A 1982 Critique on Gas Cycling Operations on Gas-Condensate Reservoirs

Abstract

Abstract 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 to the normal phase behaviour at low pressures. The process is called retrograde condensation. The 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. 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 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 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 1. 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 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.