The effect of moisture content on retention of fluorocarbon tracers on sand

Several fluorocarbon compounds have been evaluated as geothermal tracers in sand column tests using damp, dry, and untreated 'washed sea sand'. Fluorocarbons evaluated in these tests included two hydrofluorocarbon freons: trifluoromethane (R23) and 1,1,1,2-tetrafluoroethane (R134a), and five perfluorocarbons: perfluorodimethylcyclobutane, perfluoromethylcyclopentane, perfluoromethylcyclohexane, perfluorodimethylcyclohexane and perfluorotrimethylcyclohexane. Transport of the fluorocarbon compounds was explored in flowing helium at 23 and 60 °C. This work found that fluorocarbon retention is strongly affected by sand moisture content. The fluorocarbon compounds flowed with the bulk fluid when the sand was damp, but were significantly retained by the solid phase when the sand was thoroughly dried. The data suggest that the fluorocarbons may be conservative tracers in geothermal conditions up to mild superheat but they may not be conservative at geothermal conditions with a high degree of superheat.