Numerical experiments on convective heat transfer in water-saturated porous media at near-critical conditions

Abstract

Fluid and heat flow at temperatures approaching or exceeding that at the critical point (374°C for pure water, higher for saline fluids) may ·be· encountered in deep zones of geothermal systems and above cooling intrusives. In the vicinity of the critical point the density and internal energy of fluids show very strong variations for small temperature and pressure changes. This suggests that convective heat transfer from thermal buoyancy flow would be strongly enhanced at near-critical conditions. This has been confirmed in laboratory experiments.We have developed special numerical techniques for modeling porous flow at nearcritical conditions, which can handle the extreme non-linearities in water properties near the critical point Our numerical simulations show strong enhancements of convective heat transfer at near-critical conditions; however, the heat transfer rates obtained in the simulations are considerably smaller than data reported from laboratory experiments by Dunn and Hardee. We discuss possible reasons for this discrepancy and develop suggestions for additional laboratory experiments.

Application of the computer code TOUGH2 to the simulation of supercritical conditions in geothermal systems

“…Cox and Pruess (1990) created an experimental version of MULKOM using the supercritical equation of state developed by Haar et al (1984). The simulator was tested by modelling the near-critical laboratory experiment of Dunn and Hardee (1981), unfortunately with only partial success, as the very high heat flows in the experiment could not be reproduced.…”

Heat transport at boiling, near‐critical conditions

“…Numerical simulations that specifically studied the effect of near‐critical convection in a geological context have been rare (Cox & Pruess 1990; Ingebritsen & Hayba 1994). Cox & Pruess (1990) attempted to reproduce an earlier experimental study of near‐critical water convection (Dunn & Hardee 1981). Hayba & Ingebritsen (1997) studied convection in large hydrothermal systems around cooling plutons but did not carry out a detailed analysis of heat transfer.…”

“…Schubert & Straus (1977) and Straus & Schubert (1977) stressed the outstanding importance of realistic fluid properties in obtaining realistic results and stated the need for further studies. Nevertheless, the efficiency of heat transport using realistic fluid properties has been addressed by relatively few studies, notably Cox & Pruess (1990), Ingebritsen & Hayba (1994) and Fontaine & Wilcock (2007). One reason for the limited number of studies is that numerical modeling of convection at near‐critical conditions is challenging, due to the aforementioned extrema in fluid properties, the singularity in the equation of state at the critical point when expressed in terms of temperature and two‐phase flow at subcritical conditions.…”

“…Similarly, the deeper parts of continental geothermal and magmatic-hydrothermal systems can encompass the near-critical region (Cathles 1977;Norton & Knight 1977;Inge-britsen & Hayba 1994;Hayba & Ingebritsen 1997;Fournier 1999;Hurwitz et al 2003;Heinrich et al 2004). Numerical simulations that specifically studied the effect of near-critical convection in a geological context have been rare (Cox & Pruess 1990;Ingebritsen & Hayba 1994). Cox & Pruess (1990) attempted to reproduce an earlier experimental study of near-critical water convection (Dunn & Hardee 1981).…”

Fluid flow and heat transport near the critical point of H2O

“…The near-critical extrema in fluid properties create computational problems which have inhibited quantitative modeling, although Cox and Pruess [1990] attempted to reproduce the Dunn and Hardee results numerically. In the past, most users of geothermal models have had to choose between multiphase models with a temperature range of about 0-350 øC [e.g.…”