Mechanical Characterization of Shale

Written by: Jessica Lawshe

As one of the most complex natural materials on earth, shales manifest a variety of macroscale mechanical behaviors. Eagle Ford shales are carbonate-rich marine shales which exhibit textural and mineralogical variability. This variability of mineralogy and organic matter affect the bulk mechanical properties. Mechanical properties of components on the nanoscale may be determined by nanoindentation to correlate with structure and bulk properties.

Optical image of shale showing the 3 main phases
Optical image of shale with 3 main phases: pyrite framboids in highly reflective regions, carbonate grains, and a clay matrix

Shale contains three main phases: pyrite framboids, carbonate grains, and a clay matrix. These phases are readily distinguished in the mechanical property maps, measured at a speed of 1 indent/s using a nanoindentation technique. Histograms are generated using the nanoindentation mapping software.

Map and histogram of Shale
Modulus (upper left) and hardness (upper right) maps with an array of 100 x 100 indents; corresponding histograms show modulus (lower left) and hardness (lower right) distributions for 3 main phases of shale

The clay matrix, blue values on the property maps, are on the lower end of the histograms, as expected, with the carbonate phase at higher modulus and hardness. The regions of highest modulus and hardness, in red on the property maps, corresponding to the pyrite-rich areas of the sample as expected. The greater spread of pyrite data may be due to the framboid structure allowing influence from the more compliant clay matrix. These quantitative images are far more valuable for understanding and predicting the mechanical behavior of the shale phases than what might be achieved with bulk testing methods (tension, compression, microhardness, etc).