Methods Development for the Analysis of Naphthalene Sulfonates by High Performance Liquid Chromatography

Abstract

The use of naphthalene sulfonates as geothermal tracer is a recent development in tracer technology. Naphthalene sulfonates are polyaromatic fluorescent compounds found to be suitable conservative tracers in high temperature reservoirs. They have been proven to be excellent tracers because they are non-toxic, have low level of detection (ppb), absent in the reservoir, detectable by fluorescence spectroscopy, affordable and thermally stable. It has been successfully tested in various fields like the Dixie Valley geothermal system in Nevada, USA and at the PGI MakBan field in the Philippines.

Tracer tests using three naphthasulfonates will be undertaken at the Leyte Geothermal Production Field (LGPF) and Mindanao Geothermal Production Field (MGPF) to study the reservoir flow patterns. Based on the initial method development conducted, three of the di-substituted compounds (2,6-NDS, 1,5-NDS and 1,6-NDS) may be used since results showed good peak separation, fast analysis time, high purity and low cost. In case of unavailability of any one of these compounds, 2,7-NDS may be also be used. Analyses were conducted using Waters C18 reverse phase column (300 x 4.6 mm) and mobile solution consisting of 3.17 mM Na2HPO4, 6.21 mM KH2PO4, 5.0 mM C16H36NPO4 (TBAP), 25% CH3OH and 75% H2O, at flowrate of 1.2 ml/min.

Further method development was conducted using a Merck Chromolith column (30 x 4.6 mm, ôchalk-typeö). More efficient analysis of the naphthasulfonate compounds was obtained compared to the Waters C18 column. Analysis time was shortened by up to 80%, at a mobile flow rate of 2.5 ml/min. Calibration runs showed acceptable repeatability and linearity (r2>0.995) for the three compounds. Precision tests indicate acceptable recoveries of standards as samples. Recovery tests on unacidified brine samples showed acceptable recoveries, from 98-104%.