A Comparison Between the Properties of Devonian Shale and Green

20 A Comparison Between the Properties of Devonian Shale and Green River Oil Shale

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via Thermal Analysis C. S. W E N and T. F. Y E N 1

2

University of Southern California, Los Angeles, CA 90007

Differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) have been used for the investigation of Devonian shale. Activation energy values of both raw shale and kerogen concentrate were obtained, and the values are considerably higher than those of the Green River oil shale. The same is true for the heats of pyrolysis. Specific heats of Devonian shale are lower than those of Green River oil shale. Heat absorbed above 550°C is attributable to the presence of pyrite in Devonian shale. In a pyrite-free Devonian shale kerogen, the peak near 550°C had disappeared. Removal of pyrite will have a thermal decomposition peak at 450°C, which is lower than that of the Green River oil shale kerogen.

shales offer a potential crude oil resource for the United States. These oil shales represent two trillion barrels of oil in Colorado, Utah, and Wyoming, and an additional one trillion barrels of oil of Devonian shale formation in the eastern United States. Eastern Devonian shale is quite different from western Green River shale. The noticeable differences are in geological age, kerogen structure, oil content, pyrite, and other mineral compositions. In the past, little effort has been focused on Devonian shales. However, the increasing role of eastern shales has now Current address: Gulf Science and Technology Co., P . O . Drawer 2038, Pittsburgh, P A 15230. To whom correspondence should be addressed. 1

2

0-8412-0468-3 / 79 / 33-183-343$05.00/ 0 © 1979 American Chemical Society

Oblad et al.; Thermal Hydrocarbon Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1979.


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THERMAL HYDROCARBON CHEMISTRY

been recognized (J) because of their proximity to the industrial center of the United States and their unique character, and because of the shortage of oil and gas supplies. Thermal decomposition of oil shales is essential to obtain shale oil from the insoluble organic matter (kerogen) and to release gas from the gas-bearing shale. The use of the thermogravimetric analysis (TGA) to investigate the thermal properties of Green River oil shales has been studied extensively in the past few decades (2,3,4,5). However, only a few workers (6,7,8) have studied Devonian shales by this technique. Recently there is considerable interest in the production of gas from eastern Devonian shale. This chapter presents the results of a com parison between Green River oil shale and Devonian shale by using differential scanning calorimetry (DSC) and TGA. The effect of demineralization of shales upon heat of reaction also is reported. Experimental Ground shale samples (150-200 mesh) from the Green River For mation (Anvil Point, Colorado) as well as those of Devonian shale (Cottageville, West Virginia) were used. From these samples kerogen concentrates were prepared by an acid leaching method (9) in which the soluble organic matter (bitumen) had been extracted in advance. The elemental analyses of samples are listed in Table I. In the case of Devonian shale kerogen, pyrite had been removed by the electrolytic treatment (10). The DSC data were obtained with a Du Pont cell base and a Model 990 thermal analyzer. An aluminum pan containing the sample was placed on the raised platform in the DSC cell, and an empty pan was placed on the reference platform. DSC scans for the samples were obtained from 150° to 600°C at a linear heating rate of 20°C/min. Dur ing the run, a slight flow of nitrogen was maintained. The T G A data was taken with a Du Pont 951 T G A balance in conjunction with a Model 990 thermal analyzer. A platinum boat containing the sample was suspended from the quartz beam of the balance. Samples were heated to 900°C at 20°C/min with a constant nitrogen flow. Results and Discussion The thermogravimetric analysis curves of kerogen concentrates from both Green River oil shale and Devonian shale are shown in Figure 1. A sharp peak at 500°C for each sample represents the decomposition of kerogen concentrate. Although the geological age of Devonian shale (Devonian, 300 χ 10 years) is much older than Green River shale (Eocene, 60 χ 10 years), their transition temperatures for kerogen decomposition are identical. In the case of raw shales, their transition temperatures of organic decomposition are 490°C for Green River shale 6

6


20.

Table I.

Elemental Analysis of Green River Oil Shale and Devonian Shale Ash Ν H S Org.C (wt%) (wt%) (wt%) (wt%) (wt %)

Substance Green River oil shale Green River shale kerogen Devonian shale-8°' * Devonian shale-43 -° Devonian shale kerogen-8°' " Devonian shale kerogen-43°'° Pyrite-reduced Devonian shale kerogen a

d


345

Thermal Analysis of Oil Shales

WEN AND YEN

2.2 8.4 1.2 1.4 3.9 6.4 7.4

16.5 68.6 5.0 7.4 43.1 59.3 69.7

62.2 9.8 89.0 87.1 32.5 21.0 3.3

0.8 2.4 2.6 1.9 19.5 14.6 1.2

0.5 3.5 0.1 0.02 0.5 0.6 0.6

° Cottageville Devonian shale, core N o . 11940. 3445.10-3445.60 ft. 3713.78-3714.43 ft. Pyrite removed from Devonian shale kerogen^3 by electrolytic method (8). b

0

d

_ J

200

Figure 1.

1

300

1

400

«

'

500 6 0 0 T E M P E R A T U R E (°C)

' 700

— 800

1

9 0 0

TGA and DTG thermal analyses for (A) Green River shale kerogen and (B) Devonian shale kerogen (No. 43)



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and 490°C for Devonian shale, respectively. The minor trailing peak at 556°C (Figure l b ) is attributable to the pyrite decomposition from Devonian shale kerogen. The wide peak around 200°-330°C (Figure la) is probably attributable to the volatile hydrocarbons contained in the Green River shale kerogen. The peak is not present in Devonian shale kerogen. The initial temperature of thermal decomposition occurs slowly at around 400°C for both kerogen samples. In applying the kinetic analysis of Freeman and Carroll (11,12) to the T G A data (Table II), it was found that the activation energies for thermal decomposition of Devonian shale are higher than those of Green River oil shale for both raw shale and kerogen samples. Similar data on activation energies of Green River shales, presented previously (3,4,13), ranged from 40 to 48 kcal/mol for the same grade of oil shale (~ 26 gallons per ton, Fischer Assay). DSC output curves for raw shales and kerogen concentrates of the two shale samples are shown in Figure 2. It was observed that the thermal effects of the samples are mainly endothermic in nature. Values of heats of pyrolysis can be expressed as calories per unit weight of starting shale sample, as given in Table III. The results of the values concerning heats of pyrolysis are compared in Table III with the experi mental work of Sohns et al. (14), Wise et al. (15), and Cook (16) at retorting temperatures. There is a close agreement with the results of those workers. The minor differences are not surprising when one realizes that the mechanism of oil shale pyrolysis is highly complex (17,18)—it is probably that the observed thermal behavior is the result of several competing reactions. In the case of Green River shale kerogen, the first wide endothermic peak (temperatures ranging from 160° to 330°C) in the DSC curve (Figure 2a) could be attributable to the combined heat of temperature sensation, decomposition, and product volatilization. The second section of the DSC curve (temperature ranging from 330° to 4 0 0 ° C ) shows a small constant endothermic region which may be Table II. Activation Energy for Thermal Decomposition of Raw Shale and Kerogen Concentrate from Green River Oil Shale and Devonian Shale α

Kerogen Concentrate

Raw Shale Green River

Devonian

Green River

Devonian

46.2 (40-48)°

74.6 57.1

50.0 —

64.9

a 6 c d

d

Average activation energy (kcal/g-mol). Shale N o . 43. Data collected from Refs. 3, 4, and 13. Ref. 8 Chattanooga black shale, Oak Ridge, Tennessee.


b

Thermal Analysis of Oil Shahs


WEN AND YEN

American Chemical Society Library 1155 16th St. N. W. Oblad et al.; Thermal Hydrocarbon Chemistry Washington, D. C. 20036 Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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Table III.

Heats of Pyrolysis for Green River Oil Shale and Devonian Shale (Unit: cal/g)

Raw Shale

Kerogen Devonian

Green River Sohns* Wise* Cook

e

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β b c d

163

158

Green River

Devonian

This Work

This Work

Cook

This Work

No. 8

No. 43

164

186

293

283

330

314

d

c

From Ref. 13. From Ref. 14. From Ref. 15. Average value for N o . 8 and N o . 43.

attributable to the thermal cancellations from some endothermic and exothermic reactions. Though the overall pyrolysis of oil shale has always been reported as an endothermic reaction, several exothermic reactions (e.g., carbon residue formation and hydrogen transfer within pyro-bitumen) might occur during the pyrolysis. The sharp peak (temperature above 4 0 0 ° C ) in the DSC curve of kerogen (Figure 2) is attributed mainly to the thermal decomposition of the kerogen matrix. The specific heats of raw shales and kerogen for both samples (Table IV) are calculated from the relation with the DSC data (Equation 1),

L

p

~

Hm

U

T

j

where C = specific heat of the sample ( c a l / g - ° C ) , Ε = cell calibration coefficient at a given temperature ( dimensionless ), A = y-axis sensitivity ( mcal/min-in. ), A == difference in y-axis deflection between sample and blank baselines at temperature of interest (in.), H = heating rate ( ° C / m i n ) , m = sample mass (mg). Values of C for Devonian shale are lower than those of Green River shale. Most of the heat absorbed in Devonian shale above 550°C (Figure 2b and Table III) was attributable to the pyrite present in Devonian shale. Devonian raw shale contains 2-3% (by weight) of pyrite (19). Pyrite is strongly associated with kerogen matrices in Devonian shales. It is extremely difficult to separate it from the shale organic material. The DSC curves of Devonian shale kerogen, pyrite mineral, and pyritereduced kerogen are compared in Figure 3. The major DSC peak of pyrite centered around 550°-560°C had been eliminated in the pyritereduced kerogen. The heat of pyrite decomposition from Devonian shale p

q s

y

r

p


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WEN AND YEN

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Table IV. Specific Heats for Green River Oil Shale and Devonian Shale C


T(°C) 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 α

Green River Raw Shale

Green River Shale Kerogen

0.067 0.106 0.143 0.197 0.185 0.203 0.258 0.320 0.346 0.345 0.334 0.304 0.413 0.525 0.768 1.068 1.669 1.194

0.486 0.615 0.706 0.833 0.860 0.836 0.681 0.492 0.341 0.262 0.303 0.283 0.308 0.493 0.987 1.674 2.148 1.011

p

(cal/g-°C)

Devonian Shale'

Devonian Shale Kerogen-8

Devonian Shale Kerogen-43

0.005 0.012 0.019 0.027 0.032 0.063 0.078 0.105 0.104 0.106 0.111 0.105 0.105 0.121 0.239 0.455 0.657 0.814

0.218 0.242 0.297 0.320 0.365 0.586 0.554 0.457 0.334 0.189 0.148 0.258 0.818 1.359 2.303 2.287 1.595 0.863

0.167 0.158 0.206 0.306 0.506 0.669 0.614 0.515 0.408 0.328 0.317 0.438 0.793 1.343 2.190 2.645 3.061 2.074

Average value for Devonian shale No. 8 and No. 43.

kerogen was estimated to be 67.6 cal/g. This value agrees with the enthalpy of pyrite (64.5 cal/g) reported by the U.S. Geological Survey (20). Though transition temperatures of decomposition for both kerogen concentrates are identical (at 5 0 0 ° C ) , Devonian shale kerogen is much older in geological age and has higher aromaticity than does Green River shale kerogen (19). The higher activation energy obtained from Devo nian shale kerogen (64.9 kcal/g-mol vs. 50 kcal/g-mol see Table II) may be attributable to the difference of aromatic bond breaking rather than a rupture of the saturated aliphatic bonds. This may provide some informa tion on the internal structural difference between the two types of kerogen. The thermal properties of Devonian shale are quite different from those of Green River oil shale. The associated pyrite in kerogen concen trate may contribute greatly to the effect on thermal degradation of Devonian gas-bearing shale. For the first time DSC was applied to determining thermal properties of Devonian shale as well as Green River oil shale.


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CHEMISTRY


THERMAL HYDROCARBON

—J

500

!

550

1

600

TEMPERATURE (°C) Figure 3. DSC thermal analysis of pyrite in (A) pyrite mineral (Rico, Colorado), (B) Devonian shale kerogen, and (C) pyrite-reduced kerogen of Devonian shale


20.

WEN AND YEN


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Acknowledgment This work was supported by the U.S. Department of Energy under contracts M E R C EY-77-G-21-8104, L E R C E(29-2)-3619, A.G.A. BR-48-21, and GRI 5010-362-0035.


Literature Cited 1. "Enhanced Recovery of Unconventional Gas, The Program, Vol. II"; DOE, 1978. 2. Heady, H. Am. Mineral 1952, 37, 804. 3. Allred, V. D. Chem. Eng. Prog. 1966, 62, 55. 4. Arnold, C., Jr. In "Industrial and Laboratory Pyrolyses"; Albright, L. F., Crynes, B. L., Eds.; ACS Symp. Ser. 1976, 32, 492. 5. Weitkamp, A. W.; Gutberlet, L. C. Ind. Eng. Chem., Proc. Des. Dev. 1970, 9, 386. 6. Scrima, D. Α.; Meyer, W. C.; Yen, T. F. "Chemistry of Marine Sediments"; Yen, T. F., Ed.; Ann Arbor Science, 1977; Chapter 14. 7. Claypool, G. E.; Threlkeld, C. N. Preprints, First Eastern Gas Shale Sym posium, 1977, 438-449. 8. Herrell, A. Y.; Arnold, C., Jr. Thermochim. Acta, 1976, 17, 165. 9. Robinson, W. E. "Organic Geochemistry"; Eglinton, G., Murphy, M. T. J., Eds.; Springer-Verlag: New York, 1969, Chapter 6. 10. Wen, C. S.; Kwan, J.; Yen, T. F. Fuel, 1976, 55, 75. 11. Freeman, E. S.; Carroll, B. J. Phys. Chem. 1958, 62, 394. 12. Daniels, T. "Thermal Analysis"; John Wiley and Sons: New York, 1973; 60-82. 13. Hill, G. R.; Dougan, P. Q. Colo. Sch. Mines 1967, 62, 75. 14. Sohns, H . W.; Mitchell, L. E.; Cox, R. J.; Barnet, W. I.; Murphy, W. I. R. Ind. Eng. Chem. 1951, 43, 33. 15. Wise, R. L.; Miller, R. C.; Sohns, H. W. U.S. Bureau of Mines, 1971, RI 7482. 16. Cook, E. W. Q. Colo. Sch. Mines 1970, 65, 133. 17. Wen, C. S.; Yen, T. F. Chem. Eng. Sci. 1977, 32, 346. 18. Fausett, D. W.; George, J. H . ; Carpenter, H. C. U.S. Bureau of Mines, 1974, RI 7889. 19. Yen, T. F.; Wen, C. S.; Tang, J. I. S.; Kwan, J. T.; Young, D. K.; Chow, E. Preprints, First Eastern Gas Shale Symposium, 1977, 414-430. 20. Robie, R. Α.; Waldbaum, D. R. U.S., Geol. Surv. Bull. 1968, 1259. RECEIVED June 30, 1978.


A Comparison Between the Properties of Devonian Shale and Green

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