EXPERIMENTAL DETERMINATION OF THE THERMAL STABILITY OF THE ORGANIC TRACERS 1,5-NDS AND 2-NSA UNDER GEOTHERMAL CONDITIONS

Abstract

Naphthalene sulfonates are widely used in the geothermal industry as tracers. Experimental results show however, that these chemicals do have their temperature limits. Knowledge of the rate of thermal breakdown is essential to ensure successful tracer tests. In addition, knowledge of the breakdown products could be useful in interpreting fluid temperature history. The main goal of this project is to broaden the understanding of the processes taking place when these tracers are exposed to conditions found in the geothermal environment.
The experiments were conducted to measure the rate of the thermal breakdown reactions. Two tracers: 1,5-naphthalene disulfonate (1,5-NDS, least thermal stability of the compounds) and 2-naphthalene sulfonate (2-NSA, most thermally stable) were used. Aqueous solutions containing the tracers were sealed inside glass ampoules using an oxy-acetelyene torch and placed in cold-seal autoclave reactors. The reactors were exposed to 200°C and 300°C for different exposure times from 20 hours to 166 hours. The experimental pH was varied and ionic strength ranged between 0.001 and 0.05 M. Tracers were analysed by high performance liquid chromatography (HPLC) using fluorescence detection.
The rate constants were estimated by fitting a line through the data points. Results show that rate constants at 200°C are pH and ionic strength dependent. Increases in ionic strength slows the breakdown reactions while decreased pH increases the breakdown rate. Results at 300°C show similar trends, however, the overall rates of reaction are more rapid.