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

岩盐，一种被归类为蒸发物的沉积岩，是由于内陆海或任何封闭水体蒸发而形成的，可以在自然界中以层状或圆顶状的形式发现。盐丘通常在其边缘圈闭石油、天然气和其他矿物。盐穴是在地下盐矿中形成的大型洞穴或腔室，这些洞穴或腔室是由地质作用自然形成的，或人为形成的。自20世纪70年代初以来，它们一直被用作储存不同类型的碳氢化合物，包括美国的战略石油储备。盐穴可以作为二氧化碳、核废料和石油钻井作业废料的长期安全储存库。钻穿岩盐会带来许多挑战，包括长期井筒稳定性/完整性、侧向压力导致的套管坍塌以及钻井液-盐相互作用。模拟任何材料（包括岩石等地质材料）断裂行为的准确性取决于工程模型和裂纹体几何表示的保真度。岩盐在蠕变过程中的各向异性响应依赖于应力和温度，累积蠕变应变影响裂缝成核。当岩盐地层与热源和应力变化相互作用时，这种行为带来了许多挑战。对三维多晶岩石中裂缝何时、何地以及如何演化的改进和定量理解具有许多重要的技术意义，对钻井建模、地热能提取、碳封存、机器-岩石相互作用和炸药穿透都有潜在的好处。该项目(i)影响研究界并促进技术转让；（ii）让少数族裔/女本科生参与前沿工程研究；（iii）通过高中生参与研究，激发下一代科学家对土木工程领域的兴趣，并有助于提高下一代土木工程教育工作者和专业人员的质量。现有现象学蠕变模型的一个主要局限性是岩盐的各向异性响应没有在微观结构水平上得到体现。虽然存在捕获岩盐各向异性的晶体塑性模型，但它们采用的经验流速方程适用于较窄的温度和应变速率范围。目前，文献中明显缺乏用于三维岩石试样蠕变和断裂的晶体取向敏感模型，以及对三维晶体结构的直接测量。因此，本研究将结合无损三维x射线衍射（3DXRD）、三维同步加速器微计算机断层扫描（SMT）原位实验测量和三维晶体塑性建模，以增强当前对多晶岩石蠕变和裂纹形成和扩展机制的理解，在许多方面是前所未有的。通过实验测量岩石微观结构中的晶格应变的能力最近已被PI证明。这些增强的实验技术为我们提供了更丰富的数据集来校准考虑各向异性的微观结构本构模型。通过采用基于位错机制的晶体塑性模型，我们计划阐明在产生观察到的蠕变和断裂响应的岩盐晶体中运行的控制机制。

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.