Coal expansion strain calculation method based on optical fiber sensor and gas injection

Abstract

The invention discloses a coal expansion strain calculation method based on an optical fiber sensor and gas injection, and belongs to the technical field of monitoring of CO2 geological storage and oil and gas resource development. The method comprises the following steps: firstly, spirally packaging distributed optical fibers on the surface of a coal sample, and simulating formation temperature and pressure conditions through a triaxial stress system; then, compressed gas is injected, and full-field strain distribution is monitored in real time through the Rayleigh scattering optical fiber sensing technology; analyzing an edge effect and a confining pressure inhibition rule in the strain data; and finally, constructing a coal expansion model to quantify strain spatio-temporal evolution characteristics. Compared with a traditional method, the monitoring scheme is higher in resolution and higher in real-time performance, the migration state of the sealed gas can be dynamically tracked, and reliable data support can be provided for deep coal bed gas sealing safety assessment, injection parameter optimization and leakage early warning.

Description

TECHNICAL FIELD

[0001] The present application relates to CO2 geological storage monitoring technology, in particular to a coal body swelling strain calculation method based on optical fiber sensor and gas injection. BACKGROUND

[0002] The existing deep coal gas storage safety monitoring technology has defects such as passivity and hysteresis, mainly focuses on injection stage monitoring, and the swelling deformation test mostly uses the method of sticking strain gauges on the local coal sample, lacking long-term dynamic full-strain field monitoring after gas injection. These methods usually take a single sampling point or a limited area as the monitoring object, and obtain local strain or pressure data by sticking strain gauges on the surface of the coal sample or arranging pressure sensors in the gas injection path. In addition, the existing experiments mostly focus on short-term monitoring in the injection stage, and although the triaxial stress simulation and gas injection means can partially restore the formation conditions, they lack systematic and continuous monitoring capability for the long-term and full-field dynamic strain response of the medium after gas injection.

[0003] The traditional monitoring method is limited by the spatial coverage range and single sampling of point sensors, and it is difficult to capture the full-field strain distribution and non-uniform deformation characteristics of the medium induced by gas injection. The strain gauge can only reflect the local response near the sticking point and cannot realize high-resolution and full-field continuous strain monitoring; while the pressure sensor can monitor the change of pore pressure, but it cannot directly relate to the swelling strain distribution of the medium. Therefore, the distributed optical fiber sensing technology takes optical fiber as the carrier and realizes signal acquisition and transmission through optical wave sensing, which can track the deformation response of the rock in real time and has the advantages of permanent deployment, real-time monitoring and low cost. This technology can accurately measure the gas plume migration strain signal, correlate the migration rate, quantify the deformation area, and monitor the integrity of the cap rock and wellbore and gas leakage at the same time. Therefore, studying the gas-coal-optical fiber interaction mechanism and response characteristics is the key foundation for monitoring and evaluation of the whole process of deep coal gas injection, migration and storage. SUMMARY

[0004] In order to solve the above technical problems, the present application provides a coal body swelling strain calculation method based on optical fiber sensor and gas injection.

[0005] In order to realize the above-mentioned technology, the specific steps include: A coal body swelling strain calculation method based on optical fiber sensor and gas injection includes the following steps: S1, collect the coal sample of the target area and perform pretreatment operation on the coal sample to obtain a standardized coal sample; The step of obtaining a standardized coal sample includes: According to the GB / T482-2008 standard, the in-situ deep coal sample is collected, wrapped with water absorption paper and wrapped with adhesive tape to prevent oxidation, and sealed and transported to the laboratory; the coal sample is processed into a coal column or coal powder, and is adapted to subsequent mechanical and pore structure tests; an SX2-10-12N box-type resistance furnace is used to determine the industrial components (fixed carbon, moisture, ash); the core is cleaned with ultrapure water and an ultrasonic cleaning machine to remove pore fine particles.

[0006] S2, based on the standardized coal sample, the distributed optical fiber sensor is packaged in the form of spiral winding on the surface of the coal sample to improve the transmission density of the strain signal, comprising: selecting an SMF28e single-mode bare optical fiber, winding on the surface of the coal sample in a spiral manner; 2N pre-tension is applied, and the optical fiber is fixed with a two-component epoxy resin to enhance the coupling; after curing for 24 hours, the surface is sealed with a single-component silicone rubber to realize the dual functions of isolating the pore and confining pressure medium and facilitating post-experiment non-destructive stripping; finally, the packaged coal sample is loaded into the core holder, and the optical fiber is connected with the ODiSI-6000 system through a high-pressure optical feedthrough; In the application, only the case in the Z-axis direction is considered, therefore, The normal strain component in the Z direction = fiber strain along the spiral path ; therefore, the fiber strain along the spiral path and the circumferential strain of the sample ( ) are expressed as: In the formula, is the Poisson's ratio of the coal sample core, is the wrapping angle.

[0007] S3, the packaged coal sample is placed in a triaxial stress system to apply a preset axial pressure, confining pressure and temperature; In the application, the experimental condition setting includes: ① Pressure characteristics: according to the actual buried depth (1700-2000 m) of the coal seam in the experimental area, the experimental pressure is set to 10 MPa, which matches the actual formation pressure (about 10 MPa), and ensures that the physical state of supercritical CO2 (critical point: 7.38 MPa, 31.04℃) is consistent with the deep coal seam; ② Temperature characteristics: the experimental temperature is set to 35℃, which is consistent with the temperature of the deep coal seam (about 35℃), and avoids the interference of temperature difference on the adsorption deformation behavior of the coal sample; The specific application method includes: S3.1, pipeline leak detection is performed before the experiment, the vacuum pump is connected through the core clamp bottom pore pressure port to extract vacuum for ≥8h, and the gas in the pipeline and the sample is removed; S3.2, Injecting brine / nitrogen lower than confining pressure to saturate the sample, using high-pressure injection pump to pressurize the pore fluid to the set value and stabilize for 24 h, while dynamically maintaining the confining pressure always higher than the pore pressure; S3.3, Set 4 formation pressure conditions to study the influence of pore pressure-confining pressure combination on strain response, and then conduct brine displacement experiment at a flow rate of 0.6 mL / min; S3.4, Based on Terzaghi's effective stress principle, analyze the influence of pore pressure change caused by gas injection on stress field, where the effective stress can be defined as: In the formula, represents the confining pressure; represents the pore pressure; Based on the generalized effective stress theory, we have: In the formula, is the Biot-Willis coefficient; represents the strain tensor; is the Kronecker tensor; when constant , and are negatively correlated, the increase of pore pressure will reduce the effective stress, causing the expansion of pore volume (coal swelling), resulting in the positive strain signal measured by the optical fiber sensor.

[0008] S4, Based on S3, inject subcritical CO2 (4 MPa) or supercritical CO2 (8 MPa), and collect full-field strain data in real time through Rayleigh scattering optical fiber sensing system; Set the Rayleigh scattering optical fiber layout to meet the axial equivalent sampling rate of 0.29 mm, and obtain the circumferential strain by frequency shift inversion, use the Rayleigh backscattering signal detected in the optical fiber to obtain the modulation signal through the interferometer; The change of temperature or strain is linearly related to the frequency shift: In the formula, represents the total frequency shift, and represent the changes of strain and temperature, and a and b are the calculation coefficients of strain and temperature respectively; in this study, the temperature remains constant throughout the test process, so the complete frequency shift of Rayleigh backscattering is attributed to the change of strain; Collecting full-field strain data includes: S4.1. Based on thermodynamic theory and the Gibbs interface assumption, the isothermal adsorption process treats the coal body as an isotropic elastic body, neglecting the compressive deformation caused by pressure; based on the internal energy relationship of the adsorption system, when the change in elastic energy equals the change in surface energy, the adsorption expansion strain ( This can be represented as: In the formula, The volumetric strain of coal body adsorption and expansion. γ For specific surface energy, γ = Φ / A ,in, A For specific surface area, m 2 / kg, Φ It is surface energy; Let be the elastic modulus of the solid phase, in Pa; Indicates the density of the coal body; Fugacity related to the Poisson's ratio of the main phase, in Pa; The main phase is Poisson's ratio; Indicates the shape correction factor. = 1.2; This is the ratio of the radius to the length of the micropores in the coal body. When coal is the main phase, its Poisson's ratio is difficult to measure directly. Based on Bentz's model of a porous medium with non-overlapping spherical pores, this method is used. and The relation can be derived as follows: In the formula, Poisson's ratio of the coal sample core; With a Poisson's ratio as the main phase, the coal body is the main phase in this case. Pore ​​permeability when coal is the main phase; Will and The expansion strain generated by the adsorption of supercritical CO2 in coal can be calculated by combining the relationship with the expression for adsorption expansion strain. ; S4.2 The expansion strain of coal under supercritical CO2 is composed of the adsorption expansion strain caused by the adsorption of supercritical CO2, the compressive strain caused by the injection pressure, and the thermal expansion strain caused by temperature. Taking advantage of the characteristic that coal has difficulty adsorbing helium and cannot produce adsorption expansion deformation, the deformation of coal caused by pore pressure is determined. According to Hooke's law of volumetric strain, the volumetric strain generated by supercritical pore pressure can be calculated as follows: In the formula, For volumetric strain; KBulk modulus, ; S4.3, under the assumption that the coal body is in a stress-free state and isotropic, the temperature of the coal body is raised to The thermal expansion strain generated is: In the formula, is the thermal expansion strain; is the thermal expansion coefficient; S4.4, the results of S4.1-S4.3 are added to obtain the swelling complex strain of the coal body under the action of supercritical CO2 , the expression is as follows: ; At this time, let = , then: = = the fiber strain along the spiral path , to implement the inversion of the circumferential strain .

[0009] Advantages of the present application 1. The present application proposes a monitoring method based on distributed optical fiber strain sensing (DFOSS) for the complex occurrence conditions of deep heterogeneous coal seams and the problem of multiple variables involved in the field engineering, realizes real-time detection of the strain response of gas injection, and through obtaining the original strain field distribution under different confining pressure-pore pressure combinations, reveals the influence of effective stress on the strain response, and dynamically monitors the rock strain and wetting front movement in the displacement process, completes the monitoring of the whole process of deep coal gas storage.

[0010] 2. The present application first introduces the DFOSS method for dynamic monitoring of the whole process of deep coal gas storage, uses high-resolution DFOSS technology based on Rayleigh scattering to measure rock strain. The influence of the original strain field distribution and effective stress under different confining pressures is studied, and a strain measurement system of dynamic displacement process is constructed, which can accurately capture real-time information of the displacement front. This technology makes up for the defects of limited monitoring range and insufficient long-term monitoring ability of traditional methods, and provides a new method for monitoring and characterization of reservoir gas migration process. BRIEF DESCRIPTION OF DRAWINGS

[0011] Figure 1 is the step flow chart of the present application; Figure 2 is the system of the present application for real-time monitoring of gas injection induced coal swelling strain based on optical fiber sensing; Figure 3 is the monitoring method mechanism diagram of the present application based on distributed optical fiber strain sensing DFOSS; Figure 4 The process flow chart of the optical fiber laying method of the present application is shown in Figure 1, wherein, Figure 4 (a) is a schematic diagram of single-mode quartz optical fiber spirally wound on the surface of a coal sample core; Figure 4 (b) is a schematic diagram of encapsulation with a two-component epoxy resin; Figure 4 (c) is a schematic diagram of sealing the surface of the core with a one-component silicone rubber. DETAILED DESCRIPTION

[0012] The present application will be further described in detail below with specific examples.

[0013] The present application aims at the time-varying effect problem of coal seam capacity in the supercritical CO2 storage process, and proposes a coal body swelling strain real-time monitoring system and test method based on optical fiber sensing, which can quantitatively measure the dynamic strain of the coal body in the CO2 injection process, accurately capture the displacement front information, and provide a scientific basis for storage evaluation. Numerical simulation needs to follow the following basic assumptions: (1) The coal storage layer is an ideal homogeneous porous medium, and anisotropy is ignored; (2) The temperature is constant, CO2 and CH4 in the coal seam flow isothermally, and the thermal effect caused by gas adsorption-desorption is ignored; (3) The free gas in the coal seam conforms to the ideal gas state equation; (4) The adsorbed gas follows the Fick diffusion law, and the free gas obeys the Darcy seepage law; (5) The permeability of the coal seam roof and surrounding rock is ignored, and CH4 is not considered; (6) The adsorption behavior of the CO2-CH4 two-component system conforms to the Langmuir equation; (7) The coal storage layer undergoes a small elastic deformation.

[0014] As shown in Figure 1 and Figure 2 , a coal body swelling strain calculation method based on an optical fiber sensor and gas injection includes the following steps: S1, collecting a coal sample in a target area and performing a pretreatment operation on the coal sample to obtain a standardized coal sample; In the present application, the coal sample collection, packaging, preservation and transportation are completed according to the national standard GB / T482-2008; the large coal sample in the working face is wrapped with water absorption paper and wrapped with adhesive tape to prevent oxidation, then packed into a sealed bag, and the foam in the isolation box is filled and transported to the laboratory, laying a foundation for the universality of the experimental conclusion; Further, according to the experimental requirements, the coal rock sample is standardized processed into coal powder, coal column and the like, which are respectively adapted to the scenes of mechanical property test, pore-fracture structure analysis and the like, so as to ensure that the sample meets the high-precision test requirements; Furthermore, the industrial components were determined using an SX2-10-12N box-type resistance furnace (GB / T212-2008), including: fixed carbon 73.13%-80.02%, moisture 1.36%-2.57%, and medium ash content; Furthermore, the core was cleaned using a high-power ultrasonic cleaner filled with ultrapure water (UW) to remove fine particles from the pores, and the cleaning was repeated until there was no obvious sediment at the bottom of the cleaner.

[0015] S2, such as Figure 3 As shown, based on standardized coal samples, distributed optical fiber sensors are encapsulated on the surface of the coal samples in a spiral winding manner to improve the transmission density of strain signals. The encapsulation methods include: using SMF28e single-mode bare optical fiber (bending radius on the order of centimeters, low signal loss) as the sensor, spirally winding it onto the surface of the coal sample, applying a 2N pretension, and then fixing it with epoxy resin; specifically, spirally winding single-mode quartz optical fiber onto the surface of the coal sample core ( Figure 4 a), and encapsulated with a two-component epoxy resin ( Figure 4 b) Ensure the stability of the fiber-coal sample coupling and sensor protection to improve the reliability of strain measurement; Meanwhile, before the epoxy resin is fully cured, a pre-tension force of 2 N is continuously applied to enhance the adhesion strength of the fiber-coal sample interface and prevent the fiber from breaking during the test. Furthermore, after curing for 24 hours, the core surface was sealed with a single-component silicone rubber. Figure 4 c) To achieve dual functionality: ① Isolate the pores from the confining pressure medium to avoid strain distortion and system depressurization caused by coating swelling; ② Facilitates the non-destructive removal and recovery of the sensor after the experiment; Finally, after carefully loading the packaged rock sample into the core frame, the optical fiber was passed through the high-voltage optical feedthrough and fused with the prepared optical fiber pigtail. Based on the ODiSI-6000 system, the following were performed: ① Signal quality detection; ② Selection of sensor unit observation points by contact positioning method (marking points corresponding to abnormal spectral peaks); ③ Recording of the spatial coordinates of the observation points. Furthermore, the principles of distributed fiber optic sensing include: When an optical pulse is injected into an optical fiber, its phase With transmission distance The relationship between (fiber optic gauge length) and the actual fiber optic gauge length can be expressed as: In the formula, Let be the propagation constant of light in a vacuum. ,in, Indicates the refractive index; Indicates wavelength; When external disturbances cause phase changes The total differential form is: In the formula, This indicates the change in propagation distance; This represents the change in the propagation constant; Indicates the change in refractive index; Indicates the core diameter; This indicates the change in fiber core diameter; The effect of axial stress on fiber length variation on phase (strain effect); This indicates the effect of changes in the refractive index of optical fiber on the phase (optical-elastic effect). This represents the effect of changes in fiber core diameter on phase (Poisson effect); according to the basic principles of light propagation, we have: ; Usually, the Poisson effect is very small and can be ignored. It can be approximated as: in, , For the axial strain of the optical fiber; Based on the fundamental principle of the elasto-optic effect, the change in the inverse dielectric tensor... With elastic strain tensor It can be represented as: In the formula, For the fourth-order optical tensor, where, and These represent the first and second freedom indices, respectively. and These represent the first summation index and the second summation index, respectively. Let be the refractive index tensor of the optical fiber under different stress states; from this, we can obtain: ; In the formula, It represents the change in the refractive index tensor of an optical fiber under different stress states; The change in refractive index of the optical fiber caused by strain can be expressed as: ; According to the broad sense The law is obtained in the strain-stress constitutive relation of isotropic media as follows: In the formula, Denote constitutive relations; a compliance tensor representing isotropic optical fiber material; represents stress components; wherein, represents: wherein, and are the normal stress sub-matrix and the shear stress sub-matrix of the compliance matrix, respectively, and are represented as: ; wherein, , are the Poisson's ratio and the Young's modulus of the optical fiber material, respectively; represents the shear modulus, ; When the optical fiber is subjected to axial stress, the normal stress , and other stresses are 0, and the stress-strain constitutive relationship can be characterized as: wherein, , and represent the normal strain components in the X, Y and Z directions, respectively; , and represent the normal stress components in the X, Y and Z directions, respectively; When the optical fiber Z-axis is subjected to stress, the strain can be represented as: wherein, is the normal stress of the optical fiber, representing the mechanical load distribution along the length direction (Z-axis) of the optical fiber; is the cross-sectional area along the axial direction of the optical fiber; According to the photoelastic effect, when the optical fiber is subjected to external stress waves and causes strain, it will cause the change of , and the strain can be represented as: wherein, , , are the changes of the refractive index of the optical fiber in the X, Y and Z directions, respectively; and are the first photoelastic coefficient and the second photoelastic coefficient of the optical fiber, respectively; ; According to the basic principle of light propagation in a single-mode optical fiber, the change of the effective refractive index The main source is the transverse (X and Y direction) refractive index disturbance, and the specific relationship is: ; The expression of the approximation and the expression of are brought into the basic principle of the photoelastic effect, and the following is obtained: In the formula, represents a constant, ; In the present application, only the case in the Z-axis direction is considered, so = fiber strain along the spiral path ; then the fiber strain along the spiral path and the expression of the circumferential strain of the sample ( ) are as follows: In the formula, is the Poisson ratio of the coal sample core, is the wrapping angle; under the experimental conditions in the present application: ≈6.40 , =0.31, therefore, = 0.99 , indicating that the fiber optic sensor can effectively measure the circumferential strain component of the sample.

[0016] S3, the packaged coal sample is placed in a triaxial stress system to apply a preset axial pressure, confining pressure and temperature; In the present application, the experimental condition setting includes: ① pressure characteristics: according to the actual buried depth (1700-2000 m) of the coal seam in the experimental area, the experimental pressure is set to 10 MPa, which matches the actual stratum pressure (about 10 MPa), and ensures that the physical state of supercritical CO2 (critical point: 7.38 MPa, 31.04℃) is consistent with the deep coal seam. ② Temperature characteristics: the experimental temperature is set to 35℃, which is consistent with the temperature of the deep coal seam (about 35℃), so as to avoid the interference of temperature difference on the adsorption deformation behavior of the coal sample; The specific application method includes: S3.1, pipeline leak detection before the experiment, connect the vacuum pump through the core clamp bottom pore pressure port to vacuum for ≥8h, and remove the gas in the pipeline and the sample; S3.2, inject brine / nitrogen lower than the confining pressure to saturate the sample, use a high-pressure injection pump to pressurize the pore fluid to the set value and stabilize for 24h, while dynamically maintaining the confining pressure higher than the pore pressure; S3.3, Four formation pressure conditions were set to study the influence of pore pressure-confining pressure combination on strain response, and then brine displacement experiments were conducted at a flow rate of 0.6 mL / min; S3.4, Based on the Terzaghi effective stress principle, the influence of the change in pore pressure caused by gas injection on the stress field was analyzed, where the effective stress can be defined as: In the formula, represents the confining pressure; represents the pore pressure; Based on the generalized effective stress theory, we have: In the formula, is the Biot-Willis coefficient; represents the strain tensor; is the Kronecker tensor; when the constant is and are negatively correlated, the increase in pore pressure will reduce the effective stress, causing the pore volume to expand (coal expansion), resulting in a positive strain signal measured by the optical fiber sensor.

[0017] S4, Based on S3, inject subcritical CO2 (4 MPa) or supercritical CO2 (8 MPa), and collect full-field strain data in real time through the Rayleigh scattering optical fiber sensing system; Set the Rayleigh scattering optical fiber layout to meet the axial equivalent sampling rate of 0.29 mm, and use frequency shift inversion to obtain the circumferential strain Use the Rayleigh backscattering signal detected in the optical fiber to obtain the modulation signal through an interferometer; The change in temperature or strain is linearly related to the frequency shift: In the formula, represents the total frequency shift, and represent the changes in strain and temperature, and a and b are the calculation coefficients of strain and temperature, respectively; in this study, the temperature remained constant throughout the test process, so the complete frequency shift of Rayleigh backscattering was attributed to the change in strain; Collecting full-field strain data includes: S4.1, According to the thermodynamic theory and Gibbs interface assumption, the isothermal adsorption process regards the coal body as an isotropic elastic body, ignoring the compression deformation caused by pressure; based on the internal energy relationship in the adsorption system, when the elastic energy change is equal to the surface energy change, the adsorption swelling strain ( ) can be represented as: wherein, is the adsorption swelling volumetric strain of coal; γ is the specific surface energy, γ = Φ / A wherein, A is the specific surface area, m 2 / kg, Φ is the surface energy; is the solid phase elastic modulus, Pa; represents the density of coal; is the fugacity related to the Poisson's ratio of the host phase, with the unit of Pa; is the Poisson's ratio of the host phase; represents the shape correction factor, = 1.2; is the ratio of the radius to the length of the micropore of coal; When coal is the host phase, its Poisson's ratio is difficult to be directly measured. Based on the model of non-overlapping placed spherical pore porous medium proposed by Bentz, and , the following relationship can be obtained: wherein, is the Poisson's ratio of the coal sample core; is the Poisson's ratio of the host phase, when coal is the host phase; is the pore permeability when coal is the host phase; The relationship between and is combined with the expression of adsorption swelling strain to obtain the swelling strain of coal caused by the adsorption of supercritical CO2 ; S4.2, the swelling strain of coal under the action of supercritical CO2 is the swelling strain caused by the adsorption of supercritical CO2, the pressure strain caused by the injection pressure and the thermal swelling strain caused by the temperature. By using the characteristics that coal is difficult to adsorb helium and cannot produce adsorption swelling deformation, the deformation of coal caused by pore pressure is determined, and the volumetric strain caused by supercritical pore pressure can be obtained according to the Hooke's law of volumetric strain: wherein, is the volumetric strain; K is the volumetric swelling modulus, ; S4.3, under the assumption that coal is in a stress-free state and isotropic, the thermal swelling strain caused by the temperature rising from to is: wherein, is thermal expansion strain; is thermal expansion coefficient; S4.4, adding the results of S4.1-S4.3 to obtain the swelling complex strain of coal body under the action of supercritical CO2 , the expression is as follows: ; At this time, let = , then: = = fiber strain along the spiral path to invert the circumferential strain . The present application can monitor the coal deformation strain in real time during the whole process of deep coalbed gas injection, migration and storage, and provides a scientific calculation method for establishing a deep coalbed gas time-space migration monitoring system.

[0018] The above-described embodiments are merely preferred embodiments of the present application and are not intended to limit the concept and scope of the present application. Various modifications and improvements to the technical solutions of the present application made by those of ordinary skill in the art without departing from the design concept of the present application shall fall within the protection scope of the present application. The technical content claimed by the present application has been fully recorded in the claims.

Claims

1. A method for calculating coal body expansion strain based on fiber optic sensors and gas injection, characterized in that, Includes the following steps: S1. Collect coal samples from the target area and perform preprocessing operations on the coal samples to obtain standardized coal samples; S2. Based on standardized coal samples, a distributed optical fiber sensor is spirally encapsulated on the surface of the coal sample in a wound manner to improve the transmission density of strain signals. S3. Place the packaged coal sample in a triaxial stress system to apply preset axial pressure, confining pressure and temperature; S4. Based on S3, supercritical CO2, i.e. 8MPa CO2, is injected, and the strain data of the entire field is collected in real time through a Rayleigh scattering fiber optic sensing system to complete the calculation derivation.

2. The method for calculating coal body expansion strain based on fiber optic sensors and gas injection according to claim 1, characterized in that: The encapsulation method is as follows: SMF28e single-mode bare optical fiber is used as the sensor, which is spirally wound onto the surface of the coal sample, and then fixed with epoxy resin after applying a 2N pretension; at the same time, a 2N pretension is continuously applied, and after curing for 24 hours, the core surface is sealed with single-component silicone rubber.

3. The method for calculating coal body expansion strain based on fiber optic sensors and gas injection according to claim 1, characterized in that: In the process of encapsulating a distributed optical fiber sensor on the surface of a coal sample in a helical winding manner to improve the transmission density of the strain signal, only the case of the distributed optical fiber sensor in the Z-axis direction is considered, and the normal strain component in the Z-axis direction of the distributed optical fiber sensor is set. =Fiber strain along the helical path Then there is fiber strain along the helical path. With respect to the circumferential strain of the sample ( The expression for ) is: In the formula, The Poisson's ratio of the coal sample core. It is the corner of the enclosure; set up ≈6.40 , = 0.31, therefore, = 0.99 .

4. The method for calculating coal body expansion strain based on fiber optic sensors and gas injection according to claim 1, characterized in that: The preset axial pressure, confining pressure, and temperature are as follows: apply axial pressure and confining pressure of 10 MPa and heat to 35°C.

5. The method for calculating coal body expansion strain based on fiber optic sensors and gas injection according to claim 1, characterized in that: In S4, a Rayleigh scattering fiber layout is set to meet an axial equivalent sampling rate of 0.29 mm, and circumferential strain is inverted through frequency shift.

6. The method for calculating coal body expansion strain based on fiber optic sensors and gas injection according to claim 5, characterized in that: The frequency shift is linearly related to changes in temperature and strain. In the formula, Indicates the total frequency shift. and This represents the changes in strain and temperature, where a and b are the calculation coefficients for strain and temperature, respectively. The temperature was kept constant throughout the test.

7. The method for calculating coal body expansion strain based on fiber optic sensors and gas injection according to claim 6, characterized in that: The strain change includes: adsorption expansion strain. Volumetric strain and thermal expansion strain The expression is ,set up = Then we have: = =Fiber strain along the helical path To invert circumferential strain .

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