[fogri] Re: S wave -seismic multicomponent
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- To: fogri@xxxxxxxxxxxxx
- Date: Mon, 10 Dec 2001 13:46:09 +0800
Gambarnya nanti aku bikin dulu dalam pdf supaya tidak kebesaran utk =20
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| From: Franciscus.Sinartio / mime, , , Franciscus_Sinartio@xxxxxxxxxxx=
=2Ecom
| To: fogri / mime, , , fogri@xxxxxxxxxxxxx
| Subject: [fogri] Re: S wave -seismic multicomponent
| Date: Monday, 10 December, 2001 11:55AM
|
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| Vick,
| tolong di usahakan supaya kita bisa baca file nya PHN dong.... thank=
s
| sebelumnya...
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D for September 2001 ...
The Geophysical Corner is a regular column in the EXPLORER, edited by =20
R. Randy Ray. This month's column is titled "Multi-Component, =20
Time-Lapse Seismology for Monitoring Reservoir Production Processes."
S-Waves Detect Reservoir Flows
Improving reservoir performance and enhancing hydrocarbon recovery are =20
critical to the future of the petroleum industry -- and to do this, it =20
must be possible to characterize reservoir parameters, including fluid =20
properties, their movement and pressure changes with time.
Multi-component, time-lapse seismology has great potential for =20
monitoring fluid movements in reservoirs. The main reason is simply the =
=20
presence of fluid-filled fractures.
Shear waves (S-waves) are much more sensitive than compressional waves =20
(P-waves) to the presence of fractures or microfractures and the fluid =20
content within the fracture network. Seismic shear wave anisotropy in =20
the reservoir causes two shear modes to form (S1 and S2) and to =20
propagate with different velocities.
The faster mode (S1) propagates with its particle motion parallel to =20
the open fracture direction, perpendicular to the minimum horizontal =20
stress (S3) in the reservoir -- a phenomenon called S-wave splitting, =20
or birefringence (Figure 1).
Seismic shear wave anisotropy is key to monitoring fluid property =20
changes in fractured media.
Figure 1.
Shear-wave polarization and splitting in a fractured material. As an =20
S-wave with an arbitrary polarization direction enters an anisotropic =20
material, the wave splits into S1 and S2 components with different =20
polarizations and different velocities. The wave polarized parallel to =20
the fractures travels faster and is less attenuated that the wave =20
polarized perpendicular to the fractures. After the S-waves emerge from =
=20
the anisotropic material, they continue to propagate as two S-waves =20
with different polarization directions.
First 4-D, 9-C Seismic Survey
The first time-lapse (4-D), multi-component (9-C) seismic surveys were =20
acquired at Vacuum Field in Lea County, N.M.
At the Vacuum Field, shear wave (S-wave) and compressional wave =20
(P-wave) seismic data were used to monitor reservoir fluid property =20
changes associated with a carbon dioxide (CO2) tertiary flood in the =20
Permian San Andres Carbonate. Reservoir fluid properties -- including =20
viscosity, density, saturation and pressure changes -- occur in =20
response to CO2 injection. Changes are caused by CO2 and oil becoming a =
=20
miscible phase with the oil in place.
These fluid property changes alter the interval velocity and =20
attenuation of S-waves passing through the reservoir interval by up to =20
10 percent, but cause little (1 to 2 percent) or no measurable change =20
in P-wave velocity and attenuation on the surface seismic data.
The Reservoir Characterization Project of the Colorado School of Mines =20
(RCP) has conducted two studies at Vacuum Field:
=B7 Phase I efforts centered on monitoring the injection of CO2 from a =20
single wellbore (Benson and Davis, 2000).
=B7 Phase II is the dynamic reservoir characterization of a six-well CO2=
=20
injection program, which includes the Phase-I wellbore (producing =20
during Phase-II) (Wehner, et al, 2000).
The Vacuum Field was discovered in 1929 with the drilling of the Socony =
=20
Vacuum State 1 well in Section 13-T17S-R34E of Lea County.
The Vacuum Field produces predominately from the San Andres Formation =20
in a shallow-shelf carbonate depositional setting (Figure 2), which =20
structurally is positioned on the shelf edge of the Permian Basin's =20
Northwest Shelf. The structurally high shelf crest is located just west =
=20
of the RCP study area.
Porosity and permeability within the productive zones average 11.8 =20
percent and 22.0 md, respectively.
The San Andres gross pay zone can reach 600 feet in thickness, and is =20
divided into two main pay zones: Upper and Lower San Andres.
The Lovington Sandstone, a silty interval, segregates the two zones.
Figure 2.
Type log for the Vacuum field area. The San Andres Formation is at an =20
approximate depth of 4,300 feet and is the primary producing formation =20
in the Vacuum Field.
Reservoir Characterization
At Central Vacuum Unit (CVU), S-wave splitting is the key to monitoring =
=20
production processes associated with carbon dioxide (CO2) flooding.
Fluid property changes produce variations in the velocities of the =20
split S-waves passing through the reservoir interval. Reservoir fluids =20
change in response to CO2 and oil becoming a miscible phase in the =20
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 =20
velocities, because they are extremely sensitive to the local stress =20
field caused by the natural fracturing in all rocks, especially carbonat=
es.
Distinguishing Injected CO2 From Injected Water
S-wave splitting can distinguish between effective stress changes =20
associated with abnormal fluid pressures and fluid property change.
During Phase I of this study, a prominent S-wave splitting anomaly was =20
detected to the south of a cyclic CO2 injection well (CVU 97). This =20
anomaly corresponds to the CO2 flood bank that developed south of this =20
temporary injection well (Figure 3, Phase I).
Noticeable around the periphery to this CO2 anomaly are anisotropy =20
anomalies of opposite sign related to offset wells that were used to =20
contain the CO2 bank through water injection. The sign change of S-wave =
=20
anisotropy occurs because the relative velocities of the split S-waves r=
everse.
In the case of the miscible CO2-oil bank, the S2 velocity increased and =
=20
S1 decreased, whereas, in the case of water injection, the effective =20
stress causes S2 to decrease and S1 to increase.
Similar effects were observed during the second phase of the monitoring =
=20
study (Figure 3, Phase II). These results imply that S-wave anisotropy =20
can be used to monitor secondary (water flooding) as well as tertiary =20
(CO2) methods in a spatial context beyond the wellbore.
The greatest need of tertiary recovery operations is to monitor and =20
control the areal and vertical distribution of injected CO2 in the =20
reservoir. Controlled injection can maximize contact with the oil and =20
optimize sweep efficiency so that oil is not bypassed.
A spatial image of the tertiary flood-front was visualized by observing =
=20
time-lapse anisotropy differences. This enables the lateral sweep =20
efficiency of the reservoir to be monitored.
The vertical sweep efficiency can be detected through amplitude =20
differentials of split S-waves. S2 amplitude difference anomalies =20
between the pre- and post-surveys occur dominantly in the Lower San =20
Andres. This is highly encouraging, because S-wave anisotropy may =20
provide higher vertical resolution, enabling a visualization of changes =
=20
approaching the individual flow-unit scale.
The time-lapse seismic interpretation of the Phase II seismic data =20
showed a differential seismic anisotropy anomaly between the baseline =20
and monitoring survey that coincides with the tertiary flood bank =20
(Figure 3, Phase II). This anomaly was measured over the entire =20
reservoir interval, and is shown as a velocity anomaly where S1 =20
velocity decreased and S2 velocity increased.
Figure 4 shows the correspondence between time-lapse P-wave velocity, =20
time-lapse S-wave polarization direction and time-lapse S-wave velocity =
=20
anisotropy anomalies. Using this information, it is possible to =20
separate the effective stress changes associated with changing fluid =20
pressure from the fluid saturation changes associated with the tertiary =
=20
flood bank.
As a result, the tertiary flood bank -- and its growth over time -- can =
=20
be monitored by this technology.
Figure 3.
Time-lapse shear-wave velocity anisotropy differences. Phase I) CO2 =20
injection occurred at the CVU-97 well with a prominent S-wave =20
anisotropy anomaly detected to the south. Phase II) CO2 injection =20
occurred at the six offset injectors (indicated by triangles). In the =20
case of the miscible CO2-oil bank, the S2 velocity increased and S1 =20
velocity decreased (purple), whereas, in the case of water injection, =20
the change in effective stress causes the S2 velocity to decrease and =20
S1 velocity to increase (blue).
Figure 4.
Phase II seismic anomalies. The upper diagram shows the time-lapse =20
P-wave velocity differences while the lower diagram shows the =20
time-lapse S-wave velocity anisotropy differences. Overlain on each =20
diagram are the S-wave polarization direction differences (areas that =20
have changes in the S-wave polarization direction). Areas of the =20
reservoir that have P-wave velocity and S-wave polarization direction =20
anomalies correspond to zones of the reservoir with pressure changes. =20
Areas of the reservoir that have S-wave anisotropy anomalies correspond =
=20
to zones with fluid saturation changes.
Conclusions
The study indicated that shear wave analysis provided higher resolution =
=20
(than P-wave data) static reservoir characterization, allowing for =20
visualization of inter-well distribution of secondary porosity, =20
permeability and fracture zones.
Due to rigidity changes associated with fluid replacement in the =20
reservoir, dynamic monitoring with shear wave data provided a means to =20
actively follow the displacement of reservoir fluids with CO2.
This dynamic reservoir characterization will provide the industry with =20
the ability to be more proactive, rather than reactive, in the =20
management of reservoirs.
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