Development and validation of an analytical methodology for the determination of δ2H and δ18O in formation water based on Laser-Based infrared absorption spectroscopy

The present work describes the application of the Eurachem validation method to the determination of delta H-2 and delta O-18 in high-salinity petroleum using near-infrared laser spectrometry from Los Gatos Research (laser water isotopic analyzer). The validation protocol involved selectivity, linearity, trueness, precision and ruggedness. Although the method is not selective for methanol and ethanol, it was selective for formic acid and acetic acid, which are additives applied by the petroleum industry. The linearity for delta H-2 in the range of -2.74% to -427.5% and for delta O-18 from -0.98% to -55.5%, with a determination coefficient of 1.00, has been reached. Trueness was verified by the participation in an interlaboratory exercise promoted by the International Atomic Energy Agency (IAEA) and by paired formation water sample analysis applying laser spectrometry and isotopic ratio mass spectrometry (IRMS). The achieved precision fits with the equipment specification, and both repeatability and intermediary precision were equal to 0.50 % and 0.10 % for delta H-2 and delta O-18, respectively. An effect of salinity and MgCl2 up to 250 g L-1 and 50 g L-1, respectively, was not observed. The developed protocol was applied to ten petroleum water samples with salinity in the range of 28-217 g L-1. One of the samples fit the global meteoric water line (GMWL), and the others were found below the GMWL.