Asphaltene Stability Prediction Based on Dead Oil Properties:Experimental EvaluationDoris L. Gonzalez,*,† Francisco M. Vargas,∥ Elham Mahmoodaghdam,† Frank Lim,‡ and Nikhil Joshi§
†Schlumberger Reservoir Sampling and Analysis, 16115 Park Row, Suite 150, Houston, Texas 77084, United States∥The Petroleum Institute, Post Office Box 2533, Abu Dhabi, United Arab Emirates‡Anadarko Petroleum Corporation, 1201 Lake Robbins Drive, The Woodlands, Texas 77380, United States§Moulinex Business Services LLC, 157 Silverwood Ranch Drive, Shenandoah, Texas 77384, United States
ABSTRACT: The asphaltene content effect on crude oil properties was investigated for a series of deepwater Gulf of Mexico(GOM) fluids with asphaltene contents varying from 4 to 15 wt %. The objective of the study was to conduct flow assurancescreening tests on GOM samples collected from different sands and determine properties of the dead oil and the asphaltenestability. Densities, refractive indices, and viscosities were measured at different temperatures in dead oils with three differentasphaltene contents. The properties showed defined tendencies with the asphaltene content and with the temperature. Theapplication of the one-third rule in the calculation of properties, such as solubility parameter and viscosity of dead oil systems,was evaluated. This approach also provides an alternative to calculate the refractive index based on densities obtained from anequation of state. The analysis also shows the important role that the asphaltene content plays in determining the viscosity ofcrude oil and evaluates the possibility of predicting viscosity from refractive index, as proposed by Vargas et al. Another importantaspect to evaluate is the prediction of the asphaltene stability in the crude oil by measuring basic dead oil properties, such asdensity and refractive index. The asphaltene instability trend (ASIST) method was used to predict the asphaltene precipitationonset at reservoir conditions. In this analysis, the asphaltene stability was studied on the heaviest and lightest samples (high andlow asphaltene content) by determining the minimum quantity of precipitant required to initiate asphaltene flocculation,followed by measurement of the refractive index of the mixture at the onset conditions. The asphaltene precipitation kineticeffect was also considered in this study.
■ INTRODUCTION
The objective of this study was to conduct flow assurancescreening studies on Gulf of Mexico (GOM) samples fromdifferent sands and determine properties of the dead oil and theasphaltene stability.Asphaltenes are the heaviest components, with high
molecular weight, density, and aromaticity, in oil samples. Inthis paper, the dead oil analysis was conducted on samples withdifferent asphaltene contents of 4, 7, and 15 wt %.Asphaltenes play an important role in the rheological
behavior of reservoir fluids.5 Previous studies indicate that theviscosity of dead oils is sensitive to its asphaltene content.6
In 2001, Wang et al. developed the asphaltene instabilitytrend (ASIST) method4 for predicting asphaltene precipita-tion onset using refractive index (RI) measurements.3 In thiswork, the asphaltene stability was studied on samples with thelowest and highest asphaltene content by determining theminimum quantity of n-alkane precipitant required to initiateasphaltene flocculation, followed by measurement of the RI ofthe mixture at the onset conditions. Samples with 4 and 15.5 wt% asphaltene content were titrated with n-heptane (n-C7),n-undecane (n-C11), and n-pentadecane (n-C15), and the RI ofeach of the mixtures at their onset condition (initiation ofasphaltene flocculation) was measured at temperatures abovethe measured wax appearance temperature (WAT) of the stocktank oils (STOs). The ASIST method was used in this studyto predict the asphaltene precipitation onset pressure (AOP) at
reservoir conditions. The asphaltene precipitation kinetic effectwas also considered.
■ EXPERIMENTAL SECTIONThe following steps were followed during experimental work: (1) Thedead oil samples were pretreated by heating and agitating in anultrasonic bath at 70 °C to homogenize. (2) The SARA analysis,which includes saturates, aromatics, resins, and asphaltenes content, wasmeasured using a modified version of the American Society for Testingand Materials (ASTM) D4124 method, as implemented by SchlumbergerReservoir Sampling and Analysis. n-Heptane was used for asphalteneprecipitation. (3) The WAT of the conditioned STO samples wasmeasured using a cross polar microscope (CPM). (4) Density,viscosity, and RI of dead oils with different asphaltene contents weremeasured at temperatures ranging from 40 to 208 °F (4.4 to 97.8 °C)using an Anton Parr densitometer, Index Instruments refractometer,and the capillary and rheometer (cone−plate) techniques. Asphaltenestability study: (5) The refractive indices of the precipitants n-C7,n-C11, and n-C15 [high-performance liquid chromatography (HPLC)-grade quality] were measured at the chosen temperature of 45 °C(113 °F) and 60 °C (140 °F). (6) The precipitants, n-C7, n-C11, andn-C15, were added to the STOs to obtain mixtures with various ratiosof precipitant in the mixture. (7) The mixtures of the precipitant andoils were allowed to settle for different times: 20 min, 5 h, 24 h, and4 days (kinetic study). (8) Subsequently, a sub-sample from each ratiowas observed under a high-pressure microscope (HPM) to identify
Received: May 15, 2012Revised: August 22, 2012Published: August 28, 2012
asphaltene flocculation. With this instrument, it is possible to observeparticles with a diameter of 1.0 μm and larger. (9) Finer volume ratioswere prepared using a separate sample, and steps 6−8 were repeatedto identify the minimum volume of precipitant required for asphalteneflocculation. The minimum precipitant volume requirement was termedas the onset of the asphaltene flocculation point at the chosen temp-erature. (10) The ASIST method was used to predict the asphalteneprecipitation onset at reservoir conditions.
■ RESULTS AND DISCUSSIONThe SARA Analysis. The SARA analysis was performed on
dead oil samples with different asphaltene contents (Table 1).Wax Properties. The wax content and the WAT of STO
sample were measured to design the tests in such a way thatwax would not be precipitated during the asphaltene instabilitymeasurement. The WAT of the oils was determined as shownin Table 2.The compositional analysis of the dead oil samples is sum-
marized in Table 3.RI. RI values of the dead oils were measured as a function of
the temperature using an automatic refractometer (Table 4).
The accuracy of the equipment is ±0.0001. The temperature ofthe samples was controlled to ±0.1 °F with a circulating waterbath. Figures 1 and 2 show the effect of the temperature andasphaltene content on RI.According to Figures 1 and 2, the RI is higher for dead oils with
greater asphaltene content and lower temperatures. RI shows a
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linear decrease with an increasing temperature in the measuredrange (Figures 1 and 2); they are nearly parallel to each another.Dead Oil Density. The density of the dead oil samples was
measured at different temperatures (Table 5 and Figure 3).Density is higher for fluids with higher asphaltene content
(Figure 3) because asphaltenes are polynuclear aromatics(PNAs) and have the highest densities.Dead Oil Density versus RI. The “one-third rule” described
in the reference by Vargas et al.,1 states that a relationshipbetween density and RI is obtained from the Lorentz−Lorenz
model, where the molar refractivity (Rm) is a function of the RI,the molecular weight (MW), and the mass density (ρ).
ρ ρ= −
+=
⎛⎝⎜
⎞⎠⎟R F
RI 1RI 2
MW MWm
2
2 RI
On the basis of measurements for pure hydrocarbon com-ponents, the ratio between Rm and MW is a constant with avalue around 1/3.
ρ= ∼ ∼
R FMW
13
0.333m RI
The crude oil samples evaluated in this study show that the relation-ship between their RI and density is around 1/3 (Table 6).
Figure 3. Effect of the temperature on the density of the dead oils withdifferent asphaltene content.
Table 6. FRI/Density for Dead Oils with DifferentAsphaltene Contents at Different Temperatures
Figure 5. Viscosity behavior of the dead oils as a function of thetemperature and asphaltene content.
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Figure 4 graphically presents the one-third rule behavior forpure components and dead oils from this study.The “one-third rule” can be used to predict RI values from
the density measurements. This approach is used to calculatethe RI of dead oils using experimental oil density measurementsfrom different asphaltene contents at temperatures ranging from68 to 174 °F. Table 7 presents the calculated RI values comparedto RI measurements for fluids under evaluation. The differencebetween both is lower than 0.006 RI units (lower than 0.4%).Dead Oil Viscosity. The viscosity of the oil samples under
study was measured at different temperatures. Measured valuesare presented in Table 8 and Figures 5 and 6 for samples 1 and 2.
The viscosity of the STO is proportional to the asphaltenecontent of the sample. At low temperatures (<100 °F), theviscosity rapidly increases with the asphaltene content. At hightemperatures, the viscosity increases almost linearly with theasphaltene content.
Dead Oil Viscosity versus RI. The reference by Vargas et al.1
presents relationships that predict transport properties, suchas viscosity, diffusivity, and thermal conductivity, from RImeasurements. The original model for calculating transportproperties was presented by Riazi and Al-Otaibi in 2001.2 Theviscosity (μ) form is presented in the following equation:
μ= +A
BF
1
RI
where
= −+
FRI 1RI 2RI
2
2
The RI is measured at the corresponding viscosity temperature.In the work by Riazi and Al-Otaibi, linear relationships areobtained for various pure hydrocarbons, such as n-pentane,cyclohexane, and benzene. Constants A and B are given byRiazi and Al-Otaibi.2
The viscosity and RI measurements for the dead oil samplesunder evaluation in this study show that their relationship isapproximately linear above certain temperatures in which thefluid behaves as Newtonian (Figure 7). Large deviation isobserved in the fluid with greater asphaltene content.
Asphaltene Stability Study for Sample 1 (15.5 wt %Asphaltene Content). Asphaltene instability measurementswere conducted on STO sample 1. Mixtures of STO and n-C7and n-C15 were prepared at 60 °C. First, n-C7 was added tosub-samples of the oil in various proportions (volume percentof n-C7 in the mixture: 10, 20, 30, 40, 50, and 60 vol %). Eachmixture was allowed to stabilize for 24 h at 60 °C. One drop ofthe mixture was deposited on a glass slide, covered with acoverslip, and then observed under a microscope. The additionof n-C7 affected the quantity of solids at concentrations between20 and 30% of precipitant in the mixture, and the quantity ofsolids increased for higher concentrations of n-C7. In the nextstep, additional mixture preparation was performed with smallerintervals of n-C7 between 20 and 30% to accurately determinethe minimum volume of n-C7 required initiating asphalteneflocculation and allowed to stabilize for 24 h. Subsequently, theonset of flocculation was observed using the microscope atmagnification of 20 × 10 (200 times). The minimum particlesize distinguishable is estimated to be about 1.0 μm in diameter.The RI was measured at this point to characterize oil solvency.RI at the onset is denoted as PRI (Figure 8).Once particles were observed at certain n-C7/oil mixture
concentrations, the RI at the onset (PRI) of that mixture wasmeasured. In this case, asphaltene onset was detected at27/73% (n-C7/oil, v/v) and the PRI was measured as 1.4694.After the measurements with n-C7 were completed,
measurements were conducted with n-C15 as the precipitatingsolvent. Particle formation was observed for samples with then-C15 concentration in the range of 15−25% (v/v). Thequantity of solid particles further increased at higherconcentrations of n-C15. On the basis of the preliminaryevaluation, the second series of mixtures (volume percent ofn-C15 in the mixture: 15, 16, 17%, etc.) was prepared. Themixture was allowed to stabilize for 24 h at 60 °C and observed
Figure 6. Effect of the temperature on the viscosity.
Figure 7. Viscosity behavior of the dead oils as a function of RI.
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under the microscope. The increase in solids particles can beobserved in the microscopic views of the mixtures with aconcentration of 20% (v/v) of n-C15 (Figure 9). Thus, it wasconcluded that the asphaltenes became unstable at thisconcentration.Subsequently, the PRI of the STO and the n-C15/oil mixture
[20% (v/v) n-C15] was measured as 1.4885.Asphaltene Stability Study for Sample 4 (3.7 wt %
were conducted on STO with 3.7 wt % asphaltene (sample 4).Mixtures of STO and n-C7, n-C11, and n-C15 were prepared at45 °C.The procedure described in the previous section was followed
to determine the minimum volume of precipitant needed toinitiate asphaltene flocculation.The addition of n-C7 affected the quantity of solids at a
concentration of 37/63% (n-C7/oil, v/v) of precipitant in themixture as observed under the microscope. The quantity ofsolids increased for higher concentrations of n-C7. The PRI ofthe dead oil and the oil/n-C7 mixture was measured as 1.44631at 45 °C. Similar measurements were conducted with n-C11and n-C15 at 45 °C. Measurements were also performed at60 °C, with the results summarized in Table 9.These measurements were used in the prediction of the
asphaltene precipitation onset by the ASIST model, as shown inthe following section.
Prediction of the Asphaltene Precipitation Onset Using theASIST Method for Sample 1. ASIST is a technique developedat New Mexico Tech to predict asphaltene precipitation onsets.4
Figure 9. Mixture n-C15/oil (v/v) sample 1 at 60 °C.
Figure 10. Oil volumetric fraction for sample 1 (15.5 wt % asphaltene)at 60 °C (140 °F).
Figure 11. PRI for sample 1 (15.5 wt % asphaltene) at 60 °C (140 °F).
Figure 8. Mixture n-C7/oil (v/v) sample 1 at 60 °C.
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The model is based in a linear relationship observed between thePRI (or solubility parameter) and the square root of the molar
Figure 14. PRI temperature translation for sample 4 and extrapolationto lower vp.
Figure 15. Asphaltene onset prediction for sample 4 at 208 °F (24 hprediction).
Figure 16. AOP for live oil sample 1 (15 wt % asphaltene) using NIR and HPM techniques.
Figure 12. PRI temperature translation for sample 1 and extrapolationto lower vp.
Figure 13. Asphaltene onset prediction for sample 1 at 208 °F (24 hprediction).
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volume of precipitant liquid n-paraffins.7 The method is based onthe fact that RI is related to the solubility parameter in solutions ofnonpolar molecules and dead oils, which are mixtures of mostlynonpolar hydrocarbons.Titration results of STOs with n-alkanes are used to predict
asphaltene stability for live oil. The model is based on thefollowing experimental observations: (1) For each STO, thereexists a linear correlation between PRI and vp
1/2, in which vp is
the molar volume of n-alkane precipitant.4 (2) For each oil/n-alkane pair, PRI decreases approximately linearly with anincreasing temperature.8 Therefore, the linear correlation ofPRI versus temperature can be translated to the temperature ofinterest to establish a new PRI−vp1/2 trend at the reservoirtemperature. This new PRI−vp1/2 correlation can be extrapo-lated to lower vp values, corresponding to the composition ofthe solution gas from which the onset condition (PRI) of thelive oil or other oil/gas mixtures at the target temperature andpressure can be calculated (Figures 10−12).Calculations of the RI at reservoir conditions require con-
ventional PVT data from differential-liberation tests, gas/oilratio (GOR, Rs) and formation volume factor (Bo), composi-tional analysis of live oil, and RI of dead oil. Once the PRI asa function of the temperature, pressure, and composition ofthe live oil has been established, that value can be compared tothe RI calculated for the same fluid at the same temperatureand pressure. If the RI is greater than PRI, asphaltenes arestable. If the RI is less than PRI, asphaltene precipitation canoccur.4 Asphaltene precipitation for sample 1 is predictedaround 6300 psia, where the oil RI crosses the calculated PRIcurve (Figure 13).
Prediction of the Asphaltene Precipitation Onset Usingthe ASIST Method for Sample 4. Similarly, titration resultswith n-alkanes are used to predict asphaltene stability for live oilfor sample 4. Figure 14 presents the PRI measurements and thecorresponding lineal extrapolation and onset prediction.As shown in Figure 15, asphaltene precipitation for sample 4
is predicted to appear at 6200 psia, where the oil RI interceptswith the calculated PRI curve.
Live Fluid Measurements for Samples 1 and 4. The AOPwas also measured by pressure depressurization of the live fluidusing near-infrared (NIR), HPM, and particle size analysis(PSA) techniques. The AOP for live sample 1 was measured as4600 psia at 208 °F (Figures 16 and 17).The ASIST method measured at 24 h predicts AOP of
6600 psia, which is 2000 psia greater than NIR/HPM/PSApressures.Similarly, the AOP for live sample 4 was measured as 5000
psia at 208 °F using NIR/HPM/PSA techniques. The ASISTmethod calculated AOP of 6400 psia at 24 h, which is 1400psia greater than the NIR/HPM/PSA pressures (Figures 18and 19).This is believed to be due to the fact that the ASIST predic-
tions were based on PRI measurements made after 24 h, insteadof a shorter period of time. The precipitation rate or kinetic effect
Figure 17. AOP for live oil sample 1 (15 wt % asphaltene) using HPMand PSA techniques.
Figure 18. AOP for live oil sample 4 (3.7 wt % asphaltene) using NIRand HPM techniques.
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of the asphaltene precipitation must be considered in the experi-mental design, as explained in the following section.Kinetic of Asphaltene Flocculation for Sample 1. The
asphaltene onset measurements and predictions indicate that
the asphaltene precipitation presents a strong kinetic effect. Therate of titration is analogous to the rate of depressurization:increasing the volume fraction of precipitant decreases thesolubility parameter of the mixture, similar to decreasing thepressure of the live sample.9
To consider the kinetic effect in this study, the asphalteneprecipitation onset for sample 1 was predicted by measuringthe asphaltene onset at 20 min, 5 h, 24 h, and 4 days. Table 10
Figure 19. AOP for live oil sample 4 (3.7 wt % asphaltene) usingHPM and PSA techniques.
Table 10. RI at the Asphaltene Precipitation Onset forSample 1 (15.5% Asphaltene) at 60 °C
time ofmeasurement
n-C7/oil(v/v) n-C7 − PRI
n-C15/oil(v/v) n-C15 − PRI
20 min 30/70 1.4588 22/78 1.48275 h 28/72 1.4629 21/79 1.485124 h 27/73 1.4694 20/80 1.48854 days 25/75 1.4709 17/83 1.4902
Figure 20. Onsets in volume fraction induced by n-C7 and n-C15 at60 °C at varying aging times for sample 1 (15.5 wt % asphaltene).
Figure 21. PRI for sample 1 (15.5 wt % asphaltene) induced by n-C7and n-C15 at 60 °C and varying aging times.
Figure 22. AOP prediction for sample 1 (15.5 wt % asphaltene) usingASIST at different incubation times.
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shows the combinations in volume of oil/precipitant at 60 °Cand the corresponding PRI.Figures 20−22 show the kinetic effect on RI measurements
and the corresponding AOP predictions. Onset pressurescalculated with the 5 h ASIST approach are compared to theNIR/HPM/PSA measurements.Kinetic of Asphaltene Flocculation for Sample 4 (3.7 wt %
Asphaltene Content). The asphaltene precipitation onset for
sample 4 was predicted by measuring the asphaltene onset at20 min, 5 h, 24 h, and 4 days. Tables 11 and 12 show thecombinations in volume of oil/precipitant and the correspond-ing PRI at 45 and 60 °C.Figures 23−25 show the kinetic effect on RI measurements
and the corresponding AOP predictions. Again, onset pressurescalculated with the 5 h ASIST approach are comparable toNIR/HPM/PSA measurements.
Precipitant and Temperature Effects. Asphaltenes arerelatively more unstable to n-C7 than n-C15; more n-C7 isrequired than n-C15 to precipitate asphaltene from the oil atthe same temperature (Figure 26). This is because the molarvolume of n-C15 is almost 90% greater than n-C7. Ciminoet al.10 offered an explanation for the maximum in volume ofn-paraffins at the onset of asphaltene precipitation and claimedthat it is due to the entropy of mixing of molecules of differentsizes. Wiehe et al.11 determined that the maximum in volume ofn-paraffin, as a function of the carbon number of the n-paraffinat the onset of asphaltene precipitation, is general for crude oilsand bitumens. According to them, this phenomenon is a resultof mixing liquids of greatly different molecular sizes.The increase in the temperature increases the stability of the
crude oil with respect to asphaltenes. A greater amount of pre-cipitant is needed to precipitate asphaltene at higher temper-atures, as observed in Figure 26. The RI of the mixture at theonset decreases at higher temperatures (Figure 27).
Table 11. Asphaltene Precipitation Onset for Sample 4 at 45 °C
time of measurement n-C7/oil (v/v) n-C7 − PRI n-C11/oil (v/v) n-C11 − PRI n-C15/oil (v/v) n-C15 − PRI
20 min 46/54 1.43574 47/53 1.44868 41/59 1.460845 h 41/59 1.44174 42/58 1.45264 36/64 1.4645324 h 37/63 1.44631 38/62 1.45603 31/69 1.466694 days 33/67 1.45070 34/66 1.46052 28/72 1.46952
Table 12. Asphaltene Precipitation Onset for Sample 4 at 60 °C
time of measurement n-C7/oil (v/v) n-C7 − PRI n-C11/oil (v/v) n-C11 − PRI n-C11/oil (v/v) n-C11 − PRI
20 min 47/53 1.42855 48/52 1.44170 43/57 1.452995 h 42/58 1.43300 43/57 1.44559 38/62 1.4561724 h 38/62 1.43674 38/62 1.44845 33/67 1.459184 days 34/66 1.44284 33/67 1.45403 29/71 1.46198
Figure 23. Oil volumetric fraction and PRI for sample 4 (3.7 wt %asphaltene) at 60 °C.
Figure 24. Volumetric fraction of oil at the onset and PRI afterdifferent incubation times.
Figure 25. Prediction of asphaltene precipitation onset for sample 4(3.7 wt % asphaltene) using ASIST at different incubation times.
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Accuracy of Onset PRI Measurements. The RI of mostdead oils ranged from 1.48 to 1.53. The maximum error for PRImeasurements that would result can be estimated as 0.001. Theasphaltene prediction method is sensitive to the RI measure-ment; a change in 0.001 in RI can represent a change of500 psia in the onset. This error is generally a level higher thanthe accuracy of the refractometer (0.000 05−0.0001).8
■ CONCLUSION(1) The viscosity of the high asphaltene content dead oil showslarge deviation at low temperatures. The viscosity prediction byRiazi and Al-Otaibi, originally applied in n-C5 and cyclo-C5, isnot appropriate for relatively high-viscosity crude oils, whichtend to deviate from Newtonian behavior. (2) The one-thirdrule can be used to estimate the RI from density in petroleumsystems. (3) The asphaltene precipitation presents a strongkinetic effect. From results in this work, asphaltene stability incrude oils decreases at lower temperatures; a lower amountof precipitant is needed to reach the AOP. (4) Asphalteneinstability prediction using the ASIST method approaches theNIR/HPM/PSA lab method at short aging times. (5) Themethod is sensitive to the RI measurement; a change in 0.001in RI could represent a change of 500 psia in the onset.
■ ACKNOWLEDGMENTSThe authors thank Anadarko Petroleum Corporation andSchlumberger Reservoir Sampling and Analysis for approval topublish this work. We also thank Simon Andersen and Heng-Joo Ng for their valuable comments.
■ NOMENCLATUREAOP = asphaltene onset pressureASIST = asphaltene instability trendGOM = Gulf of MexicoGOR = gas/oil ratioHPM = high-pressure microscopen-C7 = n-heptanen-C11 = n-undecanen-C15 = n-pentadecaneNIR = near-infraredPb = bubble point pressurePSA = particle size analysisPRI = refractive index at the asphaltene precipitation onsetRI = refractive indexSARA = saturates, aromatics, resins, and asphaltenesSTO = stock tank oilvp = molar volume of n-alkane precipitants (cm3/mol)WAT = wax appearance temperature
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Figure 27. Comparison of the PRI for sample 4 (3.7 wt % asphaltene)induced by n-C7, n-C11, and n-C15 at 45 and 60 °C and varying times.
Figure 26. Comparison of the precipitant volume fraction at the onsetfor sample 4 (3.7 wt % asphaltene) induced by n-C7, n-C11, andn-C15 at 45 and 60 °C and varying times.
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