Recovery of Retrograde Condensed Liquids By Revaporization During Dry Gas Injection

Abstract

Abstract Laboratory experiments were conducted to determine the rates of revaporization of the immobile liquid that was deposited in 'Various porous media as a result of retrograde condensation during pressure depletion of methane n-butane mixtures. The influence of particle size, initial liquid saturation, dry was injection rate and immobile water saturation on the revaporizution process was studied by measuring effluent concentration profiles as a function of volume of methane injected. The revaporization rate was estimated by matching the solution of a Mathematical model, in which mass transfer is expressed by a first-order rate expression, to the experimental effluent concentration data. In general, the rate of mass transfer was found to increase with decreasing particle size of the porous medium, increased condensed liquid saturation and increased immobile water saturation. The increase in each case is attributed to increasing the interfacial area between the gas and the condensed liquid. The magnitude of the measured 1nass transfer rates indicates that thermodllna1nic equilibrium between flowing gas and liquid could be assumed. Introduction WHEN FACED with the problem of planning production operations for gas condensate reservoirs, engineers frequently wish to compare the fluid recoveries and cash flows for various exploitation schemea(1). The simplest scheme consists of continuous pressure depletion or "blow down" to abandonment pressure. This method is attractive because:it requires low initial capital investments,it provides high initial cash flows andit requires the least engineering design. The deficiencies of this process are:loss of valuable condensate liquids resulting from a continual decrease in volatility as the average molecular weight of the reservoir fluid increases during the depletion (2,3,4);large variations in loading of the gas plant during the life of the project(3); andhigh liquid accumulation in the vicinity of the production wells with resulting lowered deliverabilities(3). Another exploitation scheme which is commonly used is gas cycling 'With full pressure maintenance. In this method, gas cycling is initiated at the start of production. The average molecular weight of the reservoir fluid decreases as it is diluted with dry injection gas. After breakthrough of dry gas at the production wells, pressure depletion by blow down is allowed to occur. The advantages of this method are:displacement of dense upper-dew-point fluids by dry gas results in high recoveries for swept portions of the reservoir;the accumulation of liquids in the vicinity of the producing wells is inhibited both during the gas injection and the pressure decline phases of operation;in the case of injection into a gas cap in which there is an immobile residual oil saturation, much of the oil may be recovered by vaporization(5,6). The disadvantages(3) of this method of operation are:high initial and operating costs for gas handling facilities;low cash flows resulting from deferred gas sales (7), and make-up gas purchases. Several variations of these two basic processes have been discussed in the literature(3, 5, 8, 1, 9, 7).