Evolution law and field application of true triaxial fracture of deep coal rock under different injection modes
ZHAO Zhihong1, DENG Yinxin1, GUO Jianchun1, HE Jiale1, WU Tianyu1, ZHAO Feng1, ZHANG Ran2
(1. State Key Laboratory of Reservoir Geology and Development Engineering, Southwest Petroleum University, Chengdu, Sichuan 610500, China; 2. School of Mechanical Engineering, Xihua University, Chengdu, Sichuan 610039, China)
Abstract：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.
赵志红1，邓银鑫1，郭建春1，何家乐1，吴天宇1，赵 峰1，张 然2. 不同注入模式下深煤岩真三轴裂缝演化规律及现场应用[J]. 岩石力学与工程学报, 2025, 44(10): 2668-2678.
ZHAO Zhihong1, DENG Yinxin1, GUO Jianchun1, HE Jiale1, WU Tianyu1, ZHAO Feng1, ZHANG Ran2. Evolution law and field application of true triaxial fracture of deep coal rock under different injection modes. , 2025, 44(10): 2668-2678.
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2025, Vol. 44 
