- Email: [email protected]
Fractionation of petroleum during expulsion from kerogen
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SCIENCE ~ O l R E C T e ELSEVIER
JOURNAL OF
GEOCHEMICAL EXPLORATION
Journal of Geochemical Exploration 78--79 (2003) 417-420 www.elsevier.com/locate/jgeoexp
Abstract
Fractionation of petroleum during expulsion from kerogen Ulrich Ritter* Basin Modelling Department, SINTEF Petroleum Research, N-7465 Trondheim, S.P Andersens vei 15b, Norway
Abstract Polymer solution parameters, 6, were calculated for the most common petroleum compounds, and a relative solubility scale of hydrocarbons in kerogen was established. The scale predicts the expulsion behaviour of these compounds and can be used as a base for expulsion modelling. The study shows that, depending on source rock potential and swelling ratio, kerogens may expel petroleum that is dramatically different in composition from the one generated. The work done to date suggests that polymer solution theory is a viable concept to be applied in modelling and interpreting expulsion phenomena and may become part of a unifying theory of petroleum fractionation during migration. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Polymer solution theory; Expulsion; Petroleum fractionation; Primary migration
1. Introduction Fractionation of petroleum components between source and reservoir seems to occur in the sequence: asphaltenes>polars, resins>aromatics>branched alkanes>n-alkanes, the latter being preferentially expelled with respect to the former compound groups (Pepper and Corvi, 1995). However, satisfactory theoretical concepts of why this happens to be the case have, so far, not been brought forward. It has been shown in recent years that coals are macromolecular systems which can be studied by techniques developed in polymer science (e.g., Takanohashi et al., 1996, 2000; Cody and Painter, 1997; Otake and Suuberg, 1997). Application to primary migration (Sandvik et al., 1992) and the structure of kerogen (Larsen and Li, 1997a,b) has recently shown promising results. The purpose of this paper is there* Fax: +47-73591102. E-mail address: [email protected] (U. Ritter).
fore to evaluate polymer solution theory (Hildebrand, 1936), as one possible process of differential retention of petroleum compounds in kerogen, and to demonstrate its effects on the composition of expelled petroleum fluids.
2. Solubility and swelling in kerogen The solubility of a solvent can be defined by a solubility parameter, 3, (Hildebrand, 1936). The general rule is that as the 6 of two compounds becomes closer, the better is their mutual solubility. As a first approach, kerogen appears to have a 6 between 9.5 and 10 (cal era3) 1/2 (Larsen and Li, 1997b). Hence, hydrocarbon compounds closest to this value will be best retained by the kerogen. The systematic calculation of 6 for the most frequent compounds contained in petroleum (Table 1) suggests that aromatics as a group are better retained in kerogen than saturates. Within aromatics, benzene,
0375-6742/03/$ - see front matter © 2003 Elsevier Science B.V. All rights reserved. doi: 10.1016/S0375 -6742 (03)00061 -X
418
Abstract
Table 1 Solubilityparameters used for modelling of expulsion and definition of the different compound groups modelled Kerogen absorption Kerogen solubility parameters (~) (cal/cm3)i/2 Kerogen 9.50 CI: dry gas 7.70 Gw: wet gas, c2 to c5 7.30 SATI: light saturates, C6 to C14 7.00 AROI: aromatics, C6 to C14 7.80 SATh: heavy saturates, C15+ 6.80 AROh: heavy aromatics, C15 + 9.00 NSO: asphaltenes, resins, etc. 9.00
respective normal alkanes and heavy n-alkanes are less well retained than intermediate n-alkanes (n-C6 to n-C9). Dry gas is more easily expelled than wet gas. The solubility differences of pristane, phytane, and nC17 and n-C18, respectively, are small. Asphaltenes seem to have 6 between 7 and 9 (cal cm3) 1/2, and are thus poorly to moderately well expelled, depending on aggregate size (Rogel, 1995) if 6 of kerogen is 9.5 (cal/cm2) 1/2. Swelling (Qv) occurs as response to the uptake of solvent in a polymer due to absorption in the polymer structure. It represents the absorption potential of the kerogen towards any given compound. Published Qv values (volume of swollen/volume initial sample) representative of aromatic petroleum hydrocarbons are between 1.1 and 1.6 for coals and up to 3.1 for type-I kerogens (Otake and Suuberg, 1997; Larsen and Li, 1997a,b; Takanohashi et al., 2000). Theoretical and observational evidence suggests that Qv
phenol, cresol, toluene, are better retained than ethylbenzene and xylenes. Methylnaphthalene and diphenyl are better retained than anthracene, methylphenanthren and phenanthrene. Within saturates, cycloalkanes tend to be better retained than the HI: 524, %Ro: 0.69 GEN
Qv: 2.05
Qv: 2.32
Qv: 2.84
Expulsion Efficiency, HI: 524, %Ro: 0.69
Qv: 3.11
0% 10% 20 % 30 % 40 % 50 % 60 % 70 % 80 %
1.00 0.90 0.80 o,o
0.60 0.50 0.40. 0.30 0.20 0.10
90 %
0.00
!oo %
i
I
Qv: 2.05
HI: 224, Ro: %0.69 0%
~-:~
GEN
Qv: 2.05
10 % - - ~ ) ~ - 3o % 20% 40 % -~- ~
::::::::.:. i:,:::Z:::i: ::::::::i:i .
Qv: 2.32
Qv: 2.84
":':'::':L i:.iii:.!:iii i:i:i::::::• ~
~ !
"
:' ::: - ~-i!~i' ::: :~: ;i
500/0 60 % 70 % 80 %
90 %
100 %
•
0.90
0.~0°8° --_ 0.60 o.5o 0.40 0.30 0,20
~I!i:: ~:~:::
~::: ~:::
Qv: 2.05
[]O1
[]Gw
[]SATI
Qv: 3.11
1.00
0,10 0.O0
Legend
Qv: 2.84
Expulsion Efficiency, HI: 224, %Ro: 0.69
Qv: 3.11 .: .......... •
Qv: 2.32
~iAROI
[]SATh
Qv: 2.32
~AROh
Qv: 2.84
Qv: 3.11
[]NSO
Fig. 1. Generated and expelled petroleum compound groups at %Ro= 0.69 and corresponding expulsion efficiency. (Top) Moderately rich kerogen; (below) moderately poor kerogen. HI: Hydrogen Index (mg HC/g TOC); %Ro: modelled vitrinite reflectance. GEN: petroleum generated at 0.69%Ro. See Table 1 for definition of compound groups. Qv is the swelling ratio used for modelling. It represents the absorption potential between 6 = 9.1 and 9.9 (cal/cm3)m.
Abstract
decreases with maturity and increases with temperature.
3. Modelling of expulsion Qv can be modelled using the fact that the relationship usually approaches a bell-shaped curve. The following equation is therefore used to match a curve to available Qv and 6 couples: 11
Qv : S ~ - ~ e x p
-0.5
(1)
where d is a distribution factor that corresponds to the standard deviation of the curve; 3c and c~k are the solubility parameters for the hydrocarbon compound and the kerogen, respectively; and So is a scaling factor. Once calibrated by adjusting & and d, this function assigns a retention threshold to each compound group. A compound group is expelled once its retention threshold is passed. All results are given for a modelled maturity of 0.69%Ro, using the vitrinite reflectance model of Burnham and Sweeney (1989). Fig. 1 provides a good example of how expulsion affects the chemistry of the expelled petroleum fluids in a moderately poor and a moderately rich source rock. Since it is the purpose of this paper to demonstrate the effects of retention on composition of the expelled fluids, the model does not include cracking of generated compounds. At the given vitrinite reflectance, cracking would reduce the abundance of NSO considerably. The relatively rich kerogen shows a gradual increase of hydrocarbon compounds with increasing retention potential (Qv), with the fraction of saturates increasing unproportionately. Expulsion efficiency decreases gradually with increasing retention potential. Aromatics and NSO compounds always have considerably lower expulsion efficiency than saturates. The relatively poor source rock shows much stronger fractionation of the expelled fluids. Already at moderate values of Qv the abundance of aromatics and NSO compounds is drastically reduced. The expulsion efficiency of saturates is somewhat lower than in the richer rock, but that of aromatics and NSO is almost negligible.
419
4. Conclusions The work done to date suggests that polymer solution theory is a viable concept to be applied in modelling and interpreting expulsion phenomena. It may be part of a unifying theory of petroleum fractionation during migration. Fractionation during expulsion may change the composition of the expelled oils to the extent that it does no longer resemble the original generated oil. All other variables being constant, the mutual relation of initial potential and swelling ratio are the parameters influencing the composition of the expelled fluid most. Apart from vitrinitic coal and strongly alginitic kerogens, which have fairly well known swelling ratios, uncertainties remain with respect to the swelling ratios of other macerals. The concept presented here may help to explain the discrepancy observed between composition from heating experiments (e.g., Monin et al., 1990; Ritter et al., 1995; Behar et al., 1997) and reservoired oils, which are normally much more aliphatic and have less NSO compounds than analysed source rock extracts.
Acknowledgements The research on which this manuscript is based was supported by an NFR Strategic Institute Program: 'Prediction of hydrocarbon phases in reservoirs by use of selected hydrocarbon components' and by financial support from AGIP Stavanger/Milano.
References Behar, F., Vandenbroucke, M., Tang, Y., Marquis, F., Espitalie, J., 1997. Thermal cracking of kerogen in open and closed systems: determination of kinetic parameters and stoichiometric coefficients for oil and gas generation. Org. Geochem. 26, 321-339. Burnham, A.K., Sweeney, J.J., 1989. A chemical kinetic model of vitrinite maturation and reflectance. Geochim. Cosmochim. Acta 53, 2649-2657. Cody, G.D., Painter, EC., 1997. Modulus of swollen coal gels. Energy Fuels 11, 1044-1047. Hildebrand, J.H., 1936. The Solubility of Non-Electrolytes. Reinhold, New York. Larsen, J.W., Li, S., 1997a. Changes in the macromolecular structure of a type I kerogen during maturation. Energy Fuels 11, 897 901.
420 Larsen, J.W., Li, S., 1997b. An initial comparison of the interactions of type I and type III kerogens with organic liquids. Org. Geochem. 26, 305-309. Monin, J.C., Connan, J., Oudin, L., Durand, B., 1990. Qualitative and quantitative experimental approach of oil and gas generation: application to the North Sea source rocks. Org. Geochem. 16, 133-142. Otake, Y., Suuberg, E.M., 1997. Temperature dependence of solvent swelling and diffusion processes in coals. Energy Fuels 11, 1155-1164. Pepper, A.S., Corvi, P.J., 1995. Simple kinetic models of petroleum formation: Part III. Modelling an open system. Mar. Pet. Geol. 12, 417 452. Ritter, U., Mylar, M.B., Vinge, T., Aareskjold, K., 1995. Experi-
Abstract
mental heating and kinetic models of source rocks: comparison of different methods. Org. Geochem. 23, 1-9. Rogel, E., 1995. Studies on asphaltene aggregation via computational chemistry. Colloids Surf. A, Physicochem. Eng. Asp. 104, 85 -93. Sandvik, E.I., Young, W.A., Curry, D.J., 1992. Expulsion from hydrocarbon sources: the role of organic absorption. Org. Geochem. 19, 77-87. Takanohashi, T., Nakamura, K., Terao, Y., Iino, M., 2000. Computer simulation of solvent swelling of coal molecules: effects of different solvents. Energy Fuels 24, 393-399. Takanohashi, T., Yanagida, T., Iino, M., 1996. Extraction and swelling of low rank coals with various solvents at room temperature. Energy Fuels 10, 128 1132.
SCIENCE ~ O l R E C T e ELSEVIER
JOURNAL OF
GEOCHEMICAL EXPLORATION
Journal of Geochemical Exploration 78--79 (2003) 417-420 www.elsevier.com/locate/jgeoexp
Abstract
Fractionation of petroleum during expulsion from kerogen Ulrich Ritter* Basin Modelling Department, SINTEF Petroleum Research, N-7465 Trondheim, S.P Andersens vei 15b, Norway
Abstract Polymer solution parameters, 6, were calculated for the most common petroleum compounds, and a relative solubility scale of hydrocarbons in kerogen was established. The scale predicts the expulsion behaviour of these compounds and can be used as a base for expulsion modelling. The study shows that, depending on source rock potential and swelling ratio, kerogens may expel petroleum that is dramatically different in composition from the one generated. The work done to date suggests that polymer solution theory is a viable concept to be applied in modelling and interpreting expulsion phenomena and may become part of a unifying theory of petroleum fractionation during migration. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Polymer solution theory; Expulsion; Petroleum fractionation; Primary migration
1. Introduction Fractionation of petroleum components between source and reservoir seems to occur in the sequence: asphaltenes>polars, resins>aromatics>branched alkanes>n-alkanes, the latter being preferentially expelled with respect to the former compound groups (Pepper and Corvi, 1995). However, satisfactory theoretical concepts of why this happens to be the case have, so far, not been brought forward. It has been shown in recent years that coals are macromolecular systems which can be studied by techniques developed in polymer science (e.g., Takanohashi et al., 1996, 2000; Cody and Painter, 1997; Otake and Suuberg, 1997). Application to primary migration (Sandvik et al., 1992) and the structure of kerogen (Larsen and Li, 1997a,b) has recently shown promising results. The purpose of this paper is there* Fax: +47-73591102. E-mail address: [email protected] (U. Ritter).
fore to evaluate polymer solution theory (Hildebrand, 1936), as one possible process of differential retention of petroleum compounds in kerogen, and to demonstrate its effects on the composition of expelled petroleum fluids.
2. Solubility and swelling in kerogen The solubility of a solvent can be defined by a solubility parameter, 3, (Hildebrand, 1936). The general rule is that as the 6 of two compounds becomes closer, the better is their mutual solubility. As a first approach, kerogen appears to have a 6 between 9.5 and 10 (cal era3) 1/2 (Larsen and Li, 1997b). Hence, hydrocarbon compounds closest to this value will be best retained by the kerogen. The systematic calculation of 6 for the most frequent compounds contained in petroleum (Table 1) suggests that aromatics as a group are better retained in kerogen than saturates. Within aromatics, benzene,
0375-6742/03/$ - see front matter © 2003 Elsevier Science B.V. All rights reserved. doi: 10.1016/S0375 -6742 (03)00061 -X
418
Abstract
Table 1 Solubilityparameters used for modelling of expulsion and definition of the different compound groups modelled Kerogen absorption Kerogen solubility parameters (~) (cal/cm3)i/2 Kerogen 9.50 CI: dry gas 7.70 Gw: wet gas, c2 to c5 7.30 SATI: light saturates, C6 to C14 7.00 AROI: aromatics, C6 to C14 7.80 SATh: heavy saturates, C15+ 6.80 AROh: heavy aromatics, C15 + 9.00 NSO: asphaltenes, resins, etc. 9.00
respective normal alkanes and heavy n-alkanes are less well retained than intermediate n-alkanes (n-C6 to n-C9). Dry gas is more easily expelled than wet gas. The solubility differences of pristane, phytane, and nC17 and n-C18, respectively, are small. Asphaltenes seem to have 6 between 7 and 9 (cal cm3) 1/2, and are thus poorly to moderately well expelled, depending on aggregate size (Rogel, 1995) if 6 of kerogen is 9.5 (cal/cm2) 1/2. Swelling (Qv) occurs as response to the uptake of solvent in a polymer due to absorption in the polymer structure. It represents the absorption potential of the kerogen towards any given compound. Published Qv values (volume of swollen/volume initial sample) representative of aromatic petroleum hydrocarbons are between 1.1 and 1.6 for coals and up to 3.1 for type-I kerogens (Otake and Suuberg, 1997; Larsen and Li, 1997a,b; Takanohashi et al., 2000). Theoretical and observational evidence suggests that Qv
phenol, cresol, toluene, are better retained than ethylbenzene and xylenes. Methylnaphthalene and diphenyl are better retained than anthracene, methylphenanthren and phenanthrene. Within saturates, cycloalkanes tend to be better retained than the HI: 524, %Ro: 0.69 GEN
Qv: 2.05
Qv: 2.32
Qv: 2.84
Expulsion Efficiency, HI: 524, %Ro: 0.69
Qv: 3.11
0% 10% 20 % 30 % 40 % 50 % 60 % 70 % 80 %
1.00 0.90 0.80 o,o
0.60 0.50 0.40. 0.30 0.20 0.10
90 %
0.00
!oo %
i
I
Qv: 2.05
HI: 224, Ro: %0.69 0%
~-:~
GEN
Qv: 2.05
10 % - - ~ ) ~ - 3o % 20% 40 % -~- ~
::::::::.:. i:,:::Z:::i: ::::::::i:i .
Qv: 2.32
Qv: 2.84
":':'::':L i:.iii:.!:iii i:i:i::::::• ~
~ !
"
:' ::: - ~-i!~i' ::: :~: ;i
500/0 60 % 70 % 80 %
90 %
100 %
•
0.90
0.~0°8° --_ 0.60 o.5o 0.40 0.30 0,20
~I!i:: ~:~:::
~::: ~:::
Qv: 2.05
[]O1
[]Gw
[]SATI
Qv: 3.11
1.00
0,10 0.O0
Legend
Qv: 2.84
Expulsion Efficiency, HI: 224, %Ro: 0.69
Qv: 3.11 .: .......... •
Qv: 2.32
~iAROI
[]SATh
Qv: 2.32
~AROh
Qv: 2.84
Qv: 3.11
[]NSO
Fig. 1. Generated and expelled petroleum compound groups at %Ro= 0.69 and corresponding expulsion efficiency. (Top) Moderately rich kerogen; (below) moderately poor kerogen. HI: Hydrogen Index (mg HC/g TOC); %Ro: modelled vitrinite reflectance. GEN: petroleum generated at 0.69%Ro. See Table 1 for definition of compound groups. Qv is the swelling ratio used for modelling. It represents the absorption potential between 6 = 9.1 and 9.9 (cal/cm3)m.
Abstract
decreases with maturity and increases with temperature.
3. Modelling of expulsion Qv can be modelled using the fact that the relationship usually approaches a bell-shaped curve. The following equation is therefore used to match a curve to available Qv and 6 couples: 11
Qv : S ~ - ~ e x p
-0.5
(1)
where d is a distribution factor that corresponds to the standard deviation of the curve; 3c and c~k are the solubility parameters for the hydrocarbon compound and the kerogen, respectively; and So is a scaling factor. Once calibrated by adjusting & and d, this function assigns a retention threshold to each compound group. A compound group is expelled once its retention threshold is passed. All results are given for a modelled maturity of 0.69%Ro, using the vitrinite reflectance model of Burnham and Sweeney (1989). Fig. 1 provides a good example of how expulsion affects the chemistry of the expelled petroleum fluids in a moderately poor and a moderately rich source rock. Since it is the purpose of this paper to demonstrate the effects of retention on composition of the expelled fluids, the model does not include cracking of generated compounds. At the given vitrinite reflectance, cracking would reduce the abundance of NSO considerably. The relatively rich kerogen shows a gradual increase of hydrocarbon compounds with increasing retention potential (Qv), with the fraction of saturates increasing unproportionately. Expulsion efficiency decreases gradually with increasing retention potential. Aromatics and NSO compounds always have considerably lower expulsion efficiency than saturates. The relatively poor source rock shows much stronger fractionation of the expelled fluids. Already at moderate values of Qv the abundance of aromatics and NSO compounds is drastically reduced. The expulsion efficiency of saturates is somewhat lower than in the richer rock, but that of aromatics and NSO is almost negligible.
419
4. Conclusions The work done to date suggests that polymer solution theory is a viable concept to be applied in modelling and interpreting expulsion phenomena. It may be part of a unifying theory of petroleum fractionation during migration. Fractionation during expulsion may change the composition of the expelled oils to the extent that it does no longer resemble the original generated oil. All other variables being constant, the mutual relation of initial potential and swelling ratio are the parameters influencing the composition of the expelled fluid most. Apart from vitrinitic coal and strongly alginitic kerogens, which have fairly well known swelling ratios, uncertainties remain with respect to the swelling ratios of other macerals. The concept presented here may help to explain the discrepancy observed between composition from heating experiments (e.g., Monin et al., 1990; Ritter et al., 1995; Behar et al., 1997) and reservoired oils, which are normally much more aliphatic and have less NSO compounds than analysed source rock extracts.
Acknowledgements The research on which this manuscript is based was supported by an NFR Strategic Institute Program: 'Prediction of hydrocarbon phases in reservoirs by use of selected hydrocarbon components' and by financial support from AGIP Stavanger/Milano.
References Behar, F., Vandenbroucke, M., Tang, Y., Marquis, F., Espitalie, J., 1997. Thermal cracking of kerogen in open and closed systems: determination of kinetic parameters and stoichiometric coefficients for oil and gas generation. Org. Geochem. 26, 321-339. Burnham, A.K., Sweeney, J.J., 1989. A chemical kinetic model of vitrinite maturation and reflectance. Geochim. Cosmochim. Acta 53, 2649-2657. Cody, G.D., Painter, EC., 1997. Modulus of swollen coal gels. Energy Fuels 11, 1044-1047. Hildebrand, J.H., 1936. The Solubility of Non-Electrolytes. Reinhold, New York. Larsen, J.W., Li, S., 1997a. Changes in the macromolecular structure of a type I kerogen during maturation. Energy Fuels 11, 897 901.
420 Larsen, J.W., Li, S., 1997b. An initial comparison of the interactions of type I and type III kerogens with organic liquids. Org. Geochem. 26, 305-309. Monin, J.C., Connan, J., Oudin, L., Durand, B., 1990. Qualitative and quantitative experimental approach of oil and gas generation: application to the North Sea source rocks. Org. Geochem. 16, 133-142. Otake, Y., Suuberg, E.M., 1997. Temperature dependence of solvent swelling and diffusion processes in coals. Energy Fuels 11, 1155-1164. Pepper, A.S., Corvi, P.J., 1995. Simple kinetic models of petroleum formation: Part III. Modelling an open system. Mar. Pet. Geol. 12, 417 452. Ritter, U., Mylar, M.B., Vinge, T., Aareskjold, K., 1995. Experi-
Abstract
mental heating and kinetic models of source rocks: comparison of different methods. Org. Geochem. 23, 1-9. Rogel, E., 1995. Studies on asphaltene aggregation via computational chemistry. Colloids Surf. A, Physicochem. Eng. Asp. 104, 85 -93. Sandvik, E.I., Young, W.A., Curry, D.J., 1992. Expulsion from hydrocarbon sources: the role of organic absorption. Org. Geochem. 19, 77-87. Takanohashi, T., Nakamura, K., Terao, Y., Iino, M., 2000. Computer simulation of solvent swelling of coal molecules: effects of different solvents. Energy Fuels 24, 393-399. Takanohashi, T., Yanagida, T., Iino, M., 1996. Extraction and swelling of low rank coals with various solvents at room temperature. Energy Fuels 10, 128 1132.