Click to view article in PDF
format.
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.
|
The physical properties of
injected CO2 that affect
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: 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 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
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
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
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
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
|