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GCSeismic Model for Monitoring CO2 Sequestration*
Bob Hardage1 and Diana Sava1
Search and Discovery Article #40423 (2009)
Posted May 26, 2009
*Adapted from the Geophysical Corner column, prepared by the authors, in AAPG Explorer, May, 2009, and entitled “Seismic Steps Aid Sequestration”. Editor of Geophysical Corner is Bob A. Hardage. Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director.
1Bureau of Economic Geology, The University of Texas at Austin (mailto:[email protected])
Sequestration of CO2 in sealed brine is an important issue in industrialized countries that are concerned about the impact of excessive atmospheric CO2 on the environment. A general consensus is that long-term seismic monitoring of injected CO2 will be essential for successful CO2 sequestration programs. In this column we consider the P-wave reflectivity associated with tracking a CO2 plume in one reservoir considered for CO2 sequestration.
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The physical properties of
injected CO2 that affect seismic
An
Sealing carbonaceous shale: Δtp = 65 μs/ft, ρ = 2.633 gm/cm3.
Reservoir sandstone: Δtp = 80 μs/ft, ρ = 2.357 gm/cm3, Φ = 22 percent.
Granite basement: Δtp = 55 μs/ft, ρ = 2.70 gm/cm3.
The sandstone reservoir is
at a
Calculations Two reflectivity curves are calculated for the top and base of the reservoir: One curve describes the reflectivity of a brine-filled reservoir unit. The second curve describes the reflectivity of a reservoir that has a CO2 saturation of 100 percent. These reflectivity curves are shown as Figures 4a and 4c . The reflectivity at the brine-CO2 contact is defined by the single curve in Figure 4b .
Examination of Figure 4 shows that P-P reflectivity increases by about 20 percent at the top of the reservoir when brine is replaced by CO2. This brightening of the P-P reflection can be detected only if good-quality seismic data are acquired and if seismic data processing is carefully done. For this particular geologic layering, the P-P reflection from the interface at the base of the reservoir does not vary when brine is replaced by CO2 (Figure 4c).
Results An encouraging result is that there should be a measurable P-P reflection at any brine/CO2 contact boundary that is created within the reservoir unit. Figure 4b shows that P-P reflectivity at the brine/CO2 boundary is 3 percent to 6 percent. Comparing this fluid-contact reflectivity with the P-P reflectivity at the top and base of the reservoir indicates that a P-P reflection from a brine/CO2 interfac2 will be one-third to one-tenth the magnitude of the reflection amplitudes from the upper and lower interfaces of the sequestration interval. Again, this smaller fluid-contact reflection response can be detected only if good-quality seismic data are acquired and great care is used in processing the data.
An additional requirement is that the distance from the fluid interface to both the top and the base of the sequestration interval should be more than half the dominant wavelength of the illuminating wavefield. In amplitude-versus-offset (AVO) parlance, the top of the reservoir is a Class 4 AVO interface (Figure 4a), and the fluid-contact boundary is a Class 3 AVO interface (Figure 4b). These differing AVO behaviors allow a valuable data-processing strategy to be implemented. Two P-P seismic images need to be made: Image 1 would use only small-offset data (incidence angle range between 0 and 20 degrees), and Image 2 would utilize only large-offset data (incidence angles between 20 and 50 degrees).
In Image 1, the reflection from the top of the reservoir will be five to six times greater than the fluid-contact reflection. In Image 2, the reflection from the top of the reservoir will reduce and will be only two to three times brighter than the fluid-contact boundary. The reflectivity behaviors in these two images should allow a fluid-contact boundary to be identified.
For simplicity, this
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