Three-soft coal seam gob-side entry driving cooperative roof cutting and pressure relief method

Abstract

The invention discloses a three-soft coal seam gob-side entry driving collaborative roof cutting and pressure relief method. According to the method, a two-stage and two-fluid cooperative injection method of curing first and fracturing second is adopted. The method comprises the following steps: (a) arranging a roof cutting drill hole array on a gob-side entry driving entity coal side roof; (b) a first stage (curing): injecting a low-viscosity and fast-setting first fluid into the drill hole at a first pressure lower than the fracture pressure of the rock mass, and performing permeation curing on the wall surface of the drill hole and near-field microfractures to form a trap of cured sleeve structure; (c) curing and waiting; (d) a second stage (fracturing): injecting a high-viscosity, thixotropic and aggregate-carrying second fluid into the solidification casing pipe by using the same packer at a second pressure higher than the fracture pressure of the combination of the solidification casing pipe structure and the roof rock mass, and carrying out directional kerf fracturing; and (e) forming a through fracturing joint-cutting surface. According to the method, the problems of drilling stability and crack control in the three-soft rock mass are successfully solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of coal mining, in particular to a method for simultaneous gob-side entry driving and top cutting pressure relief in three-soft coal seams. BACKGROUND

[0002] In the process of coal resource mining, the stability control of roadway surrounding rock is the prerequisite for safe and efficient production. In the process of gob-side entry driving (i.e. one side of the roadway is solid coal and the other side is a goaf), the roadway not only bears the self-weight stress and tectonic stress of the roof, but also bears the intense mining pressure transmitted from the roof of the adjacent goaf, resulting in serious deformation of the roadway surrounding rock and extremely difficult support.

[0003] In order to solve this problem, the "top cutting pressure relief" technology is often used in engineering, that is, a pre-formed artificial structural weakening surface (cutting seam) is formed in the roof of the roadway, which cuts off the stress transmission path of the roof and guides the roof of the goaf to collapse at the cutting seam, thereby greatly reducing the mining pressure borne by the roadway.

[0004] The existing top cutting technology mainly includes deep hole explosive blasting, carbon dioxide cracking, mechanical cutting and hydraulic fracturing. Among them, deep hole explosive blasting has safety hazards and is strictly limited in gas mines; carbon dioxide cracking is costly and the process is still under exploration; mechanical cutting has the disadvantages of shallow cutting depth, large equipment and high requirements for roadway conditions.

[0005] In contrast, the hydraulic fracturing top cutting technology is widely researched and applied because of its low cost, safety and pollution-free, and the ability to achieve large-scale roof weakening. For example, in some hard roof conditions, a dense drilling combined with hydraulic fracturing scheme is adopted. For example, in some test designs (such as Huoshaomian 131705 air lane), it is proposed to construct dense drilling along the upper solid coal side and simultaneously perform water pressure cracking top cutting pressure relief on the roof of the working face. The purpose of this "dense drilling" combined with "hydraulic fracturing" is to form a network of through-going cracks in the hard and difficult-to-collapse roof to eliminate the stress concentration of the advanced mining.

[0006] However, it is well known to those skilled in the art that the above-mentioned technical solution for "hard roof" cannot be directly applied to "three-soft coal seam" (soft roof, soft coal seam, soft floor) geological conditions. Three-soft rock mass usually exhibits low strength, large plasticity, poor self-stability, rich in clay minerals and high water content. If the above background technology is directly applied to three-soft coal seams, there will be two fundamental technical problems that cannot be overcome:

[0007] 1. Loss of Borehole Stability: Existing methods rely on the formation of stable boreholes in the roof to facilitate the insertion of fracturing tools such as packers. However, in soft rock formations, during drilling, stopping, or water injection, the borehole wall rapidly undergoes plastic deformation, diameter reduction, or even large-scale collapse and blockage due to stress release and fluid infiltration. This prevents packers from being inserted into their intended positions, or even if inserted, fails to form an effective seal, completely negating the prerequisites for hydraulic fracturing operations.

[0008] 2. Uncontrolled Fracturing Process: Current methods employ conventional hydraulic fracturing, i.e., injecting high-pressure water. In hard, brittle rock masses, high-pressure water easily forms a single, continuous main fracture. However, in soft, plastic rock masses, injecting high-pressure water produces drastically different effects. First, the water rapidly reacts with clay minerals in the rock mass, leading to rock structure disintegration and a sharp deterioration in strength. Second, the plasticity and high permeability of soft rock prevent the fracturing energy from being concentrated. The high-pressure water diffuses along micro-fractures and bedding within the rock mass, forming an irregular, complex, and diffuse network of fractures or infiltration zones, rather than the desired "cut surface" that cuts off stress transmission.

[0009] Therefore, there is an urgent need to invent a completely new technical method that can solve the fundamental technical problem that existing top-cutting technology cannot be applied to borehole instability and fracturing runaway in soft coal seams. Summary of the Invention

[0010] The main objective of this invention is to overcome the shortcomings of the prior art and provide a method for coordinated roof cutting and pressure relief in roadway excavation along the goaf of three soft coal seams, thereby solving the technical problems existing in the prior art.

[0011] To achieve the above objectives, the present invention provides the following technical solution:

[0012] A method for coordinated roof cutting and pressure relief during gob-side excavation in three soft coal seams includes the following steps:

[0013] a. Drilling array layout: On the solid coal side roof of the goaf excavation roadway, a roof-cutting drilling array is arranged along the roadway axis;

[0014] b. First stage grouting: A borehole packer is used to seal the top-cutting borehole, and a first fluid is injected into the top-cutting borehole at a first preset injection pressure lower than the fracture pressure of the top rock mass; the first fluid is a borehole wall solidification grout.

[0015] c. Curing Waiting: Stop injecting the first fluid and keep the position of the borehole packer unchanged, wait for the first preset curing time, so that the first fluid can be cured in the wall surface and near-wall micro-crack zone of the top-cut borehole to form a closed curing sleeve structure.

[0016] d. Second stage fracturing: keeping the borehole packer sealed, injecting a second fluid into the solidified casing structure in the top-removing borehole at a second preset injection pressure higher than the fracture pressure of the combination of the solidified casing structure and the roof rock mass;

[0017] e. Forming a top-removing surface: continuously injecting the second fluid until the second fluid forms a mutual through and support by the aggregate of the fracturing slit surface between the adjacent boreholes of the borehole array, achieving the pre-splitting cutting of the solid coal side roof.

[0018] Further, in the step (a), the parameters of the top-removing borehole array are: the borehole diameter is 80 mm to 110 mm, the borehole depth is 15 m to 25 m, the borehole spacing is 300 mm to 800 mm, and the angle between the axis of the top-removing borehole and the roadway roof plane is 90° to 110°.

[0019] Further, in the step (b), the first fluid is a double-liquid sodium silicate-based grouting material composed of A liquid and B liquid; the A liquid is a sodium silicate solution, and the B liquid is an acidic or salt reaction agent.

[0020] Further, the initial apparent viscosity of the first fluid after mixing is 10 mPa·s to 50 mPa·s; and the first preset solidification time in the step (c) is 10 minutes to 60 minutes.

[0021] Further, in the step (b), the first preset injection pressure satisfies: wherein is the original fracture pressure of the three-soft coal seam roof rock mass.

[0022] Further, in the step (d), the second fluid is a high-molecular polymer-bentonite-fly ash composite slurry; and the mass percentage composition is: water 50% to 70%, sodium-based bentonite 10% to 20%, fly ash 15% to 30%, and high-molecular polymer 0.5% to 2.0%.

[0023] Further, the apparent viscosity of the second fluid is 100 mPa·s to 300 mPa·s, and the yield stress is 10 Pa to 30 Pa.

[0024] Further, in the step (d), the second preset injection pressure satisfies: wherein is the fracture pressure of the combination of the solidified casing structure and the roof rock mass.

[0025] Furthermore, in step (e), the injection process of the second fluid is controlled by a crack propagation control model based on the Herschel-Bulkley rheological model; the control model is used to adjust the rheological parameters of the second fluid. Injection displacement and the second preset injection pressure monitored in real time. The expansion length of the fracturing cut surface is calculated in real time. ; and through the stated The injection of the second fluid is stopped when the borehole spacing reaches 50% to 60%, thereby controlling the expansion of the cut surface. This represents the minimum shear stress required for a fluid to begin flowing. Represents the consistency coefficient. This represents the flow index.

[0026] Furthermore, steps (b) and (d) employ the same borehole packer, which remains in a predetermined position within the top-cut borehole throughout the entire process of the injection completion of step (b), the curing wait of step (c), and the injection start of step (d).

[0027] Compared with existing technologies, this invention has the following advantages: Firstly, through low-pressure permeation grouting in the first stage, a high-strength "solidified casing" is formed in situ within the soft rock borehole, completely solving the problems of borehole collapse and packer inability to seal, making subsequent high-pressure fracturing and top-cutting operations possible. Secondly, through high-viscosity, thixotropic grout fracturing in the second stage, the hydration and filtration of the fluid are suppressed, forcing the fractures to propagate in a predetermined direction. The second fluid carries aggregate (fly ash), forming a permanent rigid support in the cut, ensuring the long-term effectiveness and reliability of the top-cutting pressure relief effect. The borehole reinforcement (first stage) and top-cutting fracturing (second stage) processes are completed collaboratively within the same borehole using the same packer, avoiding repeated construction. Compared to separate reinforcement and fracturing technologies, this invention is safer, more efficient, and has controllable economic costs. The concept of "dense drilling + fracturing and roof cutting" used in the background technology for "hard roof" has been successfully applied to "three soft coal seams", which is considered to be unsuitable for extreme geological conditions, and has significant engineering application value. Attached Figure Description

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained from these drawings without creative effort.

[0029] Figure 1 is a flow chart of the present application. DETAILED DESCRIPTION Embodiment One

[0031] This embodiment details the whole process of applying the method of the present application under the geological conditions of "three-soft coal seams".

[0032] (I) Project background and technical contradiction analysis

[0033] The application background of this embodiment is set in the 231508 working face of a mine, and the roadway is a gob-side entry driving. However, unlike the "hard roof" in the background technology, the geological conditions of this embodiment are set as typical "three-soft coal seams":

[0034] · Soft roof: The immediate roof of the roadway is 3.5 m thick mudstone with a Protodyakonov hardness coefficient f = 1.2, the rock mass is broken, joint development, rich in clay minerals such as illite and montmorillonite, which can swell and soften when exposed to water.

[0035] · Soft coal seam: The coal seam being mined is soft coal with a Protodyakonov hardness coefficient f = 0.8, with developed endogenous fissures.

[0036] · Soft floor: The floor of the roadway is 2.0 m thick mudstone with the same properties as the roof.

[0037] For this "three-soft" geology, if the "intensive drilling + hydraulic fracturing" scheme in the background technology is attempted, the following problems will immediately arise:

[0038] 1. Drilling cannot be formed: Using the drilling method in the background technology scheme, the drill bit drills into the soft roof described above, and the drill hole wall will collapse rapidly. This results in the inability of the fracturing packer to be lowered, and the entire scheme fails at the first step of construction.

[0039] 2. Fracturing cannot cut the seam: Even if the background technology scheme can (hypothetically) lower the packer and inject high-pressure water, the high-pressure water will cause the clay minerals in the roof mudstone to rapidly hydrate and swell, and the rock mass strength will be lost instantly. The pressure cannot be concentrated, and a diffuse infiltration zone is formed instead of the "through-cutting seam surface" intended for cutting the roof.

[0040] To solve the above technical contradictions, the embodiment no longer uses ordinary hydraulic fracturing in the background art, but uses the "two-stage, double-fluid" collaborative injection method (reference Figure 1 ) proposed by the present application. The core of this method is "first use the first fluid to change the soft rock into 'artificial hard rock' (cured casing), and then use the second fluid to conduct controllable fracturing on the 'artificial hard rock'".

[0041] (2) Step (a): Drilling array layout

[0042] According to the concept of dense drilling in the background art, a drilling array is arranged along the solid coal side (upper side) of the 231508 roadway, which is used to define the top cutting surface.

[0043] Drilling diameter selection . This diameter is not only a conventional choice, but also because the second stage of the present application requires the injection of high-viscosity second fluid. A larger aperture is beneficial to reducing the flow resistance of high-viscosity slurry along the hole, ensuring that the pressure of the fracturing pump can be transmitted to the bottom of the hole to the maximum extent, and improving the fracturing efficiency. The drilling spacing is selected to be 600 mm. The plasticity of soft rock is stronger than that of hard rock, and the energy dissipation is faster when the fracturing crack expands in soft rock. Therefore, a smaller spacing is selected to ensure that the second-stage fracturing cut can more easily achieve through in adjacent drillings. The drilling depth is selected to be 19 m. This depth design aims to penetrate the immediate roof that needs to be cut off, reach the key rock interface (such as the 14# coal seam floor), form a complete and sufficient height of the top cutting surface, and effectively isolate the stress transmission. The drilling angle is selected to be 100°. This angle (i.e. the angle between the drilling axis and the roof plane) makes the top cutting surface slightly inclined to the gob side. This is beneficial for the roof to collapse smoothly along this pre-crack (cut) to the gob under the action of gravity and mining pressure in the future working face mining, and will not impact the side of the roadway.

[0044] (3) Step (b): First-stage grouting (drilling wall solidification)

[0045] This step is the first core of the present application, and the purpose is to solve the technical contradiction of drilling instability. A double-liquid type rapid-setting sodium silicate grouting material is selected.

[0046] Table 1: First fluid (solidified grouting material) component table

[0047]

[0048] • Mix A and B liquids through a Y-shaped mixing pipe at a volume ratio of 1:1.

[0049] • Initial apparent viscosity . This viscosity (close to water) has very low flow resistance, which allows it to seep into (rather than flush) the micro-cracks and loose areas around the drilling wall under an applied pressure.

[0050] • Gel time (curing time) The two-liquid slurry reacts rapidly. The fast setting property of 15 minutes ensures that the slurry forms a gel in the borehole wall and near-field rock mass rapidly before it is diluted or washed away by groundwater, completing the initial solidification.

[0051] ZWY-100 / 50 type grouting pump (double channel) and a specially customized double-channel port packer are used. The packer is lowered into the borehole to the predetermined position (e.g., 18 m inside the borehole, 1 m fracturing at the bottom of the borehole). The grouting pump is started, pumping A liquid and B liquid, which are mixed in the static mixer at the front end of the packer and then injected into the formation.

[0052] The original fracture pressure of the three-soft roof is about 12 MPa through field testing. The injection pressure of this step is strictly controlled at 10 MPa, ensuring that This pressure is lower than the original fracture pressure of the three-soft roof, which is the key to this step. This ensures that the injection process of the first fluid is "permeation grouting", i.e., the fluid is filling and cementing the borehole wall and the existing micro-cracks around it, rather than creating new macro-cracks (fracturing).

[0053] Continuous injection, stop injection immediately when the grouting volume reaches 50 L (first preset volume) or the pressure first reaches the 10 MPa limit.

[0054] (IV) Step (c): Curing waiting

[0055] After stopping the injection of step (b), the high-pressure pipeline is depressurized, but the packer is not removed, keeping its sealing position at 18 m inside the borehole.

[0056] • Waiting time (first preset curing time).

[0057] • This time is greater than the gel time of the slurry . This is sufficient for the first fluid (sodium silicate slurry) to fully gel and achieve initial strength in the borehole wall and near-field micro-cracks.

[0058] ​​​​At this point, a dense, annular "solidified casing," approximately 5-10 cm thick, formed in situ on the borehole wall (section 18-19 m), composed of solidified grout and soft rock particles. This casing transformed the loose, fractured, and highly permeable soft rock borehole wall into a robust, intact, and low-permeability "artificial composite rock wall." It not only solved the borehole instability problem of the prior art solution but also provided a reliable sealing and pressure-bearing foundation for the second-stage high-pressure injection, and sealed micro-fractures to prevent grout loss during the second stage.

[0059] (v) Step (d): Second-stage fracturing (directional slot fracturing)

[0060] The purpose of this step is to resolve the technical challenge of uncontrolled cracks in soft rock, building upon the foundation of "solidified casing." A composite slurry of polymer-bentonite-fly ash is selected.

[0061] Table 2: Composition and Rheological Properties of the Second Fluid (Fracturing Slurry)

[0062]

[0063] The high viscosity of 200 mPa·s (200 times that of water) results in extremely high resistance to fluid flow within the fracture. According to fluid mechanics principles, fluids tend to propagate along the path of least resistance (i.e., the shortest path). The high viscosity inhibits fluid permeation (filtration loss) into natural microfractures, forcing the high-pressure fluid to concentrate its energy and form a single, straight, and relatively wide main fracture (i.e., a "cut"), rather than the diffuse network formed by hydraulic fracturing in the prior art.

[0064] The high yield stress of 22 Pa means that the fluid only begins to flow when the shear stress exceeds 22 Pa; once the flow stops (e.g., at the fracture tip or after pumping is stopped), it immediately "gels." This characteristic gives the slurry an extremely strong "suspending" ability, enabling it to uniformly carry 15.0% fly ash aggregate to all corners of the fracture without settling near the wellhead or bottom.

[0065] Use the same packer that was not removed in step (c). Switch the fluid lines and start the high-pressure fracturing pump (e.g., KFB-80 / 25) to... A constant flow rate is used to inject a second fluid into the (solidified) borehole.

[0066] Due to the first stage of solidification, the strength and integrity of the borehole and near-field rock mass (solidified casing) have been significantly improved. Testing has shown that the fracture pressure of this "solidified casing + rock mass" combination... The pressure rises to approximately 15 MPa (higher than the original 12 MPa). After pumping begins, the orifice pressure... Rapidly rises. When the pressure reaches 18 MPa, the rock mass is broken (i.e. "cracking"). After cracking, the pressure is maintained in the range of 16-17 MPa (i.e. crack extension pressure), ensuring . The injection pressure is higher than , so that fracturing can be achieved.

[0067] (6) Step (e): Forming a cutting top surface (fracture control)

[0068] This step is the key to realizing "cooperative cutting top" of the present application, i.e. how to ensure that a through cutting joint is formed between the drill holes with a spacing of 600 mm.

[0069] In the background art, hydraulic fracturing is usually controlled in a rough way by "timing" or "quantity". However, in soft rock, the high-viscosity, non-Newtonian fluid of the present application is used, and the crack extension length is not a simple linear relationship with the injection time and injection amount . If the injection amount is insufficient, the crack cannot be through (600 mm), and the cutting top fails; if the injection amount is too much, the slurry will be wasted and excessive fracturing will occur.

[0070] To achieve precise quantitative control, the present embodiment implants a crack extension control model based on the Herschel-Bulkley (H-B) rheological model in the ground control system of the fracturing truck.

[0071] The technical principle is as follows:

[0072] 1. The second fluid (Table 2) is an H-B fluid, and the relationship between its shear stress and shear rate is: , wherein: , , .

[0073] 2. For the H-B fluid flowing in a flat crack (cutting joint) with a width of and a height of at a discharge rate of , its pressure gradient is a complex nonlinear function, which can be expressed as: .

[0074] 3. At the same time, the crack width is determined by the elastic modulus of the rock mass and the net pressure (injection pressure minus ground stress), which can be expressed as:

[0075] 4. Coupling the fluid flow model (2) and the rock deformation model (3) and combining the fluid volume conservation (considering filtration), a numerical solution describing the relationship between the fracture propagation length and the measurable pumping parameters (pump rate , ) can be established.

[0076] In field implementation, the target is to drill the boreholes with a spacing . To ensure the penetration, the fracture half-length needs to reach . During the fracturing process, the operator sets a constant displacement . In the model calculation, the control system collects the injection pressure in real time and takes it as the input, which is substituted into the above H-B control model (the rheological parameters and the geological parameters in Table 2 are preset). The model output is the fracture half-length calculated in real time, which is displayed on the screen of the control system. The shutdown decision is made by the operator who continuously observes the growth of the fracture half-length . When the fracture half-length reaches 300 mm, the system alarms, and the operator immediately stops the pump. After the cutting seam is completed, the second fluid is “instantly gelled” due to the high yield stress, and the fly ash aggregate is permanently suspended and supported in the cutting seam.

[0077] Through the H-B control model, the present application first realizes the quantitative control of the fracture propagation length of high-viscosity, non-Newtonian fluid in soft rock, ensures that the dense boreholes can be accurately connected by the controllable fracture of the present application, and forms an effective pressure relief surface with uniform width (about 5-8 mm) and sufficient support of the aggregate.

[0078] (VII) Implementation effect

[0079] After the construction of the test section of the 231508 roadway is completed, the effect is monitored.

[0080] • In terms of borehole stability: 100 construction boreholes, the first-stage solidification success rate is 100%, and no accident of failure of packer to be lowered or sealing failure due to borehole collapse occurs. It is proved that the present application effectively overcomes the problem that the background technical solution cannot be constructed.

[0081] • In terms of cutting seam penetration: after fracturing, through the borehole peephole instrument and coring inspection, a penetration cutting seam with a width of about 5-8 mm filled with gray fly ash slurry is formed between adjacent boreholes. It is proved that the second stage and the H-B control model effectively realize the directional cutting seam in soft rock.

[0082] • In terms of pressure-relief effect: during the working face mining, the stress concentration coefficient of the roadway solid coal side is reduced by 55%, and the deformation of the roadway roof is reduced by 65%. It proves that the purpose of "top cutting and pressure relief" has been achieved through the technical transformation of the application, and has been successfully realized in the "three-soft coal seam".

[0083] Example 2

[0084] This example aims to illustrate the versatility of the application, and the geological conditions are the same as in Example 1.

[0085] (I) Step (a): Drilling array layout

[0086] The drilling spacing is 300 mm. The other parameters (diameter , depth 19 m, angle 100°) are the same as in Example 1.

[0087] (II) Step (b): First-stage grouting

[0088] The first fluid uses a high-molecular polymer grouting material. The formula is: water + 0.08% PHPA (partially hydrolyzed polyacrylamide) + 0.3% . This formula also has low viscosity (about 15 mPa·s) and certain plugging and solidification ability, which can be used to stabilize the hole wall, and is an alternative to the first fluid of the application. The pressure control is still controlled at 10 MPa (lower than ).

[0089] (III) Step (c): Solidification waiting

[0090] The waiting time .

[0091] (IV) Step (d): Second-stage fracturing

[0092] The second fluid uses a cement-bentonite (CB) slurry. The formula (mass ratio) is: water 70%, cement (P.O 42.5) 10%, sodium-based bentonite 20%. This formula also has high viscosity and high yield stress, and the addition of cement makes the aggregate (bentonite particles) itself have cementitious properties, which can achieve permanent support. This is an alternative to the second fluid.

[0093] (V) Step (e): Forming a top cutting surface

[0094] The control model still uses the H-B control model. The target is set as: due to the drilling spacing , the fracture half-length is adjusted to . The implementation effect also realizes drilling stability and cutting seam penetration.

[0095] Conclusions:

[0096] The results of Example 1 and Example 2 show that the method of the present application has wide material and parameter adaptability. It can be understood by those skilled in the art that as long as the core technical concept of the present application "first stage: low pressure injection of low viscosity quick-setting fluid to solidify the hole wall" and "second stage: high pressure injection of high viscosity thixotropic fluid to directional cut" is followed, and the corresponding rheological control model is supplemented, various equivalent replacements or obvious deformations of fluid components (such as sodium silicate or polymer; bentonite-fly ash or cement-bentonite) or process parameters (such as 300mm or 600mm spacing) should fall within the protection scope of the present application.

[0097] Finally, it should be pointed out that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for coordinated roof cutting and pressure relief during goaf excavation in three soft coal seams, characterized in that, Includes the following steps: a. Drilling array layout: On the solid coal side roof of the goaf excavation roadway, a roof-cutting drilling array is arranged along the roadway axis; b. First stage grouting: A borehole packer is used to seal the top-cutting borehole, and a first fluid is injected into the top-cutting borehole at a first preset injection pressure lower than the fracture pressure of the top rock mass; the first fluid is a borehole wall solidification grout. c. Curing Waiting: Stop injecting the first fluid and keep the position of the borehole packer unchanged, wait for the first preset curing time, so that the first fluid can be cured in the wall surface and near-wall micro-crack zone of the top-cut borehole to form a closed curing sleeve structure. d. Second stage fracturing: While maintaining the sealed state of the borehole packer, inject a second fluid into the solidified casing structure within the top-cutting borehole at a second preset injection pressure higher than the fracturing pressure of the solidified casing structure combined with the roof rock mass; the second fluid is a high-viscosity, thixotropic fracturing slurry carrying aggregate; e. Forming a cutting surface: Continuously inject the second fluid until the second fluid forms a fracturing cutting surface that is interconnected between adjacent boreholes in the borehole array and supported by the aggregate, thereby achieving pre-fracture cutting of the solid coal side roof.

2. The method according to claim 1, characterized in that, In step (a), the parameters of the top-cutting borehole array are: borehole diameter of 80 mm to 110 mm, borehole depth of 15 m to 25 m, borehole spacing of 300 mm to 800 mm, and the angle between the axis of the top-cutting borehole and the roadway roof plane of 90° to 110°.

3. The method according to claim 1, characterized in that, In step (b), the first fluid is a two-component sodium silicate-based grout, which is composed of liquid A and liquid B; liquid A is a sodium silicate solution, and liquid B is an acidic or salt-based reactant.

4. The method according to claim 3, characterized in that, The initial apparent viscosity of the first fluid after mixing is 10 mPa·s to 50 mPa·s; the first preset curing time in step (c) is 10 minutes to 60 minutes.

5. The method according to claim 1, characterized in that, In step (b), the first preset injection pressure satisfy: ,in The original fracturing pressure of the roof rock mass of the three soft coal seams.

6. The method according to claim 1, characterized in that, In step (d), the second fluid is a polymer-bentonite-fly ash composite slurry; Its mass percentage composition is: 50% to 70% water, 10% to 20% sodium bentonite, 15% to 30% fly ash, and 0.5% to 2.0% polymer.

7. The method according to claim 6, characterized in that, The apparent viscosity of the second fluid is 100 mPa·s to 300 mPa·s, and the yield stress is 10 Pa to 30 Pa.

8. The method according to claim 1, characterized in that, In step (d), the second preset injection pressure satisfy: ,in The fracture pressure is the combination of the solidified casing structure and the roof rock mass.

9. The method according to claim 1, characterized in that, In step (e), the injection process of the second fluid is controlled by a crack propagation control model based on the Herschel-Bulkley rheological model; the control model is used to adjust the rheological parameters of the second fluid. Injection displacement and the second preset injection pressure monitored in real time. The expansion length of the fracturing cut surface is calculated in real time. ; and through the stated The injection of the second fluid is stopped when the borehole spacing reaches 50% to 60%, thereby controlling the expansion of the cut surface. This represents the minimum shear stress required for a fluid to begin flowing. Represents the consistency coefficient. This represents the flow index.

10. The method according to claim 1, characterized in that, Steps (b) and (d) use the same borehole packer, which remains in a predetermined position within the top-cut borehole throughout the entire process of the injection end of step (b), the curing wait of step (c), and the injection start of step (d).

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