Summary

Among natural fractures observed in core of Eagle Ford Shale, Texas, tall sub-vertical calcite-cemented fractures are the most spatially extensive. These completely cemented fractures break preferentially along one of the two cement-host rock interfaces during core handling suggesting that these fractures would be preferentially reactivated along the cement-host rock interface during well bore completion. Fracture cement adhering to one of the host rock walls would thus form a fracture skin on one face of the reactivated natural fractures. We used scanning-electron microscopy (SEM) on ion-milled samples of fracture cement and host rock to study pore space connectivity between the matrix and reactivated sub-vertical calcite cemented natural fractures. In the fracture calcite cement, we observed open micro-fractures suggesting that this cement layer is partially permeable to flow. The permeability of these micro-fractures was determined by lattice Boltzmann methods and upscaled using an effective medium method to determine the permeability of the fracture cement. The fracture cement studied here is found to have permeabilities within the nano-Darcy range.

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