We present a case study from the Caney Shale, which is an emerging hydrocarbon-bearing formation in Oklahoma. Open hole logs lead to a nominal distinction of five zones in an exploration well, two of which are designated "ductile" and three of which are designated as less ductile "reservoir" zones. Subsequently, a detailed mechanical characterization was carried out on core plugs taken from each of these zones. Results show nominally "ductile" zones are weaker in terms of UCS, tensile strength, fracture toughness, and confined compressive strength. However, by a variety of brittleness indices, it is found the nominal "ductile" regions do not behave as expected. They are weaker, but not consistently more ductile. Furthermore, some brittleness indices lead to contradictory conclusions. For example, zones which would be considered the most brittle according to the proportion of nonlinear stress-strain behavior prior to failure and/or by indices based on elastic properties would be considered the least brittle according to their internal friction angle.
Ductility is the ability of a material to sustain large inelastic deformation without loss of bearing capacity (e.g. Hajiabdolmajid and Kaiser 2003). Brittleness is most commonly defined as a lack of ductility (e.g. Hetenyi, 1950 as cited by Hucka and Das, 1974). Rock engineering inevitably deals with the behavior of rock beyond its elastic limit, thereby invoking a widespread desire to characterize whether such behavior is "brittle" or "ductile". The proper answer is rock is quasi-brittle because, like concrete (Bazant, 1999), it exhibits some brittle-like qualities and some ductile-like qualities. The purpose of this paper is not to delve into this issue, but rather to adopt a starting point wherein it is recognized the petroleum industry has, over the years, adopted a variety of metrics ascribed to characterize brittleness of rock (Bai, 2016).
Here we present a case study from the Caney Shale, which is an emerging hydrocarbon-bearing formation in Oklahoma. The opportunity arises because five subunits of the Caney were originally identified as nominally "ductile" and "reservoir" (i.e. less ductile) layers based on interpretation of open hole well logs. This was a "blind" identification, documented prior to testing of rock mechanical properties using core from the same borehole. Subsequently, a detailed mechanical characterization was carried out on core plugs taken from each of these five zones. The basic question is whether the log-based demarcation of nominally ductile zones corresponds in a predictive and useful way to mechanical properties and behavior of the rock.
