Method for predicting hydraulic fracturing pressure based on unified strength theory

Abstract

The invention discloses a method for predicting hydraulic fracturing pressure based on a unified strength theory. The method comprises the steps that in-situ stress field data, formation fluid pressure and rock mechanics parameters of a target reservoir are collected; calculating a material tension-compression ratio parameter and rock mass tensile strength according to rock mechanical parameters; according to an effective stress calculation formula in a rock mass hole elastic medium, the relative magnitude sequence of three-way effective principal stress borne by the well wall during hydraulic fracturing is determined; determining a corresponding stress mode and a calculation coefficient of the three-way effective principal stress according to a relative size sequence of the three-way effective principal stress; establishing a hydraulic fracturing pressure calculation model by adopting a unified strength theory; the fracture pressure is calculated according to the hydraulic fracturing pressure calculation model and the calculation coefficient of the three-way effective principal stress; and checking the consistency of the fracture pressure and the stress mode to determine the fracture pressure of the hydraulic fracturing rock mass. According to the method, the blank of predicting the hydraulic fracturing pressure by comprehensively considering the influence of the normal stress and the shear stress is filled; and the accuracy of predicting the fracture pressure of the hydraulic fracturing rock mass is improved.

Description

Technical Field

[0001] The invention relates to a method for predicting hydraulic fracturing pressure based on unified strength theory, and belongs to the technical field of hydraulic fracturing and production increase in oil and gas field development. Background Art

[0002] Hydraulic fracturing of rock mass is a core technology in the field of oil and gas field development. The fracture pressure of the target reservoir rock mass is a key parameter that restricts the reasonable configuration of fracturing equipment, optimizes construction parameters, and improves the economic efficiency of hydraulic fracturing. The current methods for predicting hydraulic fracturing pressure of rock mass are all based on the first strength theory or linear elastic fracture mechanics, which ignores the influence of the synergistic effect of rock mass units under shear, resulting in the obtained prediction results and the field response often being quite different. In order to improve the prediction accuracy of hydraulic fracturing fracture pressure and more accurately guide field applications, it is usually corrected by using empirical correction coefficients combined with field practice data in characteristic areas. This correction has poor objectivity and effectiveness.

[0003] The research results of rock mechanics have shown that shear stress is the basic factor of rock failure. Both the normal stress and shear stress on the shear stress surface affect the fracture of the rock mass. The hydraulic fracturing formation wall is subjected to the action of three principal stresses in the vertical direction, the maximum horizontal direction and the minimum horizontal direction. They all contribute to the fracture of the rock, and their influence should be considered comprehensively. The unified strength theory considers the different effects of all stress components on the double shear unit body. When the influence function of the two larger shear stresses acting on the double shear unit body and the normal stress on the action surface reaches a certain limit value, the rock mass will be destroyed. Wang Jixiu et al. considered the seepage effect and pore water pressure of the rock around the wellbore, and conducted an elastic-plastic analysis of the rock around the wellbore based on the unified strength theory, and studied the stress distribution of the rock around the wellbore and the elastic and plastic limit loads of the wellbore wall stability. Obviously, the unified strength theory can better characterize the hydraulic fracturing pressure of the rock mass, but no relevant research results have been seen in this regard, which is still a blank. Summary of the invention

[0004] In order to overcome the defects existing in the prior art, the present invention aims to provide a method for predicting hydraulic fracturing pressure based on the unified strength theory, which can comprehensively and objectively reflect the hydraulic fracturing mechanism under the joint action of two larger shear stresses acting on the double shear unit body and the normal stress on the action surface, filling the gap in predicting hydraulic fracturing pressure by comprehensively considering the influence of normal stress and shear stress; and improving the accuracy of predicting the fracture pressure of hydraulically fractured rock mass.

[0005] The technical solution provided by the present invention to solve the above technical problems is: a method for predicting hydraulic fracturing pressure based on unified strength theory, comprising the following steps:

[0006] A. Collect in-situ stress field data, reservoir fluid pressure and rock mechanical parameters of the target reservoir;

[0007] B. Calculate the material tension-compression ratio parameters and rock mass tensile strength based on rock mechanics parameters;

[0008] C. Determine the relative magnitude order of the three-dimensional effective principal stresses borne by the wellbore wall during hydraulic fracturing based on the calculation formula of effective stress in the pore elastic medium of the rock mass;

[0009] D. Determine the corresponding stress mode and the calculation coefficient of the three-way effective principal stress according to the relative magnitude order of the three-way effective principal stress;

[0010] E. Use unified strength theory to establish a hydraulic fracturing pressure calculation model;

[0011] F. Calculate the fracture pressure based on the hydraulic fracturing pressure calculation model and the calculation coefficients of the three-dimensional effective principal stress;

[0012] G. Conduct consistency check between fracture pressure and stress pattern to determine fracture pressure of hydraulically fractured rock mass.

[0013] A further technical solution is that the in-situ stress field data include horizontal maximum principal stress and minimum principal stress, overlying rock pressure; the rock mechanics parameters include cohesion and internal friction angle, pore elastic constant, Poisson's ratio and material characteristic parameters.

[0014] A further technical solution is that the calculation formula of the material tension-compression ratio parameter and the rock mass tensile strength is:

[0015]

[0016]

[0017] Where: C is the cohesion of the rock mass, MPa; φ is the internal friction angle of the rock mass, °; σ ten is the tensile strength of rock mass, MPa; a is the material tension-compression ratio parameter, dimensionless.

[0018] A further technical solution is that the effective stress calculation formula in the rock mass poroelastic medium includes:

[0019]

[0020] σ1=max(σ t,e ,σ r,e ,σ z,e )

[0021] σ3=min(σ t,e ,σ r,e ,σ z,e )

[0022] σ2=mid(σ t,e ,σr,e ,σ z,e )

[0023] Where: r,e , σ t,e , σ z,e are the effective total stresses acting on the wellbore wall in radial, circumferential and axial directions, MPa; σ H , σ h are the maximum horizontal principal stress and the minimum horizontal principal stress, MPa; α is the pore elastic constant of the rock mass, dimensionless; p i 、p r are the injection pressure into the wellbore and the formation fluid pressure during hydraulic fracturing, MPa; σ1, σ2, σ3 are the maximum, intermediate and minimum effective principal stresses, MPa; t0, r0, z0, t1, r1, z1, k are all intermediate parameters.

[0024] A further technical solution is that in step D, the stress modes are divided into the following six types according to the relative magnitude relationship of the three principal stresses, namely,

[0025] σ1=σ r,e >σ2=σ z,e >σ3=σ t,e

[0026] σ1=σ r,e >σ2=σ t,e >σ3=σ z,e

[0027] σ1=σ z,e >σ2=σ r,e >σ3=σ t,e

[0028] σ1=σ z,e >σ2=σ t,e >σ3=σ r,e

[0029] σ1=σ t,e >σ2=σ z,e >σ3=σ r,e

[0030] σ1=σ t,e >σ2=σ r,e >σ3=σ z,e

[0031] And the calculation coefficients of corresponding σ1, σ2, and σ3 are determined according to the six stress modes:

[0032] (max0,max1), (mid0,mid1), (min0,min1)

[0033] Wherein: max0, mid0, min0 are the coefficients of the maximum, intermediate and minimum effective principal stresses that are independent of the injection pressure, respectively, and are dimensionless; max1, mid1, min1 are the coefficients of the maximum, intermediate and minimum effective principal stresses that are related to the injection pressure, respectively, and are dimensionless.

[0034] A further technical solution is that the hydraulic fracturing pressure calculation model includes:

[0035] when hour;

[0036]

[0037] when hour;

[0038]

[0039] Where: max0, mid0, min0 are the coefficients of the maximum, middle and minimum effective principal stresses that are independent of the injection pressure, respectively, and are dimensionless; max1, mid1, min1 are the coefficients of the maximum, middle and minimum effective principal stresses that are related to the injection pressure, respectively, and are dimensionless; b is the material characteristic parameter, dimensionless; a is the material tension-compression ratio parameter, dimensionless; φ is the internal friction angle of the rock mass, °; σ ten is the tensile strength of rock mass, MPa; σ1, σ2, σ3 are the maximum, intermediate and minimum effective principal stresses, MPa respectively; p F is the rock splitting pressure, MPa.

[0040] A further technical solution is that the specific process of step G is:

[0041] The rock splitting pressure calculated in step F is compared with the injection pressure in step C. If the rock splitting pressure is not equal to the injection pressure, repeat steps C to F until the rock splitting pressure is equal to the injection pressure. The rock splitting pressure is the fracture pressure of the hydraulically fractured rock.

[0042] The invention has the following beneficial effects: it belongs to the technical field of hydraulic fracturing and production increase in oil and gas field development. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is the relationship diagram between the three-dimensional effective principal stress and the injection pressure during hydraulic fracturing;

[0044] Figure 2 It is the consistency check diagram of hydraulic fracturing fracture pressure PF-three-way effective principal stress under three stress modes;

[0045] Figure 3 This is the consistency verification diagram of hydraulic fracturing fracture pressure PF*-three-way effective principal stress under three stress modes. DETAILED DESCRIPTION

[0046] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0047] A method for predicting hydraulic fracturing pressure based on unified strength theory of the present invention comprises the following steps:

[0048] A. Collect in-situ stress field data, reservoir fluid pressure and rock mechanical parameters of the target reservoir;

[0049] A1. The in-situ stress of hydraulic fracturing rock formations (horizontal maximum principal stress and minimum principal stress, overburden rock pressure) is obtained by field stress testing, experimental testing such as differential strain or Kaiser effect, and logging data interpretation;

[0050] A2. Rock mechanical parameters of hydraulic fracturing rock formations, including cohesion and internal friction angle, poroelastic constant and Poisson's ratio, and material characteristic parameters, are obtained through core experiments in accordance with national standards;

[0051] A3. The formation pressure of hydraulic fracturing rock formations is obtained by well logging or pressure recovery.

[0052] B. Calculate the material tension-compression ratio parameters and rock mass tensile strength based on rock mechanics parameters;

[0053]

[0054] Where: C is the cohesion of the rock mass, MPa; φ is the internal friction angle of the rock mass, °; σ ten is the tensile strength of rock mass, MPa; a is the tension-compression ratio parameter of the material, dimensionless;

[0055] C. Determine the relative magnitude order of the three-dimensional effective principal stresses borne by the wellbore wall during hydraulic fracturing based on the calculation formula of effective stress in the pore elastic medium of the rock mass;

[0056] A calculation model for the three-dimensional principal stresses on the wellbore wall during hydraulic fracturing is established. The elastic mechanics method is used to calculate the circumferential (tangential), radial and axial principal stresses on the wellbore wall due to the three mechanisms of local stress, high-pressure injection of fracturing fluid into the wellbore and the seepage effect due to pressure difference during hydraulic fracturing.

[0057] The three-axis principal stress calculation model using elastic mechanics notation is as follows:

[0058]

[0059] Where: r , σ t , σ z are the combined total stresses acting on the wellbore in radial, circumferential and axial directions, MPa; σ H , σ h are the maximum horizontal principal stress and the minimum horizontal principal stress, MPa; p i 、p r are the injection pressure into the wellbore and the formation fluid pressure during hydraulic fracturing, MPa; α and ν are the pore elastic constant and Poisson's ratio of the rock mass, dimensionless;

[0060] make

[0061]

[0062] Where: α, ν are the pore elastic constant and Poisson's ratio of the rock mass, respectively, and are dimensionless; t0, r0, z0, t1, r1, z1, k are all intermediate parameters; σ H , σ h are the maximum horizontal principal stress and the minimum horizontal principal stress, MPa; σ v is the overlying rock pressure, MPa;

[0063] According to the effective stress principle, the calculation formula of effective stress in rock mass pore elastic medium is expressed as:

[0064]

[0065] Where: r,e , σ t,e , σ z,e are the effective total stresses acting on the wellbore wall in radial, circumferential and axial directions, MPa, respectively;

[0066] After calculating the effective principal stresses in the three directions of the wellbore wall, compare the relative sizes

[0067]

[0068] Where: σ1, σ2, σ3 are the maximum, intermediate and minimum effective principal stresses, MPa;

[0069] C. Determine the corresponding stress mode and the calculation coefficient of the three-way effective principal stress according to the relative magnitude order of the three-way effective principal stress;

[0070] According to the relative magnitude of the three principal stresses, it can be divided into the following six stress modes, namely,

[0071]

[0072] And the calculation coefficients of corresponding σ1, σ2, and σ3 are determined according to the six stress modes:

[0073] (max0,max1), (mid0,mid1), (min0,min1)(10)

[0074] Where: max0, mid0, min0 are the coefficients corresponding to the maximum, middle and minimum effective principal stresses in formula (7) that are independent of the injection pressure, and are dimensionless; max1, mid1, min1 are the coefficients corresponding to the maximum, middle and minimum effective principal stresses in formula (7) that are related to the injection pressure, and are dimensionless;

[0075] D. Use unified strength theory to establish a hydraulic fracturing pressure calculation model;

[0076] According to the unified strength theory, the rock mass fracture pressure of hydraulic fracturing is calculated in two categories. The rock mass fracture under any stress mode should satisfy the following calculation model. That is,

[0077] when hour;

[0078]

[0079] when hour;

[0080]

[0081] Where: max0, mid0, min0 are the coefficients of the maximum, middle and minimum effective principal stresses that are independent of the injection pressure, respectively, and are dimensionless; max1, mid1, min1 are the coefficients of the maximum, middle and minimum effective principal stresses that are related to the injection pressure, respectively, and are dimensionless; b is the material characteristic parameter, dimensionless; a is the material tension-compression ratio parameter, dimensionless; φ is the internal friction angle of the rock mass, °; σ ten is the tensile strength of rock mass, MPa; σ1, σ2, σ3 are the maximum, intermediate and minimum effective principal stresses, MPa respectively; p F is the rock splitting pressure, MPa;

[0082] F. Calculate the fracture pressure based on the hydraulic fracturing pressure calculation model and the calculation coefficients of the three-dimensional effective principal stress;

[0083] G. Check the consistency between the fracture pressure and stress pattern to determine the fracture pressure of the hydraulically fractured rock mass;

[0084] In formula (7), the injection pressure is included. In fact, the injection fluid pressure when the rock mass breaks is the formation fracture pressure. In formula (7), let p F =p i The formula for calculating the burst pressure is obtained by solving the equation.

[0085] The rock splitting pressure calculated in step F is compared with the injection pressure in step C. If the rock splitting pressure is not equal to the injection pressure, repeat steps C to F until the rock splitting pressure is equal to the injection pressure. The rock splitting pressure is the fracture pressure of the hydraulically fractured rock.

[0086] Example

[0087] A first embodiment of a method for calculating a plastic formation fracture pressure according to the present invention comprises the following steps:

[0088] A. Collect in-situ stress field data, rock mass Poisson's ratio, cohesion and internal friction angle, reservoir fluid pressure and poroelastic constants of the target reservoir.

[0089] In this case, the core Kaiser test is used to determine the horizontal biaxial principal stresses. The horizontal maximum principal stress σ H =102MPa, horizontal minimum principal stress σ h =90MPa, overburden rock pressure σ v =96MPa.

[0090] In this case, core test was used to clarify that the rock cohesion C = 25 MPa, internal friction angle φ = 30°, poroelastic constant α = 0.7, Poisson's ratio ν = 0.2, and material characteristic parameter b = 0.5.

[0091] In this case, the reservoir fluid pressure Pr = 75 MPa is obtained by logging interpretation.

[0092] B. Calculate the material tension-compression ratio parameters and strength parameters.

[0093] The material parameters are calculated as follows:

[0094]

[0095] C. Determine the relative magnitude order of the three-dimensional effective principal stresses on the well wall during hydraulic fracturing

[0096] C1. Calculation model of three-dimensional effective principal stress on wellbore wall during hydraulic fracturing;

[0097]

[0098] Calculate effective stress in rock mass;

[0099]

[0100] C2. Given hydraulic fracturing, the relative magnitude of the three-dimensional effective principal stresses of hydraulic fracturing can be compared

[0101] For example, when the hydraulic fracturing injection pressure is 120.23 MPa

[0102]

[0103] Then there is

[0104] σ1=max(σ t,e ,σ r,e ,σ z,e )=max(-12.645,36.07,7.04)=36.07MPa

[0105] σ3=min(σ t,e ,σ r,e ,σ z,e )=min(-12.645,36.07,7.04)=-12.645MPa

[0106] σ2=mid(σ t,e ,σ r,e ,σ z,e )=mid(-12.645,36.07,7.04)=7.04MPa

[0107] D. Use unified strength theory to establish a hydraulic fracturing pressure calculation model;

[0108] E. Calculate the fracture pressure based on the hydraulic fracturing pressure calculation model and the calculation coefficients of the three-dimensional effective principal stress;

[0109] E1. Classification of stress modes of hydraulically fractured rock masses;

[0110] It is assumed that the stress mode during hydraulic fracturing is the following stress mode among the six modes, namely,

[0111] σ1=σ r,e >σ2=σ t,e >σ3=σ z,e

[0112] Determine the corresponding calculation coefficients of σ1, σ2, and σ3 according to the stress mode:

[0113] max0=r0=0,max1=r1=0.3

[0114] mid0=t0=128.625, mid1=t1=-1.175

[0115] min0=z0=91.2, min1=z1=-0.7

[0116] E2. Calculation model of rock mass fracture pressure caused by hydraulic fracturing;

[0117] The three-dimensional effective pressure on the wellbore wall includes the injection pressure. According to the definition, the injection fluid pressure when the rock mass breaks is the formation fracture pressure. F =pi The formula for calculating the burst pressure is obtained by solving the equation.

[0118] According to the unified strength theory, the rock mass fracture pressure of hydraulic fracturing is calculated in two categories. The rock mass fracture under any stress mode should satisfy the following calculation model.

[0119] when

[0120]

[0121] (2) When

[0122]

[0123] Calculate the rupture pressure for each stress mode;

[0124] For each stress mode, the corresponding rupture pressure is calculated according to formula (11) and formula (12).

[0125]

[0126] The fracture pressure is used as the injection pressure to calculate the three-dimensional effective principal stress corresponding to the rock mass fracture (see the table above). The fracture pressure that is consistent with the assumed stress mode is the real fracture pressure of the rock mass, as shown in the table above. That is, the fracture pressure is 129.87MPa.

[0127] The above description is not intended to limit the present invention in any form. Although the present invention has been disclosed through the above embodiments, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are within the scope of the technical solution of the present invention.

Claims

1. A method for predicting hydraulic fracturing pressure based on unified strength theory, characterized in that: The following steps are involved: A. Collect in-situ stress field data, formation fluid pressure and rock mechanical parameters of the target reservoir; B. Calculate the material tension-compression ratio parameters and rock mass tensile strength based on rock mechanics parameters; C. Determine the relative magnitude order of the three-dimensional effective principal stresses borne by the wellbore wall during hydraulic fracturing based on the calculation formula of effective stress in the pore elastic medium of the rock mass; D. Determine the corresponding stress mode and the calculation coefficient of the three-way effective principal stress according to the relative magnitude order of the three-way effective principal stress; E. Use unified strength theory to establish a hydraulic fracturing pressure calculation model; F. Calculate the fracture pressure based on the hydraulic fracturing pressure calculation model and the calculation coefficients of the three-dimensional effective principal stress; G. Conduct consistency check between fracture pressure and stress pattern to determine fracture pressure of hydraulically fractured rock mass.

2. The method for predicting hydraulic fracturing pressure based on unified strength theory according to claim 1, characterized in that: The in-situ stress field data include the maximum horizontal principal stress and the minimum principal stress, and the overlying rock pressure; the rock mechanics parameters include cohesion and internal friction angle, pore elastic constant, Poisson's ratio and material characteristic parameters.

3. The method for predicting hydraulic fracturing pressure based on unified strength theory according to claim 1, characterized in that: The calculation formula of the material tension-compression ratio parameter and the rock mass tensile strength is: Where: C is the cohesion of the rock mass, MPa; φ is the internal friction angle of the rock mass, °; σ ten is the tensile strength of rock mass, MPa; a is the material tension-compression ratio parameter, dimensionless.

4. The method for predicting hydraulic fracturing pressure based on unified strength theory according to claim 1, characterized in that: The effective stress calculation formula in the rock mass poroelastic medium includes: σ1=max(σ t,e ,s r,e ,s z,e ) σ3=min(σ t,e ,s r,e ,s z,e ) σ2=mid(σ t,e ,s r,e ,s z,e ) Where: r,e , σ t,e , σ z,e are the effective total stresses acting on the wellbore wall in radial, circumferential and axial directions, MPa; σ H , σ h are the maximum horizontal principal stress and the minimum horizontal principal stress, MPa; α is the pore elastic constant of the rock mass, dimensionless; p i 、p r are the injection pressure into the wellbore and the formation fluid pressure during hydraulic fracturing, MPa; σ1, σ2, σ3 are the maximum, intermediate and minimum effective principal stresses, MPa; t0, r0, z0, t1, r1, z1, k are all intermediate parameters.

5. The method for predicting hydraulic fracturing pressure based on unified strength theory according to claim 1, characterized in that: In step D, the stress modes are divided into the following six types according to the relative magnitude relationship of the three principal stresses, namely, σ1=σ r,e >σ2=σ z,e >σ3=σ t,e σ1=σ r,e >σ2=σ t,e >σ3=σ z,e σ1=σ z,e >σ2=σ r,e >σ3=σ t,e σ1=σ z,e >σ2=σ t,e >σ3=σ r,e σ1=σ t,e >σ2=σ z,e >σ3=σ r,e σ1=σ t,e >σ2=σ r,e >σ3=σ z,e And the calculation coefficients of corresponding σ1, σ2, and σ3 are determined according to the six stress modes: (max0,max1), (mid0,mid1), (min0,min1) Wherein: max0, mid0, min0 are the coefficients of the maximum, intermediate and minimum effective principal stresses that are independent of the injection pressure, respectively, and are dimensionless; max1, mid1, min1 are the coefficients of the maximum, intermediate and minimum effective principal stresses that are related to the injection pressure, respectively, and are dimensionless.

6. The method for predicting hydraulic fracturing pressure based on unified strength theory according to claim 1, characterized in that: The hydraulic fracturing pressure calculation model includes: when hour; when hour; Where: max0, mid0, min0 are the coefficients of the maximum, middle and minimum effective principal stresses that are independent of the injection pressure, respectively, and are dimensionless; max1, mid1, min1 are the coefficients of the maximum, middle and minimum effective principal stresses that are related to the injection pressure, respectively, and are dimensionless; b is the material characteristic parameter, dimensionless; a is the material tension-compression ratio parameter, dimensionless; φ is the internal friction angle of the rock mass, °; σ ten is the tensile strength of rock mass, MPa; σ1, σ2, σ3 are the maximum, intermediate and minimum effective principal stresses, MPa respectively; p F is the rock splitting pressure, MPa.

7. The method for predicting hydraulic fracturing pressure based on unified strength theory according to claim 1, characterized in that: The specific process of step G is: The rock splitting pressure calculated in step F is compared with the injection pressure in step C. If the rock splitting pressure is not equal to the injection pressure, repeat steps C to F until the rock splitting pressure is equal to the injection pressure. The rock splitting pressure is the fracture pressure of the hydraulically fractured rock.

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