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uGeneral
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uZero
offset VSP survey
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uGeneral
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uZero
offset VSP survey
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uVSP
data inversion
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uGeneral
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uZero
offset VSP survey
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uVSP
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uZero
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Vertical
seismic profiling (VSP) technology although not new, is still viewed by
some as something of a novelty. Our industry has been using it somewhat
reluctantly since the early 1970’s--reluctantly, perhaps because there
may be a tendency to view surface seismic data as a sort of panacea and
ultimate resource to get subsurface information between wells. General
industry knowledge of surface seismic techniques are widely known; not
so with VSP technology. Fortunately, more and more VSP surveys are being
run today because advantages of producing high-resolution seismic images
to help tie the well bore to surface seismic data to lower overall
drilling risk are starting to be more appreciated and documented. Of
course, other types of subsurface data are used. Included are the
obvious well-log curves and the occasional use of gravity and magnetic
coverage to augment deficiencies and gaps which may be encountered in
surface seismic data sets--especially where regional and frontier work
is involved. We are always asking structural and stratigraphic questions
about the subsurface and wanting to know what lies ahead of the drill.
The writer re-visits “Look Ahead” VSP or PAB (“Prediction Ahead of Bit”)
VSP (whichever acronyms are preferable) technology and presents this
type of VSP survey as an accurate and versatile seismic method available
for imaging an appreciable distance around and ahead of the drill bit.
Figure Captions
Figure 1. Near or zero offset source surface position with offset
position for comparison. An offset VSP survey design should always
include a zero offset source location to calibrate the offset to the
well bore and to provide velocity control.
Figure 2. Comparison of predicted data
(center panel) with well results - log data (left panel) and additional
VSP data recorded after well was deepened (right panel).
Figure
3. Effects of different velocity trends applied below TD of a well. Top
panel represents available choices for handling velocity below TD.
Bottom panel shows resultant impedance log below TD with actual sonic
log for comparison.
The Zero Offset VSP
Survey as a Start
The look ahead VSP survey is not much more than special data
processing treatment of the standard zero offset VSP survey. The zero or
near offset VSP survey (these terms tend to be distance from wellhead
related and generally used interchangeably), is defined as a VSP survey
where the energy source whether a vibroseis truck, airgun array,
dynamite shothole, etc., is positioned as close as logistically possible
to the wellhead in an effort to focus downgoing energy in and around the
bore hole from near-surface to T.D and beyond. This geometry favors the
recording of reflected arrivals to the downhole geophone tool for about
an approximate 100 ft. radius (depending on dip), around the well bore
as well as reflected arrivals several thousands of feet below the T.D.
of the well. Typical objectives of this type of survey are (1) to obtain
velocity control to allow surface- seismic -time to depth conversion and
(2) to produce a processed seismic image known as a corridor stack of
the area around and including the well bore. Zero offset surveys are
either run alone or in conjunction with some type of offset vertical
seismic profiling (Figure 1). Included may be an offset VSP, static or
walk-away or a salt-proximity, survey. Offsets are designed to image
some distance laterally away from the well bore in the direction of the
energy source, image a salt dome flank or perhaps a granite intrusive
interface. Zero offset is the most common type of VSP survey. The
surveys are usually recommended by geophysicists seeking accurate
on-depth seismic correlation to tie a well to a 2D and/or 3D surface
seismic survey. Those who run VSP’s routinely have appreciation from
past experience of the limitations and pitfalls involving
surface- seismic -time to depth conversion and
seismic -reflection-character-tie challenges. They understand the
problems that may result from trying to get too much information from
just a checkshot velocity survey alone and/or a simple and limited
synthetic seismogram made from a sonic log. The checkshot velocity
survey is basically a seismic travel time study that measures almost
exclusively the downgoing energy traveling from a surface energy source
to a downhole geophone tool. The checkshot survey is valuable for
velocity control, however, it contains virtually no reflected arrivals
and cannot be used to produce an optical reflection seismic image of the
subsurface unless it is used to calibrate a sonic log that has been
transformed into a hybrid known as a synthetic seismogram. Synthetic
seismograms are popular because they are inexpensive, easy to produce
and use from available sonic log data, and they may give satisfactory
results in cases where rock velocities are already well known and more
predictable. Difficulties may arise in softer rock regimes, such as the
U.S. Gulf Coast and areas of more complex structure and stratigraphy.
Look Ahead VSP Survey Utility
Because
genuine reflected arrivals from strata a significant distance below the
T.D. of a well are recorded by the downhole geophone tool in a VSP
survey, there usually is always an opportunity to handle this
information selectively during the data processing phase of the project.
More information about the subsurface is available from a VSP if one is
willing to make a small investment of time and money to produce an
additional computed product, such as an impedance profile log. The log
is an inversion technique that takes the recorded VSP data and inverts
the process that generated the reflection response to derive the
original reflection sequence that allows the look ahead aspect inherent
in virtually every VSP survey to be utilized. The look ahead VSP survey
is an option any time one wants seismic information ahead of the T.D. of
a well to help locate a suspected overpressured zone, fault, amplitude
anomaly, or any other structural or stratigraphic feature that may be
imageable by seismic data. The prospect of getting useful information
ahead of the drill bit and then being able to use it to save time and
money is an exciting possibility, and this proven application of VSP
technology helped propel it to fame in the early 1970’s, when VSP
surveys started to be conducted in earnest in the U.S. An application of
the computed products of the look ahead VSP survey, among them the
upgoing wavefield display and how it may be used to “predict” or
indicate at what depth a target formation and its corresponding seismic
reflector via visual correlation will be encountered in the subsurface
by the drillbit, is illustrated in Figure 2. A check of the method’s
accuracy may be done if and when the well is deepened and additional VSP
data levels are recorded, compared to the actual strata encountered in
the well bore, and then compared back to values anticipated earlier
before drilling continued. A sonic log will be very helpful with the
process.
VSP Data Inversion Produces Acoustic Impedance Log Below T.D. of Well
Inversion is a process that is closely related to look ahead or
prediction ahead of the bit and entails one of the most useful tools for
this application. The process that generated the reflection response of
the recorded VSP data is inverted to derive the original reflection
sequence. Corridor stacking of a zero or near offset VSP survey can be
used to generate an acoustic impedance log (sometimes known as a pseudo
sonic log), below the total depth of a well. Acoustic impedance
variations indicative of changes in rock properties can be identified.
The information is useful in identifying a particular target below the
well, such as a salt layer, overpressured zone, or a sequence of sands
in an otherwise homogeneous shale section. Specific details of the
inversion process are beyond the intended scope of this article;
however, they basically involve describing the acoustic profile
associated with a reflectivity sequence of discrete reflection
coefficients. Additional applications of inversion include modeling to
fill gaps in logs, lateral impedance prediction in a deviated well,
aiding surface seismic inversion after wavelet correction, and porosity
studies. There are four generally considered data-processing options
regarding how the velocity profile may be extended beneath the TD of a
well (1) letting the estimated impedance wander, (2) forcing all trends
out of the inversion to remove low frequencies, (3) forcing a trend that
mimics the general increase in velocity with depth, and (4) modifying
the profile below TD to include prior knowledge (essentially “training”
the profile) of acoustic data from nearby wells (Figure 3). When
acoustic data from nearby wells is unavailable, experienced
practitioners have had limited success carefully integrating lithologic
information interpreted from gravity and magnetic profiles. Letting the
estimated impedance wander and forcing out all trends are not
recommended. Forcing a trend that mimics the general velocity increase
with depth and modifying the profile with known information from nearby
wells are the recommended options. The modification, or “training,” of
the velocity profile below TD option is the most intriguing to the
writer as it theoretically should have applications to the neural
network forward modeling studies that have been done with well logs.
VSP surveys will be performed more often as our industry strives to get
the most useful and cost-effective seismic data available in a prospect
area to help make critical drilling decisions. The shortcomings of
surface seismic profiling relating to recorded frequency bandwidth
limitations, shadow zones in areas of complex subsurface structure,
higher cost and significantly slower data-processing turnaround than the
VSP will continue to encourage explorationists to consider borehole
seismic surveys. The look ahead capability of the zero or near offset
VSP is currently an under-utilized option in our industry. Its usage
will undoubtedly increase with time because it is easy to include in a
well’s logging program and is probably the most cost-effective and
accurate way to obtain information about the nature of the rocks ahead
of the drill.
Brewer, Robert J. 2002,
VSP Data in Comparison to the Check Shot Velocity Survey: Search and
Discovery Article #40059 (2002).
Clough, R.P., Understanding VSP’s,
Techniques, Interpretation, Applications, CGGBSD, Compagnie Generale de
Geophysique (CGG) 1993.
Sheriff, Robert, E., Encyclopedic
Dictionary of Exploration Geophysics, Society of Exploration
Geophysicists, Third Edition, Second Printing, 1994.
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