Knowledge of temporal and spatial variations in fluid flow, which is of critical importance to resource recovery, requires better understanding of the changes and the rates of changes of permeability and porosity at reservoir conditions. Permeability and porosity are dynamic physical properties that are sensitive to mechanical and thermal loads. With increasing pressure and temperature, rocks undergo a transition in failure mode from localized brittle failure to non-localized plastic flow. However, the mechanisms (i.e., microcracking, crystal plasticity, pressure solution etc.) that control the brittle-ductile transition and their dependence on temperature are not well-understood. In this study, we deformed porous Indiana limestone samples with initial porosity of ~16% at temperatures of 298, 323 and 348 K under effective pressures ranging from 10 to 50 MPa. In each deformation test, pore fluid (distilled water) pressure and strain rate were kept constant at 10 MPa and 1x10-5/s. We use strain gauges and a pore pressure intensifier to track volumetric strain and the pore volume change during deformation. Simultaneous changes in permeability and sonic velocity were measured in a subset of samples. In comparison to dry samples, our data show lower yield strength and enhanced compaction in water-saturated samples, and these effects are exacerbated at elevated temperature. The initial yield envelopes (in the differential versus effective mean stress domain) indicate a strong temperature-dependence of yield strength. Furthermore, the shape of a yield envelope changes considerably with increasing temperature, indicating a transition in dominant deformation mechanisms. Initial microstructural analysis shows that both dilatant microcracking and mechanical twinning play important roles in deformation. We suggest that the increased water-weakening at elevated temperatures results from the interplay among microcracking, crystal plasticity, and likely solution transfer.
Carbonate rocks are widely abundant in the upper crust.