The fracture propagation pattern following hydraulic fracturing in deep coal rock significantly influences the effectiveness of reservoir stimulation. Understanding the characteristics and evolution of hydraulic fractures in deep coal rock under varying injection modes is essential for the efficient development of coalbed methane. To elucidate the fracture evolution patterns in deep coal rock and explore methods for forming fracture networks, large-scale true triaxial hydraulic fracturing physical simulation experiments were conducted. These experiments integrated acoustic emission monitoring with post-fracturing 3D fracture reconstruction technology. Testing was performed on the #8 deep coal seam of the Ordos Basin, utilizing various injection parameters and modes. The findings were corroborated by on-site reservoir reconstruction results in the Ordos deep coal rock area. The results indicate that: (1) the number of fractures and the fracture area ratio in deep coal rock are strongly correlated with the viscosity of the fracturing fluid. Low-viscosity fluids yield the highest fracture count and area ratio, albeit with lower fluid efficiency. As fluid viscosity increases, the fracture count decreases by 91%, the fracture area ratio decreases by 86%, and fracture complexity diminishes. (2) At low viscosities, natural fractures (e.g., cleats and bedding planes) predominantly dictate the direction of hydraulic fractures. With increasing viscosity, the minimum resistance plane, influenced by in-situ stress and rock anisotropy, becomes the primary controlling factor. (3) During variable-rate fracturing processes, a higher injection rate results in increased fracture initiation pressure in rock specimens, reduced fracture initiation time, and heightened intensity of acoustic emission ring-down counts, thereby facilitating the formation of dominant hydraulic fractures. (4) The injection mode of “high viscosity first, low viscosity later” effectively maintains fracture complexity while minimizing fluid loss, enhancing fluid efficiency, and achieving a fluid efficiency 2.06 times that of the low-viscosity-only injection mode. The application of a variable-viscosity injection mode combined with high-rate fracturing technology in the deep coal seams of the Ordos Basin demonstrates significant effectiveness, with post-fracturing daily average production per well reaching 4.4 times that achieved through conventional fracturing techniques.

The lithological composition of deep coal rock reservoirs in the Ordos Block is complex. The characteristics of hydraulic fracture propagation directly impact reservoir stimulation effectiveness. Therefore, efficient development requires an in-depth understanding of the cross-layer propagation mechanisms of fractures in deep coal rock. To clarify the cross-layer patterns and explore the controlling factors in deep coal rock, large-scale laboratory true triaxial hydraulic fracturing physical simulation experiments were conducted. These experiments, combined with CT scanning and post-fracture 3D reconstruction technology, investigated Ordos Block deep coal rock under different perforation locations, and the complexity of fractures was quantitatively characterized. Due to the well-developed weak planes such as natural fractures in coal rock, perforations in coal rock significantly reduce the breakdown pressure compared to perforations in sandstone. The complexity of perforation fractures in coal rock is far greater than in sandstone. Quantitative characterization of fracture complexity shows that the number of perforation fractures in coal rock fracturing reached 450% of that in sandstone, and the fracture area ratio reached 131.7%. Under high-rate and high-viscosity fracturing conditions, dominant hydraulic fractures tend to form, while the well-developed natural fractures in the coal rock interact with each other, resulting in a complex fracture network. Perforations in coal rock can effectively connect adjacent sandstone layers through cross-layer propagation, whereas perforations in sandstone form dominant hydraulic fractures without connecting the adjacent coal rock layers. The findings can provide operational guidance for optimizing field fracturing operations.