Abstract

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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