Method for predicting formation hydraulic fracturing pressure based on rock mass unit double-shear failure criterion

Abstract

The invention relates to a method for predicting formation hydraulic fracturing pressure based on a rock mass unit double-shear failure criterion, and belongs to the field of reservoir hydraulic fracturing yield increase transformation. The invention discloses a method for predicting formation hydraulic fracturing pressure based on a rock mass unit double-shear failure criterion. The method comprises the steps that an in-situ stress field, rock mechanical parameters and reservoir fluid pressure of a target formation are obtained; calculating a coefficient of a hydraulic fracturing three-way effective principal stress state; the hydraulic fracturing pressure of the rock mass is calculated according to the coefficient of the three-way effective principal stress state of the rock mass during hydraulic fracturing; and the corresponding hydraulic fracturing pressure is obtained through repeated calculation, and the hydraulic fracturing pressures of the rock mass at different positions around the well are compared. The method can comprehensively and objectively reflect a stratum rock mass hydraulic fracturing mechanism of two groups of shear stresses acting on the double-shear unit body and normal stresses on corresponding surfaces, and fills the blank of predicting the stratum hydraulic fracturing pressure by comprehensively considering the influence of the normal stresses and the shear stresses; and the accuracy of predicting the formation hydraulic fracturing pressure is improved.

Description

Technical Field

[0001] The invention relates to a method for predicting formation hydraulic fracturing pressure based on double shear failure criterion of rock mass unit, and belongs to the field of reservoir hydraulic fracturing production enhancement and transformation. Background Art

[0002] Hydraulic fracturing is the core technology in the field of oil and gas field development. The formation hydraulic fracturing pressure is the key parameter basis and constraint condition for the reasonable configuration of fracturing equipment and the optimization of construction parameters. The theoretical prediction of hydraulic fracturing pressure in engineering is almost always based on homogeneous isotropic linear elastic materials and the first strength theory. The first strength theory only considers the effect of the effective minimum principal stress, that is, when the effective minimum principal stress of the rock mass reaches the tensile strength of the rock, brittle fracture occurs, and the corresponding fluid pressure is the hydraulic fracturing pressure. The results obtained by these hydraulic fracturing pressure prediction methods are often quite different from engineering practice. In order to more accurately guide the application of mines, they are usually corrected in combination with the correction coefficient of engineering practice data. In fact, when hydraulic fracturing the formation rock mass, it is subjected to three principal stresses in the vertical and horizontal directions. They all contribute to the fracture of the rock mass, and their influence should be considered comprehensively. A large number of rock mechanics research results have shown that shear stress is the basic factor of rock mass failure, and the normal stress on the shear stress surface also affects the strength of the rock mass. Considering all stress components on the rock mass double shear unit and their different effects on material failure, when the influence function of the two sets of shear stresses acting on the rock mass double shear unit and the normal stress on the corresponding surface reaches a certain limit value, the rock mass will fail, which is the unit double shear failure criterion. Obviously, the unit double shear failure criterion can better characterize the hydraulic fracturing pressure of the formation rock mass, but no relevant research results have been seen, and it is still blank. Summary of the invention

[0003] In order to overcome the defects in the prior art, the present invention aims to provide a method for predicting the hydraulic fracturing pressure of the formation based on the double shear failure criterion of the rock mass unit, which can comprehensively and objectively reflect the hydraulic fracturing mechanism of the two sets of shear stresses acting on the double shear unit body and the normal stress on the corresponding surface, filling the gap of comprehensively considering the influence of normal stress and shear stress on predicting hydraulic fracturing pressure, and improving the accuracy of predicting hydraulic fracturing pressure.

[0004] The technical solution provided by the present invention to solve the above technical problems is: a method for predicting formation hydraulic fracturing pressure based on the double shear failure criterion of rock mass unit, comprising the following steps:

[0005] Step 1: Obtain the in-situ stress field, rock mechanics parameters, and reservoir fluid pressure of the target reservoir;

[0006] Step 2: Calculate the coefficients of the three-dimensional effective principal stress state of the rock mass during hydraulic fracturing based on the in-situ stress field and rock mechanics parameters of the target reservoir;

[0007] Step 3, calculating the hydraulic fracturing pressure according to the coefficients of the three-dimensional effective principal stress state of hydraulic fracturing;

[0008] Step 4: Given any polar angle θ=0° to 90°, repeat steps 2 to 3 to calculate the corresponding hydraulic fracturing pressure, and compare the hydraulic fracturing pressures of the formations at different positions around the well.

[0009] A further technical solution is that the in-situ stress field of the target reservoir includes a horizontal maximum principal stress, a horizontal minimum principal stress and overlying rock pressure.

[0010] A further technical solution is that the maximum horizontal principal stress, the minimum horizontal principal stress and the overlying rock pressure are obtained by using field stress testing, experimental testing such as differential strain or Kaiser effect, and logging data interpretation methods.

[0011] A further technical solution is that the rock mechanics parameters include cohesion, internal friction angle, poroelastic constant and Poisson's ratio of the hydraulically fractured rock formation.

[0012] A further technical solution is that the cohesion, internal friction angle, poroelastic constant and Poisson's ratio of the hydraulic fracturing rock formation are obtained through core experiments.

[0013] A further technical solution is that the reservoir fluid pressure is obtained by well logging or pressure recovery method.

[0014] A further technical solution is that the calculation formula in step 2 is:

[0015]

[0016] Where: m1, m2, m3, n1, n2, n3 are the coefficients of the three-dimensional effective principal stress state of hydraulic fracturing, dimensionless; σ H is the maximum horizontal principal stress, MPa; σ h is the minimum principal stress, MPa; σ v is the overlying rock pressure, MPa; p r is the reservoir fluid pressure, MPa; α is the pore elastic constant of the rock mass, dimensionless; ν is the Poisson's ratio of the rock mass, dimensionless; θ is the polar angle, °.

[0017] A further technical solution is that the calculation formula in step three is:

[0018]

[0019] Where: m1, m2, m3, n1, n2, n3 are the coefficients of the three-dimensional effective principal stress state of hydraulic fracturing, dimensionless; C, φ are the cohesion of the rock mass, MPa and the internal friction angle, °; pF is the hydraulic fracturing pressure, MPa.

[0020] The present invention has the following beneficial effects: the present invention aims at the hydraulic fracturing problem of vertical open hole completion, applies elastic mechanics theory to calculate the wellbore stress concentration effect, hydraulic fracturing fluid injection into the wellbore and reservoir seepage effect to determine the effective three-dimensional principal stress of the wellbore wall for hydraulic fracturing, adopts the unit double shear failure criterion to calculate the injection pressure (i.e., hydraulic fracturing pressure) during hydraulic fracturing of rock mass, and invents a method for predicting hydraulic fracturing pressure based on the unit double shear failure criterion, which overcomes the deficiency of currently only using the minimum principal stress based on the tensile strength theory to predict the hydraulic fracturing fracture pressure, can more scientifically and accurately quantitatively predict the hydraulic fracturing pressure, and fills the gap in this field. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the hydraulic fracturing wellbore and polar coordinate system;

[0022] Figure 2 is the total stress distribution diagram of the wellbore wall under the hydraulic fracturing pressure;

[0023] Figure 3 This is the calculation result of hydraulic fracturing pressure. DETAILED DESCRIPTION

[0024] 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.

[0025] like Figure 1 As shown, a method for predicting formation hydraulic fracturing pressure based on the double shear failure criterion of rock mass unit of the present invention comprises the following steps:

[0026] Step 1: Obtain the in-situ stress field, rock mechanics parameters, and reservoir fluid pressure of the target reservoir;

[0027] The in-situ stress field of the reservoir (horizontal maximum principal stress, minimum principal stress, overburden rock pressure) is obtained by using methods such as field stress testing, experimental testing such as differential strain or Kaiser effect, and logging data interpretation;

[0028] The rock mechanical parameters (cohesion, internal friction angle, poroelastic constant, Poisson's ratio) of hydraulic fracturing rock formations were obtained through core experiments in accordance with the national standard "Standard for Engineering Rock Test Methods" (GB / T50266-2013);

[0029] Reservoir fluid pressure is obtained by logging or pressure recovery.

[0030] Step 2: Calculate the coefficients of the three-dimensional effective principal stress state of hydraulic fracturing based on the in-situ stress field and rock mechanics parameters of the target reservoir;

[0031] The present invention considers the water storage pressure fractured rock formation as a homogeneous isotropic material, and establishes the following for the vertical wellbore: Figure 1 The cylindrical coordinate system shown has a polar radius of r and a polar angle (the angle with the direction of the maximum horizontal principal stress, i.e., the x-axis) of θ.

[0032] According to the theory of elastic mechanics, the plane stress field acting on the wellbore wall during hydraulic fracturing, the plane stress field acting on the wellbore wall during fluid injection, and the plane stress field acting on the wellbore wall during hydraulic fracturing are considered respectively. The total stress distribution acting on the wellbore wall during hydraulic fracturing is calculated by using the principle of stress superposition. The specific expression is:

[0033]

[0034] Where: r , σ θ , σ z are the radial, circumferential and vertical stresses on the borehole wall, MPa.

[0035] The effective stress in the poroelastic medium is calculated according to the classic Terzaghi principle, that is, the effective stress of the wellbore wall caused by hydraulic fracturing is calculated as follows:

[0036]

[0037] Where: r,e , σ θ,e , σ z,e are the effective stresses in the radial, circumferential and vertical directions of the borehole wall, MPa, respectively.

[0038] in

[0039]

[0040] Where: m1, m2, m3, n1, n2, n3 are the coefficients of the three-dimensional effective principal stress state of hydraulic fracturing, dimensionless; σ H is the maximum horizontal principal stress, MPa; σ h is the minimum principal stress, MPa; σ v is the overlying rock pressure, MPa; p r is the reservoir fluid pressure, MPa; α is the pore elastic constant of the rock mass, dimensionless; ν is the Poisson's ratio of the rock mass, dimensionless; θ is the polar angle, °

[0041] Step 3, calculating the hydraulic fracturing pressure according to the coefficients of the three-dimensional effective principal stress state of hydraulic fracturing;

[0042] The calculation model of hydraulic fracturing pressure when the three-dimensional stress state unit body is in a destroyed state is derived by using the unit double shear failure criterion as follows.

[0043]

[0044] Where: m1, m2, m3, n1, n2, n3 are the coefficients of the three-dimensional effective principal stress state of hydraulic fracturing, dimensionless; C, φ are the cohesion of the rock mass, MPa and the internal friction angle, °; p F is the hydraulic fracturing pressure, MPa;

[0045] Step 4: Given any angle of polar angle θ=0° to 90°, repeat steps 2-3 to calculate the corresponding hydraulic fracturing pressure, and compare the hydraulic fracturing pressures of the formations at different positions around the well. Usually, the calculated fracture pressure when the polar angle θ=0° is the actual hydraulic fracturing pressure.

[0046] Example

[0047] The present invention provides a method for predicting hydraulic fracturing pressure based on the double shear failure criterion of rock mass unit, comprising the following steps:

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

[0049] A1. The in-situ stress field of the target reservoir is interpreted by well logging data to obtain the maximum horizontal principal stress σ H =102MPa, minimum principal stress σ h =90MPa, overburden rock pressure σ v =96MPa;

[0050] A2. The rock mechanical parameters of hydraulic fracturing rock formations are obtained through core experiments according to national standards, including rock mass cohesion C = 5 MPa, internal friction angle φ = 30°, poroelastic constant α = 0.35, and Poisson's ratio ν = 0.2;

[0051] A3. Reservoir fluid pressure is obtained by well logging interpretation method. r =60MPa.

[0052] B. Calculate the coefficients of the three-dimensional effective principal stress state of hydraulic fracturing based on the in-situ stress field and rock mechanics parameters of the target reservoir;

[0053] β o =2(σ H -σ h )cos(2θ)=2(102-90)×cos(2×0)=24

[0054]

[0055] Where: m1, m2, m3, n1, n2, n3 are the coefficients of the three-dimensional effective principal stress state of hydraulic fracturing, dimensionless; σ H is the maximum horizontal principal stress, MPa; σ h is the minimum principal stress, MPa; σ v is the overlying rock pressure, MPa; p r is the reservoir fluid pressure, MPa; α is the pore elastic constant of the rock mass, dimensionless; ν is the Poisson's ratio of the rock mass, dimensionless; θ is the polar angle, °;

[0056] C. Calculate the hydraulic fracturing pressure based on the coefficients of the three-dimensional effective principal stress state of hydraulic fracturing;

[0057] The calculation model of hydraulic fracturing pressure when the three-dimensional stress state unit body is in a destroyed state is derived by using the unit double shear failure criterion as follows.

[0058]

[0059] Where: m1, m2, m3, n1, n2, n3 are the coefficients of the three-dimensional effective principal stress state of hydraulic fracturing, dimensionless; C, φ are the cohesion of the rock mass, MPa and the internal friction angle, °; p F is the hydraulic fracturing pressure, MPa;

[0060] D. Given any angle of polar angle θ = 0° to 90°, repeat steps BC to calculate the corresponding bursting pressure. Figure 3 ; and compare the hydraulic fracturing pressure of the formation at different locations around the well.

[0061] 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 formation hydraulic fracturing pressure based on double shear failure criterion of rock mass unit, characterized in that: The following steps are involved: Step 1: Obtain the in-situ stress field, rock mechanics parameters, and reservoir fluid pressure of the target reservoir; Step 2: Calculate the coefficients of the three-dimensional effective principal stress state of the rock mass during hydraulic fracturing based on the in-situ stress field and rock mechanics parameters of the target reservoir; Step 3, calculating the hydraulic fracturing pressure according to the coefficients of the three-dimensional effective principal stress state of hydraulic fracturing; Step 4: Given any polar angle θ=0° to 90°, repeat steps 2 to 3 to calculate the corresponding hydraulic fracturing pressure, and compare the hydraulic fracturing pressures of the formations at different positions around the well.

2. A method for predicting formation hydraulic fracturing pressure based on double shear failure criterion of rock mass unit according to claim 1, characterized in that: The in-situ stress field of the target reservoir includes horizontal maximum principal stress, horizontal minimum principal stress and overlying rock pressure.

3. The method for predicting formation hydraulic fracturing pressure based on the double shear failure criterion of rock mass unit according to claim 2, characterized in that: The maximum horizontal principal stress, the minimum horizontal principal stress and the overlying rock pressure are obtained by using field stress testing, experimental testing such as differential strain or Kaiser effect, and logging data interpretation methods.

4. The method for predicting formation hydraulic fracturing pressure based on the double shear failure criterion of rock mass unit according to claim 1, characterized in that: The rock mechanics parameters include the cohesion, internal friction angle, poroelastic constant and Poisson's ratio of the hydraulically fractured rock formation.

5. The method for predicting formation hydraulic fracturing pressure based on the double shear failure criterion of rock mass unit according to claim 4, characterized in that: The cohesion, internal friction angle, poroelastic constant and Poisson's ratio of the hydraulic fracturing rock formation are obtained through core experiments.

6. The method for predicting formation hydraulic fracturing pressure based on double shear failure criterion of rock mass unit according to claim 1, characterized in that: The reservoir fluid pressure is obtained by well logging or pressure recovery method.

7. The method for predicting formation hydraulic fracturing pressure based on double shear failure criterion of rock mass unit according to claim 1, characterized in that: The calculation formula in step 2 is: b o =2(σ H -s h )cos(2θ) Where: m1, m2, m3, n1, n2, n3 are the coefficients of the three-dimensional effective principal stress state of hydraulic fracturing, dimensionless; σ H is the maximum horizontal principal stress, MPa; σ h is the minimum principal stress, MPa; σ v is the overlying rock pressure, MPa; p r is the reservoir fluid pressure, MPa; α is the pore elastic constant of the rock mass, dimensionless; ν is the Poisson's ratio of the rock mass, dimensionless; θ is the polar angle, °.

8. The method for predicting formation hydraulic fracturing pressure based on double shear failure criterion of rock mass unit according to claim 1, characterized in that: The calculation formula in step 3 is: Where: m1, m2, m3, n1, n2, n3 are the coefficients of the three-dimensional effective principal stress state of hydraulic fracturing, dimensionless; C, φ are the cohesion of the rock mass, MPa and the internal friction angle, °; p F is the hydraulic fracturing pressure, MPa.

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