Borehole Seismic Survey
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Borehole Seismic Introduction
Borehole Seismic Tool and Acquisition
VSP Processing
Sonic Calibration and Synthetic Seismogram
VSP Examples
Kieu Nguyen Binh
HCMC-2010
#1
Borehole Seismic
Introduction
What is Borehole Seismic Survey?
A VSP or Vertical Seismic Profile is the process of recording
seismic data in a wellbore environment.
Vertical Well Zero Offset VSP Survey:
 Seismic source near the rig (hang by crane)
 The geophones are vertical in the earth (borehole)
 At single geophone depth > Source fire > Geophone record
signal
 Record for many geophone depth (VSP level)
The seismic ray paths (both direct and reflected rays) are vertical.
The direct ray (Downgoing) measures the time-depth information
The reflected ray (upgoing) gives a seismic image along the well
trajectory.
Why run VSP?
Well Logs
Surface Seismic Data
Time
Question: Where is my reservoir top on my seismic section?
 VSP provides Time-Depth curve
Reservoir
Depth
Depth
Time
VSP
Seismic Signal in the Borehole
Airgun Source
Time
Single level Receiver
Seismic Events
The total signal recorded by the geophone after each
firing of the gun is clearly very complex, containing a
large number of downgoing and upgoing (reflected)
signals.
1
1000m
1500m
1500m
9
5
7
v = 3000m/s
d = 2.2 g/cm3
Time
Upgoing
Downgoing
7
v = 2500m/s
3
d = 2.2 g/cm
4
1000m
6
8
2
8
v = 2000m/s 3
3
d = 2.2 g/cm
11
6
10
11
1
Depth
1
3
10
2
9
5
One-Way Time vs. Two Way Time
Surface Seismic
VSP
TWT = OWT + TT
Time
Reflection (TWT)
TWT
Airgun Source
TT
TT
TT
OWT
Depth
TT
OWT
Downgoing vs Upgoing Wavefield Reflection coefficients
INCIDENT
REFLECTION COEFFICIENT:
REFLECTED
Z1 =
interface
rho1 x vel1
R=
Z2 = rho2 x vel2
TRANSMITTED
Z2-Z1
Z2+Z1
(Vertical incidence)
rho
vel
Bulk density
Velocity
Z=rho x vel Acoust Impedance
REFLECTED AMPLITUDE = R * INCIDENT AMPLITUDE
Usual range within earth < 0.1
Water to Air : R= -1
Examples
Water to Limestone : 0.6
Shale to Limestone : < 0.1
Downgoing vs Upgoing Wavefield Relative Amplitude
Every level is a stack 3, 5 or more shots.
Stacking to increase the signal to noise ratio
More complicated example with
compressional and shear energy
Borehole Seismic Survey Types
Ps
Pp
CHECKSHOT
3D VSP
ZERO OFFSET
WALKABOVE
or VI-VSP
CROSS WELL
SALT
PROX
OFFSET
WALKAWAY
Checkshot Survey
Measurements:
 Geophone position in the borehole coarsely
- Small number of levels (50 to 100m level spacing)
 Measure travel time
- Waveform is not used
Applications:
 Time-Depth curve
 Formation velocity
 Sonic calibration -> Synthetic Seismogram (Geogram)
 Depth-to-Time conversion -> Well Tie
Zero Offset VSP
Measurements:
 Geophone position in the borehole closely
- Large number of levels (15 to 20m spacing)
 Measure both travel times and waveforms
Applications:
in addition to applications of checkshot survey
 1D Seismic Response - Corridor Stack
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- Calibration of surface seismic interpretation
Look ahead (Over pressure prediction)
Multiple pattern identification
Q (attenuation) Estimation
Phase analysis of surface seismic……..etc
Checkshot survey verses VSP survey
~ 100 m level spacing
and key depth points
A checkshot survey is only interested
in the first break time.
VSP processing looks are the entire
waveform.
~ 20 m level spacing
Checkshot
verses VSP
for Interval Velocity
Depth
INTV = Delta
Delta Time
VSP can give a higher
resolution interval velocity
curve, however, it is limited
by the transit time pick
accuracy.
Other VSP survey types
For a Deviated Well VSP (also known as a Vertical
Incidence VSP or VI-VSP), the ray path is also
vertical, so accurate OWT and TWT values can be
derived. It also gives a seismic image below the well
trajectory.
An offset VSP will have non-vertical ray paths. Nonvertical ray paths are affected by refraction and
anisotropy. These effects increase with ray-path
angle, and are difficult to correct for. Offset VSP can
give a seismic image away from the well path.
If accurate time-depth data is required, the survey should
have vertical ray paths (VI-VSP or ZO-VSP).
The applications of Offset VSP and Walkaway VSP are
for seismic imaging. Additional applications for WVSP are anisotropy and AVO measurement. If these
surveys are done, then a VI-VSP or ZO-VSP are
also normally done in conjunction.
Vertical Incident VSP
or Walkabove VSP
Depth Model
Surface
Seismic
As for Checkshot
survey
Independent
Pp & Ps Seismic
answer beneath
the well
Geophones
WALKABOVE
Depth (m)
Sources
Multiple Pattern
Identification
Time (s)
VSP
Image
Fault and Dip
Identification
…
Offset Source VSP
Surface seismic correlation
 2D seismic image
 Lateral extension of reflectors
Fault and dip identification
Shear wave velocities & imaging
Ps
Pp
OFFSET
Walkaway VSP
Measurements:
 Geophone fixed in the borehole
 Multiple source locations
 Record reflection around the borehole
Application:
 Surface seismic correlation
 Fault and dip identification
 P & S velocities and imaging
 Anisotropy and AVO analysis
 Surface survey design
Walkaway VSP
Common receiver gather
Common shot gather
One level with walkaway can give an image, but need at least 5 levels to do
up-down wavefield separation.
Typically use 8 or more simultaneous levels
3D VSP
3-D VSP
3D VSP
Well 1
Very High 3D
Sub-Surface
Imaging Resolution
Well 2
Well 1
Well 2
Surface
seismic 3D
PSDM
3D-VSP Depth
Migration
Obscured areas
All Walkaway
applications
in a 3D sense
…
3D VSP
80m
Profile Footprint at Target
Plan View
Ekofisk 3D VSP Geometry circa 1994
50 “lines” and ~ 10,000 shots
3D-VSP example (2005)
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13,400 shots
Western Geco source boat
Shooting every 10 secs at 5 knots
3.2 km spiral
3D-VSP and 7 Walkaway VSP lines in 48 hours
Processing using Q-Borseis software
P-wave processing delivered in 6 weeks
Riglocation
location
Rig
Borehole Seismic Techniques
Wireline
 Data acquired during a wireline
logging operation.
– After drilling is completed
– During a bit trip
– Can be acquired inside drill
pipe
Borehole Seismic Techniques
Drill Bit Seismic
 Service introduced in
the early 1990’s
 Uses the drill bit as
the energy source.
 Acquires checkshot
quality time-depth
information in real
time.
Borehole Seismic Techniques
LWD Seismic
 Service introduced in
2000-2001
 Logging tool is in a drillcollar to enable real-time
data acquisition.
 Transit-time data sent
up real-time with LWD
telemetry. The waveform
data is downloaded from
memory.
LWD Seismic Principle
Surface System
Surface source
Downhole receivers
Synchronized clocks
Waveforms recorded in downhole memory
Downhole processing
Real-time check-shot via MWD telemetry
Memory data processing after tool pullout
Source
MWD
sea floor
SeismicMWD
Tool
VSP Advantage
 Record both downgoing waves and upgoing wavefield
- Surface seismic survey records only upgoing waves
 VSP records reflected waves in the downhole
 Processing can separate down and upgoing wavefield
 Frequency analysis with borehole depth
 Q estimation from downgoing wavefield
 Accurate Time-Depth relationship and Velocity data
 Primary reflected waves pass the surface weathered layer only once
- Less attenuated
- Higher frequency content than surface seismic data
- Better vertical resolution
 Multiples are identified
Why run VSP more?
Here are more questions that VSP could give answers:
 Is there faults near my borehole?
 How far my reservoir laterally extend?
 I want to make synthetic seismograms (sonic calibration)
 I want to make a velocity model at the well location
 I need information to better process my surface seismic data
 Is there over pressured zone below TD?