Article pubs.acs.org/EF
Asphaltene Content Measurement Using an Optical Spectroscopy Technique Abdel M. Kharrat, Kentaro Indo, and Farshid Mostowfi* Schlumberger DBR Technology Center, 9450 17 Avenue, Edmonton, Alberta T6N 1M9, Canada ABSTRACT: A new spectrophotometry technique for quantifying the asphaltene content of black oil samples improves data quality, reduces measurement time, and reduces solvent volume in comparison with conventional methods. According to this method, asphaltenes are quantified by subtracting the visible spectrum of the maltenes from that of the oil. We found that the difference in spectra of oil and maltenes correlated well with the modified ASTM D6560 method for a large sample set that covered a wide range of geographic locations, which points to the existence of a global correlation between the visible spectrum of asphaltenes and their concentration. The repeatability of the measurements, even for low-asphaltene samples, was far better than that achieved with conventional methods. The measurement time was reduced to less than 3 h from 2 days in conventional measurements, and the solvent volume was reduced from 250 mL using the conventional technique to 40 mL using the optical method, thereby reducing the environmental footprint of the measurement.
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INTRODUCTION Asphaltenes are the heaviest and most polar fraction of crude oils. Asphaltenes can contain heteroatoms, such as nitrogen, sulfur, and oxygen, as well as heavy metals such as vanadium, nickel, and iron. They also contain aromatic rings. The structure of asphaltenes has been the subject of many investigations, but scientists generally agree on their aromaticity.1,2 The presence of these aromatic rings and conjugated double bonds allows asphaltene molecules to absorb light in the visible range and, consequently, have a black or dark brown color. The most general and accepted characteristic of the asphaltene fraction is its solubility. Asphaltenes have a low solubility in normal alkanes, such as heptane, hexane, and pentane. These alkanes are called precipitants or titrants because, when added to a crude oil, they cause asphaltenes to precipitate. Asphaltenes are highly soluble in aromatic solvents such as toluene and chlorinated solvents such as dichloromethane.1 Once asphaltenes are extracted and dried, they range in appearance from very dark, shiny, and needle-like to brown and gummy, depending on the titrant used for precipitation and the oil from which they are extracted. The asphaltene content of a reservoir fluid is an important factor in evaluating the chemical and physical properties of a crude oil. As such, the measurement of asphaltene content is of prime importance. Precipitation and deposition of asphaltenes can have catastrophic consequences on upstream, midstream, and downstream systems and operations.3 Asphaltenes can precipitate inside reservoirs, pipelines, or refineries. They can substantially decrease the activity of catalysts used for upgrading processes. The measurement of the asphaltene content of a reservoir fluid, along with its deposition tendency, is therefore essential in all aspects of oil production, transportation, and refining. Asphaltene separation methods predominantly rely on precipitation and flocculation with n-alkanes and subsequent collection using filtration or centrifugation.1 These methods require large quantities of solvents, large glass vessels, and many © 2013 American Chemical Society
other instruments for proper extraction. In most cases, quantification is performed by gravimetric measurement using a conventional balance.4−8 Conventional methods such as ASTM D6560,7 also known as IP 143 (or “wet chemistry” methods), suffer from many shortcomings such as long turnaround times, lack of automation, and poor repeatability and reproducibility. Typical asphaltene content measurements can require at least 2 days of wet chemistry work and up to 250 mL of solvents. Slight changes in temperature and humidity can result in considerable variations in quantification. The problem is even more severe for fluids with low asphaltene contents, particularly below 1%, where weighing small quantities of asphaltenes is difficult and humidity can interfere greatly with the asphaltene weight. Furthermore, the tests have to be performed in a laboratory environment by a qualified operator. Consequently, they are difficult to perform in harsh environments, such as in the field, in mobile laboratories, or on offshore platforms. In addition to the complexity of the setups, these methodologies have numerous limitations such as contamination and uncertainty. To overcome these limitations, researchers have been seeking alternative methods for determining asphaltene content. Spectrophotometry methods have been used for characterizing asphaltenes in a limited number of studies.9−13 Bouquet and Hamon11 showed that absorbance measurements at 750 nm of oil and corresponding maltenes could be used to quantify asphaltene concentrations. Other studies have used optical absorbance mostly for very small changes in asphaltene concentrations, such as in adsorption studies.14−17 Another method of precipitating asphaltenes has been developed using a polytetrafluoroethylene (PTFE) packed column, equipped with an evaporative light-scattering detector (ELSD).18,19 Various techniques have been developed for the measurement of Received: January 10, 2013 Revised: April 3, 2013 Published: April 3, 2013 2452
dx.doi.org/10.1021/ef400050y | Energy Fuels 2013, 27, 2452−2457
Energy & Fuels
Article
asphaltene content without the precipitation and extraction of asphaltenes. These methods are based on nuclear magnetic resonance (NMR),20 Fourier transform infrared (FTIR),21,22 near-infrared (NIR),23 mid-infrared (MIR),24 ultraviolet (UV),25 and fluorescence spectroscopies.26 These methods usually rely on a calibration curve that correlates the measurements with a gravimetric technique. Chemometrics and multivariate calibration approaches have also been used to predict asphaltene contents without extraction of the asphaltenes.27,28 Because these measurements are performed directly on crude oil (not just asphaltenes), they eliminate the need for precipitation and separation of asphaltenes using wet chemistry. However, because these measurements are conducted on a crude oil sample without separation of asphaltenes, it is difficult to determine the impact on the measurements of interfering species such as resins. Therefore, wet chemistry techniques based on precipitation and separation of asphaltenes are still widely used as the most common methods for determining asphaltene contents. Our goal in this study was to develop a rapid and repeatable method for determining the asphaltene content of a reservoir fluid that minimizes operator dependencies by simplifying the test and requires small sample and solvent volumes. The method is based on the optical spectroscopy of oil and maltenes in visible range. Because the direct measurement of asphaltene absorbance is difficult, we used the difference in absorbance of oil and the corresponding maltenes. We demonstrate that the difference between the two absorbance values correlates well with the asphaltene content of the fluids.
Figure 1. Linearity of absorbance versus concentration.
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ASPHALTENES CONCENTRATION AND OPTICAL SPECTROSCOPY The absorption of light by a compound in a solution can be described by the Beer−Lambert law, which states that the absorbance of light by a solution is correlated with the concentration of its constituents absorbance = εcl
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Sample Set. The sample set contained 26 stock-tank oils. The crude samples consisted of black oils onlyno heavy oil or condensates. The concentration of asphaltenes varied from 5%), the color of the oil is probably dominated by the color of the asphaltenes, and the effect of resins and aromatics could be minimal. For fluids with low asphaltene contents, however, the impact of maltenes might not be negligible, which could lead to a significant error in the correlation. The bottom graph in Figure 3 shows the residual error. The optical spectrum of a reservoir fluid can be reconstructed from its constituent species. A crude oil can be split into asphaltenes and maltenes (deasphalted oil). Therefore, the spectrum of oil can be reconstructed from that of the asphaltenes and that of the maltenes. However, the extraction, separation, and subsequent spectroscopy of asphaltenes are challenging. To determine the spectrum of asphaltenes, the spectrum of maltenes can be subtracted from that of the original oil, as in Figure 4. An example of the absorbance spectrum versus wavelength is presented in Figure 4.
Figure 5. Correlations of oil optical densities and maltene optical densities with asphaltene content. The straight lines show the best linear fits to the data.
Table 2. Standard Deviations for Modified IP 143 Measurements asphaltene content (%)
standard deviation (%)
>3 1−3
