Hydraulic Fracturing: Breaking Intact Rock or Stimulating Natural Fractures?

Abstract

Traditional bi-wing fracture models are based on the assumption that a tensile fracture will extend away from the treatment well once the injection pressure exceeds reservoir stresses and rock tensile strength. The hydraulic fracture growth is assumed to remain planar and follow SHmax direction, which is the path of least resistance. While this is a valid assumption in cases where the rock is homogeneous and unfractured, the fracture propagation becomes much more complex when there are pre-existing natural fractures, as is usually the case in most unconventional reservoirs. Numerical simulation of hydraulic fracturing in naturally fractured reservoirs indicates the propagation of an induced hydraulic fracture can be halted at the intersection with natural fractures and the fracture propagation may continue along the pre-existing fractures. Focal mechanism analysis of microseismic events also repeatedly shows the dominance of shear failure, which is characteristic of natural fracture stimulation, versus volumetric or tensile failure.

In this study, we investigated the stimulation of natural fractures, versus propagation of induced hydraulic fractures, for several cases in different US shale plays. We were specifically interested in understanding whether the recorded microseismic events represent stimulation of pre-existing fractures or failure of intact rock. Knowing this is of great practical importance in that it provides insights into the dominant stimulation mechanism so the completion parameters can be adjusted accordingly to improve completion efficiency. For this purpose we applied an inverse method to estimate friction coefficient for the recorded failures using fracture orientation and reservoir stresses as inputs. By comparing the estimated friction coefficients with the reported values in literature for similar rocks we made conclusions whether the observed events represented intact rock failure or slip on pre-existing fractures. The results of this study show the dominant impact of natural fractures on stimulation efficiency in unconventional plays.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018