A method for ground T-branch horizontal well hard roof mine pressure disaster control

Abstract

This invention discloses a method for mitigating mine pressure disasters in hard roof strata of a T-shaped branch horizontal well on the surface. First, a casing is drilled in a vertical shaft with a two-hole structure. Then, branch horizontal holes a and b are drilled in the casing using a directional drilling tool. Next, hydraulic fracturing is performed in stages along branch horizontal holes a and b, followed by perforation fracturing of the vertical shaft, ultimately forming a complex hydraulic fracture network. The hydraulic fracture network formed by this method can efficiently achieve advanced mitigation of mine pressure disasters in the hard, critical overlying strata of the coal seam mining face, preventing the manifestation of strong mine pressure disasters in the hard overlying strata during coal seam mining, ensuring the orderly progress of coal mining operations and efficient coal production.

Description

Technical Field

[0001] This invention belongs to the field of coal mine pressure disaster management technology, and relates to a method for managing mine pressure disasters caused by hard roof in a T-shaped branch horizontal well on the ground. Background Technology

[0002] my country has abundant coal reserves, but with the significant increase in mining intensity, the geological conditions of coal mines are becoming increasingly complex. The coal mining process is characterized by mine pressure hazards arising from the hard roof strata overlying the coal seam at the coal seam working face. These hard roof strata are characterized by high strength, great thickness, good integrity, and tensile strength higher than ordinary strata. During the advancement of the coal seam working face, the roof in this stratum often remains suspended without collapsing. However, when the suspension reaches its limit, a large-scale, instantaneous roof collapse occurs, causing severe pressure on the coal seam working face. This seriously restricts safe and efficient underground coal mining and severely threatens the safety of life and property. Therefore, before coal mining, it is essential to manage the mine pressure hazards caused by the hard roof strata overlying the coal seam working face to reduce the severe economic losses resulting from these hazards.

[0003] The main method for controlling mine pressure disasters in coal mining areas is the segmented hydraulic fracturing roof weakening technology. Depending on whether the treatment site is underground or on the surface, it can be divided into: underground directional long-bore segmented hydraulic fracturing technology and surface horizontal well segmented fracturing technology. The underground directional long-bore segmented hydraulic fracturing technology for roof pressure disaster control involves a relatively long directional borehole trajectory, extending along the entire strike of the coal seam mining face. However, this technology requires advance excavation of construction roadways, making it difficult to address the challenges of tight mining schedules and conflicts. Using surface horizontal well segmented fracturing technology for roof stress control can achieve advanced stress control in roadway and coal seam working face excavation or mining, effectively solving the problem of tension and conflict between coal mining operations. However, the coal seams in coal mining areas are relatively shallow, mostly around several hundred meters deep, and the overlying hard rock strata are even shallower. Therefore, the horizontal section length of the surface horizontal wells drilled using surface horizontal well segmented fracturing is only several hundred meters, making it difficult to extend to the entire coal seam working face of thousands of meters. Thus, stress control of the hard rock strata corresponding to the coal seam mining face requires the installation of two horizontal wells. Because two horizontal wells need to be drilled, the cost of this technology for stress control is relatively high. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a method for managing mine pressure disasters caused by hard roofs in surface T-shaped branch horizontal wells. This method resolves the tension and conflict between mining operations and the required drilling of additional surface horizontal wells, which is necessary for mine pressure disaster management using the aforementioned underground directional long borehole segmented hydraulic fracturing technology.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] A method for mitigating mine pressure disasters caused by hard roof in a T-shaped branch horizontal well on the surface includes the following steps:

[0007] Step 1: Determine the main and key overlying rock strata on the coal seam mining face, and accurately determine the vertical distance between the coal seam mining face and its overlying main and key rock strata, as well as the distance between the two from the ground surface;

[0008] Step 2: Use a surveying and setting-out instrument to measure the geodetic coordinates of the four corner points of the coal seam mining face boundary. Calculate the geodetic coordinates of the center point A of the coal seam mining face based on the measured geodetic coordinate data of the four corner points of the coal seam mining face boundary. Use the surveying and setting-out instrument to accurately delineate the position of coordinate point B projected onto the ground directly above center point A, and determine this coordinate point B as the wellhead position of the vertical well to be drilled.

[0009] Step 3: Based on the vertical distance parameters of the coal seam mining face and the main key rock strata from the ground obtained in Step 1, drill a vertical well at coordinate point B determined in Step 2. This vertical well has a two-section borehole structure. The first section is drilled to a depth of 5-10m into stable bedrock, and the first section casing is run in and cemented with pure cement slurry. The second section is drilled to a depth below the main key rock strata, and the second section casing is run in and cemented with pure cement slurry.

[0010] Step 4: In the vertical well drilled in Step 3, a directional drilling tool is installed for side-drilling of the branch horizontal hole. Then, a Φ171.5mm drill bit assembly is installed to side-drill the branch horizontal hole a at side-drilling point a on the second casing. The landing point of the horizontal section of the branch horizontal hole a is located within the main key rock layer. The drilling depth of the horizontal section of the branch horizontal hole a reaches the right boundary of the main key rock layer. A pressure-bearing alkaline soluble bridge plug is installed 5-10m below the side-drilling point a of the branch horizontal hole a.

[0011] Step 5: Perform casing and cementing operations on the branch horizontal hole a drilled in Step 4: Run casing and tubing string into branch horizontal hole a. The casing and tubing string, from top to bottom, consist of: Φ89mm drill pipe, drop tool, one Φ139.7mm branch horizontal hole short casing, stage clamp, and Φ139.7mm branch horizontal hole casing. Run the tubing string into branch horizontal hole a to a depth of 1-2m from the bottom of the hole for normal circulation of cementing slurry. Lower the branch horizontal hole short casing to the side drilling point a of branch horizontal hole a. Run the tubing string into the designed hole. After reaching a deep position, pure cement slurry cementing operation is carried out; after cementing and setting, the inner annulus hole of the drill pipe is opened on the surface and a steel ball is lowered in. When the steel ball is lowered to the release tool, high-pressure clean water is injected into the drill pipe. At this time, the pressure at the release tool increases instantly, causing the release tool to separate from the short casing of the branch horizontal hole. The drill pipe is pulled out on the surface, and a Φ120mm PDC drill bit is lowered to drill into the internal structure of the milling and grinding stage hoop, so that the entire branch horizontal hole a is unobstructed. Then, an alkaline soluble sealing bridge plug is lowered at the orifice of the branch horizontal hole at drilling point a on the side of branch horizontal hole a to temporarily plug the branch horizontal hole a.

[0012] Step 6: Based on the vertical well drilled in Step 3, first run a directional drilling tool for side drilling of the branch horizontal hole, then run a Φ171.5mm drill bit assembly, and side drill the branch horizontal hole b at side drilling point b on the second casing. Side drilling point b is located above side drilling point a. The landing point of the horizontal section of the branch horizontal hole b is located in the main key rock layer. The drilling depth of the horizontal section of the branch horizontal hole b reaches the left boundary end of the strike of the main key rock layer.

[0013] Step 7: Perform casing and cementing operations on the branch horizontal hole b drilled in Step 6 above. The process is the same as in Step 5, and finally, the branch horizontal hole b is temporarily plugged with a bridge plug.

[0014] Step 8: Lower the Φ89mm bare drill pipe assembly to 1-3m above the side drilling point b. On the surface, adjust the mud pump to the maximum discharge rate and inject high-velocity alkaline circulating liquid into the bottom of the well to flush and clean the drill cuttings deposited on the alkaline soluble bridge plug and the bridge plug that act as a temporary seal. At the same time, it will quickly dissolve the alkaline soluble bridge plug and the bridge plug in the vertical well, branch horizontal hole a, and branch horizontal hole b, thereby making the vertical well, branch horizontal hole a, and branch horizontal hole b unobstructed.

[0015] Step 9 involves sequentially performing hydraulic fracturing operations on branch horizontal holes a, branch horizontal holes b, and the vertical well, creating a large number of hydraulic fractures. The hydraulic fractures formed by the perforation fracturing of the two branch horizontal holes and the vertical well can efficiently control the strong mine pressure disaster of the hard main and key rock strata on the coal seam mining face.

[0016] The present invention also includes the following technical features:

[0017] Specifically, in step 3, in the two-stage borehole structure of the vertical well, the first stage is drilled using a Φ346.1mm roller cone drill bit, and a Φ273.1mm J55 grade steel casing is lowered in. Low-grade 32.5 cement mortar with a specific gravity of 1.7~2.0 g / cm³ is used. 3 Cementing was performed using pure cement slurry. The second section was drilled with a Φ241.3mm PDC drill bit, and a Φ193.7mm N80 grade steel casing was run in. The second section casing run at the pre-designed side-drilling points for the two branch horizontal holes was a Φ193.7mm fiberglass casing, prepared with high-grade 42.5 cement mortar with a specific gravity of 1.8~2.10g / cm³. 3 For cementing with pure cement slurry, after the second cementing is completed and allowed to set for 72 hours, a pressure test of 15~20 MPa must be conducted inside the casing to check the sealing of the second casing cementing. The pressure test time is 30 minutes, and the pressure drop must be less than or equal to 0.5 MPa.

[0018] Specifically, in step 5, after the tubing string is lowered to the designed borehole depth, a pure cement slurry cementing operation is performed, using high-grade 42.5 cement ash with a specific gravity of 1.8~2.10 g / cm³. 3 Pure cement slurry positive circulation cementing includes:

[0019] First, the designed amount of pure cement slurry is injected into the tubing string. This amount is sufficient to just fill the annular space from the stage collar to the bottom section of branch horizontal hole a. Then, clean water containing a red tracer is continuously injected into the tubing string. When the injected pure cement slurry flows back into the annular space between the open hole and casing of branch horizontal hole a, approaching the stage collar, a ball-dropping operation is performed at the surface wellhead. The instantaneous pressure increase when the steel ball reaches the stage collar triggers its function. At this point, the pure cement slurry flows back into the annulus between the open hole and casing of branch horizontal hole a. At the same time, the water injected from the ground begins to flow from the holes on both sides of the stage collar to the annular space between the open hole of branch horizontal hole a and the casing of branch horizontal hole. The injected water flows back to the ground from the holes of the stage collar. When a red visual trace is found on the ground, it indicates that the water has been discharged back to the ground. At this time, the annular space inside the drill pipe and the annular space between the drill pipe and the second open casing of the vertical well are sealed on the ground to achieve a pressurized state in the annular space between the pipe string and branch horizontal hole a, so that the pure cement slurry injected into the stage collar and the bottom section of branch horizontal hole a can be allowed to solidify for 72 hours under pressurized conditions.

[0020] Specifically, in step 9, when performing segmented perforation hydraulic fracturing of branch horizontal holes a and b, the fracturing tool string is re-entered into branch horizontal holes a and b using a directional tool to carry out segmented hydraulic fracturing.

[0021] The process of the fracturing tool string entering the branch horizontal hole a is as follows: First, the directional drilling tool is lowered to the side drill point a. When the directional drilling tool reaches the side drill point a, its slope end face is in any orientation. Then, the fracturing tool string is lowered to the bottom of the directional drilling tool. The directional drilling tool is slowly rotated on the ground. When the slowly rotating directional drilling tool reaches the orientation of the side drill point a, the fracturing tool string enters the branch horizontal hole a. After the fracturing tool string enters the branch horizontal hole a, it continues to be lowered to the bottom of the branch horizontal hole a. Then, a retreating segmented perforation hydraulic fracturing method is used to ultimately form a large number of hydraulic fractures.

[0022] Specifically, the fracturing tool string is lowered into the branch horizontal hole b in the same way as in the branch horizontal hole a. After the fracturing tool string enters the branch horizontal hole b, it continues to be lowered to the bottom of the branch horizontal hole b. Then, a backward segmented perforation hydraulic fracturing is used to ultimately form a large number of hydraulic fractures. The hydraulic fractures can effectively relieve pressure, thereby achieving the purpose of mine pressure disaster management.

[0023] Specifically, the landing points of the branch horizontal holes a and b, when projected horizontally to the vertical well section of equal depth, constitute a blind zone for mine pressure disaster management in the main critical rock strata. This blind zone is managed by implementing perforation fracturing in the vertical well. Implementing perforation fracturing in the vertical well includes: lowering a fracturing tool string into the vertical well section of the main critical rock strata to perform perforation fracturing within the vertical well section. The hydraulic fractures formed by this perforation fracturing can effectively manage the blind zone that cannot be managed by the segmented hydraulic fracturing of the two branch horizontal holes.

[0024] Compared with the prior art, the present invention has the following technical effects:

[0025] 1. This invention only requires drilling one surface vertical well, and then using side drilling points designed at specific locations within the vertical well to perform side drilling to open branch holes and segmented hydraulic fracturing construction, which can achieve the control of mine pressure disasters that may be caused by the overlying key strata on the coal seam mining face.

[0026] 2. This invention only requires drilling one surface vertical well above the coal seam mining face, avoiding repeated drilling of vertical well sections, thus reducing the cost of mine pressure disaster management.

[0027] 3. This invention relies on T-shaped branch horizontal wells to weaken and treat the hard top plate on both sides of the borehole, avoiding the problems of excessively high water-to-vertical ratio, high drilling difficulty in the horizontal section, and inability to achieve full coverage of the working face caused by using a single horizontal well when the rock strata are shallow.

[0028] 4. This invention employs a method where two branch holes are drilled sequentially within a vertical well. After drilling and casing cementing of each branch hole are completed, an alkaline soluble bridge plug is used to temporarily plug each successfully drilled branch hole, ensuring normal drilling circulation for the next branch hole. Once all branch holes are successfully drilled, a high-speed alkaline solvent is used to unseal them. Finally, segmented hydraulic fracturing is carried out on each branch hole and the blind spot for mine pressure disaster management in the vertical well section. After the segmented fracturing is completed, no fracturing drainage work is required. Compared with existing technologies, this method can effectively ensure that segmented hydraulic fracturing can be achieved for each branch horizontal hole.

[0029] 5. This invention employs a combination of Φ89mm drill pipe, a drop tool, a Φ139.7mm horizontal branch hole short casing, a stage clamp, and a Φ139.7mm horizontal branch hole casing, which effectively seals only the casing strings installed in each branch hole. This ensures that the casing strings installed in the branch holes are sealed and solidified under pressure. At the same time, the Φ89mm drill pipe strings installed in the vertical well section outside the branch holes can be effectively retrieved and reused.

[0030] 6. After the drilling and fracturing of each branch horizontal hole in this invention are completed, the blind zone of the vertical well with the same depth as the landing point is treated by segmented hydraulic fracturing. Compared with the existing technology that ignores the treatment of blind zone, this method can achieve all-round, full-space range treatment of mine pressure disaster without blank zone.

[0031] 7. This invention addresses the potential mine pressure disasters caused by hard rock strata overlying the main and critical roof of coal seam mining faces on the surface. It shifts the construction environment from underground coal mines to the surface, effectively improving the working environment for workers. At the same time, being located on the surface enables proactive management of mine pressure disasters before the roadways are excavated.

[0032] 8. The present invention arranges the horizontal section of the side-drilled well in the middle of the working face, which is conducive to the balanced expansion of the fracture to both sides of the wellbore, promotes the uniform weakening and treatment of the hard roof, and avoids stress concentration during the coal mining process. Attached Figure Description

[0033] Figure 1 This is a map showing the distribution of key rock strata on the coal seam mining face that may cause mine pressure disasters due to the presence of hard overburden.

[0034] Figure 2 A schematic diagram for determining the geodetic coordinates of the wellhead on the surface of a vertical well.

[0035] Figure 3 A diagram showing the relative spatial location of the surface vertical well depth and the main key rock strata;

[0036] Figure 4 Schematic diagram of drilling and completion of horizontal boreholes on the right side of the main key rock stratum a;

[0037] Figure 5 A schematic diagram of the structure for inserting a sleeve string into a branch horizontal hole;

[0038] Figure 6 Schematic diagram of drilling and completion of horizontal boreholes on the left side of the main key rock stratum b;

[0039] Figure 7 A diagram of the entire wellbore structure after the alkaline soluble bridge plugs at the bottom of the vertical well and the orifices of the branch horizontal holes have dissolved.

[0040] Figure 8 This is a schematic diagram showing the completion of hydraulic fracturing in stages throughout the well.

[0041] Figure 9 This is a magnified schematic diagram of a section of the main key rock strata after hydraulic fracturing was completed throughout the well.

[0042] The meanings of the labels in the diagram are as follows:

[0043] 1. Surface, 2. Key rock strata, 3. Coal seam mining face, 4. Center point A, 5. Coordinate point B, 6. First-stage casing, 7. Second-stage casing, 8. Alkaline soluble bridge plug, 9. Side drill point a, 10. Landing point, 11. Branch horizontal hole a, 12. Branch horizontal hole casing, 13. Stage hoop, 14. Bridge plug, 15. Side drill point b, 16. Branch horizontal hole b, 17. Packer, 18. Blind zone, 19. Branch horizontal hole short casing, 20. Drop tool, 21. Drill rod, 22. Hydraulic fracture. Detailed Implementation

[0044] This invention proposes a method for mitigating mine pressure disasters caused by hard roofs in T-shaped branch horizontal wells on the surface. The method primarily involves selecting the coordinates of the midpoint of the ground at the corresponding hard roof strata above the coal seam mining face as the wellhead location. A vertical well is drilled at this wellhead coordinates. Within the vertical well, the preferred drilling depths for the two branch horizontal holes are determined. Then, branch horizontal holes are drilled on both sides along the strike of the hard, critical roof strata. The included angle between the two branch horizontal holes drilled within the hard, critical roof strata is 180°. Simultaneously, the shapes of the two branch horizontal holes and the vertical well are similar. Similar to a T-shape, this T-shaped well design includes casing cementing for both the vertical well and the two branch horizontal holes. However, the casing near the side-drilling point is made of fiberglass, which is easy to use for side-drilling of the branch horizontal holes. Each branch horizontal hole is designed to use a staged hoop cementing process. After the drilling, casing, and cementing of the branch horizontal holes are completed, the perforation and fracturing of the two branch horizontal holes are carried out in a unified manner. Finally, hydraulic perforation fracturing is carried out in the branch hole fracturing blind zone in the section of the vertical well located in the key rock strata. Ultimately, the goal of controlling the mine pressure disaster in the hard key rock strata of the coal seam mining face is achieved efficiently.

[0045] The following are specific embodiments of the present invention. It should be noted that the present invention is not limited to the following specific embodiments. All equivalent modifications made based on the technical solutions of this application fall within the protection scope of the present invention.

[0046] Example:

[0047] like Figures 1 to 9 As shown in the figure, this embodiment provides a method for mitigating rock pressure disasters caused by hard roof in a T-shaped branch horizontal well on the surface. This method is mainly used to proactively mitigate rock pressure disasters caused by hard main and key rock strata in the overlying roof of the coal seam mining face, in order to prevent the manifestation of strong rock pressure disasters caused by hard rock strata in the overlying roof during the mining of the coal seam. The implementation process of this method is as follows:

[0048] Step 1: Using the theoretical calculation formula of the key rock strata of the overlying roof of the coal seam mining face 3, determine the most important key rock strata that may cause mine pressure disaster due to the hardness of the overlying roof of the coal seam mining face 3, namely the main key rock strata 2. Figure 1 As shown, the vertical distance between the coal seam mining face 3 and the overlying key rock strata 2, as well as their distance from the ground, are precisely determined to provide a basis for the subsequent design of the vertical well drilling trajectory and the design of the side drilling points of the two branch horizontal holes.

[0049] Step 2: Use a surveying and setting-out instrument to measure the geodetic coordinates of the four corner points of the boundary of the coal seam mining face 3 to determine the boundary range of the coal seam mining face 3 area. Then, calculate the geodetic coordinates of the center point A4 of the coal seam mining face 3 based on the measured geodetic coordinate data of the four corner points of the boundary of the coal seam mining face 3. Finally, use the surveying and setting-out instrument to accurately delineate the coordinate point B5 projected onto the ground directly above the center point A4 of the coal seam mining face 3. Determine this coordinate point B5 as the wellhead location of the vertical shaft to be drilled. Figure 2 As shown;

[0050] Step 3: Based on the vertical distance parameters of the coal seam mining face 3 and the hard main key strata 2 of the roof obtained in Step 1, drill a vertical well at the ground coordinate point B5 given in Step 2. Figure 3 As shown, this vertical well is designed with a two-stage borehole structure. The first stage uses a Φ346.1mm roller cone drill bit for initial drilling, reaching a depth of 5-10m in stable bedrock. A Φ273.1mm J55 grade steel casing (stage 6) is then lowered into the borehole. Low-grade 32.5 cement mortar with a specific gravity of 1.7-2.0 g / cm³ is used for the initial drilling. 3The well was cemented with pure cement slurry. The second section was drilled using a Φ241.3mm PDC drill bit, reaching a depth below the main critical rock layer 2. A Φ193.7mm N80 grade steel casing (section 7) was then run. The casing at the pre-designed side-drilling points for the two branch horizontal holes was made of Φ193.7mm fiberglass, facilitating easier side-drilling with casing openings for the subsequent two branch horizontal holes. High-grade 42.5 cement ash with a specific gravity of 1.8~2.10 g / cm³ was used. 3 For cement slurry cementing, after the second cementing operation is completed and allowed to set for 72 hours, a pressure test of 15-20 MPa must be conducted inside the casing to check the cementing sealing of the second casing 7. The pressure test time is 30 minutes, and the pressure drop must be less than or equal to 0.5 MPa to be considered reasonable. Otherwise, remedial cementing operations must be carried out. A pressure-bearing alkaline soluble bridge plug 8 is installed 5-10 meters below the side drilling point a9 of the designed branch horizontal hole a11 to ensure the normal circulation drilling of the subsequent two branch horizontal holes.

[0051] Step 4: Based on the vertical well drilled in Step 3, firstly, a directional drilling tool is installed for side-drilling of the branch horizontal hole. Then, a Φ171.5mm drill bit assembly is installed at side-drilling point a9 to drill the branch horizontal hole a11 using fiberglass casing. The landing point 10 of the horizontal section of branch horizontal hole a11 is located within the main key rock layer 2. The drilling depth of the horizontal section of branch horizontal hole a11 reaches the right boundary of the strike of the main key rock layer 2. Figure 4 As shown.

[0052] Step 5: Perform casing installation and cementing operations on the branch horizontal hole a11 drilled in Step 4. The branch horizontal hole a11 is designed to have a Φ139.7mm N80 grade branch horizontal hole casing 12 installed. The casing and other auxiliary tools in the hole, arranged from top to bottom, consist of: Φ89mm drill pipe 21 + drop tool 20 + one Φ139.7mm branch horizontal hole short casing 19 + stage clamp 13 + Φ139.7mm branch horizontal hole casing 12. Figure 5 As shown. The aforementioned tubing string is lowered into the branch horizontal hole a11, 1-2m from the bottom of the hole, for normal circulation of cement slurry. The short casing 19 of the branch horizontal hole, Φ139.7mm, is lowered to the side-drilling point a9 of the branch horizontal hole a11. After the tubing string is lowered to the designed hole depth, pure cement slurry cementing operation begins, using high-grade 42.5 cement ash with a specific gravity of 1.8-2.10 g / cm³. 3Pure cement slurry is used for positive circulation cementing. The required cement slurry volume is calculated precisely in advance. First, the designed amount of pure cement slurry is injected into the tubing string. This designed amount of pure cement slurry is sufficient to fill the annular space from the stage collar 13 to the bottom section of the branch horizontal hole a11. Then, clean water containing red tracers is continuously injected into the tubing string. When the injected pure cement slurry flows back into the annular space between the branch horizontal hole a11 open hole and the branch horizontal hole casing 12, approaching the stage collar 13, a ball dropping operation is performed at the surface wellhead. The instantaneous pressure increase when the steel ball reaches the stage collar 13 triggers its function. At this point, the pure cement slurry is located between the branch horizontal hole a11 open hole and the branch horizontal hole... The annular space between casing 12 flows back to the stage collar 13. At the same time, the clean water injected from the ground 1 begins to flow from the holes on both sides of the stage collar 13 to the annular space between the open hole of the branch horizontal hole a11 and the casing 12 of the branch horizontal hole. The injected clean water flows back from the holes of the stage collar 13 to the ground 1. When a red visual trace is found on the ground 1, it indicates that the clean water has been discharged back to the ground 1. At this time, the annular space inside the drill pipe 21 and the annular space between the drill pipe 21 and the second open casing 7 of the vertical well are sealed on the ground 1. This achieves the pressure-locked state of the annular space between the above-mentioned pipe string and the branch horizontal hole a11, so that the pure cement slurry injected into the bottom section of the stage collar 13 and the branch horizontal hole a11 can be allowed to solidify for 72 hours under pressure. After the cooling period, a larger steel ball is lowered into the inner annular hole of drill rod 21 on ground 1. When the larger steel ball reaches the release tool 20, high-pressure water is injected into the drill rod. At this moment, the pressure at the release tool 20 increases instantaneously, causing the release tool 20 to separate from the Φ139.7mm branch horizontal hole short sleeve 19. Drill rod 21 is then pulled out on ground 1, and a Φ120mm PDC drill bit is lowered to drill into the internal structure of the milling and grinding stage clamp 13, making the entire branch horizontal hole a11 unobstructed. Then, an alkaline soluble sealing bridge plug 14 is lowered into the branch horizontal hole orifice at drilling point a9 on the side of branch horizontal hole a11 to temporarily block branch horizontal hole a11. Figure 4 As shown.

[0053] Step 6: Based on the vertical well drilled in Step 3, firstly, a directional drilling tool is installed for side-drilling of the branch horizontal hole. Then, a Φ171.5mm drill bit assembly is installed and drilled using fiberglass casing at side-drilling point b15 to open the branch horizontal hole b16. The side-drilling point b15 is located above side-drilling point a9. The landing point of the horizontal section of the branch horizontal hole b16 is within the main key rock layer 2. The drilling depth of the horizontal section of the branch horizontal hole b16 reaches the left boundary of the main key rock layer 2.

[0054] Step 7: Perform casing installation and cementing operations on the branch horizontal hole b16 drilled in Step 6. The process is the same as in Step 5, and the final result is as follows. Figure 6 As shown.

[0055] Step 8: Then, lower the Φ89mm bare drill pipe assembly to 1-3m above the side drilling point b15. On the surface 1, adjust the mud pump to maximum flow rate and inject high-velocity alkaline circulating liquid into the well bottom to flush and clean the drill cuttings deposited at the alkaline soluble bridge plugs 8 and 14, which act as temporary seals. Simultaneously, rapidly dissolve the alkaline soluble bridge plugs 8 and 14 at the vertical well, branch horizontal holes a11, and branch horizontal holes b16, thereby ensuring the vertical well, branch horizontal holes a11, and branch horizontal holes b16 are unobstructed. Figure 7 As shown, this facilitates the smooth insertion of the fracturing tool string during subsequent construction.

[0056] Step 9: Perform segmented perforation hydraulic fracturing of branch horizontal borehole a11, segmented perforation hydraulic fracturing of branch horizontal borehole b16, and perforation fracturing of the vertical well in sequence. Use a directional drilling rig to re-enter branch horizontal boreholes a11 and b16 for segmented hydraulic fracturing. The process of the fracturing tool string entering the branch horizontal borehole is as follows: Taking branch horizontal borehole a11 as an example, first, lower the directional drilling rig to the side drilling point a9. When the directional drilling rig reaches the side drilling point a9, its slope end face is in any orientation. Then, lower the fracturing tool string to the bottom of the directional drilling rig. Then, slowly rotate the directional drilling rig at the ground 1. When the slowly rotating directional drilling rig reaches the orientation of the side drilling point a9, the fracturing tool string enters branch horizontal borehole a11. After entering branch horizontal borehole a11, continue to lower the fracturing tool string to the bottom of branch horizontal borehole a11. Then, use a retreating segmented perforation hydraulic fracturing method to ultimately form a large number of hydraulic fractures 22. The fracturing tool string is lowered into branch horizontal hole b16 in the same manner as in branch horizontal hole a11. After entering branch horizontal hole b16, the fracturing tool string continues to be lowered to the bottom of branch horizontal hole b16. Then, a retreating segmented perforation hydraulic fracturing method is used to ultimately form a large number of hydraulic fractures 22. The hydraulic fractures 22 formed by the above fracturing operation can effectively relieve pressure, thereby achieving the purpose of mine pressure disaster control. The purpose of perforation fracturing in vertical wells is to address the blind zone 18 of mine pressure disaster management in the main critical rock stratum 2 of the same depth vertical well section from the landing point 10 of the branch horizontal holes in the horizontal projection. The fracturing tool string is lowered into the vertical well section without the need for a directional fracturing device, relying solely on its own weight. Within the vertical well section of the main critical rock stratum 2, hydraulic fracturing is performed on both sides of the strike of the main critical rock stratum 2. The hydraulic fractures 22 formed by the fracturing within the vertical well section effectively address the blind zone 18 where the two horizontal branch holes cannot be used for segmented hydraulic fracturing. Based on the hydraulic fractures formed by the perforation hydraulic fracturing of the two branch horizontal holes and the vertical well, strong mine pressure disaster management can be efficiently achieved in the hard main critical rock stratum of the overlying roof on the coal seam mining face. Figure 8 and Figure 9 As shown.

Claims

1. A method for mitigating mine pressure disasters caused by hard roof in a T-shaped branch horizontal well on the surface, characterized in that, Includes the following steps: Step 1: Determine the main key rock strata (2) overlying the coal seam mining face (3), and accurately determine the vertical distance between the coal seam mining face (3) and the main key rock strata (2) overlying it, as well as the distance between the two from the ground (1). Step 2: Use a measuring point layout instrument to measure the geodetic coordinates of the four corner points of the boundary of the coal seam mining face (3). Calculate the geodetic coordinates of the center point A (4) of the coal seam mining face (3) based on the measured geodetic coordinate data of the four corner points of the boundary of the coal seam mining face (3). Use the measuring point layout instrument to accurately delineate the position of coordinate point B (5) projected onto the ground (1) directly above the center point A (4). Determine the coordinate point B (5) as the wellhead position of the vertical well to be drilled. Step 3: Based on the vertical distance parameters of the coal seam mining face (3) and the main key rock layer (2) from the ground (1) obtained in Step 1, drill a vertical well at the coordinate point B (5) determined in Step 2. The vertical well has a two-hole structure. The first hole is drilled to a depth of 5-10m in the stable bedrock, and the first casing (6) is installed and cemented with pure cement slurry. The second hole is drilled to a depth below the main key rock layer (2), and the second casing (7) is installed and cemented with pure cement slurry. Step 4: In the vertical well drilled in step 3, a directional drilling tool is installed for side drilling of the branch horizontal hole. Then, a Φ171.5mm drill bit assembly is installed. The branch horizontal hole a (11) is drilled at the side drilling point a (9) on the second casing (7). The landing point (10) of the horizontal section of the branch horizontal hole a (11) is located in the main key rock layer (2). The drilling depth of the horizontal section of the branch horizontal hole a (11) reaches the right boundary of the main key rock layer (2). A pressure-bearing alkaline soluble bridge plug (8) is installed 5-10m below the side drilling point a (9) of the branch horizontal hole a (11). Step 5, Perform casing and cementing operations on the branch horizontal hole a (11) drilled in Step 4: Run casing and tubing string into branch horizontal hole a (11). The casing and tubing string run into the hole from top to bottom are: Φ89mm drill pipe (21), drop tool (20), 1 Φ139.7mm branch horizontal hole short casing (19), stage clamp (13), Φ139.7mm branch horizontal hole casing (12); Run the above tubing string into branch horizontal hole a (11) to a distance of 1~2m from the bottom of the hole for normal circulation of cementing slurry. Lower the branch horizontal hole short casing (19) to the side drilling point a (9) of branch horizontal hole a (11); After the above tubing string is lowered to the designed hole depth, Cementing operation with pure cement slurry was carried out. After cementing and solidification, the inner annulus of the drill pipe (21) was opened on the ground (1) and a steel ball was lowered in. When the steel ball was lowered to the drop tool (20), high-pressure clean water was injected into the drill pipe. At this time, the pressure at the drop tool (20) increased instantly, causing the drop tool (20) to separate from the short casing (19) of the branch horizontal hole. The drill pipe (21) was pulled out on the ground (1), and a Φ120mm PDC drill bit was lowered to drill into the internal structure of the milling grade collar (13) to make the entire branch horizontal hole a (11) unobstructed. Then, an alkaline soluble sealing bridge plug (14) was lowered into the branch horizontal hole orifice at the drilling point a (9) on the side of the branch horizontal hole a (11) to temporarily plug the branch horizontal hole a (11). Step 6: Based on the vertical well drilled in Step 3, firstly, a directional tool is installed in the well for side drilling of the branch horizontal hole. Then, a Φ171.5mm drill bit assembly is installed. The branch horizontal hole b (16) is drilled at the side drilling point b (15) on the second casing (7). The side drilling point b (15) is located above the side drilling point a (9). The landing point of the horizontal section of the branch horizontal hole b (16) is located in the main key rock layer (2). The drilling depth of the horizontal section of the branch horizontal hole b (16) reaches the left boundary end of the main key rock layer (2). Step 7: Perform casing and cementing operations on the branch horizontal hole b (16) drilled in Step 6 above. The process is the same as in Step 5, and finally the bridge plug (14) is used to temporarily plug the branch horizontal hole b (16). Step 8: Lower the Φ89mm bare drill pipe assembly to 1-3m above the side drilling point b (15). On the ground (1), adjust the mud pump discharge to the maximum discharge and inject high-velocity alkaline circulating liquid into the bottom of the well to flush and clean the drill cuttings deposited on the alkaline soluble bridge plugs (8) and bridge plugs (14) that serve as temporary seals. At the same time, quickly dissolve the alkaline soluble bridge plugs (8) and bridge plugs (14) in the vertical well, branch horizontal hole a (11), and branch horizontal hole b (16), thereby making the vertical well, branch horizontal hole a (11), and branch horizontal hole b (16) unobstructed. Step 9, sequentially carry out hydraulic fracturing of branch horizontal hole a (11), hydraulic fracturing of branch horizontal hole b (16), and perforation fracturing of vertical well, forming a large number of hydraulic fractures (22). The hydraulic fractures (22) formed by the perforation fracturing of the two branch horizontal holes and the vertical well can efficiently realize the treatment of strong mine pressure disasters in the hard main key rock strata (2) of the overlying roof on the coal seam mining face. When performing segmented perforation hydraulic fracturing of branch horizontal holes a (11) and branch horizontal holes b (16) in step 9, the fracturing tool string is re-entered into branch horizontal holes a (11) and branch horizontal holes b (16) using the incline device to carry out segmented hydraulic fracturing construction. The process of fracturing tool string entering branch horizontal hole a (11): First, the directional tool is lowered to the side drilling point a (9). When the directional tool is lowered to the side drilling point a (9), its slope end face is in any position. Then, the fracturing tool string is lowered to the bottom of the directional tool. The directional tool is slowly rotated at the ground (1). When the slowly rotating directional tool is rotated to the side drilling point a (9), the fracturing tool string enters the branch horizontal hole a (11). After the fracturing tool string enters the branch horizontal hole a (11), the fracturing tool string is lowered to the bottom of the branch horizontal hole a (11). Then, the retreating segmented perforation hydraulic fracturing is used to finally form a large number of hydraulic fractures (22). The fracturing tool string is lowered into the branch horizontal hole b (16) in the same way as that into the branch horizontal hole a (11). After the fracturing tool string enters the branch horizontal hole b (16), it continues to be lowered to the bottom of the branch horizontal hole b (16). Then, a backward segmented perforation hydraulic fracturing is used to finally form a large number of hydraulic fractures (22). The hydraulic fractures (22) can effectively relieve pressure, thereby achieving the purpose of mine pressure disaster control. The landing point (10) of the branch horizontal holes a (11) and b (16) has a blind zone (18) for the treatment of mine pressure disasters in the main key rock layer (2) on the horizontal projection to the same depth of the vertical well section. This blind zone (18) is treated by perforating and fracturing the vertical well. The perforating and fracturing of the vertical well includes: lowering the fracturing tool string into the vertical well section of the main key rock layer (2) to carry out the perforating and fracturing construction in the vertical well section. The hydraulic fracture (22) formed can effectively treat the blind zone (18) that cannot be treated by the two branch horizontal holes segmented hydraulic fracturing.

2. The method for controlling mine pressure disasters on the hard roof of a T-shaped branch horizontal well on the ground as described in claim 1, characterized in that, In step 3, in the vertical well two-stage borehole structure, the first stage is drilled using a Φ346.1mm roller cone drill bit, and a Φ273.1mm J55 grade first-stage casing (6) is lowered in. Low-grade 32.5 cement ash with a specific gravity of 1.7~2.0g / cm³ is used. 3 Cementing with pure cement slurry; the second section was drilled using a Φ241.3mm PDC drill bit, and a Φ193.7mm N80 grade second section casing (7) was run in. The second section casing (7) run in at the pre-designed side drilling point of the two branch horizontal holes was made of Φ193.7mm fiberglass casing, and was prepared with high-grade 42.5 cement ash with a specific gravity of 1.8~2.10g / cm³. 3 Cementing with pure cement slurry; after the cementing of the second section is completed and the curing is allowed to proceed for 72 hours, a test of 15~20 MPa pressure must be conducted inside the casing to check the cementing and sealing of the second section casing (7). The pressure test time is 30 minutes and the pressure drop must be less than or equal to 0.5 MPa.

3. The method for controlling mine pressure disasters on the hard roof of a T-shaped branch horizontal well on the ground as described in claim 1, characterized in that, In step 5, after the tubing string is lowered to the designed borehole depth, pure cement slurry cementing is performed, using high-grade 42.5 cement mortar with a specific gravity of 1.8~2.10 g / cm³. 3 Pure cement slurry positive circulation cementing includes: First, the designed amount of pure cement slurry is injected into the tubing string. The designed amount of pure cement slurry is sufficient to fill the annular space from the grader collar (13) to the bottom section of the branch horizontal hole a (11). Then, clean water containing red traces is continuously injected into the tubing string. When the injected pure cement slurry flows back into the annular space between the open hole of the branch horizontal hole a (11) and the casing of the branch horizontal hole (12) and approaches the grader collar (13), a ball dropping operation is performed at the surface wellhead. The pressure increases instantaneously when the steel ball moves to the grader collar (13), triggering the effect of the grader collar (13). At this time, the pure cement slurry flows back into the annular space between the open hole of the branch horizontal hole a (11) and the casing of the branch horizontal hole (12) and reaches the position of the grader collar (13). At the same time, the water injected from the ground (1) begins to flow from the holes on both sides of the graded hoop (13) to the annular space between the open hole of the branch horizontal hole a (11) and the casing (12) of the branch horizontal hole. The injected water flows back from the holes of the graded hoop (13) to the ground (1). When a red visual trace is found on the ground (1), it indicates that the water has been discharged back to the ground (1). At this time, the annular space inside the drill pipe (21) and the annular space between the drill pipe (21) and the second open casing (7) of the vertical well are sealed on the ground (1) to achieve a pressurized state in the annular space between the pipe string and the branch horizontal hole a (11). The pure cement slurry injected into the bottom section of the graded hoop (13) and the branch horizontal hole a (11) is allowed to solidify for 72 hours under pressurized conditions.

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