p/Z Behavior for Geothermal Steam Reservoirs
Authors: Brigham William E. , Morrow William B. 
Published: December 2007

Abstract
Certain of the natural geothermal-energy reservoirs are of the type called "vapor dominated." These reservoirs contain steam in the top of the reservoir and may contain boiling water below. Some simplifying assumptions were made to predict the pressure and temperature vs production history of pressure and temperature vs production history of such reservoirs. These predictions are compared with normal hydrocarbon gas reservoirs using the standard p/Z plots.The results show that the presence of a boiling water phase will have a considerable effect on the pressure behavior of such systems. Further, the pressure behavior of such systems. Further, the porosity of the system will have a marked effect. porosity of the system will have a marked effect. Extrapolations of early data will be optimistic if the porosity is low and pessimistic if the porosity is high. In all cases, the steam zone will remain at the original temperature, though the temperature of the boiling water drops as the pressure declines.

Introduction
Two basic types of geothermal reservoirs are being used commercially worldwide to produce electric power. One type produces hot water from the reservoir. This water is partially flashed at the surface, producing steam to drive the turbo-generators. The two largest installations of this type are at Wairekei in New Zealand and Cerro Prieto in Mexico, just south of the Imperial Valley of California. The other type of reservoir has been called vapor dominated. The fluid is slightly superheated steam at reservoir conditions and nearly all the produced fluid is steam, with small amounts of inert gases. The two major installations of this type are at the Geysers in northern California and at Lardarello in Tuscany, Italy.Although these latter reservoirs contain large volumes of steam as vapor, there is a possibility that they also contain boiling water at great depths. The purpose of this paper is to investigate the behavior of steam and steam-water systems as they are produced. We would like to know whether the pressure vs production characteristics of these pressure vs production characteristics of these systems differ from each other enough to give clues as to the original nature of the reservoir. Such information might be extremely useful in predicting the reserves of such systems.We will look at three basic systems. The first is a system completely filled with steam but no water present. The second is a system where there is present. The second is a system where there is water on the bottom and steam on top. As steam is produced, some of the water will boil and the liquid produced, some of the water will boil and the liquid level will drop with production. The third system also will contain liquid water on the bottom, but we will assume that the liquid boils throughout the liquid system and that the liquid level will not drop. In this system a steam saturation builds up within the boiling liquid zone.We will assume that for all these reservoirs the fluid influx is negligible. We recognize that in an actual reservoir system, the fluid influx rate might be important compared with the production rate, but this simplifying no-influx assumption usually should be used in first-step analyses of reservoirs. Further, there is evidence that this assumption is valid for at least the Geysers and the Wairekei reservoirs.

MATERIAL AND ENERGY BALANCES
Normally in oil and gas reservoirs, only a material balance is necessary. Although boiling and condensation occur in such reservoirs, the heat effects are so small that an energy balance is not necessary. The reservoirs remain essentially isothermal.

STEAM ONLY
If only steam is in the reservoir (with no water), the same isothermal characteristics hold as for oil and gas reservoirs. This is because the heat capacity of the rock is so large compared with that of steam. Thus, a steam reservoir can be treated in the same way as an ordinary gas reservoir; we can plot p/Z vs cumulative production and get a plot p/Z vs cumulative production and get a straight line. The intercept on the abscissa is equal to the original fluid in place. The equation is
(1)
SPEJ
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