A prediction method for jointed rock mass subjected to hydraulic fracturing

Abstract

The invention relates to a prediction method for a jointed rock mass subjected to hydraulic fracturing, and the steps include: S11: conveying water in a water tank to the jointed rock mass through a high-pressure pump; S12: conveying a fracturing fluid to the jointed rock mass through a water injection conduit, so that cracks are formed in the jointed rock mass; S13: monitoring the injection rate of the fracturing fluid through a pressure gauge and a flow meter, and respectively monitoring the pressure and flow of the fracturing fluid in real time through the pressure gauge and the flow meter to obtain pressure data and flow data; S14: irradiating the jointed rock mass through an LED light source lamp; S15: photographing the jointed rock mass through an ultra-high-speed camera to obtain image data; S16: transmitting the pressure data, the flow data and the image data to a computer, thereby realizing real-time monitoring of the hydraulic fracturing process of the jointed rock mass, thereby obtaining crack extension parameters, until the cracks penetrate the jointed rock mass; S17: establishing a simplified crack model of the jointed rock mass according to the obtained crack extension parameters, and predicting the initiation and extension of the jointed rock mass.

Description

Technical Field

[0001] The invention relates to the technical field of geotechnical engineering, and in particular to a method for predicting jointed rock mass caused by hydraulic fracturing. Background Art

[0002] Jointed rock mass is a common engineering geological body in large-scale hydropower projects and energy storage power stations. It is generally composed of rocks and structural surfaces, and has obvious discontinuity, heterogeneity and anisotropy. Due to its special engineering properties, it inevitably involves the problem of thermal water-hydraulic multi-field coupling. Under the complex geological environment of high water pressure and high ground stress, engineering disturbance induces the reactivation of primary joint cracks, resulting in frequent engineering disasters such as rock cracking, sudden increase in seepage, and sudden mud and water inrush. The occurrence of such engineering disasters is closely related to the crack evolution process in which the internal stress of the rock mass is concentrated at the tip of the joint under the condition of multi-field coupling, which in turn promotes the initiation, development and even localization of the tip crack. Accurately knowing the dynamic evolution process of primary joint crack reactivation under the condition of multi-field coupling of jointed rock mass is crucial for accurately evaluating the safety and stability analysis of deep engineering.

[0003] The initiation and extension of cracks along primary joints is one of the typical failure modes of fractured rock mass in geotechnical engineering. Advanced observation techniques including CT, XRD and SEM are used to capture the dynamic evolution of cracks in fractured rock mass, but due to the opacity of the rock mass itself, the results of observing crack evolution are not accurate enough. Different materials are also used in existing technologies to simulate rock mass, but they are quite different from the characteristics of real rock mass. It is even more difficult to predict the initiation and extension trends of cracks in jointed rock mass, which makes it impossible to effectively avoid potential safety hazards in engineering applications.

[0004] To this end, the present application provides a method for predicting hydraulically fractured jointed rock masses. Summary of the invention

[0005] In view of the above shortcomings and deficiencies of the prior art, the present invention provides a method for predicting jointed rock mass by hydraulic fracturing, which solves the problem that the observed crack evolution results are not accurate enough due to the opacity of the rock mass itself in the prior art. The prior art also uses different materials to simulate rock mass, but they are all very different from the real rock mass characteristics, and it is more difficult to predict the trends of crack initiation and expansion of jointed rock mass cracks, resulting in the technical problem that potential safety hazards in engineering applications cannot be effectively avoided.

[0006] In order to achieve the above object, the present invention provides a method for predicting jointed rock mass by hydraulic fracturing, and the technical solution adopted includes:

[0007] S11: placing the jointed rock mass in a pressure platform, and conveying water in the water tank to the jointed rock mass through a high-pressure pump;

[0008] S12: mixing the tracer, the proppant and the water in the water tank through the water injection pipe to form a fracturing fluid, and transporting the fracturing fluid to the jointed rock mass to form cracks in the jointed rock mass;

[0009] S13: monitoring the injection rate of the fracturing fluid through a pressure gauge and a flow meter, so as to inject the fracturing fluid into the jointed rock mass at a fixed injection rate, and monitoring the pressure and flow of the fracturing fluid in real time through the pressure gauge and the flow meter, respectively, to obtain pressure data and flow data;

[0010] S14: irradiating the jointed rock mass with an LED light source;

[0011] S15: photographing the jointed rock mass under the illumination of the LED light source by an ultra-high-speed camera to obtain image data;

[0012] S16: transmitting the pressure data, flow data and image data to a computer, thereby realizing real-time monitoring of the hydraulic fracturing process of the jointed rock mass, until the cracks penetrate the jointed rock mass, and obtaining crack extension parameters during the real-time monitoring process;

[0013] S17: A simplified crack model of the jointed rock mass is established according to the obtained crack propagation parameters, and the crack initiation and propagation of the jointed rock mass are predicted.

[0014] In step S11, the specific steps of preparing the jointed rock mass include:

[0015] S111: adding 1% to 2.5% by mass of modified acid anhydride to the epoxy resin and stirring evenly to obtain a prefabricated resin liquid;

[0016] S112: adding a saturated rosin solution with a mass ratio of 0.2% to 0.5% to the prefabricated resin liquid, stirring evenly to obtain a resin liquid;

[0017] S113: pouring the resin liquid into a prefabricated joint mold, and obtaining a jointed rock mass after solidification, demoulding, and drilling.

[0018] In the above-mentioned preparation process of the jointed rock mass, the mass ratio of the modified acid anhydride to the saturated rosin solution is:

[0019] Modified anhydride: saturated rosin solution = 1:0.3.

[0020] In step S17, the specific prediction steps include:

[0021] S171: determining crack propagation parameters according to crack propagation parameters obtained through monitoring and establishing a simplified crack model of the jointed rock mass;

[0022] In step S171, the crack extension parameters mainly include:

[0023] It is half of the inherent crack length, in mm; is the Poisson's ratio of the jointed rock mass; σ1 is the stress borne by the volume unit in the vertical direction, unit N; σ3 is the stress borne by the volume unit in the horizontal direction, unit N; is the angle between the inherent crack and the vertical direction, unit: °; E0 is the elastic modulus, unit: Pa; K n is the normal stiffness of the inherent crack, in N / m; K s is the tangential stiffness of the inherent crack, in N / m.

[0024] S172: Calculate the normal stress acting around the crack and the tangential stress acting around the crack according to the crack extension parameters;

[0025] In step S172, the normal stress acting around the crack and the tangential stress around the crack are expressed as:

[0026] ;

[0027] ;

[0028] In the formula, σ xx is the normal stress acting around the crack, in N; σ xy is the tangential stress acting around the crack, in N; σ1 is the stress borne by the volume unit in the vertical direction, in N; σ3 is the stress borne by the volume unit in the horizontal direction, in N; K n is the normal stiffness of the inherent crack, in N / m; K s is the tangential stiffness of the inherent crack, in N / m; is the angle between the inherent crack and the vertical direction, unit: °; is the Poisson's ratio of the jointed rock mass; E0 is the elastic modulus, in Pa; It is half of the inherent crack length, unit: mm.

[0029] S173: Based on the normal stress and tangential stress around the crack, determine the sliding driving force that causes the crack to propagate and the driving force that causes the crack to occur after the crack propagates;

[0030] In step S173, the sliding driving force that causes the crack and the driving force that causes the crack to occur are respectively:

[0031] ;

[0032] ;

[0033] Where, T s The sliding driving force that causes the crack, in N; T nThe driving force that causes cracks to form, in N; is the friction coefficient of the crack surface; is the stress singularity in the direction opposite to the crack direction, in N; θ is the angle between the crack and the vertical direction, in °; σ xx is the normal stress acting around the crack, in N; σ xy is the tangential stress acting around the crack, in N; H is the cohesive force on the crack surface of the tangential stress acting on the crack, in Pa; It is the tensile strength on the crack surface, in Pa.

[0034] S174: obtaining a tangential displacement of the inherent crack surface and a normal displacement of the inherent crack surface according to the sliding driving force that causes the crack to form and the driving force that causes the crack to form;

[0035] In step S174, the tangential displacement of the inherent crack surface and the normal displacement of the inherent crack surface are respectively:

[0036] ;

[0037] Where U s (x) is the tangential displacement of the inherent crack surface, in mm; U n (x) is the normal displacement of the inherent crack surface, in mm; l is the crack growth length, in mm; is half of the inherent crack length, in mm; x is the distance from the calculation point to the crack center, in mm; α1 is the correction factor for the inherent crack; α2 is the correction factor for the crack shape; δ is the maximum displacement of the center point of the inherent crack, in mm.

[0038] S175: According to the tangential displacement of the inherent crack surface and the normal displacement of the inherent crack surface, combined with the crack growth length and the inherent crack length, the total energy caused by the crack surface driving force is obtained;

[0039] In step S175, the total energy caused by the crack surface driving force is expressed as:

[0040] ;

[0041] Where W is the total energy caused by the crack surface driving force, unit J; U s (x) is the tangential displacement of the inherent crack surface, in mm; U n (x) is the normal displacement of the inherent crack surface, in mm; T n The driving force that causes cracks, in N; T s It is the sliding driving force that causes the crack, unit is N; S1 is the inherent crack surface; S2 is the crack surface.

[0042] S176: Based on the total energy caused by the crack surface driving force, the thickness, material and stress intensity of the jointed rock mass are introduced to obtain the stress intensity factor related to the normal displacement and the stress intensity factor related to the tangential displacement;

[0043] In step S176, the stress intensity factors associated with the normal displacement and the stress intensity factors associated with the tangential displacement are:

[0044] ;

[0045] In the formula, is the stress intensity factor related to the normal displacement; is the stress intensity factor related to the tangential displacement; β n Material coefficient related to normal displacement; β s Material coefficients related to tangential displacement; l is the crack extension length, in mm; is half of the inherent crack length, unit: mm; δ n is the normal displacement after the maximum displacement is decomposed, in mm; δ s is the tangential displacement after the maximum displacement is decomposed, in mm; E0 is the elastic modulus, in Pa; F is the crack interaction factor.

[0046] S177: Finally, the crack tip growth formula is obtained based on the stress intensity factor related to the normal displacement and the stress intensity factor related to the tangential displacement, and then the stress intensity factor of crack extension at the crack tip during the crack growth process is calculated.

[0047] In step S177, the crack tip growth formula is:

[0048] ;

[0049] In the formula, K I is the stress intensity factor of crack propagation at the crack tip during crack growth; l is the crack extension length, in mm; It is half of the inherent crack length, in mm; is the angle between the inherent crack and the vertical direction, unit: °; F is the crack interaction factor; β n is the material coefficient related to the normal displacement; T s is the driving force that causes the crack to slide, in N; σ3 is the stress borne by the volume unit in the horizontal direction, in N.

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

[0051] 1. In order to better capture the crack propagation process of jointed rock mass, the present invention prepares a jointed rock mass that is more in line with the characteristics of real rock mass. The obtained jointed rock mass has the characteristics of high transparency, high strength, and reasonable brittleness at room temperature, which is more in line with the characteristics of real rock mass.

[0052] 2. In the prediction method of jointed rock mass in the present invention, since tracers are provided, the dynamic trend of cracks in the jointed rock mass can be better monitored and displayed, and the dynamic evolution process of the cracking of inherent cracks in the jointed rock mass and the reactivation that promotes the expansion of cracks can be reproduced.

[0053] 3. The present invention takes into account the influence between adjacent cracks in jointed rock mass based on the composite material fracture criterion of elastic-plastic fracture mechanics theory, solves the technical problem of promoting crack extension length, and derives a crack tip growth formula for calculating the crack extension length around parallel cracks within the framework of the composite fracture criterion. The theoretical value obtained by using an analytical mathematical solution for calculating the crack growth rate is compared with the experimental results, and the results obtained are more consistent, which further proves the accuracy of the experimental results obtained by using the crack tip growth formula, and the prediction of the initiation and expansion trend of cracks in jointed rock mass is more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 It is a structural schematic diagram of a simplified crack model of a jointed rock mass of the present invention;

[0055] Figure 2 It is a structural schematic diagram of a simplified crack model with a drilled hole in a jointed rock mass of the present invention;

[0056] Figure 3 for Figure 2 A side view of the jointed rock mass in one direction;

[0057] Figure 4 It is a connection schematic diagram of the hydraulic fracturing monitoring system of the present invention;

[0058] Figure 5 It is a schematic diagram of initial stress calculation of the present invention;

[0059] Figure 6 It is a schematic diagram of the analysis of the tip stress field of the present invention;

[0060] Figure 7 It is a schematic diagram of the analysis of the crack extension force of the present invention;

[0061] Figure 8 It is a schematic diagram of the analysis of crack extension displacement of the present invention;

[0062] Fig. 9 It is a result comparison diagram of the theoretical value and experimental value of the parallel crack growth length of the present invention.

[0063] Description of reference numerals:

[0064] 1. Jointed rock mass; 2. Water tank; 3. High-pressure pump; 4. Pressure gauge; 5. Proppant; 6. Tracer; 7. Flow meter; 8. Ultra-high-speed camera; 9. LED light source; 10. Computer; 11. Water injection pipe; 12. Cable; 13. Pressure platform; 14. Mica sheet; 15. Drilling. DETAILED DESCRIPTION

[0065] In order to better understand the above technical solution, exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided to enable a clearer and more thorough understanding of the present invention and to fully convey the scope of the present invention to those skilled in the art.

[0066] Example

[0067] Exemplarily, in order to better capture the initiation and expansion process of cracks in inherent jointed rock mass, the present invention prepares a jointed rock mass that can replace the real rock mass, which meets the requirements of crack expansion and prediction during the hydraulic fracturing experiment. The present application has the characteristics of a real rock mass through a self-made replaceable jointed rock mass. Secondly, a hydraulic fracturing experiment is carried out on the jointed rock mass. During the monitoring and prediction process, the initiation and expansion process of the crack is successfully captured, and the length trend of the crack expansion is determined. The specific details are as follows.

[0068] 1. The specific implementation process of the preparation of jointed rock mass for hydraulic fracturing is as follows:

[0069] This embodiment provides a preparation process of the jointed rock mass in the prediction method step, such as Figure 1 , Figure 2 , Figure 3 and Figure 4As shown, the entire preparation process is carried out at a temperature below 15°C, pure resin liquid is prepared, epoxy resin is used as aggregate, modified anhydride is used as curing agent, and saturated solution of rosin (RSS) is added as an accelerator to adjust the brittleness. The prefabricated joint surface is replaced by a round mica sheet 14 with a diameter of 15mm and a thickness of 1mm. It is determined that the pure resin liquid is first added with 1% modified anhydride curing agent and stirred for 1-2 minutes. Subsequently, 0.3% saturated solution of rosin as an accelerator is added and stirred evenly. In the preparation process of the jointed rock mass 1, specific materials and processes are used, and specific polymer reinforcement materials and transparent epoxy resin are selected. The mixed resin liquid is placed in a vacuum container for degassing treatment. By vacuuming, bubbles and volatile impurities in the material are removed, so that the impurities in the jointed rock mass 1 are reduced and the transparency is greatly improved. And the connection method of optimizing joints is introduced to improve the overall strength of the jointed rock mass 1. The joint surface is polished, cleaned and other treatments are performed to improve the flatness and roughness of the joint surface, increase the contact area between the bonding material and the joint surface, improve the connection strength of the joint, and drill 15 holes in the jointed rock mass after demoulding to obtain the final jointed rock mass that can be used for prediction. At the same time, before using epoxy resin glue to perfuse the joint surface, a layer of acrylic emulsion interface agent can be applied on the joint surface to enhance the bonding force between the epoxy resin and the joint surface to form a stronger structure. At the same time, the bonding material is also modified. In order to ensure that the curing agent and the accelerator react fully to enhance the transparency of the sample, the curing agent and the accelerator need to be added in sequence. After 3-6 adjustments and attempts, the optimal ratio of 1% modified anhydride curing agent and 0.3% accelerator in the resin mixture was determined. The transmittance of the rock samples was measured by a transmittance meter and reached 90%. In the mechanical property test on a triaxial testing machine, the compressive strength reached 130 MPa. The fracture toughness was measured by a fracture toughness testing device and recorded by a high-speed camera. The crack direction of the jointed rock mass 1 was clearer when it fractured, which was more similar to the fracture mode of the real rock mass in actual engineering. The prepared jointed rock mass met the characteristics of the rock mass.

[0070] 2. The specific implementation process of monitoring the jointed rock mass during hydraulic fracturing is as follows:

[0071] Start the hydraulic fracturing system. The hydraulic fracturing system consists of a water tank 2, a high-pressure pump 3, a pressure gauge 4, a proppant 5, a tracer 6, a flow meter 7, a water injection conduit 11 and a pressure platform 13. The water tank 2 is connected to the high-pressure pump 3, and the high-pressure pump 3 is connected to a pressure gauge 4, which is used to control the pressure in the hydraulic fracturing system to prevent the fluid pressure from exceeding the set value and causing system failure or damage. The set value range is between 20MPa-60MPa. The tracer 6 is connected to the proppant 5 through the water injection conduit 11 and connected to the flow meter 7. The flow meter 7 monitors the flow rate and controls the injection rate. The fracturing fluid is injected through the water injection conduit 11 at a rate of 20ml / min, and the injection rate is controlled to be constant according to the display data of the flow meter 7. The fracturing fluid is pumped into the replaceable jointed rock mass 1 by a high-pressure pump booster pump. Start the monitoring system. The monitoring system consists of an ultra-high-speed camera, an LED light source 9 and a computer 10, which are connected by a cable 12. An ultra-high-speed camera is used to photograph the prepared replaceable jointed rock mass 1, and continuous photographs of the dynamic trend of potassium permanganate monitored and photographed are recorded and transmitted to a computer to monitor the dynamic expansion of hydraulic fracturing cracks in the replaceable jointed rock mass 1 in real time, thereby obtaining the required crack expansion parameters, so as to facilitate the subsequent establishment of a simplified crack model of the jointed rock mass based on the obtained crack expansion parameters, and ultimately predict the initiation and expansion of the jointed rock mass.

[0072] Exemplarily, the proppant 5 is a polymer microsphere having good transparency and fluidity, which can effectively fill rock cracks and provide stable supporting force.

[0073] Exemplarily, the fracturing fluid is distilled water, and a tracer 6 is added to the distilled water to realize visualization of dynamic crack expansion. The tracer 6 is inorganic potassium permanganate, which has good stability and is not easily adsorbed by the layered transparent jointed rock mass 1.

[0074] Exemplarily, there are two ultra-high-speed cameras, which are placed on the left and right sides of the layered transparent jointed rock mass 1, and two LED light sources 9 are placed in front and behind the two ultra-high-speed cameras, respectively, to achieve clear all-round and multi-angle shooting, avoid the limitations of a single viewing angle and verify the monitoring accuracy with each other.

[0075] For example, the ultra-high-speed camera refers to the KIRANA 5M ultra-high-speed camera, which has a maximum resolution of 924×768 pixels and a shooting rate of up to 5×10^6 frames per second. The camera can continuously shoot 180 images and supports multiple trigger modes, which can accurately record the entire process of cracking and expansion of the jointed rock mass during the hydraulic fracturing of the jointed rock mass 1.

[0076] Exemplarily, the LED light source lamp 9 refers to the Qianyanlang L-100D-W flicker-free LED light source lamp.

[0077] 3. The specific prediction process of jointed rock mass by hydraulic fracturing is as follows:

[0078] For better understanding, this embodiment establishes a simplified crack model based on the prepared jointed rock mass 1, such as Figure 5 and Figure 6 As shown in the figure, the initial stress calculation diagram is shown to assist in the derivation and calculation of the crack extension length. During the hydraulic fracturing process, it is assumed that a representative volume unit is subjected to stress σ1 in the vertical direction and stress σ3 in the horizontal direction of the volume unit. The parallel adjacent cracks are located in the middle, and the angle between this pair of inherent cracks and the vertical direction is .

[0079] Then, on this pair of parallel cracks, the normal stress around the crack and the tangential stress around the crack are expressed as:

[0080] ;

[0081] ;

[0082] In the formula, σ xx is the normal stress acting around the crack, in N; σ xy is the tangential stress acting around the crack, in N; σ1 is the stress borne by the volume unit in the vertical direction, in N; σ3 is the stress borne by the volume unit in the horizontal direction, in N; K n is the normal stiffness of the inherent crack, in N / m; K s is the tangential stiffness of the inherent crack, in N / m; is the angle between the inherent crack and the vertical direction, unit: °; is the Poisson's ratio of the jointed rock mass; E0 is the elastic modulus, in Pa; It is half of the inherent crack length, unit: mm.

[0083] The sliding driving force that causes the crack is expressed as:

[0084] ;

[0085] Where, T S The sliding driving force that causes the crack, in N; σ xy is the tangential stress acting on the crack, in N; σ xx is the normal stress acting around the crack, unit is N; f represents the sliding friction coefficient of the crack surface, which is the ratio of the friction force on the crack surface to the normal pressure on the crack surface.

[0086] At the same time, due to the stress singularity at the crack tip, according to the elastic-plastic mechanics theory, the shear stress around the crack tip relative to the crack direction can be determined by the sliding driving force that causes the crack:

[0087] ;

[0088] In the formula is the stress singularity in the direction opposite to the crack direction, in N; T S The sliding driving force that causes the crack, in N; is the angle between the inherent crack and the vertical direction, unit: °; is half of the inherent crack length, in mm; r is a parameter determined by experimental regression analysis.

[0089] On the other hand, as the compressive stress of the representative volume unit increases, the internal cracks gradually close and friction sliding occurs, resulting in the generation of cracks, such as Figure 6 As shown in Figure 2, it is assumed that the strain of the crack at infinity is zero and the elastic energy of the crack at infinity is U0.

[0090] During the crack propagation process, the total energy caused by the driving force acting on the crack surface and the trigger energy related to the stress change are W and U respectively. The total energy of the representative volume unit can be expressed as:

[0091] ;

[0092] Where P is the total energy of the representative volume unit, unit J; U0 is the elastic energy of the crack at infinity, unit J; U is the trigger energy related to the stress change, unit J; W is the total energy caused by the driving force on the crack surface, unit J.

[0093] After crack propagation, Figure 7 As shown in Figure 2, based on the normal stress and tangential stress around the crack, the sliding driving force that causes the crack and the driving force that causes the crack to occur are obtained as follows:

[0094] ;

[0095] ;

[0096] Where, T s The sliding driving force that causes the crack, in N; T n The driving force that causes cracks to form, in N; is the friction coefficient of the crack surface; is the stress singularity in the direction opposite to the crack direction, in N; θ is the angle between the crack and the vertical direction, in °; σ xxis the normal stress acting around the crack, in N; σ xy is the tangential stress acting around the crack, in N; H is the cohesive force on the crack surface of the tangential stress acting on the crack, in Pa.

[0097] Assume that the process is elastic, where represents the friction coefficient of the crack surface, H is the tangential stress acting on the crack and the cohesive force of the crack surface. When the initial microcrack begins to slide, the total energy driven by the driving force on the crack surface is:

[0098] ;

[0099] Where U s (x) is the tangential displacement of the inherent crack surface, in mm; W is the total energy caused by the driving force on the crack surface, in J; U n (x) is the normal displacement of the inherent crack surface, in mm; T S The sliding driving force that causes the crack, in N; T n It is the driving force that causes cracks to form, in N; S1 is the inherent crack surface; S2 is the crack surface.

[0100] In addition, due to the complexity of the actual cracking mode, the prior art cannot determine the relative displacement of each point on the crack surface in real time. Therefore, in the prediction method of the present invention, the length calculation of the inherent crack and the crack is further simplified, and the inherent crack and the crack are regarded as a length of 2 ( l + ) of the regenerated cracks, such as Figure 8 Then, the tangential displacement U of the inherent crack surface s (x) and the normal displacement U of the inherent crack surface n (x) can be expressed as follows:

[0101] ;

[0102] In the formula, l is the crack growth length, in mm; is half of the inherent crack length, in mm; x is the distance from the calculation point to the crack center, in mm; α1 is the correction factor for the inherent crack; α2 is the correction factor for the crack shape; δ is the maximum displacement of the center point of the inherent crack, in mm.

[0103] Then the simplified total energy formula driven by the crack surface driving force is obtained:

[0104]

[0105] After simplification, in the formula of the total energy driven by the crack surface driving force, the maximum displacement δ of the intrinsic crack center can be decomposed into the tangential displacement after the maximum displacement decomposition and the normal displacement after the maximum displacement decomposition (δ s , δ n ), the corresponding elastic energy change caused by the displacement change is expressed as follows:

[0106] ;

[0107] Where U n The change in elastic energy caused by the change in normal displacement, unit J; U s is the change of elastic energy caused by the change of tangential displacement, unit J; E0 is the elastic modulus, unit Pa; B is the thickness of the jointed rock mass, unit mm; δ s is the tangential displacement after the maximum displacement is decomposed, in mm; δ n is the normal displacement after the maximum displacement is decomposed, in mm; σ3 is the stress borne by the volume unit in the horizontal direction, in N; α1 is the correction factor for the inherent crack; α2 is the correction factor for the crack shape; α3 is a constant related to the jointed rock mass of the experimental material; T s The sliding driving force that causes the crack, in N;

[0108] The trigger energy associated with stress change is obtained by the following formula. Based on the above assumptions and keeping the minimum energy of sliding displacement, the calculation formula of the maximum displacement δ of the center point of the inherent crack is obtained:

[0109] ;

[0110] ;

[0111] Where U is the trigger energy associated with stress change, unit J; U n The change in elastic energy caused by the change in normal displacement, unit J; U s is the change of elastic energy caused by the change of tangential displacement, unit J; α1 is the correction factor of inherent crack; α2 is the correction factor of crack shape; α3 is a constant related to the jointed rock mass of the experimental material; B is the thickness of the jointed rock mass, unit mm; T s is the sliding driving force of the crack, in N; E0 is the elastic modulus, in Pa; δ is the maximum displacement of the center point of the inherent crack, in mm;

[0112] It is further used to calculate the stress intensity factor related to the normal displacement , and the stress intensity factor related to the tangential displacement in the crack The specific formula is as follows:

[0113] ;

[0114] In the formula, is the stress intensity factor related to the normal displacement; is the stress intensity factor related to the tangential displacement; β n is the material coefficient related to the normal displacement; β s is the material coefficient related to the tangential displacement; l is the crack extension length, in mm; is half of the crack length, unit: mm; δ n is the normal displacement after the maximum displacement is decomposed, in mm; δ s is the tangential displacement after the maximum displacement is decomposed, in mm; E0 is the elastic modulus, in Pa; F is the crack interaction factor;

[0115] The stress intensity factor of crack propagation at the crack tip during crack growth is:

[0116] ;

[0117] Finally, the crack tip growth formula is derived as follows:

[0118] ;

[0119] In the formula, K I is the stress intensity factor of crack propagation at the crack tip during crack growth; l is the crack extension length, in mm; It is half of the inherent crack length, in mm; is the angle between the inherent crack and the vertical direction, unit: °; F is the crack interaction factor; β n Material coefficient related to normal displacement; T s is the driving force that causes the crack to slide, in N; σ3 is the stress borne by the volume unit in the horizontal direction, in N.

[0120] The crack tip growth formula adopts the fracture mechanics theory, taking into account the stress field and energy release rate at the crack tip, and can theoretically predict the crack initiation and expansion behavior of the jointed rock mass. The physical simplified crack model established takes into account the material properties of the rock mass, such as elastic modulus, strength, etc., the geometric characteristics of the joints, such as direction, spacing, roughness, etc., and external load conditions and other factors, providing a basis for the reliability of the crack tip growth formula. When the theoretical value is consistent with the experimental results, it shows that the crack tip growth formula can accurately capture the key factors and mechanisms of crack initiation and expansion in the jointed rock mass, and then facilitate the acquisition of the parameters of crack initiation and expansion in the jointed rock mass, effectively avoiding potential safety hazards in engineering applications.

[0121] First, according to the crack extension parameters, the lengths of the two parallel cracks are both 2 is 15 mm, d is 15 mm, the normal stiffness of the inherent crack K n and the tangential stiffness K of the inherent crack s They are 4.9N / m and 0.196N / m respectively, and the interaction coefficient of the stress intensity factor is taken as 1.04. The elastic modulus E0 is about 6.3GPa, and the Poisson's ratio of the jointed rock mass is is 0.2. Other parameters such as π, β n and β s They are 0.392, 0.4 and 1.0 respectively, and the stress F is 50MPa. The length of the crack extension is calculated by the derived crack tip growth formula to be about 4.18mm, and the measured crack extension length is 4.32mm, with an error of less than 5%, that is, the calculated theoretical value is consistent with the experimental result, which further proves the accuracy of the experimental result obtained by using the crack tip growth formula.

[0122] According to the above calculation process, if Fig. 9 As shown, compared with σ p is 10%, σ p is 30%, σ p is 50%, σ p is 70%, σ p is 80% and σ p The theoretical value and experimental results are 90% to show the crack growth length. It can be seen that the overall trend between the theoretical value and the actual result of the crack growth length shows good consistency. More specifically, the length of the crack growth initially shows a gentle growth and then increases sharply. p A significant threshold can be observed around 50% of the points. p Before about 50%, there is no difference between the theoretical value and the experimental result.

[0123] In summary, in order to better capture and predict the crack propagation process of the jointed rock mass 1, the present invention prepares a jointed rock mass 1 that conforms to the real lithology, and hydraulic fracturing is performed on the prepared replaceable jointed rock mass 1 without contacting the rock sample. Through hydraulic fracturing combined with computer observation of the trend of the tracer, the crack initiation and propagation of the jointed rock mass 1 are monitored in real time to obtain crack propagation parameters, and combined with the crack tip growth formula in the crack growth process finally derived, the stress intensity factor of the crack propagation at the crack tip in the crack growth process can be calculated, thereby solving the technical problem that the simulated rock mass has a large difference with the real rock mass characteristics, the initiation, propagation and prediction of the cracks in the jointed rock mass 1 are more difficult to obtain, resulting in the inability to effectively avoid the potential safety hazards in engineering applications, and reasonably analyzing the starting point of the jointed rock mass 1 under the loading state and the crack propagation law, and qualitatively analyzing the jointed rock mass 1 to reveal the hydraulic fracturing mechanism of the jointed rock mass 1.

[0124] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may alter, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. A method for predicting hydraulically fractured jointed rock mass, characterized in that: The specific steps include: S11: placing the jointed rock mass in a pressure platform, and conveying water in the water tank to the jointed rock mass through a high-pressure pump; S12: mixing the tracer, the proppant and the water in the water tank through the water injection pipe to form a fracturing fluid, and transporting the fracturing fluid to the jointed rock mass to form cracks in the jointed rock mass; S13: monitoring the injection rate of the fracturing fluid through a pressure gauge and a flow meter, so as to inject the fracturing fluid into the jointed rock mass at a fixed injection rate, and monitoring the pressure and flow of the fracturing fluid in real time through the pressure gauge and the flow meter, respectively, to obtain pressure data and flow data; S14: irradiating the jointed rock mass with an LED light source; S15: photographing the jointed rock mass under the illumination of the LED light source by an ultra-high-speed camera to obtain image data; S16: transmitting the pressure data, flow data and image data to a computer, thereby realizing real-time monitoring of the hydraulic fracturing process of the jointed rock mass, and obtaining crack extension parameters during the real-time monitoring process until the crack penetrates the jointed rock mass; S17: establishing a simplified crack model of the jointed rock mass according to the obtained crack propagation parameters, and predicting the crack initiation and propagation of the jointed rock mass; The preparation steps of the jointed rock mass include: S111: adding 1% to 2.5% by mass of modified acid anhydride to the epoxy resin and stirring evenly to obtain a prefabricated resin liquid; S112: adding a saturated rosin solution with a mass ratio of 0.2% to 0.5% to the prefabricated resin liquid, stirring evenly to obtain a resin liquid; S113: pouring the resin liquid into a prefabricated joint mold, and obtaining a jointed rock mass after solidification, demoulding, and drilling.

2. The method for predicting jointed rock mass by hydraulic fracturing according to claim 1, characterized in that: The mass ratio of the modified acid anhydride to the saturated rosin solution is: Modified anhydride: saturated rosin solution = 1:0.

3.

3. The method for predicting hydraulically fractured jointed rock mass according to claim 1, characterized in that: The prediction step in step S17 includes: S171: determining crack propagation parameters according to crack propagation parameters obtained through monitoring and establishing a simplified crack model of the jointed rock mass; S172: Calculating the normal stress acting around the crack and the tangential stress acting around the crack according to the determined crack extension parameter; S173: Based on the normal stress acting around the crack and the tangential stress acting around the crack, determine the sliding driving force that causes the crack to propagate and the driving force that causes the crack to occur after the crack propagates; S174: obtaining a tangential displacement of the inherent crack surface and a normal displacement of the inherent crack surface according to the sliding driving force that causes the crack to form and the driving force that causes the crack to form; S175: According to the tangential displacement of the inherent crack surface and the normal displacement of the inherent crack surface, combined with the crack growth length and the inherent crack length, the total energy caused by the crack surface driving force is obtained; S176: Based on the total energy caused by the crack surface driving force, the thickness, material and stress intensity of the jointed rock mass are introduced to obtain the stress intensity factor related to the shear displacement and the stress intensity factor related to the normal displacement; S177: Finally, the crack tip growth formula is obtained based on the stress intensity factor related to the normal displacement and the stress intensity factor related to the tangential displacement, and then the stress intensity factor of crack extension at the crack tip during the crack growth process is calculated based on the crack tip growth formula.

4. The method for predicting jointed rock mass by hydraulic fracturing according to claim 3, characterized in that: In step S172, the normal stress acting around the crack and the tangential stress acting around the crack are respectively expressed as: ; ; In the formula, σ xx is the normal stress acting around the crack, in N; σ xy is the tangential stress acting around the crack, in N; σ1 is the stress borne by the volume unit in the vertical direction, in N; σ3 is the stress borne by the volume unit in the horizontal direction, in N; K n is the normal stiffness of the inherent crack, in N / m; K s is the tangential stiffness of the inherent crack, in N / m; is the angle between the inherent crack and the vertical direction, unit: °; is the Poisson's ratio of the jointed rock mass; E0 is the elastic modulus, in Pa; It is half of the inherent crack length, unit: mm.

5. The method for predicting jointed rock mass by hydraulic fracturing according to claim 3, characterized in that: In step S173, the sliding driving force that causes the crack and the driving force that causes the crack to be generated are respectively: ; ; Where, T s The sliding driving force that causes the crack, in N; T n The driving force that causes cracks to form, in N; is the friction coefficient of the crack surface; is the stress singularity in the direction opposite to the crack direction, in N; θ is the angle between the crack and the vertical direction, in °; σ xx is the normal stress acting around the crack, in N; σ xy is the tangential stress acting around the crack, in N; H is the cohesive force on the crack surface of the tangential stress acting on the crack, in Pa; It is the tensile strength on the crack surface, in Pa.

6. The method for predicting jointed rock mass by hydraulic fracturing according to claim 3, characterized in that: In step S174, the tangential displacement of the inherent crack surface and the normal displacement of the inherent crack surface are respectively: ; Where U s (x) is the tangential displacement of the inherent crack surface, in mm; U n (x) is the normal displacement of the inherent crack surface, in mm; l is the crack growth length, in mm; is half of the inherent crack length, in mm; x is the distance from the calculation point to the crack center, in mm; α1 is the correction factor for the inherent crack; α2 is the correction factor for the crack shape; δ is the maximum displacement of the center point of the inherent crack, in mm.

7. The method for predicting jointed rock mass by hydraulic fracturing according to claim 3, characterized in that: In step S175, the total energy caused by the crack surface driving force is expressed as: ; Where W is the total energy caused by the crack surface driving force, unit J; U s (x) is the tangential displacement of the inherent crack surface, in mm; U n (x) is the normal displacement of the inherent crack surface, in mm; T n The driving force that causes cracks, in N; T s It is the sliding driving force that causes the crack, unit is N; S1 is the inherent crack surface; S2 is the crack surface.

8. The method for predicting jointed rock mass by hydraulic fracturing according to claim 3, characterized in that: In step S176, the stress intensity factor associated with the normal displacement and the stress intensity factor associated with the tangential displacement are respectively: ; In the formula, is the stress intensity factor related to the normal displacement; is the stress intensity factor related to the tangential displacement; β n Material coefficient related to normal displacement; β s Material coefficients related to tangential displacement; l is the crack extension length, in mm; is half of the inherent crack length, unit: mm; δ n is the normal displacement after the maximum displacement is decomposed, in mm; δ s is the tangential displacement after the maximum displacement is decomposed, in mm; E0 is the elastic modulus, in Pa; F is the crack interaction factor.

9. The method for predicting jointed rock mass by hydraulic fracturing according to claim 3, characterized in that: In step S177, the crack tip growth formula is: ; In the formula, K I is the stress intensity factor of crack propagation at the crack tip during crack growth; l is the crack extension length, in mm; It is half of the inherent crack length, in mm; is the angle between the inherent crack and the vertical direction, unit: °; F is the crack interaction factor; β n is the material coefficient related to the normal displacement; T s is the driving force that causes the crack to slide, in N; σ3 is the stress borne by the volume unit in the horizontal direction, in N.

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