OCTOBER, 2010

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGYRESEARCH CENTER FOR TECH. & INDUSTRIAL EQUIPMENT

VIETSOVPETRO PETROLEUMJOINT VENTURE

VERTICAL SEIMIC PROFILE PROCESSING AND INTERPRETATION training course

Borehole Seismic Survey

1 Borehole Seismic Introduction

2 Borehole Seismic Tool and Acquisition

3 VSP Processing

4 Sonic Calibration and Synthetic Seismogram

5 VSP Examples

Kieu Nguyen Binh

HCMC-2010

# 5

VSP Examples

VSP match to the Surface Seismic

VSP is good quality and zero phase.

After a gross static shift, the match to surface seismic is OK near the top, but less good deeper down.

The residual mismatch is caused by time and phase shifts in the surface seismic, that result from the low Q.

What can be done:

– From the VSP, we can estimate the effects of the Q and attempt to remove these effects from the surface seismic.

VSP match to the Surface Seismic

XCOR from 1.2 to 2.4 sec

VSP resultSurface Seismic

and VSP corridor

stack

VSP – Surface Seismic merge

Good match at 1300 ms. Not so good deeper down.

VSP is zero phase along the entire well interval

Surface seismic is not zero phase and changes with depth?.

VSP Corridor Stack

Surface

Seismic

VSP-SS match with non-ZP VSP corridor stack

By processing the VSP to be non-zero phase at the bottom, we can get a

good match to the surface seismic

VSP Corridor Stack

Surface

Seismic

Zero Phase DeconvolutionBefore

Decon

After

Decon

ZP Decon – Operator design on top traceBefore

Decon

After

Decon

The VSP is now not zero phase

VSP

Resolution

Vibroseis sweep 8-90 hz

The zero offset VSP has coherent

energy from 8-90 hz.

The corridor stack is filtered to 8-45

hz to match the surface seismic.

8-90 hz VSP corridor stack

8-45 hz VSP corridor stack

Surface seismic

Multi-pathing

Basement

Fault

Open Hole

Slotted Liner

Casing

Top Basement

YY

NO

MULTI-PATH?

MULTI-PATH?

Multi-pathing may be defined as the

phenomena where downgoing arrivals

on a VSP level are not all following the

same path.

Multipathing is usually characterized

by higher than expected interval

velocities on a VSP. The higher

velocity may be seen as high negative

drift on the sonic in good hole

conditions or even physically unreal

high velocities. Direct arrival may also

appear to bifurcate even though the

source signature seen on the

reference surface hydrophone remains

stable

Higher VSP Velocities

Multi-pathing #1

Q- estimation

The deeper traces in a VSP have less high frequency information

Q is a measure of the loss of higher frequencies as a function of OWT

Q-analysis from VSP

Q is a measure of the loss of the higher frequency signal with depth. It is caused by formation layering.

VSP can directly measure Q

A low value of Q means high attenuation.

A low value of Q also means the surface seismic is less likely to be zero phase.

1500-2350 m

Q=25

3200-3950 m

Q=81

High gradient = low Q = high attenuation

Frequency spectrumVSP waveform

Downgoing after upgoing removal

Direct downgoing compressional

Need to remove upgoing before Q

estimation

Loss of frequency with depth =>

low Q

Multi-spectral Ratio Method

Multi-Spectral Ratios

method is a more

statistical method that

estimates the Q for

every possible receiver

pair.

Red-yellow dots

indicate greater

confidence in spectral

slope. The confidence-

weighted average

Qp=52. The estimates

with greatest

confidence lie between

Qp=45-65Q estimates versus receiver pair midpoint

Q filtering

Q filtering compensates the data for the frequency dependant attenuation.

Although it restores the balance of frequencies, it can also introduce phase rotation to the data,

No Q After Q=200 filter After Q=100 filter

VSP Inversion

• Inversion is the inverse procedure to synthetic

seismogram

• Result is not unique, since the input data is band-limited

• Corridor stack inverted to give acoustic impedance

curve

• Need low frequency content

• Flat layer below TD

Acoustic Impedance Inversion

VSP Inversion used to assist

with overpressure prediction.

Inversion shows drop-off in

velocity below TD.

For best results a full

mechanical earth model (MEM)

should be constructed before

drilling the well

VSP Inversion needs flat

structure.

Inverted Impedance

Sonic Velocity

Geophones with a low frequency

response are essential.

Processed VSP data

TD

Final VSP Inversions on 18-11-96 (Intermediate TD 3807)

VSP inversion lookahead 140 msec TWT, or 200 metres

VSP summary for Look-ahead

VSP requires high frequency data to be get best resolution when identifying a target boundary.

VSP also requires low frequency data to get velocity trend, and pore pressure below TD.

The frequency content is dependent on:

the VSP tool.

the near surface ground conditions (loose sand, hard rock etc), and formation

type of energy source

Obtain nearby well logs and VSP, to know the expected VSP response.

VSP Limitation

VSP data is band-limited. Typically from 5-80 hertz ?

VSP has no information below 5 hz.

Standard seismic tools are limited to10hz.

VSP inversion is good at extracting “relative changes” in velocity.

It is not good at extracting “absolute values” in velocity.

Extracting long period velocity variations is not possible with VSP.

Walkaway VSP can overcome this limitation.

Both VSP and WVSP inversion assume a 1D earth velocity model.

Depth Prediction

below TD from VSP

Tw

o W

ay T

ime

(sec)

Extend T/D curve to meet

the peak extrapolation:

Depth = 16082ft TVD

Depth (ft)

China ExampleTD is about 200 metres above the coal beds

Run VSP Inversion on the VSP to predict distance ahead

VSP

Corridor

Synthetic from

Inversion Reflectivity Acoustic Impedance

Look-ahead Depth Accuracy

Velocity Errors: +/- 100 metre/sec will be +/- 5 metres.

The velocity error will decrease, as the lookahead distance decreases.

TWT pick errors: +/- 2.5 msec will be +/- 5 metres.

TVD 3712m = 2.701 sec TWT

TVD 3512m = 2.606 sec TWT

Implies average velocity

before TD = 4210 m/secEvent ahead of TD is picked at 2.804 sec

2.804 - 2.701 = 0.103 sec ahead = 217 metres ahead

3712 + 217 = 3929 metres depth

Event depth

For good quality

data +/- 10 m

accuracy is

possible

Depth

China Example … logs after drilling ahead

Logs from datum

=

Top coal at 3930

Walkaway VSP

Walkaway VSP

Example above

Sources from +3000m to -3000m.

16 level Receiver array at 3500m

Migrated Image out to 500m at 2.8 secs

Direct P arrivalP reflections

Migration

Example from

a Walkaway VSP

Input X and Z data

Output Down/Up, P and S

Time migration

Thank You!