Reservoir Characterization Problems Requiring Core-Scale Characterization Solutions: Sometimes You Just Have to Know What Is Happening Every Foot

Alan P. Byrnes
Kansas Geological Survey, University of Kansas, Lawrence, KS

Different reservoir systems are examined illustrating situations where critical reservoir properties must be evaluated at the one-foot scale (0.3 m) to understand and model reservoir performance. Systems requiring fine-scale properties include the Cretaceous Niobrara Chalk; Pennsylvanian oomoldic limestones; Silurian, Mississippian and Permian carbonates interpreted to be tidal flat to intertidal deposits; and Cretaceous low-permeability sandstones. In all these systems porosity, permeability, capillary pressure, and relative permeability exhibit high frequency vertical variability requiring discrete analysis for accurate formation evaluation.

With fine-scale properties for a location it is possible to upscale each property at that location, however, upscaling of properties generally “decouples” petrophysical relations defined at finer scales (e.g., core scale). Upscaled property relationships can also exhibit unacceptably wide variance because of the non-unique solution an upscaled property represents relative to the comprised fine-scale properties. Defining the representative elementary volume scale common to all properties requires testing the scale-dependence of the complete process not just by changing cell dimensions but also scaling reservoir properties. Obtaining a model solution for one process does not guarantee the model will be suitable for another process. Further, population of a coarse-scale geomodel is complicated by the non-unique solution for how one or more upscaled properties relate to other properties.

These fine-scale reservoir models lend insight into the complications of heterogeneous lithologies at differing spatial scales and underscore the difficulty of upscaling properties for reservoir simulation. Analysis also indicates that for some architectures capillary pressure and relative permeability anisotropy may need to be considered.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas