Experimental Determination of Rate Constants for the Breakdown of the Organic Tracers 2-NSA, 2, 6-NDS, 2, 7-NDS, 1, 5-NDS and 1, 6-NDS 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. The main goal of this project is to expand the understanding of the processes taking place when these tracers are exposed to geothermal temperature and pressure conditions. Two series of experiments were conducted to measure the rate of the thermal breakdown reactions. The sulfonates studied include: 1,5-naphthalene disulfonate (1,5-NDS); 1,6-naphthalene disulfonate (1,6-NDS); 2,6-naphthalene disulfonate (2,6-NDS); 2,7-naphthalene disulfonate (2,7-NDS); and 2-napthalene sulfonate (2-NSA). Aqueous solutions containing the relevant tracer were sealed inside glass ampoules using an oxy-acetelyene torch and placed in cold-seal autoclave reactors. In the first experiment the reactors were exposed to 200°C and 330°C for different exposure times from 10 hours to 1 week. The experimental pH was varied and ionic strength ranged between 0.001 and 0.05 M. In the second series of experiments, 1,5-NDS and 2-NSA were exposed to 200°C, 225°C, 250°C, 270°C, 300°C and 330°C for exposure times varying from 20 hours to 3 months. Tracers were analysed by high performance liquid chromatography (HPLC) using fluorescence detection. This is one of the first rigorous experimental kinetic studies involving 1,5-NDS at different pH’s and ionic strengths. The rate constants were estimated by linear regression through the experimental data. Results show that rate constants at 200°C are both pH and ionic strength dependent. Increased ionic strength slows the breakdown reactions while lower pH increases the breakdown rate. Results at 300°C show similar trends, however, the overall rates of decay are more rapid. 1-napthelene sulfonate was detected as stable 1,5-NDS breakdown product.