HR: 11:05h
AN: GC12A-04    [Abstracts]
TI: Seismic monitoring of CO2 plumes in deep saline aquifers: results from laboratory experiments
AU: * Schuett, H
EM: schuett@gfz-potsdam.de
AF: Geo-Research-Center Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Wigand, M
EM: marcusw@lanl.gov
AF: Los Alamos National Laboratory, P. O. Box 1663 Mail Stop E537, Los Alamos, NM 87545 United States
AU: Spangenberg, E
EM: erik@gfz-potsdam.de
AF: Geo-Research-Center Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Borm, G
EM: gborm@gfz-potsdam.de
AF: Geo-Research-Center Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AB: Geophysical monitoring of geological CO2 sequestration is required to track the location of the CO2 plume, to verify the injected mass, to assess the integrity of the cap rock and to ensure that the wells are not leaking. Any geophysical monitoring program will certainly comprise seismic methods, e. g. surface seismics, crosshole, or VSP. These methods have proved to render useful information where fluid substitution processes are involved, e. g. in enhanced oil recovery projects. The large contrast in density and bulk modulus between brine and CO2 (gaseous or supercritical) makes it possible to detect the CO2 plume in deep saline aquifers, which was successfully shown during the injection of CO2 into the Utsira formation as part of the Sleipner Project (Torp & Gale, 2004). In order to further characterize the CO2 plume, e. g. with respect to local variations of the CO2 saturation, the geophysical "signature" of different saturation and pressure states have to be established through measurements on representative reservoir rocks. As a first step we conducted laboratory measurements of seismic properties at full brine and full CO2 saturation, respectively, on a set of 5 sandstone samples from outcrops in Germany. The samples cover a porosity range from 14 % to 22%. The experiments were conducted in a triaxial cell at pressures and temperatures that are representative for deep saline aquifers. We found that the seismic velocities are clearly affected by the saturation state. The magnitude of the fluid substitution effect on vp depends on the porosity: the higher the porosity the higher the velocity change. The compressional wave velocity decreases typically by -5 % to -10 % when brine is displaced by CO2 within the porosity range of our samples. This can be explained by a decrease of the effective bulk modulus of the saturated rock (Gassmann, 1951). A further analysis of the velocity data indicates that the displacement process is incomplete; i. e. a residual brine saturation of approximately 10 % to 30 % is left in the sample. The inverse scenario (brine displacing CO2), in contrast, leads to an almost complete fluid substitution. These effects reflect probably the stability of the advancing interface between the fluids which depends on their viscosity ratio. The shear wave velocity increases typically by 1 % to 2 % when CO2 displaces brine. This can be interpreted as density effect, while the shear modulus is nearly independent of the saturand (Gassmann, 1951). The seismic wave attenuation, particularly the ratio Qp/Qs, is highly sensitive to the saturation state: the ratio changes as much as -20 % to -80 %. Although attenuation data are more difficult to derive from field measurements than velocity data, the attenuation may be useful as additional seismic attribute for plume characterization. References Gassmann, F. (1951): Ueber die Elastizitaet poroeser Medien, Vierteljahrsschrift der Naturforschenden Gesellschaft Zuerich, vol. 96, 1-23 Torp, T., J. Gale (2004): Demonstrating storage of CO2 in geologic reservoirs: The Sleipner and SACS projects, Energy, 29, 1361-1369
UR: http://www.co2sink.org/
DE: 1600 GLOBAL CHANGE
DE: 5102 Acoustic properties
SC: Global Climate Change [GC]
MN: Fall Meeting 2005