The Thermal Stability of the Naphthalene Sulfonic and Naphthalene Disulfonic Acids under Geothermal Conditions: Experimental Results and a Field-Based Example

Abstract

The thermal stability of six naphthalene sulfonic acid tracers (2,6-NDS, 2,7-NDS, 1,5-NDS, 1,6-NDS, 1-NSA, 2-NSA) was investigated using a one-pass, continuous flow, fluid - rock interaction simulator. The pore fluid utilised was pH modified, low pressure separator brine from Nga Awa Purua power station at Rotokawa with 100 ìg/kg of the six tracers added. The simulator contained crushed and clean fragments of greywacke sourced from well RK24. The temperature range investigated was 270oC – 390oC Results show that 2,6 and 2,7-NDS remain stable to about 340oC while 1,5-NDS begins to degrade as early as 280oC. 1,6-NDS shows degradation beginning at 330oC. For the NSA, the concentration of 1-NSA increases until 330oC after which, it begins to degrade while 2-NSA concentration increases until 380oC. The results are interpreted using three reaction mechanisms: desulfonation, isomerisation and naphthalene pyrolysis. A field-based example of tracer degradation is described from the Rotokawa geothermal field. A suite of NDS tracers was injected and returns were not found after a monitoring period of one year. Reanalysis, in light of the experimental results, showed the presence of 2-NSA which was not one of the original tracers injected. These results provide field-based evidence of the thermal degradation of naphthalene sulfonic acid tracers and imply the use of tracer mixtures for use as geothermometers.