Evgenii Kanin, Dmitry Garagash, Andrei Osiptsov 

This paper is concerned with an analysis of the near tip region of a propagating fluid-driven fracture in a saturated permeable rock. The study attempts to accurately resolve the coupling between the physical processes - rock breakage, fluid pressure drop in the viscous fluid flow in the fracture, and fluid exchange between fracture and the rock - that exert influence on the hydraulic fracture propagation, yet occur over length scales often too small to be efficiently captured in existing coarse grid numerical models. We consider three fluid balance mechanisms: storage in the fracture, pore fluid leak-in from the rock into the fracture as the result of dynamic suction at the dilating crack tip, and fluid leak-off from the fracture into the rock as the fluid pressure in the fracture recovers with distance away from the tip. This process leads to the formation of a pore fluid circulation cell adjacent to the propagating fracture tip. We obtain the general numerical solution for the fracture opening and fluid pressure in the semi-infinite steadily propagating fracture model and fully characterize the solution within the problem parametric space. This allows to identify the parametric regimes of fracture propagation, assess the impact of pore fluid leak-in and the associated near-tip circulation cavity on the solution, and explore limitations of the widely-used, pressure-independent Carter’s leak-off model. The obtained solution can be further used as a tip element in a numerical realization of a solution for a transient growth of a finite fracture (e.g., within the Planar3D approach).