Microscale numerical modeling is an effective approach for understanding salt creep. However, it is quite challenging because of evolving salt grain boundaries that are discontinuous, and dynamic contacts between these deformable salt mineral grains as a result of compaction, chemical reaction, fluid flow, and heat transfer. In this study, we apply the microscale mechanical model that was developed previously based on the numerical manifold method (NMM, Hu & Rutqvist, 2020a) to analyze creep of salt at the microscale. In our model, we consider three types of microscale mechanical behavior in two stages during compaction of salt aggregates that potentially contribute to creep at a larger scale. These are: initial compaction governed by reorganization of the halite aggregate, and secondary compaction as results of pressure solution and microfracture growth. We rigorously consider dynamic contacts between a number of salt grains with realistic geometric representation of the grain boundaries. These arbitrarily shaped salt grains can undergo large displacement and/or large deformation. We account for pressure solution along grain boundaries by removing mass along the boundaries. Then we determine microfracture growth based on high-stress bands during the dynamic processes of compaction, and investigate the impact of microfracture growth by and on the dynamic contact alteration. Based on the microscale modeling, we capture the overall change of porosity over time that can be related to core-scale experimental results qualitatively, thus providing insights for understanding creep behavior of salt.
Creep behavior of salt is imperative to the long-term safety of nuclear waste disposal in salt rocks. To date, a number of constitutive models based on laboratory experiments have been proposed for salt creep. Generally, two deformation types at the laboratory core scale are considered: viscoelastic and viscoplastic deformation. The viscoelastic deformation is associated with pressure solution and reorganization of grain aggregates. The viscoplastic deformation is explained by plastic flow, microfracture growth, and healing processes (Dusseault et al., 1987; Mraz et al., 1991; Costa et al., 2005; Firme et al., 2018).