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1 Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309; matthew.pranter{at}colorado.edu
2 Shell, EP Americas, 701 Poydras St., New Orleans, Louisiana 70139; colette.hirstius{at}shell.com
3 Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309; david.budd{at}colorado.edu
Matt Pranter is an assistant professor at the University of Colorado at Boulder. He received his B.S. degrees in geology and geological engineering from Oklahoma State University and the Colorado School of Mines, respectively, his M.S. degree in geology from Baylor University, and his Ph.D. in geology from the Colorado School of Mines. He was previously a senior research geologist with ExxonMobil Upstream Research Company and a geologist with Conoco Inc. His research interests are in reservoir geology and geophysics, sedimentary geology, and reservoir modeling.Colette B. Hirstius received her B.S. degree in geology from Tulane University (1996) and her M.S. degree in geology from the University of Colorado at Boulder (2003). She is a geologist with Shell in the E&P Americas group involved in asset analysis and reservoir modeling in the deepwater Gulf of Mexico. Colette previously worked for Mobil and ExxonMobil on projects in the Gulf of Mexico and Venezuela.
David A. Budd received degrees in geology from the College of Wooster, Duke University, and the University of Texas at Austin. Between 1983 and 1986, he was with ARCO Exploration and Production Technology, where his primary duties involved reservoir characterization studies. Since 1987, he has been a professor in the Department of Geological Sciences at the University of Colorado, Boulder. His research focuses on the diagenesis of carbonates, especially on the application of diagenesis to understanding pore-system evolution and porosity heterogeneity in carbonate reservoirs and aquifers.
Petrophysical data from dolomite outcrops of the Mississippian Madison Formation at Sheep Canyon, Wyoming, exhibit three scales of lateral variability in single rock fabric units. These include a near-random component (nugget effect), a short-range structure, and a long-range cyclic trend (hole effect). The nugget effect is high and accounts for 31–39 and 48–50% of the variance in porosity and permeability, respectively. Short-range lateral variability is reflected by correlation lengths of 6.5–16 ft (2–5.5 m). Laterally, long-range periodicities are equivalent to approximately 10% of the petrophysical variance and have wavelengths of 31 and 140 ft (9.5 and 42.6 m) for porosity and permeability (55 ft [16.8 m] for log10 permeability), respectively.
Cross sectional and plan-view petrophysical models and streamline simulations explore the effects of these scales of heterogeneity on fluid flow. Although short-range variability accounts for most of the petrophysical heterogeneity, the longer range trends can significantly affect fluid-flow behavior. Results indicate that breakthrough time and sweep efficiency vary depending on the magnitude of the lateral, long-range, petrophysical variability that exists in a dolomite reservoir. As the component of the long-range periodicity (hole effect) increases from approximately 0 to 25% of the total petrophysical variability, a corresponding increase in breakthrough time and sweep efficiency occurs. However, as the magnitude of the lateral, long-range, petrophysical variability increases beyond 25% of the total petrophysical variability (e.g., from 25 to 50%), a corresponding reduction in breakthrough time occurs because the spatial continuity of permeability is greater. Results indicate that heterogeneity caused by lateral petrophysical cyclicity should be incorporated into dolomite reservoir models for hole effect magnitudes that are greater than 10% of the petrophysical variance. To properly characterize and model these scales of variability in a petroleum reservoir, outcrop analogs are essential to provide accurate quantitative descriptions of lateral variability in dolomite rock fabrics.
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