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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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The original acquisition plan called for at least one velocity point for each square mile. As the benefits of velocity data became apparent, the effort was increased. Velocity points exceeded one per square mile in many drilled areas of the field. The first phase of the project included both checkshots and VSPs while subsequent phases recorded virtually all VSPs. Zero offset
VSP surveys utilizing a 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
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 velocity data with original well log suites, especially where a drilling rig was absent and a mast truck was utilized. Slim (1-11/16-in.OD) downhole geophone tools were used on some surveys where borehole conditions and equipment prevented utilization of the regular tools. Pressure control equipment was used on several occasions. In-field
data processing, to produce a corridor stack from the field records, was
employed on virtually every 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 velocity volume was accomplished by using the Keystone Natural Resources Velocity Project (KNRVP) software suite. KNRVP allows nearly instantaneous creation and update of a 3D velocity volume by incorporating all checkshot, VSP, and 2D/3D seismic velocity information (such as stacking or RMS velocities). KNRVP provides unique time-to-depth functions for any XY location within the boundaries of the velocity volume and thereby produces a unique time-depth function for every well. At Elk Hills, this capability means immediate updating of the velocity functions for the field's nearly 5,000 wells. The database feeding the computer aided exploration interpretation software can be updated immediately using the software output. Exact well log correlations with the 3D seismic can then be carried out. Final incremental well shifts involving synthetics and stretching/squeezing of the well data to fit the 3D seismic data are used to create a very detailed and accurate 3D velocity volume that matches seismic interpretation and well picks exactly. KNRVP has built-in functionality to avoid "impossible" velocity inversions that hamper other software packages when dealing with many data points and/or very closely spaced data points.
Utilizing a cost effective and aggressive velocity acquisition program along with innovative analysis has enabled interpreters to incorporate all geophysical and geological data into a comprehensive picture and to predict accurately drill depths to targeted horizons. A good example of this predictive capability is the 351-17G well. The 351-17G deep test well was drilled on the south side of the Elk Hills field and was programmed to test a previously undrilled subthrust fault block. Figure 8 shows the location of the 351-17G well with respect to Elk Hills. Figure 9 is a seismic backdrop cross section including the final drilled well. A predicted velocity function was derived from the KNRVP velocity volume created with the substantial checkshot and VSP data previously acquired. Figure 10 compares the pre-drill software velocity function with the post-drill velocity function from the VSP. The pre-drill predicted velocity function is remarkably close to the true velocity function defined by the VSP. Some
practitioners have reportedly recommended that ideally one VSP |