Calculated Effect of Pressure Maintenance on Oil Recovery

Abstract

Abstract The application of Muskat's differential equations for predicting theperformance of a solution gas-drive reservoir to the Fullerton field indicatesthat a recovery may be expected of 14,030 bbl per acre under primary depletion, and 15,910 and 17,190 bbl per acre, respectively, under the twopressure-maintenance programs analyzed. In this analysis it is assumed that fluid properties are the same throughoutthe reservoir, that the core data used are typical of the Clear Fork limestonein the locality, and that injected gas is completely diffused through the payzones. The economic study of the three recovery programs considered indicates that theone in which 60 pct of the produced gas is returned to 'the formation is themost profitable. Introduction The purpose of this paper is to consider effect of pressure maintenance by gasinjection on the ultimate recovery of oil from a solution gas-drive reservoir.This prediction of reservoir behavior is the result calculations based on theequations presented by M. Muskat in the Journal of Applied Physics, March1945. Reservoir The reservoir analyzed is the Fullerton field in Andrews County, West Texas. Itsituated on a broad anticlinal structure with approximately 330'Ft of closure, and ends approximately eight miles north and south and five miles east andwest. The areal extent is taken as 16,642 acres. The producing horizon is the Lower Clear Fork limestone of Permian age, occurring at depths from 6650 to 7350'Ft. It is divided into four zones. Forthe purposes of this study, the fluid characteristics in all zones are assumedto be the same. Therefore an equivalent zone is assumed of thickness equal tothe sum of the thicknesses of the four zones, or an average of 216 ft. The reservoir contains 3,586,991 acre feet of producing limestone, with aninitial volume of stock tank oil in place of 918,403,100 bbl. The average connate water saturation is 24.0 pct. The permeability ranges from6.7 md. to 12.6 md., and the porosity ranges from 7.7 pct to 11.4 pct, exceptfor occasional cavernous streaks. Based on these values, the limestone may beconsidered intergranular. Production histories and material-balance studies have indicated that thedriving mechanism is solution gas. Water levels have been established. Theory It is possible to express mathematically the change in residual oil saturationwith pressure decline in a solution gas-drive reservoir as being equal to thechange in the physical properties of the fluids present in the reservoir asthis pressure decline occurs. T.P. 2231