The complexity of hydraulic fracture network generation during the fracturing of shale reservoirs is a key indicator of the effectiveness of fracture stimulation. To obtain as large a reservoir stimulation volume as possible, this paper reviews articles on the study of hydraulic fracture propagation mechanism during hydraulic fracturing, analyses the factors affecting hydraulic fracture propagation, and classifies them into two categories: geological factors and engineering factors. In particular, the geological factors affecting hydraulic fracture propagation are classified into five categories: mineral composition of the shale, connections between mineral grains, defects in the shale, geostress, and temperature. Various influencing factors act together, resulting in the hydraulic fracture propagation path is difficult to predict. Therefore, this paper firstly explores the hydraulic fracture propagation pattern under the action of single geological factors and specifies its action mechanism; secondly, it also analyses the hydraulic fracture propagation pattern under the combined action of multiple geological factors and analyses its action mechanism. It is clear that relatively high brittle mineral content and temperature, low stress anisotropy and cementation strength, and a more developed natural fracture network are conducive to the generation of a complex fracture network. By analyzing the influence mechanism of single factors and multiple factors, the influence mechanism of geological factors on hydraulic fracture propagation is identified, guiding the optimal design of hydraulic fracturing.

The characterization of artificial fracture propagation law in the fracturing process of shale reservoirs is the basis for evaluating the fracture conductivity and a key indicator of the reservoir stimulated effect. In order to improve the fracture stimulated volume of shale reservoirs, this paper systematically discusses the current status of research on artificial fracture propagation law from the research methods and main control factors and provides an outlook on its future development direction. The analysis finds that the study of fracture propagation law by using indoor physical simulation experiments has the advantages of simple operation and intuitive image, and the introduction of auxiliary technologies such as acoustic emission monitoring and CT scanning into indoor physical model experiments can correct the experimental results so as to better reveal the propagation mechanism of artificial fractures. At present, the numerical simulation methods commonly used to study the propagation law of artificial fractures include the finite element method, extended finite element method, discrete element method, boundary element method and phase field method, etc. The models established based on these numerical simulation methods have their own advantages and applicability, so the numerical algorithms can be integrated and the numerical methods selected to model and solve the different characteristics of the propagation law of artificial fractures in different regions at different times can greatly improve the accuracy of the model solution and better characterize the propagation law of artificial fractures. The propagation law of artificial fracture in the fracturing process is mainly influenced by geological factors and engineering factors, so when conducting research, geological factors should be taken as the basis, and through detailed study of geological factors, the selection of the fracturing process can be guided and engineering influencing factors can be optimized.

Hydraulic fracturing technology is important in developing low-porosity and low-permeability oil and gas reservoirs. However, the presence of discontinuities such as natural fractures (NF), lithological interfaces, and bedding (B) in rocks seriously affects the propagation paths of hydraulic fractures (HF) and their geometrical morphology. Therefore, this study proposes a numerical model based on the finite element method (FEM) for indoor hydraulic fracturing scale and simulates the interaction between hydraulic fractures and bedding during the fracturing process. The results show that when the hydraulic fracture extends to the bedding, five types of propagation behaviors may occur: arresting, turning, turning and then crossing, turning while crossing, and crossing directly, which are controlled by factors such as the geostress, the mechanical properties of the rock matrix and bedding, the bedding inclination, and its density, etc. During the fracturing process, the increase of elastic modulus ratio between the rock matrix and bedding and bedding inclination angle is conducive to the hydraulic fracture turning along the bedding; the increase of principal stress difference and bedding density is conducive to the hydraulic fracture directly passing through the bedding. When the inclination angle of the bedding distributes from 15° to 60°, the hydraulic fractures will first turn along the bedding when they extend to the bedding, and then extend for a certain distance before passing through the bedding and the distance of extension is positively correlated with the inclination angle of the bedding. The hydraulic fracture extension criterion in the bedding was established by using the ratio of elastic modulus of the rock matrix and bedding, geostress, and bedding inclination, and the results of the study will guide the design and optimization of fracturing in shale with bedding development.

Numerous discontinuities, such as bedding planes and natural fractures, occur in reservoir rocks and significantly influence the propagation behavior of hydraulic fractures during reservoir stimulation. To elucidate the mechanisms underlying the influence of natural fracture networks in reservoir rocks on the propagation of hydraulic fractures, a finite element–discrete fracture model is employed to establish a fluid–solid coupled finite element–discrete fracture model. This model is used to investigate the propagation behavior of hydraulic fractures in fractured reservoirs and their underlying mechanisms. The results indicate that when distant from natural fracture networks, hydraulic fractures typically propagate along the direction of the maximum principal stress. Upon approaching natural fracture networks, the propagation path of hydraulic fractures is altered, leading to localized deflection. The mechanical properties of the rock matrix versus those of natural fracture networks, in situ stress, fracturing fluid viscosity, and injection rate significantly influence the propagation of hydraulic fractures in fractured reservoirs. Increased mechanical disparity between the rock matrix and natural fractures promotes deflection along natural fracture networks, resulting in the formation of complex fracture networks. However, increased in situ stress, fracturing fluid viscosity, and injection rate facilitate direct penetration of natural fractures by hydraulic fractures, yielding the formation of simple, long, straight primary fractures. Furthermore, the propagation distance of hydraulic fractures along the direction of the maximum principal stress is positively correlated with the in situ stress, fracturing fluid viscosity, and injection rate. The findings of this study provide theoretical guidance for optimizing fracturing design.