We present a state-of-the-art microfluidic technique for measuring the asphaltene content in crude oil samples. The automated microfluidic system improves data quality, reduces turnaround time, and minimizes solvent-volume consumption compared with conventional wet-chemistry measurements. The method is based on conventional precipitation techniques, such as the ASTM D6560, in which n-heptane is used for precipitation. However, instead of using gravimetric techniques commonly utilized in conventional methods, we use a novel spectrophotometry method for quantifying the asphaltenes. The optical technique is based on the spectral difference of crude oil before and after precipitation of asphaltenes using the titrant. We show a strong linear correlation between the optical method and the wet-chemistry technique for a broad selection of samples. The sample set included more than 50 crude oils with asphaltene contents as high as 15 wt.-%.
The unique microfluidic platform developed for this study uses a 200 µL sample loop to deliver the crude sample into a microfluidic chaotic mixer, where the sample is mixed with toluene. After dilution, we injected the sample into a 2.5-mm path-length flow cell, where a visible spectrum of the oil is recorded. To measure the spectrum of the maltenes, the sample is mixed with n-heptane in the microfluidic mixer at a predefined volume ratio. The mixture then flows into a reactor channel, where the aggregation process takes place and asphaltene molecules grow into large aggregates. After precipitation, the mixture is passed through a 200-nm-pore membrane, where asphaltenes are trapped and maltenes permeate through the membrane. The spectrum of maltenes is then measured using the 2.5-mm path-length flow cell and a UV-VIS spectrometer.
Using the microfluidic technique, a complete measurement takes only 40 minutes, a considerable improvement over conventional wet-chemistry techniques that require a minimum of 2 days. Furthermore, conventional wet-chemistry measurements are highly operator dependent. We show that, for our optical technique, the repeatability of measurement is better than ±0.1 wt.-%. The microfluidic measurements require only 200 µL of sample and 40 mL of solvent, which reduces the environmental impact of measurement. Finally, the small footprint of the apparatus makes it highly desirable for wellsite and offshore applications.
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