Liquid carbon dioxide phase change pressure pulsation change simulation experiment device for bedding drilling and design method of liquid carbon dioxide phase change pressure pulsation change simulation experiment device

Abstract

The invention discloses a bedding drilling liquid carbon dioxide phase change pressure pulsation change simulation experiment device, which comprises a limited space liquid carbon dioxide phase change pressure pulsation experiment container assembly, and a rigid annular constant temperature medium cavity assembly is wrapped outside the limited space liquid carbon dioxide phase change pressure pulsation experiment container assembly. And the constant-temperature medium circulator is connected with the rigid annular constant-temperature medium cavity assembly through a pipeline. The method can accurately invert the pressure pulsation rule of the drill hole in the process of injecting the liquid carbon dioxide in the underground coal mine to displace the gas. The invention further discloses a design method of the bedding drilling liquid carbon dioxide phase change pressure pulsation change simulation experiment device.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas disaster control and extraction promotion in high-gas and low-permeability coal seams in the coal mining industry, and specifically relates to a simulation experimental device for phase change pressure pulsation change of liquid carbon dioxide in a layered borehole, and also relates to a design method for the simulation experimental device for phase change pressure pulsation change of liquid carbon dioxide in a layered borehole. Background Art

[0002] A series of coal seam gas extraction promotion technologies such as deep hole blasting and hydraulic cutting have emerged. Liquid carbon dioxide, with its low viscosity and strong permeability characteristics, is more conducive to replacing the adsorbed gas in the coal seam and driving the coal seam's original free gas and the displaced free gas to the extraction pipeline, thereby improving the extraction effect of coal seam gas.

[0003] It is found in a large number of on-site industrial practices that under a specific combination of injection parameters, the pressure in the injection hole will show pulsating changes. At this stage, the gas concentration in the extraction pipeline, the mixed volume of pipeline extraction and the pure amount of gas extraction are better than those 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 of the target coal seam are the scientific basis for achieving the above goals; however, there is currently no similar experimental simulation equipment or device in this field that can conduct pulsation experiments on the phase change pressure of liquid carbon dioxide in confined spaces; for this reason, based on similar principles, taking the coal mine underground layer injection borehole as the 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 purpose of the present invention is to provide a simulation experimental device for the phase change pressure pulsation change of liquid carbon dioxide in a layered borehole, which can accurately invert the borehole pressure pulsation law during the injection of liquid carbon dioxide to displace gas in a coal mine.

[0005] The second purpose of the present invention is to provide a method for designing a simulation experimental device for the phase change pressure pulsation change of liquid carbon dioxide in the layered borehole, which can intuitively and vividly observe the characteristics of the borehole pressure pulsation change, and explore its formation mechanism based on the factors affecting the borehole pressure pulsation, providing a scientific basis for the selection of technical parameters for the injection of liquid carbon dioxide in underground coal seam drilling and the mastery and application of the laws of borehole pressure change.

[0006] The first technical solution adopted by the present invention is a simulation experimental device for the phase change pressure pulsation change of liquid carbon dioxide in a layered drilling, which includes a confined space liquid carbon dioxide phase change pressure pulsation experimental container assembly, the confined space liquid carbon dioxide phase change pressure pulsation experimental container assembly is outsourced 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 present invention is also characterized in that:

[0008] The confined space liquid carbon dioxide phase change pressure pulsation experimental container assembly includes three sections of confined space liquid carbon dioxide phase change pressure pulsation experimental containers, each section of the confined space liquid carbon dioxide phase change pressure pulsation experimental container is a tubular container; each section of the confined space liquid carbon dioxide phase change pressure pulsation experimental container is connected through a threaded connector, the head end of the first section of the confined space liquid carbon dioxide phase change pressure pulsation experimental container is provided with an inlet, and the tail end of the third section of the confined space liquid carbon dioxide phase change pressure pulsation experimental container is provided with an outlet;

[0009] The rigid annular constant temperature medium cavity assembly includes three outer rigid annular constant temperature medium cavities, and the outer wall of each section of the confined space liquid carbon dioxide phase change pressure pulsation experimental container is wrapped with an outer rigid annular constant temperature medium cavity; the first section of the outer rigid annular constant temperature medium cavity is provided with a cavity liquid outlet, and the third section of the outer rigid annular constant temperature medium cavity is provided with a liquid inlet, the cavity liquid outlet is connected to the constant temperature medium circulator through the cavity liquid outlet pipeline, and the liquid inlet is connected to the constant temperature medium circulator through the cavity liquid inlet pipeline; every two adjacent outer rigid annular constant temperature medium cavities are connected through a medium transmission connecting pipe, and 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 a plurality of temperature sensors, and a plurality of temperature sensor probes are arranged inside each confined space liquid carbon dioxide phase change pressure pulsation experiment container; all temperature sensors are connected to the temperature sensor data acquisition line;

[0011] It also includes a plurality of pressure sensors, and a plurality of pressure sensor probes are arranged inside each confined space liquid carbon dioxide phase change pressure pulsation experimental container; all the pressure sensors are connected to the pressure sensor data acquisition line.

[0012] The second technical solution adopted by the present invention is a design method of a simulation experimental device for phase change pressure pulsation change of liquid carbon dioxide in a layered borehole, comprising the following steps:

[0013] S1. Based on the similarity criteria, the expressions of geometric similarity, motion similarity, dynamic similarity, convective heat transfer similarity, injection pressure similarity and seepage flow similarity between the prototype and the model are obtained;

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

[0015] S3, determine the specific values ​​of the similar scales of geometry, motion, dynamics, convective heat transfer, injection pressure and seepage flow in S1;

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

[0017] The present invention is also characterized in that:

[0018] Step S1 is specifically as follows:

[0019] Step S1.1, obtain the geometric similarity expression of the prototype and the model, specifically:

[0020] In order to make the characteristic dimensions of the model and the prototype meet a certain ratio, use l p Indicates the axial length of the prototype action section, l m Indicates the axial length of the model, D p Indicates the prototype diameter, D m Represents the diameter of the model, D' p Denotes the diameter of the pipe at the carbon dioxide outlet in the prototype, D' m represents the diameter of the model inlet; the model cannot fully simulate the process of two-phase CO2 seepage from the underground injection hole through the cracks around the hole to the far field of the coal seam, so it is simplified to the container outlet; based on geometric similarity, the geometrically similar linear scale, area scale and volume scale are obtained as shown in formula (1):

[0021]

[0022] Where: A p and A m Represent the radial cross-sectional area of ​​the prototype and model, m 2 ; V p and V m Represent the volume of the prototype and model, m 3 ; C D It is the similarity ratio between the characteristic length of the prototype and the characteristic length of the model, and its value is called the similarity multiple of the two.

[0023] Step S1.2, derive the similarity expression of the motion of the prototype and the model, specifically:

[0024] In the study, t p It represents the time of liquid carbon dioxide particle flow in the prototype, t m represents the time of the flow of liquid carbon dioxide particles in the model. The similarity ratio between the two is shown in formula (2):

[0025]

[0026] Where: C t is the time similarity ratio between the prototype and the model, and its value is called the similarity multiple of the two;

[0027] According to 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 formula (3):

[0028]

[0029] Where: and represent the average velocity of liquid carbon dioxide particle flow in the prototype and model, m / s; is the fluid velocity similarity scale between the prototype and the model; a p and a m Represents the liquid carbon dioxide particle flow acceleration in the prototype and model, m / s 2 ; C a Similarity scale of fluid acceleration between prototype and model;

[0030] Step S1.3, obtain the similar expression of the dynamics of the prototype and the model, specifically:

[0031]

[0032] Where: F p and F m Respectively represent the forces acting on the liquid carbon dioxide particles in the prototype and model, N; m p and m m Respectively represent the mass of liquid carbon dioxide in the borehole and the experimental container, kg; C F is the similarity scale of forces between prototype and model fluids;

[0033] Step S1.4, obtain the similar expressions of convective heat transfer between the prototype and the model, specifically:

[0034]

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

[0036] Simplifying formula (5), we get formula (6):

[0037]

[0038] Step S1.5, obtain the similar expressions of the injection pressures of the prototype and the model, specifically:

[0039] The pressure of the numerator in the Euler criterion is expressed as the pressure difference △P z Instead, the injection pressures of the prototype and the model are similar and satisfy the relationship of formula (7), which is expressed as follows:

[0040]

[0041] Where: △P z-p and △P z-m are the pressure differences of the prototype and the model when liquid carbon dioxide is injected, MPa; C △Pz is the similarity ratio of the pressure difference between the prototype and the model when liquid carbon dioxide is injected, dimensionless; P0 is the original pressure in the prototype or model, MPa;

[0042] Step S1.6, obtain the similar expressions of the seepage flow of the prototype and the model, specifically:

[0043] Under the continuous injection pressure, is the real-time flow rate of the two-phase carbon dioxide in the process of far-field seepage from the inner wall of the borehole to the coal seam, and the amount of carbon dioxide that penetrates into the coal seam within the time △t is the real-time integral value. This process is called "pressurized outflow". Therefore, the initial condition is that the two-phase carbon dioxide seepage is similar. In the downhole test, the bottom of the borehole is closed, and the two-phase carbon dioxide enters the coal seam through the cracks on the borehole wall. In the physical similarity simulation experiment, there are no cracks on the wall of the confined space liquid carbon dioxide phase change pressure pulsation experiment container, and the container outlet is used to simulate the two-phase carbon dioxide seepage channel on the downhole borehole wall.

[0044] Q out-p represents the average permeation rate of two-phase carbon dioxide in the prototype, expressed as Q out-m represents the average seepage flow of carbon dioxide in the model. The similarity between the two is shown in formula (8):

[0045]

[0046] Where: A 1-p Represents the area of ​​two-phase carbon dioxide seepage in the prototype, m 2 ; A 1-m Represents the area of ​​the model outlet, m 2 ; represents the permeation rate of two-phase carbon dioxide in the prototype, m / s; Represents the rate of two-phase carbon dioxide permeation in the model, m / s.

[0047] Step S2 is specifically as follows:

[0048] Step S2.1: Accurately select the similarity criteria number, specifically:

[0049]

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

[0051] Step S2.2, obtain a similar scale of the phase change pressure of liquid carbon dioxide, specifically:

[0052] In this model and the prototype, the temperature when injecting carbon dioxide is -30°C, and the physical properties of liquid carbon dioxide are considered to be consistent, as shown in formula (10):

[0053]

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

[0055]

[0056] According to the similarity scale of the convective heat transfer coefficient obtained in formula (11), the final expression of the similarity scale of the heat transfer between the prototype and the model in formula (6) is obtained, as shown in formula (12):

[0057]

[0058] As for the pressure caused by the endothermic phase change of liquid carbon dioxide, it is necessary to derive the similarity ratio of the convective heat transfer between the prototype and the model based on the Nusselt number, that is, the value of the similarity criterion number π3. Combined with the ideal gas state equation, since the injection temperature of liquid carbon dioxide in the prototype and the model is the same, for this reason, the ratio of the values ​​of temperature (T), molar mass of gas (M) and molar gas constant (R) in the ideal gas state equation is regarded as a constant, and K is used. △P It is represented as shown in formula (13); Furthermore, the carbon dioxide injected into the prototype and the model are both in liquid state and have the same temperature value, and the specific heat capacity values ​​of the two are the same. Therefore, according to the heat calculation formula, the mathematical expression of the specific heat capacity is obtained as shown in formula (14):

[0059]

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

[0061]

[0062] Step S3 is as follows: select the inner diameter of the confined space liquid carbon dioxide phase change pressure pulsation experiment container as the characteristic scale, determine the corresponding characteristic scale in the model according to the characteristic scale of the prototype, and then obtain the geometric similarity scale C D , and then the similar scale values ​​of motion, power, convective heat transfer, injection pressure and seepage flow are calculated in turn.

[0063] Step S4.1, obtain the equivalent radius of the outer rigid annular constant temperature medium cavity, specifically:

[0064] Assuming that the container of the confined space liquid carbon dioxide phase change pressure pulsation experiment is filled with low-temperature liquid carbon dioxide of a certain temperature value and all phase changes occur, the required heat E can be calculated based on the latent heat of vaporization of the liquid carbon dioxide in the container. all ; Since the heat required by the container is all 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 formula (16), the smaller 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 medium cavity, the larger the required medium mass is calculated. Because the specific heat capacity of the medium in the cavity is a constant, the calculated cavity volume is the maximum value, and the calculated medium flow rate is the minimum value, as follows:

[0065] E all =C 介质 ·m all ΔT (16)

[0066] Where: C 介质 is the specific heat of the medium in the outer rigid annular constant temperature medium cavity, KJ / (kg·K); m all is the mass of the medium required in the outer rigid annular constant temperature medium cavity, kg; △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 constant temperature medium cavity, K;

[0067] In addition, the outer rigid annular constant temperature medium cavity is equivalent to a cylinder. According to the axial length of the confined space liquid carbon dioxide phase change pressure pulsation experimental container and the equivalent area formula of the ring, the equivalent outer diameter of the outer rigid annular constant temperature medium cavity is calculated, as shown in formula (17):

[0068]

[0069] Where: S 圆环 is the equivalent area of ​​the ring, m 2 ; S 内 is the area of ​​the inner circle in the ring, m 2 ; S 外 is the area of ​​the outer circle in the ring, m 2 .

[0070] Step S4.2, calculate the medium flow rate in the outer rigid annular constant temperature medium cavity, specifically:

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

[0072]

[0073] Where: F 介质 is the force on the medium when it flows, N; L st is the length of the outer rigid annular constant temperature medium cavity, m;

[0074] According to the power calculation formula and formula (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 formula (19):

[0075]

[0076] Where: P 温温介质循环器 is the output power of the constant temperature medium circulator, W.

[0077] The beneficial effects of the present 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 wrapped outside the container. The confined space liquid carbon dioxide phase change pressure pulsation experimental container is used to simulate the underground layer injection hole, and the heat provided by the fluid medium in the annular constant temperature medium cavity is used to simulate the underground coal seam providing heat to the liquid carbon dioxide in the hole. Through the design of the above-mentioned equipment, it is possible to intuitively observe the characteristics of the borehole pressure pulsation change, and to explore its formation mechanism based on the influencing factors of the borehole pressure pulsation, thereby providing a scientific basis for the selection of technical parameters for the injection of liquid carbon dioxide into underground coal seam boreholes and the mastery and application of the laws of borehole pressure change.

[0079] (2) The method of the present invention develops and constructs a set of experimental equipment that can accurately invert the borehole pressure pulsation law during the process of injecting liquid carbon dioxide to displace gas in coal mines 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, aiming 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 on borehole pressure pulsation changes; ④ simulate the influence of liquid carbon dioxide evaporation on borehole pressure pulsation changes; ⑤ explore the optimal injection parameter combination for borehole pressure pulsation changes; the method of the present invention provides a scientific basis for exploring a complete process and technical parameter system for continuous, efficient and stable extraction of coal seam gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0080] Figure 1 It is a schematic diagram of the structure of an experimental device for simulating phase change and pressure pulsation changes of liquid carbon dioxide in a layered borehole.

[0081] In the figure, 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 liquid outlet, 7. outer rigid annular constant temperature medium cavity medium transmission connecting hole, 8. liquid inlet, 9. medium transmission connecting 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 liquid inlet pipeline, 16. cavity liquid outlet pipeline. DETAILED DESCRIPTION

[0082] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0083] The present invention provides a simulation experimental device for the phase change pressure pulsation change of liquid carbon dioxide in a layered borehole, such as Figure 1 As shown, it includes a confined space liquid carbon dioxide phase change pressure pulsation experiment container assembly, which is surrounded 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 sections of confined space liquid carbon dioxide phase change pressure pulsation experimental containers 2, each section of the confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is a tubular container; each section of the confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is connected through a threaded connector 4, the head end of the first section of the confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is provided with an inlet 1, and the tail end of the third section of the confined space liquid carbon dioxide phase change pressure pulsation experimental container 2 is provided with an outlet 5;

[0085] The rigid annular constant temperature medium cavity assembly includes three outer rigid annular constant temperature medium cavities 3, and the outer wall of each section of the 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 section of the outer rigid annular constant temperature medium cavity 3 is provided with a cavity outlet 6, and the third section of the 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 the cavity outlet pipeline 16, and the liquid inlet 8 is connected to the constant temperature medium circulator 10 through the cavity inlet pipeline 15; every two adjacent outer rigid annular constant temperature medium cavities 3 are connected through a medium transmission connecting pipe 9, and 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 a plurality of temperature sensors 12. Each confined space liquid carbon dioxide phase change pressure pulsation experiment container 2 is provided with a plurality of probes of the temperature sensors 12. All the temperature sensors 12 are connected to the temperature sensor data acquisition line 11.

[0087] It also includes a plurality of pressure sensors 13 , and probes of a plurality of pressure sensors 13 are arranged inside each confined space liquid carbon dioxide phase change pressure pulsation experiment container 2 ; all the pressure sensors 13 are connected to the pressure sensor data acquisition line 14 .

[0088] The present invention provides a design method for a simulation experimental device for the phase change pressure pulsation change of liquid carbon dioxide in a layered borehole, wherein a confined space liquid carbon dioxide phase change pressure pulsation experimental container is used as a model, and a downhole layered borehole is used as a prototype. The similarity design includes the determination of similarity criteria, the selection of similarity criteria, and the determination of similarity scales; and the design of a rigid annular constant temperature medium cavity wrapped around the container mainly involves the determination of the equivalent diameter of the wall and the minimum flow rate of the medium in the cavity. In the text, the subscript P represents the physical quantity of the downhole drilling prototype, and the subscript m represents the physical quantity of the same name corresponding to the prototype in the experimental model, and the following steps are included:

[0089] S1. Based on the similarity criteria, the expressions of geometric similarity, motion similarity, dynamic similarity, convective heat transfer similarity, injection pressure similarity and seepage flow similarity between the prototype and the model are obtained;

[0090] Step S1 is specifically as follows:

[0091] From the perspective of fluid motion similarity, commonly used motion similarity criteria include Froude's criterion, Reynolds' criterion, Euler's criterion, Cauchy's criterion, Weber's criterion and Strouhal's criterion. Combined with the flow state of the fluid in the present invention, liquid carbon dioxide enters the borehole under the action of injection pressure, which belongs to a pressurized forced pipe flow, and the influence of gravity is not considered, and the Froude's criterion can be ignored. Liquid carbon dioxide is a viscous fluid, and when the model and the prototype have similar motion, the Reynolds criterion numbers of the two must be equal. In addition, in order to ensure the dynamic similarity of the fluid in the model and the prototype, the injection pressure of the two flows must be similar. Liquid carbon dioxide undergoes an endothermic phase change during the flow process, and the velocity, pressure, temperature and density of any flow microelement in the fluid will change. Therefore, the Euler criterion and Strouhal criterion must also be met in this experiment. Since this simulation does not involve the elastic force and surface tension of the fluid and the injection pressure is a constant value, the Cauchy criterion and Weber criterion are not considered.

[0092] Step S1.1, obtain the geometric similarity expression of the prototype and the model, specifically:

[0093] In order to make the characteristic dimensions of the model and the prototype meet a certain ratio, use l p Indicates the axial length of the prototype action section, l m Indicates the axial length of the model, D p Indicates the prototype diameter, D m Represents the diameter of the model, D' p Denotes the diameter of the pipe at the carbon dioxide outlet in the prototype, D' m represents the diameter of the model inlet; the model cannot fully simulate the process of two-phase CO2 seepage from the underground injection hole through the cracks around the hole to the far field of the coal seam, so it is simplified to the container outlet; based on geometric similarity, the geometrically similar linear scale, area scale and volume scale are obtained as shown in formula (1):

[0094]

[0095] Where: A p and A m Represent the radial cross-sectional area of ​​the prototype and model, m 2 ; V p and V m Represent the volume of the prototype and model, m 3 ; C D It is the similarity ratio between the characteristic length of the prototype and the characteristic length of the model, and its value is called the similarity multiple of the two.

[0096] Step S1.2, derive the similarity expression of the motion of the prototype and the model, specifically:

[0097] Motion similarity can be expressed using displacement, time and velocity, while geometric similarity and time similarity are the prerequisites for ensuring motion similarity. p It represents the time of liquid carbon dioxide particle flow in the prototype, t m represents the time of the flow of liquid carbon dioxide particles in the model. The similarity ratio between the two is shown in formula (2):

[0098]

[0099] Where: C t is the time similarity ratio between the prototype and the model, and its value is called the similarity multiple of the two;

[0100] According to 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 formula (3):

[0101]

[0102] Where: and represent the average velocity of liquid carbon dioxide particle flow in the prototype and model, m / s; is the fluid velocity similarity scale between the prototype and the model; a p and a m Represents the liquid carbon dioxide particle flow acceleration in the prototype and model, m / s 2 ; C a Similarity scale of fluid acceleration between prototype and model;

[0103] Step S1.3, obtain the similar expression of the dynamics of the prototype and the model, specifically:

[0104] Dynamic similarity means that the same forces acting on the fluid element of two systems with similar boundaries have the same scale, and the same forces acting on the fluid particles are consistent in magnitude and in a certain proportion, so as to ensure similar motion. The forces acting on the fluid element include gravity, viscous friction, inertia and pressure. According to Newton's second law, the similarity scale of the forces in the prototype and the model is shown in formula 4:

[0105]

[0106] Where: F p and F m Respectively represent the forces acting on the liquid carbon dioxide particles in the prototype and model, N; m p and mm Respectively represent the mass of liquid carbon dioxide in the borehole and the experimental container, kg; C F is the similarity scale of forces between prototype and model fluids;

[0107] Step S1.4, obtain the similar expressions of convective heat transfer between the prototype and the model, specifically:

[0108] Convective heat transfer similarity is a boundary condition in prototype and model experiments. The heat conduction and local heat radiation losses between the inner and outer walls of the confined space liquid carbon dioxide phase change pressure pulsation experiment container are ignored. Therefore, the essence of ensuring the similarity of wall forces is to ensure that the heat conduction Φ in the model m The thermal conductivity Φ in the prototype p The sizes calculated according to similar scales are consistent 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 belongs to the process of forced convection heat transfer in the circular tube. Therefore, to ensure the similarity of convection heat transfer between the model and the prototype, the Nusselt criterion, Reynolds criterion and Prandtl criterion must be followed. The similarity of convection heat transfer is regarded as the boundary condition of similar fluid motion, and its essence is the similarity of heat transfer between the model and the prototype.

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

[0111]

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

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

[0114]

[0115] Step S1.5, obtain the similar expressions of the injection pressures of the prototype and the model, specifically:

[0116] The similarity of injection pressure is the initial condition in the prototype and model experiments, that is, the initial injection pressure will affect the entire subsequent motion state of liquid carbon dioxide. The similarity of injection pressure in the present invention is derived by the similarity of the forces on the fluid particles in the prototype and model. The pressure of the numerator in the Euler criterion is expressed by the pressure difference △P z Instead, the injection pressures of the prototype and the model are similar and satisfy the relationship of formula (7), which is expressed as follows:

[0117]

[0118] Where: △P z-p and △P z-m are the pressure differences of the prototype and the model when liquid carbon dioxide is injected, MPa; C △Pz is the similarity ratio of the pressure difference between the prototype and the model when liquid carbon dioxide is injected, dimensionless; P0 is the original pressure in the prototype or model, MPa;

[0119] Step S1.6, obtain the similar expressions of the seepage flow of the prototype and the model, specifically:

[0120] Under the continuous injection pressure, is the real-time flow rate of the two-phase carbon dioxide in the process of far-field seepage from the inner wall of the borehole to the coal seam, and the amount of carbon dioxide that penetrates into the coal seam within the time △t is the real-time integral value. This process is called "pressurized outflow". Therefore, the initial condition is that the two-phase carbon dioxide seepage is similar. In the downhole test, the bottom of the borehole is closed, and the two-phase carbon dioxide enters the coal seam through the cracks on the borehole wall. In the physical similarity simulation experiment, there are no cracks on the wall of the confined space liquid carbon dioxide phase change pressure pulsation experiment container, and the container outlet is used to simulate the two-phase carbon dioxide seepage channel on the downhole borehole wall.

[0121] Q out-p represents the average permeation rate of two-phase carbon dioxide in the prototype, expressed as Q out-m represents the average seepage flow of carbon dioxide in the model. The similarity between the two is shown in formula (8):

[0122]

[0123] Where: A 1-p Represents the area of ​​two-phase carbon dioxide seepage in the prototype, m 2 ; A 1-m Represents the area of ​​the model outlet, m 2 ; represents the permeation rate of two-phase carbon dioxide in the prototype, m / s; Represents the rate of two-phase carbon dioxide permeation in the model, m / s.

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

[0125] Step S2.1: Accurately select the similarity criteria number, specifically:

[0126] According to the above analysis of fluid flow similarity and convective heat transfer similarity, it is shown that in order for the model and the prototype in the present invention to remain similar, the similarity criteria related to fluid flow and the similarity criteria related to heat transfer must be followed. The corresponding similarity criterion numbers are Reynolds number, Euler number, Strouhal number, Nusselt number and Prandtl number, respectively. The expressions of the above similarity criterion numbers are shown in Formula 9.

[0127]

[0128] Where: α l is the thermal diffusivity of the fluid, m 2 / s; P' is the fluid dynamic pressure. In the present invention, the pressure difference △P z Indicates, MPa;

[0129] Step S2.2, obtain a similar scale of the phase change pressure of liquid carbon dioxide, specifically:

[0130] In the model and prototype of the present invention, the temperature when injecting carbon dioxide is -30°C, and it is believed that the physical properties of liquid carbon dioxide remain consistent, that is, as shown in formula (10):

[0131]

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

[0133]

[0134] According to the similarity scale of the convective heat transfer coefficient obtained in formula (11), the final expression of the similarity scale of the heat transfer between the prototype and the model in formula (6) is obtained, as shown in formula (12):

[0135]

[0136] As for the pressure caused by the endothermic phase change of liquid carbon dioxide, it is necessary to derive the similarity ratio of the convective heat transfer between the prototype and the model based on the Nusselt number, that is, the value of the similarity criterion number π3. Combined with the ideal gas state equation, since the injection temperature of liquid carbon dioxide in the prototype and the model is the same, for this reason, the ratio of the values ​​of temperature (T), molar mass of gas (M) and molar gas constant (R) in the ideal gas state equation is regarded as a constant, and K is used. △PIt is represented as shown in formula (13); Furthermore, the carbon dioxide injected into the prototype and the model are both in liquid state and have the same temperature value, and the specific heat capacity values ​​of the two are the same. Therefore, according to the heat calculation formula, the mathematical expression of the specific heat capacity is obtained as shown in formula (14):

[0137]

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

[0139]

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

[0141] Step S3 is specifically as follows:

[0142] By selecting the similarity criterion number and calculating the similarity scale of each parameter, the similarity scale of the physical quantities such as the geometric dimensions, the flow time, velocity, acceleration, dynamic pressure of the fluid particles, injection pressure, convection heat transfer, and phase change pressure in the prototype and model can be obtained. The calculation of the geometric similarity scale is the basis for obtaining the similarity scales of the other physical quantities. Therefore, before determining the similarity scales of each physical quantity in the prototype and model, the geometric similarity scale of the prototype and model must be determined first.

[0143] It is obviously unrealistic to fully realize the scaled model of the prototype in the present invention. Therefore, when designing the geometric dimensions, the inner diameter of the confined space liquid carbon dioxide phase change pressure pulsation experiment container is selected as the characteristic scale, and the pressure resistance of the container wall is focused on. The heat transfer performance is taken into account while meeting the safety requirements. Therefore, the container wall thickness needs to be optimized according to the relevant design specifications for pressure pipes.

[0144] In addition, if the geometric dimensions of the model are as consistent as possible with the prototype, the experimental difficulty will increase and the experimental cost will increase greatly. If the geometric dimensions of the model are too small, the experimental error will increase and the real law cannot be accurately reflected. Therefore, after considering the comprehensive objective factors such as the difficulty of obtaining the raw materials required for the processing of the confined space liquid carbon dioxide phase change pressure pulsation experimental container, the economic efficiency of the experiment, the limitation of the experimental site, and the convenience of calculating the similarity scale between the prototype and the model, the corresponding characteristic dimensions in the model can be determined according to the characteristic dimensions of the prototype, and then the geometric similarity scale C can be obtained. D , and then the similar scale values ​​of motion, power, convective heat transfer, injection pressure and seepage flow are calculated in turn.

[0145] S4. Calculate the equivalent radius of the rigid annular constant temperature medium cavity outside 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 constant temperature medium cavity, specifically:

[0147] The confined space liquid carbon dioxide phase change pressure pulsation experimental container serves as the "place" for the experiment, and the heat required for the phase change of liquid carbon dioxide is provided by the flow heat transfer of the medium in the rigid annular constant temperature medium cavity wrapped outside the container. The outer rigid annular constant temperature medium cavity ensures the constant temperature in the cavity through the circulation method of continuous input of hot medium and continuous output of cold medium, and conducts convection heat exchange with the liquid carbon dioxide in the confined space liquid carbon dioxide phase change pressure pulsation experimental container. To this end, in order to ensure that the heat transferred from the outer rigid annular constant temperature 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 outer rigid annular constant temperature medium cavity must be reasonably designed, and the medium therein must reach a certain flow rate.

[0148] In the present invention, the medium flow direction in the outer rigid annular constant temperature medium cavity is opposite to the injection direction of the liquid carbon dioxide in the confined space liquid carbon dioxide phase change pressure pulsation experimental container, so as to ensure a better convective heat exchange effect. Among them: the liquid inlet 8 of the outer rigid annular constant temperature medium cavity 3 is connected to the liquid inlet pipeline 15. Since the cavity is manufactured in sections, when the constant temperature medium flows in the outer rigid annular constant temperature medium cavity 3, it needs to 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, and finally enter the cavity liquid outlet pipeline 16 from the cavity liquid outlet 6. The medium that completes the heat exchange inside the outer rigid annular constant temperature medium cavity 3 flows back to the constant temperature medium circulator 10, thus completing the complete medium flow loop.

[0149] Therefore, in order to ensure that the medium in the annular constant temperature medium cavity can continuously and stably provide sufficient heat, the extreme value thinking is used for reverse calculation. Assuming that the confined space liquid carbon dioxide phase change pressure pulsation experiment container is filled with low-temperature liquid carbon dioxide of a certain temperature value and all phase changes occur, the required heat E can be calculated in combination with the latent heat of vaporization of the liquid carbon dioxide in the container. all ; Since the heat required by the container is all 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 formula (16), the smaller 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 medium cavity, the larger the required medium mass is calculated. Because the specific heat capacity of the medium in the cavity is a constant, the calculated cavity volume is the maximum value, and the calculated medium flow rate is the minimum value, as follows:

[0150] Eall =C 介质 ·m all ΔT (16)

[0151] Where: C 介质 is the specific heat of the medium in the outer rigid annular constant temperature medium cavity, KJ / (kg·K); m all is the mass of the medium required in the outer rigid annular constant temperature medium cavity, kg; △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 constant temperature medium cavity, K;

[0152] In addition, the outer rigid annular constant temperature medium cavity is equivalent to a cylinder. According to the axial length of the confined space liquid carbon dioxide phase change pressure pulsation experimental container and the equivalent area formula of the ring, the equivalent outer diameter of the outer rigid annular constant temperature medium cavity can be calculated, as shown in formula (17):

[0153]

[0154] Where: S 圆环 is the equivalent area of ​​the ring, m 2 ; S 内 is the area of ​​the inner circle in the ring, m 2 ; S 外 is the area of ​​the outer circle in the ring, m 2 ;

[0155] After calculating the equivalent radius of the outer rigid annular constant temperature medium cavity, the redundancy coefficient is formulated by comprehensively considering factors such as heat transfer loss, fluid filling rate in the cavity, and processing convenience, and finally the outer diameter of the outer rigid annular constant temperature medium cavity is determined.

[0156] Step S4.2, calculate the medium flow rate in the outer rigid annular constant temperature medium cavity, specifically:

[0157] Assuming that all the heat provided by the medium in the rigid annular constant temperature medium cavity is used for heat absorption by the phase change of liquid carbon dioxide, the heat loss between it and the external environment is not considered because the outer wall of the rigid annular constant temperature medium cavity is also wrapped with open self-adhesive rubber-plastic insulation cotton, and the heat loss of the constant temperature medium when passing through the wall thickness of the liquid carbon dioxide phase change pressure pulsation experiment container in the confined space is not considered. Under this condition, E in formula (16) all All of them are maintained by the continuous flow of the medium, that is, the internal energy (heat) required for phase change is consistent with the work done by the medium flow. Based on the energy conservation and work calculation formula, the force on the medium flow is obtained, as shown in formula (18):

[0158]

[0159] Where: F介质 is the force on the medium when it flows, N; L st is the length of the outer rigid annular constant temperature medium cavity, m;

[0160] Since the medium flow 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 formula (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 formula (19):

[0161]

[0162] Where: P 温温介质循环器 is the output power of the constant temperature medium circulator, W.

[0163] From formula (19), it can be obtained that the minimum flow rate of the medium in the limited space liquid carbon dioxide phase change pressure pulsation experiment container is completely filled with low-temperature liquid carbon dioxide, and the temperature in the outer rigid annular constant temperature medium cavity is a certain set value and completely changes to gas state. It can be seen that when conducting experiments simulating different coal seam temperatures, it is only necessary to ensure that the medium flow rate in the outer rigid annular constant temperature medium cavity satisfies v>v 介质 It can satisfy the heat required for physically similar simulation experiments.

[0164] Example 1

[0165] The working face of Shuanglong Coal Mine in Huangling Mining Area, Shaanxi Province is buried at a depth of 120 to 460 m, with a coal seam thickness of 3.0 to 3.2 m, an average of 3.1 m, and a coal seam inclination of 2 to 5°; the gas content of the coal seam is 0.82 to 4.7 m 3 / t, the initial velocity of gas emission is 10-13 mmHg, the coal body solidity 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, which is a coal seam that is difficult to extract gas. This coal seam adopts the method of drilling holes in the layer and injecting liquid carbon dioxide to pre-extract coal seam gas before mining. The diameter of the injection hole is 95 mm and the hole depth is 100 m. The hole is sealed with a sealer. The initial sealing depth is 9.9 m and the sealing length is 29 m.

[0166] In order to restore the pulsation change of the borehole pressure during the injection of liquid carbon dioxide in the working face of Shuanglong Coal Mine in Huangling Mining Area, it is determined according to the following steps:

[0167] If the geometric dimensions of the model are kept as consistent as possible with the prototype, the experimental difficulty and cost will increase greatly. If the model size is too small, the experimental error will increase and the real law cannot be accurately reflected. To this end, taking into account the difficulty of obtaining raw materials, the economic efficiency of the experiment, the limitation of the experimental site, and the convenience of calculating the similarity ratio 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 48mm and the length is set to 5000mm to simulate the borehole with a diameter of 96mm in the well; according to the pressure pipeline design specifications and standards, the wall thickness of the confined space liquid carbon dioxide phase change pressure pulsation experimental container is set to 11mm, so the outer diameter of the container is 70mm. Therefore, according to formula (1), the geometric linear similarity ratio C of the prototype and the model can be obtained: D =2, the geometric area similarity ratio is 4, and the geometric volume similarity ratio is 8.

[0168] In addition, since liquid carbon dioxide is a viscous fluid, in order to make the model and the prototype move similarly, the Reynolds numbers of the two must be consistent. According to formula (9), the velocity similarity ratio between the prototype and the model can be obtained: According to formula (3), the time similarity ratio C between the prototype and the model can be obtained: t =4; According to formula (8), the similarity scale of carbon dioxide seepage flow between the prototype and the model is 2; According to formula (12), the similarity scale of heat transfer between the prototype and the model is 2. According to the obtained geometric linear similarity scale value, velocity similarity scale value and time similarity scale value, combined with formula (11) and formula (15), the similarity criterion number can be obtained as shown in Table 1 below:

[0169] Table 1 Similarity criterion number values

[0170]

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

[0172] Table 2 Similar scale values ​​of physical quantities with the same name in the prototype and the model

[0173]

[0174] As mentioned above, the inner diameter of the confined space liquid carbon dioxide phase change pressure pulsation experiment container is 48mm and the length is 5000mm, so the volume of the container is 0.00904m 3Assuming that the temperature of the liquid carbon dioxide injected into the container is -30°C, the minimum pressure to keep it in liquid phase is 1.43MPa. Because when the temperature of liquid carbon dioxide is fixed, the latent heat of vaporization will decrease with the increase of pressure. Therefore, by selecting the latent heat of vaporization of carbon dioxide (303.48KJ / kg) under the temperature and pressure conditions of -30°C and 1.43MPa, the maximum amount of heat required for the complete phase change of the liquid carbon dioxide in the container can be obtained. In addition, the density of liquid carbon dioxide under this temperature and pressure condition is 1167.6kg / m 3 , so the mass of the container filled with -30℃ liquid carbon dioxide is 10.6kg, and the heat required for the complete phase change is 3216.9KJ, which is E in formula (16): all =3216.9KJ.

[0175] The medium in the outer rigid annular constant temperature medium cavity is pure water, whose specific heat capacity is 4.19KJ / (kg·K). According to formula (16), E all and C 介质 are all constant values. The smaller the temperature difference △T is, the more the required medium mass m is. all The larger the m all The larger the value, the smaller the required flow rate of the medium will be. Therefore, in order to obtain the minimum flow rate, it is necessary to obtain m all The maximum value, that is, the minimum value of the temperature difference △T needs to be obtained; for this reason, since five groups of injection temperature differences of 60℃, 65℃, 70℃, 75℃ and 80℃ are set in this embodiment, the minimum value △T=60℃, that is, 333.15K, is selected; thus, the mass m of pure water required for the outer rigid annular constant temperature medium cavity can be obtained all =2.3kg, the density of water is 1000kg / m 3 , which is converted into a volume of 0.0023m 3 .

[0176] Assuming the outer rigid annular constant temperature medium cavity is cylindrical, excluding the length of the threaded connector and other parts, the actual effective length of the effective section is 4617mm. Therefore, the equivalent area of ​​the radial section (circular ring) of the outer rigid annular constant temperature medium cavity is 0.0005m 2According to formula (17), it can be concluded that the equivalent radius of the ring is about 37 mm, the equivalent diameter is 74 mm, and the outer diameter of the confined space liquid carbon dioxide phase change pressure pulsation experiment container is 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 in the actual processing of the cavity, the cavity medium filling rate, and the processing convenience, the surplus coefficient 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 of the confined space liquid carbon dioxide phase change pressure pulsation experiment container is 94 mm.

[0177] In addition, according to formula (18), the force F exerted on the medium when it flows in the outer rigid annular constant temperature medium cavity can be obtained: 介质 =773290N. In formula (19), P 恒温介质循环器 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 under the condition of a liquid injection temperature difference of 60°C, the minimum flow rate of pure water in the rigid annular constant temperature medium cavity is 2.9m / s; that is, as long as the medium flow rate 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 experiment container can be guaranteed.

[0178] Example 2

[0179] The device for simulating the phase change pressure pulsation of liquid carbon dioxide in a layered borehole comprises a confined space liquid carbon dioxide phase change pressure pulsation experiment container assembly, the confined space liquid carbon dioxide phase change pressure pulsation experiment container assembly is enclosed by a rigid annular constant temperature medium cavity assembly, and also comprises a constant temperature medium circulator 10, and the constant temperature medium circulator 10 is connected to the rigid annular constant temperature medium cavity assembly through a pipeline.

[0180] Example 3

[0181] The device for simulating the phase change pressure pulsation of liquid carbon dioxide in a layered borehole comprises a confined space liquid carbon dioxide phase change pressure pulsation experiment container assembly, the confined space liquid carbon dioxide phase change pressure pulsation experiment container assembly is enclosed by a rigid annular constant temperature medium cavity assembly, and also comprises a constant temperature medium circulator 10, and the constant temperature medium circulator 10 is connected to the rigid annular constant temperature medium cavity assembly through a pipeline.

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

[0183] The rigid annular constant temperature medium cavity assembly includes three outer rigid annular constant temperature medium cavities 3, and the outer wall of each section of the 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 section of the outer rigid annular constant temperature medium cavity 3 is provided with a cavity outlet 6, and the third section of the 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 the cavity outlet pipeline 16, and the liquid inlet 8 is connected to the constant temperature medium circulator 10 through the cavity inlet pipeline 15; every two adjacent outer rigid annular constant temperature medium cavities 3 are connected through a medium transmission connecting pipe 9, and 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 a plurality of temperature sensors 12. Each confined space liquid carbon dioxide phase change pressure pulsation experiment container 2 is provided with a plurality of probes of the temperature sensors 12. All the temperature sensors 12 are connected to the temperature sensor data acquisition line 11.

[0185] It also includes a plurality of pressure sensors 13 , and probes of a plurality of pressure sensors 13 are arranged inside each confined space liquid carbon dioxide phase change pressure pulsation experiment container 2 ; all the pressure sensors 13 are connected to the pressure sensor data acquisition line 14 .

Claims

1. A simulation experimental device for phase change pressure pulsation of liquid carbon dioxide in a layered borehole, characterized in that: The invention comprises a confined space liquid carbon dioxide phase change pressure pulsation experiment container assembly, the confined space liquid carbon dioxide phase change pressure pulsation experiment container assembly is covered with a rigid annular constant temperature medium cavity assembly, and also comprises a constant temperature medium circulator (10), the constant temperature medium circulator (10) is connected to the rigid annular constant temperature medium cavity assembly through a pipeline.

2. The device for simulating the phase change pressure pulsation of liquid carbon dioxide in a layered borehole according to claim 1 is characterized in that: The confined space liquid carbon dioxide phase change pressure pulsation experimental container assembly comprises three sections of confined space liquid carbon dioxide phase change pressure pulsation experimental containers (2), each section of the confined space liquid carbon dioxide phase change pressure pulsation experimental container (2) is a tubular container; each section of the confined space liquid carbon dioxide phase change pressure pulsation experimental container (2) is connected through a threaded connector (4), the head end of the first section of the confined space liquid carbon dioxide phase change pressure pulsation experimental container (2) is provided with an inlet (1), and the tail end of the third section of the confined space liquid carbon dioxide phase change pressure pulsation experimental container (2) is provided with an outlet (5); The rigid annular constant temperature medium cavity assembly comprises three outer rigid annular constant temperature medium cavities (3), and each section of the 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 section of the outer rigid annular constant temperature medium cavity (3) is provided with a cavity liquid outlet (6), and the third section of the outer rigid annular constant temperature medium cavity (3) is provided with a liquid inlet (8); the cavity liquid outlet (6) is connected to a constant temperature medium circulator (10) through a cavity liquid outlet pipeline (16), and the liquid inlet (8) is connected to 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 connecting pipe (9); It also includes a plurality of temperature sensors (12), and probes of a plurality of temperature sensors (12) are arranged inside each confined space liquid carbon dioxide phase change pressure pulsation experimental container (2); all the temperature sensors (12) are connected to the temperature sensor data acquisition line (11); It also includes a plurality of pressure sensors (13), and probes of a plurality of pressure sensors (13) are arranged inside each confined space liquid carbon dioxide phase change pressure pulsation experimental container (2); all the pressure sensors (13) are connected to the pressure sensor data acquisition line (14).

3. The design method of the experimental device for simulating the phase change and pressure pulsation of liquid carbon dioxide in the layer drilling process is characterized by: The following steps are involved: S1. Based on the similarity criteria, the expressions of geometric similarity, motion similarity, dynamic similarity, convective heat transfer similarity, injection pressure similarity and seepage flow similarity between the prototype and the model are obtained; S2. Accurately select the similarity criterion number and obtain the similarity scale of the phase transition pressure of liquid carbon dioxide; S3, determine the specific values ​​of the similar scales of geometry, motion, dynamics, convective heat transfer, injection pressure and seepage flow in S1; S4. Calculate the equivalent radius of the rigid annular constant temperature medium cavity outside the container and the minimum flow velocity of the medium in the cavity.

4. The design method of the simulation experimental device for phase change pressure pulsation change of liquid carbon dioxide in the layer drilling according to claim 3 is characterized in that: Step S1 is specifically as follows: Step S1.1, obtain the geometric similarity expression of the prototype and the model, specifically: In order to make the characteristic dimensions of the model and the prototype meet a certain ratio, use l p Indicates the axial length of the prototype action section, l m Indicates the axial length of the model, D p Indicates the prototype diameter, D m Represents the diameter of the model, D' p Denotes the diameter of the pipe at the carbon dioxide outlet in the prototype, D' m represents the diameter of the model inlet; the model cannot fully simulate the process of two-phase CO2 seepage from the underground injection hole through the cracks around the hole to the far field of the coal seam, so it is simplified to the container outlet; based on geometric similarity, the geometrically similar linear scale, area scale and volume scale are obtained as shown in formula (1): Where: A p and A m Represent the radial cross-sectional area of ​​the prototype and model, m 2 ; V p and V m Represent the volume of the prototype and model, m 3 ; C D is the similarity ratio between the characteristic length of the prototype and the characteristic length of the model, and its value is called the similarity multiple of the two; Step S1.2, derive the similarity expression of the motion of the prototype and the model, specifically: In the study, t p It represents the time of liquid carbon dioxide particle flow in the prototype, t m represents the time of the liquid carbon dioxide particle flow in the model. The similarity ratio between the two is shown in formula (2): Where: C t is the time similarity ratio between the prototype and the model, and its value is called the similarity multiple of the two; According to 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 formula (3): Where: and represent the average velocity of liquid carbon dioxide particle flow in the prototype and model, m / s; is the fluid velocity similarity scale between the prototype and the model; a p and a m Represents the liquid carbon dioxide particle flow acceleration in the prototype and model, m / s 2 ; C a Similarity scale of fluid acceleration between prototype and model; Step S1.3, obtain the similar expression of the dynamics of the prototype and the model, specifically: Where: F p and F m Respectively represent the forces acting on the liquid carbon dioxide particles in the prototype and model, N; m p and m m Respectively represent the mass of liquid carbon dioxide in the borehole and the experimental container, kg; C F is the similarity scale of forces between prototype and model fluids; Step S1.4, obtain the similar expressions of convective heat transfer between the prototype and the model, specifically: Where: h 1-p is the surface heat transfer coefficient of convective heat transfer in the prototype, W / (m 2 ·K);h 1-m is the surface heat transfer coefficient of convective heat transfer in the model, W / (m 2 ·K); T st is the temperature of the inner wall of the container in the model, °C; Simplifying formula (5), we get formula (6): Step S1.5, obtain the similar expressions of the injection pressures of the prototype and the model, specifically: The pressure of the numerator in the Euler criterion is expressed as the pressure difference △P z Instead, the injection pressures of the prototype and the model are similar and satisfy the relationship of formula (7), which is expressed as follows: Where: △P z-p and △P z-m are the pressure differences of the prototype and the model when liquid carbon dioxide is injected, MPa; C △Pz is the similarity ratio of the pressure difference between the prototype and the model when liquid carbon dioxide is injected, dimensionless; P0 is the original pressure in the prototype or model, MPa; Step S1.6, obtain the similar expressions of the seepage flow of the prototype and the model, specifically: Under the continuous injection pressure, is the real-time flow rate of the two-phase carbon dioxide in the process of far-field seepage from the inner wall of the borehole to the coal seam, and the amount of carbon dioxide that penetrates into the coal seam within the time △t is the real-time integral value. This process is called "pressure outflow". Therefore, the initial condition is that the two-phase carbon dioxide seepage is similar. In the downhole test, the bottom of the borehole is closed, and the two-phase carbon dioxide enters the coal seam through the cracks on the borehole wall. In the physical similarity simulation experiment, there is no crack on the wall of the confined space liquid carbon dioxide phase change pressure pulsation experiment container, and the container outlet is used to simulate the two-phase carbon dioxide seepage channel on the downhole borehole wall. Q out-p represents the average permeation rate of two-phase carbon dioxide in the prototype, expressed as Q out-m represents the average seepage flow of carbon dioxide in the model. The similarity between the two is shown in formula (8): Where: A 1-p Represents the area of ​​two-phase carbon dioxide seepage in the prototype, m 2 ; A 1-m Represents the area of ​​the model outlet, m 2 ; represents the permeation rate of two-phase carbon dioxide in the prototype, m / s; Represents the rate of two-phase carbon dioxide permeation in the model, m / s.

5. The design method of the simulation experimental device for phase change pressure pulsation change of liquid carbon dioxide in the layer drilling according to claim 3 is characterized in that: Step S2 is specifically as follows: Step S2.1: Accurately select the similarity criteria number, specifically: Where: α l is the thermal diffusivity of the fluid, m 2 / s; P' is the fluid dynamic pressure, MPa; Step S2.2, obtain a similar scale of the phase change pressure of liquid carbon dioxide, specifically: In this model and the prototype, the temperature when injecting carbon dioxide is -30°C, and the physical properties of liquid carbon dioxide are considered to be consistent, as shown in formula (10): When the fluid flow and convective heat transfer in the model and prototype are similar, on the basis of satisfying the selected similarity criteria, the proportional relationship between the physical quantities derived from each similarity criterion number is shown in formula (11): According to the similarity scale of the convective heat transfer coefficient obtained in formula (11), the final expression of the similarity scale of the heat transfer between the prototype and the model in formula (6) is obtained, as shown in formula (12): As for the pressure caused by the endothermic phase change of liquid carbon dioxide, it is necessary to derive the similarity ratio of the convective heat transfer between the prototype and the model based on the Nusselt number, that is, the value of the similarity criterion number π3. Combined with the ideal gas state equation, since the injection temperature of liquid carbon dioxide in the prototype and the model is the same, for this reason, the ratio of the values ​​of temperature (T), molar mass of gas (M) and molar gas constant (R) in the ideal gas state equation is regarded as a constant, and K is used. △P It is represented as shown in formula (13); Furthermore, the carbon dioxide injected into the prototype and the model are both in liquid state and have the same temperature value, and the specific heat capacity values ​​of the two are the same. Therefore, according to the heat calculation formula, the mathematical expression of the specific heat capacity is obtained as shown in formula (14): By combining equations (12) to (14), we can obtain the proportional relationship between the evaporation phase change pressure of liquid carbon dioxide in the prototype and the model, as shown in equation (15):

6. The design method of the simulation experimental device for phase change pressure pulsation change of liquid carbon dioxide in the layer drilling according to claim 3 is characterized in that: Step S3 is as follows: select the inner diameter of the confined space liquid carbon dioxide phase change pressure pulsation experiment container as the characteristic scale, determine the corresponding characteristic scale in the model according to the characteristic scale of the prototype, and then obtain the geometric similarity scale C D , and then the similar scale values ​​of motion, power, convective heat transfer, injection pressure and seepage flow are calculated in turn.

7. The design method of the simulation experimental device for phase change pressure pulsation change of liquid carbon dioxide in the layer drilling according to claim 3 is characterized in that: Step S4.1, obtain the equivalent radius of the outer rigid annular constant temperature medium cavity, specifically: Assuming that the container of the confined space liquid carbon dioxide phase change pressure pulsation experiment is filled with low-temperature liquid carbon dioxide of a certain temperature value and all phase changes occur, the required heat E can be calculated based on the latent heat of vaporization of the liquid carbon dioxide in the container. all ; Since the heat required by the container is all 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 formula (16), the smaller 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 medium cavity, the larger the required medium mass is calculated. Because the specific heat capacity of the medium in the cavity is a constant, the calculated cavity volume is the maximum value, and the calculated medium flow rate is the minimum value, as follows: E all =C 介质 ·m all ·ΔT (16) Where: C 介质 is the specific heat of the medium in the outer rigid annular constant temperature medium cavity, KJ / (kg·K); m all is the mass of the medium required in the outer rigid annular constant temperature medium cavity, kg; △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 constant temperature medium cavity, K; In addition, the outer rigid annular constant temperature medium cavity is equivalent to a cylinder. According to the axial length of the confined space liquid carbon dioxide phase change pressure pulsation experimental container and the equivalent area formula of the ring, the equivalent outer diameter of the outer rigid annular constant temperature medium cavity is calculated, as shown in formula (17): Where: S 圆环 is the equivalent area of ​​the ring, m 2 ; S 内 is the area of ​​the inner circle in the ring, m 2 ; S 外 is the area of ​​the outer circle in the ring, m 2 ; Step S4.2, calculate 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 all used for the phase change heat absorption of liquid carbon dioxide, the force on the medium flow is obtained according to the energy conservation and work calculation formula, as shown in formula (18): Where: F 介质 is the force on the medium when it flows, N; L st is the length of the outer rigid annular constant temperature medium cavity, m; According to the power calculation formula and formula (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 formula (19): Where: P 温温介质循环器 is the output power of the constant temperature medium circulator, W.

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