25th International Meeting on Organic Geochemistry IMOG 2011

P-352
Experimental simulation of gravity/density segregation by
centrifugation
Fenglou Zou, Kentaro Indo, Eric Lehne
DBR Technology Center, Schlumberger, Edmonton, Alberta, Canada (corresponding author:fzou@slb.com)
Compositional variations in reservoir fluids with depth
are common and have been observed in many
reservoirs throughout the world. This phenomenon is
not limited to thick reservoirs, but is also observed
over relatively short vertical columns. It also occurs in
reservoirs containing gas condensates or volatile oils,
as well as heavy oil reservoirs [1, 2].
Fluid gradients can originate from gravitational or
density segregation, thermal gradients, water
washing, biodegradation, fluid-rock interactions,
multiple charges to the reservoir, or leaky seals.
Compositional gradients can also occur in large oil
columns that are in apparent pressure
communication. Such pools are described as
segregated oil columns, showing characteristics of
changes in API gravity, gas-to-oil ratio (GOR),
composition or asphaltene content.
We present the results of experiments to simulate
gravitational segregation using a centrifuge technique
under reservoir pressure and temperature conditions
[3]. It is believed that asphaltene content strongly
determines the degree of segregation [4, 5]. We
compared compositional segregation characteristics
of two different oils: a black oil (oil #1) with 30 °API
and 5% asphaltene content, and a light oil (oil #2) with
40 °API and less than 0.5% asphaltenes. Oil #1 is
from marine shale source rock with Rc (calculated
vitrinite reflectance from methylphenanthrene index)
of 0.68, and #2 is a Tertiary oil from terrigenous shale
source rock with Rc of 0.83. Different cuts after
centrifugation, e.g. simulated reservoir depth, were
analyzed with UV-Vis spectroscopy, 2D GC-FID and
GC-FID and GC-MS.
GORs of the centrifuged cuts of both oil #1 and #2
do not show significant differences. However, the
analytical results from UV-Vis spectroscopy, 2D GC-
FID and GC-MS indicate that oil #2 exhibits vertical
compositional variation, while oil #1 lacks
segregation. The grading in oil #2 is obviously seen
from cut1 to cut7 (from top to bottom) that is at
equivalent field depth of 545 feet based on a
formula reported by Ratulowski et al. [3]. 2D GC-FID
as well as GC-MS analytical results indicate that the
main variation in oil #2 is in the abundance of
polycyclic aromatics, which complements the
analytical results from UV-Vis spectroscopy.
The study indicates that gravitational gradients are
not strictly related to asphaltene content and GOR,
but are also influenced by other oil properties, such
as thermal maturity and resin/aromatics ratios.
Figure 1. Compositional gradients in oil #2
observed by UV-Vis spectroscopy and 2D GC in
relation to simulated reservoir depth.
References
[1] Mullins, O.C., Betancourt, S.S., et al., 2007,
Proceedings - SPE <strong>International</strong> Symposium on
Oilfield Chemistry, 502-507.
[2] Fujisawa, G., Betancourt, S.S., et al., 2004,
Proceedings - SPE Annual Technical Conference and
Exhibition , 59-64.
[3] Ratulowski, J., Fues, A.N., Westrich, J.T., Sieler,
J.J., 2003, SPE 84777, 168-175.
[4] Indo, K., Ratulowski, J., Dindoruk, B., Gao, J., Zuo,
J., Mullins, O.C., 2009, Energy & Fuels 23, 4460-
4469.
[5] Hirschberg, A., 1988, SPE13171, 89-94.
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