Device and method for testing deformation of liquid carbon dioxide frozen and thawed coal body

Abstract

The invention relates to the technical field of coal seam anti-reflection gas extraction, and discloses a liquid carbon dioxide freeze-thaw coal body deformation testing device and method.The liquid carbon dioxide freeze-thaw coal body deformation testing device comprises a liquid supply component, a freeze-thaw component and a data monitoring component; the liquid supply component comprises a Dewar tank for storing liquid carbon dioxide, the freezing and thawing component comprises a freezing and thawing tank, a standard cylindrical coal body test piece is placed in the freezing and thawing tank, the Dewar tank is connected with the freezing and thawing tank, and the data monitoring component comprises a temperature detection unit, a pressure detection unit and a strain detection unit. The temperature detection unit is connected to the surface of the standard cylindrical coal body test piece, the pressure detection unit is connected to the freezing and thawing tank, and the strain detection unit is connected to the surface of the standard cylindrical coal body test piece; according to the method, the dynamic influence rule of the multi-field coupling effect on the coal deformation characteristic in the permeability increasing process of the low-permeability coal seam caused by the freezing and thawing effect of the liquid carbon dioxide can be effectively explained.

Description

Technical Field

[0001] This invention relates to the field of coal seam permeability enhancement and gas extraction technology, and in particular to a device and method for testing the deformation of coal seams under freeze-thaw conditions using liquid carbon dioxide. Background Technology

[0002] Coalbed methane (CBM) is an unconventional natural gas resource and a crucial component of my country's energy mix. Compared to traditional fossil fuels for power generation, CBM is a cleaner energy source and significantly reduces the greenhouse effect. The rational development of CBM not only addresses the growing shortage of traditional energy sources but also effectively reduces the occurrence of mine gas disasters, playing a vital role in improving coal mine safety and mitigating environmental issues. However, my country's complex geological structure and the generally low permeability, high adsorption, and microporosity of coal seams restrict gas extraction, resulting in unsatisfactory gas drainage effects.

[0003] To improve coal seam gas extraction efficiency, hydraulic treatment measures have been widely applied. While these technologies have achieved certain results in increasing coal seam permeability, they are prone to causing localized stress concentration, water pollution, high water consumption, and a "water-locking effect." Liquid carbon dioxide injection into the coal seam to improve permeability has addressed these problems to some extent, and its application efficiency is also higher compared to other anhydrous fracturing agents. However, research on the dynamic effects of freeze-thaw cycles on coal seam deformation and damage, as well as the regularity of the influence of carbon dioxide phase changes on the coal seam, remains insufficient. Furthermore, the effect of coal seam deformation caused by freeze-thaw cycles on enhancing coal seam permeability still needs further investigation.

[0004] Conventional liquid carbon dioxide freeze-thaw coal devices and methods focus on the freeze-thaw damage process of coal, but cannot monitor and record important parameters of coal during the freeze-thaw process. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a device and method for testing the deformation of coal seams by freezing and thawing with liquid carbon dioxide. This addresses issues such as the large mining depth, high formation stress, high gas pressure, and high mining difficulty of low-permeability coal seams in my country, as well as the lack of a definitive conclusion regarding the dynamic permeability enhancement principle of liquid carbon dioxide on coal seams. It can effectively explain the damage and destruction effects of the multi-field coupling effect involved in the permeability enhancement process of low-permeability coal seams by liquid carbon dioxide on the pore and fracture structures of the coal body, and the dynamic influence of the phase transformation reaction of liquid carbon dioxide and the effects of multiple composite stresses on the deformation characteristics of the coal body.

[0006] The first aspect of this invention provides a liquid carbon dioxide freeze-thaw coal deformation testing device, comprising: a liquid supply component, a freeze-thaw component, and a data monitoring component. The liquid supply component includes a Dewar flask for storing liquid carbon dioxide. The freeze-thaw component includes a freeze-thaw tank containing a standard cylindrical coal specimen. The Dewar flask is connected to the freeze-thaw tank to deliver the liquid carbon dioxide from the Dewar flask to the freeze-thaw tank in either a gaseous or liquid state. The data monitoring component includes a temperature detection unit, a pressure detection unit, and a strain detection unit. The temperature detection unit is connected to the surface of the standard cylindrical coal specimen to monitor the surface temperature of the standard cylindrical coal specimen and the temperature inside the freeze-thaw tank during the freeze-thaw process. The pressure detection unit is connected to the freeze-thaw tank to detect the pressure of the freeze-thaw tank. The strain detection unit is connected to the surface of the standard cylindrical coal specimen to detect the deformation of the standard cylindrical coal specimen.

[0007] Optionally, the freeze-thaw tank has a liquid inlet at the bottom, which is connected to the Dewar tank.

[0008] Optionally, the freeze-thaw tank is equipped with a pressure relief valve. During the liquid inlet process, the pressure relief valve is opened appropriately to ensure that the pressure inside the freeze-thaw tank is lower than the pressure inside the Dewar tank when liquid carbon dioxide enters from the Dewar tank.

[0009] Optionally, a pressure boosting valve is connected to the Dewar canister to ensure that the pressure inside the Dewar canister is not less than 0.7 MPa.

[0010] Optionally, a pressure relief assembly is connected to the Dewar canister to ensure that the pressure inside the Dewar canister does not exceed 2.76 MPa.

[0011] Optionally, the strain detection unit includes strain gauges, with one strain gauge attached along the cleavage direction and one perpendicular to the cleavage direction of the standard cylindrical coal specimen.

[0012] Optionally, the data monitoring component also includes a temperature compensation unit, which is connected inside the freeze-thaw tank and is used to measure the amount of deformation of the strain gauge itself due to thermal expansion and contraction caused by temperature.

[0013] A second aspect of the present invention provides a testing method using the above-described liquid carbon dioxide freeze-thaw coal deformation testing device, comprising:

[0014] Place the standard cylindrical coal body specimen in the freeze-thaw tank, connect the Dewar canister to the freeze-thaw tank, first supply gaseous carbon dioxide from the Dewar canister to the freeze-thaw tank, and ensure that the system pressure in the freeze-thaw tank is stable at 1.0 MPa for 1 minute.

[0015] Liquid carbon dioxide was slowly poured into the freeze-thaw tank until it submerged the standard cylindrical coal specimen, and the coal was frozen for 30 minutes.

[0016] The surface temperature of the standard cylindrical coal specimen, the pressure of the freeze-thaw tank, and the deformation of the standard cylindrical coal specimen are monitored by the data monitoring component during the freeze-thaw process.

[0017] Optionally, the Dewar flask can be connected to the freeze-thaw tank, ensuring that the pressure inside the freeze-thaw tank is within the specified range.

[0018] Within the range of 2.0MPa to 2.3MPa.

[0019] Optionally, after the coal body has been frozen for 30 minutes, the freeze-thaw tank needs to be depressurized to release the remaining liquid carbon dioxide. The depressurization rate should be maintained at 0.01 MPa / s, and the depressurization time should be maintained at 50-60 minutes.

[0020] The technical solution provided by the embodiments of the present invention has the following advantages compared with the prior art:

[0021] This invention provides a device for testing the deformation of coal subjected to freeze-thaw cycles using liquid carbon dioxide. Based on the principle that the temperature cycling effect of liquid carbon dioxide causes irreversible damage and deformation to the coal, the device simulates the freeze-thaw process using carbon dioxide through a liquid supply component and a freeze-thaw component. After the coal is frozen and thawed with liquid carbon dioxide, it causes damage, extending existing cracks and creating new ones. The porosity of the coal is also increased, with increased pore volume and connectivity. This allows for smoother flow of gas molecules within the coal, significantly improving its permeability. The device uses a temperature detection unit, a pressure detection unit, and a strain detection unit in the data monitoring component to achieve real-time and accurate monitoring of the temperature field, pressure field, and strain of the coal during the freeze-thaw process. It also allows for dynamic monitoring. The freeze-thaw effect after liquid carbon dioxide injection causes changes in the deformation of the coal body, and the phase characteristics of carbon dioxide can be monitored in real time throughout the entire process of liquid carbon dioxide freeze-thaw coal body. Therefore, it can specifically explain the different effects of carbon dioxide with different phase characteristics on the deformation of the coal body. Thus, it can analyze the mechanism of liquid carbon dioxide permeability enhancement technology in improving coal seam permeability from the perspective of coal body deformation. It has a certain role in explaining the principle of liquid carbon dioxide permeability enhancement coal seam. Through the above data, a real-time relationship of multiple parameters such as time-strain-temperature-pressure can be established, thereby explaining the destructive deformation effect of liquid carbon dioxide phase change on the coal body during the liquid carbon dioxide freeze-thaw process. It makes up for the shortcomings of conventional liquid carbon dioxide freeze-thaw experiments that cannot detect the phase change characteristics of liquid carbon dioxide and cannot monitor key data in real time. Attached Figure Description

[0022] Figure 1 A schematic diagram of the structure of the liquid carbon dioxide freeze-thaw coal deformation testing device provided in an embodiment of the present invention;

[0023] Figure 2 A graph illustrating the variation of coal body deformation under single freeze-thaw cycles using liquid carbon dioxide is provided in this embodiment of the invention.

[0024] Figure 3 A diagram illustrating the temperature variation of the surface of coal subjected to freeze-thaw cycles with liquid carbon dioxide, provided in an embodiment of the present invention.

[0025] Figure 4 A diagram illustrating the pressure variation of a liquid carbon dioxide system provided in an embodiment of the present invention;

[0026] Figure 5 is a diagram showing the phase change characteristics of liquid carbon dioxide freeze-thawed coal provided in an embodiment of the present invention. Figure 5a This diagram illustrates the overall pattern of phase changes in coal during the freeze-thaw cycle with liquid carbon dioxide. Figure 5b for Figure 5a A magnified schematic diagram of a local structure;

[0027] Figure 6 The graph shows the variation of coal deformation under different water saturation levels during the freeze-thaw cycle of liquid carbon dioxide, as provided in this embodiment of the invention.

[0028] Explanation of reference numerals in the attached figures:

[0029] 1-Dewar tank, 2-Liquid phase valve, 3-Gas phase valve, 4-Pressure booster valve, 5-Gas phase cryogenic metal hose, 6-Liquid phase cryogenic metal hose, 7-Copper one-to-two adapter, 8-Cryogenic metal hose manifold, 9-Normal open inlet valve, 10-Inlet, 11-Freeze-thaw tank, 12-Temperature detection unit, 13-Pressure detection unit, 14-Pressure relief valve, 15-Paperless recorder, 16-Strain detection unit, 17-Data monitoring and recording system. Detailed Implementation

[0030] The following detailed description of a specific embodiment of the present invention is provided in conjunction with the accompanying drawings. However, it should be understood that the scope of protection of the present invention is not limited to the specific embodiment.

[0031] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the technical solution of this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0032] There is still a lack of research on the dynamic effects of the freeze-thaw effect on coal seam deformation and damage caused by the process of injecting liquid carbon dioxide into the coal seam, as well as the regularity of the influence of the phase characteristics change of carbon dioxide on the coal seam. At the same time, the effect of the deformation effect of coal seam caused by the freeze-thaw of liquid carbon dioxide on enhancing the permeability of the coal seam still needs to be studied.

[0033] Conventional liquid carbon dioxide freeze-thaw coal devices and methods focus on the freeze-thaw damage process of coal, but cannot monitor and record important parameters of coal during the freeze-thaw process.

[0034] To address these issues, this invention provides a device and method for testing the deformation of coal seams by freezing and thawing with liquid carbon dioxide. This addresses the challenges of deep mining of low-permeability coal seams in my country, including high formation stress, high gas pressure, and high mining difficulty, as well as the lack of a definitive conclusion regarding the dynamic permeability enhancement principle of liquid carbon dioxide on coal seams. The invention effectively explains the damage and destruction effects of the multi-field coupling effect involved in the permeability enhancement process of low-permeability coal seams by liquid carbon dioxide on the pore and fracture structures of the coal body, and the dynamic influence of the phase transformation reaction of liquid carbon dioxide and the effects of multiple composite stresses on the deformation characteristics of the coal body.

[0035] At least one embodiment of the present invention provides a device and method for testing the deformation of coal bodies subjected to freeze-thaw cycles using liquid carbon dioxide. The device includes a liquid supply component, a freeze-thaw component, and a data monitoring component. The liquid supply component includes a Dewar flask for storing liquid carbon dioxide. The freeze-thaw component includes a freeze-thaw tank containing a standard cylindrical coal body specimen. The Dewar flask is connected to the freeze-thaw tank to deliver the liquid carbon dioxide from the Dewar flask into the freeze-thaw tank in either a gaseous or liquid state. The data monitoring component includes a temperature detection unit, a pressure detection unit, and a strain detection unit. The temperature detection unit is connected to the surface of the standard cylindrical coal body specimen to monitor the surface temperature of the specimen and the temperature inside the freeze-thaw tank during the freeze-thaw process. The pressure detection unit is connected to the freeze-thaw tank to detect the pressure within the tank. The strain detection unit is connected to the surface of the standard cylindrical coal body specimen to detect the deformation of the specimen.

[0036] In the liquid carbon dioxide freeze-thaw coal deformation testing device provided in the above embodiments of the present invention, the liquid supply component and the freeze-thaw component simulate the carbon dioxide freeze-thaw medium. After the liquid carbon dioxide freezes and thaws the coal, it will cause damage to the coal, thereby extending the original cracks in the coal and generating new cracks. The porosity of the coal is also increased, and the pore volume and connectivity are increased. This makes the flow of gas molecules in the coal smoother, thus significantly improving the permeability of the coal. The temperature detection unit, pressure detection unit, and strain detection unit in the data monitoring component realize the real-time and accurate monitoring of the temperature field, pressure field, and coal strain of the coal during the liquid carbon dioxide freeze-thaw process. At the same time, it can dynamically monitor the freeze-thaw effect of liquid carbon dioxide injection on the coal. The deformation caused by liquid carbon dioxide can be monitored in real time throughout the entire process of liquid carbon dioxide freeze-thaw coal body. Therefore, it can specifically explain the different effects of carbon dioxide with different phase characteristics on the deformation of coal body. Thus, it can analyze the mechanism of liquid carbon dioxide permeability enhancement technology in improving coal seam permeability from the perspective of coal body deformation. It has a certain role in explaining the principle of liquid carbon dioxide permeability enhancement coal seam. Through the above data, a real-time relationship of multiple parameters such as time-strain-temperature-pressure can be established, thereby explaining the destructive deformation effect of liquid carbon dioxide phase change on coal body during liquid carbon dioxide freeze-thaw process. It makes up for the deficiencies of conventional liquid carbon dioxide freeze-thaw experiments that cannot detect the phase change characteristics of liquid carbon dioxide and cannot monitor key data in real time.

[0037] The present invention will be described below through several specific embodiments. To keep the following description of the embodiments clear and concise, detailed descriptions of known functions and components may be omitted. When any component of an embodiment of the present invention appears in more than one drawing, the component may be represented by the same reference numerals in each drawing.

[0038] refer to Figure 1 , Figure 1 This is a schematic diagram of the liquid carbon dioxide freeze-thaw coal deformation testing device provided in an embodiment of the present invention, as shown below. Figure 1As shown, one embodiment of the present invention provides a liquid carbon dioxide freeze-thaw coal deformation testing device, comprising: a liquid supply component, a freeze-thaw component, and a data monitoring component. The liquid supply component includes a Dewar jar 1 for storing liquid carbon dioxide. The freeze-thaw component includes a freeze-thaw tank 11, in which a standard cylindrical coal specimen is placed. The Dewar jar 1 is connected to the freeze-thaw tank 11 to deliver the liquid carbon dioxide in the Dewar jar 1 to the freeze-thaw tank 11 in gaseous or liquid form. The data monitoring component includes a temperature detection unit 12, a pressure detection unit 13, and a strain detection unit 16. The temperature detection unit 12 is connected to the surface of the standard cylindrical coal specimen to monitor the surface temperature of the standard cylindrical coal specimen and the temperature inside the freeze-thaw tank 11 during the freeze-thaw process. The pressure detection unit 13 is connected to the freeze-thaw tank 11 to detect the pressure of the freeze-thaw tank 11. The strain detection unit 16 is connected to the surface of the standard cylindrical coal specimen to detect the deformation of the standard cylindrical coal specimen.

[0039] It should be understood that Dewar 1 is a 175L liquid carbon dioxide Dewar 1. Dewar 1 has a liquid phase valve 2 and a gas phase valve 3. Liquid phase valve 2 is connected to a liquid phase cryogenic metal hose 6, and gas phase valve 3 is connected to a gas phase cryogenic metal hose 5. These are then connected via a copper one-to-two adapter 7 to a cryogenic metal hose manifold 8. The cryogenic metal hose manifold 8 is connected to a freeze-thaw tank 11 for filling the freeze-thaw tank 11 with liquid or gaseous carbon dioxide. When only gas phase valve 3 is open, only gaseous CO2 is supplied. When liquid phase valve 2 is opened, gas phase valve 3 must be closed simultaneously. The freeze-thaw tank 11 must be sealed, and one end of the freeze-thaw tank 11 must be depressurized. At this time, the liquid CO2 will slowly... The liquid slowly flows into the freeze-thaw tank 11 to achieve liquid inlet. The Dewar tank 1 is also equipped with a pressure gauge to monitor the remaining liquid volume. Six metal gaskets can be placed in the small flange on the top of the freeze-thaw tank 11. Each metal gasket has four 1mm diameter wire connection holes for connecting the strain gauge detection unit 16 to the strain gauge inside the freeze-thaw tank 11. 703 silicone rubber is used to enhance the sealing effect. The temperature detection unit 12 in the data monitoring component is a nickel-chromium K-type thermocouple with an operating temperature range of -200 to 1300℃, an output signal of Pt100, and a probe length of 20cm, which directly contacts the coal inside the tank. The system monitors temperature changes around the coal surface and connects to a paperless recorder 13 for real-time temperature data recording. The pressure detection unit 13 in the data monitoring system is connected to the inlet of the pressure detection unit 13 at the top of the freeze-thaw tank 11. This pressure detection unit is a 3051 type direct-mount capacitive pressure transmitter with a range of 0–10 MPa and an output signal of 4–20 mA. The strain detection unit 16 in the data monitoring components is a DH3820N distributed stress-strain measuring instrument, including a DH3800 power supply / controller and a DH3820N... The package includes a data acquisition unit (16 channels), a 5-meter Cat5e shielded network cable, a 5-meter signal input cable, a 10-meter power and communication cable, a power adapter and power cord. The power and communication cable connects the output of the RS-485 communication interface of the DH3800 power supply / controller to the input of the RS-485 communication interface of the DH3820N data acquisition unit. The power adapter and power cord power the DH3800 power supply / controller. The signal input cable connects the strain gauges to the DH3820N data acquisition unit, with each signal input cable providing 4 signal channels. The Cat5e shielded network cable connects the DH3800 power supply / controller to the host computer and facilitates data reading.

[0040] This invention provides a device for testing the deformation of coal subjected to freeze-thaw cycles using liquid carbon dioxide. Based on the principle that the temperature cycling effect of liquid carbon dioxide causes irreversible damage and deformation to the coal, the device simulates the freeze-thaw process using carbon dioxide through a liquid supply component and a freeze-thaw component. After the coal is frozen and thawed with liquid carbon dioxide, it causes damage, extending existing cracks and creating new ones. The porosity of the coal is also increased, with increased pore volume and connectivity. This allows for smoother flow of gas molecules within the coal, significantly improving its permeability. The device uses a temperature detection unit, a pressure detection unit, and a strain detection unit in the data monitoring component to achieve real-time and accurate monitoring of the temperature field, pressure field, and strain of the coal during the freeze-thaw process. It also allows for dynamic monitoring. The freeze-thaw effect after liquid carbon dioxide injection causes changes in the deformation of the coal body, and the phase characteristics of carbon dioxide can be monitored in real time throughout the entire process of liquid carbon dioxide freeze-thaw coal body. Therefore, it can specifically explain the different effects of carbon dioxide with different phase characteristics on the deformation of the coal body. Thus, it can analyze the mechanism of liquid carbon dioxide permeability enhancement technology in improving coal seam permeability from the perspective of coal body deformation. It has a certain role in explaining the principle of liquid carbon dioxide permeability enhancement coal seam. Through the above data, a real-time relationship of multiple parameters such as time-strain-temperature-pressure can be established, thereby explaining the destructive deformation effect of liquid carbon dioxide phase change on the coal body during the liquid carbon dioxide freeze-thaw process. It makes up for the shortcomings of conventional liquid carbon dioxide freeze-thaw experiments that cannot detect the phase change characteristics of liquid carbon dioxide and cannot monitor key data in real time.

[0041] In this embodiment of the invention, the freeze-thaw tank 11 is provided with a liquid inlet 10 at the bottom, which is connected to the Dewar tank 1. It should be understood that the freeze-thaw tank 11 is used to place a standard cylindrical coal body with a diameter of φ25×50mm for the freeze-thaw reaction of liquid carbon dioxide. Due to the volatile and unstable nature of liquid CO2, the liquid inlet 10 needs to be set at the bottom of the freeze-thaw tank 11. Liquid CO2 can accumulate slowly and is not easy to vaporize. The liquid inlet 10 at the bottom of the freeze-thaw tank 11 is connected to the normally open liquid inlet valve 7 and then to the freeze-thaw tank 11 for the entry of gas and liquid.

[0042] The freeze-thaw tank 11 is equipped with a pressure relief valve 14. During the liquid inlet process, the pressure relief valve 14 is opened appropriately to ensure that the pressure inside the freeze-thaw tank 11 is lower than the pressure inside the Dewar 1 when liquid carbon dioxide enters from the Dewar 1. After the Dewar 1 delivers liquid carbon dioxide, the pressure in the Dewar 1 and the pressure in the freeze-thaw tank 11 will be consistent. At this time, the pressure relief valve 14 is opened to ensure that the pressure in the freeze-thaw tank 11 is lower than the pressure in the Dewar 1, thereby allowing the liquid carbon dioxide to enter the freeze-thaw tank 11 smoothly. The liquid inlet is maintained for 10 minutes. In addition, the pressure inside the freeze-thaw tank 11 is kept within the range of 2.0 MPa to 2.3 MPa to maintain a stable liquid inlet rate, ensure the stable conduct of the carbon dioxide freeze-thaw test, and ensure the validity of the subsequent data.

[0043] It should be noted that a pressure boosting valve 4 is connected to the Dewar 1. The pressure boosting valve 4 is kept open to ensure that the pressure inside the Dewar 1 is not lower than 0.7 MPa, so as to avoid the formation of dry ice inside and hindering the delivery of liquid. If the pressure is lower than 0.7 MPa, dry ice is likely to form inside the Dewar 1, blocking the entire device and pipeline, thus making it impossible to successfully simulate the carbon dioxide freeze-thaw test. At the same time, the pressure inside the Dewar 1 should be kept higher than 2.5 MPa, so that the carbon dioxide gas can be used smoothly, and it is easier to produce gaseous carbon dioxide and liquid carbon dioxide.

[0044] Specifically, the carbon dioxide inside Dewar 1 will slowly vaporize. To prevent excessive pressure inside Dewar 1 from causing a decrease in its insulation performance and an increase in the amount of carbon dioxide liquid evaporation, a pressure relief assembly is connected to Dewar 1. It should be noted that the pressure relief assembly is connected to Dewar 1 to ensure that the pressure inside Dewar 1 does not exceed 2.76 MPa. When the pressure inside Dewar 1 exceeds 2.76 MPa, the pressure relief assembly will immediately open to unload the pressure. In addition, a vent valve is also connected to the top of Dewar 1. The vent valve is used to release residual gas or liquid each time the liquid is replaced.

[0045] In this embodiment of the invention, the Dewar jar 1 is provided with an insulation layer, and liquid carbon dioxide is stored in the insulation layer. The insulation layer can prevent the liquid carbon dioxide from rapidly vaporizing due to temperature rise, thereby ensuring the smooth progress of the carbon dioxide freeze-thaw test.

[0046] Specifically, the strain detection unit 16 includes strain gauges. One strain gauge is attached along the cleavage direction and perpendicular to the cleavage direction of the standard cylindrical coal specimen, and another strain gauge is attached along both the cleavage and perpendicular directions. This allows for accurate measurement of the axial and circumferential strain of the coal body. This measurement is related to the location of the original fractures in the coal body. Furthermore, since the volumetric strain of the coal body can be calculated based on the measured axial and circumferential strain, it is necessary to express the strain generated by the coal body after freeze-thaw cycles through the volumetric strain. In this embodiment, the strain gauge is a BA120-3AA150-P200 type strain gauge with a resistance of 120.2 ± 0.1 Ω and a sensitivity coefficient of... The strain gauge is 2.10±1%, and the applicable temperature range is -80℃~150℃. The strain gauge and signal input line adopt a three-wire 1 / 4 bridge connection method with a bridge voltage of 2V. The end of the signal input line uses φ0.5mm copper enameled wire to connect to the strain gauge and passes through the flange through hole to connect to the coal surface in the freeze-thaw tank 11. One strain gauge is attached along the coal cutting direction and one perpendicular to the cutting direction. Sandpaper is used to polish the signal sensing surface of the strain gauge to prevent oxidation. 50% alcohol is used to wipe the signal sensing surface of the strain gauge. CC-33A room temperature curing instant adhesive is used to bond the coal and the strain gauge. The strain gauge and the coal are completely bonded by pressing and letting stand for 1 minute.

[0047] As a preferred implementation, the data monitoring component also includes a temperature compensation unit, which is connected inside the freeze-thaw tank 11. The temperature compensation unit is used to measure the deformation of the strain gauge itself due to temperature contraction and thermal expansion. To eliminate the tensile and contractile strain values ​​of the strain gauge itself caused by temperature fluctuations, an independent signal channel is required as a temperature compensation channel. The temperature compensation channel adopts a three-wire 1 / 4 bridge connection method. The temperature compensation unit, i.e., the temperature compensation sheet, is completely bonded to a 10mm×10mm×0.5mm square quartz sheet and placed in the freeze-thaw tank 11 to prevent it from being affected by any external force. The temperature compensation sheet is made of quartz glass and measures the strain of the quartz glass sheet. Because the quartz glass sheet has a very low coefficient of linear expansion, it will not produce a large strain under the influence of temperature. Since normal strain gauges inevitably undergo some shape changes after being affected by temperature due to their material properties, the temperature compensation sheet actually measures the deformation of the strain gauge itself due to temperature contraction and thermal expansion. Therefore, this part of the influence is removed by temperature compensation, which makes the data measured by the working gauge more accurate.

[0048] The testing method using the above-mentioned liquid carbon dioxide freeze-thaw coal deformation testing device includes:

[0049] Place the standard cylindrical coal body specimen in the freeze-thaw tank 11, connect the Dewar tank 1 to the freeze-thaw tank 11, first supply gaseous carbon dioxide from the Dewar tank 1 to the freeze-thaw tank 11, and ensure that the system pressure in the freeze-thaw tank 11 is stable to 1.0 MPa for 1 minute.

[0050] Liquid carbon dioxide is slowly poured into the freeze-thaw tank 11 until the standard cylindrical coal specimen is submerged in liquid carbon dioxide, and the coal is frozen for 30 minutes.

[0051] The surface temperature of the standard cylindrical coal specimen, the pressure of the freeze-thaw tank 11, and the deformation of the standard cylindrical coal specimen are monitored by the data monitoring component during the freeze-thaw process.

[0052] Optionally, the Dewar jar 1 can be connected to the freeze-thaw tank 11, and the pressure inside the freeze-thaw tank 11 must be within the range of 2.3MPa to 2.5MPa.

[0053] Optionally, after the coal body has been frozen for 30 minutes, the freeze-thaw tank 11 needs to be depressurized to release the remaining liquid carbon dioxide. The depressurization rate should be maintained at 0.01 MPa / s and the depressurization time should be maintained at 50-60 minutes.

[0054] The present invention provides a testing method for testing coal deformation using the aforementioned liquid carbon dioxide freeze-thaw coal deformation testing device. This method is based on the principle that the temperature cycling effect of liquid carbon dioxide causes irreversible damage and deformation of the coal body. It can significantly improve the permeability of the coal body. Using this method, the temperature field, pressure field, and coal strain of the coal body during the liquid carbon dioxide freeze-thaw process can be accurately monitored in real time. Simultaneously, it can clearly understand the real-time changes in the specific phase characteristics of liquid carbon dioxide during the freeze-thaw experiment, and establishes a real-time relationship of multi-parameter, multi-field coupling of time, strain, temperature, pressure, and liquid carbon dioxide phase characteristics. This explains the destructive deformation effect of liquid carbon dioxide phase change on the coal body during the freeze-thaw process, overcoming the shortcomings of conventional liquid carbon dioxide freeze-thaw experiments in detecting the phase change characteristics of liquid carbon dioxide and monitoring key data in real time. Furthermore, the method details the process and control methods for the entry and exit of liquid carbon dioxide within the container, supplementing key aspects neglected in conventional liquid carbon dioxide freeze-thaw experiments. In addition, the device and method proposed in this invention are significant for revealing the permeability enhancement mechanism of coal seams in L-carbon dioxide-ECBM engineering applications.

[0055] The following two specific embodiments are disclosed in detail.

[0056] Example 1

[0057] The method using the above-mentioned liquid carbon dioxide freeze-thaw coal deformation testing device includes the following steps:

[0058] Step 1: Connect all devices and pipelines. After attaching the strain gauge to the prepared φ25×50mm cylindrical coal body, place it vertically in the middle of the freeze-thaw tank 11. Attach the temperature compensation plate to the quartz and place it inside the freeze-thaw tank 11 to prevent external interference. Turn on the real-time data monitoring software on the main control terminal to record strain, temperature, and pressure data. Check the stability of the data signals of the coal body through the main control terminal. After debugging, immediately install the flange and freeze-thaw tank 11. After checking the airtightness of the entire device, close the device's pressure relief valve and open the normally open liquid inlet valve 7 of the freeze-thaw tank 11.

[0059] Step 2: Slowly open the vapor phase valve 3 on the top of the 175L liquid carbon dioxide Dewar canister, and constantly observe the changes in the reading of the pressure detection unit 13. Once the system pressure rises to above 1.2MPa, immediately close the vapor phase valve 3 and slowly adjust the system pressure to stabilize at 1.2MPa for 1 minute to allow the carbon dioxide gas to fully diffuse into the space inside the Dewar canister 1.

[0060] Step 3: After the carbon dioxide gas has diffused stably for 1 minute, immediately open the liquid phase valve 2 at the top of the 175L liquid carbon dioxide Dewar tank and appropriately control the pressure relief valve 12. In order to maintain a stable liquid inflow rate and keep the system pressure within the range of 2.3MPa to 2.5MPa, the liquid carbon dioxide slowly flows into the freeze-thaw tank 11. When the surface temperature of the coal body drops to -16℃ and no longer drops rapidly, it is considered that the liquid carbon dioxide in the freeze-thaw tank 11 has submerged the coal body. At this time, immediately close the pressure relief valve 12 and the liquid phase valve 2 of the Dewar tank 1 to carry out a 30-minute coal freezing process.

[0061] Step 4: After freezing for 30 minutes, slowly open the pressure relief valve 12 to release the carbon dioxide gas-liquid mixture. To prevent the liquid carbon dioxide in the freeze-thaw tank 11 from converting into dry ice and interfering with the normal acquisition of data, the pressure relief valve 12 should be slightly adjusted to keep the discharge rate at 0.001 MPa / s.

[0062] Step 5: When the pressure of the device drops to 0MPa, that is, the carbon dioxide in the system is completely emptied, immediately remove the flange and freeze-thaw tank 11, and carefully place the standard cylindrical coal body specimen and temperature compensation plate in the indoor environment for 30 minutes of room temperature coal thawing process.

[0063] Step 6: After the standard cylindrical coal specimen has melted, the operating software on the main unit outputs temperature, pressure, and coal strain data. This concludes the single coal freeze-thaw cycle and data monitoring. The variation law of coal deformation during a single freeze-thaw cycle with liquid carbon dioxide is shown in [the relevant documentation]. Figure 2 The changes in coal surface temperature and system pressure are respectively shown in the figures. Figure 3 , Figure 4 .

[0064] Depend on Figure 2The deformation characteristics of coal are known, where the positive and negative values ​​of strain represent the expansion and contraction of the test material, and the numerical value represents the degree of change from the original shape. The magnitude of the strain directly reflects the degree of damage to the coal. The deformation characteristic curve of coal under liquid carbon dioxide freeze-thaw action generally shows a trend of first decreasing, then increasing, and then stabilizing, which can be divided into (I) the coal matrix shrinkage deformation stage and (II) the residual strain recovery stage. After being affected by the liquid carbon dioxide freeze-thaw effect, the coal first undergoes shrinkage deformation, followed by gradual recovery of the shrinkage deformation and the generation of certain residual strain. This indicates that the deformation damage caused by the liquid carbon dioxide freeze-thaw effect on the coal is a process of the coal matrix first shrinking, then expanding and recovering, resulting in irreversible damage.

[0065] Depend on Figure 3 It can be seen that during the freeze-thaw process of liquid carbon dioxide, the surface temperature of the coal body generally shows a trend of first decreasing, then increasing, and then stabilizing. This is because the depressurization rate is slowed down to prevent the rapid formation of dry ice after the freezing process from affecting monitoring results, thus influencing both temperature and pressure. This indicates that the moisture in the coal body inevitably undergoes repeated water-ice phase transitions during the freeze-thaw process. During the freeze-thaw process, liquid carbon dioxide creates a significant gradient effect on the temperature field of the upper surface of the coal body, causing varying degrees of shrinkage or expansion of the heterogeneous matrix. This leads to compression and breakage between the matrices, inducing the generation and development of pores and fractures. Simultaneously, the freezing of moisture expands the effective area of ​​the pores and fractures, further causing deformation of the coal body.

[0066] Depend on Figure 4 It can be seen that the system pressure first increases and then decreases, reaching a maximum of 3.8 MPa. The subsequent decrease is due to the need to discharge the remaining carbon dioxide after the freezing is completed. This proves that liquid carbon dioxide does not exist in a stable liquid form during the entire freezing process. The vaporization effect of liquid carbon dioxide is involved in real time, which may cause the coal body to adsorb a small amount of gas and produce a slight expansion and deformation.

[0067] Depend on Figure 5a and 5b It can be seen that the overall phase characteristics of liquid carbon dioxide during the freeze-thaw process are: gas-liquid-gas. This indicates that the freeze-thaw effect of liquid carbon dioxide is a complex gas-liquid two-phase and multi-field coupling process, and carbon dioxide with different phase characteristics has certain differences in its dominant effect on coal.

[0068] Example 2

[0069] Experiment on deformation of coal under different water saturation levels due to freeze-thaw cycles with liquid carbon dioxide

[0070] Before conducting experiments to test the deformation of coal bodies subjected to freeze-thaw cycles with liquid carbon dioxide at different water saturation levels, the following definition is made:

[0071] m = S0(ms -m d )+m d

[0072] In the formula, m is the target drying mass; S0 is the preset moisture saturation; m s The mass of a fully saturated coal body; m d This refers to the quality of the dried coal.

[0073] Coal bodies with different saturation levels of 0%, 40%, 80%, and 100% should be prepared. The preparation steps include:

[0074] (1) Place the coal body in a vacuum drying oven (constant temperature 100℃), take it out and weigh it every 1 hour until the mass error of the last two coal samples is less than 0.1%, and record the mass md of the dried coal sample.

[0075] (2) Place the coal sample in a vacuum saturation device with a vacuum pressure of -0.1MPa to saturate it with water. Take out the coal sample and weigh it every 6 hours until the mass of the coal sample no longer increases. It is considered to be water saturated. Record the mass ms of the fully saturated coal sample.

[0076] (3) Place the fully saturated coal sample in a vacuum drying oven (constant temperature 100℃) for drying. During this period, take it out and weigh it continuously. The weighing time is adjusted according to actual needs until the target drying quality is reached. Then take out the coal sample and immediately put it into a sealed bag to cool naturally to room temperature for later use.

[0077] (4) Repeat steps (1) to (3) to prepare coal bodies with different water saturation.

[0078] After coal bodies with different water saturations were prepared, experiments were conducted to test the deformation of the coal bodies under liquid carbon dioxide freeze-thaw cycles at different water saturations. The experimental results are shown in […]. Figure 6 .

[0079] Depend on Figure 6 It can be seen that the changes in coal body with different water saturation levels during the freeze-thaw process with liquid carbon dioxide are basically consistent. Specifically, as the water saturation gradually increases, the minimum strain of the coal body also gradually rises, and the rate of strain reduction slows down in the initial freezing stage. However, during the thawing process, the residual strain of the coal body gradually increases with increasing water saturation. This indicates that water saturation is one of the key parameters affecting the strain characteristics of the coal body in this experiment. The freeze-thaw effect caused by water is a key factor leading to deformation and damage of the coal body. The freezing of water expands the pore volume, and the plastic zone inside the pores gradually expands outward during the freezing process. During the thawing process, a large amount of residual plastic strain accumulates, resulting in irreversible damage to the coal body.

[0080] Experimental analysis results demonstrate that the apparatus and method of this invention effectively damages coal during the freeze-thaw process with liquid carbon dioxide. Furthermore, the dominant effect of liquid carbon dioxide on coal deformation can be analyzed based on different phase changes. The permeability-enhancing mechanism of liquid carbon dioxide-induced fracturing in coal is revealed from the perspective of coal strain, and the deformation and damage mechanism of coal subjected to liquid carbon dioxide freeze-thaw is systematically elucidated. These findings not only solve the problem of real-time monitoring of key parameters and phase change analysis of liquid carbon dioxide in traditional freeze-thaw experiments, but also provide guidance for revealing the deformation characteristics of coal subjected to liquid carbon dioxide freeze-thaw and its induced permeability changes in engineering theory. This provides effective assistance for pressure relief and permeability enhancement, and safe extraction in low-permeability coal seams.

[0081] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A device for testing the deformation of coal seams under freeze-thaw conditions using liquid carbon dioxide, characterized in that, include: Liquid supply components include a Dewar flask (1) for storing liquid carbon dioxide; The freeze-thaw unit includes a freeze-thaw tank (11), in which a standard cylindrical coal body specimen is placed. The Dewar jar (1) is connected to the freeze-thaw tank (11) to transport liquid carbon dioxide in the Dewar jar (1) into the freeze-thaw tank (11) in gaseous or liquid form. The data monitoring component includes a temperature detection unit (12), a pressure detection unit (13), and a strain detection unit (16). The temperature detection unit (12) is connected to the surface of the standard cylindrical coal specimen to monitor the surface temperature of the standard cylindrical coal specimen and the temperature inside the freeze-thaw tank (11) during the freeze-thaw process. The pressure detection unit (13) is connected to the freeze-thaw tank (11) to detect the pressure of the freeze-thaw tank (11). The strain detection unit (16) is connected to the surface of the standard cylindrical coal specimen to detect the deformation of the standard cylindrical coal specimen.

2. The liquid carbon dioxide freeze-thaw coal deformation testing device as described in claim 1, characterized in that, The freeze-thaw tank (11) is provided with a liquid inlet (10) at the bottom, and the liquid inlet (10) is connected to the Dewar tank (1).

3. The liquid carbon dioxide freeze-thaw coal deformation testing device as described in claim 2, characterized in that, The freeze-thaw tank (11) is connected to a pressure relief valve (14), which makes the pressure inside the freeze-thaw tank (11) lower than the pressure inside the Dewar tank (1) when liquid carbon dioxide enters the freeze-thaw tank (11) from the Dewar tank (1).

4. The liquid carbon dioxide freeze-thaw coal deformation testing device as described in claim 1 or 3, characterized in that, A pressure boosting valve (4) is connected to the Dewar canister (1) to ensure that the pressure inside the Dewar canister (1) is not lower than 0.7 MPa.

5. The liquid carbon dioxide freeze-thaw coal deformation testing device as described in claim 4, characterized in that, The dewar can (1) is connected to a pressure relief assembly to ensure that the pressure inside the dewar can (1) does not exceed 2.76 MPa.

6. The liquid carbon dioxide freeze-thaw coal deformation testing device as described in claim 1, 3, or 5, characterized in that, The strain detection unit (16) includes strain gauges, with one strain gauge attached to each of the cleavage direction and the perpendicular cleavage direction of the standard cylindrical coal body specimen.

7. The liquid carbon dioxide freeze-thaw coal deformation testing device as described in claim 6, characterized in that, The data monitoring component also includes a temperature compensation unit, which is connected inside the freeze-thaw tank (11). The temperature compensation unit is used to measure the deformation of the strain gauge due to thermal expansion and contraction caused by temperature.

8. The testing method using the liquid carbon dioxide freeze-thaw coal deformation testing device according to claim 1, characterized in that, include: Place the standard cylindrical coal body specimen in the freeze-thaw tank (11), connect the Dewar tank (1) to the freeze-thaw tank (11), first transfer gaseous carbon dioxide from the Dewar tank (1) to the freeze-thaw tank (11), and ensure that the system pressure in the freeze-thaw tank (11) is stable to above 1.0 MPa for 1 minute; Liquid carbon dioxide was slowly poured into the freeze-thaw tank (11) until the standard cylindrical coal specimen was submerged in liquid carbon dioxide, and the coal was frozen for 30 minutes. The surface temperature of the standard cylindrical coal specimen, the pressure of the freeze-thaw tank (11), and the deformation of the standard cylindrical coal specimen during the freeze-thaw process are monitored by the data monitoring component.

9. The method for testing the deformation of coal seams under freeze-thaw conditions with liquid carbon dioxide as described in claim 8, characterized in that, Connect the Dewar canister (1) to the freeze-thaw tank (11), and ensure that the pressure inside the freeze-thaw tank (11) is within the range of 2.0MPa to 2.3MPa.

10. The method for testing the deformation of coal seams under liquid carbon dioxide freeze-thaw as described in claim 9, characterized in that, After the coal body has been frozen for 30 minutes, the freeze-thaw tank (11) needs to be depressurized to release the liquid carbon dioxide. The depressurization rate should be maintained at 0.01 MPa / s and the depressurization time should be maintained at 50 min-60 min.

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