Stress loading system and method for true triaxial fracturing physical model test device

Abstract

The invention provides a stress loading system and method for a true triaxial fracturing physical model test device, and the system comprises three groups of loading mechanisms which are respectively arranged in three directions of a rock test piece; the friction force reducing mechanism is arranged on the loading mechanism and can reduce the friction force between the loading mechanism and the rock test piece; the loading mechanism is also selectively provided with a pressure detection mechanism, and the pressure detection mechanism is detachably connected to the loading mechanism.

Description

Technical Field

[0001] The invention relates to a stress loading system and method for a true triaxial fracturing physical model test device, belonging to the technical field of oil and gas exploration equipment. Background Art

[0002] Indoor fracturing physical simulation test is an important means to study the expansion law of hydraulic fractures. The true triaxial fracturing simulation test system can simulate the stress conditions of underground reservoirs by loading three-dimensional principal stress, and is an important equipment for conducting indoor fracturing physical simulation tests. Taking the existing technology "three-dimensional rigid loading rock true triaxial instrument" as an example, at present, the three-dimensional principal stress loading device of this system is composed of a hydraulic loading end and a fixed end in each direction. During loading, the rock specimen is placed between the loading end and the fixed end, and the hydraulic loading end squeezes the rock unidirectionally to apply stress.

[0003] However, in practice, it is found that this loading system that applies stress through unidirectional extrusion has certain defects. For one of the three directions, as stress is applied during loading, the rock tends to shrink and deform under the action of extrusion, and the contact surface between the rock and the loading plate tends to produce higher friction under the action of pressure. The friction generated by compression in one direction will hinder the shrinkage deformation of the other two directions. Therefore, during three-way loading, (because friction must be overcome), the pressure applied to the rock by the hydraulic loading device cannot be fully transmitted to the fixed end (the hydraulic loading pressure is equal to the sum of the pressure and friction between the fixed end and the rock), and in the same direction, the stress of the rock has a stress gradient. In this case, the actual loading stress condition does not match the set loading stress condition, which affects the results of the physical model test. Summary of the invention

[0004] In view of the above-mentioned technical problems existing in the prior art, the present invention proposes a stress loading system for a true triaxial fracturing model test device, in which the hydraulic loading device and the loading plate in the fracturing model device maintain a sliding state, and when loading triaxial stress, it is no longer affected by friction, so that the rock specimen is uniformly stressed in the same direction, making the fracturing model test results more real and reliable.

[0005] The present invention proposes a stress loading system for a true triaxial fracturing physical model test device, comprising:

[0006] three groups of loading mechanisms, the three groups of loading mechanisms being arranged in three directions of the rock specimen respectively; and

[0007] A friction reducing mechanism is arranged on the loading mechanism, and the friction reducing mechanism can reduce the friction between the loading mechanism and the rock specimen.

[0008] Providing a friction reducing mechanism on the loading mechanism can reduce the friction between the loading mechanism and the rock specimen, thereby reducing the above-mentioned influence and improving the accuracy of the experiment.

[0009] A further improvement of the present invention is that a pressure detection mechanism is selectively provided on the loading mechanism, and the pressure detection mechanism is detachably connected to the loading mechanism.

[0010] The pressure detection mechanism can detect the pressure difference between the loading end and the fixed end, thereby determining the influence of friction and improving the accuracy of the experiment.

[0011] A further improvement of the present invention is that the loading mechanism comprises a loading end and a fixing end, and the loading end and the fixing end are respectively arranged on two sides of the rock specimen;

[0012] The loading end is connected to a hydraulic servo loading device, and the hydraulic servo loading device applies pressure to the rock specimen through the loading end; the fixed end remains fixed.

[0013] A further improvement of the present invention is that a loading plate is provided between the loading end and the rock specimen, and a fixing plate is provided between the fixing end and the rock specimen.

[0014] A further improvement of the present invention is that a group of friction reducing mechanisms are arranged between the loading end and the loading plate, and a group of friction reducing mechanisms are arranged between the fixed plate and the fixed end.

[0015] A loading plate and a fixing plate are respectively arranged between the loading end and the fixing end of the rock specimen, and the loading plate and the fixing plate surround the six sides of the rock specimen to form a space for accommodating the rock specimen; different loading plates and fixing plates do not contact each other to avoid the loading effect being affected by the force on the loading plates and the fixing plates. Normally, the loading plate and the fixing plate are equal to or slightly smaller than one side of the rock specimen.

[0016] The loading end and the loading plate are split structures, and the fixed plate and the fixed end are also split structures, which is convenient for adding or reducing pressure detection mechanisms and friction reduction mechanisms between the fixed plate and the fixed end, and between the loading plate and the loading end.

[0017] A further improvement of the present invention is that the pressure detection mechanism is selectively installed between the loading end and the loading plate, and between the fixed end and the fixed plate.

[0018] A further improvement of the present invention is that the friction reducing mechanism comprises a smooth metal plate, and the outer surface of the smooth metal plate is provided with a lubricating coating.

[0019] Smooth metal plates can reduce friction, and lubricating coatings can reduce friction further.

[0020] A further improvement of the present invention is that the friction reducing mechanism comprises a deformable plate, which can transmit force in one direction and deform in other directions at the same time, and the rock specimen is displaced by the force through the deformation, thereby eliminating the influence of friction.

[0021] A further improvement of the present invention is that the friction reducing mechanism comprises a rolling plate, on which a plurality of rollers or balls are arranged, and the balls roll against the loading plate or the fixing plate.

[0022] According to another aspect of the present invention, a method is provided for conducting a test using the stress loading system of the true triaxial fracturing physical model test device.

[0023] A further improvement of the present invention is that the method comprises:

[0024] Collect downhole rock samples or target reservoir outcrops to prepare rock specimens, process the size of the rock specimens, and prepare them according to the maximum size of the true triaxial hydraulic fracturing equipment;

[0025] Installing a pressure detection mechanism on the loading mechanism;

[0026] Install and adjust the friction reducing mechanism;

[0027] The rock specimen is placed on the test frame, triaxial stress is applied, and the pressure values ​​on the opposite sides of the rock are compared.

[0028] If the pressure gauge value between the loading end and the rock is greater than the pressure between the fixed end and the rock by more than 5%, return to readjust the friction reduction mechanism and improve the lubrication measures;

[0029] If the two pressure values ​​are less than 5%, the test can be carried out according to the normal fracturing simulation test procedures after unloading and removing the pressure gauge.

[0030] Compared with the prior art, the advantages of the present invention are:

[0031] The present invention discloses a stress loading system for a true triaxial fracturing physical model test device, in which a hydraulic loading device in the fracturing physical model device maintains a sliding state with a loading plate. When three-dimensional stress is loaded, it is no longer affected by friction, so that the rock specimen is uniformly stressed in the same direction, making the fracturing physical model test result more real and reliable.

[0032] According to the stress loading system and method of the true triaxial fracturing model test device described in the invention, compared with previous systems, the hydraulic press / fixed end and the loading plate in the fracturing model device maintain a sliding state, and when loading three-dimensional stress, it is no longer affected by friction, so that the rock specimen is evenly stressed in the same direction, making the fracturing model test results more realistic and reliable. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, in which:

[0034] Figure 1 Shown is a structural schematic diagram of a stress loading system of a true triaxial fracturing model test device according to an embodiment of the present invention.

[0035] The drawings are not drawn to scale.

[0036] The meanings of the reference numerals in the accompanying drawings are as follows:

[0037] 1. Loading end, 2. Fixed end, 3. Rock specimen, 4. Loading plate, 5. Fixed plate. DETAILED DESCRIPTION

[0038] In order to make the technical solutions and advantages of the present invention more clearly understood, the exemplary embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than an exhaustive list of all the embodiments. In addition, the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0039] In the prior art, the loading system that applies stress through unidirectional extrusion has certain defects. For one of the three directions, when loading, as stress is applied, the rock tends to produce shrinkage deformation under the action of extrusion, and the contact surface between the rock and the loading plate tends to produce higher friction under the action of pressure. The friction generated by compression in one direction will hinder the shrinkage deformation of the other two directions.

[0040] Therefore, during three-way loading, (because friction must be overcome), the pressure applied by the hydraulic loading device on the rock cannot be completely transmitted to the fixed end (the hydraulic loading pressure is equal to the sum of the pressure and friction between the fixed end and the rock), and in the same direction, the stress of the rock has a stress gradient. In this case, the actual loading stress condition does not match the set loading stress condition, which affects the physical model test results.

[0041] This embodiment proposes a stress loading system for a true triaxial fracturing model test device, in which the hydraulic loading device in the fracturing model device maintains a sliding state with the loading plate 4, and is no longer affected by friction when loading triaxial stress, so that the rock specimen 3 is uniformly stressed in the same direction, making the fracturing model test results more realistic and reliable.

[0042] In such Figure 1 In the embodiment shown, the stress loading system of the true triaxial fracturing physical model test device includes:

[0043] Three groups of loading mechanisms are respectively arranged in three directions of X-axis, Y-axis and Z-axis, and the three directions are perpendicular to each other. The three groups of loading mechanisms are respectively arranged to apply pressure in three directions of the rock sample.

[0044] A friction reducing mechanism disposed on the loading mechanism, the friction reducing mechanism being capable of reducing the friction force applied by the loading mechanism to the rock sample;

[0045] The true triaxial fracturing simulation test system composed of three groups of loading components can simulate the stress conditions of underground reservoirs by loading three-dimensional principal stress. However, the three-dimensional principal stress loading device is composed of a hydraulic loading end 1 and a fixed end 2 in each direction. During loading, the rock specimen 3 is placed between the loading end 1 and the fixed end 2, and the hydraulic loading end 1 squeezes the rock unidirectionally to apply stress.

[0046] The greater the applied pressure, the greater the friction between the loading end 1 and the rock specimen 3. This friction will affect the effect of applying pressure on other surfaces. For example, the friction between the loading end 1 and the rock specimen 3 in the X direction will affect the force applied by the loading end 1 on the rock specimen 3 in the Y direction. When this force is transmitted to the fixed end 2 of the Y axis, it will be partially offset by the friction in the X direction, thereby causing the result of the physical model test.

[0047] In this embodiment, a friction reducing mechanism is also provided on the loading mechanism, which can reduce the friction between the loading mechanism and the rock specimen 3, thereby reducing the above-mentioned influence and improving the accuracy of the experiment.

[0048] The stress loading system of a true triaxial fracturing model test device described in this embodiment maintains a sliding state between the hydraulic loading device and the loading plate in the fracturing model device. When loading triaxial stress, it is no longer affected by friction, so that the rock specimen is uniformly stressed in the same direction, making the fracturing model test results more realistic and reliable.

[0049] In one embodiment, a pressure detection mechanism is selectively provided on the loading mechanism, and the pressure detection mechanism is detachably connected to the loading mechanism.

[0050] In this embodiment, the pressure detection device is selectively installed, that is, it is installed or not installed according to actual needs. The pressure detection device is configured to be detachable and installable.

[0051] In the true triaxial fracturing physical model test device according to this embodiment, three groups of loading mechanisms apply pressure to the rock sample in three different directions, thereby forming the main body of the true triaxial fracturing physical model test device. The friction reducing mechanism can reduce the friction between the loading mechanism and the rock sample, thereby reducing the influence of friction on the loading of other axes.

[0052] The pressure detection mechanism can detect the pressure in each direction of the three groups of loading mechanisms before the test, and compare it with the applied pressure value to determine the size of the friction force, and further determine whether the friction reduction mechanism is effective, and then further adjust the friction reduction mechanism to achieve the required test accuracy.

[0053] In one embodiment, the loading mechanism includes a hydraulic servo loading device, wherein the pressure output mechanism of the hydraulic servo loading device is provided with a loading end 1 connected to the rock specimen 3; a fixed end 2 is provided on the other side of the rock specimen 3, i.e., on the side opposite to the loading end 1. The fixed end 2 and the loading end 1 are respectively provided at two ends of the rock specimen 3, the fixed end 2 remains fixed, and the loading end 1 applies pressure to the rock specimen 3 under the drive of the hydraulic servo loading device.

[0054] The fixed end 2 remains fixed, and a fixed plate 5 is arranged between the fixed end 2 and the rock specimen 3. The loading end 1 can move with the hydraulic servo loading device, thereby applying pressure to the rock specimen 3. A loading plate 4 is arranged between the loading end 1 and the rock specimen 3.

[0055] In the stress loading system of the triaxial fracturing physical model test device according to the present embodiment, a loading plate 4 and a fixing plate 5 are respectively arranged between the loading end 1 and the fixing end 2 with the rock specimen 3, and the loading plate 4 and the fixing plate 5 surround the six faces of the rock specimen 3 to form a space for accommodating the rock specimen 3; different loading plates 4 and fixing plates 5 do not contact each other to avoid the loading effect being affected by the force on the loading plates 4 and the fixing plates 5. Normally, the loading plates 4 and the fixing plates 5 are equal to or slightly smaller than one face of the rock specimen 3.

[0056] The loading end 1 and the loading plate 4 are split structures, and the fixed plate 5 and the fixed end 2 are also split structures, which is convenient for adding or reducing pressure detection mechanisms and friction reduction mechanisms between the fixed plate 5 and the fixed end 2, and between the loading plate 4 and the loading end 1.

[0057] In one embodiment, a group of friction reducing mechanisms are arranged between the loading end 1 and the loading plate 4 , and a group of friction reducing mechanisms are arranged between the fixing plate 5 and the fixing end 2 .

[0058] For example, a first friction reducing mechanism is set between the loading end 1 of the X-axis and the loading plate 4, and a second friction reducing mechanism is set between the fixed end 2 of the X-axis and the fixed plate 5; a second friction reducing mechanism is set between the loading end 1 of the Y-axis and the loading plate 4, and a second friction reducing mechanism is set between the fixed end 2 of the Y-axis and the fixed plate 5; a third friction reducing mechanism is set between the loading end 1 of the Z-axis and the loading plate 4, and a third friction reducing mechanism is set between the fixed end 2 of the Z-axis and the fixed plate 5.

[0059] In a preferred embodiment, the pressure detection mechanism is selectively installed between the loading end 1 and the loading plate 4 , and between the fixing end 2 and the fixing plate 5 .

[0060] Preferably, the pressure detection mechanism is arranged between the fixed end 2 and the fixed plate 5, and the pressure between the fixed end 2 and the rock specimen 3 can be detected, and the pressure detection mechanism may be arranged between the loading end 1 and the loading plate 4, or may not be arranged. When the pressure detection mechanism is not arranged between the loading end 1 and the loading plate 4, the pressure value of the hydraulic servo loading device can be compared with the detection value of the pressure detection mechanism of the fixed end 2. If the pressure value of the hydraulic servo loading device is inaccurate, it is necessary to arrange the pressure detection mechanism between the loading end 1 and the loading plate 4 for comparison with the pressure detection mechanism of the fixed end 2.

[0061] In one embodiment, the friction reducing mechanism comprises a smooth metal plate, and a lubricating coating is provided on the outer surface of the smooth metal plate. The lubricating coating is preferably facing the side of the loading plate 4 or the fixing plate 5, so as to ensure that the friction between the loading plate 4 or the fixing plate 5 is kept small.

[0062] The surface of the smooth metal plate is polished to reduce friction.

[0063] In another embodiment, the friction reducing mechanism includes a deformable plate, which can transmit force in one direction (the axial direction of the loading mechanism) and can also deform in other directions. The deformation enables the rock specimen 3 to undergo a slight displacement as the force acts, thereby eliminating the influence of friction.

[0064] In the stress loading system of the triaxial fracturing physical model test device according to this embodiment, the rock specimen 3 can be displaced to a certain extent by the deformable plate with other opposite forces, so that no friction is generated. One side of the deformable plate is connected to the loading mechanism, and the other side is connected to the loading plate 4 or the fixed plate 5.

[0065] In this embodiment, the deformable plate is any structure that can transmit force and can be deformed.

[0066] In a specific embodiment, the deformable board includes two hard boards, a deformable layer is arranged between the two hard boards, and the deformable layer can be a fluid, such as water, and the edge is surrounded by a flexible material with low elasticity and can be deformed. Since water or an incompressible fluid is incompressible and the flexible material at the edge is inelastic, the deformable board can transmit force without being affected by elasticity. Since water can flow and the flexible material can be deformed, the shape can be changed, thereby eliminating the influence of friction.

[0067] In another embodiment, the friction reducing mechanism includes a rolling plate, which includes a plate body, on which a roller or a ball is arranged, and one side of the ball is arranged toward the loading plate 4 or the fixed plate 5, and the ball rolls with the loading plate 4 or the fixed plate 5.

[0068] In this embodiment, the sliding friction force is converted into rolling friction, thereby reducing the friction force.

[0069] A lubricating layer is provided on the ball or roller.

[0070] According to the stress loading system and method of the true triaxial fracturing model test device described in this embodiment, compared with previous systems, the hydraulic press / fixed end and the loading plate in the fracturing model device maintain a sliding state, and when loading three-dimensional stress, it is no longer affected by friction, so that the rock specimen is evenly stressed in the same direction, making the fracturing model test results more realistic and reliable.

[0071] In the above embodiments, the lubricating layer may be lubricating oil or other lubricants (such as lubricating powder, etc.).

[0072] According to another aspect of the present invention, a method is provided, wherein the method uses the stress loading system according to the true triaxial fracturing model test device to perform a test.

[0073] In one embodiment, the method comprises the following steps:

[0074] Collect downhole rock samples or target reservoir outcrops to prepare rock specimens 3, and process the size of the rock specimens 3 according to the maximum accommodating size of true triaxial hydraulic fracturing equipment;

[0075] Installing a pressure detection mechanism on the loading mechanism;

[0076] Install and adjust the friction reducing mechanism;

[0077] Place the rock specimen 3 on the test stand, apply triaxial stress, and compare the pressure values ​​on the opposite sides of the rock.

[0078] If the pressure gauge value between the loading plate 4 at the pressurized end and the rock is significantly greater than the pressure gauge value between the loading plate 4 at the fixed end 2 and the rock, for example, the pressure value between the loading end 1 and the rock exceeds the pressure value between the fixed end 2 and the rock by 5%, then return to readjust the friction reduction mechanism and improve the lubrication measures;

[0079] If the two pressure values ​​are equal or the difference is small, for example, the pressure value between the loading end 1 and the rock does not exceed the pressure value between the fixed end 2 and the rock by 5%, then after unloading and removing the pressure gauge, the test can be carried out according to the normal fracturing model test steps.

[0080] The following is an explanation through specific embodiments.

[0081] Example 1

[0082] The method comprises:

[0083] Step 1: Collect downhole rock samples or target reservoir outcrops to prepare rock specimens 3, the size of which can be prepared according to the maximum accommodating size of true triaxial hydraulic fracturing equipment;

[0084] Step 2: Before the first test, pressure gauges are installed between the upper and lower sections of the rock and the loading plate 4 (the best way is to install pressure gauges between the six end faces of the rock and the loading plate 4) to check the pressure on both sides of the rock;

[0085] Step 3: spraying lubricating coatings on the contact surfaces between the hydraulic loading device and the loading plate 4 of the true triaxial fracturing model device and the contact surfaces between the fixed end 2 and the loading plate 4, including applying lubricating oil, lubricating coatings, metal plates, etc.;

[0086] Step 4: placing the rock specimen 3 on a true triaxial fracturing system test stand, applying triaxial stress, and comparing the pressure values ​​on opposite sides of the rock;

[0087] Step 5: If the pressure gauge value between the loading plate 4 at the pressurized end and the rock is significantly greater than the pressure gauge value between the loading plate 4 at the fixed end 2 and the rock, return to step 3 for rectification and improvement of lubrication measures;

[0088] Step 6. If the two pressure values ​​are equal, after unloading and removing the pressure gauge, the test can be carried out according to the normal fracturing simulation test steps.

[0089] Example 2

[0090] The present invention provides a stress loading method and system for a true triaxial fracturing physical model test device, comprising the following steps:

[0091] Step 1: Use an annular sand wire cutting machine to cut limestone to prepare a 300mm×300mm×300mm specimen;

[0092] Step 2: Place the rock specimen 3 in a physical model test device using an integrated loading plate 4, and install a pressure gauge between the upper and lower end surfaces of the rock and the loading plate 4 to check the pressure on both sides of the rock;

[0093] Step 3: Apply a pressure of 3000 kN through the hydraulic servo loading system. The pressure gauge at the lower end of the rock shows 3000 kN, and the pressure gauge at the upper end of the rock shows 2200 kN. At this time, the friction force reaches 800 kN, proving that the loading of this system is greatly affected by the friction force.

[0094] Step 4: Use a split-type physical model test device and a key friction mechanism of a smooth metal plate, and spray a lubricating coating on the contact surface between the hydraulic loading system of the true triaxial fracturing physical model device and the smooth metal plate and the contact surface between the fixed end 2 and the smooth metal plate;

[0095] Step 5: Apply a pressure of 3000 kN through the hydraulic servo loading system. The pressure gauge at the lower end of the rock shows 3000 kN, and the pressure gauge at the upper end of the rock shows 2950 kN. The difference between the upper and lower pressure values ​​is significantly reduced, which significantly reduces the influence of friction on loading.

[0096] Step 6: Conduct fracturing physical model test.

[0097] Example 3

[0098] The present invention provides a stress loading method and system for a true triaxial fracturing physical model test device, comprising the following steps:

[0099] Step 1: Use an annular sand wire cutting machine to cut limestone to prepare a 300mm×300mm×300mm specimen;

[0100] Step 2: Place the rock specimen 3 in a physical model test device using an integrated loading plate 4, and install a pressure gauge between the upper and lower end surfaces of the rock and the loading plate 4 to check the pressure on both sides of the rock;

[0101] Step 3: Apply a pressure of 3000 kN through the hydraulic servo loading system. The pressure gauge at the lower end of the rock shows 3000 kN, and the pressure gauge at the upper end of the rock shows 2200 kN. At this time, the friction force reaches 800 kN, proving that the loading of this system is greatly affected by the friction force.

[0102] Step 4: Use a split-type physical model test device and a key friction mechanism of a smooth metal plate, and spray a lubricating coating on the contact surface between the hydraulic loading system of the true triaxial fracturing physical model device and the smooth metal plate and the contact surface between the fixed end 2 and the smooth metal plate;

[0103] Step 5: Apply 3000 kN of pressure through the hydraulic servo loading system. The pressure gauge at the lower end of the rock shows 3000 kN, and the pressure gauge at the upper end of the rock shows 2700 kN. The difference between the upper and lower pressure values ​​is significantly reduced, but the pressure difference is still above 5%;

[0104] Re-spray the lubrication layer and test again with the pressure detection device. Apply 3000kN of pressure through the hydraulic servo loading system. The pressure gauge at the lower end of the rock shows 3000kN, and the pressure gauge at the upper end of the rock shows 2940kN. The difference between the upper and lower pressure values ​​is significantly reduced, which significantly reduces the influence of friction on loading.

[0105] Step 6: Conduct fracturing physical model test.

[0106] Example 4

[0107] The present invention provides a stress loading method and system for a true triaxial fracturing physical model test device, comprising the following steps:

[0108] Step 1: Use an annular sand wire cutting machine to cut limestone to prepare a 300mm×300mm×300mm specimen;

[0109] Step 2: Place the rock specimen 3 in a physical model test device using an integrated loading plate 4, and install a pressure gauge between the upper and lower end surfaces of the rock and the loading plate 4 to check the pressure on both sides of the rock;

[0110] Step 3: Apply a pressure of 3000 kN through the hydraulic servo loading system. The pressure gauge at the lower end of the rock shows 3000 kN, and the pressure gauge at the upper end of the rock shows 2200 kN. At this time, the friction force reaches 800 kN, proving that the loading of this system is greatly affected by the friction force.

[0111] Step 4: Use a split-type physical model test device, and use a deformable plate as the friction reduction mechanism.

[0112] Step 5: Apply a pressure of 3000 kN through the hydraulic servo loading system. The pressure gauge at the lower end of the rock shows 3000 kN, and the pressure gauge at the upper end of the rock shows 2900 kN. The difference between the upper and lower pressure values ​​is significantly reduced, which significantly reduces the influence of friction on loading.

[0113] Step 6: Conduct fracturing physical model test.

[0114] Example 5

[0115] The present invention provides a stress loading method and system for a true triaxial fracturing physical model test device, comprising the following steps:

[0116] Step 1: Use an annular sand wire cutting machine to cut limestone to prepare a 300mm×300mm×300mm specimen;

[0117] Step 2: Place the rock specimen 3 in a physical model test device using an integrated loading plate 4, and install a pressure gauge between the upper and lower end surfaces of the rock and the loading plate 4 to check the pressure on both sides of the rock;

[0118] Step 3: Apply a pressure of 3000 kN through the hydraulic servo loading system. The pressure gauge at the lower end of the rock shows 3000 kN, and the pressure gauge at the upper end of the rock shows 2200 kN. At this time, the friction force reaches 800 kN, proving that the loading of this system is greatly affected by the friction force.

[0119] Step 4: Use a split-type physical model test device, and use a rolling plate as the friction reduction mechanism.

[0120] Step 5: Apply a pressure of 3000 kN through the hydraulic servo loading system. The pressure gauge at the lower end of the rock shows 3000 kN, and the pressure gauge at the upper end of the rock shows 2850 kN. The difference between the pressure values ​​at the upper and lower ends is significantly reduced, which significantly reduces the influence of friction on loading.

[0121] Step 6: Conduct fracturing physical model test.

[0122] According to the stress loading system and method of the true triaxial fracturing physical model test device described in this embodiment, compared with the previous system, the hydraulic press / fixed end 2 and the loading plate 4 in the fracturing physical model device maintain a sliding state, and when loading three-dimensional stress, it is no longer affected by friction, so that the rock specimen 3 is uniformly stressed in the same direction, making the fracturing physical model test results more real and reliable.

[0123] It should be understood that the embodiments disclosed in the present invention are not limited to the specific structures, processing steps or materials disclosed herein, but should be extended to equivalent substitutions of these features understood by ordinary technicians in the relevant field. It should also be understood that the terms used herein are only used for the purpose of describing specific embodiments and are not meant to be limiting.

[0124] In the description of the present invention, unless otherwise specified, "plurality" means two or more than two; the orientations or positional relationships indicated by the terms "upper", "lower", "left", "right", "inner", "outer", "front end", "rear end", "head", "tail", etc. are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", "third", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0125] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "connected" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0126] Certain terms are used throughout this specification to refer to specific system components. As will be appreciated by those skilled in the art, different names may be used to refer to the same component, and thus this specification is not intended to distinguish between components that differ only in name and not in function. References to "one embodiment" or "an embodiment" in the specification mean that a particular feature, structure, or characteristic described in conjunction with the embodiment is included in at least one embodiment of the invention. Therefore, the phrases "one embodiment" or "an embodiment" appearing in various places throughout the specification do not necessarily all refer to the same embodiment.

[0127] The embodiments of the present invention are given for the purpose of illustration and description, and are not intended to be exhaustive or to limit the invention to the disclosed forms. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiments are selected and described in order to better illustrate the principles and practical applications of the present invention and to enable those of ordinary skill in the art to understand the present invention and thereby design various embodiments with various modifications suitable for specific uses.

[0128] Although the preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they are aware of the basic creative concepts. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and / or modifications that fall within the scope of the present invention, and changes and / or modifications made according to the embodiments of the present invention should be included within the scope of protection of the present invention.

Claims

1. A stress loading system for a true triaxial fracturing physical model test device, characterized in that: include: Three groups of loading mechanisms, the three groups of loading mechanisms are arranged in three directions of the rock specimen (3) respectively; as well as A friction reducing mechanism is arranged on the loading mechanism, and the friction reducing mechanism can reduce the friction between the loading mechanism and the rock specimen (3).

2. The stress loading system of the true triaxial fracturing physical model test device according to claim 1 is characterized in that: The loading mechanism is also selectively provided with a pressure detection mechanism, and the pressure detection mechanism is detachably connected to the loading mechanism.

3. The stress loading system of the true triaxial fracturing physical model test device according to claim 2 is characterized in that: The loading mechanism comprises a loading end (1) and a fixing end (2), wherein the loading end (1) and the fixing end (2) are respectively arranged on two sides of a rock specimen (3); The loading end (1) is connected to a hydraulic servo loading device, and the hydraulic servo loading device applies pressure to the rock specimen (3) through the loading end (1); the fixed end (2) remains fixed.

4. The stress loading system of the true triaxial fracturing physical model test device according to claim 3 is characterized in that: A loading plate (4) is provided between the loading end (1) and the rock specimen (3), and a fixing plate (5) is provided between the fixing end (2) and the rock specimen (3).

5. The stress loading system of the true triaxial fracturing physical model test device according to claim 4 is characterized in that: A group of friction reducing mechanisms is arranged between the loading end (1) and the loading plate (4), and a group of friction reducing mechanisms is arranged between the fixing plate (5) and the fixing end (2).

6. The stress loading system of the true triaxial fracturing physical model test device according to claim 5 is characterized in that: The pressure detection mechanism is selectively installed between the loading end (1) and the loading plate (4), and between the fixing end (2) and the fixing plate (5).

7. The stress loading system of the true triaxial fracturing physical model test device according to any one of claims 1 to 6, characterized in that: The friction reducing mechanism comprises a smooth metal plate, the outer surface of which is provided with a lubricating coating.

8. The stress loading system of the true triaxial fracturing physical model test device according to any one of claims 1 to 6, characterized in that: The friction reducing mechanism comprises a deformable plate, which can transmit force in one direction and can also deform in other directions. The deformation causes the rock specimen (3) to move in response to the force, thereby eliminating the influence of friction.

9. The stress loading system of the true triaxial fracturing physical model test device according to any one of claims 1 to 6, characterized in that: The friction reducing mechanism comprises a rolling plate, on which a plurality of rollers or balls are arranged, and the balls roll against the loading plate (4) or the fixing plate (5).

10. A method, characterized in that The test was conducted using a stress loading system according to the true triaxial fracturing model test apparatus.

11. The method according to claim 10, characterized in that: include: Collecting downhole rock samples or target reservoir outcrops to prepare rock specimens (3), and processing the size of the rock specimens (3) according to the maximum accommodating size of true triaxial hydraulic fracturing equipment; Installing a pressure detection mechanism on the loading mechanism; Install and adjust the friction reducing mechanism; The rock specimen (3) is placed on a test stand, subjected to triaxial stress, and the pressure values ​​on the two opposite sides of the rock are compared. If the pressure gauge value between the loading end (1) and the rock is greater than the pressure between the fixed end (2) and the rock by more than 5%, return to readjust the friction reduction mechanism and improve the lubrication measures; If the two pressure values ​​are less than 5%, the test can be carried out according to the normal fracturing simulation test procedures after unloading and removing the pressure gauge.

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