Energy & Fuels 2004, 18, 1603-1604
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Communications Kerogen Chemistry 1. Sorption of Water by Type II Kerogens at Room Temperature John W. Larsen* and Michael T. Aida Department of Chemistry, 6 E. Packer Avenue, Lehigh University, Bethlehem, Pennsylvania 18015 Received May 20, 2004. Revised Manuscript Received July 22, 2004 The reactions between kerogens and water at high temperatures (>300 °C) have been extensively studied, because oil is liberated and kerogen-water reactions may have a role in petroleum formation.1 We have been unable to find any data on the possible dissolution of water in kerogens at the lower temperatures, where water properties are very different from those of near-critical water. We report here isopiestic measurements of water uptake by four different Type II kerogens, all of which absorb water. Water uptake is larger and faster by kerogens that contain residual minerals. Kerogen-liquid interactions follow Regular Solution Theory reasonably well.2-4 Solvents having solubility parameters (δ) approximately equal to those of kerogens (9.5-10 cal1/2 cm-3/2) readily dissolve in and swell kerogens.2-4 At 350 °C, the solubility parameter of water is 9.1 cal1/2 cm-3/2 and it should readily dissolve in kerogens, so that it can react with the kerogen.5 At room temperature, water has a solubility parameter of δ ) 24 cal1/2 cm-3/2 and should not be soluble in kerogens, thereby limiting kerogen-water reactions to the kerogen’s external surface. Kerogens are hydrophobic and are not wet well by water at room temperature.1 If water is not present within kerogens under the geological conditions of petroleum formation, then kerogen-water chemical reactions, even though they are possible, are unlikely to have a role in petroleum formation from kerogen. We sought data on water solubility in kerogens at temperatures below ∼160 °C and found none in the literature. Because of its importance to kerogen geochemistry, we decided to examine the low-temperature solubility of water in kerogens experimentally, despite their hydrophobic nature and the prediction of Regular Solution Theory. When kerogen-bearing shales are heated under water to 325 or 350 °C, oil is liberated and excess carbon dioxide is formed.1,6 This process has been studied extensively * Author to whom correspondence should be addressed. Present address: The Energy Institute, 209 Academic Projects Building, The Pennsylvania State University, University Park, PA 16802-1303. Telephone: 610-758-3489. Fax: 610-758-3461. E-mail address: jwlarsen1@ juno.com. (1) Lewan, M. D. Geochim. Cosmochim. Acta 1997, 61, 3691-3723. (2) Larsen, J. W.; Li, S. Energy Fuels 1994, 8, 932-936. (3) Larsen, J. W.; Li, S. Energy Fuels 1997, 11, 897-901. (4) Larsen, J. W.; Parikh, H.; Michels, R. Org. Geochem. 2002, 33, 1143-1152. (5) Larsen, J. W. Prepr. Pap.sAm. Chem. Soc., Div. Fuel. Chem. 1999, 44 (2), 393-396.
Figure 1. Weight increase in kerogen samples exposed to water in an evacuated desiccator at room temperature: ([) Bakken, (2) Kimmeridge, (9) Paris Basin C01123, and (b) Paris Basin C01124.
(for reference, see the papers of Lewan1 and Seewald7), because of its possible role in petroleum formation. Redox buffering by minerals may be important.7 Water properties at 350 °C and at the lower temperatures of petroleum formation (60-160 °C) are so different that water is best considered to be two different materials with different properties and reactivities at the two different temperatures, although it has the same chemical composition.8 We exposed four kerogens to water vapor in an evacuated desiccator for 250 h and followed the change in mass of the kerogen samples. The results are shown in Figure 1. All four kerogens steadily increase in weight. They absorb water at room temperature. All four kerogen samples were isolated from the shales by dissolution of the minerals in aqueous HCl, followed by aqueous HF, following a standard procedure.9 Two of (6) Behar, F.; Lewan, M. D.; Lorant, F.; Vandenbrouke, M. Org. Geochem. 2003, 34, 575-600. Lewan, M. D.; Rubie, T. E. Org. Geochem. 2002, 33, 1457-1475. Ruble, T. E.; Lewan, M. D.; Philp, R. P. AAPG Bull. 2001, 85, 1333-1371. Seewald, J. S.; Benitez-Nelson, B. C.; Whelan, J. K. Geochim. Cosmochim. Acta 1998, 62, 1599-1617. Andresen, B.; Throndson, T.; Barth, T.; Bolstad, J. Org. Geochem. 1994, 21, 1229-1242. Price, L. C.; Wenger, L. M. Org. Geochem. 1992, 19, 141-159. (7) Seewald, J. S. Nature 2003, 426, 327-333. Seewald, J. S. Geochim. Cosmochim. Acta 2001, 65, 1641-1664. (8) Siskin, M.; Katritzky, A. R. Chem. Rev. 2001, 101, 825-836. (9) Saxby, J. D. Chem. Geol. 1970, 6, 173-184.
10.1021/ef049875f CCC: $27.50 © 2004 American Chemical Society Published on Web 08/21/2004
1604 Energy & Fuels, Vol. 18, No. 5, 2004
Communications
Table 1. Mineral Content of Kerogens, Using Thermogravimetric Analysis (TGA) Amount Remaining at 1000 °C (wt %) kerogen
1st run
2nd run
average
Bakken NDGS-105 Kimmeridge Paris Basin CO1123 Paris Basin CO1124
2.3 26.5 1.4 37.8
2.0 31.0 1.9 37.8
2.2 28.7 1.7 37.8
the samples (Bakken NDGS-105 and Paris Basin CO1124) were then treated with CrCl2, following the procedure of Acholla and Orr to remove pyrites.10 The isolated kerogens were dried to constant weight under vacuum. To determine their mineral content, samples were heated to 1000 °C in a stream of air under thermogravimetric analysis (TGA), which is a procedure that burns off the organic material and leaves only ash. The results are given in Table 1. To measure water uptake, samples of the kerogen were placed in small aluminum trays in a desiccator that contained water. The desiccator was closed and evacuated using a water aspirator. Periodically, the samples were removed from the desiccator, quickly placed in tared weighing bottles, weighed, and returned to the desiccator, and then the experiment was continued. Studies of the Paris Basin kerogens, including full characterization data, have been published.4 The immature Bakken shale was obtained from Lehigh Price, and its geological provenance and characterization are available.11 The data shown in Figure 1 demonstrate that kerogens do absorb water at low temperatures. It is not surprising that the weight increase in mineral-rich kerogens is greater than that in kerogens with a small mineral content. The minerals themselves may be absorbing or (10) Acholla, F. V.; Orr, W. L. Energy Fuels 1993, 7, 406-410.
adsorbing water and the mineral/organic interface may provide a more rapid diffusion pathway than any that exist in pure kerogen. Siskin has discussed the importance of kerogen/mineral interactions.12 It is noteworthy that the rate of water uptake may be increasing with time. One can visualize a situation in which hydration of polar groups would “loosen” the macromolecular structure and enhance diffusion rates. The data provide no information about the location of the water in the kerogen or why these primarily hydrocarbon materials absorb water. We suspect that the polar functionalities in these kerogens are involved in both diffusion and water absorption. The most important point is that water apparently has sufficient access to Type II kerogens at low temperature to justify including kerogen-water reactions in petroleum formation. Acknowledgment. This material is based in part on work supported by the National Science Foundation, under Grant No. 9820862. Grateful acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We are grateful to ExxonMobil Research Co. Leigh Price, and Raymond Michels for supplying samples. EF049875F (11) Meissner, F. F. Petroleum Geology of the Bakken Formation Williston Basin, North Dakota and Montana. In Petroleum Geochemistry and Basin Evaluation; Demaison, G., Murris, R. J., Eds.; AAPG: Tulsa, OK, 1984; pp 159-179. Price, L. C.; Ging, T.; Daws, T.; Pawlewicz, M.; Anders, D. Organic Metamorphism in the Mississippian-Devonian Bakken Shale North Dakota Portion of the Williston Basin. In Hydrocarbon Source Rocks of the Greater Rocky Mountain Region; Woodward, J., Meissner, F. F., Clayton, J. L., Eds.; Rocky Mountain Association of Geologists: Denver, CO, 1984; pp 83-133. Price, L. C.; Daws, T.; Pawlewicz, M. J. Pet. Geol. 1986, 9, 125-162. (12) Siskin, M.; Brons, G.; Payack, J. F., Jr. Energy Fuels 1987, 1, 248-252.
