Geological heterogeneity is recognized as a major control on reservoir production and constraint on many aspects of quantitative hydrogeology. Hydrogeologists and reservoir geologists need to characterize groundwater flow through many different types of geological media for different purposes. In this introductory paper, an updated perspective is provided on the current status of the long effort to understand the effect of geological heterogeneity on flow using numerical simulations. A summary is given of continuum vs. discrete paradigms, and zonal vs. geostatistical approaches, all of which are used to structure model domains. Using these methods and modern simulation tools, flow modelers now have greater opportunities to account for the increasingly detailed understanding of heterogeneous aquifer and reservoir systems.
One way of doing this would be to apply a broader interpretation of the idea of hydrofacies, long used by hydrogeologists. Simulating flow through heterogeneous geologic media requires that numerical models capture important aspects of the structure of the flow domain. Hydrofacies are reinterpreted here as scale-dependent hydrogeologic units with a particular representative elementary volume (REV) or structure of a specific size and shape. As such, they can be delineated in indurated sedimentary or even fractured aquifer systems, independently of lithofacies, as well as in the unlithified settings in which they have traditionally been used. This reconsideration of what constitutes hydrofacies, the building blocks for representing geological heterogeneity in flow models, may be of some use in the types of settings described in this special issue.