Shale compressibility evaluation method based on crack initiation-propagation-joint network index

Abstract

The application discloses a shale compressibility evaluation method based on a crack initiation-expansion-joint network index, and relates to the field of shale mechanics. The method comprises the following steps: performing Brazilian splitting, fracture toughness and triaxial compression experiments on a target shale sample respectively to obtain a tensile strength value, a fracture toughness value and a mechanical deformation parameter of the target shale sample; calculating a crack initiation index, a crack expansion index and a crack joint network index based on the tensile strength value, the fracture toughness value and the mechanical deformation parameter; the crack initiation index is calculated based on normalization of the tensile strength value, the crack expansion index is calculated based on normalization of the fracture toughness value, and the crack joint network index is calculated based on normalization of the mechanical deformation parameter; and according to the crack initiation index, the crack expansion index and the crack joint network index, a crack initiation-expansion-joint network ternary comprehensive quantitative evaluation is performed on the mechanical properties of the target shale sample based on a preset evaluation standard. The application can improve the evaluation accuracy of shale compressibility.

Description

Technical Field

[0001] This application relates to the field of rock mechanics, and in particular to a method for evaluating the compressibility of shale based on the initiation-propagation-fracture network index. Background Technology

[0002] Shale is a type of rock, and rocks include shale. Shale reservoirs generally have characteristics such as well-developed bedding, strong heterogeneity, and significant anisotropy. Their stress failure behavior is significantly different from that of "homogeneous rocks". Therefore, a systematic understanding of the fracture mechanism and fracture development law of shale is an important foundation for revealing the nature of shale's fracture-forming ability and carrying out compressibility evaluation research. Compressibility evaluation is one of the indicators in mechanical property evaluation.

[0003] Studies on shale fracture modes have revealed that: the resistance to crack propagation varies significantly under different shale orientations; the combination of weak bedding planes and loading direction is a crucial factor controlling shale fracture modes; spatial heterogeneity and material anisotropy jointly influence crack propagation paths and macroscopic equivalent fracture toughness under Brazilian splitting conditions; the type I fracture toughness and tensile strength of layered shale change significantly with the loading angle; thus, shale fracture is not a simple single extensional or shear fracture, but rather exhibits multiple forms under bedding control, including tensional fracture, shear fracture, and a combination of tension and shear fracture, with the final failure mode showing a significant direction dependence.

[0004] As our understanding of shale fracturing deepens, we've discovered that the resistance to fracture propagation under "dynamic" conditions is not entirely consistent with "static" results. This means that the fracture response under fracturing or impact disturbance conditions cannot be simply replaced by "static parameters." Therefore, our understanding of shale fracturing modes has gradually shifted from focusing solely on the "failure outcome" to focusing on the "fracture process."

[0005] The closest existing technical document, CN119273194A, proposes a single-well evaluation method for oil and gas reservoir engineering quality. Based on the brittleness concept and a comprehensive compressibility index fitting formula, it essentially transforms conventional parameters describing the mechanical properties of "rock" into dynamic and static values ​​through a weighted approach, inverting static reservoir parameters obtained in the laboratory onto well logging. This result performs well for highly homogeneous rocks such as sandstone, but in strongly heterogeneous shale reservoirs, it generally suffers from mismatches between evaluation results and actual fracturing effects, as well as poor applicability, limiting its widespread application (Dou Liangbin et al., 2021; Yan et al., 2024). Essentially, existing evaluation systems mostly focus on the qualitative judgment of whether a reservoir "can be fractured," failing to quantitatively answer "what it will look like"—that is, the morphological characteristics and propagation patterns of the fracture network after fracturing, which is the core factor determining fracturing stimulation and production capacity enhancement.

[0006] Existing technical document CN119273194A belongs to the field of single-well evaluation technology for oil and gas reservoir engineering quality. Its core service is the optimization of development plans and the selection of stratigraphic layers, falling under the category of reservoir engineering quality evaluation. It addresses the lack of comprehensive quantitative indicators and low evaluation accuracy in reservoir engineering quality evaluation; however, it cannot evaluate the fracture-proneness of deep shale reservoirs. This solution was applied in the early stages of development of a "conventional sandstone" oil and gas field. When it was necessary to select commercially viable well locations from multiple exploratory wells, or to determine which reservoir sections within a single well were worth prioritizing for development, the provided single-well evaluation method was applied. It quantitatively assesses reservoir engineering quality parameters such as porosity, permeability, and oil saturation. The ultimate goal is to provide a basis for decision-making in development plan formulation (e.g., whether to adopt water injection development, and which extraction technology to choose) and stratigraphic layer selection (e.g., which sandstone group to prioritize for extraction). The entire technical chain revolves around whether the reservoir itself has development value and how to efficiently develop these reservoir resources, representing a typical application of reservoir engineering quality evaluation. It cannot be used to evaluate "the impact of rock compressibility differences (i.e. heterogeneity) at different locations within a horizontal section of several kilometers on fracturing operations before fracturing operations in deep shale gas horizontal wells, in order to predict the direction, length, complexity, and morphological characteristics of the fractures after fracturing."

[0007] Existing technical document CN119273194A lacks a graded and pattern-based evaluation method, only outputting a single comprehensive score, which cannot guide on-site fracturing and stimulation. These methods introduce parameters such as in-situ stress, natural fractures, and fracture toughness on top of brittleness indices, with relatively clear physical meanings. However, the weighting values ​​largely rely on experience and lack a unified mechanical theoretical basis. Their core is to calculate a brittleness index using parameters such as Young's modulus and Poisson's ratio for macroscopic assessment of rock compressibility. They do not address the specific construction of fracture initiation, propagation, and network pattern indices, nor do they include content on integrating these three into a "ternary index" system for shale classification and compressibility evaluation.

[0008] Existing technical literature: The paper "Mechanical Properties and Mechanical Layer Division of Interbedded Ordovician Gypsum-Salt-Carbonate Rocks in the Ordos Basin" only provides conventional mechanical parameter testing methods and does not provide a ternary evaluation system. The core content focuses on the mechanical property testing of interbedded rocks, the acquisition and obtaining of stress-strain curves, and the calculation of conventional elastic parameters, which is essentially a basic experimental research method in this field. Although the paper mentions relevant mechanical moduli, it only uses them to describe the differences in rock fracture modes and fails to establish a systematic "fracture initiation-propagation-fracture network" ternary evaluation system. It also fails to form an evaluation method that can be practically applied and lacks the necessary field application verification. Summary of the Invention

[0009] The purpose of this application is to provide a method and related apparatus for evaluating the compressibility of shale based on the initiation-propagation-fracture network index, which can improve the accuracy of rock compressibility evaluation.

[0010] To achieve the above objectives, this application provides the following solution: a method for evaluating the compressibility of shale based on the initiation-propagation-fracture network index, which involves conducting Brazilian splitting tests, fracture toughness tests, and triaxial compression tests on target shale samples to obtain the tensile strength, fracture toughness, and mechanical deformation parameters of the target shale samples; including the following steps:

[0011] Step (1): Based on the tensile strength value, fracture toughness value and mechanical deformation parameters, calculate the crack initiation index, crack propagation index and crack network index;

[0012] The crack initiation index is calculated based on the normalized tensile strength value, and the calculation formula for the crack initiation index is as follows:

[0013]

[0014] Where T is the tensile strength of the oil layer where the target shale sample is located in the test area, T max It is the maximum tensile strength in the oil reservoir of the test area, T min It is the minimum tensile strength in the oil layer of the test area;

[0015] The crack propagation index is calculated based on the normalized fracture toughness value, and the calculation formula for the crack propagation index is as follows:

[0016]

[0017] Where K is the fracture toughness of the oil layer where the target shale sample is located in the test area, K max It is the maximum fracture toughness in the oil reservoir of the test area, K min It is the lowest fracture toughness in the oil layer of the test area;

[0018] The calculation formula for the crack network index is based on the normalized calculation of mechanical deformation parameters:

[0019]

[0020] Among them, E F E represents the mechanical deformation modulus of the oil layer containing the target shale sample in the test area. Fmax It is the maximum mechanical deformation modulus in the oil reservoir of the test area, E Fmin It is the minimum mechanical deformation modulus in the oil layer of the test area.

[0021] Step (2): Based on the inherent mechanical properties of rock and external stress conditions, the rock fracture mode control mechanism is constructed using Young's mode parameters and differential stress coefficient to describe the differences in rock fracture modes.

[0022]

[0023] The mechanical modulus based on the construction needs to satisfy three conditions, given by the following formula; () indicates that the mechanical deformation modulus is a function of Young's modulus parameter and the differential stress coefficient;

[0024]

[0025]

[0026]

[0027] Using a trial function:

[0028]

[0029] in, The compressive strength of the rock. For confining pressure, and , k is the differential stress coefficient. The larger the Young's modulus E, the larger the differential stress coefficient k, and the larger the mechanical deformation modulus E. F The larger the value, the easier it is for a single high-angle crack to develop after the rock fractures, and the easier the crack propagates; conversely, the smaller the mechanical deformation modulus E, the easier it is for the rock to develop a single high-angle crack. F A smaller mechanical modulus E makes it easier to form dispersed, intersecting cracks, creating a crack network, and preventing crack propagation; however, when the mechanical deformation modulus E... F When the rock is very small, it undergoes plastic deformation and is not prone to cracking; where E is Young's modulus and g is the mechanical deformation modulus. F The normalization coefficient is denoted by , where e is the natural base.

[0030] Step (3): Based on the calculated crack initiation index, crack propagation index, and crack network index, and based on the preset evaluation criteria, perform a three-dimensional comprehensive quantitative evaluation of the mechanical properties of the target shale sample, including crack initiation, propagation, and crack network.

[0031] The process for determining the above mechanical deformation parameters is as follows:

[0032] The stress-strain curve of the target shale sample was determined based on triaxial compression experiments;

[0033] Based on the rock stress-strain curve, the mechanical deformation parameters of the target shale sample are determined; the mechanical deformation parameters include Young's modulus, Poisson's ratio, and compressive strength.

[0034] The above evaluation criteria specifically include:

[0035] When the average crack initiation index (FCI) Average When the fracture initiation index (FCI) is >60%, the target shale sample is determined to be a low-tension shale; when the average fracture initiation index (FCI) is >60%, the target shale sample is determined to be a low-tension shale. Average When the fracture initiation index (FCI) is less than 60%, the target shale sample is determined to be high-tensile shale; Average This represents the average value of the fracture initiation index for multiple target shale samples;

[0036] When the average crack propagation index FGI Average When the fracture density is >60%, the target shale sample is identified as low-toughness shale; when the average fracture propagation index (FGI) is >60%, the target shale sample is identified as low-toughness shale. Average When the fracture propagation index (FGI) is less than 60%, the target shale sample is determined to be high-toughness shale; Average This represents the average value of the fracture propagation index from multiple target shale samples;

[0037] When 30% < Fragmentation-Meridian Index (FMI) < 45%, the target shale sample is identified as medium-modulus shale; when FMI > 45%, the target shale sample is identified as high-modulus shale.

[0038] Mechanical deformation modulus E F <10GPa indicates indistinct macroscopic rock fracturing, small crack aperture, and weak fracture-forming ability; mechanical deformation modulus is between 10GPa. <E F Under stress loads below 15 GPa, strain tends to disperse, leading to dispersed fractures and the formation of complex, low-angle fracture networks; the mechanical deformation modulus E F >15GPa, strain concentration, easy to develop high-angle through-crack network cracks.

[0039] This application discloses the following technical effects:

[0040] This application provides a rock compressibility evaluation method based on the crack initiation-propagation-fracture network index. By conducting Brazilian splitting, fracture toughness and triaxial compression tests on target shale samples, key mechanical parameters are obtained, and crack initiation, propagation and network pattern indices are calculated based on these parameters. Finally, a comprehensive quantitative evaluation is performed according to preset standards, which effectively solves the problem that existing evaluation methods are difficult to accurately assess shale compressibility. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0042] Figure 1 This is an application environment diagram of a rock compressibility evaluation method based on the crack initiation-propagation-fracture network index in Example 1;

[0043] Figure 2 A schematic flowchart of a rock compressibility evaluation method based on the crack initiation-propagation-fracture network index provided in Example 1;

[0044] Figure 3 This is a graph showing the tensile strength and FCI variations of each oil layer in the Gulong Shale case provided in Example 2;

[0045] Figure 4 The fracture toughness and FGI variation diagrams for each oil layer provided in Example 2;

[0046] Figure 5 This is a schematic diagram of different deformation unit crack network patterns provided in Example 2;

[0047] Figure 6 The deformation modulus of different oil layers in well rc1 provided in Example 2;

[0048] Figure 7 This is a comparison chart of the deformation modulus of different wells in the same oil layer provided in Example 2;

[0049] Figure 8 This is a schematic diagram of the deformation modulus and FMI of each oil layer provided in Example 2;

[0050] Figure 9 This is a diagram illustrating the three-level evaluation framework for mechanical deformation initiation, propagation, and fracture network in the Gulong Shale of this application.

[0051] Figure 10 This is an analysis diagram of four mechanical models of shale in the Gulong area provided in Example 2;

[0052] Figure 11 The image shows the axial stress-time curve and fracture diagram at key scanning moments of the S1 specimen in Example 3.

[0053] Figure 12 The image shows the axial stress-time curve and fracture diagram at key scanning moments for specimen S2 in Example 3.

[0054] Figure 13 The image shows the axial stress-time curve and fracture diagram at key scanning moments for specimen S3 in Example 3.

[0055] Figure 14 This is a diagram showing the distribution of BI in each section of the Gulong Shale (Q1-Q9) in Example 3, and a comparison with the mean of the sections.

[0056] Figure 15This is a schematic diagram of the functional modules of the rock compressibility evaluation device based on the crack initiation-propagation-crack network index of this application.

[0057] Figure 16 This is a schematic diagram of the structure of a computer device provided in this application. Detailed Implementation

[0058] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0059] Compressibility is the core factor determining the effectiveness of fracturing. Domestic and international research shows that the mechanical behavior of shale is closely related to its mineral composition, porosity, and bedding characteristics. Generally, higher porosity or more developed bedding results in lower compressive strength. Furthermore, as a typical anisotropic material, shale exhibits significant differences in its mechanical response across different load directions. To accurately describe this characteristic, domestic scholars have established various nonlinear and anisotropic mechanical models to simulate the deformation and failure of shale under different stress conditions, providing crucial theoretical support for hydraulic fracturing and drilling engineering design. Regarding fracture propagation, domestic scholars have systematically analyzed the fracture propagation path and mechanical behavior of shale under different fracturing modes through a combination of experiments and numerical simulations, and constructed corresponding predictive models, laying a theoretical foundation for optimizing fracturing schemes and improving oil recovery.

[0060] Therefore, the purpose of this application is to provide a shale compressibility evaluation method based on the initiation-propagation-fracture network index, which can improve the accuracy of shale compressibility evaluation.

[0061] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, this application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0062] Example 1 provides a rock compressibility evaluation method based on the crack initiation-propagation-fracture network index, which can be applied to, for example... Figure 1 In the application environment shown, terminal 102 communicates with server 104 via a network. A data storage system can store the data that server 104 needs to process. The data storage system can be set up independently, integrated into server 104, or placed in the cloud or on another server.

[0063] The terminal 102 can be, but is not limited to, various desktop computers, laptops, smartphones, tablets, IoT devices, and portable wearable devices. IoT devices can include smart speakers, smart TVs, smart air conditioners, and smart in-vehicle devices. Portable wearable devices can include smartwatches, smart bracelets, and head-mounted devices. The server 104 can be implemented using a standalone server or a server cluster composed of multiple servers, or it can be a cloud server.

[0064] In Example 1, as Figure 2 As shown, a rock compressibility evaluation method based on the crack initiation-propagation-crack network index is provided. This method is executed by computer equipment, specifically by a terminal or server alone, or by both a terminal and a server. In this embodiment, the method is applied to... Figure 1 Taking server 104 as an example, the explanation includes the following steps 1 to 3. Wherein:

[0065] Step (1): The target shale sample was subjected to Brazilian splitting test, fracture toughness test and triaxial compression test respectively to obtain the tensile strength value, fracture toughness value and mechanical deformation parameters of the target shale sample;

[0066] Step (2): Based on the tensile strength value, fracture toughness value and mechanical deformation parameters, calculate the crack initiation index, crack propagation index and crack network index; wherein, the crack initiation index is calculated based on the normalized tensile strength value, the crack propagation index is calculated based on the normalized fracture toughness value, and the crack network index is calculated based on the normalized mechanical deformation parameters.

[0067] Step (3): Based on the calculated crack initiation index, crack propagation index and crack network index, and based on the preset evaluation criteria, perform a three-dimensional comprehensive quantitative evaluation of the mechanical properties of the target shale sample, including crack initiation, propagation and crack network.

[0068] In this embodiment, the mechanical properties of the Gulong Shale in the test area are analyzed. Based on the Brazilian splitting test, the tensile strength of the target shale sample in the test area is obtained to quantitatively evaluate the crack initiation level of the rock; based on the fracture toughness test, the fracture toughness strength of the target shale sample in the test area is obtained to quantitatively evaluate the crack propagation of the rock; based on the triaxial compression test, the mechanical deformation modulus E of the target shale sample is obtained. F Quantitative evaluation of rock fracture network patterns yields parameters including compressive strength, cohesion, internal friction angle, stress-strain curve, Young's modulus, and Poisson's ratio.

[0069] Specifically, when performing steps (1) to (3), the specific steps can be as follows:

[0070] Specifically, step (1) includes the following steps:

[0071] S1: Obtain target shale samples from the test area and determine their bulk density; each shale sample has the same height and thickness. Under the same experimental conditions, rock mechanics experiments were conducted on 101 target shale samples from the test area.

[0072] S2: The tensile strength of the target shale sample is determined based on the Brazilian splitting test; the fracture toughness of the target shale sample is determined based on the fracture toughness test; the stress-strain curve of the target shale sample is determined based on the triaxial compression test, and the mechanical parameters of the target shale sample are determined using the rock stress-strain curve; the mechanical parameters include Young's modulus, Poisson's ratio, and compressive strength, etc.

[0073] Specifically, step (2) includes the following steps:

[0074] The tensile strength of the target shale sample was determined based on the Brazilian splitting experiment, and a fracture initiation index (FCI) was established based on the tensile strength value.

[0075] The fracture toughness value of the target shale sample was determined based on the fracture toughness test, and the fracture propagation index (FGI) was established based on the fracture toughness value.

[0076] The mechanical parameters of the target shale sample were determined based on triaxial compression tests. These parameters included Young's modulus, Poisson's ratio, and compressive strength. A coupled mechanical deformation modulus E, based on Young's modulus and differential stress coefficient, was then constructed. F And establish the crack network index (FMI).

[0077] Among them, such as Figure 9 As shown, based on the tensile strength values ​​tested in each oil layer, the fracture initiation index (FCI) is defined, and the calculation formula is as follows:

[0078]

[0079] Where T is the tensile strength of the oil layer where the target shale sample is located in the test area, T max It is the maximum tensile strength in the oil reservoir of the test area, T min This represents the minimum tensile strength in the oil reservoir of the test area. A higher fracture initiation index indicates a higher level of fracture initiation relative to other oil reservoirs, and consequently, a lower tensile strength.

[0080] Based on the fracture toughness strength values ​​tested in each oil layer, the fracture propagation index (FGI) is defined, and the calculation formula is as follows:

[0081]

[0082] Where K is the fracture toughness of the oil layer where the target shale sample is located in the test area, K maxIt is the maximum fracture toughness in the oil reservoir of the test area, K min This represents the minimum fracture toughness in the oil reservoir of the test area. A larger fracture propagation index indicates a higher level of fracture propagation relative to other oil reservoirs, and correspondingly lower fracture toughness.

[0083] Based on the mechanical deformation modulus E of each oil layer test F The fracture network index (FMI) of shale is defined, and the calculation formula can be as follows:

[0084]

[0085] Among them, E F E represents the mechanical deformation modulus of the oil layer containing the target shale sample in the test area. Fmax It is the maximum mechanical deformation modulus in the oil reservoir of the test area, E Fmin It is the minimum mechanical deformation modulus in the oil layer of the test area.

[0086] Specifically, based on the study of the control mechanism of rock fracture modes by inherent mechanical properties and external stress conditions, a mechanical modulus describing the differences in rock fracture modes is constructed using Young's mode parameters and differential stress coefficients:

[0087]

[0088] The mechanical modulus based on the construction needs to satisfy three conditions, given by the following formula.

[0089]

[0090]

[0091]

[0092] Using a trial function:

[0093]

[0094] in, The compressive strength of the rock. For confining pressure, and , k is the differential stress coefficient. The larger the Young's modulus E, the larger the differential stress coefficient k, and the larger the mechanical deformation modulus E. F The larger the value, the easier it is for a single high-angle crack to develop after the rock fractures, and the easier the crack propagates; conversely, the smaller the mechanical deformation modulus E, the easier it is for the rock to develop a single high-angle crack. F A smaller mechanical modulus E makes it easier to form dispersed, intersecting cracks, creating a crack network, and preventing crack propagation; however, when the mechanical deformation modulus E... F When the rock is very small, it undergoes plastic deformation and is not prone to cracking.

[0095] Specifically, performing step (3) comprises the following steps:

[0096] Shale is classified into high-tensile-resistance and low-tensile-resistance shale based on the fracture initiation index FCI. The average fracture initiation index FCI Average > 60% is defined as low-tensile-resistance shale, wherein rocks are prone to generate cracks or microfractures under the action of tensile force, which affects reservoir channels and fluidity of oil and gas; FCI Average < 60% is defined as high-tensile-resistance shale, wherein the generation of fractures is inhibited.

[0097] Shale is classified into high-toughness and low-toughness shale based on the fracture propagation index FGI. The average fracture propagation index FGI Average > 60% is defined as low-toughness shale, wherein fractures formed by rock rupture are prone to propagate; FGI Average < 60% is defined as high-toughness shale, wherein fracture propagation is effectively inhibited, and rocks can resist large external force without excessive rupture.

[0098] The mechanical deformation modulus E is obtained based on mechanical parameters of different horizons F , and the deformation characteristics of shale are classified into three deformation modes (as shown in Figure 3 ). Deformation mode I corresponds to mechanical deformation modulus E F < 10 GPa, which is characterized by unobvious macroscopic rock rupture, small formed fracture opening and weak fracture-making capacity; for deformation mode II, the mechanical deformation modulus satisfies 10 GPa < E F < 15 GPa, under stress load, strain is prone to disperse, dispersed rupture develops, and complex low-angle fracture networks are easily formed; for deformation mode III, the mechanical deformation modulus E F > 15 GPa, strain is concentrated, and high-angle connected fracture network fractures are prone to develop.

[0099] Shale is classified into low-fracture-network, medium-fracture-network and high-fracture-network shale based on the fracture network index FMI. FMI < 30% is defined as low-fracture-network shale, which is not prone to form connected fractures or has weak fracture-making capacity; 30% < FMI < 45% is defined as medium-fracture-network shale, wherein strain is prone to disperse, dispersed rupture develops, and complex fracture networks are easily formed; FMI > 45% is defined as high-fracture-network shale, which is prone to develop connected fractures with large opening and has strong fracture-making capacity.

[0100] Finally, by comprehensively considering the three relative evaluation indexes, a fracability evaluation method based on the initiation-propagation-fracture network ternary index is constructed to evaluate shale at different horizons in the test area.

[0101] Example 2: The present application further provides an embodiment, which is specifically as follows:

[0102] The Songliao Basin is one of the world's richest continental sandstone-type oil and gas basins. Gulong shale oil is a typical continental shale oil, with oil flows observed from the edge to the center of the Gulong Depression in the Songliao Basin; high yields have been achieved in interbedded, layered, and pure shale types; the lower part of the Lower Cretaceous Qing 2 Member to the Qing 1 Member is oil-bearing throughout, indicating enormous shale oil resource potential. However, due to the complex bedding structure and mineral composition of the Gulong shale, and its low permeability, the fracture network may be uneven, leading to significant differences in fracturing effectiveness, which affects oil production and development efficiency.

[0103] The compressibility characteristics of shale in the Gulong area were evaluated based on the three-element index method of initiation-propagation-fracture network in Example 1.

[0104] First, shale samples from all oil layers in the Gulong area (Q1-Q9) were obtained. Based on the core samples, triaxial compression tests, tensile strength tests, and fracture toughness tests were conducted. A total of 101 samples were tested, and key mechanical parameters such as Young's modulus, Poisson's ratio, compressive strength, tensile strength, and fracture toughness were obtained.

[0105] (1) Evaluation of crack initiation index:

[0106] Tensile strength of rock is an important parameter describing crack initiation. The lower the tensile strength, the easier it is for cracks to initiate, and vice versa. Twenty-six Brazilian splitting tests were conducted on layers Q1 to Q9. The tensile strength was generally distributed between 2 MPa and 10.4 MPa, with significant differences in tensile strength distribution among different layers.

[0107] The differences in relative crack initiation degree among layers Q1-Q9 were evaluated based on the crack initiation index FCI (e.g., Figure 3 The tensile strength distribution in layers Q1-Q3, Q6, and Q9 is uniform but low, with an overall FCI (Fractional Crack Initiation Index) greater than 50%, indicating a high susceptibility to cracking. In contrast, the tensile strength distribution in layers Q4 (FCI between 0-80%), Q5 (FCI between 30%-85%), and Q8 (FCI between 40%-80%) is dispersed, reflecting the significant heterogeneity of the shale. (The average fracture initiation index, FCI, is not specified in the original text.) Average >60%, classified as low-tension shale, is prone to fracturing and is mainly distributed in the Q1-Q3, Q6, Q7, and Q9 layers; FCI Average Rocks with a density less than 60% are classified as high-tension shale, which are less prone to fracturing and are mainly distributed in the Q4, Q5, and Q8 layers. Specific data are shown in Table 1.

[0108] Table 1. Statistical table of tensile strength and fracture initiation index (FCI) of different strata in the Gulong Shale.

[0109]

[0110] (2) Evaluation of crack propagation index:

[0111] Fracture toughness is an important parameter characterizing the propagation of rock fractures; the lower the strength, the easier the fracture propagates, and vice versa. Twelve tests were conducted on the fracture toughness (perpendicular to bedding direction) of shale to obtain the fracture toughness of each well.

[0112] The differences in relative crack initiation degree among layers Q1-Q9 were evaluated based on the crack propagation index (FGI). Figure 4 The overall average fracture propagation index (FGI) for all strata Average >60% of the rock is classified as low-toughness shale, meaning that fractures tend to propagate easily after initiation. In contrast, the average fracture propagation index (FGI) of the Q2 and Q4-Q9 layers is significantly higher. Average >80%, Q1 (FGI) Average =62.38%) and Q3 (FGI) Average The average crack propagation index of the strata (60%) was relatively low. See Table 2 for specific data.

[0113] Table 2. Statistical table of fracture toughness and fracture propagation index (FGI) of different strata in the Gulong Shale.

[0114]

[0115] (3) Evaluation of crack mesh index:

[0116] ① Classification of shale mechanical deformation modes: Based on the mechanical parameters of different stratigraphic layers, the mechanical deformation modulus E is obtained. F Shale deformation characteristics can be divided into three deformation models ( Figure 5 Deformation mode I is the mechanical deformation modulus E. F <10GPa, characterized by indistinct macroscopic rock fracturing, small crack aperture, and weak crack-forming ability; Deformation Mode II, with mechanical deformation modulus between 10GPa. <E F Under stress loads below 15 GPa, strain tends to disperse, leading to dispersed fractures and the formation of complex, low-angle fracture networks; deformation mode III, mechanical deformation modulus E F >15GPa, strain concentration, easy to develop high-angle through-crack network cracks.

[0117] Table 3 Mechanical parameters and mechanical deformation modulus E of shale at different strata F Statistical table

[0118]

[0119] Table 4 Mechanical parameters and mechanical deformation modulus E of shale at different strata F Statistical Table (Continued)

[0120]

[0121] Table 5 Mechanical parameters and mechanical deformation modulus E of shale in different intervals F Statistical Table (Continued)

[0122]

[0123] ② Differences in mechanical deformation modes of shale in different intervals:

[0124] Figure 6 It describes the rock mechanical deformation characteristics of different intervals in the same well (Well rc1). From the figure, E of Q2 and Q7 F is generally less than 10GPa, and the fracture network index FMI is less than 30%, which is called low-modulus shale, and it is not easy to form connected fractures or has weak fracture generation capacity; E of Q9 F is between 10GPa and 15GPa, 30% < FMI < 45%, which is called medium-modulus shale. Strain is easily dispersed, dispersed fractures are developed, and it is easy to form complex fracture networks; for Q1, Q3, Q4, Q5, Q6, and Q8, E F is generally greater than 15GPa, which belongs to mechanical deformation mode III, with FMI > 45%, and is called high-modulus shale. Connected fractures with large openings are prone to develop, and the fracture generation capacity is strong.

[0125] From Figure 7 it can be seen that E of different wells in the same interval F and the distribution characteristics of FMI have significant differences, indicating that the heterogeneity of the study area is strong; fracture deformation modes I, II and III are developed in intervals Q1, Q3, Q5, Q6; fracture deformation modes I and II are developed in interval Q2; fracture deformation modes II and III are developed in intervals Q4 and Q9; fracture deformation modes I and III are developed in interval Q7; fracture deformation mode III is developed in interval Q8.

[0126] From Figure 8 it can be seen that by obtaining E of different wells in the same interval F and the average value of FMI for horizontal comparison, for Q2 (low-modulus shale), FMI Average =16.69% <30%, E FAverage =6.61GPa <10GPa, which belongs to deformation mode I, and it is not easy to form connected fractures or has weak fracture generation capacity; for Q1, Q3 and Q6 (medium-modulus shale), E FAverage is between 10GPa-15GPa, FMI Average is between 30%-45%, which belongs to deformation mode II, and it is easy to form fracture network characteristics; for Q4, Q5, Q7, Q8 and Q9 (high-modulus shale), E FAverage > 15GPa, FMI Average > 45% belongs to deformation mode III, and high-angle connected fractures are prone to develop.

[0127] (4) Evaluation of the differences in compressibility of different oil layers in the Gulong Shale:

[0128] like Figure 10 As shown, based on the mechanical properties of different wells and oil layers in the Gulong Shale, the compressibility of the Gulong Shale is divided into four types: low tensile-low toughness-low model (Q2), low tensile-low toughness-medium model (Q1, Q3, Q6), low tensile-low toughness-high model (Q7, Q9), and high tensile-low toughness-high model (Q4, Q5, Q8).

[0129] The Q2 layer (low tensile strength, low toughness, low model) is mostly developed in the shallow reservoir section of the Gulong area, with moderate bedding cementation. Low tensile strength and low fracture toughness together reduce the difficulty of fracture initiation and propagation. The low deformation modulus corresponds to mechanical deformation mode I, and the macroscopic fracture characteristics of the rock are not significant, often forming a network of microcracks with small apertures and poor connectivity. Enhancing the conductivity of microcracks and promoting competitive propagation of multiple fractures are needed to achieve the desired effect.

[0130] Layers Q1, Q3, and Q6 (low tensile-low toughness-medium model) exhibit shale fractures that are prone to initiation and propagation, with moderate deformation modulus (deformation mode II). They also show low-angle intersecting fractures and no obvious macroscopically penetrating main fractures. Development can induce the propagation of branch fractures, increasing the complexity of the fracture network.

[0131] The Q7 and Q9 layers (low tensile-low toughness-high model) are the core sections of high-quality shale reservoirs in the Gulong area. They are mostly thick-layered high-quality reservoirs with good intra-layer homogeneity. Fractures are easy to initiate and propagate, with high deformation modulus (deformation mode III). They are prone to forming large-aperture, high-angle continuous main fractures, accompanied by branch fractures, resulting in a high degree of fracture network complexity.

[0132] In the Q4, Q5, and Q8 layers (high tensile strength-low toughness-high model), the high tensile strength increases the difficulty of crack initiation (low crack density), and the low fracture toughness promotes rapid crack propagation after initiation; it has a high deformation modulus (deformation mode III), which easily forms longitudinally penetrating extended cracks.

[0133] The evaluation results of the Gulong Shale based on the ternary index model are consistent with the actual compressibility evaluation results. Imaging logging data shows that the natural fractures and foliation fractures in the Q1-Q3 layers are poorly developed and have extremely poor connectivity. The natural fractures are mainly concentrated in the Q5, Q7, Q8, and Q9 oil layers. The mineral composition of the Q1-Q9 layers shows a vertical differentiation characteristic of being viscous at the top and brittle at the bottom. Among them, the Q1-Q2 layers have strong plasticity and poor compressibility, while the Q8-Q9 layers have high brittleness and excellent fracture propagation, making them the optimal fracturing target layers in this area.

[0134] Example 3: This example uses the "ternary index" evaluation method of Example 1 to further evaluate the mechanical properties of the shale in each oil layer of the Gulong area. Combined with the reconstruction results of real-time in-situ loading CT test, the fracture development patterns and structural evolution characteristics corresponding to different index types are further analyzed from the macro-micro scale.

[0135] 3.1 Evaluation of compressibility of the Gulong Shale reservoir based on the "ternary index":

[0136] 3.1.1 Classification of Gulong Shale based on the "ternary index" mechanical model:

[0137] Based on the tested Gulong Shale dataset and the index classification results of Example 2, this embodiment summarizes and statistically analyzes the fracture initiation index, fracture propagation index, and fracture network index of each stratum (Tables 6 and 7), and conducts a differential evaluation of the compressibility of Gulong Shale accordingly.

[0138] Table 6 Mechanical properties of different wells and oil layers

[0139]

[0140] Table 7 Mechanical Properties of Different Oil Reservoirs in Different Wells (Continued)

[0141]

[0142] Based on the evaluation results of the "ternary index" of different strata of Gulong Shale in Example 2, the compressibility of Gulong Shale was divided into four types (Table 8): "low tensile-low toughness-low modulus" type (Q2), "low tensile-low toughness-medium modulus" type (Q1, Q3, Q6), "low tensile-low toughness-high modulus" type (Q7, Q9) and "high tensile-low toughness-high modulus" type (Q4, Q5, Q8).

[0143] The Q2 layer (“low tensile-low toughness-low modulus” type) is mostly developed in the shallow reservoir section of the Gulong area, with moderate bedding cementation. Low tensile strength and low fracture toughness together reduce the difficulty of fracture initiation and propagation. The low deformation modulus corresponds to mechanical deformation mode I, and the macroscopic fracture characteristics of the rock are not significant, often forming a network of microcracks with small apertures and poor connectivity. Enhancing the conductivity of microcracks and promoting competitive propagation of multiple fractures are needed to achieve the desired effect.

[0144] Layers Q1, Q3, and Q6 (“low tensile-low toughness-medium modulus” type) are characterized by easily initiating and propagating fractures in shale, exhibiting a medium deformation modulus (deformation mode II), low-angle intersecting fractures, and no obvious macroscopically penetrating main fractures. Development can induce the propagation of branch fractures, increasing the complexity of the fracture network.

[0145] The Q7 and Q9 layers (“low tensile-low toughness-high modulus” type) are the core sections of high-quality shale reservoirs in the Gulong area. They are mostly thick-layered high-quality reservoirs with good intra-layer homogeneity. Fractures are easy to initiate and propagate, with high deformation modulus (deformation mode III). They are prone to forming large-aperture, high-angle continuous main fractures, accompanied by branch fractures, resulting in a high degree of fracture network complexity.

[0146] Layers Q4, Q5, and Q8 (“high tensile-low toughness-high modulus” type) have high tensile strength, which increases the difficulty of crack initiation (low crack density), and low fracture toughness, which promotes rapid crack propagation after initiation; they also have high deformation modulus (deformation mode III), which makes it easy to form longitudinal through-cracks.

[0147] Table 8. Characteristics of four mechanical models of shale in Gulong District

[0148]

[0149] 3.1.2 Microscopic fracture evolution characteristics of different shale mechanical models:

[0150] The above is the classification result of the Gulong Shale mechanical model based on the "ternary index". To further reveal the microscopic fracture structure characteristics and evolution differences corresponding to different mechanical models, this embodiment loads CT combined with digital core reconstruction results to analyze the fracture development patterns of different types of mechanical models. Three representative layers, Q8 (S1), Q9 (S2), and Q6 (S3) of well rc1, are selected as research objects, corresponding to three mechanical models: "high tensile strength - low toughness - high modulus", "low tensile strength - low toughness - high modulus", and "low tensile strength - low toughness - medium modulus", respectively. The statistical results of their digital core reconstruction parameters are shown in Table 9 below.

[0151] (1) Evolution characteristics of "high tensile strength - low toughness - high modulus" type cracks (S1):

[0152] like Figure 11 As shown, sample S1 had a certain microcrack foundation in the initial loading stage, and obvious crack initiation began in the 100-200s stage; the stress curve showed obvious sawtooth fluctuations in the 200-350s stage, reflecting the local stress release and redistribution during crack propagation; by 550s, a through-crack had formed in the sample, and it entered the post-peak failure stage. Combined with the fracture morphology diagram, it can be seen that the crack in sample S1 gradually evolved from early local initiation to later through-crack control by a single main crack. Its macro-micro evolution characteristics are characterized by identifiable initial damage, early crack initiation, rapid propagation, and obvious strain localization. Overall, it shows that crack initiation is difficult, propagation ability is strong, and concentrated through-crack characteristics.

[0153] Table 9 shows the connectivity parameters of sample S1 at different loading times, where N effCharacterizes the number of effective connected components, reflecting the degree of dispersion of fracture connected units; V eff Characterizes the total volume of the effective connected volume, reflecting the overall scale of crack development; V max The volume of the largest connected component is used to characterize the degree of development of major connectivity breaks; C=V max / V eff The value of C represents the degree to which the largest connected component dominates the overall fracture network; a larger C indicates that the fracture structure is more concentrated under the control of the main fracture. int =V eff -V max This characterizes the internal residual pore volume excluding the largest connected volume; a larger value indicates a more developed dispersed cracks or secondary connected volumes. Combined with... Figure 11 The macroscopic stress response can be summarized into three stages based on the changes in the connected structure of S1.

[0154] (I) Cracking initiation and dispersion propagation stage (1.5s to 200s): N at 100s eff For 3171, V eff It is 0.1428mm. 3 V max It is 0.0425mm 3 The value of C is 0.298, indicating that the overall fracture connectivity is still weak at this stage, the dominant main connecting element has not yet been established, and the fracture system is still mainly characterized by local discrete expansion. By 200s, N... eff Increased to 17339, V eff Increased to 0.8441mm 3 The fractured connected units and the overall connected volume grow rapidly in sync; however, C only changes to 0.307, and V... int Increased to 0.5849mm 3 This indicates that the increase in connected volume mainly comes from the synchronous expansion of multiple dispersed connected bodies, rather than the continuous penetration and centralized control of a single main fracture.

[0155] (II) Merging, Connectivity, and Centralization Abrupt Transition Phase (200s to 350s): Significant abrupt changes occur in the connectivity structure between 200s and 350s. At 350s, V... eff The thickness increased to 2.4045 mm. 3 And V max It is 2.3941mm 3 This causes C to rise sharply from 0.307 to 0.996; correspondingly, V int From 0.5849mm 3 Reduced to 0.0104mm 3 Meanwhile, N effThe decrease from 17339 to 387 indicates that the previously dispersed connected components rapidly merged and were integrated into the main connected component during this stage, resulting in a significant reduction in the number of connected domains and a sharp increase in concentration. This "C jumps and approaches 1 and N..." eff The combined characteristics of "sudden drop" can be used as a quantitative criterion for establishing the main crack connection path.

[0156] (III) Post-connection expansion and post-peak rearrangement stage (350s to 550s): After the connection is formed, the main connected component remains dominant. At 450s, V eff It is 2.7095mm 3 V max It is 2.6923mm 3 The value of C is 0.994, indicating that the crack volume growth is still mainly contributed by the main connected volume. Up to 550 s, V... eff Further increased to 5.6331mm 3 V max It is 5.5903mm 3 Furthermore, C remains at 0.992, indicating that the further increase in crack volume in the post-peak stage is mainly reflected in the expansion process of the existing through-crack. At the same time, Neff decreases to 30, indicating that the crack structure in the post-peak stage has become more concentrated, and the internal structure of the sample has changed from the dispersed development of multiple connected bodies in the early stage to the through-crack dominated by a single main crack in the later stage.

[0157] Table 9. Characteristic parameters of fracture connectivity evolution of sample S1 at different times.

[0158]

[0159] (2) Evolution characteristics of "low tensile strength - low toughness - high modulus" type cracks (S2):

[0160] like Figure 12 As shown, sample S2 exhibited significant primary cracks or initial damage foundations during the initial loading stage. However, the early cracks did not directly propagate along the existing path. Instead, they underwent compaction, local closure, and dispersion / reorganization processes during the 100-400s stage. From 400-500s, the loading phenomenon in the stress curve gradually intensified, crack activity accelerated significantly, and the main crack re-established and rapidly propagated axially. By 750s, a highly penetrating main crack had formed within the sample, entering the post-peak failure stage. Combined with the fracture morphology diagram, it can be seen that the cracks in sample S2 evolved from compaction adjustment under the existing damage foundation in the early stages to penetrating failure controlled by a single main crack in the later stages. Its macro-micro evolution characteristics are characterized by significant initial damage, prominent early compaction and reorganization, and the re-concentration and rapid penetration of the main crack in the later stages. Overall, it exhibits characteristics of easy crack initiation, low propagation resistance, and concentrated penetration.

[0161] Table 10 shows the connectivity parameters of sample S2 at different loading times. The changes in connectivity of S2 can be summarized into three stages.

[0162] (I) Initial defects and early connectivity stage (1.5s to 400s): At 1.5s, sample S2 already had a relatively obvious initial crack foundation, C=0.816, V max =0.1574mm³, in V eff A relatively high proportion of the fractures are 0.1929 mm³, and the fracture structure exhibits strong characteristics of an initial principal fracture, which is consistent with... Figure 13 This corresponds to the axial cracks already visible before the medium-pressure loading. However, these initial cracks did not directly continue to propagate during subsequent loading. As loading progressed, the sample was first subjected to compaction, and the early visible cracks closed, compressed, or had reduced connectivity under pressure, thus reducing the controlling effect of the original main crack. At 100s, N... eff It surged to 11518, while C plummeted to 0.070, and V... int The increase to 0.5141 mm³ indicates that the previously relatively concentrated fracture structure was dispersed during compaction, and the fracture system shifted from being dominated by primary fractures in the early stages to developing in parallel with numerous discrete connected structures. Subsequently, from 200 to 300 s, C continued to decrease to 0.014 and 0.009, V... max Only 0.0034 mm³ and 0.0014 mm³ remained. The original axial cracks had essentially lost their dominant position under continuous pressure, and crack activity was mainly characterized by small-scale, dispersed propagation. By 400 s, although V... eff The concentration rose to 0.7234 mm³, but C was still only 0.083, indicating that the overall structure at this stage was still in a state of dispersed reorganization after compaction and had not yet re-established a stable main fracture.

[0163] (II) Main seam establishment and rapid expansion stage (400s to 500s): 400-500s is the critical stage in which the S2 connected structure transitions from dispersed to concentrated. By 500s, V... eff It increased from 0.7234 mm³ to 1.8109 mm³, while V max The crack density jumped from 0.0598 mm³ to 1.2671 mm³, while C rapidly increased from 0.083 to 0.700. This indicates that the previously dispersed cracks began to converge again along the dominant path, and the main connected body regained control. At this point, crack propagation no longer primarily relied on the synchronous increase of numerous small connected bodies, but rather entered a rapid propagation stage controlled by the main crack. Figure 13 The axial cracks showed a significant increase in intensity around the 500s.

[0164] (III) Merging and Concentration Phase (500s to 750s): After 500s, the crack pattern of S2 further concentrates. By 750s, Veff It is 2.0838 mm³, V max The thickness is 2.0501 mm³, which is very close to the thickness of the other two, with C increasing to 0.984; meanwhile, N... eff From 14801 to 365, V int The fracture volume also decreased to 0.0337 mm³. This indicates that the vast majority of the effective connected volume in the later stage was occupied by a single main connected volume, and the fracture pattern shifted from a dispersed evolution in the middle stage to a highly concentrated one in the later stage. The continued increase in fracture volume in the post-peak stage was mainly manifested in the opening, extension, and local rearrangement of existing main fractures, rather than the generation of a large number of new dispersed fractures. Ultimately, S2 formed a main fracture-controlled pattern with strong penetration and high concentration.

[0165] Table 10. Characteristic parameters of fracture connectivity evolution in sample S2 at different times.

[0166]

[0167] (3) Evolution characteristics of "low tensile-low toughness-intermediate modulus" type cracks (S3):

[0168] like Figure 13 As shown, sample S3 exhibited only scattered microcracks and weak damage characteristics in the initial loading stage, and gradually entered a process of dispersed crack initiation and interconnected growth in the 100-300s stage. In the 300-500s stage, the stress curve showed significant fluctuations, and crack activity continued to increase. However, the propagation process did not converge to be controlled by a single main crack, but rather showed the simultaneous development of multiple cracks and branches after the main crack bud formed. By 750s, multiple through cracks had formed in the sample and intersected with each other, entering the post-peak network failure stage. Combined with the fracture morphology diagram, it can be seen that the cracks in sample S3 gradually evolved from early dispersed initiation to multiple cracks extending in parallel and intersecting in the middle and late stages. Its macro-micro evolution characteristics are characterized by weak initial damage, continuous crack accumulation, multi-branch joint propagation, and obvious network characteristics. Overall, it exhibits characteristics of easy crack initiation, low propagation resistance, and complex network development.

[0169] Table 11 shows the connectivity parameters of sample S3, and the changes in connectivity of S3 can be summarized into three stages.

[0170] (I) Crack Initiation and Dispersion Propagation Stage (1.5s to 300s): The early stage of S3 shows relatively obvious dispersion development characteristics. At 1.5s, V... eff Only 0.0796 mm³, V max =0.0136mm³, C=0.171, the cracks are mainly scattered and weakly connected structures, and no obvious controlling crack has yet formed. Up to 100s, N eff It has increased to 10024, V effThe thickness increased to 0.6719 mm³, but the carbon density only increased to 0.235, indicating that cracks still primarily initiated from multiple points and propagated in a dispersed manner. By 200s, V... max Increased to 0.8100 mm³, C rose to 0.569; to 300 s, V eff Further increased to 4.1655 mm³, V max The volume reached 3.2404 m³, and the concentration (C) increased to 0.778. During this stage, the scale of fracture connectivity continued to increase, the largest connected body gradually strengthened, and the main fracture began to take shape, but the overall structure still retained many branches and auxiliary connected bodies.

[0171] (II) Stable expansion and parallel multi-crack stage (300s to 500s): After entering the 300-500s range, S3 did not rapidly transition to a highly concentrated stage like S1 and S2. Instead, by 400s, V eff The concentration decreased from 4.1655 mm³ to 2.6818 mm³, and C dropped from 0.778 to 0.733; by 500 s, V eff It further decreased to 2.1145 mm³, C continued to decrease to 0.691, while N... eff The level remained above 10,000. This means that although the main crack had formed, its controlling effect did not continue to strengthen; surrounding secondary cracks and branches continued to move synchronously, and the crack structure did not further converge towards a single main crack. The most prominent feature of this stage is the presence of a main crack and the parallel expansion of multiple cracks. Figure 13 The persistent stress fluctuations also correspond to this evolutionary process.

[0172] (III) Post-peak penetration and bulking stage (500s to 750s): By 750s, the crack volume in S3 increases significantly, V eff The volume jumped to 6.4756 mm³, V max Increased to 3.8998mm 3 However, unlike S1 and S2, V int Meanwhile, the fracture volume increased significantly to 2.5759 mm³, while C did not increase further, but instead decreased to 0.602. This indicates that the increase in fracture volume in the post-peak stage is not primarily dependent on the absolute control of a single main fracture, but is accompanied by the participation of a large number of secondary fractures and branches. At the same time, N... eff Although the level dropped from tens of thousands in the early stage to 1313, it was still significantly higher than the later levels of S1 and S2, indicating that although some cracks merged, the overall structure still retained strong multi-connected characteristics. Ultimately, S3 formed a network-like failure structure with multiple intersecting cracks and primary and secondary cracks jointly controlling it, rather than a concentrated failure mode where a single primary crack was completely dominant.

[0173] Table 11. Characteristic parameters of crack connectivity evolution in sample S3 at different times.

[0174]

[0175] Overall, the three typical samples, S1, S2, and S3, exhibit significant differences in fracture initiation timing, propagation patterns, and connectivity evolution characteristics: S1 is characterized by rapid merging and connection of the main fractures; S2 shows compaction and reorganization based on initial damage followed by concentrated connection; and S3 is dominated by parallel propagation of multiple fractures and fracture network evolution. The study of microscopic fracture evolution patterns under different shale mechanical models provides a theoretical basis for fracturing selection and evaluation of the stimulation potential of different strata in the Gulong Shale.

[0176] Imaging logging data further indicates that the development of natural fractures and bedding fractures in the Q1-Q3 intervals is generally weak, with poor fracture connectivity, making it difficult to form an effective natural fracture network. In contrast, natural fractures are mainly concentrated in the Q5, Q7, Q8, and Q9 oil-bearing intervals. This suggests that there are significant differences in fracture development characteristics among different strata of the Gulong Shale. The upper intervals have insufficient fracture development and poor connectivity due to reservoir heterogeneity, while the middle and lower intervals have a better foundation for natural fracture development, providing favorable structural conditions for subsequent pressure fracture propagation. Meanwhile, the Q1-Q9 intervals exhibit obvious vertical differentiation in mineral composition, showing an overall pattern of "more ductile in the upper part and more brittle in the lower part." Among them, the Q1-Q2 layer has a relatively high clay mineral content and strong rock plasticity, making it more prone to plastic deformation rather than brittle fracture under pressure, thus its fracturing potential is relatively weak. In contrast, the Q8-Q9 layer has a high brittle mineral content, significantly enhancing rock brittleness, making cracks easier to initiate and propagate, exhibiting strong fracture-forming ability and superior fracture propagation. Therefore, it is considered a favorable target layer for fracturing in this area. Thus, the evaluation results based on the "ternary index" are consistent with the actual compressibility analysis characteristics.

[0177] 3.2 Comparative Analysis of Fracturability Evaluation Based on Traditional Brittleness Indicators and "Ternary Indicators":

[0178] 3.2.1 Evaluation of the compressibility of Gulong Shale based on brittleness index:

[0179] Currently, traditional brittleness index methods are mainly used in fracturing engineering to evaluate the compressibility of shale. Among them, the Brittleness Index (BI), proposed by existing technology, converts the combined effect of Young's modulus and Poisson's ratio into a dimensionless index with a unified scale, and is widely used to evaluate the mechanical properties and fracturability of rock reservoirs. This method can achieve a unified comparison of elastic brittleness characteristics under multiple layers and multiple sample conditions, thereby identifying the mechanical differences between different layers. In this embodiment, the BI of each sample is calculated based on this method, and the mean and dispersion range of the layers are statistically analyzed to characterize the brittleness differences of each oil layer in the Gulong Shale.

[0180] Existing techniques for the mechanical parameters of North American Barnett Shale propose using the average normalized Young's modulus and Poisson's ratio as a brittleness index to quantitatively characterize the relative brittleness of the shale.

[0181]

[0182] In the formula: E is Young's modulus (GPa); ν is Poisson's ratio (dimensionless); E max E min These represent the maximum and minimum values ​​of Young's modulus within the dataset, respectively; ν max ν min These represent the maximum and minimum values ​​of Poisson's ratio, respectively.

[0183] As shown in the formula, the BI index considers the influence of both Young's modulus and Poisson's ratio on the brittle characteristics of rocks. A higher Young's modulus indicates a stronger resistance to elastic deformation, making it easier to accumulate elastic energy under load and typically exhibiting a more pronounced brittle response. Conversely, a lower Poisson's ratio indicates a relatively weaker lateral deformation capacity, making it less likely to dissipate energy through plastic deformation under stress, thus increasing the tendency towards brittle fracture. Based on this understanding, when a rock has a high Young's modulus and a low Poisson's ratio, its BI value is correspondingly higher, reflecting stronger brittle characteristics; conversely, when the Young's modulus is low or the Poisson's ratio is high, the BI value is relatively low, indicating that the rock is more likely to exhibit ductile deformation characteristics under load. Therefore, based on the formula, the BI values ​​of Gulong Shale at different strata were obtained, as shown in Tables 12 and 13.

[0184] Table 12 Elastic parameters and brittleness index characteristics of samples from different stratigraphic layers of the Gulong Shale.

[0185]

[0186] Table 13 Elastic parameters and brittleness index characteristics of samples from different stratigraphic layers of the Gulong Shale (continued)

[0187]

[0188] Based on the above, a higher BI value indicates greater rock brittleness, which is more conducive to crack formation and propagation. The engineering applications of this index are summarized, indicating that when BI > 40%, the rock is generally considered to have relatively obvious brittle characteristics; when BI > 60%, it corresponds to extremely brittleness, and this value is often used as an empirical threshold in engineering evaluation. Based on this, this embodiment further uses the BI index to evaluate the compressibility of shale in the Gulong area.

[0189] from Figure 14As shown in Tables 12 and 13, a comparative analysis of different strata (Q1-Q9) of the Gulong Shale based on the BI index reveals significant differences in brittle characteristics among the strata, indicating inconsistencies in elastic response and brittle fracture tendency across different strata. Overall, the BI values ​​of the shale in the study area exhibit a gradual transition from relatively weakly brittle to highly brittle, resulting in significant differences in fracture response capabilities under hydraulic fracturing in different strata.

[0190] Looking at the average BI values ​​of the segments, Q1 and Q2 have average BI values ​​of approximately 42.48% and 42.08%, respectively, only slightly above the 40% threshold. While these two segments exhibit some brittleness, they remain relatively weak overall. Q3 and Q6 have average BI values ​​of approximately 47.65% and 45.18%, respectively, also falling within the 40%-60% range, indicating a slightly higher brittleness level than Q1 and Q2, but still not reaching the level of highly brittle segments. Individual sample distributions show that while some samples in these four segments have higher BI values, most samples remain below 60%, with Q1 and Q6 exhibiting greater dispersion and stronger intra-segment heterogeneity. Therefore, Q1, Q2, Q3, and Q6 can be classified as segments with some brittleness but generally weak, exhibiting relatively limited crack initiation and propagation capabilities during fracturing.

[0191] The average BI for Q4 and Q5 are approximately 58.43% and 58.48% respectively, both approaching the 60% threshold. The fragility levels of these two segments are significantly higher than the aforementioned lower-value segments, indicating they have entered the medium to high fragility range. Figure 14 As can be seen, most samples in Q4 and Q5 fall within the medium-to-high value range, with some exceeding 60%, reflecting that these two intervals already possess a good foundation for brittle fracture. The average BI of Q9 is approximately 60.21%, generally reaching or slightly exceeding the 60% threshold, indicating its brittleness level is approaching that of a highly brittle interval. Compared to Q4 and Q5, Q9 exhibits stronger overall brittleness, but some differences still exist between samples. Therefore, Q4, Q5, and Q9 can be considered favorable intervals in the Gulong Shale representing the transition from medium to highly brittle fracture, possessing good potential for fracturing and re-emergence.

[0192] The average BI of Q7 and Q8 are approximately 65.45% and 67.10% respectively, which are significantly higher than the 60% threshold, making them the two most brittle segments among all segments. Figure 14Individual samples from these two layers are predominantly distributed in the higher ranges, indicating their prominent brittleness characteristics. Specifically, samples from Q8 are more concentrated, showing better overall stability; while Q7, although having a higher average value, exhibits a slightly wider sample dispersion, suggesting some intralayer variation. Overall, Q7 and Q8 are more conducive to elastic energy accumulation and brittle fracture, making them more susceptible to crack initiation and propagation under fracturing conditions, and more likely to form effective fracture systems. These can be considered the preferred brittleness-dominant layers in this region.

[0193] Comprehensive analysis shows that the BI values ​​of the various layers of the Gulong Shale can be roughly divided into three categories: Q1, Q2, Q3 and Q6 are mainly located in the 40%-60% range, belonging to layers with certain brittleness but overall weakness; Q4, Q5 and Q9 are close to or reach the 60% threshold, belonging to medium to high brittleness layers; Q7 and Q8 significantly exceed 60%, and are high brittleness dominant layers.

[0194] 3.2.2 Analysis of the differences between the BI indicator and the "three-element indicator" evaluation:

[0195] Based on the BI index, the compressibility of each layer of the Gulong Shale was evaluated, and its results were further compared with those of the "ternary index." The comparison showed that the BI index and the "ternary index" exhibited some consistency in the evaluation of the Gulong Shale's compressibility; that is, Q1, Q2, Q3, and Q6 were generally relatively weak, while Q4, Q5, Q7, Q8, and Q9 were generally better. This indicates that the traditional BI index can reflect the differences in compressibility characteristics of different layers of the Gulong Shale to a certain extent. However, since it is essentially a comprehensive brittleness index based on elastic parameters, it is difficult to fully reflect its fracturing potential. In contrast, the "ternary index" decomposes shale compressibility into three aspects: fracture initiation, propagation, and fracture formation mode. This allows for a more comprehensive explanation of the differences between different layers, while maintaining a generally consistent overall trend.

[0196] Q1, Q2, Q3, and Q6 all fall within the 40%-60% range in the BI evaluation, and are generally classified as brittle to weak strata, with no significant differences between strata. However, in the "ternary index" evaluation, Q2 exhibits a "low tensile-low toughness-low modulus" pattern, indicating relatively low resistance to fracture initiation and propagation, but weak overall fracture-forming capacity. Q1, Q3, and Q6, on the other hand, exhibit a "low tensile-low toughness-medium modulus" pattern, suggesting similarities to Q2 in fracture initiation and propagation, but a greater tendency to form dispersed fractures. In other words, for strata with similar BI values, the fracture-forming capacity of shale may differ, leading to varying compressibility.

[0197] Q4, Q5, and Q9 are classified as medium to high brittle zones in the BI evaluation, indicating that these zones generally have good fracturing potential. The "ternary index" results show that Q4 and Q5 belong to the "high tensile-low toughness-high modulus" type, characterized by relatively difficult fracture initiation, but strong fracture propagation ability once initiated, and a greater likelihood of forming well-connected fractures. Q9, on the other hand, belongs to the "low tensile-low toughness-high modulus" type, indicating that it not only has strong fracture propagation ability and good fracture formation conditions, but also a lower fracture initiation threshold, which is more conducive to fracture initiation and continuous propagation during fracturing. Therefore, in terms of overall fracturing effect, Q9 is superior to Q4 and Q5, and also superior to some zones belonging to the high modulus type.

[0198] Both Q7 and Q8 exhibited high brittleness dominance in the BI evaluation, with Q8 having the highest BI value. Based on traditional brittleness indicators, it should possess the best fracturing potential. However, the "ternary index" results show that Q7 belongs to the "low tensile-low toughness-high modulus" type, while Q8 belongs to the "high tensile-low toughness-high modulus" type. Although both have strong fracture propagation ability and good fracture generation patterns, Q8 is not superior in fracture initiation. Further comparison with Q9 reveals that Q9 also exhibits "low toughness-high modulus" characteristics, but its fracture initiation resistance is lower than Q8, making it more favorable for fracturing initiation and propagation. Therefore, if only the BI value is used for segment ranking, Q8 can be identified as the optimal segment; however, based on the comprehensive evaluation of the "ternary index," Q9, due to its combination of low initiation resistance, low propagation resistance, and high fracture generation ability, has a better overall fracturing effect than Q8 and is more suitable as a priority fracturing segment.

[0199] Overall, compressibility assessment based on the "brittleness concept" is merely a descriptive assessment of the outcome and cannot characterize the fracture-forming features of shale. Therefore, there are instances where assessment results are similar, but compressibility effects vary significantly. In contrast, the "ternary index" assessment method based on the "fracture-forming concept" can not only quantitatively characterize compressibility but also more clearly reveal the gradual evolution of fractures from initiation to propagation and then to fracture pattern formation. This represents a paradigm shift from "outcome description" to "deformation mechanism," providing a new theoretical perspective for shale reservoir compressibility assessment.

[0200] This embodiment analyzes the differences in compressibility of different strata of the Gulong Shale. Combining the evaluation results of the "ternary index" and the time-series reconstruction of digital cores from different mechanical models, the fracture-forming characteristics of shale at different strata are comprehensively characterized from two aspects: macroscopic fracture response and microscopic fracture evolution. The results are compared with those of the traditional brittleness index (BI) evaluation, revealing the superiority of the compressibility evaluation method based on the "fracture-forming concept".

[0201] (1) Based on the evaluation results of the “three-element index”, the shale in the Gulong area is divided into four mechanical models: “low tension-low toughness-low modulus” type (Q2 layer), which is easy to initiate and propagate fractures, belongs to deformation mode I, with small fracture aperture or weak fracture-making ability; “low tension-low toughness-medium modulus” type (Q1, Q3, Q6 layers), which is easy to initiate and propagate fractures, belongs to deformation mode II, and develops low-angle cross fracture network; “low tension-low toughness-high modulus” type (Q7, Q9 layers), which is easy to initiate and propagate fractures, belongs to deformation mode III, and develops large aperture and high-angle through fracture network; “high tension-low toughness-high modulus” type (Q4, Q5, Q8 layers), which is difficult to initiate fractures, has low fracture density but is easy to propagate, belongs to deformation unit III, and develops large aperture and high-angle through shear fracture.

[0202] (2) Combining the digital core time-series reconstruction results, fracture evolution connectivity parameters, and axial stress-time response curves, the fracture evolution process of shale samples under different mechanical models was analyzed. The results show that there are significant differences in the number of effective interconnections, effective volume, proportion of the largest interconnection, and final fracture morphology among samples under different mechanical models. Among them, the S1 sample ("high tensile-low toughness-high modulus" type) shows that the stress required for fracture initiation is large, the fractures merge rapidly, and finally form a through main fracture; the S2 sample ("low tensile-low toughness-high modulus" type) shows that the fracture initiation difficulty is low, with a long period of dispersed fracture accumulation and expansion, and the establishment of the through main fracture is relatively delayed; the S3 sample ("low tensile-low toughness-medium modulus" type) shows that the main fracture expands and is accompanied by more obvious branch fracture proliferation, eventually forming a more complex fracture network.

[0203] (3) The evaluation results based on the BI index show that there are significant differences in the brittleness characteristics of different oil layers in the Gulong Shale: the Q1-Q3 and Q6 layers are generally weaker, while the Q4-Q5 and Q7-Q9 layers are generally better. Compared with the evaluation results of the "ternary index", the two methods have good consistency in the overall trend of compressibility of the layers and can identify the basic distribution characteristics of low-value layers and dominant layers. However, the BI index mainly characterizes the overall brittleness of the rock based on elastic parameters and can reflect the overall difference in compressibility of layers. It is suitable for the preliminary compressibility evaluation of reservoirs. The "ternary index" can further reveal the differences in compressibility layers from three aspects: fracture initiation, propagation and fracture formation mode, thus providing a more powerful basis for the selection of fracture layers.

[0204] Example 4: Based on the same inventive concept, this example also provides a rock compressibility evaluation device based on the fracture initiation-propagation-fracture network index for implementing the rock compressibility evaluation method based on the fracture initiation-propagation-fracture network index described above. The solution provided by this device is similar to the solution described in the above method. Therefore, the specific limitations of one or more embodiments of a rock compressibility evaluation device based on the fracture initiation-propagation-fracture network index provided below can be found in the limitations of the rock compressibility evaluation method based on the fracture initiation-propagation-fracture network index described above, and will not be repeated here.

[0205] In an embodiment, such as Figure 15 As shown, a rock compressibility evaluation device based on the crack initiation-propagation-fracture network index is provided, comprising:

[0206] The parameter acquisition module 1101 is used to perform Brazilian splitting test, fracture toughness test and triaxial compression test on the target shale sample to obtain the tensile strength value, fracture toughness value and mechanical deformation parameters of the target shale sample.

[0207] The index calculation module 1102 is used to calculate the crack initiation index, crack propagation index, and crack network index based on the tensile strength value, fracture toughness value, and mechanical deformation parameters; wherein, the crack initiation index is calculated based on the normalized tensile strength value, the crack propagation index is calculated based on the normalized fracture toughness value, and the crack network index is calculated based on the normalized mechanical deformation parameters.

[0208] Evaluation module 1103 is used to perform a three-dimensional comprehensive quantitative evaluation of the mechanical properties of the target shale sample based on the calculated crack initiation index, crack propagation index, and crack network index, and on a preset evaluation standard.

[0209] Example 5: In this example, a computer device is provided. This computer device can be a server or a terminal, and its internal structure diagram can be as follows. Figure 16As shown, the computer device includes a processor, memory, input / output (I / O) interfaces, and a communication interface. The processor, memory, and I / O interfaces are connected via a system bus, and the communication interface is also connected to the system bus via the I / O interfaces. The processor provides computational and control capabilities. The memory includes non-volatile storage media and internal memory. The non-volatile storage media stores the operating system, computer programs, and a database. The internal memory provides the environment for the operation of the operating system and computer programs in the non-volatile storage media. The database stores rock compressibility evaluation results. The I / O interfaces are used for information exchange between the processor and external devices. The communication interface is used for communication with external terminals via a network connection. When the computer program is executed by the processor, it implements a rock compressibility evaluation method based on the fracture initiation-propagation-fracture network index.

[0210] Those skilled in the art will understand that Figure 16 The structure shown is merely a block diagram of a portion of the structure related to the present application and does not constitute a limitation on the computer device to which the present application is applied. Specific computer devices may include more or fewer components than those shown in the figure, or combine certain components, or have different component arrangements.

[0211] In this embodiment, a computer device is also provided, including a memory and a processor. The memory stores a computer program, and the processor executes the computer program to implement the steps in the above-described method embodiments.

[0212] In one exemplary embodiment, a computer-readable storage medium is provided storing a computer program that, when executed by a processor, implements the steps in the above-described method embodiments.

[0213] It should be noted that the user information (including but not limited to user device information, user personal information, etc.) and data (including but not limited to data used for analysis, data stored, data displayed, etc.) involved in this application are all information and data authorized by the user or fully authorized by all parties, and the collection, use and processing of the relevant data must comply with relevant regulations.

[0214] Those skilled in the art will understand that all or part of the processes in the above embodiments can be implemented by a computer program instructing related hardware. The computer program can be stored in a non-volatile computer-readable storage medium. When executed, the computer program can include the processes of the embodiments described above. Any references to memory, databases, or other media used in the embodiments provided in this application can include at least one of non-volatile and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory can include random access memory (RAM) or external cache memory, etc. By way of illustration and not limitation, RAM can take many forms, such as Static Random Access Memory (SRAM) or Dynamic Random Access Memory (DRAM).

[0215] The databases involved in the embodiments provided in this application may include at least one type of relational database and non-relational database. Non-relational databases may include, but are not limited to, blockchain-based distributed databases. The processors involved in the embodiments provided in this application may be general-purpose processors, central processing units, graphics processing units, digital signal processors, programmable logic devices, quantum computing-based data processing logic devices, etc., and are not limited to these.

[0216] In summary, this application has the following technical effects:

[0217] This application provides a method and related apparatus for evaluating rock compressibility based on the fracture initiation-propagation-fracture network index. For shale samples from all oil layers (Q1-Q9) in the Gulong area, triaxial compression tests, tensile strength tests, and fracture toughness tests were conducted on core samples. A total of 101 samples were tested, obtaining key mechanical parameters such as Young's modulus, Poisson's ratio, compressive strength, tensile strength, and fracture toughness. A physical model of relative tensile strength index was established based on the tensile strength value, and a physical model of relative fracture toughness index was established based on the fracture toughness value. The mechanical parameters of the target shale samples were determined based on the triaxial compression test, and a coupled mechanical deformation modulus E of Young's modulus and differential stress coefficient was established. F A physical model for the relative mechanical deformation modulus index was established. Considering three relative evaluation indices, a ternary index method for compressibility evaluation based on fracture initiation, propagation, and fracture network was constructed to evaluate the compressibility of shale in all oil layers (Q1-Q9) in the Gulong area. This application addresses the shortcomings of existing evaluation methods and provides a new technical approach for the study of shale compressibility.

[0218] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0219] This document uses specific examples to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the methods and core ideas of this application. Furthermore, those skilled in the art will recognize that, based on the ideas of this application, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A method for evaluating the compressibility of shale based on the initiation-propagation-fracture network index, comprising conducting Brazilian splitting tests, fracture toughness tests, and triaxial compression tests on target shale samples to obtain the tensile strength, fracture toughness, and mechanical deformation parameters of the target shale samples; characterized in that, Includes the following steps: Step (1): Based on the tensile strength value, fracture toughness value and mechanical deformation parameters, calculate the crack initiation index, crack propagation index and crack network index; The crack initiation index is calculated based on the normalized tensile strength value, and the calculation formula for the crack initiation index is as follows: Where T is the tensile strength of the oil layer where the target shale sample is located in the test area, T max It is the maximum tensile strength in the oil reservoir of the test area, T min It is the minimum tensile strength in the oil layer of the test area; The crack propagation index is calculated based on the normalized fracture toughness value, and the calculation formula for the crack propagation index is as follows: Where K is the fracture toughness of the oil layer where the target shale sample is located in the test area, K max It is the maximum fracture toughness in the oil reservoir of the test area, K min It is the lowest fracture toughness in the oil layer of the test area; The calculation formula for the crack network index is based on the normalized calculation of mechanical deformation parameters: Among them, E F E represents the mechanical deformation modulus of the oil layer containing the target shale sample in the test area. Fmax It is the maximum mechanical deformation modulus in the oil reservoir of the test area, E Fmin It is the minimum mechanical deformation modulus in the oil layer of the test area; Step (2): Based on the inherent mechanical properties of rock and external stress conditions, the rock fracture mode control mechanism is constructed using Young's mode parameters and differential stress coefficients to describe the differences in rock fracture modes. The mechanical deformation modulus based on the construction needs to satisfy three conditions, given by the following formula; () indicates that the mechanical deformation modulus is a function of Young's modulus parameter and the differential stress coefficient; Using a trial function: in, The compressive strength of the rock. For confining pressure, and , k is the differential stress coefficient. The larger the Young's modulus E, the larger the differential stress coefficient k, and the larger the mechanical deformation modulus E. F The larger the value, the easier it is for a single high-angle crack to develop after the rock fractures, and the easier the crack propagates; conversely, the smaller the mechanical deformation modulus E, the easier it is for the rock to develop a single high-angle crack. F A smaller mechanical modulus E makes it easier to form dispersed, intersecting cracks, creating a crack network, and preventing crack propagation; however, when the mechanical deformation modulus E... F When the rock is very small, it undergoes plastic deformation and is not prone to cracking; where E is Young's modulus and g is the mechanical deformation modulus. F The normalization coefficient, where e is the natural base; Step (3): Based on the calculated crack initiation index, crack propagation index, and crack network index, and based on the preset evaluation criteria, perform a three-dimensional comprehensive quantitative evaluation of the mechanical properties of the target shale sample, including crack initiation, propagation, and crack network.

2. The shale compressibility evaluation method based on the initiation-propagation-fracture network index according to claim 1, characterized in that, The process for determining the mechanical deformation parameters is as follows: The stress-strain curve of the target shale sample was determined based on triaxial compression experiments; Based on the rock stress-strain curve, the mechanical deformation parameters of the target shale sample were determined; The mechanical deformation parameters include Young's modulus, Poisson's ratio, and compressive strength.

3. The shale compressibility evaluation method based on the initiation-propagation-fracture network index according to claim 1, characterized in that, The evaluation criteria specifically include: When the average crack initiation index (FCI) Average When the fracture initiation index (FCI) is >60%, the target shale sample is determined to be a low-tension shale; when the average fracture initiation index (FCI) is >60%, the target shale sample is determined to be a low-tension shale. Average When the fracture initiation index (FCI) is less than 60%, the target shale sample is determined to be high-tensile shale; Average This represents the average value of the fracture initiation index for multiple target shale samples; When the average crack propagation index FGI Average When the fracture density is >60%, the target shale sample is identified as low-toughness shale; when the average fracture propagation index (FGI) is >60%, the target shale sample is identified as low-toughness shale. Average When the fracture propagation index (FGI) is less than 60%, the target shale sample is determined to be high-toughness shale; Average This represents the average value of the fracture propagation index from multiple target shale samples; When 30% < Fragmentation-Meridian Index (FMI) < 45%, the target shale sample is identified as medium-modulus shale; when FMI > 45%, the target shale sample is identified as high-modulus shale. Mechanical deformation modulus E F <10GPa indicates indistinct macroscopic rock fracturing, small crack aperture, and weak fracture-forming ability; mechanical deformation modulus is between 10GPa. <E F Under stress loads below 15 GPa, strain tends to disperse, leading to dispersed fractures and the formation of complex, low-angle fracture networks; the mechanical deformation modulus E F >15GPa, strain concentration, easy to develop high-angle through-crack network cracks.

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