Prudhoe Bay is a mixed-wet reservoir where about half the oil recovery is attributable to gravity drainage. Gas/oil relative permeability data show that gravity-drainage recovery efficiency is poorer for more fine-grained sandstone and increases as the grain size increases. Gravity-drainage efficiency also increases with connate-water saturation. Dependence of recovery efficiency on grain size is related to changes in sorting. An effective grain size, defined by inverting the Carman-Kozeny relation, provides a useful parameter for correlating recovery efficiency. This estimate correlates well with visual estimates and direct measurements on disaggregated core. Grain size is also found to be a more effective parameter for correlating trapped gas than porosity, a common alternative. Lithology impacts trapped-gas level with finer-grained, more poorly sorted rock having higher trapped gas. Trapped gas decreases with increasing microporosity. Because little gas is trapped in microporosity, a zero-slope generalization of the Land curve better represents trapped-gas data.
As a result of the size and economic importance of Prudhoe Bay and because of the variety of oil recovery methods operating in the reservoir, data for a variety of recovery mechanisms have been collected. Much of the work to date on understanding relative permeability of Prudhoe Bay has focused on water/oil largely because it is an EOR target, but gas/oil is at least as important. The oil recovery by gravity drainage constitutes approximately half the production and potential reserves of the field. Understanding gravity drainage is important for forecasting recovery efficiency in the future and in managing the relative contributions of gravity drainage, waterflooding, and EOR recovery processes. Because macroscopic recovery efficiency is generally high for the gravity-drainage process, variations in microscopic efficiency have an even larger impact on overall recovery efficiency than in waterflooding. Gas relative permeability and trapped-gas measurements are important to predicting miscible gas usage and recovery efficiency.