Rock Brittleness

Definition:

• Brittle rocks undergo little or no ductile deformation past the yield point (or elastic limit) of the rock.
• Brittle rocks absorb relatively little energy before fracturing.
• Brittle rocks have a strong tendency to fracture.
• Brittle rocks have a higher angle of internal friction

Brittleness in Mining Industry:

Some authors in the mining industry define brittleness index B (loosely defined, but the concept is also called brittleness ratio, brittleness coefficient, or ductility number) as the ratio of uniaxial compressive strength to tensile strength.

\[ B = \frac{\mathrm{compressive}\ \mathrm{strength}}{\mathrm{tensile}\ \mathrm{strength}} = \frac{\sigma_\mathrm{C}}{\sigma_\mathrm{T}}
\]

Altindag (2003) also gives:

\[ B = \frac{\sigma_\mathrm{C} - \sigma_\mathrm{T}}{\sigma_\mathrm{C} + \sigma_\mathrm{T}} \]

Altindag (2002 and 2003) further showed that the most useful measure may be the mean average of compressive and tensile strength:

\[ B = \tfrac12 \times (\sigma_\mathrm{C} + \sigma_\mathrm{T})  \]

Tensile strength is usually correlated with compressive strength, and it may be possible to use just one of these measures as a proxy for brittleness. This is good, because some (most?) labs only measure compressive strength as a standard test, e.g. in routine triaxial rig tests.

Brittleness in Geophysics:

Rickman et al. 2008 proposed using Young's modulus E and Poisson's ratio ν to estimate brittleness. This is appealing to development geophyisicists because elastic moduli are readily available from logs and accessible from seismic data via seismic inversion. Two recent examples are Sharma & Chopra 2012 and Gray et al. 2012. Gray et al. gave the following equations for 'brittleness index' B:

\[ B=50\% \times \left(\frac{E_{\mathrm{min}}-E}{E_{\mathrm{min}}-E_{\mathrm{max}}}+\frac{\nu_{\mathrm{max}}-\nu}{\nu_{\mathrm{max}}-\nu_{\mathrm{min}}}\right) \]

However, this approach remains skeptical, which assumes that a shale's brittleness is (a) a tangible rock property and (b) a simple function of elastic moduli. Computing shale brittleness from elastic properties is not physically meaningful, stated by Lev Vernik stated at the SEG Annual Meeting in 2012.

http://subsurfwiki.org/wiki/Brittleness