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!