Estimation of Ultimate Recovery from Solution Gas-Drive Reservoirs

Abstract

Published in Petroleum Transactions, AIME, Volume 213, 1958, pages 132–138. Introduction In the past few years several articles and papers presenting results of solution gas-drive depletion calculations have appeared in the literature. Such calculations are of interest to the oil industry, for investment decisions must often be made before much is known about a reservoir. At other times, an estimate of the possible benefits to be realized from alternate production methods is desirable, and theoretical depletion calculations can serve as a floor or reference level from which to work. In any case, an estimate of ultimate oil recovery based upon engineering data is commonly required. An engineer confronted with the problem of obtaining, for a specific reservoir system, an estimate of ultimate oil recovery by solution gas-drive depletion usually will be forced to perform the calculations himself. This is despite the quantity of data in the literature. Rarely will either experience or the literature provide results from a reservoir system similar in all important respects to the one under consideration, and calculated results are not so plentiful that satisfactory interpolation procedures can be devised. Performing the calculations, however, is a tedious, time-consuming task unless an electronic computer is available, and, in practice, time and manpower are not always available for this purpose. A quick, simple, consistent method was needed for reducing the uncertainty in estimated oil recovery from solution gas-drive reservoirs when only minimum information about the reservoir system is available. Procedure Method of Calculation The usual requisite assumptions were made so that the material balance equation could be used to calculate data for the charts. The following assumptions were made:the reservoir is homogeneous and isotropic;oil recovery is due entirely to solution gas drive and neither a gas cap nor a water drive nor gravity drainage is present;the initial reservoir pressure is the bubble-point pressure of the reservoir fluid;initial total liquid saturation is 100 per cent of pore space;interstitial water saturation remains at the initial value as the reservoir pressure declines from the bubble-point pressure to atmospheric pressure;equilibrium gas saturation is 5 per cent of pore space; andoil and gas saturations are uniformly distributed throughout the reservoir at all times. There are no saturation gradients due to a wellbore, nor is the geometry of the reservoir system considered.