Collaborative Research: Salt Rock Microstructure and Deformation
Salt rock is used for secure storage of oil, high-pressure gas, and nuclear waste because of its ability to creep and form barriers to gas and fluid flow. However, creep processes near subsurface storage facilities can lead to crack debonding, opening, closure and rebonding that compromise the integrity of flow barriers and stability of engineered structures. Mathematical descriptions of cracking and healing are incompletely developed. This award will support fundamental rock physics and mechanics research to link deformation by crack debonding, opening and rebonding to microstructures and mechanical variables that govern the time-dependent behavior of salt rock over geological space and time scales. Deformation of salt occurs rapidly at laboratory conditions making it a good analog to study damage and healing in other crystalline materials and porous media. Project findings are expected to advance quantitative relations enabling accurate modeling and prediction of behaviors in engineering projects. The work will advance fundamental understanding of cracking and healing in other rocks that deform by the same processes, especially around earthquake faults and zones of induced seismicity. The team of geoscience and engineering researchers will collaborate with other programs to broaden the participation of women and other under-represented groups in research and engineering.Most continuum damage models for salt are based on the concept of dilatancy boundary, and do not account for crack-induced anisotropy of elastic properties. Micro-mechanics and homogenization principles were successfully employed to model salt rock elastoplastic behavior, but were not used to test scale invariance of microstructure to transitions in deformation regimes. The goal of this project is to link salt deformation regimes and rheological behaviors with the density and orientation distribution of microstructures. Deformation regimes at the scale of a Representative Elementary Volume will be decomposed into independent processes (e.g., cracking), each related to different microstructure descriptors (e.g., grain aspect ratio). Macroscopic thermodynamic variables will be formally related to the microstructure descriptors to test the space and time scale invariance of transition points in deformation regimes. Specific research objectives are to: (1) Identify topological components and subcomponents of salt microstructure; (2) Determine the energy cost of intra- and inter-granular crack debonding, opening, closure and rebonding; (3) Compute energy thresholds associated with transitions between deformation regimes at the grain and REV scales; (4) Test the time and scale dependence of microstructure organization during transitions; (5) Design rock salt microstructural systems that optimize healing properties in subsurface storage environments.