Artificial lacustrine shale specimen for hydraulic fracturing experiment and manufacturing method thereof

Abstract

The application provides an artificial lacustrine shale sample for hydraulic fracturing experiment and a manufacturing method thereof, and belongs to the field of oil and gas field development.The artificial lacustrine shale sample is poured by layers of concrete slurry, and comprises multiple concrete layers from bottom to top; the concrete slurry of different layers has the same or different proportions; no simulation layer is arranged or a simulation layer is arranged between the adjacent two concrete layers.The application realizes effective simulation of the structure and properties of the lacustrine shale natural rock sample, can meet various requirements of the rock sample shape for the hydraulic fracturing experiment, and further improves the accuracy of the fracturing experiment; moreover, the application has simple, clear and operable principle, and is easy to be batch-produced.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas field development, specifically relating to an artificial lacustrine shale sample for hydraulic fracturing experiments and its preparation method. Background Technology

[0002] With the continuous advancement of shale gas development technology in my country, the theory and key technologies for the effective large-scale development of shallow marine shale gas within a depth of 3500 meters have been largely mastered. However, breakthroughs in the development of lacustrine sedimentary terrestrial shale gas have been rare. Compared with marine shale, lacustrine shale has a high clay content, poor matrix brittleness, complex fracture networks making fracturing difficult, strong vertical heterogeneity of the reservoir, large differences in lithology and stress between layers, and well-developed interlayers of marl, siltstone, etc., making hydraulic fracture propagation across layers difficult and hindering the achievement of large reservoir stimulation volumes. This results in poor volumetric fracturing effects in lacustrine shale. Therefore, research on the propagation laws of hydraulic fractures across layers in lacustrine shale is of great guiding significance for achieving efficient development of lacustrine shale oil and gas.

[0003] Currently, large-scale true triaxial hydraulic fracturing experiments are one of the most commonly used methods for studying the propagation of hydraulic fractures across layers. Conducting these experiments generally requires the preparation of large-sized rock specimens, with the most common specimen size being 30cm × 30cm × 30cm. For marine shale, pre-cut outcrop rock samples or equivalent rock specimens cast in concrete are typically used. However, for lacustrine shale, due to significant differences in lithological development between layers and weak cementation at some layer interfaces, most outcrop rock samples of lacustrine shale exposed at the surface are severely weathered and have poor integrity, making it difficult to cut rock specimens that meet the size requirements for hydraulic fracturing experiments.

[0004] Chinese patent publication CN108801739B discloses a method for preparing artificial physical core samples. This method uses a mixture of quartz, cement, and a water-reducing agent to simulate the brittle mineral composition of natural sandstone cores; and clay to simulate the clay mineral composition of natural sandstone cores. By adjusting the proportions of quartz, cement, water-reducing agent, and clay, the artificial physical core samples are made identical to natural sandstone in terms of mechanical properties (including compressive strength, elastic modulus, and Poisson's ratio), whole-rock mineral composition, and clay mineral composition. This patent provides a technical measure for preparing artificial sandstone samples through material proportioning; however, it does not consider the vertical lithological heterogeneity and interlayer interface development characteristics of lacustrine shale reservoirs, thus failing to provide a targeted design for the preparation of artificial lacustrine shale samples.

[0005] A novel method for preparing artificial rock samples of dispersed argillaceous cemented loose sandstone (Progress in Geophysics, December 2013) published in China explores this method and investigates the effects of quartz grain size, clay content, cementing agent dosage, and pressing pressure on the physical properties of the artificial rock samples. The results show that the prepared artificial rock samples exhibit good structural similarity to dispersed argillaceous cemented loose sandstone, with good pore throat sorting. The desired porosity, pore structure, and permeability can be obtained by controlling quartz grain size, argillaceous (clay) content, and pressing pressure. The artificial rock samples prepared using this method have advantages such as good inertness (stable physicochemical properties), high success rate, and low cost, providing satisfactory experimental samples for research on the physical properties and mechanisms of related reservoirs. While this paper primarily addresses improving the success rate of core preparation through technological advancements, it does not consider the vertical lithological heterogeneity and interlayer interface development characteristics of lacustrine shale reservoirs, thus failing to provide a targeted design for artificial lacustrine shale sample preparation.

[0006] The Chinese publicly available literature, "Methods for Preparing Loose Sandstone Samples in the Bohai Sea Area" (Science, Technology and Engineering, April 2015), addresses the diverse cementation types and significant permeability differences in reservoir samples from the 32-6 and 36-1 oilfields in the Bohai Sea area. It proposes three methods for preparing loose sandstone samples: outcrop rock extraction, artificial pressing, and hydraulic pressing. The literature tests the samples obtained by these three methods on four aspects: permeability, mineral composition, rock mechanics, and degree of cementation. The samples are compared with natural reservoir samples, and the feasibility of using these three methods to prepare loose sandstone samples as a substitute for relevant experimental methods in the target block is explored. This literature mainly proposes a design approach to improve the quality of sample preparation for loose sandstone strata in the Bohai Sea area. However, it does not address the differences in strata in other regions, particularly the vertical lithological heterogeneity and interlayer interface development characteristics of lacustrine shale reservoirs, thus failing to provide a targeted design for the preparation of artificial lacustrine shale samples.

[0007] The above analysis shows that the existing method for preparing concrete rock specimens for hydraulic fracturing experiments of marine shale is not suitable for lacustrine shale. This is because the existing method for preparing concrete rock specimens for hydraulic fracturing experiments of marine shale involves integrated casting, resulting in consistent overall lithology and mechanical properties of the rock specimens. Furthermore, there are no interlayer interfaces within the specimens. Therefore, this method cannot accurately characterize the vertical lithological heterogeneity and interlayer interface development characteristics of lacustrine shale reservoirs.

[0008] In summary, there is an urgent need to invent an artificial lacustrine shale sample for hydraulic fracturing experiments and its preparation method, so as to meet the needs of hydraulic fracturing experimental research on lacustrine shale and provide a strong guarantee for revealing the cross-layer propagation law of hydraulic fractures in lacustrine shale. Summary of the Invention

[0009] The purpose of this invention is to solve the problems existing in the prior art and provide an artificial lacustrine shale sample for hydraulic fracturing experiments and its preparation method, which can effectively simulate the structure and properties of natural lacustrine shale rock samples and meet the various requirements of hydraulic fracturing experiments for the shape of rock samples.

[0010] This invention is achieved through the following technical solution:

[0011] In a first aspect, the present invention provides an artificial lacustrine shale specimen for hydraulic fracturing experiments, the artificial lacustrine shale specimen being formed by layered concrete slurry casting, comprising multiple concrete layers from bottom to top.

[0012] The concrete grout mix ratios for different layers may be the same or different;

[0013] No simulation layer or a simulation layer is set between two adjacent concrete layers.

[0014] A further improvement of the present invention is that:

[0015] The simulation layer is either a weakly cemented simulation layer or a medium-cemented simulation layer.

[0016] A further improvement of the present invention is that:

[0017] If the interlayer bonding strength between two adjacent concrete layers is weak, then a weak bonding simulation layer is laid between the two concrete layers.

[0018] If the interlayer bonding strength between two adjacent concrete layers is medium, then a medium-bonding simulation layer is laid between the two concrete layers.

[0019] If the interlayer bonding strength between two adjacent concrete layers is strong, then no layer is laid between the two concrete layers.

[0020] Preferably, the weak bonding simulation layer comprises one or more A4 sheets of paper;

[0021] The medium-bonded simulated layer comprises one or more layers of Xuan paper.

[0022] Preferably, the simulated layer is laid in the middle of the interlayer interface of the concrete layer.

[0023] In a second aspect, the present invention provides a method for fabricating artificial lacustrine shale specimens for hydraulic fracturing experiments. The method involves pouring concrete slurry with different proportions in layers from bottom to top to create concrete layers with different Young's moduli and fracture toughness. During the layered pouring process, a simulated layer is laid between adjacent concrete layers or not, depending on the interlayer interface strength of the actual reservoir.

[0024] A further improvement of the present invention is that:

[0025] The method includes:

[0026] (1) Obtain the number and thickness of rock layers, mechanical parameters and experimental parameters of each rock layer in the artificial lacustrine shale specimen; at the same time, determine the interlayer bonding strength between adjacent rock layers;

[0027] (2) Select and assemble the mold:

[0028] (3) Prepare concrete grout for pouring each concrete layer separately;

[0029] (4) Cast in layers and lay the simulation layer on the interface between the layers where the simulation layer needs to be laid;

[0030] (5) After the concrete slurry has completely solidified, remove the preparation mold to obtain the rock sample;

[0031] (6) Drill holes in the rock sample and bury simulated wells.

[0032] A further improvement of the present invention is that:

[0033] The operation of determining the interlayer bonding strength between adjacent rock layers in step (1) includes:

[0034] The interlayer bonding strength between adjacent rock layers is determined by observing outcrop rock layers and downhole rock cores.

[0035] A further improvement of the present invention is that:

[0036] The operation of step (4) includes:

[0037] According to the designed thickness of each rock layer, concrete slurry corresponding to each rock layer is poured in layers, and a simulation layer is laid on the interface between the layers where the simulation layer needs to be laid.

[0038] A further improvement of the present invention is that:

[0039] The operation of laying the simulation layer on the interlayer interface where the simulation layer needs to be laid includes:

[0040] If the interlayer bonding strength between two adjacent concrete layers is weak, then after the concrete layer has solidified, a weak bonding simulation layer should be laid on the concrete layer, and then another concrete layer should be poured.

[0041] If the interlayer bonding strength between two adjacent concrete layers is medium, then after the concrete layer has solidified, a medium bonding simulation layer is laid on the concrete layer, and then another concrete layer is poured.

[0042] If the interlayer bonding strength between two adjacent concrete layers is strong, then after the concrete layer has solidified, the next concrete layer can be poured directly on top of that concrete layer.

[0043] A further improvement of the present invention is that:

[0044] The drilling operation in step (6) on the rock sample includes:

[0045] A hole was drilled at the center of the concrete layer in the middle of the rock sample, in a direction parallel to the interlayer interface, to obtain a simulated wellbore.

[0046] A further improvement of the present invention is that:

[0047] The simulated wellbore is a cylindrical structure with one open end and the other closed end.

[0048] Multiple simulated perforations are evenly distributed around the circumference on the wall of the simulated wellbore, and the simulated perforations are located 20 mm above the closed end.

[0049] An annular isolation strip is provided on the outer wall of the simulated wellbore, and the annular isolation strip is located above the simulated perforation hole.

[0050] A further improvement of the present invention is that:

[0051] The operation of burying the simulated wellbore in step (6) includes:

[0052] The simulated wellbore is suspended in the simulated wellbore;

[0053] Injecting adhesive into the annulus between the simulated wellbore and the simulated borehole achieves solid sealing of the simulated wellbore;

[0054] After the adhesive has completely solidified, seal the opening end of the simulated well.

[0055] Compared with the prior art, the beneficial effects of the present invention are:

[0056] This invention enables effective simulation of the structure and properties of natural lacustrine shale samples, meeting various requirements for the shape of rock samples in hydraulic fracturing experiments, thereby improving the accuracy of fracturing experiments. Moreover, the principle of this invention is simple, clear, highly operable, and easy to mass-produce. Attached Figure Description

[0057] Figure 1 A schematic diagram of an artificial lacustrine shale specimen used in a hydraulic fracturing experiment;

[0058] Figure 2(a) shows the actual fracture propagation morphology after hydraulic fracturing of a lacustrine shale outcrop sample;

[0059] Figure 2(b) shows the actual fracture propagation morphology with a larger cut area after hydraulic fracturing of a lacustrine shale outcrop sample;

[0060] Figure 3 Reconstructed image of hydraulic fracture propagation morphology in a lacustrine shale outcrop sample;

[0061] Figure 4(a) shows the actual crack propagation morphology of the artificial lacustrine shale specimen after hydraulic fracturing experiment according to the present invention;

[0062] Figure 4(b) shows the actual crack propagation morphology with a larger cut-out range after hydraulic fracturing experiment on the artificial lacustrine shale specimen of the present invention;

[0063] Figure 5 This is a reconstruction diagram of the hydraulic crack propagation morphology of the artificial lacustrine shale specimen of the present invention. Detailed Implementation

[0064] The present invention will now be described in further detail with reference to the accompanying drawings:

[0065] To address the technical challenge of obtaining large-size outcrop samples of lacustrine shale in laboratory hydraulic fracturing experiments, this invention provides an artificial lacustrine shale specimen for hydraulic fracturing experiments and its fabrication method. This artificial rock specimen can accurately reflect the longitudinal lithological heterogeneity and interlayer interface development characteristics of lacustrine shale reservoirs.

[0066] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0067] This invention provides an artificial lacustrine shale specimen for hydraulic fracturing experiments. The artificial lacustrine shale specimen is formed by layered pouring of concrete slurry, comprising multiple concrete layers from bottom to top. The concrete slurry mix proportions of different layers may be the same or different, and a simulation layer may be present or absent between adjacent concrete layers. The simulation layer is a weakly cemented simulation layer or a medium-cemented simulation layer.

[0068] The concrete slurry is made of a mixture of cement and quartz sand.

[0069] The following are examples of artificial lacustrine shale samples used in hydraulic fracturing experiments according to the present invention:

[0070] Example 1

[0071] To accurately reflect the vertical lithological heterogeneity of lacustrine shale reservoirs, concrete slurries of different proportions of cement and quartz sand were poured in layers to create concrete layers with different Young's moduli and fracture toughness. These layers were used to characterize real rock strata of different lithologies, and the interlayer interface strengths between the concrete layers were either the same or different.

[0072] In addition, during the layered casting process, based on the interlayer interface development characteristics of the real reservoir, the following three methods are used to equivalently simulate the weak cementation, medium cementation, and strong cementation states of the interlayer interface: ① laying A4 paper between the rock layers of the artificial rock sample; ② laying Xuan paper between the rock layers of the artificial rock sample; ③ not laying any material between the rock layers of the artificial rock sample.

[0073] Specifically, if the interlayer bonding strength between two adjacent concrete layers is weak, a weak bonding simulation layer is laid between the two concrete layers.

[0074] If the interlayer bonding strength between two adjacent concrete layers is medium, then a medium-bonding simulation layer is laid between the two concrete layers.

[0075] If the interlayer bonding strength between two adjacent concrete layers is strong, then no layer is laid between the two concrete layers.

[0076] Preferably, the weak bonding simulation layer can be made of materials such as A4 paper;

[0077] The medium-bonded simulated layer can be made of materials such as Xuan paper.

[0078]

Example 2

[0079] Different paper thicknesses can characterize different interlayer interface strengths. The thicker the paper used, the weaker the interlayer interface strength. Multiple sheets of Xuan paper or multiple A4 paper can be used to further reduce the interlayer interface strength.

[0080] Furthermore, both the weak-bonding simulation layer and the medium-bonding simulation layer are laid in the middle of the interlayer interface of the concrete layer, i.e., centered. The area of ​​the weak-bonding simulation layer and the medium-bonding simulation layer is set according to actual needs. Preferably, the area of ​​the weak-bonding simulation layer and the medium-bonding simulation layer is 1 / 4 of the interlayer interface area.

[0081] The present invention also provides a method for preparing artificial lacustrine shale specimens for hydraulic fracturing experiments. The method involves pouring concrete slurry with different proportions in layers from bottom to top to create concrete layers with different Young's moduli and fracture toughness, which are used to characterize real rock strata of different lithologies. The interlayer interface strength between the concrete layers may be the same or different. At the same time, during the layered pouring process, a simulated layer may or may not be laid between adjacent concrete layers according to the interlayer interface strength of the real reservoir.

[0082] An example of this method is as follows:

[0083]

Example 3

[0084] (1) Obtain the number and thickness of rock layers, mechanical parameters and experimental parameters of each rock layer in the artificial lacustrine shale specimen; at the same time, determine the interlayer bonding strength between adjacent rock layers.

[0085] Based on the lithological development characteristics of the real lacustrine shale reservoirs in the designated study area and the on-site hydraulic fracturing construction parameters, the classical hydraulic fracturing experimental similarity criteria were used (see: De Pater CJ, Cleary MP, Quinn TS, et al. Experimental verification of dimensional analysis for hydraulic fracturing[J]. SPE Production & Facilities, 1994, 9(04): 230-238. Fu Haifeng, Wang Zhen, Xu Yun, Liu Yunzhi, Xiu Nailing, Yan Yuzhong, Guan Baoshan. Full three-dimensional simulation study of vertical extension of hydraulic fractures[C]. Proceedings of the National Natural Gas Academic Conference (04 Engineering Technology), 2018:1-9.) to calculate the number and thickness of rock layers to be cast for the artificial lacustrine shale specimen, the mechanical parameters of each rock layer (including Young's modulus, Poisson's ratio and fracture toughness value), and the experimental parameters (including injection rate, fluid viscosity, wellbore diameter, construction time and confining pressure, etc.).

[0086] At the same time, the strength of cementation at the interlayer interface is determined by observing the outcrop strata and the downhole core, that is, whether the cementation strength at the interlayer interface between each layer is weak, medium or strong.

[0087] (2) Select and assemble the mold:

[0088] Based on the design dimensions of the artificial lacustrine shale specimen, select a suitable preparation mold and complete its assembly. Various existing preparation molds for preparing rock samples can be used, which will not be elaborated here.

[0089] (3) Prepare concrete grout for pouring each concrete layer separately;

[0090] Based on the mechanical parameters of each rock layer in the cast specimens and the reference table of concrete mix proportions, concrete slurries with different proportions of cement and quartz sand were prepared.

[0091] Based on similarity criteria and actual stratum parameters, the mechanical parameters of each rock layer in the cast specimen were calculated. Then, according to the concrete mix design reference table, concrete slurry with different proportions of cement and quartz sand was prepared. The concrete mix design reference table was obtained based on experimental tests and experience, such as Table 2 given in Example 4 below. These were all achieved using existing methods and will not be described in detail here.

[0092] (4) Cast in layers and lay the simulation layer on the interface between the layers where the simulation layer needs to be laid:

[0093] According to the designed thickness of each rock layer, concrete slurry corresponding to each rock layer is poured in layers, and a simulated layer is laid on the interlayer interface where a simulated layer needs to be laid to simulate the interlayer interface with different bonding strengths.

[0094] Specifically, if the interlayer bonding strength between two adjacent concrete layers is weak, then after the concrete layer has solidified, a weak bonding simulation layer is laid in the middle of the interface of the concrete layer, and then the next concrete layer is poured.

[0095] If the interlayer bonding strength between two adjacent concrete layers is medium, then after the concrete layer has solidified, a medium bonding simulation layer is laid in the middle of the interface of the concrete layer, and then the next concrete layer is poured.

[0096] If the interlayer bonding strength between two adjacent concrete layers is strong, then no layer is laid between the two concrete layers. That is, after the concrete layer has solidified, the next concrete layer is poured directly on top of the previous concrete layer.

[0097] (5) After the pouring is completed, let it stand for a few days until the concrete slurry has completely solidified, then remove the preparation mold and obtain the rock sample;

[0098] (6) Drill holes in the rock sample and bury simulated wellbore 10, seal with glue to cement the well, and perform subsequent maintenance to complete the sample preparation.

[0099] Depending on the actual needs, holes are drilled in the rock sample to obtain simulated wellbore holes. Generally, holes are drilled in the center of the concrete layer in the middle of the rock sample, along a direction parallel to the interlayer interface, to obtain simulated wellbore holes. Then, simulated wellbore 10 is placed in the simulated wellbore holes, and then the simulated wellbore 10 is sealed with glue. After that, post-construction curing is carried out to complete the sample preparation.

[0100] An application example of the method of the present invention is as follows:

[0101]

Example 4

[0102] Taking the target reservoir of a lacustrine shale gas well in the Sichuan Basin as an example, this reservoir mainly consists of five strata. From bottom to top, the lithologies of these strata are: mudstone, shale, limestone, mudstone, siltstone, and mudstone, with thicknesses of 4m, 4m, 8m, 4m, and 4m respectively. To clearly describe the positional relationships between the strata of different lithologies, they are referred to as "Layer 1," "Layer 2," "Layer 3," "Layer 4," and "Layer 5" from bottom to top. Observation of outcrops and downhole cores reveals that the interlayer interfaces between Layer 2 and Layer 3, and between Layer 4 and Layer 5, are weakly cemented, while the remaining interlayer interfaces exhibit strong cementation.

[0103] The specific method for preparing artificial lacustrine shale specimens for hydraulic fracturing experiments, which have similar characteristics to the lacustrine shale reservoirs described above, is as follows:

[0104] (1) Determine the number, thickness, and rock mechanical parameters of the artificial lacustrine shale specimens.

[0105] Based on the requirements of the selected true triaxial hydraulic fracturing physical simulation experimental equipment, this embodiment requires the preparation of square artificial lacustrine shale specimens with dimensions of 30cm×30cm×30cm. According to the aforementioned lithological development characteristics of the actual reservoir, the prepared artificial rock sample is divided into 5 layers. Based on the principle of proportional reduction, the thicknesses of the first to fifth layers are set to 5cm, 5cm, 10cm, 5cm, and 5cm, respectively.

[0106] According to the similarity criteria of hydraulic fracturing experiments (see: De Pater CJ, Cleary MP, Quinn TS, et al. Experimental verification of dimensional analysis for hydraulic fracturing[J]. SPE Production & Facilities, 1994, 9(04): 230-238. Fu Haifeng, Wang Zhen, Xu Yun, Liu Yunzhi, Xiu Nailing, Yan Yuzhong, Guan Baoshan. Full three-dimensional simulation study of vertical extension of hydraulic fracture[C]. Proceedings of the National Natural Gas Academic Conference (04 Engineering Technology), 2018:1-9.), the three dimensionless indices in formulas (1) to (3) corresponding to the experimental parameters should be basically consistent with the three dimensionless indices in formulas (1) to (3) corresponding to the field construction parameters, and the dimensionless index κ of the hydraulic fracture extension state in formula (4) under the two parameter conditions should be in the same range, that is, in any of the following ranges: κ≤1, 1<κ<4 or κ≥4.

[0107] Based on the above criteria and on-site construction parameters, rock mechanics parameters and other experimental parameters of each set of artificial rock layers were designed (the experimental parameters were obtained by using the on-site construction parameters according to the above similarity criteria), as shown in Table 1. The crack propagation state index in Table 1 is the dimensionless index κ of hydraulic crack propagation state in the formula (4) below.

[0108]

[0109] Table 1

[0110] (1)

[0111] (2)

[0112] (3)

[0113] (4)

[0114] In the formula:

[0115] The dimensionless exponent of fracture toughness is dimensionless.

[0116] The dimensionless exponent of the elastic modulus is dimensionless.

[0117] is the dimensionless exponent of geostress, and is dimensionless;

[0118] κ The dimensionless exponent represents the hydraulic fracture propagation state; it is dimensionless.

[0119] v It is Poisson's ratio and has no dimensions;

[0120] K IC For fracture toughness, MPa•m 0.5 ;

[0121] E Young's modulus, GPa;

[0122] r w Let be the radius of the wellbore, in meters (m).

[0123] μ The viscosity of the fracturing fluid is mPa•s;

[0124] Q The fracturing fluid injection rate, m 3 / min;

[0125] σ cFor confining pressure, the minimum horizontal ground stress is generally taken as MPa;

[0126] t The time is the construction time, in minutes.

[0127] The above formulas are all existing formulas and will not be repeated here.

[0128] (2) Select a suitable sample mold and complete the assembly.

[0129] A cubic mold with internal dimensions of 30cm × 30cm × 30cm was assembled using steel plates with a wall thickness of 2cm. This existing mold includes a base plate and four cover plates perpendicular to the base plate, but no top cover, and is used to pour concrete grout into the mold. Before pouring the concrete grout into the mold, a layer of grease was evenly applied to the inner surface of the mold to ensure that the rock sample could be easily separated from the base and cover plate walls when disassembling the mold after casting.

[0130] (3) Prepare concrete grout and complete the layered pouring of rock samples.

[0131] Prepare sufficient quartz sand and cement, and prepare the concrete slurry for each layer according to the concrete slurry preparation ratio requirements in Table 2, and carry out the layered pouring work.

[0132] The existing methods can be used to prepare the concrete grout, so they will not be elaborated here.

[0133]

[0134] Table 2

[0135] The specific steps for layered pouring are as follows:

[0136] S1, First layer pouring

[0137] Slowly pour the prepared Type III concrete slurry into the mold until it reaches a height of 5cm. Place the entire mold on a vibrating table and turn on the vibrator to fully expel the air from the concrete slurry. Remove the mold and let it stand for a period of time to obtain the first layer 1.

[0138] S2, Second layer pouring

[0139] After the first layer of concrete slurry has basically solidified, continue to slowly inject the prepared Type I concrete slurry into the mold until it reaches a height of 10cm. Place the mold on a vibration table and vibrate it to fully expel the gas in the concrete slurry. Remove the mold and let it stand for a period of time to obtain the second layer 2. The interface between the first layer 1 and the second layer 2 is the first rock sample interlayer interface 6.

[0140] S3, Third Layer Pouring

[0141] After the second layer of concrete slurry has basically solidified, A4 paper 11 is laid on its interface (i.e., the interlayer interface between the second and third layers) to simulate the weak cementation interlayer interface (if it is of medium cementation strength, Xuan paper is laid). Then, the prepared Type III concrete slurry is slowly injected into the mold until it reaches a height of 20cm. The mold is then placed on a vibration table and vibrated fully. The mold is removed and left to stand for a period of time to obtain the third layer 3. The interface between the third layer 3 and the second layer 2 is the second rock sample interlayer interface 7.

[0142] The specimens of this invention are used to simulate rock samples with natural cracks in actual strata. However, most natural cracks do not penetrate the entire stratum. Therefore, the simulation layer does not need to cover the entire interlayer interface. Moreover, if the paper covers the entire interlayer interface, the strength of the cast rock sample will be poor, making it difficult to ensure the integrity of the rock sample, which in turn makes the experiment impossible to carry out.

[0143] S4, Fourth Layer Pouring

[0144] After the third layer of concrete slurry has basically solidified, continue to slowly inject the prepared Type II concrete slurry into the mold until it reaches a height of 25cm. Place the mold on a vibration table and vibrate it fully. Remove the mold and let it stand for a period of time to obtain the fourth layer 4. The interface between the fourth layer 4 and the third layer 3 is the third rock sample interlayer interface 8.

[0145] S5, Fifth Layer Pouring

[0146] After the fourth layer of concrete slurry has basically solidified, A4 paper 11 is laid on its interface to simulate the weak cementation interlayer interface (if it is of medium cementation strength, Xuan paper is laid). Then, the prepared Type III concrete slurry is slowly injected into the mold to a height of 30cm. The mold is placed on a vibration table and vibrated fully. The mold is removed and left to stand for a period of time to obtain the fifth layer 5. The interface between the fifth layer 5 and the fourth layer 4 is the fourth rock sample interlayer interface 9.

[0147] (4) Allow the mold to stand still until the rock sample has completely solidified, then remove the mold:

[0148] The poured mold is placed in a curing box to allow the concrete slurry to fully solidify. Once a certain strength is reached, the mold is removed to obtain an artificial rock sample.

[0149] (5) Drill holes in the rock sample and bury the well pipe 10, seal the well with glue, and perform subsequent maintenance to complete the sample preparation.

[0150] like Figure 1As shown, a 28mm diameter drill bit is used to drill a hole in the center of the third artificially cast rock layer, parallel to the interlayer interface, to obtain a simulated wellbore. Then, anchoring adhesive 12 is used to seal the simulated wellbore 10 to the drilled simulated wellbore. The anchoring adhesive is a liquid. During sealing, the simulated wellbore 10 is first suspended inside the simulated wellbore (the length and diameter of the simulated wellbore are slightly larger than the length and diameter of the simulated wellbore 10; during sealing, the open end of the simulated wellbore 10 is held by hand or with a tool, and its closed end is inserted into the simulated wellbore, keeping the simulated wellbore 10 suspended). Then, anchoring adhesive is injected into the annulus between the simulated wellbore 10 and the simulated wellbore, thus sealing the simulated wellbore 10. The anchoring adhesive 12 can be YD-800 type epoxy resin anchoring adhesive.

[0151] The simulated wellbore 10 described in this embodiment has the following specifications: outer diameter 20mm, inner diameter 10mm, and length 200mm. The bottom of the simulated wellbore 10 is closed, i.e., one end is an open end and the other end is a closed end. Four small circular holes with a diameter of 2mm are drilled 20mm away from its closed end, which are evenly distributed on the circumference to simulate perforation holes. The open end of the simulated wellbore 10 on the surface of the specimen is flat and smooth, and can be sealed and connected to the fracturing fluid pump injection pipeline.

[0152] Furthermore, during the cementing process, to prevent the adhesive from flowing into the lower part and causing blockage of the simulated perforation, an annular isolation strip 13 is provided on the outer wall of the simulated wellbore 10 above the perforation. In this embodiment, the annular isolation strip is made of multiple layers of electrical adhesive tape, which fills the annulus, thus preventing the adhesive from flowing into the lower part of the annular isolation strip. In other words, the annular isolation strip prevents the adhesive from flowing into the simulated perforation.

[0153] After the anchoring adhesive 12 has completely solidified, seal the opening end of the simulated well barrel 10. For example, you can use waste newspaper to plug the well opening of the simulated well barrel 10. This can prevent foreign objects from falling in and clogging the simulated well barrel 10. Then carry out daily watering and maintenance to prevent the rock sample from cracking.

[0154] This completes the sample preparation process for the artificial lacustrine shale specimens. Finally, the cured rock samples are completely wrapped in plastic wrap for use in experiments.

[0155] Figure 2(a) to Figure 3 Compared with Figure 4(a) Figure 5 The figures show the results of hydraulic fracturing experiments conducted on lacustrine shale outcrop samples and artificial lacustrine shale specimens prepared using the method of this invention, under the same experimental conditions. The rock samples in Figure 2(a) and Figure 2(b) are dissected to different degrees, showing different fracture surfaces. Similarly, the rock samples in Figure 4(a) and Figure 4(b) are dissected to different degrees, showing different fracture surfaces.

[0156] Comparison reveals that, under the same experimental conditions, the hydraulic fracture propagation morphology across layers in lacustrine shale outcrop samples and artificial lacustrine rock samples is basically the same, as detailed below:

[0157] contrast Figure 3 and Figure 5 It can be seen that, under the same experimental conditions, lacustrine shale outcrop samples (such as...) Figure 3 (as shown) and artificial lacustrine rock samples (such as) Figure 5 As shown, the hydraulic fracture first extends longitudinally for a certain distance, forming... Figure 5 The main hydraulic fracture surface 14 in the middle stops longitudinally expanding after encountering the weak cemented interlayer interface, and turns to extend along the weak cemented interlayer interface, forming the activated weak interlayer surface 15.

[0158] In summary, the artificial lacustrine shale specimens prepared using the method of this invention can be used as an equivalent substitute for outcrop lacustrine shale samples in conducting physical simulation studies of hydraulic fracture propagation across layers, providing a useful reference for subsequent experimental studies on physical simulation of hydraulic fracturing in lacustrine shale.

[0159] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0160] In the description of this invention, unless otherwise stated, the terms "upper," "lower," "left," "right," "inner," "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0161] Finally, it should be noted that the above technical solution is only one embodiment of the present invention. For those skilled in the art, based on the application methods and principles disclosed in the present invention, it is easy to make various types of improvements or modifications, and not limited to the methods described in the above specific embodiments of the present invention. Therefore, the methods described above are only preferred and have no limiting significance.

Claims

1. An artificial lacustrine shale specimen for hydraulic fracturing experiments, characterized in that: The artificial lacustrine shale specimen was formed by layered concrete grouting, consisting of multiple concrete layers from bottom to top; the mechanical parameters of each layer were calculated based on similarity criteria and actual stratigraphic parameters. The concrete grout mix ratios for different layers may be the same or different; No simulation layer or a simulation layer is set between two adjacent concrete layers; The simulation layer is a weakly cemented simulation layer or a medium-cemented simulation layer; If the interlayer bonding strength between two adjacent concrete layers is weak, then a weak bonding simulation layer is laid between the two concrete layers. If the interlayer bonding strength between two adjacent concrete layers is medium, then a medium-bonding simulation layer is laid between the two concrete layers. If the interlayer bonding strength between two adjacent concrete layers is strong, then no layer is laid between the two concrete layers. The weak bonding simulation layer comprises one or more A4 papers; The medium-bonded simulated layer comprises one or more layers of Xuan paper; The simulated layer is laid in the middle of the interlayer interface of the solidified concrete layer.

2. A method for fabricating an artificial lacustrine shale specimen for hydraulic fracturing experiments, based on the artificial lacustrine shale specimen for hydraulic fracturing experiments as described in claim 1, characterized in that: The method involves pouring concrete slurry with different proportions in layers from bottom to top to create concrete layers with different Young's modulus and fracture toughness. During the layered pouring process, a simulated layer may or may not be laid between adjacent concrete layers, depending on the interlayer interface strength of the actual reservoir. The method includes: (1) Obtain the number and thickness of rock layers, mechanical parameters and experimental parameters of each rock layer in the artificial lacustrine shale specimen; at the same time, determine the interlayer bonding strength between adjacent rock layers; (2) Select and assemble the mold: (3) Prepare concrete grout for pouring each concrete layer separately; the mechanical parameters of each layer are calculated based on similarity criteria and actual stratum parameters; (4) Cast in layers and lay the simulation layer on the interface between the layers where the simulation layer needs to be laid; (5) After the concrete slurry has completely solidified, remove the preparation mold to obtain the rock sample; (6) Drill holes in the rock sample and bury simulated wellbores; The operation of step (4) includes: according to the design thickness of each rock layer, using the concrete slurry corresponding to each rock layer to pour in layers, and laying the simulation layer on the interlayer interface where the simulation layer needs to be laid; The operation of laying the simulation layer on the interlayer interface where the simulation layer needs to be laid includes: If the interfacial bond strength between two adjacent concrete layers is weak, a weak bond simulation layer is laid on the concrete layer after it has solidified, and then another concrete layer is poured. The weak bond simulation layer includes one or more layers of A4 paper. If the interfacial bond strength between two adjacent concrete layers is medium, a medium bond simulation layer is laid on the concrete layer after it has solidified, and then another concrete layer is poured. The medium bond simulation layer includes one or more layers of Xuan paper. The simulation layer is laid in the middle of the interfacial bond of the concrete layers. If the interfacial bond strength between two adjacent concrete layers is strong, a concrete layer is poured directly on the concrete layer after it has solidified.

3. The method for preparing artificial lacustrine shale specimens for hydraulic fracturing experiments according to claim 2, characterized in that: The operation of determining the interlayer bonding strength between adjacent rock layers in step (1) includes: The interlayer bonding strength between adjacent rock layers is determined by observing outcrop rock layers and downhole rock cores.

4. The method for preparing artificial lacustrine shale specimens for hydraulic fracturing experiments according to claim 2, characterized in that: The drilling operation in step (6) on the rock sample includes: A hole was drilled at the center of the concrete layer in the middle of the rock sample, in a direction parallel to the interlayer interface, to obtain a simulated wellbore.

5. The method for preparing artificial lacustrine shale specimens for hydraulic fracturing experiments according to claim 2, characterized in that: The simulated wellbore is a cylindrical structure with one open end and the other closed end. Multiple simulated perforations are evenly distributed around the circumference on the wall of the simulated wellbore, and the simulated perforations are located 20 mm above the closed end. An annular isolation strip is provided on the outer wall of the simulated wellbore, and the annular isolation strip is located above the simulated perforation hole.

6. The method for preparing artificial lacustrine shale specimens for hydraulic fracturing experiments according to claim 2, characterized in that: The operation of burying the simulated wellbore in step (6) includes: The simulated wellbore is suspended in the simulated wellbore; Injecting adhesive into the annulus between the simulated wellbore and the simulated borehole achieves solid sealing of the simulated wellbore; After the adhesive has completely solidified, seal the opening end of the simulated well.

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