Water weakening in porous carbonate rocks: equilibrium and disequilibrium pore fluid effects

Abstract:

Carbonates are some of the most abundant rock forming minerals in the crust; carbonate dominated rocks form the bedrock of many populated areas and host seismogenic fault systems as well as energy resource reservoirs and aquifers. In many ways, the mechanical behaviors of silicate rocks and carbonate rocks are analogous; however, the ways in which the behaviors differ can be substantial. In particular, the ease of crystal plastic deformation at upper crustal conditions and the chemical effects of hydrothermal fluids can lead to significant divergence in the mechanical behavior of carbonate rocks compared to more commonly studied silicate rocks. To explore the chemical effects of pore fluids on the deformation of porous carbonate rocks, we ran hydrostatic and axial deformations in traditional triaxial configuration on Indiana Limestone (>98% calcite, ~16% porosity) at effective confining pressures from 10 to 50 MPa, temperatures from 23 to 75°C, and saturated with equilibrium and disequilibrium pore fluids. In-situ permeability measurements were made during tests. Over these pressure and temperature conditions, the inelastic behavior and failure modes of limestone samples change from localized shear failure to shear-enhanced compaction to cataclastic dilation. We find that elevated temperature and pore fluid saturation both have a weakening effect on the limestone, reducing yield strength and favoring yield by shear-enhanced compaction. Permeability is negatively correlated with axial deformation at all conditions. In addition, we find that the chemistry of the pore fluid has a significant effect on the mechanical behavior of the limestone, with disequilibrium pore fluid causing more pronounced weakening than equilibrium pore fluids. Microstructural observation of deformed samples shows that failure behaviors are controlled by the competition between microcracking and crystal plasticity.