Preparation method of hydraulic fracturing model sample with adjustable and controllable interface

Abstract

The invention discloses a preparation method of a hydraulic fracturing model sample with an adjustable and controllable interface. The preparation method of the hydraulic fracturing model sample comprises the following steps: testing physical and mechanical properties of reservoir rock; based on the obtained physical and mechanical properties of the reservoir rock, determining an optimal raw material ratio adopted for preparing the model sample; preparing a pouring material based on the optimal proportion of the model sample raw materials; the obtained pouring material is subjected to layered pouring, and the model sample is obtained; the pouring interval time t corresponding to every two adjacent layers of pouring materials exists between every two adjacent layers of pouring materials, and t is larger than or equal to 3 hours. Compared with the prior art, the preparation method of the interface-adjustable and controllable hydraulic fracturing model sample has the advantages that the preparation method is simple, easy to operate and good in generalization performance, the strength of each layer of rock of the model sample can be adjusted and controlled, the interface shear strength can be adjusted and controlled, the interface permeability can be adjusted and controlled, and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of unconventional reservoir stimulation, and particularly to a method for preparing a hydraulic fracturing model specimen with adjustable and controllable interfaces. Background Art

[0002] Hydraulic fracturing is a key production-increasing process in shale gas extraction. There are often a large number of natural fractures, joints, fissures and other discontinuities in shale oil / gas reservoirs. The existence of these discontinuous interfaces will greatly affect the propagation effect and morphology of hydraulic fractures. Therefore, clarifying the interaction law between hydraulic fractures and interfaces is of great significance for hydraulic fracturing design and subsequent evaluation of fracture-induced permeability enhancement.

[0003] In the prior art, on the one hand, it is difficult and costly to in-situ prepare specimens with interfaces, and the specimen preparation process will cause certain disturbances to the interfaces, resulting in the inability to control the properties and morphology of the original specimen interfaces, and the discreteness of test results is relatively large. On the other hand, many scholars have carried out laboratory hydraulic fracturing tests by artificially preparing specimens with interfaces. These methods mainly use prefabricated soft materials to simulate natural fractures and interfaces, or form microcracks or interfaces in specimens in advance by loading. However, the above methods cannot quantitatively control the strength and morphology of the interfaces, and it is difficult to precisely regulate the interface strength and permeability within a certain range. The repeatability of specimen preparation and fracturing test results is not ideal.

[0004] Therefore, how to provide a method for preparing a hydraulic fracturing model specimen that can achieve the technical effect of controllable and adjustable interface strength and permeability is a technical problem that those skilled in the art need to solve urgently. Summary of the Invention

[0005] In view of the problems existing in the prior art, the technical problem to be solved by the present invention is to provide a method for preparing a hydraulic fracturing model specimen that can achieve the technical effect of controllable and adjustable interface strength and permeability.

[0006] To achieve the above object, the present invention provides a method for preparing a hydraulic fracturing model specimen with adjustable and controllable interfaces. The method for preparing the hydraulic fracturing model specimen includes: S1. Testing the physical and mechanical properties of reservoir rocks;

[0007] S2. Determining the optimal raw material ratio for preparing the model specimen based on the obtained physical and mechanical properties of the reservoir rocks; S3. Preparing a casting material based on the optimal raw material ratio for preparing the model specimen;

[0008] S4. Layer-casting the obtained casting material to obtain the model specimen; there is a corresponding casting interval time t between every two adjacent layers of the casting material, and t≥3 hours.

[0009] In the first aspect, the physical and mechanical properties of the reservoir rock include the uniaxial compressive strength of each layer of rock, the shear strength of the interface between interbedded rocks, and the permeability of the interface between interbedded rocks.

[0010] In the first aspect, based on the obtained physical and mechanical properties of the reservoir rock, determining the optimal raw material ratio for preparing the model specimen specifically includes: selecting cement, aggregate, and water as the raw materials for the model specimen; designing different ratios of the raw materials and preparing different initially cast materials; using the obtained different initially cast materials to prepare standard cylindrical specimens and respectively testing to obtain the uniaxial compressive strength; comparing the uniaxial compressive strength obtained through testing with the uniaxial compressive strength of each layer of the reservoir rock, and selecting the ratio of the raw materials that is closest to the uniaxial compressive strength of each layer of the reservoir rock to respectively obtain the optimal raw material ratio for each layer of the reservoir rock.

[0011] In the first aspect, preparing the cast material based on the optimal raw material ratio for preparing the model specimen specifically includes: preparing the cast material based on the obtained optimal raw material ratio for each layer of the reservoir rock to respectively obtain the cast material for each layer of the reservoir rock.

[0012] In the first aspect, layering and casting the obtained cast material to obtain the model specimen includes: layer-by-layer casting the obtained cast material for each layer of the reservoir rock, such that the cast material of each layer is cast on the corresponding layer to obtain the model specimen; during the layer-by-layer casting, there is a corresponding optimal casting interval time between each adjacent two layers of the cast material.

[0013] In the first aspect, layer-by-layer casting the obtained cast material for each layer of the reservoir rock, such that the cast material of each layer is cast on the corresponding layer to obtain the model specimen; during the layer-by-layer casting, there is a corresponding optimal casting interval time between each adjacent two layers of the cast material; sand laying treatment is performed between each adjacent two layers of the cast material, and there is a corresponding optimal sand laying parameter for the sand laying treatment between each adjacent two layers of the cast material.

[0014] In the first aspect, the determination of the optimal casting interval time specifically includes: before layer-by-layer casting, casting the cast material for two adjacent layers corresponding to the reservoir rock, obtaining a number of first test specimens by changing the casting interval time between the two layers of cast material, performing shear tests on the first test specimens under different normal stresses, obtaining the interface shear strength of the first test specimens at different casting interval times, and determining the optimal casting interval time for the two adjacent layers of cast material corresponding to the reservoir rock.

[0015] In a first aspect, the determination of the optimal sand laying parameters specifically includes: before layer-by-layer pouring, sand is laid between the pouring materials corresponding to two adjacent layers of the reservoir rock during the pouring of two layers; when laying the sand, by changing the sand laying parameters, a second test specimen is obtained, the interfacial permeability of the second test specimen is measured, the interfacial permeabilities under different sand laying parameters are obtained, and the optimal sand laying parameters are determined; the sand laying parameters include the thickness, mesh number, and shape of the sand laying.

[0016] In a first aspect, the sand laying specifically includes: during layer-by-layer pouring, after pouring one layer of the pouring material and before pouring the adjacent layer of the pouring material, sand is laid on the surface of the poured layer of the pouring material.

[0017] In a first aspect, the step of obtaining the model specimen by layer-by-layer pouring of the obtained pouring material further includes; for the model specimen with an inclined interface, after the model specimen is poured, the model specimen is processed by wire cutting to obtain the model specimen with an inclined interface.

[0018] Beneficial effects:

[0019] The present invention provides a method for preparing a hydraulic fracturing model specimen with adjustable and controllable interfaces, which is used to prepare a model specimen of reservoir rock; before the test, the similarity ratio between the wellbore size of the reservoir rock and the wellbore size of the model specimen is determined. By conducting a compression test on the original reservoir specimen, the uniaxial compressive strength of each layer of the reservoir rock is divided by the similarity ratio to obtain the uniaxial compressive strength of the similar model specimen, and this is used as the determination standard for the uniaxial compressive strength of the model specimen. By adjusting the ratio between the raw materials used in the model specimen, the raw material ratio of the model specimen with a uniaxial compressive strength close to or the same as the determined standard uniaxial compressive strength is selected, that is, the optimal raw material ratio for the model specimen; by testing the interlayer interface shear strength and interlayer interface permeability of each layer of the reservoir rock, the test results are used as the interface shear strength and interface permeability standards for the model specimen. Different casting mortars are prepared through the obtained optimal raw material ratio of the model specimen, and stratified casting is carried out using different casting mortars. By changing the casting interval time between adjacent two layers of casting mortar, a first test specimen is prepared, and its interface shear strength is tested. The tested interface shear strength is compared with the interlayer interface shear strength of each layer of the reservoir rock, and the casting interval time corresponding to the closest interface shear strength is selected as the casting interval time for the adjacent two layers of casting mortar in the model specimen corresponding to the reservoir rock. During this process, the adjustable and controllable interlayer interface shear strength of each layer can be achieved by controlling the casting interval time; when the simulated model specimen has requirements for interface permeability, by changing the sand laying parameters between adjacent two layers of casting mortar, a second test specimen is prepared, and its interface permeability is tested. The tested interface permeability is compared with the interlayer interface permeability of each layer of the reservoir rock, and the sand laying parameters corresponding to the closest interface permeability are selected as the sand laying parameters between adjacent two layers of casting mortar in the model specimen considering permeability. During this process, the adjustable and controllable interlayer rock interface permeability can be achieved by controlling the sand laying parameters; based on the determined optimal raw material ratio of the model specimen, the casting interval time during stratified casting, and the sand laying parameters when sand laying is required, stratified casting is carried out using the prepared casting mortar to obtain the model specimen; when the reservoir rock to be simulated is multi-layered, each casting mortar during casting has different uniaxial compressive strengths; when the reservoir rock to be simulated is a reservoir with natural fractures, each casting mortar during casting has the same uniaxial compressive strength. Description of the Drawings

[0020] In order to more clearly illustrate the technical solutions in the embodiments of this specification or the prior art, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0021] Figure 1 It is a flow diagram of a method for preparing a hydraulic fracturing model specimen with adjustable and controllable interface of the present invention.

[0022] Figure 2 It is a broken line graph of shear strength under single full pouring and layered pouring with different pouring intervals. Specific embodiments

[0023] Next, the technical solutions in the embodiments of this specification will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of this specification. Obviously, the described embodiments are only a part of the embodiments of this specification, rather than all the embodiments. Based on the embodiments in this specification, all other embodiments obtained by those of ordinary skill in the art belong to the scope of protection of the present invention.

[0024] Embodiment 1

[0025] As Figure 1 shown, Embodiment 1 of the present invention provides a method for preparing a hydraulic fracturing model specimen with adjustable and controllable interface. The method for preparing the hydraulic fracturing model specimen includes: S1, testing the physical and mechanical properties of reservoir rocks; S2, determining the optimal raw material ratio for preparing the model specimen based on the obtained physical and mechanical properties of the reservoir rocks; S3, preparing a casting material based on the optimal raw material ratio for preparing the model specimen; S4, performing layered casting on the obtained casting material to obtain the model specimen; there is a corresponding casting interval time t between every two adjacent layers of the casting material, and t≥3 hours.

[0026] The present invention provides a method for preparing a hydraulic fracturing model specimen with adjustable and controllable interfaces, which is used to prepare a model specimen of reservoir rock; before the test, determine the similarity ratio between the wellbore size of the reservoir rock and the wellbore size of the model specimen. By conducting a compression test on the original reservoir specimen, divide the uniaxial compressive strength of each layer of the reservoir rock obtained by the similarity ratio to obtain the uniaxial compressive strength of the similar model specimen, and use this as the determination standard for the uniaxial compressive strength of the model specimen. By adjusting the ratio between the raw materials used for the model specimen, select the raw material ratio of the model specimen whose uniaxial compressive strength is close to or the same as the determined standard uniaxial compressive strength, that is, the optimal raw material ratio of the model specimen; by testing the interlayer interface shear strength and interlayer interface permeability of each layer of the reservoir rock, use the test results as the interface shear strength and interface permeability standards of the model specimen. Prepare different casting mortars through the obtained optimal raw material ratio of the model specimen, use different casting mortars for layered casting, and by changing the casting interval time between adjacent two layers of casting mortar, prepare the first test specimen and test its interface shear strength. Compare the tested interface shear strength with the interlayer interface shear strength of each layer of the reservoir rock, and select the casting interval time corresponding to the closest interface shear strength as the casting interval time of the casting mortar between adjacent two layers in the model specimen corresponding to the reservoir rock. During this process, the adjustable and controllable interlayer interface shear strength of each layer can be achieved by controlling the casting interval time; when the simulated model specimen has requirements for the interface permeability, by changing the sand laying parameters between adjacent two layers of casting mortar, prepare the second test specimen and test its interface permeability. Compare the tested interface permeability with the interlayer interface permeability of each layer of the reservoir rock, and select the sand laying parameters corresponding to the closest interface permeability as the sand laying parameters between adjacent two layers of casting mortar in the model specimen that needs to consider permeability. During this process, the adjustable and controllable interface permeability of the interlayer rock can be achieved by controlling the sand laying parameters; based on the determined optimal raw material ratio of the model specimen, the casting interval time during layered casting, and the sand laying parameters when sand laying is required, use the prepared casting mortar for layered casting to obtain the model specimen; when the reservoir rock to be simulated is multi-layered, each casting mortar during casting has different uniaxial compressive strengths; when the reservoir rock to be simulated is a reservoir with natural fractures, each casting mortar during casting has the same uniaxial compressive strength.

[0027] In some possible implementation manners, the physical and mechanical properties of the reservoir rock include the uniaxial compressive strength of each layer of rock, the interlayer interface shear strength of the interlayer rock, and the interlayer interface permeability of the interlayer rock.

[0028] Specifically, the physical and mechanical properties of the reservoir rock that needs to be simulated in the test are used as the mechanical standard of the model sample; one type of reservoir rock is a reservoir composed of multiple lithology rocks, each layer of rock forms a single interface or multiple interfaces, and each layer of rock has a different uniaxial compressive strength. For example, a reservoir rock composed of two layers of rock has a single interface, and a reservoir rock composed of three layers of rock has two interfaces, and so on; another type of reservoir rock is a reservoir containing natural fractures, which form a single interface or multiple interfaces through natural fractures, and the uniaxial compressive strength is the same.

[0029] In some possible implementations, based on the obtained physical and mechanical properties of the reservoir rock, determining the optimal raw material ratio for preparing the model sample specifically includes: selecting cement, aggregate and water as the raw materials for the model sample; designing different ratios of the raw materials and preparing different initial casting materials; preparing standard cylindrical specimens using the obtained different initial casting materials and testing them to obtain the uniaxial compressive strength; comparing the uniaxial compressive strength obtained by the test with the uniaxial compressive strength of each layer of the reservoir rock, selecting the ratio of the raw materials that is closest to the uniaxial compressive strength of each layer of the reservoir rock, and obtaining the optimal raw material ratio for each layer of the reservoir rock.

[0030] Specifically, suitable cement, aggregate and water are selected as raw materials for the model specimen, the aggregate includes quartz sand and materials similar to the quartz sand, a series of casting schemes with different raw material ratios for the model specimen are designed, different raw materials are weighed and mixed according to the ratio, quartz sand and cement are added to the mixer in sequence during mixing, and then water is added and fully mixed to obtain cement mortar, the cement mortar is poured into the mold, vibrated and compacted, demolded and marked, cured in a curing box, the surface of the specimen is polished and fully dried, and then a uniaxial compressive strength test is performed to obtain the uniaxial compressive strength of the cement mortar specimens with different ratios; before the test, the wellbore size of the reservoir rock and the size of the model specimen are determined. The similarity ratio of the wellbore size is obtained by conducting compression tests on the original reservoir samples, and the uniaxial compressive strength of each layer of the reservoir rock is divided by the similarity ratio to obtain the uniaxial compressive strength of the similar model sample, and this is used as the standard for determining the uniaxial compressive strength of the model sample. By adjusting the ratio between the raw materials of the model sample, the model sample raw material ratio with a uniaxial compressive strength close to or the same as the uniaxial compressive strength of the determined standard is selected, that is, the best raw material ratio used for the model sample, that is, through the similarity principle, the model sample raw material ratio corresponding to the uniaxial compressive strength closest to each layer of the reservoir rock to be simulated is selected as the best raw material ratio used for the model sample.

[0031] In some possible implementation manners, the casting material prepared based on the optimal raw material ratio used for preparing the model specimen specifically includes: preparing the casting material based on the optimal raw material ratio of each layer of the reservoir rock obtained, and separately obtaining the casting materials for each layer of the reservoir rock; the step of obtaining the model specimen by casting the obtained casting materials in layers includes: casting the obtained casting materials for each layer of the reservoir rock layer by layer, so that the casting material of each layer is cast on the corresponding layer to obtain the model specimen; during the layer-by-layer casting, there is a corresponding optimal casting interval time between every two adjacent layers of the casting materials; the step of obtaining the model specimen by casting the obtained casting materials in layers further includes: casting the obtained casting materials for each layer of the reservoir rock layer by layer, so that the casting material of each layer is cast on the corresponding layer to obtain the model specimen; during the layer-by-layer casting, there is a corresponding optimal casting interval time between every two adjacent layers of the casting materials; sand laying treatment is performed between every two adjacent layers of the casting materials, and there is a corresponding optimal sand laying parameter for the sand laying treatment between every two adjacent layers of the casting materials.

[0032] Specifically, prepare the casting materials corresponding to the rocks of the corresponding number of layers according to the obtained model raw material ratio, and perform layer-by-layer casting; during the layer-by-layer casting process, first cast the first layer of specimen according to the raw material ratio of the model specimen of the first layer of rock. After the casting of the first layer of specimen is completed, then cast the second layer of specimen according to the raw material ratio of the model specimen of the second layer of rock at an interval corresponding to the casting interval time. After the casting of the second layer of specimen is completed, then cast the third layer of specimen according to the raw material ratio of the model specimen of the third layer of rock at an interval corresponding to the casting interval time. Proceed with layer-by-layer casting in this step in turn to obtain the model specimen. The casting interval time affects the interface shear strength obtained after the casting of adjacent two layers of casting materials. By controlling the casting interval time, the adjustable and controllable interface shear strength between each layer of rocks can be realized; based on the Mohr-Coulomb criterion, the shear strength parameters of the interface at different interval times, that is, the cohesion and the internal friction angle, can be calculated from the peak shear strength. When the indoor fracturing model specimen has requirements for the interface permeability, during the layer-by-layer casting, sand laying treatment can be carried out between the adjacent two layers of casting materials at the interface with interface permeability requirements. By laying sand on the surface of the already cast material after casting one half of the casting material and before casting the other half of the casting material, different interface permeabilities correspond to different sand laying parameters. Therefore, the interface penetration characteristics can be adjusted by changing sand laying parameters such as the sand laying thickness, the sand laying mesh number, and the sand laying shape, that is, the adjustable and controllable interface permeability of the interbedded rock interface can be realized by controlling the sand laying parameters. After determining the optimal casting interval time and the optimal sand laying parameters, based on the determined optimal casting interval time and the optimal sand laying parameters, and based on the size of the required fracturing model specimen, the interbedded thickness, and the number of interfaces, prepare the corresponding molds, and use the optimal model specimen raw material ratio of each layer of rock to prepare the casting mortar of each layer of rock, and perform layer-by-layer casting in the corresponding molds until the model specimen with the required number of interfaces is cast; after the casting of each model specimen is completed, place it in an environment with a temperature of 20±5°C and let it stand for about 24h, then demold and number it, and perform water tank curing, and the height of the water in the water tank should be 1 cm higher than the model specimen; after curing to the age time, take out the model specimen and polish the surface of the model specimen smooth with sandpaper.

[0033] In some possible implementation manners, the determination of the optimal casting interval time specifically includes: before the layer-by-layer casting, cast the casting materials of the adjacent two layers corresponding to the two layers of the reservoir rock, and by changing the casting interval time between the two layers of casting materials, obtain a plurality of first test specimens, perform shear tests on the first test specimens under different normal stresses, obtain the interface shear strength of the first test specimens at different casting interval times, and determine the optimal casting interval time of the adjacent two layers of casting materials in the reservoir rock.

[0034] Specifically, prepare the casting materials corresponding to the rocks of the corresponding number of layers according to the obtained model raw material ratio, and perform layer-by-layer casting; before layer-by-layer casting, first cast the first layer of specimen according to the raw material ratio of the model specimen of one layer of rock, and then cast the second layer of specimen according to the raw material ratio of the model specimen of the adjacent layer of rock at different intervals, to obtain a number of first test specimens with a single interface. In addition, a number of parallel first test specimens are cast at each interval time, and the shear test is carried out on the number of first test specimens under different normal stresses. The average value of the shear strength of the number of parallel first test specimens cast at each interval time is taken to obtain the variation law of the shear strength of the first test specimens with the casting interval time. Through this variation law, when the shear strength of the single interface in the first test specimens is closest to the shear strength of the corresponding interface of the reservoir rock, the casting interval time between the adjacent two layers of casting materials in the first test specimens can be obtained, and it is used as the optimal casting interval time for the corresponding two layers of casting materials of the reservoir rock; it can be seen that the present invention can realize the adjustable and controllable shear strength of the interface between layers of rock by controlling the casting interval time; based on the Mohr-Coulomb criterion, the shear strength parameter values of the interface at different interval times, that is, the cohesion and the internal friction angle, can be calculated from the peak shear strength.

[0035] In some possible implementation manners, the determination of the optimal sand laying parameters specifically includes: before layer-by-layer casting, sand is laid between the casting materials corresponding to the adjacent two layers of the reservoir rock; when laying the sand, by changing the parameters of the sand laying, a second test specimen is obtained, the interface permeability of the second test specimen is tested, the interface permeability under different sand laying parameters is obtained, and the optimal sand laying parameters between the corresponding adjacent two layers of casting materials in the reservoir rock are determined; the sand laying parameters include the thickness, mesh number and shape of the sand laying; the sand laying specifically includes: after casting one layer of the casting material and before casting the adjacent layer of the casting material, sand is laid on the surface of the cast layer of the casting material.

[0036] Specifically, when the indoor fracturing model specimen has requirements for the interface permeability, sand can be laid on the surface of the cast specimen before casting the other half of the specimen, and the interface penetration characteristics can be adjusted by changing the sand laying thickness, sand laying mesh number and sand laying shape. Based on the shear-seepage coupling device, the interface permeability of the second test specimen is measured, the variation law of the interface permeability with the sand laying parameters is obtained, and the sand laying parameters corresponding to the closest interface permeability are selected as the optimal sand laying parameters between the adjacent two layers of casting mortar in the model specimen with requirements for permeability; in addition, whether to perform sand laying treatment between each adjacent two layers of casting materials is determined by whether the indoor fracturing model specimen has requirements for the interface permeability; it can be seen that the present invention can realize the adjustable and controllable interface permeability of the interbedded rock by controlling the sand laying parameters.

[0037] In some possible implementation manners, the step of performing layered pouring on the obtained pouring material to obtain the model specimen further includes: for the model specimen with an inclined interface, after the model specimen is poured, the model specimen is processed by wire cutting to obtain the model specimen with an inclined interface.

[0038] Specifically, the interface inclination angle of the interlayer specimen with an inclined interface can be adjusted and set.

[0039] In summary, the method for preparing a hydraulic fracturing model specimen with adjustable and controllable interfaces according to the present invention is simple and easy to operate, has good popularization, can prepare a hydraulic fracturing model specimen with multiple parallel interfaces, can prepare pouring mortar with different strengths by changing the ratio of raw materials used in the model specimen, and realizes the adjustable and controllable strength of each layer of rock in the model specimen; in addition, the present invention can also form interfaces with different shear strengths by controlling the pouring interval time, realizes the adjustable and controllable shear strength of the interfaces, and realizes the adjustable and controllable interface permeability by changing the sand laying thickness, sand laying mesh number, and sand laying shape.

[0040] In order to further elaborate on the technical solution of the present application to support the technical problem to be solved by the present application, the following is a specific example description of the preparation method, such as Example 1.

[0041] Example 1

[0042] Aggregate: quartz sand;

[0043] Cement: P42.5 ordinary Portland cement;

[0044] A method for preparing a hydraulic fracturing model specimen with adjustable and controllable interfaces according to the present invention, the method for preparing the hydraulic fracturing model specimen is as follows:

[0045] According to the requirements of indoor hydraulic fracturing equipment, the size of the fracturing model specimen is 200mm×200mm×200mm. The reservoir to be simulated contains two interfaces. The strength of each layer of rock in the model specimen determined according to the similarity theory is 28MPa. The cohesive force of the high-strength interface is 7MPa, the cohesive force of the medium-strength interface is 3MPa, the cohesive force of the low-strength interface is 1MPa, the interface inclination angle is 0°, and the interface permeability is not considered;

[0046] For the design model experiment, different proportions of raw materials were used, as shown in Table 1. The cement mortar was obtained by weighing and mixing according to the proportions, then poured into the mold, vibrated compactly, demolded and marked with numbers to obtain model specimens. The specimens were cured in a curing box for 14 days, the surfaces of the model specimens were polished flat and fully dried, and uniaxial tests were carried out. The uniaxial compressive strengths of the cement mortar with different proportions are shown in Table 1. The raw material proportion of the model specimen was selected as quartz sand:cement:water = 1:1:0.45 as the best raw materials for the model specimen.

[0047] The casting material was prepared with the best raw material proportion of the model specimen, quartz sand:cement:water = 1:1:0.45.

[0048] The obtained casting material was poured. One group of complete specimens and four groups of first test specimens were set. The four groups of first test specimens were all cast layer by layer at a single interface, and the casting intervals between two adjacent layers of the four groups of first test specimens were 3 hours, 12 hours, 24 hours and 72 hours respectively. The casting scheme is shown in Table 2. Shear tests were carried out on the interfaces of one group of complete specimens and four groups of first test specimens to obtain the interface shear strengths at different casting intervals. The shear strength results are shown in Figure 2 , and based on the Mohr-Coulomb criterion, the shear strength parameter values of the interface at different intervals, namely cohesion and internal friction angle, were calculated from the peak shear strength. The shear characteristic results are shown in Table 2. Combining the high-strength interface cohesion, medium-strength interface cohesion and low-strength interface cohesion determined by the similarity theory, the casting intervals corresponding to the preparation of high-strength, medium-strength and low-strength interfaces were determined to be 3h, 12h and 24h from low to high.

[0049] Based on the best casting interval time and the best sand laying parameters, the obtained casting material for each layer of the reservoir rock was cast layer by layer to obtain model specimens.

[0050] According to the determined optimal raw material ratio of the three-layer rock, the weighed amounts of each raw material for the middle interlayer are 4 kg of quartz sand, 4 kg of cement, and 1.8 kg of pure water respectively. The weighed amounts of each raw material for the upper and lower layers are 2 kg of quartz sand, 2 kg of cement, and 0.9 kg of pure water respectively. The interval pouring times for different layers of low, medium, and high-strength interfaces are set to 24 hours, 12 h, and 3 h respectively. When approaching the pouring interval time, remix and stir the raw materials for the next layer to be poured. When stirring each time, add quartz sand and cement to the mixer in sequence, dry-mix evenly, and then add pure water and stir thoroughly. Pour the well-stirred bottom-layer cement mortar evenly into the mold box, making the height of the cement mortar slightly higher than the designed interface position. Vibration table vibration and manual ramming are used to make the cement mortar evenly dense. Let it stand for a predetermined interval time, and use a spatula to scrape and level the surface to the scribed mark position, and clean the cement residue debris in the test mold. After reaching the predetermined interval time, pour the freshly stirred upper-layer cement mortar evenly into the mold box, and repeat this process until the pouring of the uppermost layer of cement mortar is completed. After the pouring is completed, use a straightedge to scrape and level the upper surface to make it slightly higher than the upper boundary of the mold to obtain a model specimen;

[0051] Let the model specimen stand in an environment with a temperature of 20 ± 5 °C for about 24 h, demold and number it, and cure it in a water tank for 18 d. The water height in the water tank should be 1 cm higher than the model specimen. After curing to the age time, take out the model specimen and polish the surface of the model specimen smooth with sandpaper.

[0052] Table 1 Different ratios of raw materials used in the model specimen and test results of cement mortar with different ratios

[0053]

[0054] Table 2 Pouring scheme and shear test results

[0055]

[0056]

[0057] It can be seen from Table 1 that when the mass ratio of quartz sand, cement, and water is 1:1:0.45, the uniaxial compressive strength is closest to the similar uniaxial compressive strength of each layer of the reservoir rock to be simulated, that is, closest to the standard of the uniaxial compressive strength of the determined model specimen. Therefore, the selected optimal mass ratio of the raw materials for the model specimen is quartz sand:cement:water = 1:1:0.45 as the optimal ratio of the raw materials for the model specimen; Through Table 2 combined with Figure 2It can be seen that the interface shear characteristics of the first test specimens with pouring intervals of 3 hours, 12 hours, and 24 hours are successively closest to the shear characteristics of the high-strength interface, medium-strength interface, and low-strength interface of reservoir rocks determined according to similarity theory. Therefore, 3 hours, 12 hours, and 24 hours are selected as the pouring intervals for the high-strength interface, medium-strength interface, and low-strength interface of the optimal raw material ratio adopted by the model specimens. To sum up, it can be seen that the interface strength controllability of the model specimens prepared by the method for preparing a hydraulically fractured model specimen with adjustable and controllable interface of the present invention is good, the preparation method is clear, simple and feasible, and has good popularization. The uniaxial compressive strength and interface shear strength of the model specimens can be regulated by changing the mass ratio of the raw materials adopted by the model specimens and the pouring interval of layered pouring.

[0058] The preferred specific embodiments of the present invention have been described in detail above. It should be understood that those of ordinary skill in the art can make many modifications and variations based on the concept of the present invention without creative work. Therefore, all technical solutions that can be obtained by those skilled in the art in the technical field of the present invention based on the concept of the present invention through logical analysis, reasoning or limited experiments on the basis of the prior art should be within the protection scope determined by the claims.

Claims

1. A method for preparing a hydraulic fracturing model sample with an adjustable and controllable interface, characterized in that: The method for preparing a hydraulic fracturing model sample comprises: S1. Test the physical and mechanical properties of reservoir rocks; S2. Determine the optimal raw material ratio for preparing the model sample based on the obtained physical and mechanical properties of the reservoir rock; S3, preparing a casting material based on the optimal raw material ratio used to prepare the model sample; S4. Casting the obtained casting material in layers to obtain the model sample; there is a corresponding casting interval time t between each two adjacent layers of the casting material, and t≥3 hours.

2. A method for preparing a hydraulic fracturing model sample with adjustable and controllable interface as claimed in claim 1, characterized in that: The physical and mechanical properties of the reservoir rock include the uniaxial compressive strength of each layer of rock, the shear strength of the interface between the interlayer rock layers, and the permeability of the interface between the interlayer rock layers.

3. A method for preparing a hydraulic fracturing model sample with adjustable and controllable interface as claimed in claim 2, characterized in that: Based on the obtained physical and mechanical properties of the reservoir rock, the optimal raw material ratio for preparing the model sample is determined, specifically including: Cement, aggregate and water were selected as raw materials for the model specimens; Design different proportions of the raw materials and prepare different initial pouring materials; Using the obtained different initial casting materials, standard cylindrical specimens are prepared and tested to obtain the uniaxial compressive strength respectively; The uniaxial compressive strength obtained by the test is compared with the uniaxial compressive strength of each layer of the reservoir rock, and the ratio of the raw materials closest to the uniaxial compressive strength of each layer of the reservoir rock is selected to obtain the optimal raw material ratio for each layer of the reservoir rock.

4. A method for preparing a hydraulic fracturing model sample with adjustable and controllable interface as claimed in claim 3, characterized in that: The casting material prepared based on the optimal raw material ratio used to prepare the model sample specifically includes: Based on the obtained optimal raw material ratio of each layer of the reservoir rock, casting materials are prepared to obtain casting materials for each layer of the reservoir rock.

5. The method for preparing a hydraulic fracturing model sample with adjustable and controllable interface according to claim 4, characterized in that: The method of pouring the obtained casting material in layers to obtain the model sample includes: pouring the casting material of each layer of the reservoir rock layer by layer, so that each layer of the casting material is poured on the corresponding layer to obtain the model sample; during the layer-by-layer pouring, there is an optimal pouring interval time corresponding to each two adjacent layers of the casting material.

6. A method for preparing a hydraulic fracturing model sample with adjustable and controllable interface as claimed in claim 4, characterized in that: The step of casting the obtained casting material in layers to obtain the model sample further comprises: The casting materials of each layer of the reservoir rock are cast layer by layer, so that each layer of the casting materials is cast on the corresponding layer to obtain a model sample; in the layer-by-layer casting, there is an optimal casting interval time corresponding to each two adjacent layers of the casting materials; the casting materials are sanded between each two adjacent layers, and the sanding treatment between each two adjacent layers of the casting materials has an optimal sanding parameter corresponding to it.

7. A method for preparing a hydraulic fracturing model sample with adjustable and controllable interface as claimed in claim 5 or 6, characterized in that: The determination of the optimal pouring interval time specifically includes: Before pouring layer by layer, two layers of the pouring material corresponding to the two adjacent layers of the reservoir rock are poured, and a plurality of first test specimens are obtained by changing the pouring interval time between the two layers of the pouring material. Shear tests are performed on the first test specimens under different normal stresses to obtain the interface shear strength of the first test specimens under different pouring interval times, and determine the optimal pouring interval time between the two adjacent layers of the pouring material corresponding to the reservoir rock.

8. The method for preparing a hydraulic fracturing model sample with adjustable and controllable interface according to claim 6, characterized in that: The determination of the optimal sand laying parameters specifically includes: Before pouring layer by layer, sand is spread between the casting materials of the two adjacent layers corresponding to the two layers of the reservoir rock; when spreading the sand, a second test sample is obtained by changing the parameters of the sand spreading, and the interface permeability of the second test sample is tested to obtain the interface permeability under different sand spreading parameters, and the optimal sand spreading parameters between the two adjacent layers of the casting materials corresponding to the reservoir rock are determined; the sand spreading parameters include the thickness, mesh size and shape of the sand spreading.

9. A method for preparing a hydraulic fracturing model sample with adjustable and controllable interface as claimed in claim 8, characterized in that: The sand spreading specifically includes: after pouring a layer of the pouring material and before pouring an adjacent layer of the pouring material, spreading sand on the surface of the poured layer of the pouring material.

10. A method for preparing a hydraulic fracturing model sample with adjustable and controllable interface as claimed in claim 5 or 6, characterized in that: The method of casting the obtained casting material in layers to obtain the model sample also includes: for the model sample containing an inclined interface, after the casting of the model sample is completed, processing the model sample by wire cutting to obtain the model sample containing an inclined interface.

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