A method for unloading and enhancing permeability of horizontal seam in coal seams under periodic pressure

Abstract

The present invention belongs to the technical field of coal seam permeability enhancement and efficient gas extraction, and provides a method for coal seam horizontal cutting to relieve pressure and enhance permeability based on periodic pressure, including: collecting coal mine production background data and selecting high-gas, low-permeability coal roadway excavation working faces, real-time roof pressure step measurement and pressure cycle prediction, coal mining working face selection, long-hole drilling along the seam, and hydraulic horizontal cutting. The present invention uses high-pressure water jets to perform fixed-point backward horizontal cutting within the range of the extraction drilling coal seam, thereby increasing the number and connectivity of internal diversion channels in the coal seam, and the seams generated in the coal reservoir after cutting form sufficient pressure relief space. Then, by utilizing the increased load on the coal mining working face during periodic pressure and the inherent stress of the coal body itself, the unloading movement space created by the horizontal cutting is used to form a fracture network, further causing the coal body to break and expand, thereby reducing the coal seam gas pressure and improving the coal seam permeability.

Description

Technical Field

[0001] The present invention relates to the technical field of coal seam permeability enhancement and efficient gas extraction, and in particular to a coal seam horizontal slit pressure relief and permeability enhancement method based on periodic pressure action. Background Art

[0002] As shallow coal reserves are depleted, mining operations are increasingly entering deeper areas of coal seams. The gas content in coal seams is directly proportional to the thickness of the overlying bedrock, and the amount of gas emitted during mining has gradually increased from low-gas mines to high-gas mines. Furthermore, deep mines are generally plagued by the technical challenge of poor extraction efficiency due to poor coal seam permeability. This coal mine gas problem will continue to hinder coal mine safety in my country. The essence of the coal fracturing and destruction process is to disrupt and crack the coal's essential structure, achieving a fracturing effect. This process is complex and dynamic. The key to preventing and controlling gas accidents lies in employing technical methods to decompress the coal seams. These technical measures primarily include protective layer decompression mining, hydraulic fracturing (hydraulic fracturing, hydraulic slitting, hydraulic punching), deephole pre-splitting blasting, and CO2 blasting.

[0003] The above methods basically focus on fracturing by expanding from the hole to the surrounding areas, but ignore the inherent stress of the coal body itself, and do not make use of the weight of the old roof, the load of the overlying rock strata, and the downward impact load formed by the collapse of the roof within a certain span during the coal mine cycle. In particular, the protective layer mining principle is not used in the coal roadway, and the horizontal space is not created and utilized as a movement compensation space in a larger range of the coal body to achieve overall fracturing of the coal seam, resulting in poor results. Summary of the Invention

[0004] A method for unloading and enhancing permeability of horizontal seams in coal seams under periodic pressure, comprising:

[0005] Collecting coal mine production background information and selecting a high-gas, low-permeability coal roadway excavation working face based on the coal mine production background information;

[0006] Determine the real-time roof pressure step distance of the high-gas and low-permeability coal roadway excavation working face, calculate roof pressure data based on the real-time roof pressure step distance collected historically, and predict the pressure cycle based on the pressure data;

[0007] In the pressure cycle, a coal mining face on one side of the leading stress concentration zone is selected;

[0008] Drilling to a pre-designed position using a drill rod at the coal mining face to obtain a long borehole along the layer;

[0009] After the drill is withdrawn, the drill rod is replaced with a horizontal slotting device, and the drill is advanced to the starting slotting position set in the long borehole in the bed. The high-pressure water jet of the horizontal slotting device is used to perform fixed-point backward horizontal cutting within the range of the drilled coal seam to obtain a movement compensation space;

[0010] Obtaining a fracture network according to the increased load on the coal mining face during periodic pressure, the inherent stress of the coal body, and the movement compensation space;

[0011] Based on the fracture network, the coal body is controlled to be over-pressurized and to undergo crushing and expansion, thereby obtaining a target coal seam with pressure relief and increased permeability.

[0012] Preferably, the coal mine production background data includes: distribution characteristics of the ground stress field, coal seam density and thickness, gas content and pressure, and coal seam permeability.

[0013] Preferably, the pressure data includes: the mine pressure time in the first period, the pressure cycle during historical mining, the pressure step distance and the periodic pressure peak value.

[0014] Preferably, the cutting water pressure range of the horizontal slitting device is 80 to 100 MPa.

[0015] The present invention discloses the following technical effects:

[0016] The present invention provides a method for unloading and increasing the permeability of coal seams by horizontal cutting under periodic pressure. The method solves the shortcomings of traditional coal seam permeability increasing technology by combining the increased load on the working face during periodic pressure, the huge stress on the coal seam formed by the inherent stress of the coal body itself, and the hydraulic cutting technology, and realizes the function of increasing the permeability of coal seams by utilizing the inherent stress of the coal body itself; the horizontal and vertical tensile shear forces formed by the high-energy water body of the horizontal cutting device inside the coal rock body solve the problems of high coal seam gas pressure and low coal seam permeability, and realizes the development of coal rock cracks. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A schematic diagram of a process for unloading and enhancing permeability of horizontal seams in coal seams under periodic pressure provided by an embodiment of the present invention;

[0018] Figure 2 A schematic diagram of the basic top-periodic fractured rock layer structure and load distribution provided by an embodiment of the present invention;

[0019] Figure 3 This is a diagram showing the effect of creating compensation space for hydraulic horizontal cutting of coal seams provided by an embodiment of the present invention;

[0020] Figure 4 This is a diagram showing the effect of unloading and enhancing permeability of horizontal seams in coal seams under periodic pressure provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0021] Figure 1 A schematic diagram of a process flow for unloading and enhancing permeability of coal seam horizontal slits under periodic pressure provided by an embodiment of the present invention is shown in FIG. Figure 1 As shown, the present invention provides a method for unloading pressure and increasing permeability of horizontal seams in coal seams under periodic pressure, comprising:

[0022] Step 100: Collecting coal mine production background data and selecting a high-gas, low-permeability coal roadway excavation working face based on the coal mine production background data;

[0023] Step 200: Determine the real-time roof pressure step distance of the high-gas, low-permeability coal roadway excavation working face, calculate roof pressure data based on the real-time roof pressure step distances collected historically, and predict the pressure cycle based on the pressure data;

[0024] Step 300: selecting a coal mining face on one side of the leading stress concentration zone within a pressure cycle;

[0025] Step 400: Drilling to a pre-designed position using a drill rod in the coal mining face to obtain a long borehole along the seam;

[0026] Step 500: After the drill is withdrawn, the drill rod is replaced with a horizontal slotting device, and the drill is advanced to the starting slot position set in the long borehole along the seam. The high-pressure water jet of the horizontal slotting device is used to perform a fixed-point backward horizontal cutting within the range of the drilled coal seam to obtain a movement compensation space;

[0027] Step 600: Obtain a fracture network based on the increased load on the coal mining face during periodic pressure, the inherent stress of the coal body, and the movement compensation space;

[0028] Step 700: Based on the fracture network, control the coal body to be over-pressurized and to cause expansion and breakage, so as to obtain the target coal seam with pressure relief and permeability enhancement.

[0029] Furthermore, the coal mine production background data includes: distribution characteristics of the ground stress field, coal seam density and thickness, gas content and pressure, and coal seam permeability.

[0030] Specifically, the pressure data includes: the mine pressure time in the first period, the pressure cycle during historical mining, the pressure step distance and the periodic pressure peak value.

[0031] Optionally, the cutting water pressure range of the horizontal cutting device is: 80 to 100 MPa.

[0032] Specifically, the steps for implementing the pressure relief and permeability enhancement of horizontal seams in coal seams under periodic pressure are as follows:

[0033] S1: Obtain background information on coal mine production, including the distribution characteristics of the ground stress field, coal seam density and thickness, gas content and pressure, and coal seam permeability. Based on the coal mine geological survey, select the coal tunneling working face with high gas content and low permeability;

[0034] S2: Detect roof pressure step distance in real time at the working face of the mine site. Measure the periodic pressure time based on theoretical formulas, obtain data such as the roof pressure time of the working face in the first period, the pressure cycle during historical mining, the pressure step distance, and the periodic pressure peak value, so as to predict the next pressure.

[0035] S3: During the periodic pressure, the old roof rock beams will break and collapse along the coal wall or even within the coal wall under the action of their own weight and the load of the overlying rock strata. The load on the working face will generally increase, and the roof pressure will increase periodically. The load on the working face during the periodic pressure and the inherent stress of the coal body will be used to compensate for the increased load on the working face during the periodic pressure.

[0036] S4: Select the coal mining face close to the advanced stress concentration area during periodic pressure;

[0037] S5: Use the drill rod to drill to the designed position to form a long borehole in the bedding. After the drill is withdrawn, the drill bit is replaced with a horizontal slotting device, and the drill is again drilled to the designed starting slot position in the long borehole in the bedding;

[0038] S6: After the horizontal slitting device is placed, the slitting device starts to operate;

[0039] S7: Use 80-100 MPa high-pressure water to horizontally cut the coal body on both sides of the drilled coal hole section;

[0040] S8: Using high-pressure water jets to impact the coal and rock mass will not only impact the coal and rock mass, but also cause impact damage to the weak areas inside it;

[0041] S9: After the water jet impact damage, the fragile area inside the coal rock body continues to expand, the shear strength is greatly reduced, and the shear stress generated by the water jet will also have a destructive effect on the fragile area;

[0042] S10: After continuous impact and shearing by high-pressure water jets, cracks continue to develop inside the coal and rock mass. The high-energy water will generate horizontal and vertical tensile shear forces inside the coal and rock mass. When the tensile shear forces exceed the stress limit of the coal mass, the cracks inside the coal and rock mass will further develop.

[0043] S11: High-energy water enters the coal body through cracks. The cracks inside the coal body continue to expand under the action of the wedge force formed by the pore pressure water. When the tensile stress exceeds the tensile failure limit of the coal body, the coal body will fail.

[0044] S12: When the high-speed water jet reaches the coal body, it first generates water hammer pressure, which breaks the coal rock and initiates shear fractures. Simultaneously, the coal body, which has a lower tensile strength, develops numerous tensile cracks under the action of tensile stress. Subsequently, due to the continued action of stagnation pressure, the coal body further breaks and spalls, forming erosion pits, and then forming a horizontal slot perpendicular to the drill hole.

[0045] S13: After the slotting is completed, the drill rod is pulled out to create and utilize the horizontal space formed by the horizontal slotting as a movement compensation space within a larger range of the coal body;

[0046] S14: During periodic pressure, the collapse of the old roof will cause a sudden increase in working face pressure, forming stress perpendicular to the coal seam. Considering the inherent stress of the coal seam itself and the changes in coal seam stress caused by hydraulic fracturing in the early stage, cracks in the coal body continue to expand and develop under the wedge force formed by pore pressure water, forming horizontal grooves, creating horizontal space as a compensation space for the movement of the coal body within a larger range. Eventually, the coal body is subjected to extreme pressure, resulting in overall cracking and even gradual collapse of the coal seam.

[0047] S15: While further effectively releasing the potential, it also alleviates the local stress concentration in the coal seam to the greatest extent, promotes the reduction of coal reservoir pressure, and completes the decompression of the coal seam.

[0048] S16: When the next cycle of pressure comes, repeat the above process to change the original stress state of the coal body and the crack opening, thereby enhancing the permeability of the coal seam and improving the gas extraction efficiency.

[0049] Specifically, Figure 2 Corresponding to S1-S4, during the period of cyclic pressure, the old roof rock beam will break and collapse along the coal wall or even within the coal wall under the action of its own weight and the load of the overlying strata. The load on the working face will generally increase, and the roof pressure will increase periodically, with the help of the increased load on the working face during cyclic pressure and the inherent stress of the coal body itself; the arrows represent the load of the overlying strata. Figure 3 For S5-S13, a drill rod is used to drill to the designed position of the coal seam to form a long borehole. After the drill is withdrawn, the drill bit is replaced with a horizontal slotter. High-pressure water jets are ejected from both ends of the horizontal slotter to act on the coal body (the arrows in the figure indicate the double-groove direction, and the cutting direction is horizontal), and then horizontal slotting can begin. After each slotting is completed, the drill is withdrawn to the next slotting position, and the horizontal slotting operation is repeated until an effective horizontal compensation space is created in the coal seam. Figure 4Corresponding to S14-S16, in the early stage, high-pressure water jets were used to perform fixed-point backward horizontal cutting within the range of the extracted drilled coal seam, which increased the number and connectivity (micro-cracks) of the internal diversion channels in the coal seam. The seams generated in the coal reservoir after the cutting formed sufficient pressure relief space. The increased load on the coal mining face during cyclic pressure and the inherent stress of the coal body were then used. With the help of the unloading and movement space created by the horizontal cutting, a crack network was formed, which further caused the coal body to break and expand. Finally, the coal body was compressed to the limit, resulting in overall cracking and even the gradual collapse of the coal seam.

[0050] The beneficial effects of the present invention are as follows:

[0051] This method uses the principle of protective layer mining, with the help of the increased load on the working face during periodic pressure, combined with the inherent stress of the coal body itself, to form a huge stress effect on the coal seam, and then uses hydraulic cutting technology to create an effective horizontal compensation space for spatial compensation, overcoming the shortcomings of traditional coal seam permeability enhancement technology (blasting fracturing, hydraulic fracturing, etc. are based on the coal seam with the hole as the center to spread to the surrounding area for fracturing, while ignoring the inherent stress of the coal body itself); after the continuous impact and shearing effect of high-pressure water jets, a horizontal compensation space is cut out, and the coal seam above the horizontal slot produces a displacement The lower coal seam loses its vertical stress, while stress persists in the coal seams to the left and right of the seam. This causes the lower coal seam to expand and shift upward, leading to the continuous development of internal cracks in the coal rock mass. High-energy water exerts horizontal and vertical tensile and shear forces within the coal rock mass. When these forces exceed the coal mass's stress limit, further cracks develop within the coal rock mass, significantly reducing coal seam gas pressure and improving coal seam permeability. This method can also reduce the gas content in adjacent protected high-gas or outburst-prone coal seams, eliminating their outburst hazard and facilitating subsequent coal seam mining. This method is safe, easy to control, and highly repeatable. It features periodic roof pressure and efficient, clean, low-heat, and low-vibration water jet impact crushing of coal and rock materials. This method has important theoretical and practical significance for further improving extraction processes, refining two prevention and control theoretical systems, promoting the development of mine dynamic disaster science, increasing coal roadway excavation speeds, clarifying the mechanism of coal seam fracture in coal roadways, reducing coal mine production costs, and reducing the incidence of safety accidents.

Claims

1. A method for increasing permeability by unloading pressure on horizontal seams in coal seams under periodic pressure, characterized in that: include: Collecting coal mine production background information and selecting a high-gas, low-permeability coal roadway excavation working face based on the coal mine production background information; Determine the real-time roof pressure step distance of the high-gas, low-permeability coal roadway excavation working face, calculate pressure data based on the real-time roof pressure step distance collected historically, and predict the pressure cycle based on the pressure data; the pressure data includes: the mine pressure time in the first period, the pressure cycle during historical mining, the pressure step distance, and the periodic pressure peak value; In the pressure cycle, a coal mining face on one side of the leading stress concentration zone is selected; Drilling to a pre-designed position using a drill rod at the coal mining face to obtain a long borehole along the layer; After the drill is withdrawn, the drill rod is replaced with a horizontal slotting device, and the drill is advanced to the starting slotting position set in the long borehole in the bed. The high-pressure water jet of the horizontal slotting device is used to perform fixed-point backward horizontal cutting within the range of the drilled coal seam to obtain a movement compensation space; Obtaining a fracture network based on the increased load on the coal mining face during periodic pressure, the inherent stress of the coal body where the coal mining face is located, and the movement compensation space; Based on the fracture network, the coal body is controlled to be over-pressurized and to undergo crushing and expansion, thereby obtaining a target coal seam with pressure relief and increased permeability.

2. The method for increasing permeability of coal seam horizontal slits under periodic pressure according to claim 1 is characterized in that: The coal mine production background data includes: distribution characteristics of the ground stress field, coal seam density and thickness, gas content and pressure, and coal seam permeability.

3. The method for increasing permeability of coal seam horizontal slits under periodic pressure according to claim 1 is characterized in that: The cutting water pressure range of the horizontal slitting device is 80 to 100 MPa.

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