Sample preparation method and device applicable to shear-sliding test of fractured tight sandstone

Abstract

This application relates to the technical field of rock mechanics and rock physical simulation tests, and discloses a sample preparation method and device for shear-slip tests applicable to fractured tight sandstone. The sample preparation method mainly includes the dissolution and re-casting of the clastic components of tight sandstone. The device includes an embedded design of a compound bourdon tube in the drilled holes of the prepared specimen. The specimen made by using the sample preparation method proposed by the present invention can not only meet the variable comparison study of shear-slip tests on fractured tight sandstone under single variable control, but also maximize the consistency of the mechanical properties of the specimen with those of tight sandstone, ensuring the accuracy of the research results and effectively solving the problem that the two cannot be achieved simultaneously by traditional sample preparation methods. The test device described in the present invention can effectively avoid the unnatural changes in the slit width and slit surface roughness of the shear-slip and dislocation parts at both ends of the specimen in traditional sample preparation, further ensuring the accuracy of the test results.

Description

Technical Field

[0001] This application relates to the technical field of rock mechanics and rock physical simulation tests, and particularly relates to a sample preparation method and device suitable for shear-slip tests of fractured tight sandstone. Background Art

[0002] The development of tight sandstone oil and gas reservoirs, CO2 geological sequestration, and the construction of rock mass projects will all encounter the phenomenon of shear slip (shearing) of fractures. The natural productivity of tight sandstone oil and gas reservoirs is low or there is no natural productivity. Fractures and hydraulic fracturing are respectively important factors and production measures for high production in tight sandstone oil and gas reservoirs. When conducting hydraulic fracturing for exploitation, a large amount of liquid needs to be injected into the formation. While increasing the pore pressure of the formation, the fluid injection will cause the normal stress on the fracture surfaces of natural fractures and / or artificial hydraulic fractures in the wellbore formation to continuously decrease, thereby causing these fractures to undergo shear slip along the fracture surfaces and the fractures to open, improving the connectivity and productivity of the tight sandstone reservoir. The process of CO2 sequestration into sandstone lenses is also a process of continuous fluid injection into the formation, which will cause the shear slip instability of natural fractures. During the construction of a dam, fracture shear slip will lead to the instability of the dam foundation and the occurrence of a dam-break accident. During coal mine excavation, fracture shear slip in sandstone-type surrounding rock will lead to the occurrence of coal mine water inrush accidents. Therefore, studying problems such as the fracture surface deformation during the shear slip of tight sandstone fractures helps to deeply understand the seepage characteristics and flow conductivity of fractures, promote the development of tight sandstone oil and gas reservoirs and the geological sequestration of CO2, and ensure the safety of rock mass project construction.

[0003] Rock physical tests are one of the necessary means to solve these deformation problems, and a suitable specimen is the basis for the success of rock physical tests. Currently, there are two sample preparation methods for shear-slip tests of fractured tight sandstone. The first is to directly conduct a Brazilian splitting test on a cylindrical tight sandstone specimen to obtain a fractured tight sandstone specimen. The second is to use organic materials such as organic resin to pour and manufacture a fractured specimen. For the specimens with straight splitting fractures made by the Brazilian splitting test, the surface morphology of each straight splitting fracture is different, so it is difficult to be used for comparative test research with controlled variables; organic materials can manufacture multiple fractured specimens with the same fracture surface morphology, so it can ensure that comparative tests with controlled variables are carried out on the premise of consistent fracture surface morphology, but the specimens made of organic materials have a large difference in mechanical properties from real rocks.

[0004] In addition, existing direct shear slip test devices either directly conduct direct shear tests on regular specimens or create shear slip displacements of fractures by cutting rock blocks of equal length at diagonal positions on both sides of regular specimens or using equivalent methods (such as the copper sheet padding method). The former method requires considering the sealing of the shear box during shear slip and usually has a relatively short slip amount during the test. Regardless of which method is used, the effective seepage size of the fracture in the test continuously decreases with the increase of the shear slip distance. However, the actual situation in the formation is that the effective seepage size of rock fractures continuously increases with the increase of the shear slip distance. Therefore, this kind of test device that does not conform to the actual situation used in the past will inevitably lead to unnatural change characteristics of the fracture width and fracture surface roughness at the specimen ends (such as the copper sheet padding and cutting parts), thereby increasing the uncertainty of the test results. Summary of the Invention

[0005] Aiming at the above deficiencies existing in the prior art, the purpose of the present invention is to provide a sample preparation method and device applicable to the shear slip test of fractured tight sandstone.

[0006] To achieve the above purpose, on the one hand, the present invention relates to a sample preparation method applicable to the shear slip test of fractured tight sandstone, including the following steps:

[0007] S1. Making a standard rock sample without fractures: Cutting the outcrop or core of tight sandstone to make a cylindrical sample that meets the requirements of rock mechanics tests;

[0008] S2. Creating fractures: Uniformly applying a 0.1 mm coating of epoxy resin around the cylindrical sample, making a marking line along the axis of the outer surface of the cylindrical sample, engraving a 1 mm shallow groove along the marking line, and then putting the rock sample into a disk with a wedge-shaped indenter for a constant displacement splitting test;

[0009] S3. Constructing a digital model of the fracture surface: Scanning the fracture surface to obtain the point cloud data of the fracture surface topography;

[0010] S4. Printing the fracture surface model: Based on the obtained point cloud data of the fracture surface topography, printing a fracture surface model with the same fracture surface topography, and placing the printed fracture surface model with the fracture surface side facing up into a pre-prepared steel groove;

[0011] S5. Crushing and dispersing the rock: Using the same rock as that used in making the sample in S1 for sufficient crushing and dispersing, and separating with a filter screen to collect the crushed debris components;

[0012] S6. Re-casting of the debris component: the collected debris component and the calcite powder and / or clay and / or the residual original rock powder collected by the filter are fully stirred and mixed according to the actual proportion of the debris component and the filler in the original rock, and then mixed with water to form a "slurry"; the "slurry" is poured layer by layer into the steel tank with the printed seam surface model, and the top surface is tamped and smoothed to obtain a "dense sandstone" sample with half the same seam surface morphology;

[0013] S7. Preparation of half specimens: The fracture surface morphology of the dense sandstone cylindrical sample with fractures is reversely copied, and the half specimens are prepared through the same production process, and the two halves of the specimens are assembled to form a complete cast cylindrical specimen.

[0014] Furthermore, the sample preparation method suitable for shear-slip test of dense sandstone containing cracks also includes sample curing, wherein the prepared sample is left to stand at a constant temperature for more than 24 hours, and then cured at normal temperature and pressure for at least one month, and then the outer surface of the sample is cut and polished to make a semi-cylindrical sample of standard size.

[0015] Furthermore, in S1, the rock cylindrical sample can also be made into a cubic sample; in S2, in the constant displacement splitting test, the sample should be split along the marked line without significant debris flaking, otherwise it is an unqualified sample.

[0016] Furthermore, in S3, a non-contact three-dimensional profiler is used to scan the crack surface.

[0017] Furthermore, in S4, a seam surface model having the same seam surface morphology is printed using a 3D printer and a polymer resin.

[0018] Furthermore, in S6, the mass ratio of the mixture of the debris component and the calcite powder to water is 1:3 to 1:6.

[0019] Furthermore, the sample preparation method applicable to shear-slip tests of tight sandstone containing fractures can also be applied to shear-slip tests of other rocks containing fractures, including sandstone, conglomerate and shale.

[0020] On the other hand, the present invention also relates to a test device for preparing samples using the sample preparation method of the present application suitable for shear-slip tests on dense sandstone containing cracks, namely, vertical holes are drilled at the center positions of the upper bottom surface and the lower bottom surface of the cast cylindrical sample, and elastic components are embedded in the vertical holes; rock blocks with the same length as the drilled holes are cut at the diagonal positions at both ends of the sample, and the cut parts are filled with silicone rubber of the same volume and with built-in springs.

[0021] In a preferred embodiment of the present invention, the vertical drilled hole may have a diameter of 2 mm and a length of 30 mm.

[0022] Further, the elastic member may be, for example, a composite spring tube, a spring, or a material having telescopic deformation properties.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) The specimen prepared by the sample preparation method of the present invention can not only meet the requirements of the shear-slip test of fractured tight sandstone and other for single-variable comparative research, but also maximize the consistency of the mechanical properties of the specimen with those of the tight sandstone, solving the problem that the two cannot be achieved simultaneously in the traditional related sample preparation methods.

[0025] (2) The improved test device of the present invention avoids the unnatural changes in the slit width and the roughness of the slit surface at the shear-slip and dislocation parts at both ends of the specimen in the traditional sample preparation, ensuring the accuracy of the test results. Embedding an elastic member in the vertical borehole can ensure that the spring tube expands and contracts as the specimen undergoes axial shear slip, realizing the simulation of the shear-slip process of fractured tight sandstone under real formation conditions; the cut-off part is filled with silicone rubber of the same volume and with an internal spring, which can ensure the elasticity and compressibility of the silicone rubber while preventing it from squeezing into the fissures and affecting subsequent tests. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] By reading the following detailed description of the non-limiting embodiments with reference to the accompanying drawings, other features, objects, and advantages of the present invention will become more apparent:

[0027] Figure 1 It is a flowchart for sample preparation in Example 1;

[0028] Figure 2 It is a schematic diagram of the specific sample preparation process in Example 1;

[0029] Markings in the figure: 1 - steel groove, 2 - debris particles, 3 - fracture surface model, 4 - composite spring tube, 5 - silicone rubber, 6 - internal spring, 7 - borehole. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The present invention will be described in detail below with reference to specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that those of ordinary skill in the art can make several modifications and improvements without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0031] In the description of the present invention, it should be understood that the orientation relationships indicated by terms such as "upper" and "lower" are based on the orientation or position relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or component referred to must have a specific orientation, be constructed and operated in a specific orientation. Terms such as "cylindrical" are only used to illustrate the shape of the specimen. The specific dimensional parameters adopted in this embodiment are only for illustrating the technical solution of the present invention and do not limit the protection scope of the present invention.

[0032] Taking the tight sandstone in the Chang 6 section of Huangling area in Ordos Basin as an example, and taking the production of a standard cylindrical sample with a size of 50mm×100mm as an example, the sample preparation method and device of the present invention are introduced in detail as follows.

[0033] As Figure 1 and 2 shown, a sample preparation method suitable for shear-slip rock physics experiments on fractured tight sandstone includes the following steps:

[0034] S1, fabricating a tight sandstone cylindrical sample without fractures: cutting the outcrop or core tight sandstone in the Chang 6 section to fabricate a standard cylindrical sample with a diameter and height of 50mm×100mm respectively;

[0035] S2, creating fractures: evenly applying a 0.1mm coating of epoxy resin around the cylindrical sample fabricated in step 1, making a marking line along the axis of the outer surface of the cylindrical sample, engraving a 1mm shallow groove along the marking line, and then placing the rock sample (standard rock sample) into a disk with a wedge-shaped indenter for a constant displacement splitting test. When the specimen cracks along the marking line and there is no significant debris spalling, it is considered a qualified specimen;

[0036] S3, constructing a digital model of the fracture surface: using an ST400 type non-contact three-dimensional profilometer to scan the fracture surface to obtain the point cloud data of the fracture surface topography;

[0037] S4, printing the fracture surface model: based on the obtained point cloud data of the fracture surface topography, using an SLA660 type 3D printer and SLA high molecular resin to print a fracture surface model 3 with the same fracture surface topography, and placing the printed fracture surface model (with the fracture surface side facing up) into a pre-prepared steel groove 1, which is used as the bottom film for the next pouring;

[0038] S5. Crushing and disintegrating tight sandstone: Use a disintegrator to fully crush and disintegrate the same tight sandstone in the Chang 6 section, completely crush the tight sandstone into loose mineral and rock fragment particles, and use a filter screen to separate and collect the crushed debris particles (mineral and rock fragment particles). The size of the filter screen used should not be less than 0.01 mm; the quartz, feldspar, and rock fragments in the collected debris particles 2 naturally meet the proportion content of the debris particles in the tight sandstone of the Chang 6 section, that is, the quartz content is 42.0%, the plagioclase content is 34.0%, and the sedimentary rock and metamorphic rock fragment content is 24.0%. "Fully" means completely crushing the tight sandstone into loose mineral and rock fragment particles, and the collected debris particles (quartz, feldspar, and rock fragments) naturally meet the proportion content of the original rock debris particles. For example, if the proportion content of quartz, feldspar, and rock fragments in the original rock is 5:4:1, then the composition of the collected debris particles also meets this proportion content;

[0039] S6. Re-casting of debris components: Since the filling material of the Chang 6 tight sandstone is clay and calcite with a content ratio of about 1:1, first, uniformly mix the clay and calcite powder according to the proportion content of 1:1, and then fully stir and mix the tight sandstone debris collected in the previous step with the mixture according to the proportion content of 9:1 of the Chang 6 section debris to filling material, and at the same time add a certain mass ratio of water to form a "slurry"; pour the "slurry" layer by layer into a steel tank with a printed fracture surface model and use a vibrating pump to tamp and level the top surface to obtain a Chang 6 "tight sandstone" specimen (half) with the same fracture surface morphology.

[0040] S7. Specimen curing: Let the fabricated specimen stand at a constant temperature of 24 °C for more than 24 hours, then cure it under normal temperature and pressure conditions indoors for at least one month. After that, cut and polish the outer surface of the specimen to make a semi-cylindrical specimen with a radius and height of 25 mm × 100 mm for subsequent shear-sliding tests.

[0041] S8. Error analysis: Scan the fracture surfaces in the 3D-printed fracture surface model and the fabricated specimen to obtain the corresponding point cloud data, compare the average height difference of the centerlines of the point cloud data of the straight split fracture surface fabricated in step 2, analyze the errors of the fabricated specimen, and check whether the fracture surface of the fabricated specimen is consistent with the fracture surface of the straight split fracture surface. If the error value is less than 5%, the fabricated specimen is considered qualified; otherwise, the specimen is unusable.

[0042] S9. Fabrication of semi-specimens: Reverse copy the fracture surface morphology obtained in step 3, and complete the fabrication of semi-specimens through the same fabrication process. Combine the two semi-specimens to form a complete Chang 6 "tight sandstone" cylindrical specimen with a diameter and height of 50 mm × 100 mm. Based on the above method, any number of Chang 6 fractured "tight sandstone" specimens with the same fracture surface morphology can be fabricated.

[0043] Drilling optimization device: Vertically drill holes at the upper and lower central positions of the fabricated cylindrical sample. The inner diameter of the drilled hole is 2 mm and the hole length is 30 mm. Use a cutting machine to cut 30-mm rock blocks at the diagonal positions at the upper and lower ends of the fabricated sample, and embed the composite spring tube 4 into the vertical drill hole 7. The composite spring tube can ensure that the spring tube deforms with the expansion and contraction when the sample undergoes axial shear slip, and can simulate the shear-sliding process of rock fractures under real formation conditions. At the same time, the cut-off parts are filled with silicone rubber 5 with the same volume and an internal spring 6 to ensure the elastic compressibility of the silicone rubber while preventing it from squeezing into the cracks and affecting subsequent tests.

[0044] Figure 2 In (a) is a schematic diagram of a dense sandstone cylindrical sample containing a straight split crack. The dense sandstone is taken from outcrops or cores, and the stratigraphic horizon is not limited. For example, the Chang 6 section of the Yanchang Formation in the Ordos Basin mentioned in the invention examples can also be the dense sandstone in the Xujiahe Formation of the Sichuan Basin, the Bashijiqik Formation in the Kuqa Depression, etc. The sample is split along the axis to make a crack, and the crack surface is scanned and a digital model of the crack surface is constructed. (b) is a schematic diagram of the disintegration of dense sandstone. The same dense sandstone is crushed and disintegrated, and the disintegrated debris particles are collected (in actual collection, the particles of different components after disintegration do not need to be distinguished, and the figure is only for illustration). (c) is a schematic diagram of the steel trough used for casting the sample. The bottom of the steel trough is paved with a physical model of the crack surface 3D-printed based on the digital model of the crack surface. (d) is a schematic diagram of casting the sample. The disintegrated debris particles, binder, and water are mixed together according to the component ratio of the actual dense sandstone, and a square sample is cast in the steel trough and cured. (e) is a schematic diagram of the structure of the sample after drilling. The two pairs of semi-square samples after curing are cut and polished to make a cylindrical sample, and a hole is drilled at the central position of the cylindrical sample. 30-mm rock blocks are cut at the diagonal positions on the upper and lower sides of the cylindrical sample. (f) is a schematic diagram of the final structure of the sample and device of the present invention. The cut parts at the diagonal positions on the upper and lower sides are filled with silicone rubber with an internal spring.

[0045] The sample preparation method and device proposed by the present invention are also applicable to shear-sliding tests of other cracked rocks such as glutenite, conglomerate, and shale.

[0046] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various deformations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A sample preparation method applicable to the shear-sliding test of fractured tight sandstone, characterized in that, It includes the following steps: S1. Making a standard rock sample without cracks: Cutting a dense sandstone outcrop or core to make a cylindrical sample suitable for rock mechanics tests; S2. Creating a crack: Uniformly applying a 0.1 mm coating of epoxy resin on the outer periphery of the cylindrical sample, making a marking line along the axis of the outer surface of the cylindrical sample, engraving a 1 mm shallow groove along the marking line, and then putting the rock sample into a disc with a wedge-shaped indenter for a constant displacement splitting test; S3. Constructing a digital model of the fracture surface: Scanning the fracture surface to obtain the point cloud data of the fracture surface morphology; S4. Printing the fracture surface model: Based on the obtained point cloud data of the fracture surface morphology, printing a fracture surface model with the same fracture surface morphology, and placing the printed fracture surface model with the fracture surface side up into a pre-prepared steel trough; S5. Rock crushing and disintegration: Using the same rock as that used in making the sample in S1 for sufficient crushing and disintegration, separating with a filter screen, and collecting the crushed debris components; S6. Re-casting of the debris components: Mixing the collected debris components with calcite powder and / or clay and / or the residual original rock powder collected by the filter screen thoroughly according to the proportional content of the actual debris components and filling materials in the original rock, then mixing with water to form a "slurry"; Pouring the "slurry" layer by layer into the steel trough with the printed fracture surface model, tamping it, and leveling the top surface to obtain a "dense sandstone" specimen with the same fracture surface morphology on one half; S7. Making of the half-specimen: Replicating the fracture surface morphology of the cracked dense sandstone cylindrical sample in reverse, completing the making of the half-specimen through the same production process, and piecing together the two half-specimens to form a complete cast cylindrical sample; A device applicable to the shear slip test of cracked dense sandstone, including: Vertically drilling holes at the central positions of the upper and lower bottom surfaces of the made cast cylindrical sample, and embedding elastic components into the vertical drill holes; Cutting rock blocks with the same length as the drill holes at the diagonal positions at both ends of the specimen, and filling the cut parts with silicone rubber of the same volume and with an in-built spring.

2. The sample preparation method applicable to the shear-sliding test of fractured tight sandstone according to claim 1, wherein It also includes specimen curing. Let the made specimen stand still at a constant temperature for more than 24 hours, then cure it under normal temperature and pressure conditions for at least one month, and then cut and polish the outer surface of the specimen to make a semi-cylindrical sample of standard size.

3. The sample preparation method applicable to the shear-slip test of fractured tight sandstone according to claim 1, characterized in that, In S1, the rock cylindrical sample can also be made into a cube sample; In S2, for the constant displacement splitting test, the specimen should crack along the marking line and there should be no significant debris spalling, otherwise it is an unqualified specimen.

4. The sample preparation method applicable to the shear-sliding test of fractured tight sandstone according to claim 1, characterized in that, In S3, a non-contact three-dimensional profiler is used to scan the fracture surface.

5. The sample preparation method applicable to the shear-slip test of fractured tight sandstone according to claim 1, characterized in that, In S4, a 3D printer and a polymer resin are used to print a fracture surface model with the same fracture surface morphology.

6. The sample preparation method applicable to the shear-slip test of fractured tight sandstone according to claim 1, characterized in that, In S6, the mass ratio of the mixture of the debris components and calcite powder to water is 1:3 to 1:

6.

7. The sample preparation method applicable to the shear-slip test of fractured tight sandstone according to claim 1, characterized in that, It can also be applicable to the shear slip tests of other cracked rocks, and the other cracked rocks include conglomerate, breccia and shale.

8. The sample preparation method applicable to the shear-slip test of fractured tight sandstone according to claim 1, characterized in that, The aperture of the vertical drill hole is 2 mm and the length is 30 mm.

9. The sample preparation method applicable to the shear-slip test of fractured tight sandstone according to claim 1, characterized in that, The elastic component is a composite spring tube or a spring or a material with telescopic deformation properties.

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