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Figure Captions
 Figure 3.
This source point map shows the location of over 5,000
vibrator points that were used to record over 1,040,000 traces in eight
receiver wells. Color indicates the source point elevations. This 3D/3C VSP recorded near Bakersfield, California in September, 2000 is the
largest 3D VSP ever recorded.
 Figure 4.
Raw data recorded with a 4,000 ft long 80 level 3C down hole
receiver array. The data is not filtered or muted and is displayed with
trace-by-trace scaling. The scale across the top is depth in m.
 Figure 5.
This is a comparison of the image obtained from surface
seismic data and  borehole seismic data. The surface data was recorded
simultaneously with the borehole data using the same dynamite shots. The
image on the left is from the surface seismic data at the receiver well.
The data has a 40 m CDP spacing. The image on the right was generated
from the borehole seismic data and covers exactly the same time range
and location as the surface seismic image. The frequency content of the
borehole seismic is about twice that of the surface seismic and the
spatial sampling of the borehole seismic is 5 times greater than
the
surface seismic which allows for detailed imaging of lateral changes in
the reservoir properties.
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Leaps in Data Acquisition Technology
In the past 18 months a
new type of borehole seismic receiver array has been introduced that
currently has 80 3-C geophone levels in a single borehole. The design
can be modified to allow as many as 400 to1,000 three-component geophone
levels when fully deployed.
The fundamental
difference between the new and the old borehole arrays is that the new
array is deployed on production tubing wherre the old style of receiver
arrays are deployed using wireline technology. The newly developed
borehole array currently has a geophone spacing of 50 feet, but can be
tailored to any desired spacing. Using a geophone spacing of 50 feet,
the length of the 80-level array is 4,000 feet and the length of a
400-level array is 20,000 feet. Thus, most boreholes can now be filled
from top to bottom with clamped three-component geophones. Geophones can
easily be deployed in horizontal wells because they are conveyed on
standard production tubing using the same method used to deploy electric
submersible pumps.
3-D Borehole Seismic Coverage
The advantage of
deploying a large number of borehole seismic receivers is that a large
amount of reflection coverage can be obtained per seismic shot, thus
making borehole seismic method commercially feasible.
Figure 1 compares the amount of data recorded with a small borehole
seismic array as compared to a large 80 level borehole seismic array.
The large increase in reflection coverage per shot that is gained with
large borehole receiver arrays quickly translates to an improved image
quality, because rig time and shot effort are reduced to a minimum and
datasets large enough for a 3-D image can be economically recorded.
The size and shape of the
seismic image provided by 3-D VSP data is controlled by the source
locations and path of the borehole. In a vertical well with shots around
the borehole the image is usually cone-shaped and the diameter of the
cone in map view is roughly equal to the depth of the image (Figure
2). By combining data from several wells extensive 3-D images can be
generated.
3-D VSP Examples
Using an 80-level 3-C
array, our company has recorded the four largest 3-D VSPs in the oil and
gas industry. The most recent examples include:
- A 372,000 trace 3-D VSP
survey recorded in four days in West Texas in February 2001.
- A 1,040,000 trace,
eight-well 3-D VSP recorded south of Bakersfield, Calif., in September
2000 (Figure
3).
- A 350,000 trace 3-D VSP in
Alberta, Canada in October 2000.
- A 152,000 trace 3-D VSP
recorded for PanCanadian Petroleum in the Weyburn Field in
Saskatchewan, Canada, in December 1999.
An example shot record
from the Weyburn Survey is shown in
Figure 4. It illustrates the strong, high frequency reflections that
can be recovered in the quiet, downhole environment. In these surveys
much smaller scale reservoir features, including faults and pinch outs,
were mapped with higher resolution than had been possible to map using
surface seismic methods.
Using the recorded
bandwidth of 10-220 Hz in the Weyburn 3-D VSP survey a resolution of
better than five meters (15 feet) was evident in the final images (Figure
5). In the Edison field survey in California, 150 Hz 3-D VSP data
was recorded in the same area in which surface seismic data did not
exceed 25 Hz, and the borehole seismic image contained much higher
signal to noise ratio features corresponding to a maximum image
frequency well over 100 Hz.
It has been demonstrated
that 3-D VSP data recorded with the 80 level array can be used to image
the entire drainage volume around the well at more than twice the
resolution that can be obtained from a surface seismic survey. In order
to maximize the use of subsurface 3-D imaging using borehole seismic
measurements for development and production application, the data can be
processed in the field -- and an initial image delivered within one to
two days. The improved borehole seismic instrumentation is now driving
the development of new, innovative and high resolution processing
technologies for borehole seismic data.
Advantages
The principal advantage
of borehole seismic data is that the frequency content is consistently
much higher than surface seismic data recorded over the same location. A
good rule of thumb is that a borehole seismic image has twice the
frequency content of the surface seismic data. Higher frequency means
higher resolution and less uncertainty in drilling decisions.
The frequency content of
borehole seismic data is higher than surface seismic data because the
wave field only passes through the attenuating near-surface layer one
time rather than twice when both the sources and receivers are at the
surface of the earth. Additionally, the geophones are strongly coupled
to the earth via the geophone clamping mechanism.
Images from 3-D borehole
surveys are typically generated directly in depth through prestack depth
migration. This allows for an exact tie to depth since the time-depth
relationship is precisely known at the receiver boreholes. Interpreters
can directly tie seismic data to log properties since logs are always in
depth. A perfect tie to depth minimizes uncertainty in extrapolating
reservoir properties derived from well logs into the seismic volume.
Conclusions
Borehole seismic methods
now provide commercially feasible 3-D/3-C high-resolution images for
reservoir characterization. New designs in borehole geophone deployment
equipment allows hundreds of three-component clamped geophones to be
deployed in boreholes instead of the old five and 20 three-component
phones that were deployed using older conventional wire line technology.
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