25th International Meeting on Organic Geochemistry IMOG 2011

P-091
Investigation of oil stability under geological conditions using
kinetics of C8 hydrocarbons cracking derived from MSSV
pyrolysis
Regina Binotto 1 , Rosane Fontes 1 , Henrique Penteado 1 , Denise Bohrer 2
1 Petrobras Research Center, Rio de Janeiro, Brazil, 2 Federal University of Santa Maria, Santa Maria, Brazil
(corresponding author:binotto@petrobras.com.br)
One of the most fundamental problems in
petroleum systems modeling for deep offshore
prospects is to determine the spatial (depth) and
temperature limits of oil occurrence [1]. Reservoirs
that were originally filled with oil might be subjected to
further burial, and liquid petroleum may be cracked to
gas. Oil-to-gas cracking is governed by a large
number of chemical reactions that are not known in
detail, but are recognized as being irreversible firstorder
reactions following the Arrhenius law.
Oil-to-gas cracking kinetics applied to petroleum
systems modeling were derived from cracking
experiments using different pyrolysis techniques [2].
Whole oils, light fractions or individual compounds
have been submitted to artificial cracking using
different methods, thus making a comparison of their
relative kinetic behavior not straightforward.
The objective of this study was to improve the
understanding of oil-to-gas cracking and the extreme
geological conditions for oil preservation by deriving
kinetics through experimental cracking of compounds
present in the light fraction of petroleum. A
methodology was developed using MSSV (microscale
sealed vessel) coupled with gas
chromatography with flame ionization detector
(MSSV-GC-FID) to assess the thermal degradation of
C8 pure hydrocarbons that occur among the lightest
and most stable compounds in natural oils.
The selected compounds are representative of
each class of light hydrocarbons: a normal-chain
alkane (n-octane), a branched alkane (2,3,4trimethylpentane),
a cyclic alkane (ethylcyclohexane),
and an aromatic (o-xylene).
By performing the same analytical protocol in these
pyrolysis experiments, kinetic parameters were
obtained (Ea, activation energy, and the frequency
factor A) for the thermal degradation of the four
selected C8 compounds. This approach allows a
direct comparison of the derived kinetics, since
differences due to various analytical setups were
avoided. MSSV Pyrolysis experiments were carried
out in an anhydrous closed system under isothermal
conditions from 0 to 7 hours (470-530°C). The global
rate constants were determined based on the reactant
conversions obtained at various temperatures. For all
temperatures, bulk decomposition of the studied
substances obeys first-order kinetics and the resulting
apparent activation energy (Ea, kcal/mol) and the
corresponding frequency factor (A, s -1 ) derived from
the Arrhenius diagram are: n-octane (57; 5.62x10 12 ),
2,3,4-trimethylpentane (54; 6.71x10 11 ),
ethylcyclohexane (49; 1.76x10 10 ) and o-xylene (40;
3.96x10 6 ).
These kinetic parameters were used to
extrapolate the behavior of compounds at low
temperatures over geologic time. First, the
extrapolation of laboratory cracking rate constants
showed that the Arrhenius plots cross at 200-300°C
(Fig. 1). This means that o-xylene, which is more
stable than the other compounds at higher
temperatures, becomes the most labile below 200-
300°C. This reverse behavior, i.e. the lower stability of
aromatics under geological conditions, explains why
oils found in deep reservoirs are more depleted in
aromatics and consequently enriched in n-alkanes [3].
ln k (s -1 )
0
-10
-20
-30
-40
Lab
Conditions
(470-530°C)
~300°C
~200°C
n-octane ethylcyclohexane
2,3,4-trim ethylpentane o-xilene
-50
0,6 0,7 0,8 0,9 1 1,1 1,2 1,3 1,4
1/RT (kcal/mol)
Natural
Conditions
(120-200°C)
Fig. 1. Comparison of the Arrhenius diagram obtained for noctane,
2,3,4-trimethylpentane, ethylcyclohexane and oxylene
degradation.
Geological simulations using the acquired
kinetics and a constant heating rate of 1°C/ million
years indicated that the temperature ―windows‖ of
compound cracking (between 10% and 90%
conversion) are: n-octane 171-202°C, 2,3,4trimethylpentane
163-195°C, ethylcyclohexane 144-
177°C, and o-xylene 126-164°C.
References
[1] Welte, et al., Springer-Verlag, 1997.
[2] Waples, D.W., Org. Geochemistry, 31, 553-575, 2000.
[3] Behar et al., Energy & Fuels, 13, 471-481, 1999.
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