hydraulic fracturing nozzle to hydraulically fracture the top rock mass. After the hydraulic fracturing of the top rock mass is completed, the fourth directional valve is closed, and the hydraulic fracturing channel is opened. At this time, high-pressure water is output through the hydraulic fracturing channel to perform segmented hydraulic fracturing on the side rock mass. After the hydraulic fracturing is completed, the high-pressure pump and the sixth directional valve are closed.

[0025] The above-mentioned method for modifying and fracturing the hard coal-rock composite structure of coal-bearing strata is characterized in that: in step six, the hydraulic pump is started, the fifth directional valve and the second directional valve are opened, the first directional valve and the liquid nitrogen fracturing channel are closed, and nitrogen is injected into the liquid nitrogen chamber through the hydraulic pump. At this time, high-pressure liquid nitrogen is output through the liquid nitrogen fracturing nozzle to perform liquid nitrogen fracturing on the top rock mass. After the liquid nitrogen fracturing of the top rock mass is completed, the second directional valve is closed, and the liquid nitrogen fracturing channel is opened. At this time, high-pressure liquid nitrogen is output through the liquid nitrogen fracturing channel to perform segmented liquid nitrogen fracturing on the side rock mass. After the liquid nitrogen fracturing is completed, the hydraulic pump and the fifth directional valve are closed.

[0026] The above-mentioned method for modifying and fracturing the hard coal-rock composite structure of coal-bearing strata is characterized in that: step five and step six can be carried out simultaneously, and the high-pressure pump and the hydraulic pump are started at the same time, the first directional valve, the second directional valve, the third directional valve, the fourth directional valve, the fifth directional valve and the sixth directional valve are opened, and the water pressure fracturing nozzle, the liquid nitrogen fracturing nozzle, the liquid nitrogen fracturing channel and the water pressure fracturing channel are closed. At this time, water and liquid nitrogen are respectively injected into the water pressure chamber and the liquid nitrogen chamber. At this time, water pressure and liquid nitrogen are output from the mixed fracturing nozzle to perform water pressure and liquid nitrogen mixed fracturing on the rock mass; after the mixed fracturing is completed, the high-pressure pump, the hydraulic pump, the fifth directional valve and the sixth directional valve are closed.

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] 1. The present invention can be used for hydraulic fracturing or liquid nitrogen fracturing alone, or the two can be used in conjunction with each other to perform fracturing treatment on rocks at different depths, thereby cutting off the stress propagation path, reducing surrounding rock stress, increasing rock layer permeability, and controlling the collapse of the hard roof, effectively preventing the safety hazards caused by surrounding rock stress concentration, large-area roof collapse, and gas outburst. After the hard overlying rock strata are modified and fractured, the weakened hard roof breaks and collapses in time under the action of mining stress. After the roof breaks, the overall support pressure level is reduced, and the stress distribution is more uniform, which is convenient for promotion and use.

[0029] 2. The high-frequency vibrator of the present invention is beneficial to weakening the integrity of the rock mass. The initial cracks are convenient for the implementation of hydraulic fracturing, providing a good working environment for hydraulic fracturing and helping to reduce the cost of rock fracturing. Hydraulic fracturing allows the pre-cracks to be further expanded, extended and connected under the action of water pressure to form large-scale directional hydraulic fractures, thereby achieving initial modification and fracturing. The temperature of the original rock stress field can be changed under the action of liquid nitrogen, and the liquid nitrogen cold shock can weaken the strength of the rock mass. At the same time, the water-ice phase change can aggravate the degree of rock damage and strengthen the hydraulic fracturing effect, thereby achieving the purpose of further intensifying the modification and fracturing of the hard coal-rock composite structure of the roof.

[0030] 3. The method adopted in the present invention can be coordinated with monitoring means such as mine microseismic equipment and borehole peep instruments to monitor the modification and fracturing process, so that construction parameters such as water pressure and hydraulic pressure can be adjusted and controlled in real time according to the monitoring feedback information during the construction process. After the system modification and fracturing is completed, the borehole peep instrument is used to detect the modification and fracturing effect of the rock formation in the current borehole and adjacent boreholes to verify the modification and fracturing effect.

[0031] To sum up, the present invention is novel and reasonable in design. It introduces liquid nitrogen fracturing and mechanically assisted fracturing on the basis of hydraulic fracturing and cleverly combines them. At the same time, it can independently adjust the fracturing means according to different working conditions, enriching the hard roof fracturing methods. It has the advantages of small engineering workload, low pollution, high safety and high work efficiency, strengthens the cracking effect, improves underground work efficiency, provides guarantee for safe and efficient underground mining, and is easy to promote and use.

[0032] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of a large-area suspended ceiling of a hard top plate of the present invention.

[0034] Figure 2 It is a schematic diagram of the modification and fracturing of the hard overburden rock formation according to the present invention.

[0035] Figure 3 It is a schematic diagram of the overall structure of the fracturing device of the present invention.

[0036] Figure 4 It is a flowchart of the method of the present invention.

[0037] Description of reference numerals:

[0038] 1- directional drilling; 2- fracturing device; 3- high pressure pump;

[0039] 4- hydraulic pump; 5- mounting seat; 6- outer tube;

[0040] 7-slit mouth; 8-T-shaped partition; 9-water pressure chamber;

[0041] 10-liquid nitrogen chamber; 11-water injection pipe; 12-nitrogen injection pipe;

[0042] 13-sixth directional valve; 14-fifth directional valve; 15-second packer;

[0043] 16-first packer; 17-hydraulic fracturing channel; 18-liquid nitrogen fracturing channel;

[0044] 19- vibrator; 20- water pressure fracturing nozzle; 21- liquid nitrogen fracturing nozzle;

[0045] 22-mixing cracking nozzle; 23-fourth directional valve; 24-second directional valve;

[0046] 25 - the third directional valve; 26 - the first directional valve. DETAILED DESCRIPTION

[0047] like Figures 2 to 3As shown, a composite structure modification and fracturing system for hard coal rock in coal-bearing strata of the present invention comprises a fracturing device 2 arranged in a directional borehole 1 extending into the fracturing area, and a high-pressure pump 3 and a hydraulic pump 4 arranged in a coal seam tunnel and injecting a high-pressure fracturing agent into the fracturing device 2. The fracturing device 2 comprises a mounting seat 5, an outer tube 6 and a fracturing nozzle 7 connected in sequence. A T-shaped partition plate 8 is arranged in the outer tube 6 to divide the inner cavity of the tube into a water pressure chamber 9 and a liquid nitrogen chamber 10. The mounting seat 5 is connected to a There is a water injection pipe 11 for connecting the water pressure chamber 9 and the high pressure pump 3, and a nitrogen injection pipe 12 for connecting the liquid nitrogen chamber 10 and the hydraulic pump 4. The fracturing nozzle 7 is provided with a water pressure fracturing nozzle 20, a liquid nitrogen fracturing nozzle 21 and a mixed fracturing nozzle 22 located between the water pressure fracturing nozzle 20 and the liquid nitrogen fracturing nozzle 21. The transverse partition of the T-shaped partition 8 extends into the fracturing nozzle 7. A first fracturing nozzle 22 is provided at the intersection of the top of the transverse partition and the liquid nitrogen fracturing nozzle 21 and the mixed fracturing nozzle 22. a directional valve 26, a second directional valve 24 is arranged at the intersection of the side wall of the transverse partition and the liquid nitrogen fracturing nozzle 21, a third directional valve 25 is arranged at the intersection of the top of the transverse partition and the hydraulic fracturing nozzle 20 and the mixed fracturing nozzle 22, a fourth directional valve 23 is arranged at the intersection of the side wall of the transverse partition and the hydraulic fracturing nozzle 20, a fifth directional valve 14 is arranged at the connection position between the nitrogen injection pipe 12 and the liquid nitrogen chamber 10, a sixth directional valve 13 is arranged at the connection position between the water injection pipe 11 and the hydraulic chamber 9, a vibrator 19 is installed on the outer wall of the outer tube 6 at the connection position between the outer tube 6 and the fracturing nozzle 7, a first packer 16 and a second packer 15 are sleeved on the outer side of the outer tube 6, a hydraulic fracturing channel 17 is opened on the outer tube 6 between the first packer 16 and the second packer 15 corresponding to the hydraulic chamber 9, and a liquid nitrogen fracturing channel 18 is opened on the outer tube 6 between the first packer 16 and the second packer 15 corresponding to the liquid nitrogen chamber 10.

[0048] In this embodiment, there are multiple hydraulic fracturing channels 17, and multiple hydraulic fracturing channels 17 are opened on the outer tube 6 between the first packer 16 and the second packer 15 corresponding to the hydraulic chamber 9 along the length direction of the outer tube 6. There are multiple liquid nitrogen fracturing channels 18, and multiple liquid nitrogen fracturing channels 18 are opened on the outer tube 6 between the first packer 16 and the second packer 15 corresponding to the liquid nitrogen chamber 10 along the length direction of the outer tube 6.

[0049] In this embodiment, an industrial computer, a borehole peep instrument, and microseismic equipment and an acoustic emission instrument installed in the overburden layer are also included. The high-pressure pump 3 and the hydraulic pump 4 are controlled by the industrial computer.

[0050] In this embodiment, there are slits at the locations of the fracturing nozzle 7, the hydraulic fracturing channel 17 and the liquid nitrogen fracturing channel 18 in the directional drill hole 1.

[0051] It should be noted that fracturing is performed on rocks at different depths to cut off the stress propagation path, reduce surrounding rock stress, increase rock permeability, control the collapse of the hard roof, and effectively prevent the safety hazards caused by surrounding rock stress concentration, large-area roof collapse, and gas outburst. After the hard overburden is modified and fractured, the weakened hard roof will break and collapse in time under the action of mining stress. After the roof is broken, the overall support pressure level is reduced and the stress distribution is more uniform; the high-frequency vibrator is conducive to weakening the integrity of the rock mass, and the initial cracks are convenient for hydraulic fracturing, providing a good working environment for hydraulic fracturing, which helps to reduce the cost of rock fracturing. Hydraulic fracturing allows the pre-cracks to be further expanded, extended, and connected under the action of water pressure, forming large-scale directional hydraulic fractures, thereby achieving initial modification and fracturing. The temperature of the original rock stress field can be changed under the action of liquid nitrogen, and the cold shock of liquid nitrogen can weaken the strength of the rock mass. At the same time, the water-ice phase change can aggravate the degree of rock damage and strengthen the hydraulic fracturing effect, so as to achieve the purpose of further aggravating the modification and fracturing of the hard coal-rock composite structure of the roof; the modification and fracturing process can be monitored in conjunction with monitoring means such as mine microseismic equipment and borehole peep instruments, so that the water pressure, hydraulic pressure and other construction parameters can be adjusted and controlled in real time according to the monitoring feedback information during the construction process. After the system modification and fracturing is completed, the borehole peep instrument is used to detect the modification and fracturing effect of the rock formation in the current borehole and adjacent boreholes, and the modification and fracturing effect is verified.

[0052] like Figure 4 A method for modifying and fracturing a hard coal-rock composite structure in a coal-bearing stratum is shown, comprising the following steps:

[0053] Step 1: Determine the location of the coal seam overburden modification and fracturing area: collect mine geological materials and physical and mechanical parameters of the rock formation, analyze the detection data of the on-site microseismic equipment and acoustic emission instrument, and determine the location of the coal seam overburden modification and fracturing area;

[0054] Step 2: Determine drilling parameters: According to the actual working conditions of the mine and the collected data, the drilling parameters are determined in combination with the numerical simulation of the actual working conditions. The drilling parameters include the spacing, aperture, angle and depth of the directional drilling holes 1;

[0055] Step 3: Obtain a directional borehole: Use a drilling rig to drill a directional borehole 1 into the roof rock layer, and use the drill pipe rotary head to rotate and cut at the designed position to generate cracks in the rock mass;

[0056] Step 4: Install the fracturing device and seal the hole: install the fracturing device 2 at the designed position of the directional drilling hole 1, seal the hole with a packer, and connect the fracturing device 2 with the high-pressure pump 3 and the hydraulic pump 4 in the coal seam tunnel;

[0057] Step 5: Calculate the rock fracture pressure and perform the initial water fracturing: According to the formula P = 1.04 × 10 -6ρgh+2.34, calculate the rock fracture pressure P, in MPa, where ρ is the average density of the overburden, in 10 3 kg·m -3 , g is the acceleration of gravity, h is the burial depth, the unit is m;

[0058] According to the rock formation fracture pressure, the high-pressure pump 3 is started to supply high-pressure water to the fracturing device 2, and the fracture is further expanded under the action of the directional high water pressure until it is penetrated, thereby completing the initial water fracturing;

[0059] At the same time, the vibrator 19 is started to assist in vibration, so as to facilitate the generation of rock damage and cracks. The vibration frequency of the vibrator 19 is the natural frequency of the roof rock formation;

[0060] Step 6: Liquid nitrogen strengthens cracking of rock mass: After hydraulic fracturing is completed, the water content of the rock formation increases, which is conducive to liquid nitrogen cracking of the rock mass. The hydraulic pump 4 is started to provide high-pressure liquid nitrogen to the fracturing device 2. The pressure of the injected liquid nitrogen is not greater than the water pressure. After the liquid nitrogen is injected, cold shock, water-ice phase change frost heave force and liquid nitrogen gasification expansion force are generated inside the coal rock mass, so that a porous structure is gradually formed inside the coal rock mass. The multiple pores are interconnected to form a fracture network and finally change the internal structure of the coal mass, so that the pores of the coal mass become larger and the fractures increase, thereby further weakening the hard rock mass and strengthening the cracking of the rock mass;

[0061] Step 7, determine whether the modified fracturing effect of the hard coal-rock composite structure of the coal-bearing stratum is achieved: observe the water seepage of the nearby boreholes and roof, and combine the monitoring results of the microseismic equipment, acoustic emission instrument and borehole peep instrument to evaluate the modified fracturing effect of the hard coal-rock composite structure of the coal-bearing stratum. If the fracturing effect is ideal, the modified fracturing effect of the hard coal-rock composite structure of the coal-bearing stratum is completed; if the fracturing effect is not ideal, repeat steps 5 and 6 until the modified fracturing effect of the hard coal-rock composite structure of the coal-bearing stratum is achieved.

[0062] In this embodiment, in step 2, the spacing of the directional drilling holes 1 is 10m to 30m; the aperture of the directional drilling holes 1 is 76mm, 89mm or 114mm; the angle of the directional drilling holes 1 is 30° to 60°, and the depth D of the directional drilling holes 1 is calculated according to the formula D=L+S, wherein L is the impact range of the working face mining, and S is the single-wing expansion distance of the hydraulic fracture.

[0063] In this embodiment, in step five, the high-pressure pump 3 is started, the sixth directional valve 13 and the fourth directional valve 23 are opened, the third directional valve 25 and the hydraulic fracturing channel 17 are closed, and water is injected into the hydraulic chamber 9 through the high-pressure pump. At this time, the high-pressure water is output through the hydraulic fracturing nozzle 20 to hydraulically fracture the top rock mass. After the hydraulic fracturing of the top rock mass is completed, the fourth directional valve 23 is closed, and the hydraulic fracturing channel 17 is opened. At this time, the high-pressure water is output through the hydraulic fracturing channel 17 to perform segmented hydraulic fracturing on the side rock mass. After the hydraulic fracturing is completed, the high-pressure pump 3 and the sixth directional valve 13 are closed. ; In step six, the hydraulic pump 4 is started, the fifth directional valve 14 and the second directional valve 24 are opened, the first directional valve 26 and the liquid nitrogen fracturing channel 18 are closed, and nitrogen is injected into the liquid nitrogen chamber 10 through the hydraulic pump 4. At this time, high-pressure liquid nitrogen is output through the liquid nitrogen fracturing nozzle 21 to perform liquid nitrogen fracturing on the top rock mass. After the liquid nitrogen fracturing of the top rock mass is completed, the second directional valve 24 is closed, and the liquid nitrogen fracturing channel 18 is opened. At this time, high-pressure liquid nitrogen is output through the liquid nitrogen fracturing channel 18 to perform segmented liquid nitrogen fracturing on the side rock mass. After the liquid nitrogen fracturing is completed, the hydraulic pump 4 and the fifth directional valve 14 are closed. After hydraulic fracturing, the water content of the rock formation increases, which is helpful for liquid nitrogen to crack the rock. Therefore, liquid nitrogen is injected into the borehole after hydraulic fracturing to produce cold shock and crystal phase change on the rock, further weakening the rock and enhancing the fracturing effect. During the rock fracturing process, a microseismic monitor can be used to monitor the fracturing process in real time. After the fracturing is completed, the rock fracturing effect can be observed through microseismic and borehole peep instruments, and the fracturing effect can be evaluated with multiple mechanical and physical parameters. If the fracturing effect is not ideal, hydraulic pressure and liquid nitrogen cycle fracturing can be carried out again until the fracturing effect is achieved.

[0064] In this embodiment, step five and step six can be performed simultaneously, and the high-pressure pump and the hydraulic pump are started at the same time, the first directional valve 26, the second directional valve 24, the third directional valve 25, the fourth directional valve 23, the fifth directional valve 14 and the sixth directional valve 13 are opened, and the water pressure fracturing nozzle 20, the liquid nitrogen fracturing nozzle 21, the liquid nitrogen fracturing channel 18 and the water pressure fracturing channel 17 are closed. At this time, water and liquid nitrogen are respectively injected into the water pressure chamber 9 and the liquid nitrogen chamber 10. At this time, water pressure and liquid nitrogen are output from the mixed fracturing nozzle 22 to perform water pressure and liquid nitrogen mixed fracturing on the rock mass; after the mixed fracturing is completed, the high-pressure pump, the hydraulic pump, the fifth directional valve 14 and the sixth directional valve 13 are closed.

[0065] When the present invention is used, according to different working conditions and different overlying rock properties, the method can be used to perform hydraulic fracturing or liquid nitrogen fracturing alone, or the two can be used in coordination to perform cyclic fracturing. During the fracturing process, the fracturing sequence can be adjusted according to actual conditions. The two methods can also be combined to perform fracturing treatment on roof rock layers at different depths by mixing water pressure and liquid nitrogen, so as to achieve the purpose of in-situ modification and fracturing of the hard coal-rock composite structure of the coal-bearing strata.

[0066] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent structural change made to the above embodiment based on the technical essence of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A composite structure modification and fracturing system for hard coal rock in coal-bearing strata, characterized by: The invention comprises a fracturing device (2) arranged in a directional drill hole (1) extending into a fracturing area, and a high-pressure pump (3) and a hydraulic pump (4) arranged in a coal seam tunnel and injecting a high-pressure fracturing agent into the fracturing device (2). The fracturing device (2) comprises a mounting seat (5), an outer tube (6) and a fracturing nozzle (7) which are connected in sequence. A T-shaped partition (8) is arranged in the outer tube (6) to divide the inner cavity of the tube into a water pressure cavity (9) and a liquid nitrogen cavity (10). The mounting seat (5) is connected to a water injection pipe (10) for connecting the water pressure cavity (9) and the high-pressure pump (3). 1), and a nitrogen injection pipe (12) for connecting the liquid nitrogen chamber (10) and the hydraulic pump (4); the fracturing nozzle (7) is provided with a water pressure fracturing nozzle (20), a liquid nitrogen fracturing nozzle (21), and a mixed fracturing nozzle (22) located between the water pressure fracturing nozzle (20) and the liquid nitrogen fracturing nozzle (21); a transverse partition of the T-shaped partition (8) extends into the fracturing nozzle (7); a first directional valve (26) is provided at the top of the transverse partition and the intersection position of the liquid nitrogen fracturing nozzle (21) and the mixed fracturing nozzle (22); A second directional valve (24) is provided at the intersection of the side wall of the transverse partition and the liquid nitrogen fracturing nozzle (21); a third directional valve (25) is provided at the intersection of the top of the transverse partition and the water pressure fracturing nozzle (20) and the mixed fracturing nozzle (22); a fourth directional valve (23) is provided at the intersection of the side wall of the transverse partition and the water pressure fracturing nozzle (20); a fifth directional valve (14) is provided at the connection position between the nitrogen injection pipe (12) and the liquid nitrogen chamber (10); and a fifth directional valve (14) is provided at the connection position between the water injection pipe (11) and the water pressure chamber (9). A sixth directional valve (13) is provided with a vibrator (19) on the outer wall of the connection position between the outer tube (6) and the fracturing nozzle (7); a first packer (16) and a second packer (15) are sleeved on the outer side of the outer tube (6); a hydraulic fracturing channel (17) is provided on the outer tube (6) between the first packer (16) and the second packer (15) corresponding to the hydraulic chamber (9); and a liquid nitrogen fracturing channel (18) is provided on the outer tube (6) between the first packer (16) and the second packer (15) corresponding to the liquid nitrogen chamber (10).

2. A composite structure modification and fracturing system for hard coal rock in coal-bearing strata according to claim 1, characterized in that: The number of the hydraulic fracturing channels (17) is multiple, and the multiple hydraulic fracturing channels (17) are opened on the outer tube (6) between the first packer (16) and the second packer (15) corresponding to the hydraulic chamber (9) along the length direction of the outer tube (6); the number of the liquid nitrogen fracturing channels (18) is multiple, and the multiple liquid nitrogen fracturing channels (18) are opened on the outer tube (6) between the first packer (16) and the second packer (15) corresponding to the liquid nitrogen chamber (10) along the length direction of the outer tube (6).

3. A composite structure modification and fracturing system for hard coal rock in coal-bearing strata according to claim 1, characterized in that: It also includes an industrial computer, a borehole peep instrument, and microseismic equipment and an acoustic emission instrument installed in the overburden layer. The high-pressure pump (3) and the hydraulic pump (4) are controlled by the industrial computer.

4. A composite structure modification and fracturing system for hard coal rock in coal-bearing strata according to claim 1, characterized in that: The directional drill hole (1) has slots at the locations of the fracturing nozzle (7), the hydraulic fracturing channel (17) and the liquid nitrogen fracturing channel (18).

5. A method for modifying and fracturing hard coal-rock composite structures in coal-bearing strata using the system as claimed in claim 3, characterized in that: The method comprises the following steps: Step 1: Determine the location of the coal seam overburden modification and fracturing area: collect mine geological materials and physical and mechanical parameters of the rock formation, analyze the detection data of the on-site microseismic equipment and acoustic emission instrument, and determine the location of the coal seam overburden modification and fracturing area; Step 2: Determine drilling parameters: Determine drilling parameters based on the actual working conditions of the mine and the collected data in combination with numerical simulation of the actual working conditions, wherein the drilling parameters include the spacing, aperture, angle and depth of the directional drilling (1); Step 3: Obtain a directional borehole: Use a drilling rig to drill a directional borehole (1) into the roof rock layer, and use the drill rod rotary head to rotate and cut at the designed position to generate cracks in the rock mass; Step 4: Install the fracturing device and seal the hole: install the fracturing device (2) at the designed position of the directional drilling hole (1), seal the borehole with a packer, and connect the fracturing device (2) with the high-pressure pump (3) and the hydraulic pump (4) in the coal seam tunnel; Step 5: Calculate the rock fracture pressure and perform the initial water fracturing: According to the formula P = 1.04 × 10 -6 ρgh+2.34, calculate the rock fracture pressure P, in MPa, where ρ is the average density of the overburden, in 10 3 kg·m -3 , g is the acceleration of gravity, h is the burial depth, the unit is m; A high-pressure pump (3) is started according to the rock formation fracture pressure to supply high-pressure water to the fracturing device (2), and the fracture is further expanded under the action of the directional high water pressure until it is penetrated, thereby completing the initial water fracturing; At the same time, a vibrator (19) is started to assist in vibration, so as to facilitate the generation of rock formation damage and cracks, and the vibration frequency of the vibrator (19) is the natural frequency of the roof rock formation; Step 6: Liquid nitrogen strengthens cracking of rock mass: After hydraulic fracturing is completed, the water content of the rock formation increases, which is conducive to liquid nitrogen cracking of the rock mass. The hydraulic pump (4) is started to provide high-pressure liquid nitrogen to the fracturing device (2). The pressure of the injected liquid nitrogen is not greater than the water pressure. After the liquid nitrogen is injected, cold shock, water-ice phase change frost heave force and liquid nitrogen gasification expansion force are generated inside the coal rock mass, so that a porous structure is gradually formed inside the coal rock mass. The multiple pores are interconnected to form a fracture network and finally change the internal structure of the coal mass, so that the pores of the coal mass become larger and the fractures increase, thereby further weakening the hard rock mass and strengthening the cracking of the rock mass; Step 7, determine whether the modified fracturing effect of the hard coal-rock composite structure of the coal-bearing stratum is achieved: observe the water seepage of the nearby boreholes and roof, and combine the monitoring results of the microseismic equipment, acoustic emission instrument and borehole peep instrument to evaluate the modified fracturing effect of the hard coal-rock composite structure of the coal-bearing stratum. If the fracturing effect is ideal, the modified fracturing effect of the hard coal-rock composite structure of the coal-bearing stratum is completed; if the fracturing effect is not ideal, repeat steps 5 and 6 until the modified fracturing effect of the hard coal-rock composite structure of the coal-bearing stratum is achieved.

6. The method for modifying and fracturing hard coal-rock composite structures in coal-bearing strata according to claim 5, characterized in that: In step 2, the spacing of the directional drill holes (1) is 10m to 30m; the aperture of the directional drill holes (1) is 76mm, 89mm or 114mm; the angle of the directional drill holes (1) is 30° to 60°, and the depth D of the directional drill holes (1) is calculated according to the formula D=L+S, wherein L is the impact range of the working face mining, and S is the single-wing expansion distance of the hydraulic fracture.

7. The method for modifying and fracturing hard coal-rock composite structures in coal-bearing strata according to claim 5, characterized in that: In step five, the high-pressure pump (3) is started, the sixth directional valve (13) and the fourth directional valve (23) are opened, the third directional valve (25) and the hydraulic fracturing channel (17) are closed, and water is injected into the hydraulic chamber (9) through the high-pressure pump. At this time, the high-pressure water is output through the hydraulic fracturing nozzle (20) to hydraulically fracture the top rock mass. After the hydraulic fracturing of the top rock mass is completed, the fourth directional valve (23) is closed, and the hydraulic fracturing channel (17) is opened. At this time, the high-pressure water is output through the hydraulic fracturing channel (17) to perform segmented hydraulic fracturing on the side rock mass. After the hydraulic fracturing is completed, the high-pressure pump (3) and the sixth directional valve (13) are closed.

8. The method for modifying and fracturing the hard coal-rock composite structure in coal-bearing strata according to claim 5, characterized in that: In step six, the hydraulic pump (4) is started, the fifth directional valve (14) and the second directional valve (24) are opened, the first directional valve (26) and the liquid nitrogen fracturing channel (18) are closed, and nitrogen is injected into the liquid nitrogen chamber (10) through the hydraulic pump (4). At this time, high-pressure liquid nitrogen is output through the liquid nitrogen fracturing nozzle (21) to perform liquid nitrogen fracturing on the top rock mass. After the liquid nitrogen fracturing of the top rock mass is completed, the second directional valve (24) is closed, and the liquid nitrogen fracturing channel (18) is opened. At this time, high-pressure liquid nitrogen is output through the liquid nitrogen fracturing channel (18) to perform segmented liquid nitrogen fracturing on the side rock mass. After the liquid nitrogen fracturing is completed, the hydraulic pump (4) and the fifth directional valve (14) are closed.

9. The method for modifying and fracturing hard coal-rock composite structures in coal-bearing strata according to claim 5, characterized in that: Step five and step six can be performed simultaneously, and the high-pressure pump and the hydraulic pump are started simultaneously, the first directional valve (26), the second directional valve (24), the third directional valve (25), the fourth directional valve (23), the fifth directional valve (14) and the sixth directional valve (13) are opened, and the water pressure fracturing nozzle (20), the liquid nitrogen fracturing nozzle (21), the liquid nitrogen fracturing channel (18) and the water pressure fracturing channel (17) are closed. At this time, water and liquid nitrogen are respectively injected into the water pressure chamber (9) and the liquid nitrogen chamber (10). At this time, water pressure and liquid nitrogen are output from the mixed fracturing nozzle (22) to perform water pressure and liquid nitrogen mixed fracturing on the rock mass; after the mixed fracturing is completed, the high-pressure pump, the hydraulic pump, the fifth directional valve (14) and the sixth directional valve (13) are closed.

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