Investigating the effect of the shale bedding structure on hydraulic fracture propagation behavior on the basis of a coupled thermal–hydraulic–mechanical numerical model

The interaction process among hydraulic fractures and natural fractures, bedding planes, and other discontinuities during shale fracturing determines the complexity of the fracture network that is formed. However, the current conclusions and understanding of the mechanisms underlying the interaction between hydraulic and natural fractures, as well as their primary controlling factors, fail to meet the requirements of hydraulic fracturing operations, thereby restricting the efficient development of shale gas resources. Therefore, in this study, a coupled thermal‒hydraulic‒mechanical finite element numerical model that is based on the maximum tensile stress and the Mohr‒Coulomb criterion is established, thereby considering rock deformation, fluid flow, and heat transfer. The reliability of this model is validated on the basis of previous research. This model is subsequently employed to simulate the propagation behavior of hydraulic fractures in shale with well-developed bedding. The results indicate that when hydraulic fractures propagate to the bedding, five propagation modes may occur: arrest, diversion, diversion and crossing, crossing and diversion, and direct crossing. These modes are controlled by factors such as the mechanical properties of the shale matrix and bedding, geostress, bedding dip angle, temperature, and fracturing fluid injection rate. During fracture propagation, increases in the elastic modulus ratio between the rock matrix and the bedding, the bedding dip angle, and the temperature are favorable for hydraulic fractures turning along the bedding, whereas increases in the difference in vertical stress and the injection rate are favorable for hydraulic fractures directly crossing the bedding. Second, on the basis of four influencing factors, namely, the shale matrix and bedding elastic modulus ratio, bedding dip angle, difference in vertical stress, and temperature, propagation criteria for hydraulic fractures along the bedding under various combinations of influencing factors are established. The results provide theoretical reference data for the design and optimization of fracturing in shale with well-developed bedding.