A method for monitoring full stress on fault plane

Abstract

The present invention discloses a method for monitoring the full stress of a fault surface, comprising embedding a stress monitoring device into the center of an inclined surface of a body to be measured, calculating six independent stress components using strain values ​​of the body to be measured in different directions collected by the stress monitoring device, and calculating the normal stress and shear stress of the fault surface using the six independent stress components, thereby achieving full stress monitoring of the fault surface. The present invention embeds a stress monitoring device with six sets of strain rosettes and a matrix into the inclined surface of the body to be measured, and achieves full stress monitoring of the fault surface using the stress values ​​measured by the six sets of strain rosettes, thereby accurately obtaining stress variation characteristics when the fault surface slips and becomes unstable, providing a basis for preventing and controlling disasters induced by fault surface slip and instability.

Description

Technical Field

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

[0002] Shale gas extraction hydraulic fracturing technology creates and expands a fracture network by injecting fracturing fluid under high pressure, thereby increasing the permeability of shale gas reservoirs and improving the extraction efficiency of shale gas resources. Since oil and gas reservoirs contain a large number of faults of varying sizes and types, when undisturbed, the faults remain stable under the action of in-situ stress. However, hydraulic fracturing to extract shale gas resources produces a huge disturbance, which can easily cause the fault stress to change sharply and become slip-instable, which may in turn induce earthquakes. Existing fault plane shear stress monitoring sensors can only monitor the shear stress on the fault plane, but not the normal stress of the fault plane. Therefore, the full 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] Patent number CN201721848493.X discloses a fault plane shear stress monitoring sensor. This sensor can only monitor the linear strain in three directions at the front and back of the gypsum board at the fault through a strain gauge rosette. The shear strain is then calculated, and the shear stress is then determined using physical equations to achieve shear stress monitoring of the fault plane. However, in theoretical analysis methods, the Mohr-Coulomb criterion or an extended Mohr-Coulomb criterion is often used to determine the conditions for fault slip instability. Fault plane failure is not only related to changes in its shear stress, but also closely related to its normal stress. When applied to stress monitoring at the fault plane, this sensor can only monitor the shear stress on the fault plane, not the normal stress of the fault plane. The full stress of the fault plane cannot be accurately determined, and the conditions for fault slip instability cannot be determined, resulting in significant limitations. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for monitoring the full stress of a fault surface. The method uses multiple sets of strain rosettes in specific orientations to form a full stress monitoring device for the fault surface. This method solves the problem that the stress monitoring sensor of the fault surface can only monitor the shear stress but not the normal stress of the fault surface, and the full stress of the fault surface cannot be accurately obtained. In addition, more accurate stress change characteristics when the fault surface slips and becomes unstable can be obtained, which can provide a basis for preventing and controlling disasters induced by fault surface slip and instability.

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

[0006] A method for monitoring full stress on a fault plane includes embedding a stress monitoring device into the center of an inclined surface of a measured object, calculating six independent stress components using strain values ​​in different directions of the measured object collected by the stress monitoring device, and calculating the normal stress and shear stress of the fault plane using the six independent stress components, thereby achieving full stress monitoring of the fault plane.

[0007] The stress monitoring device includes a cylindrical base and six sets of strain rosettes mounted on the base. 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 rosette 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 measured object can be calculated.

[0010] Furthermore, the six sets of strain rosettes are formed into a pair, measuring 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.

[0011] Furthermore, the strain rosette is bonded to the outer surface of the cylindrical substrate via 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 from the strain values ​​of the object to be measured in different directions collected by the stress monitoring device, including:

[0013] Step 1: Each set of rosettes collects three strain values ​​ε0. , 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 τ θz , strain value ε 0° , ε 45° , ε 90° and stress σ θ , σ z , τ θz The relationship is:

[0014]

[0015] Where, 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 rosettes, σ θ , σ z , τ θz are the circumferential normal stress, axial normal stress, and shear stress at each set of rosette measuring points, respectively; E is the elastic modulus of the cylindrical substrate at the measuring point; υ is the Poisson's ratio of the cylindrical substrate at the measuring point; G is the shear modulus of the cylindrical substrate at the measuring point;

[0016] Step 2: Circumferential normal stress σ calculated according to 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] Where, 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 rosettes, respectively. 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, respectively; E is the elastic modulus of the cylindrical substrate at the measuring point; υ is the Poisson's ratio of the cylindrical substrate at the measuring point; and 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 rosette and the three-dimensional stress components.

[0020] Let the strain values ​​in nine different directions obtained by the six groups of rosettes be ε′1, ε′2, ε′3, ε′4, ε′5, ε′6, ε′7, ε′8, and ε′9, respectively. 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 ; where 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 Definition 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 on 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] Where p x 、p y 、p z are the stress components of the total stress p in the directions of the three coordinate axes; l, m, and 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. The 18 stress values ​​measured by the six sets of strain rosettes enable full stress monitoring of the fault surface. Accurate stress variation characteristics during fault surface slip and instability are obtained, providing a basis for preventing and controlling disasters induced by fault surface slip and instability. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This 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 surface, including vertically embedding a stress monitoring device into the center of the inclined surface of the object to be measured, such as Figure 1 As shown, six independent stress components are calculated from the strain values ​​of the object to be measured in different directions collected by the stress monitoring device. 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 the object and can deform synchronously, thereby realizing effective low-stress monitoring.

[0036] like Figure 2 As shown, the stress monitoring device includes a base and a strain rosette mounted on the base. The base is a cylindrical structure that facilitates the establishment of 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 coordinate axis origin. Through the established coordinate system, the direction of the strain value in the strain rosette can be quickly obtained, facilitating the subsequent calculation of the total stress.

[0037] The strain gauge rosettes of this embodiment include six groups, with two groups serving as backup to prevent damage due to installation, strain gauge quality, or age. The strain gauge rosettes specifically include strain gauge rosettes A1, A2, B1, B2, C1, and C2. Strain gauge rosettes with the same letter are backups, and the strain gauges are installed in the same orientation. Strain gauge rosettes A1, B1, and C1 are installed on the same annular ring of a cylinder. Strain gauge rosette A1 is located on the y-axis of the coordinate system o-xyz. Strain gauge rosette B1 is 60° away from strain gauge rosette A1. In this embodiment, strain gauge rosette B1 is installed on the side of a cylindrical ring that is deflected 60° from the y-axis toward the x-axis. Strain gauge rosette C1 is then installed again, deflected 60° in the same direction. Strain gauge rosettes A2, B2, and C2 are 180° away from their corresponding strain gauge rosettes A1, B1, and C1, i.e., they are located at opposite ends of the same diameter. Figure 2 As shown in (b), six groups of strain rosettes are evenly distributed on the surface of the cylindrical base, 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] To ensure that the full stress monitoring device can accurately monitor strain values ​​and avoid the loss of strain data and the inability to calculate stress values ​​due to damage to a set of strain rosettes or a strain gauge, two sets of strain rosettes are arranged on the same circumferential diameter, with the strain gauges in the two sets arranged in the same direction. Similarly, the spare strain gauges of this embodiment can also be arranged on another circular ring at equal intervals along the z-axis, or two sets of spare strain rosettes can be provided.

[0039] Each rosette consists of three resistive strain gauges, with adjacent resistive strain gauges spaced 45° apart. Taking rosette A1 as an example, the 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 and second strain gauges, forming a 45° angle with each gauge.

[0040] There are 18 resistance strain gauges in total in the six groups of strain rosettes. Each resistance strain gauge is numbered separately, such as Figure 2As shown in Figure (c), six sets of strain rosettes are bonded to the outer surface of a cylindrical substrate via colloid. The elastic constants (elastic modulus and Poisson's ratio) of the colloid, cylindrical substrate, and object to be measured are consistent. The elastic modulus is measured by conducting uniaxial compression tests on standard cylindrical or cubic specimens to be measured. The Poisson's ratio can be obtained by taking the average value of the ratio of the axial strain to the radial strain measured by each set of strain rosettes. The deformation on the fault plane of the object to be measured is monitored by the six sets 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. The two strain values ​​in the same direction are averaged; thus, strain values ​​in nine different directions are obtained. The outer circumference of the cylindrical substrate is considered to be in a plane stress state. 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] Where, 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 rosettes, σ θ , σ z , τ θz are the circumferential normal stress, axial normal stress, and shear stress at each set of rosette measuring points; E is the elastic modulus of the cylindrical substrate at the measuring point; υ is the Poisson's ratio of the cylindrical substrate at the measuring point; G is the shear modulus of the cylindrical substrate at the measuring point.

[0045] Calculate the stress σ 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] Where, 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 rosettes, respectively. 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 rosette measuring points; E is the elastic modulus of the cylindrical substrate 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 gauge rosettes can be used to generate three equations, and a total of 18 equations can be obtained from the six sets of strain gauge rosettes. Since the two strain values ​​in the same direction are averaged and then used in the calculation, the six sets of strain gauge rosettes can generate nine independent equations, which can then be solved for the six independent stress components at the measuring point.

[0050] Step 3: Use the least squares method to obtain the specific relationship between the strain measured by the strain rosette and the three-dimensional stress components. Let the strain values ​​in nine different directions obtained by the six groups of strain rosettes be ε′1, ε′2, ε′3, ε′4, ε′5, ε′6, ε′7, ε′8, and ε′9, respectively. 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′6 ε′7ε′8 ε′9] T ; where σ′ i The strain values ​​ε′ in 9 different directions obtained by 6 sets of strain rosettes i The corresponding stress value.

[0051] Six independent stress components of three-dimensional stress at the measuring point σ x , σ y , σ z , τ xy , τ yz , τ zx Definition matrix: σ=[σ x σ y σ z τ xy τ yz τ zx ] T .

[0052] Since adjacent rosettes are arranged equidistantly in space and the strain gauges are arranged in the same direction, the weighting factors of the strain values ​​in nine different directions obtained by the six groups of rosettes are equal. 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 inclined fault model of the entire object to be measured, the full stress monitoring device can be regarded as a microelement. By using the first three-step monitoring calculation method through the full stress monitoring device, the 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 small tetrahedron DABC with three planes passing through point D and parallel to the coordinate plane, as shown in the following example: Figure 3 (a) shows the component p of the total stress p on plane ABC in the direction of the coordinate axis. 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] Where p x 、p y 、pz The 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 called N, where l, m, and n are the direction cosines of the external normal N on 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 embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any modification and replacement based on the technical solution and inventive concept provided by the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for monitoring full stress on a fault plane, characterized by: 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, and calculating the normal stress and shear stress of the fault plane through the six independent stress components, thereby realizing full stress monitoring of the fault plane; The stress monitoring device includes a cylindrical base structure and six sets of strain rosettes mounted on the base structure. Each set of strain rosettes includes three strain gauges, which respectively collect axial strain values, 45° direction strain values, and circumferential strain values. The base structure is a cylindrical structure. The coordinate system o-xyz is established with the axial direction of the cylindrical structure as the Z-axis direction and the center of the bottom surface as the coordinate axis origin. The six sets of strain rosettes are strain rosette A1, strain rosette A2, strain rosette B1, strain rosette B2, strain rosette C1, and strain rosette C2. The six sets of strain rosettes are paired and 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. The strain rosette A1 Strain rosettes B1, C1 are installed on the same circular ring of the cylinder. Strain rosette A1 is located on the y-axis of the coordinate system o-xyz. The angles between strain rosette B1 and strain rosette A1 are 60°. Strain rosette B1 is installed on the side of the cylindrical ring with the y-axis deflected 60° toward the positive x-axis. Strain rosette C1 is installed with the deflection direction deflected 60° again. Strain rosettes A2, B2, and C2 are 180° apart from the corresponding strain rosettes A1, B1, and C1. The six groups of strain rosettes are evenly distributed on the surface of the cylindrical base, and the angles between each group of strain rosettes are 60°. In the coordinate system o-xyz, the angles between the six groups of strain rosettes and the coordinate x-axis are θ A1 =90°、θ B1 =30°,θ C1 =-30°,θ A2 =-90°、θ B2 =-150°,θ C2 =-210°; The six independent stress components are calculated by the strain values ​​of the tested object in different directions collected by the stress monitoring device, including: Step 1: Each group of strain rosettes collects 3 strain values ​​ε 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 τ θz ; Strain value ε 0° , ε 45° , ε 90° and stress σ θ , σ z , τ θz The relationship is: Where, 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 rosettes, σ θ , σ z , τ θz are the circumferential normal stress, axial normal stress, and shear stress at each set of rosette measuring points, respectively; E is the elastic modulus of the cylindrical substrate at the measuring point; υ is the Poisson's ratio of the cylindrical substrate at the measuring point; G is the shear modulus of the cylindrical substrate at the measuring point; Step 2: Circumferential normal stress σ calculated according to 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 ; Where, 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 rosettes, respectively. 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 rosette measuring points; E is the elastic modulus of the cylindrical substrate at the measuring point; υ is the Poisson's ratio of the cylindrical substrate at the measuring point; θ is the angle between the rosette and the x-axis; Step 3: Use the least squares method to establish a functional relationship between the strain value measured by the rosette and the three-dimensional stress components. Let the strain values ​​in 9 different directions obtained by 6 groups of rosettes be ε1′, ε2′, ε3′, ε4′, ε5′, ε6′, ε7′, ε8′, ε9′, and define the matrix: σ′=[σ1′σ2′σ3′σ4′σ5′σ6′σ7′σ8′σ9′] T =E[ε1′ε2′ε3′ε4′ε5′ε6′ε7′ε8′ε9′] T ; Among them, σ i ′ is the strain value ε in 9 different directions obtained by 6 sets of strain rosettes i ′ corresponds to the 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 Definition 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-v 2 ) 2 ; a5+a6=E[2(ε1′+ε4′+ε7′)-2v(ε2′+ε5′+ε8′)-v(ε3′+ε6′6+ε9′)];a5-a6=E(1-υ 2 (4ε1′+2ε3′-2ε4′-ε6′6-2ε7′-ε9′); 2. A method for monitoring full stress on a fault surface 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.

3. The method for monitoring full stress of a fault plane according to claim 1, characterized in that: The normal and shear stresses 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: Where p x 、p y 、p z are the stress components of the total stress p in the directions of the three coordinate axes; l, m, and 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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