uGeneral
statementf
uFigure
captions
uFirst
4-D, 9-C seismic survey
uReservoir
characterization
uInjected
CO2
vs . injected water
uConclusions
uReferences
uGeneral
statementf
uFigure
captions
uFirst
4-D, 9-C seismic survey
uReservoir
characterization
uInjected
CO2
vs . injected water
uConclusions
uReferences
uGeneral
statementf
uFigure
captions
uFirst
4-D, 9-C seismic survey
uReservoir
characterization
uInjected
CO2
vs . injected water
uConclusions
uReferences
uGeneral
statementf
uFigure
captions
uFirst
4-D, 9-C seismic survey
uReservoir
characterization
uInjected
CO2
vs . injected water
uConclusions
uReferences
uGeneral
statementf
uFigure
captions
uFirst
4-D, 9-C seismic survey
uReservoir
characterization
uInjected
CO2
vs . injected water
uConclusions
uReferences
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Figure Captions
Figure 1.
Shear-wave polarization and splitting in a fractured material.
As
an S-wave with an arbitrary polarization direction enters an anisotropic
material, the wave splits into S1 and S2
components with different polarizations and different velocities. The
wave polarized parallel to the fractures travels faster and is less
attenuated that the wave polarized perpendicular to the fractures. After
the S-waves emerge from the anisotropic material, they continue to
propagate as two S-waves with different polarization directions.
First 4-D, 9-C Seismic Survey
The first time-lapse (4-D), multi-component
(9-C) seismic surveys were acquired at Vacuum Field in Lea County, New
Mexico. At the Vacuum Field, shear wave (S-wave) and compressional wave
(P-wave) seismic data were used to monitor reservoir fluid property
changes associated with a carbon dioxide (CO2) tertiary flood
in the Permian San Andres carbonate. Reservoir fluid properties --
including viscosity, density, saturation, and pressure changes -- occur
in response to CO2 injection. Changes are caused by CO2
and oil becoming a miscible phase with the oil in place.
These fluid property changes alter the
interval velocity and attenuation of S-waves passing through the
reservoir interval by up to 10 percent, but cause little (1 to 2
percent) or no measurable change in P-wave velocity and attenuation on
the surface seismic data. The Reservoir Characterization Project of the
Colorado School of Mines (RCP) has conducted two studies at Vacuum
Field:
-
Phase I efforts centered on
monitoring the injection of CO2 from a single wellbore
(Benson and Davis, 2000).
-
Phase II is the dynamic
reservoir characterization of a six-well CO2 injection
program, which includes the Phase-I wellbore (producing during
Phase-II) (Wehner, et al, 2000).
The Vacuum Field was discovered in 1929 with
the drilling of the Socony Vacuum State 1 well in Section 13-T17S-R34E
of Lea County. The Vacuum Field produces predominantly from the San
Andres Formation in a shallow-shelf carbonate depositional setting (Figure
2), which structurally is positioned on the shelf edge of the
Northwest Shelf of the Permian Basin. The structurally high shelf crest
is located just west of the RCP study area.
Porosity and permeability within the
productive zones average 11.8 percent and 22.0 md, respectively. The San
Andres gross pay zone can reach 600 feet in thickness, and is divided
into two main pay zones: Upper and Lower San Andres. The Lovington
Sandstone, a silty interval, segregates the two zones.
Reservoir Characterization
At Central Vacuum Unit (CVU), S-wave splitting
is the key to monitoring production processes associated with carbon
dioxide (CO2) flooding. Fluid property changes produce
variations in the velocities of the split S-waves passing through the
reservoir interval. Reservoir fluids change in response to CO2
and oil becoming a miscible phase in the presence of in-situ fluids.
Injected CO2 also can create areas of anomalous reservoir
pressure. Both fluid and pressure changes are detected by S-wave
splitting and velocities, because they are extremely sensitive to the
local stress field caused by the natural fracturing in all rocks,
especially carbonates.
Distinguishing Injected CO2 From Injected Water
S-wave splitting can distinguish between
effective stress changes associated with abnormal fluid pressures and
fluid property change. During Phase I of this study, a prominent S-wave
splitting anomaly was detected to the south of a cyclic CO2
injection well (CVU 97). This anomaly corresponds to the CO2
flood bank that developed south of this temporary injection well (Figure
3, Phase I).
Noticeable around the periphery to this CO2
anomaly are anisotropy anomalies of opposite sign related to offset
wells that were used to contain the CO2 bank through water
injection. The sign change of S-wave anisotropy occurs because the
relative velocities of the split S-waves reverse. In the case of the
miscible CO2-oil bank, the S2 velocity increased
and S1 decreased, whereas, in the case of water injection,
the effective stress causes S2 to decrease and S1
to increase. Similar effects were observed during the second phase of
the monitoring study (Figure
3, Phase II). These results imply that S-wave anisotropy can be used
to monitor secondary (water flooding) as well as tertiary (CO2)
methods in a spatial context beyond the wellbore.
The greatest need of tertiary recovery
operations is to monitor and control the areal and vertical distribution
of injected CO2 in the reservoir. Controlled injection can
maximize contact with the oil and optimize sweep efficiency so that oil
is not bypassed. A spatial image of the tertiary flood-front was
visualized by observing time-lapse anisotropy differences. This enables
the lateral sweep efficiency of the reservoir to be monitored.
The vertical sweep efficiency can be detected
through amplitude differentials of split S-waves. S2
amplitude difference anomalies between the pre- and post-surveys occur
dominantly in the Lower San Andres. This is highly encouraging, because
S-wave anisotropy may provide higher vertical resolution, enabling a
visualization of changes approaching the individual flow-unit scale.
The time-lapse seismic interpretation of the
Phase II seismic data showed a differential seismic anisotropy anomaly
between the baseline and monitoring survey that coincides with the
tertiary flood bank (Figure
3, Phase II). This anomaly was measured over the entire reservoir
interval, and is shown as a velocity anomaly where S1
velocity decreased and S2 velocity increased.
Figure 4 shows the correspondence between time-lapse P-wave
velocity, time-lapse S-wave polarization direction and time-lapse S-wave
velocity anisotropy anomalies. Using this information, it is possible to
separate the effective stress changes associated with changing fluid
pressure from the fluid saturation changes associated with the tertiary
flood bank. As a result, the tertiary flood bank -- and its growth over
time -- can be monitored by this technology.
Conclusions
The study indicated that shear wave analysis
provided higher resolution than (P-wave data) static reservoir
characterization, allowing for visualization of inter-well distribution
of secondary porosity, permeability, and fracture zones. Due to rigidity
changes associated with fluid replacement in the reservoir, dynamic
monitoring with shear wave data provided a means to follow actively the
displacement of reservoir fluids with CO2. This dynamic
reservoir characterization will provide the industry with the ability to
be more proactive, rather than reactive, in the management of
reservoirs.
References
Benson, R. D., and Davis, T. L., 2000, Time-Lapse Seismic
Monitoring and Dynamic Reservoir Characterization, Central Vacuum Unit,
Lea County, New Mexico, SPE Reservoir Evaluation & Engineering, v. 3,
no. 1, p. 88 - 97.
Wehner, S. C., Raines, M. A., Davis, T. L., and Benson,
R. D., 2000, Dynamic Reservoir Characterization at Central Vacuum Unit,
2000 SPE Annual Technical Conference and Exhibition, Dallas, Texas, 1-4
Oct., SPE 63134.
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