Click to view figures in PDF format

Characterization and Modeling of Multiple Scales of Lateral Petrophysical Heterogeneity within Dolomite Rock Fabrics as Determined from Outcrop Analogs

Colette B. Hirstius, Matthew J. Pranter, David A. Budd
University of Colorado, Department of Geological Sciences, Boulder, CO

Different scales of lateral petrophysical variability often exist within rock fabric facies of dolomite reservoirs. To properly characterize and model the spatial variability of petrophysical properties that effect fluid flow and storage within dolomites, an accurate quantitative description of lateral variability within dolomite rock fabrics is essential. Outcrop analogs of subsurface reservoirs provide critical information to address lateral variability that is not available from typical subsurface data.

Porosity and permeability measurements were acquired for 1250 core plug samples from Mississippian dolomites (Madison Formation, Wyoming). Four lateral transects that range in length from 14 to 165 meters, and twelve vertical transects, averaging 5 meters in length, were obtained within individual rock fabrics of dolomitized lower and upper shoreface facies. Variography shows three distinct scales of lateral petrophysical variability, including a significant hole-effect. Short-range lateral variability is reflected by correlation distances of 2 to 5.5 meters and is incorporated with a spherical model. The nugget effect is high and accounts for approximately 50% of the variance. Lateral petrophysical oscillations, modeled with hole-effect variograms, are present with periodicities of 42.5 and 9.2 meters for permeability and porosity, respectively.

Streamline simulations of stochastic cross-sectional and plan-view petrophysical models explore the effects of these heterogeneities on fluid flow. Results show that permeability models with a nested variogram structure including a small magnitude (10-25%) long-range oscillatory structure have greater breakthrough times, higher in sweep efficiencies, and higher degrees of tortuosity. As the magnitude of the petrophysical cyclicity increases from 25-50%, breakthrough time decreases as cyclicity dominates the system’s variance.

Figure 1 - Multi-facies outcrop model depicting results of streamline simulations at breakthrough time (BTT) for permeability models based on nested variograms with increasing magnitude of the hole-effect

Figure 2 - Plain-view model depicting results of streamline simulations at breakthrough time (BTT) for permeability models based on nested variograms with increasing magnitude of the long-rage, hole effect