Energy & Fuels 1990,4, 132-146
132
Art ic 1es Mathematical Model of Oil Generation, Degradation, and Expulsion Robert L. Braun* and Alan K. Burnham Lawrence Livermore National Laboratory, L-207, P.O. Box 808, Livermore, California 94550 Received September 1I , 1989. Revised Manuscript Received December 28, 1989 We present a mathematical model (PYROL) for simulating oil generation, degradation, and other chemical reactions occurring during pyrolysis of petroleum source rocks over a specified history of temperature and hydrostatic pressure. The model also simulates compaction of the source rock and expulsion of a liquid water phase, a hydrocarbon-rich liquid phase, and a vapor phase. The governing equations for PYROL consist of the time derivatives of 150 variables: 32 vapor species, 32 liquid species, 19 solid species, and 67 other variables (including pore pressure, pore volume, and diagnostics). These ordinary differential equations are expressed in terms of 100 chemical reaction rates and 32 vaporization/condensation relations. A modified Redlich-Kwong-Soave equation of state is used in calculating the vapor/liquid equilibria and PVT behavior. The model is validated by comparison with a wide variety of pyrolysis experiments in open and closed systems. It is then applied to conditions of interest for petroleum generation over geologic time periods. The results for type I kerogen indicate that oil expulsion efficiency is strongly dependent on total organic carbon (TOC), decreasing from 600 to 200 mg of C5+oil/g of TOC as carbon content decreases from 10 to 1wt % at a constant heating rate of 3 "C/My. Conversely, it is essentially independent of heating rate for 10 wt % TOC and only moderately dependent on heating rate for lower carbon contents.
Introduction Traditional petroleum exploration has used geophysical techniques to locate traps where petroleum (oil and gas) may have accumulated. However, the existence of a trap does not ensure that petroleum will be found. An objective of geochemistry is to increase the probability of finding oil by helping to predict whether a given trap is likely to contain oil. Generally, the petroleum source rock is deeper and hotter than the reservoir. Once formed, oil may either be expelled from the source rock to accumulate in a trap (reservoir) or be converted to gas as it undergoes further burial. Our efforts are directed toward improving the reliability of integrated basin analysis, a technique that incorporates many aspects of geology, geophysics, geochemistry, and hydrology in an attempt to model in a deterministic fashion when and where oil is generated, migrates, and accumulates.' It involves calculating the thermal history of the basin, combining that thermal history with chemical kinetic expressions to predict when and where oil and gas are generated, and then estimating from various physical parameters when and where the oil moves. It is important to know not only if oil and gas were formed, but also if expulsion from the source rock occurred at the right time and geometry to fill a trap. (A trap may be formed before, after, or during expulsion by crustal folding, faulting, or erosion and redeposition.) The gas/oil ratio of the expelled organic phase depends on the extent of maturation, and predicting the amount of gas expelled would help predict whether there is enough gas to fill the available reservoir (1) Petroleum Geochemistry and Basin Evaluation; Demaison, G., Murris, R. J., Eds.;AAPG Memoir 35; AAPG: Tulsa, OK, 1984.
0887-0624/90/2504-Ol32$02.50/0
capacity. The modeling must be done accurately, or the results will be counterproductive. We2 and other^^!^ have demonstrated recently that chemical kinetics based on the Arrhenius equation extrapolate fairly well from laboratory to geological temperatures. However, there are still concerns about which, if any, type of laboratory experiment (e.g., programmed micropyrolysis or hydrous pyrolysis) and which rate laws give paramekrs that extrapolate quantitatively to geologic conditions. Definitive comparisons of different laboratory pyrolysis experiments and successful geological application depend on a firm understanding of the expulsion mechanisms in both the experiments and nature. As a corollary, we can be more confident in our extrapolation to geological conditions if we understand the difference between various laboratory experiments. Our objective, therefore, was to develop a model that could describe the chemical- and mass-transport processes in various experiments as well as under geological conditions. Although many simple kinetic expressions have been developed for oil evolution during laboratory pyrolysis of various oil shales and petroleum source rocks, no comprehensive synthesis of the individual parts of the process (i.e., oil generation, vaporization, degradation, and other chemical reactions and transport limitations) was done prior to our general kinetic model of oil shale pyroly~is.~ In that model (PYROL) we calculated the rate of oil ev(2) Sweeney, J. J.; Burnham, A. K.; Braun, R. L. AAF'G Bull. 1987, 71, 967-985. (3) Tissot, B.; Pelet, R.; Ungerer, P. AAF'G Bull. 1987, 71, 145-1466, (4) Quigley, T. M.; Mackenzie, A. S.; Gray, J. R. In Migration of Hydrocarbons in Sedimentary Basins; Doligez, B., Ed.; Technip: Paris, 1987; pp 649-665. (5)Burnham, A. K.; Braun, R. L. In Situ 1985,9, 1-23.
0 1990 American Chemical Society
Model of Oil Generation, Degradation, and Expulsion Table I. Chemical Reactions in
Energy & Fuels, Vol. 4, No. 2, 1990 133
PYROL
T a b l e 11. Chemical Species (kg/ms)
Kerogen Pvrofvsis early CO2 later CO1
kerogen
+ char 1+ H 2 0 + NH3 later oil + char 1 + H 2 0 + NH3 + CO + CHI + CH,
early oil
Oil Coking and Hvdronenation
+
+
char 1 Hz
semicoke + CHI
cokable oil
HZ +uncokable oil + H 2 0 + NH3
t-
1-1 1 12-22 23 24 25 26 27 28 29 30
Oil and Gas Cracking crackable oil
---)
lighter oil
u
CH,
+ char 1 + Hz + CH4 + CH, + CO
1 2 3
char 3 + CH4
+
4
Char Pvrolvsis and Hvdrogenation char 2 +char 3 Hz char 1 --* char 2 HZ CHI NH3 Hz + C H I + H 2 0 NHs Hz -* CH4 HzO
+
+ +
+ +
+
+
+
+ NH3
Mineral Decomoosition mineral water +H z 0
carbonate minerals -* CO2
olution for an open system from the rates of oil generation, oil thermal degradation by coking and cracking, generation of other gases, porosity change, and temperature and pressure change, assuming ideal-gas behavior. Vapor/ liquid equilibria were expressed in terms of Raoult's law. That model was used quite successfully to simulate most of the important results of the laboratory pyrolysis experiments of Stouts and Burnham and Singleton,' at pressures up to 2.7 MPa. While the previous version of PYROL was able to model many laboratory experiments, its vaporization procedure becomes invalid at geologically relevant pressures. To calculate expulsion for the geological case, we need to know the dominant mechanism. Although diffusion, solubility, and capillary forces can be important in special cases, the most important expulsion mechanism from major source rocks is now generally agreed to be bulk flow caused by rock compaction and generation of pore f l ~ i d s . ~This ? ~ is similar in some respects to the gaseous bulk flow expulsion mechanism in the previous version of PYROL. However, it is necessary to adopt equations that can calculate the volumes of both the liquid and gaseous phases a t temperatures between 100 and 200 "C and pressures up to 200 MPa. To add the new capability of modeling geological petroleum expulsion while maintaining our ability to model a variety of open- and closed-vessel laboratory pyrolysis experiments, we have incorporated more rigorous equations of state into PYROL that are valid over a wide range of temperature and pressure. Additional reactions have been incorporated, and the kinetics and stoichiometry of other reactions have been improved. The reacting material can be in an open, closed, or leaky system, and a simple mechanical model for rock compaction has been added. With simultaneous modeling of chemistry, fluid equilibria, PVT relationships, and rock compaction, this pseudo-one-di(6) Stout, N. D.; Koskinas, G. J.; Raley, J. H.; Santor, S. D.; Opila, R. L.; Rothman, A. J. Colo. Sch. Mines Q.1976, 71,153-172. (7) Burnham, A. K.; Singleton, M. F. In Geochemistry and Chemistry of Oil Shale; Miknis, F. P., McKay, J. F., Eds.; ACS Symposium Series 230; American Chemical Society: Washington, DC, 1983; pp 335-351. (8) Tissot, B. In Migration of Hydrocarbons in Sedimentary Basins; Doligez, B., Ed.; Technip: Paris, 1987; pp 1-19. (9) Durand, B. Org. Geochem. 1987,13, 445-459.
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Fluid Species cokable oil species (CH1.15N0.0~00.02) uncokable oil species (CH1.82N0.0100.01)
HZO N2
COP
co
H, CH, CH.z
(-C3H7)
unused
Solid Species kerogen precursor 1 of oil, char 1, H2, H20, NH3 kerogen precursor 2 of oil, char 1, H2, HzO, NH, kerogen precursor of CH4 kerogen precursor of CH, kerogen precursor of CO kerogen precursor 1 of C 0 2 kerogen precursor 2 of C02 semicoke (CHo.9No.0500,0~) char (CH0.63N0.0500.02) char (CH0.23N0.0300.02)
char
(CH0.07N0,0300.02)
unused unused dolomite (first COz) other mineral carbonate mineral water 1 mineral water 2 mineral water 3 remainder
T a b l e 111. O t h e r Variables Integrated by PYROL cokable oil expelled, kg/m3 uncokable oil expelled, kg/m3 water expelled, kg/m3 noncondensible gas expelled, mol/m3 cumulative oil loss due to coking of oil, kg/m3 cumulative oil loss due to cracking of liquid oil, kg/m3 cumulative oil loss due to cracking of oil vapor, kg/m3 cumulative oil gain due to hydrogenation of semicoke, kg/m3 37 cumulative oil gain due to hydrogenation of oil, kg/m3 38 pressure 39 raw shale basis, kg/m3 40 porosity 41-51 cokable oil net generation (kg/m3) from bitumen, kerogen pyrolysis, and hydrogenation of cokable oil 52-62 uncokable oil net generation (kg/m3) from bitumen, kerogen pyrolysis, and hydrogenation of cokable oil 63 concentration of liquid water phase, kg/m3 64 virtual kerogen that decomposes to early virtual bitumen 65 virtual kerogen that decomposes to later virtual bitumen 66 early virtual bitumen 67 later virtual bitumen
1-11 12-22 23 24-32 33 34 35 36
mensional model can be used for studying and possibly predicting the timing and efficiency of oil expulsion from a source rock. The present paper considers parameters only for Green River shale, a typical lacustrine source rock. Future work will address parameters for marine source rocks.
Model Description The chemical reactions in PYROL are summarized in Table I. These are discussed in detail later and are presented in concise form here only to give a global view of the chemistry. The governing equations for PYROL consist of the time derivatives of 150 variables: 32 gas species, 32 liquid species, and 19 solid species listed in Table I1 and 67 additional variables listed in Table 111. These ordinary differential equations (ODE'S) are inte-
Braun and Burnham
134 Energy & Fuels, Vol. 4 , No. 2, 1990 Table IV. List of Reactions a n d Kinetic Parameters for Green River Shale' E(d, A,b s-l cal/mol 1-1 - _ _1 coking of liquid oil 3.0 X 10" 45000 0 12-22 cracking of cokable oil vapor 0.0 0.0 0 23-33 cracking of cokable oil liquid 1.0 x 1012 55 000 34-44 cracking of uncokable oil vapor 1.0 x 1012 55 000 45-55 cracking of uncokable oil liquid 0.0 0 56-66 hydrogenation of cokable oil vapor 0.0 0 67-77 hydrogenation of cokable oil liquid hydrogenation of semicoke to 0 0.0 78 cokable oil hvdroeenation of char 1 to CH, + 6.0 x 10-lo 19 870 79 80 81 82 83 84 85
.N H +~ H ~ O - NHI, + H,O
+ pyrolysis of semicoke to char 1 + H2 pyrolysis of char 1 to char 2 + H2 + CH4 pyrolysis of char 2 to char 3 + H2 cracking of CH, to CH, + char 3 hvdroeenation of char 2 to CH,
pyrolysis of kerogen to early oil, char 1, HzO, H2, NH3 pyrolysis of kerogen to later oil, 86 char 1, H20, H,, NH3 87 pyrolysis of kerogen to CHI pyrolysis of kerogen to CH, 88 89 pyrolysis of kerogen to CO 90 pyrolysis of kerogen to early C02 pyrolysis of kerogen to later C02 91 92 thermal decomposition of dolomite thermal decomposition of other 93 carbonate 94 release of mineral water 1 95 release of mineral water 2 96 release of mineral water 3 97-128 vaporization/condensation 129 pyrolysis of virtual kerogen to early virtual bitumen 130 pyrolysis of virtual kerogen to later virtual bitumen 131 pyrolysis of early virtual bitumen 132 pyrolysis of later virtual bitumen
2.6 x
1044
8.0 x 10'2 3.2
X
correlation of cracking rate with carbon number for normal hydrocarbons. Most of the reactions in Table IV follow the general first-order rate law - a w l a t = kw
where W is the concentration of the reacting species and k is the Arrhenius rate constant
k = A exp(-E/RT)
- a w l a t = Lw where14
10l2 56 850 (6 820)
Jmk e x p ( - l t k dt)D(E) dE
3.1 x 1013 77 500 (8 140) 1.2 x 10'2 57 000 5.0 X 10l2 47000 (0) 1.0 x 1013 51 000 (0)
1.0 x 1.0x 1.0 x 1.0 x 1.0 x 1.7 X
10'3 1013 1013 1013 10'2 1O'O
0.0
2.6 X 2.6 X 2.6 X 1.0 X 1.0 X
51 000 (0) 51000 (0) 51 000 (0) 43 700 (4 370) 47 000 (3 570) 57800 (0) 0 (0)
10" loo6 10" 1Ol8 10l2
20 OOO 20 000 20 000 51000 45000 (0)
1.0 X 10l2 45000 (0) 5.0 X 10l2 47000 (0) 1.0 X 1013 51 000 (0)
OThe kinetics are developed from data referenced in the text. The only free parameters were the A's for coking and char 1 hydrogenation. Some A's have units of s-l Pa-' (reactions 56-78) or s-l Pa-2 (reactions 79 and 80).
grated by LSODE, the Livermore Solver for Ordinary Differential Equations.lo The ODE'S are expressions of the 100 chemical reactions and 32 vaporization/condensation relations listed in Table IV. The model assumes a uniformly reacting material (no concentration or thermal gradients) and the absence of molecular oxygen. The properties of the oil species are given in Table V. The normal boiling points of n-alkanes are given for reference but are not used in the calculations. Native bitumen is treated as an initial oil content with the distribution shown. The initial distribution of the generated oil is taken from Bissell" with slight adjustments toward an overall heavier oil. We distinguish two oil types: cokable oil can undergo pyrolysis to predominantly a carbonaceous residue, while uncokable oil can undergo pyrolysis to predominantly smaller oil species and gas. These coking and cracking reactions are discussed separately and will serve to more completely define the two oil types in PYROL. The relative rates of coking and cracking for the 11boiling point ranges were calculated from the Voge and Good12 (10) Hindmarsh, A. C. ACM-SIGNUM Newsl. 1980, 15, 10-11. (11)Bissell, E. R.; Burnham, A. K.; Braun, R. L. Ind. Eng. Chem. Process Des. Deu. 1985, 24, 381-386. (12) Voge, H. H.; Good, G. M. J . Am. Chem. SOC.1949, 71,593-597.
(2)
Those reactions in Table IV having a specified (zero or nonzero) value of C T ~are permitted to have a Gaussian distribution of activation energies.13 They follow the rate law
7 950 51000 (0)
(1)
0 Jm
exp(-Jtk
dt)D(E) dE
(3)
]
(4)
and the Gaussian distribution function is given by D(E) =
(27r)-'"flE-'
exp[-(E - E 0 ) ~ / 2 f l ~ ~ ](5)
UE is zero for any of these reactions, then k = iz. Some of the reactions also incorporate other factors in eq 1or 3 to account for pressure, molecular size, and inhibition effects. We will now discuss the stoichiometry and kinetics of the chemical reactions for Green River shale. The expression given for the net rate of each reaction is used in formulating the time derivative of each variable that is being integrated and, with other information, in calculating the rate of fluid expulsion. The subscripts refer to the chemical species (i) in Table I1 and the chemical reaction G) in Table IV. The text following each net reaction discusses the data used in deriving the molar stoichiometric coefficients and reaction kinetics. The computer code itself uses mass stoichiometric coefficients to rigorously maintain an overall mass balance. Our formulations for a few of the reactions, however, are not conducive to a strict elemental balance (e.g., cracking of oil and CH,). For these reactions, small discrepancies in the elemental balance may be introduced but are probably of insignificant importance. Decomposition of Total Organic Matter.
If
native cokable bitumen
native uncokable bitumen
0.204CH1.,5N0.0500,0Z + 0.536CH1.82N0.0100.01 + cokable oil
uncokable oil
0.1595CH0,63N,o500,02+ 0.003CO + 0.013C02 char 1
0.0025CH4 + 0.036CH,
+
+ 0.008H20 + 0.0013NH3 (6)
The above equation expresses the total organic matter in terms of native bitumen and products from kerogen pyrolysis. PYROL also includes a bitumen intermediate species for kerogen pyrolysis, but we defer its discussion until later. The native bitumen in eq 6 is initially distributed among our oil components as indicated in Table V. This native bitumen amounts to 4.6 mol % of the total organic carbon and has a H/C ratio that agrees with the value of 1.7 reported by Robinson and Cummins.15 The (13) Anthony, D. B.; Howard, J. B. AIChE J. 1976,22, 625-656. (14) Braun, R. L.; Burnham, A. K. Energy Fuels 1987, 1, 153-161.
Model of Oil Generation, Degradation, and Expulsion
av no. of carbon atoms 6 8 10 12.6 15 19 23 27 33 40 50
1
2 3 4 5 6 7 8 9 10 11
av mol wt, dmol 86 114 142 177 212 261 317 380 464 562 703
Energy & Fuels, Vol. 4,No. 2, 1990 135
Table V. Properties of the Eleven Oil Fractions distribn init normal of native distribn of bp, " C bitumen, wt % oil, wt % 69 0.0 2 126 0.0 4 174 0.7 6 7 226 5.0 271 16.0 8 20.3 323 9 374 10.8 10 422 12 10.2 14.7 476 13 14 525 13.0 594 9.3 15
re1 cokable amt, wt%
fia
3.2 9.6 16.0 20.8 24.0 27.2 30.3 35.2 38.3 41.6 44.8
1.0 2.1 3.8 6.5 10.0 15.8 24.2 35.4 53.8 80.8 153.8
Relative rate of degradation.
maximum possible oil yield was chosen to be 110% of Fischer assay, based on fluidized-bed pyrolysis results."J6 Therefore, to get the stoichiometric coefficients for the above reaction, the Fischer assay stoichiometry measured by Singleton et al." was modified to account for the experimental degradation of 10% of the generated oil. No hydrogen and only a small amount of methane are direct products of kerogen pyrolysis. These gases are largely produced by oil degradation and semicoke pyrolysis reactions discussed later. To allow for different kinetics for oil and gas generation, kerogen pyrolysis is treated as seven separate first-order reactions:
r, = k j @
(j = 85,
..., 91 for i = 1, ..., 7, respectively) (7)
At time zero, the kerogen is partitioned into the initial concentrations of the seven reactants. The kinetics parameters are given in Table IV. Oil generation kinetics were determined by Rock-Eva1 analysis.18 Two reactions are permitted for oil generation, with 5 wt 90 of the potential oil generated by the early r e a ~ t i o n .Gas ~ generation data for Green River shale indicate that CH, (CZ-C,) follows the same kinetics as the principal oil generation.lg Kinetics for COz generation were derived from the gas generation data of Huss and Burnham.20 Although each of the seven reactions can be assigned a distribution of activation energies, only COz generation has a significant distribution for this shale. Coking of Cokable Oil to Semicoke and Methane.
By examining the change of oil composition with the yield loss of coking, we first determined the effective composition of the oil that cokes. The cokable oil (CHl,15No,05Oo.oz)is more aromatic and higher in nitrogen than Fischer assay oil (CH1,63N0,02100.0z). It was therefore assumed that onll. part of the oil, approximately equal to the 13CNMR aromaticity of about 25%, could coke. The relative amount of each oil fraction that could coke (shown in Table V) was then determined from the relative nitrogen contents estimated from published data on nitrogen content of various boiling fractions.22 The amount of CHI produced by oil coking was determined from previous data,21but with corrections for the additional CH4 generated at slow heating rates by char pyrolysis. Our reaction for oil coking is actually only part of the net coking reaction studied by Campbell et a1.21The remainder of the reaction is considered as a separate reaction, the pyrolysis of semicoke, in the next section. In the rate expression, f , is the relative coking rate given in Table V. Although our activation energy of 45 kcal/mol used for coking is significantly higher than the value of 35 kcal/mol assumed by Campbell et a1.,21our distribution of frequency factors causes the effective activation energies to be in substantial agreement. In the development of the rate expression, it was assumed that oil coking is inhibited by hydrogen. The base frequency factor was empirically determined by comparison of the calculated and measured oil yield from Fischer assay. Although the above expression treats coking of both vapor and liquid species, PYROL can exclude vapor-phase coking, if desired. Pyrolysis of Semicoke to Char 1 and Hydrogen. CH0,9N0.0500,0Z
-+
r, = f i k j
1
+ O.O002P~,
The coking reaction is based on published data on the change of oil yield and gas generation with heating rate.21 (15) Robinson, W. E.; Cummins, J. J. J. Chem. Eng. Data 1960, 5, 74-80. (16) Wallman, P. H.; Tamm, P. W.; Spars, B. G. Oil Shale, Tar Sands and Related Materials; Stauffer, H.C., Ed.; ACS Symposium Series 163; American Chemical Society: Washington, DC, 1981; pp 93-113. (17) Singleton, M. F.; Koskinas, G. J.; Burnham, A. K.; Raley, J. H. Lawrence Liuermore Natl. Lab. Rep. UCRL-53273, Rev. 1; Livermore, CA, 1986. (18) Burnham, A. K.; Braun, R. L.; Gregg, H. R.; Samoun, A. M. Energy Fuels 1987, I , 452-458. (19) Oh, M. S.; Coburn, T. T.; Crawford, R. W.; Burnham, A. K. 1988. Study of gas evolution during oil shale pyrolysis by TQMS. In Proceedings at the International Conference on Oil Shale and Shale Oil; Zhu Yajie, Ed.; Chemical Industry Press: Beijing, 1988; pp 295-302. (20) Huss, E. B.; Burnham, A. K. Fuel 1982, 61, 1188-1196. (21) Campbell, J. H.;Koskinas, G. J.; Stout, N. D. I n Situ 1978, 2, 1-47.
CH0.63N0.0500.02-k 0.135H~ (lo)
r81
=&81m
(11)
This is the other half of the oil coking reaction. The principal reason for dividing the oil coking reaction in this manner is to be able to cause the production of methane and hydrogen from oil degradation to occur at slightly different temperatures, as suggested by experimental results for Green River shale. The temperatures predicted by PYROL for the production of C 0 2 and CH, (mainly from kerogen pyrolysis), CH, (mainly from oil coking), and Hz (mainly from semicoke pyrolysis) a t a heating rate of 10 OC/min are compared in Figure 1 with recent unpublished data from our pyrolysis-TQMS apparatus. A similar calculation at 4 "C/min gave T,, values close to those reported by Oh et al.19 for the same apparatus. The continued production of C H I and H, at higher temperatures is due to char pyrolysis and will be discussed next. (22) Baughman, G. L. Synthetic Fuels Data Handbook, 2nd ed.; Cameron Engineers, Inc.: Denver, CO, 1978; p 118.
Braun and Burnham
136 Energy & Fuels, Vol. 4, No. 2, 1990
Pyrolysis of Char 1 and Char 2. CH0.63N0.0500,02
--+
0.945CHO.23N0.0300.02 + 0*055CH4+ 0.064H2 + 0.022NH3 (12)
CHo.~3No.o30o.oz -* CH0.07N0.0300.02 + 0.08H2 (13)
zjv
0' = 82, 83 for i = 9, 10, respectively) (14) rj = The char composition and reaction stoichiometry is based on measurements of Huss and Burnham.?o Our rate parameters are based on gas evolution experiments of Campbell et al.,23which are in substantial agreement with the results of Oh et al.19 Campbell clearly demonstrated the need for a distribution of activation energies for simulating char pyrolysis. Allowing such a distribution for a reactant that is itself a product of another reaction requires the special mathematical treatment that we have developed.14 Our rate parameters for char 1 pyrolysis were slightly modified from Campbell's parameters for secondary methane evolution (i.e., increase in activation energy and compensating increase in frequency factor) so that the reaction rate relative to that of other methane sources would be preserved a t low temperatures. Cracking of Oil to Lighter Oil Species, Char 1, and Gas. [oilIi [oilIii + CH0&,0&&,02 + CO + H2 + CH4 + CH, (15)
t
-
rj = fpfikjWI;OrL
where or
(16) 300
0' = 12, ..., 22 or 23, ..., 33 for i
= 1, ..., 11, respectively)
0' = 34, ..., 44 or 45, ..., 55 for i =
12, ..., 22, respectively) The chemical reaction is not written in a balanced form, since the stoichiometry is a function of pressure and reacting oil species. The nonoil products are in the following maw ratio with respect to the CH, product (from Burnham and Taylor,24with minor modifications): char 1 = 0.632, CO = 0.0566, H2 = 0.0314, and CH, = 0.166. Data of Voge and Good12 for cracking of hexadecane indicate that the char and gas product ratios are essentially independent of pressure, but that the amount of oil products relative to CH, increases with pressure. From data of Doue and GuiochonZ5we derived the following expression for the mass ratio with respect to the CH, product for each of the ligher oil species (ii = 1, ..., i - 1)produced by cracking a heavier oil species (i):
gii = 0.58
+ 1.211 - exp[-P/(6
X
106)])Mii/M1
(17)
Absolute mass stoichiometries are then calculated by normalizing the sum of all product masses to be equal to the reactant mass. A p o s s i b l e shortcoming of o u r cracking stoichiometry is that we do not account for formation of products that are heavier than the reactant. This assumption may not be valid a t high pressures. Zhou,26for example, studied the thermolysis of dodecane and found that 21.7 mol % of the reacted dodecane had been converted to C13+in 5 h at 400 "C and 9.2 MPa. Weres et al.27suggest that, at (23) Campbell, J. H.; Gallegos, G.; Gregg, M. Fuel 1980,59, 727-732. (24) Burnham, A. K.; Taylor, J. R. Lawrence Livermore Natl. Lab. Rep. UCID-21243; Livermore, CA, 1979; Available NTIS, Springfield, VA. (25) Doue, F.; Guiochon, G. J . Chim. Phys. 1968, 65, 395-409. (26) Zhou, P.; Crynes, B. L. Ind. Eng. Chem. Process Des. Deu. 1986, 25, 508-514. (27) Weres, 0.; Newton, A. S.; Tsao, L. Org. Geochem. 1988, 12, 433-444.
400 Temperature,
500
600
OC
Figure 1. (a) Measured and (b) calculated gas evolution profiles a t a heating rate of 10 OC/min. T h e measured profiles are unpublished results of John G. Reynolds. T h e calculated gas evolution comes from both primary and secondary pyrolysis reactions.
the temperatures of petroleum source beds and reservoirs, alkylation of olefins may be faster than cracking. This would result in heavier, branched alkanes and a low concentration of alkenes. As yet, however, there are insufficient data to develop an accurate model for heavier cracking products. We plan to examine alkylation on a more fundamental level with a detailed, free-radical model of hydrocarbon pyrolysis.28 Our rate expression f e q 16) is based on a review of t h e literature on thermal cracking kinetics for pure hydrocarbons and crude Although PYROL includes separate reactions for vapor and liquid cracking, we have found no strong evidence that different rate constants, as distinct from stoichiometry, should be used. The relative cracking rates for the eleven oil species, f,, are given in Table V and have already been discussed. Concerning the effect of pressure, most evidence for laboratory experimental conditions is that the cracking rate increases with pressure up to at least 10 MPa.25,30 Therefore, we use a pressure-dependent factor: f p = expl(l.9 X lO-')P exp[-(2.5 X 10-8)P]1
(18)
This causes the cracking rate to have a maximum at 40 MPa that is 16 times the rate at atmospheric pressure and then to gradually decrease at higher pressures. This was patterned after the pressure dependence reported by Fa(28) Axelsson, E. I.; Brezinski, K.; Dryer, F. L.; Pitz, W. J.; Westbrook, C. K. In Twenty-first Symposium (International)on Combustion; The Combustion Institute: Pittsburgh, 1986; pp 783-793. (29) Braun, R. L.; Burnham, A. K. Lawrence Livermore Natl. Lab. Rep. UCID-21507;Livermore, CA, 1988, Available NTIS, Springfield, VA. (30) Fabuss, B. M.; Smith, J. 0.; Satterfield, C. N. In Advances in Petroleum Chemistry and Refining; McKetta, J. J., Ed.; Interscience: New York, 1964; pp 157-201.
Model of Oil Generation, Degradation, and Expulsion buss et al.30 but uses a somewhat more conservative increase in cracking rate than they reported for hexane and heptane. Note in Table IV that Aj is set to zero for cracking reactions of the cokable oil species, since that gave the best agreement with a variety of pyrolysis experiments with Green River shale (to be discussed later). Preliminary calculations of hydrocarbon cracking, using the free-radical code developed for modeling hydrocarbon combustion,28 suggest that the initial increase in cracking rate with pressure occurs only at higher temperatures, but we have insufficient evidence at this time to warrant modifying our approach. Cracking of CH, to Char 3 and Methane. CH, 0.4CH0.07N0,0300.02 + 0.6CH4 (19)
-
(20) + %9) The reaction kinetics are based on cracking of C2-C4 paraffins.29 Note that the pressure dependence used for oil cracking is also used here. This reaction results in a net volume generation, since the actual formula of CH, is best approximated by C3H7. Hydrogenation of Cokable Oil to Uncokable Oil and Gas. r84
= f&84(@9
-
+ 0*405H2 CH1.82NO,O10O,O1+ O.O4NH3 + 0.01H20 (21)
CH1.15N0.0500.02
rj = k j v o r L P H l
0' = 56, ..., 66 or 67, ..., 77 for i =
Energy & Fuels, Vol. 4, No. 2, 1990 137 dropyrolysis identify two distinct stages: an initial rapid direct hydrogenation of the coal followed by a much slower hydrogenation of the residual carbon.31 Our char 1 reaction is probably similar to the initial coal hydrogenation, but the wide spectrum of reactivities for that reaction precluded an a priori specification of the rate of our reaction. Therefore, we used CH4 and H2 yields from pyrolysis experiments at 2.7 MPa to approximate the rate coefficient. Our char 2 reaction should be very similar to the slower coal reaction, and we modeled its rate from data given by Johnson for hydrogenation of coal char.32 The rate parameters for reactions 79 and 80 are given in Table IV. The equilibrium coefficients (in units Pa-') are K79 = 2.5 x 10-13exp(22050/RT) and Km = 2.85 X exp(22050/R T). Carbonate Decomposition. mineral carbonate
rj = kj@
This reaction was included to allow an additional sink of hydrogen and to allow the possibility of breaking the semicoke molecules into smaller fragments. As with oil hydrogenation, we saw no evidence to justify nonzero rates of this reaction for the hydrogen partial pressures of interest in this paper. Hydrogenation of Char 1 and Char 2. CH0,63N0,0500,02+ 1.78H2 CH4 + O.O5NH3 + 0.02HZO (25) CH4 + O.O3NH3 + 0.02Hz0 CHo.23NO.0300.02+ 1.95H2 (26)
' 1 +pH'KjPH, IV
(j= 79,80fori =
9, 10, respectively) (27) Hydrogenation of char appears to be the reaction that best correlates with the quantity of hydrogen ultimately expelled as a product, for the conditions of pressure and heating rate that were tested here. Studies of coal hy-
(28) (29)
These reactions can be used to model any two mineral carbonates that decompose according to first-order nonequilibrium reactions. For the Green River shale and thermal histories addressed in this study, only dolomite decomposition was used. The rate parameters were taken from Campbell.33 Release of Mineral Water. mineral water
1-
rj = k,@[
21
-
H20
(30)
0' = 94,95, 96 for i = 16, 17, 18, respectively) (31)
Evolution of mineral water from Green River shale results mainly from analcime dehydration. We use three reactions in order to allow an activation energy distribution for the equilibrium coefficients to account for a range of volatilities in the dehydrating mineral: Kj = ( @ / e 0 ) ( 6 . 4
X
10l2)exp(-Ej/RT)
(32)
where Ej = 18250, 20 000, and 21 750 cal/mol. This equation results in a decrease in volatility with decreasing fraction of mineral water remaining. The preexponential factor and mean activation energy in eq 32 were taken from Oh.34 The same chemical rate parameters were used for the three reactions, the single frequency factor being determined by matching the Burnham and Singleton7 pyrolysis experiments. This formulation for release of bound water assumes that pore diffusion and external mass transfer are not rate-limiting steps. Bitumen Intermediate Species for Kerogen Pyrolysis. virtual kerogen
-+
r, = k.[
C02
0' = 92, 93 for i = 14, 15, respectively)
11, ..., 22, respectively) (22)
PYROL includes oil hydrogenation in order to allow the possibility of increased oil production by conversion of some of the oil to a less degradable form. Using reasonable rate parameters, we saw no compelling evidence for invoking this reaction for the pyrolysis experiments tested in this paper or for the conditions of interest geologically. Thereafter, the frequency factors were set to zero. Hydrogenation of oil may be an important reaction, however, for processes using high hydrogen pressure and it should then be used. This reaction could also be modified to account for the reaction of cokable oil and water to form uncokable oil and carbon dioxide, if that reaction were demonstrated to be important. Hydrogenation of Semicoke to Cokable Oil.
-
rj = &jWr
