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VSP Data in
Comparison to the Check Shot Velocity
Survey*
By
Robert J. Brewer1
Search and Discovery Article #40059 (2002)
*Adapted for online
presentation from two articles (comprising a series) by the author in AAPG
Explorer, entitled respectively, “VSP is a Check Shot Step Up,” or “The
Check Shot Velocity
Survey: Is It Enough?” (February, 2000), and “VSP Survey
Meets Accuracy Demands,” or “Additional Computed Product Utility” (March, 2000).
Appreciation is expressed to the author, M. Ray
Thomasson, former Chairman of the AAPG Geophysical Integration Committee, and
Larry Nation, AAPG Communications Director, for their support of this online
version.
1Halliburton Energy Services, Houston ([email protected]).
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General Statement
The idea
of lowering a geophone down a well bore to get a better handle on rock
The presence of a drilled well presents a truly unique opportunity to:
· Investigate a target formation more closely with acoustic measurements. · Minimize subsurface attenuation phenomena. · Measure depth accurately. · Overcome the formidable limitation of all surface geophysical measurements -- the lack of accurate depth control.
Sonic Logs and Check Shots
Geophysicists are familiar with the What may not be clearly acknowledged are how limited check shot data are -- and how very limited sonic logs travel times are inconsistently aiding the time to depth conversion process. The sonic log excels as a formation boundary and indirect porosity measurement log, but it can only see one-two feet into the formation under good downhole conditions -- and can be subject to cycle skipping and washed-out zones.
When the
sonic log is used to produce a synthetic seismogram for surface seismic
correlation purposes, one hopes that a check shot
The check
shot
A check-shot-corrected sonic log also makes it easier to determine interval velocities between key formations, since familiar formation boundaries can be readily recognized from the sonic log. If density log information is also available, a more accurate synthetic seismogram log integration usually results.
A check
shot
Borehole
seismic data are the most effective correlation bridge available between
the well bore and the surface seismic data. Borehole seismic data that
include the check shot Some History on VSP
The
vertical seismic profile (VSP) is a truly remarkable, versatile, and,
unfortunately, under-utilized innovation--under-utilized perhaps because
of its greater cost than the more routine check shot
The effective utility of the VSP was developed by the Soviets in the 1960s, made its way into Europe, and finally arrived in earnest in the United States in the 1970s. The VSP was quite an industry sensation when it started to be used in this country because of its "a look ahead" of the drill bit capability and its use as an aid in predicting the depth at which a target formation would be encountered after drilling continued.
The Look Ahead or Prediction Ahead of the Bit (PAB) VSP, which is actually an inversion routine performed during the data processing of ideally zero-offset VSP survey data, has proven itself as a useful exploration tool over the years. It has been used to predict the depth of overpressured zones ahead of drilling offshore wells and to locate granite-sediment and salt-sediment interfaces.
A zero- or near-offset VSP survey has the energy source positioned as close as possible to the well head to focus the energy down and ahead of the well bore -- and is the preferred geometry for well correlation as opposed to the offset VSP survey configuration, which positions the energy source away from the well head to image a distance laterally away from the well.
Look ahead offset VSP surveys also have been used recently successfully to locate subsurface features such as pinnacle reefs in East Texas. The look-ahead VSP survey may seem like quite a leap of faith to the uninitiated -- until one realizes that all surface seismic data (2-D and 3-D) are all look-ahead, as all measurements are made at the surface! VSP Perspective
The VSP is
simply a precision level step change up from the check shot
The basic
difference between the check shot survey and the VSP is that the VSP
measures nearly all seismic waveforms in the well bore (up-going and
down-going energy), whereas the check shot
Check shot
The VSP, like the check shot survey, also measures down-going energy. The smaller measurement interval (level interval) required by the VSP is necessary also to record the reflected energy in the well bore. The basic computed product of the VSP is known as a corridor stack, which in appearance resembles the synthetic seismogram. In reality it is a vastly superior well correlation tool, because it contains actual seismic reflection data as well as the down-going wave field.
The
down-going wave field is all that a check shot
VSP surveys are routinely performed in many parts of the world -- especially in Europe, because of the recognized superiority and versatility of the VSP over the simpler and less expensive check shot survey. More and more VSP surveys are being conducted -- especially offset surveys and a more detailed variation of the offset survey called the walk-away VSP survey -- as the advantages become clearer and survey reliability increases.
Pre-survey ray trace modeling has gained wide acceptance and is used to design more accurately offset VSP surveys and offset energy source placement. The computed product of the offset VSP is known as a VSP/CDP transform -- basically a high-resolution, mini-seismic section resembling a surface seismic CDP (Common Depth Point) stack display. The VSP/CDP transform has been converted or "transformed" from its original recorded one-way time to two-way time and displayed at a convenient scale to match the surface seismic data it is to be correlated with (Figure 3). The VSP/CDP transform data set can be migrated, filtered, and processed just like any surface seismic data set.
Because VSP data has a broader bandwidth and contains high frequency events, subtle features like small faults, stratigraphic changes, and amplitude anomalies can be observed in the vicinity of the well bore, whereas they are not discernible on the surface seismic coverage in the same area (Figure 4). Note the marked improvement in resolution that the VSP/CDP transform yields in this example (Figure 5): It is a VSP/CDP transform display made from a vertical incident VSP survey, recorded to provide better resolution over a 2-D reconstruction line from a 3-D seismic volume prior to deepening this directional well bore.
A vertical
incident VSP survey requires the energy source to be positioned at the
surface directly over the downhole geophone tool. Vertical incident
geometry is generally preferred over the rig source option and has
proven to be a more accurate way to obtain Downhole VSP Tools
Downhole tool design has improved significantly over the last 20 years. Three-component geophone configurations are routine -- the tools have evolved from single component analog designs to digital multi-tool designs or actual downhole geophone arrays composed of up to 24 or more individual tools or satellites.
Multi-station tools greatly reduce the historic bane of bore hole seismic surveys -- rig time consumption -- and record higher quality data. Slim (1-inch and 11/16-inch O.D.) downhole geophone tools have proven their versatility and have made it possible to record high quality VSP data in producing wells.
Logging While Drilling (LWD) sonic, check shot and VSP tools are available to meet the real-time demands of directional drilling. LWD tools designed to record borehole seismic data are becoming increasingly more sophisticated as LWD logging replaces conventional wireline logging on many directional wells.
Reduced Risk and Saved Money
The two most important benefits of running a VSP survey are reduced risk and saved drilling dollars. The VSP survey reduces risk by measuring the seismic velocities accurately in the well bore; this allows accurate time-to-depth conversion of the surface seismic data. VSP data also has been used to help reprocess older seismic data to yield more clearly interpretable results.
The
accurate
Money is
saved early on with VSP surveys conducted in the first wells drilled in
a play by increasing the accuracy of the interpretation and mapping
process -- and later, as more wells are drilled and the
Most of
the VSP surveys performed are of the zero- or near-offset type, which is
primarily used for
The Future
The sonic
log and the check shot |