Introduction

A discussion of gas-cycling operations in a gas-condensate reservoir cannot be complete without covering the full range of operations from drilling and well completion, reservoir analysis, surface handling of gas and liquids, LPG plant operation, gas compression, gas sales and reinjection. All of these factors are interrelated in a decision to start cycling and even more important, is the time to start and stop injection and the quantities of dry gas to be injected. The following discussion however, strictly deals with the reservoir aspects of cycling.

It is recognised that in certain gas reservoirs at high pressures, oil condenses out of the gas phase with reduction in reservoir pressure contrary phase with reduction in reservoir pressure contrary to the normal phase behaviour at low pressures. The process is called retrograde condensation. The production process called cycling aims at preventing production process called cycling aims at preventing further dropout of retrograde liquid as well as revaporizing the liquid phase so that it can be produced with the gas. produced with the gas. Figure 1. is a typical phase diagram of a gas-condensate type reservoir. The pressure at which the first drop of liquid appears in the gas phase is called the dew point. Below the dew point pressure, further reduction in the reservoir pressure, further reduction in the reservoir pressure will increase the volume percent of pressure will increase the volume percent of liquids in the reservoir until the maximum liquids have been dropped out of the gas phase. Equilibrium volumes of the liquid and the gas phases at any pressure and temperature depend upon the composition pressure and temperature depend upon the composition of the mixture. The laboratory data collected by constant volume isothermal pressure reduction shows that at an intermediate pressure marked by point "A" in Figure I. the liquid volume begins to point "A" in Figure I. the liquid volume begins to decrease due to revaporization. However, not all of the liquids revaporize, even when the pressure is reduced to atmospheric pressure. From the-reservoir point of view, it was assumed that the liquids point of view, it was assumed that the liquids dropped in the reservoir neither flow nor revaporize. Therefore, actual reservoir behaviour was calculated assuming a phase diagram shown by the dotted line on Figure 1.

Figure 2. shows a performance curve for a gas-condensate reservoir. As the reservoir pressure decreases below the dew point, the ratio of liquids in the well stream starts to decrease (curve A). A liquid phase is created in the reservoir and its ratio with the gas phase gradually increases (curve B) As a result, the saturation of liquids in the reservoir begins to increase (curve c). Since the liquid saturation never exceeds the critical saturation, only the gas phase is produced at the surface. The loss of liquids in the reservoir results in a lower ultimate recovery factor for the condensate than the gas as shown in Figure 3. The management of gas-condensate reservoirs aims at tapping the condensate shown by the shaded area in Figure 3.

Ideally therefore, if the reservoir pressure can be maintained at or above the dew point, a 100 percent withdrawal of condensate should be possible. Actually, the condensate recovery factors possible. Actually, the condensate recovery factors by depletion range between 20–40% of inplace liquids in the reservoir. This recovery factor can only be increased with cycling to between 60–75% in the best known reservoirs. This less than perfect recovery is due to reservoir heterogeneity perfect recovery is due to reservoir heterogeneity and relative positions of the injection and producing wells. In order to determine the producing wells. In order to determine the efficiency with which condensate is recovered from a reservoir as a result of cycling, effects of several factors must be considered.

FACTORS AFFECTING RECOVERY EFFICIENCY
1. AREAL SWEEP

One of the primary parameters which must be defined in predicting any displacement process is the areal portion of the reservoir that is contacted by the displacing fluid. Earlier methods of calculating the areal sweep efficiency were evolved by engineers involved in water-flood predictions. predictions. P. 285

This content is only available via PDF.
You can access this article if you purchase or spend a download.