Test system and method for low-temperature anhydrous fracturing adsorption saturated coal rock reservoir

Abstract

The invention discloses a test system and method for a low-temperature anhydrous fracturing adsorption saturated coal rock reservoir, the system comprises a reaction kettle and a liquid nitrogen storage tank, and an injection pipe on the liquid nitrogen storage tank is connected with a self-pressurization liquid nitrogen tank; a port of the injection pipe is formed in the upper part of the interior of the liquid nitrogen storage tank; the interior of the liquid nitrogen storage tank is connected with the supercharging device; a bottom outlet of the liquid nitrogen storage tank is connected with the vacuum pump and the reaction kettle respectively; a fracturing pipe connector is vertically arranged in the middle of the reaction kettle cover, the fracturing pipe is located in the sample, the lower end is blocked, and a side hole is formed in the side wall; the upper end, outside the reaction kettle, of the fracturing pipe connector is connected with a liquid nitrogen storage tank, a vacuum pump and a seventh control valve; and the reaction kettle is connected with the methane tank (adsorption saturation). According to the invention, liquid nitrogen is used as a low-temperature fluid, and a coal body sample with balanced adsorption is fractured under large displacement and high pressure, so that a coal rock fracture process, a temperature conduction process and a monitoring process in a liquid nitrogen fracturing process 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 test system and method for low-temperature waterless fracturing adsorption saturated coal rock reservoir. 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 to use inert fluids (such as liquid nitrogen, CO 2 The technology of injecting hydraulic fracturing fluid (such as water, etc.) into coal seams to replace conventional hydraulic fracturing fluids to produce complex fracture networks is expected to become a new technology for reservoir transformation in the future because it has perfect compatibility with coal seams and does not react with minerals in coal. There is no liquid residue after gasification, no water sensitivity, water lock and other reservoir damage. It also has low rock-breaking pressure, complex fracture network, and synergistic effect of pressurization-displacement.

[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 glass fiber reinforced plastic 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 instead of water-based fracturing fluids is practically feasible.

[0005] In recent years, the use of liquid nitrogen (including supercritical CO 2 Rich results have been achieved in the fracturing of coal rock mass by liquid nitrogen (etc.), showing the good transformation effect and application potential of anhydrous fluid fracturing of coal mass. After liquid nitrogen is injected into the coal mass, 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 fractur e 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 dozens of hours or even days), and the degree of transformation of the fractures and the permeability effect are far from expectations. However, supercritical CO 2The permeability of the coal body decreases sharply after injection due to coal expansion. Waterless fracturing methods on site face a series of problems such as failure to inject and poor permeability enhancement efficiency. How to press low-temperature fluid into coal rock for fracturing is the key. The liquid nitrogen fracturing method, which has a good transformation effect in the laboratory, is not effective in actual applications, which limits the effect of waterless fracturing. Two factors are mainly considered: First, due to the low temperature of liquid nitrogen (-176℃), the state under reservoir conditions cannot be simulated in the laboratory, and the test effect is amplified; second, due to the lack of corresponding pressure simulation equipment and reliable fracturing equipment, the current waterless fracturing method is subject to the limitations of temperature and supercritical conditions. Most of the previous liquid nitrogen fracturing studies were nitrogen or high-pressure nitrogen fracturing. In fact, the strongest effect on reservoir transformation is the fracturing effect of fluids, which is the key to creating long fractures and increasing the diversion area, but it cannot be simulated and realized in the laboratory. Therefore, there is currently no reliable fracturing device for waterless fracturing technology represented by low-temperature liquid nitrogen fracturing. There is an urgent need for a test system for low-temperature waterless fracturing adsorption saturated coal rock reservoirs, in order to provide new engineering ideas for liquid nitrogen fracturing technology for 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 a test system for low-temperature waterless fracturing and adsorption of saturated coal rock reservoirs. The present invention uses liquid nitrogen as a low-temperature fluid to fracture a coal sample in adsorption equilibrium 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 system 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 test system for low-temperature waterless fracturing adsorption of saturated coal rock reservoirs, comprising a reactor and a liquid nitrogen storage tank, wherein an injection pipe on the liquid nitrogen storage tank is connected to a self-pressurizing liquid nitrogen tank; a liquid nitrogen opening valve and a pressure gauge are provided on the self-pressurizing liquid nitrogen tank; a port of the injection pipe is arranged above the interior of the liquid nitrogen storage tank; a vacuum insulation chamber is arranged outside the liquid nitrogen storage tank, the vacuum insulation chamber is connected to a vacuum pump, the interior of the liquid nitrogen storage tank is connected to a pressurizing device, and the bottom outlet of the liquid nitrogen storage tank is respectively connected to the vacuum pump and the upper end of the fracturing pipe of the reactor; a test sample is contained in the reactor, and a heating device is arranged outside the reactor; a vertically arranged The interface of the fracturing pipe is arranged, the lower end of the fracturing pipe is sealed inside the sample and a side hole is opened on the side wall; the upper end of the interface of the fracturing pipe outside the reactor is also connected to a vacuum pump and is provided with a liquid nitrogen outlet, and a seventh control valve is provided at the liquid nitrogen outlet; a first pressure sensor is also provided on the upper part of the fracturing pipe, and a temperature detector is provided in the reactor, the temperature detector includes a plurality of temperature probes vertically arranged on both sides of the fracturing pipe, and a plurality of temperature measuring points are provided on the temperature probe along the length direction thereof; the heating device, the temperature probe and the first pressure sensor are all electrically connected to the temperature and pressure monitoring system; the reactor is also connected to the methane tank through the fifth control valve.

[0008] Preferably, the fracturing pipe is an annulus casing, which comprises an outer pipe and an inner pipe, the lower end of the outer pipe is closed, the inner pipe is coaxially sleeved in the outer pipe, an air hole is provided between the inner pipe and the outer pipe, and the side hole is opened on the outer pipe wall.

[0009] 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, and the vacuum insulation cavity is connected to the vacuum pump through the second control valve; the injection pipe extends from the top and is arranged at the lower part of the interior of the tank body, and the top of the tank body is connected to the boosting device; the bottom outlet of the tank body is connected to the vacuum pump and the sixth control valve respectively through the first control valve, and the sixth control valve is connected to the upper end of the inner tube of the fracturing tube.

[0010] Preferably, the first pressure sensor is installed between the sixth control valve and the inner tube of the fracturing pipe; and a fourth control valve for emptying the tank is also provided above the liquid nitrogen storage tank.

[0011] Preferably, the vacuum pump is connected to the upper end of the outer tube through a third control valve; a liquid nitrogen outlet is also provided at the upper end of the outer tube, a second pressure sensor and a seventh control valve are connected to the liquid nitrogen outlet, and the second pressure sensor is connected to the temperature and pressure monitoring system.

[0012] Preferably, the boosting 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 corresponding control valve and the air compressor, the air outlet end of the gas booster pump is connected to the air inlet end of the high-pressure nitrogen tank, and the air outlet end of the high-pressure nitrogen tank is connected to the liquid nitrogen storage tank through a pressure regulating valve.

[0013] Preferably, the reactor comprises a reactor body and a reactor cover, the reactor cover is detachably mounted at an opening above the reactor body, and the interface of the fracturing pipe and the temperature probe are mounted on the reactor cover and extend downward into the reactor body.

[0014] Preferably, a polytetrafluoroethylene sealing ring is provided at the opening of the kettle body, and a groove matching with the polytetrafluoroethylene sealing ring is provided below the kettle cover; an O-ring is provided at the connection between the fracturing tube and the kettle cover.

[0015] Correspondingly, the present invention also proposes a test method for low-temperature waterless fracturing adsorption of saturated coal rock reservoirs based on the above-mentioned test system for low-temperature waterless fracturing adsorption of saturated coal rock reservoirs, which specifically includes the following steps: S1. Sample preparation and installation: S1-1. Sample preparation: Drill a simulated wellbore in the middle of the test sample to place the pressure pipe; and drill holes at a certain distance from the center wellbore on both sides of the sample to place the temperature probe of the temperature detector. Four measuring points are arranged on each temperature probe to monitor the temperature change of the test sample in real time during the fracturing process; S1-2. Sample installation and equipment connection: Install the test sample in the reaction axe; connect each control device in the test system to the computer control system for data synchronization; S1-3. Heating the reactor; S2. Vacuum the instrument: S2-1. Open the first valve and the second valve on the liquid nitrogen storage tank, close other valves in the system, and start the vacuum pump to evacuate the inside of the liquid nitrogen storage tank and the vacuum insulation cavity; S2-2. Open the third control valve on the reactor and close the other valves, start the vacuum pump to evacuate the reactor; S3. Methane adsorption: Open the methane gas tank and the fifth control valve to allow the coal sample to adsorb methane to an adsorption equilibrium; S4. Add liquid nitrogen: Open the opening valve of the self-pressurized liquid nitrogen tank and the fourth control valve on the liquid nitrogen storage tank, and close all other valves, inject liquid nitrogen into the liquid nitrogen storage tank, and after the liquid nitrogen flows out of the fourth control valve, close all valves; S5. The temperature and pressure monitoring system uses a temperature detector and two pressure sensors to detect the simulated in-situ reservoir temperature conditions and measure temperature and pressure; S6. Test tube cooling and sample fracturing: S6-1. Open the sixth control valve and the first control valve on the liquid nitrogen storage tank, and the seventh control valve at the liquid nitrogen outlet; S6-2. When liquid nitrogen is observed to be uniformly sprayed from the liquid nitrogen outlet, close the seventh control valve; S6-3. When the two pressure sensors detect insufficient pressure, the booster pump device is turned on to pressurize the liquid nitrogen storage tank. The air compressor is used to compress the air to provide power, and the normal pressure nitrogen in the nitrogen bottle is pressurized and the pressurized high-pressure nitrogen is stored in the high-pressure nitrogen tank through the gas booster pump. Finally, the required high-pressure nitrogen is introduced into the liquid nitrogen storage tank through the pressure regulating valve to pressurize the liquid nitrogen that has been injected; S7. End of experiment, instrument removal and data processing: S7-1. After the sample is fracturing, open the seventh control valve and close all other valves, discharge the liquid nitrogen in the reactor until the pressure is zero; open the reactor to take out the sample, and clean the reactor for standby use; S7-2. Display the data collected by the control system in real time and import it into the data processing software; S7-3. Open or export the test data from WORD or EXCEL.

[0016] Furthermore, when the test sample is installed in the reaction axe in step S1-2, gaskets of different sizes are added to the bottom of the kettle of the reaction axe according to experimental needs to meet the requirements of fracturing experiments of samples of different heights and sizes.

[0017] The beneficial effects of the present invention are: 1. The present invention connects the methane gas tank to the reactor through the fifth control valve that acts as a safety valve, and the reactor is also connected to a vacuum pump and a fracturing system, which can ensure that the coal sample fully adsorbs the gas, making the experimental results more accurate and reliable, so that the gas adsorption experiment can be combined with the in-situ fracturing experiment, and the adsorption analysis mechanism after the coal sample fully adsorbs methane under no confining pressure and the waterless fracturing adsorption displacement permeability enhancement technology and conditions can be explored, which to a certain extent overcomes the influence of time on the escape of methane gas in the test coal sample.

[0018] 2. The fracturing pipe of the present invention is an annular casing, which includes an outer tube and an inner tube. It 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.

[0019] 3. The present invention can effectively improve the sealing performance between the kettle body and the kettle cover by the cooperation of the polytetrafluoroethylene sealing ring and the card groove, and the cooperation of the O-ring and the fracturing tube can significantly improve the sealing performance between the fracturing tube and the kettle cover.

[0020] 4. The temperature detector of the present invention includes two temperature probes vertically arranged on both sides of the fracturing pipe, and multiple temperature measuring points are arranged on the temperature probes along the length direction thereof, and the first pressure sensor is connected to the top of the fracturing pipe. The present invention can monitor the temperature of the sample and the change of the liquid nitrogen output pressure in real time during the fracturing process, and the second pressure sensor is connected in front of the seventh valve of the valve ring control sleeve, so that the gas phase change fracturing and well-simmering fracturing process of low-temperature fluid entering the crack can be accurately studied, as well as the cracking and expansion process of the crack under temperature stress, which provides conditions for studying the cracking and expansion of the crack under temperature stress.

[0021] 5. The reactor of the present invention is provided with a heating device outside, which provides a platform for simulating the temperature difference fracturing of rock formations at different depths.

[0022] 6. A second pressure sensor is also provided in front of the seventh control valve at the liquid nitrogen outlet of the present invention, which can perform effective pressure monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a structural diagram of the test system of the present invention; Figure 2 This is a structural diagram of the sample model of the present invention; Figure 3 This is a structural diagram of the fracturing pipe of the present invention; Figure 4 It is a structural diagram of the reaction kettle of the present invention; Figure 5 It is a structural diagram of the liquid nitrogen storage tank of the present invention. DETAILED DESCRIPTION

[0024] 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.

[0025] like Figure 1-5 As shown, the present invention proposes a test system for low-temperature waterless fracturing adsorption saturated coal rock reservoir, which 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, 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.

[0026] The test system of low-temperature waterless fracturing adsorption saturated coal rock reservoir includes a self-pressurizing liquid nitrogen tank 1, a reactor 4, a liquid nitrogen tank 2 and a methane tank 8. The self-pressurizing liquid nitrogen tank 1 is provided with a liquid nitrogen opening valve 11 and a pressure gauge 12, and the self-pressurizing liquid nitrogen tank 1 is connected to the liquid nitrogen tank 2 through an injection pipe 21. The port of the injection pipe 21 in the liquid nitrogen tank 2 extends into and is arranged near the bottom of the tank body 201 of the liquid nitrogen tank 2. The liquid nitrogen tank 2 includes a tank body 201 and a vacuum insulation layer 202. The vacuum insulation layer 202 is arranged outside the tank body 201 and forms a vacuum insulation cavity 200. The vacuum insulation cavity 200 is connected to the vacuum pump 5 through the second control valve 72. A booster device and a fourth control valve 74 for emptying the tank are also connected above the tank body 201. The outlet at the bottom of the tank body 201 is connected to the vacuum pump 5 and the sixth control valve 76 through the first control valve 71. 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 reactor 4 contains the test sample; the reactor 4 is provided with a heating device 41 outside for heating the reactor 4. The reactor cover of the reactor 4 is provided with an interface of a fracturing pipe 42 vertically in the middle. The fracturing pipe 42 is an annular casing, which includes an outer pipe 422 and an inner pipe 421. The outer pipe 422 is closed at both ends, and the inner pipe 421 is coaxially sleeved in the outer pipe 422. The upper end of the inner pipe 421 extends out of the outer pipe 422. There are air holes between the inner pipe 421 and the outer pipe 422, and side holes are provided on the outer pipe wall. Among them, the diameter of the inner pipe 421 of the fracturing pipe 42 is φ6mm, the diameter of the outer pipe 422 is φ7mm, and the length is 100mm. Double side holes are opened at 95mm from the lower end of the fracturing pipe 42 to simulate vertical well perforation fracturing.

[0028] The sixth control valve 76 is connected to the upper end of the inner tube extending out of the outer tube through the first pressure sensor 62, the vacuum pump 5 is connected to the upper end side wall of the outer tube extending out of the reactor 4 through the third control valve 73, and the second pressure sensor 78 and the seventh control valve 77 are provided at the liquid nitrogen outlet arranged at the upper end of the outer tube 422. The reactor 4 is also connected to the methane tank 8 through the fifth control valve 75. The present invention connects the methane gas tank to the reactor 4 through the fifth control valve 75 which acts as a safety valve, and the reactor 4 is also connected to the vacuum pump 5 and the fracturing system, which can ensure that the coal sample fully absorbs the gas, making the experimental results more accurate and reliable, so that the gas adsorption experiment can be combined with the in-situ fracturing experiment, and the adsorption analysis mechanism after the coal sample fully absorbs methane under the state of no confining pressure and the waterless fracturing adsorption displacement permeability enhancement technology and conditions can be explored, which overcomes the influence of time on the escape of methane gas in the test coal sample to a certain extent.

[0029] The fracturing pipe 42 of the present invention is an annular casing, which includes an outer tube 422 and an inner tube 421. It 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.

[0030] The reactor 4 is provided with a temperature detector, which includes two temperature probes 61 vertically arranged on both sides of the fracturing pipe 42, and multiple temperature measuring points are arranged on the temperature probe 61 along its length direction; the heating device 41, the temperature probe 61 and the two pressure sensors are electrically connected to the temperature and pressure monitoring system 9. Specifically, the temperature and pressure monitoring system 9 can adopt an artificial intelligence display temperature controller with PID adjustment function, and accurately control the temperature through temperature acquisition of the temperature probe 61 and control of the heating device 41, with a display accuracy of 0.1°C and a controllable range of 0-180°C. The temperature detector uses a PT100 temperature measuring probe with a range of -200 to 250°C, a test accuracy of ±0.3°C, and a thermal response time of 1s. The temperature sensor value is collected by the acquisition module of the control system, and can communicate with the computer through the RS232 interface. The two pressure sensors can be of Senex brand, with a range of 10MPa and 70MPa and an accuracy of 0.1%FS. They are mainly used to measure the inlet and outlet pressures of the model reactor 4. The pressure sensors at the model pressure measuring points have a communication function and can be wirelessly transmitted with the control system to achieve collection, display and storage.

[0031] The reactor 4 includes a reactor body 401 and a reactor cover 402. The outer edges of the reactor body 401 and the reactor cover 402 are respectively provided with mounting holes 403. The reactor cover 402 can be detachably mounted at the opening above the reactor body 401 through a bolt 404 structure penetrating the mounting holes 403. This not only improves the connection stability between the reactor body 401 and the reactor cover 402, but also makes it easier to place and remove samples in the reactor 4, making it more convenient to use.

[0032] A PTFE sealing ring 405 is provided at the opening of the kettle body 401, and a slot matching the PTFE sealing ring 405 is provided below the opening of the kettle cover 402. The fracturing tube 42 and the temperature probe 61 are installed on the kettle cover 402 and extend downward into the kettle body 401, and an O-type sealing ring 406 is provided at the connection between the fracturing tube 42 and the kettle cover 402. The cooperation between the PTFE sealing ring 405 and the slot can effectively improve the sealing performance between the kettle body 401 and the kettle cover 402, and the cooperation between the O-type sealing ring 406 and the fracturing tube 42 can significantly improve the sealing performance between the fracturing tube 42 and the kettle cover 402.

[0033] The boosting device includes a gas booster pump 32, a nitrogen bottle 34 and an air compressor 33. The air inlet end of the gas booster pump 32 is connected to the nitrogen bottle 34 and the air compressor 33 respectively. The nitrogen bottle 34 is connected to the air inlet end of the gas booster pump 32 through the corresponding control valve and the air compressor 33. The outlet end of the high-pressure nitrogen tank 31 is connected to the liquid nitrogen storage tank 2 through the pressure regulating valve 78. Specifically, the gas booster pump 32 can adopt the STK brand, GB series gas booster pump, which is 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 498Bar, a minimum inlet pressure of 32Bar, and a maximum displacement of 112L / min. The power source of the gas booster pump 32 selects 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 to meet the experimental requirements and enter the fracturing pipeline.

[0034] The present invention uses liquid nitrogen as a cryogenic fluid to fracture the coal body at a large displacement and high pressure, thereby truly simulating the coal rock fracture process, fracture network formation, fracture expansion, extension and intersection process, temperature conduction and monitoring process during liquid nitrogen fracturing under low temperature conditions.

[0035] Correspondingly, based on the above-mentioned low-temperature waterless fracturing adsorption saturated coal rock reservoir test system, the present invention also proposes a low-temperature waterless fracturing adsorption saturated coal rock reservoir test method, which specifically includes the following steps: S1. Sample preparation and installation: S1-1. Sample preparation: a simulated wellbore is drilled in the middle of the test sample for placing the fracturing pipe 42; and holes are drilled at a certain distance from the center wellbore on both sides of the sample for placing the temperature probe 61 of the temperature detector. Multiple measuring points are arranged on each temperature probe 61 to monitor the temperature change of the test sample during the fracturing process in real time.

[0036] Specifically, the diameter of the middle wellbore is 7 mm and the length of the wellbore is 60 mm. In actual implementation, the fracturing pipe 42 with a diameter of 6 mm and a length of 100 mm can be fixed in the middle of the wellbore with epoxy resin glue in a ratio of 1:3.

[0037] The holes for placing the temperature probes 61 are located on both sides of the sample and 19 mm from the center of the circle, with a hole diameter of 7 mm and a hole length of 100 mm. Each temperature probe 61 can be arranged with 4 measuring points, with 1 side point set at the bottom of the rock sample and another measuring point every 30 mm upwards, that is, a single-sided temperature probe 61 has 4 measuring points in total, and two probes have 8 measuring points in total.

[0038] S1-2. Sample installation and equipment connection: The test sample is installed in the reaction axe. Then, each control device in the test system is connected to the computer control system for data synchronization. In this embodiment, the maximum sample size that the reaction axe body can accommodate is a cylindrical rock mass of 60×125mm. Gaskets of different sizes can be added to the bottom of the kettle body 401 according to experimental needs to realize fracturing experiments of samples of different height sizes.

[0039] S2. Vacuum the instrument: S2-1. Open the first valve 71 and the second valve 72 on the liquid nitrogen storage tank 2, and close other valves of the system, start the vacuum pump 5 to evacuate the interior of the liquid nitrogen storage tank 2 and the vacuum insulation chamber 200; S2-2. Open the third control valve 73 on the reactor 4 and close the other valves, start the vacuum pump 5 to evacuate the reactor 4; S3. Add liquid nitrogen: Open the opening valve of the self-pressurized liquid nitrogen tank 1 and the fourth control valve 74 on the liquid nitrogen storage tank 2, and close all other valves, inject liquid nitrogen into the liquid nitrogen storage tank 2, and after the liquid nitrogen flows out of the fourth control valve 74, close all valves; S4. Methane adsorption: Open the methane tank 8 and the fifth control valve 75 to allow the coal sample to adsorb methane to an adsorption equilibrium; S5 temperature and pressure monitoring system 9 uses a temperature detector and two pressure sensors to detect simulated in-situ reservoir temperature conditions and temperature and pressure measurement; S6. Test tube cooling and sample fracturing: S6-1. Open the sixth control valve 76 and the first control valve on the liquid nitrogen storage tank 2, and the seventh control valve 77 at the liquid nitrogen outlet; S6-2. When liquid nitrogen is observed to be uniformly sprayed from the liquid nitrogen outlet, the seventh control valve 77 is closed; S6-3. When the two pressure sensors detect that the pressure is insufficient, the booster pump 32 device is turned on to pressurize the liquid nitrogen storage tank 2, and the air compressor 33 is used to compress the air to provide power, and the normal pressure nitrogen in the nitrogen bottle 34 is pressurized and the pressurized high-pressure nitrogen is stored in the high-pressure nitrogen tank 31 through the gas booster pump 32. Finally, the required high-pressure nitrogen is introduced into the liquid nitrogen storage tank 2 through the pressure regulating valve 78 to pressurize the liquid nitrogen that has been injected; S7. End of experiment, instrument removal and data processing: S7-1. After the sample is fracturing, open the seventh control valve 77 and close all other valves to discharge the liquid nitrogen in the reactor 4 until the pressure is zero; open the reactor 4 to remove the sample and clean the reactor 4 for standby; S7-2. Display the data collected by the control system in real time and import it into the data processing software. The present invention monitors 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 crack initiation and expansion process of the crack under temperature stress, providing conditions for studying the crack initiation and expansion of the crack under temperature stress.

[0040] S7-3. Open or export the test data from WORD or EXCEL.

[0041] The invention has strong equipment compatibility and is specially designed for low-temperature liquid nitrogen fracturing and can be applied to supercritical CO 2 Fracturing, hydraulic fracturing, water-liquid nitrogen composite fracturing and other technologies. 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 environment, and realizes multi-phase and multi-flow gas composition detection and analysis.

[0042] The present invention designs a unique high-pressure displacement device, which overcomes the long-standing technical bottleneck that low-temperature fluids cannot be used to fractur e coal and rock under high pressure, and is expected to achieve original results in basic science and field applications. 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. A test system for low-temperature waterless fracturing adsorption of saturated coal rock reservoirs, comprising a reactor and a liquid nitrogen storage tank, characterized in that: The injection pipe on the liquid nitrogen storage tank is connected to the self-pressurizing liquid nitrogen tank; the self-pressurizing liquid nitrogen tank is provided with a liquid nitrogen opening valve and a pressure gauge; the port of the injection pipe is arranged above the interior of the liquid nitrogen storage tank; a vacuum insulation chamber is arranged outside the liquid nitrogen storage tank, the vacuum insulation chamber is connected to the vacuum pump, the interior of the liquid nitrogen storage tank is connected to the pressurizing device, and the bottom outlet of the liquid nitrogen storage tank is respectively connected to the vacuum pump and the upper end of the fracturing pipe of the reactor; the reactor contains the test sample, and the outside of the reactor is provided with a heating device; the interface of the fracturing pipe is vertically arranged in the middle of the reactor cover, and the fracturing pipe is inside the sample The lower end is sealed and the side wall is provided with a side hole; the upper end of the interface of the fracturing pipe outside the reactor is also connected to a vacuum pump and is provided with a liquid nitrogen outlet, and a seventh control valve is provided at the liquid nitrogen outlet; a first pressure sensor is also provided on the upper part of the fracturing pipe, and a temperature detector is provided in the reactor, and the temperature detector includes a plurality of temperature probes vertically arranged on both sides of the fracturing pipe, and a plurality of temperature measuring points are provided on the temperature probe along its length direction; the heating device, the temperature probe and the first pressure sensor are all electrically connected to the temperature and pressure monitoring system; the reactor is also connected to the methane tank through the fifth control valve.

2. The test system for low-temperature waterless fracturing adsorption saturated coal rock reservoir according to claim 1 is characterized by: The fracturing pipe is an annular casing, which includes an outer pipe and an inner pipe. The lower end of the outer pipe is closed, and the inner pipe is coaxially sleeved in the outer pipe. An air hole is provided between the inner pipe and the outer pipe, and the side hole is opened on the outer pipe wall.

3. The test system for low-temperature waterless fracturing adsorption saturated coal rock reservoir according to claim 2 is characterized by: 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, and the vacuum insulation cavity is connected to a vacuum pump through a second control valve; 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 bottom outlet of the tank body is connected to the vacuum pump and the sixth control valve respectively through the first control valve, and the sixth control valve is connected to the upper end of the inner tube of the fracturing pipe.

4. The test system for low-temperature waterless fracturing adsorption saturated coal rock reservoir according to claim 3 is characterized by: The first pressure sensor is installed between the sixth control valve and the inner tube of the fracturing pipe; a fourth control valve for emptying the tank is also provided above the liquid nitrogen storage tank.

5. The test system for low-temperature waterless fracturing adsorption saturated coal rock reservoir according to claim 4, characterized in that: The vacuum pump is connected to the upper end of the outer tube through the third control valve; the upper end of the outer tube is also provided with a liquid nitrogen outlet, the liquid nitrogen outlet is connected to a second pressure sensor and a seventh control valve, and the second pressure sensor is connected to the temperature and pressure monitoring system.

6. The test system for low-temperature waterless fracturing adsorption saturated coal rock reservoir according to claim 5, characterized in that: The boosting device comprises 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 corresponding control valve and the air compressor. The air outlet end of the gas booster pump is connected to the air inlet end of the high-pressure nitrogen tank. The air outlet end of the high-pressure nitrogen tank is connected to the liquid nitrogen storage tank through a pressure regulating valve.

7. The test system for low-temperature waterless fracturing adsorption saturated coal rock reservoir according to claim 1, characterized in that: The reactor comprises a reactor body and a reactor cover, wherein the reactor cover is detachably mounted on an opening above the reactor body, and an interface of the fracturing pipe and a temperature probe are mounted on the reactor cover and extend downward into the reactor body.

8. The test system for low-temperature waterless fracturing adsorption saturated coal rock reservoir according to claim 1, characterized in that: A polytetrafluoroethylene sealing ring is arranged at the opening of the kettle body, and a groove matching with the polytetrafluoroethylene sealing ring is arranged below the kettle cover opening; an O-type sealing ring is arranged at the connection between the fracturing pipe and the fracturing pipe on the kettle cover.

9. The test method of low-temperature waterless fracturing adsorption saturated coal rock reservoir implemented based on the test system of low-temperature waterless fracturing adsorption saturated coal rock reservoir according to claim 6 is characterized by: The specific steps include: S1. Sample preparation and installation: S1-1. Sample preparation: Drill a simulated wellbore in the middle of the test sample to place the pressure pipe; and drill holes at a certain distance from the center wellbore on both sides of the sample to place the temperature probe of the temperature detector. Four measuring points are arranged on each temperature probe to monitor the temperature change of the test sample in real time during the fracturing process; S1-2. Sample installation and equipment connection: Install the test sample in the reaction axe; connect each control device in the test system to the computer control system for data synchronization; S1-3. Heating the reactor; S2. Vacuum the instrument: S2-1. Open the first valve and the second valve on the liquid nitrogen storage tank, close other valves in the system, and start the vacuum pump to evacuate the inside of the liquid nitrogen storage tank and the vacuum insulation cavity; S2-2. Open the third control valve on the reactor and close the other valves, start the vacuum pump to evacuate the reactor; S3. Methane adsorption: Open the methane gas tank and the fifth control valve to allow the coal sample to adsorb methane to an adsorption equilibrium; S4. Add liquid nitrogen: Open the opening valve of the self-pressurized liquid nitrogen tank and the fourth control valve on the liquid nitrogen storage tank, and close all other valves, inject liquid nitrogen into the liquid nitrogen storage tank, and after the liquid nitrogen flows out of the fourth control valve, close all valves; S5. The temperature and pressure monitoring system uses a temperature detector and two pressure sensors to detect the simulated in-situ reservoir temperature conditions and measure temperature and pressure; S6. Test tube cooling and sample fracturing: S6-1. Open the sixth control valve and the first control valve on the liquid nitrogen storage tank, and the seventh control valve at the liquid nitrogen outlet; S6-2. When liquid nitrogen is observed to be uniformly sprayed from the liquid nitrogen outlet, close the seventh control valve; S6-3. When the two pressure sensors detect insufficient pressure, the booster pump device is turned on to pressurize the liquid nitrogen storage tank. The air compressor is used to compress the air to provide power, and the normal pressure nitrogen in the nitrogen bottle is pressurized and the pressurized high-pressure nitrogen is stored in the high-pressure nitrogen tank through the gas booster pump. Finally, the required high-pressure nitrogen is introduced into the liquid nitrogen storage tank through the pressure regulating valve to pressurize the liquid nitrogen that has been injected; S7. End of experiment, instrument removal and data processing: S7-1. After the sample is fracturing, open the seventh control valve and close all other valves, discharge the liquid nitrogen in the reactor until the pressure is zero; open the reactor to take out the sample, and clean the reactor for standby use; S7-2. Display the data collected by the control system in real time and import it into the data processing software; S7-3. Open or export the test data from WORD or EXCEL.

10. The test method for low-temperature waterless fracturing adsorption saturated coal rock reservoir according to claim 9, characterized in that: When the test sample is installed in the reaction axe in step S1-2, gaskets of different sizes are added to the bottom of the kettle of the reaction axe according to experimental needs to meet the requirements of fracturing experiments of samples of different heights and sizes.

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