25th International Meeting on Organic Geochemistry IMOG 2011

P-434
Impact of a high CO2 partial pressure on the methanogenic
pathway in a high-temperature petroleum reservoir
Daisuke Mayumi 1 , Susumu Sakata 1 , Haruo Maeda 2 , Yoshihiro Miyagawa 2 , Masayuki
Ikarashi 2
1 National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan, 2 INPEX
Corporation, Tokyo, Japan (corresponding author:su-sakata@aist.go.jp)
Injection of CO2 into deep subsurface petroleum
reservoirs has been planned and implemented
worldwide for the purpose of enhanced oil recovery
(EOR) and CO2 storage. Meanwhile, the development
of microbial EOR by stimulating the activity of in situ
microbes to produce methane has been suggested
[1]. Little is known, however, about the influence of a
high CO2 partial pressure on the activity of microbes
inhabiting the reservoirs. Here, we investigated the
influence of CO2 on in situ methanogenic microbes by
conducting high temperature and pressure
incubations mimicking the reservoir conditions.
Methanogenic pathways were determined by
combination of stable isotope (SI) tracer methods and
molecular biological analyses.
Produced water and crude oil samples were
collected at the wellhead in Yabase oil field, Japan.
The incubation experiments were conducted using
microcosms comprised of the produced water, crude
oil and traces of [2- 13 C]-acetate or [ 13 C]-bicarbonate.
The microcosms were pressurized with either N2 or
N2+CO2 (90:10) at 5 MPa and then incubated at 55°C,
simulating the reservoir conditions before and after
CO2 injection. Headspace gas and incubated water
samples were periodically measured for � 13 C values
of methane and DIC (dissolved inorganic carbon) with
GC-C-IRMS. Microbial communities of the incubated
water were analyzed by the methods of 16S rRNA
gene clone libraries and group-specific real-time PCR.
Details of the sampling and experiments are as in
Mayumi et al. [2].
In all the microcosms, methane production was
observed in response to the decrease of acetate
included in the produced water. The acetate
consumed was nearly equivalent to the methane
produced. The methane production rates in the
microcosms pressurized with N2+CO2 were
significantly higher than those pressurized with N2.
Among those pressurized with N2, an immediate and
distinct increase in � 13 C was observed for methane in
the bicarbonate-labelled microcosm, while a small
and gradual increase in � 13 C was observed for
methane in the acetate-labelled microcosm. Methane
was depleted in 13 C relative to DIC by a similar
magnitude in both microcosms. Microbial community
analyses of the incubated water showed bacterial
communities dominated by the genus
Thermacetogenium, known as a thermophilic
syntrophic acetate-oxidizing bacterium, and archaeal
communities dominated by thermophilic
hydrogenotrophic methanogens belonging to the
genus Methanothermobacter. These results
consistently indicate that acetate oxidation coupled
with hydrogenotrophic methanogenesis [3] was the
major pathway in the microcosms pressurized with N2.
Among those pressurized with N2+CO2, an
immediate and distinct increase in � 13 C was observed
for methane in the acetate-labelled microcosm, while
no appreciable increase in � 13 C was observed for
methane in the bicarbonate-labelled microcosm.
Microbial community analyses of the incubated water
showed archaeal communities dominated by
acetoclastic methanogens belonging to the genus
Methanosaeta. These results consistently indicate
that acetoclastic methanogenesis was the major
pathway in the microcosms pressurized with N2+CO2.
We therefore conclude that injection of CO2 into a
high-temperature petroleum reservoir for EOR and
CO2 storage will cause a drastic change in the in situ
methanogenic pathways.
References
[1] Jones, D.M., Head, I.M., Gray, N.D., Adams, J.J.,
Rowan, A.K., Aitken, C.M. et al. (2008) Nature 451,
176-180.
[2] Mayumi D., Mochimaru H., Yoshioka H., Sakata
S., et al., Environmental Microbiology (in press)
[3] Hattori, S. (2008) Microbes and Environments 23,
118-127.
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