Establishing Gas Phase Well Performance for Gas Condensate Wells Producing Under Three-Phase Conditions

Abstract

Abstract This paper provides a new method of forecasting natural gas production of gas-condensate wells that flow under three phase conditions. Such wells include gas condensate wells that produce liquid condensate and water along with gas phase, their main production. Mathematically treating such systems can be very demanding. A new tedious, but simple method of projecting the gas phase production is proposed. In this method we integrate reservoir production data and the pressure transient data to forecast well performance without prior knowledge of relative permeability as function of saturation. Since pressure transient well test data is usually available on yearly basis, effective permeability as a function of pressure can be updated. It is the true representative of the reservoir conditions of heterogeneity, geometry, and resident fluids. The total gas production in a gas condensate reservoir is contribution of all the three regions that might exist at certain stage of depletion. Free and dissolved gas in both oil and water in Region-1, free and dissolved gas in water in Region-2, and free and dissolved gas in water in Region-3. Thus to project the production, along with the physical properties of fluids in all the three regions, their phase change with pressure also has to be handled. It is observed from the solved examples that buildup of condensate liquid phase reduces the gas production as well as water production, a favorable situations. Partially the gas production loss is recovered in form of condensate while reducing water production. Finally, few examples with simulated data are analyzed to show the use of new method. A step-by-step procedure is also devised to establish the well performance. Small operators will benefit from this method at the most, since data acquisition like relative permeability curves requires to laboratory experiments on cores. Introduction We are extending our understanding of the retrograde gas-condensate systems lately. Recently many good papers have been published that treat gas-condensate systems. Retrograde gas-condensate reservoirs are primarily gas reservoirs. A zone of liquid begins to form as the dew point pressure is reached. The liquid keeps accumulating and does not flow until the critical liquid saturation is reached. Once critical saturation of the liquid phase is reached, it begins to flow towards the wellbore along with the gas. Pressure at this point in the reservoir is termed as P*. Interestingly, this liquid may re-vaporize as the pressure further crosses the lower line on two-phase envelope of phase diagram. This behavior of re-vaporization of the oil phase is called the "Retrograde behavior." Fig.1, Fig.2, Fig.3, and Fig.4 show the schematics of such a phenomenon in vertical wells and horizontal wells.