Purpose Constructing large-scale fracture networks through hydraulic fracturing is essential for achieving the economic development of deep coal–gas reservoirs. However, these reservoirs contain widely developed cleats with infilling minerals exhibiting large variations in cementation strength, resulting in complex interactive propagation between cemented cleats and the main hydraulic fracture. Clarifying the mechanisms by which cemented cleats influence hydraulic fracture propagation is therefore critical for improving fracturing performance in deep coal–gas reservoirs.

Methods Using the discrete element particle-flow method combined with quasi-triaxial mechanical test results of No. 8 coal from the Yichuan Block in the Ordos Basin, a fully coupled fluid–solid numerical model capable of simulating the interaction between cleats and hydraulic fractures in deep coal–gas reservoirs was established. The model was used to investigate the effects of cleat dip angle, cementation strength of infilling minerals, and fracturing-fluid viscosity on hydraulic fracture propagation. The evolution of the induced stress field during the propagation of the main fracture was also analyzed to determine its role in cleat activation and fracture-network development.

Results and Conclusions The results show that, under the induced stress field during hydraulic fracturing, cleats with a cementation-strength ratio below 0.5 are activated, generating predominantly shear-type fractures. Strongly cemented high-angle cleats tend to be directly crossed by the main fracture, whereas strongly cemented low-angle cleats may obstruct propagation and cause fracture bifurcation. Cleats with dip angles close to the minimum horizontal principal stress direction are more likely to be activated and contribute to the formation of more uniform fracture networks. Low-viscosity fracturing fluids promote the development of large cluster-like complex fracture networks at intersections between the main fracture and cleats, whereas high-viscosity fluids facilitate longer main-fracture propagation and activate more weakly cemented cleats. Alternating fracturing fluids of different viscosities can enhance reservoir permeability and enlarge the stimulated reservoir volume, providing broader pathways for coal-gas flow. The findings provide theoretical support for optimizing hydraulic-fracturing design and selecting “sweet spots” in deep coal–gas reservoirs.