Factors affecting phosphonate measurement by inductively coupled plasma optical emission spectroscopy (ICP-OES) in oilfield produced waters

Abstract

ABSTRACT Phosphonate scale inhibitors are commonly used to mitigate scaling problems in oilfields. Continuous measurement of phosphonate in the produced waters in the levels of few mgL−1 using ICP-OES is usually conducted to monitor and assess phosphonate treatments, and to qualify the discharged produced waters. These measurements of phosphonates in high salt producedwaters suffer from severe matrix effects, contamination, samples preservation and dilution issues. The results in this study showed that conducting measurements when the sample total dissolved solids (TDS) was higher than the calibration standards TDS that leads to lower phosphonate results, while higher results obtained when the sample TDS was lower than the calibration standards TDS. The matrix elements Na, Mg, and Ca were found to depress the emission lines intensity of phosphorus and their effect was in the order of: Ca > Na > Mg for the lines 213.617 nm, 253.561 nm, and in the order of Mg > Ca> Na for the line 214.914 nm. Besides, it was found that diluting the samples of low phosphonate content leads to lower phosphonate results; therefore, phosphonate measurements are better be conducted on undiluted samples. The instrument robust operating conditions at low nebulizer and pump uptake rates, high RF power and with the use of baffled cyclonic spray chamber showed better sensitivity, LOD and LOQ compared to the plasma non-robust conditions. The use of internal standard and quality control QC program along with the instrument setup at robust plasma conditions allowed direct measurement of phosphonate down to 1 mgL−1 without dilution, desalination, preconcentration or matrix matching. It was also found that Ca and Fe affect phosphonate measurements due to the high phosphonate precipitation/adsorption potential on their compounds, and acid preservation methods using HCl were found to be the best method to avoid phosphonate precipitation or adsorption.