Article pubs.acs.org/ac
Measurement of Asphaltenes Using Optical Spectroscopy on a Microfluidic Platform Marc H. Schneider, Vincent J. Sieben, Abdel M. Kharrat, and Farshid Mostowfi* Schlumberger DBR Technology Center, 9450 17th Avenue, Edmonton, Alberta T6N 1M9, Canada S Supporting Information *
ABSTRACT: We present a microfluidic apparatus and method for the measurement of asphaltene content in crudeoil samples. The measurement is based on an optical absorption technique, where it was established that asphaltene coloration correlated linearly with asphaltene weight content. The initial absorbance of the oil is measured, and asphaltenes are removed from the oil by the addition of n-alkane, leading to flocculation and subsequent filtration. The absorbance of the deasphalted oil (maltenes) is then measured, and the initial asphaltene content is revealed by the change in absorbance. The asphaltene optical densities correlated linearly with conventional weight measurement results (e.g., ASTM D6560) for 38 crude-oil samples from around the world. Sample measurement repeatability was shown to be within ±2% over several months. Other aspects influencing performance of the system were evaluated, including plug dispersion, flocculation kinetics, membrane degradation, and channel clogging. The microfluidic approach described here permits asphaltene content measurement in less than 30 min as opposed to days required with traditional gravimetric techniques. This many-fold reduction in measurement time will enable more frequent characterization of crude oil samples.
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problematic oils, with frequent and accurate characterization of asphaltene behavior for each crude oil sample. Key measurements, such as asphaltene content, aid in modeling and prediction of the general behavior of these diverse aggregates and help in preventing or mitigating costly remedial techniques to redissolve or remove the problematic material. Asphaltene measurement methods predominantly rely on precipitation and subsequent collection of the aggregates.11 Typically, large quantities of solvents, large glass vessels, and many other instruments are used for proper extraction. In most cases, the quantification is performed by weight measurement using a conventional balance.10,15 The majority of steps have to be performed in a laboratory environment by a qualified operator. Consequently, it is very difficult to perform such tasks in a harsh environment such as in the field, mobile laboratories, or offshore platforms. In addition to the complexity of the setups, these methodologies have numerous limitations, such as contamination and uncertainty. The conventional methods also suffer from many performance shortcomings, such as long turnaround, lack of automation, and poor repeatability and reproducibility. Typical asphaltene content measurements require at least two days of wet chemistry work and up to 250mL of solvents.15 Slight changes in temperature and humidity could result in
he applications of microfluidics in the petroleum industry have grown considerably over the past few years. Micromodels have been traditionally used for the investigation of transport phenomena such as invasion percolation and recovery processes in planar porous structures.1−3 Advances in wettability patterning techniques and high-resolution imaging allow the creation of heterogeneous surfaces which resemble that of rock.4,5 Recently, microfluidics technology has been used as a platform of choice for reservoir fluids characterization and measurements. Measurements related to interfacial properties of the water−oil interface,6 composition,7 phase behavior, and PVT8,9 have been successfully conducted in microfluidics. One particular area of opportunity is the determination of asphaltene content for both upstream and downstream operations, which is the subject of this study. Asphaltene precipitation and deposition from reservoir fluids is a major impediment during production, transportation, sample handling, and processing of reservoir fluids. Unexpected precipitation and the subsequent potential for deposition of asphaltene aggregates can cause reservoir impairment, plugging of wells and flowlines as well as fouling issues, and processing challenges for facilities.10,11 Asphaltenes are a subcomponent of crude oil that are conventionally defined as being poorly soluble in n-alkanes (e.g., n-heptane), but highly soluble in aromatic solvents (e.g., toluene).12 Defined as a solubility class of material, asphaltene fractions can also precipitate upon a change in pressure, temperature, or composition of the oil.13,14 In all three cases, asphaltenes precipitate because of changes in solubility. Optimal flow assurance requires identification of © 2013 American Chemical Society
Received: February 15, 2013 Accepted: April 8, 2013 Published: April 24, 2013 5153
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Figure 1. (A) Concept and flow diagram. (B) Enlarged view of the chip assembly. The disposable poly(tetrafluoroethylene) (PTFE) filter membrane is sandwiched between the glass microfluidic chip and the poly(ether ether ketone) (PEEK) collection channel.
solvents, the ratio between the solvent and the oils, the precipitation and aggregation times, and the type of washing. In our laboratory, the asphaltene content was determined using a modified ASTM D6560 (IP 143) standard method,22 briefly described here. The oil sample is added to a 40-fold excess of HPLC-grade n-heptane. The mixture is heated to reflux for 2 h. The precipitated asphaltenes are filtered while the mixture is still hot, and the filter is then washed using a Soxhlet extractor to remove the nonasphaltenic materialsmaltenes and wax that may adsorb on the asphaltenes and on the filter. The washing is performed until the solvent is visually clear. Asphaltenes are then extracted from the filter with refluxing dichloromethane. The solvent is removed, and asphaltenes are stored under nitrogen for subsequent mass measurement. Microchip. A glass microfluidic chip was manufactured by Dolomite (Royston, U.K.), which integrated a y-junction for introducing the sample and the solvent, a micromixer that implements laminar flow folding to yield smaller diffusion lengths, a serpentine reactor to ensure a sufficient time delay for reaction kinetics, and an open-face filtration channel to interface with a porous membrane for asphaltene separation. The multilayered chip was fabricated using conventional techniques (i.e., lithography, isotropic etching, and temperature annealing).23 Nominal cross-sectional dimensions (depth × width) are as follows. The mixer was 125 μm × 350 μm (large channels) and 50 μm × 125 μm (small channels). The reactor was 250 μm × 370 μm. The inlet and interconnecting channels were 175 μm × 350 μm. The filtration channel was 200 μm × 600 μm and was open-faced (i.e., not capped or sealed). The total volume of the microfluidic chip was approximately 91 μL. System. The microfluidic chip was mounted in a custommade aluminum block that sandwiched a 0.2 μm hydrophobic poly(tetrafluoroethylene) (PTFE) porous filter membrane (Sartorius Stedim, France) between the chip and a custom poly(ether ether ketone) (PEEK) block as shown in Figure 1B. This holder also enabled fluidic connections to external tubing for sample delivery and collection. The PEEK block was micromachined to include a serpentine channel (open-faced for permeate collection) with a depth and width of 300 μm × 600 μm. The total volume of the holder and chip was approximately
considerable variation in measurements. The problem is more severe for fluids with low asphaltene content (particularly those smaller than 1 wt. %). Due to these limitations, researchers have been looking for alternate methods to measure asphaltene content. Optical density measurement approaches have been used in studies where asphaltene concentrations change by very small amounts, such as in adsorption studies.16−19 In this work, a microfluidic apparatus and method for asphaltene content measurement are investigated. Optical spectroscopy measurements are combined with a microfluidic fluid-handling platform to create an apparatus for asphaltene content measurement, which vastly exceeds other common measurement techniques in terms of speed, repeatability, and cost. The underlying principles of optical detection have been addressed in previous works,20,21 where it was established that asphaltene coloration correlated linearly with asphaltene weight content. The process starts by measuring the initial absorbance of diluted oil (coloration) before asphaltenes are flocculated (step 1, shown in Figure 1A). In step 2, asphaltenes are precipitated when mixed with an alkane (e.g., n-heptane). The fluid is filtered, and the absorbance of the deasphalted oil (i.e., maltenes) is measured. The initial asphaltene content is revealed by the change in absorbance (oil spectrum minus maltene spectrum). Figure 1B shows the microfluidic chip stack in an enlarged view. The rapid and repeatable results obtained by this automated system validate that microfluidic devices can vastly improve upon existing compositional measurement techniques for analysis of oil and gas samples.
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METHODS AND MATERIALS The 38 oil samples used in this study were acquired from a variety of geographical regions, and the asphaltene content ranged from 0 to 8 wt. %. High-performance liquid chromatography-grade (HPLC-grade) toluene (CAS no. 10888-3) and n-heptane (CAS no. 142-82-5) were purchased from Fisher Scientific (Ottawa, ON). Conventional Mass Measurement. The definition of asphaltenes is quite loose, and numerous asphaltene extraction methods can be found in the literature.12 The differences among these methodologies are mainly due to the nature of the 5154
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152 μL (excluding membrane). The entire assembly was housed in a temperature-controlled environment. The absorption spectrum of the permeate was measured with a 2.5 mm path length flow cell (SMA-Z-2.5-uvol, 2 μL internal volume, FIALABs, Bellevue, WA) using a tungsten halogen white-light source (LS-1, Ocean Optics, Dunedin, FL) and a UV−vis spectrophotometer (HR2000+CG-UV-NIR, Ocean Optics). Fluid flow was achieved with three syringe pumps (Mitos Duo XS, Dolomite). Depending on the run, a 100 μL, 500 μL, or 5 mL syringe size was selected to maintain smooth flow and minimize the number of syringe refills. Two pressure sensors (40PC150G2A, Honeywell, Morristown, NJ) were used to ensure correct operation and to detect membrane clogging, fouling, and aging. Two rotary valves were used (Cheminert C22-3186EH, VICI-Valco, Houston, TX): one for sample loading and injecting and one for solvent control. The sample and solvents were filtered using 10 and 2 μm in-line frit cartridges, respectively. All subsystems were connected with fluorinated ethylenepropylene (FEP) tubing (250 μm inside diameter (i.d.)). The critical dead volume of the system (from sample input to optical detection) was approximately 200 μL. The system was controlled and automated via a personal computer running LabVIEW 2011 (National Instruments, Austin, TX) and a data acquisition card (USB-6009, National Instruments). Procedure. For each experimental run, a 200 μL crude-oil sample is manually introduced into the system with a syringe and stored in a sample loop. The system is flushed with toluene to purge air from the tubing, chip, filter membrane, and optical flow cell. Spectrometer calibration is performed on the toluenesaturated flow cell, measuring bright and dark reference spectra. Oil coloration is measured; oil and toluene are injected into the chip and mixed at a ratio of 1:40 or 1:80, depending on the coloration of the oil. The diluted oil sample is passed through the filtration membrane into the optical flow cell, where its spectrum is acquired. Flow conditions are maintained until a stable absorbance spectrum can be acquired (the typical run time is 5 min). The system is flushed with toluene to displace any residual oil and then primed with heptane. Spectrometer calibration is performed on the heptane-saturated flow cell. The coloration of the deasphalted oil is then measured. This step can also be termed the maltenes measurement. Oil and heptane are injected into the chip and mixed at a ratio of 1:40, which causes the asphaltenes to precipitate. The asphaltenes are then filtered out, and the diluted maltenes (i.e., deasphalted oil diluted with heptane) are passed through the flow cell, where the absorbance spectrum is acquired. A thorough cleaning cycle is then run to eliminate oil residues from the system. Small pulsed volumes of toluene are injected to liberate the precipitated asphaltenes from the filtration membrane and to clean all components of the system. The transmembrane pressure drop is monitored to determine the end of a sufficient cleaning cycle, followed by a 3 mL flush of toluene. The data of each run are then processed. The optical density for the asphaltenes is calculated using the following equation:
absorbance values at 600 nm and the subtraction of absorbance values at 800 nm as a simple characterization of the exponential decay of the absorption edge (spectra shown in Figure 2) has
Figure 2. Spectra of oil and maltenes and their difference, which is labeled as the asphaltene spectrum.
been proposed in previous studies.20,21 Crude oils as well as their subfractions are known to exhibit an exponential decay of the absorption edge throughout the visible light range.24 The absorbance at 600 nm was chosen because it has minimal interference with petroporphyrins and fits within the dynamic range of the spectrometer. At 800 nm, where hydrocarbons have less vibrational absorbance, the spectrometer had a relatively high signal-to-noise ratio. We subtracted the absorbance at 800 nm from that at 600 nm to cancel the spectral offset dependence on the refractive index of the sample. The oil absorbance Aoil expresses the absorbance of the oil sample diluted 40-fold in toluene, while the maltene absorbance Amalt denotes the absorbance of the maltene fraction diluted 40fold in heptane. If a different mixing ratio for one of the oil measurements was applied (e.g., 1:80), the measured value was adjusted to a 1:40 equivalent value (e.g., applying a factor of 81/41, or 1.976) before eq 1 was employed.
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RESULTS AND DISCUSSION To accurately correlate the absorbance value of the asphaltenes to the measured weight content, a steady-state concentration of injected oil sample is required during both the oil measurement (toluene dilution) and the deasphalted measurement (precipitation with heptane + filtration). Dispersion. Delivering a plug of sample through a solventfilled tube will cause sample and solvent to mix at the beginning and end of the plug. The extent of this dispersion process in the tubing and microflow channels can be approximated by Taylor−Aris dispersion25,26 and is typically notable at the leading and trailing edges of the sample plug. In pressure-driven microfluidic systems, the dispersion of solutes is caused by the parabolic flow profile, and this Taylor−Aris dispersion can have a significant effect on plug broadening.27 Because the asphaltene measurement technique requires a steady-state optical signal, the flow-rate selection, tubing and channel dimensions, plug length and volume, and processing time were optimized to reduce dispersion. The detected signal versus time
A a = [Aoil (λ600nm) − Aoil (λ800nm)] − [A malt (λ600nm) − A malt (λ800nm)]
(1)
where Aoil and Amalt are the absorbance values of oil and maltene at particular wavelengths, respectively. The use of the 5155
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resulting from Taylor−Aris dispersion with a boxcar injection can be calculated from the following equations: I (t ) =
⎛ Ld − ut − Lp /2 ⎞⎤ 1 ⎡ ⎛ Ld − ut + Lp /2 ⎞ ⎢erf⎜ ⎟ − erf⎜ ⎟⎥ 2 ⎢⎣ ⎝ 2 kt 2 kt ⎠ ⎝ ⎠⎥⎦ (2)
k=
d 2u 2 + Dm 192Dm
(3)
where I(t) is the normalized signal detected at time t, Ld is the length to the detector, Lp is the total plug length, u is the mean flow velocity, k is the Taylor−Aris dispersion coefficient, d is the pipe diameter, and Dm is the molecular diffusion coefficient.28,29 Estimation of a representative diffusion coefficient for a mixture of different molecules, such as crude oil, is not obvious. The first term in eq 3 for the dispersion coefficient is inversely proportional to the molecular diffusion coefficient. For typical flow rates used in microfluidic applications, the first term dominates, which in turn implies that k inversely scales with Dm. This inverse relationship implies that dispersion effects are more significant for more slowly diffusing molecule species. Therefore, a diffusion coefficient corresponding to large molecular components of the oil must be chosen to properly represent the oil sample. Typical diffusion coefficients for the large molecular fraction of crude oil (i.e., the asphaltenes) are in the range of Dm = 5 × 10−6 cm2/s, as determined by experimental studies.30 The corresponding characteristic diffusion time across the tube diameter is approximately 1 s, which is 1 order of magnitude less than the resident/transit time of the plug (several minutes). As such, Taylor dispersion is a valid approximation for the plug broadening. The dispersion length increases with increased flow rate; however, the volumetric throughput and head pressure also increase. The processing time can therefore be decreased with higher flow rates if it is technically feasible (i.e., pressure limitations) and if it is acceptable to lose a larger fraction of the solute to a longer dispersion edge (larger sample volumes). Figure 3A shows the dispersion as measured in the optimized asphaltene system for three samples that had 1, 3, and 8 wt. % asphaltene content. The 200 μL sample plug was loaded into the sample loop and then pushed by toluene through the toluene-filled system at a flow rate of 10 μL/min. On the microfluidic chip, the sample was mixed with a stream of fresh toluene at a ratio of 1:80 and passed through the filtration unit and the optical flow cell. The absorbance measurement in the flow cell directly indicates the sample concentration present. In Figure 3A, the concentration gradient in the leading (left) and trailing (right) edge due to dispersion is clearly visible. Although a small amount of sample is “wasted” on the plug edges, most of the sample (greater than 70%) remains in the plateau region while maintaining a relatively high flow rate (800 μL/min) and thus short measurement time. On a side note, the spikes in the absorbance plot in Figure 3A every 7 min are due to the automatic refilling of the syringe pumps. Dispersion is more pronounced for the higher asphaltene content oils; however, a 5 min measurement window (deemed sufficient for an optical reading) is achievable. All three plugs show sufficient plateau duration, with stable concentrations for longer than 10 min, needed for the 5 min of oil measurement and the 5 min of deasphalted oil measurement. Sample plug
Figure 3. (A) Sample plug injection and characterization of Taylor dispersion for three oils of varying asphaltene content. (B) Kinetic study using a varied flow rate and, thus, different residence times in the reactor.
dispersion was acceptable at high flow rates; however, the aggregation kinetics are also influenced by the flow conditions. Kinetics. A combination of microfluidic chips were used in this study, with either 12 or 16 mixing units and with varying reactor lengths. We did not observe any difference in optical readings when the different mixer lengths were used and conclude that a mixer with 12 units was sufficient. After an oil sample is mixed with heptane, asphaltene precipitation begins and asphaltene molecules start to flocculate. It is important that this precipitation process is given enough time before the solution passes through the membrane so that the asphaltene flocks have grown to sufficient size (i.e., >200 nm) to be screened by the membrane. This time delay, or transit time, is achieved in a reactor unit in which the solution travels along a serpentine channel before it reaches the membrane. Tests down to the shortest possible transit time (shortest reactor channel and fastest flow rate) have been conducted, and the results are shown in Figure 3B. Also shown in Figure 3B, at ∼10 000 s transit time, is a manual optical measurement. The oil sample was manually diluted with both toluene and n-heptane at the appropriate ratio and left for 2 h on the bench. The solutions were then filtered with a 0.2 μm PTFE syringe membrane, and an absorption measurement was made using a traditional cuvette system. Shorter transit times may result in asphaltenes passing through the membrane during the maltene run and, therefore, yield higher maltene absorbance and lower asphaltene 5156
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Figure 4. (A) Photographic image of a microfluidic chip in operation during a typical maltene measurement with asphaltene flocculates or aggregates visibly collecting in the filtration channel. (B) The typical measurement cycle is 300 s at a volumetric flow rate of 410 μL/min for both oil (circles) and maltene (triangles) runs. The upper portion of the graph displays the pressure as measured at the solvent syringe. During the oil dilution run, the pressure is stable and flat, as there are no particles blocking the membrane pores. However, during the maltene run (deasphalted oil), pressure rises exponentially as the membrane filters the asphaltene aggregates. The oil sample had a moderate asphaltene content (∼3.3 wt. %).
mixer and reactor sizes) and repeatedly changed filtration membranes. The relative standard deviation is very small (±2%), and all 14 data points fall within a ±3% band, which highlights the excellent repeatability of this technique. Clogging. Clogging in the mixer structure of the microfluidic chip was carefully investigated. The small cross-channels in the mixing stage have a cross-section of 50 × 125 μm2 and present the smallest cross-section in the entire system. Any large dust fiber or particle is, therefore, likely to be trapped in the mixing stages. The sample and solvent filters used (10 and 2 μm, respectively) prevented large fibers and particles from being trapped; however, clogging by asphaltene flocks in a clean mixer channel was visually observed for several oil samples. To investigate clogging of the mixer channels, we used an extralong sample loop (2.5 mL) and filled the loop with an oil sample that had a moderate asphaltene content (3.3 wt. %). The device was run without a filter membrane until the entire plug was injected (mixed with heptane 1:40, 10 μL/min:400 μL/min) or until the device reached the equipment pressure limit of approximately 8 bar. The results are plotted in Figure 5B. The pressure signal showed a slowly increasing pressure buildup, with large fluctuations due to asphaltene aggregates becoming trapped in the micromixer and then dislodged. A maximum pressure was reached after approximately 800 μL of oil sample was injected. The clogging primarily occurred in the small cross-channels of the micromixers, as expected, visually observed in Figure 5C. The same settings were kept and the membrane was inserted to observe the pressure trend with the filtration membrane as shown in Figure 5B. As expected, the retention of asphaltene flocks by the porous membrane is the dominant mechanism of pressure rise in a typical run. After injection of 170 μL of oil sample, the maximum pressure was reached. We therefore conclude that although the micromixer channel structures are eventually restricted (after 800 μL of sample) by precipitated asphaltenes, the typical 50 μL injected sample size is 1 order of magnitude smaller, and such blockages do not have any detrimental effect on a measurement cycle.
absorbance. An extremely short transit time in the channel or slow aggregation kinetics could introduce a small error in the microfluidic measurement. However, here we show that such an effect is insignificant. Furthermore, we correlate our measurements with those from conventional wet chemistry measurements, which indirectly accounts for such minor errors. Figure 3B does not show a strong correlation between the aggregation time and asphaltene yield for the sample investigated here. This observation is supported by the literature, which suggests that the aggregate growth (from molecules to nanoaggregates to clusters to flocks) is on the order of seconds for high mixing ratios (>1:5).31,32 Measurement Cycle. Figure 4 shows a typical measurement cycle for a moderate asphaltene content (∼3.3 wt. %) oil sample. The oil sample was injected at 10 μL/min, and the solvents were injected at 400 or 800 μL/min. The oil sample plug was divided into three regions: a preinjected section (30 μL), an oil-run section (50 μL), and a maltene-run section (50 μL), and the remaining 70 μL was not used. The two measurement steps were overlaid (oil and maltene runs), with each run taking 300 s. The upper portion of Figure 4B displays the pressure as measured at the solvent syringe. During the oildilution run (circles), the pressure is stable and flat because particles are not notably blocking the membrane pores. However, during the maltene run (triangles), pressure rises exponentially as the membrane filters the asphaltene aggregates. The asphaltene absorbance (crosshairs) is calculated as the difference between the oil and maltene absorbance values. The average value between 50 and 250 s was taken as the asphaltene optical density, Aa. A video of the asphaltene precipitation during the maltenes portion of the experiment is provided in the Supporting Information. Repeatability. Repeatability was evaluated by measuring the same oil sample over several months. Figure 5A shows the asphaltene absorbance (AU) for each of the 14 measurements made on a sample with an asphaltene content of approximately ∼1 wt. %. The measurements were conducted over a period of 3 months with microfluidic chips of various designs (different 5157
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Correlation. A set of 38 crude-oil samples from locations around the world have been collected and analyzed for their asphaltene contents. Figure 6A shows the optical density, as
Figure 6. (A) Correlation of 38 oil samples. (B) Relative errors.
determined by the microfluidic system, versus the asphaltene content (wt. %), as determined by classical wet-chemistry methodology according to the modified IP143 method with Soxhlet washing. A linear-model fit and the corresponding 90% prediction band are also shown in Figure 6A. On the basis of the least-squares fit of the data presented in Figure 6A, the following relation between the measured optical density and the asphaltene weight content can be established:
Figure 5. (A) Long-term sample measurement repeatability for oil with ∼1 wt. % asphaltene content. (B) Evaluation of chip clogging: pressure versus time or volume of 3.3 wt. % oil sample injected. (C) Photograph of micromixer clogging after injection of 800 μL of oil sample. A typical run injects 50 μL of oil sample.
Ma = 5.46A a
(4)
where Ma and Aa are the asphaltene content (wt. %) and the asphaltene absorbance (AU), respectively. For each measured absorbance value, eq 4 provides the corresponding asphaltene content (wt. %), which is compared to the wet-chemistry reference measurement. The relative error for each sample is plotted in Figure 6B. The dotted line shown in Figure 6B represents the commonly assumed errors for wet-chemistry measurements (i.e., ±50% error for asphaltene content smaller than 1 wt. %, ±20% error for asphaltene content between 1 and 3 wt. %, and ±10% error for asphaltene content greater than 3 wt. %). Equation 4 provides the calibration curve for the microfluidic apparatus, which allows relating the measured asphaltene absorbance of any crude oil sample to its asphaltene content (wt. %) according to the modified IP143 method. It should be mentioned that the optical microfluidic method could be
The effectiveness of the membrane cleaning procedure (flushing with toluene) can also be monitored as the pressure drop across the membrane. As cleaning proceeds and asphaltenes are redissolved, the pressure across the membrane returns to the reference value determined on the clean membrane. If the membrane does not return to its initial performance values due to fouling or trapping of solid particles, the condition is instantly detected and a membrane change is triggered. Cleaning with toluene was effective at removing flocculates for the 38 oils analyzed in the study. However, after several weeks of extensive use, light fouling of the glass microfluidic chip was observed as channels appeared slightly brown, which was due to adsorption of polar compounds such as asphaltenes onto the glass surface. This layer could be removed with a stronger solvent, or the glass device could be replaced with a new one. 5158
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Notes
calibrated to many other standard methods. Due to the inherent uncertainty in the asphaltene content determined by the conventional wet-chemistry-based measurement technique, a large number of samples have been used to tighten the fit and to provide a solid and reliable relationship between optical measurement results and asphaltene weight content. The ideal correlation of optical absorbance to asphaltene content would require most crude oils to have a similar “average molar extinction coefficient”. Since asphaltenes are a solubility class of molecule, there are likely hundreds to thousands of compositions and structures. Furthermore, each crude oil may have a different distribution of these molecules, which collectively give rise to a weighted molar extinction coefficient. The data reported in Figure 6 suggest that the band of molar extinction coefficients across ∼40 oils is rather narrow as there is minimal scatter about the linear fit. At present, it is difficult to discern whether the slight deviations are due to errors in wetchemistry measurements or if they arise from fundamental absorbance theory. However, there is uncertainty from the measurements performed with classical wet chemistry. Classical wet-chemistry measurements determine asphaltene content by weighting the mass of the separated asphaltenes, which is prone to larger errors when the amount of separated asphaltenes is small. This results in poor repeatability at low asphaltene contents. Repeatability of the optical absorbance measurements, particularly at low absorbance or low asphaltene contents, has been excellent and is a notable strength of the microfluidic measurement apparatus.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge Dr. John Ratulowski and Dr. Oliver Mullins for fruitful discussions. We thank Dr. Artur Stankiewicz from Schlumberger and Nancy Utech and Karthik Ramanathan from Shell for providing crude samples. We also thank Joe Baddeley, Andrew Lovatt, Richard Gray, and Phil Homewood from Dolomite for their contributions.
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CONCLUSIONS The described microfluidic apparatus for asphaltene content measurement of crude oils has proven to yield asphaltene data in excellent agreement with conventional wet-chemistry measurements, but with vastly improved performance factors. The measurement time was reduced from several days to only several minutes for an asphaltene content measurement. Such quick processing time is particularly promising for oil field applications because it provides, for the first time, the possibility to analyze the oil sample multiple times and at various points during recovery, transport, and downstream processing. Combined with the excellent measurement repeatability of the device, the microfluidic apparatus therefore offers quality monitoring to the industry. Another major advantage of the microfluidic approach is that required sample and solvent volumes are significantly lower (orders of magnitude lower) as compared to those in conventional wet-chemistry analysis. This capability to analyze very small sample quantities is of particular interest to the upstream oil industry, where crude oil samples from exploratory oil wells are extremely precious and every milliliter of sample is recovered at tremendous cost. We anticipate that our novel microfluidic apparatus for asphaltene content measurement of crude oils will have a significant impact on oil sampling in terms of versatility, reliability, and cost.
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ASSOCIATED CONTENT
S Supporting Information *
Sample video of an experiment. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Phone: (780) 577-1315. E-mail: FMostowfi@slb.com. 5159
dx.doi.org/10.1021/ac400495x | Anal. Chem. 2013, 85, 5153−5160
Analytical Chemistry
Article
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