Scales of lateral petrophysical heterogeneity in dolomite lithofacies as determined from outcrop analogs:Implications for 3-D reservoir modeling

摘要

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 6.5-16ft(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 log_(10) 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.