DATA ACQUISITION
The well velocity survey technique
A well velocity survey is a type of seismic well operation performed to determine the vertical propagation time of a wave emitted at the surface by a seismic source, and then recorded by a geo• phone. In practice, since the source and the well geophone are not generally situated on the same vertical, the distance separating the verticals which pass through the well geophone and through the source must be taken into account. (Fig. 32).
The surface seismic data obtained from seismic reflection survey are set with respect to a referenceWell geophone
Figure.32: Implementation of seismic well velocity survey.
Plane (DP datum plane) and the vertical travel time estimated by the well velocity survey set to the same reference plane. In land seismic surveys, the reference plane is generally chosen at the base of the weathered zone.For well velocity shooting, the assumption can be made that the raypath is vertical as far as the reference plane, but is oblique and rectilinear from the reference plane to the well geophone. A borehole seismic survey operation is per• formed using a seismic source, a well geophone, a reference geophone or hydrophone placed near the source and a recording laboratory (Fig. 33).
The processing equipment does not require a large number of seismic channels, but must have good recording and precision dynamics as well as a short sampling interval (0.25, 0.5 or l ms).
The tool must be small, light and equipped with a good clamping system and a seismic cartridge including either a vertical geophone or a set of three geophones arranged in a triaxialconfiguration. The source (Figure. 27) used must be repetitive, preferable emitting a short signal with a clearly• defined initial pulse and a wide spectral range: an airgun is generally used in offshore operations. Onshore, the following kinds of source can be used: offshore-type sources in a mud pit, low• charge explosives
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and dropped weight impulsive sources. The use of vibrator sources is widespread despite their being unfavourable for the picking of first arrivals.The reference receiver is either a hydrophone for offshore operations, a geophone or mud pit hydrophone for onshore operations
Figure.33:Implementation of seismic well velocity survey (source configurations).(Courtesy of Schlumberger)
Operation of a seismic well survey
After setting the zero datum at the rotary table or at ground level. a well velocity survey at a given depth cornorises the following steps:
( 1) Checking the depth of the tool in the well.
The depth is chosen in relation lo the position of the velocity and density contrasts obtained on the well logs and according to the quality of the hole. h is important not to position the tool in a zone where caving has occurred. ft is essential to have a measuring point at the starting and finishing depths of the sonic log. The intermediate points are chosen close to the markers. preferably situated beneath them so as 10 avoid interference between the downgoing wave and the upgoing wave reflected off the marker.
(2) Clamping the tool and :,,luckcni.ng the cable.
If the tool moves. the position of the measuring point has to be modified. The cable must be slack to avoid cable waves. Clamping is critical to obtain good signal to noise ratio.
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(3) Recording of seismic data.
Seismic measurements are made up from the recording of several surface shots using ampli• fication factors and filters at different frequencies as to obtain field records wilh a signal to noise ratio optimized at the first arrival,
(4) Picking of first arrivals on the well geephonc TG and oo the reference receiver TR, and verifying the con ...istency of the arrival times,
(5) Tightening the cable.
Check shot
The Checkshot is the very basic type of Borehole seismic survey. In the case of a vertical well it involves positioning the source at a single fixed zero offset position, usually relatively close to the well bore. The Borehole receiver tool is positioned at various stations throughout the well, e.g. every 500-ft, formation tops and sonic log points. At least 3 shots are fired at each station. We are only interested in the first arrival time (time it takes for the signal from the source to arrive at the downhole receiver), this time enables us to obtain time/depth information used for correlation of the Surface Seismic and for the calibration of acoustic type logging tools (figure34).
Figure.34: check shot survey
Geophysicists are familiar with the velocity survey's one-way acoustic travel time as a critical component that is necessary to help convert surface seismic's two-way travel time to depth. In the absence of the check shot velocity survey, accurate velocity information can sometimes be extracted from the tried and true sonic log. Relying solely on sonic logs, however, may entail considerable risk involving interval velocity errors (figure.35).
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
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into the formation under good downhole conditions and can be subject to cycle skipping and washed- out zones.
Figure.35:check shot survey and velocity relation.
Check shots notes
It involves the recording of first arrivals along a well that penetrates fairly deep target layers.
Objective is estimating the velocity and thickness of subsurface layers.
It is performed using receivers that are placed in the borehole at known depths and a source that is placed near the well head.
It is similar to a downhole survey but using a deeper well and larger receiver spacing.
Check Shot Surveys Interpretation
Interpretation of a check-shot survey data includes the following steps: 1. Picking the first arrivals from each depth level
2. Applying any necessary corrections to these times
3. Calculating the interval velocity between each successive receivers 4. Computing the RMS velocity profile
5. Correction from slant to vertical times may be neglected because depths are large compared to shot offset.
The interval velocity between two successive receivers (Ri, Ri+1) is calculated as:
∆Z: receiver spacing ∆Tvi: difference in vertical time from datum to
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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 velocity survey and the VSP can measure large volumes of rock -- and will indicate the presence of velocity anomalies, which may be totally missed by the sonic log. These velocity anomalies must be measured and dealt with accurately when mapping the velocity fields that are so critical to an effective surface-seismic time to drill-depth conversion process (figure.36,37).
The RMS velocity to the bottom of the Nth layer is calculated as:
Vi: interval velocity within the ith interval ∆Tvi: vertical time within the ith interval
This RMS profile is comparable to the RMS profile found by velocity analysis of surface seismic data.
Figure 36: rms velocity calculated from check shot
.
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 seismic profile (VSP) should at least be considered in the logging program of every exploration and key development well being planned to minimize or eliminate the ever-present and costly danger of surface seismic time to depth conversion error (figure.38)
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Figure.37: check shot raw data and how to convert to interval and rms velocity
When the sonic log is used to produce a synthetic seismogram for surface seismic correlation purposes, one hopes that a check shot velocity survey is available from the same well to calibrate the sonic log. Calibration and correction of the sonic log often may be needed because the production of a synthetic
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seismogram from a sonic log is a hybridization and transform process that can introduce seismic travel time error if cycle skipping, tool sticking, and washed-out zone effects are present in the sonic log. The sonic log is also of very limited use in identifying interval velocity inversions -- or any abrupt rock density and velocity change that are an appreciable distance from the well (Figure40).
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.
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Figure.39: Comparison between surface seismic and well data.
Figure.40: Time depth chart calculations methods (calibrated with sonic, not calibrated with sonic, sonic used to calculate time depth).
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