# Mechanical Properties of Shale-Reservoir Rocks Based on Stress–Strain Curves and Mineral Content

Authors: Wen-Tie Sun; Zhong-Hui Li; Yi-Shan Lou; Liang Zhu; Hui-Mei Wu; Arnaud Regis Kamgue Lenwoue; Qin Liu
Journal: Geofluids
 (2022)

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## Abstract

The evaluation of rock mechanic characteristics of shale reservoir is the key for the success of fracturing. Several limitations such as complex lithology can affect the accuracy of the rock mechanic property computation. In this research, the mechanical rock properties of shale reservoirs were analyzed based on experimental tests including shale-reservoir mineral composition, uniaxial compressive strength, and triaxial compression tests. The results showed that (1) the difference in mineral composition leads to easier fracturing of sand shale reservoirs compared with pure shale reservoir. (2) Under uniaxial conditions, the rock mechanical parameters along the vertical bedding direction of sand shale reservoirs are better than pure shale reservoir parameters. The mechanical properties of shale reservoirs in parallel bedding direction are less affected by lithology. Under triaxial conditions, the confining pressure increases more than 4.0 times with the compressive strength of rocks, and the rock Young’s modulus in the parallel bedding direction is higher compared with the vertical bedding direction. (3) Rock failure mode is mainly subdivided into cutting and splitting modes, and the bedding features of rock mechanic properties highly influence the drilling and fracturing operations. This study offers guidelines for the optimization of the drilling and fracturing process based on accurate evaluation of the rock mechanical properties.


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## Body

## 1. Introduction

Shale oil resources are mainly located in the United States, Russia, and China and are characterized by low porosity and permeability. They require large-scale hydraulic fracturing to enable industrial productivity [1–7]. An efficient development of hydraulic fracturing requires that shale reservoirs possess natural fractures, and these natural fractures should be easily modified to form artificial fractures [8–11]. With the continuous development in shale-oil exploration, researches have been focusing on the mechanical properties of rocks because they are considered as important factors for safe, fast, and efficient drilling and fracturing design. Many methods including brittleness index evaluation method and stress-strain curve evaluation method are used to evaluate the mechanical properties of shale reservoir rock [12, 13].The brittleness index evaluation method is mainly based on rock mineral content or logging data calculation of rock mechanic parameters. The brittleness index calculation model is established in order to characterize the rock fracturing features. Jarvie et al. [14] calculated the brittleness index of shale by evaluating the quartz content in the shale. They demonstrated that shale with high quartz content has good fracturing performance; Rickman et al. [15] argued that Young’s modulus and Poisson’s ratio can reflect the brittleness of the shale; Diao [16] established the elastic parameter and mineral grouping method (EP&MC Method) and obtained the quantitative evaluation of single well brittleness. Xia et al. [17] improved the accuracy of the brittleness evaluation method by establishing a mathematical model of fracturing coefficient which considers the shale mineral composition, mechanical parameters, diagenesis, and confining pressure.The stress-strain curve evaluation method mainly considers that the mechanical properties of rock are affected by its own heterogeneity and the external testing environment. Based on rock mechanic experiments, Martin, Labani and Rezaee, and Xia et al. [18–20] conducted different experimental tests to analyze the variation of stress-strain curve, and they summarized the variation laws of rock mechanic properties of shale reservoir. The heterogeneity of shale reservoir itself is mainly reflected in the differences in bedding and core angles. Islam and Skalle [21] confirmed the heterogeneity of shale reservoir by analyzing the results of triaxial test and CT scanning experiment in different sampling directions.Jia et al. and Wang et al. [22, 23] demonstrated that the bedding direction and the shale reservoir degree of purity under complex stress environment affect the mechanical properties of the reservoir. Hou et al. and Wang et al. [24, 25] analyzed the impact of coring angle on failure and deformation characteristics of shale and summarized the deformation characteristics of stress-strain curve under different coring angle conditions; Li et al. [26] proposed an index that considered the prepeak and postpeak mechanical characteristics of the stress-strain curve and evaluated the brittleness characteristics of shale. They found that the shale outside testing environment of the reservoirs mainly embodies the confining pressure, temperature, and drilling fluid performance difference. Lo et al. [27] studied the elastic anisotropy of Chicopee shale under different confining pressures and proved that Chicopee still has residual anisotropy under high confining pressures due to the arrangement of mineral particles inside the shale. Niandou et al. [28] carried out conventional triaxial test loading and unloading test for Tournemire shale and demonstrated that the studied shale sample had plastic characteristics and anisotropic plastic deformation. Kuila et al. [29] studied the complex stress environment caused by shale anisotropic characteristics and demonstrated that the studied shale has high inherent anisotropy characteristics under high confining pressure. He et al. [30] investigated the variation rules of peak strain, peak strength (bias stress), and elastic modulus of shale with the formation dip angle under different confining pressures based on experimental conditions. Rutter [31] and Wong [32] provided a detailed summary of brittle failure studies of rocks and concluded that rock failure included single shear plane failure, double shear plane failure, and splitting failure. Hou et al. [24] also summarized the failure modes of rocks under different confining pressures and demonstrated that rocks under low confining pressures are dominated by splitting and double shear failure modes, while rocks under high confining pressures are dominated by shear failure. Jiang et al. [33] analyzed the pore and fracture evolution law of coal and shale reservoirs under triaxial stress by means of CT scanning and real-time acoustic emission (AE) signal monitoring test. According to the characteristics of the stress-strain curve, the fracture and pore expansion are divided into three stages: initial, expansion, and penetration. Feng et al. [34, 35] analyzed failure modes of conjugate cracks under static and dynamic loads by using energy dissipation and fractal dimension and concluded that with the strain rate increase, the dynamic strength of rock significantly increased. Chen et al., Helstrup et al. , Zhen et al., and Liang et al. [36–39] demonstrated that drilling fluid immersion has a great influence on the mechanical properties of shale. They found that water or oil wet rock in different extent changed the mechanical strength of the rock; Mohamadi and Wan [40] studied the effect of temperature on the mechanical properties of Colorado shales and demonstrated that strength of the rock increases with the temperature.Previous research results on the mechanical properties and failure modes of shale reservoirs mostly focused on one influencing factor, while in the actual geological conditions, shale reservoirs are affected by various factors such as lithology, bedding, and confining pressure. Therefore, previous researches achievements have certain one-sidedness. Songliao Basin is rich in shale oil resources [41–43]. At present, there are few studies on the rock mechanical characteristics of shale oil reservoir in Songliao Basin, and the lack of applicability of experience in other blocks restricts the exploration and development process in this area. In this study, the core analysis data of the central depression Qingshankou Formation in the southern Songliao Basin including the mineral composition analysis obtained by indoor experimental tests, stress–strain relationship, and types of shale, as well as direction and magnitude of the confining pressure of the shale reservoir rock, were used to investigate the mechanical characteristics and changes in the internal mineralogic composition of shale reservoir (brittle mineral composition, bedding) in order to provide a theoretical basis for safe and efficient exploration and development of the shale oil reservoir in the Songliao Basin.


## 2. Geological Background of Shale Samples

Shale oil resources in China are concentrated in 50 sedimentary basins, which have different types of shale oil-bearing basins, shale oil formation conditions, deposit size and quality, and source of organic matter [44, 45]. As a potential important shale oil resource, Songliao Basin requires further investigation. Currently, the exploration area of shale oil in the Qingshankou Formation in the central depression of the southern Songliao Basin is 15,000 km2 with a predicted resource quantity of more than 15 billion tons (Figure 1). With the expansion in the exploration and development in this area, several wells exhibited high-yield industrial oil flow, showing a good prospect for exploration and development [46–48].Figure 1
(a) Generalized map showing location of Songliao basins. (b) Simplified structure distribution map and the positions of the borehole in this study.The southern part of Songliao Basin is a large continental lacustrine basin with rich organic matter and mature source rocks, providing rich material basis for shale oil [49–53]. Drilling and core analysis data have demonstrated that many shale reservoir stratifications are developed, and drilling cores are mostly flaky. Shale reservoir rock mineral composition and lithologic assemblages are controlled by sedimentary facies. According to the difference of rock mineral content and lithologic combination characteristics of shale reservoir, shale can be divided into pure shale and sand shale. Sedimentary area brittleness mineral content in deep lake formation is generally less than 40%; the clay mineral content is more than 60%, and the lithology is pure type shale reservoir. Shale reservoirs in the fan delta front area generally contain 50%-80% brittle minerals and 20%-60% clay minerals. Lithology is dominated by sand shale reservoirs (Figure 2).Figure 2
Core photo of Qingshankou Formation shale drilled in the central depression of southern Songliao Basin.
(a)
Well C34-7 (pure shale type)
(b)
Well X381 (pure shale type)
(c)
Well H238 (sand shale type)


## 3. Materials and Methods

This study was conducted on typical pure shale and sand shale. The rock mechanic properties were investigated using two types of core samples (a total of 20 pieces of rock). X-ray diffraction (XRD) analysis was performed on 53 block samples. Samples were obtained from the core layer of Qingshankou Formation at a sampling depth ranging from 1800 to 2500 m.


### 3.1. X-Ray Diffraction Experimental Equipment and Methods

The analysis was conducted using XRD Terra mineral composition tester developed by Enos Corporation. The rock sample was first ground to powder and was then transferred into the rock sample tank for testing. After completion of the experiment, the obtained peak values of the sample were compared to the database card to find the mineral composition corresponding to each peak value. Finally, the mineral composition of the sample was evaluated.


### 3.2. Rock Mechanic Experimental Equipment and Methods

Uniaxial compression test is the basic test method of rock mechanical characteristic determination. Most of the classical rock mechanic theories are based on this theory which can reflect the mechanical characteristics of rock to a certain extent. In the process of field sampling, the stress state of rock sample is changed from undisturbed true triaxial stress state to uniaxial stress state. The degree of compaction among the rock skeleton compaction grains changes from compact to loose, which leads to the change of effective stress. The variation range of skeleton strength greatly varies among different lithologies, and the variation range of brittle rocks such as sandstone, carbonate, and volcanic rock is small. The strength change caused by stress release can be approximately ignored. Gypsum rock, mudstone, and other plastic lithology, due to the stress release caused by a large range of strength changes, cannot be ignored. There are no such reports in the literature, and the lithology studied in this paper does not involve plastic lithology such as gypsum rock and mudstone; so, it will not be discussed here.The strength change caused by stress release can be approximately ignored. For plastic lithology such as gypsum rock and mudstone, the strength change caused by stress release cannot be ignored. The lithology studied in this paper does not involve plastic lithology such as gypsum rock and mudstone, which is not discussed here. The test was conducted in YSD-type 2 uniaxial tester and GCTS high temperature and high pressure on the rock triaxial test system. Due to the limited number of core samples, core samples with coring angles of 0° (parallel bedding) and 90° (vertical bedding) were selected as the standard shale samples for the uniaxial and triaxial stress–strain tests.Before mechanical tests, the core samples from the Qingshankou Formation were processed into a standard cylindrical rock sample. First, the core was extracted using a diamond drill bit. Next, the two ends of the shale rock sample were ground on the grinding machine until a flat surface was obtained on both ends of the rock sample to ensure they are smooth, parallel (parallelism error is less than 0.01 mm), and perpendicular to the central axis (angle deviation is less than 0.05°). Since rock is susceptible to crack during the sampling, cooling using small water volumes and lubrication was employed. In addition, low speed core sampling was used to avoid damage caused by repeated drilling. The test was conducted under confining pressure range of 0–40 MPa.


## 3.1. X-Ray Diffraction Experimental Equipment and Methods

The analysis was conducted using XRD Terra mineral composition tester developed by Enos Corporation. The rock sample was first ground to powder and was then transferred into the rock sample tank for testing. After completion of the experiment, the obtained peak values of the sample were compared to the database card to find the mineral composition corresponding to each peak value. Finally, the mineral composition of the sample was evaluated.


## 3.2. Rock Mechanic Experimental Equipment and Methods

Uniaxial compression test is the basic test method of rock mechanical characteristic determination. Most of the classical rock mechanic theories are based on this theory which can reflect the mechanical characteristics of rock to a certain extent. In the process of field sampling, the stress state of rock sample is changed from undisturbed true triaxial stress state to uniaxial stress state. The degree of compaction among the rock skeleton compaction grains changes from compact to loose, which leads to the change of effective stress. The variation range of skeleton strength greatly varies among different lithologies, and the variation range of brittle rocks such as sandstone, carbonate, and volcanic rock is small. The strength change caused by stress release can be approximately ignored. Gypsum rock, mudstone, and other plastic lithology, due to the stress release caused by a large range of strength changes, cannot be ignored. There are no such reports in the literature, and the lithology studied in this paper does not involve plastic lithology such as gypsum rock and mudstone; so, it will not be discussed here.The strength change caused by stress release can be approximately ignored. For plastic lithology such as gypsum rock and mudstone, the strength change caused by stress release cannot be ignored. The lithology studied in this paper does not involve plastic lithology such as gypsum rock and mudstone, which is not discussed here. The test was conducted in YSD-type 2 uniaxial tester and GCTS high temperature and high pressure on the rock triaxial test system. Due to the limited number of core samples, core samples with coring angles of 0° (parallel bedding) and 90° (vertical bedding) were selected as the standard shale samples for the uniaxial and triaxial stress–strain tests.Before mechanical tests, the core samples from the Qingshankou Formation were processed into a standard cylindrical rock sample. First, the core was extracted using a diamond drill bit. Next, the two ends of the shale rock sample were ground on the grinding machine until a flat surface was obtained on both ends of the rock sample to ensure they are smooth, parallel (parallelism error is less than 0.01 mm), and perpendicular to the central axis (angle deviation is less than 0.05°). Since rock is susceptible to crack during the sampling, cooling using small water volumes and lubrication was employed. In addition, low speed core sampling was used to avoid damage caused by repeated drilling. The test was conducted under confining pressure range of 0–40 MPa.


## 4. Results

### 4.1. X-Ray Diffraction Test

According to the whole-rock mineral composition analysis of 53 shale-debris samples from the Qingshankou Formation, the mineral composition of different types of shale reservoirs differs at different levels (Figure3). The mineral composition of shale reservoir in the Qingshankou Formation can be divided into two categories: brittle minerals and clay minerals. The brittle minerals mainly include quartz, potassium feldspar, plagioclase, calcite, and pyrite; among them, quartz and feldspar are more abundant. Sand shale and pure shale reservoirs quartz and feldspar contents are, respectively, 28.1%–85.3% (average=43.72%) and 20.0%–55.3% (average=37.69%). Clay minerals include illite, chlorite, and kaolinite. Except for kaolinite, no significant difference was observed in the components content. The content of clay minerals in sand shale and pure shale reservoirs was, respectively, 9.7%–66.9% (average=51.27%) and 39.7%–72.8% (average=57.47%). The clay mineral content of sand shale reservoir is less than that of pure shale reservoir.Figure 3
The shale mineral composition of the Qingshankou Formation in the central depression of southern Songliao Basin.


### 4.2. Rock Mechanic Experiment

#### 4.2.1. Characteristics of Stress–Strain Curves under Uniaxial Compression

Figure4(a) shows the stress–strain curves of the shale under uniaxial compression. The stress–strain curves of shale-reservoir rocks are classified as typical class II curves [54, 55]. The stress–strain curves significantly differ according to the stratification, lithology, and other factors. However, the extent of the effects of these factors is not clear. Some shale rock samples went through a short pore and fracture compaction stage and quickly entered the elastic deformation stage at the early stage of elastic deformation, while others experienced a relatively long pore and fracture compaction stage. Since the elastic and yield limits are very close, the slope of the curve is almost unchanged; hence, the axial strain greatly varies with a strain in a range between 0.3% and 2.05%. The postpeak damage of most rock samples was uncontrollable, and the postpeak curve exhibited some zigzags which reflects an unstable fault propagation. Shale reservoir bedding causes a deterioration in the heterogeneity of shale reservoir, which may result in the formation of more complex fracture networks during the hydraulic fracturing process of shale reservoirs.Figure 4
Stress-strain curve of shale reservoir under uniaxial and triaxial condition.
(a)
Uniaxial condition
(b)
Triaxial condition


#### 4.2.2. Characteristics of Stress–Strain Curves under Triaxial Compression

Rock samples obtained at similar depths were selected for triaxial compression experiments to determine their mechanical characteristics. The effects of the bedding direction and difference of confining pressure on the stress–strain curve characteristics under triaxial compression were analyzed under confining pressure of 0, 15, 25, 35, and 40 MPa. The mechanical properties of the rocks were then analyzed.Figure4(b) shows the stress–strain curves of a shale reservoir under triaxial compression. The curves indicated significant brittleness characteristics in which almost no pores or crack compaction stages were observed in the sample before elastic stage. All stress–strain curves were almost straight lines before the peak. The rock compressive strength gradually increased with the confining pressure until the sample started to fail. The stress-strain curve of rock sample after failure is affected by the release of energy stored in rock and equipment, showing a certain axial strain variable, which indirectly indicates the enhancement of lateral confining pressure.


## 4.1. X-Ray Diffraction Test

According to the whole-rock mineral composition analysis of 53 shale-debris samples from the Qingshankou Formation, the mineral composition of different types of shale reservoirs differs at different levels (Figure3). The mineral composition of shale reservoir in the Qingshankou Formation can be divided into two categories: brittle minerals and clay minerals. The brittle minerals mainly include quartz, potassium feldspar, plagioclase, calcite, and pyrite; among them, quartz and feldspar are more abundant. Sand shale and pure shale reservoirs quartz and feldspar contents are, respectively, 28.1%–85.3% (average=43.72%) and 20.0%–55.3% (average=37.69%). Clay minerals include illite, chlorite, and kaolinite. Except for kaolinite, no significant difference was observed in the components content. The content of clay minerals in sand shale and pure shale reservoirs was, respectively, 9.7%–66.9% (average=51.27%) and 39.7%–72.8% (average=57.47%). The clay mineral content of sand shale reservoir is less than that of pure shale reservoir.Figure 3
The shale mineral composition of the Qingshankou Formation in the central depression of southern Songliao Basin.


## 4.2. Rock Mechanic Experiment

### 4.2.1. Characteristics of Stress–Strain Curves under Uniaxial Compression

Figure4(a) shows the stress–strain curves of the shale under uniaxial compression. The stress–strain curves of shale-reservoir rocks are classified as typical class II curves [54, 55]. The stress–strain curves significantly differ according to the stratification, lithology, and other factors. However, the extent of the effects of these factors is not clear. Some shale rock samples went through a short pore and fracture compaction stage and quickly entered the elastic deformation stage at the early stage of elastic deformation, while others experienced a relatively long pore and fracture compaction stage. Since the elastic and yield limits are very close, the slope of the curve is almost unchanged; hence, the axial strain greatly varies with a strain in a range between 0.3% and 2.05%. The postpeak damage of most rock samples was uncontrollable, and the postpeak curve exhibited some zigzags which reflects an unstable fault propagation. Shale reservoir bedding causes a deterioration in the heterogeneity of shale reservoir, which may result in the formation of more complex fracture networks during the hydraulic fracturing process of shale reservoirs.Figure 4
Stress-strain curve of shale reservoir under uniaxial and triaxial condition.
(a)
Uniaxial condition
(b)
Triaxial condition


### 4.2.2. Characteristics of Stress–Strain Curves under Triaxial Compression

Rock samples obtained at similar depths were selected for triaxial compression experiments to determine their mechanical characteristics. The effects of the bedding direction and difference of confining pressure on the stress–strain curve characteristics under triaxial compression were analyzed under confining pressure of 0, 15, 25, 35, and 40 MPa. The mechanical properties of the rocks were then analyzed.Figure4(b) shows the stress–strain curves of a shale reservoir under triaxial compression. The curves indicated significant brittleness characteristics in which almost no pores or crack compaction stages were observed in the sample before elastic stage. All stress–strain curves were almost straight lines before the peak. The rock compressive strength gradually increased with the confining pressure until the sample started to fail. The stress-strain curve of rock sample after failure is affected by the release of energy stored in rock and equipment, showing a certain axial strain variable, which indirectly indicates the enhancement of lateral confining pressure.


## 4.2.1. Characteristics of Stress–Strain Curves under Uniaxial Compression

Figure4(a) shows the stress–strain curves of the shale under uniaxial compression. The stress–strain curves of shale-reservoir rocks are classified as typical class II curves [54, 55]. The stress–strain curves significantly differ according to the stratification, lithology, and other factors. However, the extent of the effects of these factors is not clear. Some shale rock samples went through a short pore and fracture compaction stage and quickly entered the elastic deformation stage at the early stage of elastic deformation, while others experienced a relatively long pore and fracture compaction stage. Since the elastic and yield limits are very close, the slope of the curve is almost unchanged; hence, the axial strain greatly varies with a strain in a range between 0.3% and 2.05%. The postpeak damage of most rock samples was uncontrollable, and the postpeak curve exhibited some zigzags which reflects an unstable fault propagation. Shale reservoir bedding causes a deterioration in the heterogeneity of shale reservoir, which may result in the formation of more complex fracture networks during the hydraulic fracturing process of shale reservoirs.Figure 4
Stress-strain curve of shale reservoir under uniaxial and triaxial condition.
(a)
Uniaxial condition
(b)
Triaxial condition


## 4.2.2. Characteristics of Stress–Strain Curves under Triaxial Compression

Rock samples obtained at similar depths were selected for triaxial compression experiments to determine their mechanical characteristics. The effects of the bedding direction and difference of confining pressure on the stress–strain curve characteristics under triaxial compression were analyzed under confining pressure of 0, 15, 25, 35, and 40 MPa. The mechanical properties of the rocks were then analyzed.Figure4(b) shows the stress–strain curves of a shale reservoir under triaxial compression. The curves indicated significant brittleness characteristics in which almost no pores or crack compaction stages were observed in the sample before elastic stage. All stress–strain curves were almost straight lines before the peak. The rock compressive strength gradually increased with the confining pressure until the sample started to fail. The stress-strain curve of rock sample after failure is affected by the release of energy stored in rock and equipment, showing a certain axial strain variable, which indirectly indicates the enhancement of lateral confining pressure.


## 5. Discussion

### 5.1. Effects of the Differences in Lithology and Bedding on the Rock Mechanical Properties of Shale Reservoir under Uniaxial Conditions

The differences in reservoir lithology and bedding direction affect the mechanical properties of rocks at different level. Both factors often cause some inaccuracies in the research. In this study, certain limiting conditions were set during the research process, and the effect of one of the two factors on the mechanical properties of rocks was analyzed without considering the effect of the other factor. Thus, the effect of a single factor on the mechanical properties was evaluated, minimizing the interference of other factors and its effect on the research results.


#### 5.1.1. Effects of Lithologic Differences on the Mechanical Properties of Shale Reservoir Rocks

The axial direction parallel to the shale reservoir bedding is less variable; hence, the sample experiencing a transient pressure phase rapidly enters into the linear elastic deformation stage. The axial stress differences between the samples were small. The compressive strength range was 41.34–48.07 MPa (average=45.77MPa); hence, the ranges of axial strain, Poisson’s ratio, and Young’s modulus were 0.4%–0.6%, 0.12–0.28 (average=0.2), and 7.00–20.08 GPa (average=12.63GPa), respectively. The lithology of the stress–strain curve was found to be affected by small deformation or damage in the sample. No significant changes were observed in the mechanical properties of the rocks of pure shale and sand shale reservoirs with parallel bedding direction (Figure 5(a), Table 1).Figure 5
Stress-strain curves of shale reservoirs in different bedding directions under uniaxial compression.
(a)
Parallel to the bedding
(b)
Vertical beddingTable 1
Rock mechanical parameters of shale reservoir of the Qingshankou Formation under uniaxial compression in different bedding directions (parallel bedding).
Sample number
Core direction
Compressive strength (MPa)
Young’s modulus (GPa)
Poisson’s ratio
Lithology types
TY1-7-2
Parallel
48.07
20.08
0.20
Pure shale
H258-8-1
Parallel
41.34
10.81
0.28
Sand shale
TY1-3-1
Parallel
47.90
7.00
0.12
Pure shaleSignificant changes were observed in the mechanical properties of the reservoir rocks with vertical bedding direction which indirectly reflect that the relative development of microfractures and small pores in the shale reservoir and the elastic stage and yield stage on the stress-strain curve are not obvious. The mechanical properties of the reservoir rock are greatly affected by the lithology, the stress–strain curve, and reservoir type. Sand shale reservoirs have a compressive strength far larger than pure shale reservoirs (maximum value approximately equals 37.77 MPa). The axial peak strain greatly fluctuated with minimum and maximum values of 0.38% and 2.05%, respectively. Young’s modulus in the same lithologic reservoir ranges from 3.13 to 11.23 GPa (average value equals 4.88 GPa). Although small differences were observed in the compressive strength, large ones were observed in Young’s modulus (a maximum difference of 3.2 times was observed between rock samples H258-8-3 and H258-8-3). Poisson’s ratio ranges from 0.10 to 0.15 (average=0.12) (Figure 5(b), Table 2).Table 2
Rock mechanical parameters of shale reservoir of the Qingshankou Formation under uniaxial compression in different bedding directions (vertical bedding).
Sample number
Core direction
Compressive strength (MPa)
Young’s modulus (GPa)
Poisson’s ratio
Lithology types
H238-8-3
Vertical
59.80
3.57
0.12
Sand shale
H258-8-3
Vertical
62.89
11.23
0.15
Sand shale
TY1-3-2
Vertical
26.85
1.92
0.12
Pure shale
TY1-4-3
Vertical
25.12
3.13
0.10
Pure shale


#### 5.1.2. Effects of Bedding Differences on the Mechanical Properties of Shale Reservoir Rocks

Differences in the stress–strain curves of pure shale reservoirs are caused by differences in the bedding direction. Rocks with parallel bedding direction experienced a transient pressure phase and rapidly entered into the elastic deformation stage. The axial strain of this type was between 0.38% and 0.52%, while rocks with vertical bedding direction were between 1.04% and 1.4%. Certain differences were observed in the compressive strength, Poisson’s ratio, and Young’s modulus (Tables1 and 2) of the two rock types. The rock with parallel bedding direction exhibited better mechanical properties compared with vertical bedding direction. Therefore, the difference in the bedding direction significantly affects the mechanical properties of rocks (Figure 6(a), Tables 1 and 2).Figure 6
Stress-strain curves of shale reservoirs with different lithologic conditions under uniaxial compression.
(a)
Pure shale type
(b)
Sand shale typeThe compressive strength of sand shale reservoirs with vertical bedding direction is greater than the pure shale reservoir compressive strength. The rock mechanical properties of different reservoirs that have the same lithology also exhibit certain differences. This was reflected in the large difference in Young’s modulus of samples H258-8-3 and H238-8-3. On the other hand, the differences in other parameters, such as compressive strength and Poisson’s ratio, were very small (Figure6(b), Tables 1 and 2).The experimental results confirm that when the rock sample is stressed in the vertical bedding direction, it is mainly affected by the rock deposition and compaction degree, and the reservoir exhibits strong heterogeneity. The rocks slip and deform along the weak structural plane of the bedding with large radial deformation. The stratigraphic level development of pure shale reservoirs is higher than sand shale reservoirs’ stratigraphic level, and deformation can easily occur in the pure shale reservoirs. The above experimental results are consistent with those of Hou et al. and Wang et al. [24, 25], which further verifies the accuracy of the experimental results.


### 5.2. Relationship between Mineral Composition and Rock Mechanical Properties under Uniaxial Condition

Labani and Rezaee and Pan et al. [19, 56] demonstrated that there is a close relationship between rock mineral composition and rock mechanical properties and shale oil accumulation mechanism [52, 57]. Their researches demonstrated that the main factors controlling the amount of retained hydrocarbons in shale are lithologic assemblage, effective migration channels, microfracture development, and micropore characteristics, and the fracture system of shale reservoir in this area is not developed. The main reservoir space is a large number of microscopic pores in clay minerals. The clay minerals and organic matter are wrapped together in shale reservoir, and the microscopic pores in clay minerals are arranged in a directional manner, which effectively improves the seepage capacity of the reservoir. At the same time, the clay minerals are mostly flake structures (see Figure 2), which make the rock materials anisotropic, thus causing differences in rock mechanical properties due to different clay mineral components and shale reservoir types.Figure5 indicates certain differences in the rock mineral composition results. For example, the pure shale reservoir has a higher clay mineral composition and less brittle mineral content, and the core in this area exhibits brittle plasticity. Higher clay content does not stimulate reservoir fracturing. The sand shale reservoir has higher brittle mineral content, which enhances shale fracturing.By analyzing the X-ray diffraction (XRD) results, the relationship between the mineral composition and the mechanical properties of rock was summarized by using the mineral composition content detection data of 13 rock samples and the mechanical parameters measured by stress-strain curves (Figure7, Table 3). According to the analysis results in Figures 7(a)–7(c) and Table 3, the mechanical properties of sandstone shale reservoir rock have a good correlation with the content of clay minerals. The mechanical properties are mainly affected by the content of clay minerals, and the rock mechanical properties of shale reservoir are different due to the content of clay minerals in shale reservoir. The results in Figures 7(d)–7(f) and Table 4 confirm that the mechanical properties of pure shale reservoir are not significantly affected by clay mineral composition content, and the correlation of each single parameter is poor, lacking certain regularity.Figure 7
Relation diagram of clay mineral content and rock mechanical parameters in shale reservoir.
(a)
Clay mineral-Young’s modulus relationship
(b)
Clay mineral-Poisson ratio relationship
(c)
Clay minerals-Young’s modulus/Poisson’s ratio relationship
(d)
Clay mineral-Young’s modulus relationship
(e)
Clay mineral-Poisson ratio relationship
(f)
Clay minerals-Young’s modulus/Poisson’s ratio relationshipTable 3
Statistical table of mineral composition and rock mechanical properties of shale reservoir in the Qingshankou Formation (sand shale).
Sample number
Mineral content (%)
Mechanical properties
Quartz
Potash feldspar
Plagioclase
Calcite
Pyrite
Other
Clay minerals
Young’s modulus (GPa)
Poisson’s ratio
Young’s modulus/Poisson’s ratio (GPa)
H258-17
20
2
6.6
4.1
0
0
67.3
3.17
0.25
12.68
23-22-47
44.3
3
27.3
6.8
0
0
18.6
15.52
0.23
67.48
H258-24
25.7
2
7.2
4.5
0
0
60.6
4.91
0.36
13.64
H197-7
24.5
3.4
14
5.3
3.5
0
49.3
14.59
0.32
45.59
H238-8
22.9
2.9
15.1
17.2
3.6
0
38.3
13.57
0.25
54.28Table 4
Statistical table of mineral composition and rock mechanical properties of shale reservoir in the Qingshankou Formation (pure shale).
Sample number
Mineral content (%)
Mechanical properties
Quartz
Potash feldspar
Plagioclase
Calcite
Pyrite
Other
Clay minerals
Young’s modulus (GPa)
Poisson’s ratio
Young’s modulus/Poisson’s ratio (GPa)
7-62-43
25
1.2
15.6
8.9
6.1
3
40.2
8.48
0.41
20.47
17-61-66
28.1
1.1
12.9
3.4
6.4
6.3
41.8
3.25
0.32
10.15
19-37-63
30.9
0.9
8.4
6.6
4
0
49.2
15.94
0.13
125.79
20-45-67
28.2
1
8.8
9.6
4.8
0
47.6
20.08
0.20
99.39
H258-3
21.3
1.4
14.1
10.3
11.1
0
41.8
4.56
0.22
20.73
h258-10
25.2
2
15.9
8.6
4.4
0
43.9
6.70
0.26
25.77
X381-3
20.5
1.6
9.6
10.8
2.7
0
54.8
4.56
0.22
20.73
D86-11
25.8
0.4
6.2
6.6
1.7
0
59.3
8.46
0.14
60.43


### 5.3. Effects of Confining Pressure on Rock Mechanical Properties under Triaxial Conditions

Confining pressure has a great effect on rock mechanical properties [58]. Figures 8(a) and 8(b) illustrate the triaxial stress–strain curve of rock specimens under confining pressure. The axial strain in this case should be smaller (generally less than 1%). Under confining pressure, the ultimate compressive strength increases with the decrease in the number of microcracks generated in the rock sample. In addition, the confining pressure has an inhibiting effect on the expansion of microcracks, and this effect increases with the confining pressure. Moreover, the rock sample will reach the failure state with only a small deformation. The compressive strength of rock gradually increases with the confining pressure. When the confining pressure increased from 0 to 40 MPa, the compressive strength of the sand shale reservoir rock increased from 41.34 to 219.34 MPa (approximately 5.3-times). Young’s modulus also increased, but the amplitude of the increase is relatively small, whereas Poisson’s ratio showed no significant change (Table 5). The average compressive strength of pure shale reservoir rock sample increased from 47.9 to 187.96 MPa, (approximately 4.0 times), and rock mechanical properties change rule and other basic same type sand shale reservoir (Table 6).Figure 8
Stress-strain curve of shale reservoir under different confining pressures (parallel to the bedding).
(a)
Sand shale type
(b)
Pure shale typeTable 5
Rock mechanical parameters of triaxial compression in shale reservoirs of the Qingshankou Formation under different confining pressures (parallel bedding-sand shale).
Sample number
Confining pressure (MPa)
Compressive strength (MPa)
Young’s modulus (GPa)
Poisson’s ratio
H197-8-1
40
219.34
23.32
0.23
H258-5-1
35
149.77
16.60
0.22
H258-2
25
123.58
15.84
0.24
H258-7-1
15
88.09
10.05
0.35
H258-8-1
0
41.34
10.81
0.28Table 6
Rock mechanical parameters of triaxial compression in shale reservoirs of the Qingshankou Formation under different confining pressures (parallel bedding-pure shale).
Sample number
Confining pressure (MPa)
Compressive strength (MPa)
Young’s modulus (GPa)
Poisson’s ratio
TY1-2-2
40
187.96
25.68
0.26
TY1-3-3
35
143.66
23.35
0.12
TY1-7-1
25
115.72
17.69
0.25
C347-11-1
15
87.58
11.13
0.26
TY1-3-1
0
47.90
7.00
0.12


### 5.4. Effect of Bedding on the Mechanical Properties of Rocks under Triaxial Conditions

Core samples in two directions (i.e., 0°, parallel bedding, and 90°, vertical bedding) were selected to carry out the analysis of mechanical properties while ignoring the effects of lithology and confining pressure on the rock mechanical properties (Figures9(a) and 9(b) as well as Tables 7 and 8). The compressive strength and Young’s modulus of sandstone-shale and pure shale reservoirs under the same confining pressure condition are significantly higher in the direction of parallel bedding in comparison with those in the direction of vertical bedding. Shale reservoir is obviously controlled by the internal weak structure of bedding face rock mechanical properties, leading to a significant difference in the axial strain. The axial strain in case of parallel bedding direction is less than 1.1%. Typically, the axial strain of pure shale reservoirs is less than that of sand shale reservoir. The axial strain of shale reservoirs with vertical bedding direction is within the range of 1.2–1.6%. In reservoirs with parallel direction of the bedding rock, tensile fracturing occurs under the effect of axial stress along the bedding direction. The axial stress due to the vertical stratification plane, loaded bedding plane, and superposition effect leads to compact bedding plane, closed bedding plane, and small angle of the sliding deformation. Thus, reservoirs with vertical beddings have larger axial strain, higher Poisson’s ratio, and lower Young’s modulus than those of reservoirs with parallel beddings.Figure 9
Stress-strain curves of shale reservoirs in different bedding directions (confining pressure 35 MPa).
(a)
Sand shale type
(b)
Pure shale typeTable 7
Rock mechanical parameter table of shale reservoir under triaxial compression under different bedding conditions of the Qingshankou Formation (confining pressure 35 MPa sand shale).
Sample number
Bedding direction
Compressive strength (MPa)
Young’s modulus (GPa)
Poisson’s ratio
H258-4-2
Vertical
149.34
6.0
0.28
H258-5-1
Parallel
149.77
16.60
0.22
H258-6-2
Parallel
138.58
13.13
0.24Table 8
Rock mechanical parameter table of shale reservoir under triaxial compression under different bedding conditions of Qingshankou Formation (confining pressure 35 MPa pure shale).
Sample number
Bedding direction
Compressive strength (MPa)
Young’s modulus (GPa)
Poisson’s ratio
TY1-5-4
Vertical
83.96
13.21
0.31
TY1-3-4
Vertical
115.72
13.62
0.27
TY1-3-3
Parallel
143.66
23.35
0.12
C347-11-2
Parallel
137.58
22.14
0.22


### 5.5. Failure Mode of Shale Reservoir

#### 5.5.1. Uniaxial Failure Mode Analysis

Figure10 summarizes the failure modes of rock samples in vertical and parallel bedding directions under uniaxial conditions in this area. It can be seen that the failure mode fully reflects the failure mode of brittle layered rock. Figure 10(a) shows that when the included angle is 90° in the direction of vertical bedding, the rock sample is mainly damaged by shear failure at an angle along the direction of bedding, and double shear failure occurs in each laminated block. Meanwhile, the CT scan results in Figure 10(d) confirm that uniaxial fractures in the direction of vertical bedding open along the two bedding, forming parallel bedding conjugate fractures (Figure 10(c)), with relatively serious damage at the bottom of the rock sample and obvious end effect at the bottom. Figure 10(b) shows that when the angle of bedding plane angle is 0°, the rock sample cleavage failure occurs in the direction of parallel bedding without obvious block falling. Meanwhile, the CT scan results in Figure 10(d) confirm that uniaxial fractures in the direction of vertical bedding open along two bedding, forming bedding conjugate fractures.Figure 10
Typical photos of core failure modes in uniaxial compression experiments. Note: (a) H258-11-4 (vertical bedding). (b) D86-1-H2 (parallel to the bedding). (c) TY1-3-2 (vertical bedding CT scan). (d) TY1-3-1 (parallel bedding CT scans).
(a)
(b)
(c)
(d)


#### 5.5.2. Triaxial Failure Mode Analysis

The failure mode of shale under triaxial confining pressure is mainly influenced by additional confining pressure on the basis of uniaxial compression. Figures11(a)–11(c) illustrate the pictures of rock failure in different bedding directions. Vertical direction of the bedding rock top produces shear joint fellowship with bedding which are affected by bedding shear and do not extended to the bottom of the rock. The half bottom of the rock almost has no crack extension, and the sample is mainly composed of single shear failure. Figures 11(b)–11(d) demonstrate that parallel bedding direction can be obviously shear rock seam fellowship with bedding. After fracture, the crossbedding continues to expand to the bottom until the sample cracks. The shale bedding surface and microcracks are formed, and these natural weak surfaces affect the failure results during the test, causing the crack surface to be destroyed along the direction of bedding and form multiple splitting failure, also known as crossconjugate seam [34, 35].Figure 11
Typical image of core failure mode in triaxial compression experiment. Note: (a) TY1-2-4 (vertical bedding). (b) C347-11-1 (parallel to the bedding). (c) TY1-3-4 (vertical bedding CT scan). (d) TY1-3-3 (parallel bedding CT scans).
(a)
(b)
(c)
(d)From the perspective of failure effect, the splitting failure is more complete than the shear plane failure mode and produces more failure structures. The rock surface needs to have certain strength and poor homogeneity. This is because under loading, sample within the coordinate deformation increases, local area increases in the number of microcracks, and microcracks resulted in partial discharge reduce the intensity of rock body. On the contrary, the higher the degree of homogeneity, the stronger the deformation resistance of the rock sample, and the less the development of microcracks, the less energy dissipation, the higher the strength, and the stronger the plasticity.In the process of field drilling and fracturing, most of the cores have parallel bedding. The rock failure mode is mostly obvious along the direction of bedding displacement, which indicates that the bedding plane deposited in the shale reservoir causes various anisotropy changes in strength and deformation. For example, in the process of hydraulic fracturing, the fracturing fluid enters the bedding plane, and it is easy to break the bedding and difficult to form network fractures.


## 5.1. Effects of the Differences in Lithology and Bedding on the Rock Mechanical Properties of Shale Reservoir under Uniaxial Conditions

The differences in reservoir lithology and bedding direction affect the mechanical properties of rocks at different level. Both factors often cause some inaccuracies in the research. In this study, certain limiting conditions were set during the research process, and the effect of one of the two factors on the mechanical properties of rocks was analyzed without considering the effect of the other factor. Thus, the effect of a single factor on the mechanical properties was evaluated, minimizing the interference of other factors and its effect on the research results.


### 5.1.1. Effects of Lithologic Differences on the Mechanical Properties of Shale Reservoir Rocks

The axial direction parallel to the shale reservoir bedding is less variable; hence, the sample experiencing a transient pressure phase rapidly enters into the linear elastic deformation stage. The axial stress differences between the samples were small. The compressive strength range was 41.34–48.07 MPa (average=45.77MPa); hence, the ranges of axial strain, Poisson’s ratio, and Young’s modulus were 0.4%–0.6%, 0.12–0.28 (average=0.2), and 7.00–20.08 GPa (average=12.63GPa), respectively. The lithology of the stress–strain curve was found to be affected by small deformation or damage in the sample. No significant changes were observed in the mechanical properties of the rocks of pure shale and sand shale reservoirs with parallel bedding direction (Figure 5(a), Table 1).Figure 5
Stress-strain curves of shale reservoirs in different bedding directions under uniaxial compression.
(a)
Parallel to the bedding
(b)
Vertical beddingTable 1
Rock mechanical parameters of shale reservoir of the Qingshankou Formation under uniaxial compression in different bedding directions (parallel bedding).
Sample number
Core direction
Compressive strength (MPa)
Young’s modulus (GPa)
Poisson’s ratio
Lithology types
TY1-7-2
Parallel
48.07
20.08
0.20
Pure shale
H258-8-1
Parallel
41.34
10.81
0.28
Sand shale
TY1-3-1
Parallel
47.90
7.00
0.12
Pure shaleSignificant changes were observed in the mechanical properties of the reservoir rocks with vertical bedding direction which indirectly reflect that the relative development of microfractures and small pores in the shale reservoir and the elastic stage and yield stage on the stress-strain curve are not obvious. The mechanical properties of the reservoir rock are greatly affected by the lithology, the stress–strain curve, and reservoir type. Sand shale reservoirs have a compressive strength far larger than pure shale reservoirs (maximum value approximately equals 37.77 MPa). The axial peak strain greatly fluctuated with minimum and maximum values of 0.38% and 2.05%, respectively. Young’s modulus in the same lithologic reservoir ranges from 3.13 to 11.23 GPa (average value equals 4.88 GPa). Although small differences were observed in the compressive strength, large ones were observed in Young’s modulus (a maximum difference of 3.2 times was observed between rock samples H258-8-3 and H258-8-3). Poisson’s ratio ranges from 0.10 to 0.15 (average=0.12) (Figure 5(b), Table 2).Table 2
Rock mechanical parameters of shale reservoir of the Qingshankou Formation under uniaxial compression in different bedding directions (vertical bedding).
Sample number
Core direction
Compressive strength (MPa)
Young’s modulus (GPa)
Poisson’s ratio
Lithology types
H238-8-3
Vertical
59.80
3.57
0.12
Sand shale
H258-8-3
Vertical
62.89
11.23
0.15
Sand shale
TY1-3-2
Vertical
26.85
1.92
0.12
Pure shale
TY1-4-3
Vertical
25.12
3.13
0.10
Pure shale


### 5.1.2. Effects of Bedding Differences on the Mechanical Properties of Shale Reservoir Rocks

Differences in the stress–strain curves of pure shale reservoirs are caused by differences in the bedding direction. Rocks with parallel bedding direction experienced a transient pressure phase and rapidly entered into the elastic deformation stage. The axial strain of this type was between 0.38% and 0.52%, while rocks with vertical bedding direction were between 1.04% and 1.4%. Certain differences were observed in the compressive strength, Poisson’s ratio, and Young’s modulus (Tables1 and 2) of the two rock types. The rock with parallel bedding direction exhibited better mechanical properties compared with vertical bedding direction. Therefore, the difference in the bedding direction significantly affects the mechanical properties of rocks (Figure 6(a), Tables 1 and 2).Figure 6
Stress-strain curves of shale reservoirs with different lithologic conditions under uniaxial compression.
(a)
Pure shale type
(b)
Sand shale typeThe compressive strength of sand shale reservoirs with vertical bedding direction is greater than the pure shale reservoir compressive strength. The rock mechanical properties of different reservoirs that have the same lithology also exhibit certain differences. This was reflected in the large difference in Young’s modulus of samples H258-8-3 and H238-8-3. On the other hand, the differences in other parameters, such as compressive strength and Poisson’s ratio, were very small (Figure6(b), Tables 1 and 2).The experimental results confirm that when the rock sample is stressed in the vertical bedding direction, it is mainly affected by the rock deposition and compaction degree, and the reservoir exhibits strong heterogeneity. The rocks slip and deform along the weak structural plane of the bedding with large radial deformation. The stratigraphic level development of pure shale reservoirs is higher than sand shale reservoirs’ stratigraphic level, and deformation can easily occur in the pure shale reservoirs. The above experimental results are consistent with those of Hou et al. and Wang et al. [24, 25], which further verifies the accuracy of the experimental results.


## 5.1.1. Effects of Lithologic Differences on the Mechanical Properties of Shale Reservoir Rocks

The axial direction parallel to the shale reservoir bedding is less variable; hence, the sample experiencing a transient pressure phase rapidly enters into the linear elastic deformation stage. The axial stress differences between the samples were small. The compressive strength range was 41.34–48.07 MPa (average=45.77MPa); hence, the ranges of axial strain, Poisson’s ratio, and Young’s modulus were 0.4%–0.6%, 0.12–0.28 (average=0.2), and 7.00–20.08 GPa (average=12.63GPa), respectively. The lithology of the stress–strain curve was found to be affected by small deformation or damage in the sample. No significant changes were observed in the mechanical properties of the rocks of pure shale and sand shale reservoirs with parallel bedding direction (Figure 5(a), Table 1).Figure 5
Stress-strain curves of shale reservoirs in different bedding directions under uniaxial compression.
(a)
Parallel to the bedding
(b)
Vertical beddingTable 1
Rock mechanical parameters of shale reservoir of the Qingshankou Formation under uniaxial compression in different bedding directions (parallel bedding).
Sample number
Core direction
Compressive strength (MPa)
Young’s modulus (GPa)
Poisson’s ratio
Lithology types
TY1-7-2
Parallel
48.07
20.08
0.20
Pure shale
H258-8-1
Parallel
41.34
10.81
0.28
Sand shale
TY1-3-1
Parallel
47.90
7.00
0.12
Pure shaleSignificant changes were observed in the mechanical properties of the reservoir rocks with vertical bedding direction which indirectly reflect that the relative development of microfractures and small pores in the shale reservoir and the elastic stage and yield stage on the stress-strain curve are not obvious. The mechanical properties of the reservoir rock are greatly affected by the lithology, the stress–strain curve, and reservoir type. Sand shale reservoirs have a compressive strength far larger than pure shale reservoirs (maximum value approximately equals 37.77 MPa). The axial peak strain greatly fluctuated with minimum and maximum values of 0.38% and 2.05%, respectively. Young’s modulus in the same lithologic reservoir ranges from 3.13 to 11.23 GPa (average value equals 4.88 GPa). Although small differences were observed in the compressive strength, large ones were observed in Young’s modulus (a maximum difference of 3.2 times was observed between rock samples H258-8-3 and H258-8-3). Poisson’s ratio ranges from 0.10 to 0.15 (average=0.12) (Figure 5(b), Table 2).Table 2
Rock mechanical parameters of shale reservoir of the Qingshankou Formation under uniaxial compression in different bedding directions (vertical bedding).
Sample number
Core direction
Compressive strength (MPa)
Young’s modulus (GPa)
Poisson’s ratio
Lithology types
H238-8-3
Vertical
59.80
3.57
0.12
Sand shale
H258-8-3
Vertical
62.89
11.23
0.15
Sand shale
TY1-3-2
Vertical
26.85
1.92
0.12
Pure shale
TY1-4-3
Vertical
25.12
3.13
0.10
Pure shale


## 5.1.2. Effects of Bedding Differences on the Mechanical Properties of Shale Reservoir Rocks

Differences in the stress–strain curves of pure shale reservoirs are caused by differences in the bedding direction. Rocks with parallel bedding direction experienced a transient pressure phase and rapidly entered into the elastic deformation stage. The axial strain of this type was between 0.38% and 0.52%, while rocks with vertical bedding direction were between 1.04% and 1.4%. Certain differences were observed in the compressive strength, Poisson’s ratio, and Young’s modulus (Tables1 and 2) of the two rock types. The rock with parallel bedding direction exhibited better mechanical properties compared with vertical bedding direction. Therefore, the difference in the bedding direction significantly affects the mechanical properties of rocks (Figure 6(a), Tables 1 and 2).Figure 6
Stress-strain curves of shale reservoirs with different lithologic conditions under uniaxial compression.
(a)
Pure shale type
(b)
Sand shale typeThe compressive strength of sand shale reservoirs with vertical bedding direction is greater than the pure shale reservoir compressive strength. The rock mechanical properties of different reservoirs that have the same lithology also exhibit certain differences. This was reflected in the large difference in Young’s modulus of samples H258-8-3 and H238-8-3. On the other hand, the differences in other parameters, such as compressive strength and Poisson’s ratio, were very small (Figure6(b), Tables 1 and 2).The experimental results confirm that when the rock sample is stressed in the vertical bedding direction, it is mainly affected by the rock deposition and compaction degree, and the reservoir exhibits strong heterogeneity. The rocks slip and deform along the weak structural plane of the bedding with large radial deformation. The stratigraphic level development of pure shale reservoirs is higher than sand shale reservoirs’ stratigraphic level, and deformation can easily occur in the pure shale reservoirs. The above experimental results are consistent with those of Hou et al. and Wang et al. [24, 25], which further verifies the accuracy of the experimental results.


## 5.2. Relationship between Mineral Composition and Rock Mechanical Properties under Uniaxial Condition

Labani and Rezaee and Pan et al. [19, 56] demonstrated that there is a close relationship between rock mineral composition and rock mechanical properties and shale oil accumulation mechanism [52, 57]. Their researches demonstrated that the main factors controlling the amount of retained hydrocarbons in shale are lithologic assemblage, effective migration channels, microfracture development, and micropore characteristics, and the fracture system of shale reservoir in this area is not developed. The main reservoir space is a large number of microscopic pores in clay minerals. The clay minerals and organic matter are wrapped together in shale reservoir, and the microscopic pores in clay minerals are arranged in a directional manner, which effectively improves the seepage capacity of the reservoir. At the same time, the clay minerals are mostly flake structures (see Figure 2), which make the rock materials anisotropic, thus causing differences in rock mechanical properties due to different clay mineral components and shale reservoir types.Figure5 indicates certain differences in the rock mineral composition results. For example, the pure shale reservoir has a higher clay mineral composition and less brittle mineral content, and the core in this area exhibits brittle plasticity. Higher clay content does not stimulate reservoir fracturing. The sand shale reservoir has higher brittle mineral content, which enhances shale fracturing.By analyzing the X-ray diffraction (XRD) results, the relationship between the mineral composition and the mechanical properties of rock was summarized by using the mineral composition content detection data of 13 rock samples and the mechanical parameters measured by stress-strain curves (Figure7, Table 3). According to the analysis results in Figures 7(a)–7(c) and Table 3, the mechanical properties of sandstone shale reservoir rock have a good correlation with the content of clay minerals. The mechanical properties are mainly affected by the content of clay minerals, and the rock mechanical properties of shale reservoir are different due to the content of clay minerals in shale reservoir. The results in Figures 7(d)–7(f) and Table 4 confirm that the mechanical properties of pure shale reservoir are not significantly affected by clay mineral composition content, and the correlation of each single parameter is poor, lacking certain regularity.Figure 7
Relation diagram of clay mineral content and rock mechanical parameters in shale reservoir.
(a)
Clay mineral-Young’s modulus relationship
(b)
Clay mineral-Poisson ratio relationship
(c)
Clay minerals-Young’s modulus/Poisson’s ratio relationship
(d)
Clay mineral-Young’s modulus relationship
(e)
Clay mineral-Poisson ratio relationship
(f)
Clay minerals-Young’s modulus/Poisson’s ratio relationshipTable 3
Statistical table of mineral composition and rock mechanical properties of shale reservoir in the Qingshankou Formation (sand shale).
Sample number
Mineral content (%)
Mechanical properties
Quartz
Potash feldspar
Plagioclase
Calcite
Pyrite
Other
Clay minerals
Young’s modulus (GPa)
Poisson’s ratio
Young’s modulus/Poisson’s ratio (GPa)
H258-17
20
2
6.6
4.1
0
0
67.3
3.17
0.25
12.68
23-22-47
44.3
3
27.3
6.8
0
0
18.6
15.52
0.23
67.48
H258-24
25.7
2
7.2
4.5
0
0
60.6
4.91
0.36
13.64
H197-7
24.5
3.4
14
5.3
3.5
0
49.3
14.59
0.32
45.59
H238-8
22.9
2.9
15.1
17.2
3.6
0
38.3
13.57
0.25
54.28Table 4
Statistical table of mineral composition and rock mechanical properties of shale reservoir in the Qingshankou Formation (pure shale).
Sample number
Mineral content (%)
Mechanical properties
Quartz
Potash feldspar
Plagioclase
Calcite
Pyrite
Other
Clay minerals
Young’s modulus (GPa)
Poisson’s ratio
Young’s modulus/Poisson’s ratio (GPa)
7-62-43
25
1.2
15.6
8.9
6.1
3
40.2
8.48
0.41
20.47
17-61-66
28.1
1.1
12.9
3.4
6.4
6.3
41.8
3.25
0.32
10.15
19-37-63
30.9
0.9
8.4
6.6
4
0
49.2
15.94
0.13
125.79
20-45-67
28.2
1
8.8
9.6
4.8
0
47.6
20.08
0.20
99.39
H258-3
21.3
1.4
14.1
10.3
11.1
0
41.8
4.56
0.22
20.73
h258-10
25.2
2
15.9
8.6
4.4
0
43.9
6.70
0.26
25.77
X381-3
20.5
1.6
9.6
10.8
2.7
0
54.8
4.56
0.22
20.73
D86-11
25.8
0.4
6.2
6.6
1.7
0
59.3
8.46
0.14
60.43


## 5.3. Effects of Confining Pressure on Rock Mechanical Properties under Triaxial Conditions

Confining pressure has a great effect on rock mechanical properties [58]. Figures 8(a) and 8(b) illustrate the triaxial stress–strain curve of rock specimens under confining pressure. The axial strain in this case should be smaller (generally less than 1%). Under confining pressure, the ultimate compressive strength increases with the decrease in the number of microcracks generated in the rock sample. In addition, the confining pressure has an inhibiting effect on the expansion of microcracks, and this effect increases with the confining pressure. Moreover, the rock sample will reach the failure state with only a small deformation. The compressive strength of rock gradually increases with the confining pressure. When the confining pressure increased from 0 to 40 MPa, the compressive strength of the sand shale reservoir rock increased from 41.34 to 219.34 MPa (approximately 5.3-times). Young’s modulus also increased, but the amplitude of the increase is relatively small, whereas Poisson’s ratio showed no significant change (Table 5). The average compressive strength of pure shale reservoir rock sample increased from 47.9 to 187.96 MPa, (approximately 4.0 times), and rock mechanical properties change rule and other basic same type sand shale reservoir (Table 6).Figure 8
Stress-strain curve of shale reservoir under different confining pressures (parallel to the bedding).
(a)
Sand shale type
(b)
Pure shale typeTable 5
Rock mechanical parameters of triaxial compression in shale reservoirs of the Qingshankou Formation under different confining pressures (parallel bedding-sand shale).
Sample number
Confining pressure (MPa)
Compressive strength (MPa)
Young’s modulus (GPa)
Poisson’s ratio
H197-8-1
40
219.34
23.32
0.23
H258-5-1
35
149.77
16.60
0.22
H258-2
25
123.58
15.84
0.24
H258-7-1
15
88.09
10.05
0.35
H258-8-1
0
41.34
10.81
0.28Table 6
Rock mechanical parameters of triaxial compression in shale reservoirs of the Qingshankou Formation under different confining pressures (parallel bedding-pure shale).
Sample number
Confining pressure (MPa)
Compressive strength (MPa)
Young’s modulus (GPa)
Poisson’s ratio
TY1-2-2
40
187.96
25.68
0.26
TY1-3-3
35
143.66
23.35
0.12
TY1-7-1
25
115.72
17.69
0.25
C347-11-1
15
87.58
11.13
0.26
TY1-3-1
0
47.90
7.00
0.12


## 5.4. Effect of Bedding on the Mechanical Properties of Rocks under Triaxial Conditions

Core samples in two directions (i.e., 0°, parallel bedding, and 90°, vertical bedding) were selected to carry out the analysis of mechanical properties while ignoring the effects of lithology and confining pressure on the rock mechanical properties (Figures9(a) and 9(b) as well as Tables 7 and 8). The compressive strength and Young’s modulus of sandstone-shale and pure shale reservoirs under the same confining pressure condition are significantly higher in the direction of parallel bedding in comparison with those in the direction of vertical bedding. Shale reservoir is obviously controlled by the internal weak structure of bedding face rock mechanical properties, leading to a significant difference in the axial strain. The axial strain in case of parallel bedding direction is less than 1.1%. Typically, the axial strain of pure shale reservoirs is less than that of sand shale reservoir. The axial strain of shale reservoirs with vertical bedding direction is within the range of 1.2–1.6%. In reservoirs with parallel direction of the bedding rock, tensile fracturing occurs under the effect of axial stress along the bedding direction. The axial stress due to the vertical stratification plane, loaded bedding plane, and superposition effect leads to compact bedding plane, closed bedding plane, and small angle of the sliding deformation. Thus, reservoirs with vertical beddings have larger axial strain, higher Poisson’s ratio, and lower Young’s modulus than those of reservoirs with parallel beddings.Figure 9
Stress-strain curves of shale reservoirs in different bedding directions (confining pressure 35 MPa).
(a)
Sand shale type
(b)
Pure shale typeTable 7
Rock mechanical parameter table of shale reservoir under triaxial compression under different bedding conditions of the Qingshankou Formation (confining pressure 35 MPa sand shale).
Sample number
Bedding direction
Compressive strength (MPa)
Young’s modulus (GPa)
Poisson’s ratio
H258-4-2
Vertical
149.34
6.0
0.28
H258-5-1
Parallel
149.77
16.60
0.22
H258-6-2
Parallel
138.58
13.13
0.24Table 8
Rock mechanical parameter table of shale reservoir under triaxial compression under different bedding conditions of Qingshankou Formation (confining pressure 35 MPa pure shale).
Sample number
Bedding direction
Compressive strength (MPa)
Young’s modulus (GPa)
Poisson’s ratio
TY1-5-4
Vertical
83.96
13.21
0.31
TY1-3-4
Vertical
115.72
13.62
0.27
TY1-3-3
Parallel
143.66
23.35
0.12
C347-11-2
Parallel
137.58
22.14
0.22


## 5.5. Failure Mode of Shale Reservoir

### 5.5.1. Uniaxial Failure Mode Analysis

Figure10 summarizes the failure modes of rock samples in vertical and parallel bedding directions under uniaxial conditions in this area. It can be seen that the failure mode fully reflects the failure mode of brittle layered rock. Figure 10(a) shows that when the included angle is 90° in the direction of vertical bedding, the rock sample is mainly damaged by shear failure at an angle along the direction of bedding, and double shear failure occurs in each laminated block. Meanwhile, the CT scan results in Figure 10(d) confirm that uniaxial fractures in the direction of vertical bedding open along the two bedding, forming parallel bedding conjugate fractures (Figure 10(c)), with relatively serious damage at the bottom of the rock sample and obvious end effect at the bottom. Figure 10(b) shows that when the angle of bedding plane angle is 0°, the rock sample cleavage failure occurs in the direction of parallel bedding without obvious block falling. Meanwhile, the CT scan results in Figure 10(d) confirm that uniaxial fractures in the direction of vertical bedding open along two bedding, forming bedding conjugate fractures.Figure 10
Typical photos of core failure modes in uniaxial compression experiments. Note: (a) H258-11-4 (vertical bedding). (b) D86-1-H2 (parallel to the bedding). (c) TY1-3-2 (vertical bedding CT scan). (d) TY1-3-1 (parallel bedding CT scans).
(a)
(b)
(c)
(d)


### 5.5.2. Triaxial Failure Mode Analysis

The failure mode of shale under triaxial confining pressure is mainly influenced by additional confining pressure on the basis of uniaxial compression. Figures11(a)–11(c) illustrate the pictures of rock failure in different bedding directions. Vertical direction of the bedding rock top produces shear joint fellowship with bedding which are affected by bedding shear and do not extended to the bottom of the rock. The half bottom of the rock almost has no crack extension, and the sample is mainly composed of single shear failure. Figures 11(b)–11(d) demonstrate that parallel bedding direction can be obviously shear rock seam fellowship with bedding. After fracture, the crossbedding continues to expand to the bottom until the sample cracks. The shale bedding surface and microcracks are formed, and these natural weak surfaces affect the failure results during the test, causing the crack surface to be destroyed along the direction of bedding and form multiple splitting failure, also known as crossconjugate seam [34, 35].Figure 11
Typical image of core failure mode in triaxial compression experiment. Note: (a) TY1-2-4 (vertical bedding). (b) C347-11-1 (parallel to the bedding). (c) TY1-3-4 (vertical bedding CT scan). (d) TY1-3-3 (parallel bedding CT scans).
(a)
(b)
(c)
(d)From the perspective of failure effect, the splitting failure is more complete than the shear plane failure mode and produces more failure structures. The rock surface needs to have certain strength and poor homogeneity. This is because under loading, sample within the coordinate deformation increases, local area increases in the number of microcracks, and microcracks resulted in partial discharge reduce the intensity of rock body. On the contrary, the higher the degree of homogeneity, the stronger the deformation resistance of the rock sample, and the less the development of microcracks, the less energy dissipation, the higher the strength, and the stronger the plasticity.In the process of field drilling and fracturing, most of the cores have parallel bedding. The rock failure mode is mostly obvious along the direction of bedding displacement, which indicates that the bedding plane deposited in the shale reservoir causes various anisotropy changes in strength and deformation. For example, in the process of hydraulic fracturing, the fracturing fluid enters the bedding plane, and it is easy to break the bedding and difficult to form network fractures.


## 5.5.1. Uniaxial Failure Mode Analysis

Figure10 summarizes the failure modes of rock samples in vertical and parallel bedding directions under uniaxial conditions in this area. It can be seen that the failure mode fully reflects the failure mode of brittle layered rock. Figure 10(a) shows that when the included angle is 90° in the direction of vertical bedding, the rock sample is mainly damaged by shear failure at an angle along the direction of bedding, and double shear failure occurs in each laminated block. Meanwhile, the CT scan results in Figure 10(d) confirm that uniaxial fractures in the direction of vertical bedding open along the two bedding, forming parallel bedding conjugate fractures (Figure 10(c)), with relatively serious damage at the bottom of the rock sample and obvious end effect at the bottom. Figure 10(b) shows that when the angle of bedding plane angle is 0°, the rock sample cleavage failure occurs in the direction of parallel bedding without obvious block falling. Meanwhile, the CT scan results in Figure 10(d) confirm that uniaxial fractures in the direction of vertical bedding open along two bedding, forming bedding conjugate fractures.Figure 10
Typical photos of core failure modes in uniaxial compression experiments. Note: (a) H258-11-4 (vertical bedding). (b) D86-1-H2 (parallel to the bedding). (c) TY1-3-2 (vertical bedding CT scan). (d) TY1-3-1 (parallel bedding CT scans).
(a)
(b)
(c)
(d)


## 5.5.2. Triaxial Failure Mode Analysis

The failure mode of shale under triaxial confining pressure is mainly influenced by additional confining pressure on the basis of uniaxial compression. Figures11(a)–11(c) illustrate the pictures of rock failure in different bedding directions. Vertical direction of the bedding rock top produces shear joint fellowship with bedding which are affected by bedding shear and do not extended to the bottom of the rock. The half bottom of the rock almost has no crack extension, and the sample is mainly composed of single shear failure. Figures 11(b)–11(d) demonstrate that parallel bedding direction can be obviously shear rock seam fellowship with bedding. After fracture, the crossbedding continues to expand to the bottom until the sample cracks. The shale bedding surface and microcracks are formed, and these natural weak surfaces affect the failure results during the test, causing the crack surface to be destroyed along the direction of bedding and form multiple splitting failure, also known as crossconjugate seam [34, 35].Figure 11
Typical image of core failure mode in triaxial compression experiment. Note: (a) TY1-2-4 (vertical bedding). (b) C347-11-1 (parallel to the bedding). (c) TY1-3-4 (vertical bedding CT scan). (d) TY1-3-3 (parallel bedding CT scans).
(a)
(b)
(c)
(d)From the perspective of failure effect, the splitting failure is more complete than the shear plane failure mode and produces more failure structures. The rock surface needs to have certain strength and poor homogeneity. This is because under loading, sample within the coordinate deformation increases, local area increases in the number of microcracks, and microcracks resulted in partial discharge reduce the intensity of rock body. On the contrary, the higher the degree of homogeneity, the stronger the deformation resistance of the rock sample, and the less the development of microcracks, the less energy dissipation, the higher the strength, and the stronger the plasticity.In the process of field drilling and fracturing, most of the cores have parallel bedding. The rock failure mode is mostly obvious along the direction of bedding displacement, which indicates that the bedding plane deposited in the shale reservoir causes various anisotropy changes in strength and deformation. For example, in the process of hydraulic fracturing, the fracturing fluid enters the bedding plane, and it is easy to break the bedding and difficult to form network fractures.


## 6. Conclusions

In this study, the mineral composition and characteristics of the stress–strain curve of 20 cores and 53 block samples were analyzed. In addition, the mechanical properties of the samples were evaluated under uniaxial and triaxial compression. The factors affecting the mechanical properties of shale-reservoir rocks are summarized as follows:(1)
Compared to pure shale reservoirs, sand stone shale reservoirs contain more brittle minerals and less clay minerals. Therefore, effective fracturing and more complex fracture network are easy to realize in sand stone shale reservoirs(2)
Under uniaxial compression, the reservoir type in the same bedding direction significantly affects the rock mechanical properties. In vertical bedding direction, mechanical parameters, such as compressive strength and Young’s modulus, of sand shale reservoirs are greater than those of pure shale reservoirs. In parallel bedding direction, the mechanical properties of pure shale reservoir and sand shale reservoirs are similar(3)
Under triaxial compression, the compressive strength of rocks is affected by the confining pressure and increases by 4.0–5.3 times, which correspondingly increases other parameters, but the extent of the effect is not yet clear. The mechanical properties of the rocks are also affected by the differences in the bedding directions, as Young’s modulus of the rock in the parallel bedding direction is higher than that in the vertical bedding direction(4)
The failure modes of rock are mainly shear plane failure and splitting failure. Under uniaxial condition, the main failure mode is splitting along the bedding direction, and the shear failure in the vertical direction is at a certain angle to the bedding plane. Under triaxial condition, affected by the confining pressure rock within the coordinate deformation increases, microcracks increase, and rock is given priority to with single shear failure and multiple split and destruction



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*Source: 2562872-2022-06-06.xml*