CN106840911A - A kind of liquid nitrogen temperature-control pressure-control freezing-thawing damage shale experimental provision and method - Google Patents

Abstract

The invention relates to an experimental device and method for freezing and thawing damage to shale with liquid nitrogen temperature control and pressure control. Equipped with ultra-low temperature liquid nitrogen; the high-pressure nitrogen cylinder is connected to the upper end of the temperature control and pressure control device and the freeze-thaw damage device respectively through high-pressure gas pipelines, and the high-pressure nitrogen cylinder is filled with high-pressure nitrogen; the lower part of the temperature control and pressure control device is passed through a low-temperature resistant metal pipeline It is connected with the lower part of the freeze-thaw damage device; the inside of the freeze-thaw damage device is provided with a shale core, and is connected with a data acquisition system through a low-temperature strain gauge. The beneficial effects are: the present invention can conduct freeze-thaw damage experiments on shale under the action of liquid nitrogen at different temperatures and pressures, has a large adjustable temperature range, can truly simulate the damage and deterioration of formation rocks after contact with liquid nitrogen, and has a simple experimental method. The error is small, which provides a scientific basis and theoretical basis for the development of shale gas reservoirs by ultra-low temperature fluid fracturing technology.

Description

A liquid nitrogen controlled temperature and pressure controlled freeze-thaw damage shale experimental device and method

technical field

The present invention relates to the field of petroleum engineering reservoir production stimulation and reconstruction, in particular to a liquid nitrogen controlled temperature controlled pressure freeze-thaw damage experiment device and method for shale, which is used for the fracturing scheme design of ultra-low temperature liquid nitrogen fracturing technology to exploit shale gas and post-construction reservoir productivity predictions.

Background technique

Energy crisis and environmental problems are the greatest threats to human development in the world today, and environmental problems are largely caused by the irrational structure of energy consumption. The coal-based resource stock determines my country's coal-based energy consumption structure. In my country's primary energy consumption structure, coal accounts for 69%, which is about 40% higher than the world average; while natural gas accounts for only 5%. , far below the world average of 24%. Increasing the proportion of clean energy such as natural gas in the energy consumption structure is of great significance for optimizing China's energy structure and reducing _CO2 emissions per unit of GDP. In recent years, with the sustained and rapid development of my country's social economy, the demand for energy has repeatedly hit new highs, coupled with the increasing depletion of traditional oil and gas resources, resulting in an increasing dependence on foreign oil and natural gas in my country. The dependence on foreign oil soared from 1.2% in 1993 to 58.1% in 2013, and the dependence on natural gas also rose rapidly from 0.8% in 2006 to 31.6% in 2013. It is predicted that the average annual consumption of natural gas in China will reach 3000×108m ³ by 2020, while the natural gas production in China will only be about 2000×108m ³ by then. Therefore, accelerating the exploration and development of unconventional natural gas resources is of great significance to ensure the security of national energy strategy.

In 2000, the annual shale gas production in North America was only 100×108m ³ , but by 2010, the annual shale gas production in this region reached 1,500×108m ³ , an increase of 15 times in 10 years. The "shale gas revolution" initiated by North America has greatly shaken the world's traditional energy landscape, and stimulated governments and oil and gas/energy companies' passion for shale gas resources beyond imagination. The successful development experience in the United States tells us that only by fully exposing the reservoir area, releasing a large amount of adsorbed gas reserves, and communicating with the entire reservoir, can the productivity of a single well be increased and commercial development be realized. From this, a new shale gas fracturing technology "volume fracture". Volume fracturing of shale gas reservoirs refers to the continuous expansion of natural fractures in the process of hydraulic fracturing, and the continuous shear slip of brittle rocks, forming a fracture network in which natural fractures and artificial fractures are interlaced, thereby increasing the stimulated volume and improving Gas production and ultimate recovery. Since 2005, although my country has successively made breakthroughs in shale gas exploration in marine and continental organic-rich shale in many regions and times such as the Paleozoic in the southern Sichuan Basin, the Mesozoic in the northern Sichuan Basin, and the Triassic in the Ordos Basin. , but a large number of mine practices have shown that it is difficult to carry out large-scale volume fracturing of shale gas reservoirs in my country, the volume of reservoir stimulation is limited, and the production of single wells is low, which makes the prospect of this industry worrying. Comparing the accumulation and occurrence conditions of shale gas in China and the United States, it is found that the main reasons for this challenge are as follows: ① The buried depth of the reservoir is large (the burial depth of shale in the United States is generally 200-2000 meters, while most of the shale in my country is buried deep 2) The low content of brittle minerals such as quartz makes shale brittleness generally lower than that of typical shale reservoirs in the United States; 3) Large burial depth leads to high temperature and high confining pressure, which makes shale deform Obviously change from brittle to ductile; ④The horizontal stress difference in the layer is large, and the ability to form fracture network is low; ⑤Natural fractures are not developed. These characteristics greatly limit the formation of complex fracture networks and seriously affect the effect of volume fracturing. In view of these unique reservoir geomechanical characteristics of shale gas reservoirs in my country, exploring new methods to improve formation compressibility and form complex fracture networks has become a key problem that needs to be solved urgently for the efficient development of shale gas reservoirs.

Ultra-low temperature liquid nitrogen fracturing technology refers to a new type of waterless fracturing technology that injects liquid nitrogen as fracturing fluid into the reservoir on the ground. Low-temperature liquefied gases such as liquid nitrogen usually have an extremely low temperature (standard boiling point of LN ₂ -195.8°C). When in contact with rock, the temperature of the rock surface will drop sharply, causing high-speed shrinkage and deformation, and forming a large number of microcracks; in addition, the freezing process The frost heaving force generated by the water phase in the mesopores and fissures turning into ice will also cause damage to the interior of the rock. The stronger the deterioration effect of liquid nitrogen on shale, the more fully the reservoir is broken after fracturing, the more channels for fluid seepage and migration, and the better the effect of reservoir stimulation. At present, most of the research on rock freeze-thaw damage is carried out around the problem of engineering frozen soil in cold regions. The temperature is only tens of degrees, which is not suitable for the temperature gradient of hundreds of degrees of liquid nitrogen ultra-low temperature fluid.

For this reason, it is urgent to develop an experimental device that can control the freezing and thawing damage of shale with liquid nitrogen at different temperatures and pressures, so as to develop the fracturing scheme design of shale gas reservoirs for ultra-low temperature liquid nitrogen fracturing technology and the reservoir after fracturing construction. Provide the basis for capacity forecasting.

Contents of the invention

The purpose of the present invention is to aim at the above-mentioned defects existing in the prior art, and to provide a liquid nitrogen temperature-controlled pressure-controlled freeze-thaw damage experimental device and method for shale, so as to realize the damage and deterioration of shale under the action of liquid nitrogen at different temperatures and pressures, and provide Ultra-low temperature liquid nitrogen fracturing technology provides scientific basis and theoretical basis for exploiting shale gas reservoirs.

A liquid nitrogen control temperature and pressure freeze-thaw damage experimental device for shale mentioned in the present invention includes a self-pressurized liquid nitrogen tank (1), a temperature control and pressure device (2), a freeze-thaw damage device (5), a high-pressure Nitrogen cylinder (4), data acquisition system; the self-pressurized liquid nitrogen tank (1) is connected to the temperature and pressure control device (2) through a low-temperature-resistant metal pipeline (7), and the low-temperature-resistant metal pipeline (7) is installed with The liquid inlet valve (12) and the ultra-low temperature flowmeter (11), the self-pressurized liquid nitrogen tank (1) is equipped with ultra-low temperature liquid nitrogen; The pressure device (2) is connected to the upper end of the freeze-thaw damage device (5), and the high-pressure nitrogen cylinder (4) is filled with high-pressure nitrogen; ), the lower part of the temperature and pressure control device (2) is connected to the lower part of the freeze-thaw damage device (5) through a low-temperature resistant metal pipeline (7), and a drain valve (29 );

The freeze-thaw damage device (5) is equipped with one or more shale cores (6), and the shale cores (6) are equipped with low-temperature strain gauges (61), and the low-temperature strain gauges (61) and data The strain tester (9) of the acquisition system is connected, the freeze-thaw damage device upper end cover (54) is provided on the top of the freeze-thaw damage device (5), and the outlet connected to the low-temperature resistant metal pipeline (7) is provided at the bottom;

The data acquisition system includes a four-channel temperature and pressure digital display (3), an acoustic emission detector (8), a strain tester (9) and a computer (10), wherein the acoustic emission detector (8) and the strain tester ( 9) The monitoring data is recorded by the computer (10).

Preferably, the above-mentioned temperature control and pressure control device (2) is divided into inner and outer layers, the inner layer is the first cryogenic steel liner (22), the outer layer is the first polyurethane heat insulation layer (21), and the first polyurethane heat insulation layer The layer (21) wraps the upper end cover (24) of the temperature and pressure control device (2), and a heating plate (23) is arranged between the bottom inner tank and the outer layer of the temperature and pressure control device (2), 2) The upper end cover (24) of the temperature control and pressure control device on the top is equipped with an ultra-low temperature sensor (25), a pressure sensor (26) and a vent valve (28).

Preferably, the upper end cover (24) of the above-mentioned temperature control and pressure control device is composed of a top cover and a vertical cylinder, the outer ring of the top cover is provided with a first convex edge, and a screw hole is provided on the first convex edge, through which the first sealing bolt ( 241) is fixed with the temperature control and pressure control device (2); the lower surface of the first convex edge is equipped with a first graphite gasket (242), and the outer wall of the vertical tube below the first convex edge is provided with a first polyurethane low temperature resistant O-rings (243), forming a multi-stage seal.

Preferably, the freeze-thaw damage device (5) is divided into inner and outer layers, the inner layer is the second cryogenic steel liner (52), the outer layer is the second polyurethane insulation layer (51), and the second polyurethane insulation layer (51) Wrap the upper end cover (54) of the freeze-thaw damage device, and install the second ultra-low temperature sensor (55), the second pressure sensor (56), the acoustic emission Sensor (58), control pressure valve (53) and cryogenic strain gauge wire (59).

Preferably, the upper end cover (54) of the above-mentioned freeze-thaw damage device is composed of a top cover and a vertical tube, the outer ring of the top cover is provided with a second convex edge, and a screw hole is provided on the second convex edge, through which the second sealing bolt (541 ) and the freeze-thaw damage device (5); a second graphite gasket (542) is installed on the lower surface of the second convex edge, and a second polyurethane low temperature resistant O-type is installed on the outer wall of the vertical tube below the second convex edge ring (543), forming a multi-stage seal.

Preferably, the two channels of the above-mentioned four-channel temperature and pressure digital display (3) are respectively connected to the first ultra-low temperature sensor (25) and the first pressure sensor (26) above the temperature and pressure control device (2); the other two channels are respectively It is connected with the second ultra-low temperature sensor (55) and the second pressure sensor (56) above the freeze-thaw damage device (5).

Preferably, a pressure gauge (41), a decompression valve (42) and a total intake valve (43) are provided above the high-pressure nitrogen cylinder (4), and communicated with the high-pressure gas pipeline (27).

Preferably, the outer diameter of the top cover of the upper end cover (24) of the temperature control and pressure control device is larger than the diameter of the vertical cylinder.

Preferably, the outer diameter of the top cover of the upper end cover (54) of the freeze-thaw damage device is larger than the diameter of the vertical cylinder.

The experimental method of a kind of liquid nitrogen control temperature control pressure freeze-thaw damage shale experiment device mentioned in the present invention, the specific steps are as follows:

(a) Prepare the rock core to be tested, heat the shale core (6) to the preset temperature with a box-type electric furnace, check and confirm that the liquid nitrogen control temperature control pressure freeze-thaw damage test device and all valves on the pipeline are closed ;

(b) Open the inlet valve (12) of the self-pressurized liquid nitrogen tank, add a small amount of liquid nitrogen to the temperature and pressure control device (2) to pre-cool the device, and open the vent valve (28) of the temperature and pressure control device , close the vent valve after the nitrogen in the device is completely released;

(c), shale freeze-thaw damage experiment

① Open the liquid inlet valve (12) on the self-pressurized liquid nitrogen tank (1), fill liquid nitrogen into the temperature and pressure control device (2), close the liquid inlet valve; open the decompression of the high-pressure nitrogen cylinder (4) Valve (42), adjust the nitrogen pressure to the preset freeze-thaw pressure according to the pressure gauge; open the main intake valve (43) and the first valve (271), and fill the temperature and pressure control device (2) with nitrogen, when four The channel temperature and pressure digital display (3) shows that after the air pressure in the temperature control and pressure control device (2) reaches the freeze-thaw pressure, at this time, the freeze-thaw pressure is greater than the saturated vapor pressure of nitrogen at this temperature, and the purpose is to ensure that the liquid nitrogen is heated No vaporization occurs during the process; the first valve (271) is closed; the heating plate (23) is turned on to heat the liquid nitrogen; Heating the liquid nitrogen; slightly opening the vent valve (28), slowly adjusting the air pressure in the temperature control and pressure control device to the freeze-thaw pressure, eliminating the pressure rise caused by the volume expansion of the liquid nitrogen during the heating process;

② Take the shale core (6) out of the box-type electric furnace, paste the low-temperature resistant strain gauge (61) and place it in the freeze-thaw damage device (5); open the second valve (571), and feed the freeze-thaw damage device (5 ) is filled with nitrogen, when the air pressure in the freeze-thaw damage device reaches the freeze-thaw pressure, close the main intake valve (43), open the first valve (271); open the liquid discharge valve (29) of the temperature control and pressure control device, and Connector principle, liquid nitrogen flows from the temperature control and pressure control device (2) into the freeze-thaw damage device (5) until the liquid levels of the two devices are the same;

(d) During the freeze-thaw process of shale, the control pressure valve (53) monitors the gas pressure in the freeze-thaw damage device (5) in real time, eliminates the pressure fluctuation caused by the vaporization of liquid nitrogen, and ensures that the freeze-thaw pressure is a fixed value;

(e) After the shale freeze-thaw experiment is completed, open the vent valve (28) of the temperature and pressure control device to release the pressure, take out the shale core, and the experiment is completed.

The present invention compares with prior art, and its beneficial effect is:

(1) The experimental system is divided into two parts. Liquid nitrogen temperature control and pressure control and shale freeze-thaw degradation are carried out separately to avoid mutual interference between the two parts. The temperature of liquid nitrogen is precisely regulated by using a heating plate and an ultra-low temperature sensor. Realize the freeze-thaw damage experiment of shale under the action of liquid nitrogen under different cold shock temperatures;

(2) Using high-pressure nitrogen cylinders to pressurize liquid nitrogen, conduct shale damage degradation experiments under different freeze-thaw pressures, and investigate the effect of freeze-thaw pressure on shale damage under the action of liquid nitrogen;

(3) Using a box-type electric furnace to heat the core, realizing temperature changes in two directions, investigating the influence of the same temperature difference and different temperature ranges on shale damage, which truly reflects the actual formation conditions, for ultra-low temperature liquid nitrogen fracturing development page Rock gas provides experimental results with practical guiding significance;

(4) Real-time monitoring of the freeze-thaw deterioration process of shale by means of acoustic emission and strain gauge detection, providing key experimental data for the study of shale damage evolution process;

(5) Multiple rock cores can be frozen and thawed at one time. The size of the experimental rock core is 25mm in diameter and 50mm in height. The experimental method is simple, the operation is convenient, and the experimental error is small.

Description of drawings

Fig. 1 is a schematic structural diagram of a liquid nitrogen controlled temperature controlled pressure freeze-thaw damage shale experimental device of the present invention;

Fig. 2 is a schematic diagram of the upper end surface of the end cover of the temperature control and pressure control device;

Figure 3 is a sectional view of the upper surface of the end cover of the temperature and pressure control device

Fig. 4 is a schematic diagram of the upper end surface of the end cover of the freeze-thaw degradation device;

Fig. 5 is the phase diagram of nitrogen;

In the figure, 1-self-pressurized liquid nitrogen tank, 11-ultra-low temperature flowmeter, 12-liquid inlet valve, 2-temperature control and pressure control device, 21-the first polyurethane insulation layer, 22-the first cryogenic steel liner, 23-heating plate, 24 upper end cover of temperature control and pressure control device, 241-first sealing bolt, 242-first graphite gasket, 243-first polyurethane low temperature resistant O-ring, 25-first ultra-low temperature temperature sensor, 26- The first ultra-low temperature pressure sensor, 27-high pressure gas pipeline, 271-first valve, 28-vent valve, 29-drain valve, 3-four-channel temperature and pressure digital display, 4-high pressure nitrogen cylinder, 41-pressure gauge, 42 -Decompression valve, 43-General intake valve, 5-Freeze-thaw damage device, 51-Second polyurethane insulation layer, 52-Second cryogenic steel liner, 53-Control pressure valve, 54-Upper end of freeze-thaw damage device Cover, 541-the second sealing bolt, 542-the second graphite gasket, 543-the second polyurethane low temperature resistant O-ring, 55-the second ultra-low temperature sensor, 56-the second ultra-low temperature pressure sensor, 571-the second valve, 58-Acoustic emission sensor, 59-Low-temperature strain gauge wire, 6-Shale core, 61-Low-temperature strain gauge, 7-Low temperature-resistant metal pipeline, 8-Acoustic emission detector, 9-Strain tester, 10-Computer.

detailed description

The preferred embodiments of the present invention will be described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described here are only used to illustrate and explain the present invention, and are not intended to limit the present invention.

A liquid nitrogen control temperature and pressure control freeze-thaw damage shale experimental device mentioned in the present invention includes a self-pressurized liquid nitrogen tank 1, a temperature control and pressure control device 2, a freeze-thaw damage device 5, a high-pressure nitrogen cylinder 4, and data acquisition system; the self-pressurized liquid nitrogen tank 1 is connected to the temperature and pressure control device 2 through a low-temperature-resistant metal pipeline 7, and a liquid inlet valve 12 and an ultra-low temperature flowmeter 11 are installed on the low-temperature-resistant metal pipeline 7, and the self-pressurized liquid nitrogen tank 1 is equipped with ultra-low temperature liquid nitrogen; the high-pressure nitrogen bottle 4 is respectively connected to the upper end of the temperature and pressure control device 2 and the freeze-thaw damage device 5 through the high-pressure gas pipeline 27, and the high-pressure nitrogen bottle 4 is equipped with high-pressure nitrogen; the temperature control The top of the temperature control and pressure control device 2 is equipped with an upper end cover 24 of the temperature control and pressure control device. The lower part of the temperature control and pressure control device 2 is connected with the lower part of the freeze-thaw damage device 5 through a low temperature resistant metal pipeline 7. There is a drain valve 29;

The inside of the freeze-thaw damage device 5 is provided with one or more shale cores 6, the size of the shale cores is 25 mm in diameter and 50 mm in height; the shale core 6 is provided with a low-temperature strain gauge 61, and the The strain tester 9 of the data acquisition system is connected, the top of the freeze-thaw damage device 5 is provided with an upper cover 54 of the freeze-thaw damage device, and the bottom is provided with an outlet connected to the low-temperature resistant metal pipeline 7;

The data acquisition system includes a four-channel temperature and pressure digital display 3, an acoustic emission detector 8, a strain tester 9 and a computer 10, wherein the computer 10 is responsible for recording the monitoring data of the acoustic emission detector 8 and the strain tester 9.

With reference to accompanying drawings 1, 2 and 3, a kind of temperature control and pressure control device 2 mentioned in the present invention is divided into inner and outer layers, the inner layer is the first cryogenic steel liner 22, and the outer layer is the first polyurethane heat insulating layer 21 , and the first polyurethane insulation layer 21 wraps the upper end cover 24 of the temperature and pressure control device, a heating plate 23 is arranged between the bottom inner tank and the outer layer of the temperature and pressure control device 2, and the top of the temperature and pressure control device 2 The upper end cover 24 of the temperature and pressure control device is equipped with an ultra-low temperature sensor 25 , a pressure sensor 26 and a vent valve 28 .

Wherein, the upper end cover 24 of the temperature control and pressure control device is composed of a top cover and a vertical tube. The outer ring of the top cover is provided with a first convex edge, and a screw hole is provided on the first convex edge. Through the first sealing bolt 241 and the temperature control The pressing device 2 is fixed; a first graphite gasket 242 is installed on the lower surface of the first convex edge, and a first polyurethane low temperature resistant O-ring 243 is provided on the outer wall of the vertical cylinder below the first convex edge to form a multi-stage seal.

Further, the outer diameter of the top cover of the upper end cover 24 of the temperature and pressure control device is greater than the diameter of the vertical cylinder, so that after the upper end cover 24 of the temperature and pressure control device is loaded, the outer side of the vertical cylinder passes through the first polyurethane low temperature resistant O-ring 243 is sealed with the cylindrical inner wall of the temperature and pressure control device, and the protruding convex edge of the top cover is sealed with the top of the temperature and pressure control device by the first graphite gasket 242 to realize multi-stage sealing, and the effect is better.

The upper end cover 24 of the temperature control and pressure control device is connected to the high-pressure nitrogen cylinder, and the high-pressure nitrogen cylinder is used to apply air pressure to the inside of the device, thereby further applying pressure to the liquid nitrogen. When the air pressure in the device is greater than the critical pressure of liquid nitrogen (3.3958MPa), as Liquid nitrogen will not vaporize when the temperature rises.

After the temperature and pressure control device is used to adjust the liquid nitrogen to the preset temperature and pressure, take the shale core out of the core heating box and place it in the freeze-thaw damage device; open the second valve and fill the freeze-thaw damage device with nitrogen , so that the air pressure in the device reaches the freeze-thaw pressure, the purpose is to ensure that liquid nitrogen does not gasify during the process of flowing from the temperature and pressure control device to the freeze-thaw damage device; open the drain valve of the temperature and pressure control device, and The principle is that liquid nitrogen flows from the temperature control and pressure control device into the freeze-thaw damage device until the liquid levels of the two devices are the same.

Use the temperature and pressure control device to adjust the temperature and pressure of liquid nitrogen. The first valve, the second valve, the vent valve, and the liquid discharge valve are all closed. After adjusting the temperature and pressure, open the vent valve slightly to adjust the internal pressure of the temperature and pressure control device. To the freeze-thaw pressure, close the vent valve; take the shale core out of the box-type electric furnace and release it to the freeze-thaw damage device, open the second valve, raise the air pressure in the freeze-thaw damage device to the freeze-thaw pressure, open the first valve, and close Total intake valve.

In addition, with reference to accompanying drawings 1 and 4, the freeze-thaw damage device 5 mentioned in the present invention is divided into inner and outer two layers, and its inner layer is the second cryogenic steel liner 52, and the outer layer is the second polyurethane heat insulating layer 51, and the second Two polyurethane insulation layers 51 wrap the upper end cover 54 of the freeze-thaw damage device, and a second ultra-low temperature sensor 55, a second pressure sensor 56, an acoustic emission sensor 58, and a control pressure valve are installed on the upper end cover 54 of the freeze-thaw damage device. 53 and low temperature strain gauge wire 59.

Further, the upper end cover 54 of the freeze-thaw damage device is composed of a top cover and a vertical cylinder, the outer ring of the top cover is provided with a second convex edge, and the second convex edge is provided with a screw hole, through which the second sealing bolt 541 and the freeze-thaw damage The device 5 is fixed; a second graphite gasket 542 is installed on the lower surface of the second convex edge, and a second polyurethane low-temperature resistant O-ring 543 is provided on the outer wall of the vertical cylinder below the second convex edge to form a multi-stage seal.

In addition, the outer diameter of the top cover of the upper end cover 54 of the freeze-thaw damage device is larger than the diameter of the vertical cylinder, and its structure is similar to the above-mentioned upper end cover of the temperature control and pressure control device.

Furthermore, two paths of the four-channel temperature and pressure digital display 3 are respectively connected with the first ultra-low temperature sensor 25 and the first pressure sensor 26 above the temperature and pressure control device 2; Two ultra-low temperature sensors 55 and a second pressure sensor 56 are connected.

A pressure gauge 41 , a pressure reducing valve 42 and a total air intake valve 43 are arranged above the high-pressure nitrogen cylinder 4 , and communicate with the high-pressure gas pipeline 27 .

The experimental method of a kind of liquid nitrogen control temperature control pressure freeze-thaw damage shale experiment device mentioned in the present invention, the specific steps are as follows:

(a), prepare the rock core to be tested, heat the shale rock core 6 to a preset temperature using a box-type electric furnace, check and confirm that the liquid nitrogen temperature-controlled pressure-controlled freeze-thaw damage shale test device and the valves on the pipeline are in a closed state;

(b), open the inlet valve 12 of the self-pressurized liquid nitrogen tank, add a small amount of liquid nitrogen to the temperature control and pressure control device 2 to pre-cool the device, open the temperature control and pressure control device vent valve 28, and completely discharge the device Close the vent valve after nitrogen;

(c), shale freeze-thaw damage experiment

① Open the liquid inlet valve 12 on the self-pressurized liquid nitrogen tank 1, add liquid nitrogen to the temperature and pressure control device 2, and close the liquid inlet valve; Adjust the pressure to the preset freeze-thaw pressure; open the total intake valve 43 and the first valve 271, fill the temperature and pressure control device 2 with nitrogen, when the four-channel temperature and pressure digital display 3 shows that the air pressure in the temperature and pressure control device 2 reaches After the freeze-thaw pressure, at this time, the freeze-thaw pressure is greater than the saturated vapor pressure of nitrogen at this temperature, and its purpose is to ensure that the liquid nitrogen does not vaporize during heating. The saturated vapor pressure of nitrogen at different temperatures is shown in Figure 5; Close the first valve 271; open the heating plate 23 to heat the liquid nitrogen, when the temperature of the liquid nitrogen is close to the preset freeze-thaw temperature, close the heating plate 23, and use the residual heat of the heating plate to heat the liquid nitrogen; slightly open the vent valve 28, Slowly adjust the air pressure in the temperature control and pressure control device to the freeze-thaw pressure to eliminate the pressure rise caused by the volume expansion of liquid nitrogen during the heating process;

② Take the shale core 6 out of the box-type electric furnace, paste the low-temperature resistant strain gauge 61 and place it in the freeze-thaw damage device 5; open the second valve 571, fill the freeze-thaw damage device 5 with nitrogen gas, and when the freeze-thaw damage After the air pressure in the device reaches the freeze-thaw pressure, close the main air intake valve 43, open the first valve 271; open the discharge valve 29 of the temperature control and pressure control device, and according to the principle of the connector, liquid nitrogen flows into the freeze-thaw from the temperature control and pressure control device 2 Damage device 5 until the liquid levels of both devices are the same;

(d), during the freeze-thaw process of shale, the control pressure valve 53 monitors the gas pressure in the freeze-thaw damage device 5 in real time, eliminates the pressure fluctuation caused by the vaporization of liquid nitrogen, and ensures that the freeze-thaw pressure is a fixed value;

(e) After the shale freeze-thaw experiment is completed, open the vent valve 28 of the temperature control and pressure control device to release the pressure, take out the shale core, and the experiment is completed.

The present invention compares with prior art, and its beneficial effect is:

(1) The experimental system is divided into two parts. Liquid nitrogen temperature control and pressure control and shale freeze-thaw degradation are carried out separately to avoid mutual interference between the two parts. The temperature of liquid nitrogen is precisely regulated by using a heating plate and an ultra-low temperature sensor. Realize the freeze-thaw damage experiment of shale under the action of liquid nitrogen under different cold shock temperatures;

(2) Using high-pressure nitrogen cylinders to pressurize liquid nitrogen, conduct shale damage degradation experiments under different freeze-thaw pressures, and investigate the effect of freeze-thaw pressure on shale damage under the action of liquid nitrogen;

(3) Using a box-type electric furnace to heat the core, realizing temperature changes in two directions, investigating the influence of the same temperature difference and different temperature ranges on shale damage, which truly reflects the actual formation conditions, for ultra-low temperature liquid nitrogen fracturing development page Rock gas provides experimental results with practical guiding significance;

(4) Real-time monitoring of the freeze-thaw deterioration process of shale by means of acoustic emission and strain gauge detection, providing key experimental data for the study of shale damage evolution process;

(5) Multiple rock cores can be frozen and thawed at one time. The size of the experimental rock core is 25mm in diameter and 50mm in height. The experimental method is simple, the operation is convenient, and the experimental error is small.

The above descriptions are only some of the preferred embodiments of the present invention, and any person skilled in the art may modify the technical solutions described above or modify them into equivalent technical solutions. Therefore, any simple modification or equivalent replacement made according to the technical solution of the present invention falls within the protection scope of the present invention.

Claims (10)

1. A liquid nitrogen controlled temperature and pressure freeze-thaw damage shale experimental device, characterized in that it includes a self-pressurized liquid nitrogen tank (1), a temperature and pressure control device (2), a freeze-thaw damage device (5), High-pressure nitrogen cylinder (4), data acquisition system; the self-pressurized liquid nitrogen tank (1) is connected to the temperature and pressure control device (2) through a low-temperature-resistant metal pipeline (7), and installed on the low-temperature-resistant metal pipeline (7) There is a liquid inlet valve (12) and an ultra-low temperature flowmeter (11), and the self-pressurized liquid nitrogen tank (1) is equipped with ultra-low temperature liquid nitrogen; the high-pressure nitrogen cylinder (4) is connected to the temperature-controlled The pressure control device (2) is connected to the upper end of the freeze-thaw damage device (5), and the high-pressure nitrogen cylinder (4) is filled with high-pressure nitrogen; 24), the lower part of the temperature and pressure control device (2) is connected to the lower part of the freeze-thaw damage device (5) through a low-temperature resistant metal pipeline (7), and a drain valve ( 29); The freeze-thaw damage device (5) is equipped with one or more shale cores (6), and the shale cores (6) are equipped with low-temperature strain gauges (61), and the low-temperature strain gauges (61) and data The strain tester (9) of the acquisition system is connected, the freeze-thaw damage device upper end cover (54) is provided on the top of the freeze-thaw damage device (5), and the outlet connected to the low-temperature resistant metal pipeline (7) is provided at the bottom; The data acquisition system includes a four-channel temperature and pressure digital display (3), an acoustic emission detector (8), a strain tester (9) and a computer (10), wherein the acoustic emission detector (8) and the strain tester ( 9) The monitoring data is recorded by the computer (10). 2. The liquid nitrogen temperature-controlled pressure-controlled freeze-thaw damage shale experimental device according to claim 1, characterized in that: the temperature-controlled pressure device (2) is divided into inner and outer layers, and the inner layer is the first deep The cold steel liner (22), the outer layer is the first polyurethane heat insulation layer (21), and the first polyurethane heat insulation layer (21) wraps the upper end cover (24) of the temperature control and pressure control device, and the temperature control and pressure control device ( 2) A heating plate (23) is provided between the bottom liner and the outer layer, and the upper end cover (24) of the temperature control and pressure control device (2) on the top is equipped with an ultra-low temperature sensor (25), pressure sensor (26) and vent valve (28). 3. The liquid nitrogen temperature and pressure control freeze-thaw damage shale experimental device according to claim 1, characterized in that: the upper end cover (24) of the temperature control and pressure control device is composed of a top cover and a vertical cylinder, and the top cover The outer ring is provided with a first protruding edge, and the first protruding edge is provided with a screw hole, which is fixed with the temperature and pressure control device (2) by the first sealing bolt (241); the lower surface of the first protruding edge is installed with the second A graphite gasket (242), a first polyurethane low-temperature-resistant O-ring (243) is provided on the outer wall of the vertical tube below the first convex edge to form a multi-stage seal. 4. The liquid nitrogen temperature-controlled pressure-controlled freeze-thaw damage shale experimental device according to claim 1, characterized in that: the freeze-thaw damage device (5) is divided into inner and outer layers, and the inner layer is the second cryogenic Steel liner (52), the outer layer is the second polyurethane insulation layer (51), and the second polyurethane insulation layer (51) wraps the freeze-thaw damage device upper end cover (54), and the freeze-thaw damage device upper end cover on the top The second ultra-low temperature sensor (55), the second pressure sensor (56), the acoustic emission sensor (58), the control pressure valve (53) and the low temperature strain gauge wire (59) are installed on (54). 5. The liquid nitrogen temperature and pressure controlled freeze-thaw damage shale experimental device according to claim 1, characterized in that: the upper end cover (54) of the freeze-thaw damage device is composed of a top cover and a vertical cylinder, and the outer cover of the top cover The ring is provided with a second protruding edge, and the second protruding edge is provided with a screw hole, which is fixed with the freeze-thaw damage device (5) through the second sealing bolt (541); the lower surface of the second protruding edge is installed with a second graphite As for the gasket (542), a second polyurethane low-temperature-resistant O-ring (543) is provided on the outer wall of the vertical tube below the second convex edge to form a multi-stage seal. 6. The liquid nitrogen temperature-controlled pressure-controlled freeze-thaw damage shale experimental device according to claim 1, characterized in that: the two channels of the four-channel temperature-pressure digital display (3) are respectively connected to the temperature-controlled pressure device (2 ) above the first ultra-low temperature sensor (25) and the first pressure sensor (26); the other two paths are respectively connected to the second ultra-low temperature sensor (55) and the second pressure sensor (56) above the freeze-thaw damage device (5) )connect. 7. The liquid nitrogen temperature-controlled pressure-controlled freeze-thaw damage shale experimental device according to claim 1, characterized in that: a pressure gauge (41) and a pressure reducing valve (42) are arranged above the high-pressure nitrogen cylinder (4) ) and the total intake valve (43), and communicated with the high-pressure gas pipeline (27). 8. The liquid nitrogen temperature and pressure control freeze-thaw damage shale experimental device according to claim 3, characterized in that: the outer diameter of the top cover (24) of the temperature control and pressure device is larger than the diameter of the vertical cylinder . 9. The liquid nitrogen temperature-controlled pressure-controlled freeze-thaw damage shale experimental device according to claim 5, characterized in that: the outer diameter of the top cover (54) of the freeze-thaw damage device is larger than the diameter of the vertical cylinder. 10. An experimental method of the liquid nitrogen temperature-controlled pressure-controlled freeze-thaw damage shale experimental device as described in any one of claims 1-9, characterized in that the specific steps are as follows: (a) Prepare the rock core to be tested, heat the shale core (6) to the preset temperature with a box-type electric furnace, check and confirm that the liquid nitrogen control temperature control pressure freeze-thaw damage test device and all valves on the pipeline are closed ; (b) Open the inlet valve (12) of the self-pressurized liquid nitrogen tank, add a small amount of liquid nitrogen to the temperature and pressure control device (2) to pre-cool the device, and open the vent valve (28) of the temperature and pressure control device , close the vent valve after the nitrogen in the device is completely released; (c), shale freeze-thaw damage experiment ① Open the liquid inlet valve (12) on the self-pressurized liquid nitrogen tank (1), fill liquid nitrogen into the temperature and pressure control device (2), close the liquid inlet valve; open the decompression of the high-pressure nitrogen cylinder (4) Valve (42), adjust the nitrogen pressure to the preset freeze-thaw pressure according to the pressure gauge; open the main intake valve (43) and the first valve (271), and fill the temperature and pressure control device (2) with nitrogen, when four The channel temperature and pressure digital display (3) shows that after the air pressure in the temperature control and pressure control device (2) reaches the freeze-thaw pressure, at this time, the freeze-thaw pressure is greater than the saturated vapor pressure of nitrogen at this temperature, and the purpose is to ensure that the liquid nitrogen is heated No vaporization occurs during the process; the first valve (271) is closed; the heating plate (23) is turned on to heat the liquid nitrogen; Heating the liquid nitrogen; slightly opening the vent valve (28), slowly adjusting the air pressure in the temperature control and pressure control device to the freeze-thaw pressure, eliminating the pressure rise caused by the volume expansion of the liquid nitrogen during the heating process; ② Take the shale core (6) out of the box-type electric furnace, paste the low-temperature resistant strain gauge (61) and place it in the freeze-thaw damage device (5); open the second valve (571), and feed the freeze-thaw damage device (5 ) is filled with nitrogen, when the air pressure in the freeze-thaw damage device reaches the freeze-thaw pressure, close the main intake valve (43), open the first valve (271); open the liquid discharge valve (29) of the temperature control and pressure control device, and Connector principle, liquid nitrogen flows from the temperature control and pressure control device (2) into the freeze-thaw damage device (5) until the liquid levels of the two devices are the same; (d) During the freeze-thaw process of shale, the control pressure valve (53) monitors the gas pressure in the freeze-thaw damage device (5) in real time, eliminates the pressure fluctuation caused by the vaporization of liquid nitrogen, and ensures that the freeze-thaw pressure is a fixed value; (e) After the shale freeze-thaw experiment is completed, open the vent valve (28) of the temperature and pressure control device to release the pressure, take out the shale core, and the experiment is completed.