Stability and Reactivity of Organic Molecules in Geothermal Reservoirs

Abstract

The present work deals with laboratory studies of tracers for geothermal applications. Analytical methods for the detection of molecules being used as non-reactive tracers are presented and investigations of their thermal stability as well. Furthermore, the synthesis of substances is described which can undergo a reaction by hydrolysis. Additionally, these substances are explored for their application as thermosensitive tracers by lab experiments. For the characterization of several geothermal reservoir properties, tracer tests are the method of choice. Only by tracer tests it is possible to track flow paths, to estimate reservoir fluid volumes and fluid dwell times, and various other parameters that are essential for the operation of a geothermal reservoir. But the results gained from tracer tests are often distorted or misguided due to a lack of knowledge of the substance behavior in the reservoir. Also, the analytical methods may be incorrect or not sensitive to the substance. Naphthalene sulfonates are widely used as conservative tracers in geothermal applications. In this work, an analytical method was developed and validated that is able to detect different naphthalene sulfonate isomers from highly saline geothermal brines in the sub-µg/L range. The method bases on high-performance liquid chromatography in conjunction with fluorescence detection and solid phase extraction. Beside conservative tracers, thermally reacting substances are under investigation by several research groups. These are envisaged to map reservoir temperatures and to describe a thermal drawdown during operation of the geothermal plant. Previous field applications using thermosensitive tracers gave only insufficient results. An applicable thermosensitive tracer has to possess well known reaction kinetics and transport behavior in the reservoir, as well as a sensitive detectability in brines. The investigations presented in this work shows that hydrolysis is a promising reaction for geothermal applications. On the example of phenolic esters it is shown that the kinetics of this reaction can be influenced at least by a factor of 100 by varying the structural characteristics of the ester. All these influences are predictable. Matrix effects such as high salinities of 100 g/L NaCl or the presence of powdered rock showed no effect on reaction kinetics. A strong effect on the kinetics is given by the pH of the solvent. It could also be shown that the reaction is completely alkaline catalyzed within the pH range of most geothermal systems. This effect can therefore be included in the reaction equations. However, the reaction is comparatively fast for long-term tracer tests in high temperature regimes. For these applications the findings of this work may be transferred to another class of ester substances. Finally, a hydroxynaphthalene sulfoic ester was synthesized and investigated in lab experiments as a example of a practical thermosensitive tracer for geothermal field applications.