Energy & Fuels 1987,1, 452-458
452
Articles Comparison of Methods for Measuring Kerogen Pyrolysis Rates and Fitting Kinetic Parameterst Alan K. Burnham," Robert L. Braun, and Hugh R. Gregg Lawrence Livermore National Laboratory, Livermore, California 94550
Alain M. Samoun Lab Instruments, Inc., Kenwood, California 95452 Received April 20, 1987
We determine rates of product evolution during pyrolysis of several petroleum source rocks and isolated kerogens by nonisothermal techniques, including Rock Eval pyrolysis and pyrolysis-MS/MS, The resulting data are analyzed by nonlinear regression and simpler correlation techniques in terms of discrete and Gaussian-distributed activation energy models. We find that temperatures measured by standard Rock Eval analysis are too low by about 40 "C,resulting in kinetic expressions that are much too fast. Proper temperature calibration eliminates this problem. We explore the sensitivity of the kinetic parameters and extrapolation to geologic heating rates to uncertainty in the temperature calibration. We find that the discrete distribution model provides a superior fit to the laboratory data and probably a more reliable extrapolation to geological heating rates. We also assess how differences among kinetics for individual species relate to the activation energy distribution required for total hydrocarbon evolution. Kinetics from Rock Eval pyrolysis predict hydrocarbon generation rates intermediate between kerogen decomposition and oil expulsion rates during hydrous pyrolysis, but slight differences in activation energies result in similar predictions for a geological heating rate. Predictions of petroleum generation temperatures for lacustrine source rocks cover almost the same range as for marine source rocks.
Introduction Although the concept that petroleum formation is a kinetic process is well established, determining kinetic parameters for quantitative prediction has been elusive because of the complexity of the maturation process. Many experimental procedures and kinetic models have been tried.l Programmed microscale pyrolysis (e.g., Rock Eva12)and sealed-bomb hydrous pyrolysis3 are two currently popular experimental techniques, but they differ markedly in temperature-pressure history as well as the time that products have to undergo secondary reactions. Moreover, programmed pyrolysis typically measures total volatile hydrocarbons while hydrous pyrolysis measures expelled (floating) oil and bitumen (unexpelled extractable material), so it is not obvious how the kinetics from the two different experiments should relate. Hydrous pyrolysis would appear to be a better simulation, but the Rock Eval technique is much faster and cheaper. There are additional complications in choosing how to analyze the kinetic data. Lewan3has derived reasonable kinetic parameters for expelled oil from Phosphoria and Woodford shale assuming a simple first-order reaction. In contrast, other workers have found that an activation energy distribution is needed for some aspects of product evolution from coal,4Green River oil shale: and isolated Work performed under the auspices of the U.S.Department of Energy by the Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48. 0887-0624/87/2501-0452$01.50/0
kerogens of various types.2*6 We have shown that the effective activation energy derived by assuming no distribution is strongly influenced by the distribution if actually present, and ignoring even a small distribution would cause problems in extrapolating the rate to geologic temperatures.' However, the mathematical complexity and computer requirements for treating activation energy distributions have limited their use. In this paper, we have started a systematic comparison of different techniques for measuring and deriving product formation kinetics. This work includes writing an easyto-use computer program for determining either Gaussian or discrete activation energy distributions. A t this stage, most of our kinetic results are from the Rock Eval I1 instrument, which is commonly used in the petroleum industry. However, initial pyrolysis-MS/MS experiments and both p ~ b l i s h e and d ~ ~unpublishedlOJ1 ~~~ kinetic data ~~
(1) Two good reviews, by D. W. Waples and B. Horsfield, are contained in: Aduances i n Petroleum Geochemistry, Brooks, J., Web, D., Eds.; Academic: New York, 1984; Vol. l.,pp 7-68, 247-298. (2) Ungerer, P.; Espitalie, J.; Marquis, F.; Durand, B. In Thermal Modeling in Sedimentary Basins; Burrus, J., Ed.; Technip: Paris, 1986;
pp 531-546. (3) Lewan, M. D. Philos. Tram. R. SOC.London, A 1985,315,123-134. (4) Howard, J. B. In Chemistry of Coal Utilization; Elliot, M. A., Ed.; Wiley: New York, 1981; 2nd Suppl Vol.; pp 665-784. (5) Campbell, J. H.; Gallegos, G.; Gregg, M. Fuel 1980, 59, 727-732. (6) Tissot, B.; Espitalie, J. Rev. Inst. Fr. Pet. 1975, 30, 743-777. (7) Braun, R. L.; Burnham, A. K. Energy Fuels 1987, 1, 153-161. (8)Campbell, J. H.; Koskinas, G. J.; Stout, N. D. Fuel 1978, 57, 372-376.
0 1987 American Chemical Society
Kerogen Pyrolysis Rates and Kinetic Parameters
sample AP22 AP25 NAIL1 NAIL8 NAKY KIMR
PHOS WDFRD MMNG FSHN a
tot C 16.0 15.6 4.5 1.8
11.4 6.4 16.8 15.6 18.1
11.9
tot H 1.4 1.6 0.8 0.7 1.4 1.5 2.2
1.6 2.7 2.0
Energy & Fuels, Vol. 1, No. 6,1987 453
Table I. Chemical Analysis of Samples anal., % tot N acid COz org C tot S SO4 0.3 22.2 9.9 0.3 17.1 0.5 10.9 0.0 0.7 1.0 0.2 4.2 2.2 0.0 2.0 0.2 1.3 0.9 0.2 1.0 11.1 4.4 0.0 0.8 0.4 6.2 3.3 0.7 0.5 0.8 16.6 1.8 0.1 0.7 0.8 15.4 3.3 0.2 0.8 3.8 17.0 1.7 0.2 3.3 11.0 0.8 0.7 0.0
FeS
FeSz
org S
org S/org CQ
0.1 0.1
0.4
0.2 0.3
0.006 0.026
0.2 0.0 0.2 0.1
3.8 2.3 0.5
0.016 0.021 0.023 0.019
0.4 0.0
0.9 0.5
0.5 0.3 1.0 0.8 0.3 0.2
1.8
2.2
0.006
0.008
Atomic ratio calculated before rounding.
for some of our samples enable some comparisons among different techniques and the beginnings of an understanding of the origins of activation energy distributions.
Experimental Section Samples. A chemical analysis of the samples used in this study is given in Table I. The sulfur speciation was made by measuring the sulfur content, corrected to a raw shale basis, of the residue from successive water, HC1, and HN03 treatments. The two Green River shales (Eocene) come from the Anvil Points mine in the Mahogany zone near Rifle, CO. AP25 is a blend approximately representing run-of-mine shale, while AP22 comes from a single rock. Both have been used in previous experiments from this l a b ~ r a t o r y . ~The ~ ~ JKimmeridge ~ clay (Cretaceous) was provided by G. Speers (Norsk Hydro) and came from the 1800-m depth in a Norwegian North Sea well. The Phosphoria shale (Permian, Retort member) was provided by G. Claypool (USGS) and came from a quarry near Retort Mountain in SW Montana. Woodford shale (Devonian-Mississippian) was provided by M. Lewan (Amoco, sample WD-27) and comes from the Springer outcrop in Carter Co., OK. It is said to be very similar to that used in his hydrous pyrolysis s t ~ d yand , ~ our analytical procedure gave approximately the same organic sulfur to carbon ratio. New Albany shales (Devonian-Mississippian) came from both Illinois and Kentucky. J. Crockett (ISGS) provided a series of core samples from the 340-350-m depth of a well in Tazewell Co., E. The densities of the core pieces were calculated from dry and submerged weights. The two samples characterized in Table I represent the extremes of the density range (2.45 g/cm3 for NAIL1 and 2.53 g/cm3 for NAIL8). Sample NAKY came from Bullitt Co., KY, and is similar to batches used in oil shale processing research at LLNL. Samples of Maoming (MMNG) and Fushun (FSHN) oil shale were provided by Zhang Shi KO of Sinopec International. We also obtained type I and type I1 kerogens from Africa (source proprietary) for which we had no corresponding whole rock. All samples are nominally immature (vitrinite reflectance
