Gas extraction and hole sealing method for downward drilling of coal seam roof

Abstract

The application discloses a gas extraction hole sealing method for close-distance coal seam roof downward drilling, and belongs to the technical field of underground coal mine gas extraction. The method comprises the following steps: before the construction of the roof downward drilling in the close-distance coal seam roof roadway, a pre-grouting drilling is constructed in the surrounding roof aquifer area of a preset drilling position as a center, and then modified chemical grout is injected into the hole; the roof downward drilling is constructed at the preset drilling position, and then the hole is reamed at the hole mouth section position to form a hole sealing section; after the reaming is completed, a pipeline is lowered into the drilling; and the "two plugging and one injection" pressure sealing process is used for hole sealing. The application adopts a double plugging strategy of "peripheral curtain pre-grouting + internal section pressure sealing", effectively solves the double problems of "water erosion + gas leakage" of the roof downward drilling, and ensures the dense consolidation and shrinkage-free of the hole sealing body under the water-rich condition by using the waterproof grout with specific components and the "two plugging and one injection" pressure sealing process.

Description

Technical Field

[0001] This invention belongs to the field of underground gas extraction technology in coal mines, specifically relating to a method for sealing gas extraction through downward drilling from the roof of a close-range coal seam. Background Technology

[0002] Borehole gas drainage is a primary technical method for gas control in coal mines. Under conditions of mining closely spaced coal seams, in addition to using floor-to-floor downhole drilling to drain gas from adjacent seams, roof-to-floor downhole drilling is also employed to drain or depressurize the upper coal seams within the closely spaced coal seams. However, compared to floor-to-floor drilling, roof-to-floor drilling faces more complex and differentiated technical challenges during the sealing phase.

[0003] Currently, the following technical problems exist in the efficient gas extraction and sealing of downward drilling from the roof: (1) The roof strata are rich in water, and the sealing grout is severely eroded by water. The roof strata of coal seams are often rich in water, and the extraction of gas from the downward cross-strata boreholes is greatly affected by the rich water content of the roof strata. Under water-rich conditions, conventional cement-based sealing grout is easily diluted and washed away by groundwater before solidification, resulting in a decrease in the strength of the sealing grout stone body and an increase in porosity, making it difficult to guarantee the airtightness of the sealing. Even if the "two-plug-one-injection" sealing process is adopted, if the roof aquifer is not pretreated, the sealing section may still fail due to water erosion.

[0004] (2) The downward drilling angle of the top plate is large, making it difficult for the grout to remain. The downward drilling of the top plate usually has a large downward angle. Under the action of gravity, the grout flows rapidly downward along the borehole and is difficult to remain effectively in the sealing section. In conventional sealing methods, the grout is easy to lose to the depth of the borehole, resulting in incomplete filling of the sealing section and difficulty in ensuring the sealing quality.

[0005] (3) Mining-induced fractures are well-developed in the roof of closely spaced coal seams, resulting in numerous gas leakage channels. During the mining of closely spaced coal seams, mining stress causes a large number of fractures in the roof strata. These fractures are interconnected, forming gas leakage channels. Conventional borehole sealing methods only seal the inside of the borehole and are insufficient to effectively seal the mining-induced fractures in the roof around the borehole, resulting in a large amount of gas leakage along the fracture channels.

[0006] (4) The contradiction between top slab drilling and sealing and top slab water is prominent. During the construction of the bottom drilling in the top slab, top slab water seeps downward along the drill hole, which not only affects the normal progress of the sealing operation, but also continues to erode the sealing body after sealing, leading to the gradual deterioration of the sealing body and a decrease in sealing performance. Although there is a concept of "first sealing the area with water, then drilling the extraction borehole" in the existing technology, the area sealing project is large in scale and has a long cycle, and it is difficult to accurately control the sealing range.

[0007] Therefore, there is an urgent need to develop a systematic sealing method that can simultaneously solve the dual problems of "roof water erosion" and "mining-induced fracture gas leakage" for drilling downwards into the roof of close-range coal seams. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention proposes a method for sealing gas extraction through downward drilling in close-proximity coal seam roof. This method is particularly suitable for scenarios involving high-airtightness sealing and efficient gas extraction of downward cross-seam drillings constructed within roof roadways under conditions of close-proximity coal seam group mining.

[0009] The technical solution adopted in this invention is: A method for sealing gas extraction through a downward borehole drilled into the roof of a close-proximity coal seam, characterized by comprising the following steps: (1) Pre-grouting modification of the aquifer in the roof slab; Before drilling downwards into the roof of a nearby coal seam roadway, pre-grouting boreholes are constructed in the aquifer area of ​​the roof around the pre-set borehole location. Modified chemical grout is then injected into the aquifer through the pre-grouting boreholes. (2) Drilling and enlargement of the top plate; After the pre-grouting modification is completed, a bottom hole is drilled in the top slab at the preset drilling position, and then the hole is enlarged at the hole opening section to form a sealing section. (3) Pipeline is lowered; After the borehole is enlarged, gas extraction pipes, grouting pipes, and return grout pipes are lowered into the borehole from the top plate. (4) Segmented sealing; A quick-setting expanding material is injected into the bottom of the sealing section to form the first plug; after the first plug has solidified, waterproof cement grout is injected into the sealing section through the grouting pipe; after the grouting is completed, quick-setting expanding material is injected again into the top of the sealing section to form the second plug, thus completing the sealing.

[0010] Preferably, in step (1): the number of pre-grouting holes is 3 to 6, which are arranged in a ring around the preset hole position, with a hole diameter of 42 to 60 mm, and the hole depth penetrates the top slab aquifer and enters the relative water-proof layer.

[0011] Preferably, in step (1): the modified chemical grout is a polyurethane-water glass composite grout, and the grouting pressure is 1~2 MPa.

[0012] Preferably, in step (2): the diameter of the borehole in the top plate is 94~133mm, and the borehole depth penetrates the target coal seam and enters the interlayer rock layer of the bottom plate by 2~3m.

[0013] Preferably, in step (2): the hole enlargement refers to enlarging the hole within a range of 1 to 8 m below the top plate and the diameter of the enlarged hole is 30 to 50 mm larger than the original hole diameter.

[0014] Preferably, in step (3): the gas extraction pipe is located at the center of the downward borehole in the top plate and runs through the entire sealing section area along the axial direction of the downward borehole in the top plate. The lower end is 0.5~1.0m from the bottom of the downward borehole in the top plate, and the upper end extends out of the opening of the downward borehole in the top plate and connects to the gas extraction pipeline; the grouting pipe is set on one side of the gas extraction pipe along the axial direction of the downward borehole in the top plate. The lower end is 0.4~0.6m from the bottom of the sealing section, and the upper end extends out of the opening of the downward borehole in the top plate and connects to the grouting pump; the return slurry pipe is set on the other side of the gas extraction pipe along the axial direction of the downward borehole in the top plate. The lower end is 0.8~1.2m from the top of the sealing section, and the upper end extends out of the opening of the downward borehole in the top plate and connects to the return slurry collection device; the above three pipelines are arranged side by side along the axial direction of the downward borehole in the top plate and maintain a distance of not less than 50mm between them.

[0015] Preferably, the waterproof cement slurry in step (4) comprises the following raw materials in parts by weight: 90-110 parts of silicate cement, 8-15 parts of bentonite, 3-6 parts of water glass, 0.5-1.0 parts of polyacrylamide, and 40-50 parts of water.

[0016] Preferably, in step (4): the length of the first plug is not less than 1m; the grouting pressure is 1.0~1.8Mpa, and grouting is continued until uniform grout is returned from the return pipe during the grouting process, and then the return pipe valve is closed, and the grouting pressure is maintained for 15~20min; the length of the second plug is not less than 1m.

[0017] Preferably, the method for sealing gas extraction through a borehole drilled downwards from the roof of a nearby coal seam further includes the steps of pressurized curing and secondary grouting: after sealing, the grouting system pressure is maintained at 0.5~1.0MPa for pressurized curing, and the curing time is not less than 48 hours; during the curing period, if the grouting system pressure drops by more than 0.3MPa, secondary grouting is initiated, and the grouting pressure is 1.0~1.5MPa until the pressure stabilizes.

[0018] Preferably, the method for sealing gas extraction through a borehole drilled downwards from the roof of a nearby coal seam further includes the steps of cleaning the borehole and gas extraction: after curing, the residual slurry in the gas extraction pipe is cleaned by cleaning the borehole to ensure that the extraction pipeline is unobstructed; then the upper end of the gas extraction pipe is connected to the surface gas extraction pipeline, and gas extraction is started.

[0019] The beneficial technical effects of the present invention are as follows: (1) Effectively solves the problem of roof water erosion: Before drilling the extraction borehole, this invention forms an annular waterproof curtain in the aquifer of the roof through pre-grouting drilling, isolating the roof water from the sealing section. Compared with the traditional approach of "regional water sealing first", this invention adopts local annular curtain grouting, which has a small workload, short cycle, low cost, and precise and controllable water sealing effect. At the same time, the cement grout is mixed with bentonite, water glass and polyacrylamide, which have good anti-dilution and anti-erosion properties, and can maintain the long-term stability of the sealing body under the condition of rich water in the roof.

[0020] (2) Double sealing and high airtightness: This invention adopts a double sealing strategy of "pre-grouting waterproof curtain of the aquifer in the top plate + segmented pressurized sealing of the borehole": the outer waterproof curtain isolates the erosion of the sealing body by the water in the top plate and seals some mining cracks; the segmented sealing of the borehole in the inner section achieves high airtightness sealing inside the borehole. The combination of the inner and outer sections effectively solves the dual problems of "water erosion + air leakage" in the boreholes drilled downward from the top plate.

[0021] (3) Adapting to special working conditions of downward drilling in the top plate: By expanding the hole to form a sealing section, the contact area and bonding strength between the sealing grout and the hole wall are increased; the segmented sealing structure of "lower plug section - grouting section - upper plug section" effectively prevents the loss of grout in downward drilling; the anti-cement grout has good thixotropic properties and water dilution resistance, and can solidify normally under water-rich conditions in the top plate.

[0022] (4) Stable sealing structure and adaptable to hole wall deformation: The cement grout solidifies under pressure and forms a tightly fitted integral structure with the hole wall. The micro-expansion characteristics of bentonite can compensate for the volume shrinkage during the solidification process of the grout, and avoid micro-cracks between the sealing body and the hole wall. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the system structure of the efficient gas extraction and sealing method for drilling downwards from the roof of a close-range coal seam according to the present invention; Figure 2 This is a schematic diagram of the longitudinal arrangement of the sealing tube and the segmented pressurized sealing tube of the present invention; Figure 3 This is a cross-sectional view of the sealing tube of the present invention; Figure 4 This is a process flow diagram of the segmented pressurized sealing process of the present invention.

[0024] In the diagram: 1-Roof roadway, 2-Target coal seam, 3-Lower adjacent coal seam, 4-Roof aquifer, 5-Relative aquitard, 6-Annular waterproof curtain, 7-Downward cross-layer extraction borehole, 8-Sealing section, 9-Gas extraction pipe, 10-Grouting pipe, 11-Return slurry pipe, 12-Gas extraction pipeline, 13-Grouting pump, 14-Return slurry collection device, 15-Inter-layer rock strata, 16-Borehole wall, 17-Sealing section enlargement area, 18-Upper plugging section, 19-Grouting section, 20-Lower plugging section, 21-Sealing material. Detailed Implementation

[0025] like Figure 1 As shown, the present invention provides a method for sealing gas extraction through a downward borehole drilled into the roof of a nearby coal seam, comprising the following steps: Step 1: Pre-grouting modification of the aquifer in the roof slab In the near-coal seam roof roadway 1, before constructing the downward cross-seam extraction borehole 7, pre-grouting boreholes are first constructed in the roof aquifer 4 area surrounding the predetermined borehole location. The pre-grouting boreholes are arranged in a ring around the predetermined extraction borehole location (intervals are set at the positions of the annular waterproof curtain 6), with a number of 3-6 boreholes, a diameter of φ42mm-φ60mm, and a depth penetrating the roof aquifer 4 to enter the relatively impermeable layer 5. Modified chemical grout is injected into the roof aquifer 4 through the pre-grouting boreholes. This grout is a conventional polyurethane-water glass composite grout, with a polyurethane to water glass mass ratio of 1:1, and an injection pressure of 1.0-2.0 MPa. After the modified chemical grout diffuses and solidifies in the aquifer, it forms an annular waterproof curtain 6 centered on the predetermined extraction borehole, isolating the roof water from the sealing section 8 of the extraction borehole.

[0026] Step 2: Drilling and Enlargement of Extraction Holes After the pre-grouting modification is completed (and the grout has solidified for 48-72 hours), the undercut extraction borehole 7 is constructed at the predetermined drilling location. The borehole diameter is φ94mm-φ133mm, and the drilling depth penetrates the target coal seam 2 (the adjacent upper coal seam) and enters the interlayer strata 15 of the floor for approximately 2-3 meters. After the borehole construction is completed, the borehole is enlarged within a 1-8m range at the borehole opening, with the enlarged diameter being 30-50mm larger than the original borehole diameter, forming an enlarged borehole sealing section.

[0027] Step 3: Inserting the sealing tube After the borehole is enlarged, a sealing pipe is inserted into the borehole, such as... Figure 2 , Figure 3 As shown, the sealing tube comprises, from the inside out: Gas drainage pipe 9: runs through the entire sealing section 8 along the borehole axis, with the lower end extending into the target coal seam drainage area (0.5~1.0m from the bottom of the borehole) and the upper end extending out of the borehole to connect to the gas drainage pipeline 12; Grouting pipe 10: It is installed along the borehole axis on one side of gas extraction pipe 9, with the lower end extending to the bottom of the sealing section (about 0.5m from the bottom of the enlarged section) and the upper end extending out of the borehole to connect to grouting pump 13; Return slurry pipe 11: It is set along the borehole axis on the other side of the gas extraction pipe 9, with the lower end extending to about 1m from the top of the sealing section, and the upper end extending out of the borehole to connect to the return slurry collection device 14.

[0028] The three pipes are arranged side by side and fixed with pipe clamps at intervals, maintaining a distance of not less than 50mm between each other.

[0029] Step 4: Segmented pressurized sealing of the borehole The "two-plug-one-injection" pressurized sealing process is adopted, such as... Figure 2 As shown, the following are arranged sequentially from bottom to top within the sealing section: (1) Lower plug section 20: A first plug is set at the bottom of the sealing section 8. A quick-setting expansion material (polyurethane or modified polyurea resin) is injected into the bottom of the sealing section 8 to form a lower plug section with a length of not less than 1m to prevent the grout from flowing to the bottom of the borehole. (2) Grouting section 19: After the lower plug section solidifies, the waterproof cement grout is injected into the sealing section 8 through the grouting pipe 10. The grouting pressure is 1.0~1.8MPa. Grouting continues until uniform grout is returned from the return pipe 11. Then the return pipe valve is closed and the grouting pressure is kept stable for 15~20 minutes. The waterproof cement slurry is composed of the following raw materials in parts by weight: 100 parts ordinary silicate cement, 11 parts bentonite, 5 parts water glass (modulus 2.8~3.2), 0.8 parts polyacrylamide (molecular weight 3 million~5 million), and 45 parts water. The preparation method is as follows: first, mix the bentonite and water to form a bentonite slurry; then add the cement and mix evenly; finally, add the water glass and polyacrylamide, and continue mixing for 5~10 minutes to obtain the waterproof cement slurry.

[0030] (3) Upper plug section 18: After the grouting is completed, a second plug is set at the top of the sealing section 8. The same quick-setting expansion material (polyurethane or modified polyurea resin) is injected to form an upper plug section with a length of not less than 1m, thus completing the sealing.

[0031] Step 5: Pressurized curing and secondary grouting After sealing, maintain the grouting system pressure at 0.5~1.0MPa for pressurized curing for no less than 48 hours (extended compared to conventional bottom slab sealing to resist continuous water erosion from the roof). During curing, if the grouting pressure drops by more than 0.3MPa, initiate secondary grouting at a pressure of 1.0~1.5MPa until the pressure stabilizes.

[0032] Step Six: Connect the Hole Sweeping and Extraction Processes After maintenance, a cleaning drill bit is lowered into the gas extraction pipe 9 to clean the residual slurry inside, ensuring the extraction pipeline is unobstructed. The upper end of the gas extraction pipe 9 is then connected to the surface gas extraction pipeline 12, and extraction is started.

[0033] For any parts not mentioned in the above embodiments, existing technologies can be adopted or referenced.

[0034] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the above embodiments. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should be protected by the present invention.

Claims

1. A method for sealing gas extraction boreholes drilled downwards from the roof of a close-proximity coal seam, characterized in that, Includes the following steps: (1) Pre-grouting modification of the aquifer in the roof; Before drilling downwards into the roof of a nearby coal seam roadway, pre-grouting boreholes are constructed in the aquifer area of ​​the roof around the pre-set borehole location. Modified chemical grout is then injected into the aquifer through the pre-grouting boreholes. (2) Drilling and enlargement of the top plate; After the pre-grouting modification is completed, a bottom hole is drilled in the top slab at the preset drilling position, and then the hole is enlarged at the hole opening section to form a sealing section. (3) Pipeline is lowered; After the borehole is enlarged, gas extraction pipes, grouting pipes, and return grout pipes are lowered into the borehole from the top plate. (4) Segmented sealing; A quick-setting expansion material is injected into the bottom of the sealing section to form the first plug; after the first plug has solidified, cement slurry is injected into the sealing section through the grouting pipe. After grouting is completed, a second plug is formed by injecting quick-setting expansion material into the top of the sealing section, thus completing the sealing.

2. The method for sealing gas extraction through a downward borehole drilled into the roof of a close-range coal seam according to claim 1, characterized in that, In step (1): the number of pre-grouting boreholes is 3 to 6, which are arranged in a ring around the preset borehole position, with a borehole diameter of 42 to 60 mm and a borehole depth that penetrates the top slab aquifer and enters the relative water-proof layer.

3. The method for gas extraction and sealing via a downward borehole from the roof of a close-range coal seam according to claim 1, characterized in that, In step (1): the modified chemical grout is a polyurethane-water glass composite grout, and the grouting pressure is 1~2 MPa.

4. The method for sealing gas extraction through a downward borehole drilled into the roof of a close-range coal seam according to claim 1, characterized in that, In step (2): the diameter of the borehole in the top plate is 94~133mm, and the borehole depth penetrates the target coal seam and enters the interlayer rock layer of the bottom plate by 2~3m.

5. The method for sealing gas extraction through a downward borehole drilled into the roof of a close-range coal seam according to claim 1, characterized in that, In step (2): the hole enlargement refers to enlarging the hole within a range of 1 to 8 m below the top plate and the diameter of the enlarged hole is 30 to 50 mm larger than the original hole diameter.

6. The method for sealing gas extraction through a downward borehole drilled into the roof of a close-range coal seam according to claim 1, characterized in that, In step (3): the gas extraction pipe is located at the center of the downward borehole in the top plate and runs through the entire sealing section along the axial direction of the downward borehole in the top plate. The lower end is 0.5~1.0m from the bottom of the downward borehole in the top plate, and the upper end extends out of the bottom of the downward borehole in the top plate and connects to the gas extraction pipeline; the grouting pipe is set on one side of the gas extraction pipe along the axial direction of the downward borehole in the top plate. The lower end is 0.4~0.6m from the bottom of the sealing section, and the upper end extends out of the bottom of the downward borehole in the top plate and connects to the grouting pump; the return slurry pipe is set on the other side of the gas extraction pipe along the axial direction of the downward borehole in the top plate. The lower end is 0.8~1.2m from the top of the sealing section, and the upper end extends out of the bottom of the downward borehole in the top plate and connects to the return slurry collection device; the above three pipelines are arranged side by side along the axial direction of the downward borehole in the top plate, and maintain a distance of not less than 50mm between them.

7. The method for sealing gas extraction through a downward borehole drilled into the roof of a close-range coal seam according to claim 1, characterized in that, The waterproof cement slurry mentioned in step (4) includes the following raw materials in parts by weight: 90-110 parts of silicate cement, 8-15 parts of bentonite, 3-6 parts of water glass, 0.5-1.0 parts of polyacrylamide, and 40-50 parts of water.

8. A method for sealing gas extraction via a downward borehole drilled into the roof of a close-range coal seam according to claim 1, characterized in that, In step (4): the length of the first plug is not less than 1m; the grouting pressure is 1.0~1.8Mpa, and grouting is continued until uniform grout is returned from the return pipe during the grouting process, and then the return pipe valve is closed, and the grouting pressure is maintained for 15~20min; the length of the second plug is not less than 1m.

9. A method for sealing gas extraction via a downward borehole drilled into the roof of a close-proximity coal seam according to claim 1, characterized in that, It also includes the steps of pressurized curing and secondary grouting: After the hole is sealed, the grouting system pressure is maintained at 0.5~1.0MPa for pressurized curing, and the curing time is not less than 48 hours; during the curing period, if the grouting system pressure drops by more than 0.3MPa, secondary grouting is started, and the grouting pressure is 1.0~1.5MPa until the pressure stabilizes.

10. A method for sealing gas extraction via a downward borehole drilled into the roof of a close-range coal seam according to claim 9, characterized in that, It also includes the steps of borehole cleaning and gas extraction: After the maintenance is completed, the residual slurry in the gas extraction pipe is cleaned to ensure that the extraction pipeline is unobstructed; then the upper end of the gas extraction pipe is connected to the gas extraction pipeline on the surface of the well to start gas extraction.

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