Microstructure-based modeling of damage and healing in salt rock with application to geological storage

Abstract

Most mineral and energy resources such as ore, petroleum, natural gas, and geothermal energy are recovered from the earth. Nuclear waste repositories and CO2 storage systems are buried underground. Recovery of mineral resources, storage of energy, and disposal of waste involve changes in coupled mechanical and transport rock properties. The evolution of pores and cracks during thermo-hydro-chemo-mechanical coupled processes governs the variations of macroscopic properties. This research investigates the modeling of damage and healing in rocks with applications in geological storage. This presentation focuses on salt rock, which is used as a model material to study rock microstructure evolution under various stress paths, and to understand the microscopic processes that lead to macroscopic mechanical recovery. We developed two different techniques based on continuum damage mechanics (CDM) and micromechanics. The first method enriches the framework of CDM with fabric descriptors. We carried out creep tests on granular salt to infer the form of fabric tensors from microstructure observation. Net damage evolution is governed by a diffusion equation. Macroscopic and microscopic model predictions highlight the increased efficiency of healing with time and temperature. The other method is based on a self-consistent homogenization scheme, in which the viscoplastic and damage behavior of halite polycrystals is upscaled from mono-crystal slip mechanisms. The model provides micro-mechanical interpretations to important aspects of salt rock viscoplastic and fatigue behavior. We implemented the micromechanical model in a finite element program to characterize crack patterns in salt polycrystals and predict damage around a salt cavern used for high-pressure gas storage. This study is expected to improve the fundamental understanding of damage and healing in rocks, and the long-term assessment of geological storage facilities.