Double-Diffusive Convection as a Mechanism for Transferring Heat and Mass within the Salton Sea Geothermal Brine


R.O. Fournier

Geo Location:

Salton Sea, California; Imperial Valley


Stanford Geothermal Workshop







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H. C. Helgeson noted in 1968 that the salinity of the brine in the geothermal reservoir within the Salton Sea geothermal system generally increases from the top to the bottom and from the center to the sides. He also noted that pressure measurements at perforations in cased wells seemed to indicate that the formation fluids at the depths of production have a specific density about equal to 1, and that hot concentrated brines apparently exist in pressure equilibrium with comparatively cold dilute pore waters in the surrounding rocks. Since 1968 these have been no published reports that dispute these observations. However, a very high heat flux through the top of the system seems to require a substantial corn ponent of convective transfer of heat beneath an impermeable cap, whereas the apparent salinity gradient with depth seems to require little or no free convection of brine. This paradox may be resolved if double-diffusive convection is the main process that controls the depth-temperature-salinity relations. Such convection provides a mechanism for transfering heat from the bottom to the top of the hydrothermal system while maintaining vertical and horizontal salinity gradients--densities remaining close to unity. In 1981, Griffiths showed experimentally that layered double-diffusive convection cells may develop in porous media when hot saline waters underlie more dilute cooler waters. However, nagging questions remain about whether fluid densities within the Salton Sea geothermal system really adjust to unity in response to changing temperature and salinity at depths greater than about 1 km. The State 2-14 well, the Salton Sea Scientific Drill Hole, has provided one high-quality data point for a depth interval of 1,865-1,877 m, where the temperature is about 305?C. The calculated density of the pre-flashed reservoir fluid sampled from that depth is 1.0008 f: 0.0023.

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