A system and method for accurately quantifying the critical incipient velocity of particles in a complex flow field

Abstract

The present application belongs to the field of unconventional oil and gas resource efficient development technology, and particularly relates to a system and method for accurately quantifying critical startup flow velocity of particles under complex flow field. The system for accurately quantifying critical startup flow velocity of particles under complex flow field comprises a pore medium real pore structure characterization device, a pore etching model, a pore scale flow field visualization device and a data acquisition and processing device. The data acquisition and processing device comprises a high-speed image acquisition module, a two-dimensional PIV software and a calculation module. The high-speed image acquisition module is used to acquire particle images captured by a particle image velocimetry module. The two-dimensional PIV software is used to calculate a velocity vector field based on the particle images. The calculation module is used to calculate the critical startup flow velocity of target particles based on the velocity vector field. The system for accurately quantifying critical startup flow velocity of particles under complex flow field can realize simulation of complex flow field of target porous medium and accurate calculation of critical startup flow velocity.

Description

Technical Field

[0001] This invention belongs to the field of efficient development technology of unconventional oil and gas resources, specifically relating to a system and method for accurately quantifying the critical start-up velocity of particles under complex flow fields. Background Technology

[0002] Hydraulic fracturing, as a key production enhancement measure for the efficient development of unconventional oil and gas resources such as low-permeability and tight oil and gas, has developed into a relatively mature technology system after years of development. However, field practice shows that some oil and gas wells still experience problems such as rapid production decline and short effective period after fracturing, with the decline in the conductivity of artificial fractures after fracturing being one of the important reasons. Factors affecting the evolution of fracture conductivity include proppant breakage, fine particle migration and trapping, and secondary transport of fine particles from the wall spalling powder. Their common characteristic is that, under the influence of complex pore-throat structures and nonlinear flow fields, the initiation, migration, and redeposition behavior of solid particles continuously disturb the seepage channels.

[0003] The critical initiation velocity—the threshold at which particles initiate migration—is a key parameter determining whether particles can be carried away by the fluid, whether they are trapped or blocked at the pore throat, and whether the fracture conductivity can be maintained. The breakup and migration of proppant within the fracture under formation stress and flow fields is a complex multiphysics coupling process. It must consider both the fluid's traction on particles and the disturbance of the flow field caused by changes in pore structure. The critical initiation velocity for particles of different sizes determines whether they migrate and is a key parameter for predicting the evolution of fracture conductivity. If the critical initiation velocity is too high, fine particles will remain trapped for a long time, blocking the pore throat and leading to irreversible decay of seepage capacity. Therefore, accurately determining the critical initiation velocity of different particles at different pore throat locations under realistic and complex flow field conditions at the pore scale is of great significance for establishing a model of the evolution of fracturing fracture conductivity and optimizing fracturing and sand control design.

[0004] To accurately quantify the critical initiation velocity in complex flow fields at the mesoscale, three requirements must be met simultaneously: resolving the instantaneous velocity field of the fluid at the pore scale, capturing the initiation-migration trajectory of fragmented particles in real time, and achieving fluid-structure interaction. Existing experimental methods and numerical simulations have limitations in these aspects.

[0005] In existing technologies, particle initiation within porous media primarily relies on the following two types of methods:

[0006] (1) Macroscopic seepage experiment: By using a core or sand-filled model, the injection flow rate / pressure difference is slowly increased, and the changes in outlet particle concentration or pipeline pressure drop are observed, thereby "indirectly" determining the conditions for the overall start-up of particles.

[0007] (2) Numerical simulation and empirical formulas: Based on empirical resistance relationships or particle force balance, a certain "critical shear stress / critical velocity" prediction formula is given; however, such methods usually rely on idealized geometry (such as ideal circular tubes, regular pore throats) and simplified mechanical models, which are difficult to reflect the actual morphology of the flow field under pore throat narrowing, expansion, and multiple expansion-contraction structures in real porous media, and also lack direct verification on an experimental scale.

[0008] Traditional flow field measurement methods (such as manometers and flow meters) can only provide channel inlet / outlet or overall average parameters, failing to analyze the instantaneous velocity field at the pore scale, let alone simultaneously capture the velocity vector field of the particle's neighborhood at the moment of particle initiation. These methods can only provide an average overall initiation velocity, making it difficult to obtain the local critical conditions of a single particle at a specific pore throat location, and neglecting the significant velocity non-uniformity and complex structures such as local flow around, backflow, and vortices within the pores. Even with particle image velocimetry (PIV) technology, without an optical system and data processing workflow specifically designed for microscale porous channels, it is difficult to obtain local flow velocities with sufficient spatial and temporal resolution, thus limiting the accuracy and reliability of the critical initiation velocity.

[0009] In summary, current technology lacks an experimental device and method for accurately quantifying the critical initiation velocity of particles based directly on the pore-scale velocity field within the pore throat structure of real or quasi-real porous media. Summary of the Invention

[0010] To overcome the shortcomings of existing technologies in directly and accurately quantifying the critical initiation velocity of particles in complex flow fields at the pore scale, this invention provides a quantitative measurement system and method for the critical initiation velocity of solid particles in complex and nonlinear flow fields at the pore scale based on microparticle image velocimetry. This system can accurately quantify the critical initiation velocity of solid particles in porous media with particle packing within pores under fluid action, addressing the deficiencies of existing technologies.

[0011] The following technical solution was specifically adopted:

[0012] A system for accurately quantifying the critical initiation velocity of particles under complex flow fields includes a device for characterizing the real pore structure of porous media, a pore etching model, a pore-scale flow field visualization device, and a data acquisition and processing device.

[0013] The device for characterizing the real pore structure of porous media includes a triaxial stress loading module and X-ray sources and X-ray detectors respectively located on both sides of the triaxial stress loading module. The device is used to obtain the real pore structure data of the target porous medium (which may be a particle accumulation or a core / outcrop sample) under specified working conditions in order to construct a pore etching model.

[0014] The pore etching model is used to simulate the pore-throat structure of real porous media. It is preferably fabricated using materials such as transparent PMMA, which facilitates laser illumination and high-speed imaging. The pore structure of the pore etching model can be obtained by reconstructing / simplifying real pore CT and then using a two-dimensional model. Alternatively, controllable variables such as pore throat geometry can be added for mechanism comparison. The real pore CT is obtained using a real pore structure characterization device for porous media. To improve feasibility and reproducibility, the present invention preferably uses the following scale range (example, not limitation): pore throat size can be 750:250μm or 1000:500μm, with an etching depth of approximately 200μm; a pore network structure formed by multiple pore-throat channels connected in series and parallel can also be used. Furthermore, the pore etching model is preferably a detachable upper and lower plate structure to support the placement of target particles at any preset pore throat position, improving experimental flexibility and repeatability, and facilitating disassembly and cleaning for reuse.

[0015] The pore-scale flow field visualization device includes a porous media simulation module, an injection module, a temperature and pressure control module, and a particle image velocimetry module. Preferably, it also includes an outlet metering and recovery module.

[0016] The porous media simulation module includes an etching model holder for holding a pore etching model; the injection module, temperature and pressure control module, and outlet metering and recovery module are all connected to the porous media simulation module.

[0017] The particle image velocimetry module includes a synchronization controller, a high-speed camera, a laser emitter, and a combined lens. The synchronization controller is connected to the high-speed camera and the laser emitter. The high-speed camera and the combined lens are located on opposite sides of the porous medium simulation module, while the laser emitter and the combined lens are located on the same side. A microscope objective is also provided between the high-speed camera and the porous medium simulation module. The high-speed camera is connected to a data acquisition and processing device. The laser beam emitted by the laser emitter is expanded and shaped by the combined lens to form a uniform illumination area, which is used to excite fluorescent tracer particles and form a clear particle image. The synchronization controller is used to control the time synchronization of the laser double pulse and the camera exposure. The particle displacement is obtained by using a double exposure cross-frame analysis method, thereby inverting the velocity field.

[0018] The particle image velocimetry module of the present invention forms a microscopic PIV measurement platform, which can record the motion of fluorescent tracer particles in the flow field within a microsecond time scale.

[0019] Preferably, the injection module is used to inject fluorescent tracer particle solution into the porous media simulation module, and includes an injection pump, a storage container, and an ultrasonic dispersion module connected in sequence. The porous media simulation module includes an injection pipeline, and the ultrasonic dispersion module is connected to the injection pipeline. An injector is also connected to the ultrasonic dispersion module for injecting the fluorescent tracer particle solution. More preferably, the injection pump is a micro-injection pump, capable of achieving stable low-flow-rate output and supporting step-by-step acceleration. To prevent large particles from settling during transport and injection, causing concentration fluctuations, a magnetic stirring injection unit is preferably provided to continuously stir the solution during injection. The magnetic stirring injection unit can be installed on the injection pipeline. The injection module of this invention is used to construct a liquid-solid two-phase flow field and study the initiation behavior of the liquid-solid two-phase flow field.

[0020] In a further preferred embodiment, if it is necessary to construct a gas-liquid-solid three-phase flow field, the injection module may also include a gas cylinder and a gas flow meter, wherein the gas cylinder is connected to the liquid injection pipeline and the gas flow meter is located between the gas cylinder and the liquid injection pipeline.

[0021] Preferably, the temperature and pressure control module includes a confining pressure pump, a water bath thermostat, a back pressure valve, and a back pressure pump. Both the confining pressure pump and the water bath thermostat are connected to the porous media simulation module. The confining pressure pump applies confining pressure to the etching model holder to simulate an external stress environment; the water bath thermostat provides a stable temperature boundary for the etching model holder, simulating the formation temperature environment; the porous media simulation module includes an outlet pipeline, the back pressure pump is connected to the outlet pipeline, and the back pressure valve is located on the outlet pipeline. The back pressure valve and the back pressure pump are used to control the outlet back pressure and establish a stable inlet-outlet pressure difference boundary.

[0022] Preferably, the outlet metering and recovery module includes a recovery container and an electronic balance. The recovery container is connected to the liquid outlet pipeline and placed on the electronic balance. The outlet metering and recovery module is used to record the produced liquid and realize recovery metering.

[0023] Preferably, the data acquisition and processing device includes a high-speed image acquisition module, two-dimensional PIV software, and a calculation module, which can calculate the two-dimensional planar velocity field. The cross-correlation algorithm has an accuracy of up to 0.1 pixels, which is used to improve the resolution of local environmental flow velocity extraction at the moment of startup. The high-speed image acquisition module is used to acquire particle images captured by the particle image velocimetry module. The two-dimensional PIV software is used to calculate the velocity vector field based on the particle images. The calculation module is used to calculate the critical startup flow velocity of the target particle based on the velocity vector field.

[0024] The data acquisition and processing device of the present invention performs cross-correlation calculations on two or more frames of particle images recorded by the particle image velocimetry module, inverts the two-dimensional velocity vector field at the pore scale, and further obtains the critical starting flow velocity of the target particle based on the velocity vector field.

[0025] This invention also discloses a method for accurately quantifying the critical initiation velocity of particles under complex flow fields:

[0026] (1) The real pore structure of porous media is characterized and simplified using methods such as μCT;

[0027] (2) Prepare a pore etching model (microfluidic chip) with the same scale or maintaining the topological features.

[0028] (3) Preset the target particle at the designated pore throat position;

[0029] (4) The instantaneous velocity field at the pore scale was analyzed using microscopic PIV, and the process of the particles from rest to start-up was recorded simultaneously;

[0030] Thus, the "local environmental velocity in the neighborhood at the moment of particle initiation" is used as the quantitative definition and measurement result of the critical initiation velocity, achieving accurate quantification of the critical initiation velocity of particles under complex and nonlinear flow fields.

[0031] This method is based on the aforementioned system implementation that accurately quantifies the critical initiation velocity of particles in complex flow fields. When the particle velocity within the orifice throat is below the critical initiation velocity, the particle remains stationary. At this time, the velocity field of the particle velocimetry exhibits the following characteristics: a closed isotropic line with a normal velocity of 0 is formed on the particle's upstream surface, resulting in an overall "stagnant-flow-around" flow structure. In this case, the drag force is less than or equal to the static friction between the particle and the wall. When the flow velocity within the channel reaches and exceeds the critical initiation velocity of the particle, this mechanical balance is broken, and the particle either rolls or floats with the liquid flow. The system of this invention can clearly capture the disappearance of the closed isotropic line on the upstream surface, the acceleration of the lateral shear layer, and the appearance of wake vortices, among other flow field characteristics. Simultaneously, the combined effect of drag and lift breaks through the static friction critical value, causing the particle to roll or float. The system of this invention can accurately record this process and calculate the ambient flow velocity of the particle at the moment of initiation using multiple frames. In this method, the high resolution and high frequency of microscopic particle image velocimetry enable the observation of the particle initiation velocity, solving the problem of low accuracy in conventional methods.

[0032] The method specifically includes the following steps:

[0033] Step 1: Place the target porous medium sample (such as a proppant deposit or reservoir rock sample) in a triaxial stress loading module (confining pressure / axial pressure can be applied as needed to simulate the formation stress environment), and perform μCT scanning using a radiation source and radiation detector to obtain pore structure slice data; perform noise reduction, segmentation, binarization, and reconstruction on the pore structure slices to obtain the true geometric features of the pore-throat; while maintaining the pore-throat connectivity topology and main expansion and contraction features, perform two-dimensionalization and processable simplification on the true geometric features of the pore-throat, and output the etching model layout.

[0034] Pore ​​structure data was obtained through μCT scanning. After completing 3D reconstruction of the pore-skeleton, interactive threshold segmentation was used to extract the granular phase. The watershed algorithm was then used to separate the connected components of the granular phase and label each particle and pore. A pore network model was established, and the labeled pores were networked, converting the connectivity of the pores into a graph structure composed of nodes and connections, where nodes correspond to pores and connections correspond to throats. This formed a geometric representation of the pore-skeleton, providing a basis for subsequent etching model design.

[0035] Step 2: Based on the etching model layout, prepare a transparent pore etching model that is proportional or retains the topological features; preferably, the material used for preparation is transparent PMMA; preferably, the pore etching model is a detachable structure; preferably, the etching depth and pore throat size are referenced to the target structure in the micrometer range, so that it can both represent the real structure and facilitate microscopic PIV observation and quantitative analysis.

[0036] Step 3: Preset the target particle at the designated throat position of the pore etching model, and clamp the pore etching model in the etching model holder;

[0037] Step 4: Prepare a fluorescent tracer particle solution of a preset concentration using the injection module; simulate reservoir temperature and pressure conditions using the temperature and pressure control module; turn on the laser emitter and high-speed camera, and set the laser double pulse interval Δt using the synchronization controller, preferably Δt is 100-300 μs; adjust the injection flow rate of the fluorescent tracer particle solution, the camera parameters of the high-speed camera, or the laser parameters of the laser emitter so that the displacement of the fluorescent tracer particles in two adjacent frames captured by the high-speed camera is controlled within the range of 5-12 pixels, so as to balance measurement accuracy and related calculation stability.

[0038] Preferably, the present invention uses 2-5μm monodisperse polystyrene fluorescent microspheres as fluorescent tracer particles, with a mass fraction preferably of 2.0~3.0wt%, more preferably 2.5wt%. The maximum excitation wavelength is 532nm and the maximum emission wavelength is 580nm, which facilitates obtaining high-contrast fluorescence images under 532nm laser illumination.

[0039] The flow-following property of the fluorescent tracer particles selected in this invention is verified by the Stokes number: under the parameters of characteristic velocity and characteristic length, the Stokes number of the tracer particles is 0.0006, which is much less than 1, indicating that the tracer particles can follow the fluid movement well and meet the requirements of PIV measurement. At the same time, the tracer particle concentration is low and the density is close to that of the fluid, so it has little impact on the fluid properties and solid particle transport.

[0040] Step 5: Start the injection pump, begin injection at a low flow rate and gradually increase the flow rate, preferably within the range of 0.1~1.0 mL·min. -¹; After each flow rate stabilizes, the high-speed acquisition mode of the high-speed camera is used to continuously capture image sequences, preferably 500 times continuously for statistical averaging to obtain the steady-state velocity field of that flow rate.

[0041] The state of the target particles is recorded synchronously during the process, achieving continuous coverage of the entire process from static to critical start-up.

[0042] Step 6: Transmit the image sequence to the data acquisition and processing device, preprocess the image (noise reduction, contrast enhancement, outlier removal, etc.), use the double exposure cross-frame analysis method to obtain the particle displacement information, invert the velocity vector field, and identify the start time; the accuracy of the two-dimensional PIV software algorithm used can reach the level of 0.1 pixels.

[0043] The dual-exposure cross-frame analysis method is as follows:

[0044] In an extremely short time interval Δ t Within the same flow field region, two exposures are performed, and the most probable displacement of the particle population in the two frames is calculated using normalized cross-correlation matching. This displacement is related to the particle's position in Δ... t The actual displacement within a time interval corresponds to the velocity vector field.

[0045] The specific method is as follows: To overcome the shortcomings of single-point tracking analysis, a correlation analysis method that is simple in principle, highly applicable, and highly accurate is adopted. The previous frame is used as the reference image, and the next frame as the target image. Image data is extracted using the analysis nodes of the reference image frame as the center, serving as a template. The similarity between the two image regions is evaluated using equation (I). The point with the smallest displacement change on the target image is selected as the new position. The point with the smallest displacement change refers to… r ( k , l Find the position where the maximum value is obtained, and compare the displacement difference between the template position and the new position.

[0046]

[0047] In formula (I), W This indicates the analysis window, with units of... px ; i, j This represents the pixel coordinates of fluorescent particles within the analysis window, in units of... px ; k , l The relative displacement of fluorescent particles is expressed in units of 1. px ; r for( k , l The correlation coefficient of location is dimensionless. f and g These represent the two frames of the image, respectively. f mand g m These represent the contents of the analysis window. f and g Average gray level, dimensionless; R min , R max () indicates the search range of the reference template on the target image, in units of px .

[0048] According to peak r After obtaining the movement direction of the fluorescent tracer particle swarm within the analysis window, the position of the fluorescent tracer particles in Δ is obtained. t Within a time period from ( x ( t ), y ( t ), z ( t The change is () x ( t +Δ t ), y ( t +Δ t ), z ( t +Δ t When the interval between two images is Δ t When the value is sufficiently small, the real-time velocity of the fluorescent tracer particles is obtained by the ratio of displacement to time interval, as shown in equation (II):

[0049]

[0050] In formula (II), , , These represent the fluorescent tracer particles in x, y, z Velocity components in three directions, in mm·s -1 ; , , For the particle at time t Spatial location coordinates, in mm; t Time, in seconds; The time interval between two adjacent frames is expressed in seconds.

[0051] Simultaneously, the target particle's position is identified and tracked to obtain displacement-time curves. The start-up time is determined when the following condition is met: when the target particle changes from being stationary to undergoing continuous displacement over several consecutive frames with the displacement direction being consistent (rolling, sliding, or floating). Optionally, the PIV velocity field also exhibits critical morphological characteristics: the disappearance or rupture of closed isotropic lines in the stagnant region on the frontal surface, enhanced acceleration of the lateral shear layer, and enhanced wake recirculation or vortex pair structure.

[0052] To ensure the accuracy of velocity inversion within the pores, this invention preferably tests distilled water flow in a regular rectangular cross-section channel (e.g., 750×250μm) far from the disturbance zone of the pore throat structure. The measured velocity profile of PIV is compared with COMSOL numerical simulation / theoretical calculation based on the Navier-Stokes equations to verify the reliability of the measurement.

[0053] Step 7: Based on startup time velocity vector field The critical starting velocity is calculated using the neighborhood averaging method, the cross-sectional characteristic method, or the neighborhood maximum method. ,

[0054] ① Neighborhood averaging method: at startup time velocity vector field In this process, a neighborhood window is formed by setting a distance outside the projected contour of the target particle, and the average area of ​​the velocity modulus within the neighborhood window is taken as the mean. ;

[0055] ② Cross-sectional characteristic method: Take the area-averaged velocity or centerline velocity of the cross-section of the pore throat where the target particle is located as... ;

[0056] ③ Maximum Neighborhood Method: At startup time velocity vector field In this process, a neighborhood window is formed by setting a distance outside the projected contour of the target particle, and the maximum velocity within the neighborhood window is taken as the mean. This is used to characterize the control effect of local acceleration in the throat on initiation.

[0057] By changing parameters such as target particle size, target particle preset position, porous media structure (simplified scheme of real pores), confining pressure / back pressure / temperature or injection flow rate step, and repeating the above steps, the statistical distribution of critical start-up velocity is obtained, which is used to establish a quantitative relationship between "pore throat geometry - flow field nonlinearity - particle start-up threshold".

[0058] The "target particles" mentioned in the invention include, but are not limited to, in-situ generated particles (fracturing proppant particles / debris), simulated in-situ generated particles or intrusive particles (silica microparticles, polystyrene (PS) non-fluorescent particles), etc. In the system and device disclosed in this invention, the target particles can be arranged at any preset position such as the pore throat contraction, expansion, pore throat junction or flow around the flow area.

[0059] The "critical initiation velocity" described in this invention is not the average velocity corresponding to the inlet set flow rate, but rather the representative value of the local environmental velocity in the vicinity of the pore throat at the instant the target particle changes from static to continuous motion. This local environmental velocity is directly extracted from the pore-scale velocity vector field obtained by inversion from microparticle image velocimetry (micro-PIV), thereby avoiding errors in the non-uniform flow field caused by pore throat contraction and expansion, flow around, backflow, local jets, etc.

[0060] Therefore, to avoid the limitations of using traditional macroscopic average flow velocity as a critical condition and to ensure that the measurement results have clear physical meaning and repeatability, this invention defines the critical starting flow velocity based on the local real flow field at the instant of target particle initiation, preferably using the aforementioned neighborhood averaging method, cross-sectional feature method, and neighborhood maximum method. :

[0061] The determination of the target particle initiation behavior in this invention is primarily based on particle kinematics, supplemented by flow field morphology criteria for verification. Particle initiation is identified when the relative displacement of the particle's center of mass exhibits a continuous, unidirectional increase exceeding the noise threshold over several consecutive frames. Simultaneously, corresponding changes in the flow field morphology can be observed. For example, before initiation, the particle's upflow surface exhibits a stagnant zone / closed isotropic line with a "stagnant-flow-around" structure; near the critical point, the closed isotropic line breaks / disappears, the lateral shear layer accelerates and intensifies, and the wake shows significant backflow or vortex pair structures, further confirming the occurrence of the initiation event. When either of these two conditions occurs in the velocity field of the flow domain, the particle is determined to be dragged by the fluid and begins initiation.

[0062] By adopting the above technical solution, the beneficial effects of the present invention are as follows:

[0063] 1. Precise definition and quantification of critical start-up flow rate:

[0064] This invention defines the "critical start-up velocity" as the local environmental velocity in the vicinity of the pore throat at the instant of particle start-up, rather than the overall average velocity or the inlet set flow rate. By directly measuring the velocity vector field around the particle at the pore scale using microscopic PIV, the critical start-up velocity calculated in this way can truly reflect the actual drag and lift effect of the fluid on the particle, thus improving the quantitative accuracy of the start-up conditions.

[0065] 2. Explicitly consider the effects of complex, nonlinear pore flow fields:

[0066] Due to the narrowing and widening of the pore throat and abrupt changes in channel geometry, the flow within the pores exhibits a strongly non-uniform and nonlinear distribution, easily generating localized accelerated jets, backflows, and vortex structures. This invention utilizes microscopic PIV to directly observe and quantify the influence of these flow field characteristics on particle initiation, overcoming the shortcomings of traditional methods that rely solely on inlet flow rate or average velocity to determine initiation conditions. This allows for a more accurate measurement of the critical initiation velocity of particles, more closely resembling the complex flow fields in real porous media.

[0067] 3. The etched model structure is highly customizable, and the particle position is flexible and controllable:

[0068] The pore etching model of this invention can be obtained by etching the pore structure of real porous media, or by using a set combination of pore / throat sizes (such as pore / throat sizes of 750:250μm, 1000:500μm, etching depth of 200μm, etc.). By adjusting the pore / throat width ratio and geometry, various pore / throat flow field structures can be simulated. The model adopts a detachable upper and lower plate structure, which allows target particles to be placed at any preset pore / throat position and then assembled, greatly improving the flexibility of experimental design and facilitating systematic research on the influence of different pore / throat positions, particle sizes and shapes on the critical start-up flow velocity.

[0069] 4. Reproducible quantitative experimental system

[0070] This invention uses fluorescent polystyrene microspheres with a Stokes number far less than 1 as tracer particles to ensure good flow characteristics; controls the double-pulse time interval Δt within 100–300 μs to ensure reasonable particle displacement of 5–12 pixels; continuously acquires approximately 500 frames of images under each operating condition for statistical averaging; and performs velocity field calibration in the upstream section of the regular channel; thus forming a PIV measurement system with a clear parameter window and calibration procedure, ensuring the accuracy and repeatability of the critical start-up flow velocity measurement results.

[0071] 5. This invention provides a complete system for accurately quantifying the critical starting velocity of particles in complex flow fields, which can realize the simulation of complex flow fields and accurate calculation of critical starting velocity of target porous media. Attached Figure Description

[0072] Figure 1 This is a schematic diagram of the pore-scale flow field visualization device in Example 1;

[0073] Figure 2 This is a schematic diagram of the structure of the porosity characterization device for porous media in Example 1;

[0074] Figure 3 This is a schematic diagram of the pore etching model in Example 1;

[0075] Figure 4 The flow field cloud diagrams are shown at different locations where the target particle passes through the pore throat structure in Example 2.

[0076] Figure 5 The curves show the horizontal centerline velocity of the flow field in Example 2, with and without solid particles.

[0077] Among them, 1-injection pump, 2-liquid storage container, 3-ultrasonic dispersion module, 4-sampler, 5-gas cylinder, 6-gas flow meter, 7-containing pressure pump, 8-synchronization controller, 9-high-speed camera, 10-laser emitter, 11-combination lens, 12-etching model holder, 13-data acquisition and processing device, 14-water bath constant temperature device, 15-back pressure valve, 16-back pressure pump, 17-recovery container, 18-electronic balance, 19-radiation source, 20-triaxial stress loading module, 21-radiation detector, 22-pore etching model inlet, 23-pore etching model outlet. Detailed Implementation

[0078] The present invention will be further described below with reference to the accompanying drawings and embodiments, but the present invention is not limited to the scope of the embodiments described therein.

[0079] Example 1

[0080] like Figures 1-3 As shown, this embodiment discloses a system for accurately quantifying the critical initiation velocity of particles under complex flow fields, including a device for characterizing the real pore structure of porous media, a pore etching model, a pore-scale flow field visualization device, and a data acquisition and processing device 13:

[0081] The device for characterizing the real pore structure of porous media includes a triaxial stress loading module 20 and a radiation source 19 and a radiation detector 21 respectively located on both sides of the triaxial stress loading module 20.

[0082] The throat size of the pore etching model in this embodiment can be 750:250μm or 1000:500μm, the etching depth is 200μm, and it is a detachable upper and lower plate structure, with a pore etching model inlet 22 and a pore etching model outlet 23.

[0083] The pore-scale flow field visualization device includes a porous media simulation module, an injection module, a temperature and pressure control module, a particle image velocimetry module, and an outlet metering and recovery module.

[0084] The porous media simulation module includes an etching model holder 12, which is used to hold a pore etching model; the injection module, temperature and pressure control module and outlet metering and recovery module are all connected to the porous media simulation module.

[0085] The particle image velocimetry module includes a synchronization controller 8, a high-speed camera 9, a laser emitter 10, and a combination lens 11. The synchronization controller 8 is connected to the high-speed camera 9 and the laser emitter 10. The high-speed camera 9 and the combination lens 11 are located on opposite sides of the porous medium simulation module, while the laser emitter 10 and the combination lens 11 are located on the same side. A microscope objective is also provided between the high-speed camera 9 and the porous medium simulation module. The high-speed camera 9 is connected to a data acquisition and processing device 13. The laser beam emitted by the laser emitter 10 is expanded and shaped by the combination lens 11 to form a uniform illumination area, which is used to excite fluorescent tracer particles and form a clear particle image. The synchronization controller 8 is used to control the time synchronization of the laser double pulse and the camera exposure, and the particle displacement is obtained by using a double exposure cross-frame analysis method, thereby inverting the velocity field.

[0086] The injection module includes an injection pump 1, a liquid storage container 2, and an ultrasonic dispersion module 3 connected in sequence. The porous media simulation module includes an injection pipeline. The ultrasonic dispersion module 3 is connected to the injection pipeline, and an injector 4 is also connected to the ultrasonic dispersion module 3 for injecting the fluorescent tracer particle solution. The injection pump 1 is a micro-injection pump, capable of achieving stable low-flow-rate output and supporting step-by-step acceleration. The injection pipeline is also equipped with a magnetic stirring injection unit to continuously stir the solution during injection. The injection module may also include a gas cylinder 5 and a gas flow meter 6. The gas cylinder 5 is connected to the injection pipeline, and the gas flow meter 6 is located between the gas cylinder 5 and the injection pipeline.

[0087] The temperature and pressure control module includes a confining pressure pump 7, a water bath thermostat 14, a back pressure valve 15, and a back pressure pump 16. Both the confining pressure pump 7 and the water bath thermostat 14 are connected to the porous media simulation module. The confining pressure pump 7 applies confining pressure to the etching model holder 12 to simulate an external stress environment; the water bath thermostat 14 provides a stable temperature boundary for the etching model holder 12, simulating the formation temperature environment; the porous media simulation module also includes an outlet pipeline, with the back pressure pump 16 connected to it. The back pressure valve 15 is located on the outlet pipeline, and the back pressure valve 15 and the back pressure pump 16 are used to control the outlet back pressure and establish a stable inlet-outlet pressure difference boundary.

[0088] The outlet metering and recovery module includes a recovery container 17 and an electronic balance 18. The recovery container 17 is connected to the liquid outlet pipeline and placed on the electronic balance 18. The outlet metering and recovery module is used to record the produced liquid and realize recovery metering.

[0089] The data acquisition and processing device 13 includes a high-speed image acquisition module, two-dimensional PIV software, and a calculation module. The high-speed image acquisition module is used to acquire particle images captured by the particle image velocimetry module. The two-dimensional PIV software is used to calculate the velocity vector field based on the particle images. The calculation module is used to calculate the critical initiation velocity of the target particle based on the velocity vector field.

[0090] Example 2

[0091] This embodiment discloses a method for accurately quantifying the critical initiation velocity of particles in a complex flow field using the system of Embodiment 1, comprising the following steps:

[0092] Step 1: Place the target porous medium sample in the triaxial stress loading module 20 (confining pressure / axial pressure can be applied as needed to simulate the formation stress environment). Use a μCT scan via X-ray source 19 and X-ray detector 21 to obtain pore structure slice data. Perform noise reduction, segmentation, binarization, and reconstruction on the slices to obtain the true geometric features of the pores and throats. While maintaining the pore-throat connectivity topology and main expansion / contraction characteristics, perform two-dimensionalization and simplification for machinability, outputting the etching model layout. In this case, the porous medium is a quartz sand particle packing porous medium with a particle size of 8-16 mesh. The scanned pore structure, after two-dimensionalization, has a pore width of 1000 μm, a throat width of 500 μm, and a pore-throat ratio of 2:1.

[0093] Step 2: Fabricate a detachable pore etching model based on the layout, using transparent PMMA as the material; before use, immerse the chip in deionized water for ultrasonic cleaning and drying to remove surface impurities and reduce optical contamination.

[0094] Step 3: Open the detachable pore etching model, place the target particle to be tested at the preset pore throat position (such as throat inlet, pore throat junction, flow around area, backflow prone area, etc.), then seal and assemble it and fix it in the etching model holder 12 so that the area is located in the imaging field of view of the high-speed camera 9 and the laser illumination area.

[0095] Step 4: Take distilled water (or equivalent transparent working fluid) as the base solution (stored in the storage container 2), add 2μm fluorescent tracer particle solution (added through the injector 4) to make the concentration 2.5wt% (i.e., forming a 2.5wt% fluorescent tracer particle solution in the ultrasonic dispersion module 3), and add a certain mass of large-particle powder / non-fluorescent solid particles as needed for the experiment. Place the 2.5wt% fluorescent tracer particle solution in an ultrasonic water bath at about 40℃ and disperse it by shaking for about 30 minutes to ensure uniform dispersion of fluorescent particles; after dispersion, place the solution in a magnetic stirring injection system and stir continuously to prevent larger particles from settling and to ensure stable injection concentration.

[0096] Connect the injection pump 1 and the injection pipeline to the etching model holder 12. Connect the outlet pipeline of the porous media simulation module to the back pressure valve 15, the back pressure pump 16, and the recovery container 17 to form a closed test loop. If necessary, gas can be introduced for pressure holding and leak detection. Turn on the water bath constant temperature device 14 to establish a stable temperature boundary. Establish the confining pressure boundary as needed through the confining pressure pump 7, and establish the outlet back pressure boundary through the back pressure valve 15 and the back pressure pump 16.

[0097] Turn on the laser emitter 10 and the high-speed camera 9, and set the laser double pulse interval Δt through the synchronization controller 8. To ensure reliable cross-correlation calculation, Δt is preferably set to 100-300 μs (the selection of Δt here fluctuates according to the flow rate range in the channel), and adjust the injection flow rate or camera / laser parameters to control the displacement of fluorescent particles in two adjacent frames within the range of 5-12 pixels.

[0098] Step 5: Start the injection pump, beginning with a low flow rate and gradually increasing the flow rate, ranging from 0.1 to 1.0 mL / min. - ¹; After each flow rate stabilizes, a high-speed acquisition mode is used to continuously capture image sequences. 500 consecutive acquisitions are used to statistically average the steady-state velocity field of that flow rate.

[0099] The state of the target particles is recorded simultaneously during the acceleration process, achieving continuous coverage of the entire process from static to critical start-up.

[0100] Step 6: Transmit the image sequence to the data acquisition and processing device 13, preprocess the image (noise reduction, contrast enhancement, outlier removal, etc.), use the double exposure cross-frame analysis method to obtain the particle displacement information, invert the velocity vector field, and identify the start time; the accuracy of the two-dimensional PIV software algorithm used can reach the level of 0.1 pixels.

[0101] The target particles are identified and tracked to obtain displacement-time curves. The start-up time is determined when the following condition is met: when the particle changes from being stationary to continuous displacement over several consecutive frames with a consistent displacement direction (rolling, sliding, or floating). Optionally, the PIV velocity field also exhibits critical morphological characteristics: the disappearance or rupture of closed isotropic lines in the stagnant region on the frontal surface, enhanced acceleration of the lateral shear layer, and enhanced wake recirculation or vortex pair structure.

[0102] Step 7, at startup time velocity vector field In this invention, the "representative value of local environmental velocity in the particle neighborhood" is extracted as the critical starting velocity. In this embodiment, the neighborhood averaging method is used: a window is formed by setting a distance outside the particle projection contour, and the area of ​​the velocity model within the window is averaged to obtain the velocity model. .

[0103] In this way, the critical start-up velocity is directly derived from the "real flow field of the orifice throat at the moment of particle start-up", avoiding the systematic errors caused by the orifice throat contraction and expansion and local acceleration caused by the traditional method of calculating the average velocity based on the inlet flow rate.

[0104] By changing parameters such as target particle size, target particle preset position, porous media structure (simplified scheme of real pores), confining pressure / back pressure / temperature or injection flow rate step, and repeating the above steps, the statistical distribution of critical start-up velocity is obtained, which is used to establish a quantitative relationship between "pore throat geometry - flow field nonlinearity - particle start-up threshold".

[0105] Figure 4 The complete migration process of a 300µm particle within a pore-throat structure and the corresponding flow field contour map were recorded. In actual porous media flow, there is a difference in flow velocity between the channel wall and the center, and the geometric abrupt change at the junction of the pore and throat often leads to a significant change in flow velocity. Combined with the particle migration behavior, this further complicates the flow field characteristics. The system and method described in this invention can accurately monitor the local real flow field at the moment of target particle initiation. As the particle gradually moves from the pore towards the throat, the influence of particle disturbance on the flow field gradually increases, the flow field color gradually lightens, and the overall flow velocity decreases. When the particle reaches the upstream region of the throat, the fluid is subjected to a dual effect: firstly, the geometric cross-sectional area at the pore-throat junction shrinks; secondly, the particle occupies part of the throat's cross-sectional area, further compressing the effective flow area, resulting in a brief surge in flow velocity.

[0106] Figure 5 Given the horizontal centerline velocity of the flow field when the target particle is located upstream and downstream of the pore throat structure, respectively, according to the system and method described in this invention, the neighborhood maximum method can be used to determine when the flow velocity within the pore throat is greater than 7 mm·s. -1 The target particle begins to initiate. This figure shows that the flow velocity exhibits strong heterogeneity within the pore throat structural unit of the porous medium. This variable flow field demonstrates the necessity for accurate quantification of the particle initiation threshold under complex and nonlinear flow fields.

[0107] This invention, based on the pore-scale velocity vector field obtained by micro-PIV inversion, extracts the local environmental velocity in the vicinity of a particle at the moment of initiation as the critical initiation velocity, achieving precise quantification of the particle initiation threshold under complex and nonlinear flow fields. Compared with methods that determine initiation conditions solely based on inlet set flow rate or channel average velocity, this invention can explicitly consider the influence of non-uniform flow structures such as local acceleration caused by pore throat contraction and expansion, flow-around shear layers, and wake backflow on particle initiation. Furthermore, by comparing the velocity field with the numerical / theoretical profile through a regular rectangular channel segment, it improves the quantitative reliability and repeatability of the critical initiation velocity.

[0108] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A system for accurately quantifying the critical initiation velocity of particles in complex flow fields, characterized in that, Includes a device for characterizing the real pore structure of porous media, a pore etching model, a pore-scale flow field visualization device, and a data acquisition and processing device: The device for characterizing the real pore structure of porous media includes a triaxial stress loading module and a radiation source and a radiation detector respectively located on both sides of the triaxial stress loading module. The pore etching model contains target particles; The pore-scale flow field visualization device includes a porous media simulation module, an injection module, a temperature and pressure control module, and a particle image velocimetry module. The porous media simulation module includes an etching model holder for holding a pore etching model. The injection module and the temperature and pressure control module are both connected to the porous media simulation module. The injection module is used to inject a fluorescent tracer particle solution into the porous media simulation module. The particle image velocimetry module includes a synchronization controller, a high-speed camera, a laser emitter, and a combined lens. The synchronization controller is connected to the high-speed camera and the laser emitter. The high-speed camera and the combined lens are located on opposite sides of the porous media simulation module, while the laser emitter and the combined lens are located on the same side. The high-speed camera is connected to a data acquisition and processing device. The data acquisition and processing device includes a high-speed image acquisition module, two-dimensional PIV software, and a calculation module. The high-speed image acquisition module is used to acquire particle images captured by the particle image velocimetry module. The two-dimensional PIV software is used to calculate the velocity vector field based on the particle images. The calculation module is used to calculate the critical initiation velocity of the target particle based on the velocity vector field. The temperature and pressure control module includes a confining pressure pump, a water bath constant temperature device, a back pressure valve, and a back pressure pump. The confining pressure pump and the water bath constant temperature device are both connected to the porous media simulation module. The porous media simulation module also includes a liquid outlet pipeline. The back pressure pump is connected to the liquid outlet pipeline, and the back pressure valve is located on the liquid outlet pipeline.

2. The system for accurately quantifying the critical starting velocity of particles in complex flow fields according to claim 1, characterized in that, The injection module includes an injection pump, a liquid storage container, and an ultrasonic dispersion module connected in sequence. The porous media simulation module includes an injection pipeline. The ultrasonic dispersion module is connected to the injection pipeline, and an injector is also connected to the ultrasonic dispersion module.

3. The system for accurately quantifying the critical starting velocity of particles in complex flow fields according to claim 2, characterized in that, The injection pipeline is also equipped with a magnetic stirring injection unit; The injection module also includes a gas cylinder and a gas flow meter. The gas cylinder is connected to the injection pipeline, and the gas flow meter is located between the gas cylinder and the injection pipeline.

4. A method for accurately quantifying the critical initiation velocity of particles in a complex flow field using the system described in any one of claims 1 to 3, characterized in that, Includes the following steps: Step 1: Place the target porous medium sample in the triaxial stress loading module and perform μCT scanning using a radiation source and detector to obtain pore structure slice data; perform noise reduction, segmentation, binarization, and reconstruction on the pore structure slices to obtain the true geometric features of the pore-throat; while maintaining the pore-throat connectivity topology and main expansion and contraction features, perform two-dimensionalization and simplification of the true geometric features of the pore-throat to output the etching model layout; Step 2: Based on the etching model layout, prepare a transparent pore etching model that is proportional or retains the topological features; Step 3: Preset the target particle at the designated throat position of the pore etching model, and clamp the pore etching model in the etching model holder; Step 4: Prepare a fluorescent tracer particle solution of a preset concentration using the injection module; The temperature and pressure conditions of the reservoir are simulated by the temperature and pressure control module; the laser emitter and high-speed camera are turned on, and the laser double pulse interval Δt is set by the synchronous controller. Step 5: Start the injection pump, begin injection from a low flow rate and gradually increase the injection flow rate. After each flow rate stabilizes, use the high-speed acquisition mode of the high-speed camera to continuously capture image sequences. During the process, record the state of the target particles simultaneously to achieve continuous coverage of the entire process from static to critical start-up. Step 6: Transmit the image sequence to the data acquisition and processing device, preprocess the images, use the double exposure cross-frame analysis method to obtain the particle displacement information, invert the velocity vector field, and identify the start time; Step 7: Based on the velocity vector field at the start-up moment, calculate the critical start-up velocity using the neighborhood averaging method, cross-sectional feature method, or neighborhood maximum method.

5. The method for accurately quantifying the critical starting velocity of particles in complex flow fields according to claim 4, characterized in that, Step four also includes: adjusting the injection flow rate of the fluorescent tracer particle solution, the camera parameters of the high-speed camera, or the laser parameters of the laser emitter, so that the displacement of the fluorescent tracer particles in two adjacent frames captured by the high-speed camera is controlled within the range of 5 to 12 pixels.

6. The method for accurately quantifying the critical starting velocity of particles in complex flow fields according to claim 4, characterized in that, In step four, 2-5 μm monodisperse polystyrene fluorescent microspheres are used as fluorescent tracer particles. After preparing a 2.0-3.0 wt% fluorescent tracer particle solution, it is injected into the porous media simulation module. In step four, Δt is 100–300 μs; In step five, the injection flow rate ranges from 0.1 to 1.0 mL / min. - ¹.

7. The method for accurately quantifying the critical starting velocity of particles in complex flow fields according to claim 4, characterized in that, Step Six: Obtain particle displacement information using the double-exposure cross-frame analysis method. The specific steps for inverting the velocity vector field are as follows: Using the previous frame as the reference image and the next frame as the target image, image data is extracted centered on the analysis node of the reference frame as a template. The similarity between the two image regions is evaluated using equation (I). The point with the smallest displacement change on the target image is selected as the new position. The point with the smallest displacement change refers to… r ( k , l Find the position with the maximum value and compare the displacement difference between the template position and the new position: In formula (I), W This indicates the analysis window, with units of... px ; i, j This represents the pixel coordinates of fluorescent particles within the analysis window, in units of... px ; k , l The relative displacement of fluorescent particles is expressed in units of 1. px ; r for( k , l The correlation coefficient of location is dimensionless. f and g These represent the two frames of the image, respectively. f m and g m These represent the contents of the analysis window. f and g The average gray level is dimensionless. ( R min , R max () indicates the search range of the reference template on the target image, in units of px ; according to r After obtaining the direction of movement of the local region of the flow field represented by the fluorescent tracer particle swarm, the position of the fluorescent tracer particles in Δ is obtained. t Within a time period from ( x ( t ), y ( t ), z ( t The change is () x ( t +Δ t ), y ( t +Δ t ), z ( t +Δ t When the interval between two images is Δ t When the value is sufficiently small, the real-time velocity of the fluorescent tracer particles is obtained by the ratio of displacement to time interval, as shown in equation (II): (II) In formula (II), , , These represent the velocity components of the fluorescent tracer particles in the x, y, and z directions, respectively, with units of mm·s⁻¹. , , The coordinates of the particle's spatial position at time t are in mm; t is time, in seconds. The time interval between two adjacent frames is expressed in seconds.

8. The method for accurately quantifying the critical starting velocity of particles in complex flow fields according to claim 4, characterized in that, Step six, the specific steps for identifying the startup time are as follows: The start-up time is determined when the following conditions are met: when the target particle changes from being stationary to continuous displacement within several frames with the displacement direction being consistent, the PIV velocity field simultaneously exhibits critical morphological characteristics: the closed isotropic lines in the stagnant region of the frontal surface disappear or break, the lateral shear layer is accelerated and enhanced, and the wake recirculation or vortex pair structure is enhanced.

9. The method for accurately quantifying the critical starting velocity of particles in complex flow fields according to claim 4, characterized in that, In step seven, the specific method for calculating the critical start-up flow velocity is as follows: Neighborhood averaging method: In the velocity vector field at the start-up moment, a neighborhood window is formed at a set distance from the outside of the target particle's projected profile, and the area average of the velocity moduli within the neighborhood window is taken as the critical start-up velocity. Section feature method: Take the area average velocity or centerline velocity of the pore throat section where the target particle is located as the critical starting velocity; The maximum neighborhood method: In the velocity vector field at the start-up moment, a neighborhood window is formed by setting a distance outside the projected contour of the target particle, and the maximum velocity within the neighborhood window is taken as the critical start-up velocity.

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