Equilibrium Revaporization of Retrograde Condensate By Dry Gas Injection
Authors: Smith Lowell R. , Yarborough Lyman 
Published: December 2007

This paper presents results of a laboratory study of retrograde condensate recovery by revaporization into dry injection gas. Flow tests were performed in 10.6-ft long sandpacks at 100F and 1,500 psi. In three runs methane revaporized the liquid from a n-heptane-methane mixture in the presence of immobile water. Two of these tests were water-wet, and the third was totally oil-wet. In the three runs n-heptane recovery was complete after 2.5 hydrocarbon PV of injection. There was no significant performance difference between the two wettability extremes.

In a fourth experiment, a methane-hydrogen sulfide mixture revaporized a synthetic light, sour condensate. No water saturation was present. Equilibrium compositions and volumetric data were obtained for the four-component condensate. The heavy component, n-heptane, was removed alter 6 PV production. Comparison of the effluent fluid compositions with known equilibrium data shows that the flowing fluid was equilibrium vapor and that the mixing zone between equilibrium vapor and dry injection gas was short. Data indicated that complete recovery of retrograde liquid occurred after it was contacted by a sufficient quantity of dry gas.

Introduction

When pressure declines below the fluid dew point in a gas condensate reservoir, a liquid phase forms. In this process, referred to as retrograde condensation, the quantity of liquid formed is frequently small enough that the liquid is not a flowing phase. To prevent loss of valuable retrograde liquids, the process of dry gas cycling has been employed for several years as a more or less standard practice. In this procedure the reservoir pressure is maintained above the fluid dew point so that the liquid components may be produced as vapor and then separated at the surface.

Although full pressure maintenance by gas cycling seems ideal in terms of preventing liquids loss, several factors can reduce the attractiveness of such an operation. From a study of a condensate reservoir in Alberta, Canada, Havlena et al. concluded that cycling under conditions of declining pressure leads to economic advantages and to a high recovery of hydrocarbon liquids. This study considered effects of volumetric sweep efficiency, retrograde behavior of the original wet gas and revaporization characteristics of the retrograde liquid when contacted by dry gas.

The first major work concerning revaporization of liquid in a gas condensate system is that of Standing et al. Calculations based upon the PVT behavior of a recombined gas condensate fluid indicated that all retrograde liquid can be recovered if it is contacted by a sufficient quantity of dry gas. The paper considered the effect of variable permeability upon the recovery of retrograde liquid. Standing et al. concluded that recovery of heavier components in the retrograde liquid is greatest if reservoir pressure is allowed to decline below the dew point prior to dry gas injection.

Since the work of Standing et al., several laboratory studies have been reported which show that recovery of hydrocarbon liquids by vaporization into dry injected gas can contribute to increased recovery above that obtained by ordinary production practices. Vaporization from retrograde condensate, conventional oil and volatile oils reservoirs has been considered. There is little work that deals with revaporization recovery from condensate reservoirs.