Relationship between plastic flow, microcracking and solution-precipitation processes in coarse-grained rocksalt: An experimental study
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Salt caverns have the potential to be the host rock for geological storage or they must be abandoned after production. These salt caverns always contain brine after solution mining. The stress-state of the salt caverns changes during the construction of the cavern, during the active cyclic filling and depleting of the cavern for energy storage and after abandonment. The developed effective stresses can be low enough to allow for semi-brittle deformation (plastic flow with microcracking) and/or hydrofracturing. However, little is known about what the effects of brine penetration into dilated rocksalt will be on the rates and mechanisms of plastic flow, solution-precipitation and microcracking. To this extent, triaxial and unconfined compression experiments were performed on coarse-grained Zechstein salt; solution mined in the Netherlands. The experiments were performed at room temperature and after loading the samples, and the corresponding dilation, dry and wet (including the injection of saturated brine) stress-relaxation followed. The results showed that the stress exponent of dry stress-relaxation phase is noticeably higher (n=9-35); implying dislocation glide; than measured during wet stress-relaxation, in which it decreases to ±3. The latter suggests a change in dominant deformation mechanism from dislocation glide to dynamic recrystallization. Microstructural evidence indicates that diffusive mass transfer processes like recrystallization are affected by the presence of the cracks. The major rheological weakening when saturated brine is present in dilated rocksalt can be explained by various factors: subcritical crack growth, activated dynamic recrystallization and the impact of microcracks on stress enhancement and enhanced pressure solution creep rate. The results of the study imply that microcracking and the geochemical environment must be taken into account when predicting the occurrence of cavern instability (i.e. crack growth), identifying the damage zones around well bores and modelling the formation of a sealing salt plug for well abandonment.