The thermal stability of indium tracer chelated with organic ligands ethylenediaminetetraacetic acid (EDTA) and nitriiotriacetic acid (NTA) was measured for reservoir temperatures of 160, 200, and 240?C. Measurements of the soluble indium concentration was made as a function of time by neutron activation analysis. From the data, approximate thermal decomposition rates were estimated. A t 16O"C, both chelated tracers were stable over the experimenta! period of 20 days. At 2OODC, the InEDTA concentration remained constant for 16 days, after which the thermal decomposition occurred at a measured rate constant of k = 0.09 d-'. The thermal decompositiori of inNTA at 28b'C showed a first order reaction with a measured rate constant of kob, = 0.16 d-'. At 24OoC, both indium chelated tracers showed rapid decomposition with rate constants greater than 1.8 d-'. The data indicate that for geothermal reservoir with temperatures up to about 2OODC, indium chelated tracers can be used effectively for transit times of at least 20 days. These experiments were run without reservoir rock media, and do not account for concomitant loss of indium tracer by adsorption processes.

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