By MARCUS L. COUNTISS
The Geophysical Corner is a regular column in the EXPLORER, edited by R. Randy Ray. This month's column is titled "Rejuvenation of a Mature Field Through Application of a Unique Frequency Enhancement Technology."
High Frequency Targets New Pay
Editor's note: Marc Countiss is with Pogo Producing, Houston.
See enlarged image for graphic / photo credit.
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:
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.
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.