What controls the mechanical properties of shale rocks? - Part II: Brittleness

Successful stimulation of shale gas reservoirs by hydraulic fracturing operations requires prospective rocks characterized by high brittleness to prevent fast healing of natural and hydraulically induced fractures and to decrease the breakdown pressure required to (re-) initiate a fracture. We briefly reviewed existing brittleness indices (B) and applied several, partly redefined, definitions relying on composition and deformation behavior on various, mainly European black shales with different mineralogical composition, porosity and maturity. Samples were experimentally deformed at ambient and elevated pressures (P) and temperatures (T), revealing a transition from brittle to semibrittle deformation behavior with increasing pressure and temperature. At given composition and deformation conditions, B values obtained from different definitions vary considerably. The change of B with applied deformation conditions are reasonably well captured by most definitions based on the stress-strain behavior, which do not correlate with the fraction of individual phases, e.g., clay content. However, at given deformation conditions, most composition-based indices show similar variations with bulk composition as those derived from stress-strain behavior. At low P-T conditions (≲4 km depth), where samples showed pronounced post-failure weakening, B values determined from composition correlate with those calculated from pre-failure stress-strain behavior and both correlate with the static Young's modulus. In this regime, the brittleness concept can help to constrain successful hydraulic fracturing campaigns and brittleness maybe estimated from core or sonic logs at shallow depth. However, long term creep experiments are required to estimate in-situ stress anisotropy and the healing behavior of hydraulically induced fractures.