Device and method for monitoring expansion process of hydrate-containing sediment crack

Abstract

The invention provides a device and a method for monitoring an expansion process of a hydrate-containing sediment crack, the device comprises an injection system, a reaction kettle, a temperature control system, an automatic pressure return system, a vacuum-pumping system, a data processing system and a non-contact strain observation system, a visual upper plug and a visual pressure head of the reaction kettle are combined to form a visual window; the non-contact strain observation system comprises a high-speed camera, and the high-speed camera is located above the visual window and electrically connected with the data processing system. According to the method, the crack initiation and expansion rule of the hydrate-containing sediment sample in the reservoir transformation process can be intuitively obtained; the method comprises the following steps: capturing a crack initiation point position, a crack extension direction and an evolution rule of a micro-crack on an injection surface in a fracturing crack-making process within an extremely short time, and acquiring strain change on the fracturing fluid injection surface after fracturing fluid injection and before a sample cracks; in combination with an acoustic emission monitoring means, the crack distribution characteristics in the hydrate-containing sediment sample can be further analyzed.

Description

Technical Field

[0001] The present invention belongs to the technical field of marine natural gas hydrate resource development engineering, and specifically relates to a device and method for monitoring the initiation and expansion process of hydraulic fractures in hydrate-containing sediments based on a digital image correlation method non-contact strain monitoring technology. Background Art

[0002] Natural gas hydrates (hereinafter referred to as hydrates) are unconventional strategic energy with great potential, and more than 95% of them are found in the ocean. At present, global hydrate energy research has shifted from the resource exploration stage to the integrated exploration and trial production stage. The most important indicator of whether hydrates can be industrialized is whether the daily production capacity of a single well meets the industrialization standards. At present, more than ten trial productions have been carried out in the world. Although the trial production capacity and trial production cycle are constantly increasing, the daily production capacity of a single well is still 2 to 3 orders of magnitude away from the threshold of industrial production. Substantially increasing production capacity from an order of magnitude scale is the key to realizing the industrialization of hydrate production.

[0003] However, due to the shallow burial, non-diagenetic and weak consolidation characteristics of marine hydrate reservoirs, it is still a world-class problem to increase the production capacity of hydrate pilot production from an order of magnitude scale. Domestic and foreign scholars have carried out a lot of useful explorations on increasing production in marine hydrate pilot production, and evaluated the feasibility of increasing production through technologies such as depressurization-assisted thermal stimulation, complex structure wells and multi-well network design, and artificial fracture transformation around the well. The results show that the core production increase mechanism of these methods is to improve the efficiency of hydrate decomposition, expand the hydrate decomposition front, and improve the seepage environment around the well. However, due to the uncertainties such as pilot production cost, safety risks, and technological level, except for the initial application of artificial fracture transformation technology around the well in the two rounds of hydrate pilot production in my country in 2017 and 2020, other production increase technologies are currently in the stage of indoor basic research.

[0004] Many scholars at home and abroad have conducted a large number of indoor experiments on artificial fracture reservoir transformation of hydrate sediments, analyzed the main controlling factors affecting the compressibility of hydrate reservoirs and the fracturing effect, and provided certain theoretical support for the design of reservoir transformation schemes in hydrate test production. However, most of the existing experimental methods can only observe the geometric morphology of the cracks in the sample after the fracturing is completed, or analyze the distribution of cracks during the fracturing process through indirect means such as acoustic emission. At present, there is still a lack of a non-contact and intuitive method to observe the initiation and extension of cracks in the process of fracture creation of hydrate-containing sediments after the injection of fracturing fluid, which restricts the research on the transformation mechanism of hydrate reservoirs. In addition, in addition to causing the main fractures, the hydraulic fracturing process will also induce a large number of micro-cracks and form damage zones. Conventional methods lack intuitive and quantitative characterization methods for micro-crack damage zones, which is not conducive to the study of the transformation mechanism of hydrate reservoirs. Summary of the invention

[0005] In order to solve the above-mentioned defects in the prior art, the present invention proposes a device and method for monitoring the expansion process of cracks in hydrate-containing sediments, which can non-contact and intuitively observe the initiation and expansion process of cracks in hydrate-containing sediments, obtain the damage and destruction characteristics of hydrate-containing sediments during hydraulic fracturing and the evolution mechanism of artificial fractures, and reveal the main controlling factors affecting the fracturing effect of hydrate-containing sediments.

[0006] Working principle: A low-temperature and high-pressure true triaxial experimental device was built, which can prepare hydrate sediment samples in situ and implement simulated hydraulic fracturing. Black and white sand were used to prepare hydrate sediment samples. After the fracturing fluid was injected, a high-speed camera was simultaneously turned on to continuously capture images of the injection surface. The strain evolution of the hydrate sediment samples in the initiation and expansion stages of the fracturing process was obtained by the digital image correlation method. Combined with other crack monitoring methods such as acoustic emission and fracture pressure, the main controlling factors affecting the fracture formation were comprehensively analyzed.

[0007] The technical solution adopted by the present invention is: A device for monitoring the crack expansion process of hydrate-containing sediments, comprising an injection system, a reactor, a temperature control system, an automatic back pressure system, a vacuum system, a data processing system and a non-contact strain observation system, wherein the reactor is connected to the injection system, the temperature control system, the automatic back pressure system and the vacuum system respectively, and the data processing system is connected to the non-contact strain observation system; The reactor comprises a visible upper plug, a visible pressure head, an acoustic emission probe, a sample filling cavity and a simulated wellbore, a confining pressure loading system, and a temperature control cavity. The simulated wellbore is arranged in the sample filling cavity. The visible pressure head is sealed and arranged on the upper part of the sample filling cavity. The visible upper plug is arranged on the upper part of the visible pressure head. The visible upper plug and the visible pressure head are combined to form a visualization window. The acoustic emission probe is arranged in the sample filling cavity. The confining pressure loading system is arranged outside the sample filling cavity. The temperature control cavity is arranged outside the sample filling cavity. The reactor is provided with a fluid injection port and a fluid outlet. The fluid injection port is connected to the simulated wellbore, and the fluid outlet is connected to the sample filling cavity. The injection system comprises a liquid phase injection system and a gas phase injection system, the liquid phase injection system is used to inject fracturing fluid into the sample filling cavity, and the gas phase injection system is used to inject gas into the sample filling cavity; The non-contact strain observation system includes a high-speed camera, an illumination system and a synchronization controller. The high-speed camera is located above the visualization window so that the visualization window is within the field of view of the high-speed camera. The synchronization controller is electrically connected to the illumination system and the high-speed camera respectively, and the high-speed camera is electrically connected to the data processing system.

[0008] The visualization window is used to cooperate with the non-contact strain observation system to monitor the real-time crack expansion on the fracturing fluid injection surface during the fracturing process.

[0009] The acoustic emission probe is placed inside the reactor to monitor the crack propagation law inside the hydrate-containing sediment sample.

[0010] Furthermore, the liquid phase injection system includes a liquid injection pump and an intermediate container, and the liquid injection pump is connected to the intermediate container; the gas phase injection system includes a gas cylinder, a gas booster pump and a gas flow controller connected by a pipeline; the liquid phase injection system and the gas phase injection system are mixed at the mixing injection pipeline and connected to the fluid injection port of the reactor.

[0011] The liquid injection pump and the intermediate container are used to inject fracturing fluid into the reactor to simulate the fracturing transformation of the hydrate reservoir.

[0012] The gas cylinder, gas booster pump and gas flow controller are used to inject methane gas into the reactor for sample preparation of hydrate-containing sediments. The gas flow controller can be a flow valve.

[0013] Furthermore, the liquid injection pump is a constant speed and constant pressure pump, and the constant speed and constant pressure pump has the intermediate container built in, and the intermediate container is filled with fracturing fluid.

[0014] Furthermore, the temperature control system includes a temperature control module inside the reactor and an injection fluid temperature control module; the temperature control system inside the reactor is used to inject pre-cooled temperature control liquid into the temperature control cavity to maintain temperature stability during the experiment, including a refrigeration coil and a high-pressure magnetic circulation pump; the injection fluid temperature control module includes a preheater, a temperature sensor and a refrigerated water bath, the temperature sensor and the preheater are electrically connected, and the preheater and the refrigerated water bath are respectively connected to the outlet of the liquid injection pump.

[0015] The temperature control module in the reactor is mainly used to inject pre-cooled temperature control liquid into the temperature control chamber of the reactor to maintain a stable temperature during the experiment.

[0016] The injection fluid temperature control module is used to realize the pre-cooling of fracturing fluid in conventional hydrate reservoir fracturing simulation and the pre-heating of injection fluid in the process of hot fluid fracturing.

[0017] Furthermore, the confining pressure loading system is a triaxial loading system, which is used to apply a set triaxial pressure to the hydrate-containing sediment sample, and includes upper and lower loading cylinders and a side loading cylinder.

[0018] Furthermore, the data processing system includes an image acquisition card and a data processing terminal, and the image acquisition card is electrically connected to the data processing terminal and the high-speed camera respectively.

[0019] Furthermore, the lighting system includes a laser indicator light and a blue light, and the laser indicator light is focused on the center of the reactor.

[0020] The present invention also proposes a monitoring method for the device as described above, comprising the following steps: S1. According to the sediment particle size composition of the target reservoir, weigh black and white sand of a specific mass and particle size, mix them evenly, and according to the set hydrate saturation in the sediment, fully mix the prepared black and white sand sediment sample with deionized water, seal and let it stand for more than 12 hours; S2. Use ultrapure water to clean the pipes of the connecting device and check for leaks while maintaining pressure; S3, fill the sample in step S1 in the reactor, install the upper end cover of the reactor; the gas phase injection system injects methane gas into the sample filling cavity to make the pore pressure of the reactor reach the test set value; the confining pressure loading system controls the amount of confining pressure liquid in the temperature control cavity, slowly increases the sample confining pressure, and when the effective pressure reaches the set pressure value, the liquid phase injection system stops fluid injection; methane gas is introduced into the reactor to saturate the water-sand sample, and the temperature control system adjusts the temperature in the reactor to below the phase equilibrium temperature, and reacts until the gas pressure in the reactor no longer decreases; when the internal pressure of the sample remains unchanged for more than 10 hours, the sample preparation is completed; S4. Install a high-speed camera, a camera base, a blue light, and a laser indicator light on the overhead shooting bracket, adjust the relative positions of the high-speed camera and the blue light, and place them above the visualization window of the reactor; S5. Focus the laser indicator light on the center of the visualization window of the reactor, adjust the focal length and aperture of the high-speed camera to make the image clear; shoot the special calibration plate to calibrate the scale of the shooting system; S6, a high-speed camera, a laser indicator light and a blue light are connected to the synchronous controller, and the calibration plate is photographed with a high-speed camera, and the strain observation system is calibrated and calibrated in the data processing system; S7, the confining pressure loading system applies pressure to the specimen in the xyz direction so that the triaxial pressure reaches the experimental set value; S8, the injection system injects the fracturing fluid into the reactor at a preset displacement, and pre-cools or pre-heats the fracturing fluid through the injection fluid temperature control module; at the same time, the non-contact strain observation system is started to take frame-by-frame images of the top surface of the sample during the fracturing fluid injection process, and obtain relevant data such as the crack initiation point, extension direction, and expansion law; S9. According to step 8, obtain the sample crack extension image collected by the high-speed camera, and use the digital image correlation method to analyze the captured image. The specific steps are as follows: divide the previous frame image into several image sub-areas, select the pixel gray value correlation function to perform correlation search in the next frame image, find the deformed sub-area similar to the target image sub-area of ​​the previous frame, compare the speckle patterns of the two sub-areas before and after deformation, calculate the displacement vector of each pixel point, thereby obtaining the displacement field of each sub-area, and use the point-by-point local least squares fitting method to calculate the gradient of the displacement data, thereby obtaining the strain field of the sample during the fracturing process.

[0021] Furthermore, in steps S4 to S8, the visualization window is continuously purged to prevent the visualization window from fogging due to the temperature difference between the reactor and the room temperature, thereby affecting the non-contact strain observation system.

[0022] Further, step S9 includes: S91, taking the previous frame image as the reference image and the next frame image as the deformed image, selecting a pixel point to be measured in the image, and selecting a square sub-image with the pixel point to be measured as the center as a reference subset, and the grayscale function of the image in the reference subset is represented by I1(i); S92. In the deformed image, with the pixel to be tested as the center, select a square sub-image whose size is larger than the reference subset as the search subset, and select any sub-image whose size is equal to the reference subset in the search subset as the deformed subset. The grayscale function of the image in the deformed subset is represented by I2(i). S93. Select a correlation coefficient formula to evaluate the matching degree between the reference subset and the deformed subset. In this embodiment, the normalized correlation coefficient is selected for calculation: (1); In formula (1), n ​​is the number of pixels in the deformed subset and the reference subset, and are the average gray values ​​of the reference subset and the deformed subset, respectively; When the value of the normalized cross-correlation coefficient is equal to 1, it means that the two image regions are completely matched; when the normalized cross-correlation coefficient is equal to 0, it means that there is no correlation between the two images; according to the matching degree of the correlation coefficient, the position of the deformed subset of the best matching reference subset in the search subset is determined; S94. By comparing the reference subset and the determined deformed subset, the displacement of the pixel points in the two subsets is calculated. Assume that the coordinates of a point (x, y) in the reference subset in the corresponding deformed subset are ( , )Under the action of hydraulic fracturing, hydrate-containing sediments undergo deformations such as tension, compression, and shear, accompanied by the generation of cracks. The first-order displacement shape function is introduced to calculate the displacement of pixel points: (2); In formula (2), u and v are the rigid body displacements of the center point in the x and y directions respectively; , , , is the displacement gradient, , is the distance from the reference center point (x, y) to the point ( , ) distance; Solving u and v can obtain the displacement field in the deformed subset, and then calculating for each reference subset and deformed subset according to steps S91 to S94, the displacement field distribution of the entire image can be obtained; S95, based on the displacement field data of the image obtained in step S94, further obtain the strain value, and use the local least squares fitting method based on the two-dimensional linear function to calculate the displacement: (3); In formula (3), is the strain in the x direction; is the strain in the y direction; is the shear strain.

[0023] Compared with the prior art, the present invention has the following technical advantages: By using the device described in the present invention to conduct experiments, the crack initiation and expansion laws of hydrate-containing sediment samples during the reservoir transformation process can be more intuitively obtained; the application of high-speed cameras and digital image correlation method non-contact strain observation technology can capture the crack initiation point, crack expansion direction, and micro-crack evolution law on the injection surface during the fracturing process in a very short time, and can obtain the strain changes on the fracturing fluid injection surface after the fracturing fluid is injected and before the sample is cracked; combined with acoustic emission monitoring methods, the crack distribution characteristics inside the hydrate-containing sediment sample can be further analyzed. For other hydraulic fracturing simulation equipment for hydrate-containing sediments, the device described in the present invention can not only analyze the crack distribution law in the sample after compression, but also realize frame-by-frame observation and analysis of the crack initiation process, thereby facilitating a more comprehensive and accurate study of the fracturing transformation mechanism of hydrate-containing sediments. In addition, the non-contact strain observation system works outside the reactor and does not need to be prefabricated in the high-pressure and low-temperature environment of hydrate formation, which greatly reduces the risk of equipment damage. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the simulation device for visualizing fracture extension in hydrate-containing sediments; Figure 2 Schematic diagram of a true triaxial reactor for visualizing fracture propagation in hydrate-bearing sediments; Figure 3 Top view of a true triaxial reactor for visualizing fracture propagation in hydrate-bearing sediments; Figure 4 It is a schematic diagram of a non-contact strain monitoring device; Figure 5 This is a schematic diagram of the bracket of the non-contact strain monitoring device taken from above; Figure 6 is a schematic diagram of the non-contact strain monitoring principle.

[0025] In the figure: 1. Gas booster pump; 2. Gas cylinder; 3. Preheater; 4. Constant speed and constant pressure pump; 5. Magnetic circulation pump; 6. Refrigeration water bath; 7. Non-contact strain observation device; 7-1. High-speed camera; 7-2. Blue light; 7-3. Laser indicator light; 7-4. Horizontal rod; 7-5. Horizontal rod base; 7-6. Adapter plate; 7-7. Synchronous controller; 7-8. Overhead support; 8. True three-axis reactor; 8-1. Upper fastening nut; 8-2. Upper visible plug ;8-3, visible pressure head;8-4, simulated wellbore;8-5, sample filling cavity;8-6, acoustic emission probe;8-7, upper and lower loading cylinders;8-8, lower plug;8-9, lower flange;8-10, upper flange;8-11, fluid injection port;8-12, sealing rubber sleeve;8-13, side loading cylinder;8-14, temperature control cavity;8-15, cylinder support base;8-16, fluid outlet;9, vacuum system;10, automatic back pressure system. DETAILED DESCRIPTION

[0026] To facilitate those skilled in the art to understand the present invention, specific implementations of the present invention are described below with reference to the accompanying drawings.

[0027] like Figures 1 to 5 As shown, the device for monitoring the initiation and expansion process of hydraulic fractures in hydrate-containing sediments in the present invention includes an injection system, a true triaxial reactor 8, a temperature control system, a non-contact strain observation system, a vacuum system 9, and an automatic back pressure system 10. The gas cylinder 2 of the injection system is connected to the fluid injection port 8-11 of the true triaxial reactor 8 through the gas booster pump 1, and is used to maintain the pressure in the sample filling cavity 8-5 during the hydrate formation stage of the sediment; the constant speed and constant pressure pump 4 of the injection system is connected to the fluid injection port 8-11 of the true triaxial reactor 8, and is used to pump fracturing fluid into the sample filling cavity 8-5; the preheater 3 and the refrigerated water bath 6 of the temperature control system are connected to the constant speed and constant pressure pump 4 of the injection system, and are used to pre-cool or pre-heat the fracturing fluid injected into the true triaxial reactor 8; the kettle temperature control system of the temperature control system is connected to the magnetic circulation pump 5 through a circulation pipeline, and is used to circulate the refrigerant in the middle temperature control cavity 8-14 of the true triaxial reactor 8, so as to achieve the purpose of cooling the sediment filled in the sample filling cavity 8-5, so that it reaches the temperature conditions required for hydrate formation; the automatic back pressure system 10 is connected to the true triaxial reactor 8, and is used for back pressure control at the outlet.

[0028] An intermediate container filled with fracturing fluid is arranged inside the constant speed and constant pressure pump 4 .

[0029] The vacuum system 9 can adopt the existing vacuum equipment, and the vacuum system 9 is connected to the sample filling chamber 8-5, and is used to vacuum the sample filling chamber 8-5 after the sample filling is completed, so as to realize the subsequent gas injection process; the automatic back pressure system 10 can adopt the existing automatic back pressure equipment, and the automatic back pressure system 10 is connected to the fluid outlet 8-16, and is used to automatically adjust the system back pressure according to the target back pressure value and the actual back pressure value, so as to control the smooth outflow of the fluid.

[0030] The process of conducting experiments using this device is as follows: First, use pipelines to connect the true triaxial reactor 8 with the injection system, temperature control system, and automatic back pressure system 10; install the non-contact strain observation system 7 on the overhead bracket 7-8 and place it above the upper visible plug 8-2 of the true triaxial reactor 8. Adjust the overhead bracket 7-8, the horizontal rod 7-4, the horizontal rod base 7-5, and the position knob on the high-speed camera 7-1 so that the upper visible plug 8-2 of the true triaxial reactor 8 is within the field of view of the high-speed camera 7-1; turn on the laser indicator 7-3 and focus it on the center of the true triaxial reactor 8; adjust the tilt angle, focal length, aperture, etc. of the high-speed camera according to the image monitored by the high-speed camera 7-1, and turn on the blue light 7-2 for coaxial fill light so that the sample filled inside the reactor can be clearly observed. After the non-contact strain observation system 7 is installed, a high-speed camera 7-1 is used to photograph the placed calibration plate, which should be placed on the upper visible plug 8-2. After the photographing is completed, the strain observation system needs to be calibrated and calibrated in the data processing system.

[0031] The gas excess method is used to prepare in situ the hydrate-containing sediment sample suitable for the visualization observation requirement of the non-contact strain observation system 7 in the sample filling cavity 8-5 of the true triaxial reactor 8, and the process is as follows: the quartz sand or muddy silt sand simulating the sediment is evenly mixed according to the particle size distribution of the on-site sediment, and the deionized water and the formation sediment sample with known porosity are fully mixed according to a specific ratio, and the mixture is sealed and left to stand for more than 12 hours; the sediment sample that has been sealed and left to stand is placed in the sample filling cavity 8-5 of the true triaxial reactor 8, and a layer of black and white sand with a pre-configured particle size is pre-filled on the top layer of the sediment sample, or a speckle sticker is pre-placed on the top layer of the sediment sample; the airtightness check of the device is completed, the upper flange 8-10 of the reactor is installed, and methane gas is introduced into the reactor through the fluid injection port 8-11 of the true triaxial reactor 8 to saturate the water-sand sample, and the temperature in the reactor is adjusted to below the phase equilibrium temperature, and the reaction is carried out until the gas pressure in the reactor no longer decreases; when the internal pressure of the sample remains unchanged for more than 10 hours, the sample can be considered to have been prepared.

[0032] The upper and lower loading cylinders 8-7 and the side loading cylinder 8-13 of the triaxial loading system in the true triaxial reactor 8 apply pressure to the sample in the xyz directions so that the triaxial pressure reaches the experimental set value; the fracturing fluid pre-cooled by the refrigerated water bath 6 or the fracturing fluid heated by the preheater 3 is injected into the true triaxial reactor 8 through the constant speed and constant pressure pump 4 to fracture the sample; during the injection of the fracturing fluid, the non-contact strain observation system 7 and the acoustic emission probe 8-6 need to be started at the same time to synchronously monitor the initiation and extension laws of the cracks of the hydrate-containing sediment on the fracturing fluid injection surface and inside the sediment; during the injection process, the injection curve of the fracturing fluid needs to be synchronously monitored to identify the fracture pressure of the hydrate-containing sediment sample and the extension pressure of the cracks.

[0033] After the test, the acquired speckle images were processed frame by frame using the digital image correlation method to obtain the real-time strain evolution law of the fracture surface, so as to analyze the initiation and expansion law of the main cracks and the evolution characteristics of microcracks during hydraulic fracturing. Figure 6-1 , Figure 6-2 As shown in, the specific description is as follows:

[0034] 1. Take two consecutive frames as an example, the previous frame is the reference image ( Figure 6-1 ), the next frame is the deformed image ( Figure 6-2 ). Select a pixel to be tested in the image (such as Figure 6-1 , Figure 6-2 A and B in the figure), and a square sub-image is selected as the center of the test point as the reference subset. The grayscale function of the image in the reference subset is represented by I1(i);

[0035] 2. In the deformed image, with the pixel to be tested as the center, select a square sub-image whose size is larger than the reference subset as the search subset. In the search subset, select any sub-image whose size is equal to the reference subset as the deformed subset. The grayscale function of the image in the deformed subset is represented by I2(i). 3. Select a correlation coefficient formula to evaluate the matching degree between the reference subset and the deformed subset. In this embodiment, the normalized correlation coefficient is selected for calculation: (1); In formula (1), n ​​is the number of pixels in the deformed subset and the reference subset, and are the average gray values ​​of the reference subset and the deformed subset, respectively.

[0036] When the value of the normalized cross-correlation coefficient is equal to 1, it means that the two image regions are completely matched; when the normalized cross-correlation coefficient is equal to 0, it means that there is no correlation between the two images. According to the matching degree of the correlation coefficient, the position of the deformed subset of the best matching reference subset in the search subset is determined.

[0037] 4. By comparing the reference subset and the determined deformed subset, the displacement of the pixels in the two subsets is calculated. In this embodiment, it is assumed that the coordinates of a point (x, y) in the reference subset in the corresponding deformed subset are ( , )Under the action of hydraulic fracturing, hydrate-containing sediments will undergo deformations such as tension, compression, and shear, accompanied by the generation of cracks. Therefore, it is necessary to introduce a first-order displacement shape function to calculate the displacement of pixel points: (2); In formula (2), u and v are the rigid body displacements of the center point in the x and y directions respectively; , , , is the displacement gradient, , is the distance from the reference center point (x, y) to the point ( , ) distance.

[0038] By solving u and v, the displacement field in the deformed subset can be obtained. Then, the displacement field distribution of the entire image can be obtained by calculating each reference subset and deformed subset according to the above method.

[0039] 5. Based on the displacement field data of the image, the strain value is further obtained. In this embodiment, the local least squares fitting method based on the two-dimensional linear function is selected to calculate the displacement: (3); In formula (3), is the strain in the x direction; is the strain in the y direction; is the shear strain.

[0040] The above-described embodiments of the present invention do not constitute a limitation on the protection scope of the present invention. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention shall be included in the protection scope of the claims of the present invention.

Claims

1. A device for monitoring the crack propagation process of hydrate-containing sediments, characterized in that: It includes an injection system, a reactor, a temperature control system, an automatic back pressure system, a vacuum system, a data processing system and a non-contact strain observation system, wherein the reactor is connected to the injection system, the temperature control system, the automatic back pressure system and the vacuum system respectively, and the data processing system is connected to the non-contact strain observation system; The reactor comprises a visible upper plug, a visible pressure head, an acoustic emission probe, a sample filling cavity and a simulated wellbore, a confining pressure loading system, and a temperature control cavity. The simulated wellbore is arranged in the sample filling cavity. The visible pressure head is sealed and arranged on the upper part of the sample filling cavity. The visible upper plug is arranged on the upper part of the visible pressure head. The visible upper plug and the visible pressure head are combined to form a visualization window. The acoustic emission probe is arranged in the sample filling cavity. The confining pressure loading system is arranged outside the sample filling cavity. The temperature control cavity is arranged outside the sample filling cavity. The reactor is provided with a fluid injection port and a fluid outlet. The fluid injection port is connected to the simulated wellbore, and the fluid outlet is connected to the sample filling cavity. The injection system includes a liquid phase injection system and a gas phase injection system, the liquid phase injection system is used to inject fracturing fluid into the sample filling cavity, and the gas phase injection system is used to inject gas into the sample filling cavity; The non-contact strain observation system includes a high-speed camera, an illumination system and a synchronization controller. The high-speed camera is located above the visualization window so that the visualization window is within the field of view of the high-speed camera. The synchronization controller is electrically connected to the illumination system and the high-speed camera respectively, and the high-speed camera is electrically connected to the data processing system.

2. The device according to claim 1, characterized in that The liquid phase injection system includes a liquid injection pump and an intermediate container, and the liquid injection pump is connected to the intermediate container; the gas phase injection system includes a gas cylinder, a gas booster pump and a gas flow controller connected by a pipeline; the liquid phase injection system and the gas phase injection system are mixed at the mixing injection pipeline and connected to the fluid injection port of the reactor.

3. The device according to claim 2, characterized in that The liquid injection pump is a constant speed and constant pressure pump, and the constant speed and constant pressure pump is internally provided with the intermediate container, and the intermediate container is filled with fracturing fluid.

4. The device according to claim 2, characterized in that The temperature control system includes a temperature control module inside the reactor and an injection fluid temperature control module; the temperature control system inside the reactor is used to inject pre-cooled temperature control liquid into the temperature control cavity to maintain temperature stability during the experiment, including a refrigeration coil and a high-pressure magnetic circulation pump; the injection fluid temperature control module is used to pre-cool or pre-heat the fracturing fluid, including a preheater, a temperature sensor and a refrigerated water bath, the temperature sensor and the preheater are electrically connected, and the preheater and the refrigerated water bath are respectively connected to the liquid injection pump.

5. The device according to claim 1, characterized in that The confining pressure loading system is a triaxial loading system, which is used to apply a set triaxial pressure to the hydrate-containing sediment sample, and includes upper and lower loading cylinders and a side loading cylinder.

6. The device according to claim 1, characterized in that The data processing system comprises an image acquisition card and a data processing terminal, and the image acquisition card is electrically connected to the data processing terminal and the high-speed camera respectively.

7. The device according to claim 1, characterized in that The lighting system comprises a laser indicator light and a blue light, and the laser indicator light is focused on the center of the reaction kettle.

8. The monitoring method of the device according to any one of claims 1 to 7, characterized in that: The following steps are involved: S1. According to the sediment particle size composition of the target reservoir, weigh black and white sand of specific mass and particle size, mix them evenly, calculate the amount of deionized water required to generate hydrates according to the set hydrate saturation in the sediment, fully mix the deionized water with the prepared black and white sand sediment sample, seal and let it stand for more than 12 hours; S2. Use ultrapure water to clean the pipes of the connecting device and check for leaks while maintaining pressure; S3, fill the sample in step S1 in the reactor, install the upper end cover of the reactor; the gas phase injection system injects methane gas into the sample filling cavity to make the pore pressure of the reactor reach the test set value; the confining pressure loading system controls the amount of confining pressure liquid in the temperature control cavity, slowly increases the sample confining pressure, and stops fluid injection when the effective pressure reaches the set pressure value; methane gas is introduced into the reactor to saturate the water-sand sample, and the temperature control system adjusts the temperature in the reactor to below the phase equilibrium temperature, and reacts until the gas pressure in the reactor no longer decreases; when the internal pressure of the sample remains unchanged for more than 10 hours, the sample preparation is completed; S4. Install a high-speed camera, a camera base, a blue light, and a laser indicator light on the overhead shooting bracket, adjust the relative positions of the high-speed camera and the blue light, and place them above the visualization window of the reactor; S5. Focus the laser indicator on the center of the visualization window of the reactor, and adjust the focus and aperture of the high-speed camera to make the image clear; S6, a high-speed camera, a laser indicator light and a blue light are connected to the synchronous controller, and the calibration plate is photographed with a high-speed camera, and the strain observation system is calibrated and calibrated in the data processing system; S7, the confining pressure loading system applies pressure to the specimen in the xyz direction so that the triaxial pressure reaches the experimental set value; S8, the injection system injects the fracturing fluid into the reactor at a preset displacement, and pre-cools or pre-heats the fracturing fluid through the injection fluid temperature control module; at the same time, the non-contact strain observation system is started to take frame-by-frame images of the top surface of the sample during the fracturing fluid injection process, and obtain relevant data such as the crack initiation point, extension direction, and expansion law; S9. According to step S8, a crack extension image of the sample collected by a high-speed camera is obtained, and the captured image is analyzed by using a digital image correlation method. The specific steps are as follows: the previous frame image is divided into several image sub-areas, and a pixel gray value correlation function is selected to perform a correlation search in the next frame image to find a deformed sub-area similar to the target image sub-area of ​​the previous frame, and the speckle patterns of the two sub-areas before and after deformation are compared to calculate the displacement vector of each pixel point, thereby obtaining the displacement field of each sub-area, and using a point-by-point local least squares fitting method to calculate the gradient of the displacement data, thereby obtaining the strain field of the sample during the fracturing process.

9. The monitoring method according to claim 8, characterized in that: In steps S4 to S8, the visualization window is continuously purged to prevent the visualization window from fogging due to the temperature difference between the reactor and the room temperature, thereby affecting the non-contact strain observation system.

10. The monitoring method according to claim 8, characterized in that: Step S9 includes: S91, taking the previous frame image as the reference image and the next frame image as the deformed image, selecting a pixel point to be measured in the image, and selecting a square sub-image with the pixel point to be measured as the center as a reference subset, and the grayscale function of the image in the reference subset is represented by I1(i); S92. In the deformed image, with the pixel to be tested as the center, select a square sub-image whose size is larger than the reference subset as the search subset, and select any sub-image whose size is equal to the reference subset in the search subset as the deformed subset. The grayscale function of the image in the deformed subset is represented by I2(i). S93. Select a correlation coefficient formula to evaluate the matching degree between the reference subset and the deformed subset. In this embodiment, the normalized correlation coefficient is selected for calculation: （1）； In formula (1), n ​​is the number of pixels in the deformed subset and the reference subset, and are the average gray values ​​of the reference subset and the deformed subset, respectively; When the value of the normalized cross-correlation coefficient is equal to 1, it means that the two image regions are completely matched; when the normalized cross-correlation coefficient is equal to 0, it means that there is no correlation between the two images; according to the matching degree of the correlation coefficient, the position of the deformed subset of the best matching reference subset in the search subset is determined; S94. By comparing the reference subset and the determined deformed subset, the displacement of the pixel points in the two subsets is calculated. Assume that the coordinates of a point (x, y) in the reference subset in the corresponding deformed subset are ( , )Under the action of hydraulic fracturing, hydrate-containing sediments undergo deformations such as tension, compression, and shear, accompanied by the generation of cracks. The first-order displacement shape function is introduced to calculate the displacement of pixel points: （2）； In formula (2), u and v are the rigid body displacements of the center point in the x and y directions respectively; , , , is the displacement gradient, , is the distance from the reference center point (x, y) to the point ( , ) distance; Solving u and v can obtain the displacement field in the deformed subset, and then calculating for each reference subset and deformed subset according to steps S91 to S94, the displacement field distribution of the entire image can be obtained; S95, based on the displacement field data of the image obtained in step S94, further obtain the strain value, and use the local least squares fitting method based on the two-dimensional linear function to calculate the displacement: （3）； In formula (3), is the strain in the x direction; is the strain in the y direction; is the shear strain.

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