A non-filling refined 3D printing method for large-scale complex fracture networks

Abstract

A method for fine-tuning 3D printing of large-scale complex fracture networks without filling belongs to the technical field of indoor physical simulation experiments. The method includes: conceptual model construction, reconstruction or design of fracture space structural characteristics according to the experimental purpose, and establishment of a complex fracture network conceptual model; fracture space structural characteristics include fracture opening, curvature, roughness, fracture scale, fracture occurrence, and fracture intersection position; digital model construction; printing material and parameter selection; physical model generation; physical model post-processing to obtain a large-scale cross-fracture model. The present invention is based on a method for making a fine-tuned model of a large-scale complex fracture network without filling using digital light processing 3D printing technology. The non-filling material can more finely control the fracture structure. On this basis, a transparent photosensitive resin is selected as the printing material to observe the flow of fluid inside the fracture, providing conditions for subsequent indoor experimental research on fluid seepage, heat transfer, and mass transfer laws in complex fracture networks.

Description

Technical Field

[0001] The present invention belongs to the technical field of indoor physical simulation experiments, and in particular, relates to a non-filling refined 3D printing method for large-scale complex fracture networks. Background Art

[0002] There are a large number of cracks and pores distributed inside natural rock mass. Generally, there are many pores in rock mass and they are small in size, while there are fewer cracks, they are large in size and have good permeability. Therefore, cracks play a major role in controlling rock mass seepage and material migration. In recent years, with the increasing demand for deep geological resources, the reservoir medium type that people pay attention to has gradually changed from pore type to mainly fracture type. Taking hot dry rock geothermal resources as an example, since the reservoir is dense and does not contain water in its natural state, it is necessary to form a complex fracture network in the dense rock mass through artificial reservoir creation to provide channels for hydrothermal circulation and heat acquisition. However, the underground fracture channels are intricate and cannot be directly observed, and the detailed seepage laws of fluids in fractures need to be further studied. Indoor physical simulation experiments are an effective way to analyze the seepage and energy transfer processes inside fractures, but due to the complex fracture structure, traditional technical means are difficult to accurately control the fracture microstructure and produce a fracture network physical model with complex macroscopic cross morphology. 3D printing technology provides an effective way to solve the above problems. For details, see the patent documents, application number 202011535062.4 "Method for making rock models with arbitrary crack combinations based on 3D printing", application number 201610390884.5 "Parallel crack seepage experimental system and experimental method based on 3D printing" and application number 201510568441.6 "A method for making simulated cracks for fractured rock seepage test". Application number 202011535062.4 proposes to use paraffin as the filling material and a mixture of lime, sand and water or a mixture of gypsum powder and water as the skeleton material. After printing, the filling material is melted and precipitated by heating with hot water to vacate the crack area to obtain a rock model containing cracks; while the solution proposed in application number 201610390884.5 uses polymer jet (Poly Jet) 3D printing technology, water-soluble materials as support materials, and transparent photosensitive resin as skeleton materials. In addition to the above-mentioned method of making a fractured model using filling materials, the method of removing the fracture plate in application number 201510568441.6 can also be used. Application number 202011062154.5 "A method for embrittlement of 3D printed photosensitive resin rock samples" and application number 201610844752.5 "Physical model of complex fractured rock mass based on 3D printing and modeling method". The former obtains a digital model of the fracture microstructure by CT scanning of rock samples; using 3D printing technology, photosensitive resin is selected as the skeleton material and water-soluble materials are used as the supporting materials of the fracture part to make the model, thereby realizing the reconstruction of the fractured model and the embrittlement of the rock sample. The latter is based on stereolithography (SLA) 3D printing technology, and uses 3GSM rock structure surface digital image recognition technology to conduct on-site structural surface testing to obtain the spatial distribution of fractures.

[0003] In summary, the combination of advanced three-dimensional digital CT scanning and other technologies with 3D printing technology provides new ideas for the production of complex structure experimental models. However, in the current research on rock mechanical properties and internal seepage characteristics, fused deposition modeling (FDM) and stereolithography (SLA) 3D printing technologies are commonly used to make rock sample models. Filling materials are used as crack support materials. After printing and molding, the cracks are vacated by heating and washing. Due to insufficient cleaning, this process often leads to crack blockage, making the internal structure of the crack inconsistent with the actual designed digital crack structure. Summary of the invention

[0004] In view of the above problems, the purpose of the present invention is to propose a method for making a large-scale complex fracture network unfilled refined model based on digital light processing (DLP) 3D printing technology. This method can more finely control the fracture structure without filling materials. On this basis, transparent photosensitive resin is selected as the printing material to observe the internal fluid flow, thereby providing conditions for subsequent indoor experimental research on fluid seepage, heat transfer, mass transfer laws, etc. in complex fracture networks.

[0005] To achieve the above object, the present invention adopts the following technical solution: a large-scale complex fracture network non-filling refined 3D printing method, characterized in that it includes the following steps:

[0006] Step 1: Conceptual model construction

[0007] Reconstruct or design the fracture spatial structural characteristics according to the experimental purpose, and establish a complex fracture network conceptual model; the fracture spatial structural characteristics include fracture opening, curvature, roughness, fracture scale, fracture occurrence, and fracture intersection position;

[0008] Step 2: Digital Model Construction

[0009] Based on the complex fracture network conceptual model, a corresponding 3D digital model is directly constructed by core scanning or a corresponding 3D digital model is created according to the complex fracture network conceptual model by using 3D modeling software, and after the 3D digital model is constructed or created, it is converted into an STL file that can be recognized and printed by a 3D printer;

[0010] Step 3: Printing material and parameter selection

[0011] Selecting a transparent photosensitive resin as a printing material, and determining optimal printing parameters of the selected resin material, wherein the printing parameters include layer thickness, curing time, base curing time and buffer layer;

[0012] Step 4: Physical model generation

[0013] Import the STL format file saved in step 2 into the 3D printer, and use digital light processing 3D printing technology to make the model, so as to obtain a transparent complex crack model without filled cracks;

[0014] Step 5: Physical model post-processing

[0015] The printed transparent complex crack model is rinsed with 75% alcohol, and then illuminated with a lamp to accelerate curing. After the lamp is illuminated, the model is polished, and varnish is sprayed on its surface and dried to obtain a large-sized cross-crack model.

[0016] Furthermore, in step 1, the spatial structural characteristics of the fractures are reconstructed according to the experimental purpose, and a conceptual model of a complex fracture network is established, which specifically includes:

[0017] ① Perform 3D core scanning on rock samples to obtain the fracture structure and distribution characteristics of rock samples;

[0018] ② Reconstruct the complex fracture network conceptual model of the rock sample according to the fracture structure and distribution characteristics of the rock sample.

[0019] Furthermore, in step 1, the process of designing the spatial structural characteristics of the fractures according to the experimental purpose and establishing the conceptual model of the complex fracture network is as follows:

[0020] According to the natural or artificial fracture structure characteristics of the site, the local fracture network is intercepted to form a conceptual model of the artificial fracture network;

[0021] Alternatively, based on the site-scale fracture network, the site-scale fracture model is downscaled by the Lies downscaling method to form a three-dimensional fracture network model.

[0022] Further, in step 2, the 3D modeling software is selected from SOLIDWORKS software, UG software, PROE software or CATIA software.

[0023] Furthermore, in step three, the curing time refers to the exposure time of a single layer after the model is sliced; the base curing time refers to the exposure time of the first layer (base layer) of the model attached to the printer work platform; the layer thickness accuracy is 6.25μm, and the slice layer thickness is set between 6.25-100μm according to different resin materials.

[0024] Furthermore, in step four, the model is made using digital light processing 3D printing technology to obtain a transparent complex crack model without filled cracks. The process is as follows: the two-dimensional image of the sliced ​​three-dimensional digital model is projected into the resin tank using a digital light processor DLP. The crack part of the projected two-dimensional image is the reserved blank area, and the image area outside the crack undergoes a polymerization reaction to generate a solidified layer. The layers are projected layer by layer, exposed and cured layer by layer, and stacked layer by layer to form a transparent complex crack model without filled cracks.

[0025] Furthermore, the crack opening is 1 mm.

[0026] Through the above design scheme, the present invention can bring the following beneficial effects: the method of the present invention is based on digital light processing (DLP) 3D printing technology, does not require crack filling materials, and can accurately depict fine cracks (layer thickness accuracy is 6.25μm). The constructed crack model can intuitively observe the fluid flow and mass transfer process in the cracks inside the rock mass, and provides an effective method for subsequent indoor experimental research on the seepage, heat transfer, and mass transfer laws in complex crack networks under different conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a flow chart of a method for non-filling refined 3D printing of large-scale complex fracture networks provided by an embodiment of the present invention.

[0028] Figure 2 It is a three-dimensional digital model of a large-scale cross-crack model made according to an embodiment of the present invention.

[0029] Figure 3 It is a cross-sectional view of a three-dimensional digital model of a large-scale cross-crack model made according to an embodiment of the present invention. DETAILED DESCRIPTION

[0030] The method for fine-tuning large-scale complex fracture networks without filling 3D printing includes the following steps:

[0031] Step 1: Concept model building

[0032] Combined with the purpose of the experiment, the complex fracture network model is reconstructed or designed; the reconstruction of the complex fracture network model is to realize the model construction by 3D core scanning of the rock sample; the design of the complex fracture network model is to design the microstructure characteristics of the fracture required for the experiment according to the reference or experimental purpose. For example: if the purpose is to clarify the seepage mechanism of microscopic fractures, it is necessary to determine the microstructure characteristics of the fracture model to be constructed based on the existing reported data of fracture microstructure (opening, curvature, roughness, etc.); if the purpose is the seepage mechanism of the macroscopic fracture network, it is possible to ① intercept the local fracture network according to the natural or artificial fracture structure characteristics of the site, form an artificial fracture network conceptual model, and study the seepage, heat transfer and mass transfer processes of the local fracture network; or ② according to the site-scale fracture network, the site-scale fracture model is downscaled by the Lies downscaling method to form a small-scale printable three-dimensional fracture network model, and the heat and mass transfer processes of the complex fracture network can be studied, and the research results can be upscaled and restored to the original site fracture network conditions for physical and chemical process analysis.

[0033] Step 2: Digital Model Construction

[0034] 3D digital models can be constructed through core scanning technology or manual drawing; core scanning modeling is to obtain the fracture structure and distribution characteristics of rock samples through CT scanning and other methods. After further processing of the scanned image, a 3D digital model that is exactly the same as the internal fractures of the real rock sample can be constructed; when drawing manually, information such as fracture model characteristics and dimensions can be obtained through literature excerpts, or can be designed based on the experimental purpose and existing data; the drawing software used includes SOLIDWORKS, UG, PROE, CATIA, etc. Take SOLIDWORKS as an example: in the process of building a digital model, first use the command manager toolbar to create a sketch on the plane, and give it shape and size, then create features and convert it into a three-dimensional entity. Among them, the dimensions of each part of the experimental model need to be set in advance, such as the overall size of the model, the length of the inlet and outlet pipes, and the fracture size, especially the fracture characteristics such as gap width and curvature; after the 3D digital model is built, it needs to be saved and output in STL format.

[0035] Step 3: Printing material and parameter selection

[0036] A transparent photosensitive resin is selected as the printing material; the preferred resin model is HT-800; a preliminary experiment is performed to fine-tune the recommended parameters given by the manufacturer to determine the optimal printing parameters of the resin material, that is, the parameters are adjusted so that the printed model has a good appearance, the internal cracks are complete and transparent, and the seepage process is clearly visible; the printing parameters mainly include layer thickness, curing time, base curing time and buffer layer; wherein the curing time refers to the exposure time of a single layer after the model is sliced, and the base curing time refers to the exposure time of the first layer (base layer) of the model attached to the printer work platform; the layer thickness accuracy is 6.25 μm, and the slice layer thickness is set between 6.25-100 μm according to different resin materials.

[0037] Step 4: Physical model generation

[0038] Import the STL format file saved in the second step into the 3D printer, and use the Digital Light Processing (DLP) 3D printing technology to make the model, so as to obtain a transparent complex crack model; DLP type 3D printing technology uses ultraviolet light source as the energy source, and adopts a high-resolution digital processor projector to project the two-dimensional image after slicing the three-dimensional digital model into the resin tank, project layer by layer, expose and cure layer by layer, and stack and pile up to form a complete three-dimensional model entity; this printing technology is a surface exposure rapid prototyping technology, which can realize the curing and molding of the entire plane at the same time. In the present invention, the crack part of the projected two-dimensional image is a reserved blank area, and the image area outside the crack undergoes a polymerization reaction to generate a cured layer. The cracks in the crack model formed by layer-by-layer stacking are reserved, that is, non-filled printing is achieved; this process does not require filling materials during printing, and the crack structure is formed at one time, so it will not cause crack blockage and can accurately control the crack microstructure.

[0039] Step 5: Physical model post-processing

[0040] The printed fracture model is removed from the printer work platform, rinsed with 75% alcohol, and then illuminated to accelerate curing. The illumination light is 6000k white light of LED lamp. The illumination time depends on the size of the model. In this embodiment, the complex fracture model needs to be illuminated for 4-5 hours. The illumination time can also be appropriately extended to ensure that the model is completely cured. After the illumination, the model is polished with fine sandpaper, and varnish is sprayed on its surface and dried. This can increase the gloss of the model and facilitate the subsequent seepage and heat and mass transfer experiments.

[0041] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the technical scheme in the present invention is clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the present invention is not limited to the following embodiments, and the specific implementation method can be determined according to the technical scheme of the present invention and the actual situation. In order to avoid confusing the essence of the present invention, the known methods, processes and flows are not described in detail.

[0042] See also Figure 1 The large-scale complex fracture network non-filling refined 3D printing method provided by the present invention comprises the following steps:

[0043] Step S1, conceptual model construction: for the purpose of clarifying the seepage heat transfer mechanism of the macroscopic fracture network, according to the actual fracture network obtained by the hydrological geophysical joint inversion or drone imaging method, according to the fracture network model size required by the 3D printer performance, a local fracture network is intercepted in the actual site scale fracture network to form a complex fracture model that can be 3D printed. This embodiment is an experimental model made for studying the seepage heat transfer law of cross fractures.

[0044] Step S2: Digital model construction

[0045] Combine the following Figure 2 and Figure 3 , the digital model construction is described in detail, and the specific steps are as follows (the 3D digital model construction in the embodiment takes the drawing software SOLIDWORKS as an example):

[0046] ①Open SOLIDWORKS drawing software, create a new file Part 1, draw a cross crack composed of rectangles and parallelograms with 6 single cracks in the front view sketch surface, give the overall size of 220mm long, 74mm high, and each quadrilateral is 7mm wide. Then stretch it at a given depth of 100mm to obtain a cross crack geometry prototype with an internal crack opening of 1mm; then fillet each cross part with 1mm, and shell the geometry with 3mm.

[0047] ② Create a sketch on the left view of the geometry, in the upper rectangle (Note: Figure 2 ) Draw a circle with a radius of 1.5mm in the center and cut it out with a given depth of 3mm; continue to draw a sketch on the surface, draw circles with radii of 1.5mm and 3.5mm respectively with the center point of the upper rectangle as the center, and stretch them with a given depth of 10mm. Repeat the above operation on the lower rectangle to form two upper and lower inlet and outlet pipes on the surface (Note: any water pipe can be used as both an inlet pipe and an outlet pipe.).

[0048] ③ Repeat the steps of constructing the inlet and outlet pipe features in ②, such as Figure 2As shown, two inlet and outlet water pipes are built on the right side, and 10 inlet and outlet water pipes are built on the front.

[0049] ④ Since the model is large in size, in order to prevent resin accumulation, which may cause the model to fail to solidify and crack, resulting in printing failure, holes are dug on the model surface close to the side of the printer work platform during 3D printing, that is, where no inlet and outlet water pipes are set. Geometric bodies with a thickness of about 0.1mm are cut out on the outside of each crack on the surface. The distance between each hole is approximately 3-5cm, and the number depends on the specific situation of the model. In this embodiment, a total of 17 holes are set on the model surface.

[0050] ⑤Save and output the drawn 3D digital model in STL format.

[0051] Step S3, printing material and parameter selection: Select transparent photosensitive resin as the printing material, and the preferred resin model is HT-800. This embodiment uses the MiiCraft Advance 255 model 3D printer, pours an appropriate amount of resin material, about 1.5 kg, into the 3D printer material tank, and turns on the printer to clean the glue tank for 4-5 seconds. The specific parameters are selected as follows: the slice layer thickness is set to 100μm, the curing time is set to 5s, the base curing time is set to 50s, and the buffer layer is set to 5 layers.

[0052] Step S4: Physical model generation

[0053] The STL format file saved in step S2 is imported into the 3D printer, and the model is made using the digital light processing (DLP) 3D printing technology, thereby obtaining a transparent complex crack model containing cracks. The detailed steps are as follows:

[0054] ①Open the software Utility and set the slice thickness to 100μm. Import the STL format 3D digital model file and click the landing function key to make the model close to the grid line to prevent gaps between the model and the printing platform, which may cause printing failure;

[0055] ② Set up supports on the inside and outside of each surface of the model and at the water inlet and outlet pipes to prevent the model from collapsing during printing and ensure smooth printing. Among them, the supports at the water inlet and outlet pipes can use a cross structure to increase stability to prevent the supports from being too thin, causing the water inlet and outlet pipes to collapse and resulting in printing failure.

[0056] ③ Click the printer function button to save the file as SLC format; then the software slices the model according to the set layer thickness, then searches for the printer and completes the printer connection. Then edit the print configuration file and set the print parameters: set the curing time to 5s, the base curing time to 50s, and the buffer layer to 5 layers. Finally, convert the file and start printing.

[0057] ④Import the generated 3DP format file in the printing interface and start printing.

[0058] Step S5: Physical model post-processing

[0059] After the model is printed, remove it from the printer work platform, rinse it with 75% alcohol, and then illuminate it to accelerate curing. The light is a 6000k white light LED lamp for 4-5 hours. The lighting time can also be extended appropriately to ensure that the model is completely cured. After the lighting is completed, use fine sandpaper to polish the model, spray varnish on its surface, and dry it to increase the gloss of the model. Figure 2 A three-dimensional digital model of a large-scale cross-crack model produced in this embodiment is shown. Figure 3 A cross-sectional view of a three-dimensional digital model of a large-scale cross-crack model produced in this embodiment is shown.

Claims

1. A method for fine-tuning large-scale complex fracture networks without filling, characterized in that: The following steps are involved: Step 1: Conceptual model construction Reconstruct or design the spatial structural characteristics of the fractures according to the experimental purpose and establish a conceptual model of a complex fracture network; The fracture spatial structural characteristics include fracture opening, curvature, roughness, fracture scale, fracture occurrence, and fracture intersection position; Step 2: Digital Model Construction Based on the complex fracture network conceptual model, a corresponding 3D digital model is directly constructed by core scanning or a corresponding 3D digital model is created according to the complex fracture network conceptual model by using 3D modeling software, and after the 3D digital model is constructed or created, it is converted into an STL file that can be recognized and printed by a 3D printer; Step 3: Printing material and parameter selection Selecting a transparent photosensitive resin as a printing material, and determining optimal printing parameters of the selected resin material, wherein the printing parameters include layer thickness, curing time, base curing time and buffer layer; Step 4: Physical model generation Import the STL format file saved in step 2 into the 3D printer, and use digital light processing 3D printing technology to make the model, so as to obtain a transparent complex crack model without filled cracks; Step 5: Physical model post-processing The printed transparent complex crack model is washed with 75% alcohol, and then cured by lighting. After the lighting is finished, the model is polished, and varnish is sprayed on the surface and dried, thus obtaining a large-sized cross-crack model. Among them, in step four, the model is made using digital light processing 3D printing technology to obtain a transparent complex crack model without filled cracks. The process is as follows: the two-dimensional image of the sliced ​​three-dimensional digital model is projected into the resin tank using a digital light processor DLP. The crack part of the projected two-dimensional image is the reserved blank area, and the image area outside the crack undergoes a polymerization reaction to generate a solidified layer. The layers are projected layer by layer, exposed and cured layer by layer, and stacked layer by layer to form a transparent complex crack model without filled cracks.

2. The large-scale complex fracture network non-filling refined 3D printing method according to claim 1 is characterized in that: In step 1, the spatial structural characteristics of the fractures are reconstructed according to the experimental purpose, and a conceptual model of a complex fracture network is established, which specifically includes: ① Perform 3D core scanning on rock samples to obtain the fracture structure and distribution characteristics of rock samples; ② Reconstruct the complex fracture network conceptual model of the rock sample according to the fracture structure and distribution characteristics of the rock sample.

3. The large-scale complex fracture network non-filling refined 3D printing method according to claim 1 is characterized in that: In step 1, the process of designing the spatial structural characteristics of the fractures according to the experimental purpose and establishing a conceptual model of the complex fracture network is as follows: According to the natural or artificial fracture structure characteristics of the site, the local fracture network is intercepted to form a conceptual model of the artificial fracture network; Alternatively, based on the site-scale fracture network, the site-scale fracture model is downscaled by the Lies downscaling method to form a three-dimensional fracture network model.

4. The method for non-filling and refined 3D printing of large-scale complex fracture networks according to claim 1, characterized in that: In step 2, the 3D modeling software is selected from SOLIDWORKS software, UG software, PROE software or CATIA software.

5. The method for non-filling and refined 3D printing of large-scale complex fracture networks according to claim 1, characterized in that: In step three, the curing time refers to the exposure time of a single layer after the model is sliced; the substrate curing time refers to the exposure time of the first layer of the model attached to the printer work platform; the layer thickness accuracy is 6.25μm, and the slice layer thickness is set between 6.25-100μm according to different resin materials.

6. The large-scale complex fracture network non-filling refined 3D printing method according to claim 1 is characterized by: The crack opening is 1 mm.

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