Multi-stage hydraulic fracture propagation control method

Abstract

The application provides a multistage hydraulic fracture propagation control method, comprising: obtaining the mechanical parameters of the rock mass in the tunneling area; constructing a crack mechanics model based on the mechanical parameters, calculating the critical crack initiation pressure and the optimal fracture azimuth angle, so as to determine the lowest fracturing fluid injection pressure and the corresponding drilling angle; based on the joint surface stress model and the crack propagation criterion, regulating the fracturing fluid injection pressure to guide the hydraulic fracture to propagate according to the preset propagation mode. The multistage hydraulic fracture propagation control method realizes the accurate regulation and control of multiple types of crack propagation modes, so as to achieve the controllable construction of the complex fracture network and overcome the blindness of the traditional experience design.

Description

Technical Field

[0001] This invention relates to the field of coal mine and underground engineering roadway excavation technology, and in particular to a method for controlling the propagation of multi-stage hydraulic fractures. Background Technology

[0002] In coal mine hard rock tunnel excavation, roadheaders face technical challenges such as high cutting resistance, severe tool wear, and low excavation efficiency due to the high rock strength and good integrity. While hydraulic fracturing technology, as a method for pre-weakening rock mass, is theoretically feasible, its practical application still faces the following technical bottlenecks:

[0003] (1) Single fracture propagation mode: Traditional hydraulic fracturing technology is limited by a simple water injection pressure control mechanism, mainly forming a single main fracture. The direction of fracture extension is significantly controlled by the geostress field, making it difficult to effectively activate the secondary fracture system and natural joint network. This single fracture morphology leads to a limited range of rock mass weakening and poor uniformity, which cannot effectively reduce the overall strength of the rock mass. The weakening effect is particularly unsatisfactory in hard rock strata with well-developed joints.

[0004] (2) The interaction mechanism between fractures and joints is unclear: Existing methods lack quantitative criteria for the propagation behavior of hydraulic fractures after they intersect with joint surfaces, and cannot accurately predict key behaviors such as penetration, bifurcation, and fracture arrest. Due to the lack of a mechanical criterion model based on stress intensity factor and joint surface stress state, fracturing design is blind, fracture propagation path is highly random, and the weakening effect on rock mass is unstable, often leading to ineffective injection of fracturing fluid or premature fracture arrest.

[0005] In summary, existing hydraulic fracturing technologies have significant shortcomings in terms of fracture propagation control and fracture-joint interaction mechanisms. There is an urgent need to develop a rock mass weakening method that can achieve multi-stage directional fracture propagation and controllable fracture network construction. Summary of the Invention

[0006] The purpose of this invention is to provide a multi-stage hydraulic fracture propagation control method to achieve precise control of multiple fracture propagation modes, thereby enabling the controllable construction of complex fracture networks and overcoming the blindness of traditional experience-based design.

[0007] To achieve the above objectives, the present invention provides a multi-stage hydraulic fracture propagation control method, comprising:

[0008] Obtain the mechanical parameters of the rock mass in the tunneling area;

[0009] Based on the aforementioned mechanical parameters, a fracturing initiation mechanical model is constructed to calculate the critical fracturing initiation pressure and the optimal fracture azimuth angle, thereby determining the minimum fracturing fluid injection pressure and the corresponding drilling angle.

[0010] Based on the joint surface stress model and fracture propagation criteria, the fracturing fluid injection pressure is adjusted to guide the hydraulic fractures to propagate according to a preset propagation pattern.

[0011] Optionally, the mechanical parameters include geostress field characteristics, which include the maximum principal stress, intermediate principal stress, and minimum principal stress; the fracture initiation mechanical model includes the principal stress distribution inside the borehole wall.

[0012] ,

[0013] Where θ is the azimuth angle of the stress distribution point in the polar coordinate system, σ1 is the maximum principal stress in the borehole, σ2 is the intermediate principal stress in the borehole, σ3 is the minimum principal stress in the borehole, and σ r σ is the principal stress in the radial direction inside the borehole. θ For the upward principal stress of the inner annulus of the borehole, σ z τ is the principal stress along the axial direction inside the borehole. rθ τ represents the radial-to-circumferential shear stress component inside the borehole. zθ τ represents the axial-to-circumferential shear stress component inside the borehole. rz This represents the radial shear stress component relative to the axial direction within the borehole.

[0014] Optionally, the mechanical parameters include tensile strength; the crack initiation mechanical model includes a calculation formula for the critical crack initiation pressure, which is constructed based on the maximum tensile stress failure criterion.

[0015] , where σ max σ is the critical crack initiation pressure. max The size is the same as σ3.

[0016] Let the azimuth angle θ corresponding to the critical initiation pressure be the optimal fracture azimuth angle θ0. The optimal fracture azimuth angle θ0 can be obtained by σ max Taking the partial derivative with respect to θ, we get:

[0017] ,

[0018] When initiating hydraulic fractures, factors to consider include the tensile strength of the rock mass. and pore pressure P p The borehole wall will fail under the following conditions: , where k b The elastic modulus of Biot porous materials;

[0019] The minimum fracturing fluid injection pressure is equal to the critical fracturing initiation pressure, and the borehole angle at this point is calculated based on the optimal fracture azimuth angle θ0.

[0020] Optionally, the joint surface stress model is:

[0021] ,

[0022] Where, σ n For the normal stress of the joint surface, τ n For the tangential stress of the joint surface, σ H For the maximum horizontal principal stress, σ h For the minimum horizontal principal stress, σ x σ represents the stress component in the x-direction at the crack tip. y τ is the stress component in the y-direction at the crack tip. xy Let x be the stress component at the crack tip in the x-direction relative to the y-direction, and ψ be the angle between the joint surface normal and the direction of maximum principal stress.

[0023] Optionally, the preset expansion modes include mode one, and the corresponding crack expansion criterion is:

[0024] When the normal stress σ on the joint surface n When the fracturing fluid injection pressure is exceeded, the joint surface will be in a state of pressure closure, thereby preventing the fracturing fluid from entering the joint surface and stopping the propagation of the hydraulic fracture.

[0025] Optionally, the preset expansion mode includes mode two, and the corresponding crack expansion criterion is:

[0026] If the fracturing fluid injection pressure is high enough to cause the normal stress σ on the joint surface to be high... n When the critical condition for tensile or shear failure to occur at the joint surface is reached, tensile or shear failure will occur at the joint surface, and fracturing fluid will then penetrate into the joint.

[0027] Optionally, the preset expansion modes include mode three and / or mode four, and the corresponding crack expansion criterion for mode three is:

[0028] After the hydraulic crack penetrates the joint surface, it extends to the end, inducing stress concentration. Then it continues to extend along the end and deflects in the direction of the maximum principal stress.

[0029] The corresponding crack propagation criterion for Mode 4 is:

[0030] After the hydraulic fracture extends to the joint surface, under the combined action of the stress field at the fracture tip and the fracturing fluid, the joint surface undergoes shear failure, and the hydraulic fracture merges into the joint surface. During the shear deformation of the joint surface, stress concentration occurs at local shear fracture points due to discontinuous displacement. If the fracturing fluid injection pressure meets the matrix initiation requirements at this time, the hydraulic fracture will branch out along a certain point on the joint surface and turn towards the direction of the maximum principal stress.

[0031] Optionally, the preset expansion modes include mode five, and the corresponding crack expansion criterion is:

[0032] If the joint surface does not undergo shear failure and does not induce local stress concentration, the minimum principal stress in the opened area of ​​the joint surface is uniformly distributed. Under this condition, the injection of fracturing fluid into the joint surface will cause hydraulic fractures to penetrate the joint surface at multiple locations.

[0033] Optionally, the preset expansion modes include mode six, and the corresponding crack expansion criterion is:

[0034] If the fracturing fluid injection pressure fails to induce tensile failure of the joint surface, the movement of hydraulic fractures at the joint surface will be hindered. If the fracturing fluid injection pressure reaches the critical condition of inducing shear failure of the joint surface and driving the fracture and propagation of the rock matrix at the same time, the hydraulic fracture will directly penetrate the joint surface.

[0035] Optionally, a porous fracturing process can be adopted to control the injection pressure of the fracturing fluid and the fracturing sequence at different points, so as to guide the hydraulic fractures to expand according to a preset expansion pattern and form a fracture network.

[0036] As configured above, this invention provides a multi-stage hydraulic fracture propagation control method. The main beneficial effects of this invention are as follows:

[0037] (1) This invention realizes multi-stage hydraulic fracture directional propagation and complex three-dimensional fracture network construction: This invention breaks through the limitations of existing hydraulic fracturing technology, which has a single fracture propagation mode and a limited range of rock mass weakening. It proposes a fracture propagation control method based on the "fracture-joint" interaction mechanism. The focus is on how to promote the bifurcation, cross-layer extension and multi-directional interweaving of fractures in jointed rock masses to construct a three-dimensional fracture network structure with sufficient complexity, thereby achieving effective weakening of hard rock masses;

[0038] (2) The present invention establishes a precise judgment criterion for crack propagation: In view of the problems of uncontrollable crack propagation path and fluctuating rock mass weakening effect in the prior art, a discrimination system containing six preset propagation modes is constructed, which realizes accurate prediction and dynamic control of crack initiation, extension and crack network evolution, and ensures the controllability and stability of the fracturing effect. Attached Figure Description

[0039] Those skilled in the art will understand that the accompanying drawings are provided to better understand the invention and do not constitute any limitation on the scope of the invention. Wherein:

[0040] Figure 1 This is a flowchart of a multi-stage hydraulic fracture propagation control method according to an embodiment of the present invention;

[0041] Figure 2 This is a schematic diagram illustrating the propagation of a hydraulic crack along the direction of the maximum horizontal principal stress according to an embodiment of the present invention.

[0042] Figure 3 This is a schematic diagram of Mode 1 of a preset extension mode according to an embodiment of the present invention;

[0043] Figure 4 This is a schematic diagram of Mode 2 of the preset extension mode according to an embodiment of the present invention;

[0044] Figure 5 This is a schematic diagram of Mode 3 of the preset extended mode according to an embodiment of the present invention;

[0045] Figure 6 This is a schematic diagram of Mode 4 of the preset extension mode according to an embodiment of the present invention;

[0046] Figure 7 This is a schematic diagram of mode five of the preset extension modes according to an embodiment of the present invention;

[0047] Figure 8 This is a schematic diagram of Mode Six, a preset extension mode according to an embodiment of the present invention. Detailed Implementation

[0048] In this document, unless otherwise stated, the terms “upper,” “lower,” “left,” “right,” “inner,” “outer,” “front,” “back,” “top,” “bottom,” etc., are used to indicate orientation or positional relationship based on the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a characteristic orientation and operation, and therefore should not be construed as a limitation of the invention.

[0049] The specific embodiments of the present invention will now be described in more detail with reference to the accompanying drawings. The advantages and features of the present invention will become clearer from the following description. It should be noted that the drawings are all in a very simplified form and use non-precise proportions, and are only used to facilitate and clarify the illustration of the embodiments of the present invention.

[0050] Figure 1 This is a flowchart of a multi-stage hydraulic fracture propagation control method according to an embodiment of the present invention. Please refer to it. Figure 1 This invention provides a multi-stage hydraulic fracture propagation control method, which includes steps S1, S2 and S3. Steps S1, S2 and S3 are described in detail below.

[0051] Step S1: Obtain the mechanical parameters of the rock mass in the tunneling area. For example, the mechanical parameters of the rock mass in the tunneling area can be determined through geological exploration and laboratory testing. Mechanical parameters include the characteristics of the in-situ stress field, tensile strength, joint spatial distribution, and pore pressure. Tensile strength includes the tensile strength of the rock mass. Tensile strength of joint surfaces The spatial distribution of joints is mainly used in joint surface stress models and crack propagation criteria.

[0052] Step S2: Based on the mechanical parameters, construct a fracturing initiation mechanical model, calculate the critical fracturing initiation pressure and the optimal fracture azimuth angle, and thus determine the minimum fracturing fluid injection pressure and the corresponding drilling angle.

[0053] The geostress field characteristics include the maximum principal stress, intermediate principal stress, and minimum principal stress, and the fracture initiation mechanical model includes the principal stress distribution inside the borehole wall:

[0054] (1)

[0055] Where θ is the azimuth angle of the stress distribution point in the polar coordinate system (calculated counterclockwise from the x-axis, unit: °), σ1 is the maximum principal stress in the borehole in the local coordinate system, σ2 is the intermediate principal stress in the borehole in the local coordinate system, σ3 is the minimum principal stress in the borehole in the local coordinate system, and σ r σ represents the principal stress in the radial direction inside the borehole in the polar coordinate system. θ σ represents the principal stress in the inner annulus of the borehole in polar coordinates. z τ represents the principal stress along the axial direction inside the borehole in the polar coordinate system. rθ Let τ be the radial shear stress component of the borehole relative to the circumferential direction in polar coordinates. zθ Let τ be the axial-to-circumferential shear stress component inside the borehole in polar coordinates. rz Let be the radial shear stress component of the borehole relative to the axial direction in polar coordinates. In this embodiment, the unit of force in formula (1) is MPa.

[0056] Based on the borehole stress distribution, it can be known that σ θ In planes perpendicular to each other, only circumferential stress exists; therefore, the radial stress σ r This is one of the principal stresses on the borehole wall. After knowing the circumferential, radial, and axial stresses on the borehole wall, the other two principal stresses can be calculated using the plane composite stress formula, resulting in the principal stress distribution formula (1) inside the borehole wall.

[0057] The fracturing mechanics model includes a formula for calculating the critical fracturing pressure, which is constructed based on the maximum tensile stress failure criterion. Rock masses generally exhibit compressive strength much greater than tensile strength, making them most susceptible to tensile failure under fracturing fluid. Therefore, the maximum tensile stress failure criterion can be used to calculate the magnitude of the critical fracturing pressure in the rock mass. Combining the principal stress distribution characteristics of the borehole wall (the aforementioned principal stress distribution inside the borehole wall), it can be known that the principal stress... (That is, σ3 in Formula 1 above) is the first to reach tensile strength and fail tensilely. The critical initiation pressure on the borehole wall. (Unit: MPa) and Consistency:

[0058] (2)

[0059] Where, σ max σ is the critical crack initiation pressure. max The magnitude is the same as σ3, and the unit of force in formula (2) is MPa.

[0060] Let the azimuth angle θ corresponding to the critical initiation pressure be the optimal fracture azimuth angle θ0. Critical initiation pressure It is related to the original rock stress, pore pressure, and fracturing fluid pressure. Since the original rock stress and pore pressure are inherent properties of the rock mass, it is first necessary to determine the angle corresponding to the maximum circumferential stress. The optimal crack azimuth angle, also known as the circumferential stress at the crack initiation point, can be understood as the point where, at the instant of crack initiation, the circumferential stress at the crack initiation point is exactly equal to the critical initiation pressure σ. max ,at this time Optimal crack azimuth angle It can be done right Taking the partial derivative, we get:

[0061] (3)

[0062] When hydraulic fractures initiate, the main factors to consider include the tensile strength of the rock mass. and pore pressure P p (Units are all in MPa) The borehole wall will fail under the following conditions:

[0063] (4)

[0064] Where, k b is the Biot porous elastic coefficient, obtained from the rock compressibility, and is dimensionless.

[0065] Minimum fracturing fluid injection pressure P w The magnitude is equivalent to the critical initiation pressure σ max The size of the borehole is such that the borehole angle is calculated based on the optimal crack azimuth angle θ0.

[0066] Step S3: Based on the joint surface stress model and fracture propagation criteria, adjust the fracturing fluid injection pressure to guide the hydraulic fractures to propagate according to the preset propagation mode.

[0067] The joint surface stress model and fracture propagation criterion state that, due to the limited ability of boreholes to influence the surrounding stress environment, fractures gradually veer towards the direction of the maximum principal stress of the original rock (perpendicular to the direction of the minimum principal stress) after initiation. The angle between the joint surface normal and the maximum principal stress is ψ. Hydraulic fractures propagate along the direction of the maximum horizontal principal stress. When expanding in a direction, such asFigure 2 As shown.

[0068] When a hydraulic fracture approaches a joint surface, the stress at the joint mainly consists of stress components caused by the propagation of the hydraulic fracture and stress components caused by in-situ stress. The normal and shear stress components caused by the propagation of the hydraulic fracture are related to the fracture geometry and the Type I stress intensity factor of the coal and rock mass. and Type II stress intensity factor related:

[0069] (5)

[0070] In the formula, r is the distance between the point and the tip of the crack (unit: m), σ x σ represents the stress component in the x-direction at the crack tip. y τ is the stress component in the y-direction at the crack tip. xy Let x be the stress component at the crack tip with respect to the y direction, and θ be the azimuth angle of that point.

[0071] In a geostress environment, the angle between the fracture normal and the horizontal direction can be considered as... 0°, normal stress is The shear stress is 0 MPa, thus obtaining and The size of the hydraulic crack; the angle between the hydraulic crack and the joint surface is (90-ψ), so when the hydraulic crack extends to the joint surface, this angle can be substituted into θ in formula (5) to obtain the stress state between the cracks; at this time, the normal stress and shear stress on the joint surface can be solved by superimposing the stress at the tip of the hydraulic crack and the ground stress, and the normal stresses in the horizontal and vertical directions are respectively and The magnitude of the shear stress is The normal stress σ on the joint surface under the action of both n and the tangential stress τ on the joint surface n The following is a model of joint surface stress:

[0072] (6)

[0073] Where, σ n For the normal stress of the joint surface, τ n For the tangential stress of the joint surface, σ H For the maximum horizontal principal stress, σ h The minimum horizontal principal stress is σ, and ψ is the angle between the joint surface normal and the maximum principal stress direction. In this embodiment, ψ is the angle between the joint surface normal and the maximum horizontal principal stress σ. H The angle between directions.

[0074] Once a hydraulic fracture initiates in the borehole wall and propagates to the direction of the maximum horizontal principal stress, it is no longer subjected to shear stress. The normal and shear stress components caused by cracks are only related to the Type I stress intensity factor. It is related to the coordinate position.

[0075] Furthermore, this invention provides six expansion modes, which control the fracturing fluid injection pressure in real time and guide the hydraulic fracture to expand according to one or more of the following six modes.

[0076] The preset propagation modes include mode one, and the corresponding crack propagation criterion is: when the normal stress σ on the joint surface... n When the fracturing fluid injection pressure is exceeded, the joint surface will be in a state of pressure closure, thereby preventing the fracturing fluid from entering the joint surface and stopping the propagation of the hydraulic fracture.

[0077] Specifically, when hydraulic fractures approach the joint surface, the normal stress σ on the joint surface caused by the hydraulic fractures and in-situ stress increases. n Greater than the fracturing fluid injection pressure P w At this point, the fracture surface (joint surface) is closed by pressure, and the fracturing fluid cannot penetrate the joint surface, such as... Figure 3 As shown, the hydraulic crack stops expanding at this point, and the criterion for judgment is:

[0078] ,

[0079] Furthermore, at the fracturing fluid injection pressure P w Increase until it exceeds the normal stress σ of the joint surface. n When hydraulic fractures penetrate into the joint surface, the joint surface is additionally subjected to fracturing fluid. If the injection pressure of the fracturing fluid is insufficient to overcome the normal stress on the joint surface, or if the joint surface under the action of the fracturing fluid experiences neither tensile failure nor shear failure, then the fracture will also stop propagating. The criterion for this is:

[0080] (7)

[0081] in, P represents the tensile strength of the joint surface. p Let be the pore pressure of the rock mass, c be the cohesion of the joint surface, and φ be the internal friction angle of the joint surface. It should be noted that... The 'c' in this context is different from the 'c' in Company (7). The 'c' in this context is used as a name to distinguish it from other names. distinguish.

[0082] The preset expansion modes include mode two, and the corresponding crack expansion criterion is:

[0083] If the fracturing fluid injection pressure P w Sufficiently high, causing the normal stress σ on the joint surface nWhen the critical condition for tensile or shear failure to occur at the joint surface is reached, tensile or shear failure will occur at the joint surface, and fracturing fluid will then penetrate into the joint.

[0084] Specifically, at the point where the hydraulic fracture extends to the joint surface, if the fracturing fluid injection pressure P w A relatively high normal stress satisfies the tensile or shear failure conditions of the joint surface, at which point the joint surface opens or fails in shear, and fracturing fluid seeps in. When the joint surface is large and the rock matrix on the other side of the joint surface does not fail, hydraulic fractures will merge into the joint surface and continue to propagate, such as... Figure 4 As shown, Figure 4 As can be seen, there are blue dashed lines on the joint surface, indicating that hydraulic fractures have merged into the joint surface. The criteria for identification are as follows:

[0085] (8)

[0086] The preset expansion modes include mode three, and the corresponding crack expansion criterion for mode three is:

[0087] When the joint surface size is small, the filtrate loss of fracturing fluid is correspondingly lower. After the hydraulic fracture penetrates the joint surface, it rapidly extends to the tip, inducing stress concentration, and then continues to extend along the tip and gradually deflects towards the direction of the maximum principal stress.

[0088] Specifically, under the condition that the formula is satisfied, the filtration loss of fracturing fluid is also less when the joint surface size is small. After the hydraulic fracture merges into the joint surface, it quickly extends to the end to form a stress concentration, continues to extend along the end and gradually turns towards the direction of the maximum principal stress, such as... Figure 5 As shown, Figure 5 It can be seen that the hydraulic fracture extends along σ from the end of the joint surface. H The direction of extension is determined by the following criteria:

[0089] (9)

[0090] in, Type I fracture toughness, The values ​​represent Type II fracture toughness, and all units are in MPa.

[0091] The preset expansion modes include mode four, and the corresponding crack expansion criterion for mode four is:

[0092] After the hydraulic fracture extends to the joint surface, under the combined action of the stress field at the fracture tip and the fracturing fluid, the joint surface undergoes shear failure, and the hydraulic fracture merges into the joint surface. During the shear deformation process at the joint surface, stress concentration occurs at local shear failure points due to discontinuous displacement. If the fracturing fluid injection pressure meets the matrix initiation requirements at this time, the hydraulic fracture will branch out along a certain point on the joint surface and gradually turn towards the direction of the maximum principal stress. Figure 6As shown, its discrimination criterion is:

[0093] (10)

[0094] In in-situ fracturing, under geological structures or sedimentary processes, there may be a matrix with low strength in some areas of the rock mass joint surfaces. In this case, even if the size of the fracturing fluid does not meet the shear failure criterion, this propagation mode may still occur.

[0095] The preset expansion modes include mode five, and the corresponding crack expansion criterion is:

[0096] If the joint surface does not undergo shear failure and does not induce local stress concentration, the minimum principal stress in the opened area of ​​the joint surface is uniformly distributed. Under this condition, the injection of fracturing fluid into the joint surface will cause hydraulic fractures to penetrate the joint surface at multiple locations.

[0097] Specifically, when hydraulic fractures extend to the joint surface and simultaneously meet the conditions for joint surface tension failure and hydraulic fracture propagation, if the joint surface does not experience shear failure or cause local stress concentration, the minimum principal stress in the opened area of ​​the joint surface will be equal everywhere. In this case, multiple hydraulic fractures may cross the joint surface after the fracturing fluid flows in (a single hydraulic fracture crossing the joint surface is a special case of this mode). Figure 7 As shown, the joint surface tension fracturing and hydraulic fractures traversing the joint surface subsequently occur alternately or simultaneously. The criterion for distinguishing between them is:

[0098] (11)

[0099] The preset expansion modes include mode six, and the corresponding crack expansion criterion is:

[0100] When the joint surface has a high degree of cementation and significant tensile strength, the fracturing fluid injection pressure fails to induce tensile failure at the joint surface, resulting in obstruction of hydraulic fracture migration at the joint surface. If, at this point, the fracturing fluid injection pressure simultaneously reaches the critical condition of inducing shear failure at the joint surface and driving the fracture propagation of the rock matrix, the hydraulic fracture will directly penetrate the joint surface. Figure 8 As shown, its discrimination criterion is:

[0101] (12)

[0102] Preferably, a porous fracturing process can be used to control the injection pressure of the fracturing fluid and the fracturing sequence (timing) at different points to guide the hydraulic fractures to expand according to a preset expansion pattern and form a fracture network. This preferentially induces fracture expansion patterns such as bifurcation and transverse crossing, which promote the formation of a more complex fracture network.

[0103] It is understandable that by employing porous fracturing technology, the interaction between borehole stress interference and fractures can promote fracture network weaving; by implementing porous fracturing technology, the synergistic mechanism of the interaction between borehole stress interference and fractures can induce the formation of fracture network weaving structure.

[0104] Preferably, based on the fracture network extension model, the friction resistance and hydrodynamic effects are comprehensively considered to optimize the timing of fracturing operations and the configuration of water injection parameters (such as fracturing fluid injection pressure), ensuring that the fracture network system fully covers the area affected by the advance of tunneling.

[0105] After the fracture network is formed, a roadheader can be used for rapid cutting operations, and the tunneling parameters can be dynamically adjusted based on the rock mass cutability evaluation indicators. These indicators include specific energy consumption and average cutting resistance, while the tunneling parameters include advance speed, cutter head rotation speed, oscillation speed, cutting depth, cutting path method, and cutter head operating mode. Therefore, this invention also achieves synergistic optimization of fracture network weakening and tunneling technology, eliminating the disconnect between fracturing and tunneling processes.

[0106] In this implementation, the unit of force is MPa.

[0107] As configured above, this invention provides a multi-stage hydraulic fracture propagation control method. The main beneficial effects of this invention are as follows:

[0108] (1) This invention realizes multi-stage hydraulic fracture directional propagation and complex three-dimensional fracture network construction: This invention breaks through the limitations of existing hydraulic fracturing technology, which has a single fracture propagation mode and a limited range of rock mass weakening. It proposes a fracture propagation control method based on the "fracture-joint" interaction mechanism. The focus is on how to promote the bifurcation, cross-layer extension and multi-directional interweaving of fractures in jointed rock masses to construct a three-dimensional fracture network structure with sufficient complexity, thereby achieving effective weakening of hard rock masses;

[0109] (2) The present invention establishes an accurate judgment criterion for crack propagation: In view of the problems of uncontrollable crack propagation path and fluctuating rock mass weakening effect in the prior art, by establishing an propagation mode judgment criterion based on stress intensity factor and joint surface stress state, a discrimination system containing six preset propagation modes is constructed, which realizes accurate prediction and dynamic control of crack initiation, propagation and crack network evolution, and ensures the controllability and stability of the fracturing effect.

[0110] It should be noted that references to "an embodiment," "an embodiment," "a specific embodiment," "some embodiments," etc., in the specification only indicate that the described embodiment may include a specific feature, structure, or characteristic. Furthermore, such phrases do not necessarily refer to the same embodiment. Additionally, when a specific feature, structure, or characteristic is described in conjunction with an embodiment, whether explicitly described or not, implementing such a feature, structure, or characteristic in conjunction with other embodiments is within the knowledge of those skilled in the art.

[0111] It should be noted that the various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the systems disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the descriptions are relatively simple, and relevant parts can be referred to the method section.

[0112] It should also be noted that although the present invention has been disclosed above with reference to preferred embodiments, these embodiments are not intended to limit the present invention. For any person skilled in the art, many possible variations and modifications can be made to the technical solutions of the present invention based on the disclosed technical content, or equivalent embodiments can be modified accordingly, without departing from the scope of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the present invention shall still fall within the scope of protection of the present invention.

[0113] It should also be understood that, unless otherwise specified or indicated, the terms “first,” “second,” “third,” etc., in the specification are used only to distinguish the various components, elements, and steps in the specification, and not to indicate the logical or sequential relationships between the various components, elements, and steps.

[0114] Furthermore, it should be recognized that the terminology described herein is used only to describe particular embodiments and is not intended to limit the scope of the invention. It must be noted that the singular forms “a” and “an” as used herein include plural bases unless the context clearly indicates otherwise. For example, a reference to “a step” or “an apparatus” means a reference to one or more steps or apparatuses, and may include secondary steps and secondary apparatuses. All conjunctions used should be understood in the broadest sense. Also, the word “or” should be understood to have the definition of logical “or” rather than logical “exclusive OR”, unless the context clearly indicates otherwise. Furthermore, implementation of the methods and / or devices in embodiments of the invention may include performing selected tasks manually, automatically, or in combination.

Claims

1. A method for controlling the propagation of multi-stage hydraulic fractures, characterized in that, include: Obtain the mechanical parameters of the rock mass in the tunneling area; Based on the aforementioned mechanical parameters, a fracturing initiation mechanical model is constructed to calculate the critical fracturing initiation pressure and the optimal fracture azimuth angle, thereby determining the minimum fracturing fluid injection pressure and the corresponding drilling angle. Based on the joint surface stress model and fracture propagation criteria, the fracturing fluid injection pressure is adjusted to guide the hydraulic fractures to propagate according to a preset propagation pattern.

2. The multi-stage hydraulic fracture propagation control method as described in claim 1, characterized in that, The mechanical parameters include geostress field characteristics, which include maximum principal stress, intermediate principal stress, and minimum principal stress. The fracture initiation mechanical model includes the principal stress distribution inside the borehole wall. ， Where θ is the azimuth angle of the stress distribution point in the polar coordinate system, σ1 is the maximum principal stress in the borehole, σ2 is the intermediate principal stress in the borehole, σ3 is the minimum principal stress in the borehole, and σ r σ is the principal stress in the radial direction inside the borehole. θ For the principal stress in the inner annulus of the borehole, σ z τ is the principal stress along the axial direction inside the borehole. rθ τ represents the radial-to-circumferential shear stress component inside the borehole. zθ τ represents the axial-to-circumferential shear stress component inside the borehole. rz This represents the radial shear stress component relative to the axial direction within the borehole.

3. The multi-stage hydraulic fracture propagation control method as described in claim 2, characterized in that, The mechanical parameters include tensile strength; the crack initiation mechanical model includes a critical crack initiation pressure calculation formula, which is constructed based on the maximum tensile stress failure criterion. , where σ max σ is the critical crack initiation pressure. max The size is the same as σ3. Let the azimuth angle θ corresponding to the critical initiation pressure be the optimal fracture azimuth angle θ0. The optimal fracture azimuth angle θ0 can be obtained by σ max Taking the partial derivative with respect to θ, we get: ， When initiating hydraulic fractures, factors to consider include the tensile strength of the rock mass. and pore pressure P p The borehole wall will fail under the following conditions: , where k b The elastic modulus of Biot porous materials; The minimum fracturing fluid injection pressure is equal to the critical fracturing initiation pressure, and the borehole angle at this point is calculated based on the optimal fracture azimuth angle θ0.

4. The multi-stage hydraulic fracture propagation control method as described in claim 1, characterized in that, The stress model for the joint surface is as follows: ， Where, σ n For the normal stress of the joint surface, τ n For the tangential stress of the joint surface, σ H For the maximum horizontal principal stress, σ h For the minimum horizontal principal stress, σ x σ represents the stress component in the x-direction at the crack tip. y τ is the stress component in the y-direction at the crack tip. xy Let x be the stress component at the crack tip in the x-direction relative to the y-direction, and ψ be the angle between the joint surface normal and the direction of maximum principal stress.

5. The multi-stage hydraulic fracture propagation control method as described in claim 4, characterized in that, The preset expansion modes include mode one, and the corresponding crack expansion criterion is: When the normal stress σ on the joint surface n When the fracturing fluid injection pressure is exceeded, the joint surface will be in a state of pressure closure, thereby preventing the fracturing fluid from entering the joint surface and stopping the propagation of the hydraulic fracture.

6. The multi-stage hydraulic fracture propagation control method as described in claim 4, characterized in that, The preset expansion modes include mode two, and the corresponding crack expansion criterion is: If the fracturing fluid injection pressure is high enough to cause the normal stress σ on the joint surface to be high... n When the critical condition for tensile or shear failure to occur at the joint surface is reached, tensile or shear failure will occur at the joint surface, and fracturing fluid will then penetrate into the joint.

7. The multi-stage hydraulic fracture propagation control method as described in claim 4, characterized in that, The preset expansion modes include mode three and / or mode four, and the corresponding crack expansion criterion for mode three is: After the hydraulic crack penetrates the joint surface, it extends to the end, inducing stress concentration. Then it continues to extend along the end and deflects in the direction of the maximum principal stress. The corresponding crack propagation criterion for Mode 4 is: After the hydraulic fracture extends to the joint surface, under the combined action of the stress field at the fracture tip and the fracturing fluid, the joint surface undergoes shear failure, and the hydraulic fracture merges into the joint surface. During the shear deformation of the joint surface, stress concentration occurs at local shear fracture points due to discontinuous displacement. If the fracturing fluid injection pressure meets the matrix initiation requirements at this time, the hydraulic fracture will branch out along a certain point on the joint surface and turn towards the direction of the maximum principal stress.

8. The multi-stage hydraulic fracture propagation control method as described in claim 4, characterized in that, The preset expansion modes include mode five, and the corresponding crack expansion criterion is: If the joint surface does not undergo shear failure and does not induce local stress concentration, the minimum principal stress in the opened area of ​​the joint surface is uniformly distributed. Under this condition, the injection of fracturing fluid into the joint surface will cause hydraulic fractures to penetrate the joint surface at multiple locations.

9. The multi-stage hydraulic fracture propagation control method as described in claim 4, characterized in that, The preset expansion modes include mode six, and the corresponding crack expansion criterion is: If the fracturing fluid injection pressure fails to induce tensile failure of the joint surface, the movement of hydraulic fractures at the joint surface will be hindered. If the fracturing fluid injection pressure reaches the critical condition of inducing shear failure of the joint surface and driving the fracture and propagation of the rock matrix at the same time, the hydraulic fracture will directly penetrate the joint surface.

10. The multi-stage hydraulic fracture propagation control method as described in claim 1, characterized in that, A porous fracturing process is adopted, and the injection pressure of fracturing fluid and the fracturing sequence at different points are controlled to guide the hydraulic fractures to expand according to a preset expansion pattern and form a fracture network.

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