The Caney Shale is emerging as a target for hydrocarbon production, creating opportunities to study rock mechanical origins of observed/anticipated challenges to effective and sustained stimulation. Here we examine five subunits within the Caney, two of which are dubbed "ductile" and three of which are dubbed "reservoir" rock types. The "ductile" versus "reservoir" identification is initially based on elastic properties ascertained from well logs. By comparing core-based mechanical tests, it is found nominally ductile and reservoir layers do not differ in terms of brittleness, but instead the nominally ductile layers tend to be weaker and more prone to creep deformation compared to the nominally more brittle reservoir layers. This difference in mechanical properties is shown to generate higher susceptibility to proppant embedment in the weaker and more creep-prone layers. Specifically, over a 5-year period, the creep is expected to have little impact on reservoir layers but is predicted to reduce propped fracture aperture by a factor of 2 in the ductile layers.
The Caney shale is located in southwest Oklahoma, located below the Springer shale and above the Woodford shale formations (Cardott, 2017). Recent attention has turned to the Caney as an emerging target for hydrocarbon production. Some of the main challenges are thought to be related to high clay content (Awejori et al, 2021, Radonjic et al, 2020) and the presumably ductile behavior with negative implications for effectiveness of stimulation. However, recently it has been argued (Benge et al, 2021) the Caney is not demonstrably "ductile" by the typical measures of brittleness/ductility ascribed to rocks. Hence, a more thorough mechanical study is required to highlight challenges associated with stimulation/production from the Caney so solutions can be developed to suit the specific challenges.
Firstly it is necessary to revisit what is meant by the terms "brittle" and "ductile". While such terminology is often used in a casual sense, there are at least two relevant mechanical definitions. One definition compares the peak and residual strength of a material (Bishop, 1967). Brittle materials lose strength abruptly when subjected to stress (i.e. tensile and/or shear) which exceeds the peak strength of the material, while ductile materials maintain a residual strength which can be nearly equal to the peak strength. Another definition has its roots in fracture mechanics, with brittle materials considered to ascribe to the assumption inelastic strain is confined to a very small region near a propagating crack tip (as compared to the crack length) and the energy required for crack propagation is assumed to be an intrinsic property of the material (e.g. see discussion in Papanastasiou and Atkinson, 2015).