Hypotonic coal seam hydraulic anti-reflection gas extraction promoting method

Abstract

The invention belongs to the technical field of mine gas treatment, and discloses a low-permeability coal seam hydraulic anti-reflection gas extraction promoting method which adopts a well-ground combination mode to increase drilling and fracturing efficiency on the basis of common hydraulic fracturing. The problems that fracturing cracks are closed and gas extraction attenuation is fast are solved through drilling of a ground L-shaped well and a periodic fracturing mode, and therefore the gas extraction efficiency is improved. According to the method, the staged fracturing tool string adopts a staged hydraulic fracturing mode, every time when the gas extraction pure quantity is reduced to a preset first threshold value, the staged fracturing tool string is dragged backwards to continue fracturing, the ball valve is opened after fracturing is finished, gas extraction is conducted again, and the gas extraction pure quantity is reduced to the preset first threshold value. The periodic fracturing mode not only can improve the fracturing influence range and the complexity of fracturing cracks, but also can avoid the problems that the cracks are closed in the later stage of extraction and the pure quantity of gas extraction is quickly attenuated, so that the gas extraction effect of the coal seam is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of mine gas control, and in particular relates to a method for hydraulically increasing the permeability of low-permeability coal seams and promoting gas extraction. Background Art

[0002] In order to promote coal seam gas extraction, scholars at home and abroad have developed a variety of coal seam permeability enhancement technologies. Among them, hydraulic permeability enhancement technology uses high-pressure water as a power source to expand and extend the original cracks in the coal seam, or artificially form new holes, slots, cracks, etc., to promote the displacement of the coal rock mass, thereby achieving the purpose of unloading the coal rock mass and increasing permeability.

[0003] Due to the characteristics of hydraulic permeability enhancement technology such as simple process, economy and practicality, it has been widely used in low-permeability and low-permeability coal seams in various mining areas for many years. Among them, hydraulic fracturing technology has become an important technical means to enhance permeability and promote gas extraction in low-permeability coal seams due to its advantage of a large impact range.

[0004] However, when hydraulic permeability enhancement technology is applied, the fractures in the coal seam gradually close due to stress, and gas extraction efficiency also gradually decreases. Existing solutions generally improve gas extraction efficiency by increasing the impact range of the fractures and increasing the complexity of the fractures, but they still cannot solve the problems of late fracture closure and the rapid attenuation of gas extraction.

[0005] Therefore, it is necessary to propose a method for hydraulically increasing the permeability of low-permeability coal seams to promote gas extraction, which can be used to solve the problems of hydraulic fracture closure and rapid attenuation of gas extraction. Summary of the Invention

[0006] The purpose of the present invention is to propose a method for hydraulically increasing the permeability of low-permeability coal seams to promote gas extraction. This method effectively solves the problems of fracture closure and rapid gas extraction attenuation through the drilling of L-shaped wells on the ground and periodic fracturing, thereby achieving an improvement in gas extraction efficiency.

[0007] In order to achieve the above-mentioned purpose, the present invention adopts the following technical solutions:

[0008] A method for hydraulically increasing permeability and promoting gas extraction in low-permeability coal seams comprises the following steps:

[0009] Step 1. Construct an L-shaped well on the surface and drill a hole through the mine to prepare the tunnel wall;

[0010] Step 2. Seal the bottom of the L-shaped well on the ground in the preparation tunnel and install the ball valve;

[0011] Step 3. Run a staged fracturing tool string from the ground into the L-shaped well to perform staged fracturing of the coal seam;

[0012] Step 4. Connect the surface L-shaped well to the gas extraction system underground to extract coal seam gas;

[0013] Step 5. During the gas extraction process, when the gas extraction volume drops to a preset first threshold, the ball valve is closed, the staged fracturing tool string is dragged backward, and fracturing is continued. After fracturing is completed, the ball valve is opened to perform gas extraction.

[0014] Step 6. Repeat step 5 until the increase in the pure amount of gas extraction after fracturing is less than a preset second threshold, and then remove the staged fracturing tool string from the ground.

[0015] Preferably, in step 1, the ground L-shaped well is constructed on the surface corresponding to the working face, and the horizontal section of the ground L-shaped well is located in the middle of the working face.

[0016] Preferably, in step 2, the bottom of the L-shaped well on the ground is sealed with expansive cement, and the length of the bottom sealing section is not less than 50 m.

[0017] Preferably, the compressive strength of the ball valve in step 2 is not less than 70 MPa.

[0018] Preferably, the number of fracturing stages of the staged fracturing tool string is not less than 3, and the displacement of the fracturing pump used in the staged fracturing tool string is not less than 1.5m 3 / min, the maximum pressure resistance value is not less than 105MPa, and the pressure holding time during fracturing is not less than 5h.

[0019] Preferably, the staged fracturing tool string is provided with a plurality of capsules, and the distance between two adjacent capsules is no more than 120m.

[0020] Preferably, the first threshold value preset in step 5 is 1 / 5 of the peak value of the extracted gas pure amount.

[0021] Preferably, in step 5, the distance that the staged fracturing tool string is dragged backward each time is not less than 15 m.

[0022] Preferably, the second threshold value preset in step 6 is 20%.

[0023] The present invention has the following advantages:

[0024] As described above, the present invention describes a method for promoting gas extraction by hydraulically increasing permeability in low-permeability coal seams. This method, based on ordinary hydraulic fracturing, adopts a well-ground combination method to increase drilling and fracturing efficiency, and solves the problems of fracture closure and rapid attenuation of gas extraction by drilling L-shaped wells on the ground and periodic fracturing, thereby improving gas extraction efficiency. The segmented fracturing tool string of the method of the present invention adopts a segmented hydraulic fracturing method. Whenever the pure amount of gas extraction drops to a preset first threshold, the segmented fracturing tool string is dragged backward to continue fracturing, and the ball valve is opened after the fracturing is completed to extract gas again. The periodic fracturing method can not only increase the fracturing influence range and the complexity of the fracturing cracks, but also avoid the problems of crack closure and rapid attenuation of the pure amount of gas extraction in the late extraction period, thereby improving the coal seam gas extraction effect. In the field of low-permeability coal seam gas control, the extraction efficiency of the low-permeability coal seam hydraulically increasing permeability gas extraction method of the present invention is better than other similar hydraulic permeability increasing methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The figure is a flow chart of a method for hydraulically increasing permeability and promoting gas extraction in low-permeability coal seams according to an embodiment of the present invention.

[0026] Figure 2 Schematic diagram of the construction and bottom sealing of an L-shaped well on the ground in an embodiment of the present invention.

[0027] Figure 3 This is a schematic diagram of the first fracturing section of the method for promoting gas extraction by hydraulic permeability enhancement in low-permeability coal seams in an embodiment of the present invention.

[0028] Figure 4 This is a schematic plan view of the first fracturing process using the method for hydraulically increasing permeability and promoting gas extraction in low-permeability coal seams in an embodiment of the present invention.

[0029] Figure 5 This is a schematic diagram of the second fracturing section of the method for promoting gas extraction by hydraulic permeability enhancement in low-permeability coal seams in an embodiment of the present invention.

[0030] Figure 6 This is a schematic plan view of the second fracturing process using the method for hydraulically increasing permeability and promoting gas extraction in low-permeability coal seams in an embodiment of the present invention.

[0031] Figure 7 This is a schematic diagram of the third fracturing section of the method for promoting gas extraction by hydraulic permeability enhancement in low-permeability coal seams in an embodiment of the present invention.

[0032] Figure 8 This is a schematic plan view of the third fracturing process using the method for hydraulically increasing permeability and promoting gas extraction in low-permeability coal seams in an embodiment of the present invention.

[0033] Among them, 1-surface L-shaped well, 2-bottom hole sealing section, 3-ball valve, 4-staged fracturing tool string, 41-fracturing section, 42-capsule, 5-fracturing fracture, 6-coal seam, 7-prepared tunnel. DETAILED DESCRIPTION

[0034] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0035] Example 1

[0036] The present invention provides a method for hydraulically increasing the permeability of low-permeability coal seams to promote gas extraction. The method can be applied to gas control in low-permeability coal seams. The method of the present invention adopts the method of efficient drilling of surface wells and periodic fracturing, which effectively solves the problems of fracture closure and rapid attenuation of gas extraction, thereby improving gas extraction efficiency.

[0037] like Figure 1 As shown, a method for hydraulically increasing the permeability of low-permeability coal seams to promote gas extraction specifically includes the following steps:

[0038] Step 1. Construct a surface L-shaped well 1 and drill a hole through the mine to prepare the tunnel wall.

[0039] like Figure 2 As shown, the ground L-shaped well 1 is constructed on the surface corresponding to the working face, the horizontal section of the ground well is in the middle of the working face, the drilling trajectory is designed according to the mine histogram and mining conditions, and the directional drill penetrates the coal wall of the working face corresponding to the preparation tunnel 7.

[0040] Step 2. Seal the bottom of the L-shaped well 1 on the ground in the preparation tunnel 7 and install the ball valve 3.

[0041] In this embodiment, expansive cement is preferably used to seal the bottom of the L-shaped well 1 on the ground, and the length of the bottom sealing section 2 is not less than 50 m. The compressive strength of the ball valve 3 is not less than 70 MPa.

[0042] The method of the present invention makes full use of the underground tunnel, drills through the mine underground tunnel wall, and installs a ball valve 3 at the hole mouth, which greatly facilitates the drainage and slag removal of the drill hole after hydraulic fracturing and reduces the water locking effect of coal seam gas.

[0043] In addition, compared with the method of arranging the boreholes in the rock layer, the drilling construction of the method of the present invention is all in the coal seam 6. This design can facilitate the drainage of the boreholes while reducing the possibility of the boreholes collapsing.

[0044] Step 3: Run the staged fracturing tool string 4 from the ground to the surface L-shaped well 1 to perform staged fracturing of the coal seam.

[0045] The number of fracturing segments 41 of the staged fracturing tool string 4 is not less than 3, and the displacement of the fracturing pump used in the staged fracturing tool string 4 is not less than 1.5m 3 / min, the maximum pressure resistance value is not less than 105MPa, and the pressure holding time during fracturing is not less than 5h.

[0046] The staged fracturing tool string 4 is provided with a number of capsules 42, which are respectively arranged at the front and rear ends of the fracturing section 41. The distance between two adjacent capsules 42 is no more than 120m. High-pressure water is injected into the capsules 42 to expand the capsules 42. The capsules 42 at the front and rear ends of the fracturing section 41 and the inner wall of the borehole where the fracturing section 41 is located form a relatively closed space, which helps the high-pressure water in each fracturing section 41 to diffuse to a longer distance to the coal seam 6, thereby improving the hydraulic fracturing effect on the coal seam 6.

[0047] The method of the present invention adopts a method of performing staged fracturing multiple times, and the fracturing efficiency is higher.

[0048] Step 4. Connect the surface L-shaped well 1 to the gas extraction system underground to extract coal seam gas.

[0049] Step 5. During the gas extraction process, when the pure amount of gas extraction drops to a preset first threshold, close the ball valve 3, drag the staged fracturing tool string 4 backward, and continue fracturing. After fracturing is completed, open the ball valve 3 to perform gas extraction.

[0050] In this embodiment, the preset first threshold is 1 / 5 of the peak value of the extracted gas pure amount.

[0051] In step 5, the staged fracturing tool string 4 is dragged backwards for a distance of not less than 15 m each time.

[0052] In order to overcome the problem of obvious attenuation of gas extraction efficiency when the fracturing crack 5 is closed under the action of ground stress, the method of the present invention adopts multiple pressures and multiple fracture creation methods. When it is found that the pure amount of gas extraction has dropped significantly, re-fracturing and fracture creation are performed to increase the gas migration channel.

[0053] Specifically, if a significant decrease in the pure gas extraction rate is detected during gas extraction, ball valve 3 is closed, and the staged fracturing tool string 4 is dragged backward to continue fracturing. After fracturing is complete, ball valve 3 is opened to resume gas extraction. During gas extraction, if the pure gas extraction rate drops to 1 / 5 of the peak extraction rate, the gas extraction rate is considered to have significantly decreased.

[0054] Step 6: Repeat step 5 until the increase in the pure amount of gas extraction after fracturing is less than a preset second threshold, and then remove the staged fracturing tool string 4 from the ground.

[0055] In this embodiment, the preset second threshold is 20%.

[0056] Figures 3 to 8The cross-sectional schematic diagram and plane schematic diagram obtained by the first, second and third fracturing of the low-permeability coal seam hydraulic permeability enhancement gas extraction method are shown. The method of the present invention adopts a well-ground combined approach to perform multiple staged fracturing to increase drilling and fracturing efficiency to improve the coal seam gas extraction effect.

[0057] If, after a certain period of extraction, the gas extraction yield decreases significantly, step 5 is repeated, ball valve 3 is closed, and the staged fracturing tool string 4 is continued to be pulled backward for fracturing. After fracturing is completed, ball valve 3 is opened to extract gas. This cycle continues until the gas extraction effect after fracturing is no longer noticeable, at which point the staged fracturing tool is removed from the surface. Insignificant gas extraction refers to an increase in the gas extraction yield after fracturing of less than 20%.

[0058] The method of the present invention, based on ordinary hydraulic fracturing, adopts a well-ground combination method to increase the drilling and fracturing efficiency, and solves the problems of fracture closure and rapid attenuation of gas extraction by drilling the L-shaped well 1 on the ground and cyclic fracturing, thereby improving the gas extraction efficiency. The segmented fracturing tool string 4 of the method of the present invention adopts a segmented hydraulic fracturing method. Whenever the pure amount of gas extraction drops to a preset first threshold, the segmented fracturing tool string 4 is dragged backward to continue fracturing, and the ball valve 3 is opened after the fracturing is completed to extract gas again. The cyclic fracturing method can not only increase the fracturing influence range and the complexity of the fracturing fracture 5, but also avoid the problems of fracture closure and rapid attenuation of the pure amount of gas extraction in the late extraction period, thereby improving the coal seam gas extraction effect. In the field of low-permeability coal seam gas control, the extraction efficiency of the low-permeability coal seam hydraulic permeability enhancement gas extraction method of the present invention is better than other similar hydraulic permeability enhancement methods.

[0059] Of course, the above description is only a preferred embodiment of the present invention, and the present invention is not limited to the above-mentioned embodiments. It should be noted that all equivalent substitutions and obvious deformation forms made by any technician familiar with this field under the guidance of this specification fall within the substantive scope of this specification and should be protected by the present invention.

Claims

1. A method for hydraulic permeability enhancement and gas extraction in low-permeability coal seams, characterized in that: The steps include: Step 1. Construct an L-shaped well on the surface and drill a hole through the mine to prepare the tunnel wall; Step 2. Seal the bottom of the L-shaped well on the ground in the preparation tunnel and install the ball valve; Step 3. Run a staged fracturing tool string from the ground into the L-shaped well to perform staged fracturing of the coal seam; Step 4. Connect the surface L-shaped well to the gas extraction system to extract coal seam gas underground; Step 5. During the gas extraction process, when the gas extraction volume drops to a preset first threshold, the ball valve is closed, the staged fracturing tool string is dragged backward, and fracturing is continued. After fracturing is completed, the ball valve is opened to perform gas extraction. Step 6. Repeat step 5 until the increase in the pure amount of gas extraction after fracturing is less than a preset second threshold, and then remove the staged fracturing tool string from the ground.

2. The method for promoting gas extraction by hydraulic permeability enhancement in low-permeability coal seams according to claim 1, characterized in that: In step 1, a ground L-shaped well is constructed on the surface corresponding to the working face, and the horizontal section of the ground L-shaped well is located in the middle of the working face.

3. The method for promoting gas extraction by hydraulic permeability enhancement in low-permeability coal seams according to claim 1, characterized in that: In step 2, the bottom of the L-shaped well on the ground is sealed with expansive cement, and the length of the bottom sealing section is not less than 50m.

4. The method for promoting gas extraction by hydraulic permeability enhancement in low-permeability coal seams according to claim 1, characterized in that: In step 2, the pressure resistance of the ball valve is not less than 70 MPa.

5. The method for promoting gas extraction by hydraulic permeability enhancement in low-permeability coal seams according to claim 1, characterized in that: The number of fracturing stages of the staged fracturing tool string is not less than 3, and the displacement of the fracturing pump used in the staged fracturing tool string is not less than 1.5m 3 / min, the maximum pressure resistance value is not less than 105MPa, and the pressure holding time during fracturing is not less than 5h.

6. The method for promoting gas extraction by hydraulic permeability enhancement in low-permeability coal seams according to claim 1, characterized in that: The staged fracturing tool string is provided with a plurality of capsules, and the distance between two adjacent capsules is no more than 120m.

7. The method for promoting gas extraction by hydraulic permeability enhancement in low-permeability coal seams according to claim 1, characterized in that: In step 5, the preset first threshold is 1 / 5 of the peak value of the extracted gas pure amount.

8. The method for promoting gas extraction by hydraulic permeability enhancement in low-permeability coal seams according to claim 1, characterized in that: In step 5, the staged fracturing tool string is dragged backwards for a distance of not less than 15 m each time.

9. The method for promoting gas extraction by hydraulic permeability enhancement in low-permeability coal seams according to claim 1, characterized in that: In step 6, the preset second threshold is 20%.

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