Time-lapse Crosswell Seismic Tomography
for Monitoring CO2 Sequestration* Hideki Saito,
Dai Nobuoka, andHiroyuki Azuma

OYO Corporation

International Workshop on CO2 Geological Storage, Japan ’06

February 20-21, 2006, Tokyo

Contents
1. Borehole arrangement
2. Method of seismic tomography
3. Time-lapse tomography
4. Numerical experiments
5. Conclusions

International Workshop on CO2 Geological Storage, Japan ’06

February 20-21, 2006, Tokyo

Borehole Arrangement
Well positions at the top of the target aquifer

NOB-4

Formation

D ip60m: 15°OB-2IW-1
OB-3

40m
120m

ismiceSlle
Crossw

Totionce
Syhmograp

International Workshop on CO2 Geological Storage, Japan ’06

February 20-21, 2006, Tokyo

Source – Receiver Geometry
plane view

CO2-3坑CO2-2坑OB-2

OB-3

0tomography-100-500-150

section50

100side view

front view
900

OB-3

IW-1
OB-2

receiver:900 – 1228 m@4m interval

OB-3

OB-2

source:900 – 1284 m@4m interval

1000
depth (m)
1100

1200
sourcereceiverCO2 injection point

International Workshop on CO2 Geological Storage, Japan ’06

050100 m

February 20-21, 2006, Tokyo

Data Acquisition System
Energy Source: Oyo’s OWS
Receiver: 24-channel hydrophone cable
Data acquisition: Oyo’s DAS-1 (24bit A/D)

International Workshop on CO2 Geological Storage, Japan ’06

February 20-21, 2006, Tokyo

Oyo’s OWS Downhole Source- Diameter:

108mm- Length:

3,420mm- Weight:

150kg- Pressure:

30MPa （3,000m）- Temperature:

150°C- Wire line:

7 conductor armoredTriggering
Geophonecable- Cable head:

Gearheart type 1-1/2”
7 conductor head

- Energy

max. 3,000J/shot- Shot interval:

20 – 60 sec- Electric power:

700W(AC100V)- Trigger sensor:Geophone

3,420mmElectricMotorSpringCatcherHammer

OWS
Multi-layeredDisks

φ108mmInternational Workshop on CO2 Geological Storage, Japan ’06

February 20-21, 2006, Tokyo

Observation Pattern: Source-Receiver Combinations
900OB-3
receiver:900 – 1228 m@4m interval

OB-2
source:900 – 1284 m@4m interval

1000
Depth(m)
1100

1200

International Workshop on CO2 Geological Storage, Japan ’06

February 20-21, 2006, Tokyo

Flow Chart of Tomography
900020Traveltime (msec)
40 60 80 100 120 140

BPT for BLS tomography
BLS tomogram for MS tomography

Receiver Depth (m)

950
1000
1050
1100
1150
12001250

International Workshop on CO2 Geological Storage, Japan ’06

BLS Traveltime Curves
February 20-21, 2006, Tokyo

Time-lapse Crosswell Seismic Tomographybaseline survey

BLS

before injection

Feb 2003
Jul 2003 injection started

MS1

3,200 t-CO2

Jan 2004

MS2

6,200 t-CO2

Jul 2004

monitoring surveys MS3

8,900 t-CO2

Nov 2004

MS4

10,400 t-CO2
9 months after
injection ended

Jan 2005

MS5

injection rate (t/day)

0BLS

Feb 2003

elapsed time (day)150300

Oct 2005
450

MS2
injection
startedinjection ended

10,000(t)

8,000
MS1

40

6,000 cumulative
CO2 injected
4,000

20

2,00000Jul 2003

Jan 2004

Jul 2004

International Workshop on CO2 Geological Storage, Japan ’06

Jan 2005

February 20-21, 2006, Tokyo

Reconstructed Velocity Tomograms

Depth(m)

IW-1

IW-1

IW-1

900
1000

velocity(km/sec)

1100

3.3
3.1
2.9

1200

2.7
OB-3

OB-2

OB-3

OB-2

OB-3

OB-2

BLS

MS1

MS2

before injection

3,200 t-CO2

6,200 t-CO2

International Workshop on CO2 Geological Storage, Japan ’06

2.5

February 20-21, 2006, Tokyo

BLS Tomogram
IW-1

depth(m)900

CO2 injection layer
called “Zone-2”

caprock

1000

1100

aquifer

1200
OB-3

OB-2

International Workshop on CO2 Geological Storage, Japan ’06

February 20-21, 2006, Tokyo

Velocity Difference Tomogram (BLS / MS1)
IW-1

900
top of theaquifer
1000
velocitydifference%

BLS
1100

1200

OB-3OB-20-1-2-3-4-5- 10max. velocity reduction = 3.0%MS1
3,200 t -CO2

Velocity difference = (VMS1-VBLS) / VBLSInternational Workshop on CO2 Geological Storage, Japan ’06

February 20-21, 2006, Tokyo

Velocity Difference Tomogram (BLS / MS2)
IW-1

900
top of theaquifer
1000
velocitydifference%

BLS
1100

1200

OB-3OB-2

0-1-2-3-4-5- 10max. velocity reduction = 3.5%MS2
6,200 t -CO2

Velocity difference = (VMS2-VBLS) / VBLSInternational Workshop on CO2 Geological Storage, Japan ’06

February 20-21, 2006, Tokyo

Velocity Reduction
IW-1

IW-1

900
top of theaquifer

1000

velocitydifference
%

1100

1200

OB-3

OB-2

MS1: 3,200 t-CO2max. velocity reduction = 3.0%OB-3OB-20-1-2-3-4-5- 10

MS2: 6,200 t-CO2max. velocity reduction = 3.5%

International Workshop on CO2 Geological Storage, Japan ’06

February 20-21, 2006, Tokyo

Traveltime Shift

International Workshop on CO2 Geological Storage, Japan ’06February 20-21, 2006, Tokyo

Contradictions
IW-1
OB-3

OB-2

Velocity
decrease in thecap rock ?

Nobreakthrough ?

Small velocity
reduction ?
Time-lapse Acoustic Logging

Velocity difference tomogram

International Workshop on CO2 Geological Storage, Japan ’06

February 20-21, 2006, Tokyo

Evidence of Artifact

receiverdepth(m)source = 1,100 m
1,052
1,056
1,060
1,064
1,068
BLS MS1 MS2

2 msec6,200 t-CO2

The velocity anomaly extending into the cap rock did not produce any
traveltime delays for rays passing through the area. This apparent velocity
anomaly must be an artifact or ghost.

International Workshop on CO2 Geological Storage, Japan ’06

February 20-21, 2006, Tokyo

Numerical Experiments
depth (m)900
1000

velocity(km/sec)

1100

3.3
3.1
2.9

1200
OB-3
OB-2
before injection model

OB-3
OB-2
after injection model

2.7
2.5

depth (m)900model velocity reduction = 20%

max. velocity reduction = 5.5%

1000

velocitydifference %

1100

0-2

1200
OB-3
OB-2
model difference

OB-3
OB-2
reconstructed velocity
difference tomogram

International Workshop on CO2 Geological Storage, Japan ’06-4-5- 10February 20-21, 2006, Tokyo

Conclusions
The distribution of CO2 injected into the aquifer can be imaged asarea of velocity reduction.
The velocity reduction was found to be 3.5% for MS2 (6,200 t-CO2injected), though the value obtained from acoustic logging wasmore than 20%. Judging from the result of numerical experiment,
actual velocity reduction could be much larger than the 3.5%
observed in MS2 velocity difference tomogram.
Although some anomalous velocity reduction zones were observed,
the numerical experiments revealed that those anomalies must be
artifacts or ghosts.
The velocity determination in a thin, low-velocity layer is one of
the basic problems of traveltime tomography. However, somesophisticated inversion schemes (e.g. adequate constraints) cansolve the problem.
International Workshop on CO2 Geological Storage, Japan ’06

February 20-21, 2006, TokyoAcknowledgments

This project is funded by Ministry ofEconomy, Trade and Industry (METI)of Japan.

International Workshop on CO2 Geological Storage, Japan ’06

February 20-21, 2006, Tokyo