Noble gases are ideal natural tracers to characterize the storage, migration and origin of fluids in geological environments. They are chemically and biologically inert and therefore are only partially separated by physical processes including the one related to the molecular diffusion phenomenon, so called kinetic fractionation. However, its precise quantification is difficult to achieve and its modeling is still open for debates. Thus, in this work, we have investigated the capability of simple and predictive models to estimate elemental and isotopic fractionation of noble gases in various geological fluids. To do so, molecular dynamics simulations on noble gases in water, gas and oil under sub-surface conditions were performed. These numerical results were found to be consistent with previous ones including experiments and molecular simulations when available, i.e. in water. Interestingly, it appeared that the widely used square-root law is able to provide a good prediction for elemental fractionation between noble gases in all solvents, except for helium. Such unexpected result from the theoretical point of view is explained by a peculiar relationship between the molecular mass and the molecular size of noble gas elements. Regarding the noble gas isotopic fractionation, the square-root law is shown to be unable to provide reasonable results, confirming recent experimental and numerical works. As expected, it has been noticed that the kinetic theory relation provides a reasonable estimate of the isotopic fractionation in gas whereas it deteriorates in water and in oil. Finally, using the previous findings, we propose a simple and predictive scheme for the isotopic fractionation of noble gases in all studied geo-fluids which is noticeably better than both the square-root and the kinetic theory relations.

稀有气体是表征地质环境中流体的储存、迁移和来源的理想天然示踪剂。它们在化学和生物上都是惰性的，因此只能通过物理过程进行部分分离，包括与分子扩散现象相关的过程，即所谓的动力学分馏。然而，它的精确量化很难实现，它的建模仍然存在争议。因此，在这项工作中，我们研究了简单和预测模型估计各种地质流体中惰性气体的元素和同位素分馏的能力。为此，在地下条件下对水、气和油中的惰性气体进行了分子动力学模拟。发现这些数值结果与先前的结果一致，包括可用时（即在水中）的实验和分子模拟。有趣的是，广泛使用的平方根定律似乎能够很好地预测所有溶剂中惰性气体之间的元素分馏，但氦除外。从理论的角度来看，这种意想不到的结果可以用惰性气体元素的分子质量和分子大小之间的特殊关系来解释。关于惰性气体同位素分馏，平方根定律无法提供合理的结果，证实了最近的实验和数值工作。正如预期的那样，已经注意到动力学理论关系提供了对气体中同位素分馏的合理估计，而它在水和油中会变质。