Study on the test method of the connection mechanism between hydraulic fractures and high-angle shear fractures

Abstract

The application discloses a test method for studying the connection mechanism of hydraulic fractures and high-angle shear type fractures, and belongs to the technical field of indoor test of rock mass hydraulic fracturing. The test method for studying the connection mechanism of hydraulic fractures and high-angle shear type fractures preforms a through fracture at the side of a sample close to the top, performs high-temperature thermal shock treatment on the inner side of the through fracture, thereby forming a gradient damage zone, and processes a perforation upward in the middle of the bottom surface of the sample; the treated sample is placed in an experimental device, and a plurality of acoustic emission probes are pasted on the surface of the sample; a surrounding pressure and an axial pressure are sequentially applied by using a staged loading mode, then high-pressure water fracturing is continuously carried out in the perforation at a constant rate, and the fracture pressure and the corresponding flow rate are recorded when the sample is broken; the hydraulic fracturing process is analyzed according to the flow-pressure curve obtained by the test and the information collected by the acoustic emission probes, and the connection law of the hydraulic fracturing fractures and the through fractures is obtained by combining the CT three-dimensional reconstruction technology after the test.

Description

Technical Field

[0001] This invention relates to the field of laboratory testing technology for hydraulic fracturing of rock masses, specifically to an experimental method for studying the connection mechanism between hydraulic fractures and high-angle shear fractures. Background Technology

[0002] High-angle structural fractures are the main channels for sudden water inrush at the working face of deep vertical shafts in deeply buried rock strata, seriously threatening the safety of shaft construction. Hydraulic fracturing to open this main channel and then grouting with cement-based grout materials to seal it is of great significance for eliminating the risk of water inrush at the working face and reducing the amount of water inrush at the working face.

[0003] The non-uniform damage field induced by the shear-tension composite characteristics of high-angle structural fractures has a significant guiding effect on hydraulic fracture propagation. Traditional grouting and water shut-off methods are mostly based on ideal fracture models and fail to consider the influence of the fracture interface damage gradient on the fracturing fluid migration path, resulting in difficulty in accurately controlling the sealing effect. Our research group previously established a theory of controlling fractures in vertical shaft working faces and proposed a series of technologies such as "a grouting and water shut-off method for vertical shaft working faces" and "a two-stage perforation hydraulic fracturing grouting method." However, the existing experimental system still has two major limitations: first, it has not constructed a realistic physical model of fractures with shear characteristics; second, it lacks characterization methods for the stress sensitivity characteristics of the fracture damage gradient zone. Given that the damage gradient zone is prevalent in the near field of formation fractures, the local strength weakening effect it causes can significantly change the hydraulic fracture propagation trajectory. Therefore, it is urgent to establish a visual experimental system that integrates damage gradient characteristics to provide theoretical support for optimizing grouting process parameters. Summary of the Invention

[0004] The purpose of this invention is to overcome the problems in the prior art and provide an experimental method for studying the connection mechanism between hydraulic fractures and high-angle shear fractures, and to study the turning and propagation law of hydraulic fractures in the existing fracture damage gradient region.

[0005] This invention provides an experimental method for studying the connection mechanism between hydraulic fractures and high-angle shear fractures, comprising the following steps:

[0006] Prepare a sample and pre-fabricate a through crack with an angle of 70°~90° on the side of the sample near the top. Perform high-temperature heat bombardment on the inside of the through crack to form a gradient damage zone. Then, process a perforation hole upward from the center of the bottom surface of the sample.

[0007] The treated sample is placed inside the experimental apparatus, and several acoustic emission probes are attached to the surface of the sample. The several acoustic emission probes are arranged in a ring array on the surface of the sample.

[0008] Confining pressure and axial pressure were applied sequentially using a staged loading mode, followed by continuous high-pressure water fracturing into the perforation at a constant rate. When the sample ruptured, the rupture pressure and corresponding flow rate were recorded.

[0009] Based on the flow-pressure curves obtained from the experiment and the information collected by the acoustic emission probe, the hydraulic fracturing process was analyzed. Combined with the CT three-dimensional reconstruction technology after the experiment, the connection law between hydraulic fracturing fractures and through fractures was obtained.

[0010] Preferably, a stainless steel sleeve is embedded in the perforation, and the sleeve extends into the perforation for 75% to 80% of the total perforation length, with the remaining 20% ​​to 25% of the perforation length being a bare section; an upper pad and a lower base are respectively provided above and below the sample, and the two ends of the sample, the upper pad, and the lower base are sealed with heat-shrink film.

[0011] Preferably, the sample is prepared using a thin-layer deposition-selective sintering process, employing a composite substrate of more than 90% albite powder and less than 10% quartz sand. A cylindrical standard sample with a diameter of 95mm~100mm×200mm~205mm is prepared by temperature history control. The albite powder content is constant and uniform at all heights in the sample. The mass ratio of quartz sand to albite powder between the top of the through crack and the lowest point of the exposed hole gradually decreases from bottom to top. The mass ratio of quartz sand to albite powder below the lowest point of the exposed hole remains constant and uniform.

[0012] Preferably, the through crack is obtained by wire cutting, the through crack opening is 3mm~5mm, the distance from the center point to the upper end face of the sample is 40mm~45mm, and the high temperature heat bombardment treatment temperature is 800℃~900℃.

[0013] Preferably, the confining pressure control system is activated to slowly increase the confining pressure to 5MPa~40MPa at a rate of 0.2~0.5 MPa / min and maintain it for 5~10 min; then, the axial pressure control system is activated to slowly increase the axial pressure to 5MPa~40MPa at a rate of 0.1~0.4 mm / min and maintain it for 5~10 min; subsequently, the water pressure loading system is activated to inject water and pressurize the bare hole section in the perforation through the sleeve pre-embedded in the middle of the sample at a rate of 3~5 mm / min, while recording the pressure and flow rate changes. During the water injection process, the water pressure mutation point is observed in real time to determine the occurrence of sample rupture. After the sample ruptures, the rupture pressure and water flow rate are recorded and the relevant data are saved.

[0014] Ideally, the spacing between each acoustic emission probe should be 60° to 90°.

[0015] Preferably, the perforation channel is a cylindrical channel with a diameter of Φ5~7mm×85~105mm, and the end of the perforation is at least 50mm away from the center of the through crack.

[0016] Preferably, epoxy resin is used to seal the annular space between the sleeve and the perforation wall, the sleeve is embedded in the lower base and sealed by a sealing ring.

[0017] Compared with existing technologies, the beneficial effects of this invention are as follows: by prefabricating high-angle penetrating fractures and subjecting them to high-temperature thermal bombardment to form a gradient damage zone, the invention successfully simulates for the first time in indoor tests the "shear-tension composite fracture and its surrounding non-uniform damage field," which is common in underground rock formations but previously ignored by models; through the linkage of multiple monitoring and characterization methods such as emission monitoring, flow-pressure curve analysis, and CT three-dimensional reconstruction, the invention can accurately capture the entire process of hydraulic fracture initiation, propagation, and eventual connection with natural fractures, thereby optimizing and improving the active fracturing grouting water shut-off process parameters and providing key experimental support for the prevention and control of deep well water inrush disasters. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the overall structure of the present invention.

[0019] Figure 2 This is a schematic diagram of the sample structure of the present invention.

[0020] Figure 3 This is a schematic diagram of the through crack and perforation structure opened on the sample of the present invention.

[0021] Explanation of reference numerals in the attached drawings: 1. Sample; 2. Through crack; 3. Perforation; 4. Sleeve; 5. Bare hole section; 6. Heat shrink film; 7. Upper pad; 8. Lower base; 9. Acoustic emission probe. Detailed Implementation

[0022] The following is in conjunction with the appendix Figures 1-3 To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the described embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art.

[0023] The terms "first," "second," and similar words used in the specification and claims of this patent application do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed after the word and their equivalents, without excluding other elements or objects. Terms such as "inner," "outer," "upper," "lower," "far," "near," "front," and "rear" are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly. The drawings in this invention are not strictly drawn to scale; the specific dimensions and quantity of each structure can be determined according to actual needs. The drawings described in this invention are merely structural schematic diagrams.

[0024] The present invention provides an experimental method for the connection mechanism between hydraulic fractures and high-angle shear fractures, such as... Figures 1-3 As shown, it includes:

[0025] Prepare sample 1, and pre-fabricate a through crack 2 with an angle of 70°~90° on the side of sample 1 near the top. Perform high-temperature heat bombardment on the inside of the through crack to form a gradient damage zone, and process a perforation 3 upward from the center of the bottom surface of sample 1.

[0026] The treated sample 1 is placed in the experimental apparatus, and several acoustic emission probes 9 are attached to the surface of the sample 1. The several acoustic emission probes 9 are arranged in a ring array on the surface of the sample 1.

[0027] Confining pressure and axial pressure were applied sequentially using a staged loading mode, followed by high-pressure water fracturing into perforation 3 at a constant rate. When sample 1 ruptured, the rupture pressure and corresponding flow rate were recorded.

[0028] Based on the flow-pressure curves obtained from the experiment and the information collected by the acoustic emission probe 9, the hydraulic fracturing process was analyzed. Combined with the CT three-dimensional reconstruction technology after the experiment, the connection law between the hydraulic fracturing fracture and the through fracture 2 was obtained.

[0029] In this embodiment, by prefabricating a high-angle through-crack 2 and subjecting it to high-temperature thermal bombardment to form a gradient damage zone, the "shear-tension composite crack and its surrounding non-uniform damage field," which is common in underground rock strata but was previously ignored by models, was successfully simulated for the first time in an indoor test. By linking multiple monitoring and characterization methods such as emission monitoring, flow-pressure curve analysis, and CT three-dimensional reconstruction, the entire process of hydraulic crack initiation, propagation, and eventual connection with natural cracks can be accurately captured. Based on this, the active fracturing grouting water shut-off process parameters can be optimized and improved, providing key experimental support for the prevention and control of deep well water inrush disasters.

[0030] Preferred, such as Figures 1-3As shown, a stainless steel sleeve 4 is embedded in the perforation 3. The sleeve 4 enters the perforation 3 for 75% to 80% of the total length of the perforation 3, and the remaining 20% ​​to 25% of the perforation 3 length is the bare hole section 5. An upper pad 7 and a lower base 8 are respectively set above and below the sample 1, and the two ends of the sample 1 are sealed with the upper pad 7 and the lower base 8 respectively by heat shrink film 6. The annular space between the sleeve 4 and the hole wall of the perforation 3 is sealed with epoxy resin. The sleeve 4 is embedded in the lower base 8 and sealed with a sealing ring.

[0031] In this embodiment, the hydraulic fracturing test needs to be carried out under high pressure. Any seal failure will lead to pressure leakage, which will not only cause the test to fail, but also cause the collected pressure and flow data to be distorted and unable to reflect the true fracture propagation pressure. By introducing the design of stainless steel sleeve 4 and bare hole section 5, combined with epoxy resin sealing and sealing ring structure, as well as heat shrink film 6 sealing method, a multi-seal system is constructed to ensure the sealing of the water injection channel under the complex loading environment of high confining pressure, high axial pressure and high water pressure, so that the applied pressure can be fully applied to the perforation section 3 of sample 1, thereby accurately recording the true fracture pressure when sample 1 fractures.

[0032] Preferred, such as Figure 1 As shown, Sample 1 is prepared using a thin-layer stacking-selective sintering process. It is a composite substrate with a proportion greater than 90% of albite powder and a proportion less than 10% of quartz sand. The cylindrical standard sample 1 with a diameter of Φ95mm~100mm×200mm~205mm is prepared by temperature history control. The albite powder content is constant and uniform at all heights in Sample 1. The mass ratio of quartz sand to albite powder in Sample 1 between the top of the crack 2 and the lowest end of the bare hole section 5 gradually decreases from bottom to top. The mass ratio of quartz sand to albite powder below the lowest end of the bare hole section 5 remains constant and uniform.

[0033] In this embodiment, the natural rock core sample 1 is heterogeneous, highly anisotropic, and has poor repeatability, resulting in discrete test results that are difficult to replicate and verify, and difficult to summarize patterns. The homogeneous sample 1 cannot simulate the heterogeneous characteristics of the formation. This scheme limits the preparation process of sample 1 (thin-layer stacking-selective sintering) and the specific material ratio (albite powder >90%, quartz sand <10%). It also innovatively proposes a preparation method that achieves a gradient change in the mass ratio of quartz sand in the region between the through fracture 2 and the bare borehole section 5, ensuring a high degree of consistency in the physical and mechanical properties among different samples 1, making the test results highly repeatable and comparable, facilitating serialized studies. By precisely controlling the material property gradient between the damage zone and the perforation 3, the influence of material heterogeneity on the propagation path of hydraulic fractures can be quantitatively studied, which is impossible with natural rock samples. The choice of a mixture of albite and quartz sand is closer to the mineral composition of common underground rocks, improving the geological representativeness of the test model.

[0034] Preferred, such as Figures 1-3 As shown, the through crack 2 was obtained by wire cutting. The opening of the through crack 2 is 3mm~5mm. The distance between the center point and the upper end face of the sample 1 is 40mm~45mm. The high temperature heat bombardment temperature is 800℃~900℃. The perforation 3 channel is a cylindrical channel with a diameter of Φ5~7mm×85~105mm. The distance between the end of the perforation 3 and the center of the through crack 2 is not less than 50mm.

[0035] In this embodiment, the opening (3-5mm) and location (40mm-45mm from the top) of the through crack 2, the size (Φ5mm-7mm) of the perforation 3 and its distance from the crack (≥50mm) are clearly defined to avoid introducing uncontrollable variables due to the uncertainty of key dimensions and locations, and to ensure that the boundary conditions of each test are consistent. For example, it ensures that there is sufficient distance between the through crack 2 and the perforation 3 to provide space for the full expansion of the hydraulic crack, avoids initial connection, and thus enables clear observation of the expansion trajectory.

[0036] Preferred, such as Figure 1 As shown, the confining pressure control system is activated, and the confining pressure is slowly increased to 5MPa~40MPa at a rate of 0.2~0.5 MPa / min, and maintained for 5~10 min. Then, the axial pressure control system is activated, and the axial pressure is slowly increased to 5MPa~40MPa at a rate of 0.1~0.4 mm / min, and maintained for 5~10 min. Subsequently, the water pressure loading system is activated, and water is injected into the bare hole section 5 of the perforation 3 through the sleeve 4 pre-embedded in the middle of the sample 1 at a rate of 3~5 mm / min. At the same time, the pressure and flow rate changes are recorded. During the water injection process, the water pressure change point is observed in real time to determine the occurrence of sample 1 rupture. After sample 1 ruptures, the rupture pressure and water flow rate are recorded, and the relevant data are saved.

[0037] In this embodiment, a staged loading mode was adopted. First, the confining pressure was applied and stabilized at a slow rate, then the axial pressure was applied and stabilized, and finally water injection fracturing was performed. This accurately simulated the confining pressure (geo-stress) and axial pressure (overlying strata pressure) environment of the underground rock. The step of maintaining the pressure for 5-10 minutes ensured that the stress was evenly distributed and tended to stabilize inside the sample 1, providing a real and stable stress background for subsequent hydraulic fracturing. Water injection at a constant rate can obtain a smooth pressure-time curve, which is convenient for accurately determining the rupture point.

[0038] Preferred, such as Figure 1 As shown, the spacing between each acoustic emission probe 9 is 60°~90°.

[0039] In this embodiment, multiple acoustic emission probes 9 with a spacing of 60°-90° are arranged in a ring array to collect acoustic emission signals generated during the propagation of hydraulic cracks in real time. Through sound source localization technology, the three-dimensional spatial morphology and dynamic process of crack propagation can be reversed before the sample 1 breaks, and the results can be mutually verified with the final CT scan results, providing dual data support for both process and result.

[0040] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An experimental method for studying the connection mechanism between hydraulic fractures and high-angle shear fractures, characterized in that, Includes the following steps: Prepare a sample and pre-fabricate a through crack (2) with an angle of 70°~90° on the side of the sample (1) near the top. Perform high-temperature heat bombardment on the inside of the through crack (2) to form a gradient damage zone. Then, process a perforation (3) upward from the center of the bottom surface of the sample (1). The treated sample (1) is placed in the experimental apparatus, and several acoustic emission probes (9) are attached to the surface of the sample (1). The several acoustic emission probes (9) are arranged in a ring array on the surface of the sample (1). The confining pressure and axial pressure were applied sequentially using a graded loading mode, and then high-pressure water injection was continuously performed into the perforation (3) at a constant rate. When the sample (1) ruptured, the rupture pressure and the corresponding flow rate were recorded. Based on the flow-pressure curve obtained from the experiment and the information collected by the acoustic emission probe (9), the hydraulic fracturing process was analyzed. Combined with the CT three-dimensional reconstruction technology after the experiment, the connection mechanism between the hydraulic fracturing fracture and the through fracture (2) was obtained. A stainless steel sleeve (4) is embedded in the perforation (3). The sleeve (4) enters the perforation (3) for 75% to 80% of the total length of the perforation (3), and the remaining 20% ​​to 25% of the perforation (3) length is the bare hole section (5). An upper pad (7) and a lower base (8) are respectively set on the upper and lower parts of the sample (1), and the two ends of the sample (1) are sealed with the upper pad (7) and the lower base (8) respectively by heat shrink film (6). The sample (1) is prepared by using a composite substrate of more than 90% albite powder and less than 10% quartz sand, based on a thin-layer stacking-selective sintering process. The cylindrical standard sample (1) with a diameter of 95mm~100mm×200mm~205mm is prepared by temperature process control. The albite powder is constant and uniform at all heights in the sample (1). The mass ratio of quartz sand to albite powder in the sample (1) between the top of the through crack (2) and the lowest end of the bare hole section (5) gradually decreases from bottom to top. The mass ratio of quartz sand to albite powder below the lowest end of the bare hole section (5) remains constant and uniform.

2. The experimental method for studying the connection mechanism between hydraulic fractures and high-angle shear fractures as described in claim 1, characterized in that, The through crack (2) is obtained by wire cutting. The through crack (2) has an opening of 3mm~5mm and a center point distance of 40mm~45mm from the upper end face of the sample (1). The high temperature heat bombardment treatment temperature is 800℃~900℃.

3. The experimental method for studying the connection mechanism between hydraulic fractures and high-angle shear fractures as described in claim 1, characterized in that, Start the confining pressure control system and slowly increase the confining pressure to 5MPa~40MPa at a rate of 0.2~0.5MPa / min, and maintain it for 5~10min; then start the axial pressure control system and slowly increase the axial pressure to 5MPa~40MPa at a rate of 0.1~0.4mm / min, and maintain it for 5~10min; then start the water pressure loading system and inject water into the bare hole section (5) in the perforation (3) at a rate of 3~5mm / min through the sleeve (4) pre-embedded in the middle of the sample (1), while recording the pressure and flow rate changes. During the water injection process, observe the water pressure change point in real time to determine the occurrence of the sample (1) rupture. After the sample (1) ruptures, record the rupture pressure and water flow rate, and save the relevant data.

4. The experimental method for studying the connection mechanism between hydraulic fractures and high-angle shear fractures as described in claim 1, characterized in that, The spacing between each acoustic emission probe (9) is 60°~90°.

5. The experimental method for studying the connection mechanism between hydraulic fractures and high-angle shear fractures as described in claim 1, characterized in that, The perforation (3) channel is a cylindrical channel with a diameter of Φ5~7mm×85~105mm, and the end of the perforation (3) is not less than 50mm from the center of the through crack (2).

6. The experimental method for studying the connection mechanism between hydraulic fractures and high-angle shear fractures as described in claim 1, characterized in that, The annular space between the sleeve (4) and the hole wall of the perforation (3) is sealed with epoxy resin. The sleeve (4) is embedded in the lower base (8) and sealed with a sealing ring.

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