To explore the influence of coal and rock bedding plane effects on mechanical properties, the micro-mechanical properties of coal and rock with different bedding directions are studied based on nanoindentation tests. By plotting the load-displacement curves of each group of samples, the micro-mechanical parameters of coal and rock with different bedding orientations are obtained and the failure modes of coal and rock are analyzed. Furthermore, the propagation laws of hydraulic fracturing fractures in coal and rock are discussed. The research shows that the microstructure of coal and rock is dense, the bedding structure is clear, and the bedding planes are filled with hard minerals such as quartz, accompanied by a certain amount of natural micro-cracks and developed self-generated pores. Through nanoindentation tests, it is found that the coal and rock samples have obvious anisotropy. The elastic modulus of coal and rock perpendicular to the bedding plane, parallel to the bedding plane, and at an oblique angle to the bedding plane are 5.02 GPa, 4.58 GPa, and 4.92 GPa, respectively, and the hardness is 0.38 GPa, 0.35 GPa, and 0.37 GPa, respectively. The elastic modulus and hardness of coal and rock are consistent with their macroscopic mechanical laws. In terms of the energy dissipation characteristics of failure in different directions, the coal and rock perpendicular to the bedding plane require the largest fracture energy, while the coal and rock parallel to the bedding plane have the smallest fracture energy dissipation. In addition, the fracture toughness is 0.25, 0.22, and 0.23 MPa·m^0.5, respectively. The brittleness coefficient of coal and rock varies with the differences in failure forms in different directions. Based on the nanoindentation test data, the propagation characteristics of fractures under different bedding directions are revealed. Fractures in the direction perpendicular to the bedding plane extend along natural fissures, fractures parallel to the bedding plane mainly develop along the weak bedding plane, and fractures at an oblique angle to the bedding plane are more prone to branching during the deep extension process. The research results provide scientific data support for the engineering application of coal and rock, offer important references for engineering design, construction, monitoring and optimization, and have the potential to improve the safety and economy of engineering, contributing to sustainable development.

In order to reveal the influence mechanism of acid-rock reaction in different bedding directions on fracture properties of Marine shale, SNDB samples were used to obtain the load-displacement relationship and fracture load of samples under three-point bending loading, and the fracture toughness (K_IC) and fracture energy of shale were calculated. Combined with network fracturing requirements, acid injection modes of marine shale reservoirs under different depths and stress states were initially discussed. The results show that: (1) The fracture process of the sample under three-point loading includes the stages of compaction, elastic compression, fracture, and fracture expansion. The bearing capacity of the sample is rapidly lost after fracture, and the shale is brittle. (2) In parallel, vertical, and shear stratification directions, the fracture toughness of shale decreases with the increase of acid concentration, K_IC decreases from 1.12 to 0.44 MPa· m^0.5, and fracture energy decreases from 211.50 to 56.00 J·m^-2. The fracture performance of Marine shale is significantly controlled by bedding orientation and acid concentration. (3) According to the acid filtration mechanism, the acid intrusion mode of parallel bedding shale is fracture intrusion, and the intrusion depth is large; The vertical bedding shale is a porous intrusion with shallow intrusion depth. The shear stratification sample is of mixed type. (4) Shallow shale fracturing needs to inhibit fracture propagation along the horizontal bedding direction, and acid injection is not appropriate; In deep shale fracturing, acid injection is suggested to enhance the horizontal expansion trend of fractures and improve the complexity of fractures. Due to the limitation of vertical fractures in ultra-deep shale fracturing, it is suggested that acid injection should be cautious to promote vertical fracture expansion and realize the expansion of ultra-deep reservoirs.

The anisotropy of fracture behavior of gneiss is a key constraint to the fracturing of reservoir fracture network. The differences of fracture toughness, fracture energy and fracture morphology of gneiss in different directions are studied by selecting gneiss in Daqingshan of Inner Mongolia. The results show that: The fracture process of gneiss specimens under three-points bending loading includes four stages: pore compression, elastic compression deformation, fracture and rapid unloading. The brittleness of gneiss is significant, which is beneficial for volumetric fracturing. The fracture toughness of gneiss ranges from 0.93 to 1.28 MPa·m^0.5, and the fracture performance of gneiss varies greatly in different orientations. The vertical bedding/vertical lineation > parallel bedding/vertical lineation > vertical bedding/parallel lineation > parallel bedding/parallel lineation has certain mechanical constraints on the selection of fracture propagation direction. In the vertical schistosity direction, the fracture propagation of gneiss is seriously affected by the weak plane. The shear and displacement of cracks occur at the schistosity face and form bedding-parallel fractures. In the later stage, the fractures turn to extend along the loading direction. In the parallel schistosity direction, the fractures of gneiss extend parallel to the maximum principal stress orientation. The fracture surface is flat, and the fracture tortuosity is small, which is not conducive to fracture network formation. The breaking energy consumption of samples in vertical bedding/vertical lineation, parallel bedding/vertical lineation, vertical bedding/parallel lineation, parallel bedding/parallel lineation direction is 0.47, 0.43, 0.37 and 0.25 J, respectively, and the energy consumption of fracture in each direction of gneiss varies greatly. In order to expand the reservoir volume fracturing coverage, it is recommended to drill horizontal wells parallel to gneiss lineation.

Fracture properties and cracking mechanism of hard sandstone roof are of great significance to caving. Based on the idea of acidizing fracturing, this paper applied three-point bending loading on hard sandstone SCB samples soaked in 5%, 10% and 20% hydrochloric acid solution for 7 d and 14 d respectively, and the change characteristics of fracture toughness, fracture compression work and fracture morphology before and after acidizing reaction were obtained. The influence mechanism of acidification reaction on fracture behavior of hard sandstone roof in coal mine is preliminarily clarified. The results show that the displacement-load curve shows that the slope of sandstone sample at the post-fracture stage is large, indicating that the sandstone sample has significant brittleness. Acid concentration and reaction time have little influence on sandstone brittleness, but have influence on the curve morphology of sandstone before macro fracture. The fracture toughness of sandstone samples decreases with the increase of soaking acid concentration, and the slope decreases with the longer reaction time, and the damage effect is most significant at the initial reaction stage. The fracture compression energy decreases with the increase of acid concentration and the weakening of rock toughness. With the acidizing reaction, the fracture form of sandstone samples changes from single tensile fracture to tensile fracture and shear fracture, and the fracture efficiency of hard sandstone increases after acidizing. It is concluded that the acid solution corroded the micro-plagioclase and kaolinite in sandstone to form defects and destroy the bonding strength of the minerals. In this paper, it is considered that acidification can significantly reduce the energy consumption of hard sandstone fracture and rock crushing difficulty, which is beneficial to improve the efficiency of coal mine caving.

The shale SCB samples of southern Sichuan basin with the angle θ of bedding and sample base being 0°, 30°, 45°, 60° and 90° are tested subjected to three-point bending loading. The axial load-displacement curves and fracture load were obtained, and the fracture toughness K_IC and fracture energy of shale were calculated. Combined with shale reservoir fracturing engineering, the influence of bedding orientation on fracture tortuosity of shale reservoir is discussed. The results show that: (1) As θ increases, the fracture toughness of shale samples decreases, and the average K_IC value decreases from 1.88 to 0.43 MPa·m^0.5. The bedding orientation has a significant control on the fracture toughness of shale. (2) The axial load-displacement curves of shale samples under three-point bending include defect compression, elastic compression deformation and rupture unloading stage at the initial loading stage. After the axial load increases to the critical fracture load, the shale SCB samples are unloaded rapidly, which reflects the obvious brittleness of shale. (3) According to the fracture mechanism of rock, it can be divided into tensile fracture and shearing fracture, and straight fracture and tortuous fracture according to the fracture tortuosity. When θ < 45°, the fracture propagation of shale is affected by the loading direction and bedding plane, and it is easy to form complex fracture network. On the contrary, a single straight fracture is formed along the direction of bedding. (4) There is a good linear correlation between fracture work and fracture toughness of shale, indicating that shale fracture performance can affect the selection of fracture propagation paths. (5) Selecting a large-angle intersecting bedding direction to implement perforation or pre-slicing, so that the fracturing fractures and propagation along this direction, can maximize the creation of a hydraulic fracture network and improve the efficiency of shale reservoir fracturing.

The fracture complexity of tight sandstone could work to improve the efficiency of tight gas reservoir fracturing.In this paper, three-point bending loading tests was used to characterize the fracture behavior, mechanical properties, fracture path and fracture energy characteristics of tight sandstone at room temperature(RT)and low temperature(LT)of liquid nitrogen(LN₂), and to interpret the pre-fracture mechanism of tight sandstone at LN₂.The results show that: 1)The fracture load and fracture toughness of the tight sandstone treated with LN₂ are significantly lower than those at RT, while the parameters of the samples under parallel bedding slightly increased, which may be related to the enhancement of the bedding plane by LN₂.2)After low temperature treatment with LN₂, the complexity of fracture propagation paths increases, the number of fracture increases, and the fracture bifurcation and convergence occurs.The fracture complexity of sandstone is low at RT, and the effect of LN₂ precracking on the fracture complexity of tight sandstone is significant.3)After LN₂ pre-splitting, the fracture energy of tight sandstone decreases significantly, with a maximum decrease of 46.98 %.4)It is found that the fracture paths of tight sandstone mainly develops along the mineral grain boundary and forms intergranular fracture at RT, and the rock transformation degree is low.However, the complex fracture forms are formed by intergranular fracture, transgranular fracture and intergranular fracture, and the formation of transgranular fracture and complex fracture network are promoted under low temperature.The results of this study suggest that low-temperature during fracturing of tight sandstone reservoir can effectively reduce the fracturing pressure, realize the toughening-brittle transformation of rock and improve fracture complexity.