Wax Precipitation Temperature Measurements Revisited: The Role of

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Wax Precipitation Temperature Measurements Revisited: the Role of the Degree of Sample Confinement Felipe L. Paiva, Flávio H. Marchesini, Verônica M. A. Calado, and Andre P. Galliez Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00812 • Publication Date (Web): 17 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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Wax Precipitation Temperature Measurements Revisited: the Role of the Degree of Sample Confinement Felipe L. Paiva,∗,† Flávio H. Marchesini,‡ Verônica M. A. Calado,† and André P. Galliez† †Escola de Qu´ımica, Universidade Federal do Rio de Janeiro, Rua Hor´ acio Macedo 2030, Cidade Universit´ aria, Rio de Janeiro, RJ 21941-909, Brazil ‡Department of Mechanical Engineering, Pontif´ıcia Universidade Cat´ olica do Rio de Janeiro, Rua Marquês de São Vicente 225, G´ avea, Rio de Janeiro, RJ 22453-900, Brazil E-mail: [email protected]

Abstract In this work, the Wax Precipitation Temperature (WPT) measurements of crude oils are revisited. The conventional methods used to assess the WPT of crude oils are analyzed and the results available are discussed in detail. In addition, three methods, namely polarized light microscopy, micro Differential Scanning Calorimetry (µDSC), and rheometry are employed to evaluate the WPT of two Brazilian waxy crude oils. It is shown that polarized light microscopy and µDSC are the most suitable methods to assess the WPT of crude oils and that rheometry can only be used for that purpose under very specific circumstances. In particular, the results indicate that rheometry is able to detect the WPT of crude oils only by decreasing the geometry gap to a small enough value. Moreover, it is shown that the degree of sample confinement

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has a key role on the accuracy of WPT measurements, so that care must be taken when using laboratory data to design field operations. Discussions on the origin of the effects of confinement and on how wax crystal dimensions vary with the initial cooling temperature are also provided.

Introduction It has long been known that wax crystallization during crude oil production and transportation can cause flow assurance issues that give rise to non-productive times and increase in costs. 1–10 To define strategies to circumvent these issues it is fundamental to determine the temperature at which the wax crystallization process starts in crude oils. This temperature is usually known as the Wax Precipitation Temperature (WPT) and can be defined as the maximum temperature at which the first wax crystals precipitate out of solution in view of a decrease in solubility limits of wax compounds as the oil temperature is decreased. To evaluate the Wax Precipitation Temperature of crude oils, different methods have been employed, including polarized light microscopy, Differential Scanning Calorimetry (DSC) or microcalorimetry (µDSC), viscometry or rheometry, standard methods ASTM-D2500 or ASTM-D3117, Fourier transform infrared spectroscopy (FTIR), near-infrared spectroscopy (NIR), and others. 11–23 Among these methods, polarized light microscopy, DSC, and rheometry are perhaps the most popular and have been investigated by many authors. 11–19,21–23 Rønningsen et al. 11 measured the Wax Precipitation Temperature of 17 North Sea waxy oils with different methods, including polarized light microscopy, DSC, and rheometry. By comparing the results, they found that polarized light microscopy is the method that, in general, provides the most conservative WPT measurements and that the results depend on the cooling rate and oil sample thickness. Erickson et al. 12 also investigated the different methods used to evaluate the WPT of crude oils. They pointed out that all methods require some finite mass of precipitated crystals for being able to detect the WPT and recommended polarized light microscopy as 2


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the most accurate method to obtain WPT values for field applications. Kök et al. 13 performed WPT measurements of 15 waxy oils. They recommended that both polarized light microscopy and DSC are used together to better determine the WPT of crude oils, as DSC can provide higher WPT values depending on oil composition. Besides that, the authors suggested that rheometry should be used to study the flow properties below the WPT, even though they obtained good agreement between the different methods for oils having a significant precipitation rate. Hammami et al. 17 further investigated the different methods to evaluate the WPT of crude oils and pointed out that, at the same rate of temperature change, the amount of superheating is considerably lower than the amount of supercooling, as reported by Gimzewski and Audley 24 . Therefore, they 17 suggested that the so-called WPT should be evaluated during the heating of a cold sample instead of during the cooling of a hot sample. And, in this case, the characteristic temperature of the phase transition should be termed as the Wax Dissolution Temperature (WDT) instead of WPT. Tiwary and Mehrotra 18 studied the liquid-solid phase transition of model waxy oils and come up with similar conclusions. Coutinho and Daridon 19 performed an in-depth analysis of several methods used to evaluate the WPT of crude oils, emphasizing the limitations of each method. They pointed out that, in general, the methods rely on imposing a cooling or heating rate to the sample, which can introduce errors to the measurements due to a deviation from thermal equilibrium that can lead to supercooling or superheating effects, as discussed by Hammami et al. 17 . Moreover, the authors highlighted that a detailed compositional analysis can provide insight on the accuracy of WPT measurements, as discussed by Erickson et al. 12 , and that care must be taken during sampling, sample handling, and measuring to not lose any heavy wax fraction, as pointed out by Monger-McClure et al. 15 . After these works from Rønningsen et al. 11 , Erickson et al. 12 , Kök et al. 13 , Hammami et al. 17 , and Coutinho and Daridon 19 , the literature has achieved a quite mature stage in which some conclusions can be drawn. In summary, it was established that for waxy oils,

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such as refined oils, that have a narrow range of paraffin distribution and consequently a large precipitation rate, the conventional methods usually agree on the WPT measurements and no significant issues are expected to obtain accurate WPT values. 13,19 However, for waxy oils having a wide range of paraffin distribution and a low precipitation rate, such as most crude oils, a number of precautions must be taken to obtain accurate WPT measurements. 19 It was shown that the measurements must be performed as closely as possible to the thermal equilibrium condition to avoid supercooling or superheating effects. 19 Moreover, it was found that polarized light microscopy is the method that usually provides the most conservative WPT measurements, 11,12,25 but, to obtain more reliable measurements, DSC or µDSC should be used in addition to microscopy, 13 and the results compared to WPT predictions from compositional data. 12,19 Very recently, Japper-Jaafar et al. 23 have reevaluated the effectiveness of polarized light microscopy, µDSC, and rheometry in detecting the WPT of crude oils accurately. They emphasized that even small differences in WPT measurements can lead to conflicting decisions and concluded that, in contrast to the previous works, rheometry and µDSC are the most effective methods to detect the WPT of crude oils. The authors suggested that the different findings and conclusions could be due to improvements in the sensitivity of the instruments along the last two decades. However, this apparent disagreement between the work from Japper-Jaafar et al. 23 and the previous literature 11–13,17,19 deserves more attention. There is still a lack of understanding of the real physical meaning of some rheometric measurements performed with waxy crude oils, which are usually interpreted as WPT measurements. There is a need to determine the conditions in which rheometric measurements can be used to assess the WPT of crude oils. Moreover, as far as the present authors know, there is no comprehensive discussion in the literature about the effects of confinement on WPT measurements of crude oils. Therefore, more investigation is required. Thus, in this work, the Wax Precipitation Temperature measurements of crude oils are

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revisited. We discuss the nomenclature and the different definitions found in literature, as well as analyze the conventional methods used to assess the WPT of crude oils. We, then, present the materials and methods used in this research and discuss the results obtained, pointing out how to obtain accurate WPT measurements to properly design field operations. At last, we show some concluding remarks, summarizing the main findings of this work.

Methods to evaluate the Wax Precipitation Temperature Nomenclature and definitions In the literature, the terms Wax Appearance Temperature (WAT) and Cloud Point (CP) are used interchangeably to refer to the Wax Precipitation Temperature as defined above. 11–15,17,19,26–29 However, there are some nuances in the meaning of each term that deserve attention. Besides being used to describe the maximum temperature at which the first wax crystals precipitate out of solution, the term WAT is also used to designate the temperature at which the first wax crystals appear in a waxy oil sample during a given experiment. However, depending on oil composition, applied cooling rate, effectiveness of the method, and sensitivity of the instrument used to detect wax precipitation, the first wax crystals may appear in the oil sample at a temperature below the maximum possible temperature in view of supercooling effects and/or imprecision in the measurements. In this connection, the term Cloud Point also has a different connotation than the one that seems more appropriate to describe the physical quantity related to the precipitation of wax crystals, as the term Cloud Point is sometimes used exclusively to refer to the WPT measurements performed according to standard methods, e.g. ASTM-D2500 or ASTM-D3117. Therefore, we prefer to use the term Wax Precipitation Temperature to refer to this physical quantity, which, in principle, could be predicted by a thermodynamic model from compositional data.

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There are also other temperature-related parameters called Wax Dissolution Temperature (WDT) or Wax Disappearance Temperature, Pour Point (PP), and gelation temperature (Tgel ) that deserve attention. 30 The term Wax Dissolution Temperature is used to designate the minimum temperature at which the last wax crystals are dissolved during heating of a waxy oil sample, while the term Pour Point is used to designate the lowest temperature at which the waxy oil still flows under defined conditions, usually described in standard methods, such as ASTM-D97 and ASTM-D5853. It is important noting that, according to these definitions, the WDT is a physical quantity that should be equivalent to the WPT, as both characteristic temperatures are defined in the limit where the rate of temperature change is zero, while the Pour Point is not a physical quantity of a given waxy oil, but rather a parameter used for a qualitative comparison of the “pumpability” of different oils. With regard to the gelation temperature (Tgel ), the vast majority of works in the flow assurance literature define the Tgel of a given waxy oil as: the temperature below which the storage modulus (G′ ) becomes higher than the loss modulus (G′′ ) in an oscillatory cooling ramp carried out in a rheometer, in which a constant strain amplitude and a constant frequency are applied to the sample. However, the concept of gelation temperature is associated with the gelation phenomenon, which, as discussed by Cawkwell and Charles 31 , consists on a change in rheological behavior of waxy oils from a Newtonian to a complex non-Newtonian behavior. As the onset of this change in the true rheological properties of the oil is the temperature below which the effects of the interaction between wax crystals become important, we define this temperature as the gelation temperature. It is important noting that rheometry is one of the methods used to evaluate the WPT of waxy oils and, according to the literature, 11,13,14,28,32–34 the characteristic temperature below which the waxy oil presents a non-Arrhenius viscosity-temperature dependence is defined as the WPT. However, as it is shown below, the WPT of crude oils can only be evaluated by rheometry under specific test conditions, which in turn are likely not appropriate to assess the non-Newtonian rheological properties of gelled waxy oils.

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Polarized Light Microscopy In this method, two polarizing filters, rotated 90° with respect to each other, are positioned in the light beam path of a microscope in a way that the observed sample is placed between the two filters. The oil sample is then cooled from a high enough initial temperature and when the first crystals achieve around 0.5 µm to 1 µm, bright spots appear in a dark background, indicating that the wax crystallization process has started. This temperature, at which the first wax crystals are seen in the microscope, is a measurement of the WPT. This method is based on the fact that the anisotropic wax crystals that appear in the crude oil sample rotate the plane of polarization of transmitted light, allowing for the crystals to be visible. 11,19 Polarized light microscopy has been consistently reported to be the most effective method to detect the start of wax crystallization. 11,12,15 Besides that, polarized light microscopy can provide additional valuable information on the microstructure and dimension of waxy components in their aggregated/crystallized form, by analyzing crystal fractal dimension, maximum length, and/or size distribution. 35–40 Thus, polarized light microscopy is considered one of the best methods to evaluate the WPT of crude oils.

Differential Scanning Calorimetry This method consists on cooling a waxy oil sample from a high enough initial temperature and detecting an exothermic crystallization peak in a thermogram, whose onset corresponds to a measurement of the Wax Precipitation Temperature (WPT) of the oil. This measurement relies on the fact that the phase transition of wax compounds from liquid to solid is an exothermic process in which heat is released from the sample. When compared to other methods, Differential Scanning Calorimetry can be considered a relatively easy and fast way of obtaining repeatable measurements. Thus, it became a quite popular method to evaluate the WPT of waxy oils. However, with regard to the disadvantages of DSC, it is possible to mention difficulties in obtaining a good baseline and in detecting the deviation from the baseline, 19 especially for some waxy crude oils, whose 7


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thermograms present very broad and/or modest crystallization peaks that may span over 100 ◦C in view of the complex composition of these fluids. 13,22,32,41–45 Besides that, it is important mentioning the fact that conventional DSC instruments require relatively high cooling rates to provide thermograms without noise, as the signal is the time derivative of the enthalpy change. 11 This can lead to supercooling effects on the measurements due to a departure from the thermal equilibrium condition. 19 To circumvent this issue and avoid significant supercooling effects, modern microcalorimeters can be used instead of conventional DSC instruments. 24,46,47 These instruments possess higher resolution and have been associated with better calorimetric sensitivity than conventional DSC equipment, allowing for the possibility of using lower cooling rates without compromising sensitivity and/or a greater sample mass. 24,48

Rheometry Detection of the start of wax crystallization by rheometry consists on identifying the temperature at which a minimum amount of crystals precipitate out of solution in a way that the interaction between crystals becomes important enough to affect the rheological properties being measured. It is worth noting that the transformation that is detected by the rheometer does not necessarily pertain to the onset of wax crystallization and correspond to the WPT, but rather pertains to the onset of formation of a gel network of wax crystals. So, in this method, a waxy oil sample is placed in a chosen rheometer geometry and, after achieving a stable high enough initial temperature, the sample is cooled down until a significant change in the rheological property being measured can be observed. For considering the characteristic temperature of this change in rheological properties as either a measurement of WPT or a measurement of Tgel , different conditions are required. To be considered a measurement of Tgel , a rheometer geometry with a large enough gap must be used to ensure that the measurements are gap-independent and represent true rheological properties of the oil. It is worth mentioning that Marchesini et al. 49 did not verify a change 8


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in Tgel values whether a shear rate of 2 s−1 , 20 s−1 or 200 s−1 had been used, which may indicate that Tgel measurements of crude oils do not vary with the applied shear rate. However, for considering that characteristic temperature as a measurement of WPT, very small geometry gaps must be used to ensure that a change in the rheological properties being measured can be detected at the highest possible temperature. It is important noting that when very small geometry gaps are used in rheometric measurements, the presence of crystals of the same order of magnitude of the gaps may cause violation of the continuum hypothesis employed in the rheometer theory used to calculate the rheological properties. And, in this case, only the measurements at temperatures above the detected WPT represent true rheological properties of the waxy oil at hand. Therefore, care must be taken when selecting the rheometer geometry to avoid measurement imprecision. 49 The fact that the rheometry method relies on a change in the rheological properties being measured, that, in turn, requires a minimum amount of precipitated crystals, constitutes an intrinsic limitation of this method with regard to the measurement of the start of wax crystallization. Even though this limitation should not be a problem for waxy oils presenting a large precipitation rate, for waxy oils having a small precipitation rate, this limitation can give rise to significant errors in WPT measurements. This may be the main reason why rheometry is reported to underestimate the WPT of waxy oils in many cases.

Effects of the rate of temperature change and sample confinement In view of the definitions and discussions above, it is possible to establish the conditions required to obtain more accurate measurements for the liquid-solid phase transition temperature of waxy crude oils, herein called WPT. As discussed by Coutinho and Daridon 19 , the methods used to evaluate the phase transition temperature of waxy oils usually rely on imposing a cooling or heating rate to the samples, which may introduce errors to the measurements due to supercooling or superheating effects. Different authors investigated the effects of the cooling rate on WPT measurements and observed that the values obtained in9


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crease by decreasing the cooling rate. 11,45,50–53 So, in general, the methods based on imposing a cooling rate can underestimate the WPT value by a more or less significant amount. Hammami et al. 17 emphasized that the wax crystallization process occurs in two stages, namely nucleation and crystals growth, and that the undercooled wax molecules require the existence of nucleation sites to start the formation of wax crystals, otherwise precipitation occurs at significantly lower temperatures. Therefore, and based on previous observations that the amount of superheating is considerably lower than the amount of supercooling for the same rate of temperature change, 24 Hammami et al. 17 as well as Tiwary and Mehrotra 18 suggested that the phase transition temperature should be better evaluated during the heating of a cold sample containing suspended wax crystals. However, it would be possible that a different conclusion arise with regard to the comparison between the amounts of superheating and supercooling if measurements with different degrees of sample confinement were performed. Besides that, in some cases, too conservative values can be obtained for the phase transition temperature of crude oils by applying a heating rate to the samples in view of non-negligible superheating effects. As explained by Hammami et al. 17 , these effects arise when the wax crystals are heated at a rate higher than the dissolution or melting rate. Regardless of whether a cooling or heating rate is applied to the samples, it is expected that the same value for the phase transition temperature is obtained in the limit where the rate of temperature change is zero, provided that a large enough number of nucleation sites exists in the samples submitted to the cooling process. Therefore, it is possible to conclude that to obtain more accurate measurements for the phase transition temperature of crude oils, the cooling or heating rates applied should be as small as the detection method allows. Moreover, it is possible to note that, when applying a cooling rate to the samples, the number of nucleation sites provided has a key role on the accuracy of the WPT measurements. In that regard it is important to point out that the surfaces in contact with the sample constitute a large number of nucleation sites deriving from their intrinsically microscopic roughness. So, it is expected that by decreasing the ratio between the sample volume and

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the wet area the number of nucleation sites provided increases, causing an increase in the WPT measurements up to a critical value. In fact, Rønningsen et al. 11 reported that WPT measurements with polarized light microscopy depend on oil sample thickness and that the values obtained increase by decreasing the thickness. In this connection, by performing measurements in a rheometer, Marchesini et al. 49 observed that, by decreasing the geometry gap, the characteristic temperature that marks the limit of validity of the Arrhenius viscositytemperature relationship also increases. These evidences indicate that, when submitting the oil samples to a cooling rate, to obtain more accurate values for the WPT of crude oils, the measurements should be performed in high degrees of confinement to provide a large enough number of nucleation sites to the sample to allow for wax crystallization to occur at the highest possible temperature. In the case where a heating rate is applied to the samples, it is also expected that high degrees of confinement would help to obtain more accurate values in view of more efficient heat transfer within the sample, which would avoid superheating effects.

Materials and methods Two Brazilian waxy crude oils were used in the research described in this paper. These oils are referred to as crude oil A1 and crude oil A2 throughout the text. Different experiments were performed to shed light on the composition of each crude oil. In addition, polarized light microscopy, µDSC, and rheometry are employed to evaluate the WPT and/or Tgel of both crude oils. All methods used to characterize both crude oils and to determine their WPT and Tgel are described below.

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Crude Oil Characterization API gravity analyses American Petroleum Institute (API) gravity analyses were made for both waxy crude oils A1 and A2 according to ASTM D4052. Gas Chromatography/Flame Ionization Detector (GC/FID) The waxy crude oils were analyzed by GC/FID in a Shimadzu GC-QP2010 Ultra system coupled to an RTX-1MS column. The temperature program was: 100 ◦C for 5 min; 10 ◦C/ min to 300 ◦C and hold for 15 min; followed by 10 ◦C/ min to 310 ◦C and hold for 30 min. The injector and detector temperatures were set at 290 ◦C and 320 ◦C, respectively. Moreover, xylene was used as a solvent, and the injection volume was 1 µL, while helium was employed as a carrier gas. Additionally, the linearity standard used was ASTM D5442 C12 - C60 . Fourier-Transform Infrared Spectroscopy (FTIR) Spectra of crude oil samples were recorded on a PerkinElmer Frontier FTIR/FIR spectrophotometer with a resolution of 4 cm−1 , as a result of 20 accumulated scans. Before being placed in the instrument, oil samples were painted on a KBr plate. Elemental Analysis (CHNS) Elemental analysis was performed on a Thermo Scientific Flash 2000 CHNS Organic Elemental Analyzer, onto which the samples were loaded without any kind of previous treatment, and results correspond to the average of three individual determinations. Nuclear Magnetic Resonance (NMR) and Estimation of Average Structural Parameters (ASP) The 1 H-NMR spectrum was obtained on a Varian Inova-300 spectrometer at 500 MHz, operating at 303.2 K. 128 scans were accumulated, with a line broadening of 0.3 Hz and acquisi12


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tion time of 3.27 s.

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C-NMR analysis was carried out on a Bruker Advanced NMR equipment

at 75 MHz with 5000 repetitions in the inverted gated decoupling mode at 300.1 K, with an acquisition time of 0.86 s and line broadening of 5 Hz. ASP calculation and chemical shift regions and assignments were based on the works of Hasan, Ali, and Bukhari. 54,55 The C/H ratio obtained from

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WPT and Tgel determination During sample preparation for polarized light microscopy and rheometry, the two Brazilian waxy crude oil samples were (i) shaken in their respective bottles before sampling to promote homogenization; (ii) taken to the corresponding Ti at approximately 20 ◦C/ min; (iii) cooled at 1 ◦C/ min to a final cooling temperature of 4 ◦C; (iv) tested for each experimental condition at least three times; (v) submitted to two different Ti : one that is more representative of the operational thermal conditions that both crude oils are submitted to (50 ◦C), and another intended for complete wax dissolution (80 ◦C). When Ti is equal to 50 ◦C, crude oils A1 and A2 were kept at Ti for 30 min before the beginning of each test, while for Ti equal to 80 ◦C, a time period of 15 min was used instead. The choice of utilizing a lower initial temperature is also justified by the fact that higher, more conservative values of flow properties have been obtained in this temperature range in the past, which can be of interest in terms of preventing flow assurance problems. 49,56–58 Because wax crystals could still be present after both samples were kept at 50 ◦C for 30 min, only experiments with Ti = 80 ◦C were taken into account for WPT determination through polarized light microscopy. Also, µDSC tests from 50 ◦C were not carried out because the goal was to ascertain how effective this technique is with respect to the detection of the very start of wax crystallization in crude oils A1 and A2.

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Polarized light microscopy Samples were viewed under reflected light with 20× objective lens on a Carl Zeiss AxioImager.A2m microscope equipped with a AxioCam MRc5 digital camera and a Linkam T95-PE system to control the temperature. Sample preparation for polarized light microscopy consisted of: (i) placing a drop of oil onto a microscope slide, (ii) putting another identical glass slide on top of the oil sample to spread it out, and (iii) pulling the upper-slide to the side to leave a thin film of crude oil open to the atmosphere on the lower glass slide. The cooling protocol consisted of heating samples to a temperature of 80 ◦C on the microscope stage and keeping them at this temperature for 15 min to allow for dissolution of wax crystals. Then, samples were cooled to 4 ◦C at 1 ◦C/ min. In addition to the three runs on fresh samples for WPT determination, for studies of crystal dimensions, micrographs were taken at 4 ◦C for crude oils A1 and A2; at 28.9 ◦C for crude oil A2; and also between 22.3 ◦C to 23.6 ◦C for crude oil A1. These specific temperatures between 22.3 ◦C to 28.9 ◦C correspond to the onset of ubiquitous, broad crystallization peaks detected on both crude oils by conventional DSC experiments, as determined in another work, 59 and are referred to as TDSC of each oil. AxioVision version 4.8 image acquisition and processing software was used for processing of micrographs. More specifically, an Automatic Measurement Program was used to measure parameters of wax crystal dimensions, namely Feret maximum (Fm ) and area percent (Ap ). These parameters serve as estimations of wax crystal length and area, respectively. As a cut-off criterion for calculations, we did not consider detected regions with a Fm value smaller than 1 µm. This seems to be a reasonable assumption as far as the resolution of polarized light microscopy is concerned. 11,12 µDSC Microcalorimetric experiments were done on a SETARAM EVO VII model. Sample masses of waxy crude oils A1 and A2 for this type of analysis were from around 200 mg to 300 mg. For better scanning equilibration, a quantity of undecane was placed in the reference cell 14


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because it is inert in the temperature range of interest and its specific heat is similar to that of the sample. 46 Samples were heated to a temperature of 80 ◦C at 1 ◦C/ min and kept at this temperature for 15 min to allow for dissolution of wax crystals. Then, the samples were cooled to −10 ◦C at 1 ◦C/ min and kept at −10 ◦C for 2 min. Microcalorimetry has been reported to be more sensitive for detecting the onset of wax crystallization than conventional calorimetry. 24,47 Thus, by detecting crystallization peaks that would otherwise not be revealed, more accurate calculations of the total wax content in crude oils A1 and A2 can be made. This, in turn, can be done by using an average value of 200 J g−1 for the enthalpy of paraffin crystallization (∆Hwax ). 44 Rheometry Rheometric experiments were conducted on a Discovery HR-3 Rheometer from TA Instruments fitted with a 60 mm-diameter smooth parallel plates geometry. Temperature control was achieved with a Peltier device. After zeroing the gap, crude oil samples were placed on the Peltier plate and the top plate was set to the appropriate gap in a way that the space between the plates is properly filled up. The experiments consist on thermal-cycle tests in which a constant cooling rate and a constant shear rate are applied to the waxy crude oil sample during cooling from an initial temperature Ti to 4 ◦C. Then, when 4 ◦C is reached, the sample undergoes heating at the same rate back up to the initial temperature Ti under the same shear rate. Viscosity values are then plotted as a function of temperature. The cooling/heating rate applied was 1 ◦C/ min while the shear rate was 20 s−1 . It is important noting that inhomogeneity was not found to impact the results presented. More detailed information about this kind of test and the assumptions used can be found elsewhere. 49,60 Various gap sizes were used for each crude oil with the parallel plates geometry to ascertain gap-independent rheological behavior. The gelation temperature Tgel was measured using onset-point determination on the TRIOS software from TA Instruments when large enough gaps were used. To evaluate whether each gap was large enough for obtaining gap-

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independent results, Fisher’s Least Significant Difference (LSD) test was made using the software Statistica from StatSoft. Based on a 95% level of significance, pairs of comparisons between any two gaps that yielded statistically different values for Tgel were rendered not sufficiently large for obtaining gap-independent results. To calculate the activation energy (Ea ) values of the cooling portions of thermal-cycle tests, fits to the Arrhenius equation have been made from Ti to Tgel using the Rheometers TRIOS software from TA Instruments. This has been done individually for each of the three tests performed at each gap for both crude oils and coefficients of determination for the linear fits were all above 0.998245.

Results and discussion In this section, we first present the results of the tests performed to shed light into the compositions of crude oils A1 and A2. Then, we present the results from polarized light microscopy, µDSC, and rheometry pertaining to their effectiveness in detecting the very start of wax crystallization. We also evaluate, for polarized light microscopy and rheometry, the effects on wax crystals dimension and rheological behavior of heating the samples of both oils to a lower initial temperature Ti of 50 ◦C. This temperature, in spite of not being high enough for complete wax dissolution, better reproduces oilfield conditions that both crude oils are submitted to. Besides that, we discuss the role of sample confinement in obtaining accurate WPT measurements and provide an explanation of the origin of these confinement effects. All error bars in the graphics from this Section are based on standard error calculations. If they cannot be seen for certain data points, this is due to the error bar being too small compared to the symbol used to indicate the data point.

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Energy & Fuels

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Energy & Fuels

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of analysis does not provide information about microcrystalline wax—for example, branchedchain and cyclic alkanes—or other crystallizable components. Therefore, up to this point of the present work, no conclusions pertaining to the total wax content may be drawn yet. FTIR FTIR spectra are presented in Figure 2. It is clear that the spectra from both crude oils are very similar and typical of compounds having long n-alkyl chains, given that all characteristic bending and stretching modes of normal alkanes are present. 61 These absorb mainly at 2953 cm−1 ; 2923 cm−1 ; 2853 cm−1 ; 1462 cm−1 ; 1377 cm−1 ; and 721 cm−1 , the latter one being specifically associated with long chain n-alkanes. 61 The other absorption bands may be related with a low content of aromatic compounds.

Figure 2. FTIR spectra for crude oils A1 and A2.

CHNS Results for elemental analysis are displayed in Table 1 and they consistently confirm the light, paraffinic character of the oil samples given their low

C H

ratio. Furthermore, elemental

weight percentages are individually in very close agreement between waxy crude oils A1 18


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Energy & Fuels

and A2. The very low nitrogen and sulphur contents resonate with the absence of carbonyl absorption bands at ≈ 1710 cm−1 in FTIR spectra, and indicate a low content of acidic substances in crude oils A1 and A2. 62,63 Table 1. Elemental analysis results. Waxy crude oil A1 A2

Elemental weight percentage (%) C

H

N

S

86.1 85.5

12.3 12.3

0.5 0.5

Wax Precipitation Temperature Measurements Revisited: The Role of

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