Preparation method of multi-lithology interbed shale crossing fracturing simulation rock sample, rock sample and application

Abstract

The invention provides a preparation method of a multi-lithology interbed shale crossing fracturing simulation rock sample, the rock sample and application, and belongs to the technical field of petroleum exploration and development fracturing engineering. The method comprises the following steps: acquiring a same-layer-series interlayer outcrop sample in a target area; mixing and stirring the base materials to prepare a shale rock plate, and cutting the shale rock plate into required thickness; and placing the shale plate and the interlayer outcrop sample in a mold at intervals to prepare a multi-layer mixed rock sample. The prepared rock sample can simulate target layer shale and interlayer tight sandstone and limestone with the Young modulus within 60 GPa, so that the longitudinal lithology heterogeneity of a shale reservoir is accurately represented, and the similarity and reproducibility are high; a large-size true triaxial cross-layer fracturing crack propagation physical simulation experiment under the condition of layered stress loading is carried out, and the influence of main control factors such as the thickness of the interlayer, the Young modulus ratio of the storage interlayer, the stress difference and fracturing process parameters on longitudinal propagation of the cross-layer fracturing crack can be accurately revealed.

Description

Technical Field

[0001] The invention belongs to the technical field of petroleum exploration and development fracturing engineering, and specifically relates to a method for preparing a multi-lithology interlayer shale fracturing simulation rock sample, a rock sample and an application thereof. Background Art

[0002] Horizontal well volume fracturing technology is one of the core technologies for the success of the shale oil and gas revolution in North America. Drawing on the successful experience of North America, China has developed a series of horizontal well volume fracturing technologies with "multi-stage multi-cluster bridge shot joint operation + variable viscosity slippery water + high-intensity sand fracturing" as the core after more than 10 years of technical research and application. The shale gas production in 2022 is expected to reach 240×10 8 m 3 , shale oil production exceeds 300×10 4 The total proven reserves of shale gas fields in Fuling, Changning, Weiyuan, Weirong, Luzhou and Qijiang in the Sichuan Basin exceed 3×10 12 m 3 The shale oil in the Ordos, Junggar, Bohai Bay, Songliao and northern Jiangsu basins has submitted a forecast reserve of more than 40×10 8 t. Compared with the large-scale development of shale oil and gas in multiple strata in multiple basins in North America, my country's shale oil and gas resources are of low grade, complex structure, rapid production decline, low recovery degree and high development cost. At present, the proven reserves and production of shale gas are mainly concentrated in the single stratum of the Silurian Longmaxi Formation. Although the new stratum shale gas such as the Cambrian Qiongzhusi Formation, the Jurassic Ziliujing Formation / Lianggaoshan Formation / Qianfoya Formation, the Permian Wujiaping Formation / Dalong Formation / Maokou Formation have made continuous exploration breakthroughs recently, showing good prospects for exploration and development, high-quality and efficient exploration and development still face many challenges. The EUR of a single well of continental shale oil is low, and the cost of a barrel of oil is still high. The new stratum marine shale and continental shale have developed dense sandstone, limestone, dolomite and other lithological interlayers vertically, and the thickness of a single continuous high-quality shale is relatively small. The existence of interlayers with different thicknesses and mechanical properties brings great difficulties to increasing fracture height, transformation volume and fracture-controlled reserves through through-layer fracturing. It is necessary to carry out targeted physical simulation and numerical simulation of shale fracture through-layer expansion to determine the feasibility of through-layer fracturing and optimize the design of corresponding fracturing schemes accordingly.

[0003] At present, the simulation research on the shale crack penetration extension morphology under the conditions of different vertical stress coefficients, different displacements, different fracturing fluid viscosities and different strengths is mainly carried out by using a large-scale true triaxial test system. The experimental system consists of an acoustic emission instrument, a true triaxial test frame, a triaxial hydraulic pressure source, an oil-water separator, an MTS booster and controller, a data acquisition and processing system, etc. The experimental parameters are designed based on the similarity criterion. The test rock samples are prepared by cutting the natural outcrop of the target layer, or artificial rock samples prepared by mixing cement, quartz sand, clay and other base materials uniformly and pouring them into a mold. The most commonly used rock sample size is 300mm×300mm×300mm. For example, Chinese patent publication CN115876541A provides an artificial lacustrine shale sample for hydraulic fracturing experiments and a method for making the same. The artificial lacustrine shale sample is cast in layers by concrete slurry, and includes multiple concrete layers from bottom to top; the proportions of concrete slurries in different layers are the same or different; a simulation layer is not provided or is provided between two adjacent concrete layers, thereby achieving effective simulation of the structure and properties of natural rock samples of lacustrine shale, meeting various requirements of hydraulic fracturing experiments on the appearance of rock samples, and thereby improving the accuracy of fracturing experiments. The principle of the invention is concise, clear, highly operational, and easy to mass produce. Chinese patent publication CN108801739A discloses a method for making an artificial physical core, which uses a mixture of quartz, cement, and water reducer to simulate the brittle mineral composition of natural sandstone cores; uses clay materials to simulate the clay mineral composition of natural sandstone cores, and adjusts the proportion of quartz, cement, water reducer, and clay materials to make the artificial physical core consistent with natural sandstone in 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 making artificial sandstone samples by material proportioning, but does not consider how to make shale samples and the longitudinal lithology heterogeneity and interlayer interface development characteristics of lacustrine shale reservoirs.

[0004] However, for the new strata of multi-lithology interlayered shales, due to the large differences in lithology development between layers and the weak degree of cementation at the interface of some rock layers, most of the shale outcrop rock samples exposed on the surface are severely weathered and have poor integrity, making it difficult to cut rock samples that meet the size requirements of hydraulic fracturing experiments. At the same time, due to the influence of heterogeneity, it is also difficult to obtain multiple rock samples with consistent mechanical properties and mineral components. Not only is the sampling cost high, but the reproducibility is also poor. Previous studies and mine practices have shown that the Young's modulus ratio of the reservoir and interlayer, stress difference, interlayer thickness and interface strength are the main controlling factors affecting the effect of through-layer fracturing. Artificial rock sample specimens cast by integrated concrete or layered casting have a low Young's modulus, a large difference in mineral composition, and poor equivalence with shale and interlayers. They cannot accurately characterize the longitudinal lithology heterogeneity and crack expansion law of shale reservoirs, which affects the judgment of the longitudinal expansion of through-layer fracturing cracks and the fracturing effect. Summary of the invention

[0005] The purpose of the present invention is to solve the difficulties existing in the above-mentioned prior art, and to provide a method for preparing a multi-lithology interlayer shale through-layer fracturing simulation rock sample, a rock sample and an application, to carry out targeted design and production of artificial shale rock samples, to improve the similarity and reproducibility of rock sample preparation, to meet the needs of new stratum shale fracturing crack through-layer expansion experiments and numerical simulation research, and to provide technical support for revealing the crack expansion law of through-layer fracturing and optimizing fracturing schemes.

[0006] The present invention is achieved through the following technical solutions:

[0007] The first aspect of the present invention provides a method for preparing a rock sample of multi-lithology interlayer shale through-layer fracturing simulation, obtaining an outcrop sample of the same layer and interlayer in the target area; mixing and stirring the base materials to form a shale slab and cutting it into a desired thickness; placing the shale slab and the interlayer outcrop sample in a mold at intervals to form a multi-layer mixed rock sample.

[0008] A further improvement of the present invention is:

[0009] The preparation method comprises the following steps:

[0010] Step 1, obtaining the main rock mechanical parameters of the target layer;

[0011] Step 2, obtaining natural outcrop samples of interlayers in the same layer system in the target area;

[0012] Step 3, preparing the base material into an artificial shale rock sample according to the concrete mortar method, and taking the core to test the mechanical parameters;

[0013] Step 4: According to the mechanical parameter test results of step 3, adjust the ratio of different base materials, and repeat step 3 until the tested mechanical parameters are consistent with the mechanical parameters obtained in step 1. At this time, the corresponding base material ratio is the base material ratio for preparing the target rock board;

[0014] Step 5, determining the base material ratio according to step 4 to produce a plurality of shale slabs;

[0015] Step 6, placing the shale slab made in step 5 and the interlayer natural outcrop sample obtained in step 2 in a mold, applying vitrified glue between the shale slab and the natural outcrop sample to obtain a multi-layer mixed rock sample.

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

[0017] The main rock mechanical parameters of the target layer in step 1 include: Young's modulus, Poisson's ratio, shear strength, cohesion, internal friction angle, tensile strength, fracture toughness, as well as bedding fractures, laminar fracture density, reservoir thickness, interlayer thickness and interface strength.

[0018] A further improvement of the present invention is:

[0019] In step 3, the base materials include quartz sand, cement, clay, water reducing agent, iron powder, granite powder, ultrafine calcium carbonate, tap water, vitrified glue and super glue;

[0020] Preferably, the iron powder is 200-320 mesh iron powder, and the granite powder is 200-1200 mesh granite powder.

[0021] A further improvement of the present invention is:

[0022] The mechanical parameters tested in step 3 include Young's modulus, tensile strength, shear strength and fracture toughness.

[0023] A further improvement of the present invention is:

[0024] Step 5: Determine the base material ratio according to step 4 to make multiple shale slabs, specifically:

[0025] Quartz sand, cement, clay, water reducing agent, iron powder, granite powder, ultrafine calcium carbonate, tap water, vitrified glue and super glue are mixed and stirred evenly, poured into a mold, and a layer of rice paper is placed at a certain thickness to simulate bedding joints and laminae joints to make shale slabs;

[0026] The produced shale slab is then cut into multiple shale slabs of required thickness along the bedding direction.

[0027] A further improvement of the present invention is:

[0028] Step 6, placing the shale slab made in step 5 and the interlayer natural outcrop sample obtained in step 2 in a mold, applying vitrified glue between the shale slab and the natural outcrop sample to obtain a multi-layer mixed rock sample, and the specific operation is as follows:

[0029] According to the actual thickness requirements, the shale slab produced in step 5 is first placed at the bottom of the mold, and the top is coated with vitrified glue. Then the natural outcrop sample of the interlayer obtained in step 2 is placed and the top is coated with vitrified glue. Repeat the above operations, and place the shale slab and the interlayer outcrop sample in the mold in turn. Wait for the slabs to be bonded for 48 hours to finally make a multi-layer mixed rock sample composed of an artificial shale slab and a natural outcrop sample of an interlayer.

[0030] A second aspect of the present invention provides a rock sample prepared by the above-mentioned method for preparing a multi-lithology interlayer shale cross-layer fracturing simulation rock sample.

[0031] The third aspect of the present invention provides a method for simulating a cross-layer fracturing experiment of multi-lithology interlayer shale, wherein a cross-layer fracturing simulation experiment is performed using a multi-layer mixed rock sample prepared by the above preparation method, and specifically comprises the following steps:

[0032] Step 11, obtaining the maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress parameters of the target layer;

[0033] Step 12, drilling a countersink in the middle shale layer of the mixed rock sample along the direction parallel to the bedding, solidifying the simulated wellbore in the countersink with resin glue, and reserving a certain distance below as an open hole section;

[0034] Step 14, simulating fracture propagation according to the fracturing fluid viscosity, pumping displacement and pumping program determined by the true triaxial fracturing experiment plan, and studying and analyzing the experimental results;

[0035] Step 15, preparing mixed rock samples of interlayered shales of different lithologies, and conducting a layer penetration and expansion simulation test of the mixed rock samples under different stress parameters and pumping parameters;

[0036] Step 16: According to the results of the interlayer expansion simulation test in step 15, a digital core physical model is established, and a fitting inversion is performed with the physical simulation fracture expansion morphology to establish a mathematical model for interlayer expansion of multi-lithology interlayer shale fractures;

[0037] Step 17: Perform numerical simulation of different fracturing parameters and simulation optimization of fracturing schemes according to the mathematical model established in step 16 to determine the fracturing scheme.

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

[0039] Step 13: Using the large-scale true triaxial test system to load the mixed rock sample with geostress data obtained in step 11, the specific method is as follows:

[0040] The three-dimensional stress of the shale layer is loaded according to 40% of the maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress parameters of the target layer obtained in step 11. The minimum horizontal stress loading value S hb Calculate according to formula (1):

[0041] s hb =s hbi -0.6×s hsi -2 / π(s hbi -s hsi )(cos -1 (hs / h s +h b ))-cos -1 (h si / (h si +h bi ))) (1)

[0042] Maximum horizontal principal stress loading value S Hb Calculate according to formula (2):

[0043] s Hb=s hb +s Hbi -s hbi (2)

[0044] Where: S hbi , S Hbi Respectively represent the minimum horizontal principal stress and maximum horizontal principal stress of the target interlayer, MPa; S hsi , S hs Indicates the minimum horizontal principal stress of the target shale layer and the rock sample shale layer, MPa; h si 、h bi Respectively represent the thickness of the target shale layer and interlayer, m:h s 、h b Respectively represent the thickness of the shale layer and interlayer of the rock sample, m.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] The present invention proposes a method for preparing a multi-lithology interlayer shale cross-layer fracturing simulation rock sample, a rock sample and an application thereof. The rock plate produced by adding different proportions of quartz, cement, water reducer, clay, 200-320 mesh iron powder, 200-1200 mesh granite powder, ultrafine calcium carbonate, tap water, vitrified glue and super glue can simulate the target layer shale and interlayer dense sandstone and limestone with a Young's modulus of less than 60 GPa, thereby accurately characterizing the vertical lithology heterogeneity of the shale reservoir with strong similarity and reproducibility, and solves the problem that the existing rock samples produced only by quartz, cement, water reducer and clay base materials have low Young's modulus, strong plasticity and poor equivalence. For the prepared multi-lithology interlayer mixed rock samples, large-scale true triaxial through-layer fracturing crack expansion physical simulation experiments and numerical simulation optimization under layered stress loading conditions were carried out, which can accurately reveal the influence of main controlling factors such as interlayer thickness, reservoir-interlayer Young's modulus ratio, stress difference and fracturing process parameters on the longitudinal expansion of through-layer fracturing cracks, and provide a scientific basis for optimizing shale oil and gas penetration positions, optimizing fracturing schemes, and improving fracturing transformation volume and fracturing effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 It is a schematic diagram of a mixed rock sample of multi-lithology interlayer shale through-layer fracturing simulation;

[0048] Figure 2 This is another schematic diagram of a mixed rock sample simulated by cross-layer fracturing of multi-lithology interbedded shale.

[0049] In the figure, ① is a shale plate with a thickness of 50 mm; ② is a natural outcrop sample with interlayers with a thickness of 50 mm; ③ is a shale plate with a thickness of 100 mm; ④ is a natural outcrop sample with interlayers with a thickness of 50 mm; ⑤ is a shale plate with a thickness of 50 mm; ⑥ is a shale plate with a thickness of 80 mm; ⑦ is a natural outcrop sample with interlayers with a thickness of 20 mm; ⑧ is a shale plate with a thickness of 100 mm; ⑨ is a natural outcrop sample with interlayers with a thickness of 20 mm; ⑩ is a shale plate with a thickness of 80 mm. DETAILED DESCRIPTION

[0050] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0051] [Example 1]

[0052] This embodiment provides a method for preparing a rock sample for shale fracturing simulation of multi-lithology interlayers, comprising the following steps:

[0053] Step 1: Obtain the main rock mechanical parameters of the target layer, including Young's modulus, Poisson's ratio, shear strength, cohesion, internal friction angle, tensile strength, fracture toughness, maximum horizontal principal stress, minimum horizontal principal stress, vertical principal stress, bedding fractures, laminar fracture density, reservoir thickness, interlayer thickness and interface strength, etc.;

[0054] Step 2: Obtain natural outcrops of interlayers in the same layer system in the target area, cut them into multiple samples with a length and width of 300 mm × 300 mm and a thickness of 10 to 50 mm, and the thickness sampling direction is in the vertical direction;

[0055] Step 3: prepare sufficient quartz sand, cement, clay, water reducing agent, 200-320 mesh iron powder, 200-1200 mesh granite powder, ultrafine calcium carbonate, tap water, vitrified glue and super glue and other base materials, and configure 100mm×100mm×100mm artificial shale rock samples in a certain proportion according to the existing concrete mortar configuration method. The bedding joints and laminar joints are simulated by the existing rice paper simulation method, which will not be repeated here;

[0056] Step 4, drilling a core with a diameter of 25.4 mm and a length of 40-50 mm from the artificial shale rock sample prepared in step 3, and testing its rock mechanical parameters, mainly including Young's modulus, tensile strength, shear strength and fracture toughness;

[0057] Step 5: According to the rock mechanics parameter test results of step 4, adjust the proportion of different base materials, and repeat steps 3 and 4 until the rock mechanics parameters such as Young's modulus, tensile strength, shear strength and fracture toughness are consistent with those of the target stratum shale, and the deviation is no more than 5%;

[0058] By repeating steps 3 and 4 continuously, the ratio of base materials is adjusted. When the rock mechanical parameters such as Young's modulus, tensile strength, shear strength and fracture toughness of the artificial shale sample are consistent with those of the target stratum shale (that is, the data obtained in step 1), the corresponding base material ratio is the base material ratio for preparing the target rock plate;

[0059] Step 6: According to the base material ratio determined in step 5, evenly mix the base materials such as quartz sand, cement, clay, water reducing agent, 200-320 mesh iron powder, 200-1200 mesh granite powder, ultrafine calcium carbonate, tap water, vitrified glue and super glue, and pour them into a mold with a size of 300mm×300mm×300mm. Place a layer of 300mm×300mm rice paper every 10mm to simulate bedding joints and laminae joints. Use a rubber hammer to gently hammer the mold on both sides to remove bubbles, place it under suitable humidity, and wait for the rock slab to solidify and form.

[0060] Step 7, cutting the shale slab produced in step 6 into a plurality of shale slabs of different thicknesses along the bedding direction, for example, three shale slabs of thicknesses of 100 mm, 50 mm, and 50 mm;

[0061] Step 8, according to the actual thickness requirement, first place the shale slab made in step 5 at the bottom of the mold, apply vitrified glue on the top, then place the interlayer natural outcrop sample with a thickness of 10 to 50 mm obtained in step 2, apply vitrified glue on the top, repeat the above operation, place the shale slab and interlayer outcrop sample in the mold in turn, wait for the slabs to be bonded for 48 hours, and finally make a multi-layer mixed rock sample composed of an artificial shale slab-interlayer natural outcrop sample;

[0062] Take three shale slabs with thickness of 100, 50 and 50 mm as examples, specifically:

[0063] First, place a 50 mm shale plate at the bottom of the mold, apply a certain amount of vitrified glue on the top, then place the interlayer outcrop sample with a thickness of 10 to 50 mm prepared in step 2, and also apply a certain amount of vitrified glue on the top, repeat the above operation, and sequentially place a 100 mm shale plate, an interlayer outcrop sample with a thickness of 10 to 50 mm, and a 50 mm shale plate in the mold, wait for the slabs to be bonded for 48 hours, and finally make a 5-layer mixed rock sample with a size of 300 mm × 300 mm × 300 mm, which is composed of artificial shale-interlayer natural outcrop-artificial shale-interlayer natural outcrop-artificial shale;

[0064] Step 9: Repeat steps 6 to 8 to adjust the thickness of different rock plates to produce 3 to 5 layers of mixed rock samples with different rock thickness combinations, such as Figure 1 and Figure 2As shown, there are two mixed rock samples with different lithology thickness combinations prepared.

[0065] [Example 2]

[0066] This embodiment provides a multi-lithology interlayer shale cross-layer fracturing simulation experiment method, using the multi-layer mixed rock sample prepared in Example 1 to perform a cross-layer fracturing simulation experiment, specifically including the following steps:

[0067] Step 11, obtaining the maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress parameters of the target layer;

[0068] Step 12, drilling a 150 mm long countersunk hole in the middle shale layer of the mixed rock sample along the bedding direction parallel to the bedding direction, and solidifying a 120 mm long simulated wellbore in the countersunk hole with resin glue, leaving 30 mm below as an open hole section;

[0069] Step 13, using a large-scale true triaxial test system to load the mixed rock sample with in-situ stress in layers according to the in-situ stress data obtained in step 11;

[0070] Specifically:

[0071] The three-dimensional stress of the shale layer is loaded according to 40% of the maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress parameters of the target layer obtained in step 11. The minimum horizontal stress loading value S hb According to formula (1),

[0072] s hb =s hbi -0.6×s hsi -2 / π(s hbi -s hsi )(cos -1 (h s / h s +h b ))-cos -1 (h si / (h si +h bi ))) (1)

[0073] Maximum horizontal principal stress loading value S Hb Calculate according to formula (2):

[0074] s Hb =s hb +s Hbi -s hbi (2)

[0075] Where: S hbi , S HbiRespectively represent the minimum horizontal principal stress and maximum horizontal principal stress of the target interlayer, MPa; S hsi , S hs Indicates the minimum horizontal principal stress of the target shale layer and the rock sample shale layer, MPa; h si 、h bi Respectively represent the thickness of the target shale layer and interlayer, m:h s 、h b Respectively represent the thickness of the shale layer and interlayer of the rock sample, m;

[0076] Step 14, simulating fracture propagation according to the fracturing fluid viscosity, pumping displacement and pumping program determined by the true triaxial fracturing experiment plan, and studying and analyzing the experimental results;

[0077] Step 15, preparing mixed rock samples of interlayered shales of different lithologies, and conducting a layer penetration and expansion simulation test of the mixed rock samples under different stress parameters and pumping parameters;

[0078] Step 16: According to the results of the through-layer expansion simulation test in step 15, a digital core physical model is established, and a fitting inversion is performed with the physical simulation fracture expansion morphology (specifically: using the existing three-dimensional discrete grid point fracturing numerical simulation method for mixed rock samples, repeatedly adjusting various parameters of the digital core until the fracture expansion morphology of the digital core is consistent with the results of the true triaxial fracturing physical simulation test), and a mathematical model of fracture expansion of multi-lithology interlayer shale through-layer fracturing is established;

[0079] Among them, the digital core physical model is established, specifically:

[0080] Digital cores are three-dimensional images that reflect the real pore space and components of rocks, reconstructed based on rock microstructure information. The two-dimensional information of the core is obtained through cast thin sections and particle size analysis data. Based on the two-dimensional information of the core, mathematical methods are used to reconstruct three-dimensional digital cores with different parameters that are consistent with the real core. Different components of the core are represented by different numerical values, stored as three-dimensional images, and different components are given different properties. On this basis, numerical simulation of rock physical properties can be directly performed. Establishing three-dimensional digital cores of shale gas reservoir rocks can provide a new way to study its rock physics. The specific methods are consistent with existing methods and will not be repeated here.

[0081] Among them, a mathematical model for fracture propagation of multi-lithology interlayer shale through-layer fracturing is established, specifically:

[0082] Based on the digital core physical model and fracture propagation simulation algorithm formed by fitting inversion, the small scale of the core is magnified to the large scale of the actual reservoir, which is the mathematical model of fracture propagation of multi-lithology interlayer shale through-layer fracturing. Based on this, the numerical simulation of fracture propagation of through-layer fracturing under different combinations of fracturing pumping parameters such as displacement, viscosity, time, and cluster number can be carried out.

[0083] Step 17: perform numerical simulation of different fracturing parameters and simulation optimization of fracturing schemes according to the mathematical model established in step 16, so as to maximize the transformation volume and EUR, comprehensively consider the influence of factors such as fracturing cost, and determine the fracturing scheme.

[0084] [Example 3]

[0085] Taking the target reservoir of the Jurassic shale gas well A in the Sichuan Basin as an example, the well has a vertical depth of 2933.29-2959.67m and has 5 small layers, from bottom to top: ① small layer of 5.34m thick gray-black shale interbedded with multiple thin layers of shell limestone, ② small layer of 9.15m thick gray-black shale, ③ small layer of 6.19m thick gray-black shale interbedded with 0.8m shell limestone, ④ small layer of 5.7m thick gray-black shale, and ⑤ small layer of 5.35 thick mudstone limestone and silty mudstone interbedded. The Young's modulus of gray-black shale is 7.2-38.1 GPa, and the Poisson's ratio is 0.204-0.252. The Young's modulus of shell limestone is 11.2-40.8 GPa, and the Poisson's ratio is 0.200-0.247. The Young's modulus of silty mudstone is 32.72-45.3 GPa, and the Poisson's ratio is 0.213-0.245. The vertical stress, maximum horizontal principal stress, and minimum horizontal principal stress of the shale layer are 72.1, 70.6, and 65.6 MPa, respectively. The vertical stress, maximum horizontal principal stress, and minimum horizontal principal stress of the interlayer shell limestone are 72.3, 75.9, and 70.6 MPa, respectively. The stress difference between the reservoir and interlayer is 5.0 MPa. According to the requirements of the selected true triaxial hydraulic fracturing physical simulation experimental equipment and the real reservoir lithology development characteristics, this embodiment needs to prepare 5 layers of mixed rock samples with a size of 300mm×300mm×300mm to simulate the influence of shell limestone in the interlayer of the horizontal section passing through the shale layer on the expansion of the fracturing crack through the layer. The specific method is as follows:

[0086] (1) According to the above method, the thickness of the five rock plates from top to bottom of the mixed rock sample are determined as follows: shale 50 mm, limestone 50 mm, shale 100 mm, limestone 50 mm, and shale 50 mm;

[0087] (2) Consult relevant data to obtain the natural outcrop of the target Jurassic strata and interlayers, cut them into multiple samples with a length and width of 300 mm × 300 mm and a thickness of 50 mm. The thickness sampling direction is vertical;

[0088] (3) According to the above method, quartz sand, cement, clay, 200 / 320 mesh iron powder, 200 / 1200 mesh granite powder, ultrafine calcium carbonate, water reducer, tap water, vitrified glue and super glue are prepared into a 300 mm × 300 mm × 300 mm artificial shale sample in the following proportions, which will not be described in detail here;

[0089] The Young's modulus of shale is <16GPa, and the mass ratio of each filler is 20% to 40% quartz sand + 20% to 40% cement + 20% to 40% clay + 1% to 2% water reducer + 5% to 10% ultrafine calcium carbonate + 20% to 30% tap water;

[0090] The Young's modulus of shale is 17-28GPa, and the mass ratio of each filler is 20%-25% quartz sand + 30%-40% cement + 10%-30% clay + 3%-5% water reducer + 5%-10% ultrafine calcium carbonate + 3%-5% 200 / 320 mesh iron powder + 15%-25% tap water;

[0091] The Young's modulus of shale is >28GPa, and the mass ratio of each filler is 15%~20% quartz sand + 40%~50% cement + 5%~10% clay + 5% water reducer + 5%~10% 200 / 1200 mesh granite powder + 5% 200 / 320 mesh iron powder + 1%~2% super glue + 15%~25% tap water;

[0092] (4) Cutting the shale slab prepared in step 3 into three shale slabs with thicknesses of 100 mm, 50 mm, and 50 mm along the bedding direction;

[0093] (5) The 50 mm shale plate prepared in step 4 is first placed at the bottom of the mold, and the top is coated with cement mortar containing a certain proportion of super glue, and then the interlayer outcrop sample with a thickness of 10 to 50 mm prepared in step 2 is placed, and the top is also coated with a certain amount of vitrified glue;

[0094] (6) Repeat the above step (5), place the 100 mm shale slab made in step (4), the interlayer outcrop sample with a thickness of 10 to 50 mm made in step (2), and the 50 mm shale slab made in step (4) in a mold in sequence, wait for the slabs to be bonded for 48 hours, and finally make a 5-layer mixed rock sample with a size of 300 mm × 300 mm × 300 mm, which is composed of artificial shale-interlayer natural outcrop-artificial shale-interlayer natural outcrop-artificial shale;

[0095] (7) Drill a 150 mm long countersunk hole in the middle shale layer of the mixed rock sample prepared in step 6 along the direction parallel to the bedding, and solidify a 120 mm long simulated wellbore in the countersunk hole with resin glue, leaving 30 mm below as an open hole section;

[0096] (8) A large-scale true triaxial test system was used to load the ground stress on the mixed rock samples in layers. The three-dimensional stress of the shale layer was loaded at 40% of the true stress, that is, the vertical stress, maximum horizontal principal stress and minimum horizontal principal stress of the shale layer were 28.8, 28.2 and 26.2 MPa respectively, and the minimum horizontal stress loading value of the interlayer was S hbAccording to formula (1), the maximum horizontal principal stress loading value S is 30 MPa. Hb Calculated according to formula (2), it is 35MPa.

[0097] The present invention provides a preparation and application of a simulated rock sample for through-layer fracturing of multi-lithology interlayer shale. By adjusting the proportions of fillers such as quartz sand, cement, clay, water reducer, 200-320 mesh iron powder, 200-1200 mesh granite powder, ultrafine calcium carbonate, tap water, vitrified glue and super glue, the large-scale simulated rock sample has high equivalence and reproducibility with the multi-lithology interlayer shale in terms of mechanical properties (elastic modulus and fracture toughness), whole rock mineral composition, lithology combination, etc., and then a true triaxial fracture extension physical simulation test with layered stress loading is adopted to accurately realize the simulation study of the influence of factors such as the vertical lithology combination of shale reservoirs, interlayer interface strength, and interlayer stress shielding on the through-layer fracturing effect, thereby improving the pertinence of fracturing schemes and technical measures.

[0098] The above technical solution is only one implementation mode of the present invention. For those skilled in the art, it is easy to make various types of improvements or modifications based on the principles disclosed in the present invention, and it is not limited to the technical solution described in the above specific embodiments of the present invention. Therefore, the above description is only preferred and does not have a restrictive meaning.

Claims

1. A method for preparing a simulated rock sample for interlayer shale fracturing with multiple lithologies, characterized in that: Obtain interlayer outcrop samples of the same stratum in the target area; mix and stir the base materials to form shale slabs and cut them into required thickness; place the shale slabs and interlayer outcrop samples in a mold at intervals to form multi-layer mixed rock samples.

2. The preparation method according to claim 1, characterized in that: The preparation method comprises the following steps: Step 1, obtaining the main rock mechanical parameters of the target layer; Step 2, obtaining natural outcrop samples of interlayers in the same layer system in the target area; Step 3, preparing the base material into an artificial shale rock sample according to the concrete mortar method, and taking the core to test the mechanical parameters; Step 4: According to the mechanical parameter test results of step 3, adjust the ratio of different base materials, and repeat step 3 until the tested mechanical parameters are consistent with the mechanical parameters obtained in step 1. At this time, the corresponding base material ratio is the base material ratio required for preparing shale slabs; Step 5, determining the base material ratio according to step 4 to produce a plurality of shale slabs; Step 6, placing the shale slab made in step 5 and the interlayer natural outcrop sample obtained in step 2 in a mold, applying vitrified glue between the shale slab and the natural outcrop sample to obtain a multi-layer mixed rock sample.

3. The preparation method according to claim 2, characterized in that: The main rock mechanical parameters of the target layer in step 1 include: Young's modulus, Poisson's ratio, shear strength, cohesion, internal friction angle, tensile strength, fracture toughness, as well as bedding fractures, laminar fracture density, reservoir thickness, interlayer thickness and interface strength.

4. The preparation method according to claim 2, characterized in that: In step 3, the base materials include quartz sand, cement, clay, water reducing agent, iron powder, granite powder, ultrafine calcium carbonate, tap water, vitrified glue and super glue; Preferably, the iron powder is 200-320 mesh iron powder, and the granite powder is 200-1200 mesh granite powder.

5. The preparation method according to claim 2, characterized in that: The mechanical parameters tested in step 3 include Young's modulus, tensile strength, shear strength and fracture toughness.

6. The preparation method according to claim 2, characterized in that: In step 5, the base material ratio is determined according to step 4 to produce multiple shale slabs, specifically: Quartz sand, cement, clay, water reducing agent, iron powder, granite powder, ultrafine calcium carbonate, tap water, vitrified glue and super glue are mixed and stirred evenly, poured into a mold, and a layer of rice paper is placed at a certain thickness to simulate bedding joints and laminae joints to make shale slabs; The produced shale slab is then cut into multiple shale slabs of required thickness along the bedding direction.

7. The preparation method according to claim 6, characterized in that: In step 6, the shale slab prepared in step 5 and the interlayer natural outcrop sample obtained in step 2 are placed in a mold at intervals, and vitrified glue is applied between the shale slab and the natural outcrop sample to obtain a multi-layer mixed rock sample. The specific operation is as follows: According to the actual thickness requirements, the shale slab produced in step 5 is first placed at the bottom of the mold, and the top is coated with vitrified glue. Then the natural outcrop sample of the interlayer obtained in step 2 is placed and the top is coated with vitrified glue. Repeat the above operations, and place the shale slab and the interlayer outcrop sample in the mold in turn. Wait for the slabs to be bonded for 48 hours to finally make a multi-layer mixed rock sample composed of an artificial shale slab and a natural outcrop sample of an interlayer.

8. A rock sample prepared by the method for preparing a multi-lithology interlayer shale cross-layer fracturing simulation rock sample according to any one of claims 1 to 7.

9. A multi-lithology interlayer shale fracturing simulation experimental method, characterized in that: The multi-layer mixed rock sample prepared by the preparation method according to any one of claims 1 to 7 is subjected to a through-layer fracturing simulation experiment, which specifically comprises the following steps: Step 11, obtaining the maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress parameters of the target layer; Step 12, drilling a countersink in the middle shale layer of the mixed rock sample along the direction parallel to the bedding, solidifying the simulated wellbore in the countersink with resin glue, and leaving a certain distance below as an open hole section; Step 14, simulating fracture propagation according to the fracturing fluid viscosity, pumping displacement and pumping program determined by the true triaxial fracturing experiment plan, and studying and analyzing the experimental results; Step 15, preparing mixed rock samples of interlayered shales of different lithologies, and conducting a layer penetration and expansion simulation test of the mixed rock samples under different stress parameters and pumping parameters; Step 16: According to the results of the interlayer expansion simulation test in step 15, a digital core physical model is established, and a fitting inversion is performed with the physical simulation fracture expansion morphology to establish a mathematical model for interlayer expansion of multi-lithology interlayer shale fractures; Step 17: Perform numerical simulation of different fracturing parameters and simulation optimization of fracturing schemes according to the mathematical model established in step 16 to determine the fracturing scheme.

10. The multi-lithology interlayer shale cross-layer fracturing simulation experimental method according to claim 9, characterized in that: In step 13, the ground stress data obtained in step 11 are subjected to layered ground stress loading on the mixed rock sample using a large-scale true triaxial test system, specifically: The three-dimensional stress of the shale layer is loaded according to 40% of the maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress parameters of the target layer obtained in step 11. The minimum horizontal stress loading value S hb Calculate according to formula (1): s hb =s hbi -0.6×s hsi -2 / π(s hbi -s hsi )(cos -1 (h s / h s +h b ))-cos -1 (h si / (h si +h bi ))) (1) Maximum horizontal principal stress loading value S Hb Calculate according to formula (2): s Hb =s hb +s Hbi -s hbi (2) Where: S hbi , S Hbi Respectively represent the minimum horizontal principal stress and maximum horizontal principal stress of the target interlayer, MPa; S hsi , S hs Indicates the minimum horizontal principal stress of the target shale layer and the rock sample shale layer, MPa; h si 、h bi Respectively represent the thickness of the target shale layer and interlayer, m:h s 、h b Respectively represent the thickness of the shale layer and interlayer of the rock sample, m.

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