A method for hydraulic fracturing of large-area suspended roof in coal mine goaf

Abstract

The present invention relates to the technical field of mine goaf disposal, and in particular to a method for hydraulic fracturing of large-scale suspended roof in coal mine goaf, comprising the steps of using transient electromagnetic geophysical exploration technology to determine the scope of suspended roof goaf and the scope of collapsed goaf in room-and-pillar goaf; determining the sequence of coal pillar fracturing, determining the fracturing layer and the fracturing sequence, drilling holes from the ground to the coal pillars in the suspended roof goaf; performing hydraulic fracturing according to the designed sequence; and using transient electromagnetic geophysical exploration technology to test the effect of hydraulic fracturing on suspended roof. The present invention uses transient electromagnetic geophysical exploration technology to accurately explore the scope of suspended roof goaf in room-and-pillar goaf, thus avoiding blind treatment. The present invention hydraulically fractures the coal pillars in the suspended roof goaf from the ground, destroying the overall structure of the coal pillars and causing the suspended roof goaf to collapse, thereby eliminating the hazard of suspended roof in the room-and-pillar goaf. Compared with traditional filling grouting methods, the present invention has the advantages of low cost, fast construction, safety, no pollution, and significant effect.

Description

Technical Field

[0001] This invention relates to the field of mine goaf treatment technology, and in particular to a method for large-area suspended roof hydraulic pressure cracking of coal mine goaf. Background Art

[0002] When a hard, thick roof exists above a coal seam in a coal mine, the roof may not collapse promptly after the working face is mined, resulting in large areas of overhanging roofs in the goaf. This phenomenon is more pronounced in room-and-pillar mining. Because room-and-pillar goafs leave behind numerous coal pillars, these pillars weather and undergo rheological changes over time, reducing their strength. These weakened pillars are then damaged by subsequent mining activities, triggering a chain reaction. Without the support of coal pillars, the roof of a room-and-pillar goaf will rapidly subside and fracture, causing large-scale collapses and generating dynamic disasters such as rock bursts, coal and gas outbursts, and even mine tremors and hurricanes.

[0003] Current methods for resolving large-area roof overhangs in goaf areas include blasting and grouting. Blasting involves large engineering work and explosive quantities, poor safety, high costs, and pollution of underground air, and may also affect normal mining activities. Grouting requires specific grouting materials and equipment, has a long cycle time and high cost, and is unsuitable for coal mines with multiple large-area roof overhangs. Summary of the Invention

[0004] To address the problem of large-area roof overhang in room-and-pillar goaf areas of coal mines, and to explore effective methods for managing this issue, this invention proposes a hydraulic fracturing method for large-area roof overhang in coal mine goaf areas. This method optimizes and integrates technologies such as transient electromagnetic geophysical exploration and hydraulic fracturing. Through a "probe-fracturing-verification" technical process, it ultimately eliminates large-area roof overhang in room-and-pillar goaf areas of coal mines. The specific steps include:

[0005] Step S1: Arrange transient electromagnetic probe lines above the room-pillar goaf area, and use transient electromagnetic geophysical exploration technology to detect the situation of the room-pillar goaf area and obtain the apparent resistivity cross-sectional diagram of each transient electromagnetic probe line.

[0006] Step S2: Based on the analysis of transient electromagnetic geophysical exploration results, determine the range of the suspended goaf and the range of the collapsed goaf in the room-and-pillar goaf.

[0007] S3, determine the fracturing sequence of coal pillars. In each suspended goaf area, the fracturing sequence of coal pillars is as follows: determine the center of the suspended goaf area, first fracturing the coal pillar at the boundary of the suspended goaf area that is farthest from the center of the suspended goaf area, and then fracturing the coal pillar that is closest to the previous fracturing coal pillar.

[0008] Step S4: Determine the fracturing layers and fracturing sequence. In each suspended goaf area, determine the location and number of horizontal fracturing layers for each coal pillar based on the basic conditions of the coal pillars within it. For each coal pillar, fracturing is carried out on all designed fracturing layers in a bottom-up order.

[0009] Step S5: In each suspended goaf area, drill holes from the ground towards the coal pillar of the suspended goaf area according to the coal pillar fracturing sequence.

[0010] Step S6: Lower the high-pressure water injection steel pipe with packer into the borehole, and lower the packer to the designed fracturing layer at the bottom of the coal pillar. Then connect the other end of the water injection steel pipe to the hydraulic fracturing equipment.

[0011] Step S7: Perform hydraulic fracturing on the lowest designed fracturing layer;

[0012] Step S8: Move the water injection steel pipe upward so that the packer is located at the upper layer of the designed fracturing layer, and fracturing the upper layer of the designed fracturing layer. In this way, all the designed fracturing layers are fracturing in sequence from bottom to top until the fracturing of all coal pillars in the suspended goaf is completed.

[0013] S9 again employed transient electromagnetic geophysical exploration technology to investigate the situation in the room-and-pillar goaf and verify the effectiveness of hydraulic fracturing to address the roof collapse.

[0014] Preferably, in step S3, when there are multiple coal pillars that are equidistant from the previous fracturing coal pillar, the coal pillar that is farther from the center of the overhanging goaf is fracturing first.

[0015] Preferably, in step S4, the number and location of fracturing layers are determined according to the height and hardness of the coal pillar, and a fracturing layer is designed every 0.8 to 2.0 m.

[0016] Preferably, in step S4, all fracturing layers divide the coal pillar into several segments, with the lowest segment having the smallest height, the uppermost segment having the next largest height, and the remaining middle segments having relatively larger heights.

[0017] Preferably, in step S6, the hydraulic fracturing equipment is vehicle-mounted and includes a water tank, a high-pressure water pump, a pressure sensor, a three-way valve, a pressure relief valve, and a drain outlet connected in sequence. The three ends of the three-way valve are respectively connected to the pressure sensor, the pressure relief valve, and the top of the high-pressure water injection steel pipe.

[0018] Preferably, in step S7, the high-pressure water pump is first started, and low-pressure water injection is performed first. When the packer is fully expanded, the pressure relief valve is quickly closed; high-pressure water injection is started to fracturing the coal pillar in the goaf; the pressure is gradually reduced, the high-pressure water pump is turned off, and the pressure relief valve is opened.

[0019] Beneficial effects: This invention uses transient electromagnetic geophysical exploration technology to accurately explore the extent of the overhanging goaf in room-and-pillar goaf, avoiding blind remediation. This invention performs hydraulic fracturing on the coal pillars in the overhanging goaf from the ground, forming a uniform and complex fracture network within the coal pillars, destroying the overall structure of the coal pillars, causing the overhanging goaf to collapse, thereby eliminating the overhanging hazard of room-and-pillar goaf. Compared with traditional filling and grouting methods, it has advantages such as low cost, fast construction, safety, no pollution, and significant effect. Attached Figure Description

[0020] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the following detailed description to explain the invention, but do not constitute a limitation thereof.

[0021] Figure 1 This is a schematic diagram of a method for hydraulic pressure cracking of large-area suspended roof in goaf areas in a specific embodiment of the present invention;

[0022] Figure 2 This is a diagram showing the fracturing sequence of the CK1 coal pillar in the goaf in a specific embodiment of the present invention;

[0023] Figure 3 This is a diagram illustrating the treatment effect of the hydraulic pressure cracking method for large-area suspended roof in goaf areas in a specific embodiment of the present invention.

[0024] In the attached diagram: 1-overlying strata, 2-main roof, 3-direct roof, 4-room-pillar goaf, 5-goaf space, 6-coal pillar, 7-borehole, 8-water tank, 9-high pressure water pump, 10-pressure sensor, 11-pressure relief valve, 12-drainage outlet, 13-high pressure water injection steel pipe, 14-packer, 15-three-way valve, 16-collapse zone. Detailed Implementation

[0025] The technical solution of the present invention will now be described in more detail with reference to the accompanying drawings of specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0026] A room-and-pillar goaf exists underground in a certain mine. The original coal seam mined in this goaf was No. 3 coal seam, with a burial depth of 120m and a mining thickness of 3.8m. The mined coal room ( Figure 1 The goaf 5) is 6m wide and contains residual coal pillars. Figure 1 The coal pillar 6) is 8m wide. This room-and-pillar goaf has a suspended roof and a collapsed roof section. In response to the suspended roof section, this invention proposes a hydraulic pressure cracking method for large-area suspended roof in coal mine goaf. Through the technical process of "exploration-pressure-testing", the safety hazard of large-area suspended roof in the room-and-pillar goaf of this mine is finally eliminated.

[0027] The following is combined Figures 1 to 3 This invention describes a method for large-area suspended roof hydraulic pressure cracking in coal mine goaf areas, specifically including the following steps:

[0028] Step S1: Transient electromagnetic probe lines are arranged above the room-and-pillar goaf 4 of the mine. The transient electromagnetic geophysical exploration technology is used to detect the condition of the room-and-pillar goaf 4 and obtain the apparent resistivity cross-sectional diagram of each transient electromagnetic probe line.

[0029] The basic grid of the transient electromagnetic detection line is 40m×20m (40m line spacing and 20m point spacing). A total of 29 transient electromagnetic detection lines and 1728 transient electromagnetic detection points are arranged. A transient electromagnetic instrument is used to conduct periodic detection at each transient electromagnetic detection point. After the detection results are completed, the results are statistically analyzed and the apparent resistivity cross-section of each transient electromagnetic detection line is drawn.

[0030] For the obtained apparent resistivity profile, the formation condition is judged according to the following criteria: Unmined, normal formation (e.g., ...). Figure 1 The resistivity distribution of the unbroken overlying strata 1, the basic roof 2, and the immediate roof 3 above the middle coal seam exhibits a distinct, complete, and continuous horizontal layered characteristic; while the suspended roof in the goaf shows a circular or transversely elliptical extremely high resistivity (reference). Figure 1 (As shown in the figure); when the goaf collapses, the resistivity exhibits a medium to low resistivity (refer to the case). Figure 3 (As shown in the example).

[0031] Step S2: Based on the analysis of transient electromagnetic geophysical exploration results, determine the range of the suspended roof goaf and the range of the collapsed goaf in the room-and-pillar goaf 4. The suspended roof goaf refers to the stable coal pillar area in the room-and-pillar goaf 4, and the collapsed goaf refers to the area where the roof collapses due to the instability of the coal pillar in the room-and-pillar goaf 4.

[0032] In this embodiment, a total of 7 overhanging goaf areas were detected in the mine. Each overhanging goaf area was numbered as CK1 to CK7. The number of coal pillars in each of the overhanging goaf areas CK1 to CK7 was counted, and the location of the coal pillars was marked on the ground directly above their center to facilitate subsequent drilling.

[0033] Taking the suspended goaf area CK1 as an example, such as Figure 2 As shown, there are 8 coal pillars in the suspended goaf CK1, and 8 marks are made at the corresponding locations on the ground. Other suspended goaf areas have 5-20 coal pillars respectively.

[0034] S3, Determine the fracturing sequence of coal pillars: In each suspended goaf area, the fracturing sequence of coal pillars is as follows: Determine the center of the suspended goaf area, first fracturing the coal pillar at the boundary of the suspended goaf area that is farthest from the center of the suspended goaf area, then fracturing the coal pillar that is closest to the previous fracturing coal pillar. When there are multiple coal pillars that are the same distance from the previous fracturing coal pillar, prioritize fracturing the coal pillar that is farther from the center of the suspended goaf area, until all coal pillars are fracturing.

[0035] Taking the suspended goaf area CK1 as an example, such as Figure 2 As shown, hydraulic fracturing was performed on the internal coal pillars in sequence according to markings ①-⑧.

[0036] Step S4, Determine the fracturing layers: Within each suspended goaf, determine the location and number of horizontal fracturing layers for each coal pillar based on its basic coal pillar conditions. Within each suspended goaf, fracturing is performed on each coal pillar sequentially from bottom to top, using all designed fracturing layers. The number and location of fracturing layers are determined based on the height and hardness of the coal pillar. For harder coal, the number of fracturing layers can be appropriately increased; for softer coal, the number can be appropriately reduced. Generally, a fracturing layer is designed every 0.8–2.0 m. In this embodiment, two fracturing layers are designed along the height of the coal pillar. These fracturing layers roughly divide the coal pillar 6 into three segments along its height, with heights of 1.0 m, 1.8 m, 1.0 m or 0.8 m, 1.8 m, 1.2 m respectively from bottom to top.

[0037] Step S5: In each suspended goaf area, drill holes 7 from the ground to the suspended coal pillar 6 at the marked location according to the coal pillar fracturing sequence. Drill holes 7 are drilled to the bottom surface of the coal pillar 6. The diameter of the drill holes 7 is not less than 110mm. The drill holes 7 are drilled vertically with an error of no more than 5°.

[0038] Taking the suspended goaf area CK1 as an example, such as Figure 2 As shown, boreholes 7 are drilled sequentially from the ground to the coal pillar 6 at marked locations ①-⑧. Each borehole 7 is drilled to the bottom surface of its corresponding coal pillar. The borehole diameter is φ113mm. The boreholes are made as perpendicular as possible to the horizontal cross-section of the coal pillar to ensure the fracturing effect. During the drilling process, necessary coring work is carried out to obtain formation samples. Through measurement and monitoring, it is ensured that the boreholes hit the coal pillar in the goaf.

[0039] Step S6: Lower the high-pressure water injection steel pipe 13 with packer 14 into the borehole 7, and lower the packer 14 to the designed fracturing layer at the lowest point of the coal pillar 6; then connect the other end of the high-pressure water injection steel pipe 13 to the hydraulic fracturing equipment; as shown Figure 1As shown, the hydraulic fracturing equipment is vehicle-mounted and includes a water tank 8, a high-pressure water pump 9, a pressure sensor 10, a three-way valve 15, a pressure relief valve 11, and a drain outlet 12 connected in sequence. The three ends of the three-way valve 15 are connected to the top of the high-pressure water injection steel pipe 13. The water tank 10 has a capacity of not less than 10 m³. 3 The water temperature in the water tank 10 is 10-25℃; the high-pressure water pump 9 can provide a pressure of more than 40MPa; the pressure sensor 10 can monitor the pressure of the water flow in the high-pressure water injection steel pipe 13 in real time and clearly display the reading; the high-pressure water injection steel pipe 13 is selected with a diameter of φ90mm, and a packer 14 is connected to the bottom end of the high-pressure water injection steel pipe 13.

[0040] Step S7: Perform hydraulic fracturing operation. First, start the high-pressure water pump 9 to perform low-pressure water injection, ensuring the water pressure is below 10MPa. This allows the water in the water tank 8 to be forced into the high-pressure water injection steel pipe 13, gradually flowing along the high-pressure water injection steel pipe 13 to the packer 14. The pressure in the packer 14 gradually increases and it gradually expands. When the packer 14 is fully expanded, the pressure relief valve 11 is quickly closed. The high-pressure water pump 9 starts high-pressure water injection. Observe the pressure sensor 10 and maintain the water pressure fluctuating within the range of 25MPa to 35MPa. The high-pressure water begins to fracturing the coal pillar 6.

[0041] The pressure was continuously increased for 30 to 60 minutes, causing horizontal cracks to form in coal pillar 6 and extend continuously. Then the pressure of high-pressure water pump 9 was gradually reduced, and finally high-pressure water pump 9 was turned off. The pressure relief valve 11 was opened to allow excess water to flow out through the drain outlet 12, and the packer 14 was unsealed. The fracturing work of the lowest fracturing layer was completed.

[0042] Step S8, as follows Figure 1 As shown, the water injection steel pipe 13 is moved upward so that the packer 14 is located at the upper layer of the designed fracturing layer. Referring to steps S5-S6, the upper layer of the designed fracturing layer is fracturing. In this way, all the designed fracturing layers are fracturing in sequence from bottom to top until all the fracturing layers of the coal pillar 6 are completed. This forms a uniform and complex fracture network in the coal pillar 6, destroys the overall structure of the coal pillar 6, promotes the collapse of the roof, and forms coal and rock fragments, i.e., the collapse zone 16.

[0043] The process of high-pressure water fracturing coal pillar is as follows: after high-pressure water enters the borehole, the high-pressure impact causes the coal pillar to fracture at the designed layer, forming a main fracture. The main fracture will extend outward with the continuous injection of fracturing fluid. Secondary fractures will be generated around each main fracture. The secondary fractures will continue to expand and connect, thus forming a complex fracture network that destroys the overall structure of coal pillar 6.

[0044] S9. Transient electromagnetic geophysical exploration technology is used again to detect the situation of the coal mine's pillar-type goaf, and to test the effect of hydraulic fracturing on the roof suspension. The arrangement of the transient electromagnetic probe lines (including the probe lines and probe points) is the same as in step S1. The obtained apparent resistivity cross-sectional diagram is compared with the one detected in step S1 to determine the treatment effect of the large-area roof suspension problem in the coal mine's pillar-type goaf.

[0045] The above description of the disclosed embodiments is presented in a progressive manner to enable those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for hydraulic pressure cracking of large-area suspended roofs in coal mine goaf areas, characterized in that, The following steps are involved: Step S1: Arrange transient electromagnetic probe lines above the room-pillar goaf area, and use transient electromagnetic geophysical exploration technology to detect the situation of the room-pillar goaf area and obtain the apparent resistivity cross-sectional diagram of each transient electromagnetic probe line. Step S2: Based on the analysis of transient electromagnetic geophysical exploration results, determine the range of the suspended goaf and the range of the collapsed goaf in the room-and-pillar goaf. Step S3: Determine the fracturing sequence of coal pillars. Within each suspended goaf, the fracturing sequence of coal pillars is as follows: Determine the center of the suspended goaf, first fracturing the coal pillar furthest from the center of the suspended goaf at the boundary of the suspended goaf, and then fracturing the coal pillar closest to the previous fracturing coal pillar; when there are multiple coal pillars with the same distance from the previous fracturing coal pillar, prioritize fracturing the coal pillar furthest from the center of the suspended goaf. Step S4: Determine the fracturing layers and fracturing sequence. Within each suspended goaf, determine the location and number of horizontal fracturing layers for each coal pillar based on the basic conditions of the coal pillar. For each coal pillar, fracturing is carried out on all designed fracturing layers in a bottom-up order. All fracturing layers divide the coal pillar into several segments, with the lowest segment having the smallest height, the highest segment having the next largest height, and the remaining middle segments having relatively larger heights. Step S5: In each suspended goaf area, drill holes from the ground towards the coal pillar of the suspended goaf area according to the coal pillar fracturing sequence. Step S6: Lower the high-pressure water injection steel pipe with packer into the borehole, and lower the packer to the designed fracturing layer at the bottom of the coal pillar; then connect the other end of the water injection steel pipe to the hydraulic fracturing equipment. Step S7: Perform hydraulic fracturing on the lowest designed fracturing layer; Step S8: Move the water injection steel pipe upward so that the packer is located at the upper layer of the designed fracturing layer, and fracturing the upper layer of the designed fracturing layer. In this way, all the designed fracturing layers are fracturing in sequence from bottom to top until the fracturing of all coal pillars in the suspended goaf is completed. S9 again employed transient electromagnetic geophysical exploration technology to investigate the situation in the room-and-pillar goaf and verify the effectiveness of hydraulic fracturing to address the roof collapse.

2. The method for large-area suspended roof hydraulic pressure cracking in coal mine goaf areas according to claim 1, characterized in that, In step S4, the number and location of fracturing layers are determined based on the height and hardness of the coal pillar, and a fracturing layer is designed every 0.8 to 2.0 m.

3. The method for large-area suspended roof hydraulic pressure cracking in coal mine goaf according to claim 1 or 2, characterized in that, In step S6, the hydraulic fracturing equipment is vehicle-mounted and includes a water tank, a high-pressure water pump, a pressure sensor, a three-way valve, a pressure relief valve, and a drain outlet connected in sequence. The three ends of the three-way valve are respectively connected to the pressure sensor, the pressure relief valve, and the top of the high-pressure water injection steel pipe.

4. The method for large-area suspended roof hydraulic pressure cracking in coal mine goaf areas according to claim 3, characterized in that, In step S7, first start the high-pressure water pump and perform low-pressure water injection. When the packer is fully expanded, quickly close the pressure relief valve. Start high-pressure water injection to fracturing the coal pillar in the goaf. Gradually reduce the pressure, turn off the high-pressure water pump, and open the pressure relief valve.

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