Tracer design for tracking thermal fronts in geothermal reservoirs

This paper describes the first steps in the design of new custom-made tracers for the investigation of thermal characteristics in deep geothermal reservoirs. Based on ester hydrolysis, the influences of different substituents on the reaction kinetics are studied in detail using the analog phenol acetates under the influence of varying pH and temperatures. Furthermore, a 2-ethylbutyl-2-naphthol-6-sulfonic ester is tested in laboratory experiments for its applicability as a practical thermo-sensitive tracer, considering also the influences of brine and rock matrix. The reaction is shown to be strongly thermo-sensitive while the reaction kinetics can be varied by two orders of magnitude by altering the sterical hindrance groups. These changes in the reaction kinetics are found to be predictable by using the results of this work. It has been found that the half-life can be decreased by the factor of three, if the reacting group is sterically hindered from one side and by the factor of ten, if the reacting group is hindered from both sides. Also mesomeric and inductive effects of substituents on the aromatic ring have significant effects on reaction rates. The pH-dependency on the reaction speed could be quantified and it is shown that the reaction complete alkaline catalyzed. Therefore, this reaction fulfills the many considerations of a tracer-test, such as varying absolute temperature, test durations, and temperature gradients. (C) 2012 Elsevier Ltd. All rights reserved.