Thermal Behavior of Multifluid-Saturated Formations Part 2: Effect of Vapor Saturation - Heat Pipe Concept and Apparent Thermal Conductivity

Abstract

Abstract When porous systems contain liquids at or near their vapor pressures and saturation temperatures, evaporation-condensation reactions may combine with capillary pressure gradients and greatly enhance the heat transfer capacity of the system. This behavior was simulated by the so-called Heat Pipe, a mechanism which is capable of transferring heat at rates much higher than those observed by simple conduction. A conceptual model was derived for the theoretical analysis of the heat pipe concepts in porous media. Equations were developed for expressing the heat pipe effect in the form of an additional thermal conductivity as function of the system's characteristics. The mechanism was illustrated experimentally by testing four different rock samples with two liquids of different latent heats of vaporization. Results, which were in consistent agreement with the derived theory, showed that vapor saturations resulted in high apparent thermal conductivities for the rock samples that was, in some cases, several times greater than the expected basic value. Both theoretical and experimental observations showed that the heat pipe effect is a function of permeability, porosity, latent heat of vaporization of the saturating liquid, vapor saturation, capillary pressure, and relative permeabilities and kinematic viscosities of the liquid and vapor phases. A statistical correlation was obtained expressing the heat pipe effect in terms of other easy measuring properties of the physical system. Introduction Analyses of the processes of thermal recovery of crude oil, energy extraction from geothermal steam formations, heat transfer in packed towers, and many other important engineering applications often require knowledge of thermal behavior and properties of partially vapor saturated porous media. The effect of partial vapor saturation on thermal behavior of such materials was not fully investigated. Few authors had reported briefly that the presence of some vapor saturation in porous media samples made their thermal conductivities higher than expected. These findings were mainly experimental and in some cases they were reported as experimental errors. Theoretical explanation was not available and thus these effects were not evaluated quantitatively. In the present work the effects of partial vapor saturation on the heat transfer characteristics of formations were considered and evaluated.