Squeezin' A Rock Around d' Clock

Changes in temperature, pressure, and stress affect fluid transport properties such as porosity and permeability. The response to this applied stress or change in pore pressure is seen as either a compaction or dilation of pore space. It is important to understand rock deformation processes because of the extremely relevant geological applications, which include the oil/gas exploration and groundwater industries. Many experiments have been conducted on sandstones and granites, however, there is a greater need for the understanding of calcite deformation. I am observing the way in which Indiana Limestone responds to various pressure and temperature conditions. Indiana Limestone is an excellent specimen in which to observe calcite deformation because it is relatively homogeneous, comprising more than 97% calcite.

Deformation can either be elastic, where the material returns to its original shape after the applied load has been removed, or inelastic, where deformation is non-recoverable. Inelastic deformation is further divided into brittle and plastic deformation. Brittle deformation is seen as fractures and plastic deformation is seen via crystal plasticity, in which one sees either dislocation slip or twinning. Previous deformation experiments have been conducted on Indiana Limestone at room temperature. Stress-strain curves (Vajdova et al., 2004) show the transition from brittle to ductile deformation; because crystal plasticity, part of the plastic deformation mechanism, is temperature dependent, I am interested in observing the effect of temperature. Additionally, the Vajdova experiments were conducted in unsaturated cores. My experiments include cores that are saturated with distilled water, so there will be an added comparison of the effect that water has on deformation.

I hypothesize that crystal plasticity behavior in calcite will affect the bulk deformation behavior as well as transport properties. The effect of dislocation slip and twinning on transport properties can be seen in appreciable amounts. I additionally hypothesize that as temperature increases, the effects of crystal plasticity will be more pronounced. This is because as rocks increase in temperature, they increasingly behave as a plastic. Current experiments follow the Vajdova data accordingly.

Further reading: Vajdova et al., 2004

Fig. 1: Prepared sample, pre-deformation. The core is jacketed with copper and has strain gages attached.

Fig. 2: Post-deformation twinning comparison between a sample that has experienced an effective pressure of 10 MPa and one that has undergone 50 MPa. The twinning is much more intense in the 50 MPa sample, leading me to believe that this twinning is most definitely deformation-induced (not natural).