A method for indirectly fracturing coal seams by high-voltage electric pulse combined hydraulic fracturing

Abstract

This invention discloses a method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing. It is applicable to soft, low-permeability coal seams, particularly in geological conditions where horizontal stress exceeds vertical stress and the shear strength of the coal-rock interface is low, for pressure relief, outburst mitigation, and coalbed methane extraction. This invention involves placing a horizontal well in a hard roof, implementing segmented cementing without perforation. First, high-voltage electrical pulses penetrate the roof and coal seam, creating high-voltage electrical pulse-induced fractures. Then, segmented horizontal well fracturing technology is used to fracture the coal seam, ultimately forming a complex network of fractures within the coal seam. This invention saves on the costs of horizontal well cementing and perforation while fully utilizing the advantage of high-voltage electrical pulses' ability to electrically penetrate rock. Before hydraulic fracturing, a penetrating physical channel is pre-formed within the roof and coal seam, providing directionality for subsequent hydraulic fracturing. This significantly improves the success rate of indirect horizontal well fracturing of the roof under geological conditions where horizontal stress exceeds vertical stress and the shear strength of the coal-rock interface is low.

Description

Technical Field

[0001] This invention relates to a method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing, which is particularly suitable for decompression and outburst relief and coalbed methane extraction in fractured and soft coal seams where horizontal ground stress is greater than vertical ground stress and the shear strength of the coal-rock interface is low. It belongs to the field of coal mine safety and gas extraction. Background Technology

[0002] Currently, in addition to conventional methods such as gas drainage holes, hydraulic perforation, hydraulic fracturing, and surface vertical wells, surface horizontal wells are increasingly widely used for coal seam decompression and outburst mitigation and coalbed methane development. However, for soft and fractured coal seams, directly drilling into the coal seam carries risks such as stuck drill bits and borehole collapse. This has led to the development of indirect fracturing technology using surface horizontal wells. The core idea is to drill a horizontal section of the surface horizontal well into the top and bottom plates of the coal seam or other hard rock strata, and then use water injection pressure to drive hydraulic fractures to extend into the coal seam, thereby achieving the purpose of fracturing the coal seam.

[0003] Chinese patent CN 103967472 A proposes a method for enhanced extraction of coalbed methane through staged fracturing of horizontal wells. The core idea is to control the wellbore trajectory within a certain range of mudstone or sandstone above the top boundary of the coal seam, employing ultra-deep perforation technology to directly connect the perforation to the coal seam, and then using water-carrying fracturing technology to fracture the coal seam. Chinese patent CN 110318674 A arranges the wellbore trajectory of the horizontal well within the roof of the coal seam within a certain range from the top boundary of the reserved coal pillar, with its direction consistent with the roadway excavation direction. Simultaneously, the perforation does not directly connect to the coal seam, and then supercritical CO2 is injected for fracturing, achieving roof fracturing to prevent outbursts.

[0004] However, the in-situ stress in fractured and soft coal seams is usually tectonic stress, meaning that horizontal in-situ stress is generally greater than vertical in-situ stress. According to the theoretical principle that hydraulic fractures extend along the direction of maximum in-situ stress, coupled with the weak texture and poor shear resistance of the fractured and soft coal seam-roof / floor interface, the use of indirect fracturing technology under these geological conditions is highly likely to result in hydraulic fractures failing to penetrate the layers. Instead, their length will extend horizontally or along the coal-rock interface, leading to fracturing failure. Summary of the Invention

[0005] For the decompression and outburst mitigation of fractured and soft coal seams with horizontal stress greater than vertical stress and weak coal-rock interfaces, and for coalbed methane extraction, this invention provides a high-voltage electric pulse combined with hydraulic fracturing method for indirect fracturing of coal seams, which significantly improves the fracturing success rate under the above geological conditions.

[0006] This invention addresses the challenges of indirect fracturing in coal seams with horizontal stress exceeding vertical stress and weak coal-rock interface, specifically focusing on pressure relief, outburst mitigation, and coalbed methane extraction. It tackles the problem that indirect fracturing under these geological conditions often results in hydraulic fractures extending horizontally or through the weak coal-rock interface, hindering their propagation into the coal seam. The invention proposes a method to first create a plasma channel between the roof and the coal seam using high-voltage electric pulse technology. This plasma channel then expands under heat, forming a physically connected channel—the high-voltage electric pulse-induced fracture. Finally, combined with indirect fracturing technology using horizontal wells in the roof, fracturing fluid is driven to fracture the coal seam along the high-voltage electric pulse-induced fracture. This technology effectively overcomes the difficulty in controlling the direction of hydraulic fracture propagation when fracturing soft coal seams with horizontal stress exceeding vertical stress and weak coal-rock interface using existing indirect fracturing techniques, significantly improving the success rate of indirect coal seam fracturing.

[0007] This invention provides a method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing. By arranging a horizontal well in a hard roof, segmented cementing is implemented without perforation. First, high-voltage electrical pulses are used to penetrate the roof and coal seam, pre-forming high-voltage electrical pulse fracturing fractures in the roof and coal seam before hydraulic fracturing. This fully utilizes the advantage of high-voltage electrical pulses being able to electrically penetrate rock, providing a directional effect for subsequent hydraulic fracturing. Then, segmented fracturing technology using the horizontal well is employed to fracture the coal seam, ultimately forming a complex network of fracturing fractures within the coal seam.

[0008] The above-mentioned method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing includes the following steps: S1: Drill a horizontal well in the hard roof above the coal seam; S2: Drill holes from the roadway to the bottom of the coal seam to be fracturing. The end of the hole is located directly below the fracturing section of the horizontal well in the roof. The diameter is 120mm. The spacing between the holes is set to 40~50m according to the spacing of the segmented fracturing sections. S3: A high-voltage electric pulse generator is installed on the ground. The high-voltage electrode is pulled to the horizontal well fracturing section using the tubing and made to make it in close contact with the top plate. Then, clean water is injected into the horizontal well fracturing section, and fracturing is temporarily suspended. The purpose of injecting clean water is to act as an insulating liquid. When the rise time of the high-voltage electric pulse is less than 500 ns, the breakdown field strength of the water is greater than that of the top rock layer. At this time, the top rock layer will first undergo electrical breakdown and be destroyed under the action of the high-voltage electric pulse, forming a high-voltage electric pulse-induced fracture. If clean water is not injected first, the air will undergo electrical breakdown first because the breakdown field strength of the air is lower than that of the rock, making it difficult to achieve electrical breakdown of the rock. Here, the location of the injected clean water is at the same position as the high-voltage electrode.

[0009] The pulse rise time of the high-voltage pulse generator is less than 500ns. The peak voltage and number of pulses need to be determined based on the roof lithology and the vertical distance between the horizontal well and the coal seam borehole. The principle is to be able to electrically break down the roof and coal seam within that distance.

[0010] S4: Pull the low-voltage electrode to the end of the borehole and make it in close contact with the coal seam, then inject clean water into the borehole and seal it; S5: Activate the high-voltage electric pulse generator to form a plasma channel in the roof and inside the coal seam of the fracturing section using the high-voltage electric pulse. The specific operation is as follows: continuously shove the roof and coal seam with electricity until the voltage in the high-voltage pulse device drops sharply, which indicates that a plasma channel has been formed between the roof and the coal seam, that is, the current can be smoothly conducted between the roof and the coal seam. S6: Continuously apply electric pulses and monitor microseismic events during the high-voltage electric pulse electric shock to coal and rock in real time through ground microseismic monitoring. When a large number of microseismic events occur between the roof and the coal seam, it indicates that the plasma channel between the roof and the coal seam is thermally expanded and ruptured, forming a high-voltage electric pulse-induced crack. S7: Turn off the high-voltage electric pulse generator, pump out the borehole water, remove the high and low pressure electrodes, and start the hydraulic fracturing of the roof horizontal well, eventually forming a complex fracturing network in the coal seam.

[0011] Preferably, the horizontal section of the top plate horizontal well in S1 is subjected to segmented casing installation and segmented cementing, that is, casing is only installed between each fracturing section, and cementing is performed segment by segment.

[0012] Preferably, the top plate horizontal well in S1 does not require perforation operation.

[0013] Preferably, the high-voltage electrode in S3 is a needle electrode; Preferably, the low-voltage electrode in S4 is a disc-shaped electrode; Preferably, the purpose of injecting clean water into the horizontal well and borehole in S3 and S4 is to use clean water as an insulating liquid so that plasma channels are formed between the horizontal well fracturing section and the coal seam borehole, thereby achieving directional fracturing effect.

[0014] The beneficial effects of this invention are: Compared with existing technologies, this invention implements segmented casing installation and segmented cementing for horizontal sections of horizontal wells, while avoiding perforation operations, effectively reducing completion and perforation costs. It employs high-voltage electric pulses to pre-fracture the roof and coal seam, fully utilizing the destructive power and environmentally friendly advantages of electricity, reducing water waste and formation pollution. More importantly, the high-voltage electric pulses can form a plasma channel between high- and low-voltage electrodes. The plasma channel expands upon heating, forming a high-voltage electric pulse-induced fracture that connects the roof and coal seam. This effectively overcomes the influence of tectonic stress and weak coal-rock interfaces on the subsequent hydraulic fracture propagation path, achieving directional cross-layer fracturing of the entire coal seam in the formation thickness direction. Attached Figure Description

[0015] Figure 1This is a flowchart of the method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing according to the present invention; Figure 2 This is a diagram showing the electrical breakdown of the roof and coal seam by a high-voltage electric pulse. Figure 3 This is a diagram of a horizontally mounted coal seam hydraulically fractured. In the diagram: 1—coal seam, 2—roof, 3—horizontal well, 4—segmented cement sheath, 5—sealing bridge plug, 6—borehole, 7—high-voltage electric pulse generator, 8—high-voltage cable, 9—needle-type high-voltage electrode, 10—disc-type low-voltage electrode, 11—plasma channel, 12—coal-rock interface, 13—high-voltage electric pulse fracturing fracture, 14—hydraulic fracturing device, 15—fracturing pipeline. Detailed Implementation

[0016] To make the technical problems, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail with reference to the embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention. The technical solutions of this invention are described in detail below with reference to the embodiments and accompanying drawings, but the scope of protection is not limited thereto. Example 1

[0017] This embodiment provides a method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing, including the following steps: S1: Drill a horizontal well in the hard roof above the coal seam; the horizontal section of the roof horizontal well is run in sections with casing and cementing in sections, that is, casing is only laid between each fracturing section and cemented in sections; no perforation operation is required for the roof horizontal well. See this embodiment Figure 2 and Figure 3 Drill a horizontal well 3 in the hard roof 2 above coal seam 1. The horizontal section is 2m away from the upper boundary of the coal seam and is 800m long. S2: Drill hole 6 from the roadway to the bottom of coal seam 1. The end of drill hole 6 is located directly below the fracturing section of horizontal well 3 in the roof. The spacing between drill holes is set to 50m according to the spacing between the segmented fracturing sections. In this embodiment, the borehole diameter is 120mm, and it is 50mm away from the lower boundary of the coal seam. The horizontal well is fracturing in 8 sections with a section spacing of 50m. S3: Arrange a high-voltage electric pulse generator 7 on the ground, use the oil pipe to pull the needle high-voltage electrode 9 to the fracturing section of the horizontal well 3, and make it closely contact the top plate 2. Then inject clean water into the fracturing section of the horizontal well 3 and do not fracture for the time being. S4: Pull the 100m diameter disc-type low-voltage electrode 10 to the end of the borehole 6 and make it in close contact with the coal seam 1, then inject clean water into the borehole 6 and seal the borehole. The purpose of injecting clean water into the horizontal well and borehole in steps S3 and S4 above is to use clean water as an insulating liquid so that plasma channels are formed between the horizontal well fracturing section and the coal seam borehole, thereby achieving directional fracturing effect.

[0018] S5: Turn on the high-voltage electric pulse generator 7, set the peak voltage to 200kV and the pulse rise time to 300ns, and continuously slap the roof and coal seam between the fracturing section of the horizontal well 3 and the borehole 6 until the voltage in the electrical circuit of the high-voltage electric pulse generator 7 drops sharply, indicating that a plasma channel 11 spanning the coal-rock interface 12 has been formed in the roof and coal seam between the fracturing section of the horizontal well 3 and the borehole 6. S6: Continuously apply electric pulses and monitor microseismic events between the roof and coal seam through ground microseismic monitoring. When a large number of microseismic events occur in a concentrated manner, it indicates that the plasma channel 11 is ruptured due to thermal expansion, forming a high-voltage electric pulse-induced crack 13. S7: Turn off the high-voltage electric pulse generator 7, pump out the clean water from the borehole 6, and remove the needle-type high-voltage electrode 9 and the disc-type low-voltage electrode 10. Connect the horizontal well 3 and the hydraulic fracturing device 14 using the fracturing pipeline 15, open the hydraulic fracturing device 14, seal it with the packer plug 5, and perform segmented fracturing to ultimately form a complex fracturing network in the coal seam. Microseismic monitoring is used to determine the formation of the complex fracturing network in the coal seam.

[0019] The method for fracturing coal seams in this invention avoids perforation operations, effectively reducing well completion and perforation costs. It utilizes high-voltage electric pulses to pre-fracture the roof and coal seam, fully leveraging the destructive power and environmentally friendly advantages of electricity, reducing water waste and formation pollution. More importantly, the high-voltage electric pulses can form a plasma channel between the high- and low-voltage electrodes. This plasma channel, upon heating and expansion, penetrates the roof and coal seam, creating a physical channel—the high-voltage electric pulse-induced fracture. This effectively overcomes the influence of tectonic stress and weak coal-rock interfaces on the subsequent hydraulic fracture propagation path, achieving directional, trans-layer fracturing of the entire coal seam along the formation thickness direction.

[0020] The above description is a further detailed explanation of the present invention in conjunction with specific preferred embodiments. It should not be considered that the specific embodiments of the present invention are limited to this. For those skilled in the art, several simple deductions or substitutions can be made without departing from the present invention, and all of these should be considered to fall within the scope of patent protection determined by the submitted claims.

Claims

1. A method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing, characterized in that: By deploying a horizontal well in the hard roof, segmented cementing is implemented without perforation. First, high-voltage electric pulses are used to penetrate the roof and coal seam, forming a through physical channel in the roof and coal seam before hydraulic fracturing. This fully utilizes the advantage of high-voltage electric pulses being able to electrically penetrate the rock, providing directional support for subsequent hydraulic fracturing. Then, the horizontal well segmented fracturing technology is used to fracture the coal seam, ultimately forming a complex network of fracturing fractures within the coal seam. The aforementioned method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing includes the following steps: S1: Drill a horizontal well in the hard roof above the coal seam; S2: Drill a hole from the roadway to the bottom of the coal seam to be fracturing, with the end of the hole located directly below the fracturing section of the horizontal well in the roof. S3: A high-voltage electric pulse generator is installed on the ground. The high-voltage electrode is pulled to the horizontal well fracturing section using the tubing and made to make it in close contact with the top plate. Then, clean water is injected into the horizontal well fracturing section, and fracturing is temporarily suspended. S4: Pull the low-voltage electrode to the end of the borehole and make it in close contact with the coal seam, then inject clean water into the borehole and seal it; S5: Activate the high-voltage electric pulse generator to form a plasma channel in the roof and inside the coal seam of the fracturing section using the high-voltage electric pulse. S6: Continuously apply electric pulses until the plasma channel between the roof and the coal seam expands and ruptures due to heat, forming a physically connected channel, i.e., high-voltage electric pulse-induced cracks. S7: Turn off the high-voltage electric pulse generator, remove the high and low voltage electrodes, then pump out the borehole water and start the hydraulic fracturing of the roof horizontal well, eventually forming a complex fracturing network in the coal seam.

2. The method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing according to claim 1, characterized in that: In step S1, the horizontal section of the top plate horizontal well is subjected to segmented casing installation and segmented cementing, that is, casing is only installed between each fracturing section, and cementing is performed segment by segment.

3. The method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing according to claim 1, characterized in that: The top plate horizontal well in step S1 does not require perforation.

4. The method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing according to claim 1, characterized in that: In step S2, the borehole is drilled in the lower part of the coal seam, with the end of the borehole located directly below the horizontal well fracturing section of the roof. The diameter of the borehole is 120mm, and the borehole spacing is set to 40~50m according to the segment spacing of the segmented fracturing.

5. The method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing according to claim 1, characterized in that: In step S3, the pulse rise time of the high-voltage pulse generator is less than 500ns. The peak voltage and number of pulses need to be determined based on the roof lithology and the vertical distance between the horizontal well and the coal seam borehole. The principle is to be able to electrically break down the roof and coal seam within this distance.

6. The method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing according to claim 5, characterized in that: The high-voltage electrode of the high-voltage pulse generator is a needle electrode, and the low-voltage electrode is a disc electrode.

7. The method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing according to claim 1, characterized in that: The purpose of injecting clean water into the horizontal well in step S3 and into the borehole in step S4 is to use clean water as an insulating liquid so that plasma channels are formed between the horizontal well fracturing section and the coal seam borehole, thereby achieving directional fracturing effect.

8. The method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing according to claim 1, characterized in that: In step S5, the roof and coal seam are continuously shoved until the voltage in the high-voltage pulse generator drops sharply, indicating that a plasma channel has been formed between the roof and the coal seam, meaning that the current is successfully conducting between the roof and the coal seam.

9. The method for indirect fracturing of coal seams using high-voltage electrical pulse combined with hydraulic fracturing according to claim 1, characterized in that: In step S6, microseismic events during the high-voltage electric pulse shock to coal and rock are monitored in real time using ground microseismic monitoring methods. When a large number of microseismic events occur between the roof and the coal seam, it indicates that a high-voltage electric pulse-induced crack has formed between the roof and the coal seam.

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