In-situ low-permeability reservoir pressure-drive-permeation integrated test device and method

Abstract

The application discloses a kind of waterless fracturing in-situ low-permeability reservoir pressure-drive-permeation integrated test device and method, the device includes triaxial pressure chamber system, fracturing system and seepage system, the fracturing system includes liquid nitrogen storage tank, self-pressurized liquid nitrogen tank and booster device, the self-pressurized liquid nitrogen tank is connected with liquid nitrogen storage tank by injection pipe, and the port of injection pipe is stretched to the inner bottom of liquid nitrogen storage tank;Liquid nitrogen storage tank inside is connected with booster device and is connected with the fracturing fluid inlet of triaxial pressure chamber system by corresponding control valve, and fracturing fluid inlet is connected with fracturing pipe in triaxial pressure chamber system;The bottom outlet of liquid nitrogen storage tank is connected with the fracturing fluid inlet of triaxial pressure chamber system by corresponding control valve, preheater connection.The application uses liquid nitrogen as cryogenic fluid, to large displacement and high pressure fracturing coal sample, to simulate the process of coal rock rupture in liquid nitrogen fracturing process under low temperature condition, monitoring process and seepage process under in-situ condition.

Description

Technical Field

[0001] This invention belongs to the field of waterless fracturing technology, specifically relating to an integrated test device and method for in-situ low-permeability reservoir fracturing, including pressure-drive-permeability testing. Background Technology

[0002] Reservoir permeability is a key factor limiting coalbed methane (CBM) production. Low permeability and low pressure are common characteristics of coal reservoirs in my country, with permeability decreasing exponentially with depth. This results in low and rapid decline in single-well CBM production in my country, making permeability enhancement and production increase reliant on hydraulic fracturing to connect the wellbore with natural fractures. How to perform hydraulic fracturing on low-permeability reservoirs to enhance permeability and production is a major industry challenge in CBM development. Hydraulic fracturing is currently the most commonly used production enhancement technology in CBM development, but it faces increasingly prominent problems, including huge water consumption, the presence of large amounts of chemical additives in the fracturing fluid, and the potential for artificial earthquakes that could cause property damage.

[0003] Anhydrous fracturing technology involves injecting inert fluids (such as liquid nitrogen, CO2, etc.) into coal seams to replace conventional hydraulic fracturing fluids and generate complex fracture networks. Because it has perfect compatibility with coal seams and does not react with minerals in the coal, and leaves no liquid residue after gasification, it avoids reservoir damage such as water sensitivity and water lock. Furthermore, it features low rock-breaking pressure, complex fracture networks, and synergistic effects of pressurization and displacement, and is expected to become a new technology for future reservoir stimulation.

[0004] Waterless fracturing technology, represented by liquid nitrogen fracturing, represents the future direction of technological development. As early as the 1990s, scholars proposed using the ultra-low temperature (-196℃), high compressibility ratio (1:696), high frost heave (up to 207MPa), and low viscosity characteristics of liquid nitrogen to inject high-pressure, high-volume liquid nitrogen into coal seams to replace conventional water-based fracturing fluids for fracturing and transforming low-permeability, dry reservoirs. In engineering, liquid nitrogen fracturing refers to the process of pumping liquid nitrogen to the bottom of the well at conventional fracturing rates and ultra-low temperature conditions (-195.56~-180.44℃) using appropriate equipment and processes to create artificial fractures in the formation. Nitrogen is chemically stable and completely compatible with the reservoir, without water sensitivity or water-locking damage, and is expected to fundamentally solve the reservoir damage and environmental pollution problems caused by hydraulic fracturing. Mcdaniel et al. conducted field tests on liquid nitrogen fracturing technology, using low-temperature resistant glass fiber as the fracturing string. They completed liquid nitrogen fracturing operations on a total of 5 wells, including 4 coalbed methane wells and 1 low-permeability sandstone well, with initial daily production increases of 1.22 to 6.48 times after fracturing. Grundmann et al. applied liquid nitrogen fracturing to a production well in the Devonian shale formation using fiberglass tubing and a stainless steel wellhead. Compared with nitrogen fracturing in adjacent wells, the liquid nitrogen-fracturing well had an approximately 8% higher production increase. These engineering tests confirm the practical feasibility of using liquid nitrogen to replace water-based fracturing fluid. In comparison, the main technical advantages of liquid nitrogen fracturing include:

[0005] (1) Liquid nitrogen cryogenic fracturing of rocks deteriorates the mechanical properties of rocks, significantly reduces the fracturing pressure of reservoir rocks, and increases the fracture length;

[0006] (2) The cryogenic fracturing effect of liquid nitrogen can induce the formation and propagation of secondary fractures in rocks. Its low viscosity enhances its ability to create long fractures, increases the complexity and connectivity of the fracture network, and improves the reservoir drainage area.

[0007] (3) The temperature difference between liquid nitrogen and reservoir rock is huge, generating extremely strong thermal stress (temperature difference 200℃+) and vaporization pressurization effect (liquid-to-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 of the fracture, preventing post-pressure fracture closure, and improving the conductivity of the fracture.

[0008] (4) Nitrogen has stable chemical properties, which can effectively avoid reservoir clay swelling and water lock, and the backflow is thorough and pollution-free, solving the problem of reservoir damage caused by traditional hydraulic fracturing. Given the potential advantages of liquid nitrogen fracturing, this field has been the focus of in-depth research by many scholars. At the same time, supercritical CO2, due to its efficient displacement performance and carbon burial effect, is also being studied vigorously.

[0009] In recent years, significant progress has been made in fracturing coal and rock masses using liquid nitrogen (including supercritical CO2) in the laboratory, demonstrating the good modification effect and application potential of anhydrous fluid fracturing. After liquid nitrogen is injected into the coal body, it deteriorates the mechanical properties of the rock and generates a dense fracture network. Gas diffusion and permeability are significantly improved (10-20 times), while also having a dual effect of pressurization and displacement. However, practical engineering applications have revealed problems with liquid nitrogen fracturing of coal seams, including low heat exchange efficiency (cooling rate of only 0.4-1.2℃ / min), excessively long injection and cooling fracturing times (injection time can reach tens of hours or even days), and the degree of fracture modification and permeability enhancement falling far short of expectations. Supercritical CO2 injection also suffers from a sharp decrease in permeability due to coal expansion. In-situ anhydrous fracturing methods all face a series of problems such as injection failure and poor permeability enhancement efficiency. Therefore, how to inject low-temperature fluids into coal and rock for fracturing is crucial. While liquid nitrogen fracturing has proven effective in the laboratory, its practical application has been less successful, limiting the effectiveness of waterless fracturing. This is mainly due to two factors: first, the cryogenic temperature of liquid nitrogen (-176℃) prevents the simulation of reservoir conditions in the laboratory, amplifying the experimental results; second, the lack of appropriate pressure simulation equipment and a reliable triaxial pressure chamber system restricts current waterless fracturing methods to temperature and supercritical conditions. Previous liquid nitrogen fracturing research has largely focused on nitrogen or high-pressure nitrogen fracturing, but the most significant reservoir modification effect is actually the fracturing effect of fluids, which is crucial for creating long fractures and increasing conductivity area—effects that cannot be simulated or achieved in the laboratory. Therefore, waterless fracturing technology, represented by cryogenic liquid nitrogen fracturing, currently lacks a reliable triaxial pressure chamber system. There is an urgent need for a test system for cryogenic waterless fracturing of coal and rock reservoirs, aiming to provide a new engineering approach for the technology of liquid nitrogen fracturing in low-permeability reservoirs. Summary of the Invention

[0010] To address the shortcomings of existing technologies, this invention aims to provide an integrated in-situ hydraulic-displacement-permeability testing device for waterless fracturing in low-permeability reservoirs. This invention uses liquid nitrogen as a cryogenic fluid to fracture coal samples with high displacement and high pressure, thereby realistically simulating the coal and rock fracturing process, temperature conduction, and monitoring process during liquid nitrogen fracturing under cryogenic conditions. This not only provides guidance for coalbed methane development projects but also supports carbon burial and underground gas extraction technologies, and has broad application prospects.

[0011] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0012] An integrated in-situ hydraulic-flooding-permeability testing device for waterless fracturing in low-permeability reservoirs includes a triaxial pressure chamber system, a fracturing system, and a permeation system. The fracturing system includes a liquid nitrogen storage tank, a self-pressurizing liquid nitrogen tank, and a pressurization device. The self-pressurizing liquid nitrogen tank is equipped with a seventeenth control valve and a first pressure gauge. The self-pressurizing liquid nitrogen tank is connected to the liquid nitrogen storage tank via an injection pipe equipped with a fifteenth control valve, and the port of the injection pipe extends to the bottom of the liquid nitrogen storage tank. Inside the liquid nitrogen storage tank, the pressurization device and an eleventh control valve are connected via a twelfth control valve. The eleventh control valve is connected to the fracturing fluid inlet of the triaxial pressure chamber system, and the fracturing fluid 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 fluid inlet of the triaxial pressure chamber system via a sixteenth control valve and a preheater.

[0013] The triaxial pressure chamber system contains the test sample; the inner end of the fracturing tube in the sample placed in the triaxial pressure chamber system is closed and has a side hole; the triaxial pressure chamber system is equipped with a heating device and an acoustic emission device.

[0014] The seepage system includes a backpressure system and a standard chamber system. The backpressure system includes a backpressure valve, a backpressure pump, and a gas-liquid separator. The backpressure pump is connected to the backpressure valve in sequence through the eighteenth control valve, the backpressure gauge, and the backpressure container. The backpressure valve is connected to the seepage port on the corresponding loading axis of the triaxial 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 backpressure valve. A dryer is connected to the gas port above the gas-liquid separator, and a nineteenth control valve is installed at the liquid port below.

[0015] Preferably, the fracturing tube includes a pre-embedded tube and a fracturing outlet tube. The pre-embedded tube is placed in the test sample, with its lower end closed and a side hole opened in its side wall. The lower end of the fracturing outlet tube is sleeved and fixedly connected to the pre-embedded tube, and its upper end is connected to the sample communication port on the pressure plate of the triaxial pressure chamber system.

[0016] Preferably, the fracturing outlet tube is an annular sleeve, which includes an outer tube and an inner tube. The lower end of the outer tube is connected to the pre-embedded tube, and the inner tube is coaxially sleeved on the upper end of the outer tube. The upper end of the inner tube is connected to the sample communication port.

[0017] Preferably, the liquid nitrogen storage tank includes a tank body and a vacuum insulation layer. The vacuum insulation layer is disposed around the periphery of the tank body and forms a vacuum insulation cavity. The injection pipe extends from above and is disposed inside the lower part of the tank body. A pressurization device is connected to the top of the tank body. The vacuum insulation cavity is connected to a seventh control valve on a buffer tank through a tenth control valve and a ninth control valve. A vacuum pump and a vacuum pressure monitoring gauge are also connected to the buffer tank. The seventh control valve is also connected to the fracturing liquid inlet of the triaxial pressure chamber system through an eighth control valve.

[0018] Preferably, the pressurization device includes a gas booster pump, a nitrogen cylinder, and an air compressor. The nitrogen cylinder is connected to the inlet of the gas booster pump via a first control valve, and a cylinder pressure monitoring gauge is also connected to the nitrogen cylinder. The air compressor is also connected to the inlet of the gas booster pump via the first control valve and the driving pressure monitoring gauge. The outlet of the gas booster pump is connected to the twelfth control valve of the liquid nitrogen storage tank via a third control valve, a fifth control valve, a pressure regulating valve, and an outlet valve. A storage tank pressure monitoring gauge is also connected between the third and fifth control valves, and a high-pressure gas storage tank is connected via a fourth control valve.

[0019] Preferably, the triaxial pressure chamber system includes a body, six hydraulic cylinders and pressure plates. The chambers on the six sides of the body are perpendicular to each other and connected in the middle. A hydraulic cylinder is installed at each of the six chambers, and a corresponding pressure plate is installed on the piston rod of the hydraulic cylinder. The test sample is squeezed in the center of the chamber by the six pressure plates. The fracturing tube is connected to the sample communication port on the top pressure plate and is placed in the test sample.

[0020] Preferably, the inner edge of the pressure plate is chamfered; the pressure plate in the X and Y axes is provided with multiple acoustic emission test ports for mounting acoustic emission devices; the heating device includes electric heating rods, all of which are disposed inside the pressure plate.

[0021] Preferably, the back pressure valve includes a valve body and a floating piston. The floating piston is slidably installed in the movable cavity of the valve body. The back pressure control fluid inlet is connected to one end of the movable cavity, and the other end of the movable cavity is connected to the displacement fluid inlet and the displacement fluid outlet. The floating piston is provided with a valve needle that cooperates with the displacement fluid outlet.

[0022] The upper pressure plate is provided with a fracturing fluid inlet and a sample connection port. The fracturing fluid inlet is located on one side of the upper pressure plate, and the sample connection port is located at the center of the end face of the pressure plate. The fracturing fluid inlet connects to the sample connection port. The left and right pressure plates are respectively provided with a seepage fluid inlet and a seepage fluid outlet, and the lower pressure plate is also provided with a seepage fluid outlet. The seepage fluid inlet and seepage fluid outlet are respectively connected to the displacement fluid outlet and the displacement fluid inlet.

[0023] Preferably, the standard chamber system includes a standard chamber cavity, which is connected to an external gas interface via a fourteenth control valve. The external gas interface is also connected to the fracturing liquid inlet of the triaxial pressure chamber system via a thirteenth control valve and a twenty-eighth control valve, respectively. The thirteenth control valve is an external gas cylinder opening connection valve and is used for seepage interface control.

[0024] The triaxial pressure chamber system also includes three sets of oil supply systems corresponding to the x-axis, y-axis and z-axis respectively. The three oil supply systems are respectively connected to two hydraulic cylinders symmetrically arranged in each axis. The oil supply system includes an oil supply device, which is connected to the corresponding depressurization port of the two hydraulic cylinders through the twenty-first control valve and the twenty-second control valve, and connected to the corresponding pressurization port of the two hydraulic cylinders through the twentieth control valve and the twenty-third control valve.

[0025] Accordingly, this invention also proposes an integrated test method for in-situ low-permeability reservoirs using the aforementioned integrated in-situ fracturing-flooding-permeability test device, specifically including the following steps:

[0026] S1. Sample preparation and installation:

[0027] S1-1. Before starting the experiment, turn off all valves; according to the experimental requirements, select a test sample of coal and rock of a certain size and drill a hole in its center to place the fracturing pipe;

[0028] S1-2. Retract the hydraulic cylinder to its maximum size and install the electric heating rod and acoustic emission device in appropriate positions; after the sample is placed in place, start the hydraulic cylinder to move forward into the triaxial pressure chamber system to load the sample; apply axial pressure through the six-sided pressure plates to completely seal from six sides, keeping the center position of the sample unchanged;

[0029] S1-3. Insert the fracturing tube into the center of the sample. During installation, the side hole of the fracturing tube needs to be sealed with a film or tape to prevent the sealant from clogging. Place the pre-embedded tube into the drill hole and seal it with sealant. Let it dry to prepare for the final installation of the fracturing lead tube in the experiment.

[0030] S1-4. Connect each control device in the test apparatus to the computer's control system to synchronize data;

[0031] S2. Vacuuming of instruments: Vacuuming of the liquid nitrogen tank insulation layer, liquid nitrogen tank and fracturing pipeline, opening control valves numbered 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 26 and 28;

[0032] S3. Simulation of in-situ stress and reservoir temperature conditions:

[0033] S3-1. Close all valves, open the servo hydraulic loading control system connected to the hydraulic cylinder, and use the main control console to turn on the servo loading pump to provide hydraulic pressure to drive the hydraulic cylinder;

[0034] S3-2. Select triaxial pressure loading as the loading method and set the triaxial pressure value. Use the set maximum pressure as a reference and maintain pressure stability after reaching the set pressure.

[0035] S3-3. Turn on the temperature and pressure monitoring system for precise temperature control, with a display accuracy of 0.1℃;

[0036] S4. Triaxial heating: Close all valves and turn on the heating device;

[0037] S5. Gas permeability, adsorption, and desorption testing of the seepage system begins:

[0038] S5-1. First open the thirteenth and eighteenth control valves, set the external gas interface pressure, and monitor it using a pressure gauge;

[0039] S5-2. Then open the fourteenth control valve and the twenty-eighth control valve to begin the seepage process;

[0040] S5-3. Observe the back pressure valve pressure. When the outlet pressure equals the inlet pressure, close the eighteenth control valve and open the twenty-seventh, twenty-fifth, seventeenth, and nineteenth control valves. The remaining seepage liquid or gas enters the gas-liquid separator for separation. Finally, close all valves to end the seepage.

[0041] S6. Adding liquid nitrogen: Close all valves, open the seventeenth control valve, the fifteenth control valve and the twenty-fourth 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 twenty-fourth control valve.

[0042] S7. Pipeline encounters cold: Close all valves, open control valve number sixteen and control valve number twenty-six;

[0043] S8. Sample fracturing: After the fracturing lead 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 selected as the fracturing gas, the preheater needs to be turned on for preheating.

[0044] S9. Fracturing completed, pressure released;

[0045] S10. Experiment ends, instrument dismantling and data processing:

[0046] S10-1. After the sample is fracturing, close all valves, open the twenty-first control valve and the twenty-second control valve, drain the liquid nitrogen from the triaxial pressure chamber system until the pressure is zero, open the triaxial pressure chamber system to take out the sample, and clean the triaxial pressure chamber system.

[0047] S10-2. Display and import the pressure, triaxial pressure loading pump and acoustic emission device data collected in real time by the control system in the fracturing process into the data processing software;

[0048] S10-3. Open or export the test data from Word or Excel.

[0049] The beneficial effects of this invention are as follows:

[0050] 1. This invention aims to provide an integrated in-situ hydraulic-displacement-permeability testing device for low-permeability reservoirs under waterless fracturing conditions. Simulating in-situ conditions, with low-temperature fluid fracturing simulation and displacement-permeability enhancement conditions as the core, it can reveal the multiphase and multi-fluid adsorption, transport, and desorption processes from gas-bearing coal to water-CO2 / N2-methane. It examines the fracture propagation, extension, and cross-cutting processes in the reservoir under multiple field conditions such as low temperature, phase transformation, and stress, analyzes the permeability enhancement conditions and effects of low-permeability reservoirs, and explores the displacement effect of water-nitrogen multi-fluids on methane under adsorption equilibrium conditions. This provides new technologies and ideas for permeability enhancement and gas extraction in low-permeability, gas-rich reservoirs.

[0051] 2. The invention features a unique fracturing tube structure that not only discharges the liquid nitrogen that vaporizes in the experimental device during the initial stage of fracturing, but also ensures that no high-pressure nitrogen gas is generated during the fracturing process due to the vaporization of liquid nitrogen from contact with overheated samples, thus ensuring the authenticity and accuracy of liquid nitrogen fracturing.

[0052] 3. The temperature detector of this invention includes two temperature probes vertically mounted on both sides of the fracturing tube. Multiple temperature measurement points are arranged along the length of each temperature probe, and a pressure sensor is connected to the top of the fracturing tube. This invention enables real-time monitoring of temperature and pressure changes in the sample during fracturing, allowing for precise study of the phase transformation fracturing process of cryogenic fluid entering the fracture, as well as the initiation and propagation of the fracture under temperature stress. This provides conditions for studying the initiation and propagation of fractures under temperature stress.

[0053] 4. The triaxial pressure chamber system of this invention provides a true triaxial stress environment that simulates real reservoir conditions. It can also simulate complex fracturing processes such as supercritical CO2 low temperature-high pressure-extraction, and is expected to become a forward-looking technology and equipment for improving permeability and increasing production in low-permeability coal seams. Attached Figure Description

[0054] Figure 1 This is a structural diagram of the experimental apparatus of the present invention;

[0055] Figure 2 This is a structural diagram of the triaxial pressure chamber system of the present invention;

[0056] Figure 3 This is a schematic diagram illustrating the pressurization and depressurization processes of the present invention;

[0057] Figure 4 This is a schematic diagram of the pressure plate structure of the present invention;

[0058] Figure 5 Connection structure diagram of the fracturing tube of the present invention;

[0059] Figure 6 This is a structural diagram of the liquid nitrogen storage tank of the present invention;

[0060] Figure 7 This is a structural diagram of the seepage and radiation groove on the pressure plate of the present invention;

[0061] Figure 8 This is a schematic diagram of the back pressure valve of the present invention;

[0062] Figure 9 This is a schematic diagram of the installation of the oil supply system of the present invention;

[0063] Figure 10 This is a schematic diagram of the hydraulic cylinder structure of the present invention. Detailed Implementation

[0064] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are for illustrative purposes only and are not intended to limit the scope of the invention.

[0065] like Figure 1-10 As shown, this invention proposes an integrated in-situ hydraulic fracturing-drive-permeability testing device for low-permeability reservoirs without water fracturing. This invention can simulate the effects of pure liquid nitrogen fracturing, ensuring that high-pressure nitrogen gas formed by the vaporization of liquid nitrogen upon contact with overheated samples does not participate in the fracturing process, thus ensuring the authenticity and accuracy of liquid nitrogen fracturing. It has strong compatibility, capable of simulating conventional hydraulic fracturing conditions, and can also simulate multiphase processes in gas-bearing coal bodies, including complex fracturing processes such as supercritical CO2 low-temperature-high-pressure-extraction.

[0066] This invention aims to provide an integrated in-situ hydraulic-displacement-permeability testing device for low-permeability reservoirs using waterless fracturing. Simulating in-situ conditions, with low-temperature fluid fracturing simulation and displacement-permeability enhancement conditions as its core, it can reveal the multiphase and multi-fluid adsorption, transport, and desorption processes from gas-bearing coal to water-CO2 / N2-methane. It examines the fracture propagation, extension, and cross-cutting processes in reservoirs under multiple field conditions including low temperature, phase transformation, and stress. It analyzes the permeability enhancement conditions and effects of low-permeability reservoirs and explores the displacement effect of water-nitrogen multi-fluids on methane under adsorption equilibrium conditions, providing new technologies and ideas for permeability enhancement and gas extraction in low-permeability, gas-rich reservoirs.

[0067] The waterless fracturing in-situ low-permeability reservoir integrated pressure-drive-seepage test device includes a triaxial pressure chamber system 3, a fracturing system and a seepage system. The fracturing system includes a liquid nitrogen storage tank 2, a self-pressurizing liquid nitrogen tank 1 and a pressurization device. The self-pressurizing liquid nitrogen tank 1 is equipped 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 above and is located inside the lower part 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. 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. The buffer tank 51 is also connected to a vacuum pump 52 and a vacuum pressure monitoring gauge 53. The seventh control valve A7 is also connected to the fracturing fluid inlet 311 of the triaxial pressure chamber system 3 through the eighth control valve A8. The fracturing fluid inlet 311 is connected to the fracturing tube in the triaxial pressure chamber system 3. Inside the liquid nitrogen storage tank 2, a pressurization device and an eleventh control valve A11 are connected through the twelfth control valve A12. The eleventh control valve A11 is connected to the fracturing fluid inlet 311 of the triaxial pressure chamber system 3.

[0068] This invention achieves heat insulation by setting a vacuum insulation layer 202 on the outside of the liquid nitrogen storage tank 2, thereby reducing the contact area between the tank body 201 and the outside. In actual installation, a heat insulation device 22 can be installed at the bottom of the liquid nitrogen storage tank 2, which is then mounted on a workbench, further reducing the contact area between the tank and the workbench and minimizing heat transfer. Specifically, the liquid nitrogen storage tank 2 used in this invention can have a volume of 2L and a pressure resistance of 30MPa.

[0069] The triaxial pressure chamber system 3 contains the test sample within its main model body; a heating device and an acoustic emission device are also installed within the main model body of the triaxial pressure chamber system 3; both the heating device and the pressure sensor are electrically connected to the temperature and pressure monitoring system. This invention fully considers the stress pressure reaching 40 MPa and the temperature reaching 150℃ (equivalent to burial at 4 km) during waterless fracturing, with strain testing ranging from 0 to 10 mm and deformation measurement error of ±0.5%FS. The temperature and pressure monitoring system employs an artificial intelligence display temperature controller with PID regulation function.

[0070] The pressurization device includes a gas booster pump, a nitrogen cylinder 41, and an air compressor 43. The nitrogen cylinder 41 is connected to the inlet of the gas booster pump via a first control valve A1, and a cylinder pressure gauge 42 is also connected to the nitrogen cylinder 41. The air compressor 43 is also connected to the inlet of the gas booster pump via a second control valve A2 and a drive pressure monitoring gauge 44. The outlet of the gas booster pump is connected to the twelfth control valve A12 of the liquid nitrogen storage tank 2 via a third control valve A3, a fifth control valve A5, a pressure regulating valve 47, and an outlet valve. 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 via a fourth control valve A4.

[0071] Specifically, the gas booster pump can be an STK brand GB series gas booster pump, mainly used for pressurizing gases such as methane and nitrogen. The selected model is GB60, with a pressure ratio of 60:1, a maximum outlet pressure of 498 Bar, a minimum inlet pressure of 32 Bar, and a maximum displacement of 112 L / min. The power source for the gas booster pump is a low-pressure air source with a maximum output of 0.7 MPa. This invention designs a unique high-pressure displacement device that uses nitrogen to pressurize low-pressure liquid nitrogen, aiming to pressurize the low-pressure liquid nitrogen to a high-pressure state that meets experimental requirements before it enters the fracturing tube.

[0072] The bottom outlet of liquid nitrogen storage tank 2 is connected to the fracturing fluid inlet 311 of triaxial pressure chamber system 3 via the sixteenth control valve A16 and the preheater. The fracturing fluid inlet 311 of triaxial pressure chamber system 3 is also connected to an inlet thermometer 61, an inlet pressure gauge 62 and a safety valve 63.

[0073] The triaxial pressure chamber system 3 contains the test sample; the inner end of the fracturing tube in the test sample placed in the triaxial pressure chamber system 3 is closed and has a side hole 3220; the triaxial pressure chamber system 3 is equipped with a heating device and an acoustic emission device.

[0074] The triaxial pressure chamber system 3 provides a true triaxial stress environment to simulate real reservoir conditions. It can also simulate complex fracturing processes such as supercritical CO2 low temperature-high pressure-extraction, and is expected to become a forward-looking technology and equipment for improving permeability and increasing production in low-permeability coal seams. The triaxial pressure chamber system 3 includes a model host, six hydraulic cylinders and pressure plates 31. The model host has three pressure chambers arranged in the left and right (X-axis), front and back (Y-axis), and up and down (Z-axis). The pressure chambers are perpendicular to each other and connected in the middle. Hydraulic cylinders are installed at the ports of the chambers, and corresponding pressure plates 31 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 31.

[0075] The main model of the triaxial pressure chamber system 3 of this invention is designed with six-sided loading. Hydraulic loading structures apply pressure to all six sides in the X (left-right), Y (front-back), and Z (up-down) directions, and each direction can be independently controlled, enabling a more realistic simulation of the triaxial stress state of underground reservoirs. The main model is equipped with pressure plates 31 of appropriate specifications to maintain the center position of the 300mm×300mm×300mm cubic sample. The stress value at the sample end face is automatically calculated from the hydraulic cylinder area, sample area, and loading hydraulic pressure.

[0076] The triaxial pressure chamber system 3 employs an internal piston loading structure, adopting a pressure vessel type. The sample is installed inside, and piston loading structures are installed around the sample. To ensure measurement accuracy, the stiffness of the deformation measurement components of the model host is ≥10GN / m; the full load and deformation of the deformation measurement components of the model host are ≤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, enabling both synchronous and asynchronous loading. Working in conjunction with servo actuators, it achieves precise force transmission and pressure monitoring feedback.

[0077] like Figure 10 As shown, when the hydraulic cylinder starts to pressurize, oil enters from the inlet end and pushes the piston to squeeze the outlet end; when depressurization begins, oil enters from the outlet end and squeezes the piston to release the oil.

[0078] The triaxial pressure chamber of this invention has a bearing capacity of 40 MPa, a triaxial loading pressure of 40 MPa, a control accuracy of 5%, a maximum test force of 2000 KN, an effective test force measurement range of 1–100 F.S, a test force test error of ±1%, a test force resolution of 1 / 200000, a deformation range of 0–10 mm, and a deformation measurement error of ±0.5% FS. This invention enables rapid and easy installation of the specimen; it can uniformly apply triaxial pressure to the specimen surface, effectively eliminating shear stress generated on the specimen surface due to compressive deformation.

[0079] The main body of the triaxial pressure chamber system 3 of this invention is made of high-strength steel and equipped with 6 sets of hydraulic cylinders. The maximum force is 40MPa (for a 300mm×300mm×300mm rock sample), with an error of ±0.1MPa. This enables the invention to achieve true triaxial loading control and ensures stability during the loading process.

[0080] The stress loading design for the model end face utilizes a double-acting hydraulic cylinder, with a reverse-acting hydraulic cylinder added at the rear of the main hydraulic cylinder, corresponding to its position. The reverse-acting hydraulic cylinder retracts the main hydraulic cylinder chamber, creating a larger gap between the pressure plate 31 and the core, allowing for rapid core removal. To meet the requirements of large-strain experiments, the piston stroke of the six-sided hydraulic cylinder is designed to be 10mm. This ensures that the maximum strain during the maximum sample loading deformation process is ≥5%. The double-acting and reverse-acting hydraulic cylinders are standard applications in this field, and their principles will not be elaborated upon here.

[0081] All hydraulic cylinders of this invention are made of high-strength steel. This material has high strength and toughness, good hardenability, no obvious temper brittleness, and after solution treatment, it has a high fatigue limit and resistance to repeated impacts, and good low-temperature impact toughness.

[0082] The displacement sensor installed on the hydraulic cylinder is a magnetostrictive displacement sensor, capable of detecting the displacement of the piston rod. It exhibits high resistance to impact and vibration. The selected specifications are: measuring range: 0-30mm (continuous); measurement accuracy: ±0.05%FS; measurement resolution: 0.001mm.

[0083] The pressure plates 31 on the top, bottom, and sides are made of high-rigidity mold steel to prevent deformation of the pressure plates 31 from causing measurement errors in displacement. The pressure plates 31 have approximately 10mm chamfers around their perimeter near the core sample end to prevent collisions between the six pressure plates 31 due to core deformation. The pressure plates 31 in the X-axis (left-right) and Y-axis (front-back) directions are equipped with acoustic emission test ports 317 for mounting acoustic emission devices. Specifically, each pressure plate 31 in this application has four acoustic emission test ports 317.

[0084] The pressure plate 31 along the X-axis (left-right), Y-axis (front-back), and Z-axis (up-down) directions is equipped with seepage ports and seepage radiation channels 3102. The seepage ports are connected to an external carbon dioxide, methane, or nitrogen supply system. The seepage radiation channels 3102 are located on the end face of the pressure plate 31, are concentrically arranged with the pressure plate 31, and are multiple circular designs with different diameters.

[0085] The pressure plate 31 is equipped with a fluid inlet / outlet, a pressure control interface for the fracturing fluid at the end face, and a seepage interface. The pressure control interface and the seepage interface are connected to the corresponding fluid inlet / outlet on the pressure plate 31. All the fluid inlet / outlet, the pressure control interface for the fracturing fluid at the end face, and the seepage interface use through-plate connectors with internal and external quick couplings for easy assembly and disassembly. A fracturing interface is located on the top of the pressure plate 31 in the Z-axis direction, which connects to and installs the matching fracturing tubing.

[0086] The pressure injection port of the fracturing fluid has an opening size of φ6mm, and the seepage port on the pressure plate 31 also has an opening size of 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 on the main body of the model is within a safe and effective range, meeting the pressure requirements.

[0087] The heating device includes electric heating rods, all of which are housed within the pressure plate 31. The temperature loading of the sample is entirely achieved through the pressure plate 31 surrounding the core. Three electric heating rods are machined at appropriate positions on the pressure plate 31. The electric heating rods are powered by 220V, have a diameter of 14mm, a total power of 1kW, and a maximum heating temperature of 200℃. The temperature control system controls the heating of the electric heating rods in the external flexible heating jacket, adjusting the temperature within the triaxial chamber and the system's heating power. The heating signal is transmitted through a high-precision communication instrument, thus achieving a multi-functional control method combining digital control and panel-type control.

[0088] The triaxial pressure chamber system 3 loads the three axes independently, ensuring the sample remains centered. Whether force-controlled or position-controlled, this significantly increases experimental flexibility. The true triaxial loading control software enables triaxial pressure setting, control, and display. Simultaneous loading across the three axes can achieve constant pressure loading, loading at a specific rate (pressure gradient loading), and pressure tracking control loading. Regardless of the loading method, the system maintains stable pressure after reaching the set maximum pressure, with a pressure control accuracy of 0.01 MPa.

[0089] 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 via the eighteenth control valve A18, a back pressure gauge 74, and a back pressure container 73. The back pressure valve 72 is connected to the seepage port on the corresponding loading axis of the triaxial pressure chamber system 3 via the twenty-seventh control valve A27, the seventeenth control valve A17, and the twenty-fifth control valve A25. The gas-liquid separator 76 is connected to the back pressure valve 72. A dryer 77 is connected to the gas port above the gas-liquid separator 76, and a nineteenth control valve A19 is installed at the liquid port below.

[0090] The standard chamber system includes a standard chamber cavity 71, which is connected to an external gas interface via a fourteenth control valve A14. The external gas interface is also connected to the eleventh control valve A11 via a thirteenth control valve A13 and to the fracturing fluid inlet 311 of the triaxial pressure chamber system 3 via a twenty-eighth control valve A28. The thirteenth control valve A13 is an external gas cylinder opening valve used for seepage interface control.

[0091] 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, with a pressure accuracy of 0.1%FS. The back pressure control is accurate and reliable, and it has an alarm function. The back pressure pump 75 is connected to the triaxial pressure chamber system 3 through the back pressure valve 72. The gas-liquid separator 76 is connected to the back pressure valve 72, and a drying tank is connected to the gas-liquid separator 76. The connection between the gas-liquid separator 76 and the drying tank and the back pressure valve 72 is a conventional application in this field, and its principle will not be described in detail here.

[0092] The back pressure valve 72 includes a valve body and a floating piston 722. The floating piston 722 is slidably mounted in the movable cavity of the valve body. The back pressure control fluid inlet 721 is connected to one end of the movable cavity, and the other end of the movable 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 cooperates with 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, causing the valve needle 725 to block the displacement fluid outlet 724 to prevent fluid flow. When the system pressure is higher than the top pressure, the valve needle 725 separates from the displacement fluid outlet 724, and the overpressured fluid flows out. When the flow is continuous, these two processes cycle continuously to maintain the balance between the seepage system pressure and the set value at the top.

[0093] In this application, the pressure plate 31 directly above the Z-axis is provided with a fracturing fluid inlet 311 and a sample connection port 312. The fracturing fluid inlet 311 is located on one side of the pressure plate 31, and the sample connection port 312 is located at the center of the end face of the pressure plate 31. The fracturing fluid inlet 311 connects to the sample connection port 312. The two pressure plates 31 on the left and right sides of the X-axis are respectively provided with a seepage fluid inlet 313 and a seepage fluid outlet 314. The pressure plate 31 directly below the Z-axis is also provided with a seepage fluid outlet 314. The seepage fluid inlet 313 and seepage fluid outlet 314 on the pressure plates 31 are respectively connected to the displacement fluid outlet 724 and the displacement fluid inlet 723.

[0094] At the start of seepage, seepage liquid flows into the left pressure plate 31, setting the inlet pressure, and the right seepage outlet is connected to the back pressure pump 75. Fluid flow begins the seepage process. After seepage saturation, when the seepage system meets system requirements, the outlet pressure exceeds the pressure of the back pressure valve 72. Remaining seepage liquid or gas enters the gas-liquid separator 76 through the bottom pressure plate.

[0095] This invention achieves high-precision outlet pressure control through the design of a high-precision backpressure valve 72. Pressure adjustment range: 0–50 MPa; applicable temperature range: 0–200℃; accuracy: 0.1%FS; pressure control precision reaches ±0.1 MPa.

[0096] The fracturing tube is placed in the test sample within the triaxial pressure chamber system 3. Its inner end is closed and has a side hole 3220. Specifically, the fracturing tube includes a pre-embedded tube 322 and a fracturing outlet tube 321. The pre-embedded tube 322 is placed in the test sample, its lower end is closed, and a side hole 3220 is formed in its side wall. The lower end of the fracturing outlet tube 321 is sleeved and fixedly connected to the pre-embedded tube 322, and its upper end is connected to the sample communication port 312. The fracturing outlet tube 321 is an annular sleeve, comprising an outer tube and an inner tube (the inner tube has a diameter of φ8mm, the outer tube has a diameter of φ9mm, and a length of 20cm). The lower end of the outer tube is threaded to the pre-embedded tube 322, and its upper end is closed. The inner tube is coaxially sleeved and extends through the upper end of the outer tube. The annular bushing not only removes liquid nitrogen that has vaporized in the experimental setup during the initial stage of fracturing, but also ensures that no high-pressure nitrogen gas is generated during the fracturing process due to the vaporization of liquid nitrogen from contact with overheated samples, thus ensuring the authenticity and accuracy of liquid nitrogen fracturing.

[0097] For single-well segmented fracturing wells: the pre-embedded pipe 322 is designed as a multi-segment combination. The simulated well wall, support section, sealing ring, etc. are first glued into the already drilled fracturing well in sequence. The fracturing lead-out pipe 321 can be used in conjunction with the adjustment pad to realize single-well segmented fracturing.

[0098] The triaxial pressure chamber system 3 also includes three sets of oil supply systems corresponding to the x-axis, y-axis and z-axis respectively. The three oil supply systems are respectively connected to two hydraulic cylinders symmetrically arranged in each axis. The oil supply system includes an oil supply device 33, which is connected to the pressure relief ports A and B of the two hydraulic cylinders through the twenty-first control valve A21 and the twenty-second control valve A22, and is connected to the pressure application ports A and B of the two hydraulic cylinders through the twenty-tenth control valve A20 and the twenty-third control valve A23.

[0099] The triaxial pressure chamber system 3 of this invention is connected to the fracturing test control system. The fracturing test control system can perform parameter presets, automatic measurements, manual control, automatic control, data processing, and judgment for the testing machine. It has test data storage and retrieval functions; test data can be opened or exported using WORD or EXCEL, and can be 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 equipment, control the triaxial stress loading and various parameters of the fracturing process, and collect and record triaxial pressure loading, fracturing parameters, flow rate, and other parameters in real time during the test. After the experiment, the data is exported in Excel format.

[0100] The fracturing test control system is mainly divided into two parts: true triaxial loading control and fracturing test.

[0101] The true triaxial loading control section 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 section mainly includes setting fracturing parameters and detecting fracturing fractures (if necessary), etc.

[0102] The true triaxial loading control unit can realize triaxial 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 can be maintained stable after reaching the set maximum pressure, with a pressure control accuracy of 0.01MPa.

[0103] The fracturing test section includes a calculation and analysis module, a graphical output module, a Word report output module, and an online help module, featuring a user-friendly interface for ease of use. This section is primarily used for data acquisition and control of pressure at various points in the control system, triaxial pressure loading pumps, and flow rate data. It can acquire data from each node in the process in real time, synchronize with a computer, and display the data on the data processing software. The fracturing test section can run in a Windows environment using LabVIEW programming. The instrument's workflow is displayed on the interface, enabling human-machine interaction. After the operator sets the parameters, unattended operation is achieved, with automatic acquisition of all pressure, flow rate, and other parameters.

[0104] The fracturing test section can preset parameters, perform automatic measurement, manual control, automatic control, data processing, and judgment for the testing machine. It has test data storage and retrieval functions; test data can be opened or exported using WORD or EXCEL, and can be opened or processed by other relevant control software. It can process test data, select calculation items, and automatically generate and print test report formats.

[0105] Accordingly, based on the above-mentioned waterless fracturing in-situ low-permeability reservoir integrated pressure-drive-permeability test device, the present invention also proposes a waterless fracturing in-situ low-permeability reservoir integrated pressure-drive-permeability test method, which specifically includes the following steps:

[0106] S1. Sample preparation and installation:

[0107] S1-1. Before starting the experiment, turn off all valves; according to the experimental requirements, select a test sample of coal and rock of a certain size (a large piece of coal and rock with a sample size of 30cm×30cm×30cm can be selected), and drill a hole in the center of it (the hole size is φ10mm) to place the fracturing pipe.

[0108] S1-2. Retract the hydraulic cylinder to its maximum size and install the electric heating rod and acoustic emission device in appropriate positions; after the sample is placed in place, start the hydraulic cylinder to move forward into the triaxial pressure chamber system 3 to load the sample; apply axial pressure through the six-sided pressure plates 31 to completely seal from six sides, keeping the center position of the sample unchanged;

[0109] S1-3. Insert the fracturing tube into the center of the sample. During installation, the side hole 3220 of the fracturing tube needs to be sealed with a film or tape to prevent the sealant from clogging. Place the pre-embedded tube 322 into the drill hole and seal it with sealant. Let it dry to prepare for the final installation of the fracturing lead tube 321 in the experiment.

[0110] S1-4. Connect each control device in the test apparatus to the computer's control system to synchronize data;

[0111] S2. Vacuuming of instruments: Vacuuming of the liquid nitrogen tank insulation layer, liquid nitrogen tank and fracturing pipeline, opening control valves numbered 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 26 and 28;

[0112] S3. Simulation of in-situ stress and reservoir temperature conditions:

[0113] S3-1. Close all valves, open the servo hydraulic loading control system connected to the hydraulic cylinder, and use the main control console to turn on the servo loading pump to provide hydraulic pressure to drive the hydraulic cylinder;

[0114] S3-2. Select triaxial pressure loading as the loading method and set the triaxial pressure value. Use the set maximum pressure as a reference and maintain pressure stability after reaching the set pressure.

[0115] S3-3. Turn on the temperature and pressure monitoring system for precise temperature control, with a display accuracy of 0.1℃;

[0116] S4. Triaxial heating: Close all valves and turn on the heating device;

[0117] S5. Gas permeability, adsorption, and desorption testing of the seepage system begins:

[0118] S5-1. First open the thirteenth control valve A13 and the eighteenth control valve A18, set the external gas interface pressure, and monitor it using a pressure gauge;

[0119] S5-2. Then open the fourteenth control valve A14 and the twenty-eighth control valve A28 to begin the seepage process;

[0120] S5-3. Observe the back pressure valve pressure. When the outlet pressure equals the inlet pressure, close the eighteenth control valve A18 and open the twenty-seventh control valve A27, the twenty-fifth control valve A25, the seventeenth control valve A17, and the nineteenth control valve A19. The remaining seepage liquid or gas enters the gas-liquid separator 76 for separation. Finally, close all valves to end the seepage.

[0121] S6. Adding liquid nitrogen: Close 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 close all valves after liquid nitrogen flows out of the twenty-fourth control valve A24.

[0122] S7. Pipeline encounters cold: Close all valves, open control valve A16 and control valve A26;

[0123] S8. Sample fracturing: After installing the fracturing lead pipe 321, open 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; and turn on the acoustic emission device; when carbon dioxide is selected as the fracturing gas, the preheater needs to be turned on for preheating.

[0124] S9. Fracturing completed, pressure released;

[0125] S10. Experiment ends, instrument dismantling and data processing:

[0126] S10-1. After the sample is fracturing, close all valves, open the twenty-first control valve A21 and the twenty-second control valve A22, drain the liquid nitrogen from the triaxial pressure chamber system 3 until the pressure is zero, open the triaxial pressure chamber system 3 to take out the sample, and clean the triaxial pressure chamber system 3.

[0127] S10-2. Display and import the pressure, triaxial pressure loading pump and acoustic emission device data collected in real time by the control system in the fracturing process into the data processing software;

[0128] S10-3. Open or export the test data from Word or Excel.

[0129] This invention boasts strong equipment compatibility and is specifically designed for cryogenic liquid nitrogen fracturing. It is applicable to various technologies such as supercritical CO2 fracturing, hydraulic fracturing, and water-liquid nitrogen composite fracturing. It can simulate the unique temperature and pressure environment of coal and rock, achieving controllable temperature and pressure and real-time data acquisition and recording. The equipment covers cryogenic-high pressure to high-temperature-high pressure environments, enabling the detection and analysis of multiphase and multi-flow gas components.

[0130] This invention has the advantages of strong compatibility and wide application, and can perform the following functions:

[0131] (1) The present invention utilizes an acoustic emission device to perform real-time and stable monitoring of temperature and crack initiation-propagation process during the supercritical fluid fracturing process of coal and rock;

[0132] (2) The present invention utilizes a permeation system to effectively detect gas permeability, adsorption and desorption processes in high-pressure, wide-range temperature-varying environments (between -196℃ and 150℃);

[0133] (3) The present invention is capable of detecting and analyzing multiphase and multifluid gas components in low-temperature-high-pressure to high-temperature-high-pressure displacement environments;

[0134] (4) This 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;

[0135] (5) The present invention can realize the special environmental setting of coal and rock temperature and pressure conditions and realize the controllability of environmental temperature and pressure conditions, and can realize real-time data acquisition, recording and curve display.

[0136] Obviously, the embodiments described above are only some embodiments of this application, not all embodiments. The accompanying drawings show preferred embodiments of this application, but do not limit the patent scope of this application. This application can be implemented in many different forms; rather, the purpose of providing these embodiments is to provide a more thorough and comprehensive understanding of the disclosure of this application. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing specific embodiments, or make equivalent substitutions for some of the technical features. Any equivalent structures made using the content of this application's specification and drawings, directly or indirectly applied to other related technical fields, are similarly within the scope of patent protection of this application.

Claims

1. An integrated in-situ hydraulic-flooding-seepage testing device for low-permeability reservoirs without water fracturing, comprising 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-pressurized liquid nitrogen tank, and a pressurizing device, wherein the self-pressurized liquid nitrogen tank is provided with a seventeenth control valve and a first pressure monitoring gauge, the self-pressurized liquid nitrogen tank is connected with the liquid nitrogen storage tank through an injection pipe, the injection pipe is provided with a fifteenth control valve, and the end of the injection pipe extends to the inner bottom of the liquid nitrogen storage tank; the inside of the liquid nitrogen storage tank is connected with the pressurizing device and an eleventh control valve through a twelfth control valve, the eleventh control valve is connected with a fracturing liquid inflow port of a triaxial pressure chamber system, and the fracturing liquid inflow port is connected with a fracturing pipe in the triaxial pressure chamber system; the bottom outlet of the liquid nitrogen storage tank is connected with the fracturing liquid inflow port of the triaxial pressure chamber system through a sixteenth control valve and a preheater; The triaxial pressure chamber system is provided with a test sample; the end of the fracturing pipe in the test sample in the triaxial pressure chamber system is closed and provided with a side hole, and the triaxial pressure chamber system is provided with a heating device and an acoustic emission device; The seepage system comprises a back pressure system and a standard chamber system, wherein the back pressure system comprises a back pressure valve, a back pressure pump, and a gas-liquid separator, the back pressure pump is connected with the back pressure valve through a eighteenth control valve, a back pressure pressure gauge, and a back pressure container in sequence, the back pressure valve is connected with seepage ports on corresponding loading shafts of the triaxial pressure chamber system through a twenty-seventh control valve, the seventeenth control valve, and a twenty-fifth control valve respectively; the gas-liquid separator is connected with the back pressure valve, a drier is connected with a gas inlet of the gas-liquid separator, and a nineteenth control valve is arranged on a liquid outlet of the gas-liquid separator; The fracturing pipe comprises a pre-embedded pipe and a fracturing leading pipe, the pre-embedded pipe is arranged in the test sample, the lower end of the pre-embedded pipe is closed, and a side hole is arranged on the side wall of the pre-embedded pipe; the lower end of the fracturing leading pipe is fixedly connected with the pre-embedded pipe in a sleeved mode, and the upper end of the fracturing leading pipe is connected with a test sample communication port on a triaxial pressure chamber system pressure plate; The fracturing leading pipe is an annular casing pipe, the annular casing pipe comprises an outer pipe and an inner pipe, the lower end of the outer pipe is connected with the pre-embedded pipe, the inner pipe is coaxially arranged on the upper end of the outer pipe, and the upper end of the inner pipe is connected with the test sample communication port; the diameter of the inner pipe is φ8 mm, the diameter of the outer pipe is φ9 mm, and the length of the outer pipe is 20 cm; The liquid nitrogen storage tank comprises a storage tank body and a vacuum heat insulation layer, the vacuum heat insulation layer is arranged around the periphery of the storage tank body and forms a vacuum heat insulation cavity; the injection pipe extends into the inside of the storage tank body from the upper side and is arranged below the inside of the storage tank body, and the upper side of the storage tank body is connected with the pressurizing device; the vacuum heat insulation cavity is connected with a seventh control valve on a buffer tank through a tenth control valve and a ninth control valve, the buffer tank is further connected with a vacuum pump and a vacuum pressure monitoring gauge, and the seventh control valve is further connected with the fracturing liquid inflow port of the triaxial pressure chamber system through an eighth control valve; The pressurizing device comprises a gas pressurizing pump, a nitrogen cylinder, and an air compressor, the nitrogen cylinder is connected with the gas inlet end of the gas pressurizing pump through a first control valve, and the nitrogen cylinder is further connected with a cylinder pressure monitoring gauge; the air compressor is also connected with the gas inlet end of the gas pressurizing pump through a second control valve and a driving pressure monitoring gauge in sequence; the gas outlet end of the gas pressurizing pump is connected with 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; the storage tank pressure monitoring gauge and a high-pressure gas storage tank connected through a fourth control valve are further connected between the third control valve and the fifth control valve. The power source of the gas booster pump is selected as a low-pressure air source with a maximum output of 0.7 MPa; The triaxial pressure chamber system comprises a body, six hydraulic cylinders and a pressing plate, six chambers arranged on the body are perpendicular to each other and are connected together at the middle part, the six chambers are respectively provided with the hydraulic cylinders, the piston rods of the hydraulic cylinders are respectively provided with the corresponding pressing plates, and the test sample is pressed in the central chamber by the six pressing plates; the fracturing pipe is connected with the sample communication port on the top pressing plate and is arranged in the test sample; The inner side edge of the pressing plate is correspondingly provided with a chamfer; a plurality of acoustic emission test ports for mounting acoustic emission devices are arranged on the pressing plate in the X-axis and Y-axis directions; the heating device comprises an electric heating rod, and the electric heating rod is arranged in the pressing plate; The back pressure valve comprises a valve body and a floating piston, the floating piston is slidably arranged in a movable cavity in the valve body, one end of the movable cavity is connected with the back pressure control fluid inlet, the other end of the movable cavity is connected with the displacement fluid inlet and the displacement fluid outlet, and the floating piston is provided with a valve needle matched with the displacement fluid outlet; When the pressure at the top is greater than the system pressure, the floating piston with the valve needle is pressed, so that the valve needle blocks the displacement fluid outlet to prevent fluid flow; When the system pressure is higher than the top pressure, the valve needle is separated from the displacement fluid outlet, and the fluid in the overpressure part flows out; When the continuous flow is maintained, the two processes are continuously cycled to maintain the balance between the seepage system pressure and the set value of the top; The fracturing liquid inlet and the sample communication port are arranged on the pressing plate directly above the Z-axis, the fracturing liquid inlet is arranged on one side of the pressing plate directly above the Z-axis, the sample communication port is arranged at the center of the end face of the pressing plate, and the fracturing liquid inlet is connected with the sample communication port; the seepage liquid inlet and the seepage liquid outlet are respectively arranged on the left and right pressing plates of the X-axis, and the seepage liquid outlet is also arranged on the pressing plate directly below the Z-axis; the seepage liquid inlet on the pressing plate is connected with the displacement fluid inlet; the seepage liquid outlet on the pressing plate is connected with the displacement fluid outlet; When the seepage starts, the seepage liquid flows into the left pressing plate, the inlet pressure is set, and the seepage outlet on the right side is connected with the back pressure pump; the fluid flows, the seepage process starts, and after the seepage saturation, when the seepage system meets the system requirements, the outlet pressure is greater than the back pressure valve pressure; the remaining seepage liquid or gas at the bottom pressing plate enters the gas-liquid separator; The standard chamber system comprises a standard chamber cavity, the standard chamber cavity is connected with an external gas interface through the fourteenth control valve, the external gas interface is respectively connected with the eleventh control valve through the thirteenth control valve and the fracturing liquid inlet of the triaxial pressure chamber system through the twenty-eighth control valve; the thirteenth control valve is an external gas bottle opening valve used as a seepage interface control; The triaxial pressure chamber system further comprises three sets of oil supply systems respectively arranged corresponding to the x-axis, the y-axis and the z-axis, and the three oil supply systems are respectively connected with the two symmetrically arranged hydraulic cylinders in the upward direction of each axis; the oil supply system comprises an oil supply device, the oil supply device is connected with the decompression ports of the two hydraulic cylinders through the twenty-first control valve and the twenty-second control valve, and is connected with the pressurizing ports of the two hydraulic cylinders through the twentieth control valve and the twenty-third control valve.

2. The water-free fracturing in-situ low-permeability reservoir pressure-drive-permeability integrated test device based on claim 1 realizes a water-free fracturing in-situ low-permeability reservoir pressure-drive-permeability integrated test method, characterized in that: Specifically comprising the following steps: S1. Preparation and installation of the sample: S1-1. Before starting the experiment, turn off all valves; select a certain size of test sample coal rock according to the experimental requirements, and drill a hole in the center to place the fracturing pipe; S1-2. Retract the hydraulic cylinder to the maximum size, install the electric heating rod and acoustic emission device in the appropriate position; after the sample is placed in position, start the hydraulic cylinder to advance into the triaxial pressure chamber system to load the sample; load the shaft pressure through the six-sided pressure plate, completely seal from six sides, and keep the sample center position unchanged; S1-3. Insert the fracturing pipe into the sample center, and seal the side hole of the fracturing pipe with film or tape before installation to prevent the sealant from blocking; place the pre-embedded pipe into the drilled hole and seal it with sealant and air dry to prepare for the final experiment of installing the fracturing lead pipe; S1-4. Connect each control device in the test device to the computer control system for data synchronization; S2. Instrument vacuum: vacuum the liquid nitrogen tank insulation layer, liquid nitrogen tank and fracturing pipeline, and open the seventh, eighth, ninth, tenth, eleventh, twelfth, thirteenth, fourteenth, fifteenth, sixteenth, twenty-sixth and twenty-eighth control valves; S3. Simulate in-situ ground stress and reservoir temperature conditions: S3-1. Turn off all valves, open the servo hydraulic loading control system connected to the hydraulic cylinder, and use the total control console to open the servo loading pump to provide hydraulic pressure to drive the hydraulic cylinder; S3-2. Select triaxial pressure loading and set the triaxial pressure value, with the maximum pressure set as the reference, and maintain the pressure stable after reaching the set pressure; S3-3. Open the temperature and pressure monitoring system for accurate temperature control, with a display accuracy of 0.1℃; S4. Triaxial heating: turn off all valves and open the heating device; S5. Start the seepage system to conduct gas permeability, adsorption and desorption detection work: S5-1. First open the thirteenth control valve and the eighteenth control valve, set the external gas interface pressure, and monitor it using a pressure 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, close the eighteenth control valve when the outlet pressure equals the inlet pressure, open the twenty-seventh control valve, the twenty-fifth control valve, the seventeenth control valve and the nineteenth control valve, and the remaining seepage liquid or gas enters the gas-liquid separator for separation; finally, turn off all valves and end the seepage; S6. Add liquid nitrogen: turn off all valves, open the seventeenth control valve, the fifteenth control valve and the twenty-fourth 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 the liquid nitrogen flows out of the twenty-fourth control valve; S7. Pipeline pre-cooling: turn off all valves and open the sixteenth control valve and the twenty-sixth control valve; S8. Sample fracturing: after installing the fracturing lead pipe, open the first, second, third, fourth, fifth and sixth control valves on the booster pump device, and the sixteenth control valve and the twenty-fourth control valve on the liquid nitrogen storage tank; and open the acoustic emission device; when carbon dioxide is selected as the fracturing gas, the preheater needs to be opened for preheating operation; S9. Fracturing is completed, and the pressure is released; S10. Experiment ends, instrument removal and data processing: S10-1. After the sample is fractured, turn off all valves, open the twenty-first control valve and the twenty-second control valve, discharge the liquid nitrogen in the triaxial pressure chamber system until the pressure is zero, then open the triaxial pressure chamber system to take out the sample, and clean the triaxial pressure chamber system; S10-2. Display and import the real-time collected pressure at each place, triaxial pressure loading pump and acoustic emission device, and flow data in the control system during the fracturing process into the data processing software; S10-3. Open or export the test data from WORD or EXCEL.

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