Test method for hydraulic fracturing of rock in true triaxial stress state

Abstract

The invention discloses a test method for hydraulic fracturing of rock in a true triaxial stress state, which is specifically implemented according to the following steps: step 1, processing a rock block into a rock sample, and carrying out trepanning treatment to form a sample model; 2, the sample model is placed on a true triaxial device, an X + pressure head, an X-pressure head, a Y + pressure head, a Y pressure head-, a Z + pressure head and a Z-pressure head of the true triaxial device are adjusted to be consistent in center, and the X + pressure head, the X-pressure head, the Y + pressure head, the Y pressure head-, the Z + pressure head and the Z-pressure head are connected with the water pressure applying pipe fitting; step 3, applying a load to the sample model, and controlling the true triaxial device to respectively apply a preload stage load, a second stage load and a third stage load in X, Y and Z directions by a servo motor; 4, the sample model exerts pulse circulating water pressure while maintaining third-stage load exerting; 5, unloading all loads and pressures of the sample model, and ending the test; the problem that the water pressure applying frequency and magnitude in hydraulic fracturing engineering in the prior art do not conform to the actual working condition of the well bottom is solved.

Description

Technical Field

[0001] The invention belongs to the technical field of hydraulic fracturing and relates to a test method for hydraulic fracturing of rocks under a true triaxial stress state. Background Art

[0002] Tight oil and gas resources are abundant and widely distributed, and have gradually become a hot spot for unconventional reservoir development in my country. Due to their low porosity, low permeability and complex formation conditions, they must undergo hydraulic fracturing for commercial development. With the increase in formation depth and the influence of human factors such as hydraulic grooving, the reservoir rocks are in a complex three-dimensional stress state, namely the true triaxial stress state. Therefore, exploring hydraulic fracturing under the true triaxial stress state is of great practical significance. Currently, large-scale volume fracturing construction faces problems such as difficulty in initiating fractures and high requirements for fracturing equipment. Pulse-cycle hydraulic fracturing can cause fatigue failure in rock masses, reduce fracture pressure, effectively improve mining results, and reduce requirements for fracturing equipment. Therefore, laboratory research on pulse-cycle hydraulic fracturing is of great practical significance. The present invention proposes a test method for hydraulic fracturing of rocks under true triaxial stress state. Compared with the existing test method, it proposes to use a pressure rod to apply pulsed cyclic water pressure. The advantage is that the frequency and magnitude of water pressure application are more consistent with the actual bottom hole working conditions in hydraulic fracturing projects, and the pressure rod impact speed can be changed to simulate the exchange of high and low water pressure without suspending the application of water pressure, so as to better study the pulsed cyclic hydraulic fracturing mechanism. Summary of the Invention

[0003] The purpose of the present invention is to provide a test method for hydraulic fracturing of rocks under true triaxial stress state, which solves the problem that the frequency and magnitude of water pressure applied in hydraulic fracturing projects in the prior art do not conform to the actual working conditions at the bottom of the well.

[0004] The technical solution adopted by the present invention is a test method for hydraulic fracturing of rocks under true triaxial stress state, which is specifically implemented according to the following steps: Step 1: Processing a rock block into a rock sample and performing a hole-opening process to form a sample model; Step 2: Place the sample model on a true triaxial device, adjust the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head of the true triaxial device to the same center, and connect the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head to the water pressure application pipe fittings respectively; Step 3: Apply load to the specimen model, and the servo motor controls the true three-axis device in the X, Y, and Z directions to apply load in the preload stage, the second stage, and the third stage respectively; Step 4: While maintaining the load of the third stage, the specimen model is subjected to a pulsed cyclic water pressure; Step 5: Unload all loads and pressures on the specimen model and the test ends.

[0005] The present invention is also characterized in that: Step 1 is as follows: Process the rock block into a standard cubic rock sample, drill a hydraulic fracturing hole perpendicular to the center of any face of the sample to form a sample model, and clean the sample model with acetone or alcohol. Wait for the sample model to dry before proceeding to the next step; The ratio of the diameter and depth of the hydraulic fracturing hole in the sample model is calculated using the π theorem.

[0006] Step 2 is as follows: Place the hydraulic fracturing hole of the sample model on the Y-head of the true triaxial device, and lift the Y-head until it is close to the bottom of the hole, ensuring that there is no pressure on the sample model; Place the X+ pressure head, X- pressure head, Y+ pressure head, Z+ pressure head, and Z- pressure head of the true triaxial device close to the specimen model, ensure that the X+ pressure head, X- pressure head, Y+ pressure head, Z+ pressure head, and Z- pressure head have no pressure applied to the specimen model, ensure that the center of each pressure head is consistent, connect each pressure head to the water pressure application pipe fitting, check the position of each pressure head and water pressure application pipe fitting, and then proceed to the next step.

[0007] Step 3 is as follows: Step 3.1, Preload Phase: The servo motor controls the true three-axis device to symmetrically apply a load of 9.5 to 10.5 kN to the specimen model in the X, Y, and Z directions at a rate of 0.095 to 0.105 kn / s. After the load is applied, the load is stable for 10 to 30 seconds. Step 3.2, second stage: The force control mode of the true triaxial device is used to apply the second stage load to the second preset stress state to simulate the in-situ stress of the rock before the hydraulic fracturing hole is excavated. The second preset stress state is the maximum principal stress σ1 applied in the Y direction, the intermediate principal stress σ2 applied in the X direction, and the minimum principal stress σ3 applied in the Z direction. Among them, the maximum pressure in the Y direction is not more than 4000kN, the maximum pressure in the X direction is not more than 4000kN, and the maximum pressure in the Z direction is not more than 2700kN. Step 3.3, third stage: The force control mode of the true triaxial device was used to apply the third stage load to the third preset stress state to simulate the stress change after the hydraulic fracturing hole was excavated. The third preset stress state was that the true triaxial device alone controlled the maximum principal stress σ1 in the Y direction to unload at a speed of 0.095~0.105kN / s, and unloaded to 50~60% of the second preset stress state in this direction.

[0008] Step 4 is as follows: Maintaining the stress state of the specimen model in the third stage in the X, Y, and Z directions, the servo motor controls the true three-axis device to load the hydraulic piston in the hydraulic pressure application fitting to the preset pore pressure value at a speed of 0.095-0.105 mm / s. The pressure rod in the hydraulic pressure application fitting is started, and the pressure rod is first moved to the position of the hydraulic piston so that the end of the pressure rod contacts the hydraulic piston. The pressure rod impact-related parameters are set according to the predetermined frequency and the pulse cycle water pressure peak and water pressure valley values. The pressure rod impacts the hydraulic piston until the specimen model is destroyed, and the water pressure, flow rate, and strain-related data of the hydraulic fracturing hole in the X, Y, and Z directions are recorded.

[0009] The predetermined pore pressure position is determined according to the pore pressure value of the hydraulic fracturing hole set by the force sensor; The specific manifestation of sample failure is: when the deformation of any XYZ direction of the sample model reaches 3mm, the sample is considered to be broken; The preset frequency is 0~50Hz; The peak and valley values ​​of water pressure are determined by preliminary experiments. In the preliminary experiments, the impact stroke of the pressure rod of the true triaxial device is increased by 0.1 mm increments. The functional relationship between the pressure rod stroke and the peak and valley values ​​of water pressure is established: F 压杆行程 = (water pressure 峰值 -Water pressure 谷值 ) / 2, thus obtaining the water pressure peak value and water pressure valley value.

[0010] Step 5 is as follows: Close the water pressure valve to stop the water pressure loading. Use the servo motor to control the true triaxial device to symmetrically and synchronously reduce the load stress in the X, Y, and Z directions to 0 at a speed of 0.095-0.105 kN / s. Record the stress and strain data of the true triaxial device in the X, Y, and Z directions. Remove the specimen model, observe its morphology, clean the debris from each pressure head and the specimen model in the pressure chamber, turn off the power, and complete the test.

[0011] The beneficial effects of the present invention are: The core advantage of the experimental method for hydraulic fracturing of rocks under true triaxial stress proposed in this invention lies in simulating the actual formation environment through true triaxial stress and utilizing dynamic loading technology of pressure rods to achieve highly realistic and flexible pulse cyclic water pressure application. In particular, the innovative feature of dynamically switching between high and low water pressures without interruption during the test by changing the impact velocity of the pressure rod enables this method to simulate field pulse fracturing conditions with unprecedented continuity and precision. This provides a powerful and efficient experimental means for in-depth study of the mechanisms by which pulse-cycle hydraulic fracturing reduces fracture pressure, promotes fatigue damage, and improves the transformation effect of tight reservoirs. This not only improves the engineering relevance of the experiment, but also greatly enhances the efficiency and depth of research. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 Schematic diagram of the position of the hydraulic fracturing wellbore on the YZ plane in the test method for hydraulic fracturing of rock under true triaxial stress state of the present invention; Figure 2 Schematic diagram of the position of the hydraulic fracturing wellbore on the XZ plane in the test method for hydraulic fracturing of rock under true triaxial stress state of the present invention; Figure 3 Schematic diagram of the position of the hydraulic fracturing wellbore on the XY plane in the test method for hydraulic fracturing of rock under true triaxial stress state of the present invention; Figure 4 It is a schematic diagram of the true triaxial stress on the rock sample in the test method of hydraulic fracturing of rock under the true triaxial stress state of the present invention; Figure 5 It is a water pressure-time curve diagram of a hydraulic fracturing hole in a test method for hydraulic fracturing of rock under a true triaxial stress state according to the present invention. DETAILED DESCRIPTION

[0013] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0014] The test method for hydraulic fracturing of rock under true triaxial stress state is carried out in the following steps: Step 1: Processing a rock block into a rock sample and performing a hole-opening process to form a sample model; Step 1 is as follows: like Figure 1 As shown, Figure 2 As shown, Figure 3 As shown, the rock block is processed into a standard cubic rock sample. The size of the rock sample is determined by the size of the indenter. The size of the rock sample can be determined by changing the size of the indenter. A hydraulic fracturing hole is drilled at the center of any surface of the sample in a direction perpendicular to the surface to form a sample model. The sample model is then cleaned with acetone or alcohol. Wait until the sample model is dry before proceeding to the next step. The diameter and depth of the hydraulic fracturing hole of the sample model are calculated by the π theorem and proportionally reduced compared with the actual wellbore. The diameter and depth of the hydraulic fracturing hole should be smaller than the rock sample size. Step 2: Place the sample model on a true triaxial device, adjust the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head of the true triaxial device to the same center, and connect the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head to the water pressure application pipe fittings respectively; Step 2 is as follows: Place the hydraulic fracturing hole of the sample model on the Y-head of the true triaxial device, and lift the Y-head until it is close to the bottom of the hole, ensuring that there is no pressure on the sample model; Place the X+, X-, Y+, Z+, and Z- indenters of the true triaxial device close to the specimen model, ensuring that no pressure is applied to the specimen model by the X+, X-, Y+, Z+, and Z- indenters. Ensure that the centers of the indenters are consistent. Connect the indenters to the water pressure fittings to ensure that stress is applied to the center of the specimen to avoid eccentric loading. Check the positions of the indenters and water pressure fittings before proceeding to the next step. Turn on the hydraulic power source, computer and oscilloscope, and observe whether there are any abnormalities in all displacement sensors, force sensors and water pressure application pipes. If there are no abnormalities, proceed to the next step. If there are abnormalities, repair them until there are no abnormalities, then proceed to the next step. Step 3: Apply load to the specimen model, and the servo motor controls the true three-axis device in the X, Y, and Z directions to apply load in the preload stage, the second stage, and the third stage respectively; Step 3 is as follows: Step 3.1, Preload Phase: The servo motor controls the true three-axis device to symmetrically apply a load of 9.5 to 10.5 kN to the specimen model in the X, Y, and Z directions at a rate of 0.095 to 0.105 kn / s. After the load is applied, the load is stable for 10 to 30 seconds. Step 3.2, second stage: like Figure 4 As shown, the force control mode of the true triaxial device is used to apply the second stage load to the second preset stress state to simulate the in-situ stress of the rock before the hydraulic fracturing hole is excavated. The second preset stress state is that the maximum principal stress σ1 is applied in the Y direction, the intermediate principal stress σ2 is applied in the X direction, and the minimum principal stress σ3 is applied in the Z direction. Among them, the maximum pressure in the Y direction is not more than 4000 kN, the maximum pressure in the X direction is not more than 4000 kN, and the maximum pressure in the Z direction is not more than 2700 kN. Step 3.3, third stage: The force control mode of the true triaxial device was used to apply the third stage load to the third preset stress state to simulate the stress change after the hydraulic fracturing hole was excavated. The third preset stress state was that the true triaxial device alone controlled the maximum principal stress σ1 in the Y direction to unload at a rate of 0.095-0.105 kN / s, unloading to 50-60% of the second preset stress state in this direction; Step 4: While maintaining the load of the third stage, the specimen model is subjected to a pulsed cyclic water pressure; Step 4 is as follows: Maintain the stress state of the specimen model in the third stage in the XYZ directions. The servo motor controls the true three-axis device to load the hydraulic piston in the hydraulic pressure application pipe to the preset pore pressure value at a speed of 0.095~0.105mm / s. Start the pressure rod in the hydraulic pressure application pipe and first move the pressure rod to the position of the hydraulic piston so that the end of the pressure rod contacts the hydraulic piston. Figure 5 As shown, the pressure rod impact related parameters are set according to the predetermined frequency and the pulse cycle water pressure peak and water pressure valley values. The pressure rod impacts the hydraulic piston until the sample model is destroyed, and the water pressure, flow rate, and strain related data of the hydraulic fracturing hole in the XYZ directions are recorded; The predetermined pore pressure position is determined according to the pore pressure value of the hydraulic fracturing hole set by the force sensor; The specific manifestation of sample failure is: when the deformation of any XYZ direction of the sample model reaches 3mm, the sample is considered to be broken; The preset frequency is 0~50Hz; The peak and valley values ​​of water pressure are determined by preliminary experiments. In the preliminary experiments, the impact stroke of the pressure rod of the true triaxial device is increased by 0.1 mm increments. The functional relationship between the pressure rod stroke and the peak and valley values ​​of water pressure is established: F 压杆行程 = (water pressure 峰值 -Water pressure 谷值 ) / 2, thus obtaining the peak value and valley value of water pressure; Step 5: Unload all loads and pressures on the specimen model, and the test ends; Step 5 is as follows: Close the water pressure valve to stop the water pressure loading. Use the servo motor to control the true triaxial device to symmetrically and synchronously reduce the load stress in the X, Y, and Z directions to 0 at a speed of 0.095-0.105 kN / s. Record the stress and strain data of the true triaxial device in the X, Y, and Z directions. Remove the specimen model, observe its morphology, clean the debris from each pressure head and the specimen model in the pressure chamber, turn off the power, and complete the test.

[0015] Example 1: This embodiment proposes a test method for hydraulic fracturing of rocks under true triaxial stress, which is specifically implemented according to the following steps: Step 1: Processing a rock block into a rock sample and performing a hole-opening process to form a sample model; Step 2: Place the sample model on a true triaxial device, adjust the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head of the true triaxial device to the same center, and connect the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head to the water pressure application pipe fittings respectively; Step 3: Apply load to the specimen model, and the servo motor controls the true three-axis device in the X, Y, and Z directions to apply load in the preload stage, the second stage, and the third stage respectively; Step 4: While maintaining the load of the third stage, the specimen model is subjected to a pulsed cyclic water pressure; Step 5: Unload all loads and pressures on the specimen model and the test ends.

[0016] Example 2: This embodiment proposes a test method for hydraulic fracturing of rocks under true triaxial stress, which is specifically implemented according to the following steps: Step 1: Processing a rock block into a rock sample and performing a hole-opening process to form a sample model; Step 1 is as follows: like Figure 1 As shown, Figure 2 As shown, Figure 3 As shown, the rock block is processed into a standard cubic rock sample. The size of the rock sample is determined by the size of the indenter. The size of the rock sample can be determined by changing the size of the indenter. A hydraulic fracturing hole is drilled at the center of any surface of the sample in a direction perpendicular to the surface to form a sample model. The sample model is then cleaned with acetone or alcohol. Wait until the sample model is dry before proceeding to the next step. The diameter and depth of the hydraulic fracturing hole of the sample model are calculated by the π theorem and proportionally reduced compared with the actual wellbore. The diameter and depth of the hydraulic fracturing hole should be smaller than the rock sample size. Step 2: Place the sample model on a true triaxial device, adjust the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head of the true triaxial device to the same center, and connect the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head to the water pressure application pipe fittings respectively; Step 3: Apply load to the specimen model, and the servo motor controls the true three-axis device in the X, Y, and Z directions to apply load in the preload stage, the second stage, and the third stage respectively; Step 4: While maintaining the load of the third stage, the specimen model is subjected to a pulsed cyclic water pressure; Step 5: Unload all loads and pressures on the specimen model and the test ends.

[0017] Example 3: This embodiment proposes a test method for hydraulic fracturing of rocks under true triaxial stress, which is specifically implemented according to the following steps: Step 1: Processing a rock block into a rock sample and performing a hole-opening process to form a sample model; Step 1 is as follows: like Figure 1 As shown, Figure 2 As shown, Figure 3As shown, the rock block is processed into a standard cubic rock sample. The size of the rock sample is determined by the size of the indenter. The size of the rock sample can be determined by changing the size of the indenter. A hydraulic fracturing hole is drilled at the center of any surface of the sample in a direction perpendicular to the surface to form a sample model. The sample model is then cleaned with acetone or alcohol. Wait until the sample model is dry before proceeding to the next step. The diameter and depth of the hydraulic fracturing hole of the sample model are calculated by the π theorem and proportionally reduced compared with the actual wellbore. The diameter and depth of the hydraulic fracturing hole should be smaller than the rock sample size. Step 2: Place the sample model on a true triaxial device, adjust the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head of the true triaxial device to the same center, and connect the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head to the water pressure application pipe fittings respectively; Step 2 is as follows: Place the hydraulic fracturing hole of the sample model on the Y-head of the true triaxial device, and lift the Y-head until it is close to the bottom of the hole, ensuring that there is no pressure on the sample model; Place the X+, X-, Y+, Z+, and Z- indenters of the true triaxial device close to the specimen model, ensuring that no pressure is applied to the specimen model by the X+, X-, Y+, Z+, and Z- indenters. Ensure that the centers of the indenters are consistent. Connect the indenters to the water pressure fittings to ensure that stress is applied to the center of the specimen to avoid eccentric loading. Check the positions of the indenters and water pressure fittings before proceeding to the next step. Turn on the hydraulic power source, computer and oscilloscope, and observe whether there are any abnormalities in all displacement sensors, force sensors and water pressure application pipes. If there are no abnormalities, proceed to the next step. If there are abnormalities, repair them until there are no abnormalities, then proceed to the next step. Step 3: Apply load to the specimen model, and the servo motor controls the true three-axis device in the X, Y, and Z directions to apply load in the preload stage, the second stage, and the third stage respectively; Step 4: While maintaining the load of the third stage, the specimen model is subjected to a pulsed cyclic water pressure; Step 5: Unload all loads and pressures on the specimen model and the test ends.

[0018] Example 4: This embodiment proposes a test method for hydraulic fracturing of rocks under true triaxial stress, which is specifically implemented according to the following steps: Step 1: Processing a rock block into a rock sample and performing a hole-opening process to form a sample model; Step 1 is as follows: like Figure 1 As shown, Figure 2 As shown, Figure 3 As shown, the rock block is processed into a standard cubic rock sample. The size of the rock sample is determined by the size of the indenter. The size of the rock sample can be determined by changing the size of the indenter. A hydraulic fracturing hole is drilled at the center of any surface of the sample in a direction perpendicular to the surface to form a sample model. The sample model is then cleaned with acetone or alcohol. Wait until the sample model is dry before proceeding to the next step. The diameter and depth of the hydraulic fracturing hole of the sample model are calculated by the π theorem and proportionally reduced compared with the actual wellbore. The diameter and depth of the hydraulic fracturing hole should be smaller than the rock sample size. Step 2: Place the sample model on a true triaxial device, adjust the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head of the true triaxial device to the same center, and connect the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head to the water pressure application pipe fittings respectively; Step 2 is as follows: Place the hydraulic fracturing hole of the sample model on the Y-head of the true triaxial device, and lift the Y-head until it is close to the bottom of the hole, ensuring that there is no pressure on the sample model; Place the X+, X-, Y+, Z+, and Z- indenters of the true triaxial device close to the specimen model, ensuring that no pressure is applied to the specimen model by the X+, X-, Y+, Z+, and Z- indenters. Ensure that the centers of the indenters are consistent. Connect the indenters to the water pressure fittings to ensure that stress is applied to the center of the specimen to avoid eccentric loading. Check the positions of the indenters and water pressure fittings before proceeding to the next step. Turn on the hydraulic power source, computer and oscilloscope, and observe whether there are any abnormalities in all displacement sensors, force sensors and water pressure application pipes. If there are no abnormalities, proceed to the next step. If there are abnormalities, repair them until there are no abnormalities, then proceed to the next step. Step 3: Apply load to the specimen model, and the servo motor controls the true three-axis device in the X, Y, and Z directions to apply load in the preload stage, the second stage, and the third stage respectively; Step 3 is as follows: Step 3.1, Preload Phase: The servo motor controls the true three-axis device to symmetrically apply a load of 9.5 to 10.5 kN to the specimen model in the X, Y, and Z directions at a rate of 0.095 to 0.105 kn / s. After the load is applied, the load is stable for 10 to 30 seconds. Step 3.2, second stage: like Figure 4As shown, the force control mode of the true triaxial device is used to apply the second stage load to the second preset stress state to simulate the in-situ stress of the rock before the hydraulic fracturing hole is excavated. The second preset stress state is that the maximum principal stress σ1 is applied in the Y direction, the intermediate principal stress σ2 is applied in the X direction, and the minimum principal stress σ3 is applied in the Z direction. Among them, the maximum pressure in the Y direction is not more than 4000 kN, the maximum pressure in the X direction is not more than 4000 kN, and the maximum pressure in the Z direction is not more than 2700 kN. Step 3.3, third stage: The force control mode of the true triaxial device was used to apply the third stage load to the third preset stress state to simulate the stress change after the hydraulic fracturing hole was excavated. The third preset stress state was that the true triaxial device alone controlled the maximum principal stress σ1 in the Y direction to unload at a rate of 0.095-0.105 kN / s, unloading to 50-60% of the second preset stress state in this direction; Step 4: While maintaining the load of the third stage, the specimen model is subjected to a pulsed cyclic water pressure; Step 5: Unload all loads and pressures on the specimen model and the test ends.

[0019] Example 5: This embodiment proposes a test method for hydraulic fracturing of rocks under true triaxial stress, which is specifically implemented according to the following steps: Step 1: Processing a rock block into a rock sample and performing a hole-opening process to form a sample model; Step 1 is as follows: like Figure 1 As shown, Figure 2 As shown, Figure 3 As shown, the rock block is processed into a standard cubic rock sample. The size of the rock sample is determined by the size of the indenter. The size of the rock sample can be determined by changing the size of the indenter. A hydraulic fracturing hole is drilled at the center of any surface of the sample in a direction perpendicular to the surface to form a sample model. The sample model is then cleaned with acetone or alcohol. Wait until the sample model is dry before proceeding to the next step. The diameter and depth of the hydraulic fracturing hole of the sample model are calculated by the π theorem and proportionally reduced compared with the actual wellbore. The diameter and depth of the hydraulic fracturing hole should be smaller than the rock sample size. Step 2: Place the sample model on a true triaxial device, adjust the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head of the true triaxial device to the same center, and connect the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head to the water pressure application pipe fittings respectively; Step 2 is as follows: Place the hydraulic fracturing hole of the sample model on the Y-head of the true triaxial device, and lift the Y-head until it is close to the bottom of the hole, ensuring that there is no pressure on the sample model; Place the X+, X-, Y+, Z+, and Z- indenters of the true triaxial device close to the specimen model, ensuring that no pressure is applied to the specimen model by the X+, X-, Y+, Z+, and Z- indenters. Ensure that the centers of the indenters are consistent. Connect the indenters to the water pressure fittings to ensure that stress is applied to the center of the specimen to avoid eccentric loading. Check the positions of the indenters and water pressure fittings before proceeding to the next step. Turn on the hydraulic power source, computer and oscilloscope, and observe whether there are any abnormalities in all displacement sensors, force sensors and water pressure application pipes. If there are no abnormalities, proceed to the next step. If there are abnormalities, repair them until there are no abnormalities, then proceed to the next step. Step 3: Apply load to the specimen model, and the servo motor controls the true three-axis device in the X, Y, and Z directions to apply load in the preload stage, the second stage, and the third stage respectively; Step 3 is as follows: Step 3.1, Preload Phase: The servo motor controls the true three-axis device to symmetrically apply a load of 9.5 to 10.5 kN to the specimen model in the X, Y, and Z directions at a rate of 0.095 to 0.105 kn / s. After the load is applied, the load is stable for 10 to 30 seconds. Step 3.2, second stage: like Figure 4 As shown, the force control mode of the true triaxial device is used to apply the second stage load to the second preset stress state to simulate the in-situ stress of the rock before the hydraulic fracturing hole is excavated. The second preset stress state is that the maximum principal stress σ1 is applied in the Y direction, the intermediate principal stress σ2 is applied in the X direction, and the minimum principal stress σ3 is applied in the Z direction. Among them, the maximum pressure in the Y direction is not more than 4000 kN, the maximum pressure in the X direction is not more than 4000 kN, and the maximum pressure in the Z direction is not more than 2700 kN. Step 3.3, third stage: The force control mode of the true triaxial device was used to apply the third stage load to the third preset stress state to simulate the stress change after the hydraulic fracturing hole was excavated. The third preset stress state was that the true triaxial device alone controlled the maximum principal stress σ1 in the Y direction to unload at a rate of 0.095-0.105 kN / s, unloading to 50-60% of the second preset stress state in this direction; Step 4: While maintaining the load of the third stage, the specimen model is subjected to a pulsed cyclic water pressure; Step 4 is as follows: Maintain the stress state of the specimen model in the third stage in the XYZ directions. The servo motor controls the true three-axis device to load the hydraulic piston in the hydraulic pressure application pipe to the preset pore pressure value at a speed of 0.095~0.105mm / s. Start the pressure rod in the hydraulic pressure application pipe and first move the pressure rod to the position of the hydraulic piston so that the end of the pressure rod contacts the hydraulic piston. Figure 5 As shown, the pressure rod impact related parameters are set according to the predetermined frequency and the pulse cycle water pressure peak and water pressure valley values. The pressure rod impacts the hydraulic piston until the sample model is destroyed, and the water pressure, flow rate, and strain related data of the hydraulic fracturing hole in the XYZ directions are recorded; The predetermined pore pressure position is determined according to the pore pressure value of the hydraulic fracturing hole set by the force sensor; The specific manifestation of sample failure is: when the deformation of any XYZ direction of the sample model reaches 3mm, the sample is considered to be broken; The preset frequency is 0~50Hz; The peak and valley values ​​of water pressure are determined by preliminary experiments. In the preliminary experiments, the impact stroke of the pressure rod of the true triaxial device is increased by 0.1 mm increments. The functional relationship between the pressure rod stroke and the peak and valley values ​​of water pressure is established: F 压杆行程 = (water pressure 峰值 -Water pressure 谷值 ) / 2, thus obtaining the peak value and valley value of water pressure; Step 5: Unload all loads and pressures on the specimen model and the test ends.

[0020] Example 6: This embodiment proposes a test method for hydraulic fracturing of rocks under true triaxial stress, which is specifically implemented according to the following steps: Step 1: Processing a rock block into a rock sample and performing a hole-opening process to form a sample model; Step 1 is as follows: like Figure 1 As shown, Figure 2 As shown, Figure 3 As shown, the rock block is processed into a standard cubic rock sample. The size of the rock sample is determined by the size of the indenter. The size of the rock sample can be determined by changing the size of the indenter. A hydraulic fracturing hole is drilled at the center of any surface of the sample in a direction perpendicular to the surface to form a sample model. The sample model is then cleaned with acetone or alcohol. Wait until the sample model is dry before proceeding to the next step. The diameter and depth of the hydraulic fracturing hole of the sample model are calculated by the π theorem and proportionally reduced compared with the actual wellbore. The diameter and depth of the hydraulic fracturing hole should be smaller than the rock sample size. Step 2: Place the sample model on a true triaxial device, adjust the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head of the true triaxial device to the same center, and connect the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head to the water pressure application pipe fittings respectively; Step 2 is as follows: Place the hydraulic fracturing hole of the sample model on the Y-head of the true triaxial device, and lift the Y-head until it is close to the bottom of the hole, ensuring that there is no pressure on the sample model; Place the X+, X-, Y+, Z+, and Z- indenters of the true triaxial device close to the specimen model, ensuring that no pressure is applied to the specimen model by the X+, X-, Y+, Z+, and Z- indenters. Ensure that the centers of the indenters are consistent. Connect the indenters to the water pressure fittings to ensure that stress is applied to the center of the specimen to avoid eccentric loading. Check the positions of the indenters and water pressure fittings before proceeding to the next step. Turn on the hydraulic power source, computer and oscilloscope, and observe whether there are any abnormalities in all displacement sensors, force sensors and water pressure application pipes. If there are no abnormalities, proceed to the next step. If there are abnormalities, repair them until there are no abnormalities, then proceed to the next step. Step 3: Apply load to the specimen model, and the servo motor controls the true three-axis device in the X, Y, and Z directions to apply load in the preload stage, the second stage, and the third stage respectively; Step 3 is as follows: Step 3.1, Preload Phase: The servo motor controls the true three-axis device to symmetrically apply a load of 9.5 to 10.5 kN to the specimen model in the X, Y, and Z directions at a rate of 0.095 to 0.105 kn / s. After the load is applied, the load is stable for 10 to 30 seconds. Step 3.2, second stage: like Figure 4 As shown, the force control mode of the true triaxial device is used to apply the second stage load to the second preset stress state to simulate the in-situ stress of the rock before the hydraulic fracturing hole is excavated. The second preset stress state is that the maximum principal stress σ1 is applied in the Y direction, the intermediate principal stress σ2 is applied in the X direction, and the minimum principal stress σ3 is applied in the Z direction. Among them, the maximum pressure in the Y direction is not more than 4000 kN, the maximum pressure in the X direction is not more than 4000 kN, and the maximum pressure in the Z direction is not more than 2700 kN. Step 3.3, third stage: The force control mode of the true triaxial device was used to apply the third stage load to the third preset stress state to simulate the stress change after the hydraulic fracturing hole was excavated. The third preset stress state was that the true triaxial device alone controlled the maximum principal stress σ1 in the Y direction to unload at a rate of 0.095-0.105 kN / s, unloading to 50-60% of the second preset stress state in this direction; Step 4: While maintaining the load of the third stage, the specimen model is subjected to a pulsed cyclic water pressure; Step 4 is as follows: Maintain the stress state of the specimen model in the third stage in the XYZ directions. The servo motor controls the true three-axis device to load the hydraulic piston in the hydraulic pressure application pipe to the preset pore pressure value at a speed of 0.095~0.105mm / s. Start the pressure rod in the hydraulic pressure application pipe and first move the pressure rod to the position of the hydraulic piston so that the end of the pressure rod contacts the hydraulic piston. Figure 5 As shown, the pressure rod impact related parameters are set according to the predetermined frequency and the pulse cycle water pressure peak and water pressure valley values. The pressure rod impacts the hydraulic piston until the sample model is destroyed, and the water pressure, flow rate, and strain related data of the hydraulic fracturing hole in the XYZ directions are recorded; The predetermined pore pressure position is determined according to the pore pressure value of the hydraulic fracturing hole set by the force sensor; The specific manifestation of sample failure is: when the deformation of any XYZ direction of the sample model reaches 3mm, the sample is considered to be broken; The preset frequency is 0~50Hz; The peak and valley values ​​of water pressure are determined by preliminary experiments. In the preliminary experiments, the impact stroke of the pressure rod of the true triaxial device is increased by 0.1 mm increments. The functional relationship between the pressure rod stroke and the peak and valley values ​​of water pressure is established: F 压杆行程 = (water pressure 峰值 -Water pressure 谷值 ) / 2, thus obtaining the peak value and valley value of water pressure; Step 5: Unload all loads and pressures on the specimen model and the test ends.

[0021] Step 5 is as follows: Close the water pressure valve to stop the water pressure loading. Use the servo motor to control the true triaxial device to symmetrically and synchronously reduce the load stress in the X, Y, and Z directions to 0 at a speed of 0.095-0.105 kN / s. Record the stress and strain data of the true triaxial device in the X, Y, and Z directions. Remove the specimen model, observe its morphology, clean the debris from each pressure head and the specimen model in the pressure chamber, turn off the power, and complete the test.

[0022] Example 7: This embodiment proposes a test method for hydraulic fracturing of rocks under true triaxial stress, which is specifically implemented according to the following steps: Step 1: Processing a rock block into a rock sample and performing a hole-opening process to form a sample model; Step 1 is as follows: like Figure 1 As shown, Figure 2 As shown, Figure 3 As shown, the rock block is processed into a standard cubic rock sample. The size of the rock sample is determined by the size of the indenter. The size of the rock sample can be determined by changing the size of the indenter. A hydraulic fracturing hole is drilled at the center of any surface of the sample in a direction perpendicular to the surface to form a sample model. The sample model is then cleaned with acetone or alcohol. Wait until the sample model is dry before proceeding to the next step. The diameter and depth of the hydraulic fracturing hole of the sample model are calculated by the π theorem and proportionally reduced compared with the actual wellbore. The diameter and depth of the hydraulic fracturing hole should be smaller than the rock sample size. Step 2: Place the sample model on a true triaxial device, adjust the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head of the true triaxial device to the same center, and connect the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head to the water pressure application pipe fittings respectively; Step 2 is as follows: Place the hydraulic fracturing hole of the sample model on the Y-head of the true triaxial device, and lift the Y-head until it is close to the bottom of the hole, ensuring that there is no pressure on the sample model; Place the X+, X-, Y+, Z+, and Z- indenters of the true triaxial device close to the specimen model, ensuring that no pressure is applied to the specimen model by the X+, X-, Y+, Z+, and Z- indenters. Ensure that the centers of the indenters are consistent. Connect the indenters to the water pressure fittings to ensure that stress is applied to the center of the specimen to avoid eccentric loading. Check the positions of the indenters and water pressure fittings before proceeding to the next step. Turn on the hydraulic power source, computer and oscilloscope, and observe whether there are any abnormalities in all displacement sensors, force sensors and water pressure application pipes. If there are no abnormalities, proceed to the next step. If there are abnormalities, repair them until there are no abnormalities, then proceed to the next step. Step 3: Apply load to the specimen model, and the servo motor controls the true three-axis device in the X, Y, and Z directions to apply load in the preload stage, the second stage, and the third stage respectively; Step 3 is as follows: Step 3.1, Preload Phase: The servo motor controls the true three-axis device to symmetrically apply a load of 10.5 kN to the specimen model in the X, Y, and Z directions at a rate of 0.105 kn / s. After the load is applied, the load is stable for 30 seconds. Step 3.2, second stage: like Figure 4 As shown, the force control mode of the true triaxial device is used to apply the second stage load to the second preset stress state to simulate the in-situ stress of the rock before the hydraulic fracturing hole is excavated. The second preset stress state is that the maximum principal stress σ1 is applied in the Y direction, the intermediate principal stress σ2 is applied in the X direction, and the minimum principal stress σ3 is applied in the Z direction. Among them, the maximum pressure in the Y direction is not more than 4000 kN, the maximum pressure in the X direction is not more than 4000 kN, and the maximum pressure in the Z direction is not more than 2700 kN. Step 3.3, third stage: The force control mode of the true triaxial device was used to apply the third stage load to the third preset stress state to simulate the stress change after the hydraulic fracturing hole was excavated. The third preset stress state was that the true triaxial device alone controlled the maximum principal stress σ1 in the Y direction to be unloaded at a speed of 0.105 kN / s, and unloaded to 60% of the second preset stress state in this direction; Step 4: While maintaining the load of the third stage, the specimen model is subjected to a pulsed cyclic water pressure; Step 4 is as follows: Maintaining the stress state of the specimen model in the third stage in the XYZ directions, the servo motor controls the true three-axis device to load the hydraulic piston in the hydraulic pressure application pipe to the preset pore pressure value at a speed of 0.105 mm / s. The pressure rod in the hydraulic pressure application pipe is started and first moved to the position of the hydraulic piston so that the end of the pressure rod contacts the hydraulic piston. Figure 5 As shown, the pressure rod impact related parameters are set according to the predetermined frequency and the pulse cycle water pressure peak and water pressure valley values. The pressure rod impacts the hydraulic piston until the sample model is destroyed, and the water pressure, flow rate, and strain related data of the hydraulic fracturing hole in the XYZ directions are recorded; The predetermined pore pressure position is determined according to the pore pressure value of the hydraulic fracturing hole set by the force sensor; The specific manifestation of sample failure is: when the deformation of any XYZ direction of the sample model reaches 3mm, the sample is considered to be broken; The predetermined frequency is 50 Hz; The peak and valley values ​​of water pressure are determined by preliminary experiments. In the preliminary experiments, the impact stroke of the pressure rod of the true triaxial device is increased by 0.1 mm increments. The functional relationship between the pressure rod stroke and the peak and valley values ​​of water pressure is established: F 压杆行程 = (water pressure 峰值 -Water pressure 谷值 ) / 2, thus obtaining the peak value and valley value of water pressure; Step 5: Unload all loads and pressures on the specimen model, and the test ends; Step 5 is as follows: Close the water pressure valve and stop the water pressure loading. Use the servo motor to control the true triaxial device to symmetrically and synchronously reduce the load stress in the X, Y, and Z directions to 0 at a speed of 0.105 kN / s. Record the stress and strain data of the true triaxial device in the X, Y, and Z directions. Remove the specimen model and observe its morphology. Clean the debris from each pressure head and the specimen model in the pressure chamber. Turn off the power to complete the test.

[0023] Example 8: This embodiment proposes a test method for hydraulic fracturing of rocks under true triaxial stress, which is specifically implemented according to the following steps: Step 1: Processing a rock block into a rock sample and performing a hole-opening process to form a sample model; Step 1 is as follows: like Figure 1 As shown, Figure 2 As shown, Figure 3 As shown, the rock block is processed into a standard cubic rock sample. The size of the rock sample is determined by the size of the indenter. The size of the rock sample can be determined by changing the size of the indenter. A hydraulic fracturing hole is drilled at the center of any surface of the sample in a direction perpendicular to the surface to form a sample model. The sample model is then cleaned with acetone or alcohol. Wait until the sample model is dry before proceeding to the next step. The diameter and depth of the hydraulic fracturing hole of the sample model are calculated by the π theorem and proportionally reduced compared with the actual wellbore. The diameter and depth of the hydraulic fracturing hole should be smaller than the rock sample size. Step 2: Place the sample model on a true triaxial device, adjust the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head of the true triaxial device to the same center, and connect the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head to the water pressure application pipe fittings respectively; Step 2 is as follows: Place the hydraulic fracturing hole of the sample model on the Y-head of the true triaxial device, and lift the Y-head until it is close to the bottom of the hole, ensuring that there is no pressure on the sample model; Place the X+, X-, Y+, Z+, and Z- indenters of the true triaxial device close to the specimen model, ensuring that no pressure is applied to the specimen model by the X+, X-, Y+, Z+, and Z- indenters. Ensure that the centers of the indenters are consistent. Connect the indenters to the water pressure fittings to ensure that stress is applied to the center of the specimen to avoid eccentric loading. Check the positions of the indenters and water pressure fittings before proceeding to the next step. Turn on the hydraulic power source, computer and oscilloscope, and observe whether there are any abnormalities in all displacement sensors, force sensors and water pressure application pipes. If there are no abnormalities, proceed to the next step. If there are abnormalities, repair them until there are no abnormalities, then proceed to the next step. Step 3: Apply load to the specimen model, and the servo motor controls the true three-axis device in the X, Y, and Z directions to apply load in the preload stage, the second stage, and the third stage respectively; Step 3 is as follows: Step 3.1, Preload Phase: The servo motor controls the true three-axis device to symmetrically apply a load of 9.5 kN to the specimen model in the X, Y, and Z directions with six indenters at a rate of 0.095 kn / s. After the load is applied, the load is stable for 10 seconds. Step 3.2, second stage: like Figure 4 As shown, the force control mode of the true triaxial device is used to apply the second stage load to the second preset stress state to simulate the in-situ stress of the rock before the hydraulic fracturing hole is excavated. The second preset stress state is that the maximum principal stress σ1 is applied in the Y direction, the intermediate principal stress σ2 is applied in the X direction, and the minimum principal stress σ3 is applied in the Z direction. Among them, the maximum pressure in the Y direction is not more than 4000 kN, the maximum pressure in the X direction is not more than 4000 kN, and the maximum pressure in the Z direction is not more than 2700 kN. Step 3.3, third stage: The force control mode of the true triaxial device was used to apply the third stage load to the third preset stress state to simulate the stress change after the hydraulic fracturing hole was excavated. The third preset stress state was that the true triaxial device alone controlled the maximum principal stress σ1 in the Y direction to be unloaded at a rate of 0.095 kN / s, and unloaded to 50% of the second preset stress state in this direction; Step 4: While maintaining the load of the third stage, the specimen model is subjected to a pulsed cyclic water pressure; Step 4 is as follows: Maintaining the stress state of the specimen model in the third stage in the XYZ directions, the servo motor controls the true three-axis device to load the hydraulic piston in the hydraulic pressure application pipe to the preset pore pressure value at a speed of 0.095 mm / s, and starts the pressure rod in the hydraulic pressure application pipe. First, move the pressure rod to the position of the hydraulic piston so that the end of the pressure rod contacts the hydraulic piston. Figure 5 As shown, the pressure rod impact related parameters are set according to the predetermined frequency and the pulse cycle water pressure peak and water pressure valley values. The pressure rod impacts the hydraulic piston until the sample model is destroyed, and the water pressure, flow rate, and strain related data of the hydraulic fracturing hole in the XYZ directions are recorded; The predetermined pore pressure position is determined according to the pore pressure value of the hydraulic fracturing hole set by the force sensor; The specific manifestation of sample failure is: when the deformation of any XYZ direction of the sample model reaches 3mm, the sample is considered to be broken; The predetermined frequency is 50 Hz; The peak and valley values ​​of water pressure are determined by preliminary experiments. In the preliminary experiments, the impact stroke of the pressure rod of the true triaxial device is increased by 0.1 mm increments. The functional relationship between the pressure rod stroke and the peak and valley values ​​of water pressure is established: F 压杆行程 = (water pressure 峰值 -Water pressure 谷值 ) / 2, thus obtaining the peak value and valley value of water pressure; Step 5: Unload all loads and pressures on the specimen model, and the test ends; Step 5 is as follows: Close the water pressure valve and stop the water pressure loading. Use the servo motor to control the true triaxial device to symmetrically and synchronously reduce the load stress in the X, Y, and Z directions to 0 at a speed of 0.095 kN / s. Record the stress and strain data of the true triaxial device in the X, Y, and Z directions. Remove the specimen model and observe its morphology. Clean the debris from each pressure head and the specimen model in the pressure chamber. Turn off the power to complete the test.

[0024] Example 9: This embodiment proposes a test method for hydraulic fracturing of rocks under true triaxial stress, which is specifically implemented according to the following steps: Step 1: Processing a rock block into a rock sample and performing a hole-opening process to form a sample model; Step 1 is as follows: like Figure 1 As shown, Figure 2 As shown, Figure 3 As shown, the rock block is processed into a standard cubic rock sample. The size of the rock sample is determined by the size of the indenter. The size of the rock sample can be determined by changing the size of the indenter. A hydraulic fracturing hole is drilled at the center of any surface of the sample in a direction perpendicular to the surface to form a sample model. The sample model is then cleaned with acetone or alcohol. Wait until the sample model is dry before proceeding to the next step. The diameter and depth of the hydraulic fracturing hole of the sample model are calculated by the π theorem and proportionally reduced compared with the actual wellbore. The diameter and depth of the hydraulic fracturing hole should be smaller than the rock sample size. Step 2: Place the sample model on a true triaxial device, adjust the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head of the true triaxial device to the same center, and connect the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head to the water pressure application pipe fittings respectively; Step 2 is as follows: Place the hydraulic fracturing hole of the sample model on the Y-head of the true triaxial device, and lift the Y-head until it is close to the bottom of the hole, ensuring that there is no pressure on the sample model; Place the X+, X-, Y+, Z+, and Z- indenters of the true triaxial device close to the specimen model, ensuring that no pressure is applied to the specimen model by the X+, X-, Y+, Z+, and Z- indenters. Ensure that the centers of the indenters are consistent. Connect the indenters to the water pressure fittings to ensure that stress is applied to the center of the specimen to avoid eccentric loading. Check the positions of the indenters and water pressure fittings before proceeding to the next step. Turn on the hydraulic power source, computer and oscilloscope, and observe whether there are any abnormalities in all displacement sensors, force sensors and water pressure application pipes. If there are no abnormalities, proceed to the next step. If there are abnormalities, repair them until there are no abnormalities, then proceed to the next step. Step 3: Apply load to the specimen model, and the servo motor controls the true three-axis device in the X, Y, and Z directions to apply load in the preload stage, the second stage, and the third stage respectively; Step 3 is as follows: Step 3.1, Preload Phase: The servo motor controls the true three-axis device to symmetrically apply a load of 10 kN to the specimen model in the X, Y, and Z directions with six indenters at a rate of 0.1 kn / s. After the load is applied, the load is stable for 20 seconds. Step 3.2, second stage: like Figure 4 As shown, the force control mode of the true triaxial device is used to apply the second stage load to the second preset stress state to simulate the in-situ stress of the rock before the hydraulic fracturing hole is excavated. The second preset stress state is that the maximum principal stress σ1 is applied in the Y direction, the intermediate principal stress σ2 is applied in the X direction, and the minimum principal stress σ3 is applied in the Z direction. Among them, the maximum pressure in the Y direction is not more than 4000 kN, the maximum pressure in the X direction is not more than 4000 kN, and the maximum pressure in the Z direction is not more than 2700 kN. Step 3.3, third stage: The force control mode of the true triaxial device was used to apply the third stage load to the third preset stress state to simulate the stress change after the hydraulic fracturing hole was excavated. The third preset stress state was that the true triaxial device alone controlled the maximum principal stress σ1 in the Y direction to be unloaded at a speed of 0.1 kN / s, and unloaded to 55% of the second preset stress state in this direction; Step 4: While maintaining the load of the third stage, the specimen model is subjected to a pulsed cyclic water pressure; Step 4 is as follows: Maintaining the stress state of the specimen model in the third stage in the XYZ directions, the servo motor controls the true three-axis device to load the hydraulic piston in the hydraulic pressure application pipe to the preset pore pressure value at a speed of 0.1 mm / s, and starts the pressure rod in the hydraulic pressure application pipe. First, move the pressure rod to the position of the hydraulic piston so that the end of the pressure rod contacts the hydraulic piston. Figure 5As shown, the pressure rod impact related parameters are set according to the predetermined frequency and the pulse cycle water pressure peak and water pressure valley values. The pressure rod impacts the hydraulic piston until the sample model is destroyed, and the water pressure, flow rate, and strain related data of the hydraulic fracturing hole in the XYZ directions are recorded; The predetermined pore pressure position is determined according to the pore pressure value of the hydraulic fracturing hole set by the force sensor; The specific manifestation of sample failure is: when the deformation of any XYZ direction of the sample model reaches 3mm, the sample is considered to be broken; The predetermined frequency is 25 Hz; The peak and valley values ​​of water pressure are determined by preliminary experiments. In the preliminary experiments, the impact stroke of the pressure rod of the true triaxial device is increased by 0.1 mm increments. The functional relationship between the pressure rod stroke and the peak and valley values ​​of water pressure is established: F 压杆行程 = (water pressure 峰值 -Water pressure 谷值 ) / 2, thus obtaining the peak value and valley value of water pressure; Step 5: Unload all loads and pressures on the specimen model, and the test ends; Step 5 is as follows: Close the water pressure valve and stop the water pressure loading. Use the servo motor to control the true triaxial device to symmetrically and synchronously reduce the load stress in the X, Y, and Z directions to 0 at a speed of 0.1 kN / s. Record the stress and strain data of the true triaxial device in the X, Y, and Z directions. Remove the specimen model and observe its morphology. Clean the debris from each pressure head and the specimen model in the pressure chamber. Turn off the power to complete the test.

Claims

1. A test method for hydraulic fracturing of rocks under true triaxial stress, characterized in that: Please follow the steps below to implement it: Step 1: Processing a rock block into a rock sample and performing a hole-opening process to form a sample model; Step 2: Place the sample model on a true triaxial device, adjust the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head of the true triaxial device to the same center, and connect the X+ pressure head, X- pressure head, Y+ pressure head, Y pressure head-, Z+ pressure head, and Z- pressure head to the water pressure application pipe fittings respectively; Step 3: Apply load to the specimen model, and the servo motor controls the true three-axis device in the X, Y, and Z directions to apply load in the preload stage, the second stage, and the third stage respectively; Step 4: While maintaining the load of the third stage, the specimen model is subjected to a pulsed cyclic water pressure; Step 5: Unload all loads and pressures on the specimen model and the test ends.

2. The test method for hydraulic fracturing of rocks under true triaxial stress state according to claim 1, characterized in that: The step 1 is specifically as follows: Process the rock block into a standard cubic rock sample, drill a hydraulic fracturing hole at the center of any face of the sample in a direction perpendicular to the face to form a sample model, and clean the sample model with acetone or alcohol. Wait until the sample model is dry before proceeding to the next step.

3. The test method for hydraulic fracturing of rock under true triaxial stress state according to claim 2, characterized in that: The ratio of the diameter and depth of the hydraulic fracturing hole of the sample model is calculated using the π theorem.

4. The test method for hydraulic fracturing of rock under true triaxial stress state according to claim 1, characterized in that: The step 2 is specifically as follows: Place the hydraulic fracturing hole of the sample model on the Y-head of the true triaxial device, lift the Y-head until it is close to the bottom of the hole, and ensure that the Y-head exerts no pressure on the sample model; Place the X+ pressure head, X- pressure head, Y+ pressure head, Z+ pressure head, and Z- pressure head of the true triaxial device close to the specimen model, ensure that the X+ pressure head, X- pressure head, Y+ pressure head, Z+ pressure head, and Z- pressure head have no pressure applied to the specimen model, ensure that the center of each pressure head is consistent, connect each pressure head to the water pressure application pipe fitting, check the position of each pressure head and water pressure application pipe fitting, and then proceed to the next step.

5. The test method for hydraulic fracturing of rock under true triaxial stress state according to claim 1, characterized in that: The step 3 is specifically as follows: Step 3.1, Preload Phase: The servo motor controls the true three-axis device to symmetrically apply a load of 9.5 to 10.5 kN to the specimen model in the X, Y, and Z directions at a rate of 0.095 to 0.105 kn / s. After the load is applied, the load is stable for 10 to 30 seconds. Step 3.2, second stage: The force control mode of the true triaxial device is used to apply the second stage load to the second preset stress state to simulate the in-situ stress of the rock before the hydraulic fracturing hole is excavated. The second preset stress state is the maximum principal stress σ1 applied in the Y direction, the intermediate principal stress σ2 applied in the X direction, and the minimum principal stress σ3 applied in the Z direction. Among them, the maximum pressure in the Y direction is not more than 4000kN, the maximum pressure in the X direction is not more than 4000kN, and the maximum pressure in the Z direction is not more than 2700kN. Step 3.3, third stage: The force control mode of the true triaxial device was used to apply the third stage load to the third preset stress state to simulate the stress change after the hydraulic fracturing hole was excavated. The third preset stress state was that the true triaxial device alone controlled the maximum principal stress σ1 in the Y direction to unload at a speed of 0.095~0.105kN / s, and unloaded to 50~60% of the second preset stress state in this direction.

6. The test method for hydraulic fracturing of rock under true triaxial stress state according to claim 1, characterized in that: The step 4 is specifically as follows: Maintaining the stress state of the specimen model in the third stage in the X, Y, and Z directions, the servo motor controls the true three-axis device to load the hydraulic piston in the hydraulic pressure application fitting to the preset pore pressure value at a speed of 0.095-0.105 mm / s. The pressure rod in the hydraulic pressure application fitting is started, and the pressure rod is first moved to the position of the hydraulic piston so that the end of the pressure rod contacts the hydraulic piston. The pressure rod impact-related parameters are set according to the predetermined frequency and the pulse cycle water pressure peak and water pressure valley values. The pressure rod impacts the hydraulic piston until the specimen model is destroyed, and the water pressure, flow rate, and strain-related data of the hydraulic fracturing hole in the X, Y, and Z directions are recorded.

7. The test method for hydraulic fracturing of rocks under true triaxial stress state according to claim 6, characterized in that: The predetermined pore pressure position is determined according to the pore pressure value of the hydraulic fracturing hole set by the force sensor; The specific manifestation of sample failure is: when the deformation of any XYZ direction of the sample model reaches 3mm, the sample is considered to be broken; The preset frequency is 0~50Hz; The peak and valley values ​​of water pressure are determined by preliminary experiments. In the preliminary experiments, the impact stroke of the pressure rod of the true triaxial device is increased by 0.1 mm increments. The functional relationship between the pressure rod stroke and the peak and valley values ​​of water pressure is established: F 压杆行程 = (water pressure 峰值 -Water pressure 谷值 ) / 2, thus obtaining the water pressure peak value and water pressure valley value.

8. The test method for hydraulic fracturing of rock under true triaxial stress state according to claim 1, characterized in that: The step 5 is specifically as follows: Close the water pressure valve to stop the water pressure loading. Use the servo motor to control the true triaxial device to symmetrically and synchronously reduce the load stress in the X, Y, and Z directions to 0 at a speed of 0.095-0.105 kN / s. Record the stress and strain data of the true triaxial device in the X, Y, and Z directions. Remove the specimen model, observe its morphology, clean the debris from each pressure head and the specimen model in the pressure chamber, turn off the power, and complete the test.

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