A method for enhancing permeability in strip mining areas of broken and soft coal seams

Abstract

This invention discloses a method for enhancing permeability in strip coal extraction zones of fractured and soft coal seams. This method utilizes a horizontal well strip coal extraction device for fractured and soft coal seams. The device comprises a front packer, a throttle valve, a rear packer, and a rear nozzle, coaxially connected from front to back. The method involves constructing a horizontal downhole casing in or adjacent to the top or bottom of the fractured and soft coal seam. Perforations are made at intervals on the casing. Two packers are used to isolate a sealed space within the casing, containing perforations, with another perforation located at the rear end of the rear packer. Water flowing from the throttle valve fills the sealed space and then flows into the formation along the perforations. The water then returns to the annulus between the casing and the tubing string through the perforation at the rear end of the rear packer, forming a hydraulic circulation for coal extraction. This invention can create a plastic zone at the designed location in the coal roadway, thereby enhancing permeability and relieving pressure. Combined with negative pressure gas extraction, it achieves multiple benefits, including reducing stress, elasticity, gas content, and pressure in the strip area.

Description

Technical Field

[0001] This invention belongs to the field of coal mine safety technology and relates to a method for increasing permeability in the strip mining area of ​​a soft coal seam. Background Technology

[0002] Before mining high-risk coal seams, outburst suppression must be carried out. Strip outburst suppression is a necessary task to ensure the safety of coal roadway excavation. Coal stripping is one of the commonly used coal seam outburst suppression methods. Coal stripping technologies that have been applied include vertical shaft pneumatic coal stripping and multi-branch shaft hydraulic coal stripping. However, the above technologies are all used for coal stripping in rock face, and their scope of application is limited, which cannot meet the requirements of strip outburst suppression. In horizontal well hydraulic coal stripping of soft coal seams, the above approach is no longer feasible. The reasons are: (1) Soft coal seams have poor drillability and hole formation. After the borehole is formed, casing must be installed for support. Otherwise, the borehole will collapse in a short period of time, destroying the gas extraction channel; (2) Horizontal well water jet coal stripping requires multiple coal stripping points in a single well. However, the easy collapse characteristics of soft coal seams determine that horizontal wells cannot form open holes at multiple points. Otherwise, the coal seam will collapse and block the gas extraction channel. Therefore, it is necessary to propose a coal mine strip outburst suppression method to meet the safety requirements of roadway excavation in high-risk coal seams. Summary of the Invention

[0003] To address the shortcomings of existing technologies, the present invention aims to provide a method for enhancing permeability in the strip coal extraction area of ​​fractured and soft coal seams, thereby solving the problem that existing technologies struggle to achieve hydraulic coal extraction in horizontal wells of fractured and soft coal seams.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] A method for enhancing permeability in the strip coal extraction area of ​​a fractured and soft coal seam is provided. This method utilizes a strip coal extraction device in a horizontal well of a fractured and soft coal seam. The device includes a front packer, a throttle valve, a rear packer, and a rear nozzle connected coaxially from front to back. The rear end of the rear nozzle is connected to an oil pipe, and the rear nozzle can spray water backward.

[0006] This method involves constructing horizontal wells in or adjacent to the top and bottom of soft coal seams. Casing is run in the horizontal section, and perforations are made at intervals on the casing. A front packer and a rear packer are used to isolate a sealed space within the casing, with at least one set of perforations within this sealed space. A throttle valve is installed between the front and rear packers, through which water flows. Once the sealed space is filled with water, the water flows into the formation along the perforations in the casing, then returns to the annulus between the casing and the tubing through the perforations at the rear of the rear packer, forming a hydraulic circulation for coal extraction. This process is repeated multiple times to complete the coal extraction operation, creating a plastic zone that releases elastic energy, reduces local stress, and improves permeability, thus facilitating gas extraction and outburst suppression.

[0007] The present invention also includes the following technical features:

[0008] Specifically, the method includes the following steps:

[0009] Step 1: Select the target area for permeability enhancement: The target area for permeability enhancement is called a strip. The strip area is a rectangular area with the same length as the roadway, a width of 15m to 20m along the outer sides of the roadway and the coal seam, and a height equal to the height of the coal seam.

[0010] Step 2, Design target coal extraction volume: Design the coal extraction volume based on the target permeability enhancement area selected in Step 1;

[0011] Step 3, Design and Construction of Horizontal Wells: Construct horizontal wells and run casing for cementing; the horizontal section of the horizontal well is located in the coal seam, adjacent to the coal seam roof, or adjacent to the coal seam floor; if the horizontal section of the horizontal well is located adjacent to the coal seam roof / floor, the vertical distance between the horizontal section of the horizontal well and the coal seam is no more than 2m; the length of the horizontal section of the horizontal well is equal to the length of the permeability enhancement target area; the horizontal section of the horizontal well dips upwards along the coal seam dip direction;

[0012] Step 4, Perforation Design and Construction: Design n perforation sections evenly on the horizontal section of the horizontal well. Perform dense perforation within each perforation section. The length of each perforation section should not exceed the length of the matching packer sleeve. The spacing between adjacent perforation sections should be greater than the length of the packer sleeve and between half the total length of the tool string and the total length of the packer. When using a horizontal well adjacent to the coal seam roof, use hydraulic sandblasting for directional perforation, perforating downwards to connect with the coal seam. When using a horizontal well adjacent to the coal seam floor, use hydraulic sandblasting for directional perforation, perforating upwards to connect with the coal seam.

[0013] Step 5: Lower the tool string of the strip coal extraction device into the horizontal well of the soft coal seam. The tool string is lowered to the bottom of the horizontal well hole, and the first perforation section is located between the front packer and the rear packer rubber sleeve.

[0014] Step 6: Connect the high-pressure pump to the oil pipe, turn on the ground high-pressure pump, slowly increase the pressure, start coal extraction, continue hydraulic coal extraction for more than 30 minutes, then turn off the ground high-pressure pump, and the first stage of coal extraction is completed.

[0015] Step 7: Slowly drag the tool string backward, repeating the process in Step 6, so that the front packer blocks the first perforation section, and the second perforation section is located between the two packer sleeves. Water enters through the second perforation section and exits through the third perforation section. At this time, because the first perforation section is blocked by the packer, the water flow will not flow towards the front end of the tool string under pressure. At the same time, under the action of the rearward nozzle, a high-speed jet is formed in the annulus to return coal dust, preventing blockage of the annulus and pump stalling. On the other hand, the high-speed radial jet will create a negative pressure in the radial direction, that is, a suction force is formed at the orifice of the third perforation section, which helps to discharge coal dust. Continue hydraulic coal removal for more than 30 minutes to complete the second stage of coal removal, then turn off the high-pressure pump and stop water injection.

[0016] Step 8: Continue dragging the tool string. The nth perforation section is located between the two packers. The front packer blocks the (n-1)th perforation section. Water enters from the nth perforation section and exits from the (n+1)th perforation section, completing the coal extraction operation of all perforation sections.

[0017] Step 9: After all perforation sections have completed coal extraction, lift the tool string to the vertical shaft section and wait for 12-24 hours. Then repeat steps 5-8 for a second coal extraction. After the second coal extraction is completed, wait for another 12-24 hours and then perform a third coal extraction. Repeat the above steps until the coal extraction volume reaches the design value or the coal extraction efficiency is significantly reduced. Then lift the tool string to complete the coal extraction operation.

[0018] Step 10: After the coal mining operation is completed, the coal seam gradually deforms, filling the mining area and forming a plastic zone. This releases elastic energy, reduces local stress, and improves permeability, which is beneficial for gas extraction and outburst prevention.

[0019] Specifically, the rearward nozzle includes: a nozzle body, a flow channel at the center of the nozzle body, and multiple water eye holes on the rear end face of the nozzle body, arranged around the flow channel. A compressible sealing component is provided between the water eye holes and the flow channel. Under normal circumstances, the water eye holes and the flow channel are separated by the compressible sealing component. When the water flow in the flow channel compresses the compressible sealing component, it can open the water eye holes and the flow channel, allowing water to flow backward through the water eye holes. The operating pressure of the rearward nozzle is P. sp .

[0020] Specifically, the nozzle body is provided with multiple radial mounting holes, which are connected to the flow channel and the water eye. The compressible sealing component is located in the radial mounting hole. The compressible sealing component includes a slider and a radial compression spring. One end of the radial compression spring is connected to the slider, and the other end is fixed to the bottom of the radial mounting hole. Under normal conditions, the slider blocks the water eye. When the water flow in the flow channel radially compresses the slider and the radial compression spring, the slider opens the water eye.

[0021] Specifically, an annular mounting cavity is provided in the middle of the nozzle body, which surrounds the flow channel. The rear of the annular mounting cavity is connected to the flow channel and the water eye hole. A compressible sealing assembly is provided in the annular mounting cavity. The compressible sealing assembly includes a slip ring and an axial compression spring. The rear end of the axial compression spring is connected to the slip ring, and the front end of the axial compression spring is fixed to the front end of the annular mounting cavity. Under normal conditions, the slip ring seals the water eye hole. When the water flow in the flow channel axially compresses the slip ring and the axial compression spring, the slip ring opens the water eye hole.

[0022] Specifically, 4 to 8 water holes are arranged at equal intervals around the flow channel, with each water hole parallel to the flow channel.

[0023] Specifically, when the bottom pressure P b <P sp When the compressible plugging component is in its original position, the water jet is not ejected from the orifice; when the bottom hole pressure P b ≥P sp At that time, under the action of hydraulic pressure, the compressible sealing component is pushed, exposing the water hole and starting the jet.

[0024] Specifically, the setting pressure of the front packer and the rear packer is P. ex When the bottom pressure P b ≥P ex At this time, the rubber sleeves of the front packer and the rear packer expand, squeezing the sleeve wall to complete the setting and form a sealed space between the two packers;

[0025] The length of the rubber sleeves of the front and rear packers is greater than 1m; the interval length between the rubber sleeves of the front and rear packers is between the perforation section spacing l and 2l.

[0026] Specifically, the throttle valve operating pressure P v When the bottom pressure P b <P v When the throttle valve outlet is closed, liquid can only flow through the throttle valve and cannot flow out from the side; when the bottom pressure P b >P v When the throttle valve is open, the liquid can both pass through the throttle valve and flow out from the side.

[0027] Specifically, in the horizontal well strip coal extraction device for soft coal seams, the backward nozzle operating pressure P sp The setting pressure P of the front and rear packers ex Throttle valve operating pressure P v The following relationship exists between them: P ex <P sp <P v .

[0028] Compared with the prior art, the present invention has the following technical effects:

[0029] This invention enables strip coal extraction to meet the needs of coal mine outburst prevention, achieving increased permeability in the strip area, with a wider permeability range compared to previous localized coal extraction techniques. This invention utilizes a high-power surface pump unit for water jet coal extraction, resulting in large discharge capacity and excellent extraction effect. This invention can be applied to low-yield / old coalbed methane horizontal wells, or to the rearrangement of horizontal wells to increase coalbed methane production or further improve outburst prevention. The hydraulic coal extraction environment in horizontal wells of this invention for soft coal seams is a semi-enclosed environment, i.e., within the casing. The coal seam is connected through perforations, and the coal body is peeled away layer by layer through water jet impact and scouring, achieving coal extraction.

[0030] In the initial stage of horizontal well hydraulic coal extraction, namely the local cavity formation stage, high-pressure water is generated between the two packers. This high-pressure water forms a jet along the perforation holes, impacting the coal body near the perforation holes, disrupting the coal structure within a small area, and expanding the water-coal contact area. Simultaneously, the coal's moisture content increases, reducing its mechanical strength, which is beneficial for further hydraulic coal extraction. During the seepage and erosion stage, a high water pressure is formed between the two packers, creating a pressure difference between the front and rear perforation holes. This pressure difference forms a seepage path for water, increasing the local coal moisture content and reducing its mechanical strength. Furthermore, erosion gradually forms small cavities between the two sets of perforation holes, providing channels for hydraulic circulation. During the hydraulic circulation coal extraction stage, after the small cavities are formed, water flows within them, scouring and eroding the coal body on the cavity walls, gradually forming larger cavities. During the unstable stage of the cavity structure, the formation of the cavity changes the original triaxial stress state of the coal body. The coal body loses support in the radial direction (perpendicular to the cavity wall) and forms plastic and loose zones under tangential stress. The coal body collapses and deforms, causing the cavity to shrink. After the cavity shrinks, it continues to erode under the action of hydraulic circulation, repeating the coal extraction process. Attached Figure Description

[0031] Figure 1 This is a flowchart of the method of the present invention.

[0032] Figure 2 This is a schematic diagram showing three different layout methods for the horizontal section of a horizontal well.

[0033] Figure 3 This is a schematic diagram of the strip region.

[0034] Figure 4 This is a schematic diagram of the plastic zone.

[0035] Figure 5 This is a schematic diagram of the horizontal well strip coal extraction device for soft coal seams according to the present invention.

[0036] Figure 6 This is a schematic diagram of a rearward nozzle according to the present invention.

[0037] Figure 7 This is a schematic diagram of a rearward nozzle according to the present invention.

[0038] Figure 8 This is a stress distribution cloud map and a diagram showing the relationship between the cavity radius and the plastic zone radius.

[0039] Figure 9 This is a schematic diagram of a directional perforation used to connect to a coal seam.

[0040] Figure 10 A schematic diagram showing the uniform design of n perforation sections on the horizontal section of a horizontal well.

[0041] Figure 11This is a partial schematic diagram of the perforation section.

[0042] Figure 12 This is a schematic diagram showing the position of the tool string at the bottom of the hole and the perforation section.

[0043] Figure 13 Numerical simulation results of jet velocity under different displacement and perforation orifice radii.

[0044] Figure 14 This is a graph showing the relationship between coal breaking distances under different displacement rates.

[0045] Figure 15 Numerical simulation diagrams showing the relationship between different perforation radii and coal breaking distances.

[0046] Figure 16 This is a schematic diagram of the first section of the present invention, showing the coal extraction process.

[0047] Figure 17 This is a schematic diagram of the second section of the present invention, showing the coal extraction process.

[0048] Figure 18 This is a schematic diagram of the coal extraction in the nth segment of the present invention.

[0049] Figure 19 This is a schematic diagram of the plastic zone formed after the coal extraction operation of the present invention.

[0050] The meanings of the labels in the diagram are as follows:

[0051] 1. Front packer; 2. Throttle valve; 3. Rear packer; 4. Rear nozzle; 5. Oil pipe; 41. Nozzle body; 42. Flow channel; 43. Water inlet; 44. Compressible plugging assembly; 45. Radial mounting hole; 46. Annular mounting cavity; 441. Slider; 442. Radial compression spring; 443. Slip ring; 444. Axial compression spring. Detailed Implementation

[0052] This invention provides a method for enhancing permeability in the strip mining area of ​​a soft, fractured coal seam. Strip mining in soft, fractured coal seams refers to using a segmented hydraulic circulation method in a horizontal well on the surface to mine coal at a designed location in the coal roadway, creating a plastic zone. The plastic zone covers a certain area around the coal roadway, playing a role in enhancing permeability and relieving pressure. Combined with negative pressure gas extraction, this achieves multiple effects, including reducing stress, elasticity, gas content, and pressure in the strip area.

[0053] Specifically, the aforementioned horizontal wells can be existing coalbed methane development horizontal wells (generally with casing completion) or new horizontal wells constructed with casing completion; such as Figure 2 As shown, there are three ways to arrange the horizontal section of a horizontal well: it can be arranged in the upper part of the coal seam (not exceeding 2m above the coal seam), it can be arranged in the lower part of the coal seam (not exceeding 2m below the coal seam), and it can be arranged in the coal seam.

[0054] The aforementioned strip area refers to a certain range of coal seam surrounding a coal roadway; suppressing outbursts in this strip area contributes to the safe and efficient excavation of the coal roadway and is one of the key aspects of coal mining operations; for example... Figure 3 As shown, the strip in this invention can be defined as: a coal body with a width of 15m to 20m on both sides of the coal roadway and a height equal to the total thickness of the coal seam.

[0055] The aforementioned hydraulic circulation coal extraction refers to dense perforation on the casing, followed by localized injection of high-pressure water to create hydraulic circulation. Through repeated hydraulic circulation operations, localized coal extraction is achieved from the perforated holes. Subsequently, a segmented hydraulic circulation operation is used to achieve segmented coal extraction, ultimately forming coal extraction operations along a strip area. Depending on the different strata arrangement in the horizontal section of the horizontal well, perforation is divided into three methods: ① When using a horizontal well adjacent to the coal seam roof, hydraulic blasting directional perforation is used to densely perforate downwards, penetrating the casing and rock strata to connect with the coal seam; ② When using a coal seam horizontal well, either perforation jetting or hydraulic blasting perforation can be used to uniformly and densely perforate the casing; ③ When using a horizontal well adjacent to the coal seam floor, hydraulic blasting directional perforation is used to densely perforate upwards, penetrating the casing and rock strata to connect with the coal seam.

[0056] The aforementioned plastic zone refers to the area formed after coal mining operations. Due to the removal of the in-situ coal and the disruption of the original triaxial stress state of the surrounding coal, pressure is released in one direction, leading to an increase in deviatoric stress. This stress reaches the yield limit of the coal, causing plastic deformation and releasing elastic energy. Simultaneously, damage occurs within the coal body, significantly increasing permeability. This region is called the plastic zone. Figure 4 As shown.

[0057] The following are specific embodiments of the present invention. It should be noted that the present invention is not limited to the following specific embodiments. All equivalent modifications made based on the technical solutions of this application fall within the protection scope of the present invention.

[0058] Example 1:

[0059] This embodiment provides a method for enhancing permeability in the strip coal extraction area of ​​a fractured and soft coal seam. This method is implemented using a horizontal well strip coal extraction device for fractured and soft coal seams, such as... Figure 5 As shown, the device includes a front packer, a throttle valve, a rear packer, and a rear nozzle, coaxially connected from front to back. The rear end of the rear nozzle is connected to an oil pipe. The rear nozzle can spray water backward under a certain pressure, creating a negative pressure at the upper and lower positions to facilitate the movement of the coal-water mixture towards the casing. This increases the water flow rate and velocity in the annulus, which is beneficial for pulverized coal return. Specifically, when the injection pressure is P... spWhen the pressure of the backward nozzle is reached (i.e., the operating pressure of the backward nozzle), the backward nozzle opens and sprays clean water backward. The backward nozzle includes: a nozzle body, a flow channel at the center of the nozzle body, and multiple water holes on the rear end face of the nozzle body. The multiple water holes are arranged around the flow channel, and a compressible sealing component is provided between the water holes and the flow channel. Under normal circumstances, the water holes and the flow channel are separated by the compressible sealing component. When the water flow in the flow channel compresses the compressible sealing component, it can open the water holes and the flow channel, allowing the water to flow backward through the water holes.

[0060] like Figure 6 As shown, in this embodiment, the backward nozzle has multiple radial mounting holes within the nozzle body. These radial mounting holes connect the flow channel and the water eye holes. A compressible sealing assembly is disposed within the radial mounting holes. The compressible sealing assembly includes a slider and a radial compression spring. One end of the radial compression spring is connected to the slider, and the other end is fixed to the bottom of the radial mounting hole. Under normal conditions, the slider blocks the water eye hole. When the water flow in the flow channel radially compresses the slider and the radial compression spring, the slider opens the water eye hole. More specifically, 4 to 8 water eye holes are arranged at equal intervals around the flow channel, each water eye hole being parallel to the flow channel. The flow channel is the main channel for fluid (clean water or other coal extraction medium); in this embodiment, each water eye hole corresponds to one slider. When the bottom pressure is low (P... b <P sp When the pressure at the bottom of the well is high (P), the slider is in its original position under the action of the spring (as shown by the dotted line), and the water jet does not flow; when the pressure at the bottom of the well is high (P), the slider is in its original position (as shown by the dotted line), and the water jet does not flow. b ≥P sp Under hydraulic pressure, the slider is pushed to compress the spring, exposing the water eye channel and starting the jet.

[0061] In another embodiment, such as Figure 7 As shown, an annular mounting cavity is provided in the middle of the nozzle body, surrounding the flow channel. The rear of the annular mounting cavity connects to the flow channel and the water eye. A compressible sealing assembly is located inside the annular mounting cavity. The compressible sealing assembly includes a slip ring and an axial compression spring. The rear end of the axial compression spring is connected to the slip ring, and the front end of the axial compression spring is fixed to the front end of the annular mounting cavity. Under normal conditions, the slip ring seals the water eye. When the water flow in the flow channel axially compresses the slip ring and the axial compression spring, the slip ring opens the water eye. More specifically, 4 to 8 water eyes are arranged at equal intervals around the flow channel, each water eye parallel to the flow channel. The flow channel is the main channel for fluid (clean water or other coal extraction media); all water eyes correspond to the same slip ring; when the bottom pressure is low (P... b <P sp When the pressure at the bottom of the well is high (P), the slip ring is in its original position under the action of the spring (as shown by the dotted line), and the water jet does not flow; when the pressure at the bottom of the well is high (P), the slip ring is in its original position (as shown by the dotted line), and the water jet does not flow. b ≥P spUnder hydraulic pressure, the slip ring is pushed to compress the spring, exposing the water eye channel and starting the jet.

[0062] Both the front and rear packers are hydraulic packers, mature industrial products. Their function is to ensure that when the bottom hole pressure passing through them is greater than the setting pressure (denoted as P), the packer will be able to set. b ≥P ex When the packer sleeve expands, it compresses the casing wall, completing the setting. When two packers are used together inside the casing, a pressurized, sealed space is formed between the two packers. In this invention, the packer sleeve length should be greater than 1m; the packer sleeve interval length is related to the perforation section spacing l, and the packer sleeve interval length should be between l and 2l.

[0063] Throttling valves are mature industrial products whose function is to control pressure at a certain level (denoted as the throttle valve operating pressure P). v Open the valve, i.e., when the bottom pressure P b <P v When the throttle valve outlet is closed, liquid can only flow through the throttle valve and cannot flow out from the side; when the bottom pressure P b >P v When the throttle valve is open, the liquid can both pass through the throttle valve and flow out from the side.

[0064] In a horizontal well strip coal extraction device for soft coal seams, the surface pump pressure P and the bottom hole pressure P are... b Rear nozzle operating pressure P sp Packer expansion pressure P ex Throttle valve operating pressure P v The following relationship exists between them:

[0065] ① Bottom hole pressure P b The calculation is based on the ground pump pressure P, and the calculation method is as follows:

[0066]

[0067] In the formula, Q is the flow rate displayed by the ground pump set, r is the inner diameter of the tubing string, L is the length of the tubing string, λ is the friction coefficient, which is related to the material of the inner wall of the tubing string; h is the vertical depth of the tool string; g is the acceleration due to gravity; and ρ is the density of the fracturing fluid.

[0068] ②P ex <P sp <P v ;

[0069] Different operating states of this device can be achieved by controlling the ground pump pressure P.

[0070] This embodiment describes a method for enhancing permeability in the strip mining area of ​​a broken and soft coal seam, such as... Figure 1 As shown, it includes the following steps:

[0071] Step 1: Select the anti-reflection target area:

[0072] Combined with the coal mining plan, block division and coal roadway design data, select the anti-reflection target area, which is called a strip; the range of the strip area is: a rectangular range with the same length as the roadway and a width of 15m - 20m along the coal seam direction on both sides outside the roadway and the roadway, and its height is equal to the coal seam height;

[0073] Step 2: Design the target coal extraction volume:

[0074] Design the coal extraction volume according to the anti-reflection target area selected in Step 1:

[0075]

[0076] In the formula, V is the coal extraction volume, R is the radius of the plastic zone. After coal extraction, a plastic zone is formed near the coal extraction area, and the plastic zone is beneficial to stress release and gas drainage; k is a coefficient, generally 3 < k < 6, which needs to be determined by combining actual geological conditions; L is the length of the coal extraction section, that is, the spacing between two groups of perforation holes used in the hydraulic cycle. The plastic zone refers to the area where, after coal extraction operations, due to the in-situ coal body being taken out and the original triaxial stress state of the nearby coal body being broken, pressure is relieved in one direction, resulting in an increase in deviatoric stress, reaching the yield limit of the coal body, causing plastic deformation, releasing elastic energy; at the same time, damage occurs inside the coal body, and the permeability increases significantly. This area is called the plastic zone, and the range of the plastic zone (equivalent to the strip area range).

[0077] In the above formula, the value of k is related to the mechanical parameters of the coal and can be determined by the following numerical simulation: In Figure 8 In the shown plane model, set the left and lower boundaries as symmetric boundaries, and apply in-situ stress on the right and upper boundaries; set a quarter circle with a variable radius at the lower left position of the model to simulate the taken-out coal body; obtain the stress distribution nephogram under different coal extraction radii through simulation. It is considered that when the stress exceeds the compressive strength, the coal body undergoes plastic deformation, and the range of the plastic zone is represented by the arc line at the lower left corner in the figure. Make a line graph of the simulation results to obtain the relationship between the coal extraction radius and the range of the plastic zone, and thus obtain the value range of the coefficient k. In this example, k can be taken as 4.

[0078] Step 3: Design and construct a horizontal well (this step can be omitted if an existing coalbed methane well is used):

[0079] Construct a horizontal well and cement the downhole casing; the horizontal section of the horizontal well is arranged in the coal seam, adjacent to the coal seam roof or adjacent to the coal seam floor. If it is arranged adjacent to the coal seam roof / floor, the vertical distance from the coal seam shall not be greater than 2m; the horizontal section of the horizontal well is of the same length as the anti-reflection target area; the horizontal section of the horizontal well should be inclined upward along the coal seam dip as much as possible, as Figure 2 shown.

[0080] Step 4, Perforation Design and Construction (This step is omitted if screen pipe completion is used):

[0081] N perforation sections are evenly designed on the horizontal section of the horizontal well, and dense perforation is carried out in the perforation sections, such as... Figure 9-11 As shown. The length of the perforation section is <1m, and the distance between adjacent perforation sections is 1m < l < 5m. During construction, all perforations are completed in one go. Perforation can be achieved by perforation jetting, hydraulic sandblasting, or other methods.

[0082] When using a horizontal well adjacent to the coal seam roof, hydraulic jetting is used for directional perforation, with the perforation directed downwards to connect with the coal seam; when using a horizontal well adjacent to the coal seam floor, hydraulic jetting is used for directional perforation, with the perforation directed upwards to connect with the coal seam. Figure 9 As shown.

[0083] Step 5: Lower the tool string, as shown in Part 2 of this document, "Strip Coal Extraction Device for Horizontal Wells in Soft Coal Seams." The tool string should be lowered to the bottom of the borehole, with the first perforation section located between the two packer sleeves, as shown below. Figure 12 As shown.

[0084] Step 6: Turn on the surface high-pressure pump, connect the high-pressure pump to the oil pipe, adjust the displacement Q, observe the pump pressure P, and calculate the bottom hole pressure P. b Pump pressure P and P' are controlled by adjusting the displacement. b , making P b Slowly rising; when P ex <P b <P sp At that time, the packer sets, and the pump pressure continues to increase to P. sp <P b <P v At this point, the backward nozzle activates; observe the wellhead on the ground, and the backflow should increase; continue increasing the pump pressure to P. b >P v If the water seeps back onto the surface and carries coal dust, it indicates normal operation. At this point, the water flow forms a channel between the first and second perforation sections, and coal extraction begins. Figure 16 As shown.

[0085] This invention studies the relationship between discharge rate and coal extraction range during coal extraction and provides engineering design references. Numerical simulation is used to study the jet velocity under different discharge rates and perforation radii. Referring to the research published in the literature "Zhou Zhe. Research on the Mechanism and Technology of Combined Jet Impact Crushing of Coal and Rock [D]. Chongqing University, 2017," the minimum velocity of water jet breaking coal and rock, 52.6 m / s, is used as the threshold for water jet breaking. The numerical simulation results are as follows: Figure 13 Plot a scatter plot with displacement as the X-axis and coal breaking distance as the Y-axis, and draw a trend line as follows: Figure 14 As shown.

[0086] This trend line can be used to predict coal breaking distances under different displacements:

[0087] x=aQ 2 +bQ+c

[0088] In the formula, x is the coal breaking distance, in meters; Q is the displacement, in meters. 3 / min; a, b, and c are coefficients that need to be determined based on specific research and parameters such as the radius of different perforations in actual construction.

[0089] Simultaneously, numerical simulation studies were conducted on the relationship between different perforation radii and coal breaking distances. A scatter plot and trend line were created using the perforation radius as the X-axis and the coal breaking distance as the Y-axis. Figure 15 As shown.

[0090] This trend line can be used to predict coal breaking distances for different perforation radii:

[0091] x = me nr

[0092] In the formula, x is the coal breaking distance, m; r is the perforation radius, m; m and n are coefficients, which need to be studied according to the actual engineering situation; e is a natural number.

[0093] According to the construction parameters, the hydraulic coal extraction continues for 30-90 minutes, then the ground high-pressure pump is turned off, and the first stage of coal extraction ends.

[0094] Step 7: Slowly drag the tool string backward, repeating the process in Step 6, so that the front packer blocks the first perforation section. The second perforation section is located between the two packer sleeves, with water entering through the second perforation section and exiting through the third perforation section. At this time, because the first perforation section is blocked by the packer, the water flow will not flow towards the front of the tool string under pressure. Simultaneously, under the action of the rearward nozzle, a high-speed jet is formed in the annulus, returning coal dust and preventing blockage of the annulus and pump stalling. On the other hand, the high-speed radial jet will create a negative pressure in the radial direction, that is, create suction at the orifice of the third perforation section, which helps to discharge coal dust. Continue hydraulic coal removal for 30-90 minutes to complete the second stage of coal removal. Turn off the high-pressure pump and stop water injection. Figure 17 As shown.

[0095] Step 8: Continue dragging the tool string. The nth perforation section is located between the two packers. The front packer blocks the (n-1)th perforation section. Water enters through the nth perforation section and exits through the (n+1)th perforation section, completing the coal extraction operation for all perforation sections. Figure 18 As shown.

[0096] Step 9: After all perforation sections have completed coal extraction, lift the tool string to the vertical shaft section and wait for 12-24 hours. Then repeat steps 5-8 for a second coal extraction. After the second coal extraction, wait for another 12-24 hours and then perform a third coal extraction. Repeat the above steps until the coal extraction volume reaches the design value or the coal extraction efficiency is significantly reduced. Then lift the tool string to complete the coal extraction operation.

[0097] Step 10: After the coal removal operation is completed, the coal seam gradually deforms, filling the removal area and forming a plastic zone within a certain range. This releases elastic energy, reduces local stress, and improves permeability, thus facilitating gas extraction and outburst suppression. Figure 19 As shown.

Claims

1. A method for enhancing permeability in strip mining areas of broken and soft coal seams, characterized in that, This method utilizes a horizontal well strip coal extraction device for soft coal seams. The device includes a front packer, a throttle valve, a rear packer, and a rear nozzle, which are coaxially connected from front to back. The rear end of the rear nozzle is connected to an oil pipe, and the rear nozzle can spray water backward. This method involves constructing horizontal wells in or adjacent to the top and bottom of soft coal seams. Casing is run in the horizontal section, and perforations are made at intervals on the casing. A front packer and a rear packer are used to isolate a sealed space within the casing, with at least one set of perforations within this sealed space. At least one set of perforations also exists at the rear end of the rear packer. A throttle valve is installed between the front and rear packers, through which water flows. Once the sealed space is filled with water, the water flows into the formation along the perforations in the casing, then returns to the annulus between the casing and the tubing through the perforations at the rear end of the rear packer, forming a hydraulic circulation for coal extraction. This process is repeated multiple times to complete the coal extraction operation, creating a plastic zone that releases elastic energy, reduces local stress, and improves permeability, thus facilitating gas extraction and outburst suppression. The method includes the following steps: Step 1: Select the target area for permeability enhancement: The target area for permeability enhancement is called a strip. The strip area is a rectangular area with the same length as the roadway, a width of 15m to 20m along the outer sides of the roadway and the coal seam, and a height equal to the height of the coal seam. Step 2, Design target coal extraction volume: Design the coal extraction volume based on the target permeability enhancement area selected in Step 1; Step 3, Design and Construction of Horizontal Wells: Construct horizontal wells and run casing for cementing; the horizontal section of the horizontal well is located in the coal seam, adjacent to the coal seam roof, or adjacent to the coal seam floor; if the horizontal section of the horizontal well is located adjacent to the coal seam roof / floor, the vertical distance between the horizontal section of the horizontal well and the coal seam is no more than 2m; the length of the horizontal section of the horizontal well is equal to the length of the permeability enhancement target area; the horizontal section of the horizontal well dips upwards along the coal seam dip direction; Step 4, Perforation Design and Construction: Design perforations uniformly on the horizontal section of the horizontal well. Each perforation section is densely perforated, with the length of each perforation section not exceeding the length of the packer sleeve. The spacing between adjacent perforation sections is greater than the length of the packer sleeve and falls between half the total length of the tool string and the total length of the packer. When using a horizontal well adjacent to the coal seam roof, hydraulic jetting is used for directional perforation, directional perforation downwards to connect with the coal seam. When using a horizontal well adjacent to the coal seam floor, hydraulic jetting is used for directional perforation, directional perforation upwards to connect with the coal seam. Step 5: Lower the tool string of the strip coal extraction device into the horizontal well of the soft coal seam. The tool string is lowered to the bottom of the horizontal well hole, and the first perforation section is located between the front packer and the rear packer rubber sleeve. Step 6: Connect the high-pressure pump to the oil pipe, turn on the ground high-pressure pump, slowly increase the pressure, start coal extraction, continue hydraulic coal extraction for more than 30 minutes, then turn off the ground high-pressure pump, and the first stage of coal extraction is completed. Step 7: Slowly drag the tool string backward, repeating the process in Step 6, so that the front packer blocks the first perforation section, and the second perforation section is located between the two packer sleeves. Water enters through the second perforation section and exits through the third perforation section. At this time, because the first perforation section is blocked by the packer, the water flow will not flow towards the front end of the tool string under pressure. At the same time, under the action of the rearward nozzle, a high-speed jet is formed in the annulus to return coal dust, preventing blockage of the annulus and pump stalling. On the other hand, the high-speed radial jet will create a negative pressure in the radial direction, that is, a suction force is formed at the orifice of the third perforation section, which helps to discharge coal dust. Continue hydraulic coal removal for more than 30 minutes to complete the second stage of coal removal, then turn off the high-pressure pump and stop water injection. Step 8: Continue dragging the tool string. The nth perforation section is located between the two packers. The front packer blocks the (n-1)th perforation section. Water enters from the nth perforation section and exits from the (n+1)th perforation section, completing the coal extraction operation of all perforation sections. Step 9: After all perforation sections have completed coal extraction, lift the tool string to the vertical shaft section and wait for 12-24 hours. Then repeat steps 5-8 for a second coal extraction. After the second coal extraction is completed, wait for another 12-24 hours and then perform a third coal extraction. Repeat the above steps until the coal extraction volume reaches the design value or the coal extraction efficiency is significantly reduced. Then lift the tool string to complete the coal extraction operation. Step 10: After the coal mining operation is completed, the coal seam gradually deforms, filling the mining area and forming a plastic zone. This releases elastic energy, reduces local stress, and improves permeability, which is beneficial for gas extraction and outburst prevention.

2. The method for enhancing permeability in the strip mining area of ​​a broken and soft coal seam as described in claim 1, characterized in that, The rearward nozzle includes: a nozzle body, a flow channel at the center of the nozzle body, and multiple water holes on the rear end face of the nozzle body, arranged around the flow channel. A compressible sealing component is provided between the water holes and the flow channel. Under normal circumstances, the water holes and the flow channel are separated by the compressible sealing component. When the water flow in the flow channel compresses the compressible sealing component, it can open the water holes and the flow channel, allowing water to flow backward through the water holes. The operating pressure of the rearward nozzle is... .

3. The method for enhancing permeability in the strip mining area of ​​a soft coal seam as described in claim 2, characterized in that, The nozzle body has multiple radial mounting holes, which are connected to the flow channel and the water eye. A compressible sealing component is located in the radial mounting hole. The compressible sealing component includes a slider and a radial compression spring. One end of the radial compression spring is connected to the slider, and the other end is fixed to the bottom of the radial mounting hole. Under normal conditions, the slider blocks the water eye. When the water flow in the flow channel radially compresses the slider and the radial compression spring, the slider opens the water eye.

4. The method for enhancing permeability in the strip coal mining area of ​​a broken and soft coal seam as described in claim 2, characterized in that, An annular mounting cavity is provided in the middle of the nozzle body. The annular mounting cavity surrounds the flow channel. The rear part of the annular mounting cavity is connected to the flow channel and the water eye hole. A compressible sealing assembly is provided in the annular mounting cavity. The compressible sealing assembly includes a slip ring and an axial compression spring. The rear end of the axial compression spring is connected to the slip ring, and the front end of the axial compression spring is fixed to the front end of the annular mounting cavity. Under normal conditions, the slip ring blocks the water hole when the water flow in the flow channel axially compresses the slip ring and the spring. When the water flow in the flow channel axially compresses the slip ring and the spring, the slip ring opens the water hole.

5. The method for enhancing permeability in the strip mining area of ​​a soft, fractured coal seam as described in claim 2, characterized in that, Four to eight water holes are arranged at equal intervals around the flow channel, with each water hole parallel to the flow channel.

6. The method for enhancing permeability in the strip mining area of ​​a broken and soft coal seam as described in claim 2, characterized in that, When the bottom pressure When the compressible plugging component is in its original position, the water jet does not flow from the orifice; when the bottom pressure... At that time, under the action of hydraulic pressure, the compressible sealing component is pushed, exposing the water hole and starting the jet.

7. The method for enhancing permeability in the strip coal mining area of ​​a broken and soft coal seam as described in claim 1, characterized in that, The setting pressure of the front packer and the rear packer is: When the bottom pressure At this time, the rubber sleeves of the front packer and the rear packer expand, squeezing the sleeve wall to complete the setting and form a sealed space between the two packers; The length of the rubber sleeves of the front packer and the rear packer is greater than 1. The range of the rubber sleeve length between the front and rear packers is within the perforation section spacing. and between.

8. The method for enhancing permeability in the strip mining area of ​​a soft coal seam as described in claim 1, characterized in that, The throttle valve operating pressure When the pressure at the bottom of the well When the throttle valve outlet is closed, liquid can only flow through the throttle valve and cannot flow out from the side; when the bottom pressure... When the throttle valve is open, the liquid can both pass through the throttle valve and flow out from the side.

9. The method for enhancing permeability in the strip mining area of ​​a broken and soft coal seam as described in claim 1, characterized in that, In the horizontal well strip coal extraction device for the soft coal seam, the backward nozzle operating pressure Setting pressure of the front and rear packers Throttle valve operating pressure The following relationship exists between them: .

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