The potential of SF6 as a geothermal tracer

A novel application of gaseous sulphur hexafluoride (SF6) tracer, in the evaluation of bulk water flow through an artificial geothermal reservoir, the Hot Dry Rock (HDR) reservoir at Rosemanowes quarry, south west England, is described. The potential advantages of SF6 as a tracer, and details of its deployment and analysis are outlined. In a 52 h continuous borehole pumping test in the HDR reservoir, SF6 and sodium fluorescein (NaFl) were injected simultaneously in a mass ratio of 7.5 × 10−7 SF6:NaFl and analysed in production well waters. Resultant tracer response curves demonstrate conservation of the injected tracer mass ratio throughout the first 25 h of the test. However, substantial divergence of the curves beyond 25 h reflects significant tracer fractionation, most probably a consequence of selective diffusion and enhanced SF6 residence times in some of the longer residence time flow paths of the system. These preliminary results clearly indicate the strong potential of SF6 as a water tracer in this, and in related groundwater applications.