Hydraulic fracture propagation in coal-bearing formations: The role of interface friction effects with field data and injection strategy

Astract The interaction between hydraulic fractures (HFs) and formation interfaces governs fracture propagation distance and final morphology, thereby directly influencing coalbed methane reservoir stimulation. This study investigates the vertical propagation behavior of HFs in coal-bearing strata using a coupled finite element model of fluid flow, stress, and damage. A 2D model incorporating coal–rock interfacial friction based on the geological conditions of the Shoushan No. 1 Mine is developed to analyze the effects of interfacial properties and fracturing parameters on HF propagation. The results indicate that under a constant normal stress of 4 MPa, HFs tend to directly penetrate high-friction interfaces (e.g., >0.6), forming a single fracture. Owing to the brittleness contrast, fracture tips in different lithologies exhibit asymmetric propagation. Fractures propagate faster and preferentially in rock than in coal seams, contributing to effective control of HF height within the mining area. Conversely, HFs tend to deflect along low-friction interfaces. Higher injection rates reduce the critical stress required for interfacial penetration, leading to a composite propagation mode of "penetration + deflection". These results provide guidance for optimizing injection strategies to improve coal seam stimulation.