Calculated Recoveries by Cycling from a Retrograde Reservoir of Variable Permeability

Abstract

Abstract The recovery of the heavier components from a gas cap or retrograde pool isshown to be the greatest when the sand is cycled with a dry gas at a lowpressure. This conclusion is in direct opposition to the belief that the mostefficient production program is pressure maintenance and cycling at or near thedew point. The results are calculated from:constant volume, variable compositionpressure-volume-temperature tests on a mixture of trap gas and liquid from aproducing well;published equilibrium constant data and the measuredcomposition of the dew-point material; andthe fact that in a sand sectionof homogeneous permeability, injected gas displaces reservoir gas nearlyquantitatively. The results are based on the simplifying assumption that variable permeabilitysystems may be defined by the ratio of two statistical parameters, and that gasinjected into an actual sand will behave as though the sand were composed ofmany layers, each of constant permeability. Introduction Pressure decline in gas-condensate type reservoirs is accompanied by theformation of a liquid phase throughout the reservoir. Over the past ten yearsthe processing of the material from these types of reservoirs for the heavierhydrocarbon components and the return of the light fraction to the reservoir("cycling") has become increasingly popular. It has often been stated that thepurpose of such a program is to prevent the loss of the retrograde liquid phaseformed in the reservoir. The purpose of this paper is to present the results of laboratory tests andcomputations concerned with several possible methods of producing a gas cap orcondensate type of reservoir. The results show that the recovery of heavier hydrocarbons for this type ofreservoir is not a maximum under conditions of cycling at or near the dew-pointpressure. Instead, variations in permeability and the ability of the dryinjected gas to revaporize condensate, point to cycling at a considerablyreduced pressure as the optimum production-method. In addition the papersuggests a way of evaluating sands for their permeability variation. The calculations presented in the following sections of the paper are in termsof the production of the butanes and heavier fraction from the gas gap of afield which had an original pressure of approximately 3000 psi. T.P. 2200