The mechanism of fracture initiation in shale rocks: Pottsville cap-rock-shale vs. Marcellus unconventional reservoir-shale

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Conference Proceeding

Publication Date



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 CO 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. Indentation tests were conducted on two shale formations, Pottsville, AL, and Marcellus, PA, at both micro and nanometer scale on retrieved drilled rock core samples to get the mechanical properties of the bulk and individual mineralogical phases present. Results from indentation showed Pottsville shale sample had overall better sealing properties than Marcellus shale, because of the higher bulk mechanical properties, more uniform grain size, higher rigid grain content, and the potential fracture/deformation healing and re-sealing within few months. 2

Publication Source (Journal or Book title)

53rd U.S. Rock Mechanics/Geomechanics Symposium

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