In gas-cap drives, as production proceeds and reservoir pressure declines, the expansion of the gas displaces oil downward toward the wells. This phenomenon is observed in the increase of the gas-oil ratios in successively lower wells. At the same time, by virtue of its expansion, the gas cap retards pressure decline and therefore the liberation of solution gas within the oil zone, thus improving recovery by reducing the producing gas-oil ratios of the wells. This mechanism is most effective in those reservoirs of marked structural relief, which introduces a vertical component of fluid flow, whereby gravitational segregation of the oil and free gas in the sand may occur.1 The recoveries from volumetric gas-cap reservoirs will typically be higher than the recoveries for undersaturated reservoirs and will be even higher for large gas caps, continuous uniform formations, and good gravitational segregation characteristics.
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