The influence of different fluids on the static fatigue of a porous rock: Poro-mechanical coupling versus chemical effects

Characterization of time-dependent brittle deformation is important for estimating the long-term stability of rock structures in shallow environments such as underground mines which are exposed to water or in a water vapor environment. The poro-mechanical (pore pressure) and chemical (stress corrosion reactions) effects of fluids at crack tips are responsible for the subcritical propagation of microcracks. This brittle creep results in the weakening of porous rocks over time (i.e., static fatigue). The focus of this work was to characterize using very long duration uniaxial multi-step creep tests, both the fluids effects on subcritical stress corrosion cracking in a porous rock (oolithic iron ore). Different fluids with increasing levels of chemical influence (oil, ethanol and water) were tested and both acoustic and mechanical properties were investigated. The evolution of the cumulative number of AE (Acoustic Emission) events, which reproduces the shape of the creep curves very efficiently, and static elastic properties indicate that micro-cracking plays the main role in the creep process. The poro-mechanical effect, which is predominant under partially saturated (water or ethanol) conditions, decreases the rate of subcritical cracking through capillary attraction forces. These forces harden porous rocks by modifying the effective stress state, increasing the activation energy barrier of the stress corrosion process and the fracture toughness and decreasing the stress intensity factor. The chemical effect of fluids is related to minerals developing stress corrosion reactions at crack tips which enhance subcritical cracking. Immersion in water/ethanol annihilates capillary forces and decreases the activation energy of the chemical reactions, thus increasing dilatancy, the rate of stress corrosion cracking, AE activity and rock weakening. Under saturated conditions, the time-dependent strength and time to failure increase as the chemical influence of the interstitial fluid decreases but the cumulative number and energy of AE at failure do not seem to be influenced by the chemistry of fluids. The short-term strength and deformability are also influenced by the level of chemical influence of the interstitial fluid but to a less remarkable extent because of the slow rate of chemical reactions. Water is therefore the most effective chemical agent promoting stress corrosion of iron ore among the fluids tested in our study.