CN110793864B - Method and device for measuring thermal stress of rock specimen under the action of liquid nitrogen - Google Patents

Abstract

The application provides a method and a device for measuring thermal stress of a rock test piece under the action of liquid nitrogen, wherein the method comprises the following steps: and obtaining the tensile strength of the rock test piece under the room temperature condition. A first shaft and a second shaft are preset for the rock specimen. And applying three-way confining pressure stress with different sizes to the rock test piece, injecting water into the first shaft, and plugging the second shaft so as to enable the rock test piece to be subjected to water fracturing and obtain a main crack perpendicular to the direction of the minimum confining pressure stress. And applying predetermined three-way confining pressure stress with the same magnitude to the rock test piece, injecting liquid nitrogen into the first shaft, enabling the liquid nitrogen to flow out of the second shaft, gradually reducing the confining pressure stress parallel to the main crack direction, and measuring the confining pressure stress corresponding to the first cracking signal when the cracking signal in the rock test piece is suddenly increased, wherein the confining pressure stress is critical confining pressure stress. The thermal stress of the rock test piece is the sum of the critical confining pressure stress and the tensile strength. The application can make up the blank that the thermal stress of the rock can not be effectively measured in the liquid nitrogen environment.

Description

Method and device for measuring thermal stress of rock specimen under the action of liquid nitrogen

technical field

The application belongs to the field of rock thermal stress measurement, and in particular relates to a method and a device for measuring thermal stress of a rock specimen under the action of liquid nitrogen.

Background technique

my country is rich in unconventional oil and gas resources and has great potential for development. Due to the extremely low permeability of unconventional oil and gas reservoirs, special engineering techniques are required for exploitation. At present, it is difficult for conventional hydraulic fracturing technology to effectively transform unconventional oil and gas reservoirs. Low-temperature fluids such as liquid nitrogen and liquid carbon dioxide are gradually applied to the fracturing construction of unconventional oil and gas reservoirs, which is expected to increase the production of unconventional oil and gas resources. The study of the ultra-low temperature rock-breaking mechanism of liquid nitrogen is very important for the design of liquid nitrogen fracturing construction. At present, relevant scholars generally believe that thermal stress is one of the main mechanisms of liquid nitrogen ultra-low temperature rock-breaking. The mechanism of nitrogen ultra-low temperature rock breaking is very important.

At present, the conventional method of measuring thermal stress is mainly to measure the deformation of the rock specimen by pasting strain gauges or optical fibers on the surface of the rock specimen to calculate the thermal stress. However, due to the extremely low temperature of liquid nitrogen, the conventional method of measuring thermal stress is in There are two main problems when measuring the thermal stress of rocks under the action of ultra-low temperature liquid nitrogen. First, the properties of sensors such as strain gauges and optical fibers will change greatly under the action of ultra-low temperature liquid nitrogen, which makes them unable to measure deformation normally. The adhesive of the specimen will also deform greatly under the action of ultra-low temperature of liquid nitrogen, which will interfere with the sensor to measure the strain of the specimen. Therefore, the conventional method of measuring thermal stress will fail when measuring the thermal stress of rock under the action of ultra-low temperature of liquid nitrogen. As a result, the thermal stress of rock cannot be measured effectively.

SUMMARY OF THE INVENTION

In order to overcome the above-mentioned defects of the prior art, the present invention proposes a method and a device for measuring the thermal stress of a rock specimen under the action of liquid nitrogen, which can measure the thermal stress of the rock under the action of liquid nitrogen ultra-low temperature by indirect measurement, which is used for the study of liquid nitrogen. The mechanism of nitrogen ultra-low temperature rock breaking provides the basis.

The concrete technical scheme of the present invention is:

The invention provides a method for measuring the thermal stress of a rock specimen under the action of liquid nitrogen, comprising the following steps:

Obtain the tensile strength of the rock specimen at room temperature;

Presetting a first wellbore and a second wellbore for the rock specimen;

Applying three-dimensional confining stress of different sizes to the rock specimen, injecting water into the first wellbore, and plugging the second wellbore, so that the water fracturing the rock specimen, and obtaining a Main fractures perpendicular to the direction of minimum confining compressive stress;

A predetermined three-directional confining stress of equal magnitude is applied to the rock specimen, liquid nitrogen is injected into the first wellbore, liquid nitrogen flows out of the second wellbore, and the confinement parallel to the direction of the main fracture is gradually reduced. compressive stress, measure the confining compressive stress corresponding to the first rupture signal when the rupture signal inside the rock specimen increases sharply, and the confining compressive stress is the critical confining compressive stress;

The thermal stress of the rock specimen is the sum of the critical confining compressive stress and the tensile strength.

In a preferred embodiment, the predetermined three-direction confining stress value is about 20% larger than the theoretical critical confining compressive stress value.

In a preferred embodiment, the theoretical critical confining compressive stress value is the difference between the theoretical thermal stress value and the tensile strength, wherein,

Among them, E is the elastic modulus of the rock specimen, v is the Poisson's ratio of the rock specimen, α _T is the linear elastic thermal expansion coefficient of the rock specimen, T _cool is the temperature of the rock specimen under the condition of liquid nitrogen, T _o is the initial temperature of the rock specimen at room temperature.

In a preferred embodiment, colored water is injected into the first wellbore, so that the main fractures are colored accordingly.

In a preferred embodiment, before calculating the thermal stress of the rock specimen, the rock specimen is sectioned to verify that there are no colored vertical branch fractures on both sides of the colored main fracture, then the vertical branch Cracks are thermal stress cracks generated under the action of thermal stress.

In a preferred embodiment, after the step of obtaining a main fracture perpendicular to the direction of the minimum confining compressive stress, the confining pressure needs to be released before the step of applying a predetermined three-dimensional confining compressive stress to the rock specimen, The rock specimen was allowed to stand for two weeks at room temperature, so that the water in the rock specimen was fully volatilized.

In a preferred embodiment, a Dewar flask is used to inject liquid nitrogen into the first wellbore, and at this time, low-temperature nitrogen and/or liquid nitrogen flow out of the second wellbore.

In a preferred embodiment, an acoustic emission device is used to measure the fracture signal inside the rock specimen.

In a preferred embodiment, the tensile strength of the rock specimen at room temperature is obtained by the Brazilian splitting test.

In addition, the present application also provides a method for measuring the thermal stress of a rock specimen under the action of liquid nitrogen as described in any of the above, comprising:

a rock sample, the upper part of the rock sample is provided with a drill hole extending to the inside of the rock sample;

A first wellbore and a second wellbore are preset within the borehole.

In addition, the present application also provides a device for measuring the thermal stress of a rock specimen under the action of liquid nitrogen using any of the above-mentioned methods for measuring the thermal stress of a rock specimen under the action of liquid nitrogen, including:

an acquisition module, configured to acquire the tensile strength of the rock specimen at room temperature;

a preset module configured to preset a first wellbore and a second wellbore for the rock specimen;

A water injection module is configured to apply three-dimensional confining stress to the rock specimen, inject water into the first wellbore, and plug the second wellbore, so that water fracturing the rock test piece, and a main crack perpendicular to the direction of the minimum confining compressive stress is obtained;

The measurement module is configured to apply a predetermined three-direction confining stress of equal magnitude to the rock specimen, inject liquid nitrogen into the first wellbore, the second wellbore flows out liquid nitrogen, and gradually decreases parallel to the The confining compressive stress in the direction of the main fracture is measured, and the confining compressive stress corresponding to the first rupture signal when the rupture signal inside the rock specimen increases sharply is measured, and the confining compressive stress is the critical confining compressive stress;

The calculation module is configured so that the thermal stress of the rock specimen is the sum of the critical confining compressive stress and the tensile strength.

In addition, the present application also provides a device for measuring thermal stress of a rock specimen under the action of liquid nitrogen, comprising a memory and a processor, wherein a computer program is stored in the memory, and when the computer program is executed by the processor, the following steps are implemented: The method for measuring the thermal stress of a rock specimen under the action of liquid nitrogen as described in any one of the above.

By the above technical solutions, the beneficial effects of the present application are:

Compared with the prior art, the present invention avoids the problem of failure of sensors such as strain gauges or optical fibers used in conventional measurement of thermal stress under the action of ultra-low temperature liquid nitrogen, and makes up for the blank that the thermal stress of rocks cannot be effectively measured in an ultra-low temperature environment of liquid nitrogen. , to provide a basis for studying the mechanism of liquid nitrogen ultra-low temperature rock breaking.

With reference to the following description and drawings, specific embodiments of the present application are disclosed in detail, indicating the manner in which the principles of the present application may be employed. It should be understood that the embodiments of the present application are not thereby limited in scope. Embodiments of the present application include many changes, modifications and equivalents within the spirit and scope of the appended claims.

Features described and/or illustrated for one embodiment may be used in the same or similar manner in one or more other embodiments, in combination with, or instead of features in other embodiments .

It should be emphasized that the term "comprising/comprising" when used herein refers to the presence of a feature, integer, step or component, but does not exclude the presence or addition of one or more other features, integers, steps or components.

Description of drawings

The drawings described herein are for explanatory purposes only and are not intended to limit the scope of the present disclosure in any way. In addition, the shapes and proportions of the components in the figures are only schematic and are used to help the understanding of the present application, and do not specifically limit the shapes and proportions of the components of the present application. Under the teachings of the present application, those skilled in the art can select various possible shapes and proportions according to specific conditions to implement the present application. In the attached image:

1 is a flowchart of a method for measuring thermal stress of a rock specimen under the action of liquid nitrogen according to an embodiment of the application;

2 is a schematic structural diagram of a first wellbore and a second wellbore preset for a rock specimen according to an embodiment of the present application;

3 is a schematic diagram of a pressure curve of a main fracture generated by water injection fracturing according to an embodiment of the present application;

Fig. 4(a) is a schematic diagram of the variation of confining pressure perpendicular to the direction of the main fracture during the process of injecting liquid nitrogen into the rock specimen according to the embodiment of the present application;

4(b) and 4(c) are schematic diagrams of monitoring results of the acoustic emission device according to the embodiment of the present application;

FIG. 5 is a schematic diagram of a main crack and a thermal stress crack according to an embodiment of the present application;

FIG. 6 is a block diagram of a device for measuring thermal stress of a rock specimen under the action of liquid nitrogen according to an embodiment of the present application.

Reference symbols of the above drawings: 1. Rock specimen; 2. Drilling hole; 3. First wellbore; 4. Second wellbore; 5. Epoxy resin; 6. Critical confining stress; 7. Main fracture; 8 , Thermal stress cracks (vertical branch cracks).

Detailed ways

The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, but not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

It should be noted that when an element is referred to as being "disposed on" another element, it can be directly on the other element or intervening elements may also be present. When an element is referred to as being "connected" to another element, it can be directly connected to the other element or intervening elements may also be present. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for the purpose of illustration only and do not represent the only embodiment.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the technical field to which this application belongs. The terms used herein in the specification of the present application are for the purpose of describing particular embodiments only, and are not intended to limit the present application. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

As shown in FIG. 1 , the present invention provides a method for measuring thermal stress of a rock specimen under the action of liquid nitrogen. The method includes the following steps:

S1: Obtain the tensile strength of rock specimen 1 at room temperature.

S2: Preset a first wellbore 3 and a second wellbore 4 for the rock specimen 1 .

S3: Apply three-dimensional confining stress to the rock specimen 1, inject water into the first wellbore 3, and plug the second wellbore 4, so that water fracturing the rock test piece Part 1, and a main fracture 7 perpendicular to the direction of the minimum confining compressive stress is obtained.

S4: Apply a predetermined three-way confining stress of equal magnitude to the rock specimen 1, inject liquid nitrogen into the first wellbore 3, the second wellbore 4 flows out liquid nitrogen, and gradually reduce a line parallel to the The confining compressive stress in the direction of the main fracture 7 is measured, and the confining compressive stress corresponding to the first rupture signal when the rupture signal inside the rock specimen 1 increases sharply is measured, and the confining compressive stress is the critical confining compressive stress 6 .

S5: The thermal stress of the rock specimen 1 is the sum of the critical confining compressive stress 6 and the tensile strength.

Specifically, the tensile strength σ _t of the rock specimen 1 at room temperature was obtained by the Brazilian splitting test. The choice to measure the tensile strength of rock specimen 1 at room temperature instead of measuring the tensile strength of rock specimen 1 under the action of liquid nitrogen ultra-low temperature is mainly due to the temperature difference required for the formation of thermal stress. The tensile strength is the result of thermal stress, that is, in this case, both the surface and the interior of rock specimen 1 are in an ultra-low temperature state, and there is no temperature difference. Therefore, the tensile strength of rock specimen 1 at room temperature can only be measured correctly. Calculate the thermal stress of the rock.

Then, as shown in FIG. 2, a drill hole 2 extending into the interior of the rock sample 1 is opened at the center of the top of the rock sample 1, and epoxy resin 5 is used to preset two wellbores in the drill hole 2, respectively For the first wellbore 3 and the second wellbore 4, let it stand for a week, so that the first wellbore 3 and the second wellbore 4 are firmly bonded.

Three-dimensional confining compressive stresses of different sizes are applied to the rock specimen 1, which are 2 MPa, 8 MPa and 10 MPa, respectively. The reason for this choice is that in general hydraulic fracturing is easy to form a single hydraulic fracture when the confining stress difference is greater than 5MPa. In fact, the test is not limited to 2MPa, 8MPa and 10MPa, as long as the minimum confining stress difference is greater than 5MPa.

Colored water can be used as fracturing fluid to be injected into the first wellbore 3 to block the second wellbore 4, and a main fracture 7 perpendicular to the direction of the minimum confining stress can be obtained at this time. The internal pressure curve of rock specimen 1 is shown in Figure 3, and then the confining pressure is removed, and the rock specimen 1 can be allowed to stand for two weeks at room temperature, so that the water in the fracturing fluid in the rock specimen 1 can be fully volatilized. Colored water is used as the fracturing fluid, the purpose is to make the main fracture 7 with color to be easily distinguished from the colorless thermal stress fracture 8, and the water property is relatively stable, and it will hardly affect the rock specimen 1 itself after volatilization. nature.

A predetermined three-dimensional confining stress of equal magnitude is applied to the rock specimen 1, assuming that the elastic modulus E of the rock specimen 1 is 5GPa, the Poisson's ratio v is 0.3, and the linear elastic thermal expansion coefficient α _T is 1.5×10 ^-5 /℃, Under the action of liquid nitrogen, the freezing temperature T _cool is -196 ℃, and the room temperature (initial temperature) T _o is 20 ℃. Then, according to the formula:

It can be calculated that the theoretical thermal stress value is about 23 MPa. Assuming that the tensile strength σ _t of the rock specimen 1 at room temperature is 5 MPa, the theoretical critical confining stress is the difference between the theoretical thermal stress value of 23 MPa and the tensile strength of 5 MPa. value is 18MPa. Furthermore, since the predetermined three-directional confining stress value is about 20% larger than the theoretical critical confining compressive stress value, the predetermined three-directional confining compressive stress applied here is all 22 MPa.

Then liquid nitrogen is injected into the first wellbore 3 using a Dewar flask, and liquid nitrogen (low temperature nitrogen) flows out from the second wellbore 4, so that the liquid nitrogen in the Dewar flask can be continuously injected into the rock specimen 1, which is the rock specimen 1 Create a liquid nitrogen ultra-low temperature environment inside. The use of a Dewar flask to inject liquid nitrogen into the first wellbore 3 is mainly due to the low internal pressure of the Dewar flask and the low pressure of the liquid nitrogen flowing into the rock specimen 1, and the liquid nitrogen pressure hardly affects the rock specimen 1. Internal thermal stress crack formation causes disturbance.

During this period, the acoustic emission device can be used to monitor the internal fracture signal of the rock specimen 1. Due to the large predetermined three-way confining stress applied at this time, the thermal stress is difficult to overcome the sum of the confining compressive stress and the tensile strength. There will be no thermal stress cracks 8 on the side, and the acoustic emission device fails to monitor the rupture signal. The monitoring results of the acoustic emission device are shown in Figures 4(b) and 4(c) before point A.

Furthermore, the confining compressive stress parallel to the direction of the main crack 7 is gradually reduced, as shown in Figure 4(a) after point A, when the acoustic emission device signal suddenly increases sharply, as shown in Figures 4(b) and 4(c) At the time corresponding to point B, thermal stress cracks 8 perpendicular to the main crack 7 are generated on both sides of the main crack 7. Find the confining compressive stress corresponding to the first rupture signal at this time. The confining compressive stress It is the critical confining compressive stress 6 (σ _critical ).

In order to further verify the reliability of the acoustic emission device in monitoring the occurrence of thermal stress cracks 8, the rock specimen 1 can be cut for observation, as shown in Figure 5, it is found that there are no color vertical branches on both sides of the colored main crack 7 cracks 8, the vertical branch cracks 8 can be concluded as thermal stress cracks 8 generated under the action of thermal stress.

Then the thermal stress of the rock specimen is the sum of the critical confining compressive stress σ _critical and the tensile strength σ _t .

The principle of measuring the thermal stress of the rock specimen 1 in the present invention is that the formation of thermal stress cracks needs to overcome the confining compressive stress and tensile strength. By measuring the tensile strength of the rock at room temperature and the critical confining compressive stress when the thermal stress crack is formed , the thermal stress of the rock under the ultra-low temperature of liquid nitrogen can be calculated. Compared with the prior art, the present invention avoids the problem of failure of sensors such as strain gauges or optical fibers used in conventional measurement of thermal stress under the action of ultra-low temperature liquid nitrogen, and makes up for the blank that the thermal stress of rocks cannot be effectively measured in an ultra-low temperature environment of liquid nitrogen. , to provide a basis for studying the mechanism of liquid nitrogen ultra-low temperature rock breaking.

Based on the same inventive concept, an embodiment of the present invention also provides a device for measuring thermal stress of a rock specimen under the action of liquid nitrogen, as described in the following embodiments. Since the principle of solving the problem of a device for measuring the thermal stress of a rock specimen under the action of liquid nitrogen is similar to that of a method for measuring the thermal stress of a rock specimen under the action of liquid nitrogen, the device for measuring the thermal stress of a rock specimen under the action of liquid nitrogen For the implementation, please refer to the implementation of the method for measuring thermal stress of rock specimens under the action of liquid nitrogen, and the repetition will not be repeated. As used below, the term "unit" or "module" may be a combination of software and/or hardware that implements a predetermined function. Although the apparatus described in the following embodiments is preferably implemented in software, implementations in hardware, or a combination of software and hardware, are also possible and contemplated.

As shown in Figure 6, a device for measuring the thermal stress of a rock specimen under the action of liquid nitrogen using the method for measuring the thermal stress of a rock specimen under the action of liquid nitrogen described in any one of the above includes:

The obtaining module 101 is configured to obtain the tensile strength of the rock specimen 1 at room temperature.

The preset module 102 is configured to preset the first wellbore 3 and the second wellbore 4 for the rock sample 1 .

The water injection module 103 is configured to apply three-dimensional confining stress of different sizes to the rock specimen 1, inject water into the first wellbore 3, and plug the second wellbore 4, so that the water pressure The rock specimen 1 is fractured, and a main fracture 7 perpendicular to the direction of the minimum confining compressive stress is obtained.

The measurement module 104 is configured to apply a predetermined three-directional confining stress of equal magnitude to the rock specimen 1, inject liquid nitrogen into the first wellbore 3, and the second wellbore 4 flows out of liquid nitrogen, and gradually decreases. The smaller one is the confining compressive stress parallel to the direction of the main fracture 7, and the confining compressive stress corresponding to the first rupture signal when the rupture signal inside the rock specimen 1 increases sharply is measured, and the confining compressive stress is the critical confining compressive stress. Compressive stress 6.

The calculation module 105 is configured so that the thermal stress of the rock specimen 1 is the sum of the critical confining compressive stress 6 and the tensile strength.

In a preferred embodiment, the predetermined three-direction confining stress value is about 20% larger than the theoretical critical confining compressive stress value.

Furthermore, the theoretical critical confining compressive stress value is the difference between the theoretical thermal stress value and the tensile strength, wherein,

Among them, E is the elastic modulus of the rock specimen, v is the Poisson's ratio of the rock specimen, α _T is the linear elastic thermal expansion coefficient of the rock specimen, T _cool is the temperature of the rock specimen under the condition of liquid nitrogen, T _o is the initial temperature of the rock specimen at room temperature.

In a preferred embodiment, colored water is injected into the first wellbore 3 so that the main fractures 7 are colored accordingly.

In a preferred embodiment, before calculating the thermal stress of the rock test piece 1, the rock test piece 1 is cut to verify that there are no color vertical branch cracks 8 on both sides of the colored main crack 7, then The vertical branch cracks 8 are thermal stress cracks 8 generated under the action of thermal stress.

In a preferred embodiment, after the step 7 of obtaining a main fracture perpendicular to the direction of the minimum confining compressive stress, the confining pressure needs to be relieved before the step of applying a predetermined three-dimensional confining compressive stress of equal magnitude to the rock specimen 1 , and let the rock specimen 1 stand for two weeks at room temperature, so that the water in the rock specimen 1 is fully volatilized.

In a preferred embodiment, a Dewar flask is used to inject liquid nitrogen into the first wellbore, and at this time, low-temperature nitrogen and/or liquid nitrogen flow out of the second wellbore.

In a preferred embodiment, an acoustic emission device is used to measure the fracture signal inside the rock specimen 1 .

In a preferred embodiment, the tensile strength of the rock specimen 1 at room temperature is obtained by the Brazilian splitting test.

In addition, the present invention also provides a device for measuring the thermal stress of a rock specimen under the action of liquid nitrogen, comprising a memory and a processor, wherein a computer program is stored in the memory, and when the computer program is executed by the processor, the following steps are implemented: The method for measuring the thermal stress of rock specimens under the action of liquid nitrogen as described above.

In this embodiment, the memory may include a physical device for storing information, usually after digitizing the information, it is stored in a medium using electrical, magnetic, or optical methods. The memory described in this embodiment may further include: devices that use electrical energy to store information, such as RAM, ROM, etc.; devices that use magnetic energy to store information, such as hard disks, floppy disks, magnetic tapes, magnetic core memory, magnetic bubble memory, U disk ; a device that stores information optically, such as a CD or DVD. Of course, there are other ways of memory, such as quantum memory, graphene memory, and so on.

In this embodiment, the processor may be implemented in any suitable manner. For example, the processor may take the form of, for example, a microprocessor or a processor and a computer readable medium storing computer readable program code (eg software or firmware) executable by the (micro)processor, logic gates, switches, application specific integrated Circuit (Application Specific Integrated Circuit, ASIC), programmable logic controller and embedded microcontroller form and so on.

The specific functions implemented by the processor and the memory of the server provided by the embodiments of this specification can be explained in comparison with the foregoing embodiments in this specification.

In another embodiment, a software is also provided, and the software is used to execute the technical solutions described in the above embodiments and preferred embodiments.

In another embodiment, a storage medium is also provided, and the above-mentioned software is stored in the storage medium, and the storage medium includes but is not limited to: an optical disk, a floppy disk, a hard disk, a rewritable memory, and the like.

It can be seen from the above description that the embodiments of the present invention achieve the following technical effects: the present invention avoids the problem that sensors such as strain gauges or optical fibers used in conventional thermal stress measurement operations fail under the action of ultra-low temperature of liquid nitrogen, and The blank that the thermal stress of rock cannot be effectively measured in the ultra-low temperature nitrogen environment provides a basis for studying the mechanism of ultra-low temperature rock breaking with liquid nitrogen.

Obviously, those skilled in the art should understand that each module or each step of the above-mentioned embodiments of the present invention may be implemented by a general-purpose computing device, and they may be centralized on a single computing device, or distributed in multiple computing devices. network, they can optionally be implemented with program code executable by a computing device, so that they can be stored in a storage device and executed by the computing device, and in some cases, can be different from the The illustrated or described steps are performed in order, either by fabricating them separately into individual integrated circuit modules, or by fabricating multiple modules or steps of them into a single integrated circuit module. As such, embodiments of the present invention are not limited to any particular combination of hardware and software.

All articles and references disclosed herein, including patent applications and publications, are hereby incorporated by reference for all purposes. The term "consisting essentially of" describing a combination shall include the identified element, ingredient, component or step as well as other elements, components, components or steps that do not materially affect the essential novel characteristics of the combination. Use of the terms "comprising" or "comprising" to describe combinations of elements, ingredients, components or steps herein also contemplates embodiments consisting essentially of those elements, ingredients, components or steps. By use of the term "may" herein, it is intended to indicate that "may" include any described attributes that are optional.

A plurality of elements, components, components or steps can be provided by a single integrated element, component, component or step. Alternatively, a single integrated element, component, component or step may be divided into separate multiple elements, components, components or steps. The disclosure of "a" or "an" used to describe an element, ingredient, part or step is not intended to exclude other elements, ingredients, parts or steps.

It should be understood that the above description is for purposes of illustration and not limitation. From reading the above description, many embodiments and many applications beyond the examples provided will be apparent to those skilled in the art. The scope of the present teachings should, therefore, be determined not with reference to the above description, but should instead be determined with reference to the preceding claims, along with the full scope of equivalents to which such claims are entitled. The disclosures of all articles and references, including patent applications and publications, are incorporated herein by reference for the purpose of being comprehensive. The omission of any aspect of the subject matter disclosed herein in the preceding claims is not intended to disclaim such subject matter, nor should the applicant be considered as not considering such subject matter as part of the disclosed subject matter.

Claims (11)

1. a method for measuring thermal stress of rock specimen under the action of liquid nitrogen, is characterized in that, comprises the steps: Obtain the tensile strength of the rock specimen at room temperature; Presetting a first wellbore and a second wellbore for the rock specimen; Applying three-dimensional confining stress of different sizes to the rock specimen, injecting water into the first wellbore, and plugging the second wellbore, so that the water fracturing the rock specimen, and obtaining a Main fractures perpendicular to the direction of minimum confining compressive stress; A predetermined three-directional confining stress of equal magnitude is applied to the rock specimen, liquid nitrogen is injected into the first wellbore, liquid nitrogen flows out of the second wellbore, and the confinement parallel to the direction of the main fracture is gradually reduced. compressive stress, measure the confining compressive stress corresponding to the first rupture signal when the rupture signal inside the rock specimen increases sharply, and the confining compressive stress is the critical confining compressive stress; The thermal stress of the rock specimen is the sum of the critical confining compressive stress and the tensile strength. 2 . The method for measuring thermal stress of a rock specimen under the action of liquid nitrogen according to claim 1 , wherein the predetermined three-direction confining stress value is about 20% larger than the theoretical critical confining stress value. 3 . 3. The method for measuring the thermal stress of a rock specimen under the action of liquid nitrogen according to claim 2, wherein the theoretical critical confining compressive stress value is the difference between the theoretical thermal stress value and the tensile strength, wherein,

Among them, E is the elastic modulus of the rock specimen, v is the Poisson's ratio of the rock specimen, α _T is the linear elastic thermal expansion coefficient of the rock specimen, T _cool is the temperature of the rock specimen under the condition of liquid nitrogen, T _o is the initial temperature of the rock specimen at room temperature. 4. The method for measuring the thermal stress of a rock specimen under the action of liquid nitrogen according to claim 1, wherein water with color is used to inject into the first wellbore, so that the main fracture has a corresponding color . 5. The method for measuring the thermal stress of a rock specimen under the action of liquid nitrogen according to claim 4, wherein, before calculating the thermal stress of the rock specimen, the rock specimen is cut to verify that the Colorless vertical branch cracks are produced on both sides of the main crack, and the vertical branch crack is the thermal stress crack generated under the action of thermal stress. 6. The method for measuring the thermal stress of a rock specimen under the action of liquid nitrogen according to claim 1, wherein after the step of obtaining a main fracture perpendicular to the direction of the minimum confining compressive stress, the rock specimen is subjected to Before the predetermined three-dimensional confining stress steps with equal magnitudes, the confining pressure needs to be released, and the rock specimen is allowed to stand for two weeks at room temperature, so that the water in the rock specimen is fully volatilized. 7 . The method for measuring the thermal stress of a rock specimen under the action of liquid nitrogen according to claim 1 , wherein a Dewar flask is used to inject liquid nitrogen into the first wellbore, and the second wellbore flows out at this time. 8 . Cryogenic nitrogen and/or liquid nitrogen. 8 . The method for measuring the thermal stress of a rock specimen under the action of liquid nitrogen according to claim 1 , wherein an acoustic emission device is used to measure the rupture signal inside the rock specimen. 9 . 9 . The method for measuring the thermal stress of a rock specimen under the action of liquid nitrogen according to claim 1 , wherein the tensile strength of the rock specimen at room temperature is obtained by using a Brazilian splitting test. 10 . 10. A device for measuring the thermal stress of a rock specimen under the action of liquid nitrogen using the method for measuring the thermal stress of a rock specimen under the action of liquid nitrogen according to any one of the above claims 1-9, is characterized in that, comprising: an acquisition module, configured to acquire the tensile strength of the rock specimen at room temperature; a preset module configured to preset a first wellbore and a second wellbore for the rock specimen; A water injection module is configured to apply three-dimensional confining stress to the rock specimen, inject water into the first wellbore, and plug the second wellbore, so that water fracturing the rock test piece, and a main crack perpendicular to the direction of the minimum confining compressive stress is obtained; The measurement module is configured to apply a predetermined three-direction confining stress of equal magnitude to the rock specimen, inject liquid nitrogen into the first wellbore, the second wellbore flows out liquid nitrogen, and gradually decreases parallel to the The confining compressive stress in the direction of the main fracture is measured, and the confining compressive stress corresponding to the first rupture signal when the rupture signal inside the rock specimen increases sharply is measured, and the confining compressive stress is the critical confining compressive stress; The calculation module is configured so that the thermal stress of the rock specimen is the sum of the critical confining compressive stress and the tensile strength. 11. A device for measuring thermal stress of rock specimen under the action of liquid nitrogen, characterized in that it comprises a memory and a processor, and a computer program is stored in the memory, and the computer program, when executed by the processor, realizes the following steps: The method for measuring the thermal stress of a rock specimen under the action of liquid nitrogen according to any one of claims 1 to 9.

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