"Brittleness Index" has become a popular parameter in unconventional plays, used in an attempt to describe the behavior of rocks under in-situ hydraulic stimulation. However, with a multitude of different definitions and more critical assessments of the brittleness index (BI) emerging in recent years, the shortcomings in the interpretation of the brittleness index as a fracability parameter are increasingly evident. Calculating brittleness from both rock elastic properties and mineralogy has become commonplace due to the almost ubiquitous acquisition of those data in "shale" plays. The temptation is then to directly apply them in reservoir stimulation decisions. However, taking into account many shortcomings in the computation and physical interpretation of the brittleness index, questions arise about the meaning of rock brittleness when defined as a combination of different rock properties. In this paper we look into three different brittleness index definitions based on dynamic elastic moduli and mineral composition, and critically appraise their similarities, and potential for application in hydraulic fracture programs.
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