Test method and device for natural convection heat transfer of carbon dioxide in high temperature rock

Abstract

The application belongs to the field of hot dry rock and enhanced geothermal system, and particularly relates to a test method and device for natural convection heat transfer of carbon dioxide in high-temperature rock. The test method divides a hot dry rock sample into a front section, a middle section and a rear section, and heats the front section and the rear section to form a low-temperature region and a high-temperature region respectively. Carbon dioxide is first injected into a hot dry rock fracture in a forced convection mode, and then the forced convection process is ended. At this time, the low-temperature region and the high-temperature region promote the natural convection of carbon dioxide in the fracture. Through a carbon dioxide supply source, carbon dioxide is stably supplied to the hot dry rock sample fracture to realize the continuous natural convection of carbon dioxide inside the fracture. The natural convection heat transfer coefficient is calculated by using a formula to analyze the natural convection heat transfer under different conditions. The test method fills the gap of the natural convection heat transfer test method of carbon dioxide in high-temperature rock.

Description

Technical Field

[0001] This application relates to the field of hot dry rock and enhanced geothermal system technology, and in particular to a test method and apparatus for natural convection heat transfer of carbon dioxide in high-temperature rocks. Background Technology

[0002] Geothermal resources can be classified into hydrothermal and hot dry rock types according to their formation and production conditions. Hot dry rock geothermal resources account for approximately 90% of proven geothermal resources and possess immense development potential. Because hot dry rock reservoirs are dense and contain little or no natural fractures, enhanced geothermal systems are used to develop and utilize their thermal energy. In these systems, a lower-temperature heat transfer fluid is injected from the surface into the deep, high-temperature hot dry rock reservoir fractures through an injection well. Driven by injection pressure, it flows towards the production well. As the fluid seeps through the fractures, it continuously absorbs heat from the hot dry rock, gradually increasing its temperature, and is eventually extracted back to the surface through the production well. To address the water pollution and waste issues associated with water-based enhanced geothermal systems, carbon dioxide is used instead of water as the heat extraction fluid.

[0003] Forced convection is the movement of fluid by applying external force, such as a pump or fan. Natural convection is a fluid flow phenomenon without external driving force. It is mainly caused by temperature differences, which lead to uneven density within the fluid, thus generating a driving force that causes the fluid to move from the denser end to the less dense end without external force.

[0004] In carbon dioxide-enhanced geothermal systems, forced convection of carbon dioxide is significant near the injection well due to the injection pressure. In areas farther from the injection well, the carbon dioxide fluid pressure gradually dissipates, the forced convection weakens, and natural convection becomes more apparent. Natural convection of carbon dioxide in fracture networks manifests as follows: at the end furthest from the injection well, the carbon dioxide temperature is relatively low and its density is relatively high, while near the production well, the carbon dioxide temperature is relatively high and its density is relatively low. This leads to carbon dioxide flowing from the injection well towards the production well without the driving force of pump pressure. Fully utilizing the natural convection effect formed by the significant density difference of carbon dioxide at different temperatures can greatly reduce the pumping energy consumption required for hot dry rock thermal energy extraction and improve thermal extraction efficiency. Therefore, studying the natural convection heat transfer process of carbon dioxide is particularly important. However, the laboratory experimental methods and equipment required for studying the natural convection heat transfer of carbon dioxide in hot dry rock reservoir fractures are lacking. Existing experimental methods and equipment for carbon dioxide seepage heat transfer in rock fractures can only be used to study the forced convection heat transfer process of carbon dioxide under pump pressure, and cannot be used for natural convection heat transfer experiments. Therefore, it is necessary to develop an experimental method and equipment for the natural convection heat transfer of carbon dioxide in high-temperature rocks. Summary of the Invention

[0005] This application provides a test method and apparatus for natural convection heat transfer of carbon dioxide in high-temperature rocks, realizing indoor tests of natural convection of carbon dioxide in fractures of dry hot rock reservoirs, and filling the gap in test methods for natural convection heat transfer of carbon dioxide in fractures of dry hot rock reservoirs.

[0006] Therefore, according to one aspect of this application, a method for testing the natural convection heat transfer of carbon dioxide in high-temperature rocks is provided, comprising the following steps:

[0007] S1. The hot dry rock sample is divided into three sections along the direction of its fracture extension: front section, middle section, and rear section. The front section has a fracture inlet, and the rear section has a fracture outlet. The front, middle, and rear sections are heated by heating resistors. The middle section is heated to a preset temperature to simulate the temperature environment of the hot dry rock reservoir. The front section is heated to a temperature lower than the preset temperature to form a low-temperature region. The rear section is heated to a temperature higher than the preset temperature to form a high-temperature region.

[0008] S2. Apply external force to inject carbon dioxide into the fissures of the hot dry rock sample through the fissure inlet. After the carbon dioxide flows steadily out of the fissure outlet, stop applying the external force and make the pressure difference between the fissure inlet and outlet zero to end the forced convection process. At this time, the carbon dioxide in the low-temperature region of the fissure is low in temperature and high in density, while the carbon dioxide in the middle section of the fissure is relatively high in temperature and relatively low in density, causing carbon dioxide to flow from the low-temperature region fissure to the middle section fissure. This flow process is not driven by external force and is a natural convection process. The carbon dioxide in the high-temperature region fissure is high in temperature and low in density. The carbon dioxide in the middle section of the fissure is relatively cold and relatively dense, causing it to flow from the middle section of the fissure to the high-temperature region. This flow process is not driven by external forces and is a natural convection process. The temperature difference between the high-temperature region and the low-temperature region ensures that the overall trend of natural convection of carbon dioxide in the fissure of the dry hot rock sample is from the fissure inlet to the fissure outlet. During the flow, carbon dioxide exchanges heat with the fissure surface of the dry hot rock sample, which is a natural convection heat transfer. When carbon dioxide is detected flowing out of the fissure outlet, it indicates that natural convection of carbon dioxide has occurred in the fissure of the dry hot rock sample.

[0009] S3. A carbon dioxide supply source is set at the fracture entrance. During the natural convection heat transfer stage, the amount of carbon dioxide supplied to the fracture entrance by the carbon dioxide supply source is the same as the amount of carbon dioxide flowing out of the fracture exit, so that there is a stable supply of carbon dioxide in the fracture of the hot dry rock sample and the natural convection heat transfer of carbon dioxide in the fracture of the hot dry rock sample continues. At the same time, the pressure applied by the carbon dioxide supply source to the fracture entrance is controlled to be equal to the pressure at the fracture exit, so as to avoid the occurrence of forced convection during the supply process.

[0010] S4. Monitor the starting and ending temperatures of natural convection of carbon dioxide in the fissures of the middle section;

[0011] S5. When there is a stable outflow of carbon dioxide from the fracture outlet and the temperature monitoring data is stable, the natural convective heat transfer coefficient h of carbon dioxide in the fractures of the hot dry rock reservoir is calculated using the following formula:

[0012]

[0013] Where: h is the natural convection heat transfer coefficient, in W / (m²) 2 ·℃); t b The initial temperature is expressed in °C; t e The final temperature is expressed in °C; W is the electrical power of the heating resistor used to heat the middle section, expressed in W; ε is the emissivity of the hot dry rock; A is the area of ​​the fracture surface, expressed in m². 2 C0 is the blackbody radiation coefficient, in W / (m²). 2 ·K 2 );T b The initial temperature is in Kelvin (K); T e Termination temperature, unit K;

[0014] S6. Analyze the influence of different conditions on the natural convection heat transfer coefficient using the natural convection heat transfer coefficient calculated under different conditions.

[0015] According to another aspect of this application, a test apparatus for natural convection heat transfer of carbon dioxide in high-temperature rocks is provided. Based on the above-described test method, the test apparatus includes:

[0016] The core holder includes a rubber sleeve, the interior of which forms a core chamber for holding the rock sample. The core chamber is divided into upper and lower compartments, which, from left to right, are respectively: a low-temperature auxiliary zone, a front buffer zone, a test zone, a rear buffer zone, and a high-temperature auxiliary zone. Insulation plates are installed at both ends of the low-temperature auxiliary zone, the front buffer zone, the test zone, the rear buffer zone, and the high-temperature auxiliary zone to insulate the temperature between the different zones. A gasket is installed between the upper and lower compartments of the core chamber; the thickness of the gasket can be adjusted to regulate the fracture aperture of the hot, dry rock sample. A metal cylinder is fitted over the rubber sleeve, and a connection is made between the metal cylinder and the rubber sleeve. It has a gap, which is divided into five independent confining chambers corresponding to the low-temperature auxiliary zone, the front buffer zone, the test zone, the rear buffer zone, and the high-temperature auxiliary zone. The rubber sleeve is sealed at both ends with plugs, and locking caps are installed on the outside of the plugs. The locking caps are fixed to the ends of the metal cylinder with bolts to tighten the plugs. A capillary tube is connected to the middle of the plug near the low-temperature auxiliary zone, serving as a carbon dioxide injection pipe. A capillary tube is also connected to the middle of the plug near the high-temperature auxiliary zone, serving as a carbon dioxide output pipe. A first gas path check valve is installed between the plugs and the carbon dioxide output pipe to prevent carbon dioxide backflow. The outside of the metal cylinder is wrapped with a heat insulation layer.

[0017] The pressure system is used to provide pressure to each independent confining chamber in order to apply confining pressure to the dry hot rock sample in the core chamber;

[0018] The heating system includes five heating resistors and two thermocouples. The five heating resistors are respectively installed on the inner walls of five independent confining chambers. One thermocouple enters the core chamber test area near the front buffer zone from the carbon dioxide injection pipe; the other thermocouple enters the core chamber test area near the rear buffer zone from the carbon dioxide output pipe.

[0019] The injection system is used to apply external force to inject carbon dioxide into the fissures of the hot dry rock sample through the carbon dioxide injection pipe; the injection system is also used to inject carbon dioxide into the carbon dioxide injection pipe in an amount equal to the amount of carbon dioxide flowing out of the carbon dioxide output pipe after natural convection of carbon dioxide occurs in the fissures of the hot dry rock sample, while controlling the internal pressure of the carbon dioxide injection pipe and the carbon dioxide output pipe to be equal, so as to achieve continuous natural convection.

[0020] The beneficial effects of the experimental method and apparatus for natural convection heat transfer of carbon dioxide in high-temperature rocks provided in this application are as follows:

[0021] 1. A method for experimental heat transfer of carbon dioxide through natural convection in high-temperature rocks is provided. A dry, hot rock sample is divided into a front section, a middle section, and a rear section. The front and rear sections are heated to form low-temperature and high-temperature regions, respectively. Carbon dioxide is injected into the fissures of the dry, hot rock sample by applying external force and using forced convection. The external force is then stopped, and the pressure difference between the fissure inlet and outlet is reduced to zero to terminate the forced convection process. At this point, the low-temperature and high-temperature regions promote natural convection of carbon dioxide in the fissures, achieving an overall trend of natural convection from the fissure inlet to the fissure outlet. A carbon dioxide supply source is set at the fissure inlet, supplying carbon dioxide to the fissure inlet. The amount of carbon dioxide supplied is equal to the amount flowing out of the fracture outlet, ensuring a stable supply of carbon dioxide to the fractures of the hot dry rock sample and enabling continuous natural convection heat transfer. Simultaneously, the pressure of the carbon dioxide supply source at the fracture inlet is controlled to be equal to the pressure at the fracture outlet, ensuring that the flow of carbon dioxide from the fracture inlet to the outlet is unaffected by external forces and avoiding forced convection during the supply process. By monitoring the onset and termination temperatures of natural convection within the fractures in the middle section of the hot dry rock sample, the influence of the temperatures in the designated low-temperature and high-temperature zones on temperature monitoring is avoided. The natural convection heat transfer coefficient is calculated using a formula to analyze natural convection heat transfer under different conditions. This method fills the gap in experimental methods for natural convection heat transfer of carbon dioxide in high-temperature rock fractures.

[0022] 2. A test apparatus for natural convection heat transfer of carbon dioxide in high-temperature rocks is provided. The core chamber in the core holder is divided into five zones: a low-temperature auxiliary zone, a front buffer zone, a test zone, a rear buffer zone, and a high-temperature auxiliary zone. The temperature difference created between the low-temperature and high-temperature auxiliary zones ensures that the overall trend of natural convection is from the carbon dioxide injection pipe to the carbon dioxide output pipe, avoiding the influence of temperature differences between the two ends and the outside environment on the overall trend of natural convection. The front and rear buffer zones prevent the low-temperature and high-temperature auxiliary zones from affecting temperature monitoring. By changing the thickness of the gasket between the upper and lower chambers in the core chamber, the fissure opening of the hot dry rock sample can be controlled. An injection system injects an equal amount of carbon dioxide into the carbon dioxide injection pipe as it flows out of the carbon dioxide output pipe, while simultaneously controlling the internal pressure of the carbon dioxide injection and output pipes to be equal, allowing natural convection to occur smoothly. This apparatus enables indoor experiments on natural convection of carbon dioxide in high-temperature rock fissures. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] in:

[0025] Figure 1 This is a schematic diagram of the structure of an experimental device for natural convection heat transfer of carbon dioxide in high-temperature rocks, as shown in one embodiment of this application;

[0026] Figure 2 yes Figure 1 Schematic diagram of the core holder;

[0027] Figure 3 yes Figure 1 A schematic diagram of the cross-section of the core chamber of the core holder.

[0028] Explanation of key component symbols:

[0029] 10. Core holder; 100. Core chamber; 1001. Chamber; 101. Insulation layer; 102. Bolt; 103. Locking cap; 104. Rubber sleeve; 105. Plug; 106. Insulation plate; 107. Carbon dioxide output pipe; 108. Gasket; 109. High-temperature auxiliary zone; 110. Rear buffer zone; 111. Carbon dioxide injection pipe; 112. Low-temperature auxiliary zone; 113. Front buffer zone; 114. Test zone; 115. Independent confining pressure chamber; 116. Metal cylinder; 117. First gas path check valve;

[0030] 20. Heating system; 201. Temperature control component; 202. Temperature probe; 203. Heating resistor; 204. Thermocouple;

[0031] 30. Injection system; 301. Carbon dioxide tank; 302. Constant speed and constant pressure pump; 303. Carbon dioxide recovery tank; 304. Carbon dioxide flow meter; 305. Refrigeration bath; 306. Second gas path check valve; V1. First valve; V2. Second valve; V3. Third valve;

[0032] 40. Pressure system; 401. Pressure pump; V4. Fourth valve; P. Confining pressure gauge. Detailed Implementation

[0033] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings. Preferred embodiments of this application are shown in the drawings. However, this application can be implemented in many other different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of this application.

[0034] It should be noted that when a component is referred to as being "fixed to" or "set on" another component, it can be directly on or indirectly on that other component. When a component is referred to as being "connected to" another component, it can be directly connected to or indirectly connected to that other component.

[0035] It should be understood that the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., 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 this application 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 application.

[0036] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0038] It should also be noted that in the embodiments of this application, the same reference numerals are used to represent the same component or part. For the same part in the embodiments of this application, the reference numerals may only be used to mark one part or component as an example. It should be understood that the reference numerals are also applicable to other identical parts or components.

[0039] According to one aspect of this application, embodiments of this application provide an experimental method for natural convection heat transfer of carbon dioxide in high-temperature rocks, comprising the following steps:

[0040] S1. The hot dry rock sample is divided into three sections along the direction of its fracture extension: front section, middle section, and rear section. The front section has a fracture inlet, and the rear section has a fracture outlet. The front, middle, and rear sections are heated simultaneously by heating resistors. The middle section is heated to a preset temperature to simulate the temperature environment of the hot dry rock reservoir. The front section is heated to a temperature lower than the preset temperature to form a low-temperature region. The rear section is heated to a temperature higher than the preset temperature to form a high-temperature region.

[0041] S2. Apply external force to inject carbon dioxide into the fissures of the hot dry rock sample through the fissure inlet. After the carbon dioxide flows out of the fissure stably, stop applying external force and make the pressure difference between the fissure inlet and outlet zero to end the forced convection process. At this time, the carbon dioxide in the low-temperature region of the fissure is low in temperature and high in density, while the carbon dioxide in the middle section of the fissure is relatively high in temperature and relatively low in density, causing carbon dioxide to flow from the low-temperature region fissure to the middle section fissure. This flow process is not driven by external force and is a natural convection process. The carbon dioxide in the high-temperature region fissure is high in temperature and density. The temperature of carbon dioxide in the middle section of the fracture is relatively low, while its density is relatively high. This causes carbon dioxide to flow from the middle section of the fracture to the high-temperature region. This flow process is not driven by external forces and is a natural convection process. The temperature difference between the high-temperature and low-temperature regions ensures that the overall trend of natural convection of carbon dioxide in the fracture of the dry hot rock sample is from the fracture inlet to the fracture outlet. During the flow, carbon dioxide exchanges heat with the fracture surface of the dry hot rock sample, which is a natural convection heat transfer. When carbon dioxide is detected flowing out of the fracture outlet, it indicates that natural convection of carbon dioxide has occurred in the fracture of the dry hot rock sample.

[0042] It should be noted here that a first one-way valve can be installed at the location where the carbon dioxide temperature decreases after the fissure outlet. The first one-way valve only allows carbon dioxide to flow outward from the fissure in the dry hot rock sample, preventing natural convection from occurring in the opposite direction to the natural convection of carbon dioxide in the fissure due to the decrease in temperature and increase in density after the carbon dioxide flows out of the outlet.

[0043] S3. A carbon dioxide supply source is set at the fracture entrance. During the natural convection heat transfer stage, the amount of carbon dioxide supplied to the fracture entrance by the carbon dioxide supply source is the same as the amount of carbon dioxide flowing out of the fracture exit, so that there is a stable supply of carbon dioxide in the fracture of the dry hot rock sample and the natural convection heat transfer of carbon dioxide continuously occurs in the fracture of the dry hot rock sample; at the same time, the pressure applied by the carbon dioxide supply source to the fracture entrance is controlled to be equal to the pressure at the fracture exit, so as to avoid the occurrence of forced convection during the supply process.

[0044] S4. Monitor the starting and ending temperatures of natural convection of carbon dioxide in the fissures of the middle section;

[0045] S5. When there is a stable outflow of carbon dioxide from the fracture outlet and the temperature monitoring data is stable, the natural convective heat transfer coefficient h of carbon dioxide in the fractures of the hot dry rock reservoir is calculated using the following formula:

[0046]

[0047] Where: h is the natural convection heat transfer coefficient, in W / (m²) 2 ·℃); t b The initial temperature is expressed in °C; t e The final temperature is expressed in °C; W is the electrical power of the heating resistor used to heat the middle section, expressed in W; ε is the emissivity of the hot dry rock; A is the area of ​​the fracture surface, expressed in m². 2 C0 is the blackbody radiation coefficient, in W / (m²). 2 ·K 2 );T b The initial temperature is in Kelvin (K); T e Termination temperature, unit K;

[0048] S6. Analyze the influence of different conditions on the natural convection heat transfer coefficient h calculated under different conditions (such as temperature, confining pressure, fracture aperture, fracture roughness, etc.). According to similarity theory, the Nusselt number Nu of natural convection heat transfer is a function of the Grashof number Gr and the Prandtl number Pr. Plot the experimental data on a double logarithmic coordinate system with ln Nu as the ordinate and ln(Pr·Gr) as the abscissa. Determine the coefficients C and n using regression analysis. Based on the coefficients C and n obtained from the above similarity analysis of natural convection heat transfer, derive the criterion equation.

[0049] In this embodiment, the experimental method divides the hot dry rock sample into a front section, a middle section, and a rear section, and heats the front and rear sections to form a low-temperature region and a high-temperature region, respectively. By applying external force, carbon dioxide is injected into the fissures of the hot dry rock sample through forced convection. Then, the external force is stopped, and the pressure difference between the fissure inlet and outlet is zero to end the forced convection process. At this point, the low-temperature and high-temperature regions promote natural convection of carbon dioxide in the fissures, achieving an overall natural convection trend from the fissure inlet to the fissure outlet. A carbon dioxide supply source is set at the fissure inlet, and the amount of carbon dioxide supplied to the fissure inlet by the carbon dioxide supply source is the same as that supplied to the fissure outlet. By ensuring a consistent outflow of carbon dioxide, a stable supply of carbon dioxide is maintained within the fissures of the hot dry rock sample, enabling continuous natural convection heat transfer within these fissures. Simultaneously, controlling the pressure at the fissure inlet to be equal to the pressure at the fissure outlet ensures that the flow of carbon dioxide from the fissure inlet to the outlet is unaffected by external forces, preventing forced convection during the supply process. Monitoring the initiation and termination temperatures of natural convection within the fissures in the middle section of the hot dry rock sample avoids the influence of temperature variations in designated low-temperature and high-temperature zones on temperature monitoring. The natural convection heat transfer coefficient is calculated using a formula to analyze natural convection heat transfer under different conditions. This method fills a gap in experimental methods for natural convection heat transfer of carbon dioxide in high-temperature rock fissures.

[0050] In one implementation, in step S1, the front section is heated to a temperature 20°C lower than a preset temperature; the rear section is heated to a temperature 20°C higher than a preset temperature, so that a temperature difference is formed between the front, middle and rear sections of the dry hot rock sample.

[0051] Preferably, in step S2, carbon dioxide is first injected into the fissures of the hot dry rock sample through the fissure inlet at a certain pressure, and the pressure is stopped after a stable flow of carbon dioxide is observed from the fissure outlet of the hot dry rock sample.

[0052] It is understandable that in step S2, "first injecting carbon dioxide into the fissures of the dry hot rock sample through the fissure inlet at a certain pressure" means "applying external force to inject carbon dioxide into the fissures of the dry hot rock sample through the fissure inlet in a forced convection manner", "waiting for a stable flow of carbon dioxide from the fissure outlet" means "carbon dioxide has been injected into the fissures of the dry hot rock sample", and "stopping the application of external force" means "ending the forced convection process".

[0053] Specifically, the formula for calculating the natural convection heat transfer coefficient h in step S5 includes:

[0054]

[0055] Q c =WQ r

[0056]

[0057] Where: h is the natural convection heat transfer coefficient, in W / (m²) 2 ·℃); Q r Radiative efficiency, in W; Q c Natural convection heat transfer efficiency, in W; t b The initial temperature is expressed in °C; t e Termination temperature (°C); W is the electrical power of the heating resistor (W); ε is the emissivity of the hot dry rock; A is the area of ​​the fracture surface (m²). 2 C0 is the blackbody radiation coefficient, in W / (m²). 2 ·K 2 );T b The initial temperature is in Kelvin (K); T e Termination temperature, unit K.

[0058] In a specific embodiment, in the above test method, in step S1, the middle section is divided into a test section, a front buffer section connecting the test section and the front section, and a rear buffer section connecting the test section and the rear section. The test section, the front buffer section and the rear buffer section are heated by independent heating resistors.

[0059] In step S4, the starting temperature is the carbon dioxide temperature in the crack of the test section near the front buffer section, and the ending temperature is the carbon dioxide temperature in the crack of the test section near the rear buffer section.

[0060] In step S5, W is the electrical power of the heating resistor used to heat the test section.

[0061] With the above settings, the front and rear buffer sections prevent the temperature difference between the front and rear sections from affecting the monitoring of carbon dioxide temperature in the cracks of the test section, which helps to improve the accuracy of temperature monitoring and ensures that the experimental results are accurate and reliable.

[0062] According to another aspect of this application, embodiments of this application also provide an experimental apparatus for natural convection heat transfer of carbon dioxide in high-temperature rocks, employing the experimental methods described in the above embodiments, such as... Figures 1-3 As shown, the test apparatus includes a core holder 10, a pressure system 40, a heating system 20, and an injection system 30.

[0063] The core holder 10 includes a rubber sleeve 104, the interior of which forms a core chamber 100 for placing the rock sample. The core chamber 100 is divided into upper and lower chambers 1001. From left to right, the core chamber 100 is divided into a low-temperature auxiliary zone 112, a front buffer zone 113, a test zone 114, a rear buffer zone 110, and a high-temperature auxiliary zone 109. Heat insulation plates 106 are installed at both ends of the low-temperature auxiliary zone 112, the front buffer zone 113, the test zone 114, the rear buffer zone 110, and the high-temperature auxiliary zone 109. The heat insulation plates 106 are used to insulate the temperature between the different zones. A gasket 108 is provided between the upper and lower chambers 1001 of the core chamber 100. The fissure opening of the dry hot rock sample can be adjusted by changing the thickness of the gasket 108. Figure 3 (shown as L in the image); a metal cylinder 116 is fitted over the rubber sleeve 104, with a gap between the metal cylinder 116 and the rubber sleeve 104. The gap is divided into five independent confining pressure chambers 115, each corresponding to a low-temperature auxiliary zone 112, a front buffer zone 113, a test zone 114, a rear buffer zone 110, and a high-temperature auxiliary zone 109, respectively. Plugs 105 are installed at both ends of the rubber sleeve 104 for sealing, and locking caps 103 are installed on the outside of the plugs 105. The locking caps 103 are fixed to the metal sleeve 104 by bolts 102. The end of the metal cylinder 116 is used to seal the plug 105; the plug 105 near the low temperature auxiliary zone 112 has a central opening and is connected to a capillary steel tube as a carbon dioxide injection pipe 111; the plug 105 near the high temperature auxiliary zone 109 has a central opening and is connected to a capillary steel tube as a carbon dioxide output pipe 107; a first gas path check valve 117 is provided between the plug 105 and the carbon dioxide output pipe 107 to prevent carbon dioxide backflow; the metal cylinder 116 is wrapped with a heat insulation layer 101.

[0064] The pressure system 40 is used to provide pressure to each independent confining chamber 115 to apply confining pressure to the dry hot rock sample in the core chamber 100.

[0065] The heating system 20 includes five heating resistors 203 and two thermocouples 204. The five heating resistors 203 are respectively installed on the inner walls of five independent confining chambers 115. One thermocouple 204 enters the test area 114 of the core chamber 100 from the carbon dioxide injection pipe 111 and is located near the front buffer zone 113. The other thermocouple 204 enters the test area 114 of the core chamber 100 from the carbon dioxide output pipe 107 and is located near the rear buffer zone 110.

[0066] The injection system 30 is used to apply external force to inject carbon dioxide into the fissures of the dry hot rock sample through the carbon dioxide injection pipe 111; the injection system 30 is also used to inject carbon dioxide into the carbon dioxide injection pipe 111 in an amount equal to the amount of carbon dioxide flowing out of the carbon dioxide output pipe 107 after natural convection of carbon dioxide occurs in the fissures of the dry hot rock sample, while controlling the internal pressure of the carbon dioxide injection pipe 111 and the carbon dioxide output pipe 107 to be equal, so as to achieve continuous natural convection.

[0067] In this embodiment, the experimental device divides the core chamber 100 in the core holder 10 into five regions: a low-temperature auxiliary zone 112, a front buffer zone 113, a test zone 114, a rear buffer zone 110, and a high-temperature auxiliary zone 109. The temperature difference created by the low-temperature auxiliary zone 112 and the high-temperature auxiliary zone 109 ensures that the overall trend of natural convection flows from the carbon dioxide injection pipe 111 to the carbon dioxide output pipe 107, avoiding the influence of temperature differences between the two ends and the outside environment on the overall trend of natural convection. The front buffer zone 113 and the rear buffer zone 110 prevent the low-temperature auxiliary zone 112 and the high-temperature auxiliary zone 109 from affecting temperature monitoring. By changing the thickness of the gasket 108 between the upper and lower chambers 1001 in the core chamber 100, the fissure opening of the dry hot rock sample is controlled. The injection system 30 injects an equal amount of carbon dioxide into the carbon dioxide injection pipe 111 as it flows out of the carbon dioxide output pipe 107, while simultaneously controlling the internal pressure of the carbon dioxide injection pipe 111 and the carbon dioxide output pipe 107 to be equal, ensuring smooth and continuous natural convection. This device enables indoor experiments on the natural convection of carbon dioxide in high-temperature rock fissures.

[0068] Preferably, the insulation board 106 is made of mica, which has the characteristics of high temperature and high pressure resistance, good thermal insulation, processability and electrical insulation.

[0069] The pressure system 40 includes a pressure pump 401. The output end of the pressure pump 401 is connected to five capillary steel pipes through a main pipeline. The five capillary steel pipes are connected to five independent confining pressure chambers 115 respectively. A confining pressure gauge P is installed on the main pipeline, and a fourth valve V4 is installed upstream of the confining pressure gauge P.

[0070] The heating system 20 also includes a temperature control component 201 and five temperature probes 202; the temperature control component 201 is electrically connected to five heating resistors 203 respectively, and is used to control the power of each heating resistor 203; the five temperature probes 202 are respectively set in five independent confining pressure chambers 115.

[0071] In one specific embodiment, such as Figure 1As shown, the injection system 30 includes a constant speed and constant pressure pump 302, a carbon dioxide tank 301, a carbon dioxide recovery tank 303, a gas path check valve 306, and a refrigeration bath 305; the carbon dioxide tank 301 is connected to the carbon dioxide injection pipe 111 via an injection pipe; the carbon dioxide recovery tank 303 is connected to the carbon dioxide output pipe 107 via a return pipe; a gas supply pipe connects the carbon dioxide tank 301 and the refrigeration bath 305, and a second valve V2 is installed on it; a gas supply pipe connects the refrigeration bath 305 and the carbon dioxide recovery tank 303; the refrigeration bath 305... 5. An upstream gas path one-way valve 306 is installed, which allows carbon dioxide to flow into the refrigeration bath 305 and prevents carbon dioxide from flowing out of the refrigeration bath 305; the refrigeration bath 305 is connected to the atmosphere via a pipeline and a third valve V3 is installed between it and the second valve V2; a constant speed and constant pressure pump 302 is connected to the carbon dioxide tank 301 via a pipeline; a first valve V1 is installed on the pipeline between the carbon dioxide tank 301 and the constant speed and constant pressure pump 302; during the experiment, after the device is assembled, the constant speed and constant pressure pump 302 is run, and the first valve V1 and the third valve V3 are opened. By closing valve V3 and valve V2, air in the fissures and pipes can be discharged, filling the entire pipe with carbon dioxide and preventing air from affecting subsequent experiments. During the experiment, after forced convection ends, valve V1 is closed and valve V2 is opened. The fissures in the dry hot rock sample, carbon dioxide output pipe 107, carbon dioxide recovery tank 303, cooling bath 305, carbon dioxide gas tank 301, and carbon dioxide injection pipe 111 form a closed loop, ensuring that the amount of carbon dioxide injected into carbon dioxide injection pipe 111 in the same time is the same as the amount of carbon dioxide flowing out of carbon dioxide output pipe 107. At the same time, the internal pressure of carbon dioxide injection pipe 111 and carbon dioxide output pipe 107 is controlled to be equal (in one implementation, the internal pressure of carbon dioxide injection pipe 111, carbon dioxide output pipe 107, carbon dioxide gas tank 301, and carbon dioxide recovery tank 303 can be set to be the same as atmospheric pressure), thus achieving continuous natural convection. Cooling bath 305 lowers the carbon dioxide temperature to room temperature, preventing high-temperature carbon dioxide at the outlet from affecting the carbon dioxide circulation and replenishment process.

[0072] The injection system 30 also includes a carbon dioxide flow meter 304; the carbon dioxide flow meter 304 is installed at the outlet of the carbon dioxide output pipe 107 and is used to monitor the outlet carbon dioxide flow rate. The specific implementation process of this test device is as follows:

[0073] 1. Installation of the test apparatus: according to... Figure 1Ten rock samples, six heat insulation plates 106 (the shape of the heat insulation plates 106 is adapted to the internal cross-section of the rubber sleeve 104, and a through hole for gas flow is provided in the middle), and two gaskets 108 are placed in the corresponding areas of the core chamber 100 in sequence. Then, two plugs 105 are used to plug both ends of the rubber sleeve 104, and two locking caps 103 are fixed to both ends of the metal cylinder 116 with bolts 102. Then, the heat insulation layer 101 is wrapped around the metal cylinder 116. One thermocouple 204 is inserted from the carbon dioxide injection pipe 111 into the fissure at the end of the test area 114 near the front buffer zone 113 in the core chamber 100, and another thermocouple 204 is inserted from the carbon dioxide output pipe 107 into the fissure at the end of the test area 114 near the rear buffer zone 110 in the core chamber 100. The pipes / lines of the pressure system 40, heating system 20, and injection system 30 are connected to complete the installation of the test device.

[0074] 2. Applying confining pressure: Open the fourth valve V4 and control the pressure pump 401 to inject high-temperature hydraulic oil into each independent confining pressure chamber 115. When the value on the confining pressure gauge P stabilizes at the set value, close the fourth valve V4 to complete the confining pressure application.

[0075] 3. Temperature loading: The temperature control component 201 controls the heating resistors 203 to heat (it can be understood that the temperature is conducted to the rock sample in the corresponding area of ​​the core chamber 100 through the high-temperature resistant hydraulic oil in the corresponding independent confining chamber 115), so that the front buffer zone 113, the rear buffer zone 110 and the test area 114 (that is, the middle section of the dry hot rock sample set in step S1 in the above test method embodiment) are heated to the preset temperature, for example, 100℃-200℃. The low temperature auxiliary area 112 is heated to 20℃ higher than the preset temperature, and the high temperature auxiliary area 109 is heated to 20℃ higher than the preset temperature. The temperature loading is completed when the temperature of each area measured by the temperature probe 202 reaches the corresponding target value and remains stable for 1 hour.

[0076] 4. Conduct the experiment: Run the constant speed and pressure pump 302, open the first valve V1 and the third valve V3 and close the second valve V2. Purge the fissures of the hot dry rock sample and the entire pipeline with carbon dioxide to remove air from the pipeline and prevent air from affecting subsequent experiments. Close the third valve V3 and use the constant speed and pressure pump 302 to control the carbon dioxide tank 301 to inject carbon dioxide into the fissures of the hot dry rock sample from the carbon dioxide injection pipe 111 at a certain pressure. When the carbon dioxide flow meter 304 detects a stable flow of carbon dioxide from the carbon dioxide output pipe 107, the constant speed and pressure pump 302 stops working, and the first valve V1 is closed and the second valve V2 is opened. Then, use the left thermocouple 204 to monitor the temperature of the end of the test area 114 near the front buffer zone 113, and use the right thermocouple 204 to monitor the temperature of the end of the test area 114 near the rear buffer zone 110. When there is a stable flow of carbon dioxide from the carbon dioxide output pipe 107 and the temperature monitoring data is stable, end the experiment, turn off the heating power switch and remove the confining pressure. Disassemble the device after it has cooled down naturally.

[0077] 5. Change the temperature, confining pressure, crack aperture, or crack roughness, and repeat steps 1-4 above.

[0078] 6. Analyze the influence of different conditions on the natural convection heat transfer coefficient by using the natural convection heat transfer coefficient calculated under different conditions.

[0079] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0080] The above embodiments are merely illustrative of several implementation methods of this application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A test method for natural convection heat transfer of carbon dioxide in high-temperature rocks, characterized in that, Includes the following steps: S1. The hot dry rock sample is divided into a front section, a middle section, and a rear section along the direction of its fracture extension. The front section has a fracture inlet, and the rear section has a fracture outlet. The front section, the middle section, and the rear section are heated by heating resistors. The middle section is heated to a preset temperature to simulate the temperature environment of the hot dry rock reservoir. The front section is heated to a temperature lower than the preset temperature to form a low-temperature region. The rear section is heated to a temperature higher than the preset temperature to form a high-temperature region. The middle section is divided into a test section, a front buffer section connecting the test section and the front section, and a rear buffer section connecting the test section and the rear section. The test section, the front buffer section, and the rear buffer section are heated by independent heating resistors. S2. Apply external force to inject carbon dioxide into the fissures of the hot dry rock sample through the fissure inlet. After the carbon dioxide stably flows out of the fissure outlet, stop applying external force and make the pressure difference between the fissure inlet and outlet zero to end the forced convection process. At this time, the carbon dioxide in the fissures of the low-temperature region has a low temperature and high density, while the carbon dioxide in the fissures of the middle section has a relatively high temperature and relatively low density, causing carbon dioxide to flow from the fissures of the low-temperature region to the fissures of the middle section. This flow process is not driven by external force and is a natural convection process. The carbon dioxide in the fissures of the high-temperature region has a high temperature and low density. The carbon dioxide in the middle section of the fissure is relatively cold and relatively dense, causing it to flow from the fissure to the high-temperature region. This flow is not driven by external force and is a natural convection process. The temperature difference between the high-temperature region and the low-temperature region ensures that the overall trend of natural convection of carbon dioxide in the fissure of the hot dry rock sample is from the fissure inlet to the fissure outlet. During the flow, carbon dioxide exchanges heat with the fissure surface of the hot dry rock sample, which is natural convection heat transfer. When carbon dioxide is detected flowing out of the fissure outlet, it indicates that natural convection of carbon dioxide has occurred in the fissure of the hot dry rock sample. S3. A carbon dioxide supply source is set at the fracture inlet. During the natural convection heat transfer stage, the amount of carbon dioxide supplied to the fracture inlet by the carbon dioxide supply source is the same as the amount of carbon dioxide flowing out of the fracture outlet, so that there is a stable supply of carbon dioxide in the fracture of the hot dry rock sample and that natural convection heat transfer of carbon dioxide continuously occurs in the fracture of the hot dry rock sample; at the same time, the pressure applied by the carbon dioxide supply source to the fracture inlet is controlled to be equal to the pressure at the fracture outlet, so as to avoid the occurrence of forced convection during the supply process; S4. Monitor the starting and ending temperatures of natural convection of carbon dioxide in the fissures of the middle section, wherein the starting temperature is the temperature of carbon dioxide in the fissures of the test section near the front buffer section, and the ending temperature is the temperature of carbon dioxide in the fissures of the test section near the rear buffer section. S5. When there is a stable outflow of carbon dioxide from the fracture outlet and the temperature monitoring data is stable, the natural convective heat transfer coefficient h of carbon dioxide in the fracture of the hot dry rock reservoir is calculated using the following formula: Where: h is the natural convection heat transfer coefficient, in W / (m²) 2 ·℃); t b The initial temperature is expressed in °C; t e The termination temperature is expressed in °C; W represents the electrical power of the heating resistor used to heat the test section, expressed in W. A represents the blackness of the hot, dry rock; A is the area of ​​the fracture surface, in meters. 2 C0 is the blackbody radiation coefficient, in W / (m²). 2 ·K 2 );T b The initial temperature is in Kelvin (K); T e Termination temperature, unit K; S6. Analyze the influence of different conditions on the natural convection heat transfer coefficient using the natural convection heat transfer coefficient calculated under different conditions.

2. The test method according to claim 1, characterized in that, The fissure outlet is equipped with a one-way valve, which only allows carbon dioxide to flow out of the fissure in the hot dry rock sample, preventing convection that would occur in the opposite direction to the natural convection of carbon dioxide in the fissure due to the decrease in temperature and increase in density after the carbon dioxide flows out of the fissure outlet.

3. The test method according to claim 1, characterized in that, In step S2, carbon dioxide is first injected into the fissures of the dry hot rock sample through the fissure inlet at a certain pressure, and the pressure is stopped after a stable flow of carbon dioxide is seen from the fissure outlet.

4. The test method according to claim 1, characterized in that, The formula for calculating the natural convection heat transfer coefficient h in step S5 specifically includes: Where: h is the natural convection heat transfer coefficient, in W / (m²) 2 ·℃); Q r Radiative efficiency, in W; Q c The heat transfer efficiency is expressed in W / t (watts). b The initial temperature is expressed in °C; t e The termination temperature is expressed in °C; W represents the electrical power of the heating resistor used to heat the test section, expressed in W. A represents the blackness of the hot, dry rock; A is the area of ​​the fracture surface, in meters. 2 C0 is the blackbody radiation coefficient, in W / (m²). 2 ·K 2 );T b The initial temperature is in Kelvin (K); T e Termination temperature, unit K.

5. The test method according to claim 1, characterized in that, Step S6 specifically includes: According to similarity theory, the Nusselt number Nu of natural convection heat transfer is a function of the Grashof number Gr and the Prandtl number Pr. The experimental data are plotted on a double logarithmic coordinate system with lnNu as the ordinate and ln(Pr·Gr) as the abscissa. The coefficients C and n are determined by regression analysis. Based on the coefficients C and n obtained from the similarity analysis of natural convection heat transfer above, the criterion equation is derived.

6. An experimental apparatus for natural convection heat transfer of carbon dioxide in high-temperature rocks, employing the experimental method described in any one of claims 1-5, characterized in that, The test apparatus includes: A core holder includes a rubber sleeve, the interior of which forms a core chamber for placing a rock sample. The core chamber is divided into upper and lower chambers, which, from left to right, are sequentially divided into a low-temperature auxiliary zone, a front buffer zone, a test zone, a rear buffer zone, and a high-temperature auxiliary zone. Heat insulation plates are installed at both ends of the low-temperature auxiliary zone, the front buffer zone, the test zone, the rear buffer zone, and the high-temperature auxiliary zone to insulate the temperature between the different zones. A gasket is provided between the upper and lower chambers of the core chamber; the thickness of the gasket can be adjusted to regulate the fracture aperture of the hot, dry rock sample. A metal cylinder is fitted over the rubber sleeve, with a gap between the metal cylinder and the rubber sleeve. The gap is divided into five independent confining chambers, each corresponding to the low-temperature auxiliary zone, the front buffer zone, the test zone, the rear buffer zone, and the high-temperature auxiliary zone. The rubber sleeve has plugs at both ends for sealing, and locking caps are installed on the outside of the plugs. The locking caps are fixed to the ends of the metal cylinder with bolts to tighten the plugs. A capillary tube is connected to the middle opening of the plug near the low-temperature auxiliary zone, serving as a carbon dioxide injection pipe. A capillary tube is also connected to the middle opening of the plug near the high-temperature auxiliary zone, serving as a carbon dioxide output pipe. A first gas path one-way valve is installed between the plug and the carbon dioxide output pipe to prevent carbon dioxide backflow. The metal cylinder is wrapped with a heat insulation layer. A pressure system is used to provide pressure to each of the independent confining chambers to apply confining pressure to the hot dry rock sample in the core chamber; The heating system includes five heating resistors and two thermocouples. The five heating resistors are respectively and correspondingly installed on the inner walls of the five independent confining pressure chambers. One thermocouple enters the core chamber test area from the carbon dioxide injection pipe near the front buffer zone; the other thermocouple enters the core chamber test area from the carbon dioxide output pipe near the rear buffer zone. An injection system is used to apply external force to inject carbon dioxide from the carbon dioxide injection pipe into the fissures of the hot dry rock sample; the injection system is also used to inject carbon dioxide into the carbon dioxide injection pipe in an amount equal to the amount of carbon dioxide flowing out of the carbon dioxide output pipe after natural convection of carbon dioxide occurs in the fissures of the hot dry rock sample, while controlling the internal pressure of the carbon dioxide injection pipe and the carbon dioxide output pipe to be equal, so as to achieve continuous natural convection.

7. The experimental apparatus according to claim 6, characterized in that, The injection system includes a constant-speed, constant-pressure pump, a carbon dioxide cylinder, a carbon dioxide recovery tank, a second gas path check valve, and a refrigeration bath. The carbon dioxide cylinder is connected to the carbon dioxide injection pipe via an injection pipe. The carbon dioxide recovery tank is connected to the carbon dioxide output pipe via a return pipe. A gas supply pipe connects the carbon dioxide cylinder and the refrigeration bath and is equipped with a second valve. A gas supply pipe also connects the refrigeration bath and the carbon dioxide recovery tank. A second gas path check valve is installed upstream of the refrigeration bath, allowing carbon dioxide to flow into the refrigeration bath and preventing carbon dioxide from flowing out of the refrigeration bath. A pipeline connects the refrigeration bath and the second valve to the atmosphere and is equipped with a third valve. The constant-speed, constant-pressure pump is connected to the carbon dioxide cylinder via a pipeline. A first valve is installed on the pipeline between the carbon dioxide cylinder and the constant-speed, constant-pressure pump. The refrigeration bath lowers the carbon dioxide temperature to room temperature to prevent high-temperature carbon dioxide flowing out of the carbon dioxide output pipe from affecting the carbon dioxide circulation and replenishment process.

8. The test apparatus according to claim 6, characterized in that, The pressure system includes a pressure pump, the output end of which is connected to five capillary steel pipes via a main pipeline. Each of the five capillary steel pipes is connected to one of the five independent confining pressure chambers. A confining pressure gauge is installed on the main pipeline, and a fourth valve is installed upstream of the confining pressure gauge.

9. The experimental apparatus according to claim 6, characterized in that, The heating system also includes a temperature control component and five temperature probes; the temperature control component is electrically connected to the five heating resistors respectively, and is used to control the power of each heating resistor; the five temperature probes are respectively set in the five independent confining pressure chambers.

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