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Journal of Petroleum Science and Engineering Volume 26, Issue
1-4, Pages 49–55, 2000
DOI: 10.1016/S0920-4105(00)00020-6Print ISSN: 09204105
Identification and Measurement of Petroleum PrecipitatesDynora
Vazquez and G.Ali Mansoori
Thermodynamics Research Laboratory, UniÕersity of Illinois at
Chicago, (M/C 063), Chicago, IL 60607-7052, USA
Abstract
A number of procedures to identify and measure the precipitates
that result from petroleum fluids are presented. The
fractions considered in this study include those in the
categories of asphaltenes, resins, saturates paraffinrwaxŽ .,
aromatics, inorganic minerals and diamondoids. A combination of
deposition techniques, separation by centrifuge, filtration,
gaschromatography, gel-permeation chromatography, and SARA LC-HPLCŽ
. separations, and a number of other techniques are utilized to
identify each fraction and quantify their concentrations. These
procedures provide an understanding of the overall behavior of the
species that precipitate as well as of the interactions among them.
The results of such analysis are the cornerstone of any predictive
and preventive measures for heavy organics deposition from
petroleum fluids.
Keywords: petroleum precipitates; deposition techniques;
concentration
1. Introduction
Solid precipitation during production, transporta-tion, and
storage of petroleum fluids is a commonproblem faced by the oil
industry throughout theworld. Through complex phase
transformations, dis-
Žsolved and suspended solids asphaltenes, resins,.paraffinrwax,
diamondoids, formation solids, etc.
precipitate out of solution. Such phase segregationsare
sometimes followed by flocculation of the result-ing precipitates.
In many instances, these depositionphenomena render in complete
clogging of flowlines and serious damages to storage vessels
and
processing equipment. The solution or alleviation ofthe many
technological problems posed by suchdepositions lies on a good
understanding of themulti-phase behavior of the species that
precipitate.It is also necessary to understand the
interactionsamongst these various species in the local environ-ment
where phase segregation and flocculation take
Ž .place Kawanaka et al., 1991; Mansoori, 1997 .The procedures
presented in this report are com-
plimentary to the other procedures already in theliterature. The
results of all such experimental mea-surements will be especially
useful for the better
Žmodeling and prediction Kawanaka et al., 1991;.Mansoori, 1997
of the behavior of heavy organics in
petroleum fluids. In the following sections of thisreport, the
procedures along with the results of acrude-oil analysis as
performed by each procedureare presented.
Authors email addresses: D. Vazquez ([email protected]); G.A.
Mansoori ([email protected])
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2. Analysis of suspended materials in a crude oil
Microscopic inspection of crude oil samples isgenerally
performed as a preliminary step. A micro-scope with a minimum
magnification of 200= andequipped with a photo camera may be used.
Pho-tographs are obtained by placing small samples ofthe crude oil
in a micro-chamber that is then per-fectly sealed with a glass
cover to prevent evapora-tion of the lighter fractions. This test
will determinewhether a crude oil contains suspended solids. If
not,measurements of the onsets of asphaltene precipita-
Žtion, flocculation and deposition Vuong, 1985; Es-.cobedo and
Mansoori, 1995, 1997 and other related
measurements can safely be performed. Otherwise,such
measurements cannot be performed unless thesuspended solids are
found to be without asphaltene.It may be also necessary to
establish the nature of
Ž .the droplets or bubbles present in the crude. Anumber of
procedures may be devised to study these
Ždroplets through microscopic techniques Vazquez.and Mansoori,
in press . These experiments may
demonstrate whether some of the species present incrude oil show
surface activity at brine–oil interfaceindicative of stable
water-in-oil emulsions in crudeoil. It must be noted that some of
the species con-tained in a crude oil, especially asphaltene,
mayexhibit surface activity that renders in the formationof stable
water-in-oil emulsions. This emulsion seemsto be stabilized by a
film that surrounds the brinedroplets. Generally suspended solids
in the crude oilinteract with these droplets such that lumps of
sev-eral droplets are held together by solid aggregates.The amounts
of water and sediment content of acrude oil can be measured
following the IP 359r82Ž .ASTM D4007-87 standard procedure using a
cen-
Ž .trifuge IP 359r82, 1985 .For quantitative analyses, it is
important to pre-
vent material loss due to evaporation during samplepreparation.
It is necessary to devise an appropriateprocedure to draw aliquots
from the main sample,such that uncertainties introduced by
evaporationwill be minimized.
In order to ascertain reproducible results, allaliquots must be
weighed in gas-tight vials.
The solids in a crude oil may be separated forŽfurther analysis
by two different methods Vazquez
.and Mansoori, in press . In the first method, the
suspended solids are separated using an appropriatefiltration
technique. In the second method, the solidsare separated using a
centrifuge. Then, the amountsof nC -soluble, nC -asphaltene, and
insoluble frac-5 5tions in the suspended solids are measured using
a
Ž .detailed, procedure Vazquez and Mansoori, in press .Table 1
contains the results from the fractionation
Ž .of the suspended solids in a crude oil named A .Note from
Table 1 that the suspended solids in crudeoil A are mainly
comprised of nC -soluble material5that could very well be
paraffinrwax in conjunctionwith resins and traces of oil trapped in
the solid.Note also that these suspended solids contain a
con-siderable amount of nC -asphaltenes as well as a5considerable
amount of insoluble material. The com-position of the solid
material contained in crude oilA suggests a strong interaction
among insoluble
Ž .material minerals , asphaltenes, and paraffinrwax.The
insoluble material contained in the suspendedsolids of crude oil A
was further tested and found tobe inorganic minerals from the
formation. Differen-tial scanning calorimetry is performed on the
nC -5soluble material in the suspended solids in order toestablish
its paraffinrwax molecular weight rangeŽ .Letoffe et al., 1995 .
For crude oil A, the meltingcurve indicated that the maximum of the
meltingoccurs in the range of 76.26–818C. These tempera-
Žtures correspond to nC hexatricontane; mps36. Ž .76.268C and nC
tetracontane; mps81.008C .40
Thus, the thermogram suggested the suspended solidsin crude oil
A contain mostly high-molecular-weightŽ .HMW paraffinrwax
species.
It is not surprising that the suspended solids foundin crude oil
A are primarily comprised of para-
Table 1Characterization of the suspended solids contained in
crude oil A
Fraction Suspended solids Whole crude oil
Total amount 100 2.5909"0.1156 wt.%of solids
Ž .Total volume 100 3.82"0.06% vrvof solidsnC5 solubles
81.7669"1.7794 2.1198"0.1404 nC5Ž .wt.%Asphaltenes 10.9081"1.4704
0.2815"0.0253Ž .wt.%Insoluble solids 7.3250"0.3133 0.1892"0.0032Ž
.wt.%
D. Vazquez and G.A. Mansoori Identification and Measurement of
Petroleum Precipitates
J. Petrol. Sci. & Eng'g. 26(1-4): 49-55,2000
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ffinrwax since its cloud point is above room temper-Ž .ature
;338C . However, what is important here is
the fact that this solid material also contains a sub-stantial
amount of precipitated asphaltenes. It is ap-parent from these
results that mineral particles andparaffinrwax crystals assist the
precipitation of as-phaltenes. A plausible scenario would be the
exis-tence of attractive forces between mineral particlesand
asphaltenes, which would result in the formationof larger particles
on which paraffinrwaxes maydeposit.
3. Total asphaltene content of a crude oil
The exact value of the total asphaltene content ofa crude oil
cannot be measured by any precipitationprocedure because of the
fact that it will not com-pletely precipitate by the existing
precipitation pro-cedures. However, the asphaltene that
precipitatesusing different normal paraffin hydrocarbon solventscan
be measured. Then, using such data in an appro-priate polydisperse
deposition model for asphaltene,one may be able to estimate the
true value of the
Žtotal asphaltene content of a crude oil Kawanaka et.al., 1991;
Mansoori, 1997 .
Measurement of the asphaltene content of nC -in-5solubles, could
be a challenging task. It is wellknown that some amount of resins
will precipitatealong with the asphaltene fraction and insoluble
ma-terial. It is also known that HMW solid paraffinrwaxwill
precipitate out of solution once the asphaltenestarts to
flocculate. The experimental procedure used
Žhere finds its basis in the IP143r90 ASTM D3279-. Ž .90
standard procedure IP 143r90, 1985 .The total nC -, nC -, and nC
-asphaltene con-5 7 9
tents of crude oil A were measured according to thisprocedure.
These results are summarized in Table 2.The total content of
insoluble material in crude oil A
Table 2Total nC , nC , and nC asphaltene content of crude oil A5
7 9
Paraffin solvent wt.% Asphaltene
n-Pentane 1.0833"0.0050n-Heptane 0.5167"0.0059n-Nonane
0.3982"nra
Fig. 1. Normalized amounts of asphaltenes precipitated with
vari-ous paraffin solvents. The values reported in this figure
werenormalized with respect to the content of insoluble
materialpresent in crude oil A.
is determined by the same procedure as for asphal-tene
precipitation. Fig. 1 shows the normalized totalnC -, nC -, and nC
-asphaltene content of crude oil5 7 9A along with four other crude
oils.
( )4. Gel permeation chromatography GPC studiesof precipitated
asphaltenes from a crude oil
Adsorbed resin material on the precipitated as-phaltene
particles may lead to discrepancies in theMW determination.
Therefore, a given precipitatedasphaltene fraction of a crude oil
must be purified
Žbefore the measurement Vazquez and Mansoori, in.press .
For any particular method of determination, theobserved
molecular weights suggest that asphaltenesform molecular
aggregates, even in dilute solutions.This association is influenced
by solvent polarity,asphaltene concentration and temperature. The
mostwidely used method for determining the MW ofasphaltenes is GPC.
It is understood, however, thatGPC provides relative molecular
weights and de-pends entirely on the calibration standards usedŽ
.Leontaritis and Mansoori, 1989 .
The molecular weight distribution of the nC -,5nC -, and nC
-asphaltene fractions precipitated from7 9the crude oil A were
obtained by a GPC proceduredesigned specifically for asphaltene.
Each asphaltene
D. Vazquez and G.A. Mansoori Identification and Measurement of
Petroleum Precipitates
J. Petrol. Sci. & Eng'g. 26(1-4): 49-55,2000
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Ž .fraction was dissolved in tetrahydrofuran THF at adefined
concentration. A similar solution of the wholecrude oil A was also
prepared. All samples wereequilibrated using continuous mixing for
a pre-scribed duration. Care was taken to avoid contact ofthe
samples with open air. It must also be pointedout that all samples
were filtered through appropriatemembrane filters to remove any
insoluble materialsfrom the solutions. Three injections of each
solutionwere run through a GPC system. The mobile phasefor these
analyses was THF at an appropriate flowrate. The mobile phase was
degassed by filtrationand subsequent sonication with dry helium.
Thetemperature for all the runs were kept constant andthe eluant
was analyzed. The raw GPC data wereprocessed using a size exclusion
chromatographysoftware. Calibration curves were obtained
usingnarrow-dispersity polystyrene standards.
The molecular weight distributions of the nC -,5nC -, and nC
-asphaltene fractions of the crude oil7 9A are reported in Fig. 2.
Since asphaltenes aresolubility-class compounds, properties of the
precipi-tated asphaltenes vary according to the solvent usedfor
their precipitation. The resin content in the pre-
cipitate increases as the number of carbons in theprecipitating
solvent decreases. When n-pentane isused HMW paraffins
co-precipitate with the as-phaltenes and resins. Therefore, it is
expected thatthese three types of asphaltenes will exhibit
verydifferent MW distributions. The MW distributionswill depend on
the level of interactions among as-phaltenes, resins, and
co-precipitated HMW paraf-fins.
According to Fig. 2, the MW distributions for allthe three types
of asphaltenes are bimodal. Thedistribution for the nC -asphaltenes
exhibits two dis-5
w xtinguishable peaks: one narrow HMW peak 6751Ž .and one broad
low-molecular-weight LMW peak
w x2663 . The HMW peak exhibits little tailing towardshigh
molecular weights. The shape of these twopeaks suggests the
existence of, at least, two differ-ent types of species in the
solution.
The MW distribution for the nC -asphaltenes also7exhibits two
distinct peaks: a narrow HMW peakŽ .9383 with considerable tailing
towards the high
Ž .molecular weights, and one broad LMW peak 3383 .Note that
this distribution also suggests the coexis-tence of, at least, two
types of species in the solu-
Fig. 2. Molecular weight distributions obtained by GPC for the
various asphaltene fractions derived from five different crude
oils.
D. Vazquez and G.A. Mansoori Identification and Measurement of
Petroleum Precipitates
J. Petrol. Sci. & Eng'g. 26(1-4): 49-55,2000
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53
tion. The LMW peak for the nC -asphaltenes is not9easily
observed as in the previous two cases. Oneprominent peak with a
peak molecular weight of7747 is observed with little tailing
towards highmolecular weights. The shape of this peak suggeststhat
in this case, the amount of LMW species ismuch smaller than in the
case of nC -, and nC -5 7asphaltenes. This suggests rather strong
interactionsamong asphaltene molecules such that very
HMWspeciesraggregates are formed. The average molecu-lar weights
obtained for the nC -, nC -, and nC -5 7 9asphaltenes were 2725,
6198, and 5607, respectively.
These studies also indicate that the precipitatedfractions
contain a considerable amount of LMWspecies in solution. This is
clearly indicated by thecontinuous distribution throughout the
chromatogram
Ž .until the lower exclusion limit i.e. 100 for the setupused.
When asphaltenes are dissolved in aromatic orpolar solvents, even
at low concentration, there seems
Žto be a complex equilibrium: Rogacheva et al.,.1980;
Pacheco-Sanchezand and Mansoori, 1998
MoleculeslMicelleslMicelle aggregates
Ž .In this equilibrium, the first species moleculesmay exhibit
molecular weights up to 4000; the sec-
Ž .ond species micelles could have molecular weightsŽfrom 4000
to 10,000; and the third species micelle
.aggregates may possess molecular weights up to40,000,000. The
existence of two distinct familiesdemonstrated in Fig. 2 are
attributed to the asphal-tene molecule and asphaltene micelle.
5. SARA separation
The SARA separation was originally designed forŽ
.characterization of residuals Jewell et al., 1974 .
The heavy end of a crude oil can be separated intofour distinct
fractions namely saturates, aromatics,resins, and asphaltenes using
the experimental SARAprocedure. A crude oil usually contains a
consider-able amount of volatile material that must be re-moved
prior to performing SARA separation of thecrude oil. This is
accomplished by performing avacuum batch distillation at 10 mm Hg
and roomtemperature until the weight of the residue reaches
aconstant value. The first fraction to be separated
from the vacuum residue is the nC -asphaltenes. The5filtrate
collected from the separation of the nC -5asphaltenes is commonly
known as maltenes. It con-tains the remaining three fractions,
saturates, aromat-ics and resins. These three fractions are
separatedusing open-column liquid chromatography.
Saturate hydrocarbons on percolation in a n-pen-tane eluant, are
not absorbed on activated silicaunder the conditions specified. The
saturate fractionof the oil is eluted from the column with 1 l
ofn-pentane at 5 mlrmin. The solvent is removedusing a rotary
vacuum evaporator to recover thesaturates fraction.
Aromatic hydrocarbons are adsorbed on activatedsilica in the
presence of n-pentane, and desorbed bytoluene after removal of the
saturates under theconditions specified. The aromatic fraction of
the oilis eluted from the chromatographic column usingtoluene at 5
mlrmin. The resin fraction of the oil iseluted from the
chromatographic column using a90r10 toluenerMeOH solution at 5
mlrmin. Thisfraction is reported as wt.%. Solvent removal is to
beperformed using standard laboratory procedures.
The bulk composition of crude oil A from the twoSARA separations
performed on the vacuum residueis given in Table 3. Note from Table
3 that as muchas 21.05% by weight was separated from this crude
Ž .oil at 278C and 10 mm Hg vacuum distillation .These light
ends were further analyzed by gas chro-
Ž .matography-mass spectroscopy GC-MS as will bediscussed later.
Also note that the SARA separationperformed on the vacuum residue
was successfullyaccomplished with an average recovery of about99%.
The resins content of crude oil A is very small
Table 3Results from the SARA separation of crude oil A
Fraction wt.% of whole wt.% of vacuumcrude oil residue
Light compounds 21.0500"0.1150 100Žseparated at 278C
.at 10 mm HgSaturates 55.2369"1.6849 69.1949"1.3650Aromatics
19.9590"1.5373 25.2788"1.9488
Ž .Resins or polars 2.3242"0.0108 2.9437"0.0137Ž .Asphaltenes
nC5 1.0833"0.0050 1.3753"0.0032Ž .Total recovery average 99.6534
98.7927
D. Vazquez and G.A. Mansoori Identification and Measurement of
Petroleum Precipitates
J. Petrol. Sci. & Eng'g. 26(1-4): 49-55,2000
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Ž .compared to its total asphaltene content see Fig. 1 .The
overall composition of this crude oil appears tobe a hostile
environment for the stability of itsasphaltene content.
6. Diamondoids separation and characterization
Ž .Diamondoid adamantane has the same structureas the diamond
lattice, highly symmetrical andstrain-free. It is generally
accompanied by smallamounts of alkylated adamantanes. The isolation
of asingle compound, adamantane, from such a complexmixture as
petroleum is a consequence of its highly
Žunusual physical and chemical properties Vazquez.Gurrola et
al., 1998 .
ŽAdamantane with freezing point of 2698C in.sealed capillary
crystallizes in a face-centered cubic
lattice that is quite unusual for an organic compound.The
special properties of adamantane are reflected inits mass spectrum
that is also quite unusual.
Deposition of diamondoids can be particularlyproblematic during
production and transportation ofnatural gas, gas condensates and
light crude oils.These compounds are common in petroleum fluids,in
general. However, their presence in crude oils isusually ignored
due to their low concentration. Nev-ertheless, even small
concentration of these com-pounds could lead to problems of
deposition andplugging of flow paths. The high amount of gas
Žassociated with crude oil A gas-to-oil ratios2203 3.m rm
suggests that diamondoids, if present in
appropriate amounts, may cause problem during itsproduction.
GC-MS analyses of diamondoids in petroleumfluids can be
performed on the saturates fraction ofSARA separation, rather than
on the whole crude oil.This is because the aromatic interferences
need to beremoved by isolating the saturates fraction on silicagel.
The light fraction of a crude oil, separated at278C and 10 mm Hg,
must be also analyzed byGC-MS to determine whether it contains
diamon-doids. Adamantane is expected to elute from the GCcolumn
between nC and nC ; diamantane is ex-10 11pected to elute between
nC and nC ; and tria-15 16mantane is expected to elute between nC
and19nC . The same alkyl-substituted adamantanes found20in the
saturated fraction of crude oil A were also
Table 4Alkyl substituted adamantanes found in crude oil A
Compound Mq Base peak RetentionŽ .time min
Ž . Ž .1-Methyl 150 14% 135 M-CH3 55.59adamantane
Ž . Ž .2-Methyl 150 68% 135 M-CH3 60.88adamantane
Ž . Ž .1,4-Dimethyl 164 15% 149 M-CH3 61.88adamantane
Ž . Ž .1,2-Dimethyl 164 15% 149 M-CH3 64.51adamantane
Ž . Ž .1-Ethyl 164 9% 135 M-C2H5 66.89adamantane
detected in this low-boiling fraction at similar reten-Ž .tion
times and in higher concentrations see Table 4 .
7. Conclusions
The experimental procedures presented in thisreport provide the
means for understanding theamounts and behaviors of the species
that precipitateas well as the interactions among them. A
detailedprocedure for the separation and analysis of thesuspended
solids in the crude oils is presented. Theresults of the procedure
can also reveal whether suchtests as cloud point, onset of
flocculation of asphal-tene and other onsetrthreshold of deposition
testscan be proceeded for the oil sample under study.
Total asphaltene content of crude oils are mea-sured using
various precipitating agents and its rela-tion with the nature of
the precipitating agent isestablished. This data along with the
onsetrthresholddata may be used to tune theoretical modelsŽ
.Kawanaka et al., 1991; Mansoori, 1997 of heavyorganics deposition.
GPC studies of asphaltenes sep-arated from a crude oil will produce
the size distribu-tion of asphaltene deposited as a function of
thesolvent used. Such data, along with GPC size distri-
Žbution data Leontaritis and Mansoori, 1989; Vazquez.and
Mansoori, in press for resins can be quite useful
for tuning theoretical polydisperse models of heavyŽorganics
deposition Kawanaka et al., 1991; Man-
.soori, 1997 . The SARA separation of the heavy andlight
fractions of a crude oil give the composition ofthe crude oil from
the point of view of the four basic
D. Vazquez and G.A. Mansoori Identification and Measurement of
Petroleum Precipitates
J. Petrol. Sci. & Eng'g. 26(1-4): 49-55,2000
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families of compounds, namely saturates, aromatics,resins and
asphaltenes. Interactions among these fourfamilies of compounds
have a profound effect on thestability of a crude oil that must be
understood andmodeled in the stability analysis of crude oils.
Char-acterization and concentration measurement of dia-mondoids can
be particularly useful for petroleumreservoir fluids with high
gas-to-oil ratios. The ana-lytic techniques presented in this
report can be usedto find the composition and nature of such
com-pounds present in a crude oil.
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D. Vazquez and G.A. Mansoori Identification and Measurement of
Petroleum Precipitates
J. Petrol. Sci. & Eng'g. 26(1-4): 49-55,2000