Abstract
This thesis is about borehole seismic monitoring of CO2 storage within a saline aquifer
at Ketzin, Germany. CO2 storage is part of the process ’Carbon dioxide Capture and
Storage (CCS)’. As a greenhouse gas, CO2 contributes to the global warming, therefore
efforts are made to slow down the increase of CO2 concentration in the atmosphere. CCS
is considered because ”fossil fuels are the dominant form of energy utilised in the world
(86 %) and account for 75% of current anthropogenic CO2 emissions” (IPCC (2005)).
Deep saline aquifer storage is ”the most promising and relevant CO2 sequestration
option for Europe” (Juhlin et al. (2007)). Saline aquifers are broadly distributed ”and
their storage capacity exceeds that of depleted oil and gas fields” (Juhlin et al. (2007)).
”Techniques developed for the exploration of oil and gas reservoirs, natural gas storage
sites and liquid waste disposal sites are suitable for characterising and monitoring
geological storage sites for CO2” (IPCC (2005)). Although these methods include
many of the tools ”needed to predict both short-term and long-term performance of
CO2 storage, more experience is needed to establish confidence in their effectiveness in
predicting long-term performance when adapted for CO2 storage” (IPCC (2005)).
Operated by the German Research Centre for Geosciences (GFZ), the in situ laboratory
for saline aquifer CO2 storage near the town Ketzin (35km west of Berlin) is the first
European onshore storage pilot facility. It advances ”the understanding of the science
and practical processes involved in underground storage” (Förster et al. (2006)). Within
5 years of operation, between June 2008 and April 2013, 65 kt of supercritical, food-grade
CO2 have been injected.
As part of a comprehensive geophysical monitoring program at Ketzin, the research in
this thesis focuses on the crosshole seismic, zero-offset and offset VSP measurements.
While the baseline surveys provide the structural geometry and characterisation of the
site, the repeat surveys aim at the observation of CO2 propagation in the reservoir.
Based on geological information and well cores, the reservoir is described as thin and
heterogeneous at the research site (Yang (2012)). ”The thickness of the reservoir is
generally less than 20 m”, what is ”below the resolution of conventional surface seismic
data” (Yang (2012)). Borehole seismic methods are expected to have a higher resolution
than surface seismic and are therefore are tested at the Ketzin site. Two questions
should be answered: With these methods, is it possible (1) to map the CO2 in the
reservoir layer (2) to derive geometrical and petrophysical parameters describing the
migration of CO2 in a water saturated sandstone reservoir?
The crosshole seismic measurement was designed to follow the migration of CO2 at
small scale during the injection. ”As CO2 replaces saline water in saturated reservoir
sandstones a seismic velocity reduction may occur. This velocity change can potentially
be used to monitor CO2 in sandstone aquifers using seismic tomography” (Zhang
et al. (2012)). Based on a velocity model derived from the measured data, travel time
tomography is tested on synthetic data having the same geometry as the real data. The
shape of the CO2 plume and an estimate of the velocity reduction in the reservoir is
derived for different scenarios of CO2 distribution.
The main objective of the zero-offset and offset VSP surveys was to generate highresolution
seismic images in the vicinity of the borehole. As CO2 replaces saline water in
saturated sandstones, the impedance contrast between the gas filled sandstone and the
caprock is increased, what leads to stronger reflections from top of the reservoir. This
increased reflectivity can be used to image the spreading of CO2 in the reservoir. While
the zero-offset VSP focuses on normal incidence reflectivity near the observation well,
offset VSP has the potential to generate a lateral image of the reservoir at the injection
site. A near-well corridor stack of the zero-offset VSP is compared to 3D surface seismic
data. Based on the CO2 induced amplitude changes of the repeat measurement, the
thickness of the CO2 plume is derived by a wedge modelling study and the reduction of
P-wave velocity is calculated with band limited impedance inversion. The offset VSP
measurements are imaged by Kirchhoff and Fresnel migration. The application of
two migration algorithms can help to differentiate between method-related and true
time-lapse effects when interpreting the seismic images.
It is shown that borehole seismic methods can image the distribution of CO2 in the
reservoir and contribute to the quantification of geometrical and petrophysical parameters
of the plume. In the framework of monitoring CO2 injection, borehole seismic methods
should be used as an add-on to surface seismic, in case more detailed information is
needed about the structure in the vicinity of boreholes. In addition, borehole seismic
monitoring can be applied to the observation of layers above the reservoir, for the
detection of leakage paths or the inspection of well integrity.
