The Mechanism of Fracture Initiation in Shale Rocks: Pottsville Cap-Rock-Shale vs. Marcellus Unconventional Reservoir-Shale

Shale rocks were first of interest as a source rock for hydrocarbons, and as Caprocks that provide seals for HC reservoirs. More recently their integrity has been investigated in the containment of injected CO₂ and as unconventional reservoirs for oil & gas. Hydraulic fracturing has been used to produce hydrocarbons from shale rocks for more than a decade, but the fundamental mechanism to initiate and propagate these fractures remains unclear. The shale rocks are complex and heterogeneous geological material highly impacted by depositional environment, diagenesis and earth stresses. Macro and Microstructure of shale rocks are dominated by distinct laminated layering, which causes the anisotropy in petrophysical and mechanical properties. Due to this heterogeneity, when subjected to a load, both plastic and elastic deformations manifest simultaneously at different degree in different directions, leading to a difference in failure/fracture response of the bulk rock. The objective of this paper is to gain a fundamental understanding of how and where the fractures initiate when the rock is under stress, as in the case of pressure buildup or hydraulic fracturing.