Linking Microscale Processes to Macroscale Salt Creep with a New THMC Model

ABSTRACT: Microscale grain deformation and reorganization, fracturing, and pressure solution at interfaces have been identified as the mechanisms for the creep of salt at larger scales. However, it remains uncertain how each mechanism quantitatively impacts creep in granular salt that are naturally under complex geometric and multi-physical conditions. In this study, we have developed a new microscale thermal-hydro-mechanical-chemical (THMC) model that accounts for coupled deformation, dynamic contacts, chemical reaction, and fluid transport in heated environment in granular systems where the microscale geometry can be realistically represented. The THMC model was achieved by linking a microscale mechanical code based on the numerical manifold method (NMM) to the reactive transport model Crunch. Using this first-of-its-kind microscale model, we show the results of compaction within a halite aggregate. We found that sharp corners of salt grains can dominate the contact dynamics, microfracturing, and pressure solution, thus governing the porosity loss of the system. We show that pressure solution and structural changes can be further accelerated by a thermal gradient due to changes of thermodynamics and reaction rates. We show that grain reorganization and pressure solution can occur repeatedly and continuously, thus contributing to longer-term creep of salt at larger scales. 1. INTRODUCTION Understanding the creep of salt is essential for effectively predicting the long-term evolution of nuclear waste repositories and strategic storage caverns for oil and gas in salt rocks (Urai et al., 1986). Different underlying mechanisms of creep in different stress/temperature ranges have been theoretically proposed (Urai et al., 1986). Based on laboratory tests micrograph, Urai and Spiers (2007) suggested that grain dislocation, microfracturing and pressure solution (Spiers et al., 1990; Zhang et al., 2007) are mechanisms acting during long-term creep. Identifying these mechanisms has advanced the understanding of salt creep. However, an important scientific question remains: How does each mechanism quantitatively impact creep in granular systems that involve such complex geometric and multi-physical conditions?