Vapor Pressure Lowering In Two-Phase Geothermal Systems
Authors: Chicoine S.D. , Strobel C.J. , Ramey Henry J. 
Published: December 2007

This study concerns the construction and operation of a laboratory model of a non-recharged (pressure depletion) two-phase geothermal reservoir. The primary emphasis was directed towards the significance of vapor pressure lowering phenomena caused by interfacial tension in a consolidated porous medium. Preliminary runs were made which porous medium. Preliminary runs were made which indicated vapor pressure lowering effects as large as 15 psia in a consolidated porous medium at geothermal system temperatures of 220 deg. F to 290 deg. F. Model construction and operating details are given.

Introduction

In a single-component vapor-liquid system, vaporization occurs whenever the vapor phase absolute pressure is equal to or less than the vapor pressure of the liquid at the system temperature. pressure of the liquid at the system temperature. The solid line on Fig. 1 shows the vapor pressure vs. temperature for a flat liquid surface. However, within a porous medium, interfaces should be curved and the solid line on Fig. 1 may not be appropriate. In order to understand geothermal reservoir behavior, it is important to define the vapor pressure curve for conditions correct for geothermal reservoirs.

Many factors are known to affect pressure temperature phase diagram for reservoir fluids. Among these are solution of solids, gases, and liquids in the liquid phase and the composition of both phases. Geothermal liquids are known to contain large amounts of salts in solution in some cases, and geothermal steam often contains non-condensible gases (carbon dioxide a major constituent). Another important factor is caused by the rock. Capillarity and adsorption phenomena also may affect vapor pressure of fluids in pore space of rocks. We will neglect the effects of materials in solution in the following and direct attention toward development of a geothermal model and study the vapor pressure of a pure liquid and boiling in a porous medium. Because capillary pressure is known to affect vapor pressure in a pressure is known to affect vapor pressure in a porous medium, we expect capillary pressure to be porous medium, we expect capillary pressure to be a major factor in this study.

The relationship between capillary pressure and water content for a water-wet porous medium has been studied for both drainage and imbibition for water saturations greater than the practical irreducible wetting phase saturation, S wi. The portion to the right of the dotted line in Fig. 2 portion to the right of the dotted line in Fig. 2 represents conventional capillary pressure-water saturation curves. The dotted line represents the conventional concept of irreducible wetting phase saturation. We speculate that there must be phase saturation. We speculate that there must be drainage and imbibition curves to the left of the conventional Swi.

Calhoun, et al., have provided perhaps the only quantitative information available for the low water saturation region less than Swi. The transfer of liquid from a flat surface to the pore space of a core caused by the lower vapor pore space of a core caused by the lower vapor pressure of the liquid within the core was pressure of the liquid within the core was observed. The lower vapor pressure was apparently the result of the capillary pressure caused by curved surfaces within the core. The Calhoun, et al., work was performed at 97 deg. F for high permeability cores (permeability 700 md or more). permeability cores (permeability 700 md or more).The hysteresis between drainage and imbibition in the low liquid content region on Fig. 2 could be caused by a difference in adsorption and capillarity. As liquid enters a dry core, it would form layers on the surface until liquid content became high enough for interfaces to meet and form curved surfaces. On desaturation, the liquid interfaces should remain connected to low liquid contents.

Normally, one would expect increasing temperatures to decrease capillary pressure effects because surface and interfacial tensions normally decrease with temperature increase. However, Sinnokrot, et al., found that Swi increased with temperature increase for several sandstone cores.