High-stress high-pressure large-size true triaxial fracturing test device and method

Abstract

The invention discloses a high-stress high-pressure large-size true triaxial fracturing test device and method, and relates to the technical field of geological engineering test devices. The device comprises a first gas injection system, a second gas injection system, an outlet separation system and a high-stress and high-pressure true triaxial apparatus, can apply true triaxial stress to a sample, injects different fracturing media such as supercritical CO2, liquid nitrogen and water, can control changes of multiple physical fields such as a fluid-solid field, a low-temperature field, a fracture field and a pressure field, and can be used for measuring the pressure of the sample. The device is suitable for fracturing experiments of rock samples such as clay rock, sandstone and shale, hydrate in-situ generation and fracturing can be carried out, and the device can be used for exploring the mechanical characteristics of the samples under the action of high stress, high temperature and multiple physical fields and carrying out research on the characteristics such as fracture initiation and expansion rules, volume fracturing transformation and fragmentation characteristics; in addition, the defects of a traditional true triaxial instrument loading and unloading mechanism are overcome, and the automatic control function of the instrument is further optimized.

Description

Technical Field

[0001] The present invention relates to the technical field of geological engineering test devices, and in particular to a high-stress, high-pressure, large-size true triaxial fracturing test device and method. Background Art

[0002] As a mature reservoir transformation technology, hydraulic fracturing technology is widely used in various fields such as mining engineering, water conservancy and hydropower, and oil and gas reservoir development. Fracturing technology can not only form artificial fractures in underground drilling, transform oil and gas reservoirs and underground mine structures, and expand the scope of bottom hole pressure; it can also form effective fluid seepage channels in low-permeability shale and coal seams, and improve the extraction efficiency of coalbed methane, shale gas, etc. in engineering operations.

[0003] The true triaxial apparatus is a commonly used geotechnical testing instrument that simulates the stress state of the formation and can apply true triaxial stress to the sample. Different from conventional triaxial test instruments, the true triaxial can provide three-dimensional stress with different confining pressures, which is used to study the deformation and fracturing failure behavior of the sample under more accurate underground stress application. Based on hydraulic fracturing, researchers further proposed a true triaxial coal rock sample destruction device that can inject fracturing fluid. However, there are certain problems in the actual use of the device: 1. Due to the heterogeneity and porous structure of the coal rock mass, it is difficult to obtain a truly representative test sample. The size and shape of the sample have an important influence on the accuracy and reliability of the experimental results. Patent 1 (CN111103198A) discloses a true triaxial hydraulic fracturing test device and method with CT real-time scanning, which can realize the fracturing test and CT scanning analysis of coal rock samples, but the device uses a small-scale sample of 50mm×50mm×100mm, which cannot meet the fracturing test requirements of large-scale coal rock mass, limiting the research on the fracturing effect under real field conditions and the accurate evaluation of the fracturing behavior of coal rock mass in engineering applications. 2. It is usually difficult for the test device to simulate the underground high temperature and high pressure environment, and these conditions are crucial for deep coal mining and geothermal energy development. Moreover, the device can only conduct a single test and cannot realize the comprehensive simulation of multiple physical fields. Patent 2 (CN207570982U) discloses a true triaxial seepage test structure and method for applying non-uniform loads. The system uses non-uniform loads to conduct true triaxial seepage test research, but the device only conducts seepage test research and cannot conduct multi-physical field research. The coal rock mass in actual engineering is usually affected by multiple factors (such as temperature, stress, seepage field, etc.). The device lacks a comprehensive study of the mutual coupling of factors, which affects the accurate prediction and optimization design of fracturing behavior in complex engineering scenarios. 3. Patent 3 (CN113008682A) discloses a true triaxial hydraulic fracturing simulation test device and method for natural gas hydrate reservoirs, but the device only uses pressure sensors and camera monitoring data, and cannot realize the monitoring of key parameters such as mechanical response, strain distribution, and crack extension during fracturing. 4. Patent 4 (CN114216785A) discloses a large-scale true triaxial coal-rock multi-field multi-phase variable frequency fracturing test device, but the large-scale samples are manually loaded, which is cumbersome and slow to disassemble and assemble. The operation and data processing still rely heavily on manual operation and analysis, and lack the application of automation and intelligent technology. 5. Patent 5 (CN201410089591.4) discloses a rock true triaxial test loading system. The device can realize a variety of loading tests, has a simple structure, and is easy to operate. However, the device can only perform a single rock fracturing test and cannot perform hydrate simulation tests in an in-situ environment with a closed pore pressure.

[0004] In summary, in order to make up for the shortcomings of invention patent 1 (CN111103198A), patent 2 (CN207570982U), patent 3 (CN113008682A), patent 4 (CN114216785A), and patent 5 (CN201410089591.4), the device of the present invention redesigns and optimizes the high-stress, high-pressure, large-size true triaxial fracturing test device to achieve three-dimensional stress application of large-size samples, comparative study of injection of multiple fracturing media, multi-sample fracturing process study of coal rock mass and hydrate formation, and realize multi-physical field change control (fluid-solid field, low temperature field, fracture field, and pressure field), so as to better study the crack initiation and expansion laws, volume fracturing transformation, rock fragmentation and other characteristics of coal rock and hydrate samples, and provide an accurate theoretical basis for technical research on mechanical properties, stress, and crushing methods of coal rock mass and hydrates. Summary of the invention

[0005] The purpose of the present invention is to overcome the defects in the prior art and provide a high-stress, high-pressure, large-size true triaxial fracturing test device and method.

[0006] The specific technical solutions adopted by the present invention are as follows:

[0007] In a first aspect, the present invention provides a high stress, high pressure, large-size true triaxial fracturing test device, comprising a first gas injection system, a second gas injection system, an outlet separation system, and a high stress, high pressure true triaxial instrument;

[0008] The high stress and high pressure true triaxial instrument comprises a reactor, a sample body, a pressure plate, a support plate and a reactor cover; the reactor has a sealed inner cavity, in which the sample body is placed; the sample body comprises a sample, a loading plate and a gas-liquid distribution plate; the sample is a regular hexahedron structure, and the outer sides of the six faces are respectively fitted with and covered with loading plates, and acoustic emission probes are evenly arranged on one side of the loading plate in contact with the sample; a vertical wellbore is opened in the center of the top of the sample, and the top of the wellbore is externally connected to a fracturing fluid introduction channel, and a fully enclosed rubber sleeve is sleeved on the outside of the six loading plates to fix, press and seal the loading plates with the sample; A gas-liquid distribution plate is clamped and placed between the loading plate on the bottom surface and the sample, and the bottom of the gas distribution plate is externally connected to a gas inlet channel; the upper side of the sample is externally connected to a data acquisition channel, the left side is externally connected to a seepage inlet channel, and the right side is externally connected to a seepage discharge channel; the fracturing fluid introduction channel is connected to a first gas injection system for injecting supercritical CO2 gas, the gas inlet channel is connected to a second gas injection system for injecting nitrogen or methane, each acoustic emission probe is connected to an acoustic emission and distributed optical fiber sensor through a data acquisition channel, and the seepage discharge channel is connected to an outlet separation system for three-phase separation;

[0009] The lower part, left part and rear part of the sample body are in contact with the inner wall of the reactor through support plates respectively, and the upper part, right part and front part are respectively provided with pressure plates; the pressure plate is sealingly and slidingly connected to the reactor, and can clamp the sample body together with the support plate on the opposite side to apply stress, and one end of the pressure plate is located outside the reactor and is connected to a reactor cover; the reactor cover includes a reactor cover body, a pressure column, a displacement sensor and a fourth pressure sensor; the reactor cover body is a cylindrical structure, which is detachably connected to the reactor, and a pressure column is slidably connected in the inner cavity; one end of the pressure column is coaxially and fixedly connected to the pressure plate, and the other end is connected to a ground stress loading system that can drive it to move axially, and the end connected to the ground stress loading system is provided with a displacement sensor and a fourth pressure sensor.

[0010] Preferably, the side wall surface in contact between the wellbore and the sample is covered with a sealing rubber sleeve for sealing, and one end of the fracturing fluid introduction channel sequentially penetrates the side wall of the reactor, the loading plate on the top of the sample and is sealed and connected to the top of the wellbore.

[0011] Preferably, the cross-section of the loading plate is the same as the radial cross-section of the sample.

[0012] Preferably, along the gas flow direction, the first gas injection system includes a CO2 gas cylinder, a first valve, a low-temperature water bath, a second valve, a supercritical CO2 injection pump, a preheater with a temperature sensor, a third valve and a first pressure sensor, which are connected in sequence through pipelines. The pipeline is also provided with a branch connected in parallel with the third valve, and a sand adder and a fourth valve are provided on the branch.

[0013] Preferably, along the gas flow direction, the second gas injection system includes a gas source body, a gas booster pump, a seventh valve, a gas flow sensor, a buffer container, an eighth valve, a pressure regulating valve, a ninth valve, a gas flow controller, a tenth valve, and a second pressure sensor, which are connected in sequence through pipelines; the gas source body includes two parallel branches, the first branch is connected to a nitrogen cylinder with a fifth valve, and the second branch is connected to a methane cylinder with a sixth valve.

[0014] Preferably, along the gas flow direction, the seepage discharge channel includes a third pressure sensor, an eleventh valve and a gas-liquid-solid three-phase separator connected in sequence through pipelines; the gas outlet of the gas-liquid-solid three-phase separator is connected to the gas flowmeter through a pipeline, the liquid outlet is connected to the twelfth valve and the outlet meter in sequence through a pipeline, and the solid outlet is connected to the dryer through a pipeline.

[0015] Preferably, the reactor is a hollow hexahedral structure surrounded by an upper wall of the reactor, a bottom plate of the reactor and four side walls of the reactor; the lower support plate is a horizontal cross plate, located in the middle of the inner cavity of the reactor, and the bottom is supported by a lower support fixed to the bottom plate of the reactor, and a first side support plate and a second side support plate located on the left and rear of the sample body are respectively placed above the plate surface; a number of bottom brackets are symmetrically connected below the bottom plate of the reactor, and the bottom brackets are connected as a whole through a bottom connecting plate.

[0016] Preferably, the kettle cover is detachably connected to the reactor via screws and nuts.

[0017] Preferably, each of the sensors is connected to a host computer to achieve real-time data transmission.

[0018] In a second aspect, the present invention provides a test method using any high stress, high pressure, large size true triaxial fracturing test device described in the first aspect, which is as follows:

[0019] S1: Cut the water-soil mixture into a regular hexahedron structure and grind its surface to reduce the roughness, and then dry it to obtain a sample; drill a center hole at the center of the top of the sample and clean it, insert a wellbore into the center hole after drying, and install a sealing rubber sleeve in the gap between the wellbore and the sample; place loading plates with acoustic emission probes on the outside of the six faces of the sample, and use fully enclosed rubber sleeves to cover the outside of the loading plates to fix, press and seal each loading plate with the sample to obtain the sample body;

[0020] S2: Place the sample body into the inner cavity of the reactor, connect the wellbore to the first gas injection system through the fracturing fluid introduction channel, connect the gas inlet channel to the second gas injection system, connect the seepage discharge channel to the outlet separation system, connect each acoustic emission probe to the acoustic emission and distributed optical fiber sensor through the data acquisition channel, and axially connect the outer ends of the three reactor covers to the loading cylinder of the ground stress loading system;

[0021] S3: Use hydraulic oil to control the temperature, heat the sample to the set temperature, and introduce hydraulic oil into the loading cylinder to drive the ground stress loading system to apply stress to the sample body in the x-axis direction, y-axis direction and z-axis direction, and keep it constant;

[0022] S4: Turn on the second gas injection system and inject nitrogen into the gas inlet channel to exhaust the air in the pipeline and the sample;

[0023] S5: Start the first gas injection system, pressurize the carbon dioxide to reach the required supercritical injection state, and then inject it into the wellbore through the fracturing fluid introduction channel to perform a fracturing test on the sample; if it is a fracturing fluid injection test, use a sand injector to inject the mixed fracturing fluid and sand-carrying agent into the wellbore; then evaluate the seepage capacity of the sample after the fracturing test;

[0024] S6: opening the seepage inlet channel and the seepage outlet channel, passing the seepage gas into the sample through the seepage inlet channel, and discharging the seepage gas into the outlet separation system from the seepage outlet channel, and recording the flow state according to the gas flow meter to evaluate the seepage capacity;

[0025] S7: Close all systems and valves, collect data from all sensors, take out the sample body, and perform fracturing crack and expansion analysis on the sample;

[0026] S8: If it is a hydrate fracturing test, a hydrate generation operation is performed after S4, followed by S6 and S7; the hydrate generation operation is specifically as follows:

[0027] Turn on the second gas injection system, inject pressurized methane gas into the gas inlet channel according to the required saturation, control the methane gas flow, wait for hydrates to be generated inside the sample, and monitor the gas booster pump at the same time. After the gas volume reading of the gas booster pump tends to be stable, turn off the second gas injection system to complete the hydrate generation operation.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] A high-stress, high-pressure, large-size true triaxial fracturing test device of the present invention can simulate the high-temperature and high-pressure environment at geological depth to conduct true triaxial fracturing tests on coal rock and hydrate sediment samples. The device is suitable for rock samples of types such as clay rock, sandstone, shale, etc., and can also conduct in-situ hydrate generation and fracturing experiments, explore the mechanical properties of samples under high stress, high temperature and multi-physical fields, and carry out further research on the fields of coalbed methane and shale gas. The device can apply true triaxial stress to the sample and inject different fracturing media, such as supercritical CO2, liquid nitrogen, water, etc., and can control multi-physical field changes, such as fluid-solid field, low-temperature field, crack field and pressure field, so as to better study the crack initiation and expansion law of rock, volume fracturing transformation, rock fragmentation characteristics, etc., and can provide accurate and reliable data for the research on the mechanical properties, stress, degree of fragmentation, etc. of coal rock mass and hydrate. In addition, the device can also conduct permeability test to evaluate the permeability of different fluids under different working conditions. Various test parameters are controlled, measured, displayed, processed and printed by a computer, with high integration. The device is easy and reliable to use, and can provide a better understanding of the physical and chemical properties of samples such as rocks and hydrate sediments, providing a theoretical basis for resource utilization, mining and geological engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a structural schematic diagram of a high stress, high pressure and large size true triaxial fracturing test device;

[0031] Figure 2This is a schematic diagram of the main view structure of the high stress and high pressure true triaxial apparatus;

[0032] Figure 3 It is the left view structural diagram of the high stress and high pressure true triaxial apparatus;

[0033] Figure 4 It is a schematic diagram of the distribution structure of the main body of the sample;

[0034] Figure 5 Schematic diagram of the wellbore layout (a) and the acoustic emission probe installation (b);

[0035] In the figure: 1—CO2 gas cylinder; 2—low temperature water bath; 3—supercritical CO2 injection pump; 4—preheater; 5-1—first valve; 5-2—second valve; 5-3—third valve; 5-4—fourth valve; 5-5—fifth valve; 5-6—sixth valve; 5-7—seventh valve; 5-8—eighth valve; 5-9—ninth valve; 5-10—tenth valve; 5-11—eleventh valve; 5-12—twelfth valve; 6—sand adder; 7—high stress and high pressure true triaxial instrument; 8-a—first pressure sensor; 8-b—temperature sensor; 8-c—second pressure sensor; 8-d—gas flow sensor; 8-e—third pressure sensor; 9—methane gas cylinder; 10—nitrogen gas cylinder; 11—gas booster pump; 12—buffer container; 13—pressure regulating valve; 14—gas flow controller; 15—acoustic emission and distributed optical fiber sensor; 16—export meter; 17—dryer; 18—gas Flowmeter; 19—gas-liquid-solid three-phase separator; 20—host computer; 21—upper kettle cover; 22—screws; 23—upper pressure plate; 24—fracturing fluid inlet channel; 25—seepage inlet channel; 26—loading plate; 27—first side support plate; 28—gas inlet channel; 29—nut; 30—reactor side wall; 31—lower support; 32—bottom support; 33—bottom connecting plate; 34—seepage discharge channel; 35—keep cover body; 36—first side pressure column ; 37—displacement sensor; 38—first side pressure plate; 39—fourth pressure sensor; 40—data acquisition channel; 41—sample; 42—wellbore; 43—sealing rubber sleeve; 44—fully enclosed rubber sleeve; 45—lower support plate; 46—gas distribution plate; 47—first side kettle cover; 48—second side support plate; 49—second side pressure column; 50—second side kettle cover; 51—sample body; 52—upper pressure column; 53—second side pressure plate; 54—upper wall of reactor. DETAILED DESCRIPTION

[0036] The present invention is further described and illustrated below in conjunction with the accompanying drawings and specific embodiments. The technical features of each embodiment of the present invention can be combined accordingly without conflicting with each other.

[0037] like Figure 1As shown, a high stress, high pressure, large-size true triaxial fracturing test device provided by the present invention mainly includes a first gas injection system, a second gas injection system, an outlet separation system and a high stress, high pressure true triaxial instrument 7. Among them, the first gas injection system is used for supercritical CO2 gas injection; the second gas injection system is used for nitrogen and methane gas injection; the outlet separation system is used for three-phase separation; the high stress, high pressure true triaxial instrument 7 can simulate the high temperature and high pressure environment at geological depth to carry out true triaxial fracturing tests on coal rock and hydrate sediment samples.

[0038] The structure and connection method of each component will be described in detail below.

[0039] In the present invention, Figure 2 and 3 As shown, the high stress and high pressure true triaxial tester 7 mainly includes a reactor, a sample body 51, a pressure plate, a support plate and a reactor cover. The reactor has an inner cavity with good sealing performance, and the sample body 51 is placed in the center of the inner cavity. The sample body 51 mainly includes a sample 41, a loading plate 26 and a gas-liquid distribution plate 46, as shown in FIG. Figure 4 As shown. Sample 41 is a regular hexahedron structure, including top, bottom, left, right, front, and back (as shown in FIG. Figure 2 The six surfaces of the sample 41 are respectively covered with loading plates 26. In actual use, the cross section of the loading plate 26 is the same as the radial cross section (i.e., the cross section along the central axis) of the sample 41. Acoustic emission probes are evenly arranged on one side of the loading plate 26 in contact with the sample 41.

[0040] In actual use, Figure 5 As shown in (b), in this embodiment, 16 acoustic emission sensors (as shown in S1 to S16) are used for a cubic rock sample with a side length of 300 mm, and 4 acoustic emission sensors are placed on each side, respectively located at the upper left, lower left, upper right and lower right positions of each surface, that is, a wiring groove is set in a side surface of the loading plate close to the sample 41, and acoustic emission probes are arranged at the four corners of a side surface of each loading plate close to the sample 41, and twelve wiring grooves are fixed on the inner side of each loading plate in a "cross" shape.

[0041] As a preferred embodiment of the present invention, the reactor is a hollow hexahedral structure surrounded by the reactor upper wall 54, the reactor bottom plate and four reactor side walls 30. The lower support plate 45 is a horizontal horizontal plate, located in the middle of the reactor cavity, and the bottom is supported by a lower support 31 fixed to the reactor bottom plate. The first side support plate 27 and the second side support plate 48 located on the left and rear of the sample body 51 are placed above the plate surface. A plurality of bottom brackets 32 are symmetrically connected below the reactor bottom plate, and the bottom brackets 32 are connected as a whole by a bottom connecting plate 33. The kettle cover and the reactor are detachably connected by screws 22 and nuts 29.

[0042] In the present invention, a vertical wellbore 42 is opened at the center of the top of the sample 41, and a fracturing fluid introduction channel 24 is externally connected to the top of the wellbore 42, and the fracturing fluid introduction channel 24 is connected to the first gas injection system for injecting supercritical CO2 gas. In actual use, one end of the fracturing fluid introduction channel 24 can sequentially penetrate the reactor side wall 30, the loading plate 26 at the top of the sample 41, and be sealed and connected to the top of the wellbore 42. The fully enclosed rubber sleeve 44 is sleeved on the outside of the six loading plates 26 and the loading plates 26 are fixedly pressed and sealed to the sample 41. The fully enclosed rubber sleeve 44 should cover the entire sample 41, but can be partially sleeved on the loading plate 26 to play a fixing role.

[0043] As a preferred embodiment of the present invention, the wellbore 42 does not completely penetrate the sample 41 longitudinally, and the opening depth of the wellbore 42 can be adjusted according to actual conditions. The side wall surface between the wellbore 42 and the sample 41 is covered with a sealing rubber sleeve 43 for sealing. Figure 5 As shown in (a), the wellbore 42 does not completely penetrate the sample 41 longitudinally, and the three-dimensional stress (σ1, σ2, σ3) acts on the sample, and the injected liquid flows out from the bottom of the wellbore. Figure 5 As shown in (b), acoustic emission probes are evenly arranged at four corners on each side of the loading plate 26 that contacts the sample 41.

[0044] As a preferred embodiment of the present invention, along the gas flow direction, the first gas injection system mainly includes a CO2 gas cylinder 1, a first valve 5-1, a low-temperature water bath 2, a second valve 5-2, a supercritical CO2 injection pump 3, a preheater 4 with a temperature sensor 8-b, a third valve 5-3 and a first pressure sensor 8-a, which are connected in sequence through pipelines. A branch connected in parallel with the third valve 5-3 is also provided on the pipeline, and a sand adder 6 and a fourth valve 5-4 are provided on the branch.

[0045] As a preferred embodiment of the present invention, the high stress and high pressure true triaxial instrument adopts a built-in displacement sensor to control the displacement movement of the hydraulic cylinder loading, wherein the wall wetted portion of the high stress and high pressure true triaxial instrument is made of HC276 material. A frame-type fully enclosed rubber sleeve 44 is designed inside the model system to realize the seepage test after fracturing. The fully enclosed rubber sleeve has a small hole in the vertical loading direction and is connected to the simulated wellbore of the fracturing. The wellbore and the sample and the pressure plate are connected and sealed by the fully enclosed rubber sleeve; the fully enclosed rubber sleeve is made of high temperature resistant perfluororubber material, which has strong high temperature resistance and corrosion resistance, and can adapt to carbon dioxide swelling.

[0046] In the present invention, a gas-liquid distribution plate 46 is sandwiched between the loading plate 26 at the bottom and the sample 41. The bottom of the gas distribution plate 46 is externally connected to a gas inlet channel 28, and the gas inlet channel 28 is connected to a second gas injection system for injecting nitrogen or methane. After entering through the gas inlet channel 28, the gas can be evenly distributed laterally under the action of the gas distribution plate 46 and enter the sample 41 upward.

[0047] As a preferred embodiment of the present invention, along the gas flow direction, the second gas injection system mainly includes a gas source body, a gas booster pump 11, a seventh valve 5-7, a gas flow sensor 8-d, a buffer container 12, an eighth valve 5-8, a pressure regulating valve 13, a ninth valve 5-9, a gas flow controller 14, a tenth valve 5-10, and a second pressure sensor 8-c, which are connected in sequence through pipelines. The gas source body includes two parallel branches, the first branch is connected to a nitrogen cylinder 10 having a fifth valve 5-5, and the second branch is connected to a methane cylinder 9 having a sixth valve 5-6.

[0048] In the present invention, an external data acquisition channel 40 is connected to the upper side of the sample 41 , and each acoustic emission probe can be connected to an external acoustic emission and distributed optical fiber sensor 15 through the data acquisition channel 40 .

[0049] As a preferred embodiment of the present invention, all sensors including the acoustic emission and distributed optical fiber sensor 15 can also be connected to the host computer 20 to realize real-time data transmission and display calculation. The host computer and the components connected thereto can be used as a data acquisition and control system, which mainly includes the acoustic emission and distributed optical fiber sensor 15, the host computer 20 with a computing control module, a data acquisition card, a data acquisition processing circuit, a first pressure sensor 8-a, a temperature sensor 8-b, a second pressure sensor 8-c, a gas flow sensor 8-d, and a third pressure sensor 8-e. The acoustic emission sensor is used to monitor the acoustic emission changes of the sample, the distributed optical fiber sensor uses a strain acquisition instrument to monitor the deformation signal, and the data acquisition card is placed on the host of the computing control module; one end of the data acquisition processing circuit is connected to the computing control module, and the other end is connected to the acoustic emission sensor, the distributed optical fiber sensor, the fluid pressure sensor, the temperature sensor, etc., to collect and process the strain and temperature and pressure information. Among them, the first pressure sensor 8-a is used to collect the injection pressure of the fracturing medium, the temperature sensor 8-b is used to collect the outlet temperature of the preheater, the second pressure sensor 8-c is used to collect the gas pressure at the entrance of the gas entering the channel 28, the gas flow sensor 8-d is used to collect the gas flow after being pressurized by the gas booster pump, and the third pressure sensor 8-e is used to collect the gas-liquid outlet pressure.

[0050] In the present invention, the left side of the sample 41 is connected to the seepage inlet channel 25, and the right side is connected to the seepage outlet channel 34. The seepage inlet channel 25 and the seepage outlet channel 34 are respectively arranged on two opposite sides of the sample 41, and are used to pass the seepage liquid into the sample 41 during the seepage test. Among them, the seepage outlet channel 34 is connected to the outlet separation system for three-phase separation.

[0051] As a preferred embodiment of the present invention, the seepage discharge channel 34 is used for the support evaluation after seepage and sand carrying. Along the gas flow direction, the seepage discharge channel 34 mainly includes the third pressure sensor 8-e, the eleventh valve 5-11, the twelfth valve 5-12, the outlet meter 16, the dryer 17, the gas flow meter 18 and the gas-liquid-solid three-phase separator 19. Among them, the gas-liquid-solid three-phase separator 19 is used to separate the fluid after the sand carrying experiment, the outlet meter 16 is used to measure the outlet liquid volume, and the gas flow meter 18 is used to measure the outlet gas flow rate, so as to further evaluate the flow conductivity of the sand carrying support through mass conversion and related calculations. Specifically, the front end of the seepage discharge channel 34 is connected to the third pressure sensor 8-e, the eleventh valve 5-11 and the gas-liquid-solid three-phase separator 19 in sequence; the gas outlet of the gas-liquid-solid three-phase separator 19 is connected to the gas flow meter 18 through a pipeline, the liquid outlet is connected to the twelfth valve 5-12 and the outlet meter 16 in sequence through a pipeline, and the solid outlet is connected to the dryer 17 through a pipeline.

[0052] In the present invention, the lower part, the left part and the rear part of the sample body 51 are in contact with the inner wall of the reactor through the support plate, and the upper part, the right part and the front part are respectively provided with a pressure plate. The pressure plate is sealed and slidably connected to the reactor, and can clamp the sample body 51 together with the support plate on the opposite side to apply stress. One end of the pressure plate is located outside the reactor and is connected to the reactor cover. The reactor cover mainly includes a reactor cover body 35, a pressure column, a displacement sensor 37 and a fourth pressure sensor 39. The reactor cover body 35 is a cylindrical structure, which is detachably connected to the reactor, and a pressure column is slidably connected in the inner cavity. One end of the pressure column is coaxially fixedly connected to the pressure plate, and the other end is connected to the ground stress loading system that can drive its axial movement. The end connected to the ground stress loading system is provided with a displacement sensor 37 and a fourth pressure sensor 39. In actual use, the axial force provided by the ground stress loading system causes the three end face pressure plates to move inward, thereby providing three-dimensional stress for the fracturing test of the sample 41.

[0053] Specifically, the upper part of the sample body 51 contacts the upper pressure plate 23, which is fixedly connected to the upper pressure column 52. The upper pressure column 52 can move up and down in the vertical direction inside the upper kettle cover 22. The upper kettle cover 21 is connected to the upper wall 54 of the reactor via screws 22. The right part of the sample body 51 contacts the first side pressure plate 38, which is fixedly connected to the first side pressure column 36. The first side pressure column 36 can move left and right in the horizontal direction inside the first side kettle cover 47. The first side kettle cover 47 is connected to the side wall of the reactor via screws 22. The front part of the sample body 51 contacts the second side pressure plate 53, which is fixedly connected to the second side pressure column 49. The second side pressure column 49 can move forward and backward in the horizontal direction inside the second side kettle cover 50. The second side kettle cover 50 is connected to the side wall of the reactor via screws 22.

[0054] In actual use, the ground stress loading system provides axial force to push the pressure column inward, thereby pushing the pressure plate, transferring the force to the loading plate to act on the specimen. The ground stress loading system mainly includes an independent servo oil source device, and the loading cylinder of the servo oil source device is connected to the pressure column of the high stress and high pressure true triaxial instrument through an oil pipe.

[0055] The present invention further provides a test method using the high stress, high pressure, large size true triaxial fracturing test device, the method is as follows:

[0056] S1: Cut the water-soil mixture into a regular hexahedron structure, and grind its surface to reduce the roughness, and dry it in a drying oven to obtain a sample 41. Drill a center hole at the center of the top of the sample 41 and clean it. After drying, insert the wellbore 42 into the center hole, and install a sealing rubber sleeve 43 in the gap between the wellbore 42 and the sample 41. Place loading plates 26 with acoustic emission probes on the outside of the six faces of the sample 41, and use a fully enclosed rubber sleeve 44 to cover the outside of the loading plates 26 to fix, press and seal each loading plate 26 with the sample 41 to obtain a sample body 51.

[0057] As a preferred embodiment of the present invention, a high-precision handheld impact drill is used to cut the required 300*300*300mm regular hexahedron structure sample, and then a high-precision grinder is used to grind the surface to reduce the roughness of the sample.

[0058] S2: Place the sample body 51 into the inner cavity of the reactor, connect the wellbore 42 to the first gas injection system through the fracturing fluid inlet channel 24, connect the gas inlet channel 28 to the second gas injection system, connect the seepage discharge channel 34 to the outlet separation system, connect each acoustic emission probe to the acoustic emission and distributed optical fiber sensor 15 through the data acquisition channel 40, and axially connect the outer ends of the three reactor covers to the loading cylinder of the ground stress loading system.

[0059] S3: Use hydraulic oil heating to heat the sample 41 to a set temperature, turn on the ground stress loading system, and introduce hydraulic oil into the loading cylinder to drive the ground stress loading system to apply stress to the sample body 51 in the x-axis direction, y-axis direction, and z-axis direction until the true triaxial pressure (x-axis direction, y-axis direction, and z-axis direction) reaches the stress required for the test and remains constant.

[0060] S4: Start the second gas injection system, that is, open the fifth valve 5 - 5 and inject nitrogen into the gas inlet channel 28 to exhaust the air in the pipeline and the sample 41 .

[0061] S5: Turn on the data acquisition and control system, collect medium data, turn on the first gas injection system, conduct a fracturing test of the supercritical carbon dioxide fluid medium, pressurize the carbon dioxide to reach the required supercritical injection state, and inject it into the wellbore 42 through the fracturing fluid introduction channel 24, and conduct a fracturing test on the sample 41. If it is a fracturing fluid injection test, the mixed fracturing fluid and sand-carrying agent are injected into the wellbore 42 using the sand feeder 6. Specifically, connect the fracturing container before the supercritical CO2 injection pump 3, open the fourth valve 5-4, close the third valve 5-3, conduct a sand-carrying experiment, and drive the fracturing fluid and the sand-carrying agent to mix evenly and inject them into the sample. Then, the permeability of the sample 41 after the fracturing test is evaluated.

[0062] S6: Open the seepage inlet channel 25 and the seepage outlet channel 34, pass the seepage gas into the sample 41 through the seepage inlet channel 25 for gradual seepage, and discharge it into the outlet separation system from the seepage outlet channel 34. When the flow rate displayed by the gas flowmeter 18 changes little or fluctuates little, record the flow rate at this time, so as to evaluate the seepage capacity through further calculation.

[0063] S7: After the test, close all systems and valves, collect data from all sensors, open the side kettle cover, take out the sample body 51, remove the fully enclosed rubber sleeve 44, and take out the sample 41, and perform subsequent fracturing crack and expansion analysis on the sample 41.

[0064] S8: If it is a hydrate fracturing test, the hydrate generation operation is performed after S4, followed by S6 and S7. The hydrate generation operation is as follows:

[0065] The sixth valve 5-6 is opened, the second gas injection system is turned on, and the pressurized methane gas is injected into the gas inlet channel 28 according to the required saturation, and the methane gas flow rate is controlled by the pressure regulating valve so that the methane gas enters the sample 41. When hydrate is generated inside the sample 41, the gas booster pump 11 is monitored at the same time, and when the gas volume reading of the gas booster pump 11 tends to be stable, the second gas injection system is turned off to complete the hydrate generation operation.

[0066] The present invention can apply true triaxial stress to the sample and inject different fracturing media, such as supercritical CO2, liquid nitrogen, water, etc., and can control the changes of multiple physical fields, such as fluid-solid field, low-temperature field, crack field and pressure field, etc. It is suitable for fracturing experiments of rock samples of types such as clay rock, sandstone, shale, etc., and can also perform in-situ generation and fracturing of hydrates. The device can explore the mechanical properties of the sample under high stress, high temperature and multiple physical fields, and conduct research on the laws of crack initiation and expansion, volume fracturing transformation, fragmentation characteristics, etc.; in addition, the device of the present invention improves the shortcomings of the traditional true triaxial instrument loading and unloading mechanism, and further optimizes the automatic control function of the instrument.

[0067] The above-described embodiment is only a preferred solution of the present invention, but it is not intended to limit the present invention. A person skilled in the relevant technical field may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent replacement or equivalent transformation falls within the protection scope of the present invention.

Claims

1. A high stress, high pressure, large size true triaxial fracturing test device, characterized in that: It comprises a first gas injection system, a second gas injection system, an outlet separation system and a high stress and high pressure true triaxial apparatus (7); The high stress and high pressure true triaxial instrument (7) comprises a reactor, a sample body (51), a pressure plate, a support plate and a reactor cover; the reactor has a sealed inner cavity, in which the sample body (51) is placed; the sample body (51) comprises a sample (41), a loading plate (26) and a gas-liquid distribution plate (46); the sample (41) is a regular hexahedron structure, and the outer sides of the six faces are respectively fitted with and covered with loading plates (26), and acoustic emission probes are evenly arranged on one side of the loading plate (26) in contact with the sample (41); a vertical wellbore (42) is opened at the center of the top of the sample (41), and the top of the wellbore (42) is externally connected to a fracturing fluid introduction channel (24); a fully enclosed rubber sleeve (44) is sleeved on the outside of the six loading plates (26) and connects the loading plates (26) and the sample (41) The two components are fixed, pressed and sealed; a gas-liquid distribution plate (46) is clamped and placed between the loading plate (26) located on the bottom surface and the sample (41); the bottom of the gas distribution plate (46) is externally connected to a gas inlet channel (28); the upper side of the sample (41) is externally connected to a data acquisition channel (40), the left side is externally connected to a seepage inlet channel (25), and the right side is externally connected to a seepage discharge channel (34); the fracturing fluid introduction channel (24) is connected to a first gas injection system for injecting supercritical CO2 gas, the gas inlet channel (28) is connected to a second gas injection system for injecting nitrogen or methane, each acoustic emission probe is connected to an acoustic emission and distributed optical fiber sensor (15) through a data acquisition channel (40), and the seepage discharge channel (34) is connected to an outlet separation system for three-phase separation; The lower part, left part and rear part of the sample body (51) are in contact with the inner wall of the reactor through support plates respectively, and the upper part, right part and front part are respectively provided with pressure plates; the pressure plate is sealingly and slidably connected to the reactor and can clamp the sample body (51) together with the support plate on the opposite side to apply stress, and one end of the pressure plate is located outside the reactor and is connected to a reactor cover; the reactor cover includes a reactor cover body (35), a pressure column, a displacement sensor (37) and a fourth pressure sensor (39); the reactor cover body (35) is a cylindrical structure, which is detachably connected to the reactor, and a pressure column is slidably connected in the inner cavity; one end of the pressure column is coaxially and fixedly connected to the pressure plate, and the other end is connected to a ground stress loading system that can drive it to move axially, and the end connected to the ground stress loading system is provided with a displacement sensor (37) and a fourth pressure sensor (39).

2. A high stress, high pressure, large size true triaxial fracturing test device according to claim 1, characterized in that: The side wall surface in contact between the wellbore (42) and the sample (41) is covered with a sealing rubber sleeve (43) for sealing, and one end of the fracturing fluid introduction channel (24) sequentially penetrates the side wall (30) of the reactor and the loading plate (26) at the top of the sample (41) and is sealed and connected to the top of the wellbore (42).

3. A high stress, high pressure, large size true triaxial fracturing test device according to claim 1, characterized in that: The cross section of the loading plate (26) is the same as the radial cross section of the test specimen (41).

4. A high stress, high pressure, large size true triaxial fracturing test device according to claim 1, characterized in that: Along the gas flow direction, the first gas injection system includes a CO2 gas cylinder (1), a first valve (5-1), a low-temperature water bath (2), a second valve (5-2), a supercritical CO2 injection pump (3), a preheater (4) with a temperature sensor (8-b), a third valve (5-3) and a first pressure sensor (8-a) which are sequentially connected through a pipeline. The pipeline is also provided with a branch connected in parallel with the third valve (5-3), and a sand adder (6) and a fourth valve (5-4) are provided on the branch.

5. The high stress, high pressure, large size true triaxial fracturing test device according to claim 1, characterized in that: Along the gas flow direction, the second gas injection system includes a gas source body, a gas booster pump (11), a seventh valve (5-7), a gas flow sensor (8-d), a buffer container (12), an eighth valve (5-8), a pressure regulating valve (13), a ninth valve (5-9), a gas flow controller (14), a tenth valve (5-10), and a second pressure sensor (8-c) which are sequentially connected through pipelines; the gas source body includes two parallel branches, the first branch is connected to a nitrogen cylinder (10) having a fifth valve (5-5), and the second branch is connected to a methane cylinder (9) having a sixth valve (5-6).

6. The high stress, high pressure, large size true triaxial fracturing test device according to claim 1, characterized in that: Along the gas flow direction, the seepage discharge channel (34) includes a third pressure sensor (8-e), an eleventh valve (5-11) and a gas-liquid-solid three-phase separator (19) which are connected in sequence through pipelines; the gas outlet of the gas-liquid-solid three-phase separator (19) is connected to a gas flow meter (18) through a pipeline, the liquid outlet is connected to a twelfth valve (5-12) and an outlet meter (16) in sequence through a pipeline, and the solid outlet is connected to a dryer (17) through a pipeline.

7. The high stress, high pressure, large size true triaxial fracturing test device according to claim 1, characterized in that: The reactor is a hollow hexahedral structure surrounded by an upper wall (54), a bottom plate and four side walls (30). The lower support plate (45) is a horizontal cross plate located in the middle of the inner cavity of the reactor, and the bottom is supported by a lower support (31) fixed to the bottom plate of the reactor. A first side support plate (27) and a second side support plate (48) located on the left and rear of the sample body (51) are placed above the plate surface. A plurality of bottom brackets (32) are symmetrically connected below the bottom plate of the reactor, and the bottom brackets (32) are connected as a whole via a bottom connecting plate (33).

8. The high stress, high pressure, large size true triaxial fracturing test device according to claim 1, characterized in that: The kettle cover is detachably connected to the reaction kettle via screws (22) and nuts (29).

9. The high stress, high pressure, large size true triaxial fracturing test device according to claim 1, characterized in that: Each of the sensors is connected to a host computer (20) to achieve real-time data transmission.

10. A test method using the high stress, high pressure, large size true triaxial fracturing test device according to any one of claims 1 to 9, characterized in that: The details are as follows: S1: cutting the water-soil mixture into a regular hexahedral structure and grinding its surface to reduce the roughness, and obtaining a sample (41) after drying; drilling a center hole at the center of the top of the sample (41) and cleaning it, inserting a wellbore (42) into the center hole after drying, and installing a sealing rubber sleeve (43) in the gap between the wellbore (42) and the sample (41); placing loading plates (26) with acoustic emission probes on the outside of the six surfaces of the sample (41), and using a fully enclosed rubber sleeve (44) to cover the outside of the loading plates (26) to fix, press and seal each loading plate (26) with the sample (41), and obtain a sample body (51); S2: placing the sample body (51) into the inner cavity of the reactor, connecting the wellbore (42) to the first gas injection system through the fracturing fluid introduction channel (24), connecting the gas inlet channel (28) to the second gas injection system, connecting the seepage discharge channel (34) to the outlet separation system, connecting each acoustic emission probe to the acoustic emission and distributed optical fiber sensor (15) through the data acquisition channel (40), and axially connecting the outer ends of the three reactor covers to the loading cylinder of the ground stress loading system; S3: using hydraulic oil to control the temperature, heating the sample (41) to a set temperature, and introducing hydraulic oil into the loading cylinder to drive the ground stress loading system to apply stress to the sample body (51) in the x-axis direction, the y-axis direction and the z-axis direction, and keep the stress constant; S4: starting the second gas injection system to inject nitrogen into the gas inlet channel (28) to exhaust the air in the pipeline and the sample (41); S5: starting the first gas injection system, pressurizing the carbon dioxide to reach the required supercritical injection state and then injecting it into the wellbore (42) through the fracturing fluid introduction channel (24), and performing a fracturing test on the sample (41); If it is a fracturing fluid injection test, the mixed fracturing fluid and sand-carrying agent are injected into the wellbore (42) by using a sand injector (6); then the permeability of the sample (41) after the fracturing test is evaluated; S6: opening the seepage inlet channel (25) and the seepage outlet channel (34), passing the seepage gas into the sample (41) through the seepage inlet channel (25), and discharging the seepage gas into the outlet separation system from the seepage outlet channel (34), and recording the flow state according to the gas flow meter (18) to evaluate the seepage capacity; S7: closing all systems and valves, collecting data from all sensors, taking out the sample body (51), and performing fracturing crack and expansion analysis on the sample (41); S8: If it is a hydrate fracturing test, a hydrate generation operation is performed after S4, followed by S6 and S7; the hydrate generation operation is specifically as follows: The second gas injection system is turned on, and pressurized methane gas is injected into the gas inlet channel (28) according to the required saturation, and the methane gas flow rate is controlled. Hydrates are waited for to be generated inside the sample (41), and the gas booster pump (11) is monitored at the same time. After the gas volume reading of the gas booster pump (11) tends to be stable, the second gas injection system is turned off to complete the hydrate generation operation.

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