A method for preparing and sealing a sample for hydraulic fracturing with a guide slit

Abstract

The present application relates to geotechnical engineering test technical field, disclose a kind of hydraulic fracturing sample preparation and hole sealing method containing guide slit, comprising: segmented reaming is carried out to the center hole of rock sample, and upper and lower sealing chamber and middle fracturing section with annular step are formed;Using the device with miniature camera, sealing material is accurately injected into lower annular step, and lower segment hole sealing is completed;Fracturing pipe with annular groove and sealing ring on outer wall is placed in upper sealing chamber, and sealing material is injected, and upper segment hole sealing is completed.The present application combines the physical limitation of annular step with visual injection operation, accurately controls the height of lower segment blocking body, avoids the pollution of middle fracturing section;And by the composite sealing structure of elastic sealing ring and adhesive set on fracturing pipe, the reliability and pressure-bearing capacity of upper segment sealing are significantly improved, so as to ensure the accuracy of test result and the success rate of test process.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering testing technology, specifically to a method for preparing and sealing hydraulic fracturing specimens with guide joints. Background Technology

[0002] Pulsed fracturing, as a novel hydraulic fracturing method, has shown promising application prospects in engineering practices such as roof cutting and pressure relief in coal mines due to its ability to achieve better fracturing results at lower initiation pressures. In these applications, pre-setting guide joints in the rock mass to achieve directional fracturing is key to effectively promoting roof cutting and improving pressure relief.

[0003] Therefore, conducting pulsating fracturing laboratory tests on rock samples containing pilot fractures is an important technical means to explore rock fracture mechanisms and optimize fracturing parameters. However, such tests require extremely high precision and reliability in sample preparation, and existing methods have significant technical bottlenecks. First, during the lower-level sealing, the lack of physical boundaries within the duct and the inability to observe the process often leads to overfilling of the sealing material, which then intrudes into the pilot fracture section in the middle, directly disrupting the initial conditions of the test. Second, the periodic pressure fluctuations generated by pulsating fracturing pose a severe challenge to the sealing performance of the upper pipeline. A single adhesive sealing method is highly susceptible to fatigue failure under cyclic loading, leading to interface debonding and pressure leakage. These two problems together result in poor sealing performance and poor repeatability, seriously affecting the validity and scientific rigor of the test results. Summary of the Invention

[0004] The present invention aims to address the technical problems in existing technologies when conducting pulsating fracturing laboratory tests on rock samples with guide fractures. These problems include contamination of the fracturing section and poor test repeatability due to uncontrollable lower-stage sealing processes, and the difficulty of the upper-stage sealing structure effectively resisting periodic pressure fluctuations, resulting in a low test success rate. Therefore, this invention provides a highly reliable method for preparing and sealing hydraulic fracturing samples that can precisely control the sealing range and withstand dynamic loading.

[0005] In a first aspect, this application provides a method for preparing a hydraulic fracturing specimen with a guide fracture, comprising the following steps:

[0006] A. Using wire cutting technology, a through-hole is drilled at the geometric center of the rock sample, with its axis perpendicular to the upper and lower surfaces of the sample. The borehole obtained in this step has a defined roundness and surface finish.

[0007] In one specific technical solution, the diameter of the vertical center borehole is 14-16 mm.

[0008] B. A guide slot is symmetrically machined around the periphery of the vertically centered borehole. This guide slot is used to guide the direction of crack initiation during hydraulic fracturing.

[0009] In one specific technical solution, the length of the guide slot is processed to be 4-6 mm, and the width is processed to be 1.5-2.5 mm.

[0010] C. Perform segmented reaming on the vertical center borehole. Specifically, in the upper 1 / 3 and lower 1 / 3 of the rock sample, the vertical center borehole is reamed to form an upper sealed chamber and a lower sealed chamber, respectively; simultaneously, the original borehole diameter within the middle 1 / 3 height range of the rock sample is kept unchanged to form a central fracturing section. In one specific technical solution, the diameter of the upper and lower sealed chambers is reamed to 24–26 mm.

[0011] A key technical effect of the segmented enlargement process in step C is the natural formation of two annular steps at the junction of the smaller-diameter central fracturing section and the larger-diameter upper and lower sealing chambers. This annular step structure has a clear technical function: during lower-section sealing, the lower annular step provides a physical barrier interface for the upward-filling sealing material, precisely defining the filling range of the sealing material and preventing it from entering the central fracturing section; during upper-section sealing, the upper annular step provides a bearing surface for the sealing structure of the fracturing tube.

[0012] Secondly, the present invention provides a method for sealing a hydraulic fracturing sample containing a guide slot, comprising the following steps:

[0013] a. To proceed with the next stage of sealing, first, wrap and fix the video connection cable of a miniature camera around the outer wall of an injection tube to form an integrated device, and connect this device to the anchoring adhesive injector and the video receiver. Then, insert the injection tube and the miniature camera as a single unit into the bottom of the central drilled hole of the sample body, and then inject the anchoring adhesive through the anchoring adhesive injector.

[0014] b. During the injection of the anchoring adhesive, the injection status, distribution, and filling height of the adhesive in the borehole are monitored in real time using the miniature camera and video receiver. When the filling height of the adhesive reaches the interface between the lower sealing chamber and the middle fracturing section (i.e., the lower annular step position), the injection is stopped. Then, the injection tubing and the miniature camera are removed, and the chamber is left to cure. This step achieves precise control of the lower sealing height.

[0015] c. For upper section sealing, firstly, a fracturing tube is pre-treated: 2-5 evenly distributed annular grooves are machined on its outer wall, and a rubber sealing ring is installed in each annular groove. In a specific technical solution, the depth of the annular groove is 0.8-1.5 mm and the width is 0.8-1.5 mm. Then, the pre-treated fracturing tube is installed in the upper sealing chamber. This installation process specifically includes: injecting anchoring adhesive along the tube wall into the annular space between two adjacent rubber sealing rings, and inserting the fracturing tube into the upper sealing chamber through multiple insertion and extraction cycles and additional adhesive injection. This insertion and extraction action effectively removes gas between the fracturing tube and the borehole wall, ensuring that the anchoring adhesive fills the chamber without gaps. After installation, additional anchoring adhesive is injected into any areas with poor sealing, and the chamber is allowed to cure again.

[0016] In one specific technical solution, the static curing time for both the lower sealing section and the upper sealing section is 8 to 24 hours.

[0017] This sealing method, through step c, creates an upper composite sealing structure: the rubber sealing ring on the outer wall of the fracturing tube provides radial elastic sealing capability, while the anchoring adhesive, filled and cured between the annular grooves and between the fracturing tube and the borehole wall, provides high-strength adhesive sealing and mechanical locking capability. The combination of elastic and adhesive sealing significantly improves the pressure-bearing capacity and reliability of the sealing structure during testing.

[0018] This invention provides a method for preparing and sealing hydraulic fracturing specimens with guide slots. It has the following beneficial effects:

[0019] 1. This invention provides a clear physical boundary for the subsequent sealing material by machining an annular step in the borehole; simultaneously, it utilizes a miniature camera to achieve visual monitoring of the injection process, enabling precise control of the sealing material height at a predetermined position. This combination of structure and method fundamentally solves the problem of sealing material intrusion into the fracturing section, ensuring a high degree of consistency in the initial state of the guide seam area for each sample, thus laying the foundation for repeatable scientific experiments.

[0020] 2. In the upper sealing section, this invention employs a composite sealing structure combining the elastic sealing of a rubber sealing ring with the adhesive-mechanical locking of the anchoring adhesive. This structure effectively absorbs and resists the impact of periodic pressure fluctuations during pulsating fracturing, avoiding the problem of fatigue failure at a single adhesive interface. This significantly improves the sealing reliability under high pressure and dynamic loading conditions, eliminates test interruptions due to leakage, and greatly increases the test success rate. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the guide joint structure of the hydraulic fracturing specimen of the present invention;

[0022] Figure 2 This is a schematic diagram of the middle fracturing section structure of the hydraulic fracturing specimen of the present invention;

[0023] Figure 3 This is a top view of the hydraulic fracturing specimen of the present invention;

[0024] Figure 4 This is a cross-sectional view of the hydraulic fracturing specimen AA of the present invention;

[0025] Figure 5 This is a schematic diagram of the hydraulic fracturing sample sealing device of the present invention;

[0026] Figure 6 This is a schematic diagram of the fracturing tube of the present invention;

[0027] Figure 7 This is a schematic diagram of the upper sealed chamber state when the fracturing tube of the present invention is inserted into the vertical center borehole.

[0028] The components include: 1. Guide joint; 2. Rebar adhesive injector; 3. Injection hose; 4. Video connection cable; 5. Video receiver; 6. Fracturing tube; 7. Miniature camera; and 8. Rubber sealing ring. Detailed Implementation

[0029] The technical solutions in 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 embodiments of the present invention, and not all embodiments. Based on the 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.

[0030] Example 1:

[0031] This invention provides a method for preparing a hydraulic fracturing specimen with a guide slot, comprising the following steps:

[0032] A. Select a standard-sized complete granite block and use wire cutting technology to drill a through vertical center hole with a diameter of 15mm at the geometric center of the rock sample.

[0033] B. Along the periphery of the hole drilled vertically to the center in step (a), two guide slots 1 are symmetrically machined, each guide slot 1 being 5mm long and 2mm wide.

[0034] C. Perform segmented reaming on the vertical center borehole. In the upper and lower thirds of the rock sample, enlarge the borehole to a diameter of 25 mm to form an upper and lower sealing chamber. Keep the borehole diameter of 15 mm unchanged in the middle third of the rock sample's height, forming the central fracturing section. At the junction of the central fracturing section and the upper and lower sealing chambers, an annular step is formed to define the filling range of the sealing material.

[0035] A method for sealing a hydraulic fracturing specimen with a guide slot includes the following steps:

[0036] a. Lower section sealing: First, wrap and fix the video connection cable 4 of a miniature camera 7 around the outer wall of an injection tube 3 to form an integrated device. Insert the integrated device into the bottom of the center hole of the sample body and inject high-strength epoxy anchoring adhesive through the anchoring adhesive injector 2.

[0037] b. During the injection process, the height of the upper surface of the anchoring adhesive is monitored in real time by a video receiver 5 connected to a camera. When the upper surface is detected to reach the interface between the lower sealing chamber and the middle fracturing section (i.e., the lower annular step position), the injection is stopped immediately. Subsequently, the injection hose 3 and the miniature camera 7 are removed from the central borehole, and the sample is left to cure for 12 hours.

[0038] c. Upper Section Sealing: A high-pressure stainless steel fracturing tube 6 is pretreated by machining three evenly distributed annular grooves on its outer wall. Each groove is 1.2 mm deep and 1.2 mm wide. A nitrile rubber sealing ring 8 is installed in each annular groove. Then, high-strength epoxy anchoring adhesive is injected into the annular space between two adjacent rubber sealing rings 8 along the tube wall of the fracturing tube 6. Subsequently, by repeatedly inserting and withdrawing the tube and injecting additional anchoring adhesive, the fracturing tube 6 is fully inserted into the upper sealing chamber. This operation is used to expel the gas between the fracturing tube 6 and the borehole wall, ensuring that the anchoring adhesive is tightly filled. After applying additional adhesive to any areas where the seal may be incomplete, the sample is allowed to stand for curing for another 12 hours. After curing, the entire sealing operation is completed.

[0039] Finally, a complete hydraulic fracturing specimen prepared according to the method of the present invention was obtained, which was designated as specimen 1 and used for subsequent performance testing.

[0040] Example 2:

[0041] The preparation and sealing methods provided in this embodiment are basically the same as those in Example 1, the main difference being the different process parameters used, as detailed below:

[0042] In step A of the sample preparation, the diameter of the vertical center hole is machined to 14 mm.

[0043] In step B, the length of guide slot 1 is machined to 4mm and the width is machined to 1.5mm.

[0044] In step C, the diameter of the enlarged holes in the upper and lower sealing chambers is 24 mm.

[0045] In the sealing step b above, during the pretreatment of the fracturing tube 6, two annular grooves are machined on its outer wall, each groove having a depth and width of 0.8 mm.

[0046] The static curing time after the lower and upper sections of the hole are sealed is 8 hours.

[0047] Finally, a complete hydraulic fracturing specimen prepared according to the method of the present invention was obtained, which was designated as specimen 2.

[0048] Example 3:

[0049] The preparation and sealing methods provided in this embodiment are basically the same as those in Example 1, the main difference being the different process parameters used, as detailed below:

[0050] In step A of the sample preparation, the diameter of the vertical center hole is machined to 16 mm.

[0051] In step B, the length of guide slot 1 is machined to 6mm and the width is machined to 2.5mm.

[0052] In step C, the diameter of the enlarged holes in the upper and lower sealing chambers is 26 mm.

[0053] In the sealing step b above, during the pretreatment of the fracturing tube 6, five annular grooves are machined on its outer wall, each groove having a depth and width of 1.5 mm.

[0054] The static curing time after the lower and upper sections of the hole are sealed is 24 hours.

[0055] Finally, a complete hydraulic fracturing specimen prepared according to the method of the present invention was obtained, which was designated as specimen 3.

[0056] Comparative example:

[0057] The main difference between this comparative example and Example 1 is that:

[0058] 1. In the sample preparation step, no segmented hole enlargement process is performed, and the diameter of the drilled vertical center hole is the same from top to bottom, so no annular step is formed to define the sealing area.

[0059] 2. In the next sealing step, a monitoring device with a miniature camera 7 is not used; blind injection is performed based solely on experience and theoretical calculations.

[0060] 3. In the previous sealing step, a fracturing tube 6 with a smooth outer wall was used, that is, no annular groove was machined on its outer wall, and no rubber sealing ring 8 was installed.

[0061] The remaining steps and materials used are the same as in Example 1.

[0062] Finally, a comparative sample prepared according to the existing technical method was obtained, which was designated as comparative sample 1 and used for subsequent performance testing.

[0063] Test example:

[0064] To verify the technical effectiveness of the samples prepared by the preparation method of the present invention in terms of sealing reliability and sealing hole size accuracy, especially the sealing performance under simulated pulsating fracturing conditions, performance tests were conducted on samples 1, 2, and 3 prepared in Examples 1, 2, and 3, as well as comparative sample 1 prepared in Comparative Example 1.

[0065] 1. Upper Section Sealing Performance Test (Static): Each sample was mounted on a high-pressure water pump testing system. Water was injected into the fracturing section in the middle of the sample through fracturing pipe 6, and the fracturing section was pressurized at a constant rate of 1 MPa / min. The pressure value was recorded in real time using a pressure sensor and data acquisition system. The pressure was continuously increased until leakage occurred (manifested as the pressure value could not rise and continued to drop), and the pressure value at this time was recorded as the leakage pressure. If no leakage occurred after the pressure reached 50 MPa, the pressure was maintained at this level for 30 minutes, and the pressure change during the pressure maintenance period was recorded.

[0066] 2. Lower Section Sealing Accuracy Analysis: After completing all pressure tests, each sample was axially cut using a rock cutter. The actual cured height of the lower section of the anchoring adhesive was measured using vernier calipers. This measured value was compared with the theoretical design height (i.e., 1 / 3 of the total rock sample height), and the difference between the two was calculated as the sealing height deviation.

[0067] 3. Upper Section Sealing Performance Test (Dynamic): To simulate the effect of pulsating pressure, the sample that passed the static test in step (a) was connected to a dynamic hydraulic servo system. First, the fracturing section pressure was applied to a base pressure of 30 MPa. Then, a sinusoidal pressure wave with an amplitude of 10 MPa and a frequency of 1 Hz was applied on top of this, causing the pressure to fluctuate periodically between 20 MPa and 40 MPa. The number of cycles the sample could withstand was recorded until leakage occurred. If no leakage occurred after 100 cycles, the test was terminated.

[0068] Test results:

[0069] The test results for Sample 1, Sample 2, Sample 3 and Comparative Sample 1 are shown in the table below.

[0070] Table 1 Comparison of performance test results for each sample

[0071]

[0072] Results analysis:

[0073] Test results show that the lower sealing height deviation of samples 1, 2, and 3 prepared by the method of this invention is less than 1.5 mm. This is because, during the sample preparation process, a ring-shaped step is formed at the junction of the lower sealing chamber and the middle fracturing section through segmented hole expansion. This ring-shaped step structure provides a physical barrier interface for the upward-filling sealing material. Simultaneously, during the lower sealing operation, by integrating the miniature camera 7 with the injection tubing 3, the operator can obtain real-time video information of the sealing material filling the upper surface, thus stopping the injection when the upper surface reaches the precise position of the ring-shaped step. This combination of structure and operation method allows for precise control of the lower sealing height, ensuring a high degree of consistency in the initial conditions of the fracturing section among different samples, providing a prerequisite for repeatable scientific experiments.

[0074] The data also shows that samples 1, 2, and 3 not only withstood a static pressure of 50 MPa, but more importantly, they all withstood more than 100 cycles of cyclic loading under periodic pressure fluctuations of 20 MPa-40 MPa without leakage. In contrast, comparative sample 1 leaked when the static pressure reached 23.8 MPa. This difference in pressure-bearing capacity stems from the different upper-stage sealing structures. In samples 1, 2, and 3, a rubber sealing ring 8 is installed in the annular groove on the outer wall of the fracturing tube 6. After installation, this ring provides radial elastic sealing force, which compensates for minor deformations and material relaxation that may occur under pulsating loads. The subsequently injected adhesive, after curing, forms a high-strength bond with the fracturing tube 6 and the borehole wall, and mechanically locks due to filling the annular groove. This composite structure combining elastic and adhesive sealing effectively resists the fatigue effects caused by cyclic loading during pulsating fracturing, thus ensuring the reliability of the upper-stage seal under dynamic loading conditions.

[0075] In contrast, comparative sample 1, due to its uniform diameter pore structure, cannot effectively limit the filling range of the lower sealing material, and the blind injection operation cannot achieve process control, resulting in significant deviations in the sealing height. Furthermore, its upper sealing relies solely on adhesive bonding; under hydraulic pressure, the interface between the fracturing tube 6 and the cementitious body is prone to debonding, forming leakage channels. This rigid connection, when facing cyclic loads, is highly susceptible to fatigue cracks in stress concentration areas, which propagate rapidly, leading to premature failure. In summary, the technical solution of this invention has clear technical advantages in both sealing accuracy and sealing reliability under dynamic pressure.

Claims

1. A method for preparing a hydraulic fracturing specimen with a guide slot, characterized in that, Includes the following steps: A. A through vertical central borehole is drilled at the geometric center of the rock sample using wire cutting technology; B. A guide slot (1) is symmetrically machined around the hole drilled along the vertical center. C. Perform segmented enlargement treatment on the vertical center borehole. In the upper 1 / 3 and lower 1 / 3 of the rock sample, enlarge the vertical center borehole to form an upper sealed chamber and a lower sealed chamber, while keeping the borehole in the middle 1 / 3 height range of the rock sample unchanged to form a middle fracturing section, thereby obtaining the main body of the sample. In step C, the segmented hole enlargement process involves creating an annular step at the junction of the middle fracturing section and the upper and lower sealing chambers to define the filling range of the sealing material.

2. The method for preparing a hydraulic fracturing specimen with a guide slot according to claim 1, characterized in that, In step B, the length of the guide slot (1) is processed to be 4-6 mm and the width is processed to be 1.5-2.5 mm.

3. The method for preparing a hydraulic fracturing specimen with a guide slot according to claim 1, characterized in that, In step A, the diameter of the vertical center borehole is 14-16 mm; in step C, the diameters of the upper sealing chamber and the lower sealing chamber are enlarged to 24-26 mm.

4. A method for sealing a hydraulic fracturing specimen with a guide slot, characterized in that, The method for preparing a hydraulic fracturing specimen with a guide slot as described in any one of claims 1-3 includes the following steps: a. Sealing the lower section: A sealing device consisting of an injection hose (3), a rebar adhesive injector (2), a miniature camera (7), a video connection cable (4), and a video receiver (5) is used. The injection hose (3) and the miniature camera (7) are inserted into the bottom of the central drill hole of the sample body as a whole, and then the rebar adhesive is injected. b. The injection status and distribution of the anchoring adhesive are monitored in real time by the miniature camera (7). After the anchoring adhesive is filled to the predetermined height, the injection tube (3) and the miniature camera (7) are removed and left to cure. c. Upper section sealing: After pretreatment of the fracturing pipe (6), the fracturing pipe (6) is installed in the upper sealing chamber. For the parts that are not sealed properly, additional anchoring adhesive is injected, and the pipe is left to stand for curing again. After curing, the sealing is completed.

5. The method for sealing a hydraulic fracturing specimen with a guide slot according to claim 4, characterized in that, The static curing time for sealing the lower section in step a and sealing the upper section in step c is 8 to 24 hours.

6. The method for sealing a hydraulic fracturing specimen with a guide slot according to claim 4, characterized in that, The pretreatment of the fracturing tube (6) in step c specifically involves processing 2 to 5 evenly distributed annular grooves with a depth of 0.8 to 1.5 mm and a width of 0.8 to 1.5 mm on the outer wall of the fracturing tube (6), and installing a rubber sealing ring (8) in each of the annular grooves.

7. The method for sealing a hydraulic fracturing sample with a guide slot according to claim 4, characterized in that, The installation of the fracturing tube (6) in step c specifically includes: injecting anchoring adhesive into the annular space between two adjacent rubber sealing rings (8) along the wall of the fracturing tube (6) to form a sealing layer, and inserting the fracturing tube (6) into the upper sealing chamber by multiple insertions and injections of anchoring adhesive to discharge the gas between the fracturing tube (6) and the borehole wall.

8. The method for sealing a hydraulic fracturing sample with a guide slot according to claim 4, characterized in that, In step b, real-time monitoring is performed using the miniature camera (7) to ensure that the upper surface of the anchoring adhesive reaches the interface between the lower sealed chamber and the middle fracturing section.

9. A method for sealing a hydraulic fracturing specimen with a guide slot according to claim 4, characterized in that, In step a, the video connection cable (4) of the miniature camera (7) is wrapped around and fixed to the outer wall of the injection tubing (3) to form an integrated device.

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