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Kerogen chemistry 5. Anhydride formation in, solvent swelling of, and loss of organics on demineralization of Kimmeridge shales
F U E L P RO CE SS I NG T EC H NOL O G Y 8 9 (2 0 0 8) 3 14 –3 2 1
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Kerogen chemistry 5. Anhydride formation in, solvent swelling of, and loss of organics on demineralization of Kimmeridge shales John W. Larsena,b,⁎, Carlos Islas Floresa a b
Department of Chemistry, Lehigh University, Bethlehem, PA 18015, USA The Energy Institute, The Pennsylvania State University, 209 Academic Projects Building, University Park, PA 16802-2303, USA
AR TIC LE I NFO
ABSTR ACT
Keywords: Kerogen Demineralization Maturation Keywords:
The results of three short and related, but experimentally independent, studies of 4 Kimmeridge
Kerogen
formation. After solvent swelling in tetrahydrofuran (THF), extracted organics were isolated
Demineralization
from the THF and the recovered kerogens were swollen a second time in fresh THF. The second
Maturation
solvent swelling ratios were slightly larger than the first because the presence of the extracts in
shales and their kerogens are reported. Differential scanning calorimeter (DSC) studies of the kerogens reveal that three of the four show evidence of anhydride formation when heated at 20 °C/min between 50 °C and 180 °C. There is no regular rank dependence of anhydride
the original THF lowers solvent activity thus reducing swelling. The shales were demineralized in the usual way except that methylene chloride was added to dissolve any organics that were liberated from the rock as a consequence of mineral dissolution. Small amounts of organics were found in the methylene chloride supporting Price and Clayton's conclusion that organics are expelled from the kerogen and are present in lacunae in the minerals. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
This paper describes three sets of experiments on a 4 member maturation series of Kimmeridge shales. Two might be considered to be control experiments: measurement of the amount of organics lost when the shales are demineralized and the reproducibility of sequential solvent swelling of the same sample, but these two data sets do have wider implications. Each experiment requires its own introduction. Background information on the Kimmeridge source rock and kerogen is contained in Hunt's book [1].
1.1.
occurring was demonstrated by an irreversible endotherm between 50 °C and 180 °C. The evidence that the reaction is anhydride formation is the infrared spectra of the kerogens before and after heating and the observation that the reaction is reversed by water. The thermodynamics of anhydride formation are also consistent with the DSC results. The amount of anhydride formed can be evaluated by measuring the magnitude of the irreversible endotherm caused by the endothermic reaction. With Bakken kerogens, the size of the endotherm decreases with increasing maturity. Establishing the generality of this reaction is important. To this end, DSC studies were made of 4 kerogens of different maturities isolated from Kimmeridge shales.
Anhydride formation 1.2.
We recently reported that Bakken kerogens formed carboxylic acid anhydrides when heated at 20 °C/min in a differential scanning calorimeter (DSC) [2]. That a chemical reaction was
Loss of organics on demineralization
We have been concerned for some time with the macromolecular structure of kerogens [3] and wish to analyze
⁎ Corresponding author. The Energy Institute, The Pennsylvania State University, 209 Academic Projects Building, University Park, PA 16802-2303, USA. E-mail address: [email protected] (J.W. Larsen). 0378-3820/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2007.11.019
F U E L P R O CE SS I NG T EC H NOL O G Y 8 9 (2 0 0 8 ) 3 1 4–3 2 1
kerogen–bitumen systems as sol–gel systems [4], that is systems in which the extractable material (sol) and the cross-linked polymeric material (gel) are related by chemical reactions that form one from the other. In order to do this, complete analyses of the sol and the gel are needed. The possibility of losses during the isolation of kerogen by dissolution of the rock is important [5–7] and does not seem to have been addressed in the literature except for one early study [8]. There is published work that suggests that acid treatment will result in liberating organics from the shale, organics that then would be lost with the aqueous fraction when it is discarded. First, it is known that the H/C ratio decreases during demineralization [8,9]. In addition, Price and Clayton and others have shown that time sequential extraction of ground source rocks produces families of extracts that grow heavier with increasing time [10]. They conclude that the extracted material has moved out of the kerogen and is present in “cracks and parting laminae in the rocks” and “ is ready for expulsion from the rocks” [10]. If this is so, then the nonextracting aqueous HCl and aqueous HF used to dissolve the minerals should release these organics. It is important to confirm this because the methylene chloride used by Price and Clayton for the sequential extractions can extract material from the kerogen and, by dissolving in the kerogen, plasticize it and increase diffusivities of the bitumens dissolved in the kerogens.
1.3.
Repetitive kerogen solvent swelling
There are two factors that can complicate solvent swelling of geological organic polymers. They are the rearrangement of the polymer to a new physical structure and the extraction of material from the polymer resulting in a decrease in the thermodynamic activity of the swelling liquid that results in less swelling than would be observed with the pure liquid. Both of these factors have been observed with coals [11–13]. If a sample is repetitively swollen using fresh solvent each time, the swelling ratio will usually decrease if there is a structure rearrangement and increase if material is being extracted by the swelling liquid. A glassy polymer can be “frozen” in a structure that is not its thermodynamically most stable structure. The inadequate information that we have about the physical state of kerogens at room temperature all indicate a glassy state [14–17]. When a liquid dissolves in the polymer, in this case a kerogen, the polymer is plasticized and can become rubbery. It is then no longer “frozen” in its unstable structure and can rearrange to a more stable structure. As with coals, this will normally be a more highly associated structure with a lower heat capacity and lower solvent swelling. With coals, reductions in solvent swelling ratios of as much as 50% have been observed [11]. It is Important to our understanding of kerogen chemistry to know whether Type I and II kerogens normally exist in their most stable physical structure. Solvent swelling measurements are normally carried out on extracted materials. This is because anything that is extracted from the cross-linked polymer and dissolved in the swelling liquid will lower the thermodynamic activity of that liquid. This drop in activity reduces the driving force for
315
transfer of the swelling liquid from itself to the polymer. It reduces the driving force for polymer swelling and reduces the solvent swelling ratio. Because of this, coals are routinely extracted before swelling [13]. Extraction is more difficult with kerogens because often only small amounts of material are available, especially if cores or cuttings are being studied. We have previously studied the reversibility of solvent swelling [11,18] and here we report more results including measurements of the amount of material extracted by the swelling solvent, in this case tetrahydrofuran.
1.4.
Samples
Kimmeridge clay is a major and well-studied source rock. The four samples used were EB 87, a shallow clay from between 217 and 217 ft deep, sample I from 7552–7557 ft, sample II from 9724–9732 ft, and sample III from 13,245–13,299 ft.
2.
Experimental
The samples were demineralized using the procedures recommended in Saxby's review [7] except that a layer of methylene chloride was present to dissolve and hold any organics liberated by the demineralization. Because of this, a full description of the demineralization procedure is given below. All solvents were HPLC grade supplied in sealed containers and were used without further purification. Where dryness was important, all solvent transfers were made using syringes and the glassware was scrupulously dried and sealed. 2.1.
HCl demineralization
200 mL of an aqueous HCl solution (5 N) was placed in a 500 mL two-necked glass round bottom flask with a water-cooled condenser attached to the center neck. 20 mL of methylene chloride (b.p. 40 °C) were added to the flask and the mixture was vigorously stirred using an oval shaped magnetic stirrer. 10 grams of shale were slowly added to the stirring mixture and the opened neck was closed using a glass stopper. The mixture was kept at ambient temperature under vigorous stirring for 72 h. The condenser prevents evaporative losses of methylene chloride. After the three days, the stirring was stopped and the liquid mixture was filtered by gravity into a 500 mL glass separatory funnel. The solid on the filter paper was washed with 20 mL of methylene chloride and that methylene chloride was used to wash the walls of the round bottom flask and collected. 20 mL of methylene chloride were added to the separatory funnel and the two liquid phases were separated. The two methylene chloride solutions were then combined and anhydrous magnesium sulfate was added to remove any water. The dried solution was filtered into a preweighed vial and the methylene chloride was slowly evaporated using a low dry N2 flow until the weight of the vial and contents was constant. This procedure will result in the loss of some light molecules [17]. The residue after was sealed in a vial under dry N2. The remaining rock was washed with distilled water until the washing water was neutral to blue litmus paper. It was then dried until constant weight in a vacuum oven at 40 °C (P b 1 mm Hg).
316
F U E L P RO CE SS I NG T EC H NOL O G Y 8 9 (2 0 0 8) 3 14 –3 2 1
Fig. 1 – Differential scanning calorimetry results for shallow Kimmeridge kerogen heated at 20 °C/min. Three repetitive scans of the same sample.
2.2.
HF demineralization
200 mL of a 24% weight aqueous solution of HF was placed in a 500 mL Nalgene Erlenmeyer flask. 20 mL of methylene chloride were then added and the mixture was vigorously stirred using a rod shaped magnetic stirrer. ∼ 5 grams of the kerogen from the HCl demineralization were slowly added to the stirring mixture. A Teflon stopper with a long Teflon pipe attached in order to minimize evaporation from the flask was placed in the neck of the flask. The initial height of the liquid– solid mixture was marked on the flask and the system was stirred for 72 h at ambient temperature. The height of the liquid–solid mixture was monitored and more methylene chloride was added when necessary to keep the liquid height
approximately constant. The liquid mixture was then filtered by gravity using a Teflon funnel into a 500 mL Nalgene separatory funnel. The solid was washed with 20 mL of methylene chloride and that methylene chloride was used to wash the walls of the flask. 20 mL of methylene chloride were added to the Nalgene separatory funnel, the two liquid phases were separated, and the two methylene chloride solutions were combined. After drying over anhydrous magnesium sulfate, the methylene chloride solution was filtered into a pre-weighed vial and the methylene chloride was slowly evaporated by using a low dry N2 flow until constant weight was achieved. The vial was then filled with dry N2 and sealed. Finally, any remaining rock is washed with distilled water until the washing water is neutral to blue litmus paper
Fig. 2 – Differential scanning calorimetry results for kerogen from Kimmeridge Rock I heated at 20 °C/min. Three repetitive scans of the same sample.
F U E L P R O CE SS I NG T EC H NOL O G Y 8 9 (2 0 0 8 ) 3 1 4–3 2 1
317
Fig. 3 – Differential scanning calorimetry results for kerogen from Kimmeridge Rock II heated at 20 °C/min. Three repetitive scans of the same sample.
and dried to constant weight in a vacuum oven at 40 °C (P b 1 mm Hg). 2.3.
Solvent swelling measurements
Stabilizer free dry THF was used following the procedure described by Larsen and Li [18]. After swelling equilibrium had been reached, the THF was carefully removed from the kerogen by using a syringe and the THF was allowed to evaporate in a fume hood at room temperature until constant weight was reached. This took about 3 days. Remaining THF was then removed from the kerogen by using a flow of dry N2 at room temperature overnight. Lumps were broken using a clean copper wire and the kerogen was swollen for a second time using fresh THF.
2.4.
Thermogravimetric analyses
The instrument and procedures previously described were used [2]. All measurements were duplicates.
3.
Results and discussion
3.1.
Anhydride formation
Three consecutive DSC analyses of the 4 Kimmeridge kerogens are shown in Figs. 1–4. All of the samples undergo irreversible changes. The results for sample I closely resemble those published earlier for the Bakken kerogens. There is a low temperature irreversible endotherm between 50 °C and 180 °C
Fig. 4 – Differential scanning calorimetry results for kerogen from Kimmeridge Rock III heated at 20 °C/min. Three repetitive scans of the same sample.
318
F U E L P RO CE SS I NG T EC H NOL O G Y 8 9 (2 0 0 8) 3 14 –3 2 1
Fig. 5 – Thermogravimetric Analysis Measured Weight Loss for shallow Kimmeridge kerogen heated at 20 °C/min.
followed by a second irreversible endotherm that begins at about 225 °C. It is most reasonable to assign the low temperature endotherm to anhydride formation. Sample III shows a similar low temperature endotherm, though it is impossible to determine its ending temperature because of the strong high temperature endotherm. The behavior of sample II is not intermediate between samples I and III. It exhibits two weak irreversible endotherms. These data make it clear that the maturity dependence of both anhydride formation and the high temperature endotherm is complex. Two different samples of these kerogens were studied and so the surprising response of sample II is not due to an aberrant sample. This is always a concern when working with samples as small as the 10 mg used in the DSC scans. It is also impos-
sible to separate the high and low temperature endotherms exhibited by the shallow clay. In addition, there is a reversible sharp endotherm at about 130 °C. Its origin is unknown. Additional insight is provided by the thermogravimetric weight losses recorded for duplicate samples at the same 20 °C/min scan rate as the DSC studies. The samples fall into two groups. Samples I and II have lost only about 0.5% of their weight at 180 °C. This is too little to explain the endotherm observed for sample I supporting the explanation that the endotherm is due to anhydride formation. The endotherm for sample II is so small that it could be caused by evaporative losses. Both the least and most mature samples lose significant amounts of material over the whole temperature range studied. This continuous loss would mask the end of the
Fig. 6 – Thermogravimetric Analysis Measured Weight Loss for kerogen from Kimmeridge Rock I heated at 20 °C/min.
F U E L P R O CE SS I NG T EC H NOL O G Y 8 9 (2 0 0 8 ) 3 1 4–3 2 1
319
Fig. 7 – Thermogravimetric Analysis Measured Weight Loss for kerogen from Kimmeridge Rock II heated at 20 °C/min.
initial endotherm due to anhydride formation because the decrease in heat flow is overwhelmed by an increase caused by evaporative losses. Nonetheless, sample II shows a clear maximum in heat loss at a temperature similar to that observed with the Bakken kerogens and sample III. The behavior of the shallow clay is more complex and difficult to assign with certainty (Figs. 5–8). Except for sample II, the Kimmeridge kerogens undergo an irreversible low temperature endothermic change consistent with anhydride formation. Their behavior is complex with sample II having only a weak endotherm and the shallow clay having an additional sharp endotherm. More work will be required to define all of what is occurring. Overall, the low temperature endotherm observed with the Bakken shales is seen also with the Kimmeridge kerogens.
3.2.
Solvent swelling reversibility
The volumetric solvent swelling ratios of the 4 Kimmeridge kerogens in THF are given in Table 1 together with the amount of organic material extracted by the THF during the initial swelling experiment. The swelling ratios are either the same or the second swelling is slightly larger. The larger the amount of extracted material found in the THF, the larger the difference between the swelling ratios. The data indicate that the material extracted from the kerogen does have an effect on the swelling ratio, lowering it as expected. There is no evidence for a structure rearrangement similar to that observed with coals. The results for Kimmeridge kerogen are different from those obtained with an immature Green River Type I kerogen
Fig. 8 – Thermogravimetric Analysis Measured Weight Loss for kerogen from Kimmeridge Rock III heated at 20 °C/min.
320
F U E L P RO CE SS I NG T EC H NOL O G Y 8 9 (2 0 0 8) 3 14 –3 2 1
and four Paris basin Type II kerogens. After isolation using aqueous ammonium sulfate, the Green River kerogen was swollen in chlorobenzene and held at 60 °C–70 °C overnight followed by evaporation of the chlorobenzene and careful drying [18]. This treatment was done to provide the swollen and perhaps rubbery kerogen opportunity to rearrange to a structure having a lower free energy than the initial structure of the isolated kerogen. The occurrence of a rearrangement is demonstrated by a decrease in solvent swelling. The existence of a rearrangement is evidence that the isolated kerogen is, if not glassy, then capable of rearranging only slowly. Using the swelling liquids benzene, chlorobenzene, and carbon disulfide, the second swelling was 11% to 13% less than the first, consistent with the occurrence of a structural rearrangement. Three Paris Basin kerogens were repetitively swollen in THF, methylene chloride, and pyridine after two different demineralization procedures [3]. Of the 18 samples studied, 17 swelled less the 2nd and 3rd times they were swollen than the first time. These results are also consistent with isolated kerogens being in a strained state and capable of rearranging to a more stable state when plasticized by a swelling liquid. Irreversible swelling is exhibited by both a Type I and a Type II kerogen, but the results for Kimmeridge kerogen demonstrate that this behavior is not general. It is not known what factors control this behavior.
3.3.
Loss of organics during demineralization
The data are shown in Table 1. After demineralization with aqueous HCl and aqueous HF, between 14% and 36% of the starting shale remains. Organic material is lost from the solid to the methylene chloride layer in both steps. It is unlikely that this material is extracted and we agree with Price and Clayton [10] that organic material has been expelled from the kerogen and is present either at the interface of the kerogen with the surrounding rock or in vacancies in the rock. The amounts seem small, at most 0.6% of the weight of the rock and between 2% and 3% of the material remaining after demineralization, but 0.6% of a rock that contains 6% organics is 10% of the organics. There may be cases in which the losses are experimentally significant. Organics are freed during shale demineralization. Price and Clayton separated the sequential extracts from the immature Bakken shale into compound classes and their results are contained in Table 2 [10]. There are two factors to be considered when explaining the expulsion of molecules from a kerogen. They are the thermodynamic driving force for expulsion and the diffusion rates through the kerogen. The
Table 1 – Amount of organic material (OM) extracted (mg/g clay) by CH2Cl2 during acid demineralization, volumetric swelling ratio (Q) in THF, and % of kerogen extracted into the THF during the 1st swelling Sample
HCl, OM
HF, OM
Q 1st
Q 2nd
% extracted
Shallow shale Rock I Rock II Rock III
3.7
1.8
1.5 ± 0.8
1.6 ± 1.1
3±2
0.6 2.3 3.8
2.2 1.7 2.3
1.3 ± 0.1 1.4 ± 0.1 1.3 ± 0.6
1.6 ± 0.3 1.8 ± 0.1 1.4 ± 0.6
5±3 6±3 4±3
Table 2 – Percentages of C15− saturated HCs, C15+ saturated HCs, C15− aromatic HCs, C15+ aromatic HCs, resins, and asphaltenes in each sequential extract of Bakken NDGS-105 shale (from [10] ) Fraction
C15− Sats C15+ Sats C15− Aros C15+ Aros Resins Asphaltenes
Extraction number First
Second
Third
Fourth
Fifth
Sixth
13.74 22.60 4.72 22.28 30.65 6.01
12.87 20.21 3.65 23.70 31.05 8.51
10.49 22.31 3.02 21.77 30.70 11.70
6.11 13.75 2.38 23.06 36.48 18.22
3.02 10.34 1.14 18.22 37.48 29.80
0.66 5.61 0.37 9.78 31.39 52.18
interactions between kerogens and organic liquids can be described quantitatively using Regular Solution Theory [18]. This theory is not always applicable [19], but has been successfully used in a quantitative model of petroleum formation [20]. Our use here will be qualitative. The larger the difference between the solubility parameter (δ) of a class of molecules and the solubility parameter of the kerogen, normally about 10.5 cal1/2 mole1/2 cm−3/2, the larger is the driving force for expulsion. In other words, the greater the difference in solubility parameters, the lower the solubility of the compound class in the kerogen. Very little is known about diffusivities of small molecules in kerogens. It is easy to make qualitative predictions using glassy polymers as a model. Small molecules move fastest and linear molecules move faster than branched. The conclusions arising from the application of these two models are easily arrived at and informative. The first molecules to leave a kerogen will be small linear alkanes. These will be followed by longer alkanes and branched alkanes. Alkyl substituted aromatics will be next, both because of slower diffusion and a lower driving force for expulsion. Increasing the size of the aromatic system will retard expulsion by slowing diffusion and by lowering the driving force. Finally larger aromatic polar molecules will have a small driving force for expulsion because their solubility parameters will be close to that of kerogen and their large branched structures (di- or poly substituted aromatic ring systems) will greatly slow diffusion. They are likely to be retained in the kerogen. This predicted expulsion pattern agrees closely with the Price and Clayton results on sequential extraction [10].
Acknowledgements This material is based upon work supported by the national Science Foundation under grants No. 9820862 and 9909939 and by a travel grant from the CNRS. We are grateful to Dr. Raymond Michels (Université Henri Poincaré) and Dr. Artur Stankiewiz (Shell) for supplying the kerogen samples.
REFERENCES [1] J.M. Hunt, Petroleum Geochemistry and Geology, 2nd Ed.W. H. Freeman, New York, 1995.
F U E L P R O CE SS I NG T EC H NOL O G Y 8 9 (2 0 0 8 ) 3 1 4–3 2 1
[2] J.W. Larsen, C. Islas-Flores, M.T. Aida, P. Opaprakasit, P. Painter, Energy Fuels 19 (2005) 145–151. [3] J.W. Larsen, H. Parikh, R. Michels, Org. Geochem. 33 (2002) 1143–1152. [4] L.H. Sperling, Introduction to Physical Polymer Science, Wiley – Interscience, New York, 2001. [5] J.K. Whelan, C.L. Thompson-Rizer, in: M.H. Engel, S.A. Macko (Eds.), Organic Geochemistry Principles and Applications, Plenum, NY, 1993, p. 14. [6] B. Durand, J.C. Monin, in: B. Durand (Ed.), Kerogen. Insoluble Organic Matter from Sedimentary Rocks, Editions Technip, Paris, 1980, p. 4. [7] J.D. Saxby, Chem. Geol. 6 (1970) 173–184. [8] T.E. Dancy, V.J. Giedroyc, Inst. Pet. 36 (1950) 593–603. [9] B. Durand, G. Nicaise, in: B. Durand (Ed.), Kerogen. Insoluble Organic Matter from Sedimentary Rocks, Editions Technip, Paris, 1980, p. 2. [10] L.C. Price, J.L. Clayton, Geochim. Cosmochim. Acta 56 (1992) 1213–1222. [11] J.W. Larsen, R.A. Flowers II, P.J. Hall, G. Carlson, Energy Fuels 11 (1997) 998–1002.
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[12] Y. Yun, E.M. Suuberg, Prepr. Pap. Am. Chem. Soc., Div. Fuel Chem 38 (2) (1993) 489–494; E.M. Suuberg, Y. Otake, M.J. Langer, K.T. Leung, I. Milosavjevic, Energy Fuels 8 (1994) 1247–1262. [13] T.K. Green, J. Kovac, J.W. Larsen, Fuel 63 (1984) 935–938; J.W. Larsen, J.C. Cheng, C.S. Pan, Energy Fuels 5 (1991) 57–59. [14] T.J. Parks, L.J. Lynch, D.S. Webster, D. Barrett, Energy Fuels 2 (1988) 185–190. [15] G.H.S. Lee, A.J. Berkovich, J.H. Levy, B.R. Young, M.A. Wilson, Energy Fuels 12 (1998) 262–267. [16] A.R. Palmer, G.E. Maciel, Anal. Chem. 54 (1982) 2194–2198. [17] J. C. Zeszotarski, R. C. Chromik, R. P. Vinci, M. C. Messmer, R. Michels, J. W. Larsen, Geochim. Cosmochim. Acta. in press. [18] J.W. Larsen, S. Li, Energy Fuels 8 (1994) 932–936. [19] U. Ritter, Org. Geochem. 34 (2003) 319–326. [20] S. R. Kelemen, H. Freund, M. Siskin, D. J. Curry, Y. Xiao, W. N. Olmstead, M. L. Gorbaty, A. E. U. S. Bence, Patent Appl. No. 10/426,356 Pub. No. U. S. 2004/0019437 A1, 2004.
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Kerogen chemistry 5. Anhydride formation in, solvent swelling of, and loss of organics on demineralization of Kimmeridge shales John W. Larsena,b,⁎, Carlos Islas Floresa a b
Department of Chemistry, Lehigh University, Bethlehem, PA 18015, USA The Energy Institute, The Pennsylvania State University, 209 Academic Projects Building, University Park, PA 16802-2303, USA
AR TIC LE I NFO
ABSTR ACT
Keywords: Kerogen Demineralization Maturation Keywords:
The results of three short and related, but experimentally independent, studies of 4 Kimmeridge
Kerogen
formation. After solvent swelling in tetrahydrofuran (THF), extracted organics were isolated
Demineralization
from the THF and the recovered kerogens were swollen a second time in fresh THF. The second
Maturation
solvent swelling ratios were slightly larger than the first because the presence of the extracts in
shales and their kerogens are reported. Differential scanning calorimeter (DSC) studies of the kerogens reveal that three of the four show evidence of anhydride formation when heated at 20 °C/min between 50 °C and 180 °C. There is no regular rank dependence of anhydride
the original THF lowers solvent activity thus reducing swelling. The shales were demineralized in the usual way except that methylene chloride was added to dissolve any organics that were liberated from the rock as a consequence of mineral dissolution. Small amounts of organics were found in the methylene chloride supporting Price and Clayton's conclusion that organics are expelled from the kerogen and are present in lacunae in the minerals. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
This paper describes three sets of experiments on a 4 member maturation series of Kimmeridge shales. Two might be considered to be control experiments: measurement of the amount of organics lost when the shales are demineralized and the reproducibility of sequential solvent swelling of the same sample, but these two data sets do have wider implications. Each experiment requires its own introduction. Background information on the Kimmeridge source rock and kerogen is contained in Hunt's book [1].
1.1.
occurring was demonstrated by an irreversible endotherm between 50 °C and 180 °C. The evidence that the reaction is anhydride formation is the infrared spectra of the kerogens before and after heating and the observation that the reaction is reversed by water. The thermodynamics of anhydride formation are also consistent with the DSC results. The amount of anhydride formed can be evaluated by measuring the magnitude of the irreversible endotherm caused by the endothermic reaction. With Bakken kerogens, the size of the endotherm decreases with increasing maturity. Establishing the generality of this reaction is important. To this end, DSC studies were made of 4 kerogens of different maturities isolated from Kimmeridge shales.
Anhydride formation 1.2.
We recently reported that Bakken kerogens formed carboxylic acid anhydrides when heated at 20 °C/min in a differential scanning calorimeter (DSC) [2]. That a chemical reaction was
Loss of organics on demineralization
We have been concerned for some time with the macromolecular structure of kerogens [3] and wish to analyze
⁎ Corresponding author. The Energy Institute, The Pennsylvania State University, 209 Academic Projects Building, University Park, PA 16802-2303, USA. E-mail address: [email protected] (J.W. Larsen). 0378-3820/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2007.11.019
F U E L P R O CE SS I NG T EC H NOL O G Y 8 9 (2 0 0 8 ) 3 1 4–3 2 1
kerogen–bitumen systems as sol–gel systems [4], that is systems in which the extractable material (sol) and the cross-linked polymeric material (gel) are related by chemical reactions that form one from the other. In order to do this, complete analyses of the sol and the gel are needed. The possibility of losses during the isolation of kerogen by dissolution of the rock is important [5–7] and does not seem to have been addressed in the literature except for one early study [8]. There is published work that suggests that acid treatment will result in liberating organics from the shale, organics that then would be lost with the aqueous fraction when it is discarded. First, it is known that the H/C ratio decreases during demineralization [8,9]. In addition, Price and Clayton and others have shown that time sequential extraction of ground source rocks produces families of extracts that grow heavier with increasing time [10]. They conclude that the extracted material has moved out of the kerogen and is present in “cracks and parting laminae in the rocks” and “ is ready for expulsion from the rocks” [10]. If this is so, then the nonextracting aqueous HCl and aqueous HF used to dissolve the minerals should release these organics. It is important to confirm this because the methylene chloride used by Price and Clayton for the sequential extractions can extract material from the kerogen and, by dissolving in the kerogen, plasticize it and increase diffusivities of the bitumens dissolved in the kerogens.
1.3.
Repetitive kerogen solvent swelling
There are two factors that can complicate solvent swelling of geological organic polymers. They are the rearrangement of the polymer to a new physical structure and the extraction of material from the polymer resulting in a decrease in the thermodynamic activity of the swelling liquid that results in less swelling than would be observed with the pure liquid. Both of these factors have been observed with coals [11–13]. If a sample is repetitively swollen using fresh solvent each time, the swelling ratio will usually decrease if there is a structure rearrangement and increase if material is being extracted by the swelling liquid. A glassy polymer can be “frozen” in a structure that is not its thermodynamically most stable structure. The inadequate information that we have about the physical state of kerogens at room temperature all indicate a glassy state [14–17]. When a liquid dissolves in the polymer, in this case a kerogen, the polymer is plasticized and can become rubbery. It is then no longer “frozen” in its unstable structure and can rearrange to a more stable structure. As with coals, this will normally be a more highly associated structure with a lower heat capacity and lower solvent swelling. With coals, reductions in solvent swelling ratios of as much as 50% have been observed [11]. It is Important to our understanding of kerogen chemistry to know whether Type I and II kerogens normally exist in their most stable physical structure. Solvent swelling measurements are normally carried out on extracted materials. This is because anything that is extracted from the cross-linked polymer and dissolved in the swelling liquid will lower the thermodynamic activity of that liquid. This drop in activity reduces the driving force for
315
transfer of the swelling liquid from itself to the polymer. It reduces the driving force for polymer swelling and reduces the solvent swelling ratio. Because of this, coals are routinely extracted before swelling [13]. Extraction is more difficult with kerogens because often only small amounts of material are available, especially if cores or cuttings are being studied. We have previously studied the reversibility of solvent swelling [11,18] and here we report more results including measurements of the amount of material extracted by the swelling solvent, in this case tetrahydrofuran.
1.4.
Samples
Kimmeridge clay is a major and well-studied source rock. The four samples used were EB 87, a shallow clay from between 217 and 217 ft deep, sample I from 7552–7557 ft, sample II from 9724–9732 ft, and sample III from 13,245–13,299 ft.
2.
Experimental
The samples were demineralized using the procedures recommended in Saxby's review [7] except that a layer of methylene chloride was present to dissolve and hold any organics liberated by the demineralization. Because of this, a full description of the demineralization procedure is given below. All solvents were HPLC grade supplied in sealed containers and were used without further purification. Where dryness was important, all solvent transfers were made using syringes and the glassware was scrupulously dried and sealed. 2.1.
HCl demineralization
200 mL of an aqueous HCl solution (5 N) was placed in a 500 mL two-necked glass round bottom flask with a water-cooled condenser attached to the center neck. 20 mL of methylene chloride (b.p. 40 °C) were added to the flask and the mixture was vigorously stirred using an oval shaped magnetic stirrer. 10 grams of shale were slowly added to the stirring mixture and the opened neck was closed using a glass stopper. The mixture was kept at ambient temperature under vigorous stirring for 72 h. The condenser prevents evaporative losses of methylene chloride. After the three days, the stirring was stopped and the liquid mixture was filtered by gravity into a 500 mL glass separatory funnel. The solid on the filter paper was washed with 20 mL of methylene chloride and that methylene chloride was used to wash the walls of the round bottom flask and collected. 20 mL of methylene chloride were added to the separatory funnel and the two liquid phases were separated. The two methylene chloride solutions were then combined and anhydrous magnesium sulfate was added to remove any water. The dried solution was filtered into a preweighed vial and the methylene chloride was slowly evaporated using a low dry N2 flow until the weight of the vial and contents was constant. This procedure will result in the loss of some light molecules [17]. The residue after was sealed in a vial under dry N2. The remaining rock was washed with distilled water until the washing water was neutral to blue litmus paper. It was then dried until constant weight in a vacuum oven at 40 °C (P b 1 mm Hg).
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Fig. 1 – Differential scanning calorimetry results for shallow Kimmeridge kerogen heated at 20 °C/min. Three repetitive scans of the same sample.
2.2.
HF demineralization
200 mL of a 24% weight aqueous solution of HF was placed in a 500 mL Nalgene Erlenmeyer flask. 20 mL of methylene chloride were then added and the mixture was vigorously stirred using a rod shaped magnetic stirrer. ∼ 5 grams of the kerogen from the HCl demineralization were slowly added to the stirring mixture. A Teflon stopper with a long Teflon pipe attached in order to minimize evaporation from the flask was placed in the neck of the flask. The initial height of the liquid– solid mixture was marked on the flask and the system was stirred for 72 h at ambient temperature. The height of the liquid–solid mixture was monitored and more methylene chloride was added when necessary to keep the liquid height
approximately constant. The liquid mixture was then filtered by gravity using a Teflon funnel into a 500 mL Nalgene separatory funnel. The solid was washed with 20 mL of methylene chloride and that methylene chloride was used to wash the walls of the flask. 20 mL of methylene chloride were added to the Nalgene separatory funnel, the two liquid phases were separated, and the two methylene chloride solutions were combined. After drying over anhydrous magnesium sulfate, the methylene chloride solution was filtered into a pre-weighed vial and the methylene chloride was slowly evaporated by using a low dry N2 flow until constant weight was achieved. The vial was then filled with dry N2 and sealed. Finally, any remaining rock is washed with distilled water until the washing water is neutral to blue litmus paper
Fig. 2 – Differential scanning calorimetry results for kerogen from Kimmeridge Rock I heated at 20 °C/min. Three repetitive scans of the same sample.
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317
Fig. 3 – Differential scanning calorimetry results for kerogen from Kimmeridge Rock II heated at 20 °C/min. Three repetitive scans of the same sample.
and dried to constant weight in a vacuum oven at 40 °C (P b 1 mm Hg). 2.3.
Solvent swelling measurements
Stabilizer free dry THF was used following the procedure described by Larsen and Li [18]. After swelling equilibrium had been reached, the THF was carefully removed from the kerogen by using a syringe and the THF was allowed to evaporate in a fume hood at room temperature until constant weight was reached. This took about 3 days. Remaining THF was then removed from the kerogen by using a flow of dry N2 at room temperature overnight. Lumps were broken using a clean copper wire and the kerogen was swollen for a second time using fresh THF.
2.4.
Thermogravimetric analyses
The instrument and procedures previously described were used [2]. All measurements were duplicates.
3.
Results and discussion
3.1.
Anhydride formation
Three consecutive DSC analyses of the 4 Kimmeridge kerogens are shown in Figs. 1–4. All of the samples undergo irreversible changes. The results for sample I closely resemble those published earlier for the Bakken kerogens. There is a low temperature irreversible endotherm between 50 °C and 180 °C
Fig. 4 – Differential scanning calorimetry results for kerogen from Kimmeridge Rock III heated at 20 °C/min. Three repetitive scans of the same sample.
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Fig. 5 – Thermogravimetric Analysis Measured Weight Loss for shallow Kimmeridge kerogen heated at 20 °C/min.
followed by a second irreversible endotherm that begins at about 225 °C. It is most reasonable to assign the low temperature endotherm to anhydride formation. Sample III shows a similar low temperature endotherm, though it is impossible to determine its ending temperature because of the strong high temperature endotherm. The behavior of sample II is not intermediate between samples I and III. It exhibits two weak irreversible endotherms. These data make it clear that the maturity dependence of both anhydride formation and the high temperature endotherm is complex. Two different samples of these kerogens were studied and so the surprising response of sample II is not due to an aberrant sample. This is always a concern when working with samples as small as the 10 mg used in the DSC scans. It is also impos-
sible to separate the high and low temperature endotherms exhibited by the shallow clay. In addition, there is a reversible sharp endotherm at about 130 °C. Its origin is unknown. Additional insight is provided by the thermogravimetric weight losses recorded for duplicate samples at the same 20 °C/min scan rate as the DSC studies. The samples fall into two groups. Samples I and II have lost only about 0.5% of their weight at 180 °C. This is too little to explain the endotherm observed for sample I supporting the explanation that the endotherm is due to anhydride formation. The endotherm for sample II is so small that it could be caused by evaporative losses. Both the least and most mature samples lose significant amounts of material over the whole temperature range studied. This continuous loss would mask the end of the
Fig. 6 – Thermogravimetric Analysis Measured Weight Loss for kerogen from Kimmeridge Rock I heated at 20 °C/min.
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319
Fig. 7 – Thermogravimetric Analysis Measured Weight Loss for kerogen from Kimmeridge Rock II heated at 20 °C/min.
initial endotherm due to anhydride formation because the decrease in heat flow is overwhelmed by an increase caused by evaporative losses. Nonetheless, sample II shows a clear maximum in heat loss at a temperature similar to that observed with the Bakken kerogens and sample III. The behavior of the shallow clay is more complex and difficult to assign with certainty (Figs. 5–8). Except for sample II, the Kimmeridge kerogens undergo an irreversible low temperature endothermic change consistent with anhydride formation. Their behavior is complex with sample II having only a weak endotherm and the shallow clay having an additional sharp endotherm. More work will be required to define all of what is occurring. Overall, the low temperature endotherm observed with the Bakken shales is seen also with the Kimmeridge kerogens.
3.2.
Solvent swelling reversibility
The volumetric solvent swelling ratios of the 4 Kimmeridge kerogens in THF are given in Table 1 together with the amount of organic material extracted by the THF during the initial swelling experiment. The swelling ratios are either the same or the second swelling is slightly larger. The larger the amount of extracted material found in the THF, the larger the difference between the swelling ratios. The data indicate that the material extracted from the kerogen does have an effect on the swelling ratio, lowering it as expected. There is no evidence for a structure rearrangement similar to that observed with coals. The results for Kimmeridge kerogen are different from those obtained with an immature Green River Type I kerogen
Fig. 8 – Thermogravimetric Analysis Measured Weight Loss for kerogen from Kimmeridge Rock III heated at 20 °C/min.
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and four Paris basin Type II kerogens. After isolation using aqueous ammonium sulfate, the Green River kerogen was swollen in chlorobenzene and held at 60 °C–70 °C overnight followed by evaporation of the chlorobenzene and careful drying [18]. This treatment was done to provide the swollen and perhaps rubbery kerogen opportunity to rearrange to a structure having a lower free energy than the initial structure of the isolated kerogen. The occurrence of a rearrangement is demonstrated by a decrease in solvent swelling. The existence of a rearrangement is evidence that the isolated kerogen is, if not glassy, then capable of rearranging only slowly. Using the swelling liquids benzene, chlorobenzene, and carbon disulfide, the second swelling was 11% to 13% less than the first, consistent with the occurrence of a structural rearrangement. Three Paris Basin kerogens were repetitively swollen in THF, methylene chloride, and pyridine after two different demineralization procedures [3]. Of the 18 samples studied, 17 swelled less the 2nd and 3rd times they were swollen than the first time. These results are also consistent with isolated kerogens being in a strained state and capable of rearranging to a more stable state when plasticized by a swelling liquid. Irreversible swelling is exhibited by both a Type I and a Type II kerogen, but the results for Kimmeridge kerogen demonstrate that this behavior is not general. It is not known what factors control this behavior.
3.3.
Loss of organics during demineralization
The data are shown in Table 1. After demineralization with aqueous HCl and aqueous HF, between 14% and 36% of the starting shale remains. Organic material is lost from the solid to the methylene chloride layer in both steps. It is unlikely that this material is extracted and we agree with Price and Clayton [10] that organic material has been expelled from the kerogen and is present either at the interface of the kerogen with the surrounding rock or in vacancies in the rock. The amounts seem small, at most 0.6% of the weight of the rock and between 2% and 3% of the material remaining after demineralization, but 0.6% of a rock that contains 6% organics is 10% of the organics. There may be cases in which the losses are experimentally significant. Organics are freed during shale demineralization. Price and Clayton separated the sequential extracts from the immature Bakken shale into compound classes and their results are contained in Table 2 [10]. There are two factors to be considered when explaining the expulsion of molecules from a kerogen. They are the thermodynamic driving force for expulsion and the diffusion rates through the kerogen. The
Table 1 – Amount of organic material (OM) extracted (mg/g clay) by CH2Cl2 during acid demineralization, volumetric swelling ratio (Q) in THF, and % of kerogen extracted into the THF during the 1st swelling Sample
HCl, OM
HF, OM
Q 1st
Q 2nd
% extracted
Shallow shale Rock I Rock II Rock III
3.7
1.8
1.5 ± 0.8
1.6 ± 1.1
3±2
0.6 2.3 3.8
2.2 1.7 2.3
1.3 ± 0.1 1.4 ± 0.1 1.3 ± 0.6
1.6 ± 0.3 1.8 ± 0.1 1.4 ± 0.6
5±3 6±3 4±3
Table 2 – Percentages of C15− saturated HCs, C15+ saturated HCs, C15− aromatic HCs, C15+ aromatic HCs, resins, and asphaltenes in each sequential extract of Bakken NDGS-105 shale (from [10] ) Fraction
C15− Sats C15+ Sats C15− Aros C15+ Aros Resins Asphaltenes
Extraction number First
Second
Third
Fourth
Fifth
Sixth
13.74 22.60 4.72 22.28 30.65 6.01
12.87 20.21 3.65 23.70 31.05 8.51
10.49 22.31 3.02 21.77 30.70 11.70
6.11 13.75 2.38 23.06 36.48 18.22
3.02 10.34 1.14 18.22 37.48 29.80
0.66 5.61 0.37 9.78 31.39 52.18
interactions between kerogens and organic liquids can be described quantitatively using Regular Solution Theory [18]. This theory is not always applicable [19], but has been successfully used in a quantitative model of petroleum formation [20]. Our use here will be qualitative. The larger the difference between the solubility parameter (δ) of a class of molecules and the solubility parameter of the kerogen, normally about 10.5 cal1/2 mole1/2 cm−3/2, the larger is the driving force for expulsion. In other words, the greater the difference in solubility parameters, the lower the solubility of the compound class in the kerogen. Very little is known about diffusivities of small molecules in kerogens. It is easy to make qualitative predictions using glassy polymers as a model. Small molecules move fastest and linear molecules move faster than branched. The conclusions arising from the application of these two models are easily arrived at and informative. The first molecules to leave a kerogen will be small linear alkanes. These will be followed by longer alkanes and branched alkanes. Alkyl substituted aromatics will be next, both because of slower diffusion and a lower driving force for expulsion. Increasing the size of the aromatic system will retard expulsion by slowing diffusion and by lowering the driving force. Finally larger aromatic polar molecules will have a small driving force for expulsion because their solubility parameters will be close to that of kerogen and their large branched structures (di- or poly substituted aromatic ring systems) will greatly slow diffusion. They are likely to be retained in the kerogen. This predicted expulsion pattern agrees closely with the Price and Clayton results on sequential extraction [10].
Acknowledgements This material is based upon work supported by the national Science Foundation under grants No. 9820862 and 9909939 and by a travel grant from the CNRS. We are grateful to Dr. Raymond Michels (Université Henri Poincaré) and Dr. Artur Stankiewiz (Shell) for supplying the kerogen samples.
REFERENCES [1] J.M. Hunt, Petroleum Geochemistry and Geology, 2nd Ed.W. H. Freeman, New York, 1995.
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[2] J.W. Larsen, C. Islas-Flores, M.T. Aida, P. Opaprakasit, P. Painter, Energy Fuels 19 (2005) 145–151. [3] J.W. Larsen, H. Parikh, R. Michels, Org. Geochem. 33 (2002) 1143–1152. [4] L.H. Sperling, Introduction to Physical Polymer Science, Wiley – Interscience, New York, 2001. [5] J.K. Whelan, C.L. Thompson-Rizer, in: M.H. Engel, S.A. Macko (Eds.), Organic Geochemistry Principles and Applications, Plenum, NY, 1993, p. 14. [6] B. Durand, J.C. Monin, in: B. Durand (Ed.), Kerogen. Insoluble Organic Matter from Sedimentary Rocks, Editions Technip, Paris, 1980, p. 4. [7] J.D. Saxby, Chem. Geol. 6 (1970) 173–184. [8] T.E. Dancy, V.J. Giedroyc, Inst. Pet. 36 (1950) 593–603. [9] B. Durand, G. Nicaise, in: B. Durand (Ed.), Kerogen. Insoluble Organic Matter from Sedimentary Rocks, Editions Technip, Paris, 1980, p. 2. [10] L.C. Price, J.L. Clayton, Geochim. Cosmochim. Acta 56 (1992) 1213–1222. [11] J.W. Larsen, R.A. Flowers II, P.J. Hall, G. Carlson, Energy Fuels 11 (1997) 998–1002.
321
[12] Y. Yun, E.M. Suuberg, Prepr. Pap. Am. Chem. Soc., Div. Fuel Chem 38 (2) (1993) 489–494; E.M. Suuberg, Y. Otake, M.J. Langer, K.T. Leung, I. Milosavjevic, Energy Fuels 8 (1994) 1247–1262. [13] T.K. Green, J. Kovac, J.W. Larsen, Fuel 63 (1984) 935–938; J.W. Larsen, J.C. Cheng, C.S. Pan, Energy Fuels 5 (1991) 57–59. [14] T.J. Parks, L.J. Lynch, D.S. Webster, D. Barrett, Energy Fuels 2 (1988) 185–190. [15] G.H.S. Lee, A.J. Berkovich, J.H. Levy, B.R. Young, M.A. Wilson, Energy Fuels 12 (1998) 262–267. [16] A.R. Palmer, G.E. Maciel, Anal. Chem. 54 (1982) 2194–2198. [17] J. C. Zeszotarski, R. C. Chromik, R. P. Vinci, M. C. Messmer, R. Michels, J. W. Larsen, Geochim. Cosmochim. Acta. in press. [18] J.W. Larsen, S. Li, Energy Fuels 8 (1994) 932–936. [19] U. Ritter, Org. Geochem. 34 (2003) 319–326. [20] S. R. Kelemen, H. Freund, M. Siskin, D. J. Curry, Y. Xiao, W. N. Olmstead, M. L. Gorbaty, A. E. U. S. Bence, Patent Appl. No. 10/426,356 Pub. No. U. S. 2004/0019437 A1, 2004.