Abstract:

In this study, micro-mechanisms that govern the viscous and damage behavior of salt polycrystal during creep processes are investigated. A Finite Element model is designed with POROFIS, in which surface elements represent salt grains and joint elements represent inter-granular contacts. Microscopic observations of salt thin sections serve as a basis to design the mesh, which includes voids. We compare three strategies to predict microscopic damage in the salt polycrystal: (1) inter-granular damage represented by damage propagation in joint elements; (2) intra-granular damage represented by stiffness degradation in grain surface elements; (3) damage in both surface and joint elements. We simulate creep tests in conditions typical of Compressed Air Energy Storage. The three models capture polycrystal stiffness degradation and the initiation, propagation and coalescence of cracks that originate from geometric incompatibilities and local stress concentrations. The model with damageable joints presents a more ductile behavior and captures a smooth transition between steady state and tertiary state creep. This research is expected to improve the fundamental understanding of viscous damage mechanisms in salt rock for geostorage applications, and bring new insights on numerical modeling of multi-scale damage processes in crystalline materials.

Introduction

Because of its low gas permeability, high solubility in water, favorable creep properties and fast self-healing potential, salt rock is considered as an appropriate host media for geological storage of nuclear waste, petroleum, and high-pressure gas. Fundamental deformation processes and flow properties of salt were investigated at the crystal scale (Carter and Hansen, 1983). Constitutive models were also proposed to assess the long-term performance of salt caverns at the field scale (e.g., Bérest et al., 2001; Zhu et al., 2015; Martin et al., 2015). But few numerical studies exist to correlate microscopic phenomena occurring at the grain scale to salt behavior observed at the macroscopic scale.

Microstructure-enriched models allowed better understanding the influence of grain size, orientation, shape, and boundary topology in polycrystalline materials. Finite Element (FE) models are only available for with simple grain geometries, with squared or cubic meshes (e.g., Beaudoin et al., 1995). Such methods ignore the influence of real microstructure on the macroscopic behavior of the polycrystal.

To study the deformation and strength of polycrystals at multiple scales with Finite Element Methods (FEM), discontinuities were modeled with Cohesive Zone Models (Espinosa and Zavattieri, 2003) and extended Finite Element Methods (XFEM) (Sukumar et al., 2003). Metals were extensively studied with microstructure-based FEM, but not salt. Indeed, accounting for the time-dependent viscous damage in salt polycrystals subjected to long-term creep loading remains a major challenge in FEM. Only a few studies are based on a realistic representation of microstructure and investigate inter-and intra-granular damage (Musienko and Cailletaud, 2009).

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