Abstract

In the last few years, the term “brittleness” has become part of the vernacular with all operators exploiting unconventional resources. Although it means well, the term can become a dangerous trap resulting in underperformance and missed opportunities. The rise of petroleum geomechanics as part of a geologist’s toolbox has caused a myriad of assessment protocols to become available. “Brittleness” is one such assessment protocol that is grounded in compressive failure mechanics, describing the energy transfer from load apparatus to the rock sample. A textbook example of a brittle material is one that exhibits high stiffness (high Young’s Modulus) and low compliance (low Poisson’s Ratio). Recently, this has translated into the petroleum industry as a factor to describe brittle versus ductile shale. 1D well log analysis and 3D seismic inversion processing have all shifted attention to the prediction of elastic moduli that can describe “brittleness”. Acoustic wave theory provides a fairly simple relationship between compressional and shear wave velocities and elastic moduli. This simple analysis allows very fast calculation of the moduli and subsequent mapping and interpretation. Multiple relationships between well performance, stimulation parameters, and drilling efficiency are related to these quickly acquired elastic moduli. With all of this detailed data capture and processing, why do most data-rich relationships between “brittleness” and these development outcomes typically end in weak to no positive correlation?

One explanation is that we are using an inadequate property to differentiate our target intervals in these complex, heterolithic mudrocks. I postulate that we are confusing the classic, compressional failure definition of brittle with something that should be called fracture efficiency in tensile failure. When something is efficient, it is assumed that low amounts of energy are expended to gain a particular result. In our case, the industry should look for rock that is efficient to break, not just brittle. “Brittleness” is a nebulous term and tells the geoscientist or engineer nothing about energy expense to fail in tension and maintain fracture growth.

By utilizing the elegant energy balance theory of Griffith and further elaborations by Irwin and Orowan we can approximate fracture efficiency by determining the critical strain energy release rate and subsequently, the critical stress intensity factor, or fracture toughness. The constitutive relationships that derive fracture toughness illustrate a dependency on elastic moduli. What is of note is that when Young’s Modulus decreases, fracture toughness also decreases, thereby illustrating that material with lower stiffness needs less energy paid into the system to create additional surface area in tensile growth. This results in analysis that is counterintuitive to the current understanding that brittle rock must have high Young’s Modulus. Enclosed in this discussion will be how to utilize fracture efficiency to assess target intervals and predict pressure pumping response from the formation. Direct examples from the Wolfcamp and Spraberry Shale are contained within.