Do We Need to Include Short Length-Scale Variability in Reservoir Models?

Reservoir models typically contain hundreds-of-thousands to millions of grid cells in which petrophysical properties such as porosity and permeability vary on a cell-to-cell basis. Moreover, the petrophysical properties and flow equations are discretized on the same grid. We investigate the impact of decoupling the grid used to model the petrophysical properties from the grid used to solve the flow equations. The aim is to test whether cell-to-cell variability in petrophysical properties has a significant impact on fluid flow. We find that the impact of cell-to-cell variability on predicted flow is small, and smaller than the error introduced by discretizing the flow equations on the same grid as the petrophysical properties. Grid-based reservoir models containing a large number of petrophysical property values that vary on a cell-to-cell basis can be collapsed into a much smaller number of larger, but more geometrically complex, geologic domains. Cell-to-cell variability is not necessary to capture flow in reservoir models; rather, it is the spatially correlated variability in petrophysical properties that is important. The correlated variability reflects the underlying geological heterogeneity. Reservoir modelling effort should focus on capturing the geologic domains in the most realistic and computationally efficient manner.