A calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir during hydraulic fracturing

Abstract

The present invention discloses a calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir during hydraulic fracturing, including constructing an in-situ stress field calculation model based on the non-linear elastic deformation characteristics of rocks; constructing a wellbore principal stress calculation model in the polar coordinate system considering the induced stress of the drilling wellbore during the development process, the induced stress of high-pressure liquid injection, and the additional stress increment of stable seepage in the pore medium; dividing it into three types of stress modes according to the relative magnitude relationship of the principal stresses; establishing a fracture pressure calculation model for hydraulic fracturing of the hydrate reservoir under the three types of stress modes by using the Mohr-Columb criterion; predicting and determining the fracture pressure of the hydraulic fracturing of the hydrate reservoir by checking the stress mode at the time of fracture of the hydrate reservoir, realizing the quantitative prediction of the fracture pressure of the vertical well in the hydrate reservoir during hydraulic fracturing. The present invention fills the technical gap in this field and provides technical support for the correct design and smooth implementation of the hydrate reservoir.

Description

Technical Field

[0001] The present invention relates to a calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir during hydraulic fracturing, and belongs to the field of fracturing development research in unconventional oil and gas fields. Background Art

[0002] Natural gas hydrate is considered to be the most promising alternative clean energy in the 21st century. More than 90% of the total global natural gas hydrate resources are stored in submarine clayey silt or muddy sediments. China is rich in hydrate resources. The prospective resource volume in the northern slope of the South China Sea alone is close to half of the total onshore and offshore oil and gas resources in China. The trial production of muddy silt-type natural gas hydrate carried out on the northern slope of the South China Sea from 2017 to 2020 was successful, proving that the sediments stored in submarine clayey silt also have technical recoverability, but there are still many key technical problems to be solved before industrialized exploitation. Although there is no field practice of hydraulic fracturing in hydrate reservoirs worldwide yet, Too et al. confirmed through injection pressure curves that sandy reservoirs with a natural gas hydrate saturation of 50% - 75% are fracturable. China is also vigorously promoting the exploration of using hydraulic fracturing to achieve industrialized development of natural gas hydrate reservoirs. In fact, nearly 80 years of field practice has proved that hydraulic fracturing is a necessary technical measure to effectively develop low-permeability reservoirs, and it has achieved great success in the transformation of unconventional oil and gas reservoirs in the past 30 years, thus changing the world energy pattern. Using hydraulic fracturing to transform natural gas hydrate reservoirs can significantly increase the decomposition area and decomposition rate of natural gas hydrates, resulting in increased production, and its future is promising.

[0003] The formation fracture pressure of a reservoir is a key parameter that restricts the rational allocation of fracturing equipment and the optimization of construction parameters. The traditional fracturing targets are mainly sandstone, carbonate rock, volcanic rock, etc. The industrial community usually regards them as isotropic homogeneous linear elastic materials and uses the maximum tensile stress strength criterion to predict the hydraulic fracturing fracture pressure. For example, Hubbert & Willis first proposed the H-W criterion for calculating the fracture pressure based on the linear elastic tensile failure theory; Zhou Nayun et al. made a detailed summary of the current formation fracture pressure prediction technologies; Wu Feipeng et al. subdivided the elastic mechanics fracture model and the fracture mechanics fracture model, and analyzed the basic assumptions, derivation principles, adaptability, and limitations of the existing subdivided models. Since hydrate reservoirs are buried relatively shallow, the difference in the three principal stresses is small, and the rock is mainly clayey silt or muddy sediment, its deformation characteristics do not meet the current assumption conditions of regarding the reservoir rock mass as "isotropic homogeneous linear elastic material", so the aforementioned fracture pressure prediction models and methods applicable to it cannot be applied. Summary of the Invention

[0004] In order to overcome the defects existing in the prior art, the present invention aims to provide a calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir during hydraulic fracturing.

[0005] The technical solution provided by the present invention to solve the above technical problems is: a calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir by hydraulic fracturing, comprising the following steps:

[0006] A. Collect the stress-strain characteristic coefficient, stress-strain characteristic index, Poisson's ratio, cohesion, internal friction angle, rock mass Boit constant, and pore fluid pressure of the hydrate reservoir;

[0007] B. Calculate the in-situ stress of the hydrate reservoir according to the strain characteristic index, Poisson's ratio, rock mass Boit constant, and pore fluid pressure of the hydrate reservoir;

[0008] C. Calculate the intermediate constants f1, f2, and f4 according to Poisson's ratio, cohesion, internal friction angle, and rock mass Boit constant;

[0009] D. Calculate the coefficient f3 for any polar angle θ from θ = 0° to θ = 90° according to the in-situ stress of the hydrate reservoir;

[0010] E. Calculate the fracture pressures of the three stress modes of hydraulic fracturing in the hydrate reservoir at any polar angle θ respectively;

[0011] F. Calculate the corresponding three-dimensional principal stresses with the fracture pressures in the three stress modes as the injection pressures;

[0012] G. Determine the stress mode of hydraulic fracturing in the hydrate reservoir according to the three-dimensional principal stresses, and check the corresponding stress mode. Take the result with the stress mode consistent with the response fracture pressure as the true fracture pressure.

[0013] A further technical solution is that the calculation formula in step B is:

[0014]

[0015] In the formula: k H and k h are the empirical tectonic stress coefficients in the directions of the maximum and minimum horizontal principal stresses of the reservoir, dimensionless; α is the rock mass Boit constant, dimensionless; p s is the reservoir fluid pressure, MPa; ν is the Poisson's ratio of the rock mass, dimensionless; dz is the integration variable, m; L water and L goal are the seawater depth and the target reservoir depth, m; ρ water and ρ rock are the densities of seawater and overlying rock strata varying with depth, Kg / m 3 ; σ v is the vertical principal stress received by the hydrate reservoir, MPa; g is the acceleration of gravity, m / s 2 ; σ his the horizontal component of the in-situ gravity stress of the hydrate reservoir, MPa; σ x , σ y , σ z are respectively the maximum and minimum horizontal principal stresses and the vertical principal stress of the reservoir, MPa; n is the rock mass stress-strain characteristic index, dimensionless.

[0016] A further technical solution is that the calculation formula in step C is:

[0017]

[0018] Where: C is the cohesion of the rock mass, MPa; φ is the internal friction angle, °; α is the Boit constant of the rock mass, dimensionless; ν is the Poisson's ratio of the rock mass, dimensionless.

[0019] A further technical solution is that the calculation formula in step D is:

[0020] f3 = (σ x + σ y ) - 2(σ x - σ y )cos(2θ)

[0021] Where: σ x , σ y are respectively the maximum and minimum horizontal principal stresses of the reservoir; θ is the polar angle, °.

[0022] A further technical solution is that the three stress modes of hydrate reservoir fracturing failure in step E include the first stress mode, the second stress mode, and the third stress mode;

[0023] The calculation formula for the fracture pressure of the first stress mode:

[0024]

[0025] The calculation formula for the fracture pressure of the second stress mode:

[0026]

[0027] The calculation formula for the fracture pressure of the third stress mode:

[0028]

[0029] Where: are respectively the fracture pressure of the first stress mode, the fracture pressure of the second stress mode, and the fracture pressure of the third stress mode; α is the Boit constant of the rock mass, dimensionless; p s is the reservoir fluid pressure, MPa; σ v is the vertical principal stress received by the hydrate reservoir, MPa.

[0030] A further technical solution is that the specific calculation process of the triaxial principal stress in step F is as follows:

[0031] F1. Calculate the radial and circumferential induced stresses caused by the borehole in the wellbore according to the in-situ stress of the hydrate reservoir;

[0032] F2. Calculate the additional stress increment on the wellbore during the stable seepage in the pore medium during the fracturing fluid injection process according to the Boit constant of the rock mass and the pore fluid pressure of the hydrate reservoir;

[0033] F3. Calculate the triaxial principal stress acting on the wellbore according to the stress superposition principle.

[0034] A further technical solution is that the calculation formula in step F1 is as follows:

[0035]

[0036] In the formula: σ r2 and σ θ2 are the radial and circumferential stresses generated by injecting fluid into the wellbore, in MPa; σ r1 and σ θ1 are the radial and circumferential stresses, in MPa; p i is the fluid injection pressure into the wellbore, in MPa.

[0037] A further technical solution is that the calculation formula in step F2 is as follows:

[0038]

[0039] In the formula: σ r3 and σ θ3 are the additional stress increments in the radial and circumferential directions of the wellbore during stable seepage of fracturing fluid injection, in MPa; p i is the fluid injection pressure into the wellbore, in MPa; p s is the pore fluid pressure in the reservoir, in MPa.

[0040] A further technical solution is that the calculation formula in step F3 is as follows:

[0041]

[0042] In the formula: σ r , σ θ and σ z are the radial, circumferential and vertical stress components acting on the wellbore in the polar coordinate system, in MPa; p i is the fluid injection pressure into the wellbore, in MPa.

[0043] A further technical solution is that the specific judgment criteria for determining the fracturing fracture stress mode of the hydrate reservoir according to the triaxial principal stress in step G are as follows:

[0044] When σ r >σ z >σ θ it is the first stress mode;

[0045] When σ z >σ r >σ θ it is the second stress mode;

[0046] When σ r >σ θ >σ z it is the third stress mode.

[0047] Combine the triaxial stress mode and the corresponding fracture pressure consistency to determine the true fracture pressure of the hydrate reservoir.

[0048] The present invention has the following beneficial effects: The present invention takes into account the characteristics of the hydrate reservoir medium different from the traditional "isotropic homogeneous linear elastic" medium, objectively and correctly reflects the fracture initiation mechanism during the vertical well hydraulic fracturing of the hydrate reservoir, and fills the gap in this prediction technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a stress-strain test curve graph of the hydrate reservoir core;

[0050] Figure 2 It is a graph of stress-strain characteristic coefficients and exponents;

[0051] Figure 3 It is a wellbore polar coordinate graph;

[0052] Figure 4 It is a calculation result graph of the fracturing fracture pressure of the hydrate reservoir varying with the polar angle;

[0053] Figure 5 It is a calculation result graph of the fracturing fracture pressure of the hydrate reservoir;

[0054] Figure 6 It is a triaxial stress state graph when the hydrate reservoir is fractured. DETAILED DESCRIPTION OF THE INVENTION

[0055] The technical solutions of the present invention will be described clearly and completely below with reference to the accompanying drawings. Obviously, the described embodiments are some of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0056] A calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir by hydraulic fracturing according to the present invention comprises the following steps:

[0057] A. Collect the stress-strain characteristic coefficient, stress-strain characteristic index, Poisson's ratio, cohesion, internal friction angle, rock mass Boit constant and pore fluid pressure of the hydrate reservoir;

[0058] Among them, drill the core of the hydrate reservoir, test the stress-strain curve of the hydrate reservoir on a material testing machine according to the national standard "Standard for Test Methods of Engineering Rock Mass" (GB / T 50266-2013), and determine Poisson's ratio according to the definition and the experimental curve.

[0059] Use the regression method to determine the strain characteristic coefficient and index according to the stress-strain model shown in formula (1);

[0060]

[0061] In the formula: E is the elastic modulus of the rock mass, MPa; σ and ε are the stress (MPa) and strain (dimensionless) obtained by experimental testing respectively; b and n are the stress-strain characteristic coefficient and index obtained by experimental testing, dimensionless;

[0062] Collect or obtain parameters such as the cohesion and internal friction angle of the hydrate reservoir and the rock mass Boit constant by indoor core testing. The testing method is carried out according to the national standard "Standard for Test Methods of Engineering Rock Mass" (GB / T 50266-2013);

[0063] Use well logging data or drilling prediction methods or pressure build-up methods to determine the pore fluid pressure of the hydrate reservoir;

[0064] B. Calculate the in-situ stress of the hydrate reservoir according to the strain characteristic index, Poisson's ratio, rock mass Boit constant and pore fluid pressure of the hydrate reservoir;

[0065] The in-situ stress of the hydrate reservoir includes three principal stresses, namely the vertical stress, the maximum and minimum horizontal principal stresses. It is composed of the gravity stress field and the tectonic stress field under the influence of the pore fluid pressure at this point, and satisfies the superposition principle;

[0066] Calculate the vertical principal stress of the hydrate reservoir by using the density of the overlying rock formation and the density of seawater. Among them, the density of seawater is obtained by experimental testing (usually about 1025 Kg / m 3 ), and the density of the overlying rock formation is obtained from well logging data, and its calculation formula is;

[0067]

[0068] In the formula: dz is the integration variable, m; L water 、L goalis the seawater depth and the target reservoir depth, m; ρ water , ρ rock are the seawater and the density of the overlying rock formation varying with depth, Kg / m 3 ; σ v is the vertical principal stress on the hydrate reservoir, MPa; g is the acceleration due to gravity, m / s 2 ;

[0069] According to Terzaghi's effective stress theory, based on the rock mass stress-strain model, the horizontal component of the in-situ gravity stress of the hydrate reservoir is calculated according to the generalized Hook's law. The calculation formula is;

[0070]

[0071] In the formula: α is the Biot constant of the rock mass, dimensionless; p s is the reservoir fluid pressure, MPa; ν is the Poisson's ratio of the rock mass, dimensionless;

[0072] The in-situ principal stress of the hydrate reservoir is calculated according to the stress superposition principle. That is, the in-situ principal stress field is equal to the sum of the gravity stress field and the tectonic stress field. Thus,

[0073]

[0074] In the formula: k H , k h are the empirical tectonic stress coefficients in the directions of the maximum and minimum principal stresses in the reservoir horizontally, dimensionless; σ x , σ y , σ z are the maximum and minimum principal stresses and the vertical principal stress in the reservoir horizontally, MPa; n is the stress-strain characteristic index of the rock mass, dimensionless;

[0075] C. Calculate the intermediate constants f1, f2, f4 according to the Poisson's ratio, cohesion, internal friction angle, and Biot constant of the rock mass;

[0076]

[0077] In the formula: C is the cohesion of the rock mass, MPa; φ is the internal friction angle, °; α is the Biot constant of the rock mass, dimensionless; ν is the Poisson's ratio of the rock mass, dimensionless;

[0078] D. Calculate the coefficient f3 for any polar angle θ from θ = 0° to θ = 90° according to the in-situ stress of the hydrate reservoir;

[0079] f3 = (σ x + σ y ) - 2(σ x - σ y )cos(2θ) (8)

[0080] In the formula: σ x and σ y are the maximum and minimum principal stresses in the reservoir horizontal direction respectively; θ is the polar angle, °;

[0081] E is used to calculate the fracture pressures of the three stress modes (the first stress mode, the second stress mode, and the third stress mode) of hydrate reservoir fracturing at any polar angle θ;

[0082] The formula for calculating the fracture pressure of the first stress mode:

[0083]

[0084] The formula for calculating the fracture pressure of the second stress mode:

[0085]

[0086] The formula for calculating the fracture pressure of the third stress mode:

[0087]

[0088] In the formula: are the fracture pressures of the first stress mode, the second stress mode, and the third stress mode respectively; α is the Boit constant of the rock mass, dimensionless; p s is the reservoir fluid pressure, MPa; σ v is the vertical principal stress on the hydrate reservoir, MPa;

[0089] F. The corresponding three-dimensional principal stresses are calculated with the fracture pressures under the three stress modes as the injection pressures;

[0090] F1. The radial and circumferential induced stresses caused by the borehole on the wellbore are calculated according to the in-situ stress of the hydrate reservoir;

[0091] Considering the hydrate reservoir as a homogeneous and isotropic material, a cylindrical coordinate system as shown in Figure 3 is established for the vertical wellbore. Its polar radius is r, and the polar angle (the angle with the maximum horizontal principal stress direction, i.e., the x-axis) is θ. Using the notations of elasticity: compressive stress is positive and tensile stress is negative. The radial and circumferential induced stresses on the wellbore wall are calculated according to the theory of elasticity;

[0092] The formulas for calculating the radial and circumferential induced stresses caused by the borehole on the wellbore wall are

[0093]

[0094] In the formula: σ r1 and σ θ1 are the stresses in the radial and circumferential directions respectively, MPa;

[0095] The radial and circumferential induced stresses generated by high-pressure liquid injection into the wellbore during the fracturing process are calculated according to the formula for an infinitely thick-walled cylinder;

[0096]

[0097] In the formula: σ r2 , σ θ2 are the radial and circumferential stresses generated by injecting liquid into the wellbore, respectively, in MPa;

[0098] F2. Calculate the additional stress increment at the wellbore during the fracturing liquid injection process for the steady seepage in the pore medium according to the rock mass Boit constant and the pore fluid pressure in the hydrate reservoir;

[0099] Regarding the filtration of the fracturing fluid into the reservoir during the fracturing process as steady seepage, according to the research results of Haimson&Fairhurst. The additional stress increment at the wellbore due to the change in seepage pore pressure can be calculated by the following formula:

[0100]

[0101] In the formula: σ r3 , σ θ3 are the additional radial and circumferential stress increments at the wellbore during the steady seepage of fracturing liquid injection, respectively, in MPa;

[0102] F3. Calculate the three-dimensional principal stresses acting on the wellbore according to the principle of stress superposition.

[0103] According to the principle of stress superposition, the total stress acting on the given position point on the wellbore is obtained by superimposing the stress components on the wellbore;

[0104]

[0105] In the formula: σ r , σ θ , σ z are the radial, circumferential, and vertical stress components acting on the wellbore in the polar coordinate system, respectively, in MPa.

[0106] G. Determine the fracturing fracture stress mode of the hydrate reservoir according to the three-dimensional principal stresses, and check the corresponding stress mode. Take the result where the stress mode is consistent with the response fracture pressure as the true fracture pressure.

[0107] During the hydraulic fracturing operation Figure 3 In the polar coordinate system shown, there is always a radial stress greater than the circumferential stress. It is divided into the following three modes according to the relative magnitude relationship of the three-dimensional principal stresses, namely

[0108] When σ r >σ z >σ θWhen it is in the first stress mode;

[0109] When σ z > σ r > σ θ When it is in the second stress mode;

[0110] When σ r > σ θ > σ z When it is in the third stress mode.

[0111] According to the non-linear stress-strain characteristics of the hydrate reservoir rock mass, the Mohr-Columb strength criterion should be adopted, that is

[0112] σ max = f1σ min + f2(16)

[0113] In the formula: σ max , σ min Are the maximum and minimum principal stresses respectively, in MPa.

[0114] In the hydraulic fracturing operation, the three-dimensional effective principal stresses in the pore medium are:

[0115]

[0116] In the formula: σ r,e , σ θ,e Are the effective total radial and total circumferential (tangential) stresses acting on the wellbore respectively, in MPa;

[0117] The fluid pressure at formation fracture is the formation fracture pressure, that is, at this time p F = p i . Calculate the fracture pressure under different fracture modes according to the effective stress principle and applying the Mohr-Columb criterion shown in formula (16);

[0118] Example

[0119] A hydrate reservoir in the South China Sea has a water depth of 1200 m and a rock formation thickness of 200 m. According to well logging data, the average density of the overlying rock formation is 2400 Kg / m 3 ; The empirical tectonic stress coefficients in the directions of the horizontal maximum and minimum principal stresses in this area are 0.25 and 0.1 respectively.

[0120] Use the method of the present invention for prediction, which specifically includes the following steps:

[0121] A. Core experimental tests on the mechanical properties of the hydrate reservoir rock mass

[0122] A1. Drill the hydrate reservoir core, and test the stress-strain curve of the hydrate reservoir on a material testing machine according to the national standard "Standard for Engineering Rock Mass Test Methods" (GB / T 50266-2013), as shown in the appendix Figure 1 As shown. Determine the Poisson's ratio of the rock as ν = 0.3763 according to the definition of Poisson's ratio, and determine the stress-strain characteristic index n = 0.2037 by the regression method.

[0123] A2. Collect the cohesion C = 20 MPa and the internal friction angle of the hydrate reservoir The pore elastic constant α = 0.7.

[0124] A3. Use well logging data to determine that the pore fluid pressure of the hydrate reservoir is 8 MPa.

[0125] B. Calculate the in-situ stress of the hydrate reservoir;

[0126] B1. Calculate the vertical principal stress of the hydrate reservoir using the density of the overlying rock formation in-situ and the density of seawater. The density of seawater is 1025 Kg / m according to experimental tests 3 .

[0127]

[0128] B2. Calculate the horizontal component of the gravitational stress of the hydrate reservoir;

[0129] According to Terzaghi's effective stress theory, based on the rock mass stress-strain model obtained in step A1, calculate the horizontal component of the in-situ gravitational stress of the hydrate reservoir according to the generalized Hook's law;

[0130]

[0131] B3. Calculate the in-situ three-dimensional principal stresses of the hydrate reservoir;

[0132] Calculate the in-situ principal stress of the target reservoir according to the stress superposition principle. That is, the in-situ stress field is equal to the sum of the gravitational stress field and the tectonic stress field. Thus, there is

[0133]

[0134] C. Calculate the intermediate constants f1, f2, and f4 according to the Poisson's ratio, cohesion, internal friction angle, and rock mass Boit constant;

[0135]

[0136] D. Calculate the coefficient f3 for any polar angle θ from θ = 0° to θ = 90° according to the in-situ stress of the hydrate reservoir;

[0137] f3 = (σ x + σ y ) - 2(σx -σ y )cos(2θ)=28.338-5.028cos(2θ)

[0138] The following calculations take the polar angle θ = 0° as an example.

[0139] f3=(σ x +σ y )-2(σ x -σ y )cos(2θ)=28.338-5.028cos(0)=23.31

[0140] E. Calculate the fracture pressure of the three stress modes of hydrate reservoir fracture at any polar angle θ;

[0141] The fracture pressure calculation model of hydrate reservoir vertical wells under different fracture modes is established by the following steps:

[0142] 1. Select and determine the criterion for hydrate reservoir fracture strength.

[0143] According to the nonlinear stress-strain characteristics of hydrate reservoirs, the Mohr-Columb strength criterion is adopted;

[0144] 2. Three-dimensional effective stress calculation model for hydrate reservoirs.

[0145] During hydraulic fracturing operations, the three-dimensional effective principal stresses in the porous medium are:

[0146]

[0147] 3. Calculation methods of burst pressure under three stress modes

[0148] According to the effective stress principle and the Mohr-Columb criterion shown in formula (16), the rupture pressure under different rupture modes is calculated. F =p i .

[0149] 1) First stress mode: σ max =σ r,e σ min =σ θ,e , the burst pressure is:

[0150]

[0151] 2) Second stress mode: σ min =σ θ,e σ max =σ z,e , the formation fracture pressure is:

[0152]

[0153] 3) Third stress mode: σ min = σ z,e σ max = σ r,e , and the formation fracture pressure is:

[0154]

[0155] 4) Programming calculation: The calculation results of the fracture pressures of the three fracture modes at different polar angles are shown in Appendix Figure 4 . The minimum values are obtained at the polar angle θ = 0°, that is, the corresponding fracture pressures are {21.58 22.95 35.38}.

[0156] F. Determine the fracturing fracture pressure value of the hydrate reservoir;

[0157] Taking the fracture pressures {21.58 22.95 35.38} in the three modes as the injection pressures, the corresponding three-dimensional principal stresses are calculated as shown in Table 1 below.

[0158] G. Calculate the wellbore stress-induced stress and the induced stress of injecting fluid into the wellbore;

[0159] Considering the hydrate reservoir as a homogeneous and isotropic material, establish the cylindrical coordinate system shown in Appendix Figure 3 . Denote the compressive stress as positive and the tensile stress as negative.

[0160] G1. Calculate the radial and circumferential induced stresses caused by the wellbore on the wellbore wall;

[0161]

[0162] G2. The radial and circumferential induced stresses generated by high-pressure fluid injection into the wellbore during the fracturing process are calculated according to the formula of an infinitely thick-walled cylinder

[0163]

[0164] H. Calculate the additional stress increment on the wellbore wall due to the steady seepage in the pore medium during the fracturing fluid injection process;

[0165] Regarding the filtration of the fracturing fluid into the reservoir during the fracturing process as a steady seepage, the induced stress at the wellbore wall due to the change in the seepage pore pressure can be calculated by the following formula:

[0166]

[0167] I. Superimpose the three-dimensional total stresses on the wellbore wall and determine the fracturing fracture mode of the hydrate reservoir;

[0168] I1. According to the stress superposition principle, superimpose the stress components on the wellbore wall to obtain the total stress on the wellbore wall;

[0169]

[0170] I2. Discrimination method for fracture failure modes of hydrate reservoirs.

[0171] During the hydraulic fracturing operation Figure 3 In the polar coordinate system shown, the radial stress is always greater than the circumferential stress. According to the relative magnitude relationship of the three principal stresses, it is divided into the following three modes, namely

[0172] First stress mode: σ r > σ z > σ θ

[0173] Second stress mode: σ z > σ r > σ θ

[0174] Third stress mode: σ r > σ θ > σ z

[0175] Table 1

[0176]

[0177]

[0178] J. The result where the stress mode is consistent with the corresponding fracture pressure is the true fracture pressure.

[0179] Using {21.58, 22.95, 35.38} as the injection pressure for calibration, all are in the first stress mode. Thus, the true fracture pressure of the hydrate reservoir is 21.58 MPa.

[0180] As mentioned above, it is not any form of limitation to the present invention. Although the present invention has been disclosed through the above embodiments, it is not intended to limit the present invention. Any person skilled in the art, without departing from the scope of the technical solution of the present invention, can make some changes or modifications to the above-disclosed technical content to obtain equivalent embodiments of equivalent changes. However, as long as it does not depart from the content of the technical solution of the present invention, any simple modification, equivalent change, and modification made to the above embodiments based on the technical essence of the present invention all fall within the scope of the technical solution of the present invention.

Claims

1. A calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir by hydraulic fracturing, characterized in that Including the following steps: A. Collect the stress-strain characteristic coefficient, stress-strain characteristic index, Poisson's ratio, cohesion, internal friction angle, rock mass Biot constant and pore fluid pressure of the hydrate reservoir; B. Calculate the in-situ stress of the hydrate reservoir according to the strain characteristic index, Poisson's ratio, rock mass Biot constant and pore fluid pressure of the hydrate reservoir; C. Calculate the intermediate constants f1, f2, f4 according to Poisson's ratio, cohesion, internal friction angle and rock mass Biot constant; D. Calculate the coefficient f3 of any polar angle θ from θ = 0° to θ = 90° according to the in-situ stress of the hydrate reservoir; E. Calculate the fracture pressures of the three stress modes of the hydrate reservoir fracturing rupture at any polar angle θ respectively; The three stress modes of the hydrate reservoir fracturing rupture include the first stress mode, the second stress mode and the third stress mode; The calculation formula for the fracture pressure of the first stress mode: The calculation formula for the fracture pressure of the second stress mode: The calculation formula for the fracture pressure of the third stress mode: In the formula: are the fracture pressures of the first stress mode, the second stress mode, and the third stress mode respectively; α is the Boit constant of the rock mass, dimensionless; p s is the reservoir fluid pressure, MPa; σ v is the vertical principal stress on the hydrate reservoir, MPa; F. Take the fracture pressures under the three stress modes as the injection pressures to calculate the corresponding three-dimensional principal stresses; G. Determine the stress mode of the hydrate reservoir fracturing rupture according to the three-dimensional principal stresses, and check the corresponding stress mode. Take the result with the stress mode consistent with the response fracture pressure as the true fracture pressure.

2. The calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir by hydraulic fracturing according to claim 1, characterized in that, The calculation formula in step B is: where: k H , k h are the empirical tectonic stress coefficients in the directions of the maximum and minimum principal stresses of the reservoir horizontally, dimensionless; α is the Boit constant of the rock mass, dimensionless; p s is the reservoir fluid pressure, MPa; ν is the Poisson's ratio of the rock mass, dimensionless; dz is the integration variable, m; L water 、L goal are the seawater depth and the target reservoir depth, m; ρ water and ρ rock are the densities of seawater and overlying rock formation varying with depth, Kg / m 3 ; σ v is the vertical principal stress on the hydrate reservoir, MPa; g is the acceleration of gravity, m / s 2 ; σ h is the horizontal component of the in-situ gravity stress of the hydrate reservoir, MPa; σ x , σ y , σ z are the maximum and minimum horizontal principal stresses and vertical principal stress of the reservoir, MPa; n is the rock mass stress-strain characteristic index, dimensionless.

3. The calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir by hydraulic fracturing according to claim 1, characterized in that, The calculation formula in step C is: Where: C is the cohesion of the rock mass, MPa; φ is the internal friction angle, °; α is the Biot constant of the rock mass, dimensionless; ν is the Poisson's ratio of the rock mass, dimensionless.

4. The calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir by hydraulic fracturing according to claim 1, wherein The calculation formula in step D is: f3 = (σ x + σ y ) - 2(σ x - σ y ) cos(2θ) Where: σ x and σ y are the maximum and minimum horizontal principal stresses of the reservoir respectively; θ is the polar angle, °.

5. A calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir by hydraulic fracturing according to claim 1, characterized in that The specific calculation process of the three-dimensional principal stresses in step F is: F1. Calculate the radial and circumferential induced stresses caused by the borehole on the wellbore according to the in-situ stress of the hydrate reservoir; F2. Calculate the additional stress increment on the wellbore due to the stable seepage in the pore medium during the fracturing injection process according to the Biot constant of the rock mass and the pore fluid pressure of the hydrate reservoir; F3. Calculate the three-dimensional principal stresses on the wellbore according to the stress superposition principle.

6. The calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir by hydraulic fracturing according to claim 5, characterized in that The calculation formula in step F1 is: Where: σ r2 , σ θ2 are the radial and circumferential stresses generated by injecting liquid into the wellbore, respectively, in MPa; σ r1 , σ θ1 are the radial and circumferential stresses, respectively, in MPa; p i is the liquid injection pressure into the wellbore, in MPa.

7. A calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir by hydraulic fracturing according to claim 5, characterized in that, The calculation formula in step F2 is: Where: σ r3 , σ θ3 are the increments of the radial and circumferential additional stresses on the wellbore wall during the stable seepage of the fracturing injection fluid, respectively, in MPa; p i is the injection fluid pressure into the wellbore, in MPa; p s is the pore fluid pressure in the reservoir, in MPa.

8. A calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir by hydraulic fracturing according to claim 5, characterized in that, The calculation formula in step F3 is: where: σ r , σ θ , σ z are the radial, circumferential, and vertical stress components acting on the wellbore in the polar coordinate system, respectively, in MPa; p i is the liquid injection pressure into the wellbore, in MPa.

9. The calculation method for predicting the fracture pressure of a vertical well in a hydrate reservoir by hydraulic fracturing according to claim 1, characterized in that, The specific judgment criterion for determining the stress mode of the hydrate reservoir fracturing rupture according to the three-dimensional principal stresses in step G is: When σ r > σ z > σ θ , it is the first stress mode; When σ z > σ r > σ θ , it is the second stress mode; When σ r > σ θ > σ z it is the third stress mode.

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