Water-free fracturing in-situ low-permeability reservoir pressure-displacement-permeability integrated test device and method

Abstract

The invention discloses an anhydrous fracturing in-situ low-permeability reservoir pressure-driving-seepage integrated test device and method.The device comprises a triaxial pressure chamber system, a fracturing system and a seepage system, the fracturing system comprises a liquid nitrogen storage tank, a self-pressurization liquid nitrogen tank and a pressurization device, the self-pressurization liquid nitrogen tank is connected with the liquid nitrogen storage tank through an injection pipe, and the pressurization device is connected with the fracturing system; a port of the injection pipe extends to the inner bottom of the liquid nitrogen storage tank; the interior of the liquid nitrogen storage tank is connected with a supercharging device and is connected with a fracturing liquid inlet of the triaxial pressure chamber system through a corresponding control valve, and the fracturing liquid inlet is connected with a fracturing pipe in the triaxial pressure chamber system; and a bottom outlet of the liquid nitrogen storage tank is connected with a fracturing liquid inlet of the triaxial pressure chamber system through a corresponding control valve and a preheater. According to the invention, liquid nitrogen is used as a low-temperature fluid, and a coal body sample is fractured with large displacement and high pressure, so that a coal rock fracture process and a monitoring process in a liquid nitrogen fracturing process and a seepage process under an in-situ condition are really simulated under a low-temperature condition.

Description

Technical Field

[0001] The invention belongs to the technical field of waterless fracturing, and in particular relates to a waterless fracturing in-situ low-permeability reservoir pressure-drive-permeability integrated test device and method. Background Art

[0002] my country has abundant coal-bed methane resources, which provide a resource basis for ensuring national energy security and achieving the "dual carbon" goal, but the current production capacity is not satisfactory. Reservoir permeability is the key to restricting coalbed methane production capacity, while low permeability and low pressure are common characteristics of my country's coal reservoirs. The permeability is less than 0.5×10-15 m 2 Coal seams with low permeability account for more than 70%, and the permeability decreases exponentially with increasing depth, which makes the single well production of coalbed methane in my country low and decays quickly. Increasing permeability and production must rely on fracturing transformation to connect the wellbore with natural fractures. How to perform fracturing transformation on low-permeability reservoirs to increase permeability and production is an industry problem faced by coalbed methane development. Hydraulic fracturing is the most commonly used production-increasing transformation technology in the current coalbed methane development process, but the problems faced by hydraulic fracturing are becoming increasingly prominent. There are problems such as huge water consumption in hydraulic fracturing, a large amount of chemical additives in the fracturing fluid, and it is very easy to form artificial earthquakes, causing property losses.

[0003] Waterless fracturing technology is a technology that injects inert fluids (such as liquid nitrogen, CO2, etc.) into coal seams to replace conventional hydraulic fracturing fluids to produce complex fracture networks. Because it has perfect compatibility with coal seams and does not react with minerals in coal, and there is no liquid residue after gasification, no water sensitivity, water lock and other reservoir damage, and it has low rock-breaking pressure, complex fracture network, and pressurization-displacement synergistic effect, it is expected to become a new technology for reservoir transformation in the future.

[0004] Waterless fracturing technology represented by liquid nitrogen fracturing represents the future direction of technological development. As early as the 1990s, some scholars proposed the idea of ​​using the ultra-low temperature (-196℃), high compression ratio (1:696), high frost heave force (up to 207 MPa) and low viscosity characteristics of liquid nitrogen to inject high-pressure and large-volume liquid nitrogen into coal seams to replace conventional water-based fracturing fluids for fracturing transformation of low-permeability and dry reservoirs. In engineering, liquid nitrogen fracturing refers to the process of using appropriate equipment and processes to pump liquid nitrogen to the bottom of the well at conventional fracturing displacement and ultra-low temperature (-195.56 to -180.44℃) to form artificial fracturing cracks in the formation. Nitrogen has stable chemical properties and is completely compatible with the reservoir. There is no water sensitivity or water lock damage, which is expected to fundamentally solve the problems of reservoir damage and environmental pollution caused by hydraulic fracturing. Mcdaniel et al. conducted on-site construction of liquid nitrogen fracturing technology, using low-temperature resistant glass fiber as the fracturing string, and completed liquid nitrogen fracturing operations in 5 wells, including 4 coalbed methane wells and 1 low-permeability sandstone well. The initial daily production after fracturing increased by 1.22 to 6.48 times. Grundmann et al. used fiberglass tubing and stainless steel wellheads to apply liquid nitrogen fracturing to a production well in the Devonian shale formation. Compared with nitrogen fracturing in adjacent wells, the increase in production of liquid nitrogen fracturing wells was about 8%. The above engineering tests confirmed that the use of liquid nitrogen to replace water-based fracturing fluids is practically feasible. In comparison, the technical advantages of liquid nitrogen fracturing mainly include: (1) Liquid nitrogen ultra-low temperature fractures rocks, deteriorating the mechanical properties of rocks, significantly reducing the fracture pressure of reservoir rocks, and increasing the length of fractures; (2) Liquid nitrogen low-temperature fracturing can induce the formation and expansion of secondary fractures in rocks. The low viscosity characteristics enhance its ability to create long fractures, increase the complexity and connectivity of the fracture network, and improve the reservoir leakage area. (3) The huge temperature difference between liquid nitrogen and reservoir rock generates extremely strong thermal stress (temperature difference 200°C+) and vaporization pressurization (liquid-gas ratio 1:196), which promotes further expansion of the fracture while reducing the vertical stress on the fracture surface, inducing shear slip and unconformity self-support in the fracture, preventing the fracture from closing after compression and improving the fracture conductivity; (4) The chemical properties of nitrogen are stable, which can effectively prevent clay expansion and water lock in the reservoir. The flowback is complete and pollution-free, which solves the problem of reservoir damage caused by traditional hydraulic fracturing. In view of the potential advantages of liquid nitrogen fracturing, this field has been deeply concerned and studied by many scholars. At the same time, supercritical CO2 is also being studied in full swing due to its efficient displacement performance and carbon burial effect.

[0005] In recent years, rich achievements have been made in the use of liquid nitrogen (including supercritical CO2, etc.) to fracture coal and rock in the laboratory, showing the good transformation effect and application potential of water-free fluid fracturing coal. After liquid nitrogen is injected into the coal, it will deteriorate the mechanical properties of the rock and produce a dense fracture network. The diffusion capacity and permeability of the gas have been greatly improved (10-20 times), and it has the dual role of pressurization-displacement. However, actual engineering applications have found that when liquid nitrogen is used to fracture coal seams, there are low heat exchange efficiency (the cooling rate is only 0.4-1.2 ℃ / min), the liquid nitrogen injection and cooling fracturing time is too long (the liquid nitrogen injection time is as long as tens of hours or even days), and the degree of transformation of the fractures and the permeability effect are far from expectations. The supercritical CO2 injection also causes a sharp drop in permeability due to the expansion of the coal body. The water-free fracturing methods on site are faced with a series of problems such as failure to inject and poor permeability efficiency. How to pressurize the low-temperature fluid into the coal rock for fracturing is the key. The liquid nitrogen fracturing method with good transformation effect in the laboratory is not effective in actual application, which limits the effect of waterless fracturing. There are two main factors: First, it is limited by the low temperature of liquid nitrogen (-176℃), so it is impossible to simulate the state under reservoir conditions in the laboratory, and the test effect is amplified; second, due to the lack of corresponding pressure simulation equipment and reliable triaxial pressure chamber system, the current waterless fracturing method is limited by temperature and supercritical conditions. Most of the previous liquid nitrogen fracturing research is nitrogen or high-pressure nitrogen fracturing. In fact, the strongest effect on reservoir transformation is the fracturing effect of fluid, which is the key to creating long fractures and increasing the flow area, but it cannot be simulated and realized in the laboratory. Therefore, the waterless fracturing technology represented by low-temperature liquid nitrogen fracturing currently has no reliable triaxial pressure chamber system. Now there is an urgent need for a test system for low-temperature waterless fracturing of coal rock reservoirs, in order to provide a new engineering idea for liquid nitrogen fracturing technology of low-permeability reservoirs in this field. Summary of the invention

[0006] In order to address the deficiencies in the prior art, the present invention aims to provide an integrated test device for pressure-drive-permeability of an in-situ low-permeability reservoir without water fracturing. The present invention uses liquid nitrogen as a low-temperature fluid to fracture coal samples at a large displacement and high pressure, thereby truly simulating the coal rock fracture process, temperature conduction and monitoring process during liquid nitrogen fracturing under low-temperature conditions. This not only provides guidance for coalbed methane development projects, but also provides support for carbon burial and underground gas extraction technology, and has broad application prospects.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is: A waterless fracturing in-situ low-permeability reservoir pressure-drive-seepage integrated test device comprises a triaxial pressure chamber system, a fracturing system and a seepage system, wherein the fracturing system comprises a liquid nitrogen storage tank, a self-pressurizing liquid nitrogen tank and a pressurizing device, wherein the self-pressurizing liquid nitrogen tank is provided with a seventeenth control valve and a first pressure monitoring gauge, the self-pressurizing liquid nitrogen tank is connected to the liquid nitrogen storage tank through an injection pipe, the injection pipe is provided with a fifteenth control valve, and the port of the injection pipe extends to the inner bottom of the liquid nitrogen storage tank; the inside of the liquid nitrogen storage tank is respectively connected to the pressurizing device and the eleventh control valve through a twelfth control valve, the eleventh control valve is connected to the fracturing liquid flow inlet of the triaxial pressure chamber system, and the fracturing liquid flow inlet is connected to the fracturing pipe in the triaxial pressure chamber system; the bottom outlet of the liquid nitrogen storage tank is connected to the fracturing liquid flow inlet of the triaxial pressure chamber system through a sixteenth control valve and a preheater; The triaxial pressure chamber system contains a test sample; the inner end of the fracturing pipe placed in the test sample in the triaxial pressure chamber system is closed and provided with a side hole; the triaxial pressure chamber system is provided with a heating device and an acoustic emission device; The seepage system includes a back pressure system and a standard chamber system. The back pressure system includes a back pressure valve, a back pressure pump and a gas-liquid separator. The back pressure pump is connected to the back pressure valve through the eighteenth control valve, the back pressure pressure gauge and the back pressure container in sequence. The back pressure valve is connected to the seepage port on the corresponding loading axis of the three-axis pressure chamber system through the twenty-seventh control valve, the seventeenth control valve and the twenty-fifth control valve respectively; the gas-liquid separator is connected to the back pressure valve, the upper gas port of the gas-liquid separator is connected to a dryer, and the lower liquid port is provided with a nineteenth control valve.

[0008] Preferably, the fracturing tube comprises a pre-buried tube and a fracturing outlet tube, the pre-buried tube is placed in the test sample, the lower end port of the pre-buried tube is closed, and a side hole is opened in the side wall; the lower end of the fracturing outlet tube is sleeve-connected and fixedly connected to the pre-buried tube, and the upper end is connected to the sample connecting port on the pressure plate of the triaxial pressure chamber system.

[0009] Preferably, the fracturing lead-out pipe is an annulus casing, which comprises an outer tube and an inner tube, the lower end of the outer tube is connected to the embedded tube, the inner tube is coaxially sleeved on the upper end of the outer tube, and the upper end of the inner tube is connected to the sample connecting port.

[0010] Preferably, the liquid nitrogen storage tank includes a tank body and a vacuum insulation layer, the vacuum insulation layer is arranged around the periphery of the tank body to form a vacuum insulation cavity; the injection pipe extends from the top and is arranged at the lower interior of the tank body, and the top of the tank body is connected to a booster device; the vacuum insulation cavity is connected to the seventh control valve on the buffer tank through the tenth control valve and the ninth control valve, the buffer tank is also connected to a vacuum pump and a vacuum pressure monitoring gauge, and the seventh control valve is also connected to the fracturing liquid inlet of the three-axis pressure chamber system through the eighth control valve.

[0011] Preferably, the increasing device includes a gas booster pump, a nitrogen cylinder and an air compressor. The nitrogen cylinder is connected to the air inlet end of the gas booster pump through a first control valve, and a gas cylinder pressure monitor is also connected to the nitrogen cylinder; the air compressor passes through the first control valve and the driving pressure monitor in sequence and is also at the air inlet end of the gas booster pump; the air outlet end of the gas booster pump passes through the third control valve, the fifth control valve, the pressure regulating valve, and the outlet valve in sequence and is connected to the twelfth control valve of the liquid nitrogen storage tank; a storage tank pressure monitor is also connected between the third control valve and the fifth control valve, and is connected to the high-pressure gas storage tank through the fourth control valve.

[0012] Preferably, the triaxial pressure chamber system includes a machine body, six hydraulic cylinders and a pressure plate. The chambers arranged on the six sides of the machine body are vertical in pairs and connected together in the middle. Hydraulic cylinders are installed correspondingly at the six chambers. Corresponding pressure plates are installed on the piston rods of the hydraulic cylinders. The test sample is squeezed in the center of the chamber by the six pressure plates; the fracturing tube is connected to the sample connecting port on the top pressure plate and is arranged in the test sample.

[0013] Preferably, the inner edge of the pressing plate is chamfered accordingly; a plurality of acoustic emission test ports for installing an acoustic emission device are provided on the pressing plate in the X-axis and Y-axis directions; the heating device includes electric heating rods, and the electric heating rods are all arranged in the pressing plate.

[0014] Preferably, the back pressure valve comprises a valve body and a floating piston, the floating piston is slidably mounted in an active cavity in the valve body, the back pressure control fluid inlet is connected to one end of the active cavity, the other end of the active cavity is connected to a displacement fluid inlet and a displacement fluid outlet, and the floating piston is provided with a valve needle that cooperates with the displacement fluid outlet; Among them, a fracturing liquid inlet and a sample connecting port are provided on the pressure plate directly above. The fracturing liquid inlet is opened on one side of the pressure plate directly above, and the sample connecting port is opened at the center of the end surface of the pressure plate, and the fracturing liquid inlet is connected to the sample connecting port; a seepage liquid inlet and a seepage liquid outlet are respectively provided on the left and right pressure plates, and a seepage liquid outlet is also provided on the pressure plate directly below; the seepage liquid inlet and the seepage liquid outlet are respectively connected to the displacement fluid outlet and the displacement fluid inlet.

[0015] Preferably, the standard chamber control system comprises a standard chamber cavity, the standard chamber cavity is connected to the external gas interface through a fourteenth control valve, and the external gas interface is also connected to the eleventh control valve through a thirteenth control valve and to the fracturing liquid flow inlet of the triaxial pressure chamber system through a twenty-eighth control valve; the thirteenth control valve is an external gas bottle opening connection valve, used for seepage interface control; The three-axis pressure chamber system also includes three sets of oil supply systems respectively arranged corresponding to the x-axis, y-axis and z-axis, and the three oil supply systems are respectively connected to two symmetrically arranged hydraulic cylinders in each axis; the oil supply system includes an oil supply device, which is connected to the depressurization ports corresponding to the two hydraulic cylinders through the twenty-first control valve and the twenty-second control valve, and is connected to the pressurization ports corresponding to the two hydraulic cylinders through the twentieth control valve and the twenty-third control valve.

[0016] Correspondingly, the present invention also proposes a waterless fracturing in-situ low permeability reservoir pressure-displacement-permeability integrated test method based on the above-mentioned waterless fracturing in-situ low permeability reservoir pressure-displacement-permeability integrated test device, which specifically includes the following steps: S1. Sample preparation and installation: S1-1. Before starting the experiment, turn off all valves; according to the experimental requirements, select a certain specification of test sample coal rock, and drill a hole in its center to place the fracturing pipe; S1-2. Move the hydraulic cylinder back to the maximum size, install the electric heating rod and the acoustic emission device in the appropriate position; after the sample is in place, start the hydraulic cylinder to move forward and load the sample into the triaxial pressure chamber system; load the axial pressure through the six-sided pressure plate to completely seal from the six sides and keep the center position of the sample unchanged; S1-3. Insert the fracturing tube into the center of the sample. When installing, seal the side hole of the fracturing tube with a film or tape to prevent clogging of the sealant. Place the embedded tube in the drilled hole and seal it with sealant and dry it in preparation for the installation of the fracturing outlet tube in the final experiment. S1-4. Connect each control device in the test device to the computer control system for data synchronization; S2. Vacuum the instrument: Vacuum the insulation layer of the liquid nitrogen tank, the liquid nitrogen tank and the fracturing pipeline, and open the seventh, eighth, ninth, tenth, eleventh, twelfth, thirteenth, fourteenth, fifteenth, sixteenth, twenty-sixth and twenty-eighth control valves; S3. Simulation of in-situ geostress and reservoir temperature conditions: S3-1. Close all valves, open the servo hydraulic loading control system connected to the hydraulic cylinder, and use the main control console to open the servo loading pump to provide hydraulic pressure to drive the hydraulic cylinder; S3-2. Select triaxial pressure loading as the loading method and set the triaxial pressure value. Take the set maximum pressure as the reference and maintain the pressure stable after reaching the set pressure. S3-3. Turn on the temperature and pressure monitoring system for precise temperature control, with a display accuracy of 0.1°C; S4. Three-axis heating: turn off all valves and turn on the heating device; S5. The seepage system starts to carry out gas permeability, adsorption and desorption testing: S5-1. First open the thirteenth control valve and the eighteenth control valve, set the external gas interface pressure, and use the pressure monitor to monitor; S5-2. Then open the fourteenth control valve and the twenty-eighth control valve to start the seepage process; S5-3. Observe the back pressure valve pressure. When the outlet pressure is equal to the inlet pressure, close the 18th control valve, open the 27th control valve, the 25th control valve, the 17th control valve and the 19th control valve, and the remaining seepage liquid or gas enters the gas-liquid separator for separation; finally, close all valves and the seepage ends; S6. Add liquid nitrogen: turn off all valves, open the 17th control valve, the 15th control valve and the 24th control valve on the self-pressurized liquid nitrogen tank and the liquid nitrogen storage tank, inject liquid nitrogen into the liquid nitrogen storage tank, and close all valves after liquid nitrogen flows out of the 24th control valve; S7. When the pipeline is cold: close all valves and open the 16th and 26th control valves; S8. Sample fracturing: After the fracturing outlet pipe is installed in place, open the first, second, third, fourth, fifth, and sixth control valves on the booster pump device, as well as the sixteenth and twenty-fourth control valves on the liquid nitrogen storage tank; and turn on the acoustic emission device; when carbon dioxide is used as the fracturing gas, it is necessary to turn on the preheater for preheating operation; S9. Fracturing is completed and pressure is reduced; S10. End of experiment, instrument removal and data processing: S10-1. After the sample is fractured, all valves are closed, the twenty-first control valve and the twenty-second control valve are opened, and the liquid nitrogen in the triaxial pressure chamber system is discharged until the pressure is zero, and then the triaxial pressure chamber system is opened to take out the sample and clean the triaxial pressure chamber system; S10-2. The pressure at each location, the triaxial pressure loading pump and the acoustic emission device, and the flow data collected in real time by the control system during the fracturing process are displayed and imported into the data processing software; S10-3. Open or export the test data from WORD or EXCEL.

[0017] The present invention has the following beneficial effects: 1. The present invention aims to provide an integrated test device for pressure-displacement-permeability of in-situ low-permeability reservoirs without water fracturing. Under simulated in-situ conditions, with low-temperature fluid fracturing simulation and displacement and permeability enhancement conditions as the core, it can reveal the multi-phase and multi-fluid adsorption, migration and desorption process of gas-containing coal to water-CO2 / N2-methane, investigate the crack expansion, extension and intersection process of reservoirs under multi-field conditions such as low temperature-phase change-stress, analyze the permeability enhancement conditions and effects of low-permeability reservoirs, and explore the displacement effect of water-nitrogen multi-fluids on methane under adsorption equilibrium conditions, so as to provide new technologies and new ideas for permeability enhancement and gas extraction of low-permeability gas-rich reservoirs.

[0018] 2. The unique fracturing tube structure of the present invention can not only discharge the liquid nitrogen that has been vaporized in the experimental device in the early stage, but also ensure that there is no participation of high-pressure nitrogen formed by vaporization of liquid nitrogen due to contact with overheated samples during the fracturing process, thereby ensuring the authenticity and accuracy of liquid nitrogen fracturing.

[0019] 3. The temperature detector of the present invention comprises two temperature probes vertically arranged on both sides of the fracturing pipe, and a plurality of temperature measuring points are arranged on the temperature probes along the length direction thereof, and the pressure sensor is connected to the top of the fracturing pipe. The present invention can monitor the temperature and pressure changes of the sample in the fracturing process in real time, and can accurately study the phase change fracturing process of low temperature fluid entering the crack, as well as the cracking and expansion process of the crack under temperature stress, providing conditions for studying the cracking and expansion of the crack under temperature stress.

[0020] 4. The true triaxial stress environment provided by the triaxial pressure chamber system of the present invention simulates real reservoir conditions and can also simulate composite fracturing processes such as supercritical CO2 low temperature-high pressure-extraction, and is expected to become a forward-looking technical equipment for solving the problem of increasing permeability and production of low-permeability coal seams. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a structural diagram of the test device of the present invention; Figure 2 is a structural diagram of the triaxial pressure chamber system of the present invention; Figure 3 This is a schematic diagram of pressurization and depressurization of the present invention; Figure 4 It is a schematic diagram of the structure of the pressing plate of the present invention; Figure 5 A connection structure diagram of the fracturing pipe of the present invention; Figure 6 It is a structural diagram of the liquid nitrogen storage tank of the present invention; Figure 7 It is a structural diagram of the seepage radiation groove on the pressing plate of the present invention; Figure 8 It is a structural schematic diagram of the back pressure valve of the present invention; Fig. 9 It is a schematic diagram of the installation of the oil supply system of the present invention; Fig.10 It is a schematic diagram of the structure of the hydraulic cylinder of the present invention. DETAILED DESCRIPTION

[0022] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention and are not intended to limit the scope of use of the present invention.

[0023] like Figure 1-10As shown, the present invention proposes an integrated test device for pressure-displacement-permeability of low-permeability reservoirs without water fracturing. The present invention can simulate the pure real liquid nitrogen fracturing effect, and can ensure that there is no participation of high-pressure nitrogen formed by gasification due to liquid nitrogen contacting overheated samples during the fracturing process, thereby ensuring the authenticity and accuracy of liquid nitrogen fracturing. It has strong compatibility, can simulate conventional hydraulic fracturing conditions, and can also simulate the multiphase process of gas-containing coal bodies, including supercritical CO2 low temperature-high pressure-extraction and other composite fracturing processes.

[0024] The present invention aims to provide an integrated test device for pressure-displacement-permeability of in-situ low-permeability reservoirs without water fracturing. Under simulated in-situ conditions, with low-temperature fluid fracturing simulation and displacement and permeability enhancement conditions as the core, it can reveal the multi-phase and multi-fluid adsorption, migration and desorption process of gas-containing coal to water-CO2 / N2-methane, examine the crack expansion, extension and intersection process of the reservoir under multi-field conditions such as low temperature-phase change-stress, analyze the permeability enhancement conditions and effects of low-permeability reservoirs, and explore the displacement effect of water-nitrogen multi-fluids on methane under adsorption equilibrium conditions, providing new technologies and new ideas for permeability enhancement and gas extraction of low-permeability gas-rich reservoirs.

[0025] The waterless fracturing in-situ low-permeability reservoir pressure-drive-seepage integrated test device comprises a triaxial pressure chamber system 3, a fracturing system and a seepage system. The fracturing system comprises a liquid nitrogen storage tank 2, a self-pressurizing liquid nitrogen tank 1 and a pressurizing device. The self-pressurizing liquid nitrogen tank 1 is provided with a seventeenth control valve A17 and a first pressure monitoring gauge. The self-pressurizing liquid nitrogen tank 1 is connected to the liquid nitrogen storage tank 2 through an injection pipe 21. The injection pipe 21 extends from the top and is arranged at the lower part of the interior of the storage tank body 201. A fifteenth control valve A15 is provided on the injection pipe 21, and the port of the injection pipe 21 extends to the inner bottom of the liquid nitrogen storage tank 2; a vacuum insulation chamber 200 is provided outside the liquid nitrogen storage tank 2, and the vacuum insulation chamber 200 is connected to the seventh control valve A7 on the buffer tank 51 through the tenth control valve A10 and the ninth control valve A9, and the buffer tank 51 is also connected to a vacuum pump 52 and a vacuum pressure monitor 53, and the seventh control valve A7 is also connected to the fracturing liquid inlet 311 of the three-axis pressure chamber system 3 through the eighth control valve A8, and the fracturing liquid inlet 311 is connected to the fracturing pipe in the three-axis pressure chamber system 3. The inside of the liquid nitrogen storage tank 2 is connected to the booster device and the eleventh control valve A11 through the twelfth control valve A12, and the eleventh control valve A11 is connected to the fracturing liquid inlet 311 of the three-axis pressure chamber system 3.

[0026] The present invention reduces the contact area between the tank body 201 and the outside by setting a vacuum insulation layer 202 on the outside of the liquid nitrogen storage tank 2, thereby achieving the effect of heat insulation and heat preservation. During actual installation, the liquid nitrogen storage tank 2 can be provided with a heat insulation device 22 at its bottom, and installed on a work surface through the heat insulation device 22, thereby reducing the contact area between the tank body and the work surface and reducing heat transfer. Specifically, the volume of the liquid nitrogen storage tank 2 used in the present invention can be selected to be 2L, and the pressure resistance is 30MPa.

[0027] The model body of the triaxial pressure chamber system 3 contains the test sample; the model body of the triaxial pressure chamber system 3 is provided with a heating device and an acoustic emission device; the heating device and the pressure sensor are electrically connected to the temperature and pressure monitoring system. The present invention fully considers that the stress pressure in the waterless fracturing process can reach 40mpa, the temperature can reach 150℃ (equivalent to 4km burial), the strain test: 0~10mm, and the deformation measurement error: ±0.5%FS. The temperature and pressure monitoring system adopts an artificial intelligence display temperature controller with PID adjustment function.

[0028] The increasing device includes a gas booster pump, a nitrogen bottle 41 and an air compressor 43. The nitrogen bottle 41 is connected to the air inlet end of the gas booster pump through a first control valve A1, and a gas bottle pressure gauge 42 is also connected to the nitrogen bottle 41; the air compressor 43 is also connected to the air inlet end of the gas booster pump through a second control valve A2A2 and a driving pressure monitor 44 in sequence; the air outlet end of the gas booster pump is connected to the twelfth control valve A12 of the liquid nitrogen storage tank 2 through a third control valve A3, a fifth control valve A5, a pressure regulating valve 47, and an outlet valve in sequence; a storage tank pressure gauge 45 is also connected between the third control valve A3 and the fifth control valve A5, and a high-pressure gas storage tank 46 is connected through a fourth control valve A4.

[0029] Specifically, the gas booster pump can adopt the STK brand, GB series gas booster pump, which is mainly used for pressurizing methane, nitrogen and other gases. The selected model is GB60, with a pressure ratio of 60:1, a maximum outlet pressure of 498Bar, a minimum inlet pressure of 32Bar, and a maximum displacement of 112L / min. The power source of the gas booster pump is a low-pressure air source with a maximum output of 0.7MPa. The present invention designs a unique high-pressure displacement device, which uses nitrogen to pressurize low-pressure liquid nitrogen, in order to pressurize the low-pressure liquid nitrogen to a high-pressure state that meets the experimental requirements and enter the fracturing pipe.

[0030] The bottom outlet of the liquid nitrogen storage tank 2 is connected to the fracturing liquid inlet 311 of the triaxial pressure chamber system 3 through the sixteenth control valve A16 and the preheater. The fracturing liquid inlet 311 of the triaxial pressure chamber system 3 is also connected to an inlet thermometer 61, an inlet pressure gauge 62 and a safety valve 63.

[0031] The triaxial pressure chamber system 3 contains the test sample; the inner end of the fracturing pipe placed in the test sample in the triaxial pressure chamber system 3 is closed and provided with a side hole 3220, and the triaxial pressure chamber system 3 is provided with a heating device and an acoustic emission device.

[0032] The true triaxial stress environment provided by the triaxial pressure chamber system 3 simulates the real reservoir conditions, and can also simulate the composite fracturing process such as supercritical CO2 low temperature-high pressure-extraction, and is expected to become a forward-looking technical equipment to solve the problem of increasing the permeability and production of low-permeability coal seams. The triaxial pressure chamber system 3 includes a model host, six hydraulic cylinders and a pressure plate 31. The model host is provided with three-axis pressure chambers on the left and right (X axis), front and back (Y axis), and top and bottom (Z axis). The pressure chambers are vertically connected in the middle, and hydraulic cylinders are installed at the corresponding ports of the chambers. The corresponding pressure plates 31 are installed on the piston rods of the hydraulic cylinders. The test samples are squeezed in the center of the chambers by the six pressure plates 31.

[0033] The main model of the triaxial pressure chamber system 3 of the present invention is designed to be loaded on six sides, and the six sides in the three directions of X (left and right), Y (front and back), and Z (up and down) are all pressurized by hydraulic loading structures, and can be independently controlled in one direction, so as to simulate the triaxial stress state of the underground reservoir more realistically. The main body of the model is equipped with a pressure plate 31 of corresponding specifications to keep the center position of the 300mm×300mm×300mm cube sample unchanged, and the loading hydraulic pressure is automatically converted into the sample end face stress value through the hydraulic cylinder area, sample area, and loading hydraulic pressure.

[0034] The triaxial pressure chamber system 3 adopts a piston loading structure inside the pressure chamber, adopts a pressure vessel type, installs samples inside, and installs piston loading structures around the samples to ensure measurement accuracy. The stiffness of the model host participating in the deformation measurement part is ≥10GN / m; the full load and deformation of the model host participating in the deformation measurement part is ≤0.1mm; the hydraulic cylinder drive and control of the triaxial pressure chamber system 3 are all controlled by an independent servo system + central digital system, which can realize synchronous loading and asynchronous loading, and work with the servo actuator to achieve accurate force transmission and pressure monitoring feedback.

[0035] like Fig.10 As shown, when the hydraulic cylinder starts to be pressurized, oil enters the oil inlet end to push the piston to squeeze the oil outlet end; when depressurization starts, oil enters the oil outlet end to squeeze the piston and depressurize the oil.

[0036] The triaxial pressure chamber of the present invention has a pressure bearing capacity of 40MPa, a triaxial loading pressure of 40MPa, a control accuracy of 5%, a maximum test force of 2000KN, an effective test force measurement range of 1~100F.S, a test force test error of ±1%, a test force resolution of 1 / 200000, a deformation range of 0~10mm, and a deformation measurement error of ±0.5%FS. The present invention can realize fast and easy installation of the specimen; it can evenly apply triaxial pressure to the surface of the specimen, and can effectively eliminate the shear stress generated on the surface of the specimen due to the compression deformation of the specimen.

[0037] The model body of the triaxial pressure chamber system 3 of the present invention is made of high-strength steel, and 6 sets of hydraulic cylinders are installed in a matching manner, with a maximum force of 40MPa (300mm×300mm×300mm rock sample) and an error of ±0.1MPa. The present invention can realize true triaxial loading control and ensure stability during loading. The model end face stress loading is designed as a double-acting hydraulic cylinder, and a reverse-acting hydraulic cylinder is added to the rear of the hydraulic cylinder corresponding to the position of the main hydraulic cylinder. The main hydraulic cylinder cavity is recovered by the reverse-acting hydraulic cylinder. When the main hydraulic cylinder cavity is retracted, a larger gap can be formed between the pressure plate 31 and the core, and the core can be quickly taken out. In order to ensure the large strain experimental requirements of the sample, the piston stroke of the six-sided hydraulic cylinder is designed to be 10mm. The maximum strain of the maximum specimen loading deformation process is ≥5%. Among them, double-acting hydraulic cylinders and reverse-acting hydraulic cylinders are conventional applications in this field, and their principles will not be repeated here.

[0038] All hydraulic cylinders of the present invention are made of high-strength steel, which has high strength and toughness, good hardenability, no obvious temper brittleness, high fatigue limit and multiple impact resistance after solution treatment, and good low-temperature impact toughness.

[0039] The displacement sensor installed on the hydraulic cylinder adopts magnetostrictive displacement sensor, which can detect the displacement of piston rod. It has high impact and vibration resistance. The selected specifications are: range: 0-30mm (continuous); measurement accuracy: ±0.05%FS; measurement resolution: 0.001mm.

[0040] The upper and lower and surrounding pressure plates 31 are designed with high-rigidity mold steel to avoid displacement measurement errors caused by deformation of the pressure plate 31. The pressure plate 31 is chamfered about 10mm around the end surface of the core to avoid collision between the six pressure plates 31 due to core deformation. The pressure plates 31 in the X-axis (left and right) and Y-axis (front and back) directions are provided with acoustic emission test ports 317 for installing acoustic emission devices. Among them, each pressure plate 31 in this application is provided with 4 acoustic emission test ports 317.

[0041] Seepage ports and seepage radiation grooves 3102 are provided on the pressing plate 31 in the X-axis (left and right), Y-axis (front and back), and Z-axis (up and down) directions. The seepage ports are connected to an external carbon dioxide, methane, or nitrogen supply system. The seepage radiation grooves 3102 are provided on the end surface of the pressing plate 31, are arranged concentrically with the pressing plate 31, and are circular designs of multiple different diameters.

[0042] The pressure plate 31 is provided with a fluid inlet and outlet, a pressure control interface for the end face fracturing liquid, and a seepage interface, and the pressure control interface and the seepage interface are connected to the fluid inlet and outlet on the corresponding pressure plate 31. The fluid inlet and outlet, the pressure control interface for the end face fracturing liquid, and the seepage interface are all in the form of through-plate joints + internal and external quick joints, which are convenient for quick disassembly and assembly. A fracturing interface is provided on the top of the pressure plate 31 in the Z-axis direction, and the fracturing interface is connected to the matching fracturing pipe.

[0043] The opening size of the pressure injection port of the fracturing liquid is φ6mm, and the opening size of the seepage port on the pressure plate 31 is φ6mm. According to the pressure vessel wall thickness calculation formula of GB150.1-2011 standard, the pressure bearing capacity is 40MPa, and the triaxial stress loading is (40MPa). The stress of the model body is within the safe and effective range and meets the pressure requirements.

[0044] The heating device includes electric heating rods, which are all arranged in the pressing plate 31. The temperature loading of the sample is completed through the pressing plate 31 around the core. Three electric heating rod positions are arranged and processed at appropriate positions on the pressing plate 31. The electric heating rods use a power supply of 220V, a diameter of 14mm, a total power of 3 rods of 1Kw, and a maximum heating temperature of 200℃. The temperature control system controls the heating of the electric heating rods of the external flexible heating sleeve, adjusts the temperature of the three-axis chamber and the heating power of the system, and the heating signal is transmitted through a high-precision communication instrument, thereby realizing a multifunctional control method of digital control and panel control.

[0045] The three axes of the triaxial pressure chamber system 3 are loaded independently to ensure that the sample is in the center position. Whether it is force control or position control, the flexibility of the experiment is greatly increased. The true triaxial loading control software can realize the setting, control and display of triaxial pressure. The three axes can simultaneously realize constant pressure loading, loading at a certain rate (pressure gradient loading), and pressure tracking control loading. Regardless of the loading method, the pressure at the set maximum pressure is used as the reference. After reaching the set pressure, the pressure can be maintained stable, and the pressure control accuracy is: 0.01MPa.

[0046] The seepage system includes a back pressure system and a standard chamber system. The back pressure system includes a back pressure valve 72, a back pressure pump 75 and a gas-liquid separator 76. The back pressure pump 75 is connected to the back pressure valve 72 through the eighteenth control valve A18, the back pressure gauge 74 and the back pressure container 73 in sequence. The back pressure valve 72 is connected to the seepage port on the corresponding loading axis of the three-axis pressure chamber system 3 through the twenty-seventh control valve A27, the seventeenth control valve A17 and the twenty-fifth control valve A25 respectively; the gas-liquid separator 76 is connected to the back pressure valve 72, the upper gas port of the gas-liquid separator 76 is connected to the dryer 77, and the lower liquid port is provided with the nineteenth control valve A19.

[0047] The standard chamber control system includes a standard chamber cavity 71, which is connected to the external gas interface through the fourteenth control valve A14, and the external gas interface is also connected to the eleventh control valve A11 through the thirteenth control valve A13 and to the fracturing liquid flow inlet 311 of the three-axis pressure chamber system 3 through the twenty-eighth control valve A28; the thirteenth control valve A13 is an external gas bottle opening valve, which is used for seepage interface control.

[0048] The back-pressure pump 75 is used to achieve high-precision pressure control of fluids such as N2 at low flow rates. It can maintain stable back-pressure control, automatically track to the set pressure, and is stable and convenient to use. The maximum back-pressure control is 50MPa, and the pressure accuracy is 0.1%FS. The back-pressure control is accurate and reliable, and has an alarm function. The back-pressure pump 75 is connected to the three-axis pressure chamber system 3 through the back-pressure valve 72, and the gas-liquid separator 76 is connected to the back-pressure valve 72, and the gas-liquid separator 76 is connected to a drying tank. The gas-liquid separator 76 and the drying tank connected to the back-pressure valve 72 are conventional applications in this field, and their principles will not be repeated here.

[0049] The back pressure valve 72 includes a valve body and a floating piston 722. The floating piston 722 is slidably installed in the active cavity in the valve body. The back pressure control fluid inlet 721 is connected to one end of the active cavity, and the other end of the active cavity is connected to the displacement fluid inlet 723 and the displacement fluid outlet 724. The floating piston 722 is provided with a valve needle 725 that matches the displacement fluid outlet 724. When the pressure at the top is greater than the system pressure, the floating piston 722 with the valve needle 725 is pressurized, so that the valve needle 725 blocks the displacement fluid outlet 724 to prevent the fluid from flowing; when the system pressure is higher than the top pressure, the valve needle 725 is separated from the displacement fluid outlet 724, and the overpressure part of the fluid flows out; when the flow is continuous, these two processes are continuously circulated to maintain the balance between the seepage system pressure and the fixed value set at the top.

[0050] Among them, the pressure plate 31 directly above the Z axis in the present application is provided with a fracturing liquid inlet 311 and a sample connecting port 312. The fracturing liquid inlet 311 is opened on one side of the pressure plate 31 directly above, and the sample connecting port 312 is opened at the center of the end surface of the pressure plate 31. The fracturing liquid inlet 311 is connected to the sample connecting port 312. The left and right pressure plates 31 of the X axis are respectively provided with a seepage liquid inlet 313 and a seepage liquid outlet 314, and the pressure plate 31 directly below the Z axis is also provided with a seepage liquid outlet 314. The seepage liquid inlet 313 and the seepage liquid outlet 314 on the pressure plate 31 are respectively connected to the displacement fluid outlet 724 and the displacement fluid inlet 723.

[0051] At the beginning of seepage, the seepage liquid flows into the left pressure plate 31, and the inlet pressure is set. The right seepage outlet is connected to the back pressure pump 75. The fluid flows and the seepage process begins. After the seepage is saturated, when the seepage system meets the system requirements, the outlet pressure is greater than the pressure of the back pressure valve 72. The remaining seepage liquid or gas enters the gas-liquid separator 76 at the bottom pressure plate.

[0052] The present invention can realize high-precision outlet pressure control through the design of high-precision back pressure valve 72. Pressure adjustment range: 0-50MPa; applicable temperature range: 0-200°C; accuracy 0.1%FS, pressure control accuracy reaches ±0.1MPa.

[0053] The inner end of the fracturing pipe placed in the test sample in the triaxial pressure chamber system 3 is closed and provided with a side hole 3220. Specifically, the fracturing pipe includes a pre-buried pipe 322 and a fracturing lead-out pipe 321. The pre-buried pipe 322 is placed in the test sample. The lower end of the pre-buried pipe 322 is closed, and the side wall is provided with a side hole 3220; the lower end of the fracturing lead-out pipe 321 is sleeved and fixedly connected with the pre-buried pipe 322, and the upper end is connected to the sample connecting port 312. The fracturing lead-out pipe 321 is an annular casing, which includes an outer pipe and an inner pipe (the inner pipe has a diameter of φ8mm, the outer pipe has a diameter of φ9mm, and is 20cm long). The lower end of the outer pipe is threadedly connected to the pre-buried pipe 322, the upper end of the outer pipe is closed, and the inner pipe is coaxially sleeved and penetrates and extends out of the upper end of the outer pipe. The annulus casing can not only discharge the liquid nitrogen that has been vaporized in the experimental device in the early stage, but also ensure that there is no participation of high-pressure nitrogen formed by vaporization due to liquid nitrogen contacting overheated samples during the fracturing process, thereby ensuring the authenticity and accuracy of liquid nitrogen fracturing.

[0054] For single-well staged fracturing wells: the embedded pipe 322 is designed as a multi-section combination, and the simulated well wall, support section, sealing ring, etc. are first glued and sealed in sequence into the drilled fracturing well, and the fracturing lead-out pipe 321 cooperates with the adjustment pad to achieve single-well staged fracturing.

[0055] The three-axis pressure chamber system 3 also includes three sets of oil supply systems respectively arranged corresponding to the x-axis, y-axis and z-axis, and the three oil supply systems are respectively connected to two symmetrically arranged hydraulic cylinders in each axis; the oil supply system includes an oil supply device 33, and the oil supply device 33 is connected to the depressurization ports A and B corresponding to the two hydraulic cylinders through the twenty-first control valve A21 and the twenty-second control valve A22, and is connected to the pressurization ports A and B corresponding to the two hydraulic cylinders through the twentieth control valve A20 and the twenty-third control valve A23.

[0056] The triaxial pressure chamber system 3 of the present invention is connected to the fracturing test control system. The fracturing test control system can perform parameter preset, automatic measurement, manual control, automatic control, data processing, and discrimination on the test machine. It has the function of accessing test data, and the test data can be opened or exported by WORD or EXCEL, and opened or processed by other related software. It can process test data, select calculation items, and automatically generate and print test report formats; it can control the operation of the entire set of equipment, control the parameters of the triaxial stress loading and fracturing process, and collect and record the triaxial pressure loading, fracturing parameters, flow rate and other parameters in the test process in real time, and export the data in Excel format after the experiment.

[0057] The fracturing test control system is mainly divided into two parts: true triaxial loading control and fracturing test.

[0058] The true triaxial loading control part mainly includes triaxial pressure loading and control and fracturing loading, loading mode selection and data input, setting and control; temperature setting and control; the fracturing test part mainly includes fracturing parameter setting, fracturing crack detection (if necessary), etc.

[0059] The true three-axis loading control part can realize the three-axis pressure setting, control and display. The three axes can simultaneously realize constant pressure loading, loading at a certain rate (pressure gradient loading), and pressure tracking control loading. Regardless of the loading method, the pressure is based on the set maximum pressure. After reaching the set pressure, the pressure can be maintained stable, and the pressure control accuracy is: 0.01MPa.

[0060] The fracturing test part includes calculation and analysis module, graphic output module, word report output module and online help module, etc. It has a friendly interface and is easy to use. The fracturing test part is mainly used to control the pressure at various locations of the system, data acquisition and control of the triaxial pressure loading pump, flow data acquisition, etc. It can collect data from each node in the process in real time, synchronize with the computer, and display it on the data processing software. The fracturing test part can run in a Windows environment and is programmed using LabVIEW. The instrument workflow is displayed on the interface, which can realize human-computer dialogue. After the operator sets the parameters, it can be unattended and automatically collect all pressure, flow and other parameters.

[0061] The fracturing test part can perform parameter preset, automatic measurement, manual control, automatic control, data processing, identification, etc. on the test machine. It has the function of test data access, the test data can be opened or exported by WORD or EXCEL, and opened or processed by other related control software, and can process test data, select calculation items, and automatically generate and print test report formats.

[0062] Correspondingly, based on the above-mentioned waterless fracturing in-situ low permeability reservoir pressure-displacement-permeability integrated test device, the present invention also proposes a waterless fracturing in-situ low permeability reservoir pressure-displacement-permeability integrated test method, which specifically includes the following steps: S1. Sample preparation and installation: S1-1. Before starting the experiment, turn off all valves; according to the experimental requirements, select a certain size of test coal rock sample (a large piece of coal rock with a sample size of 30cm×30cm×30cm can be selected), and drill a hole in the center (the hole size is φ10mm) to place the fracturing pipe; S1-2. Move the hydraulic cylinder back to its maximum size, install the electric heating rod and the acoustic emission device in appropriate positions; after the sample is in place, start the hydraulic cylinder to move forward and load the sample into the triaxial pressure chamber system 3; load the axial pressure through the six-sided pressure plate 31 to completely seal the six sides and keep the center position of the sample unchanged; S1-3. Insert the fracturing tube into the center of the sample. When installing, seal the side hole 3220 of the fracturing tube with a film or tape to prevent clogging with the sealant; place the embedded tube 322 in the borehole and seal it with a sealant and dry it in preparation for the final experimental installation of the fracturing outlet tube 321; S1-4. Connect each control device in the test device to the computer control system for data synchronization; S2. Vacuum the instrument: Vacuum the insulation layer of the liquid nitrogen tank, the liquid nitrogen tank and the fracturing pipeline, and open the seventh, eighth, ninth, tenth, eleventh, twelfth, thirteenth, fourteenth, fifteenth, sixteenth, twenty-sixth and twenty-eighth control valves; S3. Simulation of in-situ geostress and reservoir temperature conditions: S3-1. Close all valves, open the servo hydraulic loading control system connected to the hydraulic cylinder, and use the main control console to open the servo loading pump to provide hydraulic pressure to drive the hydraulic cylinder; S3-2. Select triaxial pressure loading as the loading method and set the triaxial pressure value. Take the set maximum pressure as the reference and maintain the pressure stable after reaching the set pressure. S3-3. Turn on the temperature and pressure monitoring system for precise temperature control, with a display accuracy of 0.1°C; S4. Three-axis heating: turn off all valves and turn on the heating device; S5. The seepage system starts to carry out gas permeability, adsorption and desorption testing: S5-1. First open the thirteenth control valve A13 and the eighteenth control valve A18, set the external gas interface pressure, and use the pressure monitor for monitoring; S5-2. Then open the fourteenth control valve A14 and the twenty-eighth control valve A28 to start the seepage process; S5-3. Observe the back pressure valve pressure. When the outlet pressure is equal to the inlet pressure, close the eighteenth control valve A18, open the twenty-seventh control valve A27, the twenty-fifth control valve A25, the seventeenth control valve A17 and the nineteenth control valve A19, and the remaining seepage liquid or gas enters the gas-liquid separator 76 for separation; finally, turn off all valves, and the seepage ends; S6. Add liquid nitrogen: turn off all valves, open the seventeenth control valve A17, the fifteenth control valve A15 and the twenty-fourth control valve A24 on the self-pressurized liquid nitrogen tank 1 and the liquid nitrogen storage tank 2, inject liquid nitrogen into the liquid nitrogen storage tank 2, and after the liquid nitrogen flows out of the twenty-fourth control valve A24, close all valves; S7. When the pipeline is cold: close all valves, open the sixteenth control valve A16 and the twenty-sixth control valve A26; S8. Sample fracturing: After the fracturing outlet pipe 321 is installed in place, the first, second, third, fourth, fifth, and sixth control valves on the booster pump device, as well as the sixteenth control valve A16 and the twenty-fourth control valve A24 on the liquid nitrogen storage tank 2 are opened; and the acoustic emission device is turned on; when carbon dioxide is selected as the fracturing gas, the preheater needs to be turned on for preheating operation; S9. Fracturing is completed and pressure is reduced; S10. End of experiment, instrument removal and data processing: S10-1. After the sample is fracturing, all valves are closed, the twenty-first control valve A21 and the twenty-second control valve A22 are opened, and the liquid nitrogen in the triaxial pressure chamber system 3 is discharged until the pressure is zero, and then the triaxial pressure chamber system 3 is opened to take out the sample, and the triaxial pressure chamber system 3 is cleaned; S10-2. The pressure at each location, the triaxial pressure loading pump and the acoustic emission device, and the flow data collected in real time by the control system during the fracturing process are displayed and imported into the data processing software; S10-3. Open or export the test data from WORD or EXCEL.

[0063] The present invention has strong equipment compatibility and is specially designed for low-temperature liquid nitrogen fracturing. It can be applied to various technologies such as supercritical CO2 fracturing, hydraulic fracturing, and water-liquid nitrogen composite fracturing. It can simulate the special temperature and pressure environment of coal and rock, realize temperature and pressure control, and collect and record data in real time. The equipment covers low-temperature-high-pressure to high-temperature-high-pressure environments, and realizes multi-phase and multi-flow gas component detection and analysis.

[0064] The present invention has the advantages of strong compatibility and wide application, and can implement the following functions: (1) The present invention utilizes an acoustic emission device to perform real-time and stable monitoring of the temperature and crack initiation-extension process of coal rock supercritical fluid fracturing; (2) The present invention utilizes a percolation system to effectively detect gas permeability, adsorption and desorption processes in a high-pressure, wide-range temperature environment (between -196°C and 150°C); (3) The present invention can detect and analyze multi-phase and multi-flow gas components in low-temperature-high-pressure to high-temperature-high-pressure displacement environments; (4) The present invention takes into account both conventional hydraulic fracturing and foam composite fracturing simulation of in-situ reservoirs and can monitor the deformation-fracture process of coal and rock; (5) The present invention can realize the special environment setting of coal rock temperature and pressure conditions and realize the controllable environmental temperature and pressure conditions, and can realize the real-time collection and recording of data and curve display.

[0065] Obviously, the embodiments described above are only some embodiments of the present application, rather than all embodiments. The preferred embodiments of the present application are given in the accompanying drawings, but they do not limit the patent scope of the present application. The present application can be implemented in many different forms. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive. Although the present application is described in detail with reference to the aforementioned embodiments, for those skilled in the art, it is still possible to modify the technical solutions recorded in the aforementioned specific implementation methods, or to perform equivalent replacement of some of the technical features therein. Any equivalent structure made using the contents of the specification and drawings of this application, directly or indirectly used in other related technical fields, is similarly within the scope of patent protection of this application.

Claims

1. An integrated test device for pressure-displacement-seepage of an in-situ low-permeability reservoir without water fracturing, including a triaxial pressure chamber system, a fracturing system and a seepage system, characterized in that: The fracturing system comprises a liquid nitrogen storage tank, a self-pressurizing liquid nitrogen tank and a pressurizing device, the self-pressurizing liquid nitrogen tank is provided with a seventeenth control valve and a first pressure monitoring gauge, the self-pressurizing liquid nitrogen tank is connected to the liquid nitrogen storage tank through an injection pipe, the injection pipe is provided with a fifteenth control valve, and the port of the injection pipe extends to the inner bottom of the liquid nitrogen storage tank; the inside of the liquid nitrogen storage tank is respectively connected to the pressurizing device and the eleventh control valve through a twelfth control valve, the eleventh control valve is connected to the fracturing liquid flow inlet of the three-axis pressure chamber system, and the fracturing liquid flow inlet is connected to the fracturing pipe in the three-axis pressure chamber system; the bottom outlet of the liquid nitrogen storage tank is connected to the fracturing liquid flow inlet of the three-axis pressure chamber system through a sixteenth control valve and a preheater; The triaxial pressure chamber system contains a test sample; the inner end of the fracturing pipe placed in the test sample in the triaxial pressure chamber system is closed and provided with a side hole; the triaxial pressure chamber system is provided with a heating device and an acoustic emission device; The seepage system includes a back pressure system and a standard chamber system. The back pressure system includes a back pressure valve, a back pressure pump and a gas-liquid separator. The back pressure pump is connected to the back pressure valve through the eighteenth control valve, the back pressure pressure gauge and the back pressure container in sequence. The back pressure valve is connected to the seepage port on the corresponding loading axis of the three-axis pressure chamber system through the twenty-seventh control valve, the seventeenth control valve and the twenty-fifth control valve respectively; the gas-liquid separator is connected to the back pressure valve, the upper gas port of the gas-liquid separator is connected to a dryer, and the lower liquid port is provided with a nineteenth control valve.

2. The waterless fracturing in-situ low permeability reservoir pressure-displacement-permeability integrated test device according to claim 1, characterized in that: The fracturing pipe comprises a pre-buried pipe and a fracturing outlet pipe. The pre-buried pipe is placed in the test sample, the lower end of the pre-buried pipe is closed, and a side hole is opened on the side wall; the lower end of the fracturing outlet pipe is sleeve-connected and fixedly connected to the pre-buried pipe, and the upper end is connected to the sample connecting port on the pressure plate of the triaxial pressure chamber system.

3. The waterless fracturing in-situ low permeability reservoir pressure-displacement-permeability integrated test device according to claim 2, characterized in that: The fracturing lead-out pipe is an annulus casing, which includes an outer pipe and an inner pipe. The lower end of the outer pipe is connected to the embedded pipe, the inner pipe is coaxially sleeved on the upper end of the outer pipe, and the upper end of the inner pipe is connected to the sample connecting port.

4. The waterless fracturing in-situ low permeability reservoir pressure-displacement-permeability integrated test device according to claim 1, characterized in that: The liquid nitrogen storage tank includes a tank body and a vacuum insulation layer, which is arranged around the periphery of the tank body to form a vacuum insulation cavity; the injection pipe extends from the top and is arranged at the bottom of the interior of the tank body, and the top of the tank body is connected to a booster device; the vacuum insulation cavity is connected to the seventh control valve on the buffer tank through the tenth control valve and the ninth control valve, and the buffer tank is also connected to a vacuum pump and a vacuum pressure monitoring gauge, and the seventh control valve is also connected to the fracturing liquid inlet of the three-axis pressure chamber system through the eighth control valve.

5. The waterless fracturing in-situ low permeability reservoir pressure-displacement-permeability integrated test device according to claim 4, characterized in that: The increasing device includes a gas booster pump, a nitrogen bottle and an air compressor. The nitrogen bottle is connected to the air inlet end of the gas booster pump through a first control valve, and a gas bottle pressure monitor is also connected to the nitrogen bottle; the air compressor is also connected to the air inlet end of the gas booster pump through a second control valve and a driving pressure monitor in sequence; the air outlet end of the gas booster pump is connected to the twelfth control valve of the liquid nitrogen storage tank through a third control valve, a fifth control valve, a pressure regulating valve, and an outlet valve in sequence; a storage tank pressure monitor is also connected between the third control valve and the fifth control valve, and is connected to a high-pressure gas storage tank through a fourth control valve.

6. The waterless fracturing in-situ low permeability reservoir pressure-displacement-permeability integrated test device according to claim 1, characterized in that: The triaxial pressure chamber system includes a machine body, six hydraulic cylinders and a pressure plate. The chambers arranged on the six sides of the machine body are vertical in pairs and connected together in the middle. Hydraulic cylinders are installed correspondingly at the six chambers. Corresponding pressure plates are installed on the piston rods of the hydraulic cylinders. The test sample is squeezed in the center of the chamber by the six pressure plates; the fracturing tube is connected to the sample connecting port on the top pressure plate and is arranged in the test sample.

7. The waterless fracturing in-situ low permeability reservoir pressure-displacement-permeability integrated test device according to claim 6, characterized in that: The inner edge of the pressing plate is correspondingly chamfered; a plurality of acoustic emission test ports for installing the acoustic emission device are provided on the pressing plate in the X-axis and Y-axis directions; the heating device includes electric heating rods, and the electric heating rods are all arranged in the pressing plate.

8. The waterless fracturing in-situ low permeability reservoir pressure-displacement-permeability integrated test device according to claim 6, characterized in that: The back pressure valve comprises a valve body and a floating piston, wherein the floating piston is slidably mounted in an active cavity in the valve body, the back pressure control fluid inlet is connected to one end of the active cavity, the other end of the active cavity is connected to a displacement fluid inlet and a displacement fluid outlet, and the floating piston is provided with a valve needle matched with the displacement fluid outlet; Among them, a fracturing liquid inlet and a sample connecting port are provided on the pressure plate directly above. The fracturing liquid inlet is opened on one side of the pressure plate directly above, and the sample connecting port is opened at the center of the end surface of the pressure plate, and the fracturing liquid inlet is connected to the sample connecting port; a seepage liquid inlet and a seepage liquid outlet are respectively provided on the left and right pressure plates, and a seepage liquid outlet is also provided on the pressure plate directly below; the seepage liquid inlet and the seepage liquid outlet are respectively connected to the displacement fluid outlet and the displacement fluid inlet.

9. The waterless fracturing in-situ low permeability reservoir pressure-displacement-permeability integrated test device according to claim 5, characterized in that: The standard chamber control system comprises a standard chamber cavity, the standard chamber cavity is connected to an external gas interface through a fourteenth control valve, and the external gas interface is also connected to the eleventh control valve through a thirteenth control valve and to the fracturing liquid inlet of the triaxial pressure chamber system through a twenty-eighth control valve; the thirteenth control valve is an external gas bottle opening connection valve, used for seepage interface control; The three-axis pressure chamber system also includes three sets of oil supply systems respectively arranged corresponding to the x-axis, y-axis and z-axis, and the three oil supply systems are respectively connected to two symmetrically arranged hydraulic cylinders in each axis; the oil supply system includes an oil supply device, which is connected to the depressurization ports corresponding to the two hydraulic cylinders through the twenty-first control valve and the twenty-second control valve, and is connected to the pressurization ports corresponding to the two hydraulic cylinders through the twentieth control valve and the twenty-third control valve.

10. A method for an integrated test of pressure-displacement-permeability for an in-situ low-permeability reservoir by waterless fracturing, implemented based on the integrated test device for pressure-displacement-permeability for an in-situ low-permeability reservoir by waterless fracturing according to claim 9, characterized in that: The specific steps include: S1. Sample preparation and installation: S1-1. Before starting the experiment, turn off all valves; according to the experimental requirements, select a certain specification of test sample coal rock, and drill a hole in its center to place the fracturing pipe; S1-2. Move the hydraulic cylinder back to the maximum size, install the electric heating rod and the acoustic emission device in the appropriate position; after the sample is in place, start the hydraulic cylinder to move forward and load the sample into the triaxial pressure chamber system; load the axial pressure through the six-sided pressure plate to completely seal from the six sides and keep the center position of the sample unchanged; S1-3. Insert the fracturing tube into the center of the sample. When installing, seal the side hole of the fracturing tube with a film or tape to prevent clogging of the sealant. Place the embedded tube in the drilled hole and seal it with sealant and dry it in preparation for the installation of the fracturing outlet tube in the final experiment. S1-4. Connect each control device in the test device to the computer control system for data synchronization; S2. Vacuum the instrument: Vacuum the insulation layer of the liquid nitrogen tank, the liquid nitrogen tank and the fracturing pipeline, and open the seventh, eighth, ninth, tenth, eleventh, twelfth, thirteenth, fourteenth, fifteenth, sixteenth, twenty-sixth and twenty-eighth control valves; S3. Simulation of in-situ geostress and reservoir temperature conditions: S3-1. Close all valves, open the servo hydraulic loading control system connected to the hydraulic cylinder, and use the main control console to open the servo loading pump to provide hydraulic pressure to drive the hydraulic cylinder; S3-2. Select triaxial pressure loading as the loading method and set the triaxial pressure value. Take the set maximum pressure as the reference and maintain the pressure stable after reaching the set pressure. S3-3. Turn on the temperature and pressure monitoring system for precise temperature control, with a display accuracy of 0.1°C; S4. Three-axis heating: turn off all valves and turn on the heating device; S5. The seepage system starts to carry out gas permeability, adsorption and desorption testing: S5-1. First open the thirteenth control valve and the eighteenth control valve, set the external gas interface pressure, and use the pressure monitor to monitor; S5-2. Then open the fourteenth control valve and the twenty-eighth control valve to start the seepage process; S5-3. Observe the back pressure valve system pressure. When the outlet pressure is equal to the inlet pressure, close the 18th control valve, open the 27th control valve, the 25th control valve, the 17th control valve and the 19th control valve, and the remaining seepage liquid or gas enters the gas-liquid separator for separation; finally, close all valves and the seepage ends; S6. Add liquid nitrogen: turn off all valves, open the 17th control valve, the 15th control valve and the 24th control valve on the self-pressurized liquid nitrogen tank and the liquid nitrogen storage tank, inject liquid nitrogen into the liquid nitrogen storage tank, and close all valves after liquid nitrogen flows out of the 24th control valve; S7. When the pipeline is cold: close all valves and open the 16th and 26th control valves; S8. Sample fracturing: After the fracturing outlet pipe is installed in place, open the first, second, third, fourth, fifth, and sixth control valves on the booster pump device, as well as the sixteenth and twenty-fourth control valves on the liquid nitrogen storage tank; and turn on the acoustic emission device; when carbon dioxide is used as the fracturing gas, it is necessary to turn on the preheater for preheating operation; S9. Fracturing is completed and pressure is reduced; S10. End of experiment, instrument removal and data processing: S10-1. After the sample is fractured, all valves are closed, the twenty-first control valve and the twenty-second control valve are opened, and the liquid nitrogen in the triaxial pressure chamber system is discharged until the pressure is zero, and then the triaxial pressure chamber system is opened to take out the sample and clean the triaxial pressure chamber system; S10-2. The pressure at each location, the triaxial pressure loading pump and the acoustic emission device, and the flow data collected in real time by the control system during the fracturing process are displayed and imported into the data processing software; S10-3. Open or export the test data from WORD or EXCEL.

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