PRAKLA-SEISMOS INFORMATION No. 42

Weil Seismies - Survey and Processing 1\

PRAKlA·SEISMDS

'J V

Introduction

In the area surrounding a weil it is often of special interest, for exploration and production, to know the lithological and structural properties of the geological layering, Meas­urements, using special seismic techniques, taken in these wells are a means to such ends, The seismic methods applied yield considerably better results than measure­ments taken on the earth's surface because multiple reflec­tions are less of a problem and the effects of the weathering zone on the seismic signal can be minimized,

Based on years of experience, PRAKLA-SEISMOS has a range of very accurate and robust instruments specifically designed for use in weil seismics, Field techniques and associated data processing techniques allow seismic velocity measurements and vertical seismic profiles to be recorded and in the same manner as weil logs, optimally processed to the final display with short turn-around times, The results produced (e, g, shape of the wavelet with respect to depth) can also be used in processing other seismic sections, recorded on the earth 's surface, to im­prove data quality,

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Extended Vertical Seismic Profiling

The technique of "Extended Vertical Seismic Profiling ", in which the shotpoints lie along a line or within an area, is especially useful for investigating the small structural characteristics around a weil and also for the determination of the true 3-D velocity field necessary for 3-D processing .

There are different methods available for processing Extended VSP data:

• Wavefront constructions

• Wave equation imaging • Reflection point mapping

The wavefront construction method as is often used by PRAKLA-SEISMOS during the interpretation of salt dome undershooting can also be applied during the interpretation of an Extended VSP . An example of this can be seen on the back cover of this brochure. The feasibility of reconstruction techniques is dependent on the extent of "illumination" of the horizon in question.

3

Field Techniques

Energy Sources

Explosives as commercially available

Airguns Type Outside diameter in mm (inch)

(single or arrays) VLF 225 (8.9) VLS 175 (6.9) VLSB 134 (5.3) VLAS 100 (3.9)

Vibrators Type Frequency range (-3 db point)

Standard Truck Vibrator VVOA 8- 115Hz Standard Crab Vibrator VVCA 6-105 Hz Broadband Crab Vibrator VVCB 10- 210Hz Shearwave Crab Vibrator VVCS 6- 80 Hz

Chamber volume in I (cu . inch)

2.0-0.6 (122-37) 2.0-0.6 (122- 37) 0.6-0.16 ( 37-10) 0.7- 0.45 ( 43-27)

Generated waves

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Taking into account the range Of instruments available and its experienced personnei, PRAKLA-SEISMOS ofters a complete program of weil seismics:

• Seismic velocity measurements • Vertical seismic profiling • Extended vertical seismic profiling • Determination of boundaries

(e.g. saltdomes)

Shots are recorded simultaneously by the receiver tool in the borehole and by geophones on the surface which thus enables vertical and horizontal profiles to be combined.

PRAKLA-SEISMOS has also developed the "Azimuth Sonde", specifically for recording of shearwaves in wells

Hydraulic Hammer HO-200 fo r shallow wells only

For its weil seismic surveys PRAKLA­SEISMOS has a large range of equipment for generating, detecting and recording seismic energy, in order to obtain optimum results considering task and surface conditions.

Receiver Tools

Type Receiver System Max. ambient. Outside Temperature Diameter

BGK standard receiver tool for combination with 1 vertical geophone ab out 180°C 95mm clamping device BGW 1 hydrophone (355°F) (3 3/ 4 , )

BGL slim hole tool 1 vertical geophone about 180°C 50mm with eccentric spring 1 hydrophone (355 °F) (2 ' )

BGKT Super deep weil tool with three channel Three-component system with receiver system and integrated clamping 1 vertical and about260°c 95mm mechanism 2 horizontal geophones (500 °F) (3%')

(optional for one channel: 1 hydrophone)

BGSA "Azimuth Sonde " 12 horizontal geophones a multichannel shear wave receiver tao I at 30 ° intervals about85°c 50mm with clamping mechanism and m9-gnetic 1 vertical geophone (185 °F) (2 ' ) compass

Recording Systems

Type N°of Sampling rate Record length Special capabilities Means of transport channels in ms in s

CBC 4 0.5 ca.5 seismic weil velocity transportable in boxes survey (check shots)

CBVA 10 0.5 up to presentation of seismic mounted in truck 16 0.8 10 data at wellsite (mounting in

i supported by container planned) HP computer

DFS IV / 24 2 upto correlation of vibrator mounted in truck CFSI 48 4 15 signals before

recording on tape

DFS V 24 0.5 upto transportable in boxes 48 1 99 96 2

4

PRAKLA-SEISMOS has many decades of world wide experience in weil seismics:

eonshore e shallow water eoffshore

even in wells with strong deviation.

Preprocessing of the data in the field consists of picking first breaks, reduction to the vertical and static correction to the datum plane etc. If required, vertical stacking and/or correlation can be done.

Field Techniques

This experience together with modern instruments enables complicated surveys, using different configurations of energy sources and detectors, to be undertaken - even ur'l.der the most difficult field conditions .

Through the use of new land seismic methods and tech­niques, such as air gun arrays and vibrators, survey and down-times can be reduced to aminimum.

Minicomputers and plotters can be used to display data during or immediately after the field acquisition.

Main Processing in PRAKLA-SEISMOS' data center is completed within short turn-around times.

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PRAKLA-SEISMOS DATACENTER .... PROCESSING

OFWELL SEISMIC DATA

5

Vertical Seismic Profiling

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Vertical Seismic Profiling In VSP seetions the downward progression of seismic waves (see above: blue line) and their reflected signals (red lines) can be observed , thus allowing the identification of multiple reflections (yellow lines) and shear waves (distin­guishable by velocity and polarization) . Both the layers penetrated by the bit and the layers ahead of the bit are displayed with high data quality.

The wavelet, extracted fram the downgoing seismic wave (including short-period multiples) , can also be applied to improve surface sections.

The correlation of the reflected wavelets , with their asso­ciated geologicallayer boundaries , is a useful tool for inter­pretation.

6

Standard Processing Sequence Input of data

a) from weil geophone

b) from farf ield geophone

Source signature deconvolution

Static corrections

Automatie computation of horizontal components . orientated parallel and perpendicular to SP-azimuth

Dynamic corrections (offset correction)

Separation of downgoing waves

Separation of upgoing waves

Suppression of guided waves

Correction to two-way travel times

Deconvolution with long operator

Frequency filtering

Stacking

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VSP, Standard Processing Standard processing of VSP data produces sections in ...... which the downgoing and upgoing wavefields are dis- JIIII' played, first together (Fig . 1 a) and later separately (Fig . 1 b + 1 cl. It is therefore possible to compare the upgoing VSP wavefield with a section recorded on the surface in the neighbourhood (Fig . 1 g) . This is normally achieved using wavelet processing (e.g . source signature deconvolution, shaping filter , wavelet compensation) .

In the following example wavelet processing has been applied to enable comparison of the signals from dynamite and air gun shots which were released in different boreholes.

There are neither problems involved in processing seismic traces recorded at unequal depth intervals nor to display this data at regular intervals (Fig . 1 f) .

Optional Processing Steps F anfi Iteri ng

Coherency filtering

Wavelet processing

Migration (extended VSP data only)

1 a: all waves

1 c: upgoing waves

Vertical Seismic Profiling Fig. 1: VSP, Presentation of Results

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1 b: downgoing waves

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1 f: interval stack 1 g: surface section (with marked borehole)

7

Vertical Seismic Profiling

VSP, Three-Component Processing When measurements are taken using 3-component­receivers, e. g. shearwave measurements, the orientation of the two horizontal receivers is normally unknown. In such cases the orientation of the receiver tool is calculated from the particle motion of the direct compressional wave. Next, the seismic traces are transformed into the horizontal components HR (in the azimuth of the shotpoint) and HT (perpendicular to the azimuth of the shotpoint).

Fig.2: Three-Component Processing ...

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Here displayed (see Fig. 2, top row) are the VSP sections of the recorded X, Y and Z components . It can be seen that due to the random orientation of the two horizontal re­ceivers, first breaks in the respective VSP sections cannot be picked . On the other hand, in the lower row the computed compontent HR enhances the signal of the direct com­pression al wave so that it can be clearly recognized. This wave is, however, not visible in the perpendicular HT component.

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Synthetic VSP In a VSP section it is often difficult to distinguish between primary and multiple reflections which occur below the deepest receiver depth. Synthetic data can be a great aid, helping the interpreter to distinguish between these two wavetypes. In a synthetic VSP, computed from a finite depth model, no reflecting horizons are defined below the base of the model; therefore signals appearing below this depth in the synthetic section can only be identified as multiples (see yellow marked area in lower section) .

The example shows recorded VSP data (Fig . 3a) and synthetic VSP data computed using the Reflectivity Method* (Fig . 3b). The input data for the synthetic VSP traces were the measured velocity and density logs, as used in the computation of normal synthetic seismograms, and the wavelet derived from the recorded VSP data; thus the trace at zero depth in the synthetic VSP section is identical to anormal synthetic seismogram. The comparable reflections in both sections are marked with colours.

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* Müller, G. , and Temme, P.: Journal 01 Geophysics , 1982, Vol. 50, pp 177 -188 3b: Synthetic VSP section

8

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Velocity Surveys

• Check Shots

• Matched Log Data

Fig. 4: Time depth graphs and results of continuous logs

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4c: Depth linear presentation of matched log data, integrated traveltime and reflection coefficients

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...... key to figures 4a and 4b (top lett and right):

[!] travel time fram seismic weil velocity survey

[!] travel time curve from velocity log*

[TI travel time curve from velocity log unmatched

[TI correction curve to match the velocity log to the seismic weil velocity survey

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* matched into seismic weil velocity survey

9

Synthetic Seismograms

Fig. 5: Processing of Log Data

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Time linearpresentations of formations, velocities, densities, acousticimpedances, impulse seismogram without multiples under consideration of transmission losses, corresponding transmission losses.

The calculation of synthetic reflection coefficients of primary events , free surface and / or interbed multiples has been carried out by PRAKLA-SEISMOS over many years.

The impulse seismogram, calculated for plane waves from continuous velocity and density logs and corrected by the seismic weil veloclty survey, is filtered , resulting in a synthetic seismogram.

Noise free synthetic seismograms aid in the identification of noise or multiple reflections in seismic sections. They are sometimes used for derivation of field parameters for a given structural model . When seen in the context of wavelet processing , the computation of synthetic seismograms and the use of VSP is becoming increasingly important.

The diagrams show the results of a seismic weil velocity survey (Fig. 4) , the processing of log data and corre­sponding synthetic seismograms (Fig . 5) .

Sb: Synthetic Seismograms Zerophase and minimum delay bandpass filters as weil as the mixed delay wavelet derived fram VSP recording have been applied to impulse seismograms without and with multiples.

10

' ITHOUrCOIISIOEIIATIO"

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Fig. 6: Extended VSP

Reconstruction of horizons using wavefront method.

f\ PRAKlA·SEISMDS

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PRAKLA-SEISMOSGMBH· BUCHHOLZERSTR.100· P.O.B.51 0530·0-3000 HANNOVER 51 PHONE: (511) 6460-0 . TELEX: 922847 + 922419 + 923250 . CABLE: PRAKLA GERMANY

© Copyright PRAKLA-SEISMOS GMBH, Hannover

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