Rock mass fracture multi-field coupling two-phase flow analysis method and test system

Abstract

The invention discloses a rock mass fracture multi-field coupling two-phase flow analysis method and a test system, and relates to the technical field of rock fracture temperature-seepage-stress coupling action mechanism research. The analysis method comprises the following steps: saturating a crack sample with a first dyeing fluid under a target temperature and pressure condition, and obtaining a single-phase saturated flow image of the crack sample; on the basis of the single-phase saturated flow image, through optical iteration inversion calculation, obtaining fracture opening two-dimensional distribution characteristics under a target condition; under the condition of the same temperature and pressure, displacing the first dyeing fluid with the second dyeing fluid or co-flowing with the first dyeing fluid to obtain a two-phase flow image of the crack sample; performing digital image processing on the two-phase flow image and the single-phase saturated flow image, and extracting phase distribution information of the second dyeing fluid; and performing fusion analysis on the phase distribution information of the second dyeing fluid and the fracture opening two-dimensional distribution characteristics, and calculating a two-phase flow structure and saturation distribution characteristics. According to the method, rock mass fracture two-phase flow process quantitative analysis can be realized under different temperature and pressure conditions.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of rock fracture temperature-seepage-stress coupling mechanism research, and particularly relates to a rock mass fracture multi-field coupling two-phase flow analysis method and a test system. BACKGROUND

[0002] In deep underground rock mass engineering, high ground temperature and high ground stress problems are extremely prominent. For example, geopressured geothermal resources are stored underground 2-3 kilometers below the surface, and the temperature and pressure of the cap rock mass are as high as 260 DEG C and 42 MPa. The seepage characteristics of fractured rock mass under the coupling of temperature and stress are a core research topic in the field of rock mechanics.

[0003] Rock mass fractures are the main channels for seepage and heat transfer. Thermal stress caused by high temperature and initial stress together promote the evolution of pore and micro-fracture structure in rock mass, resulting in changes in the seepage and heat transfer characteristics of fractured rock mass. At the same time, the changes in fluid density, viscosity and thermal effects under different temperature conditions cause the redistribution of rock mass seepage field and stress field. On the other hand, high-temperature two-phase flow has an important influence on the stress field and temperature field in rock mass through seepage pressure and heat exchange. Therefore, the stress field, seepage field and temperature field of high-temperature and high-stress rock mass interact with each other, forming a complex dynamic system of temperature-seepage-stress multi-field coupling.

[0004] The research on the behavior of multi-field coupling two-phase flow in fractured rock mass is a core basic subject of CO2 geological storage, oil and gas exploitation and deep geothermal development. For example, CO2 is injected into deep saline aquifers in a supercritical state under the action of high temperature and high pressure to displace brine. The evolution of stress field and temperature field is an important factor affecting the efficiency of CO2 storage. High pressure and high temperature water or steam is injected into oil reservoirs through injection wells to effectively supplement the external pressure difference and enhance the flow of crude oil, thereby significantly improving the efficiency of oil and gas exploitation. Geothermal development is achieved by injecting high pressure and low temperature water into high temperature fractured rock mass to form heat exchange, and then the hot water is extracted in the form of forced convection.

[0005] Due to limitations in experimental conditions, current methods for studying the multi-field coupling mechanism of temperature-seepage-stress in fractured rock masses primarily rely on theoretical models and numerical simulations, lacking strong support from experimental data. Furthermore, most of the limited multi-field coupling experiments focus on single-phase flow, while the two-phase flow mechanism has been neglected. Directly observing the two-phase flow process by replicating transparent rock fractures with epoxy resin offers advantages such as high precision, low cost, and ease of use, and has been widely applied in multi-phase flow experimental research in rock fractures. However, this method is currently limited to visualization experiments of two-phase flow under ambient temperature and pressure conditions, and cannot reveal the two-phase flow mechanism under temperature-stress coupling. Therefore, there is an urgent need to develop a visualization experimental system for two-phase flow in rock fractures under temperature-seepage-stress coupling, providing a reliable experimental means for studying the two-phase flow mechanism under multi-field coupling. Summary of the Invention

[0006] One of the objectives of this invention is to provide a multi-field coupled two-phase flow analysis method for rock mass fractures, so as to achieve quantitative analysis of the two-phase flow process in rock mass fractures under different temperature and pressure conditions.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for multi-field coupled two-phase flow analysis of rock mass fractures includes the following steps:

[0009] Under target temperature and confining pressure conditions, the fracture sample was saturated with the first dyeing fluid to obtain a single-phase saturated flow image of the fracture sample;

[0010] Based on the single-phase saturated flow image, the two-dimensional distribution characteristics of the fracture aperture under the target condition are obtained through optical iterative inversion calculation.

[0011] Under the same target temperature and confining pressure conditions, the first dyeing fluid was displaced by the second dyeing fluid or co-flowed with it to obtain a two-phase flow image of the fracture sample.

[0012] Digital image processing is performed on the two-phase flow image and the single-phase saturated flow image to extract the phase distribution information of the second stained fluid;

[0013] The phase distribution information of the second dyeing fluid is fused with the two-dimensional distribution characteristics of the fissure aperture to calculate the two-phase flow structure and saturation distribution characteristics.

[0014] Temperature and confining pressure can be referred to as "temperature and pressure".

[0015] In the above analysis method, the step of obtaining the two-dimensional distribution characteristics of the fracture aperture under the target condition through optical iterative inversion calculation based on the single-phase saturated flow image specifically includes:

[0016] S1. Measure the normal deformation of the fractured sample under different temperature and confining pressure conditions;

[0017] S2. Under the target temperature and confining pressure conditions, a single-phase flow test is conducted to determine the average initial hydraulic aperture of the fracture under these conditions. Based on the normal deformation of the fracture under the corresponding temperature and pressure conditions obtained in step S1, the average initial hydraulic aperture is corrected to obtain the average target fracture aperture for subsequent iterative calculations.

[0018] S3. Under the target temperature and confining pressure conditions, the first dyeing fluid is used to saturate the fracture sample, and the single-phase saturated flow image is captured.

[0019] S4. Input the single-phase saturated flow image into a processing program (such as MATLAB), and perform iterative inversion calculations using the Lambert-Beer law; use the average target fracture aperture obtained in step S2 as the iterative constraint. When the calculated average aperture value of the entire field is equal to the average target fracture aperture value, the iteration is completed, and the two-dimensional distribution characteristics of fracture aperture under the target conditions are output.

[0020] In the above analysis method, the image processing and fusion analysis (i.e., performing digital image processing on the two-phase flow image and the single-phase saturated flow image to extract the phase distribution information of the second dyeing fluid; fusing the phase distribution information of the second dyeing fluid with the two-dimensional distribution characteristics of the fracture aperture to calculate the two-phase flow structure and saturation distribution characteristics) specifically includes:

[0021] S5. Keep the target temperature and confining pressure unchanged, conduct a two-phase flow test, inject a second dyeing fluid for displacement or co-flow, and take a picture of the two-phase flow after the two-phase percolation process stabilizes.

[0022] S6. Perform digital image subtraction on the two-phase flow image obtained in step S5 and the single-phase saturated flow image obtained in step S3, and process the resulting image (including grayscale conversion and binarization) to obtain the phase distribution information of the second type of dyeing fluid.

[0023] S7. The phase distribution information of the second dyeing fluid is fused and analyzed with the two-dimensional distribution characteristics of the crack aperture obtained in step S4 to calculate the two-phase flow structure and saturation distribution characteristics during the two-phase flow process.

[0024] In the above analysis method, the normal deformation of the crack in step S1 is calculated based on the effective stress principle and the linear elastic constitutive relationship of the crack sample under a given temperature-confining pressure combination.

[0025] Specifically, the steps for calculating the normal deformation of the crack based on the effective stress principle and the linear elastic constitutive relation of the cracked sample are as follows:

[0026] a. Calculate the effective stress on the crack based on the effective stress principle: ,in For effective stress, For confining pressure stress, This refers to the fluid pressure within the fracture.

[0027] b. Substitute the effective stress into the linear elastic constitutive relation of the fractured sample: Calculate the normal deformation of the crack; where This represents the normal deformation of the crack. The effective stress is H, the total thickness of the cracked sample is H, and the material stiffness of the cracked sample is E.

[0028] It should be noted that the fluid pressure inside the fracture The average fluid pressure at the inlet and outlet of the fractured sample. ,in The fluid pressure at the inlet of the fractured sample. This represents the fluid pressure at the outlet of the fractured sample.

[0029] In the above analysis method, the average initial hydraulic opening of the fracture in step S2 is obtained by measuring the fluid flow rate and the pressure difference between the inlet and outlet in the single-phase flow test, and then calculating it back according to the cubic law.

[0030] Furthermore, the process of obtaining the average initial hydraulic aperture of the fracture through back-calculation includes: performing multiple measurements of flow rate and pressure difference, taking the arithmetic mean of the multiple hydraulic aperture values ​​obtained through back-calculation, and using this arithmetic mean as the average initial hydraulic aperture of the fracture (which can be used). or express).

[0031] The relationship between the hydraulic aperture of the fracture and the fluid flow rate and pressure difference is as follows:

[0032] ;

[0033] in, , , , , , These are hydraulic opening, dynamic viscosity, flow rate, pressure difference, fracture width, and fracture length, respectively.

[0034] In the above analysis method, the specific method of correction in step S2 is as follows: subtract the normal deformation of the fracture under the corresponding temperature and pressure conditions from the average initial hydraulic aperture of the fracture, and use the result as the average target fracture aperture (which can be used). or express).

[0035] In the above analysis method, the specific steps in step S4 of inputting the single-phase saturated flow image into the processing program and performing iterative inversion calculations in conjunction with Lambert-Beer's law include:

[0036] S4-1. Process the crack image after the first dyeing fluid saturation into a grayscale image and calculate the coefficient k. c That is, the product of the molar absorptivity K and the solution concentration c:

[0037] ;

[0038] in, and These represent the maximum and minimum light intensity values ​​in the grayscale image, respectively. and These are the maximum and minimum aperture values ​​in the crack, respectively;

[0039] Calculate the aperture value corresponding to a single pixel:

[0040] ;

[0041] in, Let be the light intensity value of the pixel in the i-th row and j-th column of the grayscale image of the crack. This represents the aperture value of the corresponding pixel.

[0042] S4-2. Calculate the average value of the current opening degree distribution. (i.e., the average opening value for the entire field):

[0043] ;

[0044] Where m and n are the number of pixels in the length and width directions of the image, respectively;

[0045] S4-3. Adjust the maximum crack opening value b max until the calculated average opening value is obtained. It is consistent with the average value of the target fracture aperture obtained in step S2;

[0046] S4-4. Output the overall distribution characteristics of the crack aperture (i.e., the two-dimensional distribution characteristics of the crack aperture under the target conditions).

[0047] The first dyeing fluid and the second dyeing fluid are immiscible fluids, and their refractive indices or colors differ; for example, aqueous solutions of two different dyeing agents can be used.

[0048] Another objective of this invention is to provide an experimental system for implementing the above-mentioned multi-field coupled two-phase flow analysis method for rock mass fractures. This system can conduct two-phase flow tests under different temperature and confining pressure conditions to study the experimental laws of two-phase flow under the multi-field coupling of temperature, seepage, and stress.

[0049] The test system includes:

[0050] A closed pressure vessel, comprising at least two oppositely positioned transparent observation windows;

[0051] A fracture sample carrying unit is used to fix a transparent rock fracture sample inside the sealed pressure vessel and to place the main body area of ​​the fracture sample within the field of view of the two transparent observation windows.

[0052] A thermo-pressure coupling loading unit is used to independently apply and control the temperature and confining pressure inside the sealed pressure vessel;

[0053] A fluid injection unit includes at least two sets (e.g., two sets) of fluid driving devices, which are connected to the inlet of the fracture sample via fluid pipelines for independently injecting two different fluids (e.g., two immiscible fluids with different refractive indices or colors) into the fracture.

[0054] The image acquisition unit includes an image sensor (such as an industrial CCD camera) and a light source (such as an LED parallel light source) disposed outside the transparent observation window, for acquiring flow field images (such as single-phase flow process images and two-phase flow process images) inside the fractured sample.

[0055] The data sensing and acquisition unit includes a pressure sensing device for monitoring the pressure difference between the inlet and outlet of the fracture sample, and a data acquisition unit for synchronously recording and outputting the pressure difference, temperature, confining pressure and image data, providing a data source for the analysis method.

[0056] In the above-mentioned test system, the sealed pressure vessel includes an upper plate, a lower plate, and a confining pressure cylinder connected between the two. The transparent observation window is installed at the center of the upper plate and the lower plate, respectively. The confining pressure cylinder is wrapped with an insulation layer (insulation cotton can be selected).

[0057] The image sensor is positioned facing the transparent observation window of the upper plate, and the light source is positioned facing the transparent observation window of the lower plate.

[0058] In the above-mentioned test system, the fluid driving device in the fluid injection unit is a constant flow pump (i.e., a constant flow rate pump).

[0059] The pressure sensing device in the data sensing and acquisition unit is a pressure gauge connected to the inlet and outlet of the fracture sample, used to monitor the pressure difference at both ends of the fracture, so as to determine the average value of the initial hydraulic opening of the fracture.

[0060] The data sensing and acquisition unit also includes a measuring device (such as an electronic balance) for measuring the mass of the fluid exiting the fracture sample.

[0061] In the above-mentioned test system, the temperature-pressure coupling loading unit includes:

[0062] The confining pressure loading subunit is a high-pressure fluid source (such as an ISCO plunger pump) connected to the inside of the sealed pressure vessel via a fluid pipeline, used to inject fluid into the sealed pressure vessel to provide and maintain the required confining pressure.

[0063] The temperature loading subunit is a heating device (such as an electromagnetic heating device) that is covered or disposed outside the sealed pressure vessel.

[0064] The ISCO plunger pump operates in constant pressure mode, providing a fluid injection pressure (such as water injection pressure) of no less than 10,000 psi.

[0065] The heating device is enclosed within the insulation layer.

[0066] In the above-mentioned test system, the data sensing and acquisition unit also includes a pressure sensor for monitoring the internal pressure of the sealed pressure vessel and a temperature sensor for monitoring the internal temperature of the pressure vessel.

[0067] In the above-mentioned test system, the fluid injection unit further includes a flow path control valve group, which can be configured to control the two sets of fluid drive devices to achieve co-flow mode or displacement mode injection.

[0068] Specifically, the flow path control valve assembly can be configured to achieve two flow modes:

[0069] In the co-flow mode, two sets of the fluid drive devices operate simultaneously, injecting two fluids into the fracture at a predetermined flow rate ratio.

[0070] In the displacement mode, one set of the fluid drive device first injects a first fluid to saturate the fracture, and then another set of the fluid drive device injects a second fluid to displace the first fluid.

[0071] In the above-described test system, the outlet of the fractured sample is connected to a back pressure controller or the metering device (such as an electronic balance) via a fluid pipeline. The back pressure controller can be used to safely release the pressure inside the sealed pressure vessel after the test; the metering device can be used to measure the mass of the outflowing fluid to calculate the flow rate.

[0072] In the above-mentioned test system, the transparent rock fracture sample can be sealed and contained inside the sealed pressure vessel. The transparent rock fracture sample has an upper fracture surface and a lower fracture surface made of epoxy resin replicating the morphology of real rock fractures, and is provided with a fluid inlet and a fluid outlet.

[0073] In the above-described experimental system, the fracture sample support unit includes interfaces fixed to both ends of the transparent rock fracture sample, and a sealing element for sealing the sample. Preferably, both interfaces of the fracture sample support unit are made of acrylic material; the sealing element is a rubber ring surrounding the transparent rock fracture sample.

[0074] This invention enables the conduct of a series of two-phase flow experiments in rock fractures under different temperature and pressure conditions, achieving quantitative analysis of the two-phase flow process in fractures under multi-field coupling. Combining unsaturated seepage theories in rock fractures (such as the Young-Laplace equation, the unsaturated Darcy equation, and the cubic theorem) and the seepage-deformation-heat transfer coupling theory, it reveals the two-phase flow behavior and transport laws in rock fractures under temperature-seepage-stress coupling, providing a reliable experimental method for studying the two-phase flow laws under multi-field coupling. Attached Figure Description

[0075] Figure 1 A schematic diagram of the structure of a visualization test system for two-phase flow in rock fractures under THM coupling.

[0076] Figure 2 A schematic diagram of the overall structure of a sealed pressure vessel;

[0077] Figure 3 An exploded view of the overall structure of a closed pressure vessel;

[0078] Figure 4 An exploded view of part of the structure of a closed pressure vessel;

[0079] Figure 5 This is an exploded view of the bearing element of the fractured sample.

[0080] In the picture:

[0081] 1 – Deionized water supply device; 2 – First valve

[0082] 3 – ISCO plunger pump 4 – Second valve

[0083] 5 — Sealed pressure vessel; 6 — Constant flow pump

[0084] 7 – First pressure gauge; 8 – Third valve

[0085] 9 – Back pressure controller 10 – Fourth valve

[0086] 11 - Electronic balance 12 - Fifth valve

[0087] 13 - Sixth Valve 14 - Temperature Sensor

[0088] 15 — Pressure sensor 16 — Second pressure gauge

[0089] 17—CCD camera 18—Upper observation window

[0090] 19—LED parallel light source 20—Lower observation window

[0091] 21—Electromagnetic heating device 22—Cracked sample

[0092] 23 - Interface 24 - Data Acquisition Unit

[0093] 25——upper plate 26——lower plate

[0094] 27—Containing pressure cylinder; 28—Insulation layer

[0095] 29 – Rubber ring; 30 – Interface

[0096] 31 - Top cover; 32 - Bottom plate. Detailed Implementation

[0097] To facilitate a better understanding by those skilled in the art of the improvements of this invention compared to the prior art, the invention will be further described below with reference to the accompanying drawings and embodiments. (See also...) Figures 1-5 .

[0098] This invention enables two-phase flow experiments in rock fractures under different temperature and confining pressure conditions, aiming to explore the evolution of two-phase flow behavior under the multi-field coupling of temperature, seepage, and stress. Currently, there are several experimental devices involving multi-field coupling of rocks, such as: "Rock Damage and Permeability Testing System and Method under Temperature-Stress-Circumferential Seepage Coupling" (Application No. 201310554863.9); "Large-Scale Model Test Method for Three-Field Coupling of Fractured Rock Mass (Application No. 201310310013.4); "Rock Seepage-Stress-Temperature-Chemical Coupling Rheological Testing Device and Method" (Application No. 201610061212.X).

[0099] While the aforementioned devices can perform multi-field coupled tests on rocks under different osmotic pressures, stresses, temperatures, and chemical environments, and can be used to study rheological, freeze-thaw damage, and seepage-heat transfer properties, none of them address the two-phase flow problem in rocks, especially lacking the ability to visualize and observe the two-phase flow process within fractures. Although existing technologies such as CT (computed tomography) or MRI (magnetic resonance imaging) exist for visualizing rock seepage, their application in the field of rock mechanics remains relatively limited due to the high cost of the equipment and potential health risks.

[0100] In contrast, this invention, based on light transmission technology (according to the Lambert-Beer law), not only enables the visualization and analysis of the distribution of transparent fracture aperture and two-phase flow phenomena, but also allows for in-depth exploration of the influence of dynamic changes in fracture aperture on two-phase flow behavior by combining the coupling relationship between confining pressure stress, epoxy resin stiffness, and fracture normal deformation. Although several devices have attempted to visualize fracture two-phase flow, such as: "Experimental Device for Visualizing Two-Phase Flow in Fractured Rock Mass" (Application No. 201510906972.1); "An Experimental Device for Gas-Liquid Two-Phase Flow in Transparent Fractures" (Application No. 201811447719.4); "A Device and Method for Coupling Experiments of Rock Fracture Dissolution-Two-Phase Flow" (Application No. 202110326105.6); and "A Device and Method for Visualizing the Coupling Mechanism of Rock Fracture Seepage-Dissolution" (Application No. 202311789107.4).

[0101] However, these devices are mostly limited to experiments under normal temperature and pressure conditions, making them difficult to apply to multi-field coupling environments such as high temperature and high pressure, and also unable to effectively study the effects of rock fracture deformation and fluid property changes on the dynamics of two-phase flow under temperature-stress coupling.

[0102] To address the shortcomings of the existing technologies, this invention proposes a visualization test system for two-phase flow in rock fractures under temperature-seepage-stress coupling conditions. The multi-field coupled two-phase flow visualization test device involved in this system, as well as the corresponding rock mass stress-stiffness-deformation-two-phase flow coupling analysis method, are the core innovative contents of this invention.

[0103] The specific structure and analysis methods of the experimental system are described in detail below.

[0104] This embodiment provides a visualization test system for two-phase flow in rock fractures under the coupled effects of temperature, seepage, and stress. This system can accurately simulate and observe the flow behavior and transport patterns of two-phase flow in rock fractures under the coupled effects of temperature, seepage, and stress fields. It can conduct two-phase flow tests under different temperature and confining pressure conditions to study the experimental laws of two-phase flow under the coupled effects of temperature, seepage, and stress fields. Temperature, seepage, and stress can be represented as "THM".

[0105] likeFigure 1 As shown, the test system in this embodiment includes a sealed pressure vessel 5, a fracture sample bearing unit, a thermo-pressure coupling loading unit, a fluid injection unit, an image acquisition unit, a data sensing and acquisition unit, and a transparent rock fracture sample 22.

[0106] like Figures 2-4 As shown, the sealed pressure vessel 5 (also referred to as a "pressure chamber") is used to provide a stable environment with controllable temperature and pressure for the internal sample. The vessel includes at least two transparent observation windows arranged opposite each other; in this embodiment, two observation windows are arranged symmetrically at the top and bottom.

[0107] Specifically, the sealed pressure vessel 5 includes an upper plate 25, a lower plate 26, and a confining pressure cylinder 27 connecting the two. Both the upper plate 25 and the lower plate 26 have through holes at their centers, and each is fitted with a high-strength, high-borosilicate glass observation window of the same thickness to ensure clear observation of the two-phase flow process within the fissure throughout the entire experiment.

[0108] To maintain the required temperature for the test, the outer side of the confining cylinder 27 is wrapped with an insulation layer 28 (such as insulation cotton), which can effectively reduce the loss of internal heat and ensure the stability of the temperature field.

[0109] The upper plate 25, lower plate 26, and confining cylinder 27 are preferably made of high-strength aluminum alloy through precision machining to ensure their structural integrity under high pressure. The components are connected and fixed together by several fasteners (such as screws, bolts, and nuts). During assembly, the airtightness of all sealing interfaces must be strictly checked to ensure that the pressure vessel maintains a constant internal pressure during testing.

[0110] The fracture sample support unit is used to fix the transparent rock fracture sample 22 within the sealed pressure vessel 5, ensuring that the main observation area of ​​the fracture sample 22 is within the field of view of the two transparent observation windows. The transparent rock fracture sample 22 is composed of two fracture surfaces made of high-strength transparent epoxy resin, and its inner surface morphology replicates the rough characteristics of real rock fractures. Figure 5 As shown, the fracture sample carrier unit includes a rubber ring 29 for achieving lateral sealing, several fasteners (such as screws and nuts), and an interface 30, a top cover 31, and a bottom plate 32 made of acrylic material.

[0111] The preparation method of the transparent crack sample 22 includes the following steps:

[0112] 1. Preparation of original fracture morphology: First, artificial fractures were created on the rock sample using the Brazilian splitting method. The fracture surface was washed with water and relatively intact fracture samples were selected as master samples.

[0113] 2. Making silicone molds: Based on the selected rock sample, a series of processes are carried out, including casting, spraying release agent, pouring in prepared silicone, defoaming, static curing and demolding, to obtain a negative mold that retains the original crack and rough morphology.

[0114] 3. Casting epoxy resin specimens: Prepare epoxy resin material with high strength, high transparency and high temperature resistance, pour it into the silicone mold, and then go through defoaming, standing, curing and demolding processes again to finally obtain two high transparency epoxy resin specimens with the same crack morphology as the real ones.

[0115] 4. Assembly and sealing: The two transparent crack specimens are precisely aligned and connected to the acrylic interface 30 structure. Rubber rings 29 are wrapped around them to achieve lateral restraint and sealing. Then, the whole specimen is placed on the acrylic base plate 32, the top cover 31 is closed, and uniform pressure is applied using fasteners to solidify it. Finally, silicone waterproof adhesive is applied to the inlet and outlet of the crack fluid and the side connection to ensure the sealing of the entire sample under high pressure.

[0116] In this embodiment, the transparent rock fracture sample 22 has an upper fracture surface and a lower fracture surface made of epoxy resin replicating the morphology of real rock fractures, and is provided with a fluid inlet and a fluid outlet. The transparent rock fracture sample 22 is connected and fixed through acrylic material interfaces 30 at both ends (including inlet and outlet interfaces), and is laterally sealed by rubber rings 29 surrounding it, thereby being contained in the sealed pressure vessel 5 in a completely sealed manner.

[0117] The sealed pressure vessel 5 and the transparent fracture sample 22 can adopt an arc-shaped design to avoid stress concentration and enhance the overall structural pressure resistance and durability. The fracture interface 30 has a mounting groove for installing the fracture sample 22, and its bottom wall is designed as a symmetrical downward slope structure from both sides towards the central axis, with the fluid outlet located at the lowest point of this slope structure. This geometric design facilitates the natural convergence and guidance of fluid to the outlet under the action of gravity and pressure, effectively avoiding fluid stagnation at the outlet and ensuring smooth discharge; at the same time, this flow guiding structure helps to distribute fluid pressure more evenly in the interface area, improving the stability of pressure transmission.

[0118] A thermo-pressure coupled loading unit is used to independently apply and control confining pressure and temperature loads to the sealed pressure vessel 5. This unit includes:

[0119] 1. Confining Pressure Loading Subunit: This subunit includes a high-pressure fluid source (e.g., an ISCO plunger pump 3), which is connected to the internal chamber of the sealed pressure vessel 5 via a high-pressure fluid line. This source injects fluid (such as deionized water) into the vessel to provide and precisely maintain the required confining pressure environment. The ISCO plunger pump 3 (e.g., a D-series high-precision high-pressure plunger pump, model 100DX) operates in constant pressure mode, with a maximum output pressure of not less than 10,000 psi and a standard pressure accuracy of 0.5% FS. The pump employs a dual-pump-head design, enabling continuous, pulse-free circulating fluid injection. During operation, fluid is continuously injected into the sealed pressure vessel 5 via this pump, and the readings of the connected pressure sensor 15 are monitored in real time until the internal confining pressure reaches the predetermined test value.

[0120] 2. Temperature Loading Subunit: This subunit includes a heating device (e.g., a high-efficiency, energy-saving electromagnetic heating device 21) that covers the outside of the confining cylinder 27 of the sealed pressure vessel 5 to heat the interior of the vessel. The heating device is enclosed within an insulation layer 28 to reduce heat loss. After the confining pressure is loaded and stabilized, the electromagnetic heating device 21 is activated to heat the sealed pressure vessel 5, and the temperature sensor 14 reading is monitored in real time until the internal temperature reaches and stabilizes at the predetermined test value.

[0121] The fluid injection unit, connected to the inlet of the fractured sample 22 via a fluid conduit, is used to independently inject two different fluids (e.g., two immiscible fluids with different refractive indices or colors) into the fracture to enable visualization of the two-phase flow process. This unit includes at least two fluid drive devices (e.g., two sets). The fluid drive device is a constant flow pump 6 (or constant flow rate pump), such as a Harvard Apparatus series flow pump, with a single pump flow control range of 0–200 ml / min and an accuracy of 0.25% FS. This high-precision pump can serve as an independent injection device for immiscible two-phase liquids, used to check the airtightness of the fractured sample 22 before the experiment and to perform single-phase saturation pretreatment of the fracture model. The fluid injection unit also includes a flow path control valve assembly for controlling the start, stop, and switching of the injection of the two fluid drive devices, thereby supporting the following two injection modes:

[0122] 1. Co-flow mode: Two constant flow pumps 6 operate simultaneously, injecting two fluids into the fracture at different set flow rates. By adjusting the flow rate, the distribution ratio of each phase fluid in the fracture can be controlled until the flow reaches a steady state.

[0123] 2. Displacement mode: First, a set of constant flow pump 6 (first flow pump) injects the first type of fluid into the fracture until it is completely saturated; then, another set of constant flow pump 6 (second flow pump) is started to inject the second type of fluid at a specific flow rate to gradually displace the first type of fluid, and the migration of the two-phase fluid interface and the distribution of the residual phase are observed.

[0124] The specific operating procedure is as follows: After the sealed pressure vessel 5 and the transparent fracture sample 22 are assembled and debugged, single-phase saturation is first performed: liquid A is injected into the fracture by the first flow pump until it is completely saturated. Subsequently, according to the selected mode, liquid B is injected by the second flow pump at a specific flow rate to carry out a two-phase flow test.

[0125] An image acquisition unit is used to acquire high-resolution flow field images (including single-phase and two-phase flow processes) inside the fractured sample 22 through a transparent observation window. This unit mainly includes an image sensor and a light source, which are positioned opposite each other on the upper and lower sides of the pressure vessel. Specifically, the image sensor (e.g., an ultra-high-definition 4K resolution (4096×2160 pixels) industrial CCD camera 17) faces and is located directly above the transparent observation window on the upper plate 25. This camera supports real-time image capture, video recording, and data transmission; by adjusting the lens focal length, it can clearly record the dynamic evolution of the fluid within the fracture. The light source (e.g., an LED parallel light source 19) faces and is located directly below the transparent observation window on the lower plate 26, providing uniform and stable parallel light illumination inside the sealed pressure vessel 5. This arrangement ensures that light can penetrate the upper and lower observation windows and the fractured sample 22, thus providing the necessary conditions for the upper image sensor to capture high-contrast images.

[0126] The data sensing and acquisition unit is used to synchronously monitor, record, and output multiphysics data during the experiment, including the pressure difference between the inlet and outlet of the fractured sample, confining pressure, temperature, outflow fluid mass, and flow field images, providing a data foundation for subsequent analysis. This unit mainly includes the following components:

[0127] Fracturing pressure differential sensing device: This device includes two pressure gauges (e.g., high-precision digital display micromanometers with a pressure range of -50 Pa to 10 kPa and a resolution of 1 Pa) connected to the inlet and outlet fluid lines of the fracture sample. These gauges are used to monitor the pressure changes and differences across the fracture in real time. This pressure difference data is primarily used to determine the average initial hydraulic aperture of the fracture and to provide a basis for calculating parameters such as relative permeability and energy dissipation rate.

[0128] Confining pressure and temperature monitoring device: This device includes a pressure sensor 15 (e.g., a high-temperature resistant pressure sensor with a range of -100 kPa to 60 MPa, a temperature resistance of 200 ℃, and an accuracy of 0.25% FS) and a temperature sensor 14 (e.g., a non-contact high-precision temperature sensor with a maximum operating temperature of 150 ℃ and a reading accuracy of 0.1 ℃) installed on the sealed pressure vessel 5, for real-time monitoring and recording of changes in confining pressure and temperature inside the pressure vessel.

[0129] Fluid metering device: The device includes an electronic balance 11 (e.g., a high-precision digital display electronic balance with a range of 0 to 2 kg and an accuracy of 0.01 g) installed on the flow path of the fracture sample outlet, used to weigh the mass of the outflowing fluid, and the instantaneous and cumulative flow rate of the fluid can be calculated by combining it with a timer.

[0130] Data Acquisition Unit: This device employs a multi-channel (e.g., 16-channel) high-speed data logger to synchronously acquire, integrate, and store electrical signals from all the aforementioned sensors (pressure, temperature, mass), and synchronize them with the timestamps of the image acquisition unit, providing a unified source for multi-field coupled data analysis.

[0131] Furthermore, the outlet flow path of the fractured sample 22 can be switched via a valve to selectively connect to the metering device or a back pressure controller 9. The back pressure controller 9 (e.g., a direct-flow pressure controller with a pressure control range of 0–10 MPa and an accuracy of 0.5%FS) is used to precisely control the outlet back pressure during the test and to safely and slowly release the pressure inside the pressure vessel after the test, ensuring operational safety.

[0132] The connection relationships of the core components in the experimental system are briefly described below. Functional valves are installed in all fluid pipelines as needed (see...). Figure 1 ). Figure 1 The solid lines connecting the various components represent infusion pipelines, and the dotted lines represent signal lines.

[0133] (1) Confining pressure loading and temperature control loop. The deionized water supply device 1 is connected to the inlet of the ISCO plunger pump 3 through a delivery pipeline (with a first valve 2). The outlet of the ISCO plunger pump 3 is connected to the sealed pressure vessel 5 through a delivery pipeline (with a second valve 4), forming an injection loop for the confining pressure loading and temperature control medium (deionized water).

[0134] (2) Two-phase fluid injection circuit. The sample inlet of the sealed pressure vessel 5 is connected to two sets of constant flow pumps 6 (flow pumps) through a main pipeline for injecting two different fluids (such as dyed water and another liquid). A first pressure gauge 7 (P1) is installed on the main pipeline to monitor the injection pressure. A third valve 8 is installed at the connection between the main pipeline and each branch to control the opening and closing of the injection path of each fluid.

[0135] (3) Outlet metering and control loop. The sample outlet of the sealed pressure vessel 5 is connected to the back pressure controller 9 via a main pipeline. A fourth valve 10 is installed on this pipeline to control the system back pressure and safely release pressure after the test. It is divided into two branches via another main pipeline: one branch is connected to the collection device on an electronic balance 11, and the other branch is connected to the collection device on another electronic balance 11 to measure the mass of the outflowing fluid. A fifth valve 12 is installed on this other main pipeline, and a sixth valve 13 is installed at the connection between it and the branch for flow path switching.

[0136] (4) Data Sensing and Acquisition System. Monitoring Components: Temperature sensor 14 and pressure sensor 15 are installed inside the sealed pressure vessel 5 to monitor internal environmental parameters; the second pressure gauge 16 (P2) is installed on the outlet main pipeline (this pipeline is connected to the electronic balance 11 via a branch). Image Acquisition Unit: CCD camera 17 is positioned facing the upper observation window 18 of the pressure vessel; LED parallel light source 19 is positioned facing the lower observation window 20. Data Integration: All the above sensors (pressure gauge, temperature sensor 14, pressure sensor 15, electronic balance 11), actuator (back pressure controller 9), and image acquisition unit (CCD camera 17) are connected to the data acquisition unit through signal lines to achieve synchronous data acquisition and recording.

[0137] (5) Valve configuration description. First valve 2, second valve 4, third valve 8, and sixth valve 13 are all check valves to prevent fluid backflow. Fourth valve 10 and fifth valve 12 are both back pressure valves for precise control of pipeline pressure.

[0138] This embodiment also provides a visualization test method for two-phase flow in rock fractures under temperature-seepage-stress coupling. The method is implemented using the aforementioned test system and includes the following steps:

[0139] S1. Sample preparation and system assembly.

[0140] Transparent crack specimens with realistic morphological characteristics were prepared using the epoxy resin replication method and assembled into a complete crack sample 22. The assembled crack sample 22 was placed in a pressure chamber, ensuring that its main observation area was centered on the field of view of the upper and lower transparent observation windows. All necessary signal lines and fluid lines were connected, and the sealing of each connection was checked.

[0141] S2, System preheating and pressurization.

[0142] Close the outlet valve (corresponding to the fourth valve 10) and open the inlet valve (corresponding to the first valve 2 and the second valve 4). Turn on the ISCO plunger pump 3 and the electromagnetic heating device 21 to inject fluid into the pressure chamber and heat it, while simultaneously activating the data sensing and acquisition unit. Monitor the readings of the pressure sensor 15 and the temperature sensor 14 in real time until the confining pressure and temperature inside the pressure chamber reach the predetermined test values ​​and remain stable.

[0143] S3, Optical system debugging.

[0144] Turn on the LED parallel light source 19 and the CCD camera 17, adjust the camera's focal length, aperture, and position to ensure that a clear image of the entire crack area can be captured, and complete the image calibration and focusing.

[0145] S4. System pre-test and calibration.

[0146] Maintain the pressure chamber at a predetermined temperature and pressure environment. Switch the fluid injection unit to a single-phase flow path (e.g., close the first valve 2 and the second valve 4, and open the third valve 8, the fifth valve 12, and the sixth valve 13), and inject a single-phase fluid (liquid 1 or liquid 2) into the fracture at different flow rates using a constant flow pump 6. During this process:

[0147] A pressure gauge was used to monitor the pressure difference between the inlet and outlet of the fissure, and an electronic balance was used to measure the outlet flow rate in order to verify the sealing performance and hydraulic reliability of the entire system under high pressure and high temperature.

[0148] Meanwhile, based on the acquired images, the initial aperture distribution field of the fracture was calibrated using light transmission technology (Lambert-Beer law).

[0149] S5. Two-phase flow test and data acquisition.

[0150] After confirming that the system is working properly, a two-phase flow test is conducted:

[0151] a) Displacement mode test: First, the fracture sample 22 is fully saturated with a fluid (liquid 1), and then another constant flow pump 6 is started to inject a second displacement fluid (liquid 2) at a specific flow rate.

[0152] b) Co-flow mode test: Two constant flow pumps 6 are started simultaneously to inject two fluids into the fracture at a set flow rate ratio.

[0153] Throughout the injection process, the following data were collected simultaneously: pressure difference between the inlet and outlet of the fracture (pressure gauge), change in the mass of the outlet fluid over time (electronic balance 11), confining pressure and temperature (pressure sensor 15 and temperature sensor 14), and images of the fluid distribution and transport process within the fracture (CCD camera 17).

[0154] Based on the data, the flow structure, phase distribution, capillary pressure-saturation-relative permeability relationship, and energy dissipation of the two-phase flow are analyzed, revealing the influence mechanism of temperature-stress-seepage coupling on the behavior of fracture two-phase flow.

[0155] S6. End of test and system cleanup.

[0156] After the test, shut down all equipment in sequence, including the constant flow pump 6 and the heating device. Open the pressure relief valve (corresponding to the fourth valve 10) and slowly release the pressure in the pressure chamber. After the system cools down and returns to normal pressure and temperature, disassemble the pressure chamber and the fracture sample 22, and thoroughly clean all fluid pipelines, containers, and samples for future use.

[0157] In this embodiment, both the pressure chamber and the transparent fracture sample 22 adopt an arc-shaped design to avoid stress concentration and enhance the pressure resistance and durability of the test device. The fracture interface 30 adopts an inclined angle design, which can provide stable and uniform pressure distribution and avoid the embarrassing situation where the liquid in the fracture inlet and outlet cannot be discharged. The aluminum alloy cylinder of the pressure chamber is wrapped with heat insulation cotton to ensure that the electromagnetic heating can quickly and efficiently heat the water and fracture sample 22 in the pressure chamber to the predetermined temperature. The test device of this embodiment can conduct two-phase flow tests in both co-flow and displacement modes, and measure the capillary pressure-saturation-relative permeability relationship under different flow modes. The water bath heating method used in this embodiment can not only provide a uniform and stable temperature transfer method, but also apply different degrees of confining pressure to the transparent fracture sample 22 by continuous water injection, while also facilitating the observation of visual images of the two-phase flow process.

[0158] This embodiment employs water bath heating combined with an external insulation layer 28 to achieve rapid and uniform heating and precise temperature control of the medium within the pressure chamber, while also facilitating the application of high confining pressure. Furthermore, the experimental system of this embodiment can flexibly conduct two-phase flow experiments in both co-flow and displacement modes, and can simultaneously acquire multiphysics data and high-definition images, providing a complete technical means for in-depth research on the mechanism of fractured two-phase flow under multi-field coupling.

[0159] This embodiment also provides a multi-field coupled two-phase flow analysis method for rock mass fractures (i.e., a rock mass stress-stiffness-deformation-two-phase flow coupled analysis method) to achieve quantitative analysis of the two-phase flow process in rock mass fractures under different temperature and pressure conditions. The specific analysis method is as follows.

[0160] A method for multi-field coupled two-phase flow analysis of rock mass fractures includes the following steps:

[0161] Under target temperature and confining pressure conditions, the fracture sample was saturated with the first dyeing fluid to obtain a single-phase saturated flow image of the fracture sample;

[0162] Based on single-phase saturated flow images, the two-dimensional distribution characteristics of fracture aperture under target conditions (i.e., target temperature and confining pressure conditions) are obtained through optical iterative inversion calculation.

[0163] Under the same target temperature and confining pressure conditions, a two-phase flow image of the fracture sample was obtained by using a second staining fluid to displace the first staining fluid or to co-flow with it.

[0164] Digital image processing was performed on the two-phase flow image and the single-phase saturated flow image to extract the phase distribution information of the second stained fluid;

[0165] The phase distribution information of the second dyeing fluid is fused with the two-dimensional distribution characteristics of the fracture aperture to calculate the two-phase flow structure and saturation distribution characteristics.

[0166] The first and second dyeing fluids are immiscible and differ in refractive index or color. In this embodiment, aqueous solutions of two different dyeing agents are selected as the first and second dyeing fluids.

[0167] In the above analysis method, based on the single-phase saturated flow image, the two-dimensional distribution characteristics of the fracture aperture under the target conditions are obtained through optical iterative inversion calculation, specifically including:

[0168] S1. Measure the normal deformation of the fractured sample under different temperature and confining pressure conditions;

[0169] S2. Under the target temperature and confining pressure conditions, a single-phase flow test is conducted to determine the average initial hydraulic aperture of the fracture under these conditions. Based on the normal deformation of the fracture under the corresponding temperature and pressure conditions obtained in step S1, the average initial hydraulic aperture is corrected to obtain the average target fracture aperture for subsequent iterative calculations.

[0170] S3. Under the target temperature and confining pressure conditions, use the first type of dyeing fluid to saturate the fracture sample and take a single-phase saturated flow image;

[0171] S4. Input the single-phase saturated flow image into the processing program (such as MATLAB), and perform iterative inversion calculations in conjunction with the Lambert-Beer law; use the average target fracture aperture obtained in step S2 as the iterative constraint. When the calculated average aperture value of the entire field is equal to the average target fracture aperture value, the iteration is completed, and the two-dimensional distribution characteristics of fracture aperture under the target conditions are output.

[0172] In step S1, the normal deformation of the crack is calculated based on the effective stress principle and the linear elastic constitutive relationship of the crack sample under a given temperature-confining pressure combination.

[0173] Specifically, the steps for calculating the normal deformation of the crack based on the effective stress principle and the linear elastic constitutive relation of the cracked sample are as follows:

[0174] a. Calculate the effective stress on the crack based on the effective stress principle: ,in For effective stress, For confining pressure stress, This refers to the fluid pressure within the fracture.

[0175] b. Substitute the effective stress into the linear elastic constitutive relation of the fractured sample: Calculate the normal deformation of the crack; where This represents the normal deformation of the crack. The effective stress is H, the total thickness of the cracked sample is H, and the material stiffness of the cracked sample is E.

[0176] It should be noted that the fluid pressure inside the fracture The average fluid pressure at the inlet and outlet of the fractured sample is calculated using the following formula: ,in The fluid pressure at the inlet of the fractured sample. This represents the fluid pressure at the outlet of the fractured sample.

[0177] In this embodiment, epoxy resin is selected as the material for preparing the fractured sample. To quantitatively analyze the relationship between confining pressure and fracture normal deformation, the elastic modulus of the selected epoxy resin material under different temperatures and confining pressures, i.e., the material stiffness E (its value is typically in the range of 1–5 GPa), needs to be determined beforehand through material testing or literature review. During multi-field coupled two-phase flow experiments, the confining pressure stress provided by the water injection... (Measured by a pressure sensor) The effective stress acting on the fractured sample. The influence of fluid pressure inside the fracture needs to be considered, and its calculation formula is as follows: ,in ( The value represents the average inlet and outlet water pressure of the fractured sample. Based on this effective stress and the material's elastic modulus (material stiffness), the total deformation of the fractured sample under normal load (i.e., normal deformation) can be calculated. : , where H represents the total thickness of the fractured sample. This calculation provides a theoretical basis for subsequent analysis of the stress-deformation-seepage coupling relationship.

[0178] In the above analysis method, image processing and fusion analysis (i.e., digital image processing of two-phase flow images and single-phase saturated flow images to extract the phase distribution information of the second dyeing fluid; fusing the phase distribution information of the second dyeing fluid with the two-dimensional distribution characteristics of the fracture aperture to calculate the two-phase flow structure and saturation distribution characteristics) specifically includes:

[0179] S5. Keep the target temperature and confining pressure unchanged, conduct a two-phase flow test, inject a second dyeing fluid for displacement or co-flow, and take a picture of the two-phase flow after the two-phase percolation process stabilizes.

[0180] S6. Perform digital image subtraction on the two-phase flow image obtained in step S5 and the single-phase saturated flow image obtained in step S3, and process the resulting image (including grayscale conversion and binarization) to obtain the phase distribution information of the second type of dyeing fluid.

[0181] S7. The phase distribution information of the second dyeing fluid is fused and analyzed with the two-dimensional distribution characteristics of the crack aperture obtained in step S4 to calculate the two-phase flow structure and saturation distribution characteristics during the two-phase flow process.

[0182] In the above analysis method, the average initial hydraulic opening of the fracture in step S2 is obtained by measuring the fluid flow rate and the pressure difference between the inlet and outlet in the single-phase flow test, and then calculating it back according to the cubic law.

[0183] Furthermore, the process of obtaining the average initial hydraulic aperture of the fracture through back-calculation includes: performing multiple measurements of flow rate and pressure difference, taking the arithmetic mean of the multiple hydraulic aperture values ​​obtained through back-calculation, and using this arithmetic mean as the average initial hydraulic aperture of the fracture (which can be used). or express).

[0184] The relationship between the hydraulic aperture of the fracture and the fluid flow rate and pressure difference is as follows:

[0185] ;

[0186] in, , , , , , These are hydraulic opening, dynamic viscosity, flow rate, pressure difference, fracture width, and fracture length, respectively.

[0187] In the above analysis method, the specific method of correction in step S2 is as follows: subtract the normal deformation of the fracture under the corresponding temperature and pressure conditions from the average initial hydraulic aperture of the fracture, and use the result as the average target fracture aperture (which can be used). or express).

[0188] In the above analysis method, the specific steps in step S4 of inputting the single-phase saturated flow image into the processing program and performing iterative inversion calculations in conjunction with Lambert-Beer's law include:

[0189] S4-1. Process the crack image after the first dyeing fluid saturation into a grayscale image and calculate the coefficient k. c That is, the product of the molar absorptivity K and the solution concentration c:

[0190] ;

[0191] in, and These represent the maximum and minimum light intensity values ​​in the grayscale image, respectively. and These are the maximum and minimum aperture values ​​in the crack, respectively;

[0192] Calculate the aperture value corresponding to a single pixel:

[0193] ;

[0194] in, Let be the light intensity value of the pixel in the i-th row and j-th column of the grayscale image of the crack. This represents the aperture value of the corresponding pixel.

[0195] S4-2. Calculate the average value of the current opening degree distribution. (i.e., the average opening value for the entire field):

[0196] ;

[0197] Where m and n are the number of pixels in the length and width directions of the image, respectively;

[0198] S4-3. Adjust the maximum crack opening value b max until the calculated average opening value is obtained. It is consistent with the average value of the target fracture aperture obtained in step S2;

[0199] S4-4. Output the overall distribution characteristics of the crack aperture (i.e., the two-dimensional distribution characteristics of the crack aperture under the target conditions).

[0200] Taking normal temperature and pressure conditions as an example, before conducting multi-field coupled two-phase flow tests, saturated single-phase water flow tests in rock fractures must be performed first to calibrate the initial hydraulic aperture of the fractures. The specific steps are as follows:

[0201] 1. Switch the fluid injection unit to a single-phase flow path (open the third valve, the fifth valve, and the sixth valve).

[0202] 2. Using a constant flow pump at multiple different lower flow rates ( Deionized water is injected into the fracture, and the flow rate is controlled to maintain the Reynolds number (Re, The value is always less than 10 to ensure that the flow is in a laminar state and meets the applicable conditions of the cubic law.

[0203] 3. For each injected flow ( After the flow stabilizes, the pressure difference between the inlet and outlet of the fracture is recorded synchronously using a pressure gauge. The outlet flow rate is measured using an electronic balance for calibration.

[0204] 4. According to the law of cubes, use the following formula to calculate the value corresponding to each group ( , Hydraulic opening data ( ):

[0205] ;

[0206] in , , , , , These are fluid density, dynamic viscosity, flow rate, pressure difference, crack width, and length, respectively.

[0207] 5. The hydraulic opening measured at different flow rates ( The initial average hydraulic aperture of the fractured sample can be obtained by taking the arithmetic mean of the results. .

[0208] In single-phase flow experiments, a staining aqueous solution with good absorbance (such as carmine) is prepared. This red staining solution is then passed through the fractured sample until it is completely saturated. Subsequently, a CCD camera captures the transmission image of the saturated fracture. By combining light transmission techniques (Lambert-Beer's law) and image processing methods, the aperture distribution characteristics of the transparent fracture can be calculated. The specific principles and steps are as follows:

[0209] The Lambert-Beer law states that, for monochromatic light, the intensity of transmitted light satisfies the following relationship with solution concentration and path length: ,in It is the intensity of the incident light. is the emitted light intensity, K is the molar absorptivity of the dye, a constant representing the absorbance of the solution, c is the concentration of the dye solution, and T represents the thickness of the solution (i.e., the path length of light transmission).

[0210] Based on this law, the data processing flow is as follows:

[0211] The captured color fracture image is converted to grayscale (e.g., a MATLAB program processes a fracture image saturated with dyed water into grayscale) to obtain the light intensity value of each pixel. Define a composite coefficient. , which is the product of the molar absorptivity K and the solution concentration c; where, and These represent the maximum and minimum light intensity values ​​in the grayscale image, respectively. and These represent the maximum and minimum aperture values ​​within the fracture, respectively. Since there are contact zones / points within the fracture, the minimum aperture can be assumed. ,and This is a preliminary estimate, which can be determined based on the mass conservation relationship of the fluid within the fracture. The coefficient k is then calculated. c Then, the aperture corresponding to a single pixel is represented as: ,in Let be the light intensity value of the pixel in the i-th row and j-th column of the grayscale image of the crack. This represents the aperture value of the corresponding pixel. Therefore, the average aperture value can be calculated as: , where m and n are the number of pixels in the length and width directions of the image, respectively. The calculated average aperture... Compared with the baseline average opening obtained independently by single-phase saturated water flow test (based on the cubic law) The values ​​were compared. The estimated maximum fracture opening was continuously adjusted through iterative iterations using a MATLAB program. ,until Less than the set tolerance, for example, the calculated value of the average crack aperture. Average value determined by single-phase saturated water flow test They are equal. This is the maximum opening value obtained at this point. This represents the true maximum crack aperture value, and also provides an accurate overall distribution characteristic of the crack aperture.

[0212] Specifically, before conducting two-phase flow tests on rock fractures under normal temperature and pressure conditions, red-dyed water was introduced to fully saturate the transparent fractures. Images of the transparent fractures were recorded using a CCD camera, along with the temperature and pressure values ​​in the pressure chamber. The average initial aperture of the fracture samples was calculated using the method described above. And combined with the normal deformation under the same conditions (The method for determining it is as described above), according to the formula The average aperture of the fractured sample under the current temperature and pressure conditions was calculated. The acquired crack image is then substituted into the image processing algorithm based on the Lambert-Beer law described above. The average current aperture value calculated above is then used... Using the target benchmark value as the algorithm, through iterative calculation, the detailed aperture distribution characteristics of the fracture sample under this specific temperature and confining pressure conditions are finally inverted.

[0213] Subsequently, two-phase flow experiments were conducted according to the selected mode:

[0214] 1. Displacement mode: Start another fluid drive device to inject another liquid (displacement phase) into the fissures saturated with dyed water to carry out the displacement experiment.

[0215] 2. Co-flow mode: Two fluid drive devices are activated simultaneously to inject two liquids into the fracture at a set flow rate ratio.

[0216] Once the two-phase flow process reaches a steady state, the injection is stopped and the test is terminated. Throughout the test, data such as two-phase flow images within the fracture, pressure difference between the fracture inlet and outlet, and outlet fluid flow rate are continuously recorded synchronously.

[0217] After the experiment, the collected data were processed and analyzed as follows:

[0218] 1. Image Preprocessing and Difference Extraction: Using MATLAB, the image of the stabilized two-phase flow was registered and subtracted from the baseline image (image of saturated single-phase stained water) before the experiment to highlight the differences in the distribution of the two fluid phases. Subsequently, irrelevant regions of the image were cropped, and grayscale conversion, image enhancement, and binarization were performed to distinguish the two fluid phases.

[0219] 2. Two-phase parameter quantification: The binarized fluid distribution image obtained in step 1 is spatially superimposed and compared with the fracture aperture distribution map obtained before the experiment. Based on this, the evolution law of the two-phase flow structure and saturation distribution characteristics during the two-phase flow process can be quantitatively calculated. Combined with flow rate and pressure data, the relationship between energy dissipation rate and saturation-capillary pressure-relative permeability can be analyzed.

[0220] Based on the above-mentioned test system and method, by changing the parameters of the temperature-pressure coupled loading unit and the fluid injection unit, a series of rock mass fracture two-phase flow tests under different temperatures, confining pressures and injection conditions (such as flow rate and flow ratio) were repeatedly carried out.

[0221] By comprehensively analyzing multiphysics data (including two-phase flow images, pressure, flow rate, and temperature) obtained under different working conditions, and combining them with theories of unsaturated seepage in rock fractures (such as the Young-Laplace equation, the unsaturated Darcy equation, and the cubic theorem) and multi-field coupling theories (seepage-deformation-heat transfer coupling theory), the influence of temperature-seepage-stress coupling on the two-phase flow behavior (such as flow structure and phase saturation distribution) and transport laws (such as capillary pressure-saturation-relative permeability relationship and energy dissipation mechanism) in fractures can be systematically revealed. This provides a reliable experimental means for studying the laws of two-phase flow under multi-field coupling.

[0222] In addition, the present invention can be implemented in other ways, and any obvious substitutions without departing from the concept of the present invention are within the protection scope of the present invention.

[0223] To facilitate understanding by those skilled in the art of the improvements of this invention over the prior art, some of the accompanying drawings and descriptions have been simplified, and for clarity, some other elements have been omitted from this application. Those skilled in the art should realize that these omitted elements may also constitute the content of this invention.

Claims

1. A method for multi-field coupled two-phase flow analysis of rock mass fractures, characterized in that, Includes the following steps: Under target temperature and confining pressure conditions, the fracture sample was saturated with the first dyeing fluid to obtain a single-phase saturated flow image of the fracture sample; Based on the single-phase saturated flow image, the two-dimensional distribution characteristics of the fracture aperture under the target condition are obtained through optical iterative inversion calculation. Under the same target temperature and confining pressure conditions, the first dyeing fluid was displaced by the second dyeing fluid or co-flowed with it to obtain a two-phase flow image of the fracture sample. Digital image processing is performed on the two-phase flow image and the single-phase saturated flow image to extract the phase distribution information of the second stained fluid; The phase distribution information of the second dyeing fluid is fused with the two-dimensional distribution characteristics of the fissure aperture to calculate the two-phase flow structure and saturation distribution characteristics.

2. The method for multi-field coupled two-phase flow analysis of rock mass fractures according to claim 1, characterized in that, The method of obtaining the two-dimensional distribution characteristics of fracture aperture through optical iterative inversion calculation based on single-phase saturated flow images specifically includes: S1. Measure the normal deformation of the fractured sample under different temperature and confining pressure conditions; S2. Under the target temperature and confining pressure conditions, a single-phase flow test is conducted to determine the average initial hydraulic aperture of the fracture under these conditions. Based on the normal deformation of the fracture under the corresponding temperature and pressure conditions obtained in step S1, the average initial hydraulic aperture is corrected to obtain the average target fracture aperture for subsequent iterative calculations. S3. Under the target temperature and confining pressure conditions, the first dyeing fluid is used to saturate the fracture sample, and the single-phase saturated flow image is captured. S4. Input the single-phase saturated flow image into the processing program and perform iterative inversion calculation in conjunction with the Lambert-Beer law; use the average value of the target fracture aperture obtained in step S2 as the iterative constraint condition. When the calculated average aperture value of the whole field is equal to the average value of the target fracture aperture, the iteration is completed, and the two-dimensional distribution characteristics of the fracture aperture under the target condition are output.

3. The method for multi-field coupled two-phase flow analysis of rock mass fractures according to claim 2, characterized in that, The image processing and fusion analysis specifically includes: S5. Keep the target temperature and confining pressure unchanged, conduct a two-phase flow test, inject a second dyeing fluid for displacement or co-flow, and take a picture of the two-phase flow after the two-phase percolation process stabilizes. S6. Perform digital image subtraction on the two-phase flow image obtained in step S5 and the single-phase saturated flow image obtained in step S3, and process the resulting image to obtain the phase distribution information of the second dyeing fluid. S7. The phase distribution information of the second dyeing fluid is fused and analyzed with the two-dimensional distribution characteristics of the crack aperture obtained in step S4 to calculate the two-phase flow structure and saturation distribution characteristics during the two-phase flow process.

4. The method for multi-field coupled two-phase flow analysis of rock mass fractures according to claim 2, characterized in that, The average initial hydraulic opening of the fracture mentioned in step S2 is obtained by measuring the fluid flow rate and the pressure difference between the inlet and outlet in the single-phase flow test, and then calculating it back according to the cubic law.

5. The method for multi-field coupled two-phase flow analysis of rock mass fractures according to claim 4, characterized in that, The process of obtaining the average initial hydraulic opening of the fracture through back calculation includes: performing multiple measurements of flow rate and pressure difference, taking the arithmetic mean of the multiple hydraulic opening values ​​obtained through back calculation, and using the arithmetic mean as the average initial hydraulic opening of the fracture.

6. The method for multi-field coupled two-phase flow analysis of rock mass fractures according to claim 2, characterized in that, The specific method for correction in step S2 is as follows: subtract the normal deformation of the fracture under the corresponding temperature and pressure conditions from the average initial hydraulic aperture of the fracture, and use the result as the average target fracture aperture.

7. The method for multi-field coupled two-phase flow analysis of rock mass fractures according to any one of claims 1-3, characterized in that, The first dyeing fluid and the second dyeing fluid are immiscible fluids with differences in refractive index or color.

8. An experimental system for implementing the multi-field coupled two-phase flow analysis method for rock mass fractures as described in any one of claims 1-7, characterized in that, include: A closed pressure vessel, comprising at least two oppositely positioned transparent observation windows; A fracture sample carrying unit is used to fix a transparent rock fracture sample inside the sealed pressure vessel and to place the main body area of ​​the fracture sample within the field of view of the two transparent observation windows. A thermo-pressure coupling loading unit is used to independently apply and control the temperature and confining pressure inside the sealed pressure vessel; A fluid injection unit includes at least two fluid driving devices, which are connected to the inlet of the fracture sample via fluid pipelines and are used to independently inject two different fluids into the fracture. An image acquisition unit includes an image sensor and a light source disposed outside the transparent observation window, used to acquire flow field images inside the fractured sample; The data sensing and acquisition unit includes a pressure sensing device for monitoring the pressure difference between the inlet and outlet of the fracture sample, and a data acquisition unit for synchronously recording the pressure difference, temperature, confining pressure and image data.

9. The testing system according to claim 8, characterized in that, The temperature-pressure coupling loading unit includes: The confining pressure loading subunit is a high-pressure fluid source that is connected to the interior of the sealed pressure vessel through a fluid pipeline; The temperature loading subunit is a heating device that is covered or disposed outside the sealed pressure vessel.

10. The testing system according to claim 8, characterized in that: The fluid injection unit also includes a flow path control valve group, which is configured to control the two sets of fluid drive devices to achieve co-flow mode or displacement mode injection.

Images