Preparation method of bionic microfluidic device for studying fluid migration in carbonate rock micro-nano cracks

Abstract

The application provides a preparation method of a biomimetic microfluidic device for studying fluid migration in carbonate rock micro-nano cracks, comprising the following steps: step one, analyzing and classifying scanning electron microscope images of carbonate rock core slices by a random forest algorithm to generate a chip design drawing; step two, based on the chip design drawing, adopting soft lithography technology, copying crack structures through a photoresist template and polydimethylsiloxane, and bonding with a glass substrate to form a microfluidic chip; step three, injecting a precursor liquid containing calcite nanocrystal seeds into the flow channel of the microfluidic chip, fixing the crystal seeds by ultraviolet irradiation, and realizing calcium carbonate mineral precipitation by injecting a saturated equilibrium liquid to form a biomimetic microfluidic device. The application can be used for coupling research of all fluids and carbonate rock cracks, and provides a method for restoring the structure of carbonate rock cracks to the greatest extent and a method for cloning the properties of rock surfaces.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of microfluidic chip manufacturing technology and carbonate rock fracture fluid migration research, and particularly relates to a preparation method of a biomimetic microfluidic device for studying fluid migration in carbonate rock micro-nano fractures. BACKGROUND

[0002] Carbonate rock fractures are an important topic in oil and gas migration and reservoir reconstruction research. Traditional research methods such as core experiments and nuclear magnetic resonance (NMR) technology are difficult to accurately characterize the fluid migration process in micro-nano fractures due to the limitations of time and spatial resolution. In addition, existing microfluidic chips are usually manufactured by regular geometric structure processing methods, and their fracture morphology and surface physical and chemical properties are significantly different from those of real carbonate rock fractures, making it difficult to fully reflect the complexity of the actual underground reservoir environment.

[0003] In order to solve the above problems, a biomimetic microfluidic device capable of highly restoring the morphology and surface physical and chemical properties of carbonate rock fractures is designed, which is of great significance for exploring complex physical fields, chemical fields and fluid-mineral coupling research. SUMMARY

[0004] The application provides a preparation method of a biomimetic microfluidic device, which can truly reproduce the structure and surface physical and chemical properties of natural carbonate rock fractures.

[0005] The technical scheme of the application is as follows:

[0006] A preparation method of a biomimetic microfluidic device for studying fluid migration in carbonate rock micro-nano fractures, comprising the following steps:

[0007] Step one, analyze and classify the scanning electron microscope images of carbonate rock core slices by random forest algorithm to generate chip design drawings;

[0008] Step two, based on the chip design drawings, use soft lithography technology to reproduce the fracture structure through a photoresist template and polydimethylsiloxane, and bond with a glass substrate to form a microfluidic chip;

[0009] Step three, inject a precursor solution containing calcite nanocrystals into the flow channel of the microfluidic chip, fix the crystals by ultraviolet light irradiation, and realize calcium carbonate mineral precipitation by injecting a saturated equilibrium solution to form a biomimetic microfluidic device.

[0010] Further, the step one is specifically:

[0011] Cut a slice from the carbonate rock core, vacuumize and spray conductive carbon film on it, analyze the fracture and mineral composition by scanning electron microscope and energy dispersive X-ray spectroscopy, and confirm the parent rock type and mineral distribution.

[0012] The preliminary profile of the crack is extracted by using a two-dimensional Gaussian curvature matrix to smooth the crack boundary and enhance its clarity through a Gaussian filter;

[0013] The crack and the surrounding mineral type and pore characteristics are marked to construct a training set for a random forest algorithm;

[0014] The image pixels are classified by the random forest algorithm to realize semantic segmentation of minerals, cracks and pores, and the training parameters are dynamically adjusted to improve the classification accuracy, thereby forming mineral and crack distribution data.

[0015] The main boundary and multi-scale features of the crack are extracted to generate a chip design drawing.

[0016] Further, the step two is specifically:

[0017] An SU-8 photoresist template is prepared, and a crack structure mold is formed through a coating, exposure, development and hardening process;

[0018] Components A (prepolymer) and component B (crosslinking agent) of polydimethylsilane (PDMS) are mixed at a mass ratio of 10:1, poured into the mold, vacuumized and degassed, and then cured under heating;

[0019] The cured PDMS and the glass surface to be bonded are cleaned in a plasma treatment machine and then bonded together after plasma cleaning to form a microfluidic chip.

[0020] Further, the preparation of the SU-8 photoresist template and the formation of the crack structure mold through the coating, exposure, development and hardening process are specifically:

[0021] Coating: after cleaning and drying the silicon wafer, the SU-8 photoresist is coated in the coating machine at low and high speeds, and left to stand for 5 minutes;

[0022] Soft baking: the silicon wafer after coating is placed on the leveled heating table, heated at 65℃ for 2 minutes, then heated at 95℃ for 10 minutes, and then lowered to room temperature;

[0023] Exposure: the mask plate and the silicon wafer after soft baking are attached together, and exposed to 365nm ultraviolet light for 75 seconds to make the photoresist photosensitive and cured;

[0024] Post-baking: the silicon wafer after exposure and the mask plate are separated, and the heating plate is used again to heat at 65℃ for 2 minutes, then heated at 95℃ for 10 minutes, and then the silicon wafer after exposure is lowered to room temperature with the heating plate;

[0025] Development: the unexposed area is dissolved using a developing solution, and the silicon wafer is blown dry with nitrogen after cleaning;

[0026] Hard bake: after developing, the silicon wafer is heated by a heating plate at 95 DEG C for 5 min, then at 175 DEG C for 20 min, at 65 DEG C for 5 min and cooled at room temperature to form a crack structure mold.

[0027] Further, the step three is specifically:

[0028] Injecting sodium hydroxide solution into the microfluidic chip flow channel to clean the channel surface, and carrying out silanization treatment by 3-propyl acrylate;

[0029] Injecting the pre-solution containing calcite nanocrystals into the microfluidic chip flow channel after silanization treatment, and fixing the crystal seeds on the crack surface by ultraviolet irradiation;

[0030] Injecting the mixed solution of sodium bicarbonate solution and calcium chloride solution to carry out calcium carbonate mineral precipitation, and optimizing the physical and chemical properties of the crack surface.

[0031] Further, the preparation method of the pre-solution is:

[0032] Mixing 5mg of calcite seeds, 20ul of polyethylene glycol diacrylate, 5ul of photoinitiator, 40ul of polyethylene glycol, 2ul of acrylic acid and 33ul of deionized water uniformly, and forming a uniform solution by ultrasonic oscillation dispersion.

[0033] Further, the sodium bicarbonate solution and the calcium chloride solution are mixed in a ratio of 1:2.84 to obtain an equilibrium solution with a saturation degree of 1.

[0034] Compared with the prior art, the present application has the following advantages:

[0035] The present application can be used for the coupling research of all fluids and carbonate rock cracks, and provides a method for restoring carbonate rock crack structures to the greatest extent and a rock surface property cloning method, so that the cracks in the chip are consistent with the underground carbonate rock crack structures in terms of structure and surface physical and chemical properties, and the flow channel surface properties are controlled according to different mineral growth modes. A small amount of mineral pore is marked into a random forest model for classification and extraction of difficult-to-identify pore structures, and a soft lithography technology is used to re-engrave the extracted crack structure to form a structure consistent with the carbonate rock crack microchannel.

[0036] The present application adopts a calcite nanocrystal in-situ mineral growth method. This method can manufacture a micro model with adjustable pore structure and surface mineral, facilitate the development of contrast tests, and deeply explore the fluid-mineral coupling effect, such as the fractionation of condensate oil in crack migration, acidizing fracturing of the formation, CCUS storage effect, etc. The present application not only has economic and convenient nature, but also has great expandable space and practical value. BRIEF DESCRIPTION OF DRAWINGS

[0037] The accompanying drawings illustrate various embodiments generally by way of example rather than limitation, and are used, together with the specification and claims, to explain embodiments of the invention. Where appropriate, the same reference numerals are used in all drawings to refer to the same or similar parts. Such embodiments are illustrative and are not intended to be exhaustive or exclusive embodiments of the apparatus or method.

[0038] Figure 1 A schematic diagram of the chip design of the present invention is shown;

[0039] Figure 2 A schematic diagram of the experimental apparatus of the present invention is shown; wherein, 1-air compressor, 2-constant pressure pump, 3-microscope, 4-microfluidic chip, 5-heating module, 6-centrifuge tube, 7-computer;

[0040] Figure 3 A schematic diagram of the completed bionic chip structure of the present invention is shown. Detailed Implementation

[0041] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0042] The embodiments of this invention mainly consist of three parts: The first part is to extract the structure of fractures in carbonate rocks. The second part is to create a pattern based on the extracted fracture structure and fabricate a mask, and then use soft photolithography to prepare a microfluidic base chip with the fracture structure. The third part is to grow minerals on the flow channels.

[0043] Part 1: Extraction of Natural Crack Structures

[0044] A thin rock section with sides of 1.5cm x 3cm and a thickness of 1.5mm was cut from a carbonate rock core. After vacuum carbonization, SEM-EDS was used to confirm that the parent rock of the fractures was carbonate rock, and the mineral distribution characteristics around the fractures were obtained. To obtain multi-scale fracture network images, 54 regional SEM images of the rock thin section were obtained and stitched together using a large field of view.

[0045] Then, minerals, organic matter, intergranular pores, intragranular dissolution pores, microcracks, and large cracks are manually labeled in the SEM images of the large thin sections of the rock, and a training set is established for the random forest algorithm. Finally, this is extended to the classification of pore structure features of the entire image. The detailed process is as follows:

[0046] Step 1: Gaussian Image Processing

[0047] Because the spatial distribution of carbonate minerals, organic matter, and pores / fractures is highly heterogeneous, different filters can make the pixel boundaries of the carbonate fracture blurred clear by convolution Gaussian function. The mean of Gaussian distribution of two adjacent positions is expressed by σ, and σ is selected as 10.

[0048] Step two: select edge segmentation method.

[0049] The image edge is segmented by Hessian of Gaussian Eigenvalues in 2D.

[0050] Step three: label mineral and pore types for training classifier

[0051] According to the development characteristics of carbonate minerals and fracture systems, the minerals mainly include dolomite, calcite, feldspar, and quartz, and the pore-fracture system mainly includes intergranular pores, intragranular dissolution pores, micron-scale fractures, and centimeter-scale fractures. They are labeled on the image for the random forest training set.

[0052] Step four: interactive training

[0053] The pixel features of semantic segmentation are used as the basis for random forest classification of all pixels, and the training library of semantic segmentation is continuously supplemented according to the prediction results. Finally, the maximum probability plane distribution of different types of minerals, pores, and fractures is formed.

[0054] Step five: form chip drawing

[0055] Considering that the largest fracture contains multi-scale structural information and mainly contributes to seepage, the main fracture boundary is identified to form a chip design drawing. In order to facilitate the comparison with real core seepage in the later stage, the largest chip and fracture are 1:1 in size, and the internal depth is 9 μm, as shown in Figure 1 .

[0056] Second part: complete the manufacture of basic chip structure

[0057] The microfluidic chip processing method is a soft lithography technology, and the chip processing process of the present application will be described in detail below.

[0058] The specific process of soft lithography technology includes eight steps.

[0059] Step one: coat: clean the polished side of the silicon wafer with isopropyl alcohol and deionized water, respectively, and use clean gas to blow dry the surface, use a heating table to dry the silicon wafer, remove the moisture of the silicon wafer, and ensure that the silicon wafer is clean and dry.

[0060] Put the clean silicon wafer in the center of the spin coater tray, turn on the vacuum pump, and make sure the silicon wafer is tightly attached to the tray. Pour SU-82035 photoresist onto the center of the silicon wafer, cover the tray, and start the spin coater. Set the two-stage speed: low speed 500 rpm / s, time: 10 s; high speed 2500 rpm / s, time 30 s. When the spin coater stops running, wait for 5 minutes.

[0061] The pattern of the chip is mainly determined by the pattern of the mask plate. Compared with traditional regular chips, the present chip intercepts real core cracks, and realizes the flow state of crude oil in the core.

[0062] Step two: soft bake. Place the spin-coated silicon wafer on the leveled heating plate, heat at 65°C for 2 minutes, then adjust the temperature of the heating plate to 95°C and heat for 10 minutes. Then, reduce the temperature of the heating plate to room temperature.

[0063] Step three: exposure. Turn on the ultraviolet exposure machine and let it work for a period of time. When the ultraviolet exposure energy is stable, use a joule meter to measure the UV energy to make the energy value consistent with the required exposure energy. Place the mask plate and the soft-baked silicon wafer together for exposure. After testing, use a 365 nm wavelength ultraviolet exposure machine for 75 seconds.

[0064] Step four: post-exposure bake. Carefully separate the exposed silicon wafer and the mask plate, and use the heating plate to heat the silicon wafer again. The heating temperature and time are: 65°C for 2 minutes, 95°C for 10 minutes, and then let the exposed silicon wafer cool down to room temperature with the heating plate.

[0065] Step five: development. Use developing solution (PGMEA) to develop the post-baked silicon wafer, and use isopropyl alcohol and deionized water alternately to clean it. The unexposed photoresist is washed off the silicon wafer. Finally, use deionized water to clean and use clean nitrogen to blow off the remaining water on the silicon wafer.

[0066] Step six: hard bake. After cleaning, the template needs to be hard-baked. First, use the heating plate to heat at 95°C for 5 minutes, then at 175°C for 20 minutes, at 65°C for 5 minutes, and cool down to room temperature.

[0067] Step seven: mix component A (prepolymer) and component B (crosslinking agent) of PDMS according to the mass ratio of 10:1, use a stirring rod to mix them thoroughly, and the fully mixed PDMS is filled with small bubbles. Finally, use a vacuum tank to remove the bubbles in the PDMS.

[0068] Put the SU-8 template in a culture dish, pour the mixed and fully degassed PDMS into the SU-8 template, and put it in a vacuum tank for 10 minutes. Put the culture dish on the heating plate to heat at 65℃ for 4 hours. After the PDMS is solidified, use a scalpel to cut the PDMS along the edge of the chip. Use a 5mm puncher to punch the inlet and outlet of the chip.

[0069] Step eight: bonding: After the chip with flow channel and the glass to be bonded are treated with dust-free tape, they are put into a plasma cleaning machine for plasma treatment. After treatment, the PDMS and the glass are gently attached together to complete the bonding.

[0070] Step nine: In order to improve the connection strength of the pipeline on the chip, after bonding, use silicone rubber to process the inlet and outlet of the flow channel, which is convenient for the subsequent connection of the pipeline of the experimental device.

[0071] Third part: in-situ growth of minerals in the flow channel of the chip

[0072] Step one: build the experimental device, as shown in the experimental device schematic diagram Figure 2 : mainly including pressure controller, flow sensor, microscope, microfluidic chip, heating module, etc. The fluid is driven by the pressure controller, the microscope is used to observe the fluid flow process in real time, and the heating module is used to control the temperature of the chip.

[0073] The experimental device built is shown in the attached Figure 2 .

[0074] Step two: channel surface functionalization.

[0075] First, use a constant pressure pump to inject 150mbar of 1mol / L sodium hydroxide solution into the pre-prepared base chip for 1 hour to clean the inside of the chip.

[0076] Then rinse the channel with deionized water to ensure that the residual sodium hydroxide solution is removed. Then inject 3-(trimethoxysilyl) propyl acrylate into the microchannel for 5 minutes to functionalize the glass surface with silane groups, and the free radical polymer group will be grafted onto the silica surface.

[0077] Finally, rinse the channel with ethanol and deionized water to remove unreacted silane reagents and ensure the effectiveness of surface functionalization.

[0078] Step three:

[0079] After channel functionalization and drying, inject the pre-injection liquid into the microchannel and keep it for 1 hour to prepare for photoinitiated carboxylic acid radical polymerization.

[0080] The preparation method of the pre-liquid is as follows: 5 mg of calcite crystal seeds, 20 μL of polyethylene glycol diacrylate, 5 μL of a photoinitiator, 40 μL of polyethylene glycol (Mn≈4000, 50% solution), 2 μL of acrylic acid, and 33 μL of deionized water are ultrasonically oscillated for 8 hours to fully disperse the calcite nanocrystal seeds.

[0081] Then, 365 nm ultraviolet light is irradiated for 1 min, UV exposure causes the nanocrystal seeds to be fixed on the inner surface of the microchannel, and polar carboxylic acid groups are generated, which are extremely easy to fix Ca 2+ and CO3 2- .

[0082] Finally, the microchannel is rinsed with deionized water to remove the uncured precursor solution.

[0083] Considering that the growth of minerals is affected by temperature, the growth temperature of the calcium carbonate mineral in the experiment is set to 25°C, and the temperature is controlled by a temperature control heating pad and an infrared temperature detector. The solubility of calcium carbonate at 25°C is 6.1×10 -5 M, so a 0.88 mol / L sodium bicarbonate solution and a 2.5 mol / L calcium chloride solution are mixed at a ratio of 1:2.84 to obtain an equilibrium solution with a saturation degree of 1. The equilibrium solution is injected into the previous chip at a speed of 0.1 mL / min, and the injection is repeated every 2 min, for a total of 3 times.

[0084] The inside of the finished biomimetic chip is shown in FIG. 4. Figure 3 .

[0085] The above describes only the preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can make equivalent replacements or changes to the technical solutions and inventive concepts of the present application within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A method for fabricating a biomimetic microfluidic device for studying fluid transport in micro- and nano-cracks of carbonate rocks, characterized in that, Includes the following steps: Step 1: Analyze and classify scanning electron microscope images of carbonate rock core thin sections using the random forest algorithm to generate chip design drawings; Step 2: Based on the chip design drawings, soft lithography is used to replicate the crack structure through a photoresist template and polydimethylsiloxane, and then bonded to a glass substrate to form a microfluidic chip; Step 3: Inject a precursor liquid containing calcite nanocrystal seeds into the microfluidic chip channel, fix the seeds by ultraviolet light irradiation, and precipitate calcium carbonate minerals by injecting a saturated equilibrium liquid to form a biomimetic microfluidic device. Step one specifically involves: Thin sections were cut from carbonate rock cores, vacuumed and coated with conductive carbon film. The fractures and mineral composition were analyzed using scanning electron microscopy and energy dispersive X-ray spectroscopy to confirm the parent rock type and mineral distribution. The crack boundary is smoothed and its clarity is enhanced by using a Gaussian filter, and the preliminary contour of the crack is extracted using a two-dimensional Gaussian curvature matrix. Mark the cracks and surrounding mineral types and pore characteristics to construct a training set for the random forest algorithm; The random forest algorithm is used to classify image pixels, realize semantic segmentation of minerals, cracks and pores, and dynamically adjust training parameters to improve classification accuracy, thereby forming mineral and crack distribution data. Extract the main boundary and multi-scale features of the crack to generate chip design drawings; Step two specifically involves: SU-8 photoresist template was prepared, and a crack structure mold was formed by coating, exposure, development and hardening processes. The prepolymer of polydimethylsiloxane and the crosslinking agent are mixed at a mass ratio of 10:1, poured into a mold, vacuumed to remove air, and then cured by heating. The cured polydimethylsiloxane and the glass surfaces to be bonded are subjected to plasma treatment in a cleaning machine. After plasma cleaning, they are bonded together to form a microfluidic chip.

2. The method for preparing the biomimetic microfluidic device for studying fluid transport in micro- and nano-cracks of carbonate rocks according to claim 1, characterized in that, The preparation of the SU-8 photoresist template, which involves forming a crack structure mold through processes of coating, exposure, development, and hardening, specifically includes: Spin coat: After cleaning and drying the silicon wafer, apply SU-8 photoresist in a spin coater at low and high speeds, and let it stand for 5 minutes; Soft baking: Place the silicon wafer after homogenization on the leveled heating table, heat at 65°C for 2 minutes, then heat at 95°C for 10 minutes, and then cool to room temperature; Exposure: The photomask and the softened silicon wafer are bonded together and exposed to 365nm ultraviolet light for 75 seconds to photosensitively cure the photoresist; Post-baking: Separate the exposed silicon wafer and mask, and heat again at 65°C for 2 minutes using a hot plate, followed by heating at 95°C for 10 minutes. The exposed silicon wafer was then cooled to room temperature by a heating plate. Development: Dissolve the unexposed areas with developer, clean, and then dry the silicon wafer with nitrogen gas; Hard baking: The developed silicon wafer is heated at 95°C for 5 minutes, then at 175°C for 20 minutes, and then at 65°C for 5 minutes before being cooled to room temperature to form a crack structure mold.

3. The method for preparing the biomimetic microfluidic device for studying fluid transport in micro- and nano-cracks of carbonate rocks according to claim 1, characterized in that, Step three specifically involves: Sodium hydroxide solution was injected into the microfluidic chip channel to clean the channel surface, and then silanized with propyl 3-acrylate. A precursor liquid containing calcite nanocrystal seeds was injected into the flow channel of a microfluidic chip after silanization, and the seeds were fixed to the crack surface by ultraviolet light irradiation. Injecting a mixed solution of sodium bicarbonate and calcium chloride to precipitate calcium carbonate minerals optimizes the physicochemical properties of the crack surface.

4. The method for preparing the biomimetic microfluidic device for studying fluid transport in micro- and nano-cracks of carbonate rocks according to claim 1, characterized in that, The preparation method of the precursor fluid is as follows: 5 mg of calcite seed crystals, 20 μL of polyethylene glycol diacrylate, 5 μL of photoinitiator, 40 μL of polyethylene glycol, 2 μL of acrylic acid, and 33 μL of deionized water were mixed evenly and dispersed by ultrasonic vibration to form a homogeneous solution.

5. The method for preparing the biomimetic microfluidic device for studying fluid transport in micro- and nano-cracks of carbonate rocks according to claim 3, characterized in that, The sodium bicarbonate solution and calcium chloride solution were mixed in a ratio of 1:2.84 to obtain an equilibrium solution with a saturation of 1.

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