A method for strengthening deep thick coal seam gas extraction based on horizontal well cavitation

Abstract

This invention discloses a method for enhancing gas extraction from deep, extra-thick coal seams based on horizontal well drilling. Through the coordinated use of surface drilling, horizontal wells, and auxiliary layers, the pressure-relief cavity created by the horizontal well has a large influence range. Under deep stress, the coal seam fractures are fully developed, providing a broad channel for gas extraction. The presence of the auxiliary layer is equivalent to mining a protective layer beneath the deep, thick coal seam, further reducing pressure and increasing permeability significantly. Furthermore, the surface drilling and horizontal well drilling solves the problems of borehole collapse, high drilling difficulty, and high construction risks associated with enhanced gas extraction in underground engineering, improving gas extraction concentration and utilization rate. This method maximizes permeability enhancement in deep, thick coal seams while minimizing the number of boreholes and construction difficulty, achieving safe and efficient gas extraction from deep, thick coal seams. This method also has wide applicability.

Description

Technical Field

[0001] This invention relates to a method for enhancing gas extraction in deep, thick coal seams, specifically a method for enhancing gas extraction in deep, extra-thick coal seams based on horizontal well drilling, belonging to the field of deep mine gas control technology. Background Technology

[0002] Thick coal seams account for approximately 45% of my country's total coal reserves and production, making them the mainstay for high-yield and efficient coal mining. However, with the gradual depletion of shallow coal resources, more and more mines are entering the deep coal seam mining stage. Compared to shallow coal seams, deep coal seams have more complex geological conditions and stress fields. During mining, the amount of gas emitted is enormous, and the poor permeability of the coal seam makes gas extraction difficult, leading to more complex gas hazard situations. Currently, the commonly used method is to use various boreholes underground for gas extraction. This method has the following limitations: (1) Underground space is limited, construction is difficult and time-consuming, requiring planning and deployment well in advance, and the contradiction between excavation and extraction is prominent; it is difficult to meet the needs of rapid control of gas in thick coal seams; (2) Underground construction conditions are poor, and drilling technology is limited, resulting in a small impact range of underground drilling, requiring a large number of boreholes to achieve the gas control effect, and the construction cost is high; (3) Under the complex stress of deep thick coal seams, drilling is prone to hole collapse and drill bit burial, making hole formation difficult and construction risky, and also affecting the gas extraction effect in the later stage. Therefore, the control of gas in deep thick coal seams has become one of the urgent problems to be solved in the coal industry in recent years. Summary of the Invention

[0003] To address the problems existing in the prior art, this invention provides a method for enhancing gas extraction from deep, extra-thick coal seams based on horizontal well drilling. By coordinating surface drilling, horizontal wells, and auxiliary layers, this method maximizes permeability enhancement in deep, thick coal seams while minimizing the number of boreholes and construction difficulty, thereby achieving safe and efficient gas extraction from deep, thick coal seams.

[0004] To achieve the above objectives, the technical solution adopted by this invention is: a method for enhancing gas extraction from deep, extra-thick coal seams based on horizontal well drilling, the specific steps of which are as follows:

[0005] A. Based on the dip direction of the deep thick coal seam, select the ground position corresponding to the higher elevation of the coal seam to start drilling the first well. The first well is perpendicular to the ground and penetrates the coal seam a certain distance before retracting the drill. Then, drill the first horizontal well from the inner wall of the first well along the top of the coal seam and parallel to the dip direction toward the lower elevation of the coal seam.

[0006] B. The first horizontal well is constructed to create a cavity, which will eventually be transformed into a pressure relief cavity. Under the action of complex stress in the deep part, the pressure relief cavity is subjected to stress compression from all directions, which relieves the pressure on the coal body around the cavity and causes the fractures to develop continuously, which facilitates gas extraction. Then the first well is connected to the surface extraction pumping station for gas extraction.

[0007] C. Along the direction of the first horizontal well, construct a second well on the surface at a certain distance from the first well, ensuring the second well is connected to the first horizontal well. Below the deep, thick coal seam, select a coal seam or soft rock layer without outburst risk as the first auxiliary layer. After the second well penetrates the coal seam and reaches the first auxiliary layer, perform slotting on the surrounding first auxiliary layer from within the second well. Determine the slotting thickness of the first auxiliary layer based on the distance between the first auxiliary layer and the deep, thick coal seam, ensuring that the expansion and deformation of the top of the deep, thick coal seam after slotting is not excessive. The permeability is less than 0.3%, ensuring that the deep thick coal seam is fully depressurized and permeable. After the fracturing is completed, the in-situ stress of the overlying deep thick coal seam is redistributed to reach a new equilibrium. During this process, the change in in-situ stress leads to the development and expansion of fractures in the deep thick coal seam, and the permeability of the coal seam is greatly improved, creating conditions for efficient gas extraction. At the same time, since the first horizontal well is laid along the dip of the coal seam, the wastewater generated during the cavity creation process of the first horizontal well flows into the first auxiliary layer under the action of gravity, avoiding the presence of wastewater in the deep thick coal seam, which would affect gas desorption and flow.

[0008] D. After completing step C, construct a second horizontal well from the inner wall of the second well along the top of the coal seam and parallel to the inclined direction towards the lower elevation of the coal seam; after completion, construct a cavity in the second horizontal well, and finally transform the second horizontal well into a pressure relief cavity; under the action of complex stress in the deep, the pressure relief cavity is subjected to stress compression from all directions, causing the coal body around the cavity to be depressurized, and the fractures to develop continuously, which facilitates gas extraction; then connect the second well to the surface extraction pump station for gas extraction.

[0009] E. Repeat steps C and D multiple times, drilling wells, auxiliary layers, and horizontal wells in sequence along the dip direction of the deep thick coal seam until the horizontal well drilling and enhanced gas extraction project in the mining area is completed.

[0010] Furthermore, the first well was withdrawn after penetrating the coal seam by 0.5 to 1 meter.

[0011] Furthermore, each horizontal well is 150-200m in length.

[0012] Furthermore, each horizontal well was drilled using a 350mm diameter drill bit.

[0013] Furthermore, the transformation of the horizontal well into a pressure relief cave is specifically carried out by: using a 500mm diameter drill bit, a 1200mm diameter drill bit, a 1500mm diameter drill bit, and a water jet drill bit in sequence to create a cavity, ultimately transforming the horizontal well into a pressure relief cave with a diameter of not less than 1.5m.

[0014] Compared to existing technologies, the pressure relief caverns created by horizontal well modification reach a diameter of 1.5m, with a large impact area. Under deep stress, the coal seam fractures are fully developed, providing a broad channel for gas drainage. Simultaneously, the presence of the auxiliary layer is equivalent to mining a protective layer beneath the deep, thick coal seam, leading to a redistribution of in-situ stress to achieve a new equilibrium. This further reduces pressure and enhances permeability in the deep, thick coal seam, resulting in uniform pressure relief and a significant increase in permeability. Furthermore, the horizontal wells are arranged with a gradual downward inclination from higher elevations according to the coal seam dip angle. This allows wastewater generated during the horizontal well creation process to flow into the auxiliary layer, preventing wastewater from remaining in the deep, thick coal seam and affecting gas desorption and flow. This avoids the "water lock" effect of hydraulic well creation and expansion, further enhancing gas drainage from deep, thick coal seams. This method, through the construction of horizontal wells via surface drilling, solves the problems of borehole collapse, drill bit burial, high drilling difficulty, and high construction risks in underground enhanced gas drainage projects. It shortens the time required to achieve gas drainage standards, improves gas drainage concentration and gas utilization rate. Therefore, this method, which combines surface drilling, horizontal wells, and auxiliary layers, maximizes permeability enhancement in deep thick coal seams while minimizing the number of boreholes and construction difficulty, achieving safe and efficient gas drainage from deep thick coal seams. Moreover, this method has wide applicability. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the overall layout of the present invention.

[0016] In the diagram: 1. First well; 2. Deep thick coal seam; 3. First horizontal well; 4. Second well; 5. First auxiliary layer; 6. Second horizontal well; 7. Third well; 8. Second auxiliary layer; 9. Third horizontal well; 10. Fourth well; 11. Third auxiliary layer; 12. Fourth horizontal well. Detailed Implementation

[0017] The present invention will be further described below.

[0018] like Figure 1 As shown, the specific steps of the present invention are as follows:

[0019] A. Based on the dip direction of the deep thick coal seam 2, select the ground position corresponding to the higher elevation of the coal seam to start drilling the first well 1. The first well 1 is perpendicular to the ground and penetrates the coal seam by 0.5~1m before retracting the drill. Then, start drilling the first horizontal well 3 from the inner wall of the first well 1 along the top of the coal seam and parallel to the dip direction toward the lower elevation of the coal seam. The first horizontal well 3 is drilled using a drill bit with a diameter of 350mm.

[0020] B. The first horizontal well 3 will be constructed using a cavity-building process to transform it into a pressure-relief cavity. Specifically, based on the existing 350mm diameter first horizontal well 3, a cavity will be built using 500mm, 1200mm, and 1500mm diameter drill bits, followed by a hydraulic jet drill bit, ultimately transforming the horizontal well into a pressure-relief cavity with a diameter of not less than 1.5m. Under the influence of complex deep stress, the pressure-relief cavity will be subjected to stress compression from all directions, causing the coal body around the cavity to depressurize and fractures to develop, facilitating gas extraction. Then, the first well 1 will be connected to the surface extraction pump station for gas extraction.

[0021] C. Along the direction of the first horizontal well 3, construct a second well 4 on the ground at a certain distance from the first well 1, ensuring the second well 4 is connected to the first horizontal well 3. Below the deep thick coal seam 2, select a coal seam or soft rock layer without outburst risk as the first auxiliary layer 5. After the second well 4 penetrates the coal seam and reaches the first auxiliary layer 5, perform slotting on the surrounding first auxiliary layer 5 from within the second well 4. Determine the slotting thickness of the first auxiliary layer 5 based on the distance between the first auxiliary layer 5 and the deep thick coal seam 2, ensuring the top of the deep thick coal seam 2 expands after slotting. The deformation amount is not less than 0.3%, ensuring that the deep thick coal seam 2 is fully depressurized and permeable. After the cutting is completed, the in-situ stress of the overlying deep thick coal seam 2 is redistributed to reach a new equilibrium. During this process, the change in in-situ stress leads to the development and expansion of fractures in the deep thick coal seam 2, and the permeability of the coal seam is greatly improved, creating conditions for efficient gas extraction. At the same time, since the first horizontal well 3 is laid along the dip of the coal seam, the sewage generated during the cavity creation process of the first horizontal well 3 flows into the first auxiliary layer 5 under the action of gravity, avoiding the presence of sewage in the deep thick coal seam 2, which would affect gas desorption and flow.

[0022] D. After completing step C, construct the second horizontal well 6 from the inner wall of the second well 4 along the top of the coal seam and parallel to the inclined direction towards the lower elevation of the coal seam. The second horizontal well 6 will be drilled using a 350mm diameter drill bit. After completion, the second horizontal well 6 will be transformed into a pressure relief cavity. Specifically, based on the original 350mm diameter second horizontal well 6, a cavity will be created sequentially using 500mm, 1200mm, and 1500mm diameter drill bits, followed by a hydraulic jet drill bit. This will ultimately transform the horizontal well into a pressure relief cavity with a diameter of not less than 1.5m. Under the influence of complex deep stress, this pressure relief cavity will be subjected to stress compression from all directions, causing the coal body around the cavity to depressurize and fractures to develop, facilitating gas extraction. Then, the second well 4 will be connected to the surface extraction pump station for gas extraction.

[0023] E. Repeat steps C and D multiple times, constructing wells, auxiliary layers, and horizontal wells sequentially along the dip direction of the deep thick coal seam. The length of each horizontal well is 150-200m, until the horizontal well drilling and enhanced gas extraction project in the mining area is completed.

[0024] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for enhancing gas extraction from deep, extra-thick coal seams based on horizontal well drilling, characterized in that, The specific steps are as follows: A. Based on the dip direction of the deep thick coal seam, select the ground position corresponding to the higher elevation of the coal seam to start drilling the first well. The first well is perpendicular to the ground and penetrates the coal seam a certain distance before retracting the drill. Then, drill the first horizontal well from the inner wall of the first well along the top of the coal seam and parallel to the dip direction toward the lower elevation of the coal seam. B. The first horizontal well is constructed to create a cavity, which will eventually be transformed into a pressure relief cavity. Under the action of complex stress in the deep part, the pressure relief cavity is subjected to stress compression from all directions, which relieves the pressure on the coal body around the cavity and causes the fractures to develop continuously, which facilitates gas extraction. Then the first well is connected to the surface extraction pumping station for gas extraction. C. Along the direction of the first horizontal well, a second well is constructed on the ground at a certain distance from the first well, and the second well is connected to the first horizontal well. A coal seam or soft rock layer without outburst risk is selected below the deep thick coal seam as the first auxiliary layer. After the second well penetrates the coal seam and reaches the first auxiliary layer, the surrounding first auxiliary layer is cut from the second well. The thickness of the first auxiliary layer is determined based on the distance between the first auxiliary layer and the deep thick coal seam, ensuring that the expansion deformation of the top of the deep thick coal seam after the cutting is not less than 0.3%, thus ensuring that the deep thick coal seam is fully depressurized and permeable. After the cutting is completed, the in-situ stress of the overlying deep thick coal seam is redistributed to reach a new equilibrium. During this process, the change in in-situ stress leads to the development and expansion of fractures in the deep thick coal seam, significantly increasing the permeability of the coal seam and creating conditions for efficient gas extraction. At the same time, since the first horizontal well is laid along the dip of the coal seam, the wastewater generated during the cavity creation process of the first horizontal well flows into the first auxiliary layer under the action of gravity, preventing the wastewater from existing in the deep thick coal seam and affecting gas desorption and flow. D. After completing step C, construct a second horizontal well from the inner wall of the second well along the top of the coal seam and parallel to the inclined direction towards the lower elevation of the coal seam; after completion, construct a cavity in the second horizontal well, and finally transform the second horizontal well into a pressure relief cavity; under the action of complex stress in the deep, the pressure relief cavity is subjected to stress compression from all directions, causing the coal body around the cavity to be depressurized, and the fractures to develop continuously, which facilitates gas extraction; then connect the second well to the surface extraction pump station for gas extraction. E. Repeat steps C and D multiple times, drilling wells, auxiliary layers, and horizontal wells in sequence along the dip direction of the deep thick coal seam until the horizontal well drilling and enhanced gas extraction project in the mining area is completed.

2. The method for enhancing gas extraction from deep, extra-thick coal seams based on horizontal well drilling as described in claim 1, characterized in that, The first well was withdrawn after penetrating the coal seam by 0.5 to 1 meter.

3. The method for enhancing gas extraction from deep, extra-thick coal seams based on horizontal well drilling as described in claim 1, characterized in that, Each horizontal well is 150-200m in length.

4. The method for enhancing gas extraction from deep, extra-thick coal seams based on horizontal well drilling as described in claim 1, characterized in that, Each horizontal well was drilled using a 350mm diameter drill bit.

5. The method for enhancing gas extraction from deep, extra-thick coal seams based on horizontal well drilling as described in claim 4, characterized in that, The specific steps for transforming a horizontal well into a pressure relief cavern are as follows: Based on the existing first horizontal well with a diameter of 350mm, drill bits with diameters of 500mm, 1200mm, 1500mm, and water jet drill bits are used to create a cavern, ultimately transforming the horizontal well into a pressure relief cavern with a diameter of not less than 1.5m.

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