Double-well hydraulic fracturing crack propagation visualization method based on spiral optical fiber monitoring

Abstract

The invention discloses a double-well hydraulic fracturing crack extension visualization method based on spiral optical fiber monitoring. The method comprises the steps that firstly, an equidistant spiral support is manufactured through a 3D printing method; the optical fiber is pasted on the support; pouring an artificial sample by using cement; then carrying out water injection fracturing operation, and recording data of optical fiber strain changing along with time and optical fiber length; mapping strain values of different length positions of the optical fiber along with time to the spiral optical fiber through Matlab; and finally, extracting strain abrupt change points on the optical fibers, connecting the strain abrupt change points on the upper and lower spiral optical fibers of the same shaft at similar time and in similar directions into lines, finally connecting the lines into facets to form a geometric shape of the crack, and then displaying the facets in sequence based on the occurrence time of the strain abrupt change points, so that the visualization of the crack expansion process is realized. According to the invention, the formation process of the crack can be clearly and visually displayed through a unique and reasonable optical fiber laying method and an effective means of visualizing the crack expansion process.

Description

Technical Field

[0001] The present invention belongs to the technical field of distributed optical fiber monitoring, and specifically relates to the dynamic monitoring and visual interpretation of fracture propagation in physical simulation of hydraulic fracturing in petroleum engineering. Background Art

[0002] In many fields such as rock mechanics, petroleum engineering, and building structures, the phenomenon of fracture propagation has a crucial impact on the safety, stability, and durability of structures. For example, in oil exploitation, the propagation of reservoir fractures affects oil and gas production and exploitation efficiency; the development of fractures in rock structures may trigger geological disasters. Traditional fracture monitoring technologies have significant limitations. The strain monitoring methods based on strain gauges and displacement sensors can only monitor the strain value at a single point, and the installation is complex and the spatial resolution is low. It is easily interfered by environmental factors, resulting in large deviations in measurement results. The ultrasonic detection and electromagnetic induction technologies based on the principles of acoustics and electromagnetics are limited by the medium characteristics, structural complexity, and conductivity of the monitoring object, and it is difficult to accurately and comprehensively monitor the fracture propagation. Summary of the Invention

[0003] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a visualization method for fracture propagation in dual-well hydraulic fracturing based on spiral optical fiber monitoring, which combines 3D printing technology and distributed optical fiber monitoring technology. By arranging optical fibers in an equidistant spiral shape, it can accurately monitor the strain during the whole process of hydraulic fracture initiation and propagation in the fracturing physical model, and can capture the strain information of each point inside the rock sample to the greatest extent.

[0004] The purpose of the present invention is achieved through the following technical solutions: A visualization method for fracture propagation in dual-well hydraulic fracturing based on spiral optical fiber monitoring includes the following steps:

[0005] First step: Print multiple spiral optical fiber brackets with equidistant spirals by 3D printing. The inner diameter of the spiral optical fiber bracket is larger than the wellbore diameter, and the outer diameter is smaller than the maximum size of the hydraulic fracturing sample.

[0006] Second step: Preparation and pretreatment of optical fibers: Paste the single-mode bare optical fiber along the inner side wall of the spiral optical fiber bracket; during the pasting process, leave a length of 3 - 5 cm for the free end of the single-mode bare optical fiber located at the center of the spiral optical fiber bracket, and make a circle with a diameter of 0.5 - 1 cm; leave a length of 300 cm for the free end of the single-mode bare optical fiber located at the outermost side of the spiral optical fiber bracket for connection with an external optical fiber monitoring instrument.

[0007] Step 3: Manufacture of artificial specimens and fiber optic cable layout: Place two brackets with fiber optic cables attached into a mold, with the positions of the two fiber optic cables corresponding vertically; Pass an artificial wellbore through the centers of the two layers of brackets, with the bottom of the artificial wellbore close to the bottom of the mold and the top extending out of the mold; Lead out the free ends of the fiber optic cables for connection to external fiber optic monitoring instruments, then pour mixed mortar into the mold, demold after standing for 24 hours, and cure for 30 days or more in accordance with the relevant specifications for cement specimen curing to obtain specimens;

[0008] Step 4: Fiber optic integrity monitoring and fiber optic parameter determination: Use a fiber optic fusion splicing device to splice the free ends led out from the fiber optic cables to fiber optic jumpers, and connect the fiber optic monitoring instrument through the fiber optic jumpers; Create strain disturbances at the positions where the fiber optic cables are connected on the surface of the specimen, and confirm the effective monitoring length and fiber optic monitoring positions of the fiber optic cables through the fiber optic monitoring instrument;

[0009] Then input the fiber optic monitoring parameters into the fiber optic monitoring instrument, and continue the test after preparation;

[0010] Step 5: Conduct hydraulic fracturing test: Lift the specimen into the true triaxial hydraulic fracturing confining pressure loading cavity, connect the artificial wellbore on the specimen and the injection pipeline of the fracturing machine with a high-pressure connector, and debug the true triaxial hydraulic fracturing equipment and fiber optic monitoring instrument for injection;

[0011] Apply triaxial stress to the rock sample: Inject fracturing fluid into the artificial wellbore to form triaxial stress on the rock sample; At the same time as starting to inject the fracturing fluid, start the fiber optic monitoring instrument to record the strain response along the entire fiber optic cable, and obtain the strain data at different times and different distances on the helical fiber optic cable; The strain data is a two-dimensional matrix, where each column corresponds to the strain data of all monitoring positions on the entire fiber optic cable within the monitoring range of a time point, and each row corresponds to the fiber optic strain values at different time points at a certain monitoring position;

[0012] Step 6: Construction of helical lines in Matlab: Based on the given arc length range θ min to θ max , use the integral function arcLengthIntegral to calculate the arc lengths of the helical fiber optic cables at each monitoring position, and map the arc lengths to the range from θ min to θ max to obtain the values of the angle θ corresponding to each monitoring position;

[0013] After obtaining the angle θ at each monitoring position, calculate the coordinates x and y of the spiral in the Cartesian coordinate system according to the basic formula for the conversion between polar coordinates and rectangular coordinates, and at the same time set the z values of the two spirals for constructing the spiral in three-dimensional space;

[0014] Step 7, Data Import and Processing: Use the load function in MATLAB to load the fiber optic strain data into the workspace, and start to organize and expand the data to obtain the maximum strain value and the maximum value position index maxwz at each moment in each interval.

[0015] Step 8, Constructing Fractures Based on Strain Mutation Points: In the time series loop, when it is detected that the maximum strain on the helical fiber optic is significantly increased compared to the maximum strain in the previous time step at a certain time step, start to construct fractures.

[0016] Step 9, Extract all monitoring positions, then first connect the fracture break points of the helical fiber optics around the same wellbore to form line segments, and then connect the line segments to form a surface. When all line segments are connected, form fracture patches. All fracture patches form a fracture surface, and then display the fracture patches in chronological order to form the above-mentioned visualization of the dynamic expansion of hydraulic fracturing fractures.

[0017] The beneficial effects of the present invention are as follows: The main purpose of the present invention is to overcome the difficulties in accurately visualizing the fracture formation process and the strain distribution of rock samples during the fracture propagation research. The advantages of the present invention are specifically reflected in the following aspects:

[0018] (1) Integrate 3D printing technology and distributed fiber optic monitoring technology, use equidistant spiral-shaped fiber optic layout to monitor the strain of the whole process of hydraulic fracture initiation and propagation during the fracturing physical model process with high precision, and can capture the strain information of each point inside the rock sample to the greatest extent, providing a new characterization idea for in-depth analysis of the geometric shape and dynamic evolution process of the fracture.

[0019] (2) Compile an inversion program for fracture propagation morphology based on fiber optic strain data. According to the parameters of the spiral support (inner diameter, outer diameter, pitch, etc.) and the length position and strain intensity where the fiber optic strain undergoes mutation, characterize the three-dimensional fracture geometry and evolution characteristics of the fracturing physical model, accurately interpret the characteristics such as hydraulic fracture initiation and propagation, and realize the high-precision visualization characterization of the three-dimensional space fractures of the fracturing physical model sample. Description of the Drawings

[0020] Figure 1 It is a flow chart of the method for visualizing the propagation of double-well hydraulic fracturing fractures based on helical fiber optic monitoring of the present invention;

[0021] Figure 2 It is a schematic diagram of the fiber optic support structure of the present invention;

[0022] Figure 3 It is a schematic diagram of the layout method of the helical fiber optic support of the present invention;

[0023] Figure 4 It is a schematic diagram of the mold after placing the fiber optic support;

[0024] Figure 5 It is a waterfall diagram of the strain response monitored by a helical optical fiber;

[0025] Figure 6 It is the response curve of the helical optical fiber;

[0026] Figure 7 It is a mapping diagram of the strain response values monitored by the helical optical fiber;

[0027] Figure 8 It is the inversion of the fracture morphology of hydraulic fracturing based on the strain response of the helical optical fiber;

[0028] Figure 9 It is a visualization diagram of the fractures of dual-well hydraulic fracturing. Specific embodiments

[0029] The present invention aims to overcome the defects of existing strain monitoring methods, use distributed optical fiber sensing technology to monitor the strain in the hydraulic fracturing physical model specimen, so as to indirectly reflect the dynamic crack propagation, and thus form a new hydraulic fracturing physical model crack monitoring scheme based on optical fiber strain monitoring means. Through an innovative layout method, combined with the helical spatial position calculation equation, it is ensured that the optical fiber strain data can accurately reflect the crack position and propagation dynamics, and improve the accuracy of hydraulic fracturing physical model crack characterization. The present invention can achieve accurate monitoring and clear and intuitive visualization of the crack propagation process, providing a more accurate, convenient and efficient technical means for the research and engineering applications related to the crack propagation of hydraulic fracturing in petroleum engineering, and has significant innovation, practicability and application value.

[0030] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0031] As Figure 1 shown, a method for visualizing the propagation of dual-well hydraulic fracturing cracks based on helical optical fiber monitoring of the present invention includes the following steps:

[0032] First step, use 3D printing to print multiple helical optical fiber brackets with equally spaced spiral lines, as Figure 2 shown; set the inner diameter d of the bracket (as shown by 1 in Figure 2 ), the outer diameter D (as shown by 2 in Figure 2 ), and the pitch P (as shown by 3 in Figure 2 ) according to the size of the rock sample. The inner diameter of the helical optical fiber bracket should be larger than the wellbore diameter, and the outer diameter should be slightly smaller than the maximum size of the hydraulic fracturing specimen (the side length of the cubic specimen and the outer diameter of the cylindrical specimen. In this embodiment, the cubic specimen is taken as an example). The fabricated optical bracket is an equally spaced spiral line with the number of turns n and the pitch P, and the center is a hollow area with a diameter d.

[0033] Step 2. Preparation and pre-treatment of optical fiber: Paste the single-mode bare optical fiber along the inner wall of the spiral optical fiber support (the side of the support close to the wellbore) with 502 strong glue. During the pasting process, leave a length of 3 - 5 cm for the free end of the single-mode bare optical fiber located at the center of the spiral optical fiber support, and make a loop with a diameter of 0.5 - 1 cm, so that the optical signal can be reflected back after reaching the end of the optical fiber, and the optical fiber monitor can monitor a stronger optical fiber signal, improving the monitoring accuracy. Leave a length of 300 cm for the free end of the single-mode bare optical fiber located at the outermost side of the spiral optical fiber support for connection with an external optical fiber monitoring instrument. As Figure 3 shown; Prepare more than 2 sets of optical fiber supports according to this method;

[0034] Step 3. Manufacture of artificial specimen and optical fiber layout: Use a specimen casting mold to make a specimen of a certain size (such as

[0035] 400 mm × 400 mm × 400 mm). Put two optical fiber supports with pasted optical fibers into the mold. The positions of the two optical fibers correspond to each other in the vertical direction. The positions and distances of the two supports can be determined according to the specimen size and the wellbore length. Generally, they are placed symmetrically up and down. For example, place the optical fibers at 1 / 4 and 3 / 4 of the mold height respectively (the positions are not strictly specified, as long as the crack propagation can be monitored). As Figure 4 shown, where 4, 5, and 6 are the first layer of the optical fiber support with pasted optical fiber, the second layer of the optical fiber support with pasted optical fiber, and the artificial rock sample of the fracturing material mold respectively; Pass the artificial wellbore through the centers of the two supports. The bottom of the artificial wellbore is close to the bottom of the mold, and the top extends out of the mold (the total length of the wellbore is less than the side length of the core. If the side length of the mold is 400 mm, the wellbore is about 10 cm away from the bottom of the mold after being placed in the mold. The specific distance can be adjusted according to actual needs to ensure that the bottom of the wellbore is inside the specimen after pouring); Prepare two artificial wellbores and optical fiber supports with pasted optical fibers according to the above method, lead out the free ends of the optical fibers used for connection with the external optical fiber monitoring instrument, and then pour the mixed mortar into the mold. Pay attention not to damage the optical fibers when pouring the mixed mortar. Demold after standing for 24 hours, and maintain it for 30 days or more according to the relevant specifications for the maintenance of cement specimens to obtain the specimen.

[0036] Step 4: Fiber optic integrity monitoring and fiber optic parameter determination: After the specimen maintenance is completed, use a fiber optic fusion device to fuse the free end of the fiber optic cable led out with a fiber optic jumper, and connect the fiber optic monitoring instrument through the fiber optic jumper; create a strain disturbance at the position where the fiber optic cable is connected on the specimen surface, and confirm the effective monitoring length and fiber optic monitoring position of the fiber optic cable through the fiber optic monitoring instrument. The fiber optic monitoring instrument monitors the signals along the entire length of the fiber optic cable. However, outside the specimen, there is still a section of fiber optic cable connected to the fiber optic monitoring instrument. Therefore, after the fiber optic cable is arranged and before the test, it is necessary to confirm which position of the fiber optic cable is the actual buried fiber optic length in order to obtain more accurate monitoring results later. The number of required fiber optic jumpers is the same as the number of spiral fiber optic brackets, that is, the fiber optic cables on one spiral fiber optic bracket are all separate fiber optic channels.

[0037] Then input the fiber optic monitoring parameters into the fiber optic monitoring instrument, including: select fiber optic channel, fiber optic length, scanning range, sampling interval, starting distance, ending distance, refractive index, spatial resolution, initial strain, strain coefficient, initial temperature, temperature coefficient, starting point of each channel, sensing resolution, sampling resolution, scanning mode (single / continuous scanning); after preparation, continue the test;

[0038] Step 5: Conduct a hydraulic fracturing test: Lift the specimen into the true triaxial hydraulic fracturing confining pressure loading cavity, connect the artificial wellbore on the specimen and the injection pipeline of the fracturing machine with a high-pressure joint, and debug the true triaxial hydraulic fracturing equipment and fiber optic monitoring instrument for injection; apply three-way stress to the rock sample: inject fracturing fluid into the artificial wellbore at a certain displacement to form three-way stress on the rock sample; at the same time as starting to inject the fracturing fluid, start the fiber optic monitoring instrument to record the strain response along the entire fiber optic cable; during the test, the true triaxial hydraulic fracturing equipment continuously collects the three-way stress values, injection pressure, and total injection fluid volume of the specimen; when observing a sharp drop in the injection pressure, stop injecting and stop the signal acquisition of the true triaxial hydraulic fracturing equipment and the fiber optic monitoring instrument.

[0039] The fiber optic monitoring instrument monitors and obtains the strain data at different times and different distances on the spiral fiber optic cable and stores it in the mat format; the strain data is a two-dimensional matrix, where each column corresponds to the strain data of all monitoring positions on the entire fiber optic cable within the monitoring range at a time point, and each row corresponds to the fiber optic strain values at different time points at a certain monitoring position of the fiber optic cable. Corresponding to the coordinate system, a spiral fiber optic monitoring strain response waterfall diagram with the abscissa being time and the ordinate being distance can be formed, as Figure 5 shown, Figure 5 (a) and (b) are the strain response waterfall diagrams of two fiber optic cables respectively, Figure 5 (a) The corresponding response curve is as Figure 6 shown.

[0040] Step 6. Construction of the spiral line in Matlab: First, determine the key parameters of the spiral optical fiber, including the pitch P, the number of turns n, and the maximum radius D. These parameters are all known parameters determined in the first step (the same as the parameters of the spiral optical fiber support equidistant from the spiral). Then, define the arc length calculation function arcLengthFunc of the spiral as follows:

[0041]

[0042] where r(θ) is the spiral radius corresponding to the rotation angle θ, and r′(θ) is the derivative of r(θ); α represents the starting angle of the spiral calculation. β represents the ending angle of the spiral calculation;

[0043] According to the given arc length range θ min to θ max and the number of points n, where n is the number of monitoring positions; use the integral function arcLengthIntegral to calculate the arc length of the spiral optical fiber at each monitoring position, and map the arc length to the range of θ min to θ max to obtain the value of the corresponding angle θ at each monitoring position; the integral function arcLengthIntegral calculates the arc length of the spiral between θ min and a certain angle θ through numerical integration, and finds the value of the angle θ corresponding to the equally spaced divided arc length values within the range of θ min to θ max ;

[0044] During the calculation, first create an arc length ratio sequence arcLengths from 0 to 1, then use the arcLengthIntegral function to find the arc length of the spiral optical fiber at each monitoring position, and then perform a normalization operation on the arc length; then use the fminbnd function within the set range of θ min to θ max to find the value of θ that minimizes the absolute value of the difference between the normalized arc length at each monitoring position and the elements in the arc length ratio sequence, and use it as the angle θ at that monitoring position to achieve the conversion from arc length calculation to angle solution;

[0045] After obtaining the angle θ at each monitoring position, calculate the coordinates x and y of the spiral in the Cartesian coordinate system according to the basic formula (2) for the conversion between polar coordinates and rectangular coordinates, and at the same time set the z values of the two spirals for constructing the spiral in three-dimensional space:

[0046]

[0047] Step 7: Data Import and Processing: Use the load function in MATLAB to load the fiber optic strain data into the workspace, and start organizing and expanding the data to obtain the maximum strain value and the index maxwz of the maximum value position at each moment for each interval.

[0048] First, generate an equally spaced new time series, and then match the time series of the original fiber optic strain data with the new time series; for missing data, that is, time points that exist in the new time series but do not appear in the original time series, fill the corresponding data rows in the new time series with zeros; for time points that exist in both the original time series and the new time series, copy the corresponding data values in the original time series to the new time series. The original collected data is not strictly equal in time intervals, so there may be missing data in the new time series. For missing data, the common filling methods are directly filling with 0 and filling with the average value of the adjacent data before and after. In this embodiment, filling with 0 is selected.

[0049] Extract the strain values representing the fiber at different positions and time points, and map the strain values to an equidistant spiral, as Figure 7 shown. The original data contains strain value data at different monitoring positions and different time points. The position of the monitoring position corresponds to the position of the spiral length. Only need to map the strain value to the corresponding length according to the monitoring position; then traverse each time period to obtain the strain values of each monitoring position on the equidistant spiral at different time points.

[0050] Divide the entire optical fiber into intervals according to the angle of the spiral. Starting from the free end at the center position of the spiral, each time the spiral rotates 180°, a interval idxCell is formed. The entire optical fiber contains nIntervals intervals; and the fiber optic strain data is also correspondingly divided into nIntervals sub-matrices for subsequent analysis of the strain characteristics of different regions; divide the entire angle range [θ min , θ max into nIntervals intervals, and use the find function to find the interval indexes and store them in the idxCell cell array idxCell{i}.

[0051] Call the processStressData function, which receives the strain data strainValues and the angle interval index idxCell as inputs, and finds the maximum value of the strain data in each angle interval and returns the maximum value and the corresponding index.

[0052] Process it through the function processStressData in combination with the divided interval idxCell; in the function processStressData, first obtain the strain value data matrix strainValues of each interval, then initialize an output matrix modifiedData, whose size is the same as the strain value data matrix of this interval and all elements in the matrix are 0; at the same time, initialize two cell arrays maxIdx and maxwz, which are used to store the maximum strain and its corresponding index of each interval respectively;

[0053] Then, perform data processing through two nested loops. The outer loop traverses each interval index. For each interval, first obtain the corresponding index idxCell{i} of this interval, and initialize the array maxIdx{i} for storing the maximum value position of this interval at each moment and the array maxwz{i} for storing the index corresponding to the maximum value;

[0054] The inner loop is for each time step. Extract the data currentData of this interval from the data of the current time step. The data currentData is a vector containing the strain values corresponding to all monitoring positions within this interval; find the maximum strain value within this interval at the current time step through the max function, store it in maxIdx{i}(t), and use the monitoring position corresponding to the maximum strain value as the index and store it in maxwz{i}(t); then, only keep the value of the monitoring position with the maximum strain value, and set the strain values of other monitoring positions in this interval at the current time step to 0, thus completing the processing of the data for each interval; then proceed to the data of the next time step until all time steps are traversed.

[0055] Finally, obtain the processed data modifiedData, in which only the maximum strain value is retained for each interval at each moment, and the rest are 0, as well as the index maxwz of the maximum value position for each interval at each moment; after processing through the processStressData function, obtain the processed data and the index of the maximum value point for each interval, providing key data support for the subsequent operation of connecting cracks based on the monitoring positions.

[0056] Step 8: Construct cracks based on strain mutation points: In the time series loop, when the maximum strain on the spiral optical fiber detected at a certain time step shows a significant increase compared to the maximum strain at the previous time step (the specific regulation for this significant increase can be set according to the materials used), start constructing cracks; traverse the index of the maximum value point, and obtain the coordinate points on the corresponding spiral from the spiral coordinate array through the index value, including the x, y, z coordinates and the corresponding strain values. Store the time, coordinate points, and strain values at the strain mutation point in a new array, and map the strain value to the corresponding spatial position on the spiral line. Its color attribute corresponds to the magnitude of the strain value, and use the scatter3 function to mark a mutation point; judge the orientation and occurrence time of the strain mutation points on the spiral optical fiber. If the azimuth difference and time difference between two points are less than the preset threshold, connect these two strain mutation points into a line segment, as Figure 8 shown; then connect adjacent line segments into a quadrilateral patch, and connect multiple patches in sequence according to the time order of the appearance of the strain mutation points, so as to form a continuous surface and visually display the shape of the crack; when the pitch of the spiral support is smaller, the connected patches are narrower, and the characterized crack will be finer.

[0057] Step 9: Based on the monitoring and crack inversion characterization method of a single spiral optical fiber, extract the monitoring positions on all supports. Then, first connect the crack fracture points (i.e., the points with larger strain) of the spiral optical fibers around the same wellbore to form line segments, and then connect the line segments to form a surface. When all the line segments are connected, a crack patch is formed. All the crack patches form a crack surface. Then, display the crack patches in sequence according to the time sequence, forming the above-mentioned visualization of the dynamic expansion of hydraulic fracturing cracks, as Figure 9 shown, Figure 9 which is the visualization diagram of the hydraulic fracturing cracks in the double wells in this embodiment.

[0058] Those of ordinary skill in the art will realize that the embodiments described herein are to assist the reader in understanding the principles of the present invention, and it should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those of ordinary skill in the art can make various other specific deformations and combinations without departing from the essence of the present invention based on the technical revelations disclosed in the present invention, and these deformations and combinations are still within the protection scope of the present invention.

Claims

1. A visualization method for the fracture propagation of double-well hydraulic fracturing based on the monitoring of spiral optical fibers, characterized in that It includes the following steps: First step: Print multiple spiral optical fiber supports with equidistant spirals by means of 3D printing. The inner diameter of the spiral optical fiber support is larger than the wellbore diameter, and the outer diameter is smaller than the maximum size of the hydraulic fracturing specimen; Second step: Preparation and pretreatment of optical fibers: Paste the single-mode bare optical fiber along the inner side wall of the spiral optical fiber support; during the pasting process, leave a length of 3 - 5 cm for the free end of the single-mode bare optical fiber located at the center of the spiral optical fiber support, and make a loop with a diameter of 0.5 - 1 cm; leave a length of 300 cm for the free end of the single-mode bare optical fiber located at the outermost side of the spiral optical fiber support for connection with an external optical fiber monitoring instrument; Third step: Artificial specimen production and optical fiber layout: Place two optical fiber - pasted supports into a mold, and the positions of the two optical fibers correspond to each other in the vertical direction; Pass the artificial wellbore through the centers of the two layers of supports, with the bottom of the artificial wellbore close to the bottom of the mold and the top extending out of the mold; Lead out the free end of the optical fiber used for connection with the external optical fiber monitoring instrument, then pour the mixed mortar into the mold, demold after standing for 24 hours, and cure according to the relevant specifications for cement specimen curing for 30 days or more to obtain the specimen; Fourth step: Optical fiber integrity monitoring and optical fiber parameter determination: Use an optical fiber fusion device to fuse the free end led out from the optical fiber with an optical fiber jumper, and connect the optical fiber monitoring instrument through the optical fiber jumper; Create a strain perturbation at the position where the optical fiber is connected on the specimen surface, and confirm the effective monitoring length and the optical fiber monitoring position through the optical fiber monitoring instrument; Then input the optical fiber monitoring parameters into the optical fiber monitoring instrument, and continue the experiment after preparation; Fifth step: Conduct a hydraulic fracturing test: Lift the specimen into the true triaxial hydraulic fracturing confining pressure loading cavity, connect the artificial wellbore on the specimen and the injection pipeline of the fracturing machine with a high - pressure joint, and debug the true triaxial hydraulic fracturing equipment and the optical fiber monitoring instrument for injection; Apply triaxial stress to the rock sample: Inject fracturing fluid into the artificial wellbore to form triaxial stress on the rock sample; At the same time as starting to inject the fracturing fluid, start the optical fiber monitoring instrument to record the strain response of the entire optical fiber, and obtain the strain data at different times and different distances on the spiral optical fiber; The strain data is a two - dimensional matrix, where each column corresponds to the strain data of all monitoring positions on the entire optical fiber within the monitoring range of a time point, and each row corresponds to the optical fiber strain values at different time points at a certain monitoring position; Step 6: Construction of the spiral line in Matlab: According to the given arc length range θ min from θ max to θ min and using the integral function arcLengthIntegral to calculate the arc lengths of the spiral optical fibers at each monitoring position, map the arc lengths to the range from θ max to θ to obtain the values of the corresponding angle θ at each monitoring position; After obtaining the angle θ of each monitoring position, calculate the coordinates x, y of the spiral in the Cartesian coordinate system according to the basic formula for the conversion between polar coordinates and rectangular coordinates, and at the same time set the z - value of the two spirals to construct a helix in three - dimensional space; Seventh step: Data import and processing: Use the load function of MATLAB to load the optical fiber strain data into the workspace, and start to organize and expand the data to obtain the maximum strain value and the maximum value position index maxwz at each moment in each interval; Eighth step: Construct cracks based on strain mutation points: In the time - series loop, when it is detected that the maximum strain on the spiral optical fiber in a certain time step increases significantly compared with the maximum strain in the previous time step, start to construct cracks; Step 9: Extract all monitoring positions. First, connect the crack fracture points of the helical optical fibers around the same wellbore to form line segments, and then connect the line segments to form a surface. When all the line segments are connected, a crack patch is formed. All the crack patches form a crack surface. Then, display the crack patches in chronological order to form the visualization of the dynamic expansion of the above hydraulic fracturing cracks.

2. The visualization method for the fracture propagation of dual-well hydraulic fracturing based on spiral optical fiber monitoring according to claim 1, wherein The specific implementation method of Step 7 is as follows: First, generate a new equidistant time series, and then match the time series of the original optical fiber strain data with the new time series; for the missing data, that is, the time points that exist in the new time series but do not appear in the original time series, fill the corresponding data rows in the new time series with zeros; for the time points that exist in both the original time series and the new time series, copy the corresponding data values in the original time series to the new time series. Extract the strain values representing what the optical fiber senses at different positions and time points, and map the strain values to an equidistant spiral. The entire optical fiber is divided into intervals according to the angle of the helix. Starting from the free end at the center of the helix, each 180° rotation of the helix forms an interval idxCell, and the entire optical fiber contains nIntervals intervals; the optical fiber strain data is correspondingly divided into nIntervals sub-matrices; the entire angular range [θ min , θ max is divided into nIntervals intervals, and the find function is used to find the indexes of each interval and store them in the idxCell cell array idxCell{i}; Process through the function processStressData in combination with the divided interval idxCell; in the function processStressData, first obtain the strain value data matrix of each interval, and then initialize an output matrix modifiedData, whose size is the same as the strain value data matrix of this interval and all elements in the matrix are 0; at the same time, initialize two cell arrays maxIdx and maxwz, which are used to store the maximum strain value of each interval and its corresponding index respectively. Then, perform data processing through two nested loops. The outer loop traverses each interval index. For each interval, first obtain the corresponding index idxCell{i} of this interval, and initialize the array maxIdx{i} for storing the maximum value position of this interval at each moment and the array maxwz{i} for storing the index corresponding to the maximum value. The inner loop is for each time step. Extract the data currentData of this interval from the data of the current time step. The data currentData is a vector containing the strain values corresponding to all monitoring positions within this interval; find the maximum strain value within this interval at the current time step through the max function, store it in maxIdx{i}(t), and use the monitoring position corresponding to the maximum strain value as the index and store it in maxwz{i}(t); then, only keep the value of the monitoring position with the maximum strain value, and set the strain values of other monitoring positions in this interval at the current time step to 0, so as to complete the data processing for each interval; then proceed to the data of the next time step until all time steps are traversed.

3. The visualization method for the fracture propagation of the double-well hydraulic fracturing based on the spiral optical fiber monitoring according to claim 1, wherein The specific implementation method of the eighth step is as follows: traverse the index of the maximum value point, and obtain the coordinate points on the corresponding spiral from the spiral coordinate array through the index value, including the x, y, z coordinates and the corresponding strain values. Store the time, coordinate points and strain values at which the strain mutation points appear in a new array, and map the strain values to the corresponding spatial positions on the spiral line. Its color attribute corresponds to the magnitude of the strain value, and use the scatter3 function to mark a mutation point; judge the orientation and appearance time of the strain mutation points on the spiral optical fiber. If the azimuth difference and time difference between the two points are less than the preset threshold, connect these two strain mutation points into a line segment; Then connect the adjacent line segments into a quadrilateral patch, and connect multiple patches in sequence according to the time order of the appearance of the strain mutation points, so as to form a continuous surface and visually display the shape of the crack.

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