Oil Recovery by Gravity Segregation

Abstract

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose: To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology. Gravity segregation of oil from gas- or water-invaded regions is anefficient natural recovery mechanism if conditions are favorable. Favorableconditions include high vertical permeability, an advantageous relation betweenrelative permeability and oil saturation, high vertical continuity, a largedensity difference, low oil viscosity, and long oil drainage times after aregion has been invaded. Recoveries as high as 87% of the original oil in place(OOIP) have been observed in gas-invaded regions of the Hawkins field and morethan 80% OOIP has been recovered from water-invaded parts of the East Texasfield. Theory for calculating the vertical segregation of oil first was presentedby Cardwell and Parsons in 1949. Equations were derived from Darcy's law andthe continuity equation, assuming that the driving force for oil movement isthe hydrostatic head caused by density differences. Thus, the theory firstassumes that pressure gradients caused by movement of the displaced fluid, gasor water, is negligibly small. This is usually valid, especially at low oilsaturations. The second tacit assumption is that capillary pressure gradientscaused by saturation gradients are small. Calculations show that ignoringcapillary pressure gradients usually results in errors in the average oilsaturation of less than 1% PV because capillary pressure gradients becomesignificant only as the equilibrium saturation distribution above the gas/oilcontact (GOC) or water/oil contact is approached.