Hydraulic activation of natural fracture indoor shear simulation method

Abstract

The present application belongs to the field of rock mechanics and engineering technology, and discloses a kind of hydraulic activation natural fracture indoor shearing simulation method, comprising: preparing sample with natural fracture, and placing sample in fracturing fluid until reaching saturation state;Based on shear test device, initial normal stress and tangential stress are applied to the sample;Change normal load with set variable normal force control mode until crack sliding occurs, record the tangential displacement time curve and normal displacement time curve of the sample, and the time variation curve of apparent friction coefficient.The hydraulic activation natural fracture indoor shearing simulation method provided by the present application has the characteristics of simplicity, high efficiency, good practical effect and wide applicability.

Description

Technical Field

[0001] This invention relates to the field of rock mechanics and engineering technology, and in particular to a method for simulating indoor shearing of natural fractures using hydraulic activation. Background Technology

[0002] Enhanced geothermal systems are a clean energy concept proposed based on hot dry rock technology. Hydraulic fracturing to establish artificial thermal reservoirs is one of the effective methods for exploiting underground hot dry rock thermal energy. However, there is little research on the activation process of natural fractures under hydraulic fracturing. Triaxial fracturing fluid injection tests can also be used to study the activation process of natural fractures, but this test is very complex and it is difficult to maintain constant normal and tangential forces during the test. There is an urgent need for a rapid indoor simulation method for hydraulic activation of natural fractures. Summary of the Invention

[0003] This invention provides a method for simulating the indoor shear of natural fractures by hydraulic activation. It aims to simulate the effect of fracturing fluid on rocks to a certain extent by saturating the fracturing fluid and simulating the pressure effect of the fracturing fluid by unloading the load, thereby simplifying the test operation and improving the test efficiency.

[0004] To address the aforementioned technical problems, this invention provides a method for hydraulically activated indoor shear simulation of natural fractures, comprising: Prepare a sample with cracks and place the sample in fracturing fluid until it reaches saturation; An initial normal stress and tangential stress are applied to the specimen using a shear testing device; The normal load was changed using a set variable normal force control mode until the crack slipped. The tangential displacement-time curve, normal displacement-time curve, and apparent friction coefficient time variation curve of the specimen were recorded.

[0005] Furthermore, the preparation of the pattern with cracks includes: Natural cracks of varying roughness are formed within natural specimens by splitting or shearing methods.

[0006] Furthermore, the natural sample is cubic.

[0007] Furthermore, the method for obtaining the roughness of the natural crack includes: A 3D scanner was used to acquire surface morphology information of natural cracks, and several traces were extracted along the shear direction to calculate the roughness coefficient JRC. The roughness coefficient JRC is calculated using the following formula:

[0008]

[0009] In the formula: x and z are the X-axis coordinates and Z-axis coordinates of each point on the trace, respectively; L is the projected length of the trace in the shearing direction; and n is the total number of points.

[0010] Furthermore, the number of natural samples is multiple, and the multiple natural samples are distinguished according to the roughness coefficient JRC.

[0011] Furthermore, the initial normal stress and the tangential stress are determined based on the in-situ stress field of the natural sample.

[0012] Furthermore, the determination based on the in-situ stress field of the natural sample includes:

[0013]

[0014] In the formula, σ is the normal stress, τ is the tangential stress, σ1 is the maximum principal stress, σ3 is the minimum principal stress, and β is the angle between the natural crack and the principal stress.

[0015] Furthermore, the step of changing the normal load according to the set variable normal force control mode until the crack slides includes: Monotonous unloading; Cyclic loading and unloading of normal forces were used to simulate different water injection conditions on site.

[0016] Furthermore, when implementing the variable normal force control mode to simulate the change of normal load, the unloading rate, cyclic amplitude, and period should be obtained based on the actual parameters at the engineering site.

[0017] One or more technical solutions provided in the embodiments of this application have at least the following technical effects or advantages: The hydraulically activated natural fracture indoor shear simulation method provided in this application simulates the fracturing process of a natural fracture indoors by collecting in-situ natural samples, creating fractures on them, applying normal and tangential forces to the samples, and controlling the unloading of the normal force to achieve sliding along the fracture, thus simulating the fracturing effect. This method greatly simplifies the operation, improves simulation efficiency and convenience, and can reasonably and appropriately simulate the fracturing process. Attached Figure Description To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram illustrating the effects of normal and tangential forces provided in an embodiment of the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0020] It should be noted that all directional indications in the embodiments of this application are only used to explain the relative positional relationship and movement of each component in a specific posture. If the specific posture changes, the directional indications will also change accordingly.

[0021] The following disclosure provides numerous different embodiments or examples for implementing various structures of this application. To simplify the disclosure, specific examples of components and arrangements are described below. These are merely examples and are not intended to limit the scope of this application. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed. In addition, various specific examples of processes and materials are provided in this application, but those skilled in the art will recognize the application of other processes and / or the use of other materials.

[0022] This application is described below with reference to the accompanying drawings and specific embodiments.

[0023] This application provides a hydraulically activated indoor shear simulation method for natural fractures, which aims to simulate the effect of fracturing fluid on rocks to a certain extent by saturating the fracturing fluid and simulating the pressure effect of the fracturing fluid by unloading, thereby simplifying the test operation and improving the test efficiency.

[0024] To better understand the above technical solutions, the following will describe the above technical solutions in detail with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments of the present invention and the specific features in the embodiments are detailed descriptions of the technical solutions of this application, rather than limitations on the technical solutions of this application. In the absence of conflict, the embodiments of this application and the technical features in the embodiments can be combined with each other.

[0025] See Figure 1 This application provides a method for simulating the indoor shearing of water-induced natural cracks by hydraulic activation. The method involves preparing cracks using in-situ natural samples, applying normal and tangential forces, and then controlling the unloading of the normal force to achieve relative sliding of the samples. This method simulates the water-induced crack activation process indoors, greatly simplifying the simulation operation and improving operational efficiency.

[0026] The simulation method is described in detail below, including: Prepare fractured samples and immerse them in fracturing fluid until saturation. Cubic rock samples can be obtained in situ and fractures can be created within the samples using methods such as splitting or shearing. Generally, fractured rock samples are immersed in fracturing fluid until saturation, thus fully restoring their in-situ state and making the simulation closely approximate actual conditions. Generally, the fracturing fluid should be prepared according to the actual mixing ratio.

[0027] The initial normal and tangential stresses are applied to the sample using a shear testing device. The natural sample can be placed inside the shear testing device, and its internal multiaxial force-applying structure can be used to apply force along the normal and tangential directions of the fracture to the upper rock mass, thereby compressing the upper and lower rock masses on both sides of the fracture. Generally, the initial normal and tangential stresses can be determined based on the in-situ stress field of the natural sample, or they can be converted from the database of water-induced fracturing tests, or they can be selected and set according to the range of experimental experience data.

[0028] The normal load is changed by a set variable normal force control mode until the crack slides. The normal force is unloaded at a certain rate to weaken the compaction effect and relatively amplify the effect of the tangential force until the rock mass on both sides of the crack moves relative to each other. During this process, the tangential displacement time curve, normal displacement time curve, and apparent friction coefficient time change curve of the sample are recorded to complete the simulation test and data acquisition.

[0029] It is worth noting that the tangential displacement-time curve refers to the time curve of relative displacement in the normal direction. The apparent friction coefficient refers to the curve showing how the ratio of normal force to tangential force changes over time.

[0030] Generally, a series of tests can be set up, which requires setting up multiple natural rock samples with different crack roughness. Natural cracks with different roughness can be formed in the natural samples by splitting or shearing methods, and then natural rock samples are selected based on the roughness.

[0031] Specifically, the method for obtaining the roughness of the natural cracks includes: A 3D scanner was used to acquire surface morphology information of natural cracks, and several traces were extracted along the shear direction to calculate the roughness coefficient JRC.

[0032] The roughness coefficient JRC is calculated using the following formula:

[0033]

[0034] In the formula: x and z are the X-axis coordinates and Z-axis coordinates of each point on the trace, respectively; L is the projected length of the trace in the shearing direction; and n is the total number of points.

[0035] Furthermore, the determination based on the in-situ stress field of the natural sample can be the actual stress range measured on-site, empirical data, or set data that conforms to the laws of stress field. Specific conversion methods include:

[0036]

[0037] In the formula, σ is the normal stress, τ is the tangential stress, σ1 is the maximum principal stress, σ3 is the minimum principal stress, and β is the angle between the natural crack and the principal stress.

[0038] It is worth noting that the shear simulation operation can be performed once or repeatedly. That is, the normal force can be loaded and unloaded repeatedly to perform a cyclic shear simulation operation. During the cyclic shear simulation operation, the amplitude and switching frequency of the normal force in each cycle are recorded.

[0039] The process of changing the normal load using a set variable normal force control mode until the crack slides includes: Monotonic unloading; cyclic loading and unloading of normal forces to simulate different water injection conditions on site.

[0040] When implementing the variable normal force control mode to simulate changing the normal load, the unloading rate, cyclic amplitude, and period should be obtained based on the actual parameters at the engineering site.

[0041] One or more technical solutions provided in the embodiments of this application have at least the following technical effects or advantages: The hydraulically activated natural fracture indoor shear simulation method provided in this application simulates the fracturing process of a natural fracture indoors by collecting in-situ natural samples, creating fractures on them, applying normal and tangential forces to the samples, and controlling the unloading of the normal force to achieve sliding along the fracture, thus simulating the fracturing effect. This method greatly simplifies the operation, improves simulation efficiency and convenience, and can reasonably and appropriately simulate the fracturing process. In this application, unless otherwise expressly specified and limited, the terms "connection," "fixed," etc., should be interpreted broadly. For example, "fixed" can mean a fixed connection, a detachable connection, or an integral part; it can mean a mechanical connection or an electrical connection; it can mean a direct connection or an indirect connection through an intermediate medium; it can mean the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0042] In the description of this application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0043] Furthermore, the use of terms such as "first" and "second" in this application is for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the technical solutions of the various embodiments can be combined with each other, but only on the basis of being achievable by those skilled in the art. If the combination of technical solutions is contradictory or impossible to implement, such a combination of technical solutions should be considered non-existent and not within the scope of protection claimed in this application.

[0044] In the description of this invention, unless otherwise expressly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature being directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature being directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0045] In the description of this specification, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification. Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0046] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A method for simulating indoor shearing of natural cracks using hydraulic activation, characterized in that, include: Prepare a sample with natural fractures and place the sample in fracturing fluid until it reaches saturation; An initial normal stress and tangential stress are applied to the specimen using a shear testing device; The normal load was changed by a set variable normal force control mode until the crack slipped, thereby simulating the pressure effect of fracturing fluid by unloading. Then the tangential displacement time curve, normal displacement time curve, and apparent friction coefficient time change curve of the sample were recorded. The step of changing the normal load according to the set variable normal force control mode until the crack slides includes: Monotonous unloading; Cyclic loading and unloading of normal forces were used to simulate different fracturing fluid injection conditions in the field.

2. The method for simulating indoor shearing of natural cracks by hydraulic activation as described in claim 1, characterized in that, The preparation of the pattern with cracks includes: Natural cracks of varying roughness are formed within rock samples by splitting or shearing methods.

3. The method for simulating indoor shearing of natural cracks by hydraulic activation as described in claim 2, characterized in that, The rock sample was cubic.

4. The method for simulating indoor shearing of natural cracks by hydraulic activation as described in claim 2, characterized in that, The method for obtaining the roughness of the natural cracks includes: A 3D scanner was used to acquire surface morphology information of natural cracks, and several traces were extracted along the shear direction to calculate the roughness coefficient JRC. The roughness coefficient JRC is calculated using the following formula: In the formula: x and z are the X-axis coordinates and Z-axis coordinates of each point on the trace, respectively; L is the projected length of the trace in the shearing direction; and n is the total number of points.

5. The method for simulating indoor shearing of natural cracks by hydraulic activation as described in claim 4, characterized in that, The number of natural samples is multiple, and the multiple natural samples are distinguished according to the roughness coefficient JRC.

6. The method for simulating indoor shearing of natural cracks by hydraulic activation as described in claim 1, characterized in that, The initial normal stress and the tangential stress are determined based on the in-situ stress field of the natural sample.

7. The method for simulating indoor shearing of natural cracks by hydraulic activation as described in claim 6, characterized in that, The determination based on the in-situ stress field of the natural sample includes: In the formula, σ is the normal stress, τ is the tangential stress, σ1 is the maximum principal stress, σ3 is the minimum principal stress, and β is the angle between the natural crack and the principal stress.

8. The method for simulating indoor shearing of natural cracks by hydraulic activation as described in any one of claims 1-7, characterized in that, When implementing the variable normal force control mode to simulate changing the normal load, the unloading rate, cyclic amplitude, and period should be obtained based on the actual parameters at the engineering site.

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