Fracture propagation in high-permeability rocks: The key influence of fracture tip behavior

Pressure distribution at the tip of a hydraulic fracture is a key element for controlling fracture propagation. In low-permeability formations, under downhole reservoir conditions, a severe pressure drop occurs at the tip of the fracture and a lag zone develops due to fluid cavitation. Properly taking into account the controlling parameters of tip behavior has resulted in more accurate and robust fracture propagation models. However, the situation is still unclear in high permeability formations, because the formation fluid can invade the tip zone where the pressure drops below the far-field pore pressure. Moreover, the assumptions of the Carter leak-off model do not apply in this zone. This work presents a fundamental study of fracture tip behavior in high permeability formations. We consider a steadily propagating fracture, taking into account the flow within the fracture, filtrate leak-off across the fracture faces, and kinetics of filter cake growth. The flow within the reservoir due to leak-off is described by the 2D pressure diffusion equation. The formation is elastic and fracture growth is based on Linear Elastic Fracture Mechanics. Results show that the tip zone can be either a recirculation zone, in which the formation fluid enters the fracture and then get expelled back into the formation, or is completely filled with fracturing fluid. When the pressure at the tip is higher than the far-field pore pressure, a pressure wave propagates ahead of the tip with severe consequences on fracture propagation. Finally, the total energy consumption (which determines the net pressure) is primarily dissipated in viscous flow within the fracture, leak-off flow across the filter cake, and seepage flow induced by leak-off in the reservoir. The flow within the reservoir due to leak-off is described by the 2D pressure diffusion equation. The formation is elastic and fracture growth is based on linear elastic fracture mechanics. This work provides new insight for fracture propagation in highly permeable sandstones and allows us to understand some previously unexplained field and laboratory observations.