Last
month's column introduced a history of the development of South Marsh
Island Block 128 Field (figure 1) where, beginning
in 1989, interpretations used a first generation 3-D seismic volume that
led interpreters to treat seismic terminations as fault related.
This produced
interpretations with complex, highly faulted patterns that were geologically
suspect.
The addition
of a newer vintage of 3-D seismic data with target-oriented reprocessing
was an improvement, but still left some questions unresolved. Pressure
and production data indicated that some wells originally interpreted to
be producing from the same reservoir had to be separated.
This new 3-D
seismic seemed to eliminate faulting as the reason, but failed to offer
an obvious geologic solution.
The data hinted
at a more complex stratigraphic interpretation, but it became clear that
the standard bandwidth seismic data would be unable to image the thicknesses
of many of the sand units seen in the wells.
Boosting Seismic
Signal Frequency
The decision
was made to apply a new frequency enhancement technology to the newly
reprocessed 3-D seismic data set to see if the vertical resolution could
be improved.
In this case,
the algorithm works to decode the seismic "message" and extract the acoustic
reflectivity series directly from it. The operation is entirely mathematical,
with no wavelet estimation or other interpretive input applied. The primary
requirement is a seismic trace with reasonably good signal-to-noise ratio.
The high-frequency
technique considers the broad-band reflectivity series, or "earth signal,"
to be convolved with the band-limited embedded wavelet through the process
of polynomial multiplication (one-sided convolution). The new method used
here takes an alternative approach by describing one-sided convolution
as a matrix multiplication with the problem resembling a process used
to decode encrypted messages.
This way, the
earth reflectivity is not viewed as being filtered but rather "encoded,"
with the upper portion of the spectrum not removed but encrypted in the
lower end of the spectrum, which is still observable.
By treating the
seismic trace in this domain, it can be manipulated to increase the high
frequency signal without boosting the ambient noise. Consequently, the
signal emerges from beneath the noise level and is recoverable.
The resultant
signal is very similar to the original "earth signal" or unconvolved reflectivity
series, and produces a reasonable estimate of the reflectivity series
with greater resolution than the input seismic trace.
Since the entire
spectrum is encoded by the embedded wavelet, it is theoretically possible
to regain frequencies up to Nyquist frequency (half the sampled frequency)
on properly recorded and processed data.
Testing the Updip
Pinchout
The reprocessed
high-frequency version of the seismic from last month revealed an apparent
undrained reservoir in our zone of interest. Recalling that the L-10 zone
in the B-6 well was productive, and observing that we can penetrate this
reservoir updip to the B-6 take point without a break in continuity, leads
to the obvious conclusion that we have defined a new drilling target not
previously recognized.
In November 2000
a side-track of the B-6 well was spudded to test the prospect. The well
reached total depth and was logged in early December.
Logs revealed
oil pay in three zones for a total of 52 feet of net oil pay, 26 feet
of which were in the L-10 zone of interest -- with no water contact present!
Independent engineering
calculations assigned 407 MBO and 183 MMCF of new proved reserve additions
to the field, with 203 MBO coming from the zone of interest.
Figure
2 shows a frequency enhanced arbitrary 3-D extracted line, B-B', that
incorporates the new B-6 ST with the older B-6 and B-9 wells. The location
of this traverse is shown in figure 3. Again
the target horizon is indicated in the B-6 and the new B-6 ST wellbores
with the black line tracking the seismic event related to the horizon.
The discontinuity
marked by the arrow separates the B-6 and B-6 ST from the updip B-9 well.
This interpretation agrees with the separation implied by the pressure
data.
In figure
4, the normal bandwidth version of this line is displayed for comparison.
The discontinuity visible on the frequency enhanced version is also apparent
on this particular profile (highlighted by the arrow), albeit in a less
obvious state. Clearly there are places where the separation is visible
on the standard bandwidth seismic, but this is something that was never
recognized in previous investigations.
(This break in
the reflector certainly does not appear on the original processing profile
and is laterally discontinuous when viewed in detail. In any event, this
prospect was never previously identified.)
Finally we are
led to the cross section incorporating the new B-6 ST well (figure
5), which shows the correlation interpreted on the high frequency
3-D data.
Six Out of Seven
This project generated
seven new drilling opportunities, all of which turned out to be commercial
producers.
It would be misleading
to claim that all of these wells were primarily the product of high frequency
imaging. Specifically:
- Two wells were essentially
production acceleration wells, although the frequency enhanced data
helped to optimize the target locations.
- One was a side-track of
an existing well that had a completion failure and was drilled back
into the same zone.
- The remaining four wells
relied principally on the high frequency data and acoustic impedance
inversion.
Only one well
had to be sidetracked to obtain a positive result, and this well was completed
in a secondary target as a commercial producer; this could be counted
as a scientific failure, since the primary target was non-commercial.
However, six
out of seven is an acceptable success rate for any subsurface method employed.
As of May, total
daily field production rates were averaging 11,500 BOPD and 18 MMCFPD,
a 328 percent increase in oil rates and a 450 percent increase in natural
gas rates! Furthermore, an estimated 3.5 MMBO and 5 BCF of proved reserves
were added to the field.
Not a bad day's
work in a 27-year-old field.
A Valid Technique
Since the frequency
enhancement technique described herein was applied as a post stack process,
it is desirable to have the basic processing of the data set in state-of-the-art
condition to obtain the best result. Accurate statics, velocities and
migration must be applied, since errors in any of these steps affect the
high frequencies more so than the low frequencies.
Favorable results
were obtained in this example because the basic seismic data quality was
good, but inferior acquisition and processing may restrict or eliminate
the effectiveness of the method.
Although the
clear success of the drilling program supports the validity of the method,
good matches with broad band synthetics demonstrate the ability of the
technique to extract real high frequency signal.
As with all seismic
methods, there is no one "silver bullet" that will achieve all goals --
but this is another weapon in the seismic arsenal.
Although the
application of the acoustic impedance inversion has not been detailed
here, it was beneficial in the course of this program. The combination
of multiple techniques is always the best way to improve the reliability
of the prediction of a favorable result.
ACKNOWLEDGEMENTS
The author would
like to express appreciation to Pogo Producing Company, Devon Energy and
BP for their kind support. Special thanks to Geotrace Technologies, Inc.
for use of the HFI (trademark) processing. Also, thanks to Dr. Carl Zinsser
for technical guidance and Marsha Brown for outstanding graphic design.
Assistance with engineering data provided by Bill Foshag of Pogo and Johnny
Rau with Devon.