Hydraulic fracturing has been applied extensively to stimulate the production of oil, natural gas, and geothermal energy from subterranean formations of ultra-low permeability and porosity. With this technique, massive hydraulic fractures can be induced, which will inevitably cross or activate pre-existing discontinuities and therefore result in complicated fracture networks. The complexity of hydraulic fracture networks can largely disturb the efficient and economic development of such unconventional resources. Thus, accurately characterizing fracture network morphology is necessary for flow simulation and fracturing evaluation.

Moreover, the key factors affecting the morphology of post-fracturing networks involve geological conditions (e.g., temperature, in situ stress), rock properties (e.g., brittleness, permeability, bedding plane), fracturing treatment parameters (e.g., injection rate, fluid viscosity, pumping pressure), and fracturing techniques (e.g., multistage horizontal-well fracturing, progressive cyclic and pulse fracturing, and synchronous fracturing). Although various efforts have been devoted to characterizing hydraulic fracture morphology, the formation mechanism of complex fracture networks and the role of the influencing factors are yet to be fully clarified. These limitations lead to the lack of a quantitative technical basis in fracturing design and implementation.

This Special Issue aims to bring together original research and review articles highlighting recent advances and challenges in elucidating the formation mechanism of complex fracture networks. We welcome submissions focusing on theoretical derivation, numerical modeling, experimental investigations, and field studies at various scales that explore the role of multiple influencing factors and address the difficulties of qualitative and quantitative characterization of fracture networks for fracturing design and implementation.

Potential topics include but are not limited to the following:

• Qualitative and quantitative characterization of hydraulic fracture morphology
• Intersection mechanism between hydraulic fractures and natural fractures
• Stress shadow effect on hydraulic fracture behavior
• Crack competitive extension during hydraulic fracturing
• Modeling hydraulic fracture complexity in naturally fractured reservoirs
• Sensitivity analysis of fracturing parameters, such as injection rate and fluid viscosity
• Visualization of fracture network formation process
• Evaluation of hydraulic fracturing effectiveness of different fracturing techniques
• Hydraulic fracturing-induced seismicity
• Optimization of hydraulic fracturing design in oil/gas/geothermal reservoirs