A method for calculating and controlling the peak pressure of supercritical CO2 phase change

Abstract

The invention discloses a supercritical CO2 phase change peak pressure calculation and control method, which belongs to the field of supercritical CO2 phase change control technology, and includes the following steps: based on the thin-walled cylinder theory, a supercritical CO2 phase change peak pressure calculation model is constructed according to the fracturing tube parameters; a supercritical CO2 phase change pressure test experiment is carried out to obtain peak pressure data, and the supercritical CO2 phase change peak pressure calculation model is verified through the peak pressure data; based on the verified supercritical CO2 phase change peak pressure calculation model, a pressure database under the cross-influence of different parameters is established, and the optimal parameter combination is selected through the pressure database to complete the control of the supercritical CO2 phase change peak pressure. The invention solves the problem of lack of theoretical guidance in the practical application of CO2 phase change fracturing technology.

Description

Technical Field

[0001] The invention belongs to the technical field of supercritical CO2 phase change control, and in particular relates to a method for calculating and controlling supercritical CO2 phase change peak pressure. Background Art

[0002] Supercritical CO2 phase change is a new type of blasting technology. This technology uses a heating tube to quickly change the liquid CO2 at a certain pressure and temperature into a liquid-gas phase, and the volume expands 600 times instantly, thereby generating a strong physical pressure (above 200MPa). It is a green and ecological blasting technology. The control of the peak pressure of phase change in practical applications of this technology is still immature, so further theoretical analysis and verification are needed.

[0003] There are still two problems with the existing technology in blasting applications: first, the calculation of phase change peak pressure mostly relies on shear calculation formulas, and the calculation assumptions and design parameters are different from the parameters of the one-time fracturing tube structure used in actual engineering; second, the control method of phase change peak pressure is still imperfect, and most key control parameters cannot be reflected in the calculation method. Summary of the invention

[0004] The present invention provides a method for calculating and controlling supercritical CO2 phase change peak pressure, which solves the problem of lack of theoretical guidance in practical application of CO2 phase change cracking technology.

[0005] In order to solve the above technical problems, the technical solution of the present invention is: a method for calculating and controlling the supercritical CO2 phase change peak pressure, comprising the following steps:

[0006] S1. Based on the thin-walled cylinder theory and the fracture tube parameters, a supercritical CO2 phase change peak pressure calculation model is constructed;

[0007] S2. Conduct a supercritical CO2 phase change pressure test to obtain peak pressure data, and verify the supercritical CO2 phase change peak pressure calculation model through the peak pressure data;

[0008] S3. Based on the verified supercritical CO2 phase change peak pressure calculation model, a pressure database under the cross-influence of different parameters is established, and the optimal parameter combination is selected through the pressure database to complete the control of the supercritical CO2 phase change peak pressure.

[0009] Furthermore, the specific steps of step S1 are:

[0010] S11. Based on the thin-walled cylinder theory, the fracture tube is regarded as a thin-walled cylinder, and a fracture tube mechanical model is constructed;

[0011] S12. Based on the mechanical model of cracked tube, determine the principal stress-strain state of the thin-walled cylinder when it is in the plastic limit state;

[0012] S13. Describe the stress-strain relationship of the cracking pipe steel when it is subjected to burst pressure;

[0013] S14. Based on the unified strength criterion of triple shear, the equivalent stress is obtained through the principal stress-strain state;

[0014] S15. According to the stress-strain relationship and the equivalent stress, the explosion internal pressure of the thin-walled cylinder is obtained, and the calculation formula of the explosion internal pressure of the thin-walled cylinder is used as the calculation model of the supercritical CO2 phase change peak pressure.

[0015] Furthermore, the calculation formula for the bursting internal pressure of the thin-walled cylinder is:

[0016]

[0017] Among them, P b represents the bursting internal pressure of the thin-walled cylinder, α represents the tensile and compressive strength ratio of the material, b represents the influence of the principal stress on the material damage, r0 represents the initial average radius of the thin-walled cylinder, d0 represents the initial wall thickness of the thin-walled cylinder, n represents the strain hardening exponent, σ t Indicates the engineering ultimate tensile strength of the pipe.

[0018] Furthermore, the specific steps of step S2 are:

[0019] S21, drilling a hole on the fracturing pipe using a drilling machine, and installing the pressure sensor in a threaded form based on the drilling position;

[0020] S22. Fix the liquid storage tube with a steel bracket, assemble the energy release plate, sealing gasket, release head, heating tube and charging head in sequence, and perform an air tightness check;

[0021] S23. Test the circuit connectivity between the heating tube and the charging head by using an ohmmeter;

[0022] S24, connect the CO2 filling system and the filling head through the filling hose, and set the booster pump and chiller parameters through the display screen;

[0023] S25, connecting the detonation wire, and starting the data acquisition system, and completing the detonation through the detonator;

[0024] S26. Obtain peak pressure data through the data acquisition system and complete the supercritical CO2 phase change pressure test;

[0025] S27, inputting the test parameters of the supercritical CO2 phase change pressure test into the supercritical CO2 phase change peak pressure calculation model for calculation to obtain the supercritical CO2 phase change peak pressure;

[0026] S28. Verify the supercritical CO2 phase change peak pressure calculation model through peak pressure data and supercritical CO2 phase change peak pressure.

[0027] Furthermore, the specific steps of step S3 are:

[0028] S31, setting a preset number of working conditions by the control variable method, and inputting the preset number of working conditions into the verified supercritical CO2 phase change peak pressure calculation model for calculation, to obtain the supercritical CO2 phase change peak pressure and hole wall pressure change curves under different fracturing pipe parameters;

[0029] S32. The influence of the inner diameter of the fracturing tube, the wall thickness of the fracturing tube and the tensile strength on the supercritical CO2 phase change peak pressure is obtained through the variation curves of the supercritical CO2 phase change peak pressure and the hole wall pressure under different fracturing tube parameters.

[0030] S33. Based on the influence rules, adjust the values ​​of the fracture tube parameters input into the supercritical CO2 phase change peak pressure calculation model, and establish a pressure database under the cross-influence of different parameters;

[0031] S34, inputting the bursting pressure value required for the actual project, searching the pressure database through the genetic algorithm, and obtaining different parameter combinations under the required bursting pressure value;

[0032] S35. Select the optimal parameter combination from different parameter combinations to complete the control of the supercritical CO2 phase change peak pressure.

[0033] Furthermore, the calculation formula for the number of combinations of fracture pipe parameters in the pressure database in step S33 is:

[0034]

[0035] Where S represents the number of combinations of fracture tube parameters, Max(·) represents the maximum value, Min(·) represents the minimum value, a represents the inner diameter of the fracture tube, b' represents the wall thickness of the fracture tube, c represents the tensile strength, and A, B and C represent the number of fracture tube inner diameters, fracture tube wall thicknesses and tensile strengths, respectively.

[0036] Furthermore, the objective function of the genetic algorithm in step S34 is:

[0037]

[0038] Where Fitness(·) represents the objective function, x represents the fracture pipe parameter vector, P(·) represents the pressure calculation function, and P target represents the target pressure value, and |·| represents the absolute value.

[0039] The beneficial effects of the present invention are: (1) according to the actual failure mode of the one-time fracturing pipe commonly used in actual engineering, a calculation model of the supercritical CO2 phase change peak pressure is derived based on the thin-walled cylinder theory, which can accurately calculate the pressure change and enhance the understanding of the behavior of the engineering system;

[0040] (2) The verified pressure calculation model was used to analyze the influence of the fracture pipe parameters. By establishing a pressure database and analyzing the cross-influence of different parameter combinations, the control strategy of supercritical CO2 phase change can be optimized to improve the operating efficiency and safety of the system.

[0041] (3) The database provides a wealth of parameter combinations and pressure data to help engineers make scientific design and operation decisions and select the optimal parameter settings. The optimized control strategy can effectively control the phase change pressure of supercritical CO2, reduce pressure fluctuations and emergencies, and ensure stable operation of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The figure is a flow chart of the method for calculating and controlling the supercritical CO2 phase change peak pressure of the present invention.

[0043] Figure 2 It is a schematic diagram of the mechanical model of the fracture tube of the present invention.

[0044] Figure 3 This is a supercritical CO2 phase change peak pressure-time curve diagram of the present invention.

[0045] Figure 4 The graph is a variation curve of supercritical CO2 phase change peak pressure and pore wall pressure under different fracturing tube parameters of the present invention. DETAILED DESCRIPTION

[0046] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the present invention.

[0047] Example

[0048] like Figure 1 As shown, the present invention provides a method for calculating and controlling supercritical CO2 phase change peak pressure, comprising the following steps:

[0049] S1. Based on the thin-walled cylinder theory and the fracture tube parameters, a supercritical CO2 phase change peak pressure calculation model is constructed;

[0050] S2. Conduct a supercritical CO2 phase change pressure test to obtain peak pressure data, and verify the supercritical CO2 phase change peak pressure calculation model through the peak pressure data;

[0051] S3. Based on the verified supercritical CO2 phase change peak pressure calculation model, a pressure database under the cross-influence of different parameters is established, and the optimal parameter combination is selected through the pressure database to complete the control of the supercritical CO2 phase change peak pressure.

[0052] In this embodiment, the supercritical CO2 shock pressure, as the driving force of the surface vibration, has a direct impact on the vibration velocity, so an accurate description of the pressure is crucial for the derivation of the vibration velocity. Previous studies have mostly used the shear failure formula to calculate the peak pressure of the fracture pipe blasting, which is not suitable for on-site blasting pressure calculation.

[0053] Therefore, according to the geometric conditions of the fractured pipe (r / d ≥ 20) and the failure mode, such as Figure 2 As shown in (a), it is regarded as a closed large-diameter thin-walled cylinder, and the pressure formula applicable to on-site supercritical CO2 phase change explosion is derived based on it. The mechanical model of the fracture tube is as follows: Figure 2 (b) and (c), where P is the uniformly distributed supercritical CO2 phase change peak pressure, in MPa, and the instantaneous radius r of the fracturing tube and the instantaneous wall thickness d of the fracturing tube are both in mm.

[0054] The specific steps of step S1 are:

[0055] S11. Based on the thin-walled cylinder theory, the fracture tube is regarded as a thin-walled cylinder, and a fracture tube mechanical model is constructed;

[0056] In this embodiment, a fracturing tube mechanical model is constructed according to the uniformly distributed supercritical CO2 phase change peak pressure p, the instantaneous radius r of the fracturing tube, and the instantaneous wall thickness d of the fracturing tube.

[0057] S12. Based on the mechanical model of cracked tube, determine the principal stress-strain state of the thin-walled cylinder when it is in the plastic limit state;

[0058] S13. Describe the stress-strain relationship of the cracking pipe steel when it is subjected to burst pressure;

[0059] S14. Based on the unified strength criterion of triple shear, the equivalent stress is obtained through the principal stress-strain state;

[0060] S15. According to the stress-strain relationship and the equivalent stress, the explosion internal pressure of the thin-walled cylinder is obtained, and the calculation formula of the explosion internal pressure of the thin-walled cylinder is used as the calculation model of the supercritical CO2 phase change peak pressure.

[0061] In this embodiment, under the action of the explosion internal pressure, an infinitesimal differential unit of the outer wall of a thin-walled cylinder is considered, and its stress-strain state is expressed in a cylindrical coordinate system (r, θ, z). When the thin-walled cylinder is in the plastic limit state, the Barlow formula and finite strain theory are used to determine its principal stress-strain state.

[0062] The expression of the principal stress-strain state is:

[0063]

[0064]

[0065] Among them, σ1, σ2 and σ3 represent the first principal stress, the second principal stress and the third principal stress respectively, σ θ , σ z and σ r They represent the hoop stress, axial stress and radial stress of the thin-walled cylinder, ε1, ε2 and ε3 represent the first principal strain, the second principal strain and the third principal strain, respectively. θ , ε z and ε r They represent the hoop strain, axial strain and radial strain of the thin-walled cylinder respectively, r0 represents the initial average radius of the thin-walled cylinder, and d0 represents the initial wall thickness of the thin-walled cylinder.

[0066] In this embodiment, considering the influence of the strain hardening phenomenon of the material when the material reaches the limit state under the action of internal pressure, a pure power rate curve is selected to describe the stress-strain relationship of the cracking pipe steel when subjected to the blasting pressure as follows:

[0067] σ=Kε n

[0068]

[0069] σ s =e n σ t

[0070]

[0071] Where σ and ε represent the uniaxial true stress and uniaxial true strain under simple tension, K represents the strength parameter, n represents the strain hardening exponent, and σ s Indicates the tensile yield limit of the pipe, σ t Indicates the engineering ultimate tensile strength of the pipe.

[0072] In this embodiment, the unequal tensile and compressive strength characteristics of steel (SD effect) and the influence of intermediate principal stress are considered, and the three-shear unified strength criterion is introduced:

[0073] (1+b)(σ1-σ3)σt =(ασ1-σ3)(σ1-σ3)+b(ασ1-σ2)(σ1-σ2)+b(ασ2-σ3)(σ2-σ3)

[0074] α=σ t / σ c

[0075] 0≤b≤α / (2+α)

[0076] Among them, α represents the tensile and compressive strength ratio of the material, b represents the influence of the principal stress on the material failure, and σ c Indicates the compressive strength of steel.

[0077] In this embodiment, the stress state of the thin-walled cylinder is substituted into the three-shear unified strength criterion to obtain the equivalent stress as follows:

[0078]

[0079] Among them, σ eq represents equivalent stress;

[0080] In this embodiment, in the plastic deformation stage, the material volume is incompressible, so ε1+ε2+ε3=0. Substituting this formula into the principal stress-strain state, we get

[0081] Based on the assumption of plastic work, we can get: eq ε eq =σε=σ1ε1.

[0082] The formula σ eq ε eq =σε=σ1ε1 and equivalent stress σ eq Combined, the expression of the first principal strain ε1 is obtained as

[0083]

[0084] Substituting the expression of the first principal strain ε1 into In the equation, the geometric relationship of the thin-walled cylinder is obtained as follows:

[0085] Simultaneous stress-strain relationship, equivalent stress σ eq And the geometric relationship of the thin-walled cylinder, the bursting internal pressure of the thin-walled cylinder is obtained as:

[0086] Derivation of the explosion internal pressure of a thin-walled cylinder according to the plastic instability condition: The equivalent effect can be obtained as:

[0087] Will Substituting it into the explosion internal pressure of the thin-walled cylinder, the final explosion internal pressure of the thin-walled cylinder is obtained, which is the calculation model of the supercritical CO2 phase change peak pressure.

[0088] The calculation formula of the explosion internal pressure of the thin-walled cylinder is:

[0089]

[0090] Among them, P b represents the bursting internal pressure of the thin-walled cylinder, α represents the tensile and compressive strength ratio of the material, b represents the influence of the principal stress on the material damage, r0 represents the initial average radius of the thin-walled cylinder, d0 represents the initial wall thickness of the thin-walled cylinder, n represents the strain hardening exponent, σ t Indicates the engineering ultimate tensile strength of the pipe.

[0091] The specific steps of step S2 are:

[0092] S21. Perform a supercritical CO2 phase change pressure test test through a test device to obtain peak pressure data and complete the supercritical CO2 phase change pressure test test;

[0093] S22, inputting the test parameters of the supercritical CO2 phase change pressure test into the supercritical CO2 phase change peak pressure calculation model for calculation to obtain the supercritical CO2 phase change peak pressure;

[0094] S23. Verify the supercritical CO2 phase change peak pressure calculation model through peak pressure data and supercritical CO2 phase change peak pressure.

[0095] In this embodiment, a supercritical CO2 phase change pressure test experiment is carried out to obtain a supercritical CO2 phase change peak pressure-time curve, that is, peak pressure data.

[0096] In this embodiment, the steel type of the fracturing pipe used in the test is Q235 steel, whose elastic modulus E=207Gpa, yield strength is 235MPa, tensile strength is 370MPa, engineering ultimate tensile strength is 500MPa, the initial radius r0 of the fracturing pipe is 11mm, and the initial thickness d0 of the fracturing pipe under each working condition is 1.9mm, 1.9mm, 2.6mm, 2.6mm, 2.6mm, 3.4mm, 3.4mm and 3.4mm respectively. The supercritical CO2 phase change peak pressure-time curve is obtained as shown in Figure 3 shown.

[0097] Considering that the three-shear unified strength criterion can be degenerated into or approximate multiple existing yield criteria, for example, when α = 1, b = 0, it can be degenerated into the Tresca yield criterion; when α = 1, b = 1 / 3, it can be approximated into the Von-Mises yield criterion.

[0098] Therefore, when the radius of the fracturing tube is 11 mm and the material used is Q235 steel, the peak pressure value of the supercritical CO2 phase change obtained in the experiment is used as a reference, and the theoretical pressure calculation value degenerated into Tresca and the theoretical pressure calculation value approximating Von-Mises are used for comparison. The comparison is shown in Table 1. Table 1 is a statistical table of the comparison results between the test value and the theoretical value of the bursting pressure of the fracturing tube.

[0099] Table 1

[0100]

[0101] Among them, P s , P b1 and P b2 They represent the experimental value of the bursting pressure of a thin-walled cylinder, the calculated value of the theoretical pressure degenerated into Tresca, and the calculated value of the theoretical pressure approximating Von-Mises.

[0102] It can be seen from Table 1 that the average ratios of the burst pressure calculated by the Tresca yield criterion and the Von-Mises yield criterion to the experimental value are 0.828 and 1.049, respectively. The latter obviously has higher calculation accuracy and is more suitable for the calculation of the peak pressure of supercritical CO2 phase change.

[0103] The specific steps of step S3 are:

[0104] S31, setting a preset number of working conditions by the control variable method, and inputting the preset number of working conditions into the verified supercritical CO2 phase change peak pressure calculation model for calculation, to obtain the supercritical CO2 phase change peak pressure and hole wall pressure change curves under different fracturing pipe parameters;

[0105] In this embodiment, the setting of the fracture pipe parameters is shown in Table 2.

[0106] Table 2

[0107]

[0108] The above working conditions are input into the supercritical CO2 phase change peak pressure calculation model for calculation, and the supercritical CO2 phase change peak pressure and hole wall pressure change curves under different fracture pipe parameters are obtained, as shown in Figure 4 shown.

[0109] S32. The influence of the inner diameter of the fracturing tube, the wall thickness of the fracturing tube and the tensile strength on the supercritical CO2 phase change peak pressure is obtained through the variation curves of the supercritical CO2 phase change peak pressure and the hole wall pressure under different fracturing tube parameters.

[0110] When the rock properties remain unchanged, due to the hole wall amplification effect, the hole wall pressure is 2.17 times the initial blasting pressure, but the change trends of the two are consistent. Among them, with the increase of the thickness of the fracturing tube, the blasting pressure and the hole wall pressure both show an obvious linear growth, and the latter grows faster; with the increase of the inner diameter of the fracturing tube, the blasting pressure and the hole wall pressure decrease in a nearly exponential form; in addition, increasing the tensile strength of the fracturing tube also increases the blasting pressure and the hole wall pressure, and the growth trend is nearly linear.

[0111] S33. Based on the influence rules, adjust the values ​​of the fracture tube parameters input into the supercritical CO2 phase change peak pressure calculation model, and establish a pressure database under the cross-influence of different parameters;

[0112] In this embodiment, the amount of calculated data is expanded, the value range of the inner diameter of the fracturing tube is set to 10-100 mm (a value is taken every 0.1 mm), the wall thickness of the fracturing tube is set to 0.1-10 mm (a value is taken every 0.1 mm), the tensile strength and the yield strength of the corresponding material are set according to the existing materials in the market, and some mechanical property parameters of the material are shown in Table 3, which is a statistical table of mechanical property values ​​of some steel materials available in the market.

[0113] Table 3

[0114]

[0115]

[0116] Among them, the tensile strength of steel takes the minimum value.

[0117] Then, based on the supercritical CO2 phase change peak pressure calculation model, all parameters were substituted for calculation to obtain the peak pressure calculation values ​​under different working conditions, forming a pressure database of supercritical CO2 phase change peak pressure under different influencing parameters.

[0118] S34, inputting the bursting pressure value required for the actual project, searching the pressure database through the genetic algorithm, and obtaining different parameter combinations under the required bursting pressure value;

[0119] In this embodiment, the blasting pressure value required for the actual project is input, that is, the supercritical CO2 phase change peak pressure in the pressure database, such as 150 MPa, and a genetic algorithm is used to find a high-quality solution in a large-scale parameter space by defining a fitness function, initializing the population, selecting, crossover and mutation operations.

[0120] S35. Select the optimal parameter combination from different parameter combinations to complete the control of the supercritical CO2 phase change peak pressure.

[0121] The calculation formula for the number of fracture pipe parameter combinations in the pressure database in step S33 is:

[0122]

[0123] Where S represents the number of combinations of fracture tube parameters, Max(·) represents the maximum value, Min(·) represents the minimum value, a represents the inner diameter of the fracture tube, b' represents the wall thickness of the fracture tube, c represents the tensile strength, and A, B and C represent the number of fracture tube inner diameters, fracture tube wall thicknesses and tensile strengths, respectively.

[0124] The objective function of the genetic algorithm in step S34 is:

[0125]

[0126] Where Fitness(·) represents the objective function, x represents the fracture pipe parameter vector, P(·) represents the pressure calculation function, and P target represents the target pressure value, and |·| represents the absolute value.

[0127] In summary, the present invention derives a calculation model for the supercritical CO2 phase change peak pressure based on the thin-walled cylinder theory according to the actual failure mode of the disposable fracturing tube commonly used in actual engineering, and simultaneously carries out a supercritical CO2 phase change pressure test experiment to obtain a phase change pressure time-history curve, verifies the pressure calculation model, and uses the verified pressure calculation model to analyze the influence rules of the fracturing tube parameters. Therefore, the method of the present invention solves the problem of lack of theoretical guidance in the practical application of CO2 phase change fracturing technology, can improve the accuracy, safety and efficiency of design and operation, and provides a rich combination of parameters and pressure data to help engineers make scientific design and operation decisions and select the optimal parameter settings.

Claims

1. A method for calculating and controlling supercritical CO2 phase change peak pressure, characterized in that: The following steps are involved: S1. Based on the thin-walled cylinder theory and the fracture tube parameters, a supercritical CO2 phase change peak pressure calculation model is constructed; The specific steps of step S1 are: S11. Based on the thin-walled cylinder theory, the fracture tube is regarded as a thin-walled cylinder, and a fracture tube mechanical model is constructed; S12. Based on the mechanical model of cracked tube, determine the principal stress-strain state of the thin-walled cylinder when it is in the plastic limit state; The expression of the principal stress-strain state is: Among them, σ1, σ2 and σ3 represent the first principal stress, the second principal stress and the third principal stress respectively, σ θ , σ z and σ r They represent the hoop stress, axial stress and radial stress of the thin-walled cylinder, ε1, ε2 and ε3 represent the first principal strain, the second principal strain and the third principal strain, respectively. θ , ε z and ε r They represent the hoop strain, axial strain and radial strain of the thin-walled cylinder respectively, r0 represents the initial average radius of the thin-walled cylinder, and d0 represents the initial wall thickness of the thin-walled cylinder; S13. Describe the stress-strain relationship of the cracking pipe steel when it is subjected to bursting pressure, specifically: σ=Kε n s s =e n s t Where σ and ε represent the uniaxial true stress and uniaxial true strain under simple tension, K represents the strength parameter, n represents the strain hardening exponent, and σ s Indicates the tensile yield limit of the pipe, σ t Indicates the engineering ultimate tensile strength of the pipe; S14. Based on the unified strength criterion of triple shear, the equivalent stress is obtained through the principal stress-strain state; The equivalent stress is expressed as: Among them, σ eq represents the equivalent stress, α represents the tensile and compressive strength ratio of the material, α represents the tensile and compressive strength ratio of the material, and b represents the influence of the principal stress on the material failure; S15. According to the stress-strain relationship and the equivalent stress, the bursting internal pressure of the thin-walled cylinder is obtained, and the calculation formula of the bursting internal pressure of the thin-walled cylinder is used as the supercritical CO2 phase change peak pressure calculation model; The calculation formula of the explosion internal pressure of the thin-walled cylinder is: Among them, P b Indicates the bursting internal pressure of a thin-walled cylinder; S2. Conduct a supercritical CO2 phase change pressure test to obtain peak pressure data, and verify the supercritical CO2 phase change peak pressure calculation model through the peak pressure data; S3. Based on the verified supercritical CO2 phase change peak pressure calculation model, a pressure database under the cross-influence of different parameters is established, and the optimal parameter combination is selected through the pressure database to complete the control of the supercritical CO2 phase change peak pressure.

2. The method for calculating and controlling the supercritical CO2 phase change peak pressure according to claim 1, characterized in that: The specific steps of step S2 are: S21. Perform a supercritical CO2 phase change pressure test test through a test device to obtain peak pressure data and complete the supercritical CO2 phase change pressure test test; S22, inputting the test parameters of the supercritical CO2 phase change pressure test into the supercritical CO2 phase change peak pressure calculation model for calculation to obtain the supercritical CO2 phase change peak pressure; S23. Verify the supercritical CO2 phase change peak pressure calculation model through peak pressure data and supercritical CO2 phase change peak pressure.

3. The method for calculating and controlling the supercritical CO2 phase change peak pressure according to claim 1, characterized in that: The specific steps of step S3 are: S31, setting a preset number of working conditions by the control variable method, and inputting the preset number of working conditions into the verified supercritical CO2 phase change peak pressure calculation model for calculation, to obtain the supercritical CO2 phase change peak pressure and hole wall pressure change curves under different fracturing pipe parameters; S32. The influence of the inner diameter of the fracturing tube, the wall thickness of the fracturing tube and the tensile strength on the supercritical CO2 phase change peak pressure is obtained through the variation curves of the supercritical CO2 phase change peak pressure and the hole wall pressure under different fracturing tube parameters. S33. Based on the influence rules, adjust the values ​​of the fracture tube parameters input into the supercritical CO2 phase change peak pressure calculation model, and establish a pressure database under the cross-influence of different parameters; S34, inputting the bursting pressure value required for the actual project, searching the pressure database through the genetic algorithm, and obtaining different parameter combinations under the required bursting pressure value; S35. Select the optimal parameter combination from different parameter combinations to complete the control of the supercritical CO2 phase change peak pressure.

4. The method for calculating and controlling the supercritical CO2 phase change peak pressure according to claim 3, characterized in that: The calculation formula for the number of fracture pipe parameter combinations in the pressure database in step S33 is: Where S represents the number of combinations of fracture tube parameters, Max(·) represents the maximum value, Min(·) represents the minimum value, a represents the inner diameter of the fracture tube, b' represents the wall thickness of the fracture tube, c represents the tensile strength, and A, B and C represent the number of fracture tube inner diameters, fracture tube wall thicknesses and tensile strengths, respectively.

5. The method for calculating and controlling the supercritical CO2 phase change peak pressure according to claim 4, characterized in that: The objective function of the genetic algorithm in step S34 is: Where Fitness(·) represents the objective function, x represents the fracture pipe parameter vector, P(·) represents the pressure calculation function, and P target represents the target pressure value, and |·| represents the absolute value.

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