Ground high-pressure rotary jet pressure relief method for thick and hard roof of coal seam

Abstract

This invention relates to the field of coal mining technology and discloses a high-pressure rotary jetting method for depressurizing thick, hard coal seams with topsoil. The method includes: quantitatively screening key layers using stiffness weighting and strength dual-control indicators; establishing a constraint relationship between the jet cutting radius and the top thickness, and planning a multi-level, layered, and vertically staggered borehole layout; treating the top as a fixed-support beam structure and calculating the ultimate span to determine the axial spacing of the cutting grooves; based on the principle of energy consumption per unit volume of crushing, back-calculating the dynamic parameters for matching the target cutting depth; using the fluid reaction torque generated by the eccentric nozzle to drive the tool, and achieving adaptive and stable speed control through a viscous damping mechanism. This invention, through scientific quantitative design and downhole self-balancing control, achieves precise full-thickness coverage of extremely thick, hard topsoil, eliminates depressurization blind spots, ensures cutting depth and rock-breaking efficiency under deep hard rock conditions, and effectively cuts off the stress transmission path of the suspended roof in the goaf.

Description

Technical Field

[0001] This invention relates to the field of coal mining technology, specifically a high-pressure rotary jetting method for depressurizing thick, hard coal seams with roofs. Background Technology

[0002] This type of geological structure is characterized by its large thickness, high strength, and dense structure. During mining, it easily accumulates a large amount of elastic energy, making it difficult to break and collapse in a timely manner, and readily inducing dynamic phenomena such as large-area roof pressure. Existing technologies for addressing this problem can be broadly categorized into downhole pressure relief and surface pressure relief. Downhole pressure relief mainly includes large-diameter drilling, blasting pressure relief, and borehole fracturing pressure relief methods; surface pressure relief primarily employs roof hydraulic fracturing or high-pressure jet cutting technology. Among these methods, surface high-pressure rotary jetting technology, due to its ability to prefabricate regular weak surfaces and its relatively controllable operating range, is gradually becoming an important means of managing extremely thick and hard roofs.

[0003] However, in actual engineering projects involving surface jet decompression of extremely thick and hard roofs, existing process designs often lack quantitative criteria based on rock mechanical properties. Engineers rely heavily on experience to select working layers, making it difficult to accurately pinpoint the key layers controlling roof movement from complex roof configurations. Furthermore, limited by conventional drilling and completion techniques and the attenuation characteristics of jet energy, current construction methods often employ a single horizontal well profile. For extremely thick roofs with thicknesses far exceeding the effective cutting radius of the jet, single-layer operations cannot achieve full thickness coverage vertically, resulting in large unrelieved blind zones within the roof. This fails to effectively sever the stress transmission path of the suspended roof in the goaf, impacting the final remediation effect.

[0004] Furthermore, the high ground stress in deep strata and the high strength of hard rock layers place extremely high demands on the energy density of jet rock breaking. Existing rotary jet tools mostly employ a hydraulic backflush free-rotation mode, lacking an effective speed control mechanism. In hard rock cutting operations, if the tool rotation speed is too high, the residence time of the high-pressure jet on a unit area of ​​rock will be significantly shortened, resulting in insufficient energy accumulation to overcome the rock's compressive strength. Often, only shallow scratches are formed on the wellbore, failing to create an effective groove to the designed depth. Simultaneously, the setting of groove spacing and dynamic parameters often lacks theoretical support for hard rock breaking energy consumption, making it difficult to adapt to the actual needs of deep and complex geological environments. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a high-pressure rotary jet decompression method for thick, hard coal seams. This method solves the problems of existing decompression technologies, such as difficulty in accurately locating key disaster-causing layers under extremely thick and hard roof conditions, the inability of single horizontal boreholes to achieve effective full-thickness coverage leading to decompression blind zones, and the lack of theoretical basis for setting cutting spacing and dynamic parameters, resulting in low rock-breaking efficiency.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a high-pressure rotary jet depressurization method for thick, hard coal seam roofs, comprising the following steps:

[0007] S1, quantitatively identify key layers of thick and hard roof: based on the logging data of the target block and the results of rock mechanics experiments, calculate the key layer discrimination coefficient of each rock layer in the roof, and combine the preset discrimination coefficient threshold and uniaxial compressive strength threshold to screen out the main key layer that controls the movement of the roof as the target pressure relief layer;

[0008] S2, Design a multi-level layered pressure relief drilling structure: Establish the geometric constraint relationship between the effective cutting radius of the jet and the total thickness of the top plate, calculate the number of horizontal hole layers required to cover the entire thickness of the top plate, and plan the staggered arrangement of each horizontal segment in the vertical direction and the layer spacing accordingly.

[0009] S3, Determine the axial slot spacing based on the fixed beam theory: Treat the target top rock layer as a fixed beam structure at both ends, and use the tensile strength of the rock, the thickness of a single layer and the uniformly distributed load parameters of the overlying rock layer to calculate the ultimate span that causes the rock beam to fail tensilely, and use this ultimate span as the upper limit of the axial spacing of the jet slot.

[0010] S4, Determine the dynamic parameters of high-pressure rotary jet: Set the working pressure of the high-pressure pump group and the jet velocity at the nozzle outlet to meet the rock breaking strength requirements. At the same time, based on the principle of energy accumulation in rock breaking per unit volume, combine the abrasive flow rate and the maximum allowable rotational angular velocity of the reverse thrust jet tool with the design depth.

[0011] S5, using tool structure to achieve self-rotation cutting: a jetting tool with an eccentric nozzle is lowered into the horizontal borehole. The tool is driven to rotate by the tangential reaction torque generated when the high-pressure fluid is ejected. The rotation speed is controlled by adjusting the fluid parameters or the damping structure, forming a disc-shaped groove around the well wall.

[0012] Furthermore, the key layer discrimination coefficient in step S1 is mainly used to reflect the ability of a single rock layer to resist bending deformation and its relative weight in the composite rock strata. The calculation logic of this coefficient is as follows: the bending stiffness contribution value of the rock layer is calculated by ratio to the overall cumulative bending stiffness of the top plate. The bending stiffness contribution value is determined by the product of the elastic modulus of the rock layer and the cube of its single-layer thickness; the overall cumulative bending stiffness of the top plate is determined by the sum of the products of the elastic modulus of each layer of rock in the top plate and the cube of its corresponding single-layer thickness. When screening target decompression layers, if the key layer discrimination coefficient of a certain rock stratum is greater than the preset key layer discrimination coefficient threshold, and the uniaxial compressive strength of the rock stratum is also greater than the preset uniaxial compressive strength threshold, then the rock stratum is determined to be a primary key layer.

[0013] Regarding the calculation of the number of horizontal hole layers in step S2, specifically, the total thickness of the target decompression layer and the effective cutting radius of a single high-pressure jet are obtained. The total thickness is divided by the product of twice the effective cutting radius and the effective jet coefficient, and the calculation result is rounded up to obtain the required number of horizontal hole layers. If the planned number of horizontal hole layers is greater than 1, the vertical distance between the axes of adjacent horizontal hole layers is set in a uniform distribution manner. For the staggered arrangement of each horizontal segment in the vertical direction, by setting a relative displacement between odd-numbered and even-numbered drilling layers in the horizontal direction, the vertical projection of the axis of the upper layer drilling layer on the horizontal plane is located in the horizontal interval area between two adjacent drilling layers in the lower layer, forming a spatial distribution structure of interlayer misalignment.

[0014] The calculation method for the ultimate span in step S3 is as follows: First, calculate twice the product of the tensile strength of the rock and the square of the thickness of a single layer of the target rock stratum, and use this as the numerator; second, calculate the product of the unit weight of the rock stratum and the burial depth, and use this as the uniformly distributed load of the overlying rock stratum, and use this as the denominator; finally, divide the numerator by the denominator to obtain the ratio, and perform a square root operation on the ratio to obtain the ultimate span that causes the rock beam to fail under tensile stress.

[0015] To determine the dynamic parameters of the high-pressure rotary jet in step S4, the working pressure of the high-pressure pump set must first be set to be greater than 2.5 times the uniaxial compressive strength of the target rock stratum. Based on Bernoulli's fluid equation, the nozzle outlet jet velocity is calculated using the working pressure of the high-pressure pump set, the density of the abrasive-containing fluid medium, and the nozzle velocity coefficient. The maximum permissible rotational angular velocity of the jetting tool is constrained by the following relationships: the maximum rotational angular velocity is directly proportional to the rock drillability coefficient, the 1.5 power of the working pressure of the high-pressure pump set, and the square of the nozzle outlet diameter; simultaneously, the maximum rotational angular velocity is inversely proportional to the designed target cutting depth, the uniaxial compressive strength of the rock, and the abrasive mass flow rate in the mixed fluid.

[0016] During the execution of step S5, the jetting tool used has an eccentric nozzle. The axis of this eccentric nozzle does not pass through the rotational center axis of the tool, but forms a preset deflection angle with the radial plane passing through the center of the nozzle outlet. The resulting tangential reaction torque is determined by the fluid medium density, fluid volumetric flow rate, nozzle outlet jet velocity, lever arm radius, and the sine of the deflection angle. Simultaneously, the tool controls its rotational speed through an internal damping structure. A viscous shear damping cavity is set between the rotating and stationary parts, utilizing the damping medium to generate a viscous damping torque positively correlated with the rotational angular velocity. The total resistance torque of the system is defined as the sum of the inherent mechanical friction torque and the viscous damping torque. When the tangential reaction torque generated by hydraulics equals the total resistance torque, the system reaches a dynamic torque equilibrium state, at which point the tool maintains a stable operating speed for cutting.

[0017] This invention provides a high-pressure rotary jetting method for depressurizing thick, hard coal seams. It offers the following advantages:

[0018] 1. This invention establishes a critical layer discrimination model based on stiffness weights and a fixed beam ultimate span calculation model, achieving precise positioning of the control layer of the roof and quantitative design of the slotting parameters. This method can eliminate non-critical layers with little impact on mine pressure manifestation and concentrate jet energy on the main critical layer with strong bearing capacity. Simultaneously, it determines a reasonable slotting spacing based on the tensile strength of the rock, avoiding engineering waste caused by blind construction and significantly improving the efficiency of weakening treatment for hard rock roofs.

[0019] 2. This invention solves the technical problem that a single horizontal borehole cannot cover the entire thickness of a massive roof by constructing a multi-level, layered, and vertically staggered borehole spatial layout system. This arrangement scientifically plans the borehole positions based on the effective cutting radius of the jet, utilizing interlayer staggered distribution to promote the expansion and connection of fractures in three-dimensional space, eliminating pressure relief blind zones, and thus forming a continuous and complete structural weak surface within the extra-thick and hard roof, effectively cutting off the stress transmission path of the suspended roof in the goaf.

[0020] 3. This invention establishes a dynamic parameter matching criterion based on unit volume crushing energy consumption and employs a tool control mechanism combining hydraulic self-drive and damping stabilization to achieve precise control of the deep hard rock cutting process. This mechanism utilizes fluid reaction force for drive and viscous damping to automatically balance the rotational speed, preventing insufficient cutting depth due to excessive tool speed. This ensures that the high-pressure jet has sufficient energy accumulation density on the hard rock surface, thereby forming a disc-shaped groove that meets the design depth requirements. Attached Figure Description

[0021] Figure 1 This is a flowchart of the method of the present invention;

[0022] Figure 2 This is a schematic diagram of the quantitative identification process for key strata in the top plate according to the present invention;

[0023] Figure 3 This is a flowchart illustrating the design process of the multi-level layered pressure relief drilling structure of the present invention.

[0024] Figure 4 This is a schematic diagram of the process for determining the high-pressure rotary jet dynamics parameters of the present invention;

[0025] Figure 5 This is a flowchart illustrating the principle implementation of the self-rotating sandblasting orifice tool of the present invention;

[0026] Figure 6 This is a schematic diagram of the hydraulic self-driven rotary cutting torque balance mechanism of the present invention. Detailed Implementation

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] See attached document Figure 1 , Figure 1 This is a flowchart of a method according to an embodiment of the present invention. The present invention provides a method for high-pressure rotary jetting depressurization of thick, hard coal seams, comprising the following steps:

[0029] S1, quantitatively identify key layers of thick and hard roof: based on the logging data of the target block and the results of rock mechanics experiments, calculate the key layer discrimination coefficient of each rock layer in the roof, and combine the preset discrimination coefficient threshold and uniaxial compressive strength threshold to screen out the main key layer that controls the movement of the roof as the target pressure relief layer;

[0030] S2, Design a multi-level layered pressure relief drilling structure: Establish the geometric constraint relationship between the effective cutting radius of the jet and the total thickness of the top plate, calculate the number of horizontal hole layers required to cover the entire thickness of the top plate, and plan the staggered arrangement of each horizontal segment in the vertical direction and the layer spacing accordingly.

[0031] S3, Determine the axial slot spacing based on the fixed beam theory: Treat the target top rock layer as a fixed beam structure at both ends, and use the tensile strength of the rock, the thickness of a single layer and the uniformly distributed load parameters of the overlying rock layer to calculate the ultimate span that causes the rock beam to fail tensilely, and use this ultimate span as the upper limit of the axial spacing of the jet slot.

[0032] S4, Determine the dynamic parameters of high-pressure rotary jet: Set the working pressure of the high-pressure pump group and the jet velocity at the nozzle outlet to meet the rock breaking strength requirements. At the same time, based on the principle of energy accumulation in rock breaking per unit volume, combine the abrasive flow rate and the maximum allowable rotational angular velocity of the reverse thrust jet tool with the design depth.

[0033] S5, using tool structure to achieve self-rotation cutting: a jetting tool with an eccentric nozzle is lowered into the horizontal borehole. The tool is driven to rotate by the tangential reaction torque generated when the high-pressure fluid is ejected. The rotation speed is controlled by adjusting the fluid parameters or the damping structure, forming a disc-shaped groove around the well wall.

[0034] The technical principles and execution process of the above steps will be explained in detail below with reference to specific embodiments and accompanying drawings.

[0035] See attached document Figure 2 , Figure 2This is a schematic diagram of the quantitative identification process for key strata in the roof according to an embodiment of the present invention. During implementation, considering the geological characteristics of the interplay of soft and hard rock strata and the varying strata within the composite roof structure, a quantitative calculation model that incorporates a rock mechanics stratigraphic profile and introduces stiffness weights is established to identify the main key strata controlling rock movement. This process specifically includes the following sub-steps:

[0036] S101, Establish a rock-mechanical stratigraphic profile of the roof. Using the coal seam roof as the reference surface, select strata within the fracture zone height range upwards as the research object. Based on borehole columnar sections in the geological exploration report or field logging data, identify the contact surfaces or abrupt changes in physical parameters between different lithologies, using these as natural boundary points. Discretize the continuous roof strata vertically, dividing them into... Each layer of rock is a separate stratum. ( This involves obtaining its geometric and physical / mechanical parameters. The main geometric parameter is the single-layer thickness. The main physical and mechanical parameters include the elastic modulus. and uniaxial compressive strength The above parameters can be obtained by coring downhole samples and then testing them in a laboratory using a rock mechanics testing machine, or by inversion calculations based on geophysical logging data such as sonic logging and density logging. Conventional core sampling and testing methods and logging curve interpretation methods are well-known techniques to those skilled in the art and will not be elaborated upon here.

[0037] S102, Construct a key layer discrimination coefficient calculation model. To quantify the control role of each layer in the overall roof movement, a key layer discrimination coefficient is introduced. The key layer discrimination coefficient is based on the stiffness theory in rock material mechanics, reflecting the ability of a single rock layer to resist bending deformation and its relative weight in a composite rock stratum. For the first... Key strata discrimination coefficient of rock layers The calculation formula is as follows:

[0038] ;

[0039] In the formula, For the first Key stratum discrimination coefficient of layered rock;

[0040] For the first The elastic modulus of the layered rock;

[0041] For the first The thickness of a single layer of rock;

[0042] This represents the total number of layers in the top strata.

[0043] and The first in the summation calculation The elastic modulus and single-layer thickness of the rock layer;

[0044] Numerator of Formula Characterized the first The contribution of the flexural stiffness of the layered rock, denominator term It characterizes the cumulative bending stiffness of the entire roof. Calculations using this formula can eliminate thick but soft rock layers, or strong but extremely thin rock layers, accurately identifying the layers that contribute the most to stiffness.

[0045] S103, perform dual threshold screening. Based on the calculated... Value and rock strength The target decompression layer is determined using dual control indicators. The threshold for the key layer discrimination coefficient is set at [value missing]. The uniaxial compressive strength threshold is set as In this embodiment, considering the geological conditions of the thick, hard sandstone roof, the following is selected: Select =60MPa. For all Each layer is logically judged one by one: if and only if the first layer... Layered rock simultaneously satisfy as well as When both conditions are met, the layer is determined to be the key layer controlling the movement of the top plate. This key layer is the target layer for high-pressure rotary jet cutting in subsequent steps. For layers that only meet these conditions... A weak, thick layer that meets the strength condition but not the physical condition, or a layer that meets the physical condition but not the physical condition. Thin, hard rock layers that do not meet the requirements are not considered target layers and are not subject to special jet decompression treatment. This screening logic ensures that engineering energy is concentrated on hard rock layers with bearing capacity that play a decisive role in the manifestation of mining pressure in the stope.

[0046] See attached document Figure 3 , Figure 3 This is a flowchart illustrating the design of a multi-level, layered pressure-relief drilling structure according to an embodiment of the present invention. Addressing the engineering challenge of achieving effective pressure relief across the entire thickness of massive roof slabs, typically exceeding 20 meters in thickness, where a single horizontal borehole cannot provide adequate relief, this embodiment employs a layered, discretized spatial planning strategy to construct a drilling layout structure covering the entire thickness of the roof slab. The process specifically includes the following sub-steps:

[0047] S201, Construct a layered calculation model based on the effective range of the jet. Before implementing the borehole layout, establish a quantitative relationship between the number of borehole layers, the top plate thickness, and the jet capacity. Obtain the total thickness of the target decompression layer determined by the previous steps. Based on the power of the high-pressure pump set equipped on site and the uniaxial compressive strength of the target rock layer, the effective cutting radius of a single high-pressure jet under these geological conditions was determined through ground-based simulated penetration tests or downhole test cuts. The effective cutting radius refers to the maximum radial distance that the jet stream can form a continuous, penetrating groove within the rock mass. To ensure that the pressure relief range covers the entire roof in the vertical profile and prevents pressure relief blind zones, the required number of horizontal borehole layers is determined. Determined by the following formula:

[0048] ;

[0049] In the formula, The required number of horizontal hole layers, which is a positive integer.

[0050] The total thickness of the target thick hard top plate;

[0051] The effective cutting radius of a single high-pressure jet;

[0052] The effective coefficient of the jet is used to correct for the effects of rock heterogeneity and jet energy attenuation at the far end. In this embodiment, the value is set to be between 0.8 and 0.9.

[0053] symbol This indicates a floor function, meaning that if the result is not an integer, the smallest integer greater than that result is taken. This calculation model ensures that the designed number of layers can completely cover the thickness of the roof slab.

[0054] S202, Plan the vertical spacing between floors and the aperture parameters. (This is in addition to determining the number of floors.) Then, the specific vertical locations of each borehole were determined. For In this case, the boreholes are arranged at the centerline of the top plate thickness; for In this case, the vertical spacing between adjacent horizontal pore layers is set using a uniform distribution method. The formula for calculating the vertical spacing is:

[0055] ;

[0056] In the formula, This represents the vertical distance between the axes of two adjacent horizontal holes. Based on this distance, the layers from the bottom interface of the top plate upwards are sequentially determined as layer 1 to layer 2. The opening height of the horizontal borehole, i.e., the first layer layer( The vertical distance from the horizontal hole to the bottom boundary of the top plate is This uniform distribution strategy ensures that the control ranges of each jet layer are interconnected in the vertical direction, creating a uniform stress release space.

[0057] S203, implement a staggered spatial arrangement of multiple horizontal boreholes. To avoid excessive local rock fragmentation or stress concentration caused by the overlap of multiple horizontal boreholes in the same vertical plane, the boreholes in each horizontal segment are arranged in a staggered pattern on the vertical projection plane. It is set that there is a relative displacement in the horizontal direction between odd-numbered and even-numbered borehole layers, so that the vertical projection of the axis of the borehole in the upper layer lies within the horizontal interval area between two adjacent boreholes in the lower layer, forming a quincunx-like spatial distribution structure. In this structural setting, the horizontal relative displacement is set to half the horizontal distance between adjacent boreholes in the same layer. This staggered arrangement structure can maximize the spatial expansion efficiency of the weak surfaces formed by jet cutting within the rock mass, allowing the fracture network to fully develop in three-dimensional space, thereby achieving overall weakening of the thick roof structure. For the specific construction of the horizontal boreholes, including directional drilling trajectory control and borehole structure stabilization, those skilled in the art can use conventional coal mine directional drilling rigs and measurement-while-drilling systems, which are well-known technologies in the field and will not be elaborated further here.

[0058] See attached document Figure 4 , Figure 4 This is a schematic diagram of the high-pressure rotary jet dynamics parameter determination process according to an embodiment of the present invention. After determining the spatial layout of the borehole and the axial spacing of the cutting grooves, it is necessary to further accurately set the hydrodynamic parameters for the cutting operation to ensure that the energy transmission efficiency meets the requirements for hard rock breaking. The parameter determination process specifically includes the following sub-steps:

[0059] S301, Construct a calculation model for the initial kinetic energy of the jet. To ensure that the high-pressure jet possesses the stress intensity to break through the hard matrix when it contacts the rock surface, the operating pressure of the high-pressure pump unit is set. The rock strength constraint condition must be met, that is ,in The uniaxial compressive strength of the target rock stratum is used as the basis. Based on this pressure, the jet velocity at the nozzle exit is determined according to Bernoulli's fluid equation. This velocity is a key indicator for measuring the kinetic energy of the jet impact, and its calculation formula is as follows:

[0060] ;

[0061] In the formula, The nozzle exit jet velocity;

[0062] This refers to the working pressure of the high-pressure pump unit.

[0063] The density of the fluid medium containing abrasive particles;

[0064] The nozzle velocity coefficient characterizes the resistance loss of the fluid in the internal flow channel of the nozzle, and is taken as 0.95 in this embodiment. This step establishes the strength basis of the rock-breaking energy.

[0065] S302, the upper limit of rotational angular velocity is determined based on the principle of energy consumption per unit volume of crushing. Under the premise of constant jet pressure and flow velocity, the cutting depth of the jet on the rock mainly depends on the time the jet beam acts on a unit area of ​​the rock. The rotational angular velocity is established. Cut deep into the target The inverse constraint relationship model between them. For a uniaxial compressive strength of... To achieve the predetermined design cutting depth in the rock strata, the rotational angular velocity of the jetting tool must be controlled below a critical threshold to ensure sufficient energy accumulation density. This rotational angular velocity... The calculation constraint formula is as follows:

[0066] ;

[0067] In the formula, The maximum permissible rotational angular velocity of the jetting tool;

[0068] The pump pressure set for the preceding steps;

[0069] The nozzle outlet diameter;

[0070] The mass flow rate of the abrasive in the mixed fluid;

[0071] The target grooving depth is defined in the design.

[0072] It represents the uniaxial compressive strength of the rock.

[0073] The rock drillability coefficient is a comprehensive reflection of the rock's mineral composition and cementation state's resistance to water jet abrasion. It is determined through laboratory standard rock sample penetration tests.

[0074] S303, generate the dynamic control parameter set. The pump pressure calculated above... Abrasive flow rate and the upper limit of rotational angular velocity This is converted into control commands for drilling and grouting equipment. During on-site operations, the rotational speed of the rotary motor is adjusted via a frequency converter to maintain it at the calculated speed. This ensures that the groove depth formed in hard rock environments deep underground strictly meets the design specifications. The requirement is to prevent insufficient cutting depth due to excessive rotation speed, which would prevent the top plate cantilever structure from being cut off.

[0075] See attached document Figure 5 , Figure 5 This is a flowchart illustrating the principle of a self-rotating sandblasting perforation tool according to an embodiment of the present invention. After determining the dynamic control parameters, this embodiment employs a downhole self-driven actuator based on the principle of fluid reaction force to achieve high-pressure rotary cutting. This process does not rely on a downhole motor for power supply, but directly utilizes the energy of the high-pressure fluid, and specifically includes the following sub-steps:

[0076] The S401 features an eccentric jet tool structure. It employs a specially designed high-pressure hydraulic jet tool, primarily composed of a stationary connecting section, a rotating nozzle section, an internal flow channel, and a high-pressure sealed bearing assembly. The rear end of the stationary connecting section connects to the high-pressure drill pipe, while the front end is coupled to the rotating nozzle section via a bearing. The rotating nozzle section has a non-radial eccentric nozzle on its sidewall. The axis of the eccentric nozzle does not pass through the tool's rotational center axis, but instead forms a preset deflection angle with the radial plane passing through the nozzle outlet center. In this embodiment, to ensure sufficient starting torque while also considering radial impact depth, this deflection angle... The value range is set to 15 degrees to 30 degrees.

[0077] S402, Establish a hydraulic self-driving torque conversion model. When high-pressure abrasive-containing fluid is ejected at high speed from an eccentric nozzle, according to the law of conservation of momentum, the fluid generates a reaction force on the nozzle head. Due to the deflection angle of the nozzle axis, this reaction force produces a tangential component in a plane perpendicular to the rotation axis, thus forming a driving torque that drives the nozzle head to rotate around the bearing. The formula for calculating the driving torque is:

[0078] ;

[0079] In the formula, For hydraulic driving torque;

[0080] The density of the fluid medium;

[0081] This refers to the fluid volumetric flow rate;

[0082] The nozzle exit jet velocity;

[0083] The lever arm radius is the vertical distance from the center point of the nozzle outlet to the axis of rotation of the tool.

[0084] Let be the nozzle deflection angle. This formula shows that, through structural parameters... and Fixed design, in conjunction with hydraulic parameters and By adjusting the force, controllable rotational drive can be obtained.

[0085] S403, implement speed damping stabilization control. Since simple hydraulic drive may lead to excessively high no-load speed, thus preventing the calculated depth of cut from being achieved, a passive speed control mechanism needs to be integrated into the tool. A damping cavity or mechanical friction pair is installed at the fit gap between the rotating nozzle section and the stationary connection section. By filling the damping cavity with a high-viscosity damping fluid, utilizing the shear resistance of the fluid, or adjusting the preload of the mechanical friction ring, a resistance torque that increases with the speed is generated. When the driving torque... When the tool reaches dynamic equilibrium with the resistance torque and bearing friction torque, it will maintain the calculated optimal angular velocity. Stable rotation within the range. This mechanism ensures that the jet forms a uniform, continuous, and deep disc-shaped groove around the wellbore without the need for a complex downhole electronic control system.

[0086] See attached document Figure 6 , Figure 6 This is a schematic diagram of a hydraulic self-driven rotary cutting torque balancing mechanism according to an embodiment of the present invention. To ensure that the high-pressure jetting tool maintains a constant rotational speed during deep-hole operations, thereby cutting regularly shaped and uniformly deep disc-shaped fractures in the hard roof, this embodiment constructs a torque self-balancing control mechanism based on the coupling of fluid dynamics and viscous damping. This mechanism achieves automatic rotational speed stabilization through the dynamic interaction of internal fluid drive and passive damping braking. The process specifically includes the following sub-steps:

[0087] S501 generates a constant hydraulic driving torque. A high-pressure water jet containing abrasive is injected into the tool's internal flow channel, and the fluid is accelerated and ejected through an eccentric nozzle. Based on the principle of reaction force, the jet generates tangential thrust in the rotating nozzle section. Because the input high-pressure pump pressure and flow rate remain constant during operation, and the eccentric structural parameters of the tool are fixed, a constant driving torque is generated. The value of this driving torque is determined by the fluid density, volumetric flow rate, jet velocity, lever arm radius, and nozzle deflection angle. This constant torque serves as the system's input energy source, providing continuous angular acceleration to the rotating nozzle section.

[0088] S502, construct a velocity-dependent damping braking model. To prevent the rotating nozzle from accelerating indefinitely under a constant driving torque and exceeding the critical speed, a viscous shear damping cavity is set between the rotating and stationary parts of the tool. The cavity is filled with a damping medium (such as high-viscosity silicone oil) with a specific viscosity coefficient. When the rotating nozzle rotates, it drives the damping medium to flow within the shear gap, generating a viscous drag torque that is positively correlated with the rotational angular velocity. Simultaneously, considering the inherent mechanical friction of the bearings and sealing components, the total drag torque of the system is... Defined as the sum of mechanical friction torque and viscous damping torque, its calculation model is as follows:

[0089] ;

[0090] In the formula, This represents the total resistance torque during the rotation process;

[0091] The inherent mechanical friction torque of the system mainly comes from the contact friction between the high-pressure rotary seal and the supporting bearing, and can be regarded as a constant.

[0092] This is the viscous damping coefficient, which depends on the shear area of ​​the damping cavity, the width of the shear gap, and the dynamic viscosity of the damping medium.

[0093] Let be the real-time rotational angular velocity of the tool. This model shows that as the rotational speed increases, the resistance experienced by the system will increase linearly or non-linearly.

[0094] The S503 achieves dynamic torque balance and speed lock. During the initial tool startup, due to the low speed, the total resistance torque... Less than driving torque The rotating nozzle is in an accelerated state. With... With the continuous increase of viscous damping term The torque increases rapidly. When the total resistance torque increases to equal the driving torque, the system reaches a dynamic torque equilibrium state. The torque equilibrium equation at this point is:

[0095] ;

[0096] By solving this equilibrium equation, the stable operating speed of the system can be obtained. :

[0097] ;

[0098] In the formula, This ensures a stable rotational speed under torque balance. The mechanism utilizes a physical feedback loop to automatically lock the tool speed within the designed range. If external disturbances cause a sudden increase in speed, the resistance torque immediately increases and exceeds the driving torque, resulting in a deceleration effect, and vice versa, thus guaranteeing the stability of the cutting process.

[0099] S504, Adjust the damping parameters to match the target cutting depth. For rock formations of varying hardness, the optimal cutting speed is determined by altering the viscosity of the damping medium or adjusting the effective shear area of ​​the damping cavity to correct the viscous damping coefficient. Before carrying out the operation, based on the upper limit requirement of rotational angular velocity calculated in step S4, pre-mix the damping fluid formula or adjust the position of the damping adjustment ring inside the tool to ensure that the calculated upper limit is met. Strictly less than or equal to the maximum permissible speed required by the design. This configuration ensures that even when the high-pressure pump unit is operating at full load, the tool can perform powerful cutting at a controlled low speed, guaranteeing the formation of effective artificial pressure relief fractures in deep, high-stress hard rock environments.

[0100] In this embodiment, the aforementioned high-pressure rotary jetting decompression method for thick, hard coal seams is applied to coal mining operations characterized by typical hard, extremely thick composite roofs. This embodiment aims to demonstrate how to pre-weaken hard rock strata that are difficult to collapse naturally underground through surface drilling.

[0101] At the outset of operations, engineers first established a stratigraphic profile of the roof rock based on the mine's geological exploration report and well logging data. This process did not involve processing all rock strata; rather, it required precisely identifying the key strata controlling roof movement. By analyzing the geometric parameters of each stratum within the fracture zone height range, such as single-layer thickness, and physical and mechanical parameters, such as elastic modulus and uniaxial compressive strength, the contribution weight of each rock stratum to bending stiffness was calculated. The system automatically screened out rock strata with high stiffness contribution and high strength; specifically, it selected those rock strata whose key layer discrimination coefficient exceeded a preset threshold and whose uniaxial compressive strength also exceeded a preset high-strength standard as target decompression layers. This screening mechanism ensured that subsequent fracturing energy was concentrated on the thick, hard rock capable of supporting the goaf, rather than being wasted on soft rock strata that crumbled during mining.

[0102] For a given target thick and hard roof, considering that its thickness often exceeds the effective control range of a single jet, the embodiment employs a multi-level layered pressure-relief drilling structure design. Designers calculated the number of horizontal drilling layers required to cover the entire thickness based on the effective cutting radius of the jet measured from ground simulation experiments and the total thickness of the target roof. When the target layer is extremely thick and requires multiple layers of horizontal holes, the holes in each layer are not simply aligned vertically, but rather arranged in a staggered spatial topology. Specifically, the axis of the upper layer's hole is located in the middle of two adjacent holes in the lower layer on the horizontal projection plane, forming a distribution pattern similar to a quincunx. Simultaneously, a uniform vertical spacing is maintained between each layer. This staggered arrangement strategy effectively avoids excessive overlap of the jet cutting area in the vertical direction, preventing the risk of wellbore collapse due to stress concentration, while ensuring the uniform expansion of the fracture network in three-dimensional space, achieving overall cutting of the extremely thick roof.

[0103] After determining the spatial layout of the borehole, to ensure the roof collapses as expected after mining, it is necessary to determine the spacing of the cuts along the borehole axis. This step is based on the fixed beam theory of rock mechanics, treating the rock strata to be cut as a beam structure fixed at both ends. Engineers use the tensile strength parameters of the target rock strata and the uniformly distributed load data of the overlying rock strata to deduce the ultimate span at which the rock beam will fracture and collapse. In this embodiment, the axial spacing of the jet cuts is set to be less than or equal to this ultimate span. In this way, during high-pressure jet cutting operations, an artificial weak surface is created at predetermined intervals along the horizontal borehole axis, dividing the originally continuous and complete long-span rock beam into several short-span rock blocks. When the underground working face advances, these short-span rock blocks cannot withstand the overlying load and undergo tensile failure, thus achieving timely periodic collapse.

[0104] In the specific construction phase, to create a cut to the designed depth in deep hard rock, the dynamic parameters of the high-pressure rotary jetting were rigorously matched. The output pressure of the ground-based high-pressure pump unit was set significantly higher than the uniaxial compressive strength of the target rock layer to ensure that the jet had the stress conditions to break the matrix when it contacted the rock. Simultaneously, to ensure the jet had sufficient time to grind the rock to achieve a penetrating cut depth, the rotational speed of the jetting tool was strictly limited. Based on the principle of energy consumption per unit volume of rock breaking, rotational speed is negatively correlated with the target cut depth; therefore, a maximum allowable upper limit for rotational angular velocity was calculated and set. During operation, the control system ensured that the actual rotational speed of the tool remained below this upper limit, thereby avoiding the problem of insufficient cut depth and inability to cut the roof cantilever structure due to excessive rotational speed.

[0105] This high-pressure rotary cutting operation is achieved using a self-rotating blasting perforation tool that requires no downhole power. Utilizing fluid dynamics principles, the tool features a specific deflection angle between its nozzle axis and the tool's rotational axis. When high-pressure, abrasive-laden fluid is ejected at high speed from the eccentric nozzle, a tangential reaction torque is generated, driving the nozzle to rotate around the bearing. To prevent the rotational speed from running out of control under fluid drive and exceeding the calculated maximum permissible angular velocity, a viscous damping control mechanism is integrated within the tool. High-viscosity damping fluid is filled into the gap between the rotating and stationary components, or a mechanical friction pair is installed, utilizing fluid shear resistance or frictional resistance to generate a braking torque positively correlated with the rotational speed. When the hydraulic driving torque and the internal damping torque reach dynamic equilibrium, the tool automatically stabilizes at a preset low-speed rotation. This mechanism ensures that, even in deep well environments, regular-shaped, uniformly deep disc-shaped fractures can be cut into hard roof rock formations without complex electronic control, ultimately achieving the technical goal of disrupting roof integrity and mitigating the risk of rockburst.

Claims

1. A high-pressure rotary jetting method for depressurizing thick, hard coal seam roofs, characterized in that: Includes the following steps: S1. Quantitatively identify key strata of thick and hard roof: Based on the logging data of the target block and the results of rock mechanics experiments, establish a rock mechanics stratigraphic profile of the roof, calculate the key strata discrimination coefficient of each rock layer in the roof, and combine the preset key strata discrimination coefficient threshold and uniaxial compressive strength threshold to screen out the main key strata that control the movement of the roof as the target decompression strata. S2, Design a multi-level layered pressure relief drilling structure: Establish the geometric constraint relationship between the effective cutting radius of the jet and the total thickness of the top plate, calculate the number of horizontal hole layers required to cover the entire thickness of the top plate, and plan the staggered arrangement of each horizontal segment in the vertical direction and the vertical spacing between layers. S3, Determine the axial slot spacing based on the fixed beam theory: Treat the target decompression layer as a fixed beam structure at both ends, and use the tensile strength of the rock, the thickness of a single layer and the uniformly distributed load parameters of the overlying rock layer to calculate the ultimate span that causes the rock beam to fail tensilely, and use this ultimate span as the upper limit of the axial spacing of the jet slot. S4, Determine the dynamic parameters of high-pressure rotary jet: Set the working pressure of the high-pressure pump group and the jet velocity at the nozzle outlet to meet the rock breaking strength requirements. At the same time, based on the principle of energy accumulation in rock crushing per unit volume, combine the abrasive flow rate and the design target cutting depth to back-calculate the maximum allowable rotational angular velocity of the jet tool. S5, using tool structure to achieve self-rotation cutting: a jetting tool with an eccentric nozzle is lowered into the horizontal borehole. The tangential reaction torque generated when the high-pressure fluid is ejected drives the jetting tool to rotate. The rotation speed is controlled by adjusting the fluid parameters or the damping structure, forming a disc-shaped cutting groove around the well wall. In step S1, the key layer discrimination coefficient characterizes the ability of a single rock layer to resist bending deformation and its relative weight in the composite rock layer; The value of the key layer discrimination coefficient is equal to the ratio of the bending stiffness contribution value of the rock layer to the overall cumulative bending stiffness of the top plate; wherein, the bending stiffness contribution value is determined by the product of the elastic modulus of the rock layer and the cube of the thickness of a single layer of the rock layer; the overall cumulative bending stiffness of the top plate is determined by the sum of the products of the elastic modulus of each layer of rock in the top plate and the cube of the thickness of its corresponding single layer. In step S1, the logic for selecting the main key layer controlling the roof movement as the target decompression layer is as follows: A rock stratum is determined to be a primary critical stratum if and only if the critical stratum discrimination coefficient of a certain rock stratum is greater than a preset critical stratum discrimination coefficient threshold, and the uniaxial compressive strength of the rock stratum is greater than a preset uniaxial compressive strength threshold. In step S2, the method for calculating the number of horizontal hole layers is as follows: Obtain the total thickness of the target depressurization layer and the effective cutting radius of a single high-pressure jet; Divide the total thickness by the product of twice the effective cutting radius and the effective jet coefficient, and round up the result to obtain the required number of horizontal hole layers. When the number of horizontal hole layers is greater than 1, the distance between the axes of adjacent horizontal holes in the vertical direction is set in a uniform distribution manner. In step S2, the specific planning of the staggered arrangement is as follows: The odd-numbered drilling layers and the even-numbered drilling layers are set to have a relative displacement in the horizontal direction, so that the vertical projection of the axis of the upper layer drilling layer on the horizontal plane is located in the horizontal interval area between two adjacent drilling layers in the lower layer, forming a spatial distribution structure of interlayer misalignment. In step S4, the specific requirements for setting the working pressure of the high-pressure pump set and the jet velocity at the nozzle outlet include: The working pressure of the high-pressure pump set is set to be greater than 2.5 times the uniaxial compressive strength of the target rock stratum; Based on Bernoulli's fluid equation, the nozzle outlet jet velocity is calculated using the high-pressure pump set working pressure, the density of the abrasive-containing fluid medium, and the nozzle velocity coefficient. In step S4, the upper limit of the allowable rotational angular velocity of the jetting tool is determined based on the principle of energy consumption per unit volume of crushing, and the relationship between the rotational angular velocity ω and the target cutting depth D is established. target The inverse proportional constraint relationship model between them, the formula for calculating the rotational angular velocity ω is: in, The rock drillability coefficient For the high pressure pump unit working pressure The 1.5th power, Nozzle outlet diameter The square of, To design the target grooving depth, denoted as uniaxial compressive strength of rock, and u as mass flow rate of abrasive in the mixed fluid; In step S5, the torque balance mechanism for controlling the rotational speed through the damping structure is as follows: A viscous shear damping cavity is set between the rotating and stationary parts of the jetting tool, and a viscous damping torque that is positively correlated with the rotational angular velocity is generated by the damping medium. The total resistance torque of the system is defined as the sum of the inherent mechanical friction torque and the viscous damping torque; When the tangential reaction torque generated by hydraulics is equal to the total resistance torque, the system reaches a dynamic torque balance state, and the rotational angular velocity at this time is the stable operating speed.

2. The high-pressure rotary jetting method for depressurizing thick, hard coal seam roofs according to claim 1, characterized in that, In step S3, the method for calculating the limit span is as follows: Calculate twice the product of the rock's tensile strength and the square of the thickness of a single target rock layer, and use that as the numerator; Calculate the product of the unit weight of the rock stratum and the burial depth and use it as the uniformly distributed load of the overlying rock stratum, and use it as the denominator; Divide the numerator by the denominator to obtain a ratio, and then perform a square root operation on this ratio to obtain the ultimate span that causes tensile failure of the rock beam.

3. The high-pressure rotary jetting method for depressurizing thick, hard coal seam roofs according to claim 1, characterized in that, In step S5, the mechanism for generating the tangential reaction torque is as follows: The eccentric nozzle axis of the jetting tool does not pass through the rotation center axis of the tool, but forms a preset deflection angle with the radial plane passing through the center of the nozzle outlet; The value of the tangential reaction torque is determined by the fluid medium density, fluid volumetric flow rate, nozzle outlet jet velocity, lever arm radius, and the sine value of the deflection angle.

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