3D Experimental and Computational Studies of Crystallographic Effects on Creep and Fracture in Salt Rock

University Of Tennessee Knoxville, Knoxville TN

Investigators

Abstract

Rock salt, a sedimentary rock classified as an evaporate, forms as a result of evaporation of inland seas or any enclosed body of water, and can be found in nature as bedded or domal formations. Salt domes often trap oil, gas, and other minerals around their edges. Salt caverns are large cavities or chambers that form inside underground salt deposits either naturally by the effect of geological processes or are man-made. They have been used as storage for different types of hydrocarbons since early 1970s including the US strategic petroleum reserve. Salt caverns may potentially serve as a long-term and safe repository for carbon dioxide, nuclear waste, and the waste of oil drilling operations. Drilling through rock salt poses many challenges, including long-term wellbore stability/integrity, casing collapse due to lateral pressure, and drilling fluid-salt interaction. The accuracy with which the fracture behavior of any material can be simulated, including geological materials like rock, hinges upon the fidelity of both the engineering model and the geometrical representation of the cracked body. The anisotropic response of rock salt during creep deformations is stress and temperature dependent, and the accumulated creep strain influences fracture nucleation. This behavior creates many challenges when rock salt formations interact with sources of thermal and stress changes. An improved and quantitative understanding of when, where, and how cracks evolve within 3D polycrystalline rocks has many important technological implications with potential benefits for modeling drilling, geothermal-energy extraction, carbon sequestration, machine-rock interaction, and explosive penetration. This project (i) impacts the research community and promotes technology transfer; (ii) involves minority/female undergraduate students in conducting cutting-edge engineering research; and (iii) engages the next generation of scientists through the involvement of high school students in research, sparking their interest in the field of civil engineering and helping contribute to the quality of the next generation of civil engineering educators and professionals. A key limitation of existing phenomenological creep models is that rock salt's anisotropic response is not represented at the microstructural level. While crystal plasticity models for capturing anisotropy in rock salt do exist, they employ empirical flow rate equations which are valid for a narrow range of temperature and strain rate. Presently, there is an apparent lack of crystal-orientation-sensitive models in the literature for creep and fracture in 3D rock specimens coupled with direct measurements of 3D crystal structure. Thus, this research will combine nondestructive 3D x-ray diffraction (3DXRD), 3D synchrotron micro-computed tomography (SMT) in-situ experimental measurements, and 3D crystal-plasticity modeling to enhance current understanding of creep and crack formation and growth mechanisms in polycrystalline rock is unprecedented in many regards. The ability to experimentally measure lattice strains within the microstructure of rock has recently been demonstrated by the PI. These enhanced experimental techniques provide us with a richer dataset for calibrating the microstructural constitutive model accounting for anisotropy. By employing dislocation mechanism-based crystal plasticity models, we plan to elucidate the governing mechanisms operating in the halite crystals that yield the observed creep and fracture response.