Natural geological formations are complex objects,involving geological,mechanical,physico-chemical processes occurring over very wide length scales and time scales.Phenomena ranging from the molecular scale to several hundred of kilometers may influence the overall behaviour of fluid transport in a geological formation.For example,wettability properties,themselves due to molecular effects,have a very strong impact on the water/oil displacements in oil reservoirs.Analogously,reservoir heterogeneities that cover a large range of spatial scales play an essential role to channel fluid-flows,especially when they are coupled with non linearities inherent to fluid dynamics.In order to face this complexity,and to be able to hierarchize the influence of the various relevant geological and physico-chemical phenomena,it is thus essential to handle a multiscale description of fluid transport in these reservoirs.This is an essential tool to help reservoir engineers to focus on the crucial phenomena that control the flow.This helps them to integrate data,and this results in a lowering of the uncertainties of the reservoir description that enhances economical decisions.In this paper,we present classical upscaling approaches,as well as more recent multiscale concepts.
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