PREDICTING ORIENTATIONS OF INDUCED PERMEABILITY CREATED BY HYDRAULIC FRACTURING

The primary goal of hydraulic fracturing is to enhance permeability within a rock mass to allow migration of fluids to a borehole. Except for a small area in the immediate vicinity of the borehole, the permeability enhancement is created entirely by fracture flow along tight fracture systems (no open fractures) being activated as shear fractures (mode II). In a tight fracture environment, preferential permeability follows fracture intersections, especially those where recent divergent movement has occurred.

Virtually all fractures moving during hydraulic fracturing are pre-existing fracture surfaces where the increased fluid pressure has moved them into an unstable state within the current regional stress field. As the hydraulic pressure decreases with distance from the borehole, the population of fractures activated by the increased pore pressures decreases. By calculating the orientations and displacement directions of the activated fractures, it is possible to calculate the resulting orientation of enhanced permeability.

In a theoretical uniformly fractured rock mass, the preferential enhancement would be in the current regional σ₂ direction. However, where fracture orientations are not uniformly distributed, the lack of perfectly oriented pre-existing fracture surfaces results in primary permeability enhancement orientations that diverge dramatically from this ideal. Furthermore, the change in active fracture populations with distance from the borehole also lresults in areal variations in the orientation of the resulting permeability enhancement.