En-Echelon Fracture Growth in Shales During Hydraulic Stimulation

Abstract:

The goal of a hydraulic fracture treatment program is to activate a discrete fracture network (DFN) that connects as much of the reservoir as possible and produces pathways for the extraction of hydrocarbons at the production well. The activated DFN usually grows outwards from the treatment well along the direction of the local maximum horizontal stress, SH_max. However, individual fractures in the DFN will have varying orientations that need not align with the overall azimuth or growth. The orientations of the individual fractures are determined by which pre-existing fractures generated from geological processes. The idealized cases of pure shear failures from long term deformation are likely oriented about 30 degrees from the direction of the maximum principal stress at the time of their activation and are not necessarily aligned with the current SH_max. Generally, pre-existing fracture sets do not align with SH_max ,and these multiple sets will be activated and grow in an en-echelon fashion to create a connected network that can move fluid away from the well.

We use mechanisms from microseismic data, combined with a stress inversion, to constrain the fracture orientations associated with microseismic events. We examine a number of stages from the zipper-frac completion of half pad in the Horn River Basin, where generally two fracture sets are activated during the completion. However, we observe, potentially due to subtle rotations in the stresses throughout the completion, that different fractures are activated with space. This heterogeneity in orientation relative to SH­_max is critical input for the geomechanical models on how treatment design programs influences the propagation of fluid and proppant into the reservoir.