Nitrogen foam fracturing construction parameter optimization method

Abstract

The invention discloses a nitrogen foam fracturing construction parameter optimization method. The method comprises the following steps: firstly, acquiring geological parameters, physical parameters and fracturing design parameters of a tight sandstone block reservoir; then, a fracturing and sand transportation integrated heat-fluid-solid coupling model in a wellhead-well bottom-fracturing crack is established, and values are assigned to attributes of all discrete element units according to parameters such as real reservoir ground stress and rock mechanics; finally, the construction parameters are adjusted, the reservoir transformation areas under different construction parameter conditions are calculated, and the construction parameter with the maximum reservoir transformation area under the target reservoir parameters is obtained with the maximum reservoir transformation area as the optimization target. According to the method, the time cost of construction parameter optimization can be reduced, and the precision of construction parameter optimization can be improved, so that a powerful basis is provided for nitrogen foam fracturing construction parameter optimization of the compact sandstone reservoir.

Description

Technical Field

[0001] The present invention belongs to the field of enhanced oil and gas production and transformation, and particularly relates to a method for optimizing construction parameters of nitrogen foam fracturing. Background Art

[0002] The micron-nano scale pore throats in tight sandstone gas reservoirs are well-developed, but the pore throat connectivity is poor, the capillary pressure is high, and the clay mineral content is high. Such reservoirs have geological characteristics such as low initial water saturation, resulting in easy self-absorption and difficult backflow of the aqueous phase. Therefore, water lock is an important damage form in such gas reservoirs. Since conventional hydraulic fracturing basically uses water-based fracturing fluids for fracturing construction, problems such as serious water lock, difficult liquid drainage, and great damage to fracture conductivity are likely to occur during the construction process, which seriously affects the fracturing construction effect. Once a low-permeability tight sandstone reservoir is damaged by water lock, it is more difficult to recover than medium-high permeability reservoirs. Therefore, special fracturing techniques are required for such reservoirs.

[0003] The technical principle of nitrogen foam fracturing is the same as that of conventional hydraulic fracturing. Both inject high-pressure fluids with large displacement into the reservoir to fracture the reservoir and form artificial support fractures in the reservoir. At the same time, the original fissures can be connected, so that a complex fracture network is formed in the reservoir, thereby realizing enhanced production and transformation. Compared with the conventional hydraulic fracturing method, the nitrogen foam fracturing fluid has a high viscosity and good sand-carrying performance. Nitrogen foam fracturing can carry a larger volume of proppants and place them into the support fractures. Along with the extension and distribution of the fractures, the proppants can also be placed into the newly created fractures by the nitrogen foam fracturing fluid, thus forming long and wide support fractures in tight sandstone, and maximizing the permeability of the reservoir and the conductivity of the support fractures. At present, the nitrogen foam fracturing technology for tight sandstone in China is not yet mature, and there are few reports on the related on-site operations of nitrogen foam fracturing for tight sandstone in China. The fracture propagation law after nitrogen foam fracturing construction is not clear, which causes great difficulties for on-site fracturing construction design. Therefore, it is of great significance to study the fracture extension characteristics of foam fracturing in tight sandstone by numerical simulation methods and optimize the construction parameters of nitrogen foam fracturing to improve the transformation effect of tight sandstone reservoirs. Summary of the Invention

[0004] The present invention is proposed to solve the problems existing in the prior art, and its purpose is to provide a method for optimizing construction parameters of nitrogen foam fracturing.

[0005] The present invention is achieved by the following technical solutions:

[0006] A method for optimizing construction parameters of nitrogen foam fracturing includes the following steps:

[0007] S1. Obtain the geological parameters, physical property parameters, and fracturing design parameters of the reservoir in the tight sandstone block;

[0008] S2. Considering the influence of the change in the rheological parameters of the foam fracturing fluid caused by the temperature and pressure changes during the process from the wellhead to the bottom hole and into the fracture, establish the foam rheological parameter calculation equation, the wellbore temperature and pressure calculation equation, the temperature and pressure calculation equation in the fracture, the fracture propagation equation, and the proppant transport equation respectively;

[0009] S3. Integrate the equations established in step S2, and use the discrete element method to establish a coupled thermo-hydro-mechanical model for sand transportation during fracturing from the wellhead to the bottom hole and into the fracture;

[0010] S4. Substitute the parameters obtained in step S1 into the coupled thermo-hydro-mechanical model for sand transportation during fracturing established in step S3 to obtain the simulation results of fracture propagation and the proppant placement pattern in the fracture;

[0011] S5. According to the simulation results obtained in step S4, adjust the construction parameters, calculate the reservoir stimulation area under different construction parameters, and take the maximum reservoir stimulation area as the optimization goal to obtain the construction parameters with the largest reservoir stimulation area under the target reservoir parameters, which are the optimal parameters.

[0012] In the above technical solution, the geological parameters include the reservoir and interlayer thickness, Poisson's ratio, and Young's modulus; the physical property parameters include the maximum horizontal principal stress, the minimum horizontal principal stress, and the vertical stress; the fracturing design parameters include the bedding tensile strength, the bedding cohesion, the bedding friction angle, the fracturing fluid viscosity, the fracturing fluid density, the construction displacement, the proppant particle size, the proppant density, the proppant volume fraction, the total pumping duration, and the sand-mixed fluid injection duration.

[0013] In the above technical solution, the foam rheological parameter calculation equation includes the foam state transformation equation, the foam mass change equation, the nitrogen foam fracturing fluid density equation, the nitrogen foam fracturing fluid thermal conductivity, and the foam viscosity equation;

[0014] The expression of the foam state transformation equation is:

[0015]

[0016] In the formula: V g1 is the initial state volume of the foam, with the unit of m 3 ; V g2 is the transformed state volume of the foam, with the unit of m 3 ; n is the foam flow state index, dimensionless; Z is the compression index, dimensionless; R e is the Reynolds number, dimensionless; T is the temperature under different conditions, with the unit of °C; P is the pressure under different conditions, with the unit of MPa;

[0017] The expression of the foam mass change equation is:

[0018]

[0019] In the formula: is the foam quality when the foam fracturing fluid is the continuous phase, with the unit of kg / m 3 ; is the foam quality when the foam fracturing fluid is the discontinuous phase, with the unit of kg / m 3 ; Z1 is the compression index when the foam fracturing fluid is the continuous phase, dimensionless; Z2 is the compression index when the foam fracturing fluid is the discontinuous phase, dimensionless; P1 is the pressure when the foam fracturing fluid is the continuous phase, with the unit of MPa; P2 is the pressure when the foam fracturing fluid is the discontinuous phase, with the unit of MPa; T1 is the temperature when the foam fracturing fluid is the continuous phase, with the unit of °C; T2 is the temperature when the foam fracturing fluid is the discontinuous phase, with the unit of °C;

[0020] The expression of the density equation of the nitrogen foam fracturing fluid is:

[0021]

[0022] In the formula: ρ1 is the density of the foam system in the pipeline, with the unit of kg / m 3 ; ρ 1g is the gas density, with the unit of kg / m 3 ; ρ 1l is the liquid density, with the unit of kg / m 3 ; ρ 1s is the proppant density, with the unit of kg / m 3 ; C sf is the sand concentration in the foam, %; is the foam quality, with the unit of kg / m 3 ;

[0023] The expression of the thermal conductivity coefficient of the nitrogen foam fracturing fluid is:

[0024]

[0025] In the formula: λ1 is the thermal conductivity coefficient of the foam system, with the unit of W / m·°C; λ 1g is the thermal conductivity coefficient of the gas in the foam system, dimensionless; λ 1l is the thermal conductivity coefficient of the liquid in the foam system, dimensionless; λ 1s is the thermal conductivity coefficient of the proppant in the foam system, dimensionless; C sf is the sand concentration in the foam, %; is the foam quality, with the unit of kg / m 3

[0026] The expression of the foam viscosity equation is:

[0027]

[0028] Where: μ is the viscosity of the foam system, with the unit of mPa·s; τ is the shear stress, with the unit of Mpa; γ is the shear coefficient, dimensionless; is the foam quality, with the unit of kg / m 3 ; μ e is the viscosity of the aqueous phase, with the unit of mPa·s; ν1 is the flow velocity of the foam system, with the unit of m / s; R is the radius of the spherical liquid droplet, with the unit of m.

[0029] In the above technical solution, the wellbore temperature and pressure calculation equation calculates the changes of the temperature and pressure in the wellbore with respect to the well depth and time, providing temperature and pressure data for calculating the foam rheological parameters;

[0030] The wellbore temperature and pressure calculation equation includes the temperature field equation in the tubing, the temperature field equation of the tubing wall, the temperature field equations in the annulus, casing, cement sheath and formation, and the wellbore fluid flow equation:

[0031] The expression of the temperature field equation in the tubing is:

[0032]

[0033]

[0034] Q = qΔp f (10)

[0035] The expression of the temperature field equation of the tubing wall is:

[0036]

[0037] The expression of the temperature field equations in the annulus, casing, cement sheath and formation is:

[0038]

[0039] The expression of the wellbore fluid flow equation is:

[0040]

[0041] Where: r1 is the inner diameter of the pipe, with the unit of m; Q1 is the heat generated by the fluid friction loss per unit length of the pipe and absorbed by N2, with the unit of W / m; ρ1 is the density of the foam system in the pipe, with the unit of kg / m 3; v1 is the flow rate of the foam system, with the unit of m / s; c1 is the heat capacity of the foam system, with the unit of J / (kg·°C); T1 is the temperature of the foam system, with the unit of °C; T2 is the temperature of the pipeline, with the unit of °C; h1 is the convective coefficient inside the pipeline, with the unit of W / (m2·K); z represents the coordinate in the axial direction; Re is the Reynolds number; Pr is the Prandtl number; λ1 is the thermal conductivity of the foam system, with the unit of W / m·°C; λ2 is the thermal conductivity of the pipeline, with the unit of W / m·°C; ρ2 is the density of the pipeline, with the unit of kg / m 3 ; c2 is the heat capacity of the pipeline, with the unit of J / (kg·°C); Q is the total heat generated by fluid friction loss per unit length of the pipeline, with the unit of W / m; q is the volumetric flow rate of the foam system, with the unit of m / s; Δp f is the gradient of the frictional pressure drop; r2 is the outer radius of the pipeline, with the unit of m; r3 is the inner diameter of the casing, with the unit of m; λ3 is the thermal conductivity of the foam system in the annulus, with the unit of W / m·°C; T3 is the temperature of the fluid in the annulus, with the unit of °C; Q2 is the heat generated by fluid friction loss per unit length of the pipeline and absorbed by the pipeline, with the unit of W / m; r i is the distance from the i-th element to the wellbore center in the radial direction, with the unit of m; λ i is the thermal conductivity of the i-th element in the radial direction, with the unit of W / (M·k); T i is the temperature of the i-th element in the radial direction, with the unit of °C; ρ i is the density of the i-th element in the radial direction, with the unit of kg / m 3 ; c i is the heat capacity of the i-th element in the radial direction, with the unit of J / (kg·°C); i = 3, 4, 5 respectively represent the elements in the annulus, casing, and cement sheath; i ≥ 6 represents the elements in the formation.

[0042] In the above technical solution, the temperature and pressure calculation equation in the fracture calculates the changes of temperature and pressure in the fracture with time and space, provides temperature and pressure data for calculating the rheological parameters of the foam, and provides pressure data for the extension of the fracture;

[0043] The expression of the temperature and pressure calculation equation in the fracture is:

[0044]

[0045] Where: ρ is the density of the fracturing fluid, with the unit of kg / m 3 ; q is the flow rate, with the unit of m 3 / s; w is the fracture width, with the unit of m; T is the fluid temperature, with the unit of °C; T0 is the initial fluid temperature, with the unit of °C; c i is the proportion of component i in the fluid, with the unit of %; C p,iis the specific heat of component i in the fluid, with the unit of J / (kg·K); S is the source term, with the unit of m 3 / s; q res is the heat conduction between the rock and the fluid, with the unit of W / m; q acid is the heat generated by the reaction, with the unit of W / m; q leak is the fluid filtration rate, with the unit of m / s; h is the reservoir thickness, with the unit of m.

[0046] In the above technical solution, the expression of the proppant transport equation is:

[0047]

[0048] In the formula: ρ p is the proppant density, with the unit of kg / m 3 ; ρ f is the fracturing fluid density, with the unit of kg / m 3 ; d p is the proppant particle size, with the unit of m; μ is the fracturing fluid viscosity, with the unit of mPa·s; c is the proppant volume fraction, %; ν p is the proppant flow velocity, with the unit of m / s; g is the acceleration due to gravity, with the unit of m / s 2 ; ν is the fluid flow velocity, with the unit of m / s.

[0049] Considering gravitational settlement, the proppant migration velocity consists of two parts. During the migration process, the influence caused by the interaction between proppant particles and the wall effect is used to correct the proppant transport equation with a correction factor.

[0050] In the above technical solution, the expression of the fracturing fracture extension equation is:

[0051]

[0052] In the formula: u is the fracture width, with the unit of m; K is the fluid bulk modulus, with the unit of Pa; μ is the fluid viscosity, with the unit of mPa.s; p is the fluid pressure, with the unit of Pa; t is the time, with the unit of s.

[0053] In the above technical solution, the specific process of establishing the integrated thermal-fluid-solid coupling model of fracturing sand transportation from the wellhead to the bottomhole and into the fracturing fracture using the discrete element method in step S3 is as follows:

[0054] S31. Establish a geometric model according to the geological characteristics of the target block, and preset a fracture surface in the established geometric model so that the hydraulic fracture can expand on the preset fracture surface; cut blocks and divide grids in the established geometric model to make the established geometric model more in line with the formation under actual conditions;

[0055] S32. Assign basic parameters to the geometric model, define the constitutive equations of rocks and fractures. The rock mass adopts a homogeneous isotropic elastic model, the hydraulic fracturing fracture uses the classical Mohr-Coulomb strength model, and the bedding failure criterion uses a continuous yield model.

[0056] The beneficial effects of the present invention are as follows:

[0057] The present invention provides a method for optimizing the construction parameters of nitrogen foam fracturing. Based on the discrete element method, a coupled thermo-hydro-mechanical model for fracturing sand transportation from the wellhead to the bottomhole and into the fracture is established, improving the accuracy of predicting the fracture morphology and proppant placement morphology of nitrogen foam fracturing in tight sandstone, and providing theoretical guidance for practical engineering applications. Brief Description of the Drawings

[0058] Figure 1 is the flowchart of the method of the present invention;

[0059] Figure 2 is a comparison diagram of fracture propagation morphologies of conventional hydraulic fracturing and nitrogen foam fracturing under the same geological conditions in Embodiment 1 of the present invention (a is the proppant placement nephogram of conventional water-based fracturing fluid, b is the proppant placement nephogram of nitrogen foam fracturing fluid);

[0060] Figure 3 is the reservoir stimulation area SRA diagram under the construction parameters in Embodiment 1 of the present invention (a is the construction displacement, b is the SRA under the construction displacement).

[0061] For those of ordinary skill in the art, other related drawings can be obtained based on the above drawings without creative efforts. Detailed Description of the Embodiments

[0062] In order to enable those skilled in the art of the present technology to better understand the technical solution of the present invention, the technical solution of the present invention will be further described below with reference to the drawings in the specification and through specific embodiments.

[0063] Embodiment 1

[0064] As Figure 1 shown, a method for optimizing the construction parameters of nitrogen foam fracturing includes the following steps:

[0065] S1. Obtain geological, physical property, and fracturing design parameters

[0066] Table 1 Thickness of each small layer and rock mechanical parameters of the model

[0067] Position Thickness / m Young's modulus / GPa Poisson's ratio Upper interlayer 15 22.28 0.32 Pay zone 30 29.02 0.17 Lower interlayer 15 23.58 0.31

[0068] Table 2 Stress parameters of each small layer of the model

[0069] Position Maximum horizontal principal stress / MPa Minimum horizontal principal stress / MPa Vertical stress / MPa Upper interlayer 31.37 24.72 25.63 Pay zone 26.04 19.20 24.89 Lower interlayer 31.52 24.76 26.61

[0070] Table 3 Model Fracturing Design Parameters

[0071] Other parameters Value Unit Fracturing fluid viscosity 3 mPa·s Fracturing fluid density 1000 <![CDATA[kg / m 3 > Treatment rate 3 <![CDATA[m 3 / min]]> Proppant particle size 275 μm Proppant density 1000 <![CDATA[kg / m 3 > Proppant volume fraction 15 ％ Bedding tensile strength 4 MPa Bedding cohesion 17 MPa Bedding internal friction angle 20 ° Total pumping duration 10 min Sand mixture injection duration 10 min

[0072] S2. Substitute the parameters in Tables 1 - 3 into the integrated fracturing sand transportation, heat - fluid - solid coupling model established in the present invention from wellhead to bottomhole to fracture interior. The comparison diagram of the simulation results of fracture propagation and proppant placement morphology in the fracture and the simulation results of conventional water - based fracturing fluid is as Figure 2 shown.

[0073] S3. According to the simulation results of this embodiment, draw the comparison diagram of reservoir stimulation area under different construction parameters as Figure 3 shown.

[0074] S4. It can be seen from the simulation results that the optimal construction parameters are: the construction displacement is recommended to be 3.5 m 3 / min, the construction fluid volume is recommended to be 340 m 3 , where the ratio of liquid nitrogen to water is 1:23, the sand - adding scale is 45 m 3 , and 30 / 50 - mesh proppant is used.

[0075] The finally optimized construction parameters in this embodiment are close to the on - site construction parameters, which can provide guidance for on - site construction.

[0076] The applicant declares that the above - mentioned is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A method for optimizing construction parameters of nitrogen foam fracturing, characterized in that: It includes the following steps: S1. Obtain the geological parameters, physical property parameters and fracturing design parameters of the reservoir in the tight sandstone block; S2. Considering the influence of the change of the rheological parameters of the foam system caused by the change of temperature and pressure during the process of the foam fracturing fluid from the wellhead - bottomhole - inside the fracture, establish the foam rheological parameter calculation equation, wellbore temperature and pressure calculation equation, temperature and pressure calculation equation inside the fracture, fracture extension equation and proppant transport equation respectively; S3. Integrate the equations established in step S2, and establish a coupled thermo - hydro - mechanical model for fracturing sand transportation from the wellhead - bottomhole - inside the fracture by using the discrete element method; S4. Substitute the parameters obtained in step S1 into the coupled thermo - hydro - mechanical model for fracturing sand transportation established in step S3 to obtain the simulation results of fracture propagation and the placement pattern of proppants inside the fracture; S5. According to the simulation results obtained in step S4, adjust the construction parameters, calculate the reservoir stimulation area under different construction parameters, and take the maximum reservoir stimulation area as the optimization goal to obtain the construction parameters with the largest reservoir stimulation area under the target reservoir parameters, which are the optimal parameters.

2. The method for optimizing the construction parameters of nitrogen foam fracturing according to claim 1, characterized in that: The geological parameters include the reservoir - separating layer thickness, Poisson's ratio and Young's modulus; the physical property parameters include the maximum horizontal principal stress, minimum horizontal principal stress and vertical stress; the fracturing design parameters include the bedding tensile strength, bedding cohesion, bedding friction angle, fracturing fluid viscosity, fracturing fluid density, construction displacement, proppant particle size, proppant density, proppant volume fraction, total pumping duration and mixed sand fluid injection duration.

3. The method for optimizing the construction parameters of nitrogen foam fracturing according to claim 1, wherein: The foam rheological parameter calculation equation includes the foam state transformation equation, foam mass change equation, nitrogen foam fracturing fluid density equation, nitrogen foam fracturing fluid thermal conductivity and foam viscosity equation; The expression of the foam state transformation equation is: Where: V g1 is the initial volume of the foam, with the unit of m 3 ; V g2 is the transformed volume of the foam, with the unit of m 3 ; n is the foam flow state index, dimensionless; Z is the compression index, dimensionless; R e is the Reynolds number, dimensionless; T is the temperature under different conditions, with the unit of °C; P is the pressure under different conditions, with the unit of MPa; The expression of the foam mass change equation is: Wherein: is the foam quality when the foam fracturing fluid is the continuous phase, with the unit of kg / m 3 ; is the foam quality when the foam fracturing fluid is the discontinuous phase, with the unit of kg / m 3 ; Z1 is the compression index when the foam fracturing fluid is the continuous phase, dimensionless; Z2 is the compression index when the foam fracturing fluid is the discontinuous phase, dimensionless; P1 is the pressure when the foam fracturing fluid is the continuous phase, with the unit of MPa; P2 is the pressure when the foam fracturing fluid is the discontinuous phase, with the unit of MPa; T1 is the temperature when the foam fracturing fluid is the continuous phase, with the unit of °C; T2 is the temperature when the foam fracturing fluid is the discontinuous phase, with the unit of °C; The expression of the nitrogen foam fracturing fluid density equation is: Where: ρ1 is the density of the foam system in the pipeline, with the unit of kg / m 3 ; ρ 1g is the gas density, with the unit of kg / m 3 ; ρ 1l is the liquid density, with the unit of kg / m 3 ; ρ 1s is the proppant density, with the unit of kg / m 3 ; C sf is the sand concentration in the foam, %; is the foam quality, with the unit of kg / m 3； The expression of the nitrogen foam fracturing fluid thermal conductivity is: In the formula: λ1 is the thermal conductivity of the foam system with the unit of W / m·℃; λ 1g is the thermal conductivity of the gas in the foam system, dimensionless; λ 1l Thermal conductivity of the liquid in the foam system, dimensionless; λ 1s is the thermal conductivity of the proppant in the foam system, dimensionless; C sf is the sand concentration in the foam, %; is the foam quality, with the unit of kg / m 3 The expression of the foam viscosity equation is: Where: μ is the viscosity of the foam system, with the unit of mPa·s; τ is the shear stress, with the unit of Mpa; γ is the shear coefficient, dimensionless; is the foam quality, with the unit of kg / m 3 ; μ e is the viscosity of the aqueous phase, with the unit of mPa·s; ν1 is the flow velocity of the foam system, with the unit of m / s; R is the radius of the spherical liquid droplet, with the unit of m.

4. The method for optimizing the construction parameters of nitrogen foam fracturing according to claim 1, wherein: The wellbore temperature and pressure calculation equation calculates the changes of temperature and pressure in the wellbore with respect to well depth and time, providing temperature and pressure data for calculating the foam rheological parameters; The wellbore temperature and pressure calculation equation includes the temperature field equation inside the tubing, the temperature field equation of the tubing wall, the temperature field equations in the annulus, casing, cement sheath and formation and the wellbore fluid flow equation: The expression of the temperature field equation inside the tubing is: Q = qΔp f (10) The expression of the temperature field equation of the tubing wall is: The expression of the temperature field equations in the annulus, casing, cement sheath and formation is: The expression of the wellbore fluid flow equation is: Where: r1 is the inner diameter of the pipe, in m; Q1 is the heat generated by the fluid friction loss in the pipe per unit length and absorbed by N2, in W / m; ρ1 is the density of the foam system in the pipe, in kg / m 3 ; v1 is the flow velocity of the foam system, in m / s; c1 is the heat capacity of the foam system, in J / (kg.°C); T1 is the temperature of the foam system, in °C; T2 is the temperature of the pipe, in °C; h1 is the convective coefficient inside the pipe, in W / (m2.K); z represents the coordinate in the axial direction; Re is the Reynolds number; Pr is the Prandtl number; λ1 is the thermal conductivity of the foam system, in W / m·°C; λ2 is the thermal conductivity of the pipe, in W / m·°C; ρ2 is the density of the pipe, in kg / m 3 ; c2 is the heat capacity of the pipe, in J / (kg.°C); Q is the total heat generated by fluid friction loss per unit length of the pipe, in W / m; q is the volumetric flow rate of the foam system, in m / s; Δp f is the gradient of the frictional pressure drop; r2 is the outer radius of the pipe, in m; r3 is the inner diameter of the casing, in m; λ3 is the thermal conductivity of the foam system in the annulus, in W / m·°C; T3 is the temperature of the fluid in the annulus, in °C; Q2 is the heat generated by fluid friction loss per unit length of the pipe and absorbed by the pipe, in W / m; r i is the distance from the i-th element to the wellbore center in the radial direction, in m; λ i is the thermal conductivity of the i-th element in the radial direction, with the unit of W / (M·K); T i is the temperature of the i-th element in the radial direction, with the unit of °C; ρ i is the density of the i-th element in the radial direction, with the unit of kg / m 3 ; c i is the heat capacity of the i-th element in the radial direction, with the unit of J / (kg·°C); i = 3, 4, 5 represent the elements in the annulus, casing, and cement sheath respectively; i ≥ 6 represents the elements in the formation.

5. The method for optimizing the construction parameters of nitrogen foam fracturing according to claim 1, characterized in that: The temperature and pressure calculation equation inside the fracture calculates the changes of temperature and pressure inside the fracture with respect to time and space, providing temperature and pressure data for calculating the foam rheological parameters and pressure data for fracture extension; The expression of the temperature and pressure calculation equation inside the fracture is: Where: ρ is the density of the fracturing fluid, with the unit of kg / m 3 ; q is the flow rate, with the unit of m 3 / s; w is the fracture width, with the unit of m; T is the fluid temperature, in °C; T0 is the initial fluid temperature, in °C; c i is the proportion of component i in the fluid, in %; C p,i is the specific heat of component i in the fluid, in J / (kg·K); S is the source term, in m 3 / s; q res is the heat conduction between the rock and the fluid, in W / m; q acid is the heat generated by the reaction, in W / m; q leak is the fluid filtration loss, in m / s; h is the reservoir thickness, with the unit of m.

6. The method for optimizing nitrogen foam fracturing construction parameters according to claim 1, wherein: The expression of the proppant transport equation is: Where: ρ p is the density of the proppant, with the unit of kg / m 3 ; ρ f is the density of the fracturing fluid, with the unit of kg / m 3 ; d p where d is the proppant particle size in m; μ is the viscosity of the fracturing fluid in mPa·s; c is the proppant volume fraction in %; ν p is the proppant flow rate, with the unit of m / s; g is the acceleration due to gravity, with the unit of m / s 2 ; ν is the fluid flow rate, with the unit of m / s.

7. The method for optimizing nitrogen foam fracturing construction parameters according to claim 1, characterized in that: The expression of the fracture extension equation is: In the formula: u is the fracture width, with the unit of m; K is the fluid bulk modulus, with the unit of Pa; μ is the fluid viscosity, with the unit of mPa·s; p is the fluid pressure, with the unit of Pa; t is the time, with the unit of s.

8. The method for optimizing the construction parameters of nitrogen foam fracturing according to claim 1, wherein: The specific process of establishing the integrated heat-fluid-solid coupling model for fracturing sand transportation from wellhead to bottomhole and into the fracture in step S3 by using the discrete element method is as follows: S31. Establish a geometric model according to the geological characteristics of the target block, and preset fracture surfaces in the established geometric model so that hydraulic fractures can propagate along the preset fracture surfaces; cut blocks and divide grids in the established geometric model to make the established geometric model more in line with the actual formation; S32. Assign basic parameters to the geometric model, define the constitutive equations of rocks and fractures. The rock blocks adopt a homogeneous isotropic elastic model, the hydraulic fracturing fractures use the classical Mohr-Coulomb strength model, and the bedding failure criterion uses a continuous yield model.

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