methods have been used since the beginning of exploration seismology.
Check shot surveys were used to obtain travel times and interval velocities,
and 2-D VSPs (Vertical Seismic Profiles) and 2-D high-resolution crosswell
data also have been recorded.
however, borehole seismology has be relegated to a secondary role in seismology
because it generated only 1-D or 2-D images -- and they were obtained
at a high cost relative to the amount of information they provided.
factor in borehole seismic methods was that the fundamental designs of
borehole seismic systems only allowed a small number of geophones to be
deployed in the borehole. Until about two years ago the maximum number
of three-component (3-C) clamped geophones that could be deployed in a
borehole was around 12. Thus it was expensive to record enough data to
make large surveys economically feasible.
A few experimental
3-D VSPs were recorded with a small number of geophones in the borehole.
The surveys were expensive and the seismic images were of limited quality
because of the small amount of data recorded per shot.
Leaps in Data
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.
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
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.
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).
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
- 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
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.
borehole seismic instrumentation is now driving the development of new,
innovative and high resolution processing technologies for borehole seismic
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.
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.
the geophones are strongly coupled to the earth via the geophone clamping
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
A perfect tie
to depth minimizes uncertainty in extrapolating reservoir properties derived
from well logs into the seismic volume.
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.