Objective  Creating large-scale fracture networks through hydraulic fracturing serves as an important approach to the commercial development of deep coalbed methane (CBM) reservoirs. However, these reservoirs contain extensively developed cleats. The significantly different cementation strengths of cleat fillings lead to complex interactions between cemented cleats and major hydraulic fractures during fracture propagation. Therefore, clarifying the mechanisms underlying the influence of cemented cleats on hydraulic fracture propagation is critical to improving the simulation effects of deep CBM reservoirs.

Methods  Using particle flow code (PFC) discrete element method, combined with quasi-triaxial mechanical test results of the No.8 coal seam within the Yichuan Block in the Ordos Basin, this study established a fluid-solid fully coupling numerical model that enables the simulation of the interactive propagation of cleats and hydraulic fractures in deep CBM reservoirs. Accordingly, this study investigated the mechanisms behind the influence of cleat dip angle, the cementation strength of cleat fillings, and fracturing fluid viscosity on hydraulic fracture propagation in deep CBM reservoirs. Furthermore, the role of induced stress field evolution in both cleat activation and fracture network formation during primary hydraulic fracture propagation was analyzed.

Results and Conclusions  During hydraulic fracture propagation, the induced stress fields can activate cleats with a cementation strength ratio (CSR) of less than 0.5, generating fissures formed predominantly by shear failure. Hydraulic fractures will directly run through strongly cemented, high-angle cleats. In contrast, these fractures will be blocked by strongly cemented, low-angle cleats, leading to the formation of branch fractures. Cleats with dip angles approaching the direction of the minimum horizontal principal stress are more prone to be activated and contribute to the formation of more uniform fracture networks. Hydraulic fracturing using low-viscosity fracturing fluids promotes the development of large-scale, clustered, complex fracture networks at the intersections of major fractures and cleats. In contrast, hydraulic fracturing using high-viscosity fluids assists in increasing the lengths of major fractures and activating a greater number of weakly cemented cleats. Therefore, hydraulic fracturing adopting alternating injection of fracturing fluids of different viscosities can enhance both permeability and controlled fracture volume of deep CBM reservoirs, providing broad pathways for CBM migration. The results of this study offer theoretical support for optimizing the design schemes and selecting the optimal sweet spots for hydraulic fracturing construction in deep CBM reservoirs.