Carbonate Flow-Unit Definition for Reservoir Simulation

Matthew J. Pranter¹, Neil F. Hurley²
¹University of Colorado, Boulder, Colorado
²Colorado School of Mines, Golden, Colorado

Flow units in carbonate reservoirs have been defined in many ways. Likewise, various techniques exist to identify or designate flow units within a reservoir. Among the many methods include the use of rock fabrics, porosity-permeability cutoffs, pore-throat radii (R35), plots of cumulative storage- and flow-capacity, and lithology, to name a few. Which technique to use is dependent on the abundance, type, and distribution of data that is available for reservoir characterization and modeling. Ideally, flow units are established within a sequence stratigraphic and structural framework. The construction of a representative stratigraphic or structural framework with an appropriate number of layers or flow units is essential in reservoir modeling and simulation.

Facies-keyed, petrophysics-based methods to identify hydraulic flow units within a sequence-stratigraphic framework can be especially useful. For example, flow units can be characterized within high-frequency carbonate sequences through analyses of the vertical variation of flow capacity (kh), storage capacity (h), and pore-throat radius (R35) associated with vertical successions of carbonate rock fabrics (Figure 1). Cross-sectional flow profiles constructed from log and core petrophysical data are often used to correlate flow units associated with high-frequency cycles or commonly cycle sets. However, reservoir-scale lateral petrophysical variability within flow units is often not available from most subsurface data sets and must be estimated or inferred from outcrop analog information or other sources.

In some cases, the various methods for flow unit definition can reduce or eliminate the need for additional upscaling of the geologic model prior to flow simulation. The varied approaches to flow unit definition have been tested with two-dimensional flow simulation to evaluate their utility.

Figure 1: Flow units based, in part, on plots of cumulative flow capacity versus cumulative storage capacity for a cyclic carbonate reservoir (San Andres Formation, Vacuum field, New Mexico).