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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 velocity is hardly a new concept. Geophysicists have engaged in the practice with increasing precision since the1930s -- around the time when the first geophones were designed to withstand the rigors of the borehole.
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 velocity survey's one-way acoustic
travel time as a critical component that is necessary to help convert
surface 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 The check shot velocity survey can be used to produce a corrected sonic log, allowing sonic log pitfalls to be alleviated by enabling a data processing analyst to correlate effectively and more accurately through questionable zones that were traversed by the sonic logging tool downhole.
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 velocity survey measures a much larger cylindrical volume of rock
compared to the relative soda straw volume measured by the sonic log.
The check shot survey and the more precise vertical
Borehole
Some History on VSP
The
vertical
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 VSP Perspective
The VSP is simply a precision level step change up from the check shot velocity survey.
The basic
difference between the check shot survey and the VSP is that the VSP
measures nearly all
Check shot velocity survey measurements are typically taken every 250-500 feet downhole and were designed to measure the down-going waveforms used in velocity determination. VSP measurements are much more closely spaced (50-100 feet).
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
The
down-going wave field is all that a check shot velocity survey records.
The corridor stack made from the VSP is the well bore converted to a
full reflection waveform
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
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
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
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 velocity control and image a
highly deviated borehole. Displaying the VSP/CDP Transform and the
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
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
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
The
accurate velocity information from the VSP helps make the velocity
analysis involved in processing and stacking surface
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 velocity field is better understood.
Most of
the VSP surveys performed are of the zero- or near-offset type, which is
primarily used for velocity determination and surface
The Future
The sonic
log and the check shot velocity survey have been the standard |