A calculation method for the fracture propagation behavior of hydraulic fracturing in hot dry rock

Abstract

The present invention relates to the field of hot dry rock geothermal resource development, and provides a method for calculating the expansion behavior of hot dry rock hydraulic fracturing cracks, including: obtaining basic parameters of hot dry rock; calculating the pressure of the fracturing section of the production well; calculating the permeability coefficient of the hot dry rock; constructing the relationship between the basic parameters and the crack opening, crack length, and crack permeability coefficient; calculating the shape characteristic parameters and focal length of the cracks, and performing coordinate transformation on the hydraulic fracturing cracks; constructing a heterogeneous seepage analytical element model between the pressure of the fracture section of the production well, the injection volume, and the permeability coefficient of the hot dry rock after the rock mass is fractured; and using an iterative method to solve the crack length under different pressure and flow conditions. The change process of the crack length during the hydraulic fracturing of the hot dry rock is calculated by the wellhead pressure and the injection flow during the hydraulic fracturing process.

Description

Technical Field

[0001] The invention belongs to the field of enhanced geothermal system development, and in particular relates to a method for calculating the expansion behavior of hydraulic fracturing cracks in hot dry rocks. Background Art

[0002] Hot dry rock geothermal resources refer to abnormally high-temperature rock masses buried deep in the earth with no or only a small amount of fluid inside and a temperature above 180°C. It is a green and clean energy source. The heat stored in hot dry rocks needs to be transformed into heat reservoirs through hydraulic fracturing technology to improve the permeability of the heat reservoirs, thereby forming an enhanced geothermal system to develop hot dry rock geothermal resources.

[0003] Enhanced geothermal systems usually use a pumping well and an injection well to develop hot dry rock geothermal resources. The distance between the pumping and injection wells affects the economic benefits of the enhanced geothermal system. If the distance between the pumping and injection wells is too small, the heat exchange path is short and the heat exchange efficiency is low; if the distance between the pumping and injection wells is too large, the two wells cannot be effectively connected, and geothermal resources cannot be developed. Therefore, accurate calculation of hydraulic fracturing fracture expansion behavior is of great significance for the design of pumping and injection wells in enhanced geothermal systems.

[0004] The existing method for calculating hydraulic fracturing cracks mainly uses microseismic event inversion to calculate the influence range of cracks. However, microseismic events can only be used to invert whether the rock mass has been fractured, but cannot reflect whether the cracks are connected to form effective cracks. Therefore, a method for calculating the expansion behavior of hydraulic fracturing cracks that can calculate hydraulic connectivity is urgently needed. Summary of the invention

[0005] Purpose of the invention: The method for calculating the expansion behavior of hydraulic fracturing cracks in hot dry rocks proposed in this patent generalizes the hydraulic fractures into elliptical smooth water-conducting fractures on a two-dimensional plane, calculates the shape parameters of the ellipse based on the physical and mechanical parameters of the hot dry rock mass, calculates the expansion range of the fracture based on the wellhead pressure and water injection volume during the fracturing process, considers the internal resistance of the fracture itself on the basis of the existing method for calculating the half-length of the fracture, calculates the permeability coefficient of the fracture based on the fracture width, and calculates the fracture length through an iterative method, thereby calculating the expansion behavior of the hydraulic fracturing fracture. The calculation method is fast and accurate.

[0006] Technical solution, in order to solve the above problems, the present invention proposes a method for calculating the expansion behavior of hydraulic fracturing cracks in hot dry rocks, the method comprising the following steps:

[0007] Step SS1, obtaining basic parameters of the hydraulic fracturing crack expansion behavior of hot dry rock, the basic parameters include: elastic modulus of hot dry rock, Poisson's ratio, wellbore diameter of the fracturing section of the production well, dynamic viscosity of the fracturing fluid, relationship function between the injection volume of the fracturing fluid and the friction of the well pipe, monitoring the injection volume and wellhead pressure during hydraulic fracturing; the relationship function between the injection volume of the fracturing fluid and the friction of the well pipe is calculated by testing the friction of the well pipe under different flow rates of the fracturing fluid, and obtaining the relationship function between the injection volume of the fracturing fluid and the friction of the well pipe by a binary function fitting method;

[0008] Step SS2, calculating the fracturing section pressure, fracturing section breakdown pressure, and fracturing section initial pressure of the production well according to the relationship function between the fracturing fluid injection volume and the well pipe friction obtained in step SS1 and the monitored injection volume and wellhead pressure during the hydraulic fracturing process, and calculating the permeability coefficient of the hot dry rock according to the fracturing section breakdown pressure and the injection volume;

[0009] Step SS3, based on the basic parameters obtained in step SS1, respectively constructing the relationship between the basic parameters and the fracture opening, fracture length, and fracture permeability coefficient, wherein the basic parameters include: elastic modulus of hot dry rock, Poisson's ratio, dynamic viscosity of fracturing fluid, and injection volume;

[0010] Step SS4, calculating the shape characteristic parameters and focal length of the crack according to the hot dry rock permeability calculated in step SS2 and the crack opening, crack length, and crack permeability calculated in step SS3, performing coordinate transformation on the hydraulic fracturing crack, and transforming the elliptical hydraulic fracturing crack into a unit circle in space;

[0011] Step SS5, constructing a heterogeneous seepage analytical element model between the pressure of the fracturing section of the production well, the injection volume and the permeability coefficient of the hot dry rock according to the coordinates converted in step SS4;

[0012] Step SS6, according to the heterogeneous seepage analytical metamodel established in step SS5, the initial value of the hydraulic fracturing crack length is set, and the crack length under different pressure and flow conditions is solved by an iterative method.

[0013] Furthermore, the calculation process of the production well fracturing stage pressure, fracturing stage rupture pressure, fracturing stage initial pressure and hot dry rock permeability coefficient in step SS2 is as follows:

[0014] (1) Based on the relationship function between the injection volume of the fracturing fluid and the friction of the well pipe and the injection volume and wellhead pressure during the monitored hydraulic fracturing process, the calculation process of the fracturing section pressure, the fracturing section breakdown pressure, and the initial pressure of the fracturing section of the production well is as follows:

[0015] P=P′+γ f h w -f(Q) (1)

[0016] Where, P is the calculated fracturing pressure of the production well, Pa; P′ is the monitored wellhead pressure, Pa; γ f is the density of the fracturing fluid, N / m 3 ;h w is the elevation difference from the fracturing section to the wellhead, m; f(Q) is the wellbore friction force calculated by the relationship function between the fracturing fluid injection volume and the wellbore friction, Pa; Q is the monitored injection volume, m 3 / d;

[0017] (2) According to the monitored wellhead pressure P w ', wellhead rupture pressure P f ′, the initial wellhead pressure P0′ and the injection volume at the corresponding time, and use formula (1) to calculate the fracturing stage pressure P of the production well w , Fracturing stage rupture pressure P f , initial pressure of fracturing section P0;

[0018] (3) According to the fracturing fluid density, fracturing section thickness, fracturing section fracture pressure, fracturing section initial pressure, injection volume, and wellbore of the fracturing section of the production well, the initial value of the permeability coefficient of the dry hot rock mass is constructed:

[0019]

[0020] In the formula, K (1) is the initial value of the permeability coefficient of hot dry rock, m / d; P f is the fracture pressure of the fracturing section, N / m 3 ; P0 is the initial pressure of the fracturing stage, N / m 3 ; Q0 is the injection volume of the fracturing stage before the dry hot rock mass breaks, m 3 / d;r w is the diameter of the fracturing section of the production well, m; M is the thickness of the fracturing section, m;

[0021] (4) Using the fracture pressure of the fracturing section, the amount of fluid injected before the fracture of the hot dry rock mass, the initial pressure of the fracturing section and the initial value of the permeability coefficient of the hot dry rock mass K (1) , iterative solution until K (k+1) -K (k) |≤0.001, K (k) That is the permeability coefficient K of dry hot rock mass, and the iterative equation is as follows:

[0022] K (k+1) =Aln[K (k) ]+Bln(r w )+C (3)

[0023]

[0024] Where A, B, and C are the coefficients of the iteration equation; k is the number of iterations, and k=1 at the initial moment.

[0025] Furthermore, the relationship between the hydraulic fracturing crack opening, crack length, and crack permeability coefficient constructed in step SS3 is as follows:

[0026] (1) The hydraulic fracture is generalized as an ellipse on a plane, the major axis of the ellipse is the length of the hydraulic fracture, the minor axis of the ellipse is the maximum opening of the hydraulic fracture, and the height of the fracture is the thickness of the fracture section;

[0027] (2) The relationship between the opening and length of hydraulic fracturing cracks is as follows:

[0028]

[0029] Where w is the opening of the hydraulic fracturing crack, m; η is the kinematic viscosity coefficient of the fracturing fluid, m 2 / s;ρ f is the density of the fracturing fluid, kg / m 3 ; Q is the injection volume, m 3 / d; ν is Poisson's ratio; L is the length of the hydraulic fracturing crack, m; E is the elastic modulus, Pa; M is the thickness of the fracturing section, m;

[0030] (3) The seepage in hydraulic fracturing cracks follows the characteristics of smooth fracture flow, and its permeability coefficient is as follows:

[0031]

[0032] In the formula, K f is the permeability coefficient of hydraulic fracturing fracture, m / d.

[0033] Furthermore, in step SS4, the shape characteristic parameters of the crack, the focal length of the ellipse, and the spatial coordinate conversion calculation process are as follows:

[0034] (1) The hydraulic fracturing crack is generalized as an ellipse on the plane, and its shape characteristic parameters and focal length are as follows:

[0035]

[0036] In the formula, λ is the characteristic parameter of the ellipse shape; c is the focal length of the ellipse;

[0037] (2) Based on the shape characteristic parameters λ and focal length c calculated by formula (9) and formula (10), a spatial coordinate transformation relationship is established, the wellbore coordinates of the fracturing section of the production well are transformed into the Z coordinate system, and the reference radius in the Z coordinate system is calculated. The coordinate transformation is performed so that the hydraulic fracturing crack is transformed into a circular area with a radius of 1:

[0038]

[0039] In the formula, Z w is the radius of the production well in the Z coordinate system; Z R It is the reference radius in the Z coordinate system.

[0040] Furthermore, in step SS5, the heterogeneous seepage analytical metamodel constructed is as follows:

[0041]

[0042] Where P w is the fracturing pressure of the production well, pa; Q is the injection volume, m 3 / d; K is the permeability coefficient of dry hot rock, m / d; Re is the real part function.

[0043] Furthermore, in step SS6, an iterative method is used to solve the fracture length under different production well fracturing stage pressure and flow conditions, and the calculation process is as follows:

[0044] (1) Set the initial value of the crack length L (1) , according to formula (7), calculate the corresponding initial value of crack opening w (1) According to formula (8), the initial crack opening w (1) Calculate the initial value of the fracture permeability

[0045] (2) Calculate the initial shape parameter λ according to formula (9) and formula (10): (1) and the initial focal length c (1) According to formula (11), the wellbore coordinate r of the production well fracturing section is w Transform to the new coordinate system Calculate the reference in the Z coordinate system according to formula (12)

[0046] (3) Establish an iterative equation and iteratively solve the crack length until |L (i+1) -L (i) |<0.001, L (i) is the crack length obtained;

[0047]

[0048]

[0049] In the formula, i is the number of iterations, and the initial moment i=1.

[0050] Beneficial effects: Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0051] (1) The present invention overcomes the deficiency that microseismic testing can only invert and infer rock fractures but cannot calculate the connectivity of the initiating fractures. By establishing the relationship between the injection pressure, injection flow rate and fracture length, the effective length and expansion process of the water-conducting fractures are calculated, which is more instructive for the cyclic pumping and water injection of hot dry rocks.

[0052] (2) By establishing the relationship between the fracture permeability coefficient and the fracture opening, the internal resistance of the fracture itself is taken into account, overcoming the deficiency of the traditional method of calculating the half-length of the fracture that ignores the internal resistance of the fracture, and the calculation result is more objective and accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 This is a flowchart of a method for calculating the crack expansion behavior of hot dry rock hydraulic fracturing according to an embodiment of the present invention;

[0054] Figure 2 A schematic diagram of a three-dimensional conceptual model of a method for calculating the crack expansion behavior of hot dry rock hydraulic fracturing according to an embodiment of the present invention;

[0055] Figure 3 A schematic diagram of a two-dimensional plane conceptual model of a method for calculating the crack extension behavior of hot dry rock hydraulic fracturing according to an embodiment of the present invention;

[0056] Figure 4 The relationship between the friction resistance of the fracturing fluid and the injection flow rate tested by a method for calculating the crack extension behavior of hot dry rock hydraulic fracturing according to an embodiment of the present invention;

[0057] Figure 5 The wellhead flow rate and water injection rate curve monitored during the hot dry rock hydraulic fracturing process of the embodiment of the present invention;

[0058] Figure 6 The crack expansion process of hot dry rock hydraulic fracturing calculated in an embodiment of the present invention. DETAILED DESCRIPTION

[0059] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and are not intended to limit the protection scope of the present invention.

[0060] like Figure 1 As shown, the present invention proposes a method for calculating the expansion behavior of hydraulic fracturing cracks in hot dry rocks, the method comprising the following steps:

[0061] Step SS1, obtaining basic parameters of the hydraulic fracturing crack expansion behavior of hot dry rock, the basic parameters include: elastic modulus of hot dry rock, Poisson's ratio, wellbore diameter of the fracturing section of the production well, dynamic viscosity of the fracturing fluid, relationship function between the injection volume of the fracturing fluid and the friction of the well pipe, monitoring the injection volume and wellhead pressure during hydraulic fracturing; the relationship function between the injection volume of the fracturing fluid and the friction of the well pipe is calculated by testing the friction of the well pipe under different flow rates of the fracturing fluid, and obtaining the relationship function between the injection volume of the fracturing fluid and the friction of the well pipe by a binary function fitting method;

[0062] Step SS2, calculating the fracturing section pressure, fracturing section breakdown pressure, and fracturing section initial pressure of the production well according to the relationship function between the fracturing fluid injection volume and the well pipe friction obtained in step SS1 and the monitored injection volume and wellhead pressure during the hydraulic fracturing process, and calculating the permeability coefficient of the hot dry rock according to the fracturing section breakdown pressure and the injection volume;

[0063] Step SS3, based on the basic parameters obtained in step SS1, respectively constructing the relationship between the basic parameters and the fracture opening, fracture length, and fracture permeability coefficient, wherein the basic parameters include: elastic modulus of hot dry rock, Poisson's ratio, dynamic viscosity of fracturing fluid, and injection volume;

[0064] Step SS4, calculating the shape characteristic parameters and focal length of the crack according to the hot dry rock permeability calculated in step SS2 and the crack opening, crack length, and crack permeability calculated in step SS3, performing coordinate transformation on the hydraulic fracturing crack, and transforming the elliptical hydraulic fracturing crack into a unit circle in space;

[0065] Step SS5, constructing a heterogeneous seepage analytical element model between the pressure of the fracturing section of the production well, the injection volume and the permeability coefficient of the hot dry rock according to the coordinates converted in step SS4;

[0066] Step SS6, according to the heterogeneous seepage analytical metamodel established in step SS5, the initial value of the hydraulic fracturing crack length is set, and the crack length under different pressure and flow conditions is solved by an iterative method.

[0067] Furthermore, the calculation process of the production well fracturing stage pressure, fracturing stage rupture pressure, fracturing stage initial pressure and hot dry rock permeability coefficient in step SS2 is as follows:

[0068] (1) Based on the relationship function between the injection volume of the fracturing fluid and the friction of the well pipe and the injection volume and wellhead pressure during the monitored hydraulic fracturing process, the calculation process of the fracturing section pressure, the fracturing section breakdown pressure, and the initial pressure of the fracturing section of the production well is as follows:

[0069] P=P′+γ f h w -f(Q) (1)

[0070] Where, P is the calculated fracturing pressure of the production well, Pa; P′ is the monitored wellhead pressure, Pa; γ f is the density of the fracturing fluid, N / m 3 ;h w is the elevation difference from the fracturing section to the wellhead, m; f(Q) is the wellbore friction force calculated by the relationship function between the fracturing fluid injection volume and the wellbore friction, Pa; Q is the monitored injection volume, m 3 / d;

[0071] (2) According to the monitored wellhead pressure P w ', wellhead rupture pressure P f ′, the initial wellhead pressure P0′ and the injection volume at the corresponding time, and use formula (1) to calculate the fracturing stage pressure P of the production well w , Fracturing stage rupture pressure P f , initial pressure of fracturing section P0;

[0072] (3) According to the fracturing fluid density, fracturing section thickness, fracturing section fracture pressure, fracturing section initial pressure, injection volume, and wellbore of the fracturing section of the production well, the initial value of the permeability coefficient of the dry hot rock mass is constructed:

[0073]

[0074] In the formula, K (1) is the initial value of the permeability coefficient of hot dry rock, m / d; P f is the fracture pressure of the fracturing section, N / m 3 ; P0 is the initial pressure of the fracturing stage, N / m 3 ; Q0 is the injection volume of the fracturing stage before the dry hot rock mass breaks, m 3 / d;r w is the diameter of the fracturing section of the production well, m; M is the thickness of the fracturing section, m;

[0075] (4) Using the fracture pressure of the fracturing section, the amount of fluid injected before the fracture of the hot dry rock mass, the initial pressure of the fracturing section and the initial value of the permeability coefficient of the hot dry rock mass K (1) , iterative solution until K (k+1) -K (k) |≤0.001, K (k) That is the permeability coefficient K of dry hot rock mass, and the iterative equation is as follows:

[0076] K (k+1) =Aln[K (k) ]+Bln(r w )+C (3)

[0077]

[0078] Where A, B, and C are the coefficients of the iteration equation; k is the number of iterations, and k=1 at the initial moment.

[0079] Furthermore, the relationship between the hydraulic fracturing crack opening, crack length, and crack permeability coefficient constructed in step SS3 is as follows:

[0080] (1) The hydraulic fracture is generalized as an ellipse on a plane, the major axis of the ellipse is the length of the hydraulic fracture, the minor axis of the ellipse is the maximum opening of the hydraulic fracture, and the height of the fracture is the thickness of the fracture section;

[0081] (2) The relationship between the opening and length of hydraulic fracturing cracks is as follows:

[0082]

[0083] Where w is the opening of the hydraulic fracturing crack, m; η is the kinematic viscosity coefficient of the fracturing fluid, m 2 / s;ρ f is the density of the fracturing fluid, kg / m 3 ; Q is the injection volume, m 3 / d; ν is Poisson's ratio; L is the length of the hydraulic fracturing crack, m; E is the elastic modulus, Pa; M is the thickness of the fracturing section, m;

[0084] (3) The seepage in hydraulic fracturing cracks follows the characteristics of smooth fracture flow, and its permeability coefficient is as follows:

[0085]

[0086] In the formula, K f is the permeability coefficient of hydraulic fracturing fracture, m / d.

[0087] Furthermore, in step SS4, the shape characteristic parameters of the crack, the focal length of the ellipse, and the spatial coordinate conversion calculation process are as follows:

[0088] (1) The hydraulic fracturing crack is generalized as an ellipse on the plane, and its shape characteristic parameters and focal length are as follows:

[0089]

[0090] In the formula, λ is the characteristic parameter of the ellipse shape; c is the focal length of the ellipse;

[0091] (2) Based on the shape characteristic parameters λ and focal length c calculated by formula (9) and formula (10), a spatial coordinate transformation relationship is established, the wellbore coordinates of the fracturing section of the production well are transformed into the Z coordinate system, and the reference radius in the Z coordinate system is calculated. The coordinate transformation is performed so that the hydraulic fracturing crack is transformed into a circular area with a radius of 1:

[0092]

[0093] In the formula, Z w is the radius of the production well in the Z coordinate system; Z R It is the reference radius in the Z coordinate system.

[0094] Furthermore, in step SS5, the heterogeneous seepage analytical metamodel constructed is as follows:

[0095]

[0096] Where P w is the fracturing pressure of the production well, pa; Q is the injection volume, m 3 / d; K is the permeability coefficient of dry hot rock, m / d; Re is the real part function.

[0097] Furthermore, in step SS6, an iterative method is used to solve the fracture length under different production well fracturing stage pressure and flow conditions, and the calculation process is as follows:

[0098] (1) Set the initial value of the crack length L (1) , according to formula (7), calculate the corresponding initial value of crack opening w (1) According to formula (8), the initial crack opening w (1) Calculate the initial value of the fracture permeability

[0099] (2) Calculate the initial shape parameter λ according to formula (9) and formula (10): (1) and the initial focal length c (1) According to formula (11), the wellbore coordinate r of the production well fracturing section is w Transform to the new coordinate system Calculate the reference in the Z coordinate system according to formula (12)

[0100] (3) Establish an iterative equation and iteratively solve the crack length until |L (i+1) -L (i) |<0.001, L (i) is the crack length obtained;

[0101]

[0102] In the formula, i is the number of iterations, and the initial moment i=1.

[0103] The embodiment of the present invention takes the established benchmark model as an example.

[0104] (1) The elastic modulus, Poisson's ratio, well diameter of the fracturing section of the production well, and dynamic viscosity of the fracturing fluid of the hot dry rock in the Subei Basin tested are as follows:

[0105] Table 1 Main parameters in the embodiments of the invention

[0106]

[0107] (2) Figure 4 The relationship between the flow rate of the fracturing fluid and the friction resistance of the wellbore wall is tested. Figure 5 The injection volume and wellhead pressure monitored during hydraulic fracturing are used to calculate the fracturing section pressure of the production well, and the pressure at the fracturing wellhead is corrected to the bottom pressure of the fracturing section. The corrected pressure is as follows:

[0108] Table 2 Corrected pressure of stable section in the embodiment of the invention

[0109]

[0110]

[0111] (3) The initial permeability coefficient of the hot dry rock bedrock is set to 1 m / d. The parameters in Table 1 and the pre-fracture injection volume and fracture pressure in Table 2 are substituted into Formulas (2) to (5) to calculate the permeability coefficient of the hot dry rock bedrock to be 0.018 m / d.

[0112] (4) Set the initial value of the crack length to 10 m and substitute it into formulas (12) to (13) to calculate the initial crack opening to be 0.39 mm and the initial permeability coefficient of the hydraulic fracturing crack to be 1.55×10 3 m / d.

[0113] (5) Substitute the initial value of the hydraulic fracturing crack into formula (14) and perform iterative calculation until the calculated crack length converges, which is the length of the hydraulic fracture under the stable state. Figure 6 The hydraulic fracturing crack propagation process calculated for this example.

Claims

1. A method for calculating the crack expansion behavior of hot dry rock hydraulic fracturing, characterized in that: The method comprises the following steps: Step SS1, obtaining basic parameters of the hydraulic fracturing crack expansion behavior of hot dry rock, the basic parameters include: elastic modulus of hot dry rock, Poisson's ratio, wellbore diameter of the fracturing section of the production well, dynamic viscosity of the fracturing fluid, relationship function between the injection volume of the fracturing fluid and the friction of the well pipe, monitoring the injection volume and wellhead pressure during hydraulic fracturing; the relationship function between the injection volume of the fracturing fluid and the friction of the well pipe is calculated by testing the friction of the well pipe under different flow rates of the fracturing fluid, and obtaining the relationship function between the injection volume of the fracturing fluid and the friction of the well pipe by a binary function fitting method; Step SS2, calculating the production well fracturing section pressure, fracturing section rupture pressure, and fracturing section initial pressure according to the relationship function between the fracturing fluid injection volume and the well pipe friction obtained in step SS1 and the monitored injection volume and wellhead pressure during the hydraulic fracturing process, and calculating the hot dry rock permeability coefficient according to the fracturing section rupture pressure and the injection volume; Step SS3, based on the basic parameters obtained in step SS1, respectively constructing the relationship between the basic parameters and the fracture opening, fracture length, and fracture permeability coefficient, wherein the basic parameters include: elastic modulus of hot dry rock, Poisson's ratio, dynamic viscosity of fracturing fluid, and injection volume; Step SS4, calculating the shape characteristic parameters and focal length of the crack according to the hot dry rock permeability calculated in step SS2 and the crack opening, crack length, and crack permeability calculated in step SS3, performing coordinate transformation on the hydraulic fracturing crack, and transforming the elliptical hydraulic fracturing crack into a unit circle in space; Step SS5, constructing a heterogeneous seepage analytical element model between the pressure of the fracturing section of the production well, the injection volume and the permeability coefficient of the hot dry rock according to the coordinates converted in step SS4; Step SS6, according to the heterogeneous seepage analytical metamodel established in step SS5, the initial value of the hydraulic fracturing crack length is set, and the crack length under different pressure and flow conditions is solved by an iterative method; The calculation process of the production well fracturing stage pressure, fracturing stage rupture pressure, fracturing stage initial pressure and hot dry rock permeability coefficient in step SS2 is as follows: (1) Based on the relationship function between the injection volume of the fracturing fluid and the friction of the well pipe and the injection volume and wellhead pressure during the monitored hydraulic fracturing process, the calculation process of the fracturing section pressure, the fracturing section breakdown pressure, and the initial pressure of the fracturing section of the production well is as follows: P=P′+γ f h w -f(Q) (1) Where, P is the calculated fracturing pressure of the production well, Pa; P′ is the monitored wellhead pressure, Pa; γ f is the density of the fracturing fluid, N / m 3 ;h w is the elevation difference from the fracturing section to the wellhead, m; f(Q) is the wellbore friction force calculated by the relationship function between the fracturing fluid injection volume and the wellbore friction, Pa; Q is the monitored injection volume, m 3 / d; (2) According to the monitored wellhead pressure P w ', wellhead rupture pressure P f ′, the initial wellhead pressure P0′ and the injection volume at the corresponding time, and use formula (1) to calculate the fracturing stage pressure P of the production well w , Fracturing stage rupture pressure P f , initial pressure of fracturing section P0; (3) According to the fracturing fluid density, fracturing section thickness, fracturing section fracture pressure, fracturing section initial pressure, injection volume, and wellbore of the fracturing section of the production well, the initial value of the permeability coefficient of the dry hot rock mass is constructed: In the formula, K (1) is the initial value of the permeability coefficient of hot dry rock, m / d; P f is the fracture pressure of the fracturing section, N / m 3 ; P0 is the initial pressure of the fracturing stage, N / m 3 ; Q0 is the injection volume of the fracturing stage before the dry hot rock mass breaks, m 3 / d;r w is the diameter of the fracturing section of the production well, m; M is the thickness of the fracturing section, m; (4) Using the fracture pressure of the fracturing section, the amount of fluid injected before the fracture of the hot dry rock mass, the initial pressure of the fracturing section and the initial value of the permeability coefficient of the hot dry rock mass K (1) , iterative solution until K (k+1) -K (k) |≤0.001, K (k) That is the permeability coefficient K of dry hot rock mass, and the iterative equation is as follows: K (k+1) =Aln[K (k) ]+Bln(r w )+C (3) In the formula, A, B, and C are the coefficients of the iteration equation; k is the number of iterations, and k = 1 at the initial moment; The relationship between the hydraulic fracturing crack opening, crack length, and crack permeability coefficient constructed in step SS3 is as follows: (1) The hydraulic fracture is generalized as an ellipse on a plane, the major axis of the ellipse is the length of the hydraulic fracture, the minor axis of the ellipse is the maximum opening of the hydraulic fracture, and the height of the fracture is the thickness of the fracture section; (2) The relationship between the opening and length of hydraulic fracturing cracks is as follows: Where w is the opening of the hydraulic fracturing crack, m; η is the kinematic viscosity coefficient of the fracturing fluid, m 2 / s;ρ f is the density of the fracturing fluid, kg / m 3 ; Q is the injection volume, m 3 / d; ν is Poisson's ratio; L is the length of the hydraulic fracturing crack, m; E is the elastic modulus, Pa; M is the thickness of the fracturing section, m; (3) The seepage in hydraulic fracturing cracks follows the characteristics of smooth fracture flow, and its permeability coefficient is as follows: In the formula, K f is the permeability coefficient of hydraulic fracturing fracture, m / d.

2. A method for calculating the crack expansion behavior of hot dry rock hydraulic fracturing according to claim 1, characterized in that: In step SS4, the shape characteristic parameters of the crack, the focal length of the ellipse and the spatial coordinate conversion calculation process are as follows: (1) The hydraulic fracturing crack is generalized as an ellipse on the plane, and its shape characteristic parameters and focal length are as follows: In the formula, λ is the characteristic parameter of the ellipse shape; c is the focal length of the ellipse; (2) Based on the shape characteristic parameters λ and focal length c calculated by formula (9) and formula (10), a spatial coordinate transformation relationship is established, the wellbore coordinates of the fracturing section of the production well are transformed into the Z coordinate system, and the reference radius in the Z coordinate system is calculated. The coordinate transformation is performed so that the hydraulic fracturing crack is transformed into a circular area with a radius of 1: In the formula, Z w is the radius of the production well in the Z coordinate system; R It is the reference radius in the Z coordinate system.

3. A method for calculating the expansion behavior of hot dry rock hydraulic fracturing cracks according to claim 2, characterized in that: In step SS5, the heterogeneous seepage analytical metamodel constructed is as follows: Where P w is the fracturing pressure of the production well, pa; Q is the injection volume, m 3 / d; K is the permeability coefficient of dry hot rock, m / d; Re is the real part function.

4. A method for calculating the expansion behavior of hot dry rock hydraulic fracturing cracks according to claim 3, characterized in that: In step SS6, an iterative method is used to solve the fracture length under different production well fracturing stage pressure and flow conditions, and the calculation process is as follows: (1) Set the initial value of the crack length L (1) , according to formula (7), calculate the corresponding initial value of crack opening w (1) According to formula (8), the initial crack opening w (1) Calculate the initial value of the fracture permeability (2) Calculate the initial shape parameter λ according to formula (9) and formula (10): (1) and the initial focal length c (1) According to formula (11), the wellbore coordinate r of the production well fracturing section is w Transform to the new coordinate system Calculate the reference in the Z coordinate system according to formula (12) (3) Establish an iterative equation and iteratively solve the crack length until |L (i+1) -L (i) |<0.001, L (i) is the crack length obtained; In the formula, i is the number of iterations, and the initial moment i=1.

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