L-shaped radial branch well drilling and fracturing method for coal mine rock burst prevention and control

Abstract

The application relates to the technical field of coal mine safety mining and disaster prevention and control, and discloses an L-shaped well diameter radial branch well drilling and fracturing method for coal mine rock burst prevention and control, which comprises the following steps: screening an accumulated elastic energy target layer and establishing a branch well build-up extension space path; calculating a limit extension distance based on pipe column mechanics to define a drilling and excavation engineering boundary; establishing an annular minimum rock carrying displacement model to determine a branch well drilling construction displacement; optimizing fracturing fluid based on a branch hole diameter and matching a proppant particle size according to a bridge plugging criterion; combining filtration loss and pressure resistance to construct a construction displacement safe operation window; and converting injection volume by using an extension model to drive crack network volume interweaving coverage. The limit extension distance calculation model based on pipe column mechanics quantifies the sine buckling critical load and the maximum drillable length of a flexible drill rod in a hard roof, and solves the problems of pipe column buckling locking or power transmission failure caused by blind construction of the radial branch well in a limited space.

Description

Technical Field

[0001] This invention relates to the field of coal mine safety mining and disaster prevention and control technology, specifically to an L-shaped radial branch well drilling and fracturing method for coal mine rockburst prevention and control. Background Technology

[0002] With the increasing depth of coal mining, the geostress environment is becoming increasingly complex, and rockburst has become a major dynamic disaster restricting safe production in deep coal mines. Due to their thickness and high strength, hard roof strata easily accumulate a large amount of elastic energy under mining influence. The energy released by their sudden fracture often induces severe mine tremors or rockburst manifestations. Therefore, pre-fracture and weakening treatment of the hard roof, artificially disrupting its integrity and releasing the accumulated elastic energy to block stress transmission paths, is an important means of preventing and controlling rockburst.

[0003] To achieve regional modification of the top strata, L-shaped horizontal well fracturing technology is widely used on the surface. To further increase the control range of a single well, recent engineering practices have begun to explore radial branch well operations. This involves using flexible drilling tools to drill multiple branch wells laterally into the main wellbore, forming a tree-like three-dimensional perforation network structure. This technology aims to utilize branch wells to penetrate deep into the rock formations, improving the weakening effect of hard rock formations through multi-branch synergistic expansion. Compared to traditional single vertical wells or conventional horizontal wells, it theoretically has natural advantages in terms of spatial coverage and fracture initiation guidance.

[0004] However, existing flexible drill pipes lack sufficient bending stiffness when drilling through hard rock formations, making them prone to buckling and locking. Current technologies lack quantitative mechanical assessments of the ultimate extension distance, and blind drilling often leads to drill string jamming or breakage, making it difficult to reach the designed depth. The annular space of branch wellbores is narrow, creating a conflict between fluid-carrying rock and internal tubing pressure. Insufficient displacement leads to rock cuttings accumulation and rock cutting beds, while excessive displacement increases friction and internal tubing pressure, ultimately increasing construction safety risks. There is no design method for fracturing parameters in branch wells; current methods rely on experience from large-diameter wells, lacking strict matching of proppant particle size and microfracture width, easily causing sand bridge blockage in branch wells. Furthermore, the lack of volumetric coverage calculations for fracture propagation between adjacent branch wells easily leaves unrelieved gaps between two wells. Therefore, this invention provides a drilling and fracturing method for L-shaped radial branch wells for coal mine rockburst control, addressing the shortcomings of existing technologies. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for drilling and fracturing L-shaped radial branch wells for the prevention and control of rockburst in coal mines. This method solves the technical problems of existing technologies in the treatment of rockburst in hard roofs, such as limited extension of flexible tubing, difficulty in carrying rock in the annulus of branch wells, inability to quantify and determine fracturing parameters of branch wells, and discontinuous fracture network coverage.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a drilling and fracturing method for L-shaped radial branch wells used for rockburst control in coal mines, comprising the following steps:

[0007] S1. Based on rock mechanics parameters, target layers for accumulating elastic energy are selected, and radial branch well directional drilling and extension spatial paths adapted to the geometric boundaries of the target layers are established.

[0008] S2. Based on the spatial path and the mechanical characteristics of the flexible drill string, calculate the ultimate extension distance of the branch well, and determine the maximum drillable length under the corresponding wellbore constraints through sinusoidal buckling critical load analysis, so as to define the engineering boundary of physical drilling.

[0009] S3. For mechanical rotary drilling operations carried out within the engineering boundary, determine the drilling displacement of the branch well, and establish a minimum cuttings carrying displacement model based on the annular geometry formed by the drill pipe and the well wall to ensure cuttings transportation within the confined space.

[0010] S4. Based on the wellbore diameter constraints of the radial branch well, select the fracturing fluid system and proppant, screen low friction fluid to reduce flow loss along the pipe, and match the proppant particle size to adapt to the dynamic width of the fracture according to the bridging criterion.

[0011] S5. Combining the filtration characteristics of the selected fracturing fluid with the construction pressure control level, control the fracturing operation flow rate and pressure, and construct a safe operation window for the construction flow rate that includes the lower limit of filtration loss and the upper limit of pressure resistance, so as to drive the initiation and propagation of hydraulic fractures.

[0012] S6. Within the safe operation window, control the fracturing scale and fracture geometry, use the fracture propagation model to convert the geological weakening requirements into single-hole injection volume, and drive the fracture network generated by adjacent branch wells to interweave and cover within a preset interval.

[0013] Preferably, step S1 further includes:

[0014] A rock mechanics profile of the top plate is established using well logging data. The ratio of the dynamic elastic modulus of the rock to the uniaxial compressive strength is calculated as the key layer discrimination index. Layers that simultaneously meet the key layer discrimination index and rock thickness at a set threshold are identified as target layers.

[0015] The plan consists of a combined trajectory consisting of a circular arc build-up section and a straight line stabilization extension section. The effective total length of the strata crossing is calculated using the rated build-up radius and design angle of the flexible drill bit. The build-up radius is then double-verified based on the drill bit yield limit and the geological stratum boundary.

[0016] Preferably, in step S2, the step of calculating the ultimate extension distance of the branch well further includes:

[0017] The sinusoidal buckling critical load that the drill string can withstand to maintain a stable shape is estimated based on the bending stiffness, buoyancy, and annular space radius of the flexible drill pipe.

[0018] Calculate the minimum drilling pressure required for effective rock breaking by combining the diameter of the mechanical drill bit and the hardness of the target rock formation;

[0019] Based on the difference between the sinusoidal buckling critical load and the minimum drilling pressure, and combined with the comprehensive friction coefficient of the pipe wall, the maximum extension length for the flexible drill pipe to transmit effective drilling pressure is calculated.

[0020] Preferably, in step S3, the step of determining the drilling displacement of the branch well further includes:

[0021] Based on the balance relationship between the gravity, buoyancy and fluid resistance of rock cuttings, the slip velocity of rock cuttings in a static fluid is calculated.

[0022] The annular fluid backflow velocity is set to be greater than the product of the slip velocity and the safety factor. Combined with the annular cross-sectional area defined by the wellbore diameter and the drill pipe outer diameter, the minimum pumping flow rate to overcome cuttings settling is generated.

[0023] Preferably, in step S4, the step of selecting the fracturing fluid system and the proppant further includes:

[0024] A slickwater system with an apparent viscosity lower than a set value and a drag reduction rate higher than a set threshold at the shear rate was selected to meet the rheological requirements of small-diameter, long-distance tubular columns.

[0025] A particle size matching model is established, and the smaller value between the wellbore diameter and the dynamic width of the fracture during the fracturing process is selected as the reference size. The average particle size of the proppant is limited to be less than one-sixth of the reference size.

[0026] Preferably, step S5 further includes:

[0027] Based on the theory of stress concentration at the borehole edge in elastic mechanics, and taking into account the geostress state, rock tensile strength and formation pore pressure, the critical bottom-hole pressure required to overcome the stress concentration effect around the wellbore is calculated.

[0028] The bottom-hole critical pressure is used as the reference for the initiation pressure of tensile failure in rocks, and is used to guide the pressure setting of the pumping procedure.

[0029] Preferably, in step S5, the step of constructing a safe operating window for construction discharge capacity that includes a lower limit for filtration loss and an upper limit for pressure resistance further includes:

[0030] Based on the filtration control criteria, the minimum flow rate required to maintain fracture opening and offset the infiltration of fracturing fluid into the formation matrix is ​​calculated. The minimum flow rate required is positively correlated with the fracture height and the overall filtration coefficient.

[0031] Based on the equipment's pressure resistance limit, a balance relationship between surface pressure and bottom hole pressure is established, and the maximum flow rate allowed to pass under the rated working pressure of the flexible tubing is calculated. The maximum flow rate is limited by the friction loss along the pipe flow.

[0032] The closed interval between the lower limit of filtration loss and the upper limit of pressure resistance is used as the dynamic adjustment range of the fracturing operation displacement.

[0033] Preferably, in step S6, the step of controlling the fracturing scale and fracture geometry, and using a fracture propagation model to convert the geological weakening requirement into single-hole injection volume, further includes:

[0034] The PKN fracture propagation model was used to establish the physical relationship between pumping rate, fracture dynamic size and formation filtration characteristics, and to verify the dynamic extension length of a single-wing fracture.

[0035] A volume back-calculation model was established to determine the fracture control volume based on the preset horizontal spacing of the branch wells and the volume coverage criterion.

[0036] By combining the target rock layer thickness, target fracture porosity, and fracturing fluid efficiency, the total injection volume of fracturing fluid required to fill the control volume of the fracture in a single branch well is calculated.

[0037] Preferably, the target fracture porosity is the proportion of the volume of the fracture formed by hydraulic fracturing to the total volume of the rock mass, determined based on the weakening requirements for rockburst control.

[0038] The fracturing fluid efficiency is the ratio of the volume of fluid used for fracturing to the total pumped volume, used to compensate for fluid loss caused by formation filtration and microfracture opening.

[0039] Preferably, the volume coverage criterion is as follows:

[0040] The effective control radius of the fracture is set to a preset proportional coefficient that matches the horizontal spacing between adjacent branch wells. This ensures that the fracture-affected areas formed by adjacent branch wells overlap and cover the deep rock strata, thereby eliminating untreated blank zones and connecting the fracture network.

[0041] This invention provides a drilling and fracturing method for L-shaped radial branch wells used in coal mine rockburst control. It offers the following advantages:

[0042] 1. This invention quantifies the sinusoidal buckling critical load and maximum drillable length of flexible drill pipe in a hard top plate by constructing a calculation model of the ultimate extension distance based on the mechanics of the drill string. This effectively solves the problem of drill string buckling lock-up or power transmission failure caused by blind construction in confined spaces in radial branch wells. It strictly limits drilling operations to the engineering boundaries allowed by physical mechanics, avoids the risk of drill string damage in deep operations, and improves the success rate of drilling into specific target layers.

[0043] 2. This invention establishes a fluid cuttings carrying and material parameter optimization system adapted to the characteristics of branch wellbore. By using the minimum cuttings carrying capacity model and particle size bridging criterion, it solves the problems of cuttings deposition in extremely narrow annulus and end blockage in small-diameter fracturing. In conjunction with the constructed safe operating window for construction discharge, it can ensure that the pumping pressure is always lower than the pressure resistance limit of the flexible tubing while meeting the formation filtration requirements, thus ensuring the continuity and safety of fracturing operations under long-distance, small-diameter conditions.

[0044] 3. This invention proposes a fracture morphology control method based on the volume coverage criterion, which back-calculates the rock mass weakening requirements for rock burst control into single-hole injection volume, forcing the fracture network generated by adjacent branch wells to spatially interweave and overlap. This effectively eliminates the stress concentration gaps left by the discrete control range of single holes in traditional fracturing operations, achieving continuous fracturing and large-scale pressure relief of the hard roof, and blocking the energy accumulation path of rock bursts. Attached Figure Description

[0045] Figure 1 This is a flowchart of the method steps of the present invention;

[0046] Figure 2 This is a flowchart illustrating the stress model and ultimate extension distance calculation of the flexible drill pipe according to the present invention. Detailed Implementation

[0047] 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.

[0048] 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 drilling and fracturing method for L-shaped radial branch wells used for rockburst prevention and control in coal mines, comprising the following steps:

[0049] S1. Construct a multi-level target area and directional drilling trajectory model; establish a rock mechanics profile based on well logging data; use the key layer discrimination index to screen the target layer; construct a geometric model of radial branch well directional drilling and extension trajectory and verify the tool feasibility.

[0050] S2, calculate the ultimate extension distance of the branch well based on tubing mechanics; establish an extension model based on sinusoidal buckling critical load, and calculate the ultimate extension distance of the branch well in combination with the required minimum drilling pressure;

[0051] S3, determine the drilling displacement of the branch well; establish a calculation model for the minimum cuttings removal rate to ensure that the annular return velocity meets the cuttings slip velocity requirements;

[0052] S4, the preferred fracturing fluid system and proppant; a low-viscosity slickwater system is used, and the proppant particle size is matched based on the bridging principle;

[0053] S5, control the fracturing operation flow rate and pressure; calculate the fracturing initiation pressure after mechanical drilling, and determine the optimal operation flow rate range based on the filtration control criteria and the rated pressure resistance of the tubing string;

[0054] S6 controls the fracturing scale and fracture geometry; it back-calculates the injection volume per hole based on the fracture propagation model to ensure that multiple branch wells generate a fracture network covering the target area.

[0055] When implementing the L-shaped radial branch well drilling and fracturing method for coal mine rockburst control, constructing a multi-layer target area and directional drilling trajectory model is the basis for accurate cross-layer fracturing. For step S1, this includes quantitative identification of the target area, geometric modeling of the radial branch well directional drilling trajectory, and verification of tool feasibility.

[0056] Quantitative identification of the target area requires establishing a roof rock mechanics profile based on well logging data. Specific well logging data includes geophysical logging parameters such as gamma ray (GR), density (DEN), and sonic transit time (AC). The acquisition of well logging data and the conversion and processing of basic rock mechanics parameters are conventional techniques well-known to those skilled in the art and will not be elaborated upon here. This embodiment defines a key layer discrimination index based on the aforementioned basic data. This is used to quantify the hardness and elastic energy accumulation of rock strata, and to screen target strata requiring fracturing and weakening. Key stratum discrimination index. The calculation formula is as follows:

[0057] ;

[0058] In the formula, It represents the dynamic elastic modulus of rock, reflecting the rock's ability to resist deformation; It represents the uniaxial compressive strength of rock, reflecting the rock's failure strength.

[0059] After obtaining the rock strata mechanical profile, a judgment threshold is set. (For example, a value of 500) and a minimum thickness threshold (for example, 2.0m). When the key layer discrimination index of a certain rock stratum satisfies And the thickness of the rock layer At that time, the rock stratum was determined to be the target stratum. The system will automatically record the first... The center vertical depth of each target layer and layer thickness This serves as the geometric constraint boundary for subsequent trajectory design. Here, the vertical depth of the L-shaped horizontal main shaft is set to... .

[0060] For the selected target layers, a build-up trajectory model for the radial branch well needs to be established. This is to ensure that the branch well can be constructed at a predetermined design angle. (Typically set to 60° to 90°) To accurately penetrate the target layer, this embodiment constructs a combined trajectory model consisting of a build-up section and a stabilizing extension section. This trajectory assumes the branch well starts from the main wellbore and first passes through a section with a radius of... The arc is used to create an inclination, and then a section of length is... The straight segment continues at a constant angle until it reaches the target point.

[0061] In this geometric model, the vertical penetration distance is defined as... Based on geometric relationships, the effective total length of cross-layers in the branch well Determined by the following formula:

[0062] ;

[0063] In the formula, The rated build-up radius of the selected flexible drill bit; this parameter depends on the mechanical characteristics of the selected equipment. This is the radian value corresponding to the design angle; The final included angle between the axis of the designed branch well and the axis of the main wellbore; This indicates that in a circular arc motion per unit radius, the rotation is... The proportion of displacement in the vertical direction after the angle; and This formula represents the sine and cosine values ​​of the final included angle. It clarifies the total drilling length required for a branch well, given the known geological depth difference and tool build-up capability, providing a theoretical basis for subsequent drill string length selection.

[0064] Before commencing actual drilling operations, a passability and feasibility verification must be performed based on the parameters of the selected flexible drilling tool to prevent engineering accidents. The verification process establishes inequalities involving lower and upper limits. Lower limit The upper limit is mainly determined by the yield bending radius of the flexible drill pipe or high-pressure hose material itself, ensuring that the drill bit does not undergo plastic deformation or damage during directional drilling. This is determined by the spatial boundaries of the geological strata, preventing the build-up radius from being too large and causing branch wells to be built before reaching the designed angle. It had already broken through the top or bottom boundary of the target layer.

[0065] Upper limit of geological boundary constraints The following relationship must be satisfied:

[0066] ;

[0067] In the formula, This represents the vertical depth of the top boundary of the target rock stratum. Based on the above verification, if the designed ford radius... satisfy If the geological target selection is successful, the trajectory design is deemed feasible; otherwise, the tool selection needs to be adjusted or the layout of the main wellbore needs to be replanned. This step achieves complete closed-loop control from geological target selection to engineering trajectory design.

[0068] See attached document Figure 2 In step S2, during the physical drilling of the radial branch well, in order to ensure that the drill string can reach the predetermined target depth without causing engineering accidents, a calculation method based on tubing mechanics is used to predict and control the limit extension distance of the branch well.

[0069] This embodiment employs a flexible drill pipe driving a rotating mechanical drill bit for rock breaking. Unlike conventional rigid drill pipes, the extension capacity of flexible drill pipes in confined wellbores is strictly limited by the buckling critical load and the drilling pressure transmission efficiency. As the drilling depth increases, the cumulative friction between the tubing and the wellbore gradually increases. When the frictional force in the wellbore prevents the thrust applied to the drill pipe tail end from being effectively transmitted to the drill bit, i.e., when tubing helical buckling or "self-locking" occurs, drilling reaches its limit. To mitigate this risk, an extension model based on the sinusoidal buckling critical load is established, and the ultimate extension distance of the branch well is determined through calculation. Maximum extension distance The computational model is constructed as follows:

[0070] ;

[0071] In the formula, This indicates the maximum length that the flexible drill pipe can safely extend under current geological and engineering conditions; It represents the sinusoidal buckling critical load of the flexible drill pipe in a confined space, that is, the maximum axial pressure that the drill string can withstand while maintaining a stable shape; This represents the minimum drilling pressure required to break the target rock formation, i.e., the minimum axial pressure required for the drill bit to effectively break the rock. This indicates the buoyancy of the flexible drill pipe in the drilling fluid environment; This represents the overall friction coefficient between the flexible drill pipe and the borehole wall; This represents the average inclination angle of the branch wells. For horizontal branch wells, The value is 0.

[0072] In this extended model, the minimum drilling pressure required to fracture the target rock formation. It depends on the physical properties of the rock and the mechanical parameters of the drill bit. The calculation formula is:

[0073] ;

[0074] In the formula, The diameter of the mechanical drill bit; The rock hardness of the target rock layer; This is the drill bit cutting coefficient, which reflects the rock-breaking efficiency of a specific drill bit tooth profile. This formula translates the rock mechanical properties of the formation into specific requirements for drilling pressure.

[0075] For the key parameter in the model, the sinusoidal buckling critical load Considering the confined space characteristics of flexible drill pipe within radially branched wellbores, its estimation formula is as follows:

[0076] ;

[0077] In the formula, The bending stiffness of the flexible drill pipe reflects the ability of the drill string to resist bending deformation; This is the radius of the annular space between the drill pipe and the wellbore, which is the difference between the wellbore radius and the drill pipe radius. This formula shows that the greater the stiffness of the tubing string, the greater the buoyancy, or the smaller the annular space, the higher the critical buckling load, and the more favorable it is for force transmission.

[0078] Before actual construction, the theoretical limit extension distance was obtained using the above calculation model. If the designed branch well length is less than this limit, the current drill string assembly and process parameters are deemed to meet the construction requirements; if the designed length exceeds this limit, the drill string selection needs to be adjusted (e.g., selecting a drill pipe with higher rigidity) or the overall friction coefficient needs to be optimized through the lubrication system. This is to prevent the drill string from locking up during drilling, which would prevent the drill bit from obtaining sufficient drilling pressure. The measurement of the buoyancy of the flexible drill pipe, the determination of the coefficient of friction, and the acquisition of rock hardness are standard testing or experimental methods in this field and will not be elaborated upon here.

[0079] In step S3, during the radial branch well physical drilling process using a flexible drill pipe to drive a mechanical drill bit, precise control of the drilling fluid pumping rate is required to maintain wellbore cleanliness and ensure the continuity of drilling operations. In mechanical rotary drilling mode, the drilling fluid is used to cool the drill bit to prevent overheating failure and to transport broken rock cuttings from the bottom of the well to the wellhead.

[0080] In this embodiment, the radial branch wellbore diameter is relatively small (typically 3cm to 5cm), resulting in a narrow annular flow channel. Insufficient displacement can easily lead to cuttings settling and accumulating in the annulus, potentially causing stuck pipe. Therefore, the core criterion for determining the displacement during branch well drilling is to ensure that the annular fluid return velocity is greater than the gravity slip velocity of the cuttings particles. Based on fluid dynamics principles, a minimum cuttings-carrying displacement rate is established. The calculation model and its formula are as follows:

[0081] ;

[0082] In the formula, This indicates the minimum pump flow rate required to effectively carry rock cuttings out of the wellbore; Indicates the wellbore diameter of the radial branch well; This represents the outer diameter of the flexible drill pipe. The formula calculates the cross-sectional area of ​​the annular space between the wellbore wall and the outer wall of the drill pipe, combined with the required minimum annular return velocity. Determine the volumetric flow rate.

[0083] For the minimum annular return velocity Its settings must be sufficient to overcome the settling tendency of rock fragments in the fluid. The following inequalities must be satisfied:

[0084] ;

[0085] In the formula, For safety factors, taking into account the irregularity of the wellbore, the non-spherical shape of the cuttings, and the pulsating nature of the fluid flow, a value of 1.2 to 1.5 is typically used to provide additional cuttings carrying capacity. This is the slip velocity of rock cuttings in static drilling fluid (i.e., terminal settling velocity).

[0086] In order to accurately obtain Numerical calculations were performed based on a sedimentation dynamics model of solid particles in a fluid. This model comprehensively considers the balance between gravity, buoyancy, and fluid resistance, and its calculation formula is as follows:

[0087] ;

[0088] In the formula, It is the acceleration due to gravity; The average equivalent diameter of the rock cuttings can be obtained by conducting drillability tests and rock cuttings sieve analysis on the target formation rocks. The density of the rock fragments; The density of the drilling fluid; This is the fluid resistance coefficient, which is related to particle shape and Reynolds number. For irregular rock fragments, those skilled in the art can select it based on relevant empirical charts or experimental measurements. Through the above step-by-step calculations, the final determined construction discharge rate should be maintained at... The above figures are lower than the equipment's rated output capacity and the formation's fracturing pressure, thus achieving efficient and safe mechanical rotary drilling operations.

[0089] In step S4, before carrying out fracturing operations, a material matching optimization model based on the pipe flow rheological properties and particle bridging criteria needs to be established to reduce the frictional resistance of fluid along the long and narrow tubing string and prevent proppant particles from clogging, taking into account the structural characteristics of radial branch wells with small wellbore diameters (usually 30mm to 50mm).

[0090] For the fracturing fluid system, a low-viscosity slickwater system was selected. The rheological parameters of this system needed to meet specific engineering specifications: at a shear rate of 170 s⁻¹... -1 Under the test conditions, the apparent viscosity of the fluid The pressure needs to be controlled within a range of less than 5 mPa·s, and the drag reduction rate of the fluid system must be greater than 70%. By selecting fracturing fluids with the above-mentioned rheological properties, the frictional resistance generated when the fluid flows at high speed within a flexible tubing string can be significantly reduced, thereby achieving a higher discharge rate under limited pumping pressure and ensuring that energy can be effectively transferred to deeper formations. The specific chemical composition (such as polyacrylamide polymers) and formulation process of slickwater drag reducers are common fluid engineering techniques in this field and will not be elaborated here.

[0091] In proppant selection, to ensure the proppant can smoothly pass through the diameter-constrained branch wellbore and enter the hydraulic fracture, a particle size matching model based on the bridging criterion is established. This model comprehensively considers the dual constraints of rigid wellbore constraints and elastic fracture width, requiring the average particle size of the proppant to be... The following inequality constraints must be satisfied:

[0092] ;

[0093] In the formula, Indicates the average particle size of the selected proppant; Indicates the wellbore diameter of the radial branch well; This represents the dynamic width of the fracture during fracturing operations. The formula indicates that the selection of proppant particle size is limited not only by the wellbore's physical dimensions but also by the degree of fracture opening; the smaller of these two values ​​must be used as the calculation benchmark, and the upper limit of the particle size must not exceed one-sixth of this benchmark value. This strict proportional restriction is to prevent interference caused by multiple particles passing simultaneously through narrow sections, which could lead to a sand bridging effect and blockage of the wellbore or fracture opening. In actual operation, the dynamic width of the fracture... Predictions can be made using conventional fracture simulation software based on formation rock mechanical parameters and preset pumping procedures.

[0094] During the fracturing operation in step S5, the discharge rate is... It is a key dynamic parameter that determines the propagation of fracture geometry and the success or failure of the operation. Considering the high friction characteristics of small-diameter tubing in radial branch wells, a dual-constraint control model of fracturing operation displacement and pressure is established to determine the optimal operation displacement range.

[0095] During implementation, the initiation pressure of the target rock formation was first predicted based on the stress concentration theory at the borehole edge in elastic mechanics. Mechanical drilling disrupts the original stress balance of the rock mass, creating stress concentration around the wellbore. For the borehole wall rock to undergo tensile failure and form cracks, the bottom-hole fluid pressure must overcome the stress concentration effect and the tensile strength of the rock itself. The initiation pressure after mechanical drilling... The calculation model is as follows:

[0096] ;

[0097] In the formula, This represents the critical bottom-hole pressure required for rock to fracture. and These represent the minimum and maximum horizontal principal stresses of the target formation, respectively. These two parameters are usually obtained through in-situ stress testing of hydraulic fracturing or acoustic emission experiments. Indicates the tensile strength of the rock; Indicates formation pore pressure; It represents the effective stress coefficient, used to characterize the effect of pore fluid pressure on the effective stress of the rock skeleton.

[0098] After clarifying the crack initiation pressure benchmark, a safe operating window for the construction discharge rate is established, which is the lower limit determined by the filtration loss control criteria. The upper limit determined by the equipment's withstand pressure limit Jointly defined.

[0099] For the lower limit of construction displacement The determination of the flow rate is based on the filtration control criterion. Because fracturing fluid permeates and leaches into the formation matrix at the fracture wall, the pumping rate must be significantly greater than the filtration rate of the fracturing fluid to maintain fracture opening and extension. If the flow rate is too low, a large amount of fracturing fluid will leach into the formation, leading to insufficient pressure at the fracture tip and resulting in sand plugging or fracture failure. The lower limit of the flow rate must satisfy the following inequality constraint:

[0100] ;

[0101] In the formula, Minimum pump flow rate required to prevent excessive filtration loss; The designed crack height; The comprehensive filtration loss coefficient of the fracturing fluid reflects the combined effects of fracturing fluid viscosity, formation permeability, and wall-building properties on filtration loss. This refers to the pumping time of the pre-fluid.

[0102] Construction discharge limit The determination of the pressure resistance is mainly limited by the rated pressure resistance of the flexible tubing and surface pumping equipment. In long-distance, small-diameter branch wells, fluid flow friction is significant, and the friction... With displacement It is proportional to the square of the value. To prevent tubing rupture, a balance equation between surface pressure and bottom hole pressure needs to be established:

[0103] ;

[0104] In the formula, The surface pumping pressure must be strictly controlled within the rated working pressure of the flexible tubing and wellhead equipment. This is the bottom hole construction pressure (which is usually slightly greater than the closure pressure during the fracture extension stage). Frictional losses caused by fluid flowing through the tubing are the displacement losses. The function; This represents the hydrostatic pressure generated by the fluid column in the wellbore. Using this equation, the maximum allowable discharge rate under the premise of meeting the equipment's pressure resistance safety requirements can be calculated. Fracturing operations will be carried out in the interval Dynamic adjustments are made internally to achieve a balance between efficient crack propagation and construction safety.

[0105] In step S6, when performing fracturing operations on a hard roof, in order to ensure that the fracture network generated by adjacent branch wells can achieve effective spatial interweaving and coverage in the deep formation, thereby achieving the goal of weakening the roof as a whole and eliminating the risk of rockburst, it is necessary to establish a back-calculation model of injection volume based on the fracture propagation model and the volume coverage criterion.

[0106] To quantify the geometric propagation characteristics of fractures under specific pumping conditions, a PKN fracture propagation model was used for inverse calculations. This model assumes a fixed fracture height, a length that extends over time, and an elliptical fracture cross-section. Using this model, a physical relationship was established between the pumping rate, fracture geometry, and formation filtration characteristics, including the fracture propagation length. The calculation formula is as follows:

[0107] ;

[0108] In the formula, Indicates the dynamic extension length of a single-wing crack; Indicates the pumping volume during construction; This represents the average dynamic width within the crack, a parameter that can be estimated using the rock's elastic modulus and Poisson's ratio. Indicates the height of the crack confined by the bedding interface; This represents the overall filtration loss coefficient. This formula is used to verify whether, under given displacement and filtration loss conditions, the crack has the ability to extend to the design control range.

[0109] Based on the confirmed fracture propagation capacity, and to achieve the volumetric fracturing effect of a fracture network—that is, to ensure that the fracture-sweeped areas formed by adjacent branch wells overlap, thereby avoiding untreated blank zones—a quantitative design model for the total amount of fracturing fluid injected per well was established. This quantitative design model is based on a preset control volume and fracture porosity to back-calculate the injection volume per well. The calculation formula is as follows:

[0110] ;

[0111] In the formula, This represents the total volume of fracturing fluid required to inject into a single radial branch well; This represents the horizontal spacing between the designed branch wells. The coefficient 0.6 is introduced into the formula to ensure that the effective control radius of the fracture reaches 0.6 times the spacing, thereby forming at least 20% overlap between adjacent wells and achieving connectivity of the fracture network. Indicates the thickness of the target rock layer; This represents the target fracture porosity, which is the proportion of fracture volume formed by hydraulic fracturing to the total volume of the rock mass. Based on the weakening requirements for rockburst control, this parameter is typically set to a value within [value range missing]. between; This represents the fracturing fluid efficiency, which is the ratio of the volume of fluid used for fracturing to the total pumped volume. Considering losses due to formation filtration and microfracture opening, this parameter typically ranges from 20% to 40%. This calculation model transforms geological weakening requirements into actionable engineering injection volume indicators, enabling precise design of fracturing scale.

[0112] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A drilling and fracturing method for L-shaped radial branch wells used in coal mine rockburst control, characterized in that, Includes the following steps: S1. Based on rock mechanics parameters, target layers for accumulating elastic energy are selected, and radial branch well directional drilling and extension spatial paths adapted to the geometric boundaries of the target layers are established. S2. Based on the spatial path and the mechanical characteristics of the flexible drill string, calculate the ultimate extension distance of the branch well, and determine the maximum drillable length under the corresponding wellbore constraints through sinusoidal buckling critical load analysis, so as to define the engineering boundary of physical drilling. S3. For mechanical rotary drilling operations carried out within the engineering boundary, determine the drilling displacement of the branch well, and establish a minimum cuttings carrying displacement model based on the annular geometry formed by the drill pipe and the well wall to ensure cuttings transportation within the confined space. S4. Based on the wellbore diameter constraints of the radial branch well, select the fracturing fluid system and proppant, screen low friction fluid to reduce flow loss along the pipe, and match the proppant particle size to adapt to the dynamic width of the fracture according to the bridging criterion. S5. Combining the filtration characteristics of the selected fracturing fluid with the construction pressure control level, control the fracturing operation flow rate and pressure, and construct a safe operation window for the construction flow rate that includes the lower limit of filtration loss and the upper limit of pressure resistance, so as to drive the initiation and propagation of hydraulic fractures. S6. Within the safe operation window, control the fracturing scale and fracture geometry, use the fracture propagation model to convert the geological weakening requirements into single-hole injection volume, and drive the fracture network generated by adjacent branch wells to interweave and cover within a preset interval.

2. The drilling and fracturing method for L-shaped radial branch wells used for rockburst control in coal mines according to claim 1, characterized in that, Step S1 further includes: A rock mechanics profile of the top plate is established using well logging data. The ratio of the dynamic elastic modulus of the rock to the uniaxial compressive strength is calculated as the key layer discrimination index. Layers that simultaneously meet the key layer discrimination index and rock thickness at a set threshold are identified as target layers. The plan consists of a combined trajectory consisting of a circular arc build-up section and a straight line stabilization extension section. The effective total length of the strata crossing is calculated using the rated build-up radius and design angle of the flexible drill bit. The build-up radius is then double-verified based on the drill bit yield limit and the geological stratum boundary.

3. The drilling and fracturing method for L-shaped radial branch wells used for rockburst control in coal mines according to claim 1, characterized in that, In step S2, the step of calculating the ultimate extension distance of the branch well further includes: The sinusoidal buckling critical load that the drill string can withstand to maintain a stable shape is estimated based on the bending stiffness, buoyancy, and annular space radius of the flexible drill pipe. Calculate the minimum drilling pressure required for effective rock breaking by combining the diameter of the mechanical drill bit and the hardness of the target rock formation; Based on the difference between the sinusoidal buckling critical load and the minimum drilling pressure, and combined with the comprehensive friction coefficient of the pipe wall, the maximum extension length for the flexible drill pipe to transmit effective drilling pressure is calculated.

4. The drilling and fracturing method for L-shaped radial branch wells used for rockburst control in coal mines according to claim 1, characterized in that, In step S3, the step of determining the drilling displacement of the branch well further includes: Based on the balance relationship between the gravity, buoyancy and fluid resistance of rock cuttings, the slip velocity of rock cuttings in a static fluid is calculated. The annular fluid backflow velocity is set to be greater than the product of the slip velocity and the safety factor. Combined with the annular cross-sectional area defined by the wellbore diameter and the drill pipe outer diameter, the minimum pumping flow rate to overcome cuttings settling is generated.

5. The drilling and fracturing method for L-shaped radial branch wells used for rockburst control in coal mines according to claim 1, characterized in that, In step S4, the step of selecting the fracturing fluid system and proppant further includes: A slickwater system with an apparent viscosity lower than a set value and a drag reduction rate higher than a set threshold at the shear rate was selected to meet the rheological requirements of small-diameter, long-distance tubular columns. A particle size matching model is established, and the smaller value between the wellbore diameter and the dynamic width of the fracture during the fracturing process is selected as the reference size. The average particle size of the proppant is limited to be less than one-sixth of the reference size.

6. The drilling and fracturing method for L-shaped radial branch wells used for rockburst control in coal mines according to claim 1, characterized in that, Step S5 further includes: Based on the theory of stress concentration at the borehole edge in elastic mechanics, and taking into account the geostress state, rock tensile strength and formation pore pressure, the critical bottom-hole pressure required to overcome the stress concentration effect around the wellbore is calculated. The bottom-hole critical pressure is used as the reference for the initiation pressure of tensile failure in rocks, and is used to guide the pressure setting of the pumping procedure.

7. The drilling and fracturing method for L-shaped radial branch wells used for rockburst control in coal mines according to claim 1, characterized in that, In step S5, the step of constructing a safe operating window for construction discharge capacity that includes a lower limit for filtration loss and an upper limit for pressure resistance further includes: Based on the filtration control criteria, the minimum flow rate required to maintain fracture opening and offset the infiltration of fracturing fluid into the formation matrix is ​​calculated. The minimum flow rate required is positively correlated with the fracture height and the overall filtration coefficient. Based on the equipment's pressure resistance limit, a balance relationship between surface pressure and bottom hole pressure is established, and the maximum flow rate allowed to pass under the rated working pressure of the flexible tubing is calculated. The maximum flow rate is limited by the friction loss along the pipe flow. The closed interval between the lower limit of filtration loss and the upper limit of pressure resistance is used as the dynamic adjustment range of the fracturing operation displacement.

8. The drilling and fracturing method for L-shaped radial branch wells used for rockburst control in coal mines according to claim 1, characterized in that, In step S6, the step of controlling the fracturing scale and fracture geometry, and using a fracture propagation model to convert the geological weakening requirements into single-hole injection volume, further includes: The PKN fracture propagation model was used to establish the physical relationship between pumping rate, fracture dynamic size and formation filtration characteristics, and to verify the dynamic extension length of a single-wing fracture. A volume back-calculation model was established to determine the fracture control volume based on the preset horizontal spacing of the branch wells and the volume coverage criterion. By combining the target rock layer thickness, target fracture porosity, and fracturing fluid efficiency, the total injection volume of fracturing fluid required to fill the control volume of the fracture in a single branch well is calculated.

9. The drilling and fracturing method for L-shaped radial branch wells used for rockburst control in coal mines according to claim 8, characterized in that, The target fracture porosity is the proportion of the volume of fractures formed by hydraulic fracturing to the total volume of the rock mass, determined based on the weakening requirements for rockburst control. The fracturing fluid efficiency is the ratio of the volume of fluid used for fracturing to the total pumped volume, used to compensate for fluid loss caused by formation filtration and microfracture opening.

10. The drilling and fracturing method for L-shaped radial branch wells used for rockburst control in coal mines according to claim 8, characterized in that, The volume coverage criterion is specifically as follows: The effective control radius of the fracture is set to a preset proportional coefficient that matches the horizontal spacing between adjacent branch wells. This ensures that the fracture-affected areas formed by adjacent branch wells overlap and cover the deep rock strata, thereby eliminating untreated blank zones and connecting the fracture network.

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