Fracture plane total stress monitoring method

Abstract

The invention discloses a fault plane total stress monitoring method, which comprises the following steps of: embedding a stress monitoring device into the center of an inclined plane of a to-be-measured body, calculating six independent stress components according to strain values of the to-be-measured body in different directions acquired by the stress monitoring device, and calculating normal stress and shear stress of a fault plane according to the six independent stress components. The fault plane total stress monitoring is realized; according to the invention, the stress monitoring device with six groups of strain rosettes and the base body is embedded into the inclined plane of the object to be measured, and the full stress monitoring of the fault plane is realized through the stress values measured by the six groups of strain rosettes, so that the accurate stress change characteristics during the slippage instability of the fault plane are obtained, and a basis is provided for preventing and controlling disasters induced by the slippage instability of the fault plane.

Description

Technical Field

[0001] The invention relates to the technical field of rock mechanics stress monitoring, and in particular to a fault plane full stress monitoring method. Background Art

[0002] The hydraulic fracturing technology for shale gas extraction creates and expands the fracture network by injecting fracturing fluid under high pressure, thereby increasing the permeability of the shale gas reservoir and improving the extraction efficiency of shale gas resources. Since the oil and gas reservoir contains a large number of faults of different sizes and types, the faults remain stable under the action of in-situ stress when they are not disturbed. However, when hydraulic fracturing is used to extract shale gas resources, great disturbances are generated, which can easily cause the fault stress to change sharply and cause slip instability, which may induce earthquakes. The existing fault plane shear stress monitoring sensor can monitor the shear stress on the fault plane, but cannot monitor the normal stress of the fault plane, and the total stress of the fault plane cannot be accurately obtained. Therefore, designing a fault plane full stress monitoring device and method is of great significance for studying the stress evolution law of the fault plane during hydraulic fracturing.

[0003] The invention patent with the patent number CN201721848493.X discloses a fault plane shear stress monitoring sensor. This sensor can only monitor the linear strain in three directions of the front and back points of the gypsum board at the fault through the strain rosette, and then calculate the shear strain, and then calculate the shear stress through the physical equation to realize the monitoring of the shear stress of the fault plane. However, in the theoretical analysis method, the Moore-Coulomb criterion or the extended Moore-Coulomb criterion is often used to determine the fault slip instability conditions. The damage of the fault plane is not only related to the change of its shear stress, but also closely related to its normal stress. It is applied to the stress monitoring of the fault plane, and can only monitor the shear stress on the fault plane, but not the normal stress of the fault plane. The total stress of the fault plane cannot be accurately obtained, and the fault slip instability conditions cannot be given, which has great limitations. Summary of the invention

[0004] The purpose of the present invention is to provide a method for monitoring the total stress of a fault plane. The method adopts multiple groups of strain rosettes in specific orientations to form a fault plane total stress monitoring device, so as to solve the problem that the fault plane stress monitoring sensor can only monitor the shear stress but not the normal stress of the fault plane, and the total stress of the fault plane cannot be accurately obtained. Therefore, more accurate stress change characteristics when the fault plane slips and becomes unstable can be obtained, which can provide a basis for preventing and controlling disasters induced by fault plane slips and becomes unstable.

[0005] To achieve the above object, the present invention adopts the following technical solution:

[0006] A method for monitoring the full stress of a fault plane comprises embedding a stress monitoring device into the center of the inclined surface of a body to be measured, calculating six independent stress components through strain values ​​of the body to be measured in different directions collected by the stress monitoring device, calculating the normal stress and shear stress of the fault plane through the six independent stress components, and realizing full stress monitoring of the fault plane;

[0007] The stress monitoring device includes a cylindrical substrate and six sets of strain rosettes installed on the substrate, each set of strain rosettes includes three strain gauges; the three strain gauges respectively collect axial strain values, 45° direction strain values, and circumferential strain values;

[0008] According to the relationship between strain rosettes and stress, the circumferential normal stress σ at each set of strain rosette measuring points is solved. θ , axial normal stress σ z , shear stress τ θz ; Then, according to the known stress, the six independent stress components σ are calculated using the hole edge stress distribution in elastic mechanics x , σ y , σ z , τ xy , τ xz , τ yz ;

[0009] Based on the six independent stress components, the normal stress and shear stress at any point on the fault plane of the object to be measured can be calculated.

[0010] Furthermore, the six groups of strain rosettes are paired in pairs to measure strain values ​​in the same direction; the two groups of strain rosettes in a pair are symmetrically arranged along the center line of the cylinder.

[0011] Furthermore, the strain rosette is bonded to the outer surface of the cylindrical substrate through colloid, and the elastic constants of the colloid, the cylindrical substrate and the object to be measured are the same.

[0012] Furthermore, six independent stress components are calculated by using the strain values ​​of the object to be tested in different directions collected by the stress monitoring device, including:

[0013] Step 1: Each group of strain rosettes collects three strain values ​​ε0. , ε 45 . , ε 90 . The average value of the two strain values ​​in the same direction is taken to obtain the strain values ​​in 9 different directions; the circumferential normal stress σ is calculated using the relationship between each set of strain rosettes and stress θ , axial normal stress σ z , shear stress τ θz , strain value ε 0° , ε 45° , ε 90° and stress σ θ , σ z , τ θz The relationship is:

[0014]

[0015] In the formula, ε0. , ε 45 . , ε 90 . They are the axial strain value, 45° direction strain value, and circumferential strain value measured by 3 strain gauges in each set of strain rosettes, σ θ , σ z , τ θz are the circumferential normal stress, axial normal stress, and shear stress at each set of strain rosette measuring points; E is the elastic modulus of the cylindrical matrix at the measuring point; υ is the Poisson's ratio of the cylindrical matrix at the measuring point; G is the shear modulus of the cylindrical matrix at the measuring point;

[0016] Step 2: Calculate the circumferential normal stress σ from step 1 θ , axial normal stress σ z , shear stress τ θz , the six independent stress components σ are calculated by converting the hole edge stress distribution formula in elastic mechanics x , σ y , σ z , τ xy , τ xz , τ yz ;

[0017]

[0018] In the formula, ε 0° , ε 45° , ε 90° They are the axial strain value, 45° direction strain value, and circumferential strain value measured by 3 strain gauges in each set of strain rosettes, γ θz is the shear strain value, σ θ , σ z , τ θz are the circumferential normal stress, axial normal stress and shear stress at each set of strain rosette measuring points; E is the elastic modulus of the cylindrical matrix at the measuring point; υ is the Poisson's ratio of the cylindrical matrix at the measuring point; θ is the angle between the strain rosette and the x-axis.

[0019] Furthermore, the least square method is used to establish a functional relationship between the strain value measured by the strain rosette and the three-dimensional stress component.

[0020] Suppose the strain values ​​in 9 different directions obtained by 6 groups of strain rosettes are ε′1, ε′2, ε′3, ε′4, ε′5, ε′6, ε′7, ε′8, ε′9, and define the matrix: σ′=[σ′1 σ′2 σ′3 σ′4 σ′5 σ′6 σ′7 σ′8 σ′9] T =E[ε′1 ε′2 ε′3 ε′4ε′5 ε′6 ε′7 ε′8 ε′9]T ; Among them, σ′ i The strain values ​​ε′ in 9 different directions obtained by 6 sets of strain rosettes i The corresponding stress value; E is the elastic modulus of the cylindrical substrate at the measuring point;

[0021] The three-dimensional stress of the measured point has six independent stress components σ x , σ y , σ z , τ xy , τ yz , τ zx Define the matrix: σ = [σ x σ y σ z τ xy τ yz τ zx ] T ;

[0022] The three-dimensional stress components can be obtained by the least squares method:

[0023]

[0024] in, a2=9(1-υ 2 ) 2 ; a5+a6=E[2(ε′1+ε′4+ε′7)-2υ(ε′2+ε′5+ε′8)-υ(ε′3+ε′6+ε′9)]; a5-a6=E(1-υ 2 )(4ε′1+2ε′3-2ε′4-ε′6-2ε′7-ε′9); a7=

[0025]

[0026] Furthermore, the normal stress and shear stress of the fault plane are calculated through six independent stress components, including:

[0027] The component p of the total stress p at any point D on the fault plane of the measured body in the direction of the coordinate axis is x 、p y 、p z for:

[0028] In the formula, p x 、p y 、p z The stress components of the total stress p in the directions of the three coordinate axes; l, m, n are the direction cosines of the normal N outside the plane where point D is located and the x-axis, y-axis, and z-axis respectively;

[0029] Normal stress σ N , shear stress τ N for:

[0030] The present invention embeds a stress monitoring device with six sets of strain rosettes into the inclined surface of the object to be measured, and realizes full stress monitoring of the fault surface through 18 stress values ​​measured by the six sets of strain rosettes; obtains accurate stress change characteristics when the fault surface slips and becomes unstable; and can provide a basis for preventing and controlling disasters induced by fault surface slip and instability. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic structural diagram of the fault plane full stress monitoring device of the present invention.

[0032] Figure 2 This is a schematic diagram of the installation of the fault plane full stress monitoring device of the present invention.

[0033] Figure 3 It is the stress state of any point P during monitoring of the present invention. DETAILED DESCRIPTION

[0034] This embodiment provides a method for monitoring the full stress of a fault plane, including vertically embedding a stress monitoring device into the center of the inclined surface of the object to be tested, such as Figure 1 As shown, six independent stress components are calculated from the strain values ​​of the object to be tested in different directions collected by the stress monitoring device, and the normal stress and shear stress of the fault plane are calculated through the six independent stress components, thereby realizing full stress monitoring of the fault plane.

[0035] The stress monitoring device is tightly connected to the inclined surface of the object to be measured by pre-embedding or drilling and bonding, so that the stress monitoring device does not slide or rotate relative to each other, and can deform synchronously, thereby realizing effective low-stress monitoring.

[0036] like Figure 2 As shown, the stress monitoring device includes a substrate and a strain rosette installed on the substrate. The substrate is a cylindrical structure, which is convenient for establishing a coordinate system. Specifically, a coordinate system o-xyz is established with the axial direction of the cylinder as the Z-axis direction and the center of the bottom surface as the origin of the coordinate axis. Through the established coordinate system, the direction of the strain value in the strain rosette can be quickly obtained, which is convenient for the subsequent calculation of the total stress.

[0037] The strain rosettes of this embodiment include six groups, and every two groups are in a standby state to prevent damage caused by installation or limitation of strain gauge quality or usage time. The strain rosettes specifically include strain rosette A1, strain rosette A2, strain rosette B1, strain rosette B2, strain rosette C1, and strain rosette C2, wherein strain rosettes with the same letters are standby for each other, and the installation directions of strain gauges are consistent; the strain rosettes A1, strain rosette B1, and strain rosette C1 are installed on the same ring of the cylinder, and the strain rosette A1 is located on the y-axis of the coordinate system o-xyz. The strain rosette B1 differs from the strain rosette A1 by 60°. In this embodiment, the strain rosette B1 is installed on the side of the cylindrical structure ring with the y-axis deflected 60° toward the positive direction of the x-axis, and the strain rosette C1 is installed by deflecting it again by 60° in the same deflection direction. The strain rosettes A2, strain rosette B2, strain rosette C2 differ from the corresponding strain rosettes A1, strain rosette B1, strain rosette C1 by 180°, that is, they are located at both ends of the same diameter, such as Figure 2 As shown in (b), the six groups of strain rosettes are evenly distributed on the surface of the cylindrical substrate, that is, the difference between each group of strain rosettes is 60°. In the coordinate system, the angles between the six groups of strain rosettes A1, B1, C1, A2, B2, and C2 and the coordinate axis X are θ A1 =90°、θ B1 =30°,θ C1 =-30°,θ A2 =-90°,θ B2 =-150°,θ C2 =-210°.

[0038] In order to ensure that the full stress monitoring device can accurately monitor the strain value and avoid the failure to calculate the stress value due to the loss of strain data caused by the damage of a certain group of strain rosettes or a certain strain gauge, two groups of strain rosettes are arranged on the same circumferential diameter, and the arrangement directions of the strain gauges in the two groups of strain rosettes are consistent with each other. Similarly, the spare strains of this embodiment can also be arranged on another circular ring at equal intervals along the z-axis, or two groups of spare strain rosettes can be arranged.

[0039] Each set of strain rosettes consists of three resistive strain gauges, and adjacent resistive strain gauges are spaced 45° apart. Taking strain rosette A1 as an example, the strain rosette includes a first strain gauge, a second strain gauge and a third strain gauge. The first strain gauge is parallel to the x-axis of the coordinate system, the second strain gauge is parallel to the z-axis, and the third strain gauge is located between the first strain gauge and the second strain gauge, and is at a 45° angle to the first strain gauge and the second strain gauge, respectively.

[0040] There are 18 resistance strain gauges in total in the six groups of strain gauges, and each resistance strain gauge is numbered separately, such as Figure 2As shown in (c), six groups of strain rosettes are bonded to the outer surface of the cylindrical substrate by colloid. The elastic constants (elastic modulus and Poisson's ratio) of the colloid, the cylindrical substrate and the object to be measured are consistent. The elastic modulus needs to be measured by taking a cylindrical or cubic standard specimen to be measured and conducting a uniaxial compression test. The Poisson's ratio can be obtained by taking the average value of the ratio of the axial strain and the radial strain measured by each group of strain rosettes. The deformation on the fault plane of the object to be measured is monitored by the six groups of strain rosettes, thereby realizing the monitoring of the full stress of the fault plane of the object to be measured.

[0041] Specifically, the normal stress and shear stress on the fault plane are calculated through six independent stress components:

[0042] Step 1: 18 strain values ​​are collected through six sets of strain rosettes, and the two strain values ​​in the same direction are averaged; strain values ​​in 9 different directions can be obtained. The outer circumference of the cylindrical substrate is regarded as a plane stress state, and the strain values ​​measured by the three strain gauges in each set of strain rosettes are ε0. , ε 45 . , ε 90 . With stress σ θ , σ z , τ θz The relationship is:

[0043]

[0044] In the formula, ε 0° , ε 45° , ε 90° are the axial strain value, 45° direction strain value, and circumferential strain value measured by 3 strain gauges in each set of strain rosettes, σ θ , σ z , τ θz are the circumferential normal stress, axial normal stress, and shear stress at each set of strain rosette measuring points; E is the elastic modulus of the cylindrical matrix at the measuring point; υ is the Poisson’s ratio of the cylindrical matrix at the measuring point; G is the shear modulus of the cylindrical matrix at the measuring point.

[0045] The stress σ is calculated using the relationship between each set of strain rosettes and stress θ , σ z , τ θz , a total of 9 stress values ​​can be calculated.

[0046] Step 2: Based on the 9 stress values ​​calculated in step 1, use the hole edge stress distribution formula in elastic mechanics to calculate the 6 independent stress components σ x , σ y , σ z , τ xy , τ xz , τ yz ;

[0047]

[0048] In the formula, ε0. , ε 45 . , ε 90 . They are the axial strain value, 45° direction strain value, circumferential strain value measured by 3 strain gauges in each set of strain rosettes, γ θz is the shear strain value, σ θ , σ z , τ θz are the circumferential normal stress, axial normal stress, and shear stress at each set of strain rosette measuring points; E is the elastic modulus of the cylindrical matrix at the measuring point; υ is the Poisson's ratio of the cylindrical substrate at the measuring point; θ is the angle between the strain rosette and the X-axis.

[0049] The measurement results of each set of strain rosettes can obtain 3 equations, and 6 sets of strain rosettes can obtain a total of 18 equations. Since the two strain values ​​in the same direction are averaged and used in the calculation, 9 independent equations can be obtained from 6 sets of strain rosettes, and then 6 independent stress components at the measuring point can be solved.

[0050] Step 3: The least square method is used to obtain the specific relationship between the strain measured by the strain rosette and the three-dimensional stress component. The strain values ​​in nine different directions obtained by the six groups of strain rosettes are ε′1, ε′2, ε′3, ε′4, ε′5, ε′6, ε′7, ε′8, and ε′9, respectively. The matrix is ​​defined as: σ′=[σ′1 σ′2 σ′3 σ′4 σ′5 σ′6 σ′7 σ′8 σ′9] T =E[ε′1 ε′2 ε′3 ε′4 ε′5 ε′6 ε′7ε′8 ε′9] T ; Among them, σ′ i The strain values ​​ε′ in 9 different directions obtained by 6 sets of strain rosettes i The corresponding stress value.

[0051] The three-dimensional stress at the measuring point has six independent stress components σ x , σ y , σ z , τ xy , τ yz , τ zx Define the matrix: σ = [σ x σ y σ z τ xy τ yz τ zx ] T .

[0052] Since the spatial positions of adjacent strain rosettes are arranged equidistantly and the arrangement directions of the strain gauges are consistent, the weight factors of the strain values ​​in nine different directions obtained by the six groups of strain rosettes are equal, and the three-dimensional stress components can be obtained by the least squares method;

[0053]

[0054] in, a2=9(1-υ 2 ) 2 ; a5+a6=E[2(ε′1+ε′4+ε′7)-2υ(ε′2+ε′5+ε′8)-υ(ε′3+ε′6+ε′9)]; a5-a6=E(1-υ 2 )(4ε′1+2ε′3-2ε′4-ε′6-2ε′7-ε′9);

[0055] Since the size of the full stress monitoring device is much smaller than the size of the overall inclined fault model of the object to be measured, the full stress monitoring device can be regarded as a microelement. By using the first three steps of monitoring calculation methods through the full stress monitoring device, six independent stress components of any point D on the fault plane of the object to be measured can be obtained.

[0056] like Figure 3 As shown in the figure, the stress state of any point D on the fault plane of the measured body has six independent stress components: x , σ y , σ z , τ xy =τ yx , τ xz =τ zx , τ yz =τ zy In order to obtain the stress on any inclined plane passing through point D, it is necessary to take a plane ABC near point D, parallel to this inclined plane, and form a tiny tetrahedron DABC with three planes passing through point D and parallel to the coordinate plane, such as Figure 3 (a) is shown. The component p of the total stress p on plane ABC in the direction of the coordinate axis is x 、p y 、p z ,like Figure 3 As shown in (b), when the tetrahedron DABC shrinks infinitely and approaches point D, the total stress on plane ABC becomes the total stress on the inclined plane.

[0057] The component p of the total stress p in the direction of the coordinate axis x 、p y 、p z for:

[0058] In the formula, p x 、p y 、p zThe stress components of the total stress p on plane ABC in the directions of the three coordinate axes are respectively; the external normal on plane ABC is named N, where l, m, n are the direction cosines of the external normal N of plane ABC and the x-axis, y-axis, and z-axis respectively.

[0059] Furthermore, the normal stress σ on the inclined plane ABC N , shear stress τ N for:

[0060] Therefore, if the six independent stress components at any point P on the inclined surface of the object to be measured are known, the normal stress and shear stress on any inclined surface passing through this point can be calculated.

[0061] The above description is only a preferred implementation manner of the present invention, but the protection scope of the present invention is not limited thereto, and any modification and replacement based on the technical solution and inventive concept provided by the present invention should be included in the protection scope of the present invention.

Claims

1. A method for monitoring the full stress of a fault plane, characterized in that: The method includes embedding a stress monitoring device into the center of the inclined surface of the object to be measured, calculating six independent stress components through the strain values ​​of the object to be measured in different directions collected by the stress monitoring device, calculating the normal stress and shear stress of the fault plane through the six independent stress components, and realizing full stress monitoring of the fault plane; The stress monitoring device includes a cylindrical substrate and six sets of strain rosettes installed on the substrate, each set of strain rosettes includes three strain gauges; the three strain gauges respectively collect axial strain values, 45° direction strain values, and circumferential strain values; According to the relationship between strain rosettes and stress, the circumferential normal stress σ at each set of strain rosette measuring points is solved. θ , axial normal stress σ z , shear stress τ θz ; Then, according to the known stress, the six independent stress components σ are calculated using the hole edge stress distribution in elastic mechanics x , σ y , σ z , τ xy , τ xz , τ yz ; Based on the six independent stress components, the normal stress and shear stress at any point on the fault plane of the object to be measured can be calculated.

2. A method for monitoring the total stress of a fault plane according to claim 1, characterized in that: The six sets of strain rosettes are paired in pairs to measure the strain value in the same direction; the two sets of strain rosettes in a pair are symmetrically arranged along the center line of the cylinder.

3. A fault plane full stress monitoring method according to claim 1, characterized in that: The strain rosette is bonded to the outer surface of the cylindrical substrate through colloid, and the elastic constants of the colloid, the cylindrical substrate and the object to be measured are the same.

4. A fault plane full stress monitoring method according to claim 1, characterized in that: Six independent stress components are calculated from the strain values ​​of the object to be tested in different directions collected by the stress monitoring device, including: Step 1: Collect 3 strain values ​​ε for each set of strain rosettes 0° , ε 45° , ε 90° , take the average of the two strain values ​​in the same direction to obtain the strain values ​​in 9 different directions; use the relationship between each set of strain rosettes and stress to calculate the circumferential normal stress σ θ , axial normal stress σ z , shear stress σ θz , strain value ε 0° , ε 45° , ε 90° and stress σ θ , σ z , ε θz The relationship is: In the formula, ε 0° , ε 45° , ε 90° are the axial strain value, 45° direction strain value, and circumferential strain value measured by 3 strain gauges in each set of strain rosettes, σ θ , σ z , ε θz are the circumferential normal stress, axial normal stress, and shear stress at each set of strain rosette measuring points; E is the elastic modulus of the cylindrical matrix at the measuring point; υ is the Poisson's ratio of the cylindrical matrix at the measuring point; G is the shear modulus of the cylindrical matrix at the measuring point; Step 2: Calculate the circumferential normal stress σ from step 1 θ , axial normal stress σ z , shear stress σ θz , the six independent stress components σ are calculated by converting the hole edge stress distribution formula in elastic mechanics x , σ y , σ z , τ xy , τ xz , τ yz ; In the formula, ε 0° 、v 45° , ε 90° are the axial strain value, 45° direction strain value, and circumferential strain value measured by 3 strain gauges in each set of strain rosettes, γ θz is the shear strain value, σ θ , σ z , τ θz are the circumferential normal stress, axial normal stress and shear stress at each set of strain rosette measuring points; E is the elastic modulus of the cylindrical matrix at the measuring point; υ is the Poisson's ratio of the cylindrical matrix at the measuring point; θ is the angle between the strain rosette and the x-axis.

5. A fault plane full stress monitoring method according to claim 4, characterized in that: The least square method is used to establish a functional relationship between the strain value measured by the strain rosette and the three-dimensional stress component. Suppose the strain values ​​in nine different directions obtained by the six groups of strain rosettes are ε1′, ε2′, ε3′, ε4′, ε′5, ε′6, ε′7, ε′8, and ε′9, respectively, and define the matrix: σ′=[σ′1 σ′2 σ′3 σ′4 σ′5 σ′6 σ′7 σ′8 σ′9] T =E[ε′1 ε′2 ε′3 ε′4 ε′5 ε′6ε′7 ε′8 ε′9] T ; Among them, σ′ i The strain values ​​ε′ in 9 different directions obtained by 6 sets of strain rosettes i The corresponding stress value; E is the elastic modulus of the cylindrical substrate at the measuring point; The three-dimensional stress of the measured point has six independent stress components σ x , σ y , σ z , τ xy , τ yz , τ zx Define the matrix: σ = [σ x σ y σ z τ xy τ yz τ zx ] T ; The three-dimensional stress components can be obtained by the least squares method: Among them, a2=9(1-υ 2 ) 2 ; a5+a6=E[2(ε′1+ε′4+ε′7)-2v(ε′2+ε′5+ε′8)-υ(ε′3+ε′6+ε9′)];a5-a6=E(1-υ 2 )(4e1 ′ +2e3 ′ -2ε4′-ε6′-2ε7′-ε9′); 6. A fault plane full stress monitoring method according to claim 3, characterized in that: The normal stress and shear stress on the fault plane are calculated from six independent stress components, including: The component p of the total stress p at any point D on the fault plane of the measured body in the direction of the coordinate axis is x 、p y 、p z for: In the formula, p x 、p y 、p z The stress components of the total stress p in the directions of the three coordinate axes; l, m, n are the direction cosines of the normal N outside the plane where point D is located and the x-axis, y-axis, and z-axis respectively; Normal stress σ N , shear stress τ N for:

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