Reservoir Engineering - Calculated Recoveries by Cycling from a Retrograde Reservoir of Variable Permeability (TP 2200, Petr. Tech., May 1947, with discussion

The recovery of the heavier components from a gas cap or retrograde pool is shown to be the greatest when the sand is cycled with a dry gas at a low pressure. This conclusion is in direct opposition to the belief that the most efficient production program is pressure maintenance and cycling at or near the dew point. The results are calculated from: (I) constant volume, variable composition pressure-volume-temperature tests on a mixture of trap gas and liquid from a producing well; (2) published equilibrium constant data and the measured composition of the dew-point material; and (3) the fact that in a sand section of homogeneous permeability, injected gas displaces reservoir gas nearly, quantitatively. The results are based on the simplifying assumption that variable permeability systems may be defined by the ratio of two statistical parameters, and that gas injected into an actual sand will behave as though the sand were composed of many layers, each of constant permeability. Introduction Pressure decline in gas-condensate type reservoirs is accompanied by the formation of a liquid phase throughout the reservoir. Over the past ten years the processing of the material from these types of reservoirs for the heavier hydrocarbon components and the return of the light fractions to the reservoir ("cycling") has become increasingly popular. It has often been stated that the purpose of such a program is to prevent the loss of the retrograde liquid phase formed in the reservoir. The purpose of this paper is to present the results of laboratory tests and computations concerned with several possible methods of producing a gas cap or condensate type of reservoir. The results show that the recovery of heavier hydrocarbons for this type of reservoir is not a maximum under conditions of cycling at or near the dew-point pressure. Instead, variations in permeability and the ability of the dry injected gas to revaporize condensate, point to cycling at a considerably reduced pressure as the optimum production-method. In addition the paper suggests a way of evaluating sands for their permeability variation. The calculations presented in the following sections of the paper are in terms of the production of the butanes and heavier fraction from the gas cap of a field which had an original pressure of approximately 3000 psi. Specifically, the calculations show: 1. To what extent the condensate formed in the reservoir after pressure decline can be recovered by the cycling of a dry absorber-plant gas through the sands. 2. Whether there would be greater recovery of hydrocarbons if cycling were instigated at the original reservoir pressure.