Previous Talks

Wednesday, May 8, 2013

Brittleness in shales

Amie Lucier Hows and Ronny Hofmann, Shell International Exploration and Production, Yi Yang, Stanford University


Hydraulic fracturing is essentially where engineering meets geology. The structured, standardized, computationally efficient theories on crack propagation developed for engineering are turned upside down and applied to heterogeneous, flaw-filled, layered rocks. For many engineering applications, materials experience tensile loading conditions, crack propagation is to be avoided and ductility is desired in order to prevent catastrophic brittle failure. On the other hand, hydraulic fracture engineering seeks to create large-scale, brittle failure by forcing rocks under compressional stresses into tensile failure by increasing fluid pressures above a critical fracture propagation pressure. The success of a hydraulic fracture depends on rock properties, in situ stress and temperature conditions, and the optimization of engineering parameters. The term “brittleness” has entered the exploration and production lexicon as a means to (imprecisely) describe the ideal rock properties associated with successful hydraulic fracturing and effective system permeability enhancement. Unfortunately while brittleness qualitatively connotes a certain style of deformation and fracture behavior (i.e. the opposite of ductile or plastic deformation), the quantitative definition of brittleness in the context of hydraulic fracturing is elusive. In this paper, we review a variety of existing definitions of terms related to brittleness, brittle-ductile transition, plasticity and toughness from several different disciplines including structural geology, rock mechanics, reservoir engineering, soil mechanics, and mineralogy. We then discuss which of these concepts, definitions, and measurements are most relevant for hydraulic fracture stimulation in unconventional reservoirs.

Speaker Biography

Amie Lucier Hows joined Shell in 2008 after completing her PhD at Stanford University as a member of the Stress and Crustal Mechanics research group in the Department of Geophysics. She received her undergraduate degree in Geology at Washington and Lee University in Lexington, VA. She is a research petrophysicist in the Rock Characterization and Modeling Team in Exploration Research. She investigates the relationships between geological, petrophysical, and geomechanical rock properties.