Method for preparing and fracturing simulation of multilayer system rock sample containing lithology gradual change area

Abstract

The present application relates to a kind of multilayer system rock sample preparation and fracturing simulation method containing lithology gradual change area, multilayer system rock sample preparation method includes the following steps: analyzing the characteristics of multilayer system reservoir rock, obtain the spatial distribution characteristics of the internal crack parameters of multilayer system reservoir rock, and obtain the spatial distribution characteristics of the mechanical parameters of the interlayer transition zone lithology layer of the intralayer homogeneous lithology layer of multilayer system reservoir rock;According to the spatial distribution characteristics of the crack parameters determined and the spatial distribution characteristics of the mechanical parameters, construct 3D printing model, and obtain multilayer system rock sample using 3D printing technology printing.The present application adopts rock mechanics test method, and combines 3D printing technology, and the spatial distribution characteristics of the mechanical parameters of the intralayer homogeneous lithology layer and the interlayer transition zone lithology layer of complex multilayer system reservoir rock is reconstructed, and the model applicable to indoor similar physical simulation experiment is established, effectively solve the current indoor problem that gradual change area lithology layered sample cannot be finely prepared.

Description

Technical Field

[0001] This invention relates to the field of rock sample preparation technology, specifically to a method for preparing multi-layered rock samples containing lithological gradient zones and simulating hydraulic fracturing. Background Technology

[0002] In recent years, over 60% of newly added oil and gas reserves have been low-quality reservoirs belonging to complex multi-layered reservoirs, including layered shale reservoirs, coal-bearing producing groups, and thick sandstone and mudstone reservoirs. Complex multi-layered reservoirs are characterized by numerous vertical producing layers with uneven thickness, strong intra- and inter-layer heterogeneity, and prominent anisotropy. Oil and gas wells exhibit low natural production and rapid decline rates. Integrated cross-layer fracturing technology is an effective way to improve interconnectivity between producing layers, reservoir drainage area, and resource utilization. However, hydraulic fracturing across layers often faces engineering challenges such as insufficient main fracture propagation, insufficient fracture height, and premature sand plugging. The vertical extension morphology of fractures and the mechanical mechanism of cross-layer propagation remain unclear.

[0003] Hydraulic fracturing physical simulation experiments are a crucial and direct method for studying the mechanisms of fracture initiation and propagation. The preparation of representative rock sample models at the laboratory scale is a prerequisite and foundation for conducting these experiments. Because the transition between different lithologies in complex multi-layered layered rocks is gradual in the vertical direction, these gradually transitioning lithologies in a certain way are called transition zones. Currently, researchers have conducted extensive research on rock sample preparation methods for rock masses containing pores, fractures, voids, bedding structures, seepage characteristics, and channel distribution. However, the preparation of layered rocks with complex multi-lithological combinations containing transition zones has not yet been addressed. For layered rock samples, current preparation methods often neglect the transition zone, simplifying it to a zero-thickness abrupt change surface. This lack of comprehensive consideration leads to significant differences in the properties of the obtained rock samples compared to real rocks, and the consistency between samples is also unsatisfactory. The main reasons are: ① The transition zone exhibits strong lithological heterogeneity, and current technologies cannot achieve refined parameter characterization; ② Based on parameter characterization, current core preparation techniques cannot reproduce this structure. The results of hydraulic fracturing tests on simplified layered rocks prepared using existing technologies cannot reflect the true fracture network morphology. Summary of the Invention

[0004] To address the challenge of accurately simulating the lithological gradient zones of layered rocks in existing indoor experiments, this invention provides a method for preparing multi-layered rock samples containing lithological gradient zones and simulating their fracturing.

[0005] The technical solution of this invention to solve the above-mentioned technical problems is as follows: A method for preparing multi-layered rock samples containing lithological gradient zones, comprising the following steps:

[0006] The characteristics of multi-layered reservoir rocks were analyzed to obtain the spatial distribution characteristics of fracture parameters within the multi-layered reservoir rocks, and the spatial distribution characteristics of mechanical parameters of homogeneous lithological layers within the multi-layered reservoir rocks and lithological layers in the interlayer transition zone were obtained.

[0007] Based on the spatial distribution characteristics of the determined fracture parameters and the spatial distribution characteristics of the mechanical parameters, a 3D printing model was constructed, and multi-layer rock samples were obtained by printing using 3D printing technology.

[0008] The beneficial effects of this invention are as follows: This invention employs rock mechanics testing methods and combines them with 3D printing technology to reconstruct the spatial distribution characteristics of mechanical parameters of homogeneous lithological layers within complex multi-layered reservoirs and lithological layers in interlayer transition zones. This allows for the establishment of a model suitable for indoor similar physical simulation experiments, effectively solving the current problem of the inability to precisely prepare layered lithological samples containing gradation zones in laboratory settings. This invention can precisely characterize the lithological gradation zone features between layers in complex multi-layered strata. The mechanical properties of the simulated multi-layered rock samples are close to those of real rocks, avoiding the problem of conventional methods ignoring lithological gradation zones and simplifying them to abrupt changes with zero thickness, resulting in low accuracy of the prepared physical model.

[0009] Since multi-layered reservoir rocks are layered media, they are divided into a series of strata based on vertical lithological changes. Layers with the same lithology are considered homogeneous strata within a single layer, while transitional layers between adjacent homogeneous strata are considered heterogeneous strata in the interlayer transition zone.

[0010] Based on the above technical solution, the present invention can be further improved as follows.

[0011] Furthermore, the mechanical parameter is the elastic modulus; the homogeneous lithological layer within the layer has the same elastic modulus at all points in space, and the elastic modulus of the homogeneous lithological layer within the layer is obtained by triaxial compression test; the spatial distribution of the elastic modulus of the lithological layer in the interlayer transition zone is obtained by nanoindentation test.

[0012] The beneficial effect of adopting the above-mentioned further scheme is that the homogeneous lithological layer within the layer is regarded as homogeneous and isotropic, and therefore has the same rock mechanical parameters at all points in space.

[0013] Furthermore, the elastic modulus distribution in the interlayer transition zone lithological layer space is obtained using nanoindentation experiments. Specifically, this includes: cutting the lithological layer in the interlayer transition zone layer by layer along its longitudinal direction, dividing each cutting plane into a series of control units, and performing nanoindentation experiments on each control unit to obtain the elastic modulus of each control unit, thus obtaining the elastic modulus distribution on the entire cutting plane; based on the obtained elastic modulus distribution on different cutting planes, using a linear interpolation method, calculating the elastic modulus distribution of the transition zone between different cutting planes, thereby obtaining the elastic modulus distribution in the interlayer transition zone lithological layer space.

[0014] The beneficial effect of adopting the above-mentioned further scheme is that, since the lithological layer in the interlayer transition zone is heterogeneous, the mechanical parameters at different locations in the longitudinal and horizontal directions are different and cannot be obtained by conventional triaxial compression tests. It is necessary to use nanoindentation tests to obtain the elastic modulus distribution of the entire space of the lithological layer in the interlayer transition zone.

[0015] Furthermore, the nanoindentation experiment for each control unit specifically includes: conducting tests using a lattice nanoindentation experiment, with multiple nanoindentation experiments for each control unit, statistically obtaining the probability distribution results of the elastic modulus of each control unit after the experiment, selecting the data of the elastic modulus in the interval with the largest probability distribution, and calculating the average value as the elastic modulus of the control unit.

[0016] Furthermore, when calculating the elastic modulus distribution of the transition zone between different cutting planes using the linear interpolation method, the number of linear interpolation layers is 2 to 4, and the spacing between each interpolation layer is equal.

[0017] The beneficial effect of adopting the above-mentioned further scheme is that it facilitates more accurate calculation of the elastic modulus distribution of lithological layers in the interlayer transition zone.

[0018] Furthermore, after the lithological layers in the interlayer transition zone are cut layer by layer using wire cutting technology in the longitudinal direction, the cutting planes are polished, and the distance between adjacent cutting planes is less than 0.5 mm.

[0019] The advantages of adopting the above-mentioned further scheme are: the distance between adjacent cutting planes is less than 0.5 mm, which can ensure the accuracy of the spatial distribution of rock mechanical parameters in the transition zone; and the polishing process can ensure the smoothness of the cutting surface.

[0020] Furthermore, the method for obtaining the multi-layered reservoir rock includes obtaining the downhole core from the coring well, cutting the core into a cuboid, and obtaining the multi-layered reservoir rock.

[0021] The advantages of adopting the above-mentioned further scheme are: it facilitates the acquisition of internal fracture parameters and mechanical distribution parameters of multi-layer reservoir rocks, and also facilitates subsequent hydraulic fracturing simulation experiments.

[0022] Furthermore, when using 3D printing technology, a mixture of cement, quartz sand, and water is used as the printing material, with water accounting for 10% to 12% of the total mass of cement and quartz sand. The elastic modulus varies at different spatial locations in multi-layered reservoir rocks, resulting in different mass ratios of quartz sand to cement. The elastic modulus at different spatial locations in multi-layered reservoir rocks ranges from 41 GPa to 7.1 GPa, and the mass ratio of quartz sand to cement is 1:5 to 3:1. The mass ratio of quartz sand to cement corresponding to different elastic moduli is calculated using interpolation.

[0023] Furthermore, when the elastic modulus at different spatial locations in the multi-layered reservoir rocks is 41 GPa, the mass ratio of quartz sand to cement is 1:5; when the elastic modulus at different spatial locations in the multi-layered reservoir rocks is 29 GPa, the mass ratio of quartz sand to cement is 1:4; when the elastic modulus at different spatial locations in the multi-layered reservoir rocks is 16 GPa, the mass ratio of quartz sand to cement is 1:3; and when the mass ratio of quartz sand to cement is 0:1, the mass ratio of quartz sand to cement at different spatial locations in the multi-layered reservoir rocks is... The elastic modulus is 12.7 GPa; when the elastic modulus at different spatial locations in the multi-layered reservoir rock is 11.2 GPa, the mass ratio of quartz sand to cement is 1:1; when the elastic modulus at different spatial locations in the multi-layered reservoir rock is 7.23 GPa, the mass ratio of quartz sand to cement is 2:1; when the elastic modulus at different spatial locations in the multi-layered reservoir rock is 7.1 GPa, the mass ratio of quartz sand to cement is 3:1. The mass ratio of quartz sand to cement corresponding to other elastic moduli is calculated by interpolation.

[0024] Furthermore, when using 3D printing technology, gypsum powder is used as the filler material for printing filled cracks, while water-soluble materials are used as the filler material for printing non-filled cracks.

[0025] The beneficial effect of adopting the above-mentioned further scheme is that, during the later stage of fracturing, when the fracturing fluid extends into the non-filling fracture, this type of water-soluble material can be dissolved.

[0026] A method for simulating the fracturing of multi-layered rock samples containing a lithological gradient zone includes the following steps: after the multi-layered rock sample is printed and cured, the multi-layered rock sample is wrapped with concrete to form a fracturing specimen, and a simulated wellbore is drilled along the axial direction of the fracturing specimen. The simulated wellbore is then bonded in the simulated wellbore to form a fracturing specimen.

[0027] The fracturing specimen was subjected to fracturing experiments using a true triaxial fracturing experimental apparatus.

[0028] The beneficial effects of this invention are: by conducting hydraulic fracturing experiments on complex multilayered rocks containing lithological transition zones, this invention can accurately obtain the extension law of fracturing fluid fractures in the lithological layers of the interlayer transition zone, clarify the influence mechanism of the lithological layers of the interlayer transition zone on the cross-layer extension of fracturing fluid fractures, and is of great significance for studying the mechanical behavior of hydraulic fracturing in complex multilayered rocks.

[0029] Furthermore, before wrapping the multi-layer rock sample with concrete, the multi-layer rock sample is first wrapped with waterproof tape; the concrete is a mixture of quartz sand, cement and water in a mass ratio of 1:1:1.

[0030] Furthermore, the bottom of the simulated wellbore is reserved with an open hole section for simulating the initial fracture initiation during fracturing. The open hole section is located in the homogeneous lithological layer within the layer and the lithological layer in the interlayer transition zone.

[0031] The beneficial effect of adopting the above-mentioned further scheme is that the open hole section is located in the homogeneous lithological layer within the layer and the lithological layer in the interlayer transition zone, which can be used to compare and study the influence of the lithological layer in the interlayer transition zone on the propagation morphology of hydraulic fractures when different fracture initiation layer locations are used.

[0032] Furthermore, after the fracturing specimen is prepared, a true triaxial fracturing test device is used to inject fluid into the fracturing specimen along the pipeline to fracture multi-layer rock samples; after the multi-layer rock samples are fracturing, the internal fracture morphology of the multi-layer rock samples after fracturing is obtained by combining CT scan. Attached Figure Description

[0033] Figure 1 A flowchart illustrating the method for preparing multi-layered rock samples containing lithological gradient zones;

[0034] Figure 2 The flowchart shows the simulation method for hydraulic fracturing of multi-layered rock samples containing lithological gradient zones.

[0035] Figure 3 This is a three-dimensional view of the full-diameter cylindrical core of a two-layered sandstone and mudstone sample containing an interlayer transition zone lithological layer in the example;

[0036] Figure 4 This is a side view of a two-layered sandstone and mudstone sample containing an interlayer transition zone lithological layer after being processed into a cuboid core in the example.

[0037] Figure 5 This is a schematic diagram of the cross-section of a two-layered sandstone and mudstone sample containing an interlayer transition zone lithological layer in the example.

[0038] Figure 6 A schematic diagram of the lithological layers in the interlayer transition zone containing the cross-section;

[0039] Figure 7 This is a schematic diagram showing the control unit divided into individual units using a 100×100 grid.

[0040] Figure 8 A schematic diagram of a lattice nanoindentation testing method in a control unit;

[0041] Figure 9 This is a schematic diagram of the fracturing specimen and the crack morphology after fracturing.

[0042] 1. Mudstone layer; 2. Sandstone layer; 3. Lithological layer in the interlayer transition zone; 4. Filling fracture; 5. Cross-section; 6. Concrete wrapping layer; 7. Simulated wellbore; 8. Open hole section; 9. Fracture morphology after hydraulic fracturing. Detailed Implementation

[0043] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0044] Example 1

[0045] This embodiment describes a method for preparing multi-layered rock samples containing lithological gradient zones, such as... Figure 1 As shown, it includes the following steps:

[0046] The characteristics of multi-layered reservoir rocks were analyzed to obtain the spatial distribution characteristics of fracture parameters within the multi-layered reservoir rocks. Then, the spatial distribution characteristics of mechanical parameters of homogeneous lithological layers within the multi-layered reservoir rocks and lithological layers in the interlayer transition zone were obtained.

[0047] Based on the spatial distribution characteristics of the determined fracture parameters and mechanical parameters, a 3D printing model is constructed, and multi-layered rock samples are obtained using 3D printing technology. Specifically, a fracture model is established based on the spatial distribution characteristics of the determined fracture parameters, and a three-dimensional digital rock model is established based on the spatial distribution characteristics of the mechanical parameters. The models are then input into a 3D printer using STL format to print the rock samples.

[0048] Since multi-layered reservoir rocks are layered media, they are divided into a series of strata based on vertical lithological changes. Layers with the same lithology are considered homogeneous strata within a single layer, while transitional layers between adjacent homogeneous strata are considered heterogeneous strata in the interlayer transition zone.

[0049] The mechanical parameter is the elastic modulus. The homogeneous lithological layers within each layer have the same elastic modulus at all points in space. The elastic modulus of the homogeneous lithological layers within each layer is obtained using triaxial compression tests. Specifically, the average test results of 2-3 standard plunger samples (25mm × 50mm) are taken from each homogeneous lithological layer within the layer. The spatial distribution of the elastic modulus of the lithological layers in the interlayer transition zone is obtained using nanoindentation tests. The homogeneous lithological layers within each layer are considered homogeneous and isotropic, therefore possessing the same rock mechanical parameters at all points in space. A three-dimensional digital spatial model of multi-layered reservoir rocks can be established based on the obtained elastic moduli of rocks at different locations in the three-dimensional space within and between layers.

[0050] One specific embodiment of this scheme involves obtaining the spatial elastic modulus distribution of the lithological layers in the interlayer transition zone using nanoindentation experiments. Specifically, this includes: longitudinally sectioning the lithological layers in the interlayer transition zone layer by layer, such as... Figure 6 As shown; a series of control units are divided into each cutting plane, specifically, control units can be divided into 100×100 units, with the side length of each square between 0.1 and 0.2 cm, as shown. Figure 7 As shown; nanoindentation experiments were performed on each control unit, with 100 nanoindentation tests conducted on each control unit, as follows. Figure 8 As shown, the elastic modulus of each control unit is obtained, i.e., the elastic modulus distribution across the entire cutting plane is obtained. Based on the obtained elastic modulus distributions across different cutting planes, a linear interpolation method is used to calculate the elastic modulus distribution of the transition zone between different cutting planes, thereby obtaining the spatial elastic modulus distribution of the lithological layers in the interlayer transition zone. Since the lithological layers in the interlayer transition zone are heterogeneous, their mechanical parameters differ at different locations in the longitudinal and horizontal directions, making them unsuitable for conventional triaxial compression tests. Therefore, nanoindentation tests are required to obtain the elastic modulus distribution across the entire space of the lithological layers in the interlayer transition zone.

[0051] The nanoindentation experiment for each control unit specifically includes: conducting tests using a lattice nanoindentation method, with multiple nanoindentation experiments performed on each control unit. After the experiments, the probability distribution of the elastic modulus for each control unit is statistically analyzed. The data of the elastic modulus within the interval with the highest probability distribution is selected, and the average value is taken as the elastic modulus of that control unit. For example, using the elastic modulus variation range of 2–3 GPa as an interval, the probability distribution of the elastic modulus within the control unit is calculated. Based on the probability distribution results, the data of the elastic modulus within the interval with the highest probability is selected, and the average value is taken as the elastic modulus of one control unit.

[0052] One specific scheme of this embodiment is to use a linear interpolation method to calculate the elastic modulus distribution of the transition zone between different cutting planes. The number of linear interpolation layers is 2 to 4, and the spacing between each interpolation layer is equal.

[0053] In a preferred embodiment, after longitudinally cutting the lithological layers in the interlayer transition zone layer by layer using wire cutting technology, the cutting planes are then polished, with the distance between adjacent cutting planes being less than 0.5 mm. This distance ensures the accuracy of depicting the spatial distribution of rock mechanical parameters in the transition zone, while polishing ensures the smoothness of the cutting surfaces.

[0054] One specific embodiment of this method involves obtaining the multi-layered reservoir rock by acquiring a core sample from a cored well, cutting the core into a rectangular parallelepiped with a square cross-section, the diagonal of which is approximately equal to the diameter of the cylindrical core. The required dimensions of the cut core are: a core length of 80–100 cm and a horizontal cross-section side length of 12–15 cm. Based on the lithological characteristics of the downhole core, the number and thickness of homogeneous lithological layers within the layer and the lithological layers in the interlayer transition zone are determined, facilitating subsequent calculations of mechanical parameters, etc.

[0055] This embodiment utilizes CT scanning to obtain the spatial distribution characteristics of fracture parameters in multi-layered reservoir rocks. The fracture parameters within the rock include fracture type and fracture orientation. Fracture types are categorized as either filled or unfilled, and fracture orientation includes dip, strike, and dip angle. CT scanning is used to obtain the fracture orientation characteristics. Since measuring the mechanical parameters of homogeneous lithological layers within a formation and lithological layers in the interlayer transition zone would damage the core, the downhole core is cut into rectangular sections. The core is then subjected to CT scanning, and image processing is used to extract the spatial distribution characteristics of fractures, determining the spatial distribution characteristics of fracture parameters. After the CT scan is completed, the rock mechanical parameters are then tested.

[0056] When using 3D printing technology, a mixture of cement, quartz sand, and water is used as the printing material, with water accounting for 10% to 12% of the total mass of cement and quartz sand. The elastic modulus varies at different spatial locations in multi-layered reservoir rocks, resulting in different mass ratios of quartz sand to cement. The elastic modulus at different spatial locations in multi-layered reservoir rocks ranges from 41 GPa to 7.1 GPa, and the mass ratio of quartz sand to cement is 1:5 to 3:1. The mass ratio of quartz sand to cement corresponding to different elastic moduli is calculated by interpolation.

[0057] Specifically, when the elastic modulus at different spatial locations in the multi-layered reservoir rock is 41 GPa, the mass ratio of quartz sand to cement is 1:5; when the elastic modulus at different spatial locations in the multi-layered reservoir rock is 29 GPa, the mass ratio of quartz sand to cement is 1:4; when the elastic modulus at different spatial locations in the multi-layered reservoir rock is 16 GPa, the mass ratio of quartz sand to cement is 1:3; and when the mass ratio of quartz sand to cement is 0:1, the mass ratio of quartz sand to cement at different spatial locations in the multi-layered reservoir rock is... The elastic modulus is 12.7 GPa; when the elastic modulus at different spatial locations in the multi-layered reservoir rock is 11.2 GPa, the mass ratio of quartz sand to cement is 1:1; when the elastic modulus at different spatial locations in the multi-layered reservoir rock is 7.23 GPa, the mass ratio of quartz sand to cement is 2:1; when the elastic modulus at different spatial locations in the multi-layered reservoir rock is 7.1 GPa, the mass ratio of quartz sand to cement is 3:1. The mass ratio of quartz sand to cement corresponding to other elastic moduli is calculated by interpolation.

[0058] When using 3D printing technology, gypsum powder is used as the filler material for filled fractures, while water-soluble materials are used for unfilled fractures. Later, during the fracturing process, when the fracturing fluid extends into the unfilled fractures, these water-soluble materials dissolve.

[0059] The following explanation uses a two-layered sandstone-mudstone sample as an example. Figure 3 As shown, a core sample was obtained from the coring well. The core is a full-diameter cylindrical structure, with an overall length of 80cm and a diameter of 18cm. The lower rock layer is sandstone layer 2 (thickness 40cm), and the upper rock layer is mudstone layer 1 (thickness 35cm). The interlayer lithology is a gradual transition zone from sandstone layer 2 to mudstone layer 1 (thickness 5cm). Figure 4 and Figure 5 As shown, the core was cut into a rectangular prism with a square cross-section and a side length of 12 cm, yielding multi-layered reservoir rocks. The multi-layered reservoir rocks contain a filling fracture 4 located in the lower sandstone layer 2. CT scans were first used to scan the multi-layered reservoir rocks, and image processing was used to extract the spatial distribution characteristics of the filling fracture 4 to determine a three-dimensional fracture model. Since both the upper mudstone layer 1 and the lower sandstone layer 2 are homogeneous intralayered lithologies, the elastic modulus at various points in space is the same for both layers. Triaxial compression tests were used to test the rock mechanics parameters of the two multi-layered reservoir rocks, yielding an elastic modulus of 7.1 GPa for the lower sandstone layer 2 and 29 GPa for the upper mudstone layer 1. The required mass ratio of quartz sand to cement for the lower sandstone layer 2 was 3:1, and the required mass ratio for the upper mudstone layer 1 was 1:4. To obtain the spatial distribution characteristics of the mechanical parameters of the homogeneous lithological layer within the stratum, a layer-by-layer sectioning method from bottom to top was employed, followed by nanoindentation layer by layer. Figure 6As shown. During layer-by-layer sectioning, the distance between layers is 0.5mm. Each square horizontal section plane 5 is divided into 100×100 control units, as shown... Figure 7 As shown, each control unit is a small square with a side length of 0.12 cm. For each small square control unit, a dot-matrix nanoindentation test was performed, with 100 nanoindentation tests conducted on each control unit. Figure 8 As shown. The probability distribution of the elastic modulus obtained after the experiment was statistically analyzed. The modulus distribution value with the highest probability was selected, and the average value was calculated as the elastic modulus of a control unit. Based on the elastic modulus distribution on different sections of lithological layer 3 in the interlayer transition zone, a linear interpolation method was used to calculate the elastic modulus distribution of homogeneous lithological layers within different sections. The number of linear interpolation layers was 2–4, with equal spacing between each interpolation layer. This allowed for the spatial distribution of rock mechanical parameters in lithological layer 3 throughout the interlayer transition zone. Based on the established three-dimensional digital rock model and fracture model, the data was input into a 3D printer using STL format to print rock samples.

[0060] This embodiment employs rock mechanics testing methods combined with 3D printing technology to reconstruct the spatial distribution characteristics of mechanical parameters of homogeneous lithological layers within complex multi-layered reservoirs and lithological layers in interlayer transition zones. This reconstruction establishes a model suitable for indoor similar physical simulation experiments, effectively solving the current challenge of accurately preparing layered lithological samples containing gradation zones in laboratory settings. This invention can precisely characterize the lithological gradation zones between layers in complex multi-layered strata. The simulated mechanical properties of multi-layered rock samples are close to those of real rocks, avoiding the problem of conventional methods ignoring lithological gradation zones and simplifying them to abrupt changes with zero thickness, resulting in low accuracy of the prepared physical models.

[0061] Example 2

[0062] This embodiment presents a method for simulating the hydraulic fracturing of multi-layered rock samples containing lithological gradient zones, such as... Figure 2 As shown, the process includes the following steps: After the multi-layered rock sample is printed and cured, the multi-layered rock sample is encased in concrete to form a fracturing specimen, and a simulated wellbore is drilled along the axial direction of the fracturing specimen. A simulated wellbore 7 is then bonded into the simulated wellbore to form a fracturing specimen; as shown... Figure 9 As shown, specifically, after the concrete on the multi-layered rock sample has solidified, a simulated wellbore is drilled along the core axis on the surface of the fracturing specimen, and a steel simulated wellbore is bonded to form the fracturing specimen. The simulated wellbore has an inner diameter of 2cm, an outer diameter of 25mm, and a length of 50mm, with the length ranging from 40 to 60cm; after the concrete solidifies, a concrete wrapping layer 6 is formed on the outside of the multi-layered rock sample.

[0063] Before encasing the multi-layered rock sample in concrete, it was first wrapped with waterproof tape. The concrete was a mixture of quartz sand, cement, and water in a mass ratio of 1:1:1. Since the multi-layered rock sample obtained by 3D printing has a cylindrical structure, the concrete encased the multi-layered rock sample into a cuboid specimen, and the center of the concrete coincided with the center of the cylindrical multi-layered rock sample.

[0064] Specifically, such as Figure 9 As shown, the bottom of the simulated wellbore 7 has a pre-reserved open-hole section 8 for simulating the initial fracture initiation during hydraulic fracturing. The open-hole section 8 is located between the homogeneous lithological layer within the layer and the transitional lithological layer 3 between the layers, and its length is 2–3 cm. The open-hole section 8, located between the homogeneous lithological layer within the layer and the transitional lithological layer 3 between the layers, is used to compare and study the influence of the transitional lithological layer on the hydraulic fracture propagation morphology when different fracture initiation layer locations are used.

[0065] Based on a true triaxial fracturing experimental setup, fracturing experiments were conducted on the fracturing specimen. Specifically, as follows: Figure 9 As shown, after the fracturing specimen was prepared, a true triaxial fracturing experimental setup was used to inject fluid into the fracturing specimen along the pipeline to fracturing multi-layered rock samples. After the multi-layered rock samples were fracturing, CT scans were used to obtain the internal fracture morphology of the fracturing multi-layered rock samples. The fracture morphology after fracturing is shown in Figure 9. Figure 9 As shown, the influence of homogeneous lithological layers within the layer and lithological layers in the interlayer transition zone on the hydraulic fracture initiation and propagation law is analyzed. The influence of homogeneous lithological layers within the layer and lithological layers in the interlayer transition zone on the hydraulic fracture propagation law under different fluid viscosities, discharge rates, and confining pressures is also analyzed.

[0066] This embodiment, through fracturing experiments on complex multilayered rocks containing lithological transition zones, can accurately obtain the extension law of fracturing fluid fractures in the lithological layers of the interlayer transition zone, clarify the influence mechanism of the lithological layers of the interlayer transition zone on the cross-layer extension of fracturing fluid fractures, and is of great significance for studying the mechanical behavior of hydraulic fracturing in complex multilayered rocks.

[0067] In the description of this invention, it should be understood that the terms "center", "longitudinal", "length", "thickness", "upper", "lower", "horizontal", "top", "bottom", "inner", "outer", "axial", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0068] In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0069] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0070] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0071] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing multi-layered rock samples containing lithological gradient zones, characterized in that, Includes the following steps: The characteristics of multi-layered reservoir rocks were analyzed to obtain the spatial distribution characteristics of fracture parameters within the multi-layered reservoir rocks. Then, the spatial distribution characteristics of mechanical parameters of homogeneous lithological layers within the multi-layered reservoir rocks and lithological layers in the interlayer transition zone were obtained. Based on the spatial distribution characteristics of the determined fracture parameters and the spatial distribution characteristics of the mechanical parameters, a 3D printing model was constructed, and multi-layer rock samples were obtained by printing using 3D printing technology. The mechanical parameter is the elastic modulus; the homogeneous lithological layer within the layer has the same elastic modulus at all points in space, and the elastic modulus of the homogeneous lithological layer within the layer is obtained by triaxial compression test; the spatial distribution of the elastic modulus of the lithological layer in the interlayer transition zone is obtained by nanoindentation test. The elastic modulus distribution in the interlayer transition zone lithological layer space was obtained using nanoindentation experiments. Specifically, this involved: longitudinally cutting the lithological layer in the interlayer transition zone layer by layer, dividing each cutting plane into a series of control units, and testing each control unit using lattice nanoindentation experiments to obtain the elastic modulus of each control unit, thus obtaining the elastic modulus distribution on the entire cutting plane; based on the obtained elastic modulus distribution on different cutting planes, using a linear interpolation method, calculating the elastic modulus distribution of the transition zone between different cutting planes, thereby obtaining the elastic modulus distribution in the interlayer transition zone lithological layer space.

2. The method for preparing multi-layered rock samples containing lithological gradient zones according to claim 1, characterized in that, The test of each control unit using a lattice nanoindentation experiment specifically includes: multiple nanoindentation experiments for each control unit, statistical analysis of the probability distribution of the elastic modulus of each control unit after the experiment, selection of the elastic modulus data in the interval with the largest probability distribution, and calculation of the average value as the elastic modulus of the control unit.

3. The method for preparing multi-layered rock samples containing lithological gradient zones according to claim 1, characterized in that, When calculating the elastic modulus distribution in the transition zone between different cutting planes using the linear interpolation method, the number of linear interpolation layers is 2 to 4, and the spacing between each interpolation layer is equal.

4. The method for preparing multi-layered rock samples containing lithological gradient zones according to claim 1, characterized in that, After the lithological layers in the interlayer transition zone are cut layer by layer using wire cutting technology, the cut planes are polished, and the distance between adjacent cut planes is less than 0.5 mm.

5. The method for preparing multi-layered rock samples containing lithological gradient zones according to claim 1, characterized in that, When using 3D printing technology, a mixture of cement, quartz sand, and water is used as the printing material, with water accounting for 10% to 12% of the total mass of cement and quartz sand. The elastic modulus varies at different spatial locations in multi-layered reservoir rocks, resulting in different mass ratios of quartz sand to cement. The elastic modulus at different spatial locations in multi-layered reservoir rocks ranges from 41 GPa to 7.1 GPa, and the mass ratio of quartz sand to cement is 1:5 to 3:

1. The mass ratio of quartz sand to cement corresponding to different elastic moduli is calculated by interpolation.

6. The method for preparing multi-layered rock samples containing lithological gradient zones according to claim 5, characterized in that, When the elastic modulus at different spatial locations in the multi-layered reservoir rock is 41 GPa, the mass ratio of quartz sand to cement is 1:5; when the elastic modulus at different spatial locations in the multi-layered reservoir rock is 29 GPa, the mass ratio of quartz sand to cement is 1:4; when the elastic modulus at different spatial locations in the multi-layered reservoir rock is 16 GPa, the mass ratio of quartz sand to cement is 1:3; when the mass ratio of quartz sand to cement is 0:1, the mass ratio of quartz sand to cement is 1... 2.7 GPa; when the elastic modulus at different spatial locations in the multi-layered reservoir rock is 11.2 GPa, the mass ratio of quartz sand to cement is 1:1; when the elastic modulus at different spatial locations in the multi-layered reservoir rock is 7.23 GPa, the mass ratio of quartz sand to cement is 2:1; when the elastic modulus at different spatial locations in the multi-layered reservoir rock is 7.1 GPa, the mass ratio of quartz sand to cement is 3:

1. The mass ratio of quartz sand to cement corresponding to other elastic moduli is calculated by interpolation.

7. The method for preparing multi-layered rock samples containing lithological gradient zones according to claim 1, characterized in that, When using 3D printing technology, gypsum powder is used as the filler material for printing filled cracks, while water-soluble materials are used as the filler material for printing unfilled cracks.

8. A method for simulating hydraulic fracturing of multi-layered rock samples containing lithological gradient zones, characterized in that, The process includes the following steps: after the multi-layer rock sample as described in any one of claims 1 to 7 is printed and cured, the multi-layer rock sample is wrapped with concrete to form a fracturing specimen, and a simulated wellbore is drilled along the axial direction of the fracturing specimen. The simulated wellbore is then bonded in the simulated wellbore to form a fracturing specimen. The fracturing specimen was subjected to fracturing experiments using a true triaxial fracturing experimental apparatus.

9. The method for simulating the hydraulic fracturing of multi-layered rock samples containing lithological gradient zones according to claim 8, characterized in that, Before wrapping the multi-layer rock sample with concrete, the multi-layer rock sample is first wrapped with waterproof tape; the concrete is a mixture of quartz sand, cement and water in a mass ratio of 1:1:

1.

10. The method for simulating the hydraulic fracturing of multi-layered rock samples containing lithological gradient zones according to claim 8, characterized in that, The bottom of the simulated wellbore is reserved with an open hole section for simulating the initial fracture initiation during fracturing. The open hole section is located in the homogeneous lithological layer within the layer and the lithological layer in the transition zone between layers. After the fracturing specimen is prepared, a true triaxial fracturing test device is used to inject fluid into the fracturing specimen along the pipeline to fracture the multi-layer rock sample; after the multi-layer rock sample is fracturing, the internal fracture morphology of the multi-layer rock sample after fracturing is obtained by combining CT scan.

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