Microscale Mechanical-Chemical Modeling of Granular Salt: Insights for Creep

Microscale numerical modeling is potentially an effective approach for understanding salt creep, but it is quite challenging because natural salt grain boundaries can have complex and evolving geometry, and because contacts between these deformable salt mineral grains are dynamic as a result of compaction, chemical reaction, fluid flow, and heat transfer. In this study, we have overcome these challenges and developed a new microscale mechanical-chemical (MC) model to analyze creep of salt at the microscale, accounting for coupled deformation, dynamic contacts, and chemical reaction in granular systems with realistic geometric representations. The MC model was realized by linking a new microscale mechanical code based on the numerical manifold method (NMM) to a reactive transport code named Crunch. We simulated the processes of reorganization of the salt grains, microfracturing, and pressure solution that contribute to creep at larger scales. Based on this first quantitative microscale model, we found that sharp corners of mineral grains can dominate the contact dynamics, microfracturing, and pressure solution when salt is compacted, thus governing the structural changes and porosity loss of the system. We found that pressure solution, which preferentially dissolves sharp corners and edges, can lead to relatively high porosity loss in the system, thus playing an important role in the creep of salt. Our analysis shows that the dynamic changes of salt granular systems involving grain relocation and pressure solution can occur repeatedly and continuously, thus contributing to longer-term creep of salt at larger scales. Plain Language Summary Laboratory tests have identified grain dislocation, microfracturing, and pressure solution as the microscale mechanisms that can explain the creep of salt at larger scales. However, it remains uncertain how each mechanism quantitatively impacts creep in granular systems. Microscale numerical modeling can be effective for quantifying the impacts of each microscale mechanism, but it is very challenging because the microscale salt grains are naturally discontinuous, and the multiphysics can be quite complex. In this study, we have developed a new microscale mechanical-chemical model that accounts for grain relocation, microfracturing, and pressure solution of halite grains. Based on this first-of-its-kind quantitative microscale model, we found that sharp corners of mineral grains can dominate the contact dynamics, microfracturing, and pressure solution when salt is compacted, thus governing the structural changes and porosity loss of the system. We found that pressure solution, which preferentially dissolves sharp corners and edges, can lead to relatively high porosity loss in the system. Our analysis shows that the dynamic changes of salt granular systems involving grain relocation and pressure solution can occur repeatedly and continuously, thus contributing to longer-term creep of salt at larger scales.

Automated summary

The study developed a new microscale mechanical-chemical model that showed sharp corners of mineral grains dominating the contact dynamics, microfracturing, and pressure solution in salt systems, thus influencing structural changes and porosity loss. The analysis revealed that pressure solution dissolved sharp corners and edges, leading to relatively high porosity loss, playing a crucial role in salt creep. Dynamic changes in salt granular systems involving grain relocation and pressure solution were identified as contributing factors to longer-term salt creep at larger scales.