Desenvolvimento e validação de metodologia analítica em espectroscopia de absorção a laser para determinação de δ2H e δ18O em águas de formação

Abstract

Nowadays, the analysis of formation water samples from oil fields are used for physical-chemistry characterization or dissolved salts quantification, with focus on oil production monitoring. However, because of data contamination problems through the use of this techniques, it has been necessary the development of other analytical methodologies. Among these methodologies, the determination of water isotopes composition stands out, regarding δ2H and δ18O signatures. This determination can be performed using two distinct techniques. The first one is based on mass spectrometry, a traditional method and with fewer interferences, but expensive and slow. The other technique consists on the use of laser spectroscopy, a cheap and faster method but with a significant number of interference compounds. Therefore, the objective of this work was the development of an isotope ratio determination methodology of formation water samples based on laser spectroscopy techniques and the evaluation of influence of the matrix components on the final results. First, a validation of a sweet water analysis method was performed and, then, an investigation of the matrix contribution to the isotope ratio results of formation water samples analysis was carried out. The validation test results revealed a good linearity for this method, in the range of -9,2 to -427,5 for δ2H and of -2,69 to -55,50 for δ18O, with repeatability of 0,46 for δ2H and 0,09 for δ18O. Moreover, a good performance in the analysis of four proficiency test samples was achieved, where all the measured results corresponded to z-score values under 2 in comparison with other laboratories and with an excellent performance for δ2H and an acceptable performance for δ18O. The preliminary results that were obtained for the matrix interference contribution revealed that salinity levels up to 250 g L-1 could not affect the isotope ratios results. Low concentrations of volatile organic compounds, such as 500 mg L-1 of methanol, could already affect the isotope ratios values, resulting in a variation of 14,6 for δ2H and 9,01 for δ18O. However, this type of interference has not been found in the formation water samples analyzed in this study. Finally, it could be concluded that the laser spectroscopy can be used for the determination of hydrogen and oxygen isotopes composition in formation water samples without impacting the accuracy of these results