| Title | Double-Diffusive Convective Transfer of Thermal Energy Within the Salton Sea Geothermal Brine |
|---|---|
| Authors | Fournier, R. O. |
| Year | 1988 |
| Conference | Japan International Geothermal Symposium |
| Keywords | |
| Abstract | Helgeson (1968) noted the following characteristics of the Salton Sea geothermal system: (l) the salinity of the brine in the geothermal reservoir increases from the top toward the bottom and from the sides toward the center, (2) fluid densities, determined from pressure measurements at perforations in cased wells, are about equal to 1 g cm-2 at all depths of production, (3) thermal gradients in wells in the central part of the field generally are nearly linear from the surface to a depth of about 0.5 to 1 km and range from about 250° to 380"C km-l, and (4) below about 1 km thermal gradients also are nearly linear, but commonly are about 40°C km-l • Since 1968 there have been no published reports that dispute these observations. Rex (1985) agreed that temperature and salinity achieve a halance with density 2 of the fluid equal to 1 g cm-, and stated that according to his study this phenomenon is an intrinsic property of the reservoir. The observed thermal profiles within the Salton Sea geothermal system are typical of geothermal systems that are capped by impermeable rock through which heat is transferred mainly by conduction, and that are underlain by relatively permeable rock through which heat is transferred largely by convective upflow. Helgeson (1968) recognized that this interpretation of the data presented a paradox: a high heat flow through the top part of the system and a relatively low thermal gradient below the impermeahle cap require convective transfer of thermal energy wIthin the hydrothermal system, while a hydrostatic gradient exactly consistent with unit density and systematically increasing salinity with depth preclude free convection in the geothermal reservoir. In view of this paradox, he suggested that below about 0.9 to 1.2 km the salinity of the reservoir fluid in a given well may be constant with increasing depth (but different from well to well), and that the uncertainty in the pore-fluid pressure measurements was too great to distinguish between isosalinity and isodensity conditions. Helgeson (1968) did note, however, that the model that he proposed was only one of several possible convective mechanisms of heat transfer that might he operating in the reservoir. As an example of other mechanisms, he called attention to the paper by Turner and Stommel (1964) on double-diffusive convection. It is the intent of this paper to again call attention to double-diffusive convection and to suggest that this process be given more consideration as a possible way to explain the variations in pressure, temperature, heat flow, and salinity observed throughout the hydrothermal system. |