Simulation test method of multi-coal seam cracking induced by punching under true triaxial stress

Abstract

The invention discloses a simulation test method for punching and fracturing multiple coal seams under true triaxial stress, comprising the following steps: step 1, sample preparation; step 2, preparation for installation of a multi-field coupled coal-rock dynamic disaster prevention and control technology simulation system; step 3, application of true triaxial prestress; step 4, liquid carbon dioxide punching and fracturing; step 5, recording data; the nozzle feed displacement is measured by a displacement sensor, the pressure sensor displays the real-time injected liquid carbon dioxide value, and the electromagnetic flowmeter displays the real-time flow of the injected fluid; step 6, replacing the shaped coal sample; step 7, repeating steps 4 to 5; step 8, other tests in the same group; step 9, collating test data. It can perform large-scale punching and fracturing simulation of multiple coal seams under true triaxial stress, with high control accuracy, and can perform punching and fracturing at different injection distances, and the simulation process is more realistic and reliable.

Description

Technical Field

[0001] The invention belongs to the technical field of coal seam mining simulation test, and in particular relates to a simulation test method for punching and fracturing multiple coal seams under true triaxial stress. Background Art

[0002] The main problems of existing simulation tests of coal seam fracturing by perforation are: (1) the hydraulic perforation fracturing has a complex structure of the inlet and outlet channels of the perforation fluid, and the control accuracy is not high, so perforation at different injection distances cannot be performed. (2) The model size used is small and has certain space limitations, which cannot accurately simulate the real scene of perforation fracturing of multiple coal seams under true triaxial stress. Summary of the invention

[0003] The present invention intends to provide a simulation test method for punching and fracturing of multiple coal seams under true triaxial stress, which can carry out large-scale simulation of punching and fracturing of multiple coal seams under true triaxial stress, has high control accuracy, can carry out punching and fracturing at different injection distances, and the simulation process is more realistic and reliable.

[0004] To this end, the technical solution adopted by the present invention is: a simulation test method for punching and fracturing multiple coal seams under true triaxial stress, comprising the following steps:

[0005] Step 1: Sample preparation;

[0006] The raw coal blocks used in the research were crushed by a jaw crusher, and gypsum, river sand and cement were mixed to prepare similar materials to simulate rock formations. The crushed raw coal powder was used as the coal seam, and the number of coal seams was set to 3. Each coal seam was separated by a rock layer. After each layer of coal rock was paved, it was compacted one by one until the sample was paved. The specimen box was placed on a pressing machine, and the lateral deformation limiting device of the box was used to prevent the lateral deformation of the specimen box during the pressing process. First, the similar material simulating the rock formation was placed at the bottom of the specimen box, and the similar material was pressed to a predetermined height by a pressing machine. Then, the prefabricated raw coal powder was added to the specimen box, and pressed in layers. The maximum pressing pressure could reach 10MPa, forming a rectangular specimen of 1000mm in length, 400mm in width and 400mm in height. After the pressing was completed, the specimen box was transferred from the pressing machine to the transfer rack using a truss crane.

[0007] Step 2: Prepare for installation of multi-field coupled coal-rock dynamic disaster prevention and control technology simulation system;

[0008] The multi-field coupled coal rock dynamic disaster prevention and control technology simulation system includes a main model and a transfer frame; the main model has a true triaxial simulation experiment function, and the main model includes a main high-pressure chamber module and a specimen box module. The specimen box module is located in the main high-pressure chamber module, and a punching interface is provided through the middle of the right side of the specimen box module and the main high-pressure chamber module, and a flange is installed outside the punching interface; an independent hydraulic loading device is provided in the X direction for pressurization, and the maximum loading pressure is 5000kN; there are 4 independent hydraulic loading devices in both Y and Z directions for pressurization, and the maximum loading pressure of a single hydraulic loading device is 3000kN, each group of hydraulic loading devices can be controlled separately, and different forces can be loaded in the 1000mm length direction, which can more realistically simulate the true triaxial stress state of the underground reservoir;

[0009] The specimen box is sent into the true triaxial loading system through the transfer rack, so that the stress loading pads of the specimen box correspond to the pressure heads in the true triaxial loading system one by one, and the punching cracking system is connected to the punching interface through the flange;

[0010] The punching fracturing system includes a punching feed system, a punching execution system, and a punching liquid supply and discharge system; the punching feed system includes a hydraulic servo control system and a hydraulic cylinder; the punching execution system includes a central pipe column, an outer pipe column, and a nozzle, wherein the central pipe column is fixedly installed in the outer pipe column and is coaxially arranged, the left end of the central pipe column is located outside the outer pipe column and extends into the punching interface through a flange, the nozzle is fixedly installed at the left end of the central pipe column, and a punching liquid outlet pipe is also penetrated on the flange; the punching liquid supply and discharge system includes a carbon dioxide gas source, a pressure reducing valve, a first pressure gauge, and a pressure relief valve connected in sequence through pipelines. , an electromagnetic flowmeter, a first stop valve, a one-way valve, a pressure sensor, a high-pressure pump, an accumulator, a second pressure gauge, and a second stop valve are sequentially connected in series and then connected to the pipeline between the one-way valve and the pressure sensor, and the pipeline behind the pressure sensor is laterally connected to the central pipe column; the punching liquid outlet pipe is connected to the interlayer between the central pipe column and the outer pipe column, and a laterally connected pipeline is arranged on the outer pipe column, and a bag filter is arranged on the pipeline; the hydraulic servo control system controls the central pipe column, the outer pipe column, and the nozzle to move left and right together through the hydraulic cylinder, thereby adjusting the spray distance of the nozzle, and the nozzle feed displacement is measured by the displacement sensor;

[0011] Step 3, applying true triaxial prestress;

[0012] According to the measured formation in-situ stress, the true triaxial loading system is used to apply in-situ stress to the simulated formation. During the stress loading process, the pressure head is first moved to make the pressure head contact with the loading pad, and a certain prestress is applied to achieve σ x =σ y =σ zThe hydrostatic pressure state is then applied in a step-by-step manner to load stresses in the three directions of Z, Y, and X to reach a predetermined ground stress value;

[0013] Step 4: Punching and cracking with liquid carbon dioxide;

[0014] The punching feed system is fed axially to realize the forward feeding of the nozzle. Liquid carbon dioxide will enter the hollow of the central pipe column from the pipeline and flow toward the nozzle along the central channel. The fluid forms a jet through the nozzle and punches out a channel in the coal seam. The punching fluid carries debris and is discharged through the channel between the central pipe column and the outer pipe column. When the nozzle is fed to a given stroke, the displacement sensor automatically controls the hydraulic servo control system to stop advancing, thereby achieving the punching effect. The test selected the punching speed of the liquid carbon dioxide jet as 50mm / min, 75mm / min, and 100mm / min.

[0015] Step 5: Record data;

[0016] The nozzle feed displacement is measured by the displacement sensor, the pressure sensor displays the real-time injected liquid carbon dioxide value, and the electromagnetic flowmeter displays the real-time flow rate of the injected fluid;

[0017] Step 6: Replace the briquette sample;

[0018] Repeat steps 2 to 3, and then increase the force applied by the upper pressure head alone to a new predetermined value;

[0019] Step 7. Repeat steps 4 to 5;

[0020] Step 8: Other tests in the same group: Replace the briquette sample, change the nozzle diameter and injection distance, or change the liquid CO2 injection rate and true triaxial stress conditions, and repeat steps 1 to 7;

[0021] Step 9: Organize the test data.

[0022] As a preferred embodiment of the above scheme, the shell of the main high-pressure chamber module is a high-pressure closed pressure chamber structure with an outer circle and an inner circle formed by a circular ring, a left circular end cover, a right circular end cover and bolts, and a front pad, a rear pad, an upper pad and a lower pad are respectively installed on the front, back, upper and lower sides of the inner wall of the circular ring, and the front pad, the rear pad, the upper pad and the lower pad form a rectangular cavity just for the test piece box to be placed in, an axial hydraulic cylinder is installed through the left circular end cover, a punching interface is installed through the middle part of the right circular end cover, and wiring harness pipeline lead-out holes are respectively opened on the left and right circular end covers, and a row of lifters are installed in grooves at intervals on the left and right sides of the top of the lower pad, and the lifters can protrude from the lower pad and also sink into the lower pad;

[0023] The test piece box is a rectangular test piece accommodating chamber surrounded by a left side plate, a bottom plate, a top plate, a right side plate, a front side plate, and a rear side plate in combination with bolts, and the rectangular test piece accommodating chamber is in line with the axial line of the high-pressure closed pressure chamber. A left pressure plate is installed on the left side of the rectangular test piece accommodating chamber, a plurality of upper pressure plates are installed on the top left and right in sequence, and a plurality of front pressure plates are installed on the front left and right in sequence. The axial hydraulic cylinder can pass through the left side plate and connect with the left pressure plate, each upper pressure plate is connected with the top hydraulic cylinder through an upper pad installed on the top plate, and each front pressure plate is connected with the lateral hydraulic cylinder through a side pad installed on the front side plate. A plurality of heating tubes and temperature control probes are installed with openings on the upper pressure plate, the front pressure plate, the bottom plate, and the rear side plate, and a plurality of ultrasonic probes are installed with openings on the upper pressure plate, the front pressure plate, the left pressure plate, the bottom plate, the rear side plate, and the right side plate. A row of rollers are installed at intervals on the left and right sides of the bottom of the test piece box through a lining plate. When the test piece box is pushed into the main high-pressure chamber module, the lifter is supported under the rollers.

[0024] An anti-channeling plate corresponding to the upper pressure plate is arranged directly above the bottom plate, and a central air inlet hole and a plurality of annular grooves surrounding the central air inlet hole are opened on the anti-channeling plate, and all the annular grooves are connected with the central air inlet hole through communication grooves distributed in a divergent shape, and the air inlet pipe passes horizontally through the side wall of the test piece box and is connected to the bottom of the central air inlet hole, a water-permeable baffle is installed above the anti-channeling plate, filter plates are installed at the left and right ends of the test piece, and sealing gaskets are installed at the top, bottom, front and back of the test piece.

[0025] It is further preferred that there is only one axial hydraulic cylinder with a maximum loading pressure of 5000kN; there are four groups of top hydraulic cylinders and lateral hydraulic cylinders, and each group of hydraulic cylinders is equipped with two parallel hydraulic loading systems for pressurization, one of which is a static load loading system and the other is a dynamic load loading system. The maximum loading pressure of a single hydraulic loading device is 3000kN. Each group of hydraulic loading systems independently controls a pressure plate and is arranged in the center of the corresponding pressure plate. The axial hydraulic cylinder, the top hydraulic cylinder, and the lateral hydraulic cylinder can all load dynamic and static loads.

[0026] More preferably, the annular grooves are rectangular or circular and are distributed at equal intervals.

[0027] It is further preferred that it also includes a main frame for supporting the main model, the main frame is a rectangular frame structure, the left and right ends of the main model extend out of the main frame, a transfer slide is arranged on the right side of the main frame, and the transfer slide extends to directly below the main high-pressure chamber module, and the width of the transfer slide is smaller than the inner space width of the main frame; a specimen box lifting transfer frame and a right round end cover transfer frame are slidably installed on the transfer slide, the specimen box lifting transfer frame can perform lifting and lowering movements, and is used to support the specimen box; the top of the right round end cover transfer frame is arc-shaped and is used to support the right round end cover, and after the specimen box lifting transfer frame is raised, the specimen box can be pushed horizontally into the main high-pressure chamber module, and after the specimen box lifting transfer frame is lowered, the top is lower than the bottom of the main high-pressure chamber module, so that it can slide into the bottom of the main high-pressure chamber module, so that the right round end cover transfer frame can slide to the left to the set position for the installation of the right round end cover.

[0028] Further preferably, each of the lifters adopts a double support structure that is spaced front and back and symmetrically arranged, each lifter adopts a separate hydraulic drive, and all lifters move synchronously.

[0029] The beneficial effects of the present invention are as follows: it can carry out large-scale simulation of multi-coal seam punching and fracturing under true triaxial stress, with high control accuracy, can carry out punching and fracturing at different injection distances, and the simulation process is more realistic and reliable; combined with a carbon dioxide gas source as the punching liquid supply, the punching liquid pipeline structure is simple, safe and reliable. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic diagram of the steps of the present invention.

[0031] Figure 2 It is a system composition diagram of the present invention.

[0032] Figure 3 It is a schematic diagram of the main model of the multi-field coupled coal-rock dynamic disaster prevention and control technology simulation system.

[0033] Figure 4 for Figure 3 Interior left view.

[0034] Figure 5 This is a schematic diagram of the structure of the test box.

[0035] Figure 6 for Figure 5 Interior left view.

[0036] Figure 7 This is a simplified diagram of the arrangement of the heating tube, temperature control probe, and ultrasonic probe.

[0037] Figure 8 This is a simplified diagram of an anti-channeling plate.

[0038] Fig. 9This is the state before the test box is loaded into the main high-pressure chamber module. DETAILED DESCRIPTION

[0039] The present invention will be further described below by way of embodiments and in conjunction with the accompanying drawings:

[0040] like Figure 1 As shown in the figure, a simulation test method for multi-coal seam fracturing caused by punching under true triaxial stress is required. Figure 2 The system diagram shown in FIG. 1 includes the following steps:

[0041] Step 1: Sample preparation.

[0042] The raw coal lumps used in the research were crushed by jaw crusher, and gypsum, river sand and cement were mixed to prepare similar materials to simulate rock formations.

[0043] The crushed raw coal powder is used as the coal seam, and the number of coal seams is set to 3. The coal seams are separated by rock layers. After each layer of coal and rock is paved, they are compacted one by one until the sample is paved.

[0044] The specimen box is placed on a compression molding machine, and a lateral deformation limiting device for the box is used to prevent lateral deformation of the specimen box during the compression process.

[0045] First, put similar materials simulating rock formations into the bottom of the specimen box, use a pressing machine to press the similar materials to a predetermined height, then add prefabricated raw coal powder into the specimen box, and press it in layers. The maximum pressing pressure can reach 10MPa, forming a rectangular specimen of 1000mm long, 400mm wide and 400mm high. After the pressing is completed, use a gantry crane to transfer the specimen box from the pressing machine to the transfer rack.

[0046] Step 2: Prepare for the installation of a multi-field coupled coal-rock dynamic disaster prevention and control technology simulation system.

[0047] The multi-field coupled coal-rock dynamic disaster prevention and control technology simulation system is used for punching fracture test, including a main model and a transfer frame. The main model has a true triaxial simulation experiment function, and the main model includes a main high-pressure chamber module and a specimen box module. The specimen box module is located in the main high-pressure chamber module, and a punching interface is set through the middle of the right side of the specimen box module and the main high-pressure chamber module, and a flange is installed outside the punching interface.

[0048] The main model has an independent hydraulic loading device in the X direction for pressurization, with a maximum loading pressure of 5000kN; there are 4 independent hydraulic loading devices in the Y and Z directions for pressurization, and the maximum loading pressure of a single hydraulic loading device is 3000kN. Each hydraulic loading device can be controlled separately and realize different force loading in the 1000mm length direction, which can more realistically simulate the true triaxial stress state of the underground reservoir.

[0049] The specimen box is sent into the true triaxial loading system through the transfer rack, so that the stress loading pads of the specimen box correspond one to one with the pressure heads in the true triaxial loading system, and the punching and fracturing system is connected to the punching interface through the flange.

[0050] like Figure 2 As shown, the main body model A includes a main body high pressure chamber module A-1 and a test box module A-2, and the test box module A-2 is located in the main body high pressure chamber module A-1. A punching interface a is provided through the middle of the right side of the test box module A-2 and the main body high pressure chamber module A-1, and a flange b is installed outside the punching interface.

[0051] In addition, it also includes four parts: punching feed system, punching execution system, and punching liquid supply and discharge system.

[0052] The punching feeding system is mainly composed of a hydraulic servo control system and a hydraulic cylinder c. The hydraulic servo control system is used to control the hydraulic cylinder c.

[0053] The punching execution system mainly consists of a central pipe column d, an outer pipe column e, and a nozzle (not shown in the figure). The central pipe column d is fixedly installed in the outer pipe column e and is coaxially arranged. The left end of the central pipe column d is located outside the outer pipe column e and extends through the flange to the punching interface a. The nozzle is fixedly installed at the left end of the central pipe column d, and a punching liquid outlet pipe (not shown in the figure) is also penetrated on the flange b.

[0054] The punching liquid supply and discharge system includes a carbon dioxide gas source f, a pressure reducing valve g, a first pressure gauge h, an electromagnetic flowmeter i, a first stop valve j, a check valve k, and a pressure sensor m, which are connected in sequence through pipelines. A high-pressure pump n, an accumulator p, a second pressure gauge q, and a second stop valve r are connected in series in sequence and then connected to the pipeline between the check valve k and the pressure sensor m. The pipeline behind the pressure sensor m is laterally connected to the central pipe column d; the punching liquid outlet pipe is connected to the interlayer between the central pipe column d and the outer pipe column e, and a lateral outlet pipe is provided on the outer pipe column e, and a bag filter r is provided on the pipe. The punching liquid is sent to the nozzle through the central pipe column for punching; because the nozzle maintains pressure, the punching liquid after punching carries debris and flows out through the punching liquid outlet pipe, the interlayer between the central pipe column and the outer pipe column.

[0055] The hydraulic servo control system controls the center pipe column d, the outer pipe column e, and the nozzle to move left and right together through the hydraulic cylinder c, thereby adjusting the spray distance of the nozzle. The nozzle feed displacement is measured by a displacement sensor (not shown in the figure).

[0056] Step 3: Apply true triaxial prestress.

[0057] According to the measured formation in-situ stress, the true triaxial loading system is used to apply in-situ stress to the simulated formation. During the stress loading process, the pressure head is first moved to make the pressure head contact with the loading pad, and a certain prestress is applied to achieve σ x =σ y =σ z The hydrostatic pressure state is then applied one by one in the three directions of Z, Y and X in a step-by-step manner to reach the predetermined ground stress value.

[0058] Step 4: Punching and cracking with liquid carbon dioxide.

[0059] The punching feed system is fed axially to realize forward feeding of the nozzle. Liquid carbon dioxide will enter the hollow part of the central pipe column from the pipeline and flow toward the nozzle along the central channel. The fluid forms a jet through the nozzle and will punch out a channel in the coal seam. The punching fluid carries debris and is discharged through the channel between the central pipe column and the outer pipe column. When the nozzle is fed to a given stroke, the displacement sensor automatically controls the hydraulic servo control system to stop advancing, thereby achieving the punching effect. The test selected the punching speed of the liquid carbon dioxide jet as 50mm / min, 75mm / min, and 100mm / min.

[0060] Step 5: Record data.

[0061] The nozzle feed displacement is measured by the displacement sensor, the pressure sensor displays the real-time injected liquid carbon dioxide value, and the electromagnetic flowmeter displays the real-time flow rate of the injected fluid.

[0062] Step 6: Replace the coal sample.

[0063] Repeat steps 2 to 3, and then increase the force applied by the upper pressure head alone to the new predetermined value.

[0064] Step 7. Repeat steps 4 to 5.

[0065] Step 8. Other tests in the same group: Replace the briquette sample, change the nozzle diameter and injection distance, or change the liquid CO2 injection rate and true triaxial stress conditions, and repeat steps 1 to 7.

[0066] Step 9: Organize the test data.

[0067] In order to meet different experimental needs, 5 nozzles with different calibers are designed to be interchangeable, namely 2, 4, 6, 8 and 10 mm nozzle calibers.

[0068] Specific test plan:

[0069]

[0070] like Figure 3 — Figure 4As shown in the figure, the main model of the multi-field coupled coal rock dynamic disaster prevention and control technology simulation system is mainly composed of two parts: the main high-pressure chamber module and the test box.

[0071] The shell 1 of the main high-pressure chamber module is a high-pressure closed pressure chamber structure with an outer circle and an inner circle surrounded by a circular ring 3, a left circular end cover 4, and a right circular end cover 5 combined with bolts. The front pad 2, the rear pad 9, the upper pad 10, and the lower pad 11 are installed on the front, back, top, and bottom of the inner wall of the circular ring 3. The front pad 2, the rear pad 9, the upper pad 10, and the lower pad 11 form a rectangular cavity just for the test box to be placed in.

[0072] An axial hydraulic cylinder 6 is installed through the left circular end cover 4, and a punching interface is installed through the middle of the right circular end cover 5.

[0073] The left circular end cover 4 and the right circular end cover 5 are respectively provided with wiring harness pipeline lead-out holes 7. A row of lifters 8 are installed in slots arranged at intervals on the top of the lower pad 11. The lifters 8 can protrude from the lower pad 11 or sink into the lower pad 11. Each lifter 8 adopts a double wheel structure that is spaced front and back and symmetrically arranged to achieve front and back double support, balanced force and stability. Each lifter 8 adopts a separate hydraulic drive, and all lifters 8 are controlled by a control system to move synchronously.

[0074] Combination Figure 3 — Figure 6 As shown, the specimen box is a rectangular specimen accommodating chamber formed by a left side plate 12, a bottom plate 13, a top plate 14, a right side plate 15, a front side plate 23, and a rear side plate 24 in combination with bolts. The rectangular specimen accommodating chamber is colinear with the axis of the high-pressure closed pressure chamber to ensure that the rectangular specimen is centered in the main model. A left pressure plate 16 is installed on the left side of the rectangular specimen accommodating chamber, a plurality of upper pressure plates 17 are installed on the top left and right in sequence, and a plurality of front pressure plates 18 are installed on the front left and right in sequence. The axial hydraulic cylinder 6 can pass through the left side plate 12 and connect with the left pressure plate 16. Each upper pressure plate 17 is connected with the top hydraulic cylinder 20 through the upper pad 19 installed on the top plate 14. The top hydraulic cylinder 20 has a hydraulic piston 20a. The hydraulic piston 20a acts on the upper pad 19, and then the upper pressure plate 17 applies load to the rectangular specimen. Each front pressure plate 18 is connected to a lateral hydraulic cylinder 22 through a side pad 21 installed on a front side plate 23. The lateral hydraulic cylinder 22 also has a hydraulic piston. The hydraulic piston acts on the side pad 21, and then the front pressure plate 18 applies a load to the rectangular specimen.

[0075] Combination Figure 3 — Figure 7As shown, a plurality of heating tubes 27 and temperature control probes 28 are installed in the holes of the upper pressing plate 17, the front pressing plate 18, the bottom plate 13, and the rear side plate 24, and a plurality of ultrasonic probes 29 are installed in the holes of the upper pressing plate 17, the front pressing plate 18, the left pressing plate 16, the bottom plate 13, the rear side plate 24, and the right side plate 15. A row of rollers 26 are installed at intervals on the left and right sides of the bottom of the test box through the lining plate 25. When the test box is pushed into the main high-pressure chamber module, the lifter 8 is supported under the rollers 26.

[0076] Preferably, a high-frequency vibrator is installed on the cavity of the axial hydraulic cylinder 6, and high-speed vibration is generated under the action of a high-pressure gas source. The high-frequency vibration force can be transmitted to the test piece to the right through the corresponding hydraulic cavity, hydraulic piston, and left pressure plate 16.

[0077] A channeling prevention plate 30 corresponding to the upper pressure plate 17 is arranged directly above the bottom plate 13. Figure 7 As shown, the anti-channeling plate 30 is provided with a central air inlet hole 30a and a plurality of annular grooves 30b surrounding the central air inlet hole 30a, and all the annular grooves 30b are connected to the central air inlet hole 30a through the communication grooves 30c distributed in a divergent shape, and the air inlet pipe passes through the rear side wall of the test box horizontally and is connected to the bottom of the central air inlet hole 30a. The annular grooves 30b are rectangular or circular and are distributed at equal intervals.

[0078] The air inlet pipe is transversely passed through the rear side wall of the test box and connected to the bottom of the central air inlet hole 30a, and a water-permeable baffle 31 is installed above the anti-channeling plate 30. The internal anti-channeling design, combined with the use of a water-permeable baffle, has better sealing performance and can better complete the punching experiment.

[0079] Filter plates 32 are installed at both left and right ends of the test piece, and sealing gaskets 33 are installed at the top, bottom, front and back of the test piece.

[0080] The inner cavity of the test chamber can accommodate a rectangular test piece of length 1000×width 400×height 400mm, and the internal pressure resistance of the main high-pressure chamber module is 10MPa.

[0081] There is only one axial hydraulic cylinder 6, and the maximum loading pressure is 5000kN; there are four groups of top hydraulic cylinders 20 and lateral hydraulic cylinders 22, and each group of hydraulic cylinders is equipped with two parallel hydraulic loading systems for pressurization, one of which is a static load loading system and the other is a dynamic load loading system. The maximum loading pressure of a single hydraulic loading device is 3000kN. Each group of hydraulic loading systems independently controls a pressure plate and is arranged in the center of the corresponding pressure plate. The axial hydraulic cylinder 6, the top hydraulic cylinder 20, and the lateral hydraulic cylinder 22 can all load dynamic and static loads.

[0082] like Fig. 9As shown, the multi-field coupled coal rock dynamic disaster prevention and control technology simulation system includes, in addition to the main model, a main frame 37 for supporting the main model, a transfer slide rail 36, a specimen box lifting transfer frame 34, and a right round end cover transfer frame 35. The specimen box lifting transfer frame 34 and the right round end cover transfer frame 35 are collectively referred to as transfer frames.

[0083] The main frame 37 is used to support the main model. The main frame 37 is a rectangular frame structure, and the left and right ends of the main model extend outside the main frame 37. A transfer slide 36 is provided on the right side of the main frame 37, and the transfer slide 36 extends to the right below the main high-pressure chamber module, and the width of the transfer slide 36 is less than the inner space width of the main frame 37. The specimen box lifting and transfer frame 34 and the right round end cover transfer frame 35 are slidably installed on the transfer slide 36. The specimen box lifting and transfer frame 34 can perform lifting and lowering movements and is used to support the specimen box. The top of the right round end cover transfer frame 35 is arc-shaped and is used to hold up the right round end cover 5. After the specimen box lifting and transfer frame 34 is raised, the specimen box can be pushed horizontally into the main high-pressure chamber module. After the specimen box lifting and transfer frame 34 is lowered, the top is lower than the bottom of the main high-pressure chamber module, so that the specimen box lifting and transfer frame can slide into the bottom of the main high-pressure chamber module, so that the right round end cover transfer frame 35 can slide to the left to the set position to install the right round end cover 5.

[0084] The main features of the main model:

[0085] (1) The shell of the main high-pressure chamber module is a high-pressure closed pressure chamber with an outer circle and an inner circle formed by a circular ring, a left circular end cover, a right circular end cover and bolts, which is completely different from the traditional pressure chamber structure with an outer square and an inner square surrounded by six plates; at the same time, since the test chamber itself is rectangular, in order to meet the installation requirements of the test chamber, special-shaped front, rear, upper and lower pads are creatively installed on the front, back, upper and lower sides of the inner wall of the high-pressure closed pressure chamber, and a rectangular cavity is formed by the front, rear, upper and lower pads to fit the test chamber, thereby forming an outer circle and inner square test chamber installation environment, which has stronger internal pressure resistance and better sealing ability, and can provide an internal pressure resistance of up to 10MPa, providing a better test environment for the multi-coal seam simulation test of punching fracture under true triaxial stress;

[0086] (2) A row of lifters that can protrude or sink into the lower pad are installed in grooves at intervals on the left and right of the top of the lower pad. At the same time, a row of rollers are installed at intervals on the left and right of the bottom of the test box through the lining plate. When the test box is pushed into the main high-pressure chamber module, the lifters are supported under the rollers, which can make the test box easier to push in and pull out, thereby improving the degree of automation of the installation and making large-scale simulation test operations easier and more labor-saving;

[0087] (3) Several heating tubes and temperature control probes are installed in the holes of the upper pressure plate, front pressure plate, bottom plate, and rear side plate. Several ultrasonic probes are installed in the holes of the upper pressure plate, front pressure plate, left pressure plate, bottom plate, rear side plate, and right side plate. It is also possible to carry out seepage test of fractured rock mass under three-dimensional stress-seepage-temperature multi-field coupling conditions; and in combination with the anti-channeling plate set just above the bottom plate, a permeable baffle is installed above the anti-channeling plate, filter plates are installed at the left and right ends of the specimen, and sealing pads are installed at the top, bottom, front and back of the specimen, which can prevent channeling and ensure good air permeability, and have multiple filtering and sealing effects.

Claims

1. A simulation test method for punching and fracturing multiple coal seams under true triaxial stress, characterized by: The following steps are involved: Step 1: Sample preparation; The raw coal blocks used in the research were crushed by a jaw crusher, and gypsum, river sand and cement were mixed to prepare similar materials to simulate rock formations. The crushed raw coal powder was used as the coal seam, and the number of coal seams was set to 3. Each coal seam was separated by a rock layer. After each layer of coal rock was paved, it was compacted one by one until the sample was paved. The test box was placed on a pressing machine, and the lateral deformation limiting device of the box was used to prevent the lateral deformation of the test box during the pressing process. First, the similar material simulating the rock formation was placed at the bottom of the test box, and the similar material was pressed to a predetermined height by a pressing machine. Then, the prefabricated raw coal powder was added to the test box, and the test was pressed in layers. The maximum pressing pressure can reach 10 MPa, forming a rectangular sample of 1000 mm in length, 400 mm in width and 400 mm in height. After the pressing is completed, the test box is transferred from the pressing machine to the transfer rack by a truss crane. Step 2: Prepare for installation of multi-field coupled coal-rock dynamic disaster prevention and control technology simulation system; The multi-field coupled coal rock dynamic disaster prevention and control technology simulation system includes a main model and a transfer frame; the main model has a true triaxial simulation experiment function, and the main model includes a main high-pressure chamber module and a specimen box module. The specimen box module is located in the main high-pressure chamber module, and a punching interface is provided through the middle of the right side of the specimen box module and the main high-pressure chamber module, and a flange is installed outside the punching interface; an independent hydraulic loading device is provided in the X direction for pressurization, and the maximum loading pressure is 5000kN; there are 4 independent hydraulic loading devices in both Y and Z directions for pressurization, and the maximum loading pressure of a single hydraulic loading device is 3000 kN. Each group of hydraulic loading devices can be controlled separately, and different forces can be loaded in the 1000mm length direction, which can more realistically simulate the true triaxial stress state of the underground reservoir; The specimen box is sent into the true triaxial loading system through the transfer rack, so that the stress loading pads of the specimen box body correspond one to one with the pressure head in the true triaxial loading system, and the punching cracking system is connected to the punching interface through the flange; The punching fracturing system includes a punching feed system, a punching execution system, and a punching liquid supply and discharge system; the punching feed system includes a hydraulic servo control system and a hydraulic cylinder; the punching execution system includes a central pipe column, an outer pipe column, and a nozzle, wherein the central pipe column is fixedly installed in the outer pipe column and coaxially arranged, the left end of the central pipe column is located outside the outer pipe column and extends into the punching interface through a flange, the nozzle is fixedly installed at the left end of the central pipe column, and a punching liquid outlet pipe is also penetrated on the flange; The punching liquid supply and discharge system includes a carbon dioxide gas source, a pressure reducing valve, a first pressure gauge, an electromagnetic flowmeter, a first stop valve, a one-way valve, and a pressure sensor connected in sequence through pipelines. A high-pressure pump, an accumulator, a second pressure gauge, and a second stop valve are connected in series in sequence and then connected to the pipeline between the one-way valve and the pressure sensor. The pipeline behind the pressure sensor is laterally connected to the central pipe column; the punching liquid outlet pipe is connected to the interlayer between the central pipe column and the outer pipe column, and a laterally connected pipeline is arranged on the outer pipe column, and a bag filter is arranged on the pipeline; the hydraulic servo control system controls the central pipe column, the outer pipe column, and the nozzle to move left and right together through a hydraulic cylinder, thereby adjusting the spray distance of the nozzle, and the nozzle feed displacement is measured by the displacement sensor; Step 3, applying true triaxial prestress; According to the measured ground stress, the ground stress is applied to the simulated ground using a true triaxial loading system. During the stress loading process, the pressure head is first moved to make the pressure head contact with the loading pad, and a certain prestress is applied to achieve σ x = σ y = σ z The hydrostatic pressure state is then applied in a step-by-step manner to load stresses in the three directions of Z, Y, and X to reach a predetermined ground stress value; Step 4: Punching and cracking with liquid carbon dioxide; The punching feed system is fed axially to realize the forward feeding of the nozzle. Liquid carbon dioxide will enter the hollow of the central pipe column from the pipeline and flow toward the nozzle along the central channel. The fluid forms a jet through the nozzle and punches out a channel in the coal seam. The punching fluid carries debris and is discharged through the channel between the central pipe column and the outer pipe column. When the nozzle is fed to a given stroke, the displacement sensor automatically controls the hydraulic servo control system to stop advancing, thereby achieving the punching effect. The test selected the punching speed of the liquid carbon dioxide jet as 50 mm / min, 75 mm / min, and 100 mm / min. Step 5: Record data; The nozzle feed displacement is measured by the displacement sensor, the pressure sensor displays the real-time injected liquid carbon dioxide value, and the electromagnetic flowmeter displays the real-time flow rate of the injected fluid; Step 6: Replace the briquette sample; Repeat steps 2 to 3, and then increase the force applied by the upper pressure head alone to a new predetermined value; Step 7. Repeat steps 4 to 5; Step 8. Other tests in the same group; Replace the briquette sample, change the nozzle diameter and injection distance, or change the liquid CO2 injection rate and true triaxial stress conditions, and repeat steps 1 to 7; Step 9: Arrange the test data; The shell (1) of the main high-pressure chamber module is a high-pressure closed pressure chamber structure with an outer circle and an inner circle formed by a circular ring (3), a left circular end cover (4), and a right circular end cover (5) in combination with bolts. A front pad (2), a rear pad (9), an upper pad (10), and a lower pad (11) are respectively installed on the front, back, upper, and lower sides of the inner wall of the circular ring (3). The front pad (2), the rear pad (9), the upper pad (10), and the lower pad (11) form a rectangular cavity just for the test piece box to be placed in. An axial hydraulic cylinder (6) is installed through the left circular end cover (4). A punching interface is provided through the middle of the right circular end cover (5). A wiring harness pipeline lead-out hole (7) is respectively opened through the left circular end cover (4) and the right circular end cover (5). A row of lifters (8) are installed in grooves at intervals on the left and right sides of the top of the lower pad (11). The lifters (8) can protrude from the lower pad (11) or sink into the lower pad (11). The test specimen box is a rectangular test specimen accommodating chamber formed by a left side plate (12), a bottom plate (13), a top plate (14), a right side plate (15), a front side plate (23), and a rear side plate (24) in combination with bolts, and the rectangular test specimen accommodating chamber is colinear with the axis of the high-pressure closed pressure chamber. A left pressure plate (16) is installed on the left side of the rectangular test specimen accommodating chamber, a plurality of upper pressure plates (17) are installed on the top left and right in sequence, and a plurality of front pressure plates (18) are installed on the front left and right in sequence. The axial hydraulic cylinder (6) can pass through the left side plate (12) and be connected to the left pressure plate (16). Each upper pressure plate (17) is connected to the top hydraulic cylinder (20) by passing through an upper pad (19) installed on the top plate (14). Each front pressure plate (18) is connected to the top hydraulic cylinder (20) by passing through an upper pad (19) installed on the top plate (14). The pressure plate (18) is connected to the lateral hydraulic cylinder (22) through a side pad (21) installed on the front side plate (23); a plurality of heating tubes (27) and temperature control probes (28) are installed through holes in the upper pressure plate (17), the front pressure plate (18), the bottom plate (13), and the rear side plate (24); a plurality of ultrasonic probes (29) are installed through holes in the upper pressure plate (17), the front pressure plate (18), the left pressure plate (16), the bottom plate (13), the rear side plate (24), and the right side plate (15); a row of rollers (26) are installed at intervals on the left and right sides of the bottom of the test piece box through a lining plate (25); when the test piece box is pushed into the main high-pressure chamber module, the lifter (8) is supported below the rollers (26); An anti-channeling plate (30) corresponding to the upper pressure plate (17) is arranged directly above the bottom plate (13), and the anti-channeling plate (30) is provided with a central air inlet hole (30a) and a plurality of annular grooves (30b) surrounding the central air inlet hole (30a), and all the annular grooves (30b) are connected to the central air inlet hole (30a) through communication grooves (30c) distributed in a divergent manner, and the air inlet pipe passes through the side wall of the test piece box horizontally to connect to the bottom of the central air inlet hole (30a), a water-permeable baffle (31) is installed above the anti-channeling plate (30), filter plates (32) are installed at the left and right ends of the test piece, and sealing gaskets (33) are installed at the top, bottom, front and back of the test piece.

2. The simulation test method for multi-coal seam fracturing by punching under true triaxial stress according to claim 1, characterized in that: There is only one axial hydraulic cylinder (6), and the maximum loading pressure is 5000 kN; there are four groups of top hydraulic cylinders (20) and lateral hydraulic cylinders (22), each of which is equipped with two parallel hydraulic loading systems for pressurization, one of which is a static load loading system and the other is a dynamic load loading system. The maximum loading pressure of a single hydraulic loading device is 3000 kN. Each hydraulic loading system controls a pressure plate independently and is arranged in the left and right center of the corresponding pressure plate. The axial hydraulic cylinder (6), the top hydraulic cylinder (20), and the lateral hydraulic cylinder (22) can all load static and dynamic loads.

3. The simulation test method for fracturing multiple coal seams by punching under true triaxial stress according to claim 1, characterized in that: The annular grooves (30b) are rectangular or circular and are distributed at equal intervals.

4. The simulation test method for multi-coal seam fracturing by punching under true triaxial stress according to claim 1, characterized in that: The main body frame (37) is used to support the main body model. The main body frame (37) is in a rectangular frame structure. The left and right ends of the main body model extend outside the main body frame (37). A transfer slide rail (36) is provided on the right side of the main body frame (37). The transfer slide rail (36) extends to the bottom of the main body high pressure chamber module. The width of the transfer slide rail (36) is smaller than the inner space width of the main body frame (37). A specimen box lifting transfer frame (34) and a right round end cover transfer frame (35) are slidably mounted on the transfer slide rail (36). The box lifting and transporting frame (34) can be lifted and lowered and is used to support the test box; the top of the right round end cover transporting frame (35) is arc-shaped and is used to hold up the right round end cover (5). After the test box lifting and transporting frame (34) is lifted, the test box can be pushed horizontally into the main body high pressure chamber module. After the test box lifting and transporting frame (34) is lowered, the top is lower than the bottom of the main body high pressure chamber module, so that it can slide under the main body high pressure chamber module, so that the right round end cover transporting frame (35) can slide to the left to the set position to install the right round end cover (5).

5. The simulation test method for fracturing multiple coal seams by punching under true triaxial stress according to claim 1, characterized in that: Each of the lifters (8) adopts a double support structure that is spaced front and back and symmetrically arranged, each lifter (8) adopts a separate hydraulic drive, and all lifters (8) move synchronously.

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