Vincent J. Sieben

Determination of boron in produced water using the carminic acid assay

Using the carminic acid assay, we determined the concentration of boron in oilfield waters. We investigated the effect of high concentrations of salts and dissolved metals on the assay performance. The influence of temperature, development time, reagent concentration, and water volume was studied. Ten produced and flowback water samples of different origins were measured, and the method was successfully validated against ICP-MS measurements. In water-stressed regions, produced water is a potential source of fresh water for irrigation, industrial applications, or consumption. Therefore, boron concentration must be determined and controlled to match the envisaged waste water reuse. Fast, precise, and onsite measurements are needed to minimize errors introduced by sample transportation to laboratories. We found that the optimum conditions for our application were a 5:1 mixing volume ratio (reagent to sample), a 1 g L(-1) carminic acid concentration in 99.99% sulfuric acid, and a 30 min reaction time at ambient temperature (20 °C to 23 °C). Absorption values were best measured at 610 nm and 630 nm and baseline corrected at 865 nm. Under these conditions, the sensitivity of the assay to boron was maximized while its cross-sensitivity to dissolved titanium, iron, barium and zirconium was minimized, alleviating the need for masking agents and extraction methods.

Determination of boron concentration in oilfield water with a microfluidic ion exchange resin instrument.

We developed and validated a microfluidic instrument for interference-free determination of boron in produced water. The instrument uses a boron-specific chelating resin to separate the analyte from its complex matrix. Ten produced water samples were analyzed with the instrument and the results were successfully validated against ICP-MS measurements. Removing interference effects enables precise boron measurement for wastewater even with high total dissolved solid (TDS) levels. 1,4-Piperazinediethanesulfonic acid conditions the resin and maintains the optimum pH for boron adsorption from the sample. Boron is then eluted from the resin using a 10% sulfuric acid solution and its concentration measured with the colorimetric carminic acid assay in 95% sulfuric acid. The use of a microfluidic mixer greatly enhances the sensitivity and kinetics of the carminic acid assay, by factors of 2 and 7.5, respectively, when compared against the same assay performed manually. A maximum sensitivity of 2.5mg(-1)L, a precision of 4.2% over the 0-40.0mgL(-1) measuring range, a 0.3mgL(-1) limit of detection, and a sampling rate of up to four samples per hour were achieved. Automation and microfluidics reduce the operator workload and fluid manipulation errors, translating into safer and higher-quality measurements in the field.

Rapid determination of boron in oilfield water using a microfluidic instrument

We developed and validated an interference free microfluidic instrument for rapid determination of boron concentration in produced water using the carminic acid assay. Interferences from other species are eliminated by controlling crucial aspects of the kinetics and thermodynamics of the assay. The use of a microfluidic mixer improves the reaction kinetics by reducing the diffusion time. The high surface-to-volume characteristic of microchannels efficiently dissipates the heat generated by the exothermic reaction of the carminic acid with water. We incorporate a 6.5 bar back pressure element, which offsets the onset of calcium sulfate precipitation and hydrochloric acid outgassing that often disturb optical measurement for the assay when performed at atmospheric pressure. The analyzer has a measurement rate of 1 sample per min, a precision of 5%, a limit of detection of 0.16 mg L−1 at 615 nm, and a measurement range of 0–420 mg L−1. The instrument accurately measured ten water samples that were representative of fluids encountered in the oilfield when compared to ICP-MS measurements. The fast measurement rate of the instrument enables near real-time decision of the fate of produced water in the oilfield.