Underground stress measurement device and measurement method based on distributed optical fiber sensing

Abstract

The present invention provides a distributed fiber optic sensing-based underground stress measurement device and method, comprising a metal casing installed in a borehole, with an armored optical cable fixed to its inner wall; the armored optical cable comprises at least one high-temperature-resistant pressure-sensitive optical cable and at least two high-temperature-resistant multimode optical fibers; two separators installed underground, with a sealed in-situ stress measurement section between the two separators; a water injection string for injecting high-pressure water into the underground in-situ stress measurement section; and a DPS / DTS composite modem connected to the high-temperature-resistant pressure-sensitive optical cable and the high-temperature-resistant multimode optical fiber within the armored optical cable. The present invention utilizes hydraulic fracturing to measure the two-dimensional in-situ stress field (σ2, σ1) at different depths point by point along the wellbore. Pressure and temperature sensing armored optical cables can be deployed from the bottom of the wellbore to the wellhead in a single operation, and pressure data can be corrected using temperature data to rapidly, accurately, and reliably measure the two-dimensional in-situ stress field of the rock surrounding the wellbore throughout the entire wellbore, from shallow to ultra-deep wells.

Description

Technical Field

[0001] The present invention belongs to the technical field of ground stress measurement, and in particular relates to a ground stress measurement device and a measurement method based on distributed optical fiber sensing. Background Art

[0002] Fiber-optic sensing technology, originating in 1977, has rapidly developed alongside the advancement of fiber-optic communications. It is a key indicator of a country's level of informatization. It has been widely used in military, defense, aerospace, industrial and mining enterprises, energy and environmental protection, industrial control, medicine and health, measurement and testing, construction, and household appliances, enjoying a broad market. Hundreds of fiber-optic sensing technologies exist worldwide, enabling diverse sensing capabilities for physical quantities such as temperature, pressure, flow, displacement, vibration, rotation, bending, liquid level, velocity, acceleration, sound field, current, voltage, magnetic field, and radiation.

[0003] Downhole fiber optic sensing systems can be used to measure stress, strain, pressure, temperature, noise, vibration, acoustic waves, seismic waves, flow, component analysis, and electric and magnetic fields. The system is based on a fully armored fiber optic cable structure, with both the sensor and the connection and data transmission cables made of optical fiber.

[0004] Currently, there are many methods for measuring ground stress, including direct measurement methods such as hydraulic fracturing, acoustic emission, and borehole collapse, and indirect measurement methods such as casing stress relief and strain recovery.

[0005] The direct measurement method uses instruments to directly measure and record various stresses, such as compensatory stress, restorative stress, and equilibrium stress. The in-situ stress value is then calculated based on the relationship between these stresses and the in-situ stress. The calculation process does not involve conversions between different physical quantities and does not require knowledge of the rock's physical and mechanical properties or stress-strain relationships.

[0006] Direct measurement methods primarily include the flat jack method, hydraulic fracturing, rigid inclusion stress gauges, and acoustic emission. Indirect measurement methods do not directly measure stress. Instead, they utilize certain sensing elements or media to measure and record changes in indirect physical quantities related to stress in the rock mass. These changes can be detected by measuring and recording the rock mass's deformation or strain, changes in its density, permeability, water absorption, resistance, capacitance, and elastic wave propagation velocity. These changes in indirect physical quantities are then used to calculate the rock mass's stress using known theoretical or empirical formulas. Therefore, in order to calculate stress using indirect measurement methods, it is first necessary to determine certain physical and mechanical properties of the rock mass and the relationship between the measured physical quantities and stress.

[0007] Among them, the hydraulic fracturing ground stress measurement method conducts in-situ measurements in the rock mass, and directly measures the stress state in the rock mass without obtaining relevant rock mechanics parameters. It has the advantages of small disturbance to the stress field of the original rock structure, simple equipment, easy operation, strong representativeness of the measured values, and good adaptability. It is widely used in ground stress measurement of various geotechnical engineering projects, especially in ground stress measurement of deep rock engineering projects.

[0008] Hydraulic fracturing is an effective method for measuring the in-situ stress state of deep crustal rock. The test principle of in-situ stress measurement is based on three basic assumptions: (1) crustal rocks are linearly uniform and isotropic elastic bodies; (2) when rocks are porous media, the flow of fluids in the pores conforms to Darcy's law; (3) one of the principal stress directions is parallel to the axis of the borehole. High-pressure water is injected into the closed borehole. When the pressure reaches the maximum value P i After that, the borehole wall will rupture, causing the pressure in the well to drop. In order to keep the crack open, the pressure in the hole will eventually reach a constant value. After no more injection, the pressure in the hole will drop rapidly, the crack will heal, and then the pressure drop rate will slow down. Its critical value is the instantaneous closing pressure P s After the pressure is completely released, the fluid is injected again to obtain the reopening pressure P of the crack. r And instantaneous closing pressure P s Finally, the direction of the crack is recorded by the instrument and the two-dimensional stress field (σ2, σ1) at the borehole stress measurement position is calculated according to the corresponding formula. Summary of the Invention

[0009] The present invention proposes a distributed fiber optic sensing-based underground stress measurement device composed of a metal casing installed in a borehole, an armored optical cable laid on the inner wall of the metal casing, a separator installed underground, a water injection string for injecting high-pressure water into the underground, and a distributed fiber optic pressure sensor / distributed fiber optic temperature sensor (DPS / DTS) composite modulator and demodulator placed near the wellhead. The device uses hydraulic fracturing to measure the two-dimensional geostress field (σ2, σ1) at different depths point by point along the wellbore. This method allows the deployment of pressure and temperature sensing armored optical cables from the bottom of the well to the wellhead in one go, allowing for rapid, accurate, and reliable measurement of the two-dimensional geostress field of the rock surrounding the entire well section, from shallow to ultra-deep wells, providing strong support for underground engineering implementation plans with geostress field data for the entire well section.

[0010] The purpose of the present invention is to overcome the shortcomings of existing downhole ground stress measurement technology. It proposes laying armored optical cables on the inner wall of metal casings and using a hydraulic fracturing ground stress measurement method to construct a measurement system for the distribution changes of ground stress in downhole rock formations based on distributed optical fiber sensing. This system can monitor and measure the damage or destruction that underground stress may cause to downhole casings and various downhole tools and pipelines in real time over a long period of time. This provides an indispensable means, system and method for ensuring the long-term stable, safe and reliable operation of oil and gas production wells, water injection wells and monitoring or observation wells.

[0011] To achieve the above object, the specific technical solutions of the present invention are as follows:

[0012] An underground stress measurement device based on distributed optical fiber sensing includes a metal casing installed in a borehole, the inner wall of the metal casing is provided with an armored optical cable; the armored optical cable includes at least one high-temperature resistant pressure-sensitive optical cable and at least two high-temperature resistant multimode optical fibers;

[0013] The system also includes at least two separators installed downhole, with a sealed ground stress measurement section between the two separators; a water injection string for injecting high-pressure water into the ground stress measurement section downhole, and a high-pressure pump truck for providing a high-pressure water source to the ground stress measurement section; the water injection string is connected to the high-pressure pump truck;

[0014] It also includes a DPS / DTS composite modulation and demodulation instrument placed near the wellhead, and the DPS / DTS composite modulation and demodulation instrument is respectively connected to the high-temperature resistant pressure sensitive optical cable and the high-temperature resistant multimode optical fiber in the armored optical cable.

[0015] The DPS / DTS composite modulation and demodulation instrument is a distributed optical fiber pressure sensing and distributed optical fiber temperature sensing composite modulation and demodulation instrument, which includes a data acquisition module and a modulation and demodulation module.

[0016] The high temperature resistant pressure sensitive optical cable contains high temperature resistant single mode or high temperature resistant special pressure sensitive optical fiber, and the high temperature resistant pressure optical cable and high temperature resistant multimode optical fiber are respectively encapsulated in a continuous first metal tube and a second metal tube.

[0017] The high temperature resistant pressure sensitive optical cable contains a single mode pressure sensitive optical fiber, or a high density continuous grating optical fiber with a spacing of less than 1 meter, or a three-dimensional high density array type Fabry cavity pressure sensor optical fiber with a spacing of 1 meter to 5 meters.

[0018] The first and second metal tubes are further wrapped with a single or multiple layer of protective armored steel wire.

[0019] The first metal tube is equipped with a high-temperature resistant pressure-sensitive optical cable that is tightly wrapped with a high-temperature resistant and high-strength composite material or is made by wrapping an optical fiber in one step using an injection molding machine. The cable is tightly attached to the wall and sealed in the first metal tube. A light extinguisher is installed at the tail end of the high-temperature resistant pressure-sensitive optical cable to eliminate the strong light reflected back from the tail end of the optical fiber.

[0020] The second metal tube is also provided with high temperature resistant optical fiber paste.

[0021] The separator installed underground is of pressurized expansion type, and the pressurized medium is liquid or gas.

[0022] The measuring method of the underground stress measuring device based on distributed optical fiber sensing comprises the following steps:

[0023] (a) Perform perforation operations to penetrate the metal casing at all depths where ground stress measurements are required;

[0024] (b) Slowly lower the armored optical cable into the metal casing so that it is close to the inner wall of the metal casing; at the wellhead, connect the high-temperature resistant pressure-sensitive optical cable inside the armored optical cable to the DPS signal input terminal of the DPS / DTS composite modem instrument; at the tail end of the armored optical cable, fuse the two high-temperature resistant multimode optical fibers together to form a U-shaped structure; at the top end of the armored optical cable, connect the two high-temperature resistant multimode optical fibers to the TDS two-end signal input terminal of the DPS / DTS composite modem instrument;

[0025] (c) two separators are respectively arranged above and below the depth where the in-situ stress measurement is required, and water or air is filled into the separators until the fluid exchange with the well section outside the two separators is completely cut off;

[0026] (d) injecting high-pressure water into the ground stress measurement well section between the two separators through the water injection string;

[0027] (e) Continuously increase the water injection pressure until the rock outside the metal casing begins to crack. The DPS / DTS composite modulation and demodulation instrument demodulates the phase change of the backscattered Rayleigh light caused by the pressure change on the high-temperature resistant pressure-sensitive optical cable inside the armored optical cable in the ground stress measurement section to obtain the initial cracking pressure P of the rock. i ;

[0028] (f) Continue to increase the water injection pressure to expand the rock cracks. When the cracks expand to a depth of three times the wellbore diameter, stop high-pressure water injection and keep the water pressure constant. The DPS / DTS composite modem instrument measures the closing pressure P at this time. s , then release the pressure to close the crack;

[0029] (g) During the entire pressurization process, record the pressure-time curve and flow-time curve simultaneously to determine Pi and P s value;

[0030] (h) Re-inject high-pressure water into the in-situ stress measurement section to reopen the rock cracks. The DPS / DTS composite modem instrument simultaneously measures the pressure P when the cracks reopen. r and the subsequent constant closing pressure P s ;

[0031] (i) Repeat this depressurization-repressurization process 2-3 times to improve the accuracy of the pressure measurement data;

[0032] (j) During the entire process of repeated depressurization and re-pressurization, record the pressure-time curve and flow-time curve simultaneously to determine P r and P s value;

[0033] (j) Use high-temperature resistant multimode optical fiber and DPS / DTS composite modulation and demodulation instruments to monitor and measure the temperature changes in the metal casing of the entire well section in real time. Based on the monitored and measured stress in the metal casing, the temperature change data in the well section is measured. The spectral drift obtained from the strain ε or temperature t response is similar to the drift Δλ of the resonance wave or the spectral drift Δυ of the Bragg grating. The formula is:

[0034] Δλ / λ=-Δυ / υ=K T Δt+K ε ε

[0035] Where λ and υ are the average light wavelength and frequency respectively; K T and K ε are the standard constants for temperature and strain, respectively;

[0036] The temperature value at the specific measurement location is used to correct the DPS measurement data for the drift of the scattered light spectrum in the optical fiber caused by temperature changes, thereby obtaining the true pressure value in the metal casing stress measurement section without the influence of temperature.

[0037] (k) The fracture water pressure at the separation section of the ground stress measurement depth is P0, and the rock tensile strength is T. According to the formula P i =3σ2-σ1+T-P0,P r =3σ2-σ1-P0 and P s =σ2, and the two-dimensional stress field (σ2,σ1) at the ground stress measurement location is calculated.

[0038] The two-dimensional geostresses σ2 and σ1 can be obtained from the last two formulas without knowing the tensile strength of the rock. Therefore, the measurement of in-situ rock stress by hydraulic fracturing will not involve the physical and mechanical properties of the rock, but will be determined entirely by the measured and recorded pressure values.

[0039] The underground stress measurement device and method based on distributed optical fiber sensing provided by the present invention are a low-cost, high-precision, and high-reliability method and technology for measuring stress distribution changes in rock formations throughout the entire well section and for monitoring dynamic changes. The present invention proposes a distributed fiber optic sensing-based underground stress measurement device, which uses a metal casing installed in a borehole, an armored optical cable laid on the inner wall of the metal casing, a separator installed underground, a water injection string for injecting high-pressure water into the underground, and a distributed fiber optic pressure sensor / distributed fiber optic temperature sensor (DPS / DTS) composite modulator and demodulator placed near the wellhead. The device uses a hydraulic fracturing method to measure the two-dimensional geostress field (σ2, σ1) at different depths point by point along the wellbore. This method can be used to lay pressure and temperature sensing armored optical cables from the bottom of the well to the wellhead in one go, quickly, accurately and reliably measuring the two-dimensional geostress field of the rock surrounding the entire well section, from shallow wells to ultra-deep wells. This provides strong support for underground engineering implementation plans with geostress field data for the entire well section, effectively ensuring the long-term stable, safe and reliable operation of oil and gas production wells, water injection wells and monitoring or observation wells, and providing indispensable means, systems and methods for scientific management of oil and gas reservoirs and improving oil and gas recovery. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a schematic diagram of the system structure of the present invention.

[0041] Figure 2 It is a schematic diagram of the metal casing and armored optical cable structure of the present invention.

[0042] Figure 3 It is a schematic diagram of the internal structure (cross section) of the armored optical cable of the present invention.

[0043] Figure 4 It is a schematic diagram of the pump pressure change and characteristic pressure during the fracturing process of the present invention. DETAILED DESCRIPTION

[0044] The embodiments of the present invention are described in detail below with reference to the accompanying drawings. However, these embodiments do not limit the present invention and are merely examples. The advantages of the present invention will become clearer and easier to understand through the description.

[0045] A specific embodiment of a ground stress distribution monitoring system for underground rock formations based on distributed optical fiber sensing of the present invention is as follows:

[0046] like Figure 1As shown, the underground stress measurement device based on distributed fiber optic sensing includes a metal casing 1 installed in a borehole, with an armored optical cable 2 arranged on its inner wall. A DPS / DTS composite modem 3 is placed near the wellhead. A separator 6 is installed downhole, a water injection string 7 for injecting high-pressure water into the well, and a high-pressure pump truck 9 that supplies high-pressure water to the sealed section of the downhole separator. The DPS / DTS composite modem 3 is connected to a high-temperature-resistant pressure-sensitive optical cable 4 and a high-temperature-resistant multimode optical fiber 5 inside the armored optical cable 2.

[0047] like Figure 2 As shown, the DPS / DTS composite modulation and demodulation instrument 3 includes data acquisition and modulation and demodulation functions for distributed fiber optic pressure sensing and distributed fiber optic temperature sensing (DPS / DTS). The armored optical cable 2 includes at least one high-temperature-resistant pressure-sensitive optical cable 4 and at least two high-temperature-resistant multimode optical fibers 5. The high-temperature-resistant pressure-sensitive optical cable 4 contains a high-temperature-resistant single-mode or high-temperature-resistant special pressure-sensitive optical fiber. The high-temperature-resistant pressure-sensitive optical cable 4 and the high-temperature-resistant multimode optical fiber 5 are respectively encapsulated in a continuous first metal tube 41 and a second metal tube 51. A light damper 8 is installed at the tail end of the high-temperature-resistant pressure-sensitive optical cable 4 to eliminate strong light reflected back from the tail end of the optical fiber.

[0048] The high temperature resistant pressure sensitive optical cable 4 may contain a single mode pressure sensitive optical fiber, a high density (spacing less than 1 meter) continuous grating optical fiber, or a high density array type Fabry cavity pressure sensor optical fiber with spacing between 1 meter and 5 meters.

[0049] like Figure 3 As shown, the first capillary metal tube 41 houses a high-temperature-resistant, pressure-sensitive optical cable 4, which is tightly wrapped with a high-temperature-resistant, high-strength composite material or molded using an injection molding machine to encapsulate an optical fiber. The cable is tightly adhered to the wall and sealed within the first capillary metal tube 41. The second capillary metal tube 51, which encapsulates the high-temperature-resistant multimode optical fiber 5, is also coated with a high-temperature-resistant optical fiber paste. The first and second capillary metal tubes 41, 51 are also wrapped with a single layer or multiple layers of protective steel armor wire.

[0050] The separator 6 is of pressurized expansion type, and the pressurized material may be liquid or gas.

[0051] The measuring method of the underground stress measuring device based on distributed optical fiber sensing includes the following steps:

[0052] (a) performing perforation 10 of the metal casing 1 at all depths where ground stress measurement is required;

[0053] (b) Slowly lower the armored optical cable 2 into the metal casing 1 so that it is close to the inner wall of the metal casing 1; at the wellhead, connect the high-temperature resistant pressure-sensitive optical cable 4 in the armored optical cable 2 to the DPS signal input terminal of the DPS / DTS composite modem instrument 3; at the tail end of the armored optical cable 2, fuse the two high-temperature resistant multimode optical fibers 5 together to form a U-shaped structure; at the top end of the armored optical cable 2, connect the two high-temperature resistant multimode optical fibers 5 to the DTS double-ended signal input terminal of the DPS / DTS composite modem instrument 3 ( Figure 2 );

[0054] (c) placing at least two separators 6 above and below the depth where the ground stress measurement is required, and filling the separators 6 with water or air until the fluid exchange with the well section outside the two separators 6 is completely cut off;

[0055] (d) injecting high-pressure water into the ground stress measurement well sections where the two separators 6 are respectively arranged through the water injection string 7;

[0056] (e) Continuously increase the water injection pressure until the rock outside the casing begins to crack. The DPS / DTS composite modulation and demodulation instrument 3 demodulates the phase change of the backscattered Rayleigh light caused by the pressure change on the high-temperature resistant pressure sensitive optical cable 4 inside the armored optical cable 2 in the ground stress measurement section to obtain the initial cracking pressure P of the rock. i ;

[0057] (f) Continue to increase the water injection pressure to expand the rock cracks. When the cracks expand to a depth of three times the wellbore diameter, close the high-pressure water system to maintain a constant water pressure. The DPS modem instrument on the ground measures the closing pressure P at this time. s , then release the pressure to close the crack. The closing pressure at this time is P s0 ;

[0058] (g) During the entire pressurization process, the pressure-time curve is recorded simultaneously ( Figure 4 ) and flow-time curve, determine P i and P s value;

[0059] (h) Re-inject high-pressure water into the sealed well section to reopen the rock cracks. The DPS / DTS composite modem instrument 3 simultaneously measures the pressure P when the cracks reopen. r and the subsequent constant closing pressure P s ;

[0060] (i) Repeat this depressurization-repressurization process 2-3 times to improve the accuracy of the pressure measurement data;

[0061] (j) During the entire process of repeated depressurization and re-pressurization, the pressure-time curve is recorded simultaneously ( Figure 4) and flow-time curve, determine P r and P s value;

[0062] (j) The temperature change in the casing of the entire well section is monitored and measured in real time using the high-temperature resistant multimode optical fiber 5 in the armored optical cable 2 and the DPS / DTS composite modulation and demodulation instrument 3. The pressure (strain) data measured by the DPS / DTS composite modulation and demodulation instrument 3 is temperature-corrected based on the temperature change data in the well section measured by the monitored and measured stress in the metal casing 1.

[0063] When using distributed fiber optic sensors for strain / temperature measurement, wavelength scanning interferometry is used to measure backscattered Rayleigh scattering and use it as a function of position on the fiber. Rayleigh scattering in the fiber is caused by the fluctuation of the refractive index along the length of the fiber. Although the scattering is random, for a given fiber, if the state of the fiber does not change, it will always produce reflected light of the same wavelength. This inherent characteristic is called the inherent texture information of the fiber. If a certain position of the fiber is deformed due to load or temperature, then only the wavelength of the reflected light at that position will deviate. By comparing the reflected light before and after deformation, it is possible to confirm where the deformation occurred in the fiber. Under normal conditions, the drift of the scattered light spectrum in the optical fiber is mainly caused by strain or temperature changes. The spectral drift obtained in response to strain ε or temperature t is similar to the drift Δλ of the resonance wave or the spectral drift Δυ of the Bragg grating, that is: using the formula:

[0064] Δλ / λ=-Δυ / υ=K T Δt+K ε ε

[0065] Where: λ and υ are the average light wavelength and frequency respectively; K T and K ε are the standard constants for temperature and strain, respectively. For most optical fibers with germanosilicate cores, K T =6.45μ℃ -1 , K ε =0.78.

[0066] The temperature value at the specific measurement location is used to correct the DPS measurement data for the drift of the scattered light spectrum in the optical fiber caused by temperature changes, thereby obtaining the true stress measurement pressure value in the metal casing 1 in the well section without the influence of temperature.

[0067] (k) The fracture water pressure at the separation section of the ground stress measurement depth is P0, and the rock tensile strength is T. According to the formula

[0068] P i =3σ2-σ1+T-P0,P r=3σ2-σ1-P0 and P s =σ2, and the two-dimensional stress field (σ2,σ1) at the in-situ stress measurement location is calculated. The two formulas above eliminate the need to know the rock's tensile strength to determine the two-dimensional in-situ stresses σ2 and σ1. Therefore, in-situ rock stress measurements using hydraulic fracturing do not involve the rock's physical and mechanical properties, but are determined entirely by the measured and recorded pressure values.

[0069] The downhole ground stress measurement system and its measurement method based on distributed optical fiber sensing are low-cost, high-precision, and high-reliability methods and technologies for measuring stress distribution changes in rock formations throughout the entire downhole section and for monitoring dynamic changes. The present invention proposes a distributed fiber optic sensing-based underground stress measurement device, which uses a metal casing installed in a borehole, an armored optical cable laid on the inner wall of the metal casing, a separator installed downhole, a water injection string for injecting high-pressure water into the downhole, and a distributed fiber optic pressure sensor / distributed fiber optic temperature sensor (DPS / DTS) composite modulator and demodulator placed near the wellhead. The device uses a hydraulic fracturing method to measure the two-dimensional geostress field (σ2, σ1) at different depths point by point along the wellbore. This method can be used to lay pressure and temperature sensing armored optical cables from the bottom of the well to the wellhead in one go, allowing for rapid, accurate, and reliable measurement of the two-dimensional geostress field of the rock surrounding the entire well section, from shallow to ultra-deep wells. This method provides strong support for downhole engineering implementation plans with full-section geostress field data, effectively ensuring the long-term stable, safe, and reliable operation of oil and gas production wells, water injection wells, and monitoring or observation wells, and providing indispensable means, systems, and methods for scientific management of oil and gas reservoirs and improving oil and gas recovery.

Claims

1. An underground stress measurement device based on distributed optical fiber sensing, characterized in that: It comprises a metal casing (1) installed in a drill hole, wherein the inner wall of the metal casing (1) is provided with an armored optical cable (2); the armored optical cable (2) comprises at least one high-temperature resistant pressure-sensitive optical cable (4) and at least two high-temperature resistant multimode optical fibers (5); The high temperature resistant pressure sensitive optical cable (4) contains a high temperature resistant single mode or high temperature resistant special pressure sensitive optical fiber, and the high temperature resistant pressure sensitive optical cable (4) and the high temperature resistant multimode optical fiber (5) are respectively encapsulated in a continuous first metal tube (41) and a second metal tube (51); Alternatively, the high temperature resistant pressure sensitive optical cable (4) contains a single mode pressure sensitive optical fiber, or a high density continuous grating optical fiber with a spacing of less than 1 meter, or a high density array type Fabry cavity pressure sensor optical fiber with a spacing of 1 meter to 5 meters; The first thin metal tube (41) and the second thin metal tube (51) are further wrapped with a single layer or multiple layers of protective armored steel wire; A high-temperature resistant pressure-sensitive optical cable (4) is placed in the first metal tube (41) and is tightly wrapped with a high-temperature resistant high-strength composite material or is formed by wrapping an optical fiber in one step using an injection molding machine. The cable is tightly adhered to the wall and sealed in the first metal tube (41). A light damper (8) is installed at the tail end of the high-temperature resistant pressure-sensitive optical cable (4); The second metal tube (51) is also provided with high temperature resistant optical fiber paste; It also includes at least two separators (6) installed underground, with a sealed ground stress measurement well section between the two separators (6); it also includes a water injection string (7) for injecting high-pressure water into the underground ground stress measurement well section, and a high-pressure pump truck (9) for providing a high-pressure water source to the ground stress measurement well section; the water injection string (7) is connected to the high-pressure pump truck (9); It also includes a DPS / DTS composite modulation and demodulation instrument (3) placed near the wellhead, and the DPS / DTS composite modulation and demodulation instrument (3) is respectively connected to the high-temperature resistant pressure sensitive optical cable (4) and the high-temperature resistant multimode optical fiber (5) in the armored optical cable (2).

2. The underground stress measurement device based on distributed optical fiber sensing according to claim 1, characterized in that: The DPS / DTS composite modulation and demodulation instrument (3) is a distributed optical fiber pressure sensing and distributed optical fiber temperature sensing composite modulation and demodulation instrument, comprising a data acquisition module and a modulation and demodulation module.

3. The underground stress measurement device based on distributed optical fiber sensing according to claim 1, characterized in that: The separator (6) installed underground is of pressurized expansion type, and the pressurized medium is liquid or gas.

4. The measuring method of the underground stress measuring device based on distributed optical fiber sensing according to any one of claims 1 to 3, characterized in that: The following steps are involved: (a) performing perforation (10) to penetrate the metal casing (1) at all depths where ground stress measurement is required in the well; (b) slowly lowering the high-temperature resistant pressure-sensitive optical cable (4) into the metal casing (1) so that it is in close contact with the inner wall of the metal casing (1); connecting the high-temperature resistant pressure-sensitive optical cable (4) in the armored optical cable (2) to the DPS signal input end of the DPS / DTS composite modulation and demodulation instrument (3) at the wellhead, fusing two high-temperature resistant multimode optical fibers (5) together at the tail end of the armored optical cable (2) to form a U-shaped structure, and connecting the two high-temperature resistant multimode optical fibers (5) to the DTS double-ended signal input end of the DPS / DTS composite modulation and demodulation instrument (3) at the top end of the armored optical cable (2); (c) two separators (6) are arranged above and below the depth where the ground stress measurement is required, and water or air is filled into the separators (6) until they expand and completely cut off the fluid exchange with the well section outside the two separators (6); (d) injecting high-pressure water into the ground stress measurement well section between the two separators (6) through the water injection string (7); (e) Continuously increase the water injection pressure until the rock outside the metal casing (1) begins to crack. The DPS / DTS composite modulation and demodulation instrument (3) demodulates the phase change of the backscattered Rayleigh scattered light caused by the pressure change on the armored optical cable (2) in the ground stress measurement section of the well, and obtains the initial cracking pressure of the rock. P i ; (f) Continue to increase the water injection pressure to expand the rock cracks. When the cracks expand to a depth of three times the wellbore diameter, stop the high-pressure water injection and keep the water pressure constant. The DPS / DTS composite modem instrument (3) measures the closing pressure at this time. P s , then unload the pressure to close the rock cracks; (g) During the entire pressurization process, record the pressure-time curve and flow-time curve simultaneously to determine P i and P s value; (h) Re-inject high-pressure water into the in-situ stress measurement section to reopen the cracks. The DPS / DTS composite modem instrument (3) simultaneously measures the pressure when the rock cracks reopen. P r and subsequent constant closing pressure P s ; (i) Repeat this depressurization-repressurization process 2-3 times to improve the accuracy of the pressure measurement data; (j) During the entire process of repeated depressurization and re-pressurization, record the pressure-time curve and flow-time curve simultaneously to determine P r and P s value; (j) Using high temperature resistant multimode optical fiber (5) and DPS / DTS composite modulation and demodulation instrument (3) to monitor and measure the temperature change in the metal casing (1) of the whole well section in real time, and measuring the actual temperature change data in the well section based on the stress in the metal casing (1) monitored and measured, the strain ε or temperature t The resulting spectral shift is similar to the resonant wave shift Δλ or the Bragg grating spectral shift Δυ, using the formula: Δλ / λ=−Δυ / υ=K T Δt+K ε eh, Where λ and υ are the average light wavelength and frequency respectively; K T and K ε are the standard constants for temperature and strain, respectively; The temperature value at the specific measurement location is used to correct the drift of the scattered light spectrum in the optical fiber caused by temperature change to obtain the real metal casing (1) internal stress measurement pressure value in the well section without temperature influence; (k) The fracture water pressure at the separation section of the ground stress measurement depth is P 0 ， The tensile strength of rock is T， According to the formula P i =3σ 2 −σ 1 +T−P 0 , P r =3σ 2 −σ 1 − P 0 and P s =σ 2 ， Calculate the two-dimensional stress field at the ground stress measurement location ( σ 2 , σ 1 ).

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