A multi-stage multi-cluster fracturing fracture propagation physical simulation experiment device and method

Abstract

The application discloses a multi-section multi-cluster fracturing fracture propagation physical simulation experiment device and method. The device comprises a simulation wellbore assembly and a multi-path shunt system. The simulation wellbore assembly comprises at least two fracturing cluster units arranged in series along the axial direction of a borehole, each fracturing cluster unit comprising a support fracturing pad with a perforating structure arranged on the outer peripheral surface, elastic rubber pads arranged at the axial two ends of the support fracturing pad, a clamping mechanism, a fracturing liquid inlet pipe and a rubber injection pipe. The clamping mechanism drives the compression of the elastic rubber pad by the opposite movement of the compression gasket and the clamping gasket through the locking screw of the reverse thread to realize sealing. The multi-path shunt system comprises a shunt water tank, a plurality of parallel shunt branches, a pressure detection unit and a controller. The controller controls the independent pressure maintaining and pressure compensation of each branch according to the comparison result of the real-time pressure and the set pressure. The device reduces the risk of rubber separation of the sealing element under high pressure and realizes independent liquid supply and pressure control of each cluster.

Description

Technical Field

[0001] This application relates to the field of petroleum engineering experimental technology, specifically to a physical simulation experimental device and method for multi-segment, multi-cluster fracturing crack propagation. Background Technology

[0002] Hydraulic fracturing, as a key production enhancement technology for unconventional oil and gas reservoir development, is of great significance for the development of deep shale gas and tight oil and gas reservoirs. As oil and gas resource exploration and development continues to advance into deeper formations, the application of hydraulic fracturing technology faces significant challenges due to difficulties such as deep strata, complex geological conditions, and the development of natural fractures and faults. In-depth research into hydraulic fracturing technology can better improve oil and gas recovery rates, meet the ever-increasing energy demand, and promote the sustainable development of the oil and gas industry. However, during field operations, engineers cannot directly observe the condition of underground fractures, making indoor physical simulation experiments an important approach to studying the relevant laws of multi-stage, multi-cluster fracturing.

[0003] In indoor hydraulic fracturing physical simulation experiments, traditional simulation wellbores mostly adopt open-hole structures, which can only achieve single-stage, single-cluster fracturing simulation. Some multi-stage, multi-cluster simulation wellbores use epoxy resin segment-by-segment potting technology for inter-segment sealing and isolation. Furthermore, existing experimental methods are mostly simple constant pressure or constant flow rate pressurization.

[0004] Traditional open-hole well structures cannot accurately reflect the actual conditions of multi-stage, multi-cluster fracturing in the field. The epoxy resin segmented encapsulation process is cumbersome, requiring a long curing time for each segment, and under high pressure, it is prone to debonding due to the large stress area, leading to pressure crosstalk between adjacent fracturing segments and a high failure rate. Furthermore, existing experimental methods lack real-time means to determine the fracture initiation time, making it impossible to quantitatively evaluate the competitive propagation dominance of each fracture cluster in multi-cluster fracturing, or to simulate advanced processes such as variable displacement fracturing and equilibrium fracturing. Additionally, the lack of methods to measure fracture closure pressure limits in-depth research on the relationship between fracture propagation direction and geostress. Summary of the Invention

[0005] To address the technical problems in the prior art, this application provides a physical simulation experimental device and method for multi-segment, multi-cluster hydraulic fracturing crack propagation.

[0006] This application provides a physical simulation experimental device and method for multi-segment, multi-cluster hydraulic fracturing fracture propagation, which adopts the following technical solution:

[0007] A physical simulation experimental device and method for multi-segment, multi-cluster hydraulic fracturing crack propagation includes:

[0008] A simulated wellbore assembly for installation within a borehole of a rock sample, the simulated wellbore assembly comprising at least two fracturing cluster units arranged in series along the borehole axis, each of the fracturing cluster units comprising:

[0009] A fracturing pad is provided with a perforation structure on its outer circumference to form a fracturing channel with the borehole wall; the force-bearing area of ​​the fracturing pad is limited to its outer circumferential annular region, so that the pressure-bearing area of ​​the sealing partition is smaller than the cross-sectional area of ​​the borehole during high-pressure fracturing.

[0010] Sealing partitions are respectively provided at both ends of the axial direction of the supporting fracturing pad, and the sealing partitions are elastic rubber pads;

[0011] The clamping mechanism includes a pressure washer and a clamping washer respectively disposed on the axial outer side of the elastic rubber pad, and a locking screw passing through the center of the supporting fracturing pad. The pressure washer has a first screw hole and the clamping washer has a second screw hole. The threads of the first screw hole and the second screw hole are opposite in direction. The locking screw is threadedly rotatably connected to both the first screw hole and the second screw hole. When the locking screw is tightened, it drives the pressure washer and the clamping washer to move towards each other, compressing the elastic rubber pad so that it expands radially and fits and seals against the borehole wall.

[0012] A fracturing fluid inlet pipe, one end of which is connected to the perforation structure, and the other end of which is led out from the rock sample;

[0013] An injection tube, passing through the supporting fracturing pad, is used to inject cured sealant into the annular space between the simulated wellbore assembly and the inner wall of the borehole.

[0014] In the case of adjacent fracturing cluster units, the fracturing fluid inlet pipe of the lower fracturing cluster unit is led out through a pre-set through-pipe channel on the upper fracturing cluster unit, and the gap between the through-pipe channel and the fracturing fluid inlet pipe is sealed by filling with sealing material.

[0015] The multi-channel diversion system is connected to each of the fracturing fluid inlet pipes and is used to independently supply fluid and independently control the pressure of each fracturing cluster unit.

[0016] An experimental method for physical simulation of multi-segment, multi-cluster hydraulic fracturing fracture propagation, employing the aforementioned experimental equipment for physical simulation of multi-segment, multi-cluster hydraulic fracturing fracture propagation, includes the following steps:

[0017] S1. Sample preparation: Drill a hole simulating a wellbore in the center of the rock sample;

[0018] S2. Wellbore installation: Assemble multiple fracturing cluster units in series and place them into the borehole. Tighten each clamping mechanism to make the sealing partition fit and seal with the borehole wall. Then, inject sealant between the simulated wellbore assembly and the borehole wall through the injection tube and let it cure.

[0019] S3. Stress loading: The rock sample with the wellbore installed is placed in the true triaxial loading device, and triaxial confining pressure is applied to simulate the in-situ stress state.

[0020] S4. Piping connection: Connect the fracturing fluid inlet pipe of each fracturing cluster unit to the corresponding branch of the multi-way diversion system;

[0021] S5. Fracturing Experiment: High-pressure liquid is injected into the distribution tank through an external loading pump. Each distribution branch independently supplies liquid to the corresponding fracturing cluster unit. The controller, based on the comparison between the real-time pressure of each branch and the set pressure, controls the valve group to achieve independent pressure holding and pressure replenishment for each branch.

[0022] S6. Results Analysis: After the experiment, the rock samples were cut open, and the fracture propagation morphology was observed based on the different colored dyes injected into each fracturing cluster unit.

[0023] In summary, this application includes at least one of the following beneficial technical effects:

[0024] 1. By limiting the force-bearing area of ​​the supporting fracturing pad to its outer circumferential annular region, the pressure-bearing area of ​​the sealing partition is made smaller than the cross-sectional area of ​​the borehole, reducing the axial thrust borne by the sealing partition during high-pressure fracturing. Combined with the structural design of the locking screw with reverse threads in the clamping mechanism driving the pressure pad and clamping pad to move in opposite directions to compress the elastic rubber pad, and the double seal formed by injecting cured sealant through the injection tube, the risk of sealant delamination under high pressure is reduced, improving the reliability of inter-segment sealing and the first-time success rate of the experiment.

[0025] 2. By using throttle valves, flow meters, and control valve groups consisting of pneumatic check valves and gas solenoid valves independently set in each branch of the multi-path diversion system, and in conjunction with the PLC controller, the opening and closing of the pneumatic check valves are automatically controlled based on the comparison results between the real-time pressure and the set pressure of each branch. This enables independent liquid supply, independent pressure control, and automatic pressure holding and replenishment for each fracturing cluster unit, allowing multiple fracturing clusters to be fracturing simultaneously in the same sample without interfering with each other. This can simulate the actual working conditions of multi-stage and multi-cluster fracturing in the field.

[0026] 3. By adopting a two-stage fracturing control strategy that combines sequential fracturing and synchronous fracturing, the independent initiation pressure of each cluster is measured as baseline data in the first stage. In the second stage, all clusters are pressurized simultaneously, and the fracture propagation dominance is evaluated by calculating the flow distribution ratio of each cluster in real time. This allows for quantitative analysis of the dynamic evolution process of competitive propagation among clusters and the influence of stress shadowing effect on the initiation and propagation of each cluster. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the overall system structure of a multi-segment, multi-cluster fracturing crack propagation physical simulation experimental device provided in an embodiment of this application;

[0028] Figure 2 yes Figure 1A front view schematic diagram of the simulated wellbore assembly installed inside a rock sample and placed in a true triaxial loading device.

[0029] Figure 3 yes Figure 1 A three-dimensional structural diagram of the fracturing cluster units arranged in series in the simulated wellbore assembly;

[0030] Figure 4 yes Figure 3 A schematic diagram of the cross-sectional structure of a single fracturing cluster unit;

[0031] Figure 5 This is a flowchart illustrating the two-stage fracturing control strategy provided in the embodiments of this application;

[0032] Figure 6 This is a schematic diagram of the crack initiation discrimination principle based on pressure-flow coupling response provided in the embodiments of this application;

[0033] Figure 7 This is a schematic diagram of the displacement-time relationship of the variable displacement step-by-step loading fracturing method provided in the embodiments of this application;

[0034] Figure reference numerals: 1-Simulated wellbore assembly; 10-Fracturing cluster unit; 11-Supporting fracturing pad; 111-Perforation structure; 12-Sealing partition; 121-Upper partition gasket; 122-Lower partition gasket; 13-Clamping mechanism; 131-Pressure gasket; 132-Clamping gasket; 133-Locking screw; 14-Fracturing inlet pipe; 15-Injection hose; 2-Multi-way diversion system; 21-Diversion tank; 22-Diversion branch; 221-Throttle valve; 222-Filter; 223-Flow meter; 224-Control valve assembly; 2241-Pneumatic check valve; 2242 - Gas solenoid valve; 225- Three-way valve; 226- Dyeing agent storage tank; 2261- Dyeing agent injection port; 227- Hand valve; 23- Pressure detection unit; 231- Pressure sensor; 24- Controller; 241- PLC controller; 242- PLC display; 3- Rock sample; 31- Drill hole; 4- External loading pump; 5- Gas source; 6- True triaxial loading device; 61- Reaction frame; 62- Base plate; 63- Axial pressure loading assembly; 631- Upper pressure plate; 632- Upper pressure cylinder; 64- Side pressure loading assembly; 641- Side pressure plate; 642- Side pressure cylinder. Detailed Implementation

[0035] The technical solutions in the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. The described embodiments are only possible technical implementations of the present invention, but are not limited thereto. Other embodiments obtained by those skilled in the art in conjunction with the embodiments of the present invention without creative effort are also within the protection scope of the present invention.

[0036] This application mainly adopts a combination of simulated wellbore and multi-path diversion system to realize multi-stage and multi-cluster fracturing simulation, which can accurately simulate multi-stage and multi-cluster fracturing conditions, improve the success rate of experiments and the accuracy of data. The following is a further detailed description of this application.

[0037] Example 1

[0038] Please refer to Figures 1-4 The multi-stage, multi-cluster fracturing fracture propagation physical simulation experimental device provided in this application includes a simulated wellbore assembly 1 and a multi-channel diversion system 2. The simulated wellbore assembly 1 is installed inside the borehole 31 of the rock sample 3. The multi-channel diversion system 2 is connected to each fracturing fluid inlet pipe 14 of the simulated wellbore assembly 1, enabling independent fluid supply and pressure control to each fracturing cluster unit 10. This achieves the effect of accurately simulating the actual working conditions of multi-stage, multi-cluster fracturing in the field, avoiding pressure crosstalk between stages, and independently controlling each fracturing cluster. This is because the unique structural design of the simulated wellbore assembly 1 ensures the independence and sealing of each fracturing cluster unit 10, while the multi-channel diversion system 2 provides independent fluid supply and pressure control capabilities to each fracturing cluster unit 10.

[0039] Please refer to Figure 3 and Figure 4 Specifically, the simulated wellbore assembly 1 includes at least two fracturing cluster units 10 arranged in series along the borehole 31 axial direction. Each fracturing cluster unit 10 includes a supporting fracturing pad 11, a sealing partition 12, a clamping mechanism 13, a fracturing fluid inlet pipe 14, and an injection pipe 15.

[0040] The outer circumferential surface of the fracturing support pad 11 is provided with a perforation structure 111, which forms a fracturing channel between the perforation structure 111 and the borehole wall 31. The force-bearing area of ​​the fracturing support pad 11 is limited to its outer circumferential annular region, so that during high-pressure fracturing, the pressure-bearing area of ​​the sealing partition 12 is smaller than the cross-sectional area of ​​the borehole 31. The fracturing support pad 11 can be made of metal, such as stainless steel, or high-strength engineering plastics, such as polyetheretherketone (PEEK). The perforation structure 111 can be regularly arranged circular holes or elliptical holes, and their arrangement can be uniformly distributed around the circumference or specially arranged according to experimental requirements.

[0041] During high-pressure fracturing, high-pressure fluid enters the fracturing channel between the supporting fracturing pad 11 and the borehole wall 31 through the perforation structure 111. At this time, the high-pressure fluid generates axial thrust on the sealing partition 12. Since the force-bearing area of ​​the supporting fracturing pad 11 is limited to its outer circumferential annular region, the pressure-bearing area of ​​the sealing partition 12 is only the area of ​​this annular region, not the entire cross-sectional area of ​​the borehole 31. Therefore, the axial thrust borne by the sealing partition 12 is significantly reduced. Compared with the traditional epoxy resin segmented potting process where the sealing surface bears the pressure of the entire borehole cross-sectional area, this design reduces the risk of seal debonding and improves the sealing reliability and experimental success rate under high-pressure conditions.

[0042] The sealing partition 12 consists of elastic rubber pads, which are respectively disposed at both axial ends of the supporting fracturing pad 11. The elastic rubber pads can be made of rubber, such as nitrile rubber, or silicone rubber. Their shape is typically annular, allowing for better contact with the borehole wall 31. The elastic rubber pads achieve sealing and isolation between the fracturing cluster units 10 by contacting the borehole wall 31. Specifically, the sealing partition 12 includes an upper partition rubber pad 121 disposed at the upper axial end of the supporting fracturing pad 11 and a lower partition rubber pad 122 disposed at the lower axial end of the supporting fracturing pad 11. Both the upper partition rubber pad 121 and the lower partition rubber pad 122 are annular elastic rubber pads, and their inner diameter matches the outer diameter of the supporting fracturing pad 11. Initially, their outer diameter is slightly smaller than the inner diameter of the borehole 31. When the clamping mechanism 13 applies axial compressive force, the upper partition gasket 121 and the lower partition gasket 122 are compressed axially. Due to the Poisson effect of the elastic material, the gasket expands radially, and the outer diameter increases until it fits tightly against the borehole 31 wall, thereby forming a reliable sealing partition at the upper and lower ends of the supporting fracturing pad 11 to prevent pressure crosstalk between adjacent fracturing cluster units 10.

[0043] Please refer to Figure 4The clamping mechanism 13 includes a pressure washer 131 and a clamping washer 132 respectively disposed on the axial outer side of the elastic pad, and a locking screw 133 passing through the center of the supporting fracturing pad 11. The pressure washer 131 and the clamping washer 132 can be thin metal plates, such as aluminum alloy plates. The pressure washer 131 has a first threaded hole, and the clamping washer 132 has a second threaded hole. The threads of the first threaded hole and the second threaded hole are opposite in direction. The locking screw 133 is threadedly rotatably connected to both the first threaded hole and the second threaded hole. When the locking screw 133 is tightened, because the threads of the first threaded hole and the second threaded hole are opposite in direction, the rotation of the locking screw 133 simultaneously generates axial displacements in opposite directions on the pressure washer 131 and the clamping washer 132, that is, drives the pressure washer 131 and the clamping washer 132 to move towards each other, thereby compressing the elastic pad and causing it to expand radially, thus sealing it against the wall of the borehole 31. The locking screw 133 can be a regular bolt or a special bolt with an anti-loosening structure. The head of the locking screw 133 is preferably an internal hexagonal head, which facilitates operation with an internal hexagonal wrench within the confined space of the drilled hole 31.

[0044] During use, when the operator uses a wrench to rotate the locking screw 133, because the threads of the first threaded hole of the clamping washer 131 and the second threaded hole of the clamping washer 132 are in opposite directions, each rotation of the locking screw 133 causes both the clamping washer 131 and the clamping washer 132 to move inward by one thread pitch, meaning they move in opposite directions for a total distance of two thread pitches. This bidirectional synchronous compression design allows the elastic gasket to be compressed evenly and efficiently, requiring only a single-direction rotation to complete the seal, greatly simplifying the installation process in confined drilling spaces. Simultaneously, the reverse thread structure has self-locking properties, preventing loosening due to vibration or pressure fluctuations during high-pressure fracturing, ensuring long-term seal reliability.

[0045] One end of the fracturing fluid inlet pipe 14 is connected to the perforation structure 111, and the other end is led out from the rock sample 3. The fracturing fluid inlet pipe 14 can be a metal pipe, such as a steel pipe, or a high-strength plastic pipe, such as a nylon pipe. Its function is to deliver high-pressure fluid to the perforation structure 111 for fracturing operations.

[0046] The injection tube 15 passes through the supporting fracturing pad 11 and is used to inject cured sealant into the annular space between the simulated wellbore assembly 1 and the inner wall of the borehole 31. The injection tube 15 can be a thin metal tube or a plastic tube, and its diameter is determined according to the amount of sealant injected and the injection speed.

[0047] After the simulated wellbore assembly 1 is installed into the borehole 31 and initially sealed by the clamping mechanism 13, cured sealant is injected into the annular space between the simulated wellbore assembly 1 and the inner wall of the borehole 31 through the injection tube 15. The sealant flows within the annular space and fills all gaps, forming a robust sealing layer after curing, further enhancing the sealing effect between the simulated wellbore assembly 1 and the borehole 31 wall. This dual-seal design (elastic gasket mechanical seal + cured sealant chemical seal) ensures complete isolation between the fracturing cluster units 10 under high-pressure fracturing conditions, effectively preventing inter-stage pressure cross-contamination.

[0048] Please refer to Figure 3 In adjacent fracturing cluster units 10, the fracturing fluid inlet pipe 14 of the lower fracturing cluster unit 10 passes through a pre-designed through-pipe channel on the upper fracturing cluster unit 10 and exits. The gap between the through-pipe channel and the fracturing fluid inlet pipe 14 is sealed by filling with sealing material. The sealing material can be sealant or sealing filler. Figure 3 As shown, multiple fracturing cluster units 10 are arranged in series along the axial direction of the borehole 31. The fracturing fluid inlet pipe 14 of the lowest fracturing cluster unit 10 needs to pass through the through-tube channels of all the fracturing cluster units 10 above it before it can be led out from the top of the rock sample 3. Each fracturing fluid inlet pipe 14 is sealed at the through-tube channel with sealing material to ensure the independence between each cluster. Figure 3 In the diagram, pipes of different colors represent fracturing inlet pipes 14 of different fracturing cluster units 10.

[0049] The assembly logic of the components of the simulated wellbore assembly 1 is as follows: First, the sealing partition 12 (including the upper partition gasket 121 and the lower partition gasket 122) is placed at both ends of the supporting fracturing pad 11. Then, the clamping mechanism 13 (including the clamping gasket 131, the clamping gasket 132, and the locking screw 133) is installed, and the sealing partition 12 is made to fit against the borehole wall 31 by tightening the locking screw 133. Next, the fracturing fluid inlet pipe 14 and the glue injection pipe 15 are installed, and finally, glue is injected for sealing. This assembly method ensures the sealing and independence of the fracturing cluster unit 10, allowing each fracturing cluster unit 10 to perform fracturing operations independently.

[0050] Please refer to Figure 1 Specifically, the multi-channel diversion system 2 includes a diversion tank 21, multiple parallel diversion branches 22, a pressure detection unit 23, and a controller 24.

[0051] The inlet of the distribution tank 21 is connected to the external loading pump 4, and it has multiple outlets corresponding to each fracturing cluster unit 10. The distribution tank 21 can be made of stainless steel, and its capacity is determined according to experimental requirements. The external loading pump 4 is a high-pressure plunger pump or a high-pressure injection pump, used to provide the high-pressure fluid required for fracturing. The outlet of the external loading pump 4 is connected to the inlet of the distribution tank 21 through a high-pressure pipeline, injecting the high-pressure fluid into the distribution tank 21. The distribution tank 21 serves to buffer and evenly distribute the pressure, ensuring that each branch 22 receives a stable supply pressure.

[0052] Each branch line 22 connects to one outlet of the branch water tank 21 and the corresponding fracturing inlet pipe 14. Each branch line 22 is equipped with a throttle valve 221, a filter 222, a flow meter 223, and a control valve assembly 224 sequentially along the fluid flow direction. The throttle valve 221 regulates the fluid flow rate, the filter 222 filters impurities in the liquid to ensure experimental accuracy, the flow meter 223 measures the fluid flow rate, and the control valve assembly 224 controls the fluid flow and pressure.

[0053] The control valve assembly 224 includes a pneumatically controlled check valve 2241 and a gas solenoid valve 2242. The pneumatically controlled check valve 2241 is located on the liquid line of the branch line 22, and the gas solenoid valve 2242 is located between the pneumatic control end of the pneumatically controlled check valve 2241 and the gas source 5. The pneumatically controlled check valve 2241 has a hard seal structure with a steel ball and a metal sealing surface. The pneumatically controlled check valve 2241 prevents liquid backflow, and the gas solenoid valve 2242 controls the opening and closing of the pneumatically controlled check valve 2241. The gas source 5 is a compressed air source or a nitrogen cylinder, providing the gas pressure required to drive the pneumatically controlled check valve 2241. The gas solenoid valve 2242 is a two-position three-way solenoid valve. When the PLC controller 241 sends an energizing signal, the gas solenoid valve 2242 opens, introducing high-pressure gas from the gas source 5 into the air control terminal of the pneumatic check valve 2241. This pushes the steel ball inside the pneumatic check valve 2241 to press against the metal sealing surface, closing the hydraulic circuit to maintain pressure. When the PLC controller 241 sends an de-energizing signal, the gas solenoid valve 2242 closes and discharges the gas from the air control terminal of the pneumatic check valve 2241. Under hydraulic pressure, the steel ball leaves the sealing surface, and the hydraulic circuit reopens to replenish pressure. The pneumatic check valve 2241 adopts a hard-seal structure of steel ball and metal sealing surface. Compared with rubber seals, it can maintain good sealing performance under high pressure (up to 50MPa and above), and is not prone to aging and deformation, resulting in a long service life.

[0054] The pressure detection unit 23 includes a pressure sensor 231 connected to each branch 22 via a three-way valve 225 for real-time pressure detection of the branch 22. The three-way valve 225 is installed on the branch 22 pipeline, with its three ports connected to the upstream pipeline, downstream pipeline, and pressure sensor 231, respectively. Through the three-way valve 225, the pressure sensor 231 can detect the pressure in the pipeline in real-time without interrupting the liquid flow. The pressure sensor 231 is preferably a high-pressure resistant pressure transmitter with a range of 0-100 MPa, an accuracy class of not less than 0.25, and an output signal of 4-20mA standard current signal or 0-10V standard voltage signal, facilitating acquisition and processing by the PLC controller 241.

[0055] Each branch line 22 is also equipped with a dye reservoir 226 and a hand valve 227. The dye reservoir 226 is used to add dye to the fluid, facilitating subsequent observation of fracture propagation morphology. The hand valve 227 is used to control the on / off state of the branch line 22. The dye reservoir 226 is equipped with a dye injection port 2261 for adding dye of different colors into the reservoir. Different colors of dye (such as red, green, blue, etc.) are injected into the dye reservoir 226 of each branch line 22, so that the fracturing fluid injected into each fracturing cluster unit 10 has a different color. When the rock sample 3 is dissected after the experiment, the fractures generated by each cluster can be distinguished according to the color of the dye in the fracture, so as to intuitively observe the propagation morphology, propagation direction and inter-cluster interference of each cluster fracture. The hand valve 227 is a manual ball valve or needle valve, located at the end of the branch line 22 near the fracturing fluid inlet pipe 14, for manually controlling the on / off state of the pipeline before and after the experiment, and closing the pipeline to achieve well shut-in state when the fracture closure pressure is measured.

[0056] The controller 24 includes a PLC controller 241 and a PLC display 242. The PLC controller 241 is communicatively connected to each pressure sensor 231 and each gas solenoid valve 2242. When the real-time pressure of a branch reaches the set pressure, the PLC controller 241 controls the corresponding gas solenoid valve 2242 to energize and open the gas path, driving the pneumatic check valve 2241 to close the liquid path to maintain pressure. When the real-time pressure is lower than the set pressure, the PLC controller de-energizes the gas solenoid valve 2242 to close the gas path, and the pneumatic check valve 2241 reopens to replenish pressure. The PLC display 242 is a touch screen display used to display the pressure value, flow value, and control status of each branch 22 in real time. It also provides a human-machine interface, allowing operators to set parameters such as target pressure value, pressure threshold, and control mode for each branch through the PLC display 242. The PLC controller 241 runs a closed-loop pressure control program, which collects signals from each pressure sensor 231 at a preset sampling period (e.g., 100ms) and compares them with the set pressure. Based on the comparison result, it outputs a control signal to the corresponding gas solenoid valve 2242 to realize the automatic adjustment of the pressure of each branch.

[0057] In operation, the PLC controller 241 collects pressure signals from the pressure sensors 231 of each branch 22 in real time at a preset sampling period, comparing the real-time pressure value with the set pressure value. When the real-time pressure of a branch reaches or exceeds the set pressure, the PLC controller 241 immediately sends an energizing signal to the gas solenoid valve 2242 of that branch. The gas solenoid valve 2242 opens the gas path, and the high-pressure gas from the gas source 5 drives the pneumatically controlled check valve 2241 to close the liquid path, stopping the liquid supply to that branch and achieving pressure maintenance. When the real-time pressure of that branch drops below the set pressure due to fracture expansion or other reasons, the PLC controller 241 sends an de-energizing signal to the gas solenoid valve 2242 of that branch. The gas solenoid valve 2242 closes the gas path and vents gas, while the pneumatically controlled check valve 2241 reopens under hydraulic pressure, resuming the liquid supply to that branch and achieving pressure replenishment. Through this closed-loop control, the pressure of each branch can be automatically maintained near the set pressure, achieving independent pressure control of each fracturing cluster unit 10 and avoiding mutual pressure interference between clusters.

[0058] The combination logic of the components of the multi-channel diversion system 2 is as follows: the diversion tank 21 distributes the high-pressure liquid provided by the external loading pump 4 to each diversion branch 22. Each diversion branch 22 controls and regulates the fluid through a throttle valve 221, a filter 222, a flow meter 223, and a control valve group 224. The pressure detection unit 23 detects the pressure in real time, and the controller 24 controls the control valve group 224 according to the pressure conditions to achieve independent pressure maintenance and pressure replenishment for each branch.

[0059] The overall working process of the entire experimental equipment in this embodiment is described below:

[0060] Please refer to Figure 1The working process of the multi-segment, multi-cluster fracturing fracture propagation physical simulation experimental equipment is as follows: First, the simulated wellbore assembly 1 is installed inside the borehole 31 of the rock sample 3. Multiple fracturing cluster units 10 in the simulated wellbore assembly 1 are arranged in series along the axial direction of the borehole 31. Each fracturing cluster unit 10 is sealed and isolated between segments by a sealing partition 12 and a cured sealant. Then, the rock sample 3 with the simulated wellbore assembly 1 installed is placed in the true triaxial loading device 6. Triaxial confining pressure is applied by the axial pressure loading component 63 and the lateral pressure loading component 64 to simulate the in-situ stress state of the formation. Next, the fracturing fluid inlet pipes 14 of each fracturing cluster unit 10 are connected to the corresponding branch 22 of the multi-path diversion system 2. At the start of the experiment, the external loading pump 4 injects high-pressure liquid into the diversion water tank 21. The high-pressure liquid is distributed to each branch 22 through the diversion water tank 21. Throttling valves 221 on each branch 22 regulate flow rate, filters 222 filter impurities, flow meters 223 measure flow rate, and pneumatic check valves 2241 and gas solenoid valves 2242 in the control valve group 224 achieve independent on / off switching and pressure maintenance / replenishment of each branch under the control of PLC controller 241. High-pressure liquid reaches the perforation structure 111 of each fracturing cluster unit 10 through each branch 22 and fracturing inlet pipe 14, and enters the fracturing channel between the supporting fracturing pad 11 and the borehole 31 wall from the perforation structure 111 to fracturing the rock sample 3. Pressure sensor 231 in pressure detection unit 23 detects the pressure of each branch in real time, and PLC controller 241 performs closed-loop control based on the pressure signal. Different colored dyes injected into the dye storage tanks 226 of each branch 22 enter the fractures of each cluster along with the fracturing fluid. After the experiment, rock sample 3 was cut open, and the propagation morphology of each cluster of cracks was observed based on the different colored dyes in the cracks. The competitive propagation law of cracks, stress shadow effect and inter-cluster interference mechanism were analyzed.

[0061] The implementation principle of this embodiment is as follows: This experimental equipment, through the cooperation of the simulated wellbore assembly 1 and the multi-channel diversion system 2, solves the problems of traditional simulated wellbores in simulating multi-stage, multi-cluster fracturing conditions and inter-stage sealing, while simultaneously achieving independent fluid supply and pressure control for each fracturing cluster. The design of the supporting fracturing pad 11 and the sealing partition 12 in the simulated wellbore assembly 1 ensures the sealing and independence of each fracturing cluster unit 10. The reverse thread design of the clamping mechanism 13 simplifies the installation operation and improves sealing reliability. The multi-channel diversion system 2 provides precise flow and pressure control for each fracturing cluster unit 10. Compared with existing technologies, this improves the success rate of experiments and the accuracy of data, and can better simulate the actual situation of multi-stage, multi-cluster fracturing in the field.

[0062] Example 2

[0063] Please refer to Figure 1 and Figures 5-7, the experimental method for physical simulation of multi-stage multi-cluster fracturing fracture propagation provided by the embodiments of the present application uses the multi-stage multi-cluster fracturing fracture propagation physical simulation experimental apparatus described in Embodiment 1, and includes the following steps:

[0064] S1. Specimen preparation: A borehole 31 simulating a wellbore is drilled at the center of the rock specimen 3. When drilling the borehole 31, professional drilling equipment such as a core drilling rig may be used. A suitable drill bit and drilling parameters are selected according to experimental requirements to ensure that the diameter and depth of the borehole 31 meet the requirements. After the borehole 31 is completed, rock cuttings and impurities in the borehole 31 are cleaned to ensure the cleanliness of the borehole 31.

[0065] S2. Wellbore installation: A plurality of fracturing cluster units 10 are assembled in series in sequence, placed into the borehole 31, each clamping mechanism 13 is tightened to make the sealing partition member 12 fit and seal against the wall of the borehole 31, and then sealant is injected through the glue injection pipe 15 between the simulated wellbore assembly 1 and the wall of the borehole 31 and cured. When assembling the fracturing cluster units 10, it is necessary to ensure that all components are installed correctly and tightly. When tightening the clamping mechanism 13, the operation shall be performed according to a certain torque requirement to ensure the fitting effect between the sealing partition member 12 and the wall of the borehole 31. Specifically, when the locking screw 133 is tightened, since the thread directions of the first screw hole of the pressing gasket 131 and the second screw hole of the clamping gasket 132 are opposite, the rotation of the locking screw 133 simultaneously drives the pressing gasket 131 and the clamping gasket 132 to move toward each other, uniformly compressing the upper partition rubber pad 121 and the lower partition rubber pad 122, so that they expand radially and closely fit against the wall of the borehole 31. After the sealant is injected, the sealant is waited for curing, and the curing time is determined according to the type of the sealant and environmental conditions. Meanwhile, this step further includes a tightness verification step: after the injected sealant is completely cured, a low-pressure water injection tightness test is performed on each fracturing cluster unit 10 one by one. During the test, low-pressure water is injected into the fracturing liquid inlet pipe 14 of the currently tested cluster, and it is monitored whether water flows out from the fracturing liquid inlet pipe 14 of the adjacent cluster. If no water flows out from the adjacent cluster, it is determined that the seal between the currently tested cluster and the adjacent cluster is qualified; if water flows out from the adjacent cluster, it is determined that the seal is unqualified, and the glue injection sealing treatment needs to be performed again. The next step can be carried out only after the tightness tests of all clusters are qualified.

[0066] In this embodiment, by performing the low-pressure water injection test on each fracturing cluster unit 10 one by one, sealing defects can be found and eliminated before the formal high-pressure fracturing experiment. The pressure of the low-pressure water injection test is usually set to 10%-20% of the formal fracturing pressure, which can effectively detect the tightness and will not cause damage to the rock specimen 3. This step ensures that the seal between all fracturing cluster units 10 is completely reliable, avoids experimental failure caused by unqualified sealing, and improves the one-time success rate of the experiment.

[0067] Please refer to Figure 1 and Figure 2S3, Stress Loading: The rock sample 3 with the wellbore installed is placed in the true triaxial loading device 6, and triaxial confining pressure is applied to simulate the in-situ stress state. The true triaxial loading device 6 can provide accurate triaxial pressure to simulate different in-situ stress conditions. When applying the confining pressure, appropriate pressure values ​​and loading rates should be set according to experimental requirements to ensure that the simulated in-situ stress state conforms to the actual situation.

[0068] In this embodiment, the true triaxial loading device 6 includes a reaction frame 61, a base plate 62, an axial compression loading component 63, and a lateral compression loading component 64.

[0069] The reaction frame 61 is a high-strength steel frame structure used to bear and transmit the reaction force generated during loading. The internal space of the reaction frame 61 forms a loading cavity to accommodate the rock sample 3 and each loading component. The material of the reaction frame 61 is preferably 45# steel or Q345 steel, and its structural stiffness should meet the requirement that the deformation under the maximum loading force does not exceed 0.1mm.

[0070] The base plate 62 is fixedly installed at the bottom of the reaction frame 61 to support the rock sample 3. The upper surface of the base plate 62 is precision machined to ensure a flatness of no more than 0.05 mm, ensuring that the rock sample 3 is placed stably. The base plate 62 also serves as the vertical ( The lower constraint surface (direction).

[0071] Please refer to Figure 2 The axial compression loading assembly 63 is mounted on top of the reaction frame 61 for use in the vertical direction ( Axial pressure is applied to the rock sample 3 in the direction of the load. The axial pressure loading assembly 63 includes an upper pressure plate 631 and an upper pressure cylinder 632. The upper pressure cylinder 632 is fixedly mounted on the top crossbeam of the reaction frame 61, and its piston rod extends downward. The upper pressure plate 631 is connected to the end of the piston rod of the upper pressure cylinder 632, and the lower surface of the upper pressure plate 631 contacts the upper surface of the rock sample 3. The dimensions of the upper pressure plate 631 match the dimensions of the upper surface of the rock sample 3 to ensure uniform pressure distribution. The upper pressure cylinder 632 is preferably a hydraulic cylinder with a maximum output force of not less than 500kN and a stroke of not less than 50mm, and is equipped with a servo proportional valve to achieve precise pressure control. The upper pressure plate 631 is preferably made of alloy tool steel with a surface hardness of not less than HRC55 to prevent plastic deformation during high-pressure loading.

[0072] Please refer to Figure 2The lateral pressure loading assembly 64 is installed on both sides of the reaction frame 61 to apply lateral pressure to the rock sample 3 in the horizontal direction. The lateral pressure loading assembly 64 includes two sets of symmetrically arranged lateral pressure plates 641 and lateral pressure cylinders 642. Each set of lateral pressure cylinders 642 is fixedly installed on the side wall of the reaction frame 61, with its piston rod extending horizontally. The lateral pressure plate 641 is connected to the end of the piston rod of the lateral pressure cylinder 642, and the inner surface of the lateral pressure plate 641 contacts the side of the rock sample 3. In this embodiment, the left and right lateral pressure loading assemblies 64 respectively provide the maximum horizontal principal stress. Direction and minimum horizontal principal stress Lateral pressure in the direction of the rock sample 3. The lateral pressure cylinder 642 is preferably a hydraulic cylinder with a maximum output force of not less than 500kN and a stroke of not less than 50mm. The dimensions of the lateral pressure plate 641 match the side dimensions of the rock sample 3. For cases where a third-dimensional horizontal stress needs to be applied, a second set of lateral pressure loading components 64 can be added in the front and rear directions of the reaction frame 61 to achieve true three-dimensional independent loading.

[0073] In actual operation, the rock sample 3, with the simulated wellbore assembly 1 installed, is placed on the base plate 62 and its position is adjusted to be centered. Then, the axial pressure loading assembly 63 and the lateral pressure loading assembly 64 are activated sequentially, applying triaxial confining pressure step by step according to the preset loading path. The confining pressure loading should be applied step by step according to the preset loading path, with a loading rate preferably between 0.1 MPa / s and 0.5 MPa / s, to avoid damage to the sample during loading. After the confining pressure loading is completed, it should be kept stable for at least 30 minutes to allow the internal stress of the sample to be fully homogenized. The pressure in each direction is controlled by an independent hydraulic servo system, which can realize independent adjustment of the triaxial stress. The vertical stress provided by the axial pressure loading assembly 63... Simulating the pressure of the overlying strata, the horizontal stress provided by the left and right lateral pressure loading components 64 and The maximum and minimum horizontal principal stresses are simulated separately. The magnitudes of the triaxial stresses can be set based on the actual geostress data of the target formation. For example, for deep shale gas reservoirs, typical geostress conditions are... , , .

[0074] Each hydraulic cylinder of the true triaxial loading device 6 is equipped with a pressure sensor and a displacement sensor, which can monitor the loading force and displacement in all directions in real time. The pressure sensor is preferably 0-100MPa with an accuracy class of not less than 0.5. The displacement sensor is preferably an LVDT displacement sensor with a range of not less than 50mm and a resolution of not less than 0.01mm. The signals from each sensor are connected to an independent data acquisition system to realize real-time monitoring and data recording of the loading process.

[0075] In this embodiment, the reaction frame 61 provides stable reaction support, while the axial compression loading component 63 and the lateral compression loading component 64 provide independent and controllable pressures in the vertical and horizontal directions, respectively, achieving accurate simulation of the triaxial stress state of the formation. The independent control of pressure in each direction allows the experiment to study the crack propagation law under different stress ratios, such as the stress state of a normal fault (…). ), stress state of strike-slip faults ( ) and reverse fault stress state ( ).

[0076] S4. Piping Connection: Connect the fracturing inlet pipes 14 of each fracturing cluster unit 10 to the corresponding branch pipes 22 of the multi-way diversion system 2. When connecting the pipes, ensure a secure connection to prevent leakage. Sealing joints and gaskets can be used to ensure the connection is airtight. Specifically, each fracturing inlet pipe 14 is connected to the end pipe of the corresponding branch pipe 22 via a high-pressure quick-connect fitting. After connection, a pressure test should be performed to verify the airtightness of all connection points.

[0077] Please refer to Figure 5 S5. Fracturing Experiment: High-pressure liquid is injected into the distribution tank 21 via an external loading pump 4. Each distribution branch 22 independently supplies liquid to its corresponding fracturing cluster unit 10. The controller 24, based on the comparison between the real-time pressure and the set pressure of each branch, controls the valve group 224 to achieve independent pressure maintenance and pressure replenishment for each branch. This step can employ various fracturing control strategies and methods.

[0078] A two-stage fracturing control strategy is adopted, including the first stage—the sequential fracturing stage: the PLC controller 241 controls the control valve groups 224 of each branch 22, so that each fracturing cluster unit 10 is pressurized individually one by one. For the currently pressurized fracturing cluster unit 10, the pneumatic check valve 2241 of its corresponding branch 22 is opened, while the pneumatic check valves 2241 of all other branch 22 are closed. After the current fracturing cluster unit 10 is pressurized to the point of fracture initiation, the independent initiation pressure of that cluster is recorded. (in (Assign the current cluster number), then close the pneumatic check valve 2241 of that cluster, and open the pneumatic check valve 2241 of the next cluster, sequentially completing the independent fracturing pressure measurement of all clusters. For example... Figure 5 As shown, in the first stage, let the cluster number be... , start the The pneumatic check valve 2241 of the cluster is closed, and the pneumatic check valves 2241 of all other clusters are closed, for the first cluster... Clusters were individually pressurized until cracking occurred, and the first crack was recorded. Independent initiation pressure of clusters Then judge Is it valid () (total number of fracturing clusters), if not true, then let The above process is repeated; if successful, the process proceeds to the second stage. Second stage – Synchronous fracturing stage: The PLC controller 241 simultaneously opens the pneumatic check valves 2241 of all branch lines 22, simultaneously pressurizing all fracturing cluster units 10, and observing the competitive propagation process of fractures under multi-cluster synchronous fracturing conditions. For example… Figure 5 As shown, in the second stage, the gas-controlled check valves 2241 of all clusters are opened simultaneously, all clusters are pressurized at the same time, the pressure and flow of each cluster are monitored in real time, the flow distribution ratio is calculated, the expansion dominance is evaluated, the competitive expansion process data is recorded, and finally the well shut-in / results analysis is performed.

[0079] In this embodiment, the first stage of sequential fracturing involves individually pressurizing each fracturing cluster unit 10 to obtain the independent fracturing initiation pressure of each cluster under conditions without inter-cluster interference. This provides baseline data for subsequent analysis. The second stage of synchronous fracturing simulates the actual conditions of multi-cluster synchronous fracturing by simultaneously pressurizing all fracturing cluster units 10, observing the competitive propagation process of each cluster under the stress shadow effect. By comparing the independent initiation pressure of the first stage with the synchronous fracturing data of the second stage, the influence of the stress shadow effect on the initiation and propagation of each cluster can be quantitatively analyzed.

[0080] Please refer to Figure 6 In the first and second stages, the initiation of cracks in each cluster is determined in real time using a pressure-flow coupling response method. The PLC controller 241 acquires pressure signals from the pressure sensors 231 of each branch 22 in real time. and the flow signal of flow meter 223 Calculate the pressure change rate of each branch. and flow rate When a branch simultaneously satisfies the pressure change rate (i.e., the rate of pressure appearance exceeds the preset threshold for the rate of pressure decrease) (sudden drop) and flow rate change (i.e., the rate of traffic occurrence exceeds the preset traffic increase rate threshold) When the pressure spikes (or the pressure peaks), the cluster is determined to have started cracking. The PLC controller 241 records the moment when the above conditions are met as the crack initiation moment of the cluster, and records the corresponding pressure peak as the crack initiation pressure of the cluster.

[0081] like Figure 6 As shown, the pressure curve during the fracturing pressurization process... First, it continues to rise, the flow rate curve The pressure curve increases slowly. When the cracks in rock sample 3 initiate, the pressure curve... A sudden drop (pressure change rate) occurred Negative values ​​and absolute values ​​greater than the threshold ), and the flow curve A sudden increase (rate of change of flow) Positive value and greater than the threshold The moment when a sudden drop in pressure and a sudden increase in flow occur simultaneously is the moment of crack initiation, and the corresponding pressure peak is the initiation pressure. This dual-condition discrimination method based on pressure-flow coupled response, compared with a single discrimination method relying solely on pressure signals, can effectively eliminate misjudgments caused by interference factors such as pipeline fluctuations, thus improving the accuracy and reliability of crack initiation discrimination.

[0082] During the second stage of synchronous fracturing, the propagation dominance of each fracture cluster is evaluated in real time. The PLC controller 241 collects the instantaneous flow rate of the flow meters 223 in each branch 22 in real time. Real-time calculation of the traffic allocation ratio for each cluster:

[0083]

[0084] in The sum of the instantaneous flows of all clusters. This represents the total number of fracturing clusters. Based on the flow rate distribution ratio. The clusters are sorted by their flow distribution ratio, with the cluster having the largest flow distribution ratio identified as the dominant fracture cluster and the cluster having the smallest flow distribution ratio identified as the suppressed fracture cluster. The PLC controller 241 records the flow distribution ratio of each cluster over time in real time. It is used to analyze the dynamic evolution process of inter-cluster competitive expansion and the time-varying characteristics of stress shadowing effect.

[0085] In multi-cluster simultaneous fracturing, due to differences in the geostress state, rock mechanical properties, and stress shadowing effect at the location of each cluster, the propagation rate and scale of fractures in each cluster differ, resulting in different fracturing fluid flow rates absorbed by each cluster. Flow rate distribution ratio This directly reflects the relative advantage of each cluster of cracks during the competitive propagation process. When a certain cluster of cracks... When the value is significantly higher than other clusters, it indicates that this cluster has a dominant role in fracture propagation and has absorbed more fracturing fluid; conversely, when the value is significantly lower, Clusters with smaller values ​​indicate that crack propagation is suppressed. This was achieved through real-time recording. The time-varying curves can dynamically track the evolution of the competitive propagation of each cluster of fractures, reveal the changing law of stress shadow effect over time, and provide experimental basis for optimizing the design of multi-cluster fracturing in the field.

[0086] The second stage of synchronous fracturing also includes adaptive equalization fracturing control based on flow distribution ratio. The PLC controller 241 calculates the flow distribution ratio of each cluster in real time. When the flow allocation ratio of a certain cluster is detected When the PLC controller 241 increases the opening of the corresponding throttle valve 221 or increases the set pressure of the cluster, it increases the injection flow rate of the cluster; when the flow distribution ratio of a certain cluster is detected... At this time, the PLC controller 241 reduces the opening of the corresponding throttle valve 221 or lowers the set pressure of the cluster to reduce the injection flow rate of the cluster. The preset lower limit threshold for equilibrium. This is a preset upper limit threshold for equilibrium. Through the above adaptive adjustment, the flow distribution ratio of each cluster tends to be uniform, simulating the on-site equilibrium fracturing process.

[0087] In this embodiment, during multi-cluster simultaneous fracturing, due to the stress shadowing effect and rock heterogeneity, the propagation of fractures in each cluster is often uneven. Some clusters may become dominant fractures and absorb a large amount of fracturing fluid, while other clusters are suppressed. Adaptive equilibrium fracturing control monitors the flow distribution ratio of each cluster in real time. When the flow distribution ratio of a certain cluster falls below a lower threshold... When the flow rate of a cluster exceeds the upper limit threshold, the liquid supply to that cluster is automatically increased to promote crack propagation; when the flow rate distribution ratio of a cluster exceeds the upper limit threshold... At that time, the fluid supply to the cluster is automatically reduced to suppress its excessive expansion. Through this closed-loop adaptive adjustment, the fracture expansion of each cluster tends to be more uniform, simulating the effect of in-situ equalization fracturing technology, and providing an experimental means to study the effectiveness of equalization fracturing technology and optimize parameters.

[0088] Please refer to Figure 7 The variable displacement stepwise loading fracturing method was adopted, and the fracturing experiment process was divided into three stages. Each displacement level corresponds to a preset injection displacement value, and the injection displacement value increases progressively for each displacement level to satisfy... Within each displacement level, the injection displacement of each cluster is maintained at a preset value for that level by adjusting the throttle valve 221 of each branch 22, and a constant displacement injection duration is maintained. During constant-displacement injection at each displacement level, the PLC controller 241 records the stable pressure value and flow distribution ratio of each cluster. After completing the injection at the current displacement level, the injection displacement of each cluster is increased to the preset value of the next displacement level, and the above process is repeated until the injection of all displacement levels is completed. Based on the variation law of pressure response and flow distribution ratio of each cluster under different displacement levels, the influence of injection displacement on crack competitive propagation and stress shadowing effect is analyzed.

[0089] like Figure 7 As shown, with Taking a displacement class as an example, the injection displacement values ​​for each displacement class are as follows: , , , Maintain a constant displacement injection time for each displacement class. Within each displacement level, the stable pressure and flow distribution ratio of each cluster are recorded to analyze the impact of displacement on competitive expansion.

[0090] In this embodiment, by progressively increasing the injection rate, the variation patterns of pressure response and flow distribution ratio of each fracture cluster under different injection rates can be systematically studied. Under low injection rates, the fracture propagation rate is slow, the stress shadowing effect between clusters is weak, and the flow distribution ratio may be relatively uniform. As the injection rate increases progressively, the fracture propagation rate accelerates, the stress shadowing effect intensifies, and the flow distribution ratio may show significant differentiation. By comparing experimental data at different injection rate levels, the influence of injection rate on fracture competitive propagation and stress shadowing effect can be quantitatively analyzed, providing experimental basis for injection rate optimization in field fracturing operations.

[0091] Based on the determination of fracture closure pressure using pressure decay curves, after the pressurization process of the fracturing experiment is completed, the external loading pump 4 is shut off and the manual valves 227 of all shunt branches 22 are closed, putting each cluster in a shut-in state. The PLC controller 241 continuously acquires the pressure decay signals from the pressure sensors 231 of each shunt branch 22. Pressure decay curves for each cluster The analysis identifies the inflection point where the slope of the pressure decay curve changes abruptly; the pressure value corresponding to this inflection point is the crack closure pressure of that cluster. By comparing the crack closure pressure of each cluster The relationship between the propagation direction of each cluster of cracks and the direction of in-situ stress was analyzed based on the applied triaxial confining pressure.

[0092] With the well shut in, the pressure within the fracture gradually decreases over time. When the pressure within the fracture drops to the fracture closure pressure... At this point, the crack surface begins to close, and the leakage rate of the fluid within the crack changes abruptly, leading to a change in the pressure decay curve. The slope exhibits a sudden change. By identifying the inflection point of this slope change, the crack closure pressure can be determined. Crack closure pressure This reflects the magnitude of the minimum principal stress perpendicular to the crack surface. The crack closure pressure of each cluster is compared. With the applied triaxial confining pressure ( , , ), which can determine the propagation direction of each cluster of cracks: if near Then the crack surface is perpendicular to the direction of the minimum horizontal principal stress; if near If the crack surface is horizontal, then the crack is a horizontal crack. This analytical method provides a quantitative means to study the relationship between the crack propagation direction and the direction of geostress.

[0093] S6. Results Analysis: After the experiment, rock sample 3 was dissected, and the fracture propagation morphology was observed based on the different colored dyes injected into each fracturing cluster unit 10. By observing the fracture propagation morphology, the competitive propagation law of fractures, stress shadowing effect, and inter-cluster interference mechanism can be analyzed.

[0094] The overall working process of the entire experimental method in this embodiment is described below:

[0095] Please refer to Figure 1 and Figures 5-7 The overall working process of the multi-stage, multi-cluster fracturing fracture propagation physical simulation experiment method is as follows: First, in step S1, rock sample 3 is prepared and borehole 31 is drilled. Then, in step S2, the simulated wellbore assembly 1 is installed into borehole 31, and sealing and sealing performance verification are completed. Next, in step S3, rock sample 3 is placed in true triaxial loading device 6 to apply triaxial confining pressure. In step S4, pipeline connection is completed. In step S5, fracturing experiments are conducted. First, sequential fracturing in the first stage is performed, and the independent initiation pressure of each cluster is measured one by one. Then, the second stage of synchronous fracturing is carried out, with all clusters pressurized simultaneously. The pressure and flow rate of each cluster are monitored in real time. The initiation of fractures in each cluster is determined by a pressure-flow coupling response method, and the flow rate distribution ratio is used to determine the fracture initiation. The expansion dominance of each cluster is evaluated, allowing for selective adaptive equilibrium fracturing control and variable displacement staged fracturing. After fracturing, the well is shut in, and the fracture closure pressure of each cluster is determined using pressure decay curves. Finally, in step S6, rock sample 3 is dissected, and the propagation morphology of each cluster of fractures is observed according to different colored dyes. Combined with the data such as pressure, flow rate, and flow rate distribution ratio recorded during the experiment, the competitive propagation law, stress shadow effect, inter-cluster interference mechanism, and relationship between fracture propagation direction and geostress direction of multi-segment and multi-cluster hydraulic fracturing are comprehensively analyzed.

[0096] The implementation principle of this embodiment is as follows: This experimental method, through a series of steps and various fracturing control strategies and analysis methods, achieves a physical simulation of multi-stage, multi-cluster fracturing fracture propagation. From sample preparation to final result analysis, each step is closely coordinated to ensure the accuracy and reliability of the experiment. Multiple fracturing control strategies and analysis methods can more comprehensively study the relevant laws of multi-stage, multi-cluster fracturing, such as fracture initiation, propagation dominance, equilibrium fracturing, and fracture closure pressure. Compared with existing experimental methods, it can provide a deeper understanding of various phenomena and mechanisms in the multi-stage, multi-cluster fracturing process, offering more accurate reference data for on-site fracturing operations.

[0097] The specific embodiments described above do not constitute a limitation on the scope of protection of this application. Any other corresponding changes and modifications made based on the technical concept of this application should be included within the scope of protection of this application.

Claims

1. A physical simulation experimental device for multi-segment, multi-cluster hydraulic fracturing crack propagation, characterized in that, include: A simulated wellbore assembly for installation within a borehole of a rock sample, the simulated wellbore assembly comprising at least two fracturing cluster units arranged in series along the borehole axis, each of the fracturing cluster units comprising: A fracturing pad is provided with a perforation structure on its outer circumference to form a fracturing channel with the borehole wall; the force-bearing area of ​​the fracturing pad is limited to its outer circumferential annular region, so that the pressure-bearing area of ​​the sealing partition is smaller than the cross-sectional area of ​​the borehole during high-pressure fracturing. Sealing partitions are respectively provided at both ends of the axial direction of the supporting fracturing pad, and the sealing partitions are elastic rubber pads; The clamping mechanism includes a pressure washer and a clamping washer respectively disposed on the axial outer side of the elastic rubber pad, and a locking screw passing through the center of the supporting fracturing pad. The pressure washer has a first screw hole and the clamping washer has a second screw hole. The threads of the first screw hole and the second screw hole are opposite in direction. The locking screw is threadedly rotatably connected to both the first screw hole and the second screw hole. When the locking screw is tightened, it drives the pressure washer and the clamping washer to move towards each other, compressing the elastic rubber pad so that it expands radially and fits and seals against the borehole wall. A fracturing fluid inlet pipe, one end of which is connected to the perforation structure, and the other end of which is led out from the rock sample; An injection tube, passing through the supporting fracturing pad, is used to inject cured sealant into the annular space between the simulated wellbore assembly and the inner wall of the borehole. In the case of adjacent fracturing cluster units, the fracturing fluid inlet pipe of the lower fracturing cluster unit is led out through a pre-set through-pipe channel on the upper fracturing cluster unit, and the gap between the through-pipe channel and the fracturing fluid inlet pipe is sealed by filling with sealing material. The multi-channel diversion system is connected to each of the fracturing fluid inlet pipes and is used to independently supply fluid and independently control the pressure of each fracturing cluster unit.

2. The multi-segment, multi-cluster fracturing fracture propagation physical simulation experimental device according to claim 1, characterized in that, The multiplex system includes: The diversion tank has an inlet connected to an external loading pump and multiple outlets corresponding to each fracturing cluster unit. Multiple parallel diversion branches are provided, each branch connecting one outlet of the diversion tank to the corresponding fracturing inlet pipe. Each branch is equipped with a throttle valve, a filter, a flow meter, and a control valve assembly in sequence along the fluid flow direction. The control valve assembly includes a pneumatic check valve and a gas solenoid valve. The pneumatic check valve is located on the liquid path of the diversion branch, and the gas solenoid valve is located between the pneumatic control end of the pneumatic check valve and the gas source. The pneumatic check valve has a hard seal structure with a steel ball and a metal sealing surface. The pressure detection unit includes a pressure sensor connected to each of the said branch lines via a three-way valve; Each of the aforementioned branch lines is also equipped with a dye storage tank and a manual valve; The controller includes a PLC controller and a PLC display. The PLC controller is communicatively connected to each of the pressure sensors and each of the gas solenoid valves. When the real-time pressure of a certain branch reaches the set pressure, the PLC controller controls the corresponding gas solenoid valve to be energized to open the gas path and drives the pneumatic check valve to close the liquid path to maintain pressure. When the real-time pressure is lower than the set pressure, the PLC controller controls the gas solenoid valve to be de-energized to close the gas path and the pneumatic check valve to be reopened to replenish pressure.

3. The experimental equipment for physical simulation of multi-segment, multi-cluster fracturing fracture propagation according to claim 1, and an experimental method for physical simulation of multi-segment, multi-cluster fracturing fracture propagation, characterized in that, The experimental equipment used for physical simulation of multi-segment, multi-cluster fracturing fracture propagation as described in claim 2, the experimental method includes the following steps: S1. Sample preparation: Drill a hole simulating a wellbore in the center of the rock sample; S2. Wellbore installation: Assemble multiple fracturing cluster units in series and place them into the borehole. Tighten each clamping mechanism to make the sealing partition fit and seal with the borehole wall. Then, inject sealant between the simulated wellbore assembly and the borehole wall through the injection tube and let it cure. S3. Stress loading: The rock sample with the wellbore installed is placed in the true triaxial loading device, and triaxial confining pressure is applied to simulate the in-situ stress state. S4. Piping connection: Connect the fracturing fluid inlet pipe of each fracturing cluster unit to the corresponding branch of the multi-way diversion system; S5. Fracturing Experiment: High-pressure liquid is injected into the distribution tank through an external loading pump. Each distribution branch independently supplies liquid to the corresponding fracturing cluster unit. The controller, based on the comparison between the real-time pressure of each branch and the set pressure, controls the valve group to achieve independent pressure holding and pressure replenishment for each branch. S6. Results Analysis: After the experiment, the rock samples were cut open, and the fracture propagation morphology was observed based on the different colored dyes injected into each fracturing cluster unit.

4. The multi-segment, multi-cluster fracturing fracture propagation physical simulation experimental equipment according to claim 1, and the experimental method according to claim 3, characterized in that, The fracturing experiment in step S5 employs a two-stage fracturing control strategy, including: Phase 1 – Sequential Fracturing Phase: The PLC controller controls the control valve groups of each branch, allowing each fracturing cluster unit to be pressurized individually in sequence. For the currently pressurized fracturing cluster unit, the pneumatic check valve of its corresponding branch is opened, while the pneumatic check valves of all other branch units are closed. After the current fracturing cluster unit is pressurized to the point of fracture initiation, the independent initiation pressure of the cluster is recorded. Then, the pneumatic check valve of the cluster is closed, and the pneumatic check valve of the next cluster is opened, and the independent initiation pressure of all clusters is measured in sequence. The second stage – synchronous fracturing stage: The PLC controller simultaneously opens the pneumatic check valves of all branch lines to pressurize all fracturing cluster units at the same time, and observes the competitive propagation process of fractures under multi-cluster synchronous fracturing conditions.

5. The multi-segment, multi-cluster fracturing fracture propagation physical simulation experimental device according to claim 1, and the experimental method according to claim 4, characterized in that, In the first and second stages, the initiation of cracks in each cluster is determined in real time using the following method based on pressure-flow coupling response: The PLC controller collects pressure signals from the pressure sensors of each branch circuit in real time. and the flow signal of the flow meter ; Calculate the pressure change rate of each branch. and flow rate ; A crack initiation is determined to have occurred in a cluster when a branch simultaneously meets both of the following conditions: Pressure change rate This means that the rate of pressure appearance exceeds the preset pressure drop rate threshold. A sharp drop; Flow rate of change This means that the rate of increase in traffic exceeds the preset threshold for the rate of increase in traffic. The sudden increase; The PLC controller records the moment when the above conditions are met as the crack initiation moment of the cluster, and records the corresponding pressure peak as the crack initiation pressure of the cluster.

6. The physical simulation experimental device for multi-segment, multi-cluster hydraulic fracturing fracture propagation according to claim 1, According to the experimental method of claim 4, the characteristic is that, during the synchronous fracturing process in the second stage, the propagation dominance of each fracture cluster is evaluated in real time using the following method: The PLC controller collects the instantaneous flow rate of the flow meters in each branch in real time. ,in Number the fracturing clusters; Real-time calculation of the flow allocation ratio of each cluster : in The sum of the instantaneous flows of all clusters. This represents the total number of fracturing clusters. Based on the flow allocation ratio The clusters are sorted by their size, and the cluster with the largest flow allocation ratio is determined to be the dominant fracture cluster, while the cluster with the smallest flow allocation ratio is determined to be the suppressed fracture cluster. The PLC controller records the change curve of the flow distribution ratio of each cluster over time in real time. It is used to analyze the dynamic evolution process of inter-cluster competitive expansion and the time-varying characteristics of stress shadowing effect.

7. The physical simulation experimental device for multi-segment, multi-cluster fracturing crack propagation according to claim 1, The experimental method according to claim 4 is characterized in that, The second stage of synchronous fracturing also includes adaptive equalization fracturing control based on the flow distribution ratio: The PLC controller calculates the flow distribution ratio of each cluster in real time. ; When the flow allocation ratio of a certain cluster is detected At this time, the PLC controller increases the opening of the corresponding throttle valve or increases the set pressure of the cluster to increase the injection flow rate of the cluster; wherein This is the preset lower threshold for equilibrium. When the flow allocation ratio of a certain cluster is detected At this time, the PLC controller reduces the opening of the corresponding throttle valve of the cluster or lowers the set pressure of the cluster to reduce the injection flow rate of the cluster; wherein This is the preset upper limit threshold for balance.

8. The multi-segment, multi-cluster fracturing fracture propagation physical simulation experimental device according to claim 1, and the experimental method according to claim 3, characterized in that, Step S5 employs a variable displacement, step-by-step loading fracturing method: The fracturing experiment process is divided into: Each displacement level corresponds to a preset injection displacement value, and the injection displacement value increases progressively for each displacement level to satisfy... ; Within each displacement class, the injection displacement of each cluster is maintained at a preset value for that class by adjusting the throttle valves of each branch, and a constant displacement injection duration is maintained. ; During constant displacement injection at each displacement level, the PLC controller records the stable pressure value and flow distribution ratio of each cluster; After completing the injection of the current displacement level, increase the injection displacement of each cluster to the preset value of the next displacement level, and repeat the above process until the injection of all displacement levels is completed; Based on the variation patterns of pressure response and flow distribution ratio of each cluster under different displacement levels, the influence of injection displacement on crack competitive propagation and stress shadowing effect is analyzed.

9. The multi-segment, multi-cluster fracturing fracture propagation physical simulation experimental device according to claim 1, and the experimental method according to claim 3, characterized in that, Step S5 also includes the determination of crack closure pressure based on the pressure decay curve: After the pressurization process of the fracturing experiment is completed, the external loading pump is turned off and all the hand valves of the diversion branches are closed, so that each cluster is in the shut-in state. The PLC controller continuously collects the pressure attenuation signals from the pressure sensors of each branch circuit. ; Pressure decay curves for each cluster The analysis identifies the inflection point where the slope of the pressure decay curve changes abruptly; the pressure value corresponding to this inflection point is the crack closure pressure of that cluster. ; By comparing the crack closure pressure of each cluster The relationship between the propagation direction of each cluster of cracks and the direction of in-situ stress was analyzed based on the applied triaxial confining pressure.

10. The experimental apparatus for physical simulation of multi-segment, multi-cluster fracturing crack propagation according to claim 1, and the experimental method according to claim 3, characterized in that, The wellbore installation in step S2 also includes a sealing verification step: After the sealant has cured, each fracturing cluster unit is subjected to a low-pressure water injection sealing test. During testing, low-pressure water is injected into the fracturing inlet pipe of the current test cluster, while monitoring whether water flows out of the fracturing inlet pipes of adjacent clusters. If no water flows out from adjacent clusters, the seal between the current test cluster and the adjacent clusters is deemed to be qualified; if water flows out from adjacent clusters, the seal is deemed to be unqualified and re-sealing with glue is required. Step S3 can only proceed after all clusters have passed the sealing test.

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