Recent Progress on Hydrolyzable Compounds as Thermo-Sensitive Tracers for Investigating the Thermal Drawdown of Geothermal Reservoirs

摘要

Currently, there is an increasing demand for reactive tracers with thermo-sensitive properties for an improved reservoir management in geothermal systems with thermal drawdown. This is mainly caused by the fact that the reservoir temperature is not directly accessible. To estimate the thermal state of a georeservoir, two slightly different thermo-sensitive tracer approaches are currently pursued. One uses the non-specific thermal decay kinetics of established tracers (e.g. naphthalene sulfonates, fluorescein) at high temperatures while the other approach exploits the structure-related kinetics of defined hydrolysis reactions (e.g. phenol acetates). Up to now, the successful tracer application, however, is limited to certain geothermal temperature conditions due to either the non-availability of suitable purchasable substances or the lack of knowledge on the structure-dependent kinetics. To fill these gaps and to improve the general applicability of thermo-sensitive tracers, a wide spectrum of hydrolyzable compounds with thermo-sensitive properties was synthesized in our working group. The concept of structure-dependent kinetics learnt from phenol acetates has been transferred to amides. Their thermally induced hydrolysis leads to well-known reaction products with fluorescent properties in a first-order reaction. The influence of structural elements of the molecules on the reaction speed of these amides at different reservoir temperatures was investigated. For this, the thermal decay kinetics of 18 newly synthesized amides was determined by static batch experiments in the temperature range from 203°F (95°C) to 374°F (190°C). Furthermore, the influence of pH on the hydrolysis kinetics was studied. The identified relationship between reaction speed and structural properties enables first insight into the selection and molecular design of suitable thermo-sensitive tracers for specific reservoir types and tracer tests. These novel amides with sulfonic groups and a high water solubility (>1 kg L~(-1)) differ in their substructures and thus in their reaction kinetics. The results show that they are potentially suitable for reservoir temperatures up to 400°F (ca. 200°C) and experimental durations of up to several months. Thus, they fill the "kinetic gap" between esters (only useful in low enthalpy systems) and tracers based on thermal decay (e.g. naphthalene sulfonates).