--> The Look Ahead VSP Survey: Its Utility and Future, by Robert J. Brewer #40060
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The Look Ahead Previous HitVSPNext Hit Survey: Its Utility and Future*

By

Robert J. Brewer1

Search and Discovery Article #40060 (2002)

 

*Sequel to article, Previous HitVSPNext Hit Previous HitDataNext Hit in Comparison to the Check Shot Velocity Survey, Search and Discovery Article #40059 (2002), by the author.

1Halliburton Energy Services, Houston ([email protected]).

 

 

 

uGeneral statement

uFigure captions

uZero offset Previous HitVSPNext Hit survey

uLook ahead utility

uPrevious HitVSPNext Hit Previous HitdataNext Hit inversion

uFuture

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uGeneral statement

uFigure captions

uZero offset Previous HitVSPNext Hit survey

uLook ahead utility

uPrevious HitVSPNext Hit Previous HitdataNext Hit inversion

uFuture

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uGeneral statement

uFigure captions

uZero offset Previous HitVSPNext Hit survey

uLook ahead utility

uPrevious HitVSPNext Hit Previous HitdataNext Hit inversion

uFuture

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uGeneral statement

uFigure captions

uZero offset Previous HitVSPNext Hit survey

uLook ahead utility

uPrevious HitVSPNext Hit Previous HitdataNext Hit inversion

uFuture

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uGeneral statement

uFigure captions

uZero offset Previous HitVSPNext Hit survey

uLook ahead utility

uPrevious HitVSPNext Hit Previous HitdataNext Hit inversion

uFuture

uReferences

 

 

 

 

 

 

 

 

 

 

General Statement

 Vertical seismic profiling (Previous HitVSPNext Hit) 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 Previous HitdataNext Hit 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 Previous HitVSPNext Hit technology. Fortunately, more and more Previous HitVSPNext Hit surveys are being run today because advantages of producing high-resolution seismic images to help tie the well bore to surface seismic Previous HitdataNext Hit to lower overall drilling  risk are starting to be more appreciated and documented. Of course, other types of subsurface Previous HitdataNext Hit 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 Previous HitdataNext Hit 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” Previous HitVSPNext Hit or PAB (“Prediction Ahead of Bit”) Previous HitVSPNext Hit (whichever acronyms are preferable) technology and presents this type of Previous HitVSPNext Hit 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 Previous HitVSPNext Hit 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 Previous HitdataNext Hit (center panel) with well results - log Previous HitdataNext Hit (left panel) and additional Previous HitVSPNext Hit Previous HitdataNext Hit 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 Previous HitVSPNext Hit Survey as a Start

 The look ahead Previous HitVSPNext Hit survey is not much more than special Previous HitdataNext Hit processing treatment of the standard zero offset Previous HitVSPNext Hit survey. The zero or near offset Previous HitVSPNext Hit survey (these terms tend to be distance from wellhead related and generally used interchangeably), is defined as a Previous HitVSPNext Hit 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 Previous HitVSPNext Hit, 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 Previous HitVSPNext Hit survey. The surveys are usually recommended by geophysicists seeking accurate on-depth seismic correlation to tie a well to a 2D and/or Previous Hit3DNext Hit surface seismic survey. Those who run Previous HitVSPNext Hit’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 Previous HitsyntheticNext Hit 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 Previous HitsyntheticNext Hit seismogram. Previous HitSyntheticNext Hit seismograms are popular because they are inexpensive, easy to produce and use from available sonic log Previous HitdataNext Hit, 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 Previous HitVSPNext Hit 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 Previous HitVSPNext Hit survey, there usually is always an opportunity to handle this information selectively during the Previous HitdataNext Hit processing phase of the project. More information about the subsurface is available from a Previous HitVSPNext Hit 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 Previous HitVSPNext Hit Previous HitdataNext Hit 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 Previous HitVSPNext Hit survey to be utilized. The look ahead Previous HitVSPNext Hit 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 Previous HitdataNext Hit. 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 Previous HitVSPNext Hit technology helped propel it to fame in the early 1970’s, when Previous HitVSPNext Hit surveys started to be conducted in earnest in the U.S. An application of the computed products of the look ahead Previous HitVSPNext Hit 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 Previous HitVSPNext Hit Previous HitdataNext Hit 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.

 

Previous HitVSPNext Hit Previous HitDataNext Hit 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 Previous HitVSPNext Hit Previous HitdataNext Hit is inverted to derive the original reflection sequence. Corridor stacking of a zero or near offset Previous HitVSPNext Hit 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 Previous HitdataNext Hit-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 Previous HitdataNext Hit from nearby wells (Figure 3). When acoustic Previous HitdataNext Hit 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.

 

The Future

Previous HitVSPNext Hit surveys will be performed more often as our industry strives to get the most useful and cost-effective seismic Previous HitdataNext Hit 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 Previous HitdataNext Hit-processing turnaround than the Previous HitVSPNext Hit will continue to encourage explorationists to consider borehole seismic surveys. The look ahead capability of the zero or near offset Previous HitVSPNext Hit 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.

 

Recommended References

Brewer, Robert J. 2002, Previous HitVSPNext Hit Previous HitDataNext Hit in Comparison to the Check Shot Velocity Survey: Search and Discovery Article #40059 (2002).

Clough, R.P., Understanding Previous HitVSPTop’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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