Vapor-Pressure Lowering in Geothermal Systems

The water vapor-pressure lowering phenomenon in porous media was investigated for a range of temperatures by measuring vapor pressure vs. mass of water adsorbed in consolidated sandstone cores and unconsolidated silica sands. Experimental results showed that the mass of water adsorbed on the rock surface is much more than the amount of pore steam. Results also revealed that the water adsorption is caused mainly by micropores in the porous medium. Measurement of the mass of methane and ethane adsorbed on dry rocks showed that the amount of adsorption is not great in comparison with the pore gas. It was found that adsorption data for water/sandstone core studies could be normalized with respect to temperature. Although this appears not to have been reported previously, it does agree in principle with findings for solid powders with micropores. Another interesting result was that reanalysis of previous studies of capillarity in sandstones indicates that experimental data probably were influenced mostly by adsorption. Introduction If a container is evacuated and partially filled with a liquid, at temperature, T, the equilibrium pressure, po, can be measured. This pressure po is called the saturated vapor pressure. The phase diagram for a particular pure single-component liquid can be constructed by using sets of (T, po), where po is a function of T only. Vapor-pressure lowering refers to the fact that. under some conditions, the equilibrium pressure, p, may be less than po. The liquid/vapor pressure-temperature (p-T) relationship may depend on other factors such as solution of salts or gases in a pure liquid. An investigation of vapor-pressure lowering is important in understanding the behavior of geothermal steam reservoirs. Traditionally, it has been considered that superheated steam and rock are the only two components in a dry steam geothermal reservoir. From the fundamental physical properties of fluid and rocks, however, there should exist a certain amount of liquid in addition to steam. If the quantity of additional liquid compared with the quantity of superheated steam is significant, material balance computations should account for this. Capillarity has been thought to be the main factor causing vapor-pressure lowering in a porous medium. However, in the course of this study, it became apparent that surface adsorption was the most important factor causing vapor pressure lowering in dry-steam geothermal reservoirs. For this study, adsorption isotherms were obtained for several fluids and consolidated sandstones over a range of temperatures. To reduce the number of variables, only pure single-component fluids were used. For the following, liquids" refers to simple liquids, excluding solutions and other mixtures. Theory Many factors can cause vapor-pressure lowering for pure liquids in porous media. The most important are capillarity and surface adsorption. Capillarity originates from surface tension, and describes the relationship between a liquid phase and a gas phase for a single component fluid in a capillary tube. Surface adsorption is the result of interaction between molecules of the solid surface and the gas molecules in the pore space. Capillarity considers only surface tension forces, and surface adsorption considers forces of attraction between the liquid and the solid surface. The following discussion briefly reviews surface tension and capillarity. SPEJ P. 157^