Gravity Drainage is a phenomenon seen in reservoirs with high structural relief, that are relatively homogeneous, have large hydrocarbon columns and exhibit excellent permeability. It is of interest as it is associated with higher recovery efficiencies as compared to solution and water drive mechanisms. I have seen recoveries > 63% OOIP in reservoirs that exhibit this behavior.

Essentially these favorable properties promote the segregation of gas and oil under the influence of gravity, so that liberated gas in the reservoir (i.e. below Pb) tends to flow upwards into the gas cap or form a secondary gas cap (i.e it is not produced). In this way drive energy is conserved in the reservoir, that is the highly compressible gas which forms a significant portion of the reservoir depletion energy, remains in the reservoir, promoting higher recovery efficiencies.

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