Thermodynamic calculations of calcium carbonate scaling in geothermal wells, dixie valley geothermal field, U.S.A.

Wells in the Dixie Valley geothermal field of central Nevada intercept a fracture-dominated hydrothermal system at depths of 2.5 to 3 km. The reservoir water is a dilute sodium-bicarbonate-chloride type solution thought to be in equilibrium with quartz, calcite, chlorite, and albite. Fluid sampling and chemical analysis of production during an early flow test gave remarkably low calcium concentrations. Thermodynamic calculations of mineral stability in the presence of the reservoir water indicate that five times the amount of calcium measured in fluid reaching the surface is actually in solution in the reservoir fluid. Approximately 80 percent of the calcium is lost as calcium carbonate scale on the well casing before the fluid reaches the surface. The results of thermodynamic calculations compare well with the scale-volume measurements of Benoit (1989).