Asphaltene: A General Introduction Jianxin Wang P & Sc Group PRRC, New Mexico Tech Oct. 18, 2000
Outline What is asphaltene? Why to study asphaltene? How to characterize asphaltene stability in crude oil?
Asphaltenes are the most heavy and polar components in crude oil Crude oil
Saturates
Aromatics
Resins
Asphaltenes (0-20%)
S
A
R
A
nonpolar
molecular weight, polarity & aromaticity
Separating asphaltene from crude oil ASTM D2007-80: Mixing crude oil / n-C7 (or n-C5) = 1 / 40 Aging for two days Filtering through 0.22 µm filter paper
Separated asphaltene amount is determined by precipitant molecule size 50
Asph. (wt%)
40
30
20
10
0
2
4
6
8
10
Chain-length of n-alkane precipitant
12
Separated asphaltene amount is also determined by precipitant/oil ratio 8
Asph. (wt%)
7 6 5 4 3 2
Mars-pink
1 0
2
5
10
20
50
100
n-C7 / oil ratio
200
500
Furthermore, separated asphaltene amount is also time-dependent 5.8
Mars-pink/nC7=1/10
Asph. (wt%)
5.6 5.4 5.2 5 4.8 4.6 4.4 4.2
1
2
5
10
20
50
Aging time (days)
100
200
Asphaltene is a compositional continuum
Molecular weight
n-C7 asph.
n-C5 asph.
polarity = N, S, O aromaticity =
Polarity & aromaticity Long, R.B.: "The Concept of Asphaltenes," Chemistry of Asphaltenes, J.W. Bunger and N.C. Li (eds.), ACS, Washington, DC (1981) 17-27.
Typical asphaltene molecular properties H/C = 0.8 - 1.4 Molecular weight: depends on solvent and concentration monomer = 500 - 1000 micelles = 1000 - 5000 Heteroatoms: acting as polar functional group S=0.5-10 wt%; N=0.6-2.6 wt%; O=0.3-4.8 wt% Metal elements: Ni, V, Fe
Hypothetical polycyclic structures for asphaltene molecules
OH N
S
O S N
S N S
S
Speight, J.G.: "A Chemical and Physical Explanation of Incompatibility during Refining Operations," Proc. 4th Intl. Conf. on the Stability and Handling of Liquid Fuels, US Dept. Energy, 169 (1992).
Asphaltene? Like a sea monster
Differences between asphaltene and wax
Asphaltene
Wax
Dissolves in heptane:
No
Yes
Crystalline:
No
Yes
Melting point:
No
Yes
Buckley, J.S.: "Asphaltenes: Separating Fact from Fiction," SPE Distinguished Lecture Slides (1998-99).
Outline What is asphaltene? Why to study asphaltene? How to characterize asphaltene stability in crude oil?
What if asphaltene flocculates ?
plugging reservoir
fouling production facilities
Asphaltene Flocculation poisoning refinary catalysts
altering wettability
Stability is determined by crude oil solvency Composition
Asphaltene stability
- gas injection; - phase separation; - incompatible chemicals; - mixing of crude streams; - etc.
Temperature Pressure
High asphaltene content is not necessary to associate with high risk of trouble troublesome 100
asphaltenes
80
Weight %
troublefree
resins
60
aromatics
40
saturates
20
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19
Oil # from Venezuela oil fields Carbognani, L. and Espidel, J.: "Characterization of Solid Deposits from Production Facilities. Identification of Possible Causes of Deposits Formation," Vision Technologica, Vol. 3, No. 1, 35-42.
Asphaltene trouble is associated with RI troublesome
troublefree
1.55
1.50
PRI
RI 1.45
High RI
Low RI 1.40
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19
Oil # Buckley, J.S.: "Asphaltenes: Separating Fact from Fiction," SPE Distinguished Lecture Slides (1998-99).
Outline What is asphaltene? Why to study asphaltene? How to characterize asphaltene stability in crude oil?
For a given crude/precipitant, flocculation occurs at a critical RI RI
crude oil
stable onset
maltene
PRI redissolved
alkane
flocculated
Volume fraction Buckley, J.S.: "Asphaltenes: Separating Fact from Fiction," SPE Distinguished Lecture Slides (1998-99).
Asphaltene stability varies with solvent RI High RI
In a "good" solvent, asphaltenes are not strongly attracted to one another.
In a "poor" solvent, asphaltenes attractive forces are enhanced.
Low RI
Buckley, J.S.: "Asphaltenes: Separating Fact from Fiction," SPE Distinguished Lecture Slides (1998-99).
RI measurement sample
GPR-11-37
refractometer RI accuracy = 0.0001 T: 10 ~ 70 °C
Sodium-D light beam
θ
total internal reflection
n=
1 sin(θ θ)
Procedure for onset detection Prepare a series of mixtures with varying oil/precipitant ratios in sealed vials Aging for 24 hours Observe asphaltene precipitation, if any, under microscope Once onset is detected, record volume ratio and measure the refractive index (RI) for mixture
Asphaltene flocculates under microscope (1)
3
1
2
Asphaltene flocculates under microscope (2)
PRI vs. time (Lagrave) 1.50 1.49
n-C15
1.48
n-C9
PRI 1.47 1.46
n-C5
1.45 1.44
particle onset aggregate onset 0
2
4
6
8
10
12
Time (day)
14
16
18
Volume ratios of precipitant to oil at onset Vprecipitant / Voil (ml/ml)
4
3 A-93 A-95 Lagrave Lost Hills Mars-pink Tensleep
2
1
0
4
6
8
10
12
14
Chain-length of n-alkane
16
RI at onset (PRI) for crude oils 1.52 1.50 1.48
PRI
A-93 A-95 Lagrave Lost Hills Mars-pink Tensleep
1.46 1.44 1.42 1.40
4
6
8
10
12
14
Chain-length of n-alkane
16
Simple correlation of PRI vs. precipitant 1.52 1.50
Crude oils average r = 0.992
1.48
PRI
A-93 A-95 Lagrave Lost Hills Mars-pink Tensleep
1.46 1.44 1.42 1.40 10
11
12
13
14
vp
15
16
17
Conclusions Asphaltene molecules are polyaromatics with side chains. They exist in crude oils as monomer and micelles equilibrated to each other. The sizes of asphaltene micelles fall in colloidal range. Asphaltene flocculation occurs when oil solvency is reduced. Flocculated asphaltene could have detrimental effects on oil production, transportation and refinary process. Refractive index (RI) is a simple, readily measurable parameter for characterizing crude oil solvency with respect to asphaltene stability.
