A method for accurate permeability enhancement in soft and thick coal seams by synergizing pressure relief and fracturing

Abstract

The present invention discloses a method for precise regional permeability enhancement of soft and thick coal seams by coordinated pressure relief and fracturing. The method optimizes the key parameters of hydraulic cavitation and hydraulic fracturing by calculating through a specific simulation method, realizes precise fracture creation and pressure relief and permeability enhancement of the coal seam, and optimizes the permeability enhancement effect of the coal seam. After determining the optimal parameters, segmented cavitation is performed in the coal seam according to the set optimal parameters to realize pressure relief of the coal seam, form disturbed fractures around the borehole, and improve the gas extraction effect. Then, segmented fracturing is performed in the roof rock layer of the soft coal seam by using a directional long borehole, so that the fractures extend downward into the soft coal seam, thereby forming a spatial three-dimensional fracture network in the soft coal seam. In this method, the internal stress distribution of the soft coal seam is first changed by the pressure relief cave, and then a three-dimensional fracture network is generated by the segmented fracturing and stress guidance. The segmented cavitation and segmented fracturing are combined, thereby realizing uniform pressure relief and permeability enhancement of the coal seam, and providing an advantageous channel for gas flow.

Description

Technical Field

[0001] The present invention relates to the technical field of permeability enhancement and efficient gas extraction in soft coal seams, and in particular to a method for precise permeability enhancement in a coordinated area of ​​pressure relief and fracturing in soft and thick coal seams. Background Art

[0002] Gas is one of the major sources of disasters that threaten coal mine safety. At present, the most important measure to control gas is gas extraction from underground boreholes in coal mines. However, due to the characteristics of micropores, low permeability, and strong adsorption in my country's coal seams, coal seam gas extraction is difficult, gas extraction efficiency is low, and the extraction cycle is long, which seriously affects the mining and replacement work of mines. For this reason, it is necessary to artificially transform the coal seams to improve their permeability. Traditional and commonly used coal seam permeability enhancement technologies include hydraulic cavitation, hydraulic fracturing and other measures. These measures have been widely used in traditional coal seam gas control projects and have achieved good results.

[0003] However, with the continuous development of coal mine mechanization, automation and even intelligent technology, coal mine production is constantly developing towards high yield and high efficiency, and it has also put forward new requirements for regional gas control capabilities. To this end, directional long drilling technology has been continuously developed in recent years and has gradually been popularized in coal mines. However, for soft coal seams, due to the poor permeability of coal seams, the extraction effect after long drilling construction is very poor, and reservoir transformation is required. However, the traditional permeability enhancement technology has poor effect on soft and thick coal seams after construction, which will lead to uneven coal seam gas extraction, prone to extraction blind spots, and threaten mining safety.

[0004] Therefore, how to provide a new permeability enhancement method that can significantly improve the gas extraction effect of thick coal seams by accurately enhancing the permeability of soft thick coal seams, thereby ensuring efficient recovery of coal seams, is the research direction required by the present invention. Summary of the invention

[0005] In view of the problems existing in the above-mentioned prior art, the present invention provides a method for precise permeability enhancement of the soft and thick coal seams by coordinating pressure relief and fracturing. By combining segmented cavitation and segmented fracturing, a three-dimensional fracture network can be formed in the soft coal seams, thereby achieving precise permeability enhancement of the soft and thick coal seams, thereby significantly improving the gas extraction effect of the thick coal seams and ensuring the efficient recovery of the coal seams.

[0006] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a method for accurate permeability enhancement of soft and thick coal seams by pressure relief and fracturing coordination, the specific steps are:

[0007] Step 1: Test the mechanical properties of the soft coal seam to obtain the elastic modulus, Poisson's ratio, compressive strength, tensile strength, cohesion and internal friction angle, test the gas occurrence characteristics of the soft coal seam, and obtain the gas pressure and adsorption constant; carry out numerical simulation research based on COMOSOL Multiphysics numerical simulation software to obtain the gas pressure distribution characteristics around the borehole, change the diameter of the pressure relief cave, and use the minimum gas pressure at the midpoint of the line connecting two adjacent cavitation boreholes (i.e., simulate the use of conventional directional drilling technology to construct a directional long borehole in the soft coal seam on the side wall of the air intake tunnel toward the return air tunnel) as the judgment criterion to obtain the optimal diameter of the pressure relief cave; carry out numerical simulation research based on FLAC3D numerical simulation software to obtain the deformation characteristics around the pressure relief cave, change the distance between two adjacent pressure relief caves on the same cavitation borehole, and use the displacement of the cave wall as the judgment index. When the displacement is greater than a certain value, the pressure relief cave is determined to be unstable, and the minimum value of the distance l1 between the two adjacent pressure relief caves corresponding to the critical instability of the pressure relief cave is determined as the optimal cave spacing;

[0008] Step 2: Use conventional directional drilling technology to construct a directional long borehole in the soft coal seam on the side wall of the air inlet tunnel toward the return air tunnel. The directional long borehole is used as a hole-making borehole. After the construction is completed, the drill tool is withdrawn;

[0009] Step 3: Install a high-pressure ejector on the drill, adjust the pressure and flow of the high-pressure water pump, rotate the drill in the hole-making hole of step 2 and use the drill back punching method to achieve the optimal diameter of the pressure relief cave obtained by simulation in step 1, and construct a pressure relief cave every distance l1 along the direction of the hole-making hole; then repeat steps 2 and 3, construct a hole-making hole every distance along the direction of the air inlet tunnel, and each hole-making hole is parallel to each other;

[0010] Step 4: Carry out numerical simulation research based on ABQUS numerical simulation software, change the distance h between the fracturing hole (i.e., simulate the use of conventional directional drilling technology to construct a borehole from the air intake tunnel into the roof rock layer of the soft coal seam, and the borehole is constructed in a directional long drilling manner after entering the roof rock layer, and the directional long drilling part is used as the fracturing hole) and the upper boundary of the soft coal seam, obtain the trajectory of the crack, and use the ability of the crack trajectory to pass through the roof rock layer and enter the soft coal seam as the judgment criterion to determine the reasonable distance between the fracturing hole and the upper boundary of the soft coal seam; carry out numerical simulation research based on COMOSOL Multiphysics numerical simulation software, change the distance l2 between two adjacent cracks, and use the gas pressure at each location of the soft coal seam within a set time as the judgment index less than 0.74MPa as the judgment index, and the maximum distance between the two cracks under the premise of meeting the judgment index is the reasonable crack spacing;

[0011] Step 5: According to the reasonable distance between the fracturing hole and the upper boundary of the soft coal seam in step 4, a fracturing hole is constructed from the air intake tunnel to the roof rock layer of the soft coal seam by using conventional directional drilling technology, and the fracturing hole and the hole-making hole are parallel to each other, and then a fracturing hole is constructed to the roof rock layer at intervals along the direction of the air intake tunnel, and each fracturing hole is parallel to each other;

[0012] Step 6. According to the reasonable crack spacing in step 4, two high-pressure sealing capsules are first used to seal one of the fracturing holes in sections to form a sealing section; then a high-pressure ejector is used to spray in the sealing section toward the soft coal seam below to form perforations; then the high-pressure pump is started to inject high-pressure water into the sealing section, and the perforations are hydraulically fractured to form a fracture network. Since the stress around the pressure relief cave is relatively low, the fracture network formed by the perforation of the sealing section will develop toward the adjacent pressure relief cave and connect the fracture network with the pressure relief cave; after completion, a high-pressure sealing capsule is arranged in the fracturing hole to form another sealing section, and the above steps are repeated until the construction process of the fracturing hole segmented to form a fracture network is completed; then a fracturing hole is selected and the above steps are repeated until all fracturing holes have completed the construction of the segmented formation of the fracture network.

[0013] Furthermore, the diameter of the pressure relief cave in step 1 is the equivalent diameter, and the specific calculation method is to regard the pressure relief cave as a cylinder and calculate it according to formula (1):

[0014]

[0015] Where d is the equivalent diameter of the cave, m; m is the mass of coal discharged from the cave-making section, kg; ρ is the density of the coal, kg / m 3 ; l is the length of the cavitation section, m.

[0016] Furthermore, in the step 1, the critical displacement value of the cave wall corresponding to the instability of the pressure relief cave is taken as 1 / 3 to 1 / 2 of the equivalent diameter d of the pressure relief cave.

[0017] Furthermore, the inclination angle of the hole drilling in step 2 is 2 to 8 degrees upward from the air inlet lane to facilitate slag discharge.

[0018] Furthermore, the rated pressure of the high-pressure water pump in step three is 50MPa, and the rated flow rate is 400L / min; the water pressure at the outlet of the high-pressure water pump during the construction process is 10-30MPa, which needs to be determined according to the hardness of the coal seam; the flow rate of the high-pressure water pump during the construction process is 200-300L / min, which is determined according to the slag discharge situation of the drilling hole. When the slag discharge is not smooth, the flow rate needs to be increased to ensure smooth slag discharge of the drilling hole.

[0019] Furthermore, in the step 5, the projection of each fracturing hole on the cavitation drilling plane is located in the middle of two adjacent cavitation drilling holes.

[0020] Furthermore, in step six, the water pressure that the high-pressure sealing capsule can withstand is at least 50 MPa; during the perforation process, the outlet pressure of the high-pressure water pump is not less than 40 MPa, and the time of hydraulic jetting for a single perforation is not less than 5 minutes.

[0021] Furthermore, the stopping condition of fracturing in each sealing section in step six is: observing the orifice of the cavitation borehole during the fracturing process, and when water comes out of the orifice, sealing the orifice of the borehole, and continuing water injection fracturing until another cavitation borehole adjacent to the air inlet lane also comes out of water, and then stopping fracturing.

[0022] Compared with the prior art, the present invention is based on directional long drilling technology, which can achieve precise positioning of the drilling trajectory and achieve targeted permeability enhancement; in addition, the present invention optimizes the key parameters of hydraulic cavitation and hydraulic fracturing through specific simulation methods, achieves precise seam creation and pressure relief permeability enhancement in coal seams, and optimizes the permeability enhancement effect of coal seams; after determining the optimal parameters, for soft and thick coal seams, firstly, segmented cavitation is carried out in the coal seams according to the set optimal parameters to achieve pressure relief in the coal seams, form disturbed cracks around the boreholes, and improve the gas extraction effect; and because the stress in the soft coal seams will be redistributed after cavitation, the stress around the pressure relief caves will be reduced, and then segmented fracturing is carried out in the roof rock layer of the soft coal seams by using directional long drilling. , causing the cracks to expand downward into the soft coal seam. Since the stress around the pressure relief cave in the soft coal seam is lower than the stress at other locations, during the hydraulic fracturing process of the perforations in each sealing section, the cracks entering the soft coal seam will be affected by the stress distribution and will gradually deflect toward the pressure relief cave until the cracks are connected to the pressure relief cave, thereby forming a spatial three-dimensional fracture network in the soft coal seam. This method first changes the internal stress distribution of the soft coal seam through the pressure relief cave, provides guidance for the crack expansion and development direction of the subsequent staged fracturing, and then generates a three-dimensional fracture network through staged fracturing and stress guidance. The staged cavitation and staged fracturing are combined, thereby achieving uniform pressure relief and permeability enhancement in the coal seam, providing an advantageous channel for gas flow. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the construction layout of the present invention;

[0024] Figure 2 yes Figure 1 A top view rotated 90°;

[0025] Figure 3 yes Figure 1 Schematic diagram of a cross-section of a cavitation borehole and a fracturing hole.

[0026] In the figure: 1-air inlet tunnel, 2-hole drilling, 3-pressure relief cave, 4-soft coal seam, 5-return air tunnel, 6-fracturing hole, 7-high pressure sealing capsule, 8-roof rock layer, 9-perforation, 10-crack, 11-high pressure water pump. DETAILED DESCRIPTION

[0027] The present invention will be further described below.

[0028] like Figure 1 As shown, the specific steps of the present invention are:

[0029] Step 1: Test the mechanical properties of soft coal seams to obtain elastic modulus, Poisson's ratio, compressive strength, tensile strength, cohesion and internal friction angle, test the gas storage characteristics of soft coal seams, and obtain gas pressure and adsorption constant; based on COMOSOL The numerical simulation study was carried out using Multiphysics numerical simulation software to obtain the gas pressure distribution characteristics around the borehole, change the diameter of the pressure relief cave, and take the minimum gas pressure at the midpoint of the line connecting two adjacent cavitation boreholes (i.e., simulating the use of conventional directional drilling technology to construct a directional long borehole in the soft coal seam from the side wall of the air intake tunnel toward the return air tunnel) as the judgment criterion to obtain the optimal diameter of the pressure relief cave. The numerical simulation study was carried out based on FLAC3D numerical simulation software to obtain the deformation characteristics around the pressure relief cave, change the distance between two adjacent pressure relief caves on the same cavitation borehole, and the critical displacement value of the cave wall corresponding to the instability of the pressure relief cave is taken as 1 / 3 to 1 / 2 of the equivalent diameter d of the pressure relief cave. When the displacement is greater than the critical value, the pressure relief cave is determined to be unstable, and the minimum value of the distance l1 between the two adjacent pressure relief caves corresponding to the critical instability of the pressure relief cave is determined as the optimal cave spacing. The diameter of the pressure relief cave is the equivalent diameter, and the specific calculation method is to regard the pressure relief cave as a cylinder and calculate it according to formula (1):

[0030]

[0031] Where d is the equivalent diameter of the cave, m; m is the mass of coal discharged from the cave-making section, kg; ρ is the density of the coal, kg / m 3 ; l is the length of the cavitation section, m;

[0032] Step 2: Use conventional directional drilling technology to construct a long directional borehole in the soft coal seam on the side wall of the air intake tunnel toward the return air tunnel. The long directional borehole is used as a hole-making borehole, and the drill tool is withdrawn after the construction is completed. The inclination angle of the hole-making borehole is 2 to 8 degrees from the air intake tunnel to facilitate slag discharge.

[0033] Step three, install a high-pressure ejector on the drill bit, adjust the pressure and flow of the high-pressure water pump, rotate the drill bit to use the drill back punching method in the hole-making hole of step two, reach the optimal diameter of the pressure relief cave simulated in step one, and construct a pressure relief cave at a distance l1 along the direction of the hole-making hole; then repeat steps two and three, construct a hole-making hole at a certain distance along the direction of the air inlet lane, and each hole-making hole is parallel to each other; the rated pressure of the high-pressure water pump is 50MPa, and the rated flow is 400L / min; the water pressure at the outlet of the high-pressure water pump during construction is 10~30MPa, which needs to be determined according to the hardness of the coal seam; the flow of the high-pressure water pump during construction is 200~300L / min, which is determined according to the slag discharge situation of the drilling hole. When the slag discharge is not smooth, the flow needs to be increased to ensure smooth slag discharge of the drilling hole.

[0034] Step 4: Carry out numerical simulation research based on ABQUS numerical simulation software, change the distance h between the fracturing hole (i.e., simulate the use of conventional directional drilling technology to construct a borehole from the air intake tunnel into the roof rock layer of the soft coal seam, and the borehole is constructed in a directional long drilling manner after entering the roof rock layer, and the directional long drilling part is used as the fracturing hole) and the upper boundary of the soft coal seam, obtain the trajectory of the crack, and use the ability of the crack trajectory to pass through the roof rock layer and enter the soft coal seam as the judgment criterion to determine the reasonable distance between the fracturing hole and the upper boundary of the soft coal seam; carry out numerical simulation research based on COMOSOL Multiphysics numerical simulation software, change the distance l2 between two adjacent cracks, and use the gas pressure at each location of the soft coal seam within a set time as the judgment index less than 0.74MPa as the judgment index, and the maximum distance between the two cracks under the premise of meeting the judgment index is the reasonable crack spacing;

[0035] Step 5: According to the reasonable distance between the fracturing hole and the upper boundary of the soft coal seam in step 4, a fracturing hole is constructed from the air intake tunnel to the roof rock layer of the soft coal seam by using conventional directional drilling technology, and the fracturing hole and the cavitation borehole are parallel to each other. Then, a fracturing hole is constructed into the roof rock layer at intervals along the air intake tunnel, and each fracturing hole is parallel to each other; the projection of each fracturing hole on the cavitation borehole plane is located in the middle of two adjacent cavitation boreholes. Figure 2 As shown;

[0036] Step six: According to the reasonable crack spacing in step four, first use two high-pressure sealing capsules to seal one of the fracturing holes in sections to form a sealing section; then use a high-pressure ejector to spray in the sealing section toward the soft coal seam below to form perforations; the water pressure that the high-pressure sealing capsule can withstand is at least 50MPa; during the perforation process, the outlet pressure of the high-pressure water pump is not less than 40MPa, and the hydraulic injection time of a single perforation is not less than 5 minutes. Then start the high-pressure pump, inject high-pressure water into the sealing section, and hydraulically fracture the perforations to form a fracture network. Since the stress around the pressure relief cave is relatively low, the fracture network formed by the perforation of the sealing section will develop toward the adjacent pressure relief cave and connect the fracture network with the pressure relief cave; the stopping condition for fracturing in each sealing section is: observe the orifice of the cavitation borehole during the fracturing process, and when water comes out of the orifice, seal the orifice of the borehole, and continue to inject water for fracturing until another adjacent cavitation borehole along the air inlet tunnel also comes out of water, then stop fracturing;

[0037] After completion, a high-pressure sealing capsule is placed in the fracturing hole to form a sealing section, and the above steps are repeated until the construction process of the fracturing hole segmented into a fracture network is completed; then a fracturing hole is selected and the above steps are repeated until all fracturing holes have completed the construction of segmented fracture networks. Figure 3 shown.

[0038] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for accurate permeability enhancement of soft and thick coal seams by coordinated pressure relief and fracturing, characterized in that: The specific steps are: Step 1: Test the mechanical properties of the soft coal seam to obtain the elastic modulus, Poisson's ratio, compressive strength, tensile strength, cohesion and internal friction angle, test the gas occurrence characteristics of the soft coal seam, and obtain the gas pressure and adsorption constant; carry out numerical simulation research based on COMOSOL Multiphysics numerical simulation software to obtain the gas pressure distribution characteristics around the borehole, change the diameter of the pressure relief cave, and use the minimum gas pressure at the midpoint of the line connecting two adjacent cavitation boreholes as the judgment criterion to obtain the optimal diameter of the pressure relief cave; carry out numerical simulation research based on FLAC3D numerical simulation software to obtain the deformation characteristics around the pressure relief cave, change the distance between two adjacent pressure relief caves on the same cavitation borehole, and use the displacement of the cave wall as the judgment index. When the displacement is greater than a certain value, the pressure relief cave is judged to be unstable, and the minimum value of the distance l1 between two adjacent pressure relief caves corresponding to the critical instability of the pressure relief cave is determined as the optimal cave spacing; Step 2: Use conventional directional drilling technology to construct a directional long borehole in the soft coal seam on the side wall of the air inlet tunnel toward the return air tunnel. The directional long borehole is used as a hole-making borehole. After the construction is completed, the drill tool is withdrawn; Step 3: Install a high-pressure ejector on the drill, adjust the pressure and flow of the high-pressure water pump, rotate the drill in the hole-making hole of step 2 and use the drill back punching method to achieve the optimal diameter of the pressure relief cave obtained by simulation in step 1, and construct a pressure relief cave every distance l1 along the direction of the hole-making hole; then repeat steps 2 and 3, construct a hole-making hole every distance along the direction of the air inlet tunnel, and each hole-making hole is parallel to each other; Step 4: Carry out numerical simulation research based on ABQUS numerical simulation software, change the distance h between the fracturing hole and the upper boundary of the soft coal seam, obtain the trajectory of the crack, and use the fracture trajectory to pass through the roof rock layer and enter the soft coal seam as the judgment criterion to determine the reasonable distance between the fracturing hole and the upper boundary of the soft coal seam; carry out numerical simulation research based on COMOSOL Multiphysics numerical simulation software, change the distance l2 between two adjacent fractures, and use the gas pressure at each location of the soft coal seam within a set time as less than 0.74MPa as the judgment index. The maximum distance between the two fractures under the premise of meeting the judgment index is the reasonable fracture spacing; Step 5: According to the reasonable distance between the fracturing hole and the upper boundary of the soft coal seam in step 4, a fracturing hole is constructed from the air intake tunnel to the roof rock layer of the soft coal seam by using conventional directional drilling technology, and the fracturing hole and the hole-making hole are parallel to each other, and then a fracturing hole is constructed to the roof rock layer at intervals along the direction of the air intake tunnel, and each fracturing hole is parallel to each other; Step 6. According to the reasonable crack spacing in step 4, two high-pressure sealing capsules are first used to seal one of the fracturing holes in sections to form a sealing section; then a high-pressure ejector is used to spray in the sealing section toward the soft coal seam below to form perforations; then the high-pressure pump is started to inject high-pressure water into the sealing section, and the perforations are hydraulically fractured to form a fracture network. Since the stress around the pressure relief cave is relatively low, the fracture network formed by the perforation of the sealing section will develop toward the adjacent pressure relief cave and connect the fracture network with the pressure relief cave; after completion, a high-pressure sealing capsule is arranged in the fracturing hole to form another sealing section, and the above steps are repeated until the construction process of the fracturing hole segmented to form a fracture network is completed; then a fracturing hole is selected and the above steps are repeated until all fracturing holes have completed the construction of the segmented formation of the fracture network.

2. According to claim 1, the method for accurate regional permeability enhancement of soft thick coal seam pressure relief and fracturing coordination is characterized in that: The diameter of the pressure relief cave in step 1 is the equivalent diameter, and the specific calculation method is to regard the pressure relief cave as a cylinder and calculate it according to formula (1): Where d is the equivalent diameter of the cave, m; m is the mass of coal discharged from the cave-making section, kg; ρ is the density of the coal, kg / m 3 ; l is the length of the cavitation section, m.

3. The method for accurate regional permeability enhancement of soft thick coal seam pressure relief and fracturing coordination according to claim 1 is characterized in that: In the step 1, the displacement critical value of the cave wall corresponding to the instability of the pressure relief cave is 1 / 3 to 1 / 2 of the equivalent diameter d of the pressure relief cave.

4. The method for accurate regional permeability enhancement of soft thick coal seam pressure relief and fracturing coordination according to claim 1 is characterized in that: The inclination angle of the hole drilling in step 2 is 2 to 8 degrees upward from the air inlet tunnel to facilitate slag discharge.

5. The method for accurate regional permeability enhancement of soft thick coal seam pressure relief and fracturing coordination according to claim 1 is characterized in that: The rated pressure of the high-pressure water pump in step three is 50MPa, and the rated flow rate is 400L / min; the water pressure at the outlet of the high-pressure water pump during the construction process is 10-30MPa, which needs to be determined according to the hardness of the coal seam; the flow rate of the high-pressure water pump during the construction process is 200-300L / min, which is determined according to the slag discharge situation of the drilling hole. When the slag discharge is not smooth, the flow rate needs to be increased to ensure smooth slag discharge of the drilling hole.

6. The method for accurate regional permeability enhancement of soft thick coal seam pressure relief and fracturing coordination according to claim 1 is characterized in that: In the step 5, the projection of each fracturing hole on the cavitation drilling plane is located in the middle of two adjacent cavitation drilling holes.

7. The method for accurate regional permeability enhancement of soft thick coal seam pressure relief and fracturing coordination according to claim 1 is characterized in that: In step six, the water pressure that the high-pressure sealing capsule can withstand is at least 50 MPa; during the perforation process, the outlet pressure of the high-pressure water pump is not less than 40 MPa, and the time of hydraulic jetting for a single perforation is not less than 5 minutes.

8. The method for accurate regional permeability enhancement of soft thick coal seam pressure relief and fracturing coordination according to claim 1 is characterized in that: The stopping condition of fracturing in each sealing section in step six is: observing the opening of the cavitation borehole during the fracturing process, and when water comes out of the opening, sealing the opening of the borehole, and continuing water injection fracturing until another cavitation borehole adjacent to the air inlet tunnel also comes out of water, then stopping fracturing.

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