Seam drilling liquid carbon dioxide phase transition pressure pulsation change simulation experiment device and design method

Abstract

The application discloses a simulation experiment device for pressure pulsation change of liquid carbon dioxide phase transition in a borehole along a stratum, which comprises a limited-space liquid carbon dioxide phase transition pressure pulsation experiment container assembly, and the limited-space liquid carbon dioxide phase transition pressure pulsation experiment container assembly is externally wrapped with a rigid annular constant-temperature medium cavity assembly; the device further comprises a constant-temperature medium circulator which is connected with the rigid annular constant-temperature medium cavity assembly through a pipeline. The device can accurately invert the pressure pulsation law of the borehole in the process of injecting liquid carbon dioxide to displace gas in a coal mine underground. A design method of the simulation experiment device for pressure pulsation change of liquid carbon dioxide phase transition in a borehole along a stratum is also disclosed.

Description

Technical Field

[0001] This invention belongs to the field of gas disaster control and extraction technology in high-gas, low-permeability coal seams in the coal mining industry. Specifically, it relates to a simulation experimental device for pressure pulsation changes during liquid carbon dioxide phase change in in-seam boreholes, and also to a design method for such a device. Background Technology

[0002] Deep-hole blasting, hydraulic fracturing, and other coal seam gas extraction technologies have emerged. Liquid carbon dioxide, with its low viscosity and strong permeability, is more effective at replacing adsorbed gas in coal seams and driving the original free gas and the replaced free gas into the extraction pipeline, thereby improving the extraction efficiency of coal seam gas.

[0003] Extensive field industrial practice has revealed that under specific injection parameter combinations, the pressure in the injection borehole exhibits pulsating changes. During this phase, indicators such as gas concentration, pipeline extraction mixing volume, and gas extraction purity in the extraction pipeline are all better than under normal extraction conditions. Based on this, exploring the formation mechanism and evolution law of borehole pressure pulsation, and determining the optimal injection pressure, injection flow rate, and injection temperature difference for the target coal seam are the scientific basis for achieving the above objectives. However, currently, there is no similar experimental simulation equipment or device in this field to conduct pulsating experiments on the phase change pressure of liquid carbon dioxide in confined spaces. Therefore, based on the principle of similarity, and taking the in-seam injection borehole in underground coal mines as a prototype, a design method for a confined space liquid carbon dioxide phase change pressure pulsation experimental container and its outer rigid annular constant temperature medium cavity is proposed. Summary of the Invention

[0004] The first objective of this invention is to create an experimental device for simulating the pressure pulsation changes during the phase change of liquid carbon dioxide in boreholes along the bedding plane, which can accurately invert the pressure pulsation pattern during the injection of liquid carbon dioxide to displace methane in coal mines.

[0005] The second objective of this invention is to design a method for simulating the pressure pulsation of liquid carbon dioxide phase change in in-seam boreholes. This method allows for a direct and vivid observation of the characteristics of borehole pressure pulsation, and also enables the exploration of the formation mechanism based on the influencing factors of borehole pressure pulsation. This provides a scientific basis for selecting technical parameters for injecting liquid carbon dioxide into underground coal seams and for understanding and applying the laws governing borehole pressure changes.

[0006] The first technical solution adopted in this invention is a simulation experimental device for pressure pulsation of liquid carbon dioxide phase change in in-seam drilling, which includes a confined space liquid carbon dioxide phase change pressure pulsation experimental container assembly, which is surrounded by a rigid annular constant temperature medium cavity assembly, and also includes a constant temperature medium circulator, which is connected to the rigid annular constant temperature medium cavity assembly through a pipeline.

[0007] The invention is further characterized in that:

[0008] The confined space liquid carbon dioxide phase change pressure pulsation experimental container assembly includes three confined space liquid carbon dioxide phase change pressure pulsation experimental containers, each of which is a tubular container; each confined space liquid carbon dioxide phase change pressure pulsation experimental container is connected to the other confined space liquid carbon dioxide phase change pressure pulsation experimental containers by threaded connectors, the first confined space liquid carbon dioxide phase change pressure pulsation experimental container has an inlet at its head end, and the third confined space liquid carbon dioxide phase change pressure pulsation experimental container has an outlet at its tail end;

[0009] The rigid annular constant temperature medium cavity assembly includes three outer rigid annular constant temperature medium cavities. Each confined space liquid carbon dioxide phase change pressure pulsation experimental container has an outer rigid annular constant temperature medium cavity wrapped around its outer wall. The first outer rigid annular constant temperature medium cavity has a cavity outlet, and the third outer rigid annular constant temperature medium cavity has a cavity inlet. The cavity outlet is connected to the constant temperature medium circulator through a cavity outlet pipe, and the cavity inlet is connected to the constant temperature medium circulator through a cavity inlet pipe. Every two adjacent outer rigid annular constant temperature medium cavities are connected by a medium transmission connecting pipe. The connection between the medium transmission connecting pipe and the outer rigid annular constant temperature medium cavity is the outer rigid annular constant temperature medium cavity medium transmission connecting hole.

[0010] It also includes multiple temperature sensors, with several temperature sensor probes installed inside each confined space liquid carbon dioxide phase change pressure pulsation experimental container; all temperature sensors are connected to temperature sensor data acquisition lines.

[0011] It also includes multiple pressure sensors. Each confined space liquid carbon dioxide phase change pressure pulsation experimental container is equipped with several pressure sensor probes; all pressure sensors are connected to pressure sensor data acquisition lines.

[0012] The second technical solution adopted in this invention is a design method for an experimental device simulating the pressure pulsation of liquid carbon dioxide phase change in in-seam drilling, comprising the following steps:

[0013] S1. Based on the similarity criteria, derive expressions for geometric similarity, kinematic similarity, dynamic similarity, convective heat transfer similarity, injection pressure similarity, and exudation flow rate similarity between the prototype and the model.

[0014] S2. Accurately select similarity criterion numbers and obtain the similarity scale of liquid carbon dioxide phase change pressure;

[0015] S3. Determine the specific values ​​of the similarity scale in S1 for geometry, motion, dynamics, convective heat transfer, injection pressure, and exudation flow rate;

[0016] S4. Calculate the equivalent radius of the rigid annular isothermal medium cavity surrounding the container and the minimum flow velocity of the medium in the cavity.

[0017] The invention is further characterized in that:

[0018] Step S1 is as follows:

[0019] Step S1.1: Obtain the expression for the geometric similarity between the prototype and the model, specifically:

[0020] To ensure that the feature sizes of the model and the prototype meet a certain ratio, l is used. p Indicates the axial length of the prototype's active segment, l m D represents the axial length of the model. p D represents the diameter of the prototype. m D' represents the diameter of the model. p D' represents the diameter of the pipe at the carbon dioxide outlet in the prototype. m The diameter of the model inlet is represented by ; however, the model cannot completely simulate the process of two-phase CO2 seeping into the far field of the coal seam through the periphery fractures of the injection hole in the well. Therefore, it is simplified to the outlet of the container. Based on geometric similarity, the linear scale, area scale, and volume scale of geometric similarity are obtained as shown in equation (1):

[0021]

[0022] In the formula: A p and A m Let m represent the radial cross-sectional areas of the prototype and the model, respectively. 2 V p and V m Let m represent the volume of the prototype and the model, respectively. 3 C D It is the similarity ratio between the prototype feature length and the model feature length, and its value is called the similarity multiple between the two.

[0023] Step S1.2: Derive the expression for the motion similarity between the prototype and the model, specifically:

[0024] In the study, t p The time taken for liquid carbon dioxide particles to flow in the prototype is expressed in t. m The time of flow of liquid carbon dioxide particles in the model is represented by the similarity ratio between the two as shown in equation (2):

[0025]

[0026] In the formula: C t The time similarity scale between the prototype and the model is called the similarity multiple between the two.

[0027] Based on the time similarity scale, the velocity similarity scale and acceleration similarity scale of the carbon dioxide flow between the prototype inlet and the model inlet are obtained as shown in equation (3):

[0028]

[0029] In the formula: and represents the average velocity of liquid carbon dioxide particles in the prototype and model, respectively, in m / s; A fluid velocity similarity scale between the prototype and the model; a p and a m Represent the flow acceleration of liquid carbon dioxide particles in the prototype and model, respectively, in m / s². 2 C a Fluid acceleration similarity ratio between prototype and model;

[0030] Step S1.3: Derive the expression for the dynamic similarity between the prototype and the model, specifically:

[0031]

[0032] In the formula: F p and F m Represent the forces acting on liquid carbon dioxide particles in the prototype and model, respectively, in N; m p and m m The values ​​of C represent the mass of liquid carbon dioxide in the downhole borehole and the experimental container, respectively, in kg; F The similarity scale of forces between the prototype and the model fluid;

[0033] Step S1.4: Derive the similarity expression for convective heat transfer between the prototype and the model, specifically as follows:

[0034]

[0035] Where: h 1-p The surface heat transfer coefficient for convective heat transfer in the prototype is W / (m²). 2 ·K);h 1-m The surface heat transfer coefficient for convective heat transfer in the model is W / (m²). 2 ·K); T st The temperature of the inner wall of the container in the model is given in °C.

[0036] Simplifying and generalizing equation (5), we obtain equation (6):

[0037]

[0038] Step S1.5: Derive the expression for similar injection pressure between the prototype and the model, specifically:

[0039] The pressure of the numerator term in the Euler criterion is expressed as the pressure difference ΔP. z If we replace it, the injection pressures of the prototype and the model are similar and satisfy the relationship of equation (7), which is expressed as follows:

[0040]

[0041] In the formula: △P z-p and △P z-m The pressure difference between the prototype and the model injected with liquid carbon dioxide, in MPa; C △Pz P0 is the similarity ratio of the pressure difference between the injected liquid carbon dioxide in the prototype and the model, dimensionless; P0 is the original pressure in the prototype or model, in MPa.

[0042] Step S1.6: Derive expressions for the similarity of percolation flow rates in the prototype and model, specifically:

[0043] Under continuous injection pressure, The real-time flow rate of two-phase carbon dioxide is measured as it seeps into the coal seam through the borehole wall. The amount of carbon dioxide that seeps into the coal seam within a time interval Δt is the real-time integral value. This process is called "pressurized outflow". Therefore, the similarity of the two-phase carbon dioxide seepage is the initial condition. In the downhole test, the bottom of the borehole is closed, and the two-phase carbon dioxide enters the coal seam through the cracks in the borehole wall. In the physical similarity simulation experiment, the wall of the confined space liquid carbon dioxide phase change pressure pulsation experimental container is without cracks. The container outlet is used to simulate the two-phase carbon dioxide seepage channel on the borehole wall in the downhole.

[0044] Q out-p The average percolation flow rate of carbon dioxide in the two phases of the prototype is represented by Q. out-m The average percolation flow of carbon dioxide in the representative model is shown in equation (8).

[0045]

[0046] In the formula: A 1-p The area of ​​carbon dioxide seeping out from the two phases in the prototype is represented by m. 2 A 1-m The area of ​​the model's outlet is represented in m. 2 ; The velocity of carbon dioxide seeping out in the prototype is expressed in m / s. The velocity of carbon dioxide seepage in the two phases of the model is expressed in m / s.

[0047] Step S2 is as follows:

[0048] Step S2.1: Accurately select the similarity criterion number, specifically as follows:

[0049]

[0050] In the formula: α l m is the thermal diffusivity of the fluid. 2 / s; P' is the fluid dynamic pressure, MPa;

[0051] Step S2.2: Obtain the similarity scale of the phase transition pressure of liquid carbon dioxide, specifically:

[0052] In both this model and the prototype, the temperature during carbon dioxide injection is -30℃, and the physical properties of liquid carbon dioxide are assumed to remain consistent, as shown in equation (10):

[0053]

[0054] When the fluid flow and convective heat transfer are similar in the model and the prototype, the proportional relationship between physical quantities is derived from the number of similarity criteria, as shown in Equation (11), based on satisfying the selected similarity criteria:

[0055]

[0056] Based on the similarity scale of the convective heat transfer coefficient obtained in equation (11), the final expression for the similarity scale of the prototype and model heat transfer in equation (6) is obtained, as shown in equation (12):

[0057]

[0058] The pressure caused by the endothermic phase change of liquid carbon dioxide requires determining the convective heat transfer similarity ratio between the prototype and the model based on the Nusselt number, i.e., the value of the similarity criterion number π³. Combining this with the ideal gas law, since the injection temperature of liquid carbon dioxide is the same in both the prototype and the model, the ratios of the values ​​of temperature (T), molar mass of the gas (M), and molar gas constant (R) in the ideal gas law are considered constants, expressed in K. △P As shown in equation (13); furthermore, the carbon dioxide injected into both the prototype and the model is in liquid state and has the same temperature value, so their specific heat capacity values ​​are the same. Therefore, according to the heat calculation formula, the mathematical expression for the specific heat capacity is shown in equation (14):

[0059]

[0060] By combining equations (12) to (14), the proportional relationship between the phase change pressure of liquid carbon dioxide evaporation in the prototype and the model is obtained, as shown in equation (15):

[0061]

[0062] Step S3 specifically involves: selecting the inner diameter of the confined space liquid carbon dioxide phase change pressure pulsation experimental container as the characteristic scale; determining the corresponding characteristic scale in the model based on the characteristic dimensions of the prototype; and thus obtaining the geometric similarity scale C. D Then, the similarity scale values ​​of motion, dynamics, convective heat transfer, injection pressure, and exudation flow rate are calculated sequentially.

[0063] Step S4.1: Obtain the equivalent radius of the outer rigid annular isothermal medium cavity, specifically:

[0064] Assuming the confined space liquid carbon dioxide phase change pressure pulsation experimental container is filled with cryogenic liquid carbon dioxide at a certain temperature and all of it undergoes a phase change, the required heat E can be calculated by combining the latent heat of vaporization of the liquid carbon dioxide in the container. all Since all the heat required by the container is provided by the flow of the medium in the outer rigid annular constant temperature medium cavity, and according to the heat calculation formula shown in Equation (16), the smaller the temperature difference between the liquid carbon dioxide in the confined space liquid carbon dioxide phase change pressure pulsation experimental container and the medium in the outer rigid annular medium cavity, the larger the calculated required medium mass. Since the specific heat capacity of the medium in the cavity is constant, the cavity volume calculated from this is the maximum value, and the calculated medium flow velocity is the minimum value, as follows:

[0065] E all =C 介质 ·m all ·ΔT (16)

[0066] In the formula: C 介质 The specific heat of the medium in the outer rigid annular isothermal medium cavity, in kJ / (kg·K); m all ΔT represents the required mass of medium in the outer rigid annular constant-temperature medium cavity, in kg; ΔT represents the temperature difference between the liquid carbon dioxide in the confined space liquid carbon dioxide phase change pressure pulsation experimental container and the medium in the outer rigid annular constant-temperature medium cavity, in K.

[0067] Furthermore, the outer rigid annular isothermal medium cavity is equivalent to a cylinder. Based on the axial length of the confined space liquid carbon dioxide phase change pressure pulsation experimental container and combined with the equivalent area formula of the annulus, the equivalent outer diameter of the outer rigid annular isothermal medium cavity is calculated, as shown in Equation (17):

[0068]

[0069] In the formula: S 圆环 Let m be the equivalent area of ​​the annulus. 2 S 内 Let m be the area of ​​the inner circle of the annulus. 2 S 外 Let m be the area of ​​the outer circle of the annulus. 2 .

[0070] Step S4.2: Calculate the medium flow velocity in the outer rigid annular isothermal medium cavity, specifically as follows:

[0071] Assuming that all the heat provided by the medium in the outer rigid annular isothermal medium cavity is used for the heat absorption of liquid carbon dioxide phase change, the force on the medium flow is obtained according to the energy conservation and work calculation formula, as shown in equation (18):

[0072]

[0073] In the formula: F 介质 The force exerted by the medium during flow, N; L st The length of the outer rigid annular thermostatic medium cavity, in meters;

[0074] Based on the power calculation formula and equation (18), when the medium in the outer rigid annular constant temperature medium cavity reaches a stable flow state and the temperature reaches the set value and remains stable, the minimum flow velocity of the medium in the annular constant temperature medium cavity is calculated as shown in equation (19):

[0075]

[0076] In the formula: P 温温介质循环器 , where W is the output power of the thermostatic medium circulator.

[0077] The beneficial effects of this invention are:

[0078] (1) The device of the present invention specifically includes a confined space liquid carbon dioxide phase change pressure pulsation experimental container and a rigid annular constant temperature medium cavity surrounding the container. The confined space liquid carbon dioxide phase change pressure pulsation experimental container simulates the in-seam injection hole in the well, and the heat provided by the fluid medium in the annular constant temperature medium cavity simulates the heat provided by the coal seam to the liquid carbon dioxide in the hole. Through the above-mentioned device design, the characteristics of borehole pressure pulsation can be observed intuitively and vividly, and the formation mechanism can be explored according to the influencing factors of borehole pressure pulsation. This provides a scientific basis for the selection of technical parameters for injection of liquid carbon dioxide into underground coal seams and the mastery and application of borehole pressure change law.

[0079] (2) The method of this invention, through the design of a confined space liquid carbon dioxide phase change pressure pulsation experimental container and its outer rigid annular constant temperature medium cavity, develops and builds an experimental device that can accurately invert the borehole pressure pulsation law during the injection of liquid carbon dioxide to drive methane in coal mines. The aim is to achieve the following effects: ① Simulate the time-varying law of borehole pressure and its pulsation evolution characteristics under different injection parameters; ② Simulate the influence of injection pressure on borehole pressure pulsation changes; ③ Simulate the influence of two-phase carbon dioxide seepage flow rate on borehole pressure pulsation changes; ④ Simulate the influence of liquid carbon dioxide evaporation rate on borehole pressure pulsation changes; ⑤ Explore the optimal injection parameter combination for borehole pressure pulsation changes; The method of this invention provides a scientific basis for exploring a complete process technology parameter system for continuous, efficient and stable extraction of coal seam gas. Attached Figure Description

[0080] Figure 1 This is a schematic diagram of the experimental setup for simulating the pressure pulsation during the phase change of liquid carbon dioxide in in-seam drilling.

[0081] In the diagram, 1. Inlet, 2. Confined space liquid carbon dioxide phase change pressure pulsation experimental container, 3. Outer rigid annular constant temperature medium cavity, 4. Threaded connector, 5. Outlet, 6. Cavity outlet, 7. Outer rigid annular constant temperature medium cavity medium transmission connection hole, 8. Inlet, 9. Medium transmission connection pipe, 10. Constant temperature medium circulator, 11. Temperature sensor data acquisition line, 12. Temperature sensor, 13. Pressure sensor, 14. Pressure sensor data acquisition line, 15. Cavity inlet pipe, 16. Cavity outlet pipe. Detailed Implementation

[0082] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0083] This invention provides an experimental apparatus for simulating the pressure pulsation changes during the phase transition of liquid carbon dioxide in bedding boreholes, such as... Figure 1 As shown, it includes a confined space liquid carbon dioxide phase change pressure pulsation experimental container assembly, which is enclosed by a rigid annular constant temperature medium cavity assembly, and also includes a constant temperature medium circulator 10, which is connected to the rigid annular constant temperature medium cavity assembly through a pipeline.

[0084] The confined space liquid carbon dioxide phase change pressure pulsation experimental container assembly includes three confined space liquid carbon dioxide phase change pressure pulsation experimental containers 2, each confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is a tubular container; each confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is connected to each other by a threaded connector 4, the first confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is provided with an inlet 1 at the first end, and the third confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is provided with an outlet 5 at the tail end;

[0085] The rigid annular constant temperature medium cavity assembly includes three outer rigid annular constant temperature medium cavities 3. Each confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is wrapped with an outer rigid annular constant temperature medium cavity 3. The first outer rigid annular constant temperature medium cavity 3 is provided with a cavity outlet 6, and the third outer rigid annular constant temperature medium cavity 3 is provided with a liquid inlet 8. The cavity outlet 6 is connected to the constant temperature medium circulator 10 through a cavity outlet pipe 16, and the liquid inlet 8 is connected to the constant temperature medium circulator 10 through a cavity inlet pipe 15. Every two adjacent outer rigid annular constant temperature medium cavities 3 are connected by a medium transmission connecting pipe 9. The connection between the medium transmission connecting pipe 9 and the outer rigid annular constant temperature medium cavity 3 is the outer rigid annular constant temperature medium cavity medium transmission connecting hole 7.

[0086] It also includes multiple temperature sensors 12. Each confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is equipped with several temperature sensor probes 12; all temperature sensors 12 are connected to temperature sensor data acquisition lines 11.

[0087] It also includes multiple pressure sensors 13. Each confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is equipped with several probes of pressure sensors 13; all pressure sensors 13 are connected to pressure sensor data acquisition lines 14.

[0088] This invention provides a design method for a simulation experimental device for pressure pulsation during liquid carbon dioxide phase change in a borehole along the bedding plane. The device uses a confined space liquid carbon dioxide phase change pressure pulsation experimental container as a model and a downhole borehole as a prototype. The similarity design includes determining similarity criteria, selecting similarity numbers, and defining the similarity scale. The design of the rigid annular isothermal medium cavity surrounding the container mainly involves determining the equivalent diameter of the wall and the minimum flow velocity of the medium within the cavity. In this paper, the subscript P represents the physical quantity of the downhole borehole prototype, and the subscript m represents the corresponding physical quantity in the experimental model. The method includes the following steps:

[0089] S1. Based on the similarity criteria, derive expressions for geometric similarity, kinematic similarity, dynamic similarity, convective heat transfer similarity, injection pressure similarity, and exudation flow rate similarity between the prototype and the model.

[0090] Step S1 is as follows:

[0091] From the perspective of fluid motion similarity, commonly used motion similarity criteria include the Froude criterion, Reynolds criterion, Euler criterion, Cauchy criterion, Weber criterion, and Strouhal criterion. Considering the fluid flow state in this invention, liquid carbon dioxide enters the borehole under injection pressure, which is a pressurized forced pipe flow. Since the influence of gravity is not considered, the Froude criterion can be ignored. However, as liquid carbon dioxide is a viscous fluid, the Reynolds number must be equal for the model and prototype to achieve motion similarity. Furthermore, to ensure the dynamic similarity of the fluid in the model and prototype, the injection pressures of the two flows must be similar. Since liquid carbon dioxide undergoes an endothermic phase change during flow, the velocity, pressure, temperature, and density of any fluid element will change. Therefore, the Euler and Strouhal criteria must also be satisfied in this experiment. Because this simulation does not involve the fluid's elastic force or surface tension, and the injection pressure is constant, the Cauchy and Weber criteria are not considered.

[0092] Step S1.1: Obtain the expression for the geometric similarity between the prototype and the model, specifically:

[0093] To ensure that the feature sizes of the model and the prototype meet a certain ratio, l is used. p Indicates the axial length of the prototype's active segment, l m D represents the axial length of the model. p D represents the diameter of the prototype. m D' represents the diameter of the model. p D' represents the diameter of the pipe at the carbon dioxide outlet in the prototype. m The diameter of the model inlet is represented by ; however, the model cannot completely simulate the process of two-phase CO2 seeping into the far field of the coal seam through the periphery fractures of the injection hole in the well. Therefore, it is simplified to the outlet of the container. Based on geometric similarity, the linear scale, area scale, and volume scale of geometric similarity are obtained as shown in equation (1):

[0094]

[0095] In the formula: A p and A m Let m represent the radial cross-sectional areas of the prototype and the model, respectively. 2 V p and V m Let m represent the volume of the prototype and the model, respectively. 3 C D It is the similarity ratio between the prototype feature length and the model feature length, and its value is called the similarity multiple between the two.

[0096] Step S1.2: Derive the expression for the motion similarity between the prototype and the model, specifically:

[0097] Motion similarity can be represented using displacement, time, and velocity, while geometric similarity and temporal similarity are prerequisites for ensuring motion similarity. In this study, t... p The time taken for liquid carbon dioxide particles to flow in the prototype is expressed in t. m The time of flow of liquid carbon dioxide particles in the model is represented by the similarity ratio between the two as shown in equation (2):

[0098]

[0099] In the formula: C t The time similarity scale between the prototype and the model is called the similarity multiple between the two.

[0100] Based on the time similarity scale, the velocity similarity scale and acceleration similarity scale of the carbon dioxide flow between the prototype inlet and the model inlet are obtained as shown in equation (3):

[0101]

[0102] In the formula: and represents the average velocity of liquid carbon dioxide particles in the prototype and model, respectively, in m / s; A fluid velocity similarity scale between the prototype and the model; a p and a m Represent the flow acceleration of liquid carbon dioxide particles in the prototype and model, respectively, in m / s². 2 C a Fluid acceleration similarity ratio between prototype and model;

[0103] Step S1.3: Derive the expression for the dynamic similarity between the prototype and the model, specifically:

[0104] Dynamic similarity refers to the fact that the forces acting on fluid elements in two systems with similar boundaries have the same scale, ensuring that the magnitudes of the forces acting on fluid particles are consistent and proportional, thereby guaranteeing similar motion. Forces acting on fluid elements include gravity, viscous friction, inertial force, and pressure. Based on Newton's second law, the similarity scale of forces in the prototype and model is shown in Equation 4:

[0105]

[0106] In the formula: F p and F m Represent the forces acting on liquid carbon dioxide particles in the prototype and model, respectively, in N; m p and mm The values ​​of C represent the mass of liquid carbon dioxide in the downhole borehole and the experimental container, respectively, in kg; F The similarity scale of forces between the prototype and the model fluid;

[0107] Step S1.4: Derive the similarity expression for convective heat transfer between the prototype and the model, specifically as follows:

[0108] Convective heat transfer similarity is a boundary condition in prototype and model experiments. Heat conduction and localized thermal radiation losses between the inner and outer walls of the confined liquid carbon dioxide phase change pressure pulsation experimental container are neglected. Therefore, ensuring similar wall forces essentially ensures the heat conduction Φ in the model. m Compared with the thermal conductivity Φ in the prototype p The sizes calculated using a similar scale are identical, and the direction of heat transfer is the same.

[0109] From the perspective of fluid endothermic phase change similarity, the injection of liquid carbon dioxide through the injection hole is a forced convection heat transfer process in a circular tube. Therefore, to ensure the similarity of convective heat transfer between the model and the prototype, the Nusselt criterion, Reynolds criterion, and Prandtl criterion must be observed. Treating convective heat transfer similarity as a boundary condition for similar fluid motion essentially means that the heat transfer between the model and the prototype is similar.

[0110] The heat transfer of fluid flowing over the solid surface is calculated using Newton's cooling formula. The heat loss during the heat conduction process between the inner and outer walls of the container is ignored, and the heat transfer similarity is shown in Equation (5).

[0111]

[0112] Where: h 1-p The surface heat transfer coefficient for convective heat transfer in the prototype is W / (m²). 2 ·K);h 1-m The surface heat transfer coefficient for convective heat transfer in the model is W / (m²). 2 ·K); T st The temperature of the inner wall of the container in the model is given in °C.

[0113] In this invention, the container wall temperature in the model remains consistent with the borehole inner wall temperature in the prototype. Therefore, equation (5) is simplified and summarized to obtain equation (6):

[0114]

[0115] Step S1.5: Derive the expression for similar injection pressure between the prototype and the model, specifically:

[0116] Injection pressure similarity is the initial condition in prototype and model experiments; that is, the initial injection pressure affects the entire subsequent motion state of liquid carbon dioxide. The injection pressure similarity in this invention is derived through the force similarity of fluid particles in the prototype and model. The pressure of the numerator term in the Euler criterion is expressed using the pressure difference ΔP. z If we replace it, the injection pressures of the prototype and the model are similar and satisfy the relationship of equation (7), which is expressed as follows:

[0117]

[0118] In the formula: △P z-p and △P z-m The pressure difference between the prototype and the model injected with liquid carbon dioxide, in MPa; C △Pz P0 is the similarity ratio of the pressure difference between the injected liquid carbon dioxide in the prototype and the model, dimensionless; P0 is the original pressure in the prototype or model, in MPa.

[0119] Step S1.6: Derive expressions for the similarity of percolation flow rates in the prototype and model, specifically:

[0120] Under continuous injection pressure, The real-time flow rate of two-phase carbon dioxide is measured as it seeps into the coal seam through the borehole wall. The amount of carbon dioxide that seeps into the coal seam within a time interval Δt is the real-time integral value. This process is called "pressurized outflow". Therefore, the similarity of the two-phase carbon dioxide seepage is the initial condition. In the downhole test, the bottom of the borehole is closed, and the two-phase carbon dioxide enters the coal seam through the cracks in the borehole wall. In the physical similarity simulation experiment, the wall of the confined space liquid carbon dioxide phase change pressure pulsation experimental container is without cracks. The container outlet is used to simulate the two-phase carbon dioxide seepage channel on the borehole wall in the downhole.

[0121] Q out-p The average percolation flow rate of carbon dioxide in the two phases of the prototype is represented by Q. out-m The average percolation flow of carbon dioxide in the representative model is shown in equation (8).

[0122]

[0123] In the formula: A 1-p The area of ​​carbon dioxide seeping out from the two phases in the prototype is represented by m. 2 A 1-m The area of ​​the model's outlet is represented in m. 2 ; The velocity of carbon dioxide seeping out in the prototype is expressed in m / s. The velocity of carbon dioxide seepage in the two phases of the model is expressed in m / s.

[0124] S2. Accurately select similarity criterion numbers and obtain the similarity scale of liquid carbon dioxide phase change pressure;

[0125] Step S2.1: Accurately select the similarity criterion number, specifically as follows:

[0126] Based on the foregoing analysis of fluid flow similarity and convective heat transfer similarity, it is shown that in order for the model and prototype to maintain similarity, they must comply with the similarity criteria related to fluid flow and heat transfer. The corresponding similarity criterion numbers are Reynolds number, Euler number, Strauhall number, Nusselt number and Prandtl number, respectively. The expressions for the above similarity criterion numbers are shown in Equation 9.

[0127]

[0128] In the formula: α l m is the thermal diffusivity of the fluid. 2 / s; P' is the fluid dynamic pressure, which is referred to in this invention as the pressure difference ΔP. z Indicates MPa;

[0129] Step S2.2: Obtain the similarity scale of the phase transition pressure of liquid carbon dioxide, specifically:

[0130] In both the model and prototype of this invention, the temperature during carbon dioxide injection is -30℃, and it is assumed that the physical properties of liquid carbon dioxide remain consistent, as shown in equation (10):

[0131]

[0132] When the fluid flow and convective heat transfer are similar in the model and the prototype, the proportional relationship between physical quantities is derived from the number of similarity criteria, as shown in Equation (11), based on satisfying the selected similarity criteria:

[0133]

[0134] Based on the similarity scale of the convective heat transfer coefficient obtained in equation (11), the final expression for the similarity scale of the prototype and model heat transfer in equation (6) is obtained, as shown in equation (12):

[0135]

[0136] The pressure caused by the endothermic phase change of liquid carbon dioxide requires determining the convective heat transfer similarity ratio between the prototype and the model based on the Nusselt number, i.e., the value of the similarity criterion number π³. Combining this with the ideal gas law, since the injection temperature of liquid carbon dioxide is the same in both the prototype and the model, the ratios of the values ​​of temperature (T), molar mass of the gas (M), and molar gas constant (R) in the ideal gas law are considered constants, expressed in K. △PAs shown in equation (13); furthermore, the carbon dioxide injected into both the prototype and the model is in liquid state and has the same temperature value, so their specific heat capacity values ​​are the same. Therefore, according to the heat calculation formula, the mathematical expression for the specific heat capacity is shown in equation (14):

[0137]

[0138] By combining equations (12) to (14), the proportional relationship between the phase change pressure of liquid carbon dioxide evaporation in the prototype and the model is obtained, as shown in equation (15):

[0139]

[0140] S3. Considering the convenience of liquid carbon dioxide supply, the ease of experimental operation, and the economy of the experiment, determine the specific values ​​of the similarity scale of geometry, motion, dynamics, convective heat transfer, injection pressure, and seepage flow rate in S1.

[0141] Step S3 is as follows:

[0142] By selecting similarity criteria numbers and calculating the similarity scales of various parameters, the geometric dimensions, fluid particle flow time, velocity, acceleration, dynamic pressure, injection pressure, convective heat transfer, and phase change pressure in the prototype and model can be obtained. The calculation of the geometric similarity scale is the foundation for determining the similarity scales of other physical quantities. Therefore, before determining the similarity scales of each physical quantity in the prototype and model, the geometric similarity scales of the prototype and model must first be determined.

[0143] It is obviously unrealistic to completely realize a scaled-down model of the prototype in this invention. Therefore, when designing the geometric dimensions, the inner diameter of the confined space liquid carbon dioxide phase change pressure pulsation experimental container is selected as the characteristic scale, and the pressure resistance of the container wall is given special consideration. While meeting the safety requirements, the heat transfer performance is also taken into account. Therefore, the container wall thickness needs to be optimized according to the relevant design specifications of pressure-bearing pipelines.

[0144] Furthermore, if the model's geometric dimensions are as consistent as possible with the prototype, the experimental difficulty and cost increase significantly. Conversely, if the model's geometric dimensions are too small, the experimental error increases, failing to accurately reflect the true laws. Therefore, considering factors such as the ease of obtaining raw materials for fabricating the confined space liquid carbon dioxide phase change pressure pulsation experimental container, experimental economics, experimental site limitations, and the ease of calculating the similarity scale between the prototype and the model, the corresponding characteristic dimensions in the model can be determined based on the characteristic dimensions of the prototype, thereby deriving the geometric similarity scale C. D Then, the similarity scale values ​​of motion, dynamics, convective heat transfer, injection pressure, and exudation flow rate are calculated sequentially.

[0145] S4. Calculate the equivalent radius of the rigid annular isothermal medium cavity surrounding the container and the minimum flow velocity of the medium in the cavity.

[0146] Step S4.1: Obtain the equivalent radius of the outer rigid annular isothermal medium cavity, specifically:

[0147] The confined space liquid carbon dioxide phase change pressure pulsation experimental container serves as the "site" for the experiment. The heat required for the phase change of liquid carbon dioxide is provided by the flow of heat transfer within the rigid annular isothermal medium cavity surrounding the container. The rigid annular isothermal medium cavity maintains a constant temperature within itself through a continuous circulation of hot and cold media, and engages in convective heat transfer with the liquid carbon dioxide in the confined space liquid carbon dioxide phase change pressure pulsation experimental container. Therefore, to ensure that the heat transferred from the rigid annular isothermal medium cavity to the liquid carbon dioxide in the confined space liquid carbon dioxide phase change pressure pulsation experimental container meets the design requirements, the volume of the rigid annular isothermal medium cavity must be rationally designed, and the medium within it must achieve a certain flow rate.

[0148] In this invention, the flow direction of the medium in the outer rigid annular constant temperature medium cavity is opposite to the injection direction of liquid carbon dioxide in the confined space liquid carbon dioxide phase change pressure pulsation experimental container, thereby ensuring better convective heat transfer effect. Specifically, the inlet 8 of the outer rigid annular constant temperature medium cavity 3 is connected to the inlet pipe 15. Because the cavity is manufactured in sections, when the constant temperature medium flows within the outer rigid annular constant temperature medium cavity 3, it must first pass through the medium transmission connecting pipe 9 connected to the medium transmission connecting hole 7 of the outer rigid annular constant temperature medium cavity to form a complete passage. Finally, it enters the cavity outlet pipe 16 from the cavity outlet 6. The medium that has completed heat exchange inside the outer rigid annular constant temperature medium cavity 3 flows back to the constant temperature medium circulator 10, thus completing a complete medium flow loop.

[0149] Therefore, to ensure that the medium in the annular isothermal medium cavity can continuously and stably provide sufficient heat, a reverse calculation is performed using extreme value thinking. Assuming that the confined space liquid carbon dioxide phase change pressure pulsation experimental container is filled with cryogenic liquid carbon dioxide at a certain temperature and all of it undergoes a phase change, the required heat E can be calculated by combining the latent heat of vaporization of the liquid carbon dioxide in the container. all Since all the heat required by the container is provided by the flow of the medium in the outer rigid annular constant temperature medium cavity, and according to the heat calculation formula shown in Equation (16), the smaller the temperature difference between the liquid carbon dioxide in the confined space liquid carbon dioxide phase change pressure pulsation experimental container and the medium in the outer rigid annular medium cavity, the larger the calculated required medium mass. Since the specific heat capacity of the medium in the cavity is constant, the cavity volume calculated from this is the maximum value, and the calculated medium flow velocity is the minimum value, as follows:

[0150] Eall =C 介质 ·m all ·ΔT (16)

[0151] In the formula: C 介质 The specific heat of the medium in the outer rigid annular isothermal medium cavity, in kJ / (kg·K); m all ΔT represents the required mass of medium in the outer rigid annular constant-temperature medium cavity, in kg; ΔT represents the temperature difference between the liquid carbon dioxide in the confined space liquid carbon dioxide phase change pressure pulsation experimental container and the medium in the outer rigid annular constant-temperature medium cavity, in K.

[0152] Furthermore, the rigid annular constant temperature medium cavity is equivalent to a cylinder. Based on the axial length of the confined space liquid carbon dioxide phase change pressure pulsation experimental container and combined with the formula for the equivalent area of ​​the annulus, the equivalent outer diameter of the rigid annular constant temperature medium cavity can be calculated, as shown in Equation (17):

[0153]

[0154] In the formula: S 圆环 Let m be the equivalent area of ​​the annulus. 2 S 内 Let m be the area of ​​the inner circle of the annulus. 2 S 外 Let m be the area of ​​the outer circle of the annulus. 2 ;

[0155] After calculating the equivalent radius of the outer rigid annular thermostatic medium cavity, and taking into account factors such as heat transfer loss, fluid filling rate in the cavity, and ease of processing, a redundancy factor is determined, and the outer diameter of the outer rigid annular thermostatic medium cavity is finally determined.

[0156] Step S4.2: Calculate the medium flow velocity in the outer rigid annular isothermal medium cavity, specifically as follows:

[0157] Assuming that all the heat provided by the medium in the rigid annular constant-temperature medium cavity is used for the heat absorption of liquid carbon dioxide phase change, and since the outer wall of the rigid annular constant-temperature medium cavity is also wrapped with open self-adhesive rubber and plastic insulation cotton, heat loss to the external environment is not considered, and heat transfer loss of the constant-temperature medium through the wall thickness of the confined space liquid carbon dioxide phase change pressure pulsation experimental container is also not considered. Under these conditions, E in equation (16) all The entire process relies on the continuous flow of the medium, meaning the internal energy (heat) required for the phase change is equal to the work done by the medium flow. Based on the law of conservation of energy and the formula for calculating work, the force acting on the medium flow is derived, as shown in equation (18):

[0158]

[0159] In the formula: F介质 The force exerted by the medium during flow, N; L st The length of the outer rigid annular thermostatic medium cavity, in meters;

[0160] Since the flow of the medium in the outer rigid annular constant temperature medium cavity relies on the constant temperature medium circulator to provide the flow power, according to the power calculation formula and equation (18), when the medium in the outer rigid annular constant temperature medium cavity reaches a stable flow state and the temperature reaches the set value and remains stable, the minimum flow velocity of the medium in the annular constant temperature medium cavity is calculated as shown in equation (19):

[0161]

[0162] In the formula: P 温温介质循环器 , where W is the output power of the thermostatic medium circulator.

[0163] Equation (19) yields the minimum flow velocity of the medium in the limiting case where the confined space liquid carbon dioxide phase change pressure pulsation experimental container is completely filled with cryogenic liquid carbon dioxide and completely transforms into a gaseous state at a certain set temperature within an outer rigid annular constant-temperature medium cavity. Therefore, when conducting experiments simulating different coal seam temperatures, it is only necessary to ensure that the medium flow velocity within the outer rigid annular constant-temperature medium cavity satisfies v > v 介质 This can satisfy the heat required for physical similarity simulation experiments.

[0164] Example 1

[0165] In a certain working face of Shuanglong Coal Mine in Huangling Mining Area, Shaanxi Province, the coal seam depth ranges from 120 to 460 meters, with a thickness of 3.0 to 3.2 meters, averaging 3.1 meters, and a dip angle of 2 to 5 degrees; the gas content of the coal seam ranges from 0.82 to 4.7 m³. 3 The initial gas emission velocity is 10–13 mmHg, the coal body firmness coefficient is 0.89–1.37, the coal body porosity is 2.9–5.8%, and the coal seam permeability coefficient is 0.029–0.244 mD, classifying it as a coal seam with relatively difficult gas extraction. Before mining, this coal seam employs in-seam borehole injection of liquid carbon dioxide to pre-extract coal seam gas. The injection borehole diameter is 95 mm, the borehole depth is 100 m, and the borehole is sealed using a sealing device with an initial sealing depth of 9.9 m and a sealing length of 29 m.

[0166] To reconstruct the pulsating changes in borehole pressure during liquid carbon dioxide injection at the Shuanglong Coal Mine in the Huangling Mining Area, the following steps were taken:

[0167] If the geometric dimensions of the model are kept as consistent as possible with the prototype, the experimental difficulty and cost will increase significantly. If the model size is too small, the experimental error will increase and it will not accurately reflect the real law. Therefore, considering factors such as the ease of obtaining raw materials, experimental economy, experimental site limitations, and the convenience of calculating the similarity scale between the prototype and the model, the inner diameter of the confined space liquid carbon dioxide phase change pressure pulsation experimental container is set to 48 mm and the length is set to 5000 mm to simulate a borehole with a diameter of 96 mm in the well. According to the design specifications and standards of pressure pipelines, the wall thickness of the confined space liquid carbon dioxide phase change pressure pulsation experimental container is set to 11 mm, so the outer diameter of the container is 70 mm. Therefore, according to equation (1), the geometric linear similarity scale C between the prototype and the model can be obtained. D =2, the geometric area similarity scale is 4, and the geometric volume similarity scale is 8.

[0168] Furthermore, since liquid carbon dioxide is a viscous fluid, to ensure that the motion of the model is similar to that of the prototype, the Reynolds numbers of the two must be consistent. According to equation (9), the velocity similarity ratio between the prototype and the model can be obtained. According to equation (3), the time similarity ratio C between the prototype and the model can be obtained. t =4; According to equation (8), the similarity scale of carbon dioxide seepage flow between the prototype and the model is 2; According to equation (12), the similarity scale of heat transfer between the prototype and the model is 2. Based on the obtained geometric linear similarity scale values, velocity similarity scale values, and time similarity scale values, combined with equations (11) and (15), the similarity criteria numbers are shown in Table 1 below:

[0169] Table 1. Values ​​of Similarity Criteria

[0170]

[0171] Based on the above, the similarity scale values ​​of each physical quantity with the same name in the prototype and the model can be statistically obtained, as shown in Table 2.

[0172] Table 2. Similarity scale values ​​of corresponding physical quantities in the prototype and model.

[0173]

[0174] As previously known, the inner diameter of the confined space liquid carbon dioxide phase change pressure pulsation experimental container is 48 mm, and its length is 5000 mm. Therefore, the container volume is 0.00904 m³. 3Assuming the liquid carbon dioxide injected into the container is at -30℃, and the minimum pressure required to maintain it in the liquid phase is 1.43 MPa, since the latent heat of vaporization of liquid carbon dioxide decreases with increasing pressure at a constant temperature, the latent heat of vaporization of carbon dioxide (303.48 kJ / kg) under the temperature and pressure conditions of -30℃ and 1.43 MPa can be used to derive the maximum heat required for the container to undergo a complete phase change when filled with liquid carbon dioxide. Furthermore, the density of liquid carbon dioxide under these temperature and pressure conditions is 1167.6 kg / m³. 3 Therefore, the mass of the container filled with liquid carbon dioxide at -30℃ is 10.6 kg, and the heat required for the entire phase change is 3216.9 KJ, which is E in equation (16). all =3216.9KJ.

[0175] The medium in the outer rigid annular constant temperature medium cavity is pure water, with a specific heat capacity of 4.19 KJ / (kg·K). According to formula (16), E all and C 介质 All values ​​are constants; the smaller the temperature difference ΔT, the greater the required mass of medium m. all The larger, and m all The larger the value, the lower the required flow velocity of the medium. Therefore, to obtain the minimum flow velocity, it is necessary to obtain m. all The maximum value is the minimum temperature difference ΔT that needs to be obtained. Therefore, since this embodiment sets five injection temperature differences of 60℃, 65℃, 70℃, 75℃, and 80℃, the minimum value ΔT = 60℃, or 333.15K, is selected. From this, the required mass of pure water m for the outer rigid annular constant-temperature medium cavity can be calculated. all = 2.3 kg, the density of water is 1000 kg / m³ 3 This translates to a volume of 0.0023 m³. 3 .

[0176] Assuming the outer rigid annular thermostatic medium cavity is cylindrical, and removing the length of threaded connections and other components, the actual effective working section length is 4617 mm. Therefore, the equivalent area of ​​the radial cross-section (annulus) of the outer rigid annular thermostatic medium cavity is 0.0005 m². 2According to equation (17), the equivalent radius of the ring is approximately 37 mm and the equivalent diameter is 74 mm. After deducting the outer diameter of the confined space liquid carbon dioxide phase change pressure pulsation experimental container (70 mm), the width from the outer wall of the container to the annular constant temperature medium cavity is 2 mm. Considering the heat transfer loss, cavity medium filling rate, and processing convenience during the actual processing of the cavity, the margin is set to 4.0, that is, the width of the outer rigid annular constant temperature medium cavity is 8 mm, and the corresponding stainless steel wall thickness is 4 mm. Therefore, the outer diameter of the rigid annular constant temperature medium cavity surrounding the confined space liquid carbon dioxide phase change pressure pulsation experimental container is 94 mm.

[0177] Furthermore, according to equation (18), the force F acting on the medium flowing in the outer rigid annular isothermal medium cavity can be obtained. 介质 =773290N. In equation (19), P 恒温介质循环器 It is the output power of the power pump group that continuously provides heat medium to the cavity in the experiment. In this experiment, the value is 3700W. Therefore, according to formula (19), it can be concluded that the minimum flow velocity of pure water in the outer rigid annular constant temperature medium cavity is 2.9m / s under the condition of a liquid injection temperature difference of 60℃. That is, as long as the medium flow velocity in the cavity is not less than 2.9m / s, the heat required for the phase change of liquid carbon dioxide in the confined space liquid carbon dioxide phase change pressure pulsation experimental container can be guaranteed.

[0178] Example 2

[0179] The experimental device for simulating the pressure pulsation of liquid carbon dioxide phase change in in-seam drilling includes a confined space liquid carbon dioxide phase change pressure pulsation experimental container assembly, which is enclosed by a rigid annular constant temperature medium cavity assembly, and also includes a constant temperature medium circulator 10, which is connected to the rigid annular constant temperature medium cavity assembly through pipelines.

[0180] Example 3

[0181] The experimental device for simulating the pressure pulsation of liquid carbon dioxide phase change in in-seam drilling includes a confined space liquid carbon dioxide phase change pressure pulsation experimental container assembly, which is enclosed by a rigid annular constant temperature medium cavity assembly, and also includes a constant temperature medium circulator 10, which is connected to the rigid annular constant temperature medium cavity assembly through pipelines.

[0182] The confined space liquid carbon dioxide phase change pressure pulsation experimental container assembly includes three confined space liquid carbon dioxide phase change pressure pulsation experimental containers 2, each confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is a tubular container; each confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is connected to each other by a threaded connector 4, the first confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is provided with an inlet 1 at the first end, and the third confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is provided with an outlet 5 at the tail end;

[0183] The rigid annular constant temperature medium cavity assembly includes three outer rigid annular constant temperature medium cavities 3. Each confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is wrapped with an outer rigid annular constant temperature medium cavity 3. The first outer rigid annular constant temperature medium cavity 3 is provided with a cavity outlet 6, and the third outer rigid annular constant temperature medium cavity 3 is provided with a liquid inlet 8. The cavity outlet 6 is connected to the constant temperature medium circulator 10 through a cavity outlet pipe 16, and the liquid inlet 8 is connected to the constant temperature medium circulator 10 through a cavity inlet pipe 15. Every two adjacent outer rigid annular constant temperature medium cavities 3 are connected by a medium transmission connecting pipe 9. The connection between the medium transmission connecting pipe 9 and the outer rigid annular constant temperature medium cavity 3 is the outer rigid annular constant temperature medium cavity medium transmission connecting hole 7.

[0184] It also includes multiple temperature sensors 12. Each confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is equipped with several temperature sensor probes 12; all temperature sensors 12 are connected to temperature sensor data acquisition lines 11.

[0185] It also includes multiple pressure sensors 13. Each confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is equipped with several probes of pressure sensors 13; all pressure sensors 13 are connected to pressure sensor data acquisition lines 14.

Claims

1. A simulation experiment device for phase transition pressure pulsation change of liquid carbon dioxide in bedding drilling, characterized in that, The application relates to a limited-space liquid carbon dioxide phase transition pressure pulsation experiment container assembly, a rigid annular constant-temperature medium cavity assembly, a constant-temperature medium circulator (10) and a temperature sensor data acquisition line (11). The limited-space liquid carbon dioxide phase transition pressure pulsation experiment container assembly comprises three limited-space liquid carbon dioxide phase transition pressure pulsation experiment containers (2), each of which is a tubular container; the limited-space liquid carbon dioxide phase transition pressure pulsation experiment containers (2) are connected through threaded joints (4), the first limited-space liquid carbon dioxide phase transition pressure pulsation experiment container (2) is provided with an inlet (1) at the head end, and the third limited-space liquid carbon dioxide phase transition pressure pulsation experiment container (2) is provided with an outlet (5) at the tail end. The rigid annular constant-temperature medium cavity assembly comprises three outer rigid annular constant-temperature medium cavities (3), each of which is wrapped on the outer wall of the limited-space liquid carbon dioxide phase transition pressure pulsation experiment container (2); the first outer rigid annular constant-temperature medium cavity (3) is provided with a cavity liquid outlet (6), the third outer rigid annular constant-temperature medium cavity (3) is provided with a liquid inlet (8), the cavity liquid outlet (6) is connected with the constant-temperature medium circulator (10) through a cavity liquid outlet pipeline (16), and the liquid inlet (8) is connected with the constant-temperature medium circulator (10) through a cavity liquid inlet pipeline (15); every two adjacent outer rigid annular constant-temperature medium cavities (3) are connected through a medium transmission communication pipe (9). The application further comprises a plurality of temperature sensors (12), and the inside of each limited-space liquid carbon dioxide phase transition pressure pulsation experiment container (2) is provided with a plurality of temperature sensor probes; all the temperature sensors (12) are connected with the temperature sensor data acquisition line (11). The application further comprises a plurality of pressure sensors (13), and the inside of each limited-space liquid carbon dioxide phase transition pressure pulsation experiment container (2) is provided with a plurality of pressure sensor probes; all the pressure sensors (13) are connected with the pressure sensor data acquisition line (14).

2. The method according to claim 1, wherein the method is characterized by: The application comprises the following steps: S1, obtaining the geometric similarity, motion similarity, dynamic similarity, convective heat transfer similarity, liquid injection pressure similarity and seepage flow similarity expressions of the prototype and the model according to the similarity criterion; S2, accurately selecting the similar criterion number and obtaining the liquid carbon dioxide phase transition pressure similarity scale; S3, determining the geometric similarity, motion similarity, dynamic similarity, convective heat transfer similarity, liquid injection pressure similarity and seepage flow similarity scale specific values in S1; S4, calculating the equivalent radius of the container outer rigid annular constant-temperature medium cavity and the minimum flow rate of the medium in the cavity.

3. The method according to claim 2, wherein the method is characterized by: Step S1 is specifically: Step S1.1, obtaining the geometric similarity expression of the prototype and the model, which is specifically: To make the model and prototype feature size to meet a certain proportion, using l p Lp represents the axial length of the prototype action section, l m Lm represents the axial length of the model, D p Dp represents the diameter of the prototype, D m Dm represents the diameter of the model, Dp represents the diameter of the carbon dioxide outlet pipe in the prototype, Dm represents the diameter of the model inlet; for the process of two-phase CO2 seepage through the hole peripheral fracture to the far field of the coal seam, the model cannot be completely simulated, so it is simplified as a container outlet; according to geometric similarity, the geometric similarity linear scale, area scale and volume scale are obtained as shown in formula (1): （1） In the formula: A p and A m Let m represent the radial cross-sectional areas of the prototype and the model, respectively. 2 ; V p and V m Let m represent the volume of the prototype and the model, respectively. 3 ; C D The similarity ratio between the prototype feature length and the model feature length is called the similarity multiple between the two. Step S1.2, obtaining the motion similarity expression of the prototype and the model, which is specifically: In the study, the liquid carbon dioxide was injected into the prototype at t p The time of liquid carbon dioxide particle flow in the prototype is represented by t0, and the time of liquid carbon dioxide particle flow in the model is represented by tm, and the similarity scale is shown in equation (2): t m The time of liquid carbon dioxide particle flow in the prototype is represented by t0, and the time of liquid carbon dioxide particle flow in the model is represented by tm, and the similarity scale is shown in equation (2): （2） In the formula: C t The time similarity scale of the prototype and the model, the value of which is called the similarity multiple of the two. The velocity similarity scale and acceleration similarity scale of carbon dioxide flow between the prototype inlet and the model inlet according to the time similarity scale are shown in equation (3): （3） where: and U and U represent the average velocity of liquid carbon dioxide particles in the prototype and model, respectively, m / s; is the fluid velocity similarity scale between the prototype and the model; and U and U represent the average acceleration of liquid carbon dioxide particles in the prototype and model, respectively, m / s 2 ; C a is the fluid acceleration similarity scale between the prototype and the model; Step S1.3, obtaining the expression of dynamic similarity between the prototype and the model, specifically: （4） where: and F and F' represent the forces experienced by the liquid carbon dioxide mass points in the prototype and model, respectively, N; and m and m' represent the mass of the liquid carbon dioxide in the downhole borehole and the experimental vessel, respectively, kg; C F is the force similarity scale between the prototype and model fluids; Step S1.4, obtaining the expression of convective heat transfer similarity between the prototype and the model, specifically: （5） where: h 1-p hconv is the surface heat transfer coefficient for convective heat transfer in the prototype, W / (m 2 ·K); h 1-m hmod is the surface heat transfer coefficient for convective heat transfer in the model, W / (m 2 ·K); T st Tmod is the temperature of the inner wall surface of the vessel in the model, °C; Simplifying and inducing equation (5), equation (6) is obtained: （6） Step S1.5, obtaining the expression of injection pressure similarity between the prototype and the model, specifically: The pressure of the molecule term in the Euler criterion is replaced by the pressure difference △P z Instead, the injection pressure of the prototype and the model satisfy the relationship of equation (7) as follows: （7） wherein: Δ P z-p and Δ P z-m respectively, the pressure difference of injecting liquid carbon dioxide in the prototype and the model, MPa; C △Pz is the similarity ratio of the pressure difference of injecting liquid carbon dioxide in the prototype and the model, dimensionless; P 0 is the initial pressure in the prototype or the model, MPa; Step S1.6, obtaining the expression of exudation flow rate similarity between the prototype and the model, specifically: Under the action of continuous injection pressure, The real-time flow rate of two-phase carbon dioxide seeping into the far field of coal seam through the inner wall of the borehole, △t The amount of carbon dioxide seeping into the coal seam within a certain time is the real-time integral value. This process is called "pressure outflow". Therefore, the similar two-phase carbon dioxide seepage is the initial condition. The borehole bottom is closed in the downhole test, and the two-phase carbon dioxide enters the coal seam through the borehole wall cracks. In the physical similar simulation experiment, the container wall has no cracks due to the pressure pulsation of the phase change of liquid carbon dioxide in the confined space. The outlet of the container simulates the two-phase carbon dioxide seepage channel through the borehole wall along the layer in the downhole. The average outflow rate of the two-phase carbon dioxide in the representative prototype is represented by The average outflow rate of the two-phase carbon dioxide in the representative model is represented by The similarity ratio of both is represented by Equation (8) as follows: （8） where: Arepresents the area of the two-phase carbon dioxide outflow in the prototype, m 2 ; Arepresents the area of the model outlet, m 2 ; Arepresents the outflow velocity of the two-phase carbon dioxide in the prototype, m / s; Arepresents the outflow velocity of the two-phase carbon dioxide in the model, m / s.

4. The method according to claim 2, wherein the method is characterized by: Step S2 is specifically: Step S2.1, accurately selecting the similarity criterion number, specifically: （9） wherein: α l is the thermal diffusivity of the fluid, m 2 / s; P’ is the hydrodynamic pressure, MPa; Step S2.2, obtaining the similarity scale of the phase transition pressure of liquid carbon dioxide, specifically: In the model and the prototype, the temperature is-30℃ when injecting carbon dioxide, and it is considered that the physical property parameters of liquid carbon dioxide remain consistent, i.e., as shown in equation (10): （10） When the fluid flow and convective heat transfer in the model and the prototype are similar, on the basis of satisfying the selected similarity criterion, the proportional relationship between the physical quantities is derived from each similarity criterion number, as shown in equation (11): （11） According to the similarity scale of the convective heat transfer coefficient obtained in equation (11), the final expression of the heat transfer quantity similarity scale of the prototype and the model in equation (6) is obtained, as shown in equation (12): （12） The pressure caused by the endothermic phase change of liquid carbon dioxide requires the calculation of the convective heat transfer similarity ratio between the prototype and the model, i.e., the similarity criterion number, based on the Nusselt number. π The value of 4, combined with the ideal gas law, is given because the injection temperature of liquid carbon dioxide is the same in both the prototype and the model. Therefore, for the temperature in the ideal gas law ( T ), molar mass of gas ( M ) and molar gas constant ( R The ratio of these values ​​is considered a constant. K △P As shown in equation (13); furthermore, the carbon dioxide injected into the prototype and the model are both liquid and have the same temperature, and their specific heat capacity is the same. Therefore, according to the heat calculation formula, the mathematical expression of the specific heat capacity is as shown in equation (14): （13） （14） By combining equations (12)-(14), the proportional relationship formula between the phase transition pressures of liquid carbon dioxide in the prototype and the model is obtained, as shown in equation (15): （15）。 5. The method according to claim 2, wherein the method is characterized by: The step S3 specifically comprises: selecting the inner diameter of the liquid carbon dioxide phase transition pressure pulsation experiment container in the confined space as a characteristic scale, determining the corresponding characteristic scale in the model according to the characteristic scale of the prototype, and then obtaining the geometric similarity scale C D , and then sequentially calculating the similarity scale values of motion, power, convective heat transfer, injection pressure and seepage flow.

6. The method according to claim 2, wherein the method is characterized by: Step S4.1, obtaining the equivalent radius of the outer rigid annular constant-temperature medium cavity, specifically: Assuming that the confined space liquid carbon dioxide phase transition pressure fluctuation experiment container is filled with low temperature liquid carbon dioxide at a certain temperature value and all phase transition occurs, the required heat can be calculated by combining the latent heat of vaporization of the liquid carbon dioxide in the container E all ; Since the required heat of the container is provided by the medium flow in the outer rigid ring-shaped constant temperature medium cavity, according to the heat calculation formula shown in formula (16), the smaller the temperature difference between the liquid carbon dioxide in the confined space liquid carbon dioxide phase transition pressure fluctuation experiment container and the medium in the outer rigid ring-shaped medium cavity, the greater the calculated medium mass, because the specific heat capacity of the medium in the cavity is a constant value, the calculated cavity volume is the maximum value, and the calculated medium flow rate is the minimum value, as follows: （16） wherein: C 介质 Cp is the specific heat of the medium in the outer rigid annular thermostatic medium cavity, KJ / (kg·K); M is the required medium mass in the outer rigid annular thermostatic medium cavity, kg;△ T △T is the temperature difference between the liquid carbon dioxide in the confined space liquid carbon dioxide phase change pressure pulsation experiment container and the medium in the outer rigid annular thermostatic medium cavity, K; In addition, the outer rigid annular constant-temperature medium cavity is equivalent to a cylinder, and according to the axial length of the limited space liquid carbon dioxide phase transition pressure pulsation experimental container and combined with the equivalent area formula of the annular ring, the equivalent outer diameter of the outer rigid annular constant-temperature medium cavity is calculated, as shown in equation (17): （17） wherein: S 圆环 Ae is the equivalent area of the circular ring, m 2 ; S 内 Ai is the area of the inner circle in the circular ring, m 2 ; S 外 Ao is the area of the outer circle in the circular ring, m 2 ; Step S4.2, calculating the medium flow rate in the outer rigid annular constant-temperature medium cavity, specifically: Assuming that the heat provided by the medium in the outer rigid annular constant-temperature medium cavity is entirely used for the phase transition heat absorption of liquid carbon dioxide, and according to the energy conservation and work calculation formula, the force suffered by the medium flow is obtained, as shown in equation (18): （18） wherein: F is the force experienced by the media flow, N; L is the length of the outer rigid annular constant temperature media cavity, m; According to the power calculation formula and equation (18), when the medium in the outer rigid annular constant-temperature medium cavity reaches a stable flow state and the temperature reaches the set value and remains stable, the minimum flow velocity of the medium in the annular constant-temperature medium cavity is calculated, as shown in equation (19): （19） In the formula: is the output power of the thermostatic medium circulator, W.

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