Evaluating naphthol and naphthalene as novel geothermal tracers, with potential application in supercritical systems

• Naphthalene and naphthols persist in geothermal fluids at 350 to 400 °C for at least 200 h. • 1,5-NDS degrades rapidly, supporting the need for alternative tracers in high temperature systems. • The 1-naphthol to 2-naphthol ratio may serve as a high temperature geothermometer. • Ethanol post mixing is the most effective method for preserving volatile tracer compounds. Naphthalene disulfonates, particularly 1,5-naphthalene disulfonate (1,5-NDS), are widely used as geothermal tracers, yet their behaviour under extreme thermal and flow conditions remains insufficiently understood. In this study, we examine the formation of 1,5-NDS key breakdown products: naphthalene, 1-naphthol, 2-naphthol, and 1-naphthalene sulfonate (1-NSA), under high-temperature flow-through conditions designed to simulate geothermal reservoirs. Experiments were conducted using geothermal brine at 350 °C and 400 °C across a range of flow rates, employing a custom-built flow-through reactor system. High-performance liquid chromatography (HPLC) analysis was used to quantify reactants and degradation products over time. The results reveal that both temperature and residence time exert strong control on degradation rates and product profiles. At higher temperatures and lower flow rates, the formation of volatile products such as naphthalene and naphthols was favoured, indicating a shift toward desulfonation and aromatic ring rearrangement pathways. These findings provide important constraints on the stability and behaviour of 1,5-NDS in deep, high-temperature geothermal environments and offer a basis for refining tracer selection and test design in supercritical and near-supercritical systems.