25th International Meeting on Organic Geochemistry IMOG 2011

O-67
Biogeochemical impact of CO2 exposure on reservoir rocks and
the indigenous microbial community
Ann-Kathrin Scherf, Maren Wandrey, Kornelia Zemke, Andrea Vieth-Hillebrand, Ketzin
group
German Research Centre for Geosciences - GFZ, Potsdam, Germany (corresponding author:scherf@gfzpotsdam.de)
In recent years the widespread occurrence of
microorganisms was demonstrated in the subsurface.
Low molecular weight organic acids (LMWOA) form a
potential feedstock for indigenous microbial
community and represent also intermediate products
of bacterial metabolism.
As a part of the ongoing efforts to understand the
intra-reservoir processes and potential of using deep
saline aquifers for long term storage of CO2 in Ketzin
(Northeast German Basin) CO2 is injected into a
Triassic saline aquifer at a depth between 630 and
710 m below surface since June 2008. Within the two
projects CO2SINK and CO2MAN the injection and
propagation of the CO2 is monitored and the
geochemical, geophysical and microbiological effects
are analysed in situ as well as in laboratory
experiments. Among those this long-term experiment
was set up in September of 2007 to study changes in
biogeochemical and microbiological processes and
petrophysical properties of the rocks.
Fresh, pristine reservoir rock samples from the Ketzin
site have been incubated with artificial high salinity
brine and CO2 in high pressure at 5.5 MPa and 40°C.
Generally, rock and fluid samples were analysed with
respect to geochemistry, microbial community, as well
as mineralogical and petrophysical characteristics
[1,2,3]. The focus of the research presented here, is
the amount of LMWOA in the fluid phase and its
change over time. After 15, 21, 24 and 40 months of
CO2 exposure, fluid samples were analysed using ion
chromatography to obtain information on the
alteration of the concentrations of low molecular
weight organic acids.
In the cause of the experiment formate and acetate
concentrations showed first only slight, but later on
partly exponential increase. This may reflect an
increased production of organic acids by microbial
activity, as the change of reservoir (here,
experimental) conditions due to the injection and
storage of CO2 in geological formations may induce
changes in the indigenous microbial community and
thus, the overall microbial activity. But also
mobilisation of organic molecules from mineral
phases during CO2 exposure may take place as
(supercritical) CO2 is an excellent solvent for certain
organic molecules.
To unravel changes in the microbial community and
thus, metabolic activity pristine rock samples as well
as selected incubated core pieces have been
analysed for the composition of the inhibiting
microbial community using molecular biological
methods. Here it has been shown, that the
composition of the microbial community in the
reservoir sandstone mainly consists of
chemoheterotrophic bacteria [1]. So far, only minor
changes of the microbial community composition
were observed. However, evaluating the results
regarding the timescale of the shifts in LMWOA
concentrations in the brine makes an adaptation of
the microorganisms to the modified conditions more
likely then a ―late‖ mobilisation event due to the
presence of CO2. This is supported by the results of
another set of laboratory experiments where the
mobilisation of LMWOA through the exclusive
exposure to (supercritical) CO2 has been
demonstrated [4]. In these experiments strongest
mobilisation effects were observed in the first few
hours.
Summarised we conclude that the increased
concentration of the LMWOAs formate and acetate
has its main origin in increased microbial activity and
not in mobilisation / solution effects through the
presence of brine and CO2. These results may be
extended by the comparison of PLFA profiles of fresh
and CO2-treated twin samples and furthermore,
LC-ir-MS analyses of LMWOA.
References
[1] Wandrey, M. et al., 2011. Energy Procedia 4,
3644-3650.
[2] Zemke, K. et al., 2010. <strong>International</strong> Journal on
Greenhouse Gas Control 4, 990-999.
[3] Fischer, S. et al, 2010. Chemie der Erde 70, 155-
164.
[4] Scherf, A.-K. et al., 2011. Energy Procedia 4,
4524-4531.
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