Abstract
At the Ketzin pilot site for geological CO2 storage, about 67,000 tons of CO2 were injected during
the period June 2008 – August 2013. Since August 2013, the site is in its post-closure phase. Before
and during the injection phase, a comprehensive monitoring programme was established. In the
early post-injection phase, a majority of the monitoring activities have continued. The stepwise
abandonment of the pilot site, which is planned to be accomplished in 2018, marks also the
termination of most monitoring activities.
Four 3D seismic surveys were acquired between 2005 and 2015 for characterizing the reservoir
structure and its overburden and for monitoring the propagation of the injected CO2 in the storage
formation. A baseline survey was acquired in 2005, followed by three repeat surveys in 2009, 2012
and 2015. The first and second repeat surveys revealed the lateral extension of the CO2 plume after
injecting 22 and 61 ktons, respectively. In spite of rather unfavourable time-lapse noise conditions,
typical for 4D land seismic surveys, it was possible to image the lateral extent of the CO2
plume using normalized amplitude differences at the top level of the storage horizon. In 2009 (22 ktons),
the imaged CO2 plume showed an extension of ~500 m in E-W direction and ~300 m in N-S direction. The 2012 plume was ~700 m in E-W direction and ~400 m in N
-S direction. The lateral extent of the CO2 plume (“plume footprint area”) was one of the performance criteria which was used for an assessment of the conformity between observed and simulated CO2
plume propagation (Lüth et al., 2015). Reservoir simulations performed to predict the CO2 distribution in the storage formation at the time of seismic data acquisition for the repeat surveys 2009 and 2012 suggest a larger lateral plume extent than could be observed on the seismic data. This was attributed to a
significant proportion of a relatively thin CO2
layer which is hard to detect by surface based
geophysical observations (Kempka et al., 2013). E.g., for 2009 reservoir simulations predicted a
CO2 plume footprint area of 418,000 m2. Quantifying the exact plume footprint area from the seismic data is not straight forward, as this depends strongly on the noise level of the seismic amplitude data and on the noise threshold assumed for differentiating between areas affected by CO2 and those free of a CO2 signature. Comparing the simulated plume footprint areas with the seismic amplitude signatures suggests that at the Ketzin pilot site the CO2 plume can be imaged with a thickness threshold in the order of approximately 5 m.
In autumn 2015, the third 3D repeat seismic survey, serving as the first post-injection survey, was
acquired. The survey was acquired using the same acquisition geometry as for previous surveys, 2
consisting of 33 templates with five receiver lines and twelve source profiles perpendicular to the receiver lines. A small extension of the acquisition area was designed in order to acquire new baseline data at the western margin of the region covered by previous surveys. This extension was
added in order to account for the previously observed dominantly westward migration of the CO2
plume. Seismic processing of the recently acquired data has resulted in preliminary observations
which can be summarized as follows:
- As in previous seismic repeat surveys, a clear CO2 signature is observed at the top of the
storage formation.
- No systematic amplitude changes are observed above the reservoir which might indicate
leakage, that is the reservoir is not leaking.
- Compared to the second repeat survey acquired in 2012, the lateral extent of the CO2 plume seems to have been reduced, which may be an indication for ongoing (and relatively fast) dissolution of the CO2
in the formation brine.
The preliminary results gained from the 2015 seismic survey so far will undergo further seismic processing and analysis, followed by integrated interpretations that include other geophysical measurements and fluid flow simulations.
The on-going research at Ketzin is funded within the project COMPLETE by the Federal Ministry of Education and Research. Further funding is received
by VGS, RWE, Vattenfall, Statoil, OMV
and the Norwegian CLIMIT programme.
