Asphaltene: A General Introduction

Jianxin Wang

P & Sc GroupPRRC, 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 ResinsAromatics Asphaltenes

S A R A

nonpolar molecular weight, polarity & aromaticity

(0-20%)

Separating asphaltene from crude oil

ASTM D2007-80:

Mixing crude oil / n-C7 (or n-C5) = 1 / 40Aging for two daysFiltering through 0.22 µµm filter paper

Separated asphaltene amount is determined by precipitant molecule size

2 4 6 8 10 120

10

20

30

40

50A

sph.

(w

t%)

Chain-length of n-alkane precipitant

Separated asphaltene amount is also determined by precipitant/oil ratio

2 5 10 20 50 100 200 5000

1

2

3

4

5

6

7

8A

sph.

(w

t%)

n-C7 / oil ratio

Mars-pink

Furthermore, separated asphaltene amount is also time-dependent

1 2 5 10 20 50 100 2004.2

4.4

4.6

4.8

5

5.2

5.4

5.6

5.8A

sph.

(w

t%)

Aging time (days)

Mars-pink/nC7=1/10

Asphaltene is a compositional continuum

Long, R.B.: "The Concept of Asphaltenes," Chemistry of Asphaltenes, J.W. Bunger and N.C. Li (eds.), ACS, Washington, DC (1981) 17-27.

polarity = N, S, O

aromaticity =

Polarity & aromaticity

Mol

ecul

ar w

eigh

t

n-C7 asph.

n-C5 asph.

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

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).

N S

S

S

OH

SN

S

N

O

Asphaltene?Like a sea monster

Differences between asphaltene and wax

Asphaltene Wax

Dissolves in heptane:

Crystalline:

Melting point:

No Yes

No Yes

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

foulingproduction facilities

alteringwettability

poisoning refinary catalysts

AsphalteneFlocculation

Stability is determined by crude oil solvency

Temperature

Pressure

Composition - gas injection; - phase separation; - incompatible chemicals; - mixing of crude streams; - etc.Asphaltene

stability

High asphaltene content is not necessary to associate with high risk of trouble

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.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

20

40

60

80

100

saturates

troublefree

aromatics

resins

asphaltenes

troublesomeW

eigh

t %

Oil # from Venezuela oil fields

Asphaltene trouble is associated with RI

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 191.40

1.45

1.50

1.55troublesome troublefree

Oil #

Low RI High RI

PRI

Buckley, J.S.: "Asphaltenes: Separating Fact from Fiction," SPE Distinguished Lecture Slides (1998-99).

RI

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

Volume fraction

crude oil

maltene

alkane flocculated

PRI

RI

onset

redissolved

stable

Buckley, J.S.: "Asphaltenes: Separating Fact from Fiction," SPE Distinguished Lecture Slides (1998-99).

Asphaltene stability varies with solvent RI

Buckley, J.S.: "Asphaltenes: Separating Fact from Fiction," SPE Distinguished Lecture Slides (1998-99).

In a "poor" solvent, asphaltenes attractive forces are enhanced.

In a "good" solvent, asphaltenes are not

strongly attracted to one another.

Low RI

High RI

RI measurement

RI accuracy = 0.0001T: 10 ~ 70 °C

GPR-11-37

n = 1sin(θθ)

total internal reflection

θθ

sample

Sodium-D light beam

refractometer

Procedure for onset detection

Prepare a series of mixtures with varying oil/precipitant ratios in sealed vialsAging for 24 hoursObserve asphaltene precipitation, if any, under microscopeOnce onset is detected, record volume ratio and measure the refractive index (RI) for mixture

Asphaltene flocculates under microscope (1)

3

21

Asphaltene flocculates under microscope (2)

PRI vs. time (Lagrave)

Time (day)0 2 4 6 8 10 12 14 16 18

1.44

1.45

1.46

1.47

1.48

1.49

1.50

n-C5

n-C9

n-C15

particle onset aggregate onset

PRI

Volume ratios of precipitant to oil at onset

4 6 8 10 12 14 160

1

2

3

4

Chain-length of n-alkane

Vpr

ecip

itan

t / V

oil

(ml/m

l)

A-95 Lagrave

Mars-pink Tensleep

Lost Hills

A-93

RI at onset (PRI) for crude oils

4 6 8 10 12 14 161.40

1.42

1.44

1.46

1.48

1.50

1.52

PRI

Chain-length of n-alkane

A-95 Lagrave

Mars-pink Tensleep

Lost Hills

A-93

Simple correlation of PRI vs. precipitant

10 11 12 13 14 15 16 171.40

1.42

1.44

1.46

1.48

1.50

1.52

vp

Crude oils

PRI

average r = 0.992

A-95 Lagrave

Mars-pink Tensleep

Lost Hills

A-93

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.