An experimental system and method for manufacturing complex fractures by temperature-controlled cyclic loading

Abstract

The application provides an experimental system and method for manufacturing complex fractures through temperature control and cyclic loading, and belongs to the technical field of geothermal fracturing experiments. The experimental system for manufacturing complex fractures through temperature control and cyclic loading comprises a true triaxial core holder, an injection system, a temperature control system and a servo control system. The injection system and the temperature control system are connected with the true triaxial core holder. Meanwhile, the injection system and the temperature control system are connected with the servo control system. The application controls the temperature difference generated by pumping fracturing fluid to cause fatigue damage to rocks, excites natural microfractures existing in the rocks, makes the artificially manufactured hydraulic fractures communicate with the natural fractures, and manufactures complex fracturing fracture networks. The communication between the hydraulic fractures and the natural fractures is more complex than that of the rock samples directly fractured, the rock seepage efficiency is improved, and the application can be applied to the process technology for improving the development of geothermal resources.

Description

Technical Field

[0001] This invention belongs to the field of geothermal fracturing experimental technology, specifically relating to an experimental system and method for creating complex fractures by temperature-controlled cyclic loading. Background Technology

[0002] Deep geothermal energy is mostly found in hot dry rocks, which are hot rock masses devoid of water and gas, typically granite. The main applications of hydraulic fracturing technology are in the development of unconventional oil and gas and as a measure to enhance the production of oil and gas wells. Due to the complex geological conditions, poor permeability, and significant stress differences in my country's reservoir areas, commercial exploitation has progressed slowly. By injecting fracturing fluid into the reservoir under high pressure, the permeability of the rock mass is improved, effectively solving the exploitation problem.

[0003] Hydraulic fracture is a tensile fracture caused by the internal pressure of an injected viscous fluid. It propagates in a solid medium under the pressure of the fluid. Under high pressure, fracturing fluid is injected into a tight reservoir to increase the permeability of the rock.

[0004] In fracture system development, the complex fractures formed by the interweaving of natural and artificial fractures are particularly important for improving oil and gas recovery. During hydraulic fracturing, providing a larger cumulative fracture area (a larger area allows for the diffusion of oil and gas) and increasing the connectivity of the natural fracture network (more permeable reservoirs) increases the seepage area of ​​new fractures. In the seepage zone, oil and gas in the rock matrix diffuse into the fractures and flow along the fractures into the wellbore, thereby improving oil and gas recovery.

[0005] Other scholars, using modeling and imaging tools, have discovered that employing horizontal well segmented multi-cluster fracturing technology with large-scale, high-volume, low-viscosity fracturing fluid generates complex monitoring cloud images near the horizontal well. This indicates that this fracturing measure can perform three-dimensional modification of tight reservoirs, forming complex fracture networks. This results in better production enhancement of tight oil reservoirs compared to traditional fracturing, thereby improving the efficiency of tight oil reservoir stimulation. CLCipolla et al. believe that obtaining complex fracture networks is the main objective of fracturing operations, and these complex fracture networks can increase the production efficiency of ultra-low permeability oil layers. This technique is still being adopted and refined by major oilfields.

[0006] Chinese patent publication CN112784337A discloses a method for predicting the remaining life rate of reinforced concrete beams based on the degradation of deflection verification coefficient. It determines the stiffness degradation law of the reinforced concrete beam slabs for each beam slab based on the static load test results during the initial construction and evaluation periods of the bridge structure. Furthermore, it determines the curve of deflection difference versus loading cycles, analyzes the equivalent stiffness degradation law during fatigue loading, and further determines the beam deflection degradation law, establishing the relationship between deflection degradation rate and remaining life to predict the residual life of the reinforced concrete beam. However, it only provides a theoretical evaluation method and does not involve specific destructive testing processes. Chinese patent publication CN110031345A discloses a method for evaluating the fatigue resistance of asphalt mixtures. This method evaluates the fatigue resistance of asphalt mixtures by conducting accelerated fatigue tests, including determining basic parameters, calculating the stiffness modulus and dissipated energy density of the mixture, determining the fatigue strain state point through the dissipated energy density curve, and determining the fatigue resistance of the asphalt mixture based on the number of loading cycles corresponding to the fatigue strain state point. However, its focus is mainly on accelerated fatigue testing.

[0007] In addition, most previous studies have focused on computer methods such as software simulation and imaging tool monitoring, or on the mechanical properties of rocks themselves. There is no publicly available data on physical simulation experiments of fracturing by combining temperature control with cyclic loading. Summary of the Invention

[0008] The purpose of this invention is to solve the problems existing in the prior art and provide an experimental system and method for creating complex cracks by temperature-controlled cyclic loading. This allows for a deeper study of crack propagation laws, better transformation and development of geothermal resources, and fatigue damage to rocks caused by the temperature difference generated by pumping fracturing fluid at controlled temperatures. It also stimulates the natural micro-cracks that exist within the rocks, allowing artificially created hydraulic cracks to communicate with natural cracks, thus creating a complex fracturing network and improving seepage efficiency.

[0009] This invention is achieved through the following technical solution:

[0010] In a first aspect, the present invention provides an experimental system for manufacturing complex cracks by temperature-controlled cyclic loading, the system comprising: a true triaxial core holder, an injection system, a temperature control system, and a servo control system;

[0011] The injection system and temperature control system are respectively connected to the true triaxial core holder;

[0012] Meanwhile, the injection system and the temperature control system are respectively connected to the servo control system.

[0013] A further improvement of the present invention is that the injection system includes multiple injection pumps, namely a confining pressure injection pump and a fracturing fluid injection pump;

[0014] One end of the confining pressure injection pump is connected to the confining pressure plate in the true triaxial core holder, and the other end is connected to the servo control system.

[0015] One end of the fracturing fluid injection pump is connected to the core wellbore in the true triaxial core holder, and the other end is connected to the servo control system.

[0016] Preferably, multiple heating plates are provided on the true triaxial core holder, and each heating plate is connected to a temperature control system.

[0017] Preferably, a computer is also connected between the temperature control system and the servo control system.

[0018] A second aspect of the present invention provides an experimental method for creating complex cracks by temperature-controlled cyclic loading, the method comprising:

[0019] S1: Detects natural cracks present in the rock sample;

[0020] S2: Load the rock sample into the true triaxial core holder in the above system and heat the rock sample;

[0021] S3: The rock sample is loaded using a cyclic loading method.

[0022] A further improvement of the present invention is that the operation of heating the rock sample in step S2 includes:

[0023] The rock sample is heated from room temperature to the target temperature using a segmented heating method.

[0024] A further improvement of the present invention is that the operation of heating the rock sample from room temperature to the target temperature using a segmented heating method includes:

[0025] Assume the target heating temperature is T℃;

[0026] S21: The rock sample is heated from room temperature to H1℃ and then kept at that temperature for t1.

[0027] S22: The rock sample is heated from H1℃ to H2℃ and then kept at that temperature for t2.

[0028] S23: The rock sample is heated from H2℃ to H3℃ and then kept at that temperature for t3.

[0029] S24: The rock sample is heated from H3℃ to H4℃ and then kept at that temperature for t4.

[0030] S25: After heating the rock sample from H4℃ to T℃, it is kept warm for t5.

[0031] A further improvement of the present invention is that,

[0032] H1 = 0.1T;

[0033] H2 = H1 + 0.1T;

[0034] H3 = H2 + 0.2T;

[0035] H4 = H3 + 0.5T.

[0036] A further improvement of the present invention is that,

[0037] t1 is 4 hours;

[0038] t2 is 8 hours;

[0039] t3 is 10 hours;

[0040] t4 is 12 hours;

[0041] t5 is 4 hours.

[0042] A further improvement of the present invention is that the operation of step S3 includes:

[0043] S31, apply a set confining pressure to the rock sample using a confining pressure injection pump;

[0044] S32, fracturing fluid is injected using a fracturing fluid injection pump to load the rock sample to A MPa, and then unloaded;

[0045] S33: After repeating step S32 N times, apply pressure directly until the rock sample fractures.

[0046] A further improvement of the present invention is that,

[0047] A = 0.6p b

[0048] p b =3σ h -σ H +σ f -p o

[0049] Where, σ h For the minimum horizontal principal stress, σ H For the maximum horizontal principal stress, σ f p represents the uniaxial tensile stress strength of the rock. o This represents the pressure of the overlying rock strata.

[0050] A further improvement of the present invention is that,

[0051]

[0052] The total experiment duration is a set value;

[0053] The single loading time is the time required to load a rock sample to A MPa.

[0054] Compared with the prior art, the beneficial effects of the present invention are:

[0055] This invention induces fatigue damage in rocks by controlling the temperature difference generated by pumping in fracturing fluid, thereby stimulating the natural micro-fractures that already exist within the rocks. This allows artificially created hydraulic fractures to connect with the natural fractures, creating a complex fracturing network. The connection between the hydraulic and natural fractures is more complex than that of rock samples subjected to direct fracturing, thus improving rock seepage efficiency. This invention can be applied to improve the technology for geothermal resource transformation and development. Attached Figure Description

[0056] Figure 1 A schematic diagram of the system composition of this invention;

[0057] Figure 2 Pressure curve of rock sample 1 in the application embodiment of the present invention;

[0058] Figure 3 Pressure curve of rock sample 2 in the application embodiment of the present invention;

[0059] Figure 4 The fracturing results of rock sample 1 in the application examples of this invention;

[0060] Figure 5 The fracturing results of rock sample 2 in the application examples of this invention; Detailed Implementation

[0061] The present invention will now be described in further detail with reference to the accompanying drawings:

[0062] This invention provides an experimental system for controlling temperature cyclic loading to create complex cracks, such as... Figure 1 As shown, an embodiment of the system is as follows:

[0063] Example 1

[0064] The system includes:

[0065] The system comprises a true triaxial core holder 1, an injection system, a temperature control system 2, and a servo control system 4. The injection system includes multiple injection pumps 5. Figure 1 The true triaxial core holder 1, temperature control system 2, servo control system 4, and injection pump 5 are all existing mature products, and will not be described in detail here.

[0066] The temperature control system 2 is connected to the heating plate of the true triaxial core holder 1; specifically, multiple heating plates are provided on the true triaxial core holder 1, and each heating plate is connected to the temperature control system 2.

[0067] The injection system includes multiple injection pumps 5 connected in parallel. Using multiple injection pumps in parallel can improve the injection efficiency and make the pumping rate and pressure more stable during the injection process. Figure 1 For the simplified diagram of the equipment, the injection system has four injection pumps 5. Three of the injection pumps are confining pressure injection pumps in the x, y, and z directions, respectively. The fourth injection pump is a fracturing fluid injection pump. One end of the confining pressure injection pump is connected to the confining pressure plate in the clamp, and the other end is connected to the servo control system 4. One end of the fracturing fluid injection pump is connected to the core wellbore in the true triaxial core clamp 1, and the other end is connected to the servo control system 4. Figure 1 The cross-section of the Zhongzhen triaxial core clamp 1 is the fixing device for fixing the confining pressure plate. Because the core is square and the cross-section of the clamp is circular, a device is needed to fill the gap and ensure the stability of the confining pressure plate when pressure is applied.

[0068] The servo control system 4 is used to control the injection pump and the temperature control system 2. A computer 3 is also connected between the temperature control system 2 and the servo system 4. The computer is used to control the temperature control system 2 and to control the injection and stop of the injection pump.

[0069]

Example 2

[0070] The experimental core was loaded onto a true triaxial core holder 1. Three confining pressure injection pumps were connected to the confining pressure plate on the true triaxial core holder 1. The fracturing fluid injection pump was connected to the core wellbore inside the true triaxial core holder 1. The computer 3 controlled the heating, confining pressure loading (by injecting confining pressure oil into the confining pressure plate to apply confining pressure to the rock sample, a mature existing technology that will not be elaborated here), and fracturing fluid injection to complete the fracturing experiment.

[0071] An embodiment of the experimental method for creating complex cracks by controlled temperature cyclic loading provided by the present invention is as follows:

[0072]

Example 3

[0073] The experimental methods include:

[0074] S1: Before conducting the experiment, use an acoustic emission instrument to detect the natural cracks in the rock sample. This way, after the experiment, it can be determined which cracks are propagated by pressure fracturing and which cracks are inherent to the rock sample.

[0075] S2: Load the rock sample into the true triaxial core holder in the above system, and heat it from room temperature using a segmented heating method to raise the rock sample from room temperature to the target temperature, assuming the target temperature is T℃. Specifically, this includes:

[0076] S21: The dry hot rock sample is heated from room temperature to H1℃ and then kept warm for a duration of t1, H1 = 0.1T.

[0077] S22: The dry hot rock sample is heated from H1℃ to H2℃ and then kept warm for a holding time of t2, where H2 = H1 + 0.1T;

[0078] S23: The dry hot rock sample is heated from H2℃ to H3℃ and then kept warm for a holding time of t3, where H3 = H2 + 0.2T;

[0079] S24: The dry hot rock sample is heated from H3℃ to H4℃ and then kept warm for a duration of t4, H4 = H3 + 0.5T.

[0080] S25: The dry hot rock sample is heated from H4℃ to T℃ and then kept warm for t5.

[0081] In this embodiment, setting four heating stages ensures more uniform heating of the rock core, avoiding inaccurate experimental results due to inconsistent temperatures inside and outside the core. The four stages are an empirical number derived from integrating most rock properties; adjustments can be made based on specific experimental conditions, with more or fewer heating stages.

[0082] Optionally, the heat preservation time can be set as follows:

[0083] The heat preservation time t1 is set to 4 hours;

[0084] The heat preservation time t2 is set to 8 hours;

[0085] The heat preservation time t3 is set to 10 hours;

[0086] The heat preservation time t4 is set to 12 hours;

[0087] The heat preservation time t5 is set to 4 hours.

[0088] The above-mentioned heat preservation time is a time setting suitable for most rocks, which was integrated through multiple experiments. It can also be set by the user. The heat preservation time can be determined according to the specific properties of the rock.

[0089] S3: After heating to the target temperature and performing heat preservation and stabilization treatment (the rock sample is kept at a constant temperature using a temperature control system), the rock sample is loaded using a cyclic loading method, specifically including:

[0090] S31: First, apply a predetermined confining pressure to the rock sample using a confining pressure injection pump;

[0091] S32. Inject fracturing fluid through a fracturing fluid injection pump to load the rock sample to A MPa (loading to A MPa means the inlet pressure during injection), and unload after reaching this value.

[0092] The temperature of the injected fracturing fluid is Tw. At this time, a temperature difference ΔT = T - Tw is generated between the fracturing fluid and the rock sample; where A = 0.6p b , and the formula for the fracture pressure is: p b = 3σ h -σ H +σ f -p o .

[0093] σ h is the minimum horizontal principal stress, MPa; σ H is the maximum horizontal principal stress, MPa; σ f is the unidirectional tensile stress strength of the rock, that is, the fracture pressure, MPa. p o is the overburden pressure, MPa. The magnitudes of the principal stresses of the experiment are determined by the specific values of the actual formation environment. The unidirectional tensile stress strength of the rock can be measured by existing equipment and methods, which will not be elaborated here.

[0094] S33. After repeating step S32 N times, directly pressurize until the rock fractures. The fracture pressure is B MPa, B < A, and the fracturing experiment ends;

[0095] Each time step S32 is repeated, it is loaded to A MPa again after unloading and then unloaded. At this time, in the rock sample, the injection of the fracturing fluid will generate a temperature difference ΔT = T - Tw again;

[0096]

[0097] The total duration of the experiment is set artificially, and the single loading duration is the time required to load to A MPa.

[0098] Examples for verifying the method of the present invention are as follows:

[0099]

Example 4

[0100] Before the experiment, first use an acoustic emission instrument to detect the natural fractures existing in the rock sample itself, and select two rock samples with similar fracture development degrees. During the experiment, one rock sample is experimented by the method of high-temperature direct fracturing (it can also be used Figure 1The system shown injects fracturing fluid through an injection pump for fracturing, instead of using a cyclic loading method (i.e., no multiple loading and unloading, but direct loading until the core fractures, at which point the fracturing curve drops sharply). Another sample was tested using the same high-temperature cyclic loading method. After the experiment, both rock samples were tested again using an acoustic emission instrument. By comparing the acoustic emission data of the two rock samples and the distribution of the tracer after fracturing, it can be determined that in the rock sample fracturing by the cyclic loading method of this invention, the activated natural fractures and hydraulic fractures communicate with each other to form a complex fracturing network. The communication between hydraulic fractures and natural fractures is more complex than that of the rock sample fracturing by direct fracturing.

[0101] In actual geothermal fracturing, injection is stopped after the fracturing fluid reaches the set value, and injection resumes after the pressure decreases, thus achieving cyclic loading. Actual fracturing processes can use cyclic loading without the need for staged heating, because underground rocks are inherently hot. The staged heating in the above experiment was to ensure uniform heating of the rock sample, more realistically simulating rock temperature and thus ensuring the accuracy of the experimental results.

[0102] An application embodiment of the present invention is as follows:

[0103] Example 5

[0104] This experiment used 300mm×300mm×300mm dry hot rock samples. Two samples with relatively low fracture development were selected and compared using different loading methods for fracturing. The experimental conditions were 200℃, minimum horizontal principal stress, maximum horizontal principal stress, and overlying stratum pressure of 7MPa, 13MPa, and 15MPa, respectively; fracturing fluid viscosity of 1mPa·s; and flow rate of 30ml / min. Rock sample 1 was injected using the direct loading method, while rock sample 2 was injected using the cyclic loading method of this invention.

[0105] Before the experiment, acoustic emission instruments were used to monitor the natural fractures within the two rock samples. Rock sample 1 was first heated and then subjected to fracturing using a direct loading method, and the pressure curve is shown below. Figure 2 As shown, the fracturing results are as follows: Figure 4 As shown.

[0106] Rock sample 2 was loaded into the true triaxial core holder of this system and heated from room temperature to a target temperature of 200℃.

[0107] The dry hot rock sample was heated from room temperature to 20°C and then kept at that temperature for 4 hours.

[0108] After being heated from 20℃ to 40℃, the temperature is kept warm for 8 hours.

[0109] After being heated from 40℃ to 80℃, the temperature is kept warm for 10 hours.

[0110] Heating from 80℃ to 180℃ and then holding for 12 hours;

[0111] The sample is heated from 180℃ to 200℃ and then kept warm for 4 hours.

[0112] After the heating is uniform, loading begins.

[0113] The fracturing fluid temperature injected into the rock sample was 25℃. At this temperature, a temperature difference ΔT = 200 - 25℃ = 175℃ was generated between the fracturing fluid and the rock sample. The pressure was increased to 10MPa, then unloaded. After unloading, the pressure was increased to 10MPa again and unloaded. This process was repeated 10 times. Finally, the pressure was directly increased until the rock fractured, and the fracturing experiment was completed. The actual fracturing curve of rock sample 2 is shown in the figure. Figure 3 As shown, the fracturing curve illustrates the pressure changes during fracturing, reflecting the pressure changes during cyclic loading, the process of gradually depressurizing after reaching a certain pressure value, and then repressurizing. The fracturing results of rock sample 2 are shown below. Figure 5 As shown.

[0114] from Figure 4 , Figure 5 The experimental results clearly show that rock sample 1, subjected to direct loading and fracturing using traditional methods, has only one hydraulic fracture of a single morphology, such as... Figure 4 As shown, rock sample 2, subjected to cyclic loading fracturing according to the present invention, connects two natural fractures, forming a complex fracture network, as shown in the figure. Figure 5 As shown, the cyclic loading fracturing method can produce more complex cracks, thus verifying the feasibility of this method.

[0115] This invention induces fatigue damage in rocks by controlling the temperature difference generated when fracturing fluid is pumped in, thereby stimulating naturally occurring micro-fractures within the rock and connecting artificially created hydraulic fractures with these natural fractures to produce a complex network of fracturing fractures. Based on the experimental results, the technology for geothermal resource redevelopment can be specifically improved, laying the foundation for the development of hot dry rock resources.

[0116] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0117] In the description of this invention, unless otherwise stated, the terms "upper," "lower," "left," "right," "inner," "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0118] Finally, it should be noted that the above technical solution is only one embodiment of the present invention. For those skilled in the art, based on the application methods and principles disclosed in the present invention, it is easy to make various types of improvements or modifications, and not limited to the methods described in the above specific embodiments of the present invention. Therefore, the methods described above are only preferred and have no limiting significance.

Claims

1. An experimental system for creating complex cracks by temperature-controlled cyclic loading, characterized in that: The system includes: a true triaxial core holder, an injection system, a temperature control system, and a servo control system; The injection system and temperature control system are respectively connected to the true triaxial core holder; Meanwhile, the injection system and the temperature control system are respectively connected to the servo control system; The injection system includes multiple injection pumps, namely a confining pressure injection pump and a fracturing fluid injection pump; One end of the confining pressure injection pump is connected to the confining pressure plate in the true triaxial core holder, and the other end is connected to the servo control system. One end of the fracturing fluid injection pump is connected to the core wellbore in the true triaxial core holder, and the other end is connected to the servo control system; The fracturing fluid injection pump uses a cyclic loading method to load the rock sample. That is, fracturing fluid is injected into the rock sample through the fracturing fluid injection pump to load it to a set pressure. After the set pressure is reached, the injection is stopped and the sample is unloaded. After the pressure drops to 0, the injection is continued to reach the set pressure. After N loading and unloading cycles, the rock is directly pressurized until it fractures. The temperature of the injected fracturing fluid is Tw, and the temperature difference ΔT between the fracturing fluid and the rock sample is ΔT = T - Tw, where T is the target temperature.

2. The experimental system for creating complex cracks by temperature-controlled cyclic loading according to claim 1, characterized in that: Multiple heating plates are installed on the true triaxial core holder, and each heating plate is connected to the temperature control system.

3. The experimental system for creating complex cracks by temperature-controlled cyclic loading according to claim 1, characterized in that: A computer is also connected between the temperature control system and the servo control system.

4. An experimental method for inducing complex cracks by temperature-controlled cyclic loading, employing the experimental system for inducing complex cracks by temperature-controlled cyclic loading as described in any one of claims 1-3, characterized in that: The method includes: S1: Detects natural cracks present in the rock sample; S2: Load the rock sample into the true triaxial core holder and heat the rock sample; S3: The rock sample is loaded using a cyclic loading method.

5. The experimental method for creating complex cracks by temperature-controlled cyclic loading according to claim 4, characterized in that: The heating operation of the rock sample in step S2 includes: The rock sample is heated from room temperature to the target temperature using a segmented heating method.

6. The experimental method for creating complex cracks by temperature-controlled cyclic loading according to claim 5, characterized in that: The operation of heating the rock sample from room temperature to the target temperature using a segmented heating method includes: Assume the target heating temperature is T℃; S21: The rock sample is heated from room temperature to H1℃ and then kept at that temperature for t1. S22: The rock sample is heated from H1℃ to H2℃ and then kept at that temperature for t2. S23: The rock sample is heated from H2℃ to H3℃ and then kept at that temperature for t3. S24: The rock sample is heated from H3℃ to H4℃ and then kept at that temperature for t4. S25: After heating the rock sample from H4℃ to T℃, it is kept warm for t5.

7. The experimental method for creating complex cracks by temperature-controlled cyclic loading according to claim 6, characterized in that: H1 = 0.1T; H2 = H1 + 0.1T; H3 = H2 + 0.2T; H4 = H3 + 0.5T.

8. The experimental method for creating complex cracks by temperature-controlled cyclic loading according to claim 6, characterized in that: t1 is 4 hours; t2 is 8 hours; t3 is 10 hours; t4 is 12 hours; t5 is 4 hours.

9. The experimental method for creating complex cracks by temperature-controlled cyclic loading according to claim 4, characterized in that: The operation of step S3 includes: S31, apply a set confining pressure to the rock sample using a confining pressure injection pump; S32, fracturing fluid is injected using a fracturing fluid injection pump to load the rock sample to A MPa, and then unloaded; S33: After repeating step S32 N times, apply pressure directly until the rock sample fractures.

10. The experimental method for creating complex cracks by temperature-controlled cyclic loading according to claim 9, characterized in that: A=0.6p b p b =3σ h -s H +s f -p o Where, σ h For the minimum horizontal principal stress, σ H For the maximum horizontal principal stress, σ f p represents the uniaxial tensile stress strength of the rock. o This represents the pressure of the overlying rock strata.

11. The experimental method for creating complex cracks by temperature-controlled cyclic loading according to claim 9, characterized in that: The total experiment duration is a set value; The single loading time is the time required to load a rock sample to A MPa.

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