Impacts of Petrophysical Cut-Offs in Reservoir Models

Abstract Petrophysical cut-offs are commonly applied so as to discard non-producing pay, yet, several aspects must be balanced one against the other in the process. On one hand, considering present hardware and grid size limitations, one of the main benefit of cut-offs is that the relationships between petrophysical parameters are better preserved at the upscaled model grid scale. Otherwise, some bias between porosity, permeability and water saturation distributions may result from the upscaling process, due to the non-linear dependences between those parameters. Conversely, applying cut-offs may tend to downgrade volumetrics and reservoir connectivity, and alter the actual reservoirs dynamics. The overall process must therefore aim at defining an optimized set of cut-offs providing the simplest yet accurate upscaled model. This set must be basically adapted to both rock-type and recovery mechanism. Some case studies are shown to illustrate how cut-offs can limit upscaling bias. The benefits of both "total" and "effective" porosity – saturation approaches are briefly reviewed. We illustrate how "rock-types" characteristics differ in effective and total domains, and how cut-offs must therefore be tuned in each case. Impacts of cut-offs on static volumetrics must be quantified. This can be conveniently achieved by visualizing the whole cumulative curve of "net hydrocarbon volume" vs. cut-offs values for each rock-type. We present a handy way of testing and visualizing multiple combinations of nested cut-offs, which allows to distinguish critical from redundant or secondary cut-offs.