Evaluation of Chemical Tracers for Geothermal Use

Abstract

The use of waste water reinjection schemes has led to substantial interest in tracer testing. Tracers have been used to estimate the length of time necessary for injected water to be recycled through the reservoir back to the producing well. Experience in recent years has shown that in fractured systems the transit time is surprisingly short and reinjected water has caused losses in productivity due to enthalpy decline. This paper examines the advantages and disadvantages of chemical tracer materials with a view to their use in evaluating reinjection schemes. These observations are based on laboratory experiments performed at Stanford over the last year using core material from Los Azufres geothermal field in Mexico and Iodide and Bromide tracers. Major conclusions reached were that halogen ion tracers are generally sufficiently stable at reservoi r temperatures but suffered from concentration loss as a function of time. This loss appears to be independent of concentration and temperature, and is at least partly reversible. These results highlight the need for laboratory experiments such as these with reservoir specific rock materials before performing a field test