Scales of geologic reservoir description for engineering applications

A consequence of the increased interaction between geologists and engineers in resolving reservoir problems has been an awareness on the part of geologists of the need to vary the scale of their geologic description according to particular engineering applications. Conventional geological descriptions are normally too detailed for reservoir engineering simulations and often are not in an appropriate form for relating to reservoir performance. An example is presented of two scales of description of a North Sea oil field for two different applications. The field is a Tertiary submarine slope-fan deposit consisting of thick unconsolidated channel sand facies, a lobe sand facies, and a slope claystone facies, all arranged into 12 stratigraphic units and several subunits. Permeability of the channel sands is about twice that of lobe sands, demonstrating a facies control on reservoir quality. For the purpose of calculating reservoir volumetrics, it was possible to scale up the stratigraphy, by combining similar stratigraphic units, into a simple four-layer reservoir model. Average porosity and permeability vary among the layers in this geologically based model. For the purpose of improving understanding of the reservoir, a more complex flow unit model was developed according to geological and petrophysical properties that would influence themore » flow of fluids in the reservoir. This model is partly based upon sedimentary facies distribution, but differs from a geologic facies model and is in a more suitable form for relating to reservoir performance.« less