CN115655133B - Ground stress measurement method based on optical fiber strain sensing pipe string - Google Patents

Abstract

The invention belongs to the technical field of ground stress measurement, and particularly relates to an optical fiber strain sensing pipe column and a ground stress measurement method, wherein the optical fiber strain sensing pipe column comprises a pipe column body and an optical fiber sensor, the pipe column body is provided with an inlet end for connecting a liquid injection pressurizing system, and strain can be generated by the pipe column body according to internal and external pressure changes; the optical fiber sensor is adhered to the peripheral wall of the pipe column body and can measure the strain of the pipe column body, and a plurality of sensing points which are arranged at intervals and positioning marks for marking the space positions of the sensing points are arranged on the optical fiber sensor. According to the invention, the winding type optical fiber strain sensing pipe column is adopted, the optical fiber sensor with high spatial resolution is installed in the stratum, different ground stress states of the positions of different sensing sections can be calculated according to the strain measured by different sensing points on the optical fiber strain sensing pipe column, and the real-time measurement of multiple layers of ground stress is realized. Compared with the traditional ground stress measuring method, the method is simpler to operate, the ground stress state of the continuous time-space domain can be measured, and the ground stress measuring capability is greatly improved.

Description

Ground stress measurement method based on optical fiber strain sensing string

Technical Field

The present invention belongs to the technical field of ground stress measurement, and in particular relates to a ground stress measurement method based on an optical fiber strain sensing column.

Background technique

Geostress refers to the in-situ geostress state of the stratum. In geotechnical engineering fields such as drilling engineering, mining engineering, and road engineering, geostress, as one of the key engineering parameters, has received extensive attention. Therefore, how to accurately measure and predict geostress has always been a research hotspot in the field of geotechnical engineering.

However, traditional geostress measurement can only perform static measurement at a single point, and the measurement result can only represent the geostress state at a certain time and location. After the cementing or production of the oil and gas well is completed, geostress measurement cannot be performed again; and the higher the spatial resolution of geostress measurement, the more points need to be measured, and the traditional method can only measure the geostress state at a certain point or a certain depth; in addition, the traditional geostress measurement method requires a series of complex operations such as coring, drilling, lowering sensors, hydraulic fracturing, and embedding flat jacks, which makes the measurement difficult.

Summary of the invention

The main purpose of the present invention is to propose a ground stress measurement method based on an optical fiber strain sensing column, aiming to solve the technical problem that the ground stress measurement method in the prior art can only perform single-point static measurement and has high operation difficulty.

In order to achieve the above object, the present invention provides a method for measuring ground stress based on an optical fiber strain sensing column, wherein the optical fiber strain sensing column comprises:

A pipe column body, provided with an inlet port for connecting to a liquid injection pressurization system, the pipe column body being capable of generating strain according to changes in internal and external pressures; and

The optical fiber sensor is bonded to the outer peripheral wall of the pipe column body and can measure the strain of the pipe column body. The optical fiber sensor is provided with a plurality of sensing points arranged at intervals and positioning marks for marking the spatial positions of the plurality of sensing points.

The ground stress measurement method is applied to the optical fiber strain sensing string, and the ground stress measurement method includes:

Step S10: preparing an optical fiber strain sensing column and obtaining a strain correction coefficient of the optical fiber strain sensing column;

Step S20: burying the optical fiber strain sensing string in the rock mass;

Step S30: applying different preset pressures to the optical fiber strain sensing tube column, and measuring the actual strain value of the outer wall of the optical fiber strain sensing tube column under the different preset pressures according to the strain correction coefficient;

Step S40: obtaining formation elastic parameters according to the actual strain value;

Step S50: Obtain the overlying formation pressure, and calculate the static in-situ stress of the rock mass according to the overlying formation pressure and formation elastic parameters.

In the embodiment of the present invention, the outer peripheral wall of the pipe column body is provided with a thread-shaped shallow groove, and the optical fiber sensor is located in the shallow groove and spirally wound around the outer peripheral side of the pipe column body along the shallow groove.

In an embodiment of the present invention, the optical fiber strain sensing column further includes a film wrapped on the outer wall of the column body.

In the embodiment of the present invention, step S10 includes:

Sealed fiber optic strain sensing column;

Applying a preset pressure to the optical fiber strain sensing string through a liquid injection pressurization system, and calculating the actual value of the corrected strain of the optical fiber strain sensing string according to the preset pressure;

Record the corrected strain measurement value measured at each sensing point;

The strain correction coefficient can be obtained by linearly correcting the measured value of the correction strain and the actual value of the correction strain using analytical methods or numerical simulation methods.

In the embodiment of the present invention, step S20 includes:

Drilling installation holes into the rock mass;

The optical fiber strain sensing tube column is placed in the installation hole, and cement is poured between the inner wall surface of the installation hole and the outer wall surface of the optical fiber strain sensing tube column.

In the embodiment of the present invention, step S30 includes:

Inject high-pressure liquids of different preset pressures into the optical fiber strain sensing column through a liquid injection and pressurization system, so that different internal pressures are generated inside the optical fiber strain sensing column;

The optical fiber sensor is used to sense the strain of the optical fiber strain sensing pipe string, and the strain measurement value of the optical fiber strain sensing pipe string under different internal pressures is obtained;

The actual strain value is obtained based on the strain measurement value and the strain correction coefficient.

In the embodiment of the present invention, step S40 includes:

Step S41: obtaining a strain assumption value and a formation elastic parameter assumption value according to a functional relationship between a preset pressure of the optical fiber strain sensing string, an actual strain value and a formation elastic parameter;

Step S42: Repeat step S41 until the error between the assumed strain value and the actual strain value reaches a preset error range, and the assumed value of the formation elastic parameter is the actual formation elastic parameter of the rock mass.

In the embodiment of the present invention, step S50 includes:

The ratio between the static geostress and the overlying formation pressure is obtained by using the linear relationship between the formation elastic parameters and the static geostress, and the static geostress is obtained according to the overlying formation pressure and the ratio.

In the embodiment of the present invention, the linear relationship between the formation elastic parameter and the static ground stress is:

Among them, _Ex , _Ey , _Ez , _υxy , _υxz , and _υyz are all formation elastic parameters; _σx and _σy are horizontal ground stresses; _σz is the overlying formation pressure.

Through the above technical solution, the ground stress measurement method based on optical fiber strain sensing pipe string provided by the embodiment of the present invention has the following beneficial effects:

Compared with the traditional ground stress measurement, when the fiber optic strain sensing string of the present invention is used to measure ground stress, it is possible to continuously sense the dynamic changes of ground stress in different time periods; and a winding fiber optic strain sensing string is used to install a high spatial resolution fiber optic sensor into the formation. Since the fiber optic sensor has multiple sensing points and the spatial sensing resolution is as high as 1mm, it can achieve high spatial resolution multi-point measurement of ground stress. In addition, after the fiber optic strain sensing string is buried in the formation rock mass, it can be used as a production string, a structural string, a fluid flow channel or a structural member in the rock mass, etc., to remain in the formation to achieve continuous sensing of the ground stress state in different time periods without additional operation.

Other features and advantages of the present invention will be described in detail in the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are used to provide an understanding of the present invention and constitute a part of the specification. Together with the following specific embodiments, they are used to explain the present invention, but do not constitute a limitation of the present invention. In the accompanying drawings:

FIG1 is a schematic flow chart of a method for measuring ground stress according to an embodiment of the present invention;

FIG2 is a schematic diagram of the structure of an optical fiber strain sensing column according to an embodiment of the present invention;

FIG. 3 is a response curve diagram of the actual strain value to the preset pressure according to an embodiment of the present invention.

Description of Reference Numerals

Label name Label name 10 Fiber Optic Sensor 20 String body 11 Sensing point

Detailed ways

The specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

The ground stress measurement method based on the optical fiber strain sensing string according to the present invention is described below with reference to the accompanying drawings.

As shown in FIG2 , in an embodiment of the present invention, a fiber optic strain sensing column is provided, wherein the fiber optic strain sensing column includes a column body 20 and a fiber optic sensor 10, the column body 20 is provided with an inlet end for connecting to a liquid injection pressurization system, and the column body 20 can generate strain according to changes in internal and external pressures; the fiber optic sensor 10 is bonded to the outer peripheral wall of the column body 20 and can measure the strain of the column body 20, and the fiber optic sensor 10 is provided with a plurality of sensing points 11 arranged at intervals and positioning marks for marking the spatial positions of the plurality of sensing points 11.

When the optical fiber strain sensing column of the present invention is used to measure ground stress, the dynamic changes of ground stress in different time periods can be continuously sensed; and a winding optical fiber strain sensing column is used to install a high spatial resolution optical fiber sensor 10 into the formation. Since the optical fiber sensor 10 has multiple sensing points 11 and the spatial sensing resolution is as high as 1 mm, multi-point measurement of ground stress with high spatial resolution can be achieved.

When preparing the optical fiber strain sensing column, the optical fiber sensor 10 is firstly spirally wound on the outer peripheral wall of the column body 20, and the optical fiber sensor 10 is fixed on the column body 20 by adhesives such as 502 glue or phenolic resin. At this time, the optical fiber sensor 10 is wrapped on the outer wall surface of the column body 20, and a strain correspondence relationship is established between the optical fiber sensor 10 and the column body 20, so that the optical fiber sensor 10 can synchronously sense the outer surface strain of the column body 20.

When winding the optical fiber sensor 10, two methods can be used to wind and install the optical fiber sensor 10: groove positioning or fixed spiral angle winding. The groove positioning method is: through mechanical processing, a threaded shallow groove is machined on the outer peripheral wall of the pipe column body 20, and the optical fiber sensor 10 is installed in the shallow groove and spirally wound on the outer peripheral side of the pipe column body 20 along the shallow groove. This method can achieve the installation and positioning of the optical fiber sensor 10 under the condition of slight influence on the strength of the pipe column body 20, and the shallow groove can be used to protect the optical fiber sensor 10 from the influence of the adhesive material on the external interface, thereby improving the sensing stability of the optical fiber sensor 10. The fixed spiral angle winding method is: using the optical fiber winding equipment, with a certain spiral angle, the optical fiber sensor 10 is stably fixed on the pipe column body 20 at a preset position. The advantage of the fixed spiral angle method is that it does not have an additional impact on the strength and stress distribution of the structural parts of the pipe column body 20, and the advantage of the groove positioning method is that it can better protect the stability of the optical fiber strain sensing.

Due to the physical characteristics of the optical fiber sensor 10, there are multiple sensing points 11 arranged at equal intervals on the optical fiber sensor 10. The optical fiber sensor 10 reflects the strain of the pipe column body 20 by measuring the spatial position of the sensing points 11. Therefore, the initial distribution position of the sensing points 11 on the outer peripheral wall of the pipe column body 20 should be marked to clarify the initial position of the sensing points 11, so as to obtain the strain of the pipe column body 20 in the external change of the sensing points 11. The specific operation is: after the optical fiber sensor 10 and the pipe column body 20 are bonded, the sensing point 11 is marked by the point pressure method or the liquid nitrogen point spraying method. Among them, the point pressure method uses a sharp rigid object such as a probe to press and mark a point on the optical fiber sensor 10, so that the pressed point generates local strain, and the specific spatial position of the next sensing point 11 is obtained by using the interval distance between the sensing points 11, and then the spatial position of each subsequent sensing point 11 is deduced; the liquid nitrogen point spraying method is similar to the point pressure method, and uses the liquid nitrogen point spraying method to make the optical fiber sensor 10 locally produce obvious frequency shift, thereby achieving marking.

After the optical fiber sensor 10 is installed on the pipe column body 20 , the pipe column body 20 needs to be wrapped with a film to shield the influence of the interface bonding material and ensure the measurement accuracy of the optical fiber sensor 10 .

It should be noted that in the prior art, the differential strain method, as a commonly used indoor test method, is widely used in geostress measurement. Its theoretical basis is the memory of microcracks in the stratum: when a wellbore is formed on the rock mass, the equilibrium state controlled by the far-field stress of the well wall and the drilled rock mass is destroyed, stress release occurs, and a new stress equilibrium state is formed. The large number of microcracks contained in the rock mass are affected by stress release, and displacements such as expansion, compression, and shear occur, thereby affecting the rock elastic parameters of the well wall and the rock mass. Therefore, the geostress state is contained in the rock elastic parameters. The experimental object of the traditional differential strain method is the underground core taken to the ground. First, a strain gauge is attached to the surface of the core, and then hydrostatic pressure is applied to the rock mass. The strain during the pressurization process is recorded, and the far-field stress ratio is represented by calculating the principal stress ratio, and then the far-field stress is calculated.

With the development of sensing technology, key technical indicators such as measurement accuracy and spatial resolution of fiber optic sensing technology have been greatly improved. Compared with traditional strain measurement methods, fiber optic sensing technology has the advantages of high spatial resolution, high frequency, and high precision. Its application in the field of geotechnical engineering has become one of the new research hotspots. The present invention combines fiber optic strain sensing technology with differential strain method to invent a new method for measuring ground stress, which can be applied to a variety of measurement scenarios such as indoor tests and underground measurements.

The theoretical basis of the present invention is the same as the traditional differential strain method theory, that is, the micro-cracks of the rock have memory, and the ground stress state information can be stored in the elastic parameters of the rock, and then the elastic mechanical parameters of the rock can be used to invert the ground stress. The core component of the ground stress measurement method in the present invention is the above-mentioned optical fiber strain sensing column. When measuring ground stress, it is first necessary to open a wellhead on the formation rock mass, and then bury the optical fiber strain sensing column from the wellhead into the formation, and use cementing cement to bond the optical fiber strain sensing column to the formation, thereby forming a stress transfer relationship. At this time, the strain sensed by the optical fiber sensor 10 is the strain at the interface between the column body 20 and the cement. Since the ground stress state of the formation has been released before the column is lowered, a new equilibrium state is formed, so after the optical fiber strain sensing column is lowered into the formation, no strain signal is generated. At this time, the internal pressure method is used to artificially inject high-pressure liquid into the interior of the column body 20, so that the optical fiber sensor 10 generates a strain signal that can reflect the elastic parameters of the formation.

The internal pressure method can be used to obtain the strain value at the outer surface of the pipe column body 20 under different internal pressures. It is worth noting that since the optical fiber sensor 10 has a high spatial resolution characteristic, the sensing pipe column can obtain a large number of strain sensing results at different times and in different spaces. At this time, the strain sensing results can be inverted to obtain the change curve of the formation elastic parameters with the internal pressure of the pipe column body 20, and then inverted to obtain the ground stress of a certain period of time. In addition, the invention can obtain derivative parameters such as anisotropic elastic parameters of rocks.

After obtaining the ground stress of a certain period of time, since the optical fiber strain sensing pipe string is buried in the formation rock mass, it can be used as a production pipe string, a structural pipe string, a fluid flow channel or a structural part in the rock mass and remain in the formation. When the ground stress in the formation changes over time, the outer surface of the optical fiber strain sensing pipe string will produce strain and send out corresponding strain signals. Continuous monitoring of the strain signal can invert the ground stress change, and then obtain the dynamic ground stress change results on different time axes, so as to realize the continuous sensing of the ground stress state in different time periods.

It should be noted that in order to form the internal high pressure of the tubular body 20, it is necessary to ensure that the tubular body 20 is a sealed structure, so that physical isolation needs to be formed at both ends of the tubular body 20. According to different ground stress measurement environments, the tubular body 20 can be sealed by means of isolation by a packer or end welding.

As shown in FIG. 1 , in order to further understand the technical solution of the present application, the overall working steps of the ground stress measurement method are described in detail as follows. The ground stress measurement method of the present invention includes:

Step S10: preparing an optical fiber strain sensing column and obtaining a strain correction coefficient of the optical fiber strain sensing column;

Step S20: burying the optical fiber strain sensing string in the rock mass;

Step S30: applying different preset pressures to the optical fiber strain sensing column, and measuring the actual strain value of the outer wall of the optical fiber strain sensing column under different preset pressures according to the strain correction coefficient, wherein the preset pressure ranges from about 0 to 25 MPa;

Step S40: obtaining formation elastic parameters according to the actual strain value;

Step S50: Obtain the overlying formation pressure, and calculate the static in-situ stress of the rock mass according to the overlying formation pressure and formation elastic parameters.

Compared with the traditional ground stress measurement method, the present invention adopts the optical fiber strain sensing pipe string as the sensing medium to sense the strain of the formation rock mass, and then invert the formation elastic parameters and ground stress. The ground stress measurement method of the present invention can continuously sense the dynamic changes of ground stress in different time periods; and the winding optical fiber strain sensing pipe string is used to install the optical fiber sensor 10 with high spatial resolution into the formation. Since the optical fiber sensor 10 has multiple sensing points 11 and the spatial sensing resolution is as high as 1mm, it can realize multi-point measurement of ground stress with high spatial resolution; in addition, the ground stress measurement method of the present invention has low operation difficulty. After the optical fiber strain sensing pipe string is buried in the formation rock mass, it can be used as a production pipe string, a structural pipe string, a fluid flow channel or a structural member in the rock mass, etc., and it remains in the formation. When it is necessary to measure the dynamic value of ground stress in different time periods, it is only necessary to perform a new round of inversion on the formation elastic parameters through the re-measured actual strain value, and then the dynamic ground stress change results on different time axes can be obtained, so as to realize the continuous sensing of the ground stress state in different time periods. The invention effectively solves the technical problem that the ground stress measurement method in the prior art can only perform static measurement of the ground stress in a single time period without the need for additional operations.

In the embodiment of the present invention, step S10 includes preparing an optical fiber strain sensing column and obtaining a strain correction coefficient of the optical fiber strain sensing column. Since the strain of the optical fiber body is a Rayleigh backscattered light frequency shift value, the measured value obtained by the optical fiber sensor 10 is different from the actual strain value and there is a linear relationship between the two, so it is necessary to use the strain correction coefficient to correct the measured value to the actual strain value of the outer surface of the column body 20.

At this time, the internal pressure method is required to obtain the strain correction coefficient. Specifically, a preset pressure is first applied to the optical fiber strain sensing column through the injection pressurization system, and the spatial strain of the internal pressure column is calculated according to the preset pressure using an analytical solution or a numerical simulation method to obtain the actual value of the correction strain of the optical fiber strain sensing column, and then the correction strain measurement value measured at each sensing point 11 is recorded, and finally the correction strain measurement value and the correction strain actual value are linearly corrected using an analytical method or a numerical simulation method to obtain the strain correction coefficient.

After obtaining the strain correction coefficient, the strain measurement value detected by the optical fiber sensor 10 can be converted into the actual strain value, which greatly increases the detection accuracy of the optical fiber sensor 10 on the strain of the pipe body 20, thereby making the measurement effect of the ground stress measurement method in this application more accurate.

In the embodiment of the present invention, step S20 includes:

An installation hole is drilled in the rock mass, and then the optical fiber strain sensing pipe column is placed in the installation hole, and cement is poured between the inner wall surface of the installation hole and the outer wall surface of the optical fiber strain sensing pipe column. The optical fiber strain sensing pipe column and the formation are bonded by cement, so that a seamless connection is achieved between the optical fiber strain sensing pipe column and the formation, and then a stress-strain transmission relationship is formed between the optical fiber strain sensing pipe column and the formation, so that the optical fiber strain sensing pipe column can be synchronized with the strain of the formation, so that the strain measured by the optical fiber sensor 10 can better reflect the ground stress state in the formation.

In the embodiment of the present invention, step S30 includes:

High-pressure liquids of different preset pressures are injected into the optical fiber strain sensing column through a liquid injection and pressurization system to generate different internal pressures inside the optical fiber strain sensing column; an optical fiber sensor 10 is used to sense the strain of the optical fiber strain sensing column, and the strain measurement values of the optical fiber strain sensing column under different internal pressures are obtained; finally, the actual strain value of the optical fiber strain sensing column is obtained based on the strain measurement value and the strain correction coefficient obtained previously.

At this time, after the preset pressure is applied in the optical fiber strain sensing column, the combination of the column body 20-concrete bonding layer-formation is deformed at the same time, and the formation produces strain under the preset pressure of the optical fiber strain sensing column, and a response curve of the actual strain value to the preset pressure is obtained as shown in Figure 3.

Multiple sensing points 11 on the optical fiber strain sensing column can measure multiple actual strain values, which are collectively reflected in Figure 3. Detecting multiple sensing points 11 at different spatial positions can more accurately reflect the relationship between the actual strain value and the preset pressure, thereby improving the accuracy of the measurement experiment.

In the embodiment of the present invention, step S40 is the inversion process of formation elastic parameters, including:

Step S41: according to the relationship between the preset pressure and actual strain value of the optical fiber strain sensing string and the formation elastic parameters, the strain assumption value and the formation elastic parameter assumption value can be obtained by using the finite element method;

Step S42 is: continuously repeating step S41 to obtain new strain assumption values and formation elastic parameter assumption values until the error between the strain assumption value and the actual strain value reaches the tolerance range, and it can be considered that the actual formation elastic parameter of the rock mass is the same as the formation elastic parameter assumption value.

Among them, for the inversion process of formation elastic parameters under different preset pressures, the inversion process of formation elastic parameters can be performed using two methods: numerical method and analytical method. Among them, the numerical method is to use numerical simulation software or algorithm to invert the formation elastic parameters; the analytical method is a method of inverting elastic mechanical parameters using an analytical solution model. Since the theoretical basis of the implementation of the numerical method and the analytical method is based on the above step S40, the numerical method and the analytical method will not be described in detail here.

In addition, the inversion process of formation elastic parameters can be arithmetically optimized by using optimization methods, machine learning methods, and deep learning methods. Among them, the optimization methods include evolutionary algorithms such as particle swarm method, simulated annealing method, snake optimization, and whale optimization. Common machine learning methods include random forests, support vector machines, etc. Deep learning methods include BP neural networks and neural networks combined with optimization algorithms. Within the technical concept of the present invention, the inversion process of elastic mechanics can be optimized in various ways, including combining the above-mentioned specific optimization methods in any suitable way, or other conventional optimization algorithms. The present invention will not further explain various possible algorithms. However, these algorithms should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

It should be noted that before the wellhead is opened in the formation and the optical fiber strain sensing string is buried, the formation is squeezed by the stress around the wellbore, causing the formation cracks to be in an over-closed state. After the optical fiber strain sensing string is buried in the formation from the wellbore, during the internal pressurization and expansion process, the formation cracks around the wellbore are reopened by force, resulting in the formation density becoming smaller and smaller during the pressurization process, which is reflected in Figure 3 as the increasing slope of the curve.

Specifically, the curve in Figure 3 can be divided into a first linear region, a nonlinear region and a second linear region in sequence as the preset pressure increases. When the preset pressure is in the first linear region, the formation fractures are in an over-closed state, the formation elastic parameters are constant, the slope of the curve remains unchanged, and the formation is in an isotropic state; when the preset pressure is in the nonlinear region, the formation fractures are in a gradually opening state, the formation elastic parameters change nonlinearly, and the slope of the curve gradually increases; when the preset pressure is in the second linear region, the formation fractures are in a fully open state, the formation elastic parameters are constant, the slope of the curve remains unchanged, and the formation is in an orthogonal anisotropic state.

By inverting the formation elastic parameters of the data in the first linear region in step S40, the formation elastic parameters can be obtained: E, υ, where E is the isotropic formation elastic modulus; υ is the isotropic formation Poisson's ratio.

By inverting the formation elastic parameters of the data in the second linear region in step S40, the formation elastic parameters can be obtained: _Ex , _Ey , _Ez , _υxy , _υxz , _υyz , where _Ex , _Ey , _Ez are orthotropic formation elastic moduli; _υxy , _υxz , _υyz are orthotropic formation Poisson's ratios.

In the embodiment of the present invention, step S50 includes:

Combined with the overlying formation pressure, the ratio between the static in-situ stress and the overlying formation pressure is obtained by using the linear relationship between the formation elastic parameters and the in-situ stress, and the static in-situ stress of the formation is obtained based on the overlying formation pressure and the obtained ratio.

The overlying formation pressure can be calculated by the following formula:

σ _z ＝Hρ _r g

Wherein, _σz is the overlying formation pressure, H is the depth of the sensing point 11 in the formation; _ρr is the total density of the formation, _which can be measured by density logging; and g is the gravitational acceleration.

The linear relationship between formation elastic parameters and ground stress is:

Wherein, σ _x and σ _y are horizontal geostresses. According to the above formula and the formation elastic parameters obtained in step S40: _Ex , Ey, _Ez , _vxy , _{vxz , vyz} , the ratios of σ _x to σ _z and σ _y to σ _z are finally obtained respectively, and then the specific values of horizontal geostresses σ _x and σ _y can be obtained, and the measurement of geostress (including horizontal geostress and overlying formation pressure) is completed.

In general, the present invention can calculate the different ground stresses at the locations of different sensing points 11 according to the strains measured at different sensing points 11 on the optical fiber strain sensing column, thereby realizing multi-point measurement of ground stress, and does not require the complicated operations of traditional measurement methods, greatly reducing the difficulty of measuring ground stress.

It should be noted that the geostress and formation elastic parameters in the formation are not a constant value, but are constantly changing with time. The geostress measurement method of the present invention can not only measure the static geostress of a certain period of time, but the optical fiber strain sensing tube can be used as a production string, a structural string, a fluid flow channel or a structural member in the rock mass and remain in the formation. When the geostress and formation elastic parameters in the formation change with time, the outer surface of the optical fiber strain sensing string will produce strain and continuously measure new actual strain values. A new round of inversion can be performed on the formation elastic parameters and geostress to obtain the formation elastic parameters on different time axes, and to achieve continuous sensing of the geostress state in different time periods. It effectively solves the technical problem that the geostress measurement method in the prior art can only perform static measurement of geostress in a single time period.

In the description of the present invention, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral one; it can be a mechanical connection, an electrical connection, or communication with each other; it can be a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims (7)

1. The method for measuring the ground stress based on the optical fiber strain sensing tubular column is characterized by comprising the following steps of:

The pipe column body (20) is provided with an inlet end used for being connected with a liquid injection pressurizing system, and the pipe column body (20) can generate strain according to internal and external pressure changes; and

The optical fiber sensor (10) is adhered to the peripheral wall of the tubular column body (20) and can measure the strain of the tubular column body (20), and a plurality of sensing points (11) which are arranged at intervals and positioning marks for marking the space positions of the plurality of sensing points (11) are arranged on the optical fiber sensor (10);

the method for measuring the ground stress is applied to the optical fiber strain sensing tubular column, and comprises the following steps:

Step S10: preparing an optical fiber strain sensing tubular column, and acquiring a strain correction coefficient of the optical fiber strain sensing tubular column;

step S20: burying the optical fiber strain sensing tubular column into a rock mass;

step S30: applying different preset pressures to the optical fiber strain sensing tubular column, and measuring actual strain values of the peripheral wall of the optical fiber strain sensing tubular column under the different preset pressures according to the strain correction coefficient;

Step S40: acquiring stratum elasticity parameters according to the actual strain value;

Step S50: obtaining a ratio between the overburden formation pressure and the static ground stress by utilizing a linear relation between the static ground stress and the formation elastic parameter, obtaining the overburden formation pressure, and obtaining the static ground stress according to the overburden formation pressure and the ratio;

wherein the linear relationship is:

Wherein E _x、E_y、E_z、υ_xy、υ_xz、υ_yz is stratum elasticity parameter; σ _x、σ_y is the horizontal ground stress; σ _z is the overburden pressure.

2. The method for measuring the ground stress based on the optical fiber strain sensing tubular column according to claim 1, wherein the outer peripheral wall of the tubular column body (20) is provided with a threaded shallow notch, and the optical fiber sensor (10) is positioned in the shallow notch and spirally wound on the outer peripheral side of the tubular column body (20) along the shallow notch.

3. The method of claim 2, further comprising wrapping a film around an outer wall of the column body (20).

4. The method for measuring the ground stress based on the optical fiber strain sensing tubular column according to claim 1, wherein the step S10 comprises:

sealing the fiber optic strain sensing tubing string;

applying preset pressure into the optical fiber strain sensing tubular column through a liquid injection pressurizing system, and calculating a corrected strain actual value of the optical fiber strain sensing tubular column according to the preset pressure;

recording the corrected strain measurement values measured at each of the sensing points (11);

and linearly correcting the corrected strain measured value and the corrected strain actual value by using an analytic method or a numerical simulation method to obtain a strain correction coefficient.

5. The method for measuring the ground stress based on the optical fiber strain sensing tubular column according to claim 1, wherein the step S20 comprises:

drilling a mounting hole on the rock mass;

And placing the optical fiber strain sensing pipe column in the mounting hole, and pouring cement between the inner wall surface of the mounting hole and the outer wall surface of the optical fiber strain sensing pipe column.

6. The method for measuring the ground stress based on the optical fiber strain sensing tubular column according to claim 1, wherein the step S30 comprises:

Injecting high-pressure liquid with different preset pressures into the optical fiber strain sensing tubular column through a liquid injection pressurizing system so as to enable the interior of the optical fiber strain sensing tubular column to generate different internal pressures;

Sensing the strain of the optical fiber strain sensing tubular column by adopting an optical fiber sensor (10), and obtaining strain measurement values of the optical fiber strain sensing tubular column under the action of different internal pressures;

And obtaining the actual strain value according to the strain measurement value and the strain correction coefficient.

7. The method for measuring the ground stress based on the optical fiber strain sensing tubular column according to claim 1, wherein the step S40 comprises:

Step S41: obtaining a strain assumption value and a formation elasticity parameter assumption value according to a functional relation between the preset pressure and the actual strain value of the optical fiber strain sensing tubular column and the formation elasticity parameter;

Step S42: and repeating the step S41 until the error between the strain assumption value and the actual strain value reaches a preset error range, wherein the formation elasticity parameter assumption value is the actual formation elasticity parameter of the rock mass.