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

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. INTRODUCTION The geologic and thermodynamic characteristics of the Salton Sea geothermal system were described in great detail by Helgeson [19681. He showed plots of depth versus temperature, pore-fluid pressure, and total dissolved solids for many of the wells that had been drilled at that time. A characteristic of the system is that fluids with different salinities are produced from different wells. The data strongly suggested that within the hydrothermal system the salinity increases from the sides toward the central part of the field, and from the top toward the bottom. 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. Below about 1 km, thermal gradients generally are nearly linear, but at much lower values (commonly about 40°C km-l). in the State 2-14 Salton Sea Scientific Drill Hole (SSSDH) from a depth of 1,890 to 3,170 m is about 39OC km-l [Sass et al., 19871. According to Helgeson [1968], pressures measured at perforations in various wells under static conditions fall on a curve with a 0.0295 atmosphere ft-l gradient--a pressure gradient which requires the brines to have virtually unit specific density at all depths (to at least 7,000 ft, the deepest measurement at that time). Since then there have been no published papers that dispute these observations. Rex [1985] agreed that temperature and salinityThieve a balance with specific density of the fluid equal to unity, and stated that according to his study this phenomenon is an intrinsic property of the reservoir. The observed thermal profile within the Salton Sea geothermal system is typical of geothermal systems that are capped by impermeable rock through which heat is transferred mainly by conduction, and are underlain by relatively permeable rock through which heat is transThe thermal The gradient measured with-