South Wasson Clear Fork Reservoir Modeling: Representing Stratigraphically Controlled Petrophysical Variability for Fluid-Flow Simulation

James W. Jennings, Jr., F. Jerry Lucia, and Stephen C. Ruppel
Bureau of Economic Geology
John A. and Katherine G. Jackson School of Geosciences
The University of Texas at Austin
Austin, Texas

Petrophysical properties important for fluid-flow simulation exhibit a high degree of spatial variability over a wide range of scales in most carbonate reservoirs. Most of the variability is spatially chaotic at small scales within rock-fabric flow units and difficult to predict in detail. However, mappable larger-scale heterogeneities usually exist as well. In the South Wasson Clear Fork reservoir these larger-scale variations are spatially controlled by the stratigraphy, and although they have a smaller variance and are easily obscured by the small-scale variability in data analysis and modeling, they exert a dominant control on waterflood sweep.

In this presentation we will use the South Wasson Clear Fork reservoir as an example to illustrate a new method for the construction of 3-D reservoir-flow simulation models that focuses on the accurate representation of stratigraphically controlled petrophysical variability. The new method links high-resolution sequence-stratigraphic frameworks, well logs as the primary source of subsurface porosity data, porosity-permeability relationships from core data, outcrop-derived models of small-scale spatial statistics, and a practical approach to porosity-permeability scaleup. The resulting model exhibits stratification and stacking patterns typical of shallow-water platform carbonates in the Permian Basin and elsewhere, and it has a reduced requirement for arbitrary kv/kh modifications to inhibit cross-flow in waterflood simulations.