uGeneral statement

uFigures

uModel

uConclusion

uGeneral statement

uFigures

uModel

uConclusion

uGeneral statement

uFigures

uModel

uConclusion

uGeneral statement

uFigures

uModel

uConclusion

uGeneral statement

uFigures

uModel

uConclusion

Figure Captions

Model

The physical properties of injected CO₂ that affect seismic imaging are its density and acoustic propagation velocity at the pressure and temperature of its host medium. Because CO₂ has a shear modulus of zero whether it is a gas or a liquid, shear-wave velocity in CO₂ is zero. The only velocity that has to be known for seismic modeling purposes is VP, the propagation velocity of the P-wave mode in CO₂. The density and P-wave velocity of CO₂ over a range of pressure and temperature conditions are defined by the curves displayed in Figures 1 and 2 , respectively.

An Earth model that defines reflecting interfaces at the top and base of the sandstone reservoir and at the fluid interface between CO₂ and brine internal to that reservoir is shown as Figure 3 . From available log data at this site, the Earth layers have the following petrophysical properties:

Sealing carbonaceous shale:

Δt_p = 65 μs/ft, ρ = 2.633 gm/cm³.

Reservoir sandstone:

Δt_p = 80 μs/ft, ρ = 2.357 gm/cm³, Φ = 22 percent.

Granite basement:

Δt_p = 55 μs/ft, ρ = 2.70 gm/cm³.

The sandstone reservoir is at a depth of 6,000 feet; it is important to define the depth of the injection interval in order to determine the temperature and hydrostatic pressure that act on the sequestered CO₂. This temperature and pressure, in turn, specify the density and VP values that should be used to describe the seismic properties of the in situ CO₂ (Figures 1 and 2). A factor of 0.433 psi/ft was used to convert target depth to hydrostatic pressure. In utilizing the curves in Figures 1 and 2 , the in situ temperature was assumed to be 130 degrees Fahrenheit. These assumptions lead to VP and ρ values of 1,285 ft/s and 47.0 lb/ft³, respectively, for the sequestered CO₂.

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.

Conclusion

For simplicity, this modeling assumes that the pore space in the sandstone reservoir is filled with either 100 percent brine or 100 percent CO₂. In reality, the pore space will be occupied by various percentage ratios of brine and CO₂. Our only purpose here is to emphasize that a detailed seismic modeling should be done to determine the viability and strategies of seismic monitoring of injected CO₂ before any CO₂ sequestration project is initiated. Some CO₂ plumes may require that careful and precise procedures be implemented for monitoring plume growth, as in this case. Appropriate modeling can show if a CO₂ plume in another geologic setting will be easier to image.

Return to top.