Unconventional shale reservoirs have practically no in-situ permeability, but they may be rich in natural fractures, which can positively or negatively impact hydraulic fracturing. We study a naturally fractured shale formation, with bed-perpendicular fractures, which are mostly cemented, fracture corridors and bedding-parallel calcite veins (beef). Microseismic indicates that the Stimulated Rock Volume (SRV) is heterogeneous and anisotropic, which is likely related to reactivation of the natural structural heterogeneities.

At in-situ conditions, the shale is overpressured, but the pore pressure is below the required threshold to reactivate natural fractures. During hydraulic fracturing, fractures may be locally reactivated. The local reactivation is quantified using a coupled stress-fluid pressure Finite Element model, based on a discrete fracture network constructed around a pilot area with several horizontal and vertical wells.

The models show that only a small part of the fracture network close to induced fractures is reactivated, creating an anisotropic SRV. Vertically, beef limits growth of induced fractures, while increasing the aperture and length. Furthermore, beef can generate horizontal high-permeability zones, increasing the SRV. These models, when accurately calibrated using pressure interference data, are useful to optimize well placement and completion strategies.


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