A method for monitoring rock stress based on distributed optical fiber

Abstract

The present invention relates to the technical field of optical fiber sensing and rock stress detection, and discloses a method for monitoring rock stress based on distributed optical fiber, which includes the following steps: Step 1, preparation of a cubic rock sample: Lay the optical fiber on each layer, and the laying method of the optical fiber is: Taking the wellbore in the rock as the center, pour four layers of rock samples, each layer with a height of 10 cm, and lay three layers of "U"-shaped optical fibers; Step 2, conduct an integrity inspection on the optical fiber in the rock sample; Step 3, stress loading and detection test: Extrude uniform stress loads from different directions of X, Y, and Z, and analyze the stress conduction in the rock by analyzing the deformation of the optical fiber. The present invention demonstrates the integrity of the optical fiber through a percussion experiment, determines the position of the starting section of the optical fiber, ensures the accuracy of subsequent three-direction stress detection in X, Y, and Z, realizes noise reduction at the initial end, reduces the detection error, and ensures the accuracy of the detection result.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber optic sensing and rock stress detection, and particularly relates to a method for monitoring rock stress based on distributed optical fiber. Background Art

[0002] In the field of rock mechanics, the stress conduction characteristics of rocks are of great significance for understanding the deformation and failure processes of rocks and engineering safety. Traditional rock stress measurement methods, such as strain gauges and acoustic emission monitoring, although can provide information on rock stress changes to a certain extent, usually have problems such as limited monitoring range, low spatial resolution, and complex installation.

[0003] Distributed Fiber Optic Sensing (DFOS) is a technology that uses optical fiber as a sensing medium to achieve real-time monitoring of continuous or distributed physical quantities (such as temperature, strain, etc.) along the optical fiber line. Distributed fiber optic sensing technology can achieve long-distance and large-scale continuous stress monitoring, providing high-sensitivity and high-resolution data, thus making it possible to monitor the real-time stress state inside rocks, facilitating the understanding of the stress conduction mechanism of rocks under dynamic loading; it can predict rock failure by analyzing the development law of the strain field; it can quantitatively analyze the width and development of microcracks. In the fields of hydraulic fracturing exploitation of coal and unconventional oil and gas resources (such as shale gas and tight gas), oil and gas pipeline monitoring, geological disaster warning, etc., distributed fiber optic sensing technology has begun to be applied, and its effect in real-time monitoring of rock stress changes, evaluating engineering risks, and guiding safe exploitation is particularly significant. However, although the distributed fiber optic monitoring technology has obvious advantages in the study of rock stress conduction, there are still many challenges in terms of theory, technology, and application. First, the installation and deployment of fiber optic sensors are difficult in hard rock media, especially in complex geological environments, and how to ensure the stability and long-term reliability of the sensors is a key issue. Second, the heterogeneity and anisotropy of rocks may lead to the complexity of stress transfer, which poses higher requirements for the interpretation and analysis of monitoring data. Summary of the Invention

[0004] The present invention aims to provide a method for monitoring rock stress based on distributed optical fiber, aiming to improve the measurement accuracy and more accurately reflect the actual situation of rock stress conduction.

[0005] To achieve the above object, the present invention adopts the following technical solutions: A method for monitoring rock stress based on distributed optical fiber, comprising the following steps:

[0006] Step 1. Preparation of cubic rock samples: During the pouring process of the rock samples, optical fibers are laid in each layer, and the integrity of the optical fibers is inspected before laying. The laying method of the optical fibers is as follows: Taking the wellbore in the rock as the center, four layers of rock samples are poured, each layer being 10 cm high, and three layers of "U"-shaped optical fibers are laid; a total of 8 optical fibers are laid in three layers, with two "U"-shaped optical fibers laid together, and the outer optical fibers are distributed in a semi-surrounding shape outside the inner optical fibers.

[0007] Step 2. Inspect the integrity of the optical fibers in the rock samples;

[0008] Step 3. Stress loading and detection test: Apply instantaneous point stress loads by knocking on the outside of the rock and uniform stress loads by squeezing from different directions of X, Y, and Z in the pressure chamber of the triaxial machine. Analyze the stress conduction in the rock by analyzing the deformation of the optical fibers.

[0009] Preferably, as an improvement, in Step 1, the pouring method of the rock samples includes the following steps:

[0010] S1. Prepare the mold and wellbore;

[0011] S2. Weigh the raw materials in proportion, stir evenly, and remove the bubbles by vibration for later use;

[0012] S3. Test the parameters and integrity of the optical fibers;

[0013] S4. Lay the optical fibers in each layer according to the design plan, observe and record the stationary time;

[0014] S5. Demold and cure.

[0015] Preferably, as an improvement, in Step 1, the raw materials of the rock samples are mixed with cements of different meshes.

[0016] Preferably, as an improvement, in Step 1, two optical fibers are arranged in the first layer, four optical fibers are arranged in the second layer, and two optical fibers are arranged in the third layer.

[0017] Preferably, as an improvement, in Step 1, the optical fibers are single-mode optical fibers with a diameter of 125 μm and a coating diameter of 250 μm.

[0018] Preferably, as an improvement, in Step 1, the method for inspecting the integrity of the optical fibers is as follows: Cut the optical fiber, put it into a heat shrinkable tube, wipe it with an alcohol paper, and then put it into an optical fiber fusion instrument for fusion. After the fusion is completed, connect it with a red light pen for inspection. If a red light can be clearly seen at the end of the optical fiber, it means that the optical fiber has good performance.

[0019] Preferably, as an improvement, in Step 3, pinch the optical fiber before the test, measure the strain data of the optical fiber, and determine the test range of the signal and the position where the strain responds on the optical fiber by testing the front and rear ends of the optical fiber.

[0020] Preferably, as an improvement, in step three, the fracturing test includes a confining pressure test and a hydraulic fracturing test.

[0021] Preferably, as an improvement, the confining pressure test uses staged pressurization.

[0022] The principle and advantages of this solution are as follows: In practical applications, in this technical solution, in order to improve the accuracy of rock stress monitoring, during the casting of rock samples, through a unique optical fiber laying method, more comprehensive and accurate data on the crack opening position and crack propagation degree are obtained, which is more convenient for comparative research on the crack propagation mechanism and stress conduction mechanism, and realizes the comprehensive monitoring of rock stress information. Before using the distributed optical fiber detection technology for stress detection, first, the integrity of the optical fiber is demonstrated through a tapping experiment, and the position of the starting section of the optical fiber is determined to ensure the accuracy of subsequent detection, realizing noise reduction at the initial end and reducing detection errors; then, through experimental data, the stress conduction law of the rock under different confining pressure conditions and the relationship with crack propagation behavior are analyzed in detail. The results show that: ① The optical fiber sensor can effectively capture the strain changes caused by the extrusion, expansion of microcracks and stress redistribution due to stress loading, revealing the complexity of stress conduction during the crack formation and expansion process. ② Strain concentration often indicates the formation and expansion of cracks, and the non-uniformity of strain distribution reflects the discontinuity of the internal structure of the rock. ③ Stress conduction is from the compressed surface to the inside, and due to the effect of stress release, stress will also conduct to the surface that is not compressed. The distributed optical fiber sensing technology demonstrates its advantages in monitoring rock strain with high sensitivity and high resolution, providing strong technical support for understanding the rock stress transfer mechanism. Description of the Drawings

[0023] Figure 1 It is a schematic diagram of the signal-to-noise ratio of D4 optical fiber in the embodiment of the present invention.

[0024] Figure 2 It is a corresponding position diagram of the tapping experiment in the embodiment of the present invention.

[0025] Figure 3 It is a strain waterfall diagram and position diagram of D1 optical fiber in the tapping experiment in the embodiment of the present invention.

[0026] Figure 4 It is a strain waterfall diagram and position diagram of D4 optical fiber in the tapping experiment in the embodiment of the present invention.

[0027] Figure 5 It is a strain waterfall diagram and position diagram of D7 optical fiber in the tapping experiment in the embodiment of the present invention.

[0028] Figure 6 It is a D1 waterfall diagram in the first stage of the fracturing experiment in the embodiment of the present invention.

[0029] Figure 7 It is the D4 waterfall chart of the first stage of the fracturing experiment in the embodiment of the present invention.

[0030] Figure 8 It is the D7 waterfall chart of the first stage of the fracturing experiment in the embodiment of the present invention.

[0031] Figure 9 It is the D1 waterfall chart of the second stage of the fracturing experiment in the embodiment of the present invention.

[0032] Figure 10 It is the D4 waterfall chart of the second stage of the fracturing experiment in the embodiment of the present invention.

[0033] Figure 11 It is the D7 waterfall chart of the second stage of the fracturing experiment in the embodiment of the present invention.

[0034] Figure 12 It is the D1 waterfall chart of the third stage of the fracturing experiment in the embodiment of the present invention.

[0035] Figure 13 It is the D4 waterfall chart of the third stage of the fracturing experiment in the embodiment of the present invention.

[0036] Figure 14 It is the D7 waterfall chart of the third stage of the fracturing experiment in the embodiment of the present invention.

[0037] Figure 15 It is the D1 waterfall chart of the fourth stage of the fracturing experiment in the embodiment of the present invention.

[0038] Figure 16 It is the D4 waterfall chart of the fourth stage of the fracturing experiment in the embodiment of the present invention.

[0039] Figure 17 It is the D7 waterfall chart of the fourth stage of the fracturing experiment in the embodiment of the present invention.

[0040] Figure 18 It is the first-order optical fiber strain distribution diagram in the embodiment of the present invention.

[0041] Figure 19 It is the second-order optical fiber strain distribution diagram in the embodiment of the present invention.

[0042] Figure 20 It is the third-order optical fiber strain distribution diagram in the embodiment of the present invention.

[0043] Figure 21 It is the fourth-order optical fiber strain distribution diagram in the embodiment of the present invention.

[0044] Figure 22 It is the schematic diagram of stress inward conduction in the embodiment of the present invention.

[0045] Figure 23Schematic diagram of stress upward conduction in the embodiment of the present invention.

[0046] Figure 24 Schematic diagram of stress conduction law in the embodiment of the present invention.

[0047] Figure 25 Top view of optical fiber numbers and optical fiber layout in the embodiment of the present invention. Specific embodiments

[0048] The following is a further detailed description through specific embodiments, but the embodiments of the present invention are not limited thereto. Unless otherwise specified, the technical means used in the following embodiments are conventional means well-known to those skilled in the art; the experimental methods used are all conventional methods; the materials, reagents, etc. used can all be obtained from commercial channels.

[0049] Embodiment 1

[0050] A method for monitoring rock stress based on distributed optical fiber includes the following steps:

[0051] Step 1, preparation of rock samples: Referring to the actual geological conditions of shale oil, a large-scale artificial rock sample with multiple bedding planes is made, and a cube artificial rock sample with a side length of 40 cm is poured according to the ratio of cement: 260 mesh: 150 mesh: 80 mesh: 60 mesh: water = 40: 6: 2: 20: 12: 15.

[0052] The pouring method of the rock sample includes the following steps:

[0053] S1, preparation of the mold and wellbore;

[0054] S2, weighing the raw materials according to the ratio, stirring evenly and removing the bubbles for standby;

[0055] S3, optical fiber parameter and integrity test: Use a single-mode optical fiber (SMF) with a diameter of 125 μm and a coating diameter of 250 μm; the optical fiber is cut with an optical fiber cutter, sleeved with a heat shrinkable tube, wiped with an alcohol paper, and then put into an optical fiber fusion instrument for fusion. After the fusion is completed, it is connected and inspected with a red light pen. It can be clearly seen that there is red light at the end of the optical fiber, and it can be determined that the optical fiber performance is intact;

[0056] S4, laying the optical fiber to each layer according to the design scheme, observing and recording the static time; As Figure 25 shown, the design scheme of the optical fiber layout is: taking the wellbore as the center, pouring four layers of rock samples, each layer with a height of 10 cm, and laying three layers of "U"-shaped optical fibers (layering is for convenient laying of the optical fiber). A total of 8 are laid in three layers, and two "U"-shaped optical fibers are laid together to prevent the optical fiber from being damaged due to subsequent operation errors, so as to ensure a relatively high integrity of the optical fiber.

[0057] In the first layer, two optical fibers are arranged. The total length of fiber D1 is 140.8 cm, the length of the connected pigtail is 60 cm, the side length of the "U" shape is 20 cm, and the width is 20 cm; the total length of fiber D2 is 138.9 cm, the length of the connected pigtail is 60 cm, the side length of the "U" shape is 20 cm, and the width is 25 cm.

[0058] In the second layer, four optical fibers are arranged. The total length of fiber D3 is 102.1 cm, the length of the connected pigtail is 60 cm, the side length of the "U" shape is 20 cm, and the width is 15 cm; the total length of fiber D4 is 94.3 cm, the length of the connected pigtail is 60 cm, the side length of the "U" shape is 20 cm, and the width is 20 cm; the total length of fiber D5 is 142.7 cm, the length of the connected pigtail is 60 cm, the side length of the "U" shape is 20 cm, and the width is 25 cm; the total length of fiber D6 is 142.7 cm, the length of the connected pigtail is 60 cm, the side length of the "U" shape is 20 cm, and the width is 30 cm.

[0059] In the third layer, two optical fibers are arranged. The total length of fiber D7 is 114.8 cm, the length of the connected pigtail is 60 cm, the side length of the "U" shape is 20 cm, and the width is 20 cm; the total length of fiber D8 is 114.8 cm, the length of the connected pigtail is 60 cm, the side length of the "U" shape is 20 cm, and the width is 25 cm.

[0060] S5. Remove the mold after 24 hours and cure the rock sample (for 14 days).

[0061] Step 2. Conduct integrity tests on the optical fibers in the rock sample. After the tests are completed, connect the surviving optical fibers to the distributed optical fiber monitoring system for testing. Find four points close to the arranged optical fibers on each of the side and top surfaces of the rock sample, and observe whether the joint signals are good and whether the monitoring instrument can receive the strain signals of the optical fibers by tapping.

[0062] Step 3. Fracturing experiment: Place the rock sample in the confining pressure chamber and apply pressure along the X and Y axes. In the initial stage, due to strong reflection at the tail end, there will be noise interference. It is necessary to tie a knot at the tail end of the optical fiber. When a loop is formed, the strong reflection will leak out from the loop at the tail end to reduce experimental errors.

[0063] Pinch the optical fiber by hand and use the instrument to measure the strain data of the optical fiber to test the front and rear ends of the optical fiber, so as to determine the signal test range and the position where the strain responds on the optical fiber.

[0064] 1) Confining pressure test: Conduct two groups of experiments. First, measure the confining pressure of 1 MPa as the control group, which takes 57 seconds for pressurization and 28 seconds for stabilization, and then conduct normal tests.

[0065] Apply pressure in stages. In the first stage, increase the pressure to 5 MPa, which takes 88 seconds for pressurization and 70 seconds for stabilization;

[0066] In the second stage, increase the pressure to 8 MPa, which takes 113 seconds for pressurization and 83 seconds for stabilization;

[0067] In the third stage, the pressure is increased to 10 MPa, the pressurization time is 102 seconds, and the stabilization time is 79 seconds.

[0068] In the fourth stage, the pressure is increased to 15 MPa, the pressurization time is 218 seconds, and the stabilization time is 82 seconds.

[0069] Measure the strain data of each group respectively, and based on the data detected by the distributed optical fiber, study the mechanism of rock stress conduction.

[0070] 2) Hydraulic fracturing experiment test: After the confining pressure test is completed, connect the wellbore, inject the fracturing fluid, use the pressurization operation instrument of model TC-3000, stop pressurizing when a small crack is pressed out of the rock sample, and observe the strain information distributed along the optical fiber.

[0071] When conducting the fracturing experiment, it is completed by using a true triaxial test system and a distributed optical fiber shape monitoring demodulator.

[0072] Distributed optical fiber shape monitoring demodulator: It is a high-spatial-resolution and high-precision fully distributed optical fiber sensing system based on optical fiber Rayleigh backscattering and optical frequency domain reflectometry. The distributed optical fiber shape monitoring demodulator can measure three-dimensional spatial information such as the attitude, orientation, track, and position of the optical fiber itself or the object to be measured connected to it. In addition, the system also has the functions of link monitoring and temperature / strain measurement, provides an ultra-high spatial resolution of up to 10 μm, and can diagnose various optical network faults such as optical fiber bending, fusion splicing, connection, and fracture within a maximum measurement range of 2 km. The distributed optical fiber shape monitoring demodulator can provide technical support for the online health monitoring, intelligent early warning, and feedback control of precision structures in fields such as national defense and military, life and health, intelligent manufacturing, and geotechnical engineering.

[0073] After the experiment, the data is collected and analyzed. By comparing with other traditional test methods (such as acoustic emission), verify the accuracy and reliability of the distributed optical fiber sensing technology in the study of rock stress conduction. At the same time, through comparative analysis, the crack propagation mechanism and the rock stress conduction mechanism can be obtained.

[0074] Results and analysis:

[0075] I. Data accuracy analysis:

[0076] 1. Optical fiber signal-to-noise ratio

[0077] The optical fiber signal-to-noise ratio (OSNR, Optical Signal-to-Noise Ratio) is an important parameter to measure the performance of an optical fiber communication system. It describes the ratio between the signal optical power and the noise optical power. By comparing the heights or power levels of the signal waveform and the noise waveform, it can be directly seen whether the signal is submerged by the noise.

[0078] Number the 8 laid optical fibers respectively from D1 - D8. Then observe the signal - to - noise ratio data of each optical fiber on the interface of the distributed optical fiber operating system. Take the D4 optical fiber as an example. As Figure 1 shown, the signal - to - noise ratio of the D4 optical fiber is relatively large. The peak part and power level of the signal segment are significantly higher than the average part of the noise. The influence of noise on the optical fiber signal and quality is very low, and the measured data has high accuracy and reliability. After inspection of the 8 optical fibers used in this experiment, only the signal - to - noise ratio of D2 is relatively low, and the influence of noise is large. It is very difficult to measure accurate strain data, so the data measured by the D2 optical fiber is discarded. The remaining 7 optical fibers all have good performance, and the analysis of the experimental results based on the data measured by them is reliable.

[0079] 2. Knocking experiment

[0080] Knock on three sides of the rock sample, near D1 on the first layer, near D4 on the second layer, and near D7 on the third layer. The specific knocking positions are as Figure 2 shown. Judge whether the performance of the optical fiber buried inside the rock sample is intact by whether the optical fiber receives the strain data of the real - time response generated by knocking.

[0081] Perform continuous knocking near the first knocking position and analyze the strain information received by the optical fiber. When the part of the D1 optical fiber buried inside the rock sample from 30 cm to 37 cm received a strain information ranging from - 20 to about 105 micro - strain at the 12th second. The strain waterfall diagram of the D1 optical fiber and the approximate position of the optical fiber segment (red - marked part) that received the strain information are as Figure 3 shown.

[0082] Perform continuous knocking near the second knocking position and analyze the strain information received by the optical fiber. When the part of the D4 optical fiber buried inside the rock sample from 20 cm to 50 cm received a strain information ranging from - 50 to about 350 micro - strain at about the 13th second. The strain waterfall diagram of the D1 optical fiber and the approximate position of the optical fiber segment (red - marked part) that received the strain information are as Figure 4 shown.

[0083] Perform continuous knocking near the third knocking position and analyze the strain information received by the optical fiber. When the part of the D7 optical fiber buried inside the rock sample from 20 cm to 40 cm received a strain information ranging from - 400 to about 600 micro - strain at about the 4th second. It is speculated that maybe because the distance of the optical fiber arrangement is very close to the position where the D7 optical fiber is arranged, the received strain information is larger. The strain waterfall diagram of the D7 optical fiber and the approximate position of the optical fiber segment (red - marked part) that received the strain information are as Figure 5 shown.

[0084] II. Pressure Test

[0085] 1. First-stage Pressurization

[0086] From the above percussion experiment, it can be seen that the optical fiber performances of D1, D4, and D7 are intact. The pressure on the X-axis and Y-axis is increased from 0 to 5 MPa, and the pressurization takes 88 seconds and stabilizes for 70 seconds. Select the data of the optical fiber D1 in the first layer to draw a waterfall plot (as shown in Figure 6 ), select the data of the optical fiber D4 in the second layer to draw a waterfall plot (as shown in Figure 7 ), and select the data of the optical fiber D7 in the third layer to draw a waterfall plot (as shown in Figure 8 ) for data processing and analysis.

[0087] The total length of the D1 optical fiber is 200.8 cm. From the waterfall plot of D1 in the first stage, it can be obtained that there is a very small strain along the entire length of the optical fiber at D1. Strain can be monitored during the pressurization stage within 0 - 38 seconds. As the pressure on the X-axis and Y-axis gradually increases, the strain monitored by the optical fiber gradually decreases, which does not conform to the expectation. It may be that the rock sample is continuously stressed during this stage, the value of the strain continuously decreases, stress is released, and the magnitude of the strain data monitored by the optical fiber gradually decreases. Since the experiment is a confining pressure test and the strain data measured by the optical fiber is positive, it indicates that the laid optical fiber is subjected to tensile force during pressurization.

[0088] It may be that the optical fiber section is subjected to shear force during pressurization, resulting in a small tensile effect on some optical fiber sections; stress adjustment inside the rock sample: At the initial stage of the experiment, there may be a pre-existing stress state or microstructural inhomogeneity inside the rock sample. The initial loading may cause the rearrangement of the microstructure or the redistribution of the stress state in local areas, and this adjustment may produce a dilation effect in a short time; the influence of cracks and pores: The cracks or pores existing in the rock sample may undergo a certain degree of closure or adjustment during the initial pressurization, which may produce abnormal strain readings in the initial stage.

[0089] The total length of the D4 optical fiber is 154.3 cm. From the waterfall plot of D4, it can be obtained that at 10.3 cm, 18 cm, and 33.9 cm, during the pressurization process from 25 seconds to 75 seconds, data of three sections with strain increasing from 0 to 33.9 positive strain are monitored. The strain is mainly concentrated in the area marked in the figure. During the subsequent stable time, the rock sample is continuously stressed during this stage, the value of the strain continuously decreases, stress is released, and the change in the strain data is small. From the data inference, the stress is relatively large near the three positions where the D4 optical fiber has a large strain change, and there may be small transverse cracks generated. The cracks are perpendicular to the minimum horizontal principal stress. When the cracks extend horizontally to the optical fiber, the cracks extend longitudinally along the optical fiber laying direction.

[0090] The total length of the D7 optical fiber is 174.8 cm, and the length laid inside the rock plane is 55 cm. From the waterfall diagram of the first-stage D7, it can be obtained that at the front end of the optical fiber buried in the rock formation, at the 12.7 cm position, during the pressure application process from 35 seconds to 65 seconds, a set of microstrain data gradually increasing from 0 to 33.9 positive was monitored.

[0091] 2. Pressure application in the second stage

[0092] The pressure on the X-axis and Y-axis is increased from 5 - 8 MPa, and the pressure application time is 113 seconds, with a stabilization time of 83 seconds. The data of the optical fiber D1 in the first layer is selected to draw a waterfall diagram (as Figure 9 shown); the data of the optical fiber D4 in the second layer is selected to draw a waterfall diagram (as Figure 10 shown), and the data of the optical fiber D7 in the third layer is selected to draw a waterfall diagram (as Figure 11 shown) for data processing and analysis.

[0093] As the confining pressure increases, points at 71.5 cm, 116.5 cm, and 164.8 cm on the D1 optical fiber generate strain at 250 seconds, 225 seconds, and 250 seconds respectively. Extending from this point position to both sides, the strain band becomes wider and wider, that is, the optical fiber section where strain occurs becomes longer and longer, with a total of three strain concentration areas. Its strain data changes from positive strain in the early stage to negative strain, indicating that the optical fiber is under pressure in this stage, and the behavior of the rock sample changes to a conventional compression mode, which conforms to the experimental expectation. Its strain data all gradually increases from 0 to -33.9 microstrain, showing an increasing trend.

[0094] As the confining pressure increases, the point at 99.7 cm on the D4 optical fiber receives strain information at 200 seconds. Extending from this point position to both sides, the strain band becomes wider and wider, and rapidly widens at 250 seconds, and the optical fiber section where strain occurs rapidly becomes longer, mainly concentrated in one area. Its strain data is negative, indicating that the optical fiber is under pressure in this stage. Its strain data also gradually increases from 0 to -33.9 microstrain. It is speculated that around 250 seconds, a crack may have occurred between D4 and D1, and then extended in the transverse direction, covering the range monitored by the two optical fibers, so the two optical fibers received strain information with similar data magnitudes within a relatively close time.

[0095] With the increase of confining pressure, points at 53.3 cm, 115.6 cm, and 164.4 cm on the D7 optical fiber generated strain at the 180th second. Strain bands became wider and the length of the optical fiber segments with strain increased as they extended from these points to both sides. The strain data was negative, indicating that the optical fiber was under pressure during this stage. Several obvious horizontal stress concentrations in the figure showed that stress was concentrated in these areas, resulting in significant strain values. All the strain data showed a trend of increasing strain with the increase of confining pressure. Especially at the point of 51.6 cm, the color changed rapidly from red to blue and the strain data increased rapidly, presumably due to factors such as the inhomogeneity within the rock, such as pores, fractures, or uneven hardness distribution within the rock.

[0096] 3. Third-stage pressure application

[0097] The pressure on the X-axis and Y-axis was increased from 8 - 10 MPa, taking 102 seconds for the pressure application and 79 seconds for stabilization. The data of the optical fiber D1 in the first layer was selected to plot a waterfall plot (as Figure 12 shown), the data of the optical fiber D4 in the second layer was selected to plot a waterfall plot (as Figure 13 shown), and the data of the optical fiber D7 in the third layer was selected to plot a waterfall plot (as Figure 14 shown) for data processing and analysis.

[0098] During the continuous application of confining pressure from 350 seconds to 450 seconds, points around 71 cm, 130 cm, 164 cm, and 194 cm on the D1 optical fiber accumulated relatively large negative strain. The waterfall plot showed that the overall rock sample presented a fluctuating negative strain, with three strain concentration areas, which was consistent with the "U" - shaped structure arranged in the experiment, that is, closer to the pressure application surface, the strain was larger, meeting the experimental expectations. There were more blue areas in the waterfall plot, indicating that in most of the measurement areas, the rock sample experienced the expected compression, that is, as the experiment progressed, the stress - strain relationship within the rock reached a more stable state, making the strain distribution more uniform.

[0099] During the third - stage pressure application, the strain concentration area of the D4 optical fiber was still similar to that in the second stage, but the width of the concentration area increased significantly, indicating that with the increase of the applied confining pressure, more optical fiber segments received larger strain growth data, and the strain magnitude was also larger than that in the second stage, conforming to the law that the strain magnitude increases with the increase of pressure. It can also be seen from the waterfall plot that the data received by the D4 optical fiber was very regular, indicating that the rock around this optical fiber transmitted relatively uniform negative strain information, conforming to the conventional compression behavior under continuous pressure.

[0100] In the third-stage pressurization, there are three main strain concentration zones and multiple shorter strain concentration zones around them on the D7 optical fiber. It can be seen from the three-dimensional diagram that there are also a small number of very tiny positive strain data, presumably due to the generation of cracks, which causes some optical fiber segments to be stretched. At the same time, as the pressure increases, the cracks expand, resulting in some optical fibers around the main strain concentration zones also receiving concentrated and relatively large negative strain information.

[0101] 4. Pressurization in the fourth stage

[0102] The pressurization of the X-axis and Y-axis ranges from 10 - 15 MPa, and the pressurization time is 218 seconds, with a stabilization time of 82 seconds. The data of the optical fiber D1 in the first layer is selected to plot a waterfall diagram (as Figure 15 shown), the data of the optical fiber D4 in the second layer is selected to plot a waterfall diagram (as Figure 16 shown), and the data of the optical fiber D7 in the third layer is selected to plot a waterfall diagram (as Figure 17 shown) for data processing and analysis.

[0103] During the continuous application of confining pressure from 450 seconds to 800 seconds, the strain data of the optical fiber segments on the D1 optical fiber that previously accumulated relatively large negative strain increased, and some additional negative strain concentration zones appeared, as well as two positive strain concentration zones with a relatively large range and relatively small strain data. This phenomenon may be closely related to the development and behavior of internal cracks in the rock.

[0104] During the fourth-stage pressurization, compared with the third stage, two relatively large positive strain concentration zones appeared at both ends of the D4 optical fiber, with a maximum of 135.7 positive microstrains, indicating that this section of the optical fiber was strongly stretched.

[0105] During the fourth-stage pressurization, for the D7 optical fiber, both the magnitude of the strain data and the width of the strain concentration zone increased compared with the third stage, meeting the experimental expectations under the pressurization conditions. At the same time, some of the original tiny positive strain data also increased on the original basis, which may be caused by factors such as the formation of new cracks, the expansion of old cracks, and the redistribution of stress states.

[0106] 5. Stress transfer law

[0107] Through comparative studies on the data of two optical fibers in the first layer, four optical fibers in the second layer, and two optical fibers in the third layer during four pressurization stages, and selecting two moments in each pressurization stage, a total of 6 diagrams were plotted. The first stage is as Figure 18 shown, the second stage is as Figure 19 shown, the third stage is as Figure 20 shown, and the fourth stage is as Figure 21 shown. Through the strain information received by the optical fiber at different moments, that is, through the color representing the magnitude of the strain on the optical fiber, the stress transfer law during pressurization is reflected.

[0108] Through the data analysis of four pressurization stages, the stress transfer law of rocks under different confining pressure conditions and the relationship with crack propagation behavior are revealed. The change of the stress transfer path directly affects the strain data monitored by the optical fiber, reflecting the process of internal stress redistribution in the rock. The stress conduction law can be reflected through the analysis and processing of the strain data. The following is the analysis and explanation of the strain data, stress conduction law and abnormal phenomena during the whole experiment process.

[0109] The strain data shows that the direction of stress transfer is from the compressed surface to the interior of the rock sample. When selecting the optical fiber strain distribution map at a certain moment during the third-stage pressurization to draw the stress conduction direction, as Figure 22 shown. Comparing Figure 19 and Figure 20 in the change of the optical fiber data color, especially in the X-axis direction, the change of the strain data is particularly significant. It can be observed that as the pressurization progresses, from the pressurized surface to the interior, the color gradually deepens in the X-axis direction. Connecting the strain data on the X-axis from large to small with arrows, it can be clearly seen that the stress is transmitted from the compressed surface to the interior of the rock sample.

[0110] In the initial stage of pressurization, the optical fiber detects a small positive strain along the whole length, which may be due to the tensile effect caused by the shear force on the optical fiber segment; as the confining pressure increases, the strain data gradually turns into negative strain. The strain data of the optical fiber segment where larger negative strain was previously concentrated increases, and some negative strain concentration areas are added, indicating that the optical fiber is under pressure and the rock enters the conventional compression mode. However, there are still some smaller positive strain data, and this phenomenon may be closely related to the development and behavior of internal cracks in the rock. The following is the explanation and analysis of this phenomenon:

[0111] ① Negative stress concentration section: During the compression process of the rock sample, the original microcracks or pores may gradually close under the action of pressure. This crack closing effect will cause stress to concentrate in the closed area, thus being recorded as negative stress concentration on the optical fiber sensor; as the pressure continues to increase, the compression of the rock material may reach the extreme in some areas, especially in the relatively dense central area. This extreme compression will also cause significant negative stress concentration.

[0112] ② Positive stress concentration section: During the pressurization process, new cracks may form inside the rock, or the original microcracks may produce tensile effects on the surrounding materials during the expansion process. This crack expansion can be recorded as positive stress concentration on the optical fiber sensor because the formation and expansion of cracks will cause an increase in the tension of the local material; when the cracks inside the rock sample expand to a certain extent, the surrounding area may experience stress release, especially in the area near the cracks. This stress release may be manifested as positive stress concentration on the optical fiber sensor because the material in the local area changes from the compressed state to the tensile state.

[0113] When the loading pressure is very high, around 15 MPa, a small part of relatively large positive strain data is monitored, which may be caused by:

[0114] ① Formation and development of internal cracks: During the process of applying confining pressure, new cracks may form or existing micro-cracks may expand in the rock sample due to uneven pressure distribution. The formation and expansion of such cracks can cause the surrounding rock material to experience tensile strain, i.e., the material is stretched. This usually occurs on both sides of the crack because the surrounding area of the rock needs to expand to accommodate the spatial change when the crack forms.

[0115] ② Stress redistribution: When internal cracks develop or existing cracks are adjusted due to the applied external pressure, the stress state within the rock sample will redistribute. In some cases, this redistribution may lead to stress release in local areas, resulting in positive strain. Especially when the pressure acts on the harder or denser rock areas around the cracks, these areas may expand outward, showing positive strain.

[0116] ③ Response of fiber optic sensors: The fiber optic sensors are arranged in a "U" shape in the rock sample and may respond more sensitively to strain changes at geographical locations in areas with more developed cracks or stress concentration points. If cracks develop along the path where the sensors are arranged, then these areas may show a greater concentration of positive strain.

[0117] Since no pressure is applied in the Z-axis direction, only in the X and Y-axis directions, and due to the relatively large weight of the rock sample itself while the upper bottom surface is not subjected to any pressure at all, due to the effect of stress release, the rock sample will be subjected to a stress similar to upward extrusion, making the change in strain data monitored by the optical fibers closer to the upper bottom surface within the rock sample more significant. When selecting the fiber optic strain distribution diagram at a certain moment during the second stage of pressure application to plot the stress conduction direction, as Figure 23 shown. Comparing Figure 18 and Figure 19 in the change of the fiber optic data color, it can be seen from the figure that from the central plane of the rock sample to the upper and lower bottom surfaces, the color gradually deepens, and the strain data monitored by the fiber optic increases gradually, with the increase being more obvious at the upper bottom surface. It can be summarized that during pressure application, due to the effect of stress release, stress will tend to conduct towards the non-compressed surface.

[0118] Analyzing the data of the entire experimental process, as the confining pressure increases, in the X-axis direction, the strain data changes significantly, and the overall stress conduction direction shows a law of conduction from the compressed surface towards the interior. At the same time, as the pressure application progresses, the data monitored by the fiber optic sensors near the upper and lower bottom surfaces of the rock sample increases significantly. Due to the effect of stress release, stress has a tendency to conduct towards the upper and lower bottom surfaces. The schematic diagram of the stress conduction law is as Figure 24As shown

[0119] In summary, through the distributed optical fiber monitoring technology, this technical solution first demonstrated the integrity of the optical fiber through a tapping experiment and determined the position of the starting section of the optical fiber. Then, through experimental data, it analyzed in detail the stress conduction law of the rock under different confining pressure conditions and the relationship with the crack propagation behavior. The results show that: ① The fiber optic sensor can effectively capture the strain changes caused by crack propagation and stress redistribution, revealing the complexity of stress conduction during the formation and propagation of cracks. ② Strain concentration often indicates the formation and propagation of cracks, and the non-uniformity of strain distribution reflects the discontinuity of the internal structure of the rock. ③ Stress conduction is from the compressed surface to the inside, and due to the effect of stress release, stress will also conduct to the surface that is not compressed. The distributed optical fiber sensing technology demonstrates its advantages in monitoring rock strain with high sensitivity and high resolution, providing strong technical support for understanding the rock stress transfer mechanism.

[0120] The above are only embodiments of the present invention. Specific technical solutions and / or common knowledge such as characteristics well known in the art are not described in detail herein. It should be noted that for those skilled in the art, without departing from the technical solution of the present invention, several deformations and improvements can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicality of the patent. The protection scope required by this application should be based on the content of its claims, and the specific implementation manners described in the specification can be used to interpret the content of the claims.

Claims

1. A rock stress monitoring method based on distributed optical fiber, characterized in that: The steps include: Step 1, preparation of cubic rock samples: during the pouring process of rock samples, optical fibers are laid in each layer, and the integrity of the optical fibers is checked before laying. The laying method of optical fibers is as follows: with the wellbore in the rock as the center, four layers of rock samples are poured, each layer is 10 cm high, and three layers of "U"-shaped optical fibers are laid; a total of 8 optical fibers are laid in three layers, and two optical fibers are laid together in each "U"-shaped optical fiber, and the outer optical fibers are semi-enclosed and distributed outside the inner optical fibers; two optical fibers are arranged in the first layer, four optical fibers are arranged in the second layer, and two optical fibers are arranged in the third layer; Step 2: Conduct integrity inspection on the optical fiber in the rock sample; Step 3: Stress loading and detection test: Apply instantaneous point stress load by knocking on the rock from the outside and squeeze the rock in the triaxial pressure chamber from different directions of X, Y, and Z to uniformly distribute stress load, and analyze the stress conduction in the rock by analyzing the optical fiber deformation.

2. A rock stress monitoring method based on distributed optical fiber according to claim 1, characterized in that: In step 1, the method for pouring the rock sample includes the following steps: S1. Mould and wellbore preparation; S2, weigh the raw materials according to the proportion, stir them evenly and vibrate to remove bubbles for later use; S3, optical fiber parameter and integrity test; S4. Lay the optical fiber to each layer according to the design plan, observe and record the static time; S5. Demoulding and maintenance.

3. A rock stress monitoring method based on distributed optical fiber according to claim 2, characterized in that: In step 1, the raw material of the rock sample is a mixture of cements of different mesh sizes.

4. A rock stress monitoring method based on distributed optical fiber according to claim 3, characterized in that: In step 1, the optical fiber is a single-mode optical fiber with a diameter of 125 μm and a coating diameter of 250 μm.

5. A rock stress monitoring method based on distributed optical fiber according to claim 4, characterized in that: In step one, the integrity inspection method of the optical fiber is as follows: cut the optical fiber, put it into a heat shrink tube, wipe it with alcohol paper, and put it into the optical fiber fusion splicer for fusion splicing. After the fusion splicing is completed, use a red light pen to check. If you can clearly see red light at the end of the optical fiber, the optical fiber performance is intact.

6. A rock stress monitoring method based on distributed optical fiber according to claim 5, characterized in that: In step three, the optical fiber is pinched before testing to measure the strain data of the optical fiber, and the test range of the signal and the position of the strain response on the optical fiber are determined by testing the front and rear ends of the optical fiber.

7. A rock stress monitoring method based on distributed optical fiber according to claim 6, characterized in that: In step three, the fracturing test includes confining pressure test and hydraulic fracturing test.

8. A rock stress monitoring method based on distributed optical fiber according to claim 7, characterized in that: The confining pressure test adopts staged pressurization.

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