Fluid density distribution in a high temperature, stratified thermohaline system: implications for saline hydrothermal circulation

Density distribution within the Salton Sea geothermal system, of fluids ranging from 20°C to 325X, has been computed using chemical and thermal data from geothermal production well tests and curve-fit models of Na-Ca-K chloride solution properties. Density corrections can easily be made to iO.01 g/cm3 for solute effects of each of the dominant chloride salts as well as pressure above vapor saturation. Field data on dissolved CO, is too sparse to routinely compute a correction, however this moderately, but variably abundant component can be shown to produce only minor ( < 0.01 g/cm”) errors in density estimates. Fluid density within the shallow, cool ( < 25O“C), low salinity portions of the system decreases markedly with increasing depth and temperature, from = 1.0 to = 0.85 g/cm3. A sharp interface separates these relatively dilute fluids from hypersaline brines with TDS > 20 wt%. The density of brine climbs rapidly to near 1.0 g/cm’ as the salinity increase across this interface overwhelms the thermal effect on fluid density. This steep density gradient precludes all but diffusional-conductive or perhaps double diffusive-convective mass and heat transfer in this transitional regime. Measurement uncertainties for reservoir depths, temperatures and salinities commonly exceed errors of our density model, limiting the accuracy of details in our modeled density distribution. Gross scale relationships of this salinity stratified geothermal system are, however apparent, permitting rational discussion of extrapolations to conditions beneath the presently explored reservoir and inference of dynamics during the systems’ evolution. Keyvords: density; stratification: convection; brines; Salton Sea geothermal field