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Comprehensive Use of VSP Technology at Elk Hills Field, Kern County, California*
By
R.J. Brewer1 and Don Greenfield2
Search and Discovery Article #40091 (2003)
*The viewer is referred
to two other articles about VSP, written by the senior author,
The Look Ahead VSP Survey: Its Utility and Future, Search and Discovery
Article #40060 (2002) and
VSP Data in Comparison to the Check Shot 
Velocity
Survey, Search and
Discovery Article #40059 (2002).
1Halliburton Energy Services, Houston, TX ([email protected]).
2Occidental of Elk Hills, Inc.
Introduction
To understand accurately subsurface rock formation and pay zone seismic travel times and velocities within the historic Elk Hills oil field in Kern County, San Joaquin Valley, California (Figure 1), Occidental of Elk Hills, Inc. in 1999 embarked on one of the first coordinated and comprehensive VSP survey data acquisition programs in the country. The trend-setting effort, uncommon in the United States, has helped produce more accurate surface seismic time-to-drill depth conversions. This result has been more accurate drilling prospect maps. VSP data has proven to be an effective means to lower drilling ambiguities as well as overall drilling costs in the Elk Hills field. It was concluded that rigorous integration of the all the VSP data recorded during the campaign into a entire surface and borehole seismic data set significantly improved the accuracy of a complex subsurface structural mapping process. Knowledge of the challenging stratigraphy of the area has also been enhanced. The result has been markedly improved success rates for exploration and development well drilling.
Occidental purchased the Elk Hills field from
the United States government in late 1998. As part of a comprehensive plan to
assist development drilling and to prepare for exploration drilling, Oxy
acquired an 80 square mile seismic survey and completed preliminary mapping
throughout the Elk Hills field. Previous 2D and limited 3D seismic acquisition
had proved problematic at best, yielding poor data. Near surface low velocity
air sands and extreme topographic variability (Figure
2) hampered surface
seismic data quality. Careful pre-3D acquisition testing and extensive quality
control yielded a 3D seismic volume that was magnitudes better than any previous
acquisition efforts (Figures 3 and
4).
It has long been
known that VSP data has the established advantage of being generally higher
frequency and better quality than surface seismic. The VSP’s one-way travel path
from energy source to receiver helps prevent dissipation of frequency and
amplitude. Consequently, following the 3D seismic acquisition, a comprehensive
wellbore velocity data acquisition program was initiated. Although there were
nearly 5,000 wells within the Elk Hills field, prior to the checkshot/VSP
acquisition program, there were only 11 checkshots within the Elk Hills Field
and another 6 checkshots surrounding the field (Figure 5). During the 1980's,
one or two VSPs had been acquired. However, no digital VSP records were extant;
and only checkshot data remained.
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Click to
view sequence of Figures 5 and 6 for comparison of
The
original acquisition plan called for at least one Zero offset
VSP surveys utilizing a vibroseis energy source located as close as
possible to the wellhead were acquired in selected, recently-drilled
development wells as well as in key existing wells slated for workover.
Of course, preference was given to deeper wells. VSPs were recorded in
every exploration well. Acquisition was performed in open hole wells,
cased hole wells, and both combination open/cased hole wells. Close
coordination and cooperation between exploitation/exploration, drilling,
field operations, and the contractor were essential for the ultimate
success in the acquisition effort. Figure 6 shows the density and
locations of checkshot and VSP surveys in the field area after
implementation of the borehole seismic survey program. Acquisition
through January 2003 provided seven new check shot While zero offset VSPs were the goal, many times vibrators had to be offset in order to suppress noise attributable to ground roll and "ringing" pipe. Every effort was made to keep offsets less than 500 feet. However, due to excessive noise and topographic problems, one VSP was acquired with a 1,000-foot offset. Although this offset was considered excessive, the survey was acquired to test the viability of the data. The resultant data were excellent. Whenever
possible, 50-foot level intervals were acquired with a dual station
downhole geophone tool assembly from TD to near surface. These 50-foot
intervals were chosen after careful modeling indicated that this spacing
would be sufficient to correlate with 3D surface seismic and avoid
aliasing. Dual tool deployment helped save as much as 40% rig time over
a single tool operation. Well depths typically ranged from 6,000 to
12,000 feet. Gamma ray acquisition combined with strip log correlation
was run on most surveys to tie the In-field
data processing, to produce a corridor stack from the field records, was
employed on virtually every survey to give At the beginning, postulated expenditures for the aggressive acquisition project were a concern. Monetary concerns were mitigated and cost reductions were made possible by competitive bidding along with an Oxy/contractor agreement that guaranteed an average number of VSPs per month in return for significant cost breaks. This agreement has thus far saved Oxy approximately 40% to 50% over individually contracted VSPs.
A
significant part of the exploration and development methodology employed
by Oxy involves being able to produce accurate time-depth functions at
any X-Y location encompassed by the surface seismic 3D and a time-depth
function and/or synthetic seismogram for every well in the field. This
criterion is made possible by the integration of the numerous checkshot
and VSP surveys recorded throughout the Elk Hills field and surrounding
area. The data integration and transformation to a 3D
Utilizing a
cost effective and aggressive Some practitioners have reportedly recommended that ideally one VSP survey be acquired for each 1-2 square miles of 3D surface seismic coverage. The Occidental of Elk Hills project has demonstrated that the recommendation is a good one. The elimination of drilling ambiguities is important to the optimization of a multi-well exploitation drilling program. |
Figure 1:
Location map of Elk Hills Field, Kern County, San Joaquin Valley,
California.
Figure 2:
Digital terrain model showing extreme topographic variations. Yellow and
red denotes higher elevations and blue lower elevations.
Figure 3:
Elk Hills Oil Field highlighting the locations of major structural
features, well control, and comparative seismic data.
Figure 4:
Comparison of high effort, high quality 2D seismic data versus the high
effort 3D seismic data.
Figure 5:
Figure
6: 
Figure 7:
VSP/3D seismic data correlation. VSP data was convolved with an
extracted wavelet from the 3D seismic and incorporated into the 3D
dataset to facilitate well tie formation correlations. 
Figure 8:
Elk Hills south flank location, 351-17G.
Figure 9:
Seismic backdrop cross section through the 351-17G well.
Figure 10:
Comparison of pre-drill 
