How can injected CO 2 be monitored?

t

Netherlands Institute of Applied Geoscience TNO- National Geological Survey

How can injected CO2 be monitored?

OSPAR Workshop, Trondheim, 26-27 November 2004

Barthold Schroot, geophysicist / project manager

OSPAR Trondheim 2004

Monitoring of injected CO2 2t

Outline of this presentation

• Why do we monitor?

• What do we mean by monitoring?

• Techniques available for monitoring

• What can we expect to see?Examples from:• Recent research w/r to underground storage of CO2

• Natural analogues

• Conclusions



Why do we need to monitor?

Because

• we want to be sure that CO2 stays where we put

it (in case of geological storage)• Direct environmental concern: ensure that no leakage to

shallower levels occurs• Economics / Climate Change Objectives: verify the amounts of

avoided emissions (Emissions Trading Schemes)

• we need to ensure that no other undesired effects will occur after subsurface injection • Think of geomechanical effects (subsidence or uplift of the

surface or the seabed)



What do we mean by monitoring?

• Use techniques that enable us to “see”

where the injected volume of CO2 is sitting

(or where it might be moving to)

CO2 escapescenarios







• What can we expect to see? Examples from:• Recent research w/r to underground storage of CO2


• Conclusions



What properties do we measure?

Where do we measure?

1) at or near the surface

2) in existing or new boreholes (wells)

Monitoring techniques

Physical prop.

Chemical prop.

properties:

Geophysical

Geochemical

method:seismic / acoustic

gravimetric

others (electric etc.)

seabed sediment samples

water samples

includes for example:



• Common to all techniques:we are looking for anomalies

• A ‘baseline’ reference measurement (before the start of injection) is recommendable.

• Jargon: geophysical anomalies geochemical anomalies

Monitoring techniques



(Geo)physical techniques at surface

Cap rock * integrity (leakage)

good in case of leakage to

the sea

in case of leakage to

the sea

low resolu-tion

low resolu-tion

x x x

Ground movements

x x x x x good good good

Lateral spreading

good x x low resolu-tion

low resolu-tion

x x x

Verification or mass balance

fair x x too low resolu-tion

too low resolu-tion

x x x

•A very powerful tool is the 3D surface seismic method (seismic imaging)

•Repeated surveys : time lapse seismic / 4D seismic => changes in time



Physical techniques in wells

Cap rock integrity (leakage)

good monitor above the reservoir

monitor above the reservoir

good good in area of investi-gation

good in area of investi-gation

x Lab tests

Ground movements

x x x Detection of (small)

earth-quakes

x x x x

Lateral spreading

Presence moni-toring well



possible limited area, cali-bration

for seismics

limited area, cali-bration

for seismics


Samples around

reservoir


x x x x cali-bration

for seismics

cali-bration

for seismics

x x



Geochemical sampling & analysis(at or near the surface)

Cap rock integrity (leakage)

In case of leakage to

the surface

Injected CO2

discrimination


the surface


the surface

Ground movements

x x x x

Lateral spreading

x x x x


x x x x









• Conclusions



Sleipner gas field: CO2 injection project

Courtesy NPD

Location of the Sleipner-East fieldNorthern North Sea

•Average injection of 1 Mtonnes of CO2 per year•Injection started in 1996



Sleipner: application of 4D seismic method

Courtesy Statoil

Storage location



Sleipner: repeated 3D surveys reveal presence of CO2 injected since 1996

1994 1999

2001 2002

800 m

1200 m



Changes in acoustic velocity result indifferent expression on seismic data

0

500

1000

1500

2000

2500

0 0.2 0.4 0.6 0.8 1

Vel

oci

ty (

m/s

)

Low CO2 saturationHigh CO2 saturation

Seis

mic

velo

cit

y (

m/s

)

Shear wave velocity insensitive to saturationor compressibility C of the CO2

Compressional (P) wave velocities for differentcompressibilities C of the CO2

High C

Low C



Sleipner: seismic data -> geological interpretation and modelling

Stacked CO2 saturated layers



Sleipner: Seafloor micro-gravity method



Sleipner: Instrument fixed on concrete benchmarks at seabed



Sleipner: Gravity modeling of assumed 21 million tonnes of CO2 (5 Gal detectable)

Modelledgravityanomalydue topresenceof CO2



The study of natural analogues:Naturally occurring CO2 (and CH4) seepage

CO2 bubbles

Matra mountains, Hungary

E.g. in the EU sponsored 5th FW project NASCENT about natural analoguesfor CO2 in the geological environment

www.bgs.ac.uk/nascent



One part of NASCENT project:monitoring shallow gas and methane seepagein the Southern North Sea

6°

55°

54°

53°

52°

51°

5°4°3° 7°

0 50 km

The N etherlands

North Sea

A

B

K

G

NM

Q

SR

O

P

L

D

FE

German sector

UKsector

Studya rea

In the Netherlands offshoremost reports of shallow gasare from the northernmost sector

Rational: examining expressions of shallow gas(methane) in the North Sea will result in an assessment of monitoring capabilities,also applicable to monitoringCO2



Marine acoustic and seismic surveys

Marine seismic data acquisition

Images up the 5000 meters belowsea bed

Hull mounted or floating single channel 3.5 kHz system (sub-bottom profiler)

Images the shallowest tens of meters below sea bed, but also effects in the water column



High frequency acoustic (sub bottom profiler data

Sea bottom

Pockmark

acoustic b lanking

3.5 kHz sub-bottom profiler data:Seabed pockmark associated withventing of gas

(pockmark diameter ~ 40m depth ~ 2m)

3.5 kHz data:Acoustic blanking due to gas saturation of shallow layers



Shallow enhanced reflectors on 2D seismic line (example block F7)



Multiple of thefirst gas-sand ??

Shallow enhanced reflectors on 3D seismic survey (example block E17)

Shadow zone

Phase shift

These are seismic anomaliescorresponding to gas saturation of shallowest layers.



Time-slice at 152 msec

Profile from previous slide

Shallow enhanced reflectors on 3D seismic survey (example block E17)

Glacial Channels?



Selected areas in the NASCENT project

Area 1 (A11)

Area 2 (B13)

Area 3 (F3)

Acquired new data:

• Multi-beam echo

• High frequency (acoustic) sub-bottom profiler data

• 2D seismic data

• 60 vibrocores:

• core description

• headspace gas analysis=> C1, C2 concentrationsand isotope analysis(δ13C of C1)



Multi-beam seabed imaging :vertical resolution is high (cms)

Marine acousticand seismic surveys



Vibrocoring method for sea bed sediment sampling(North Sea : 2-5 m depth)

Seabed sediment sampling



Area 1: A seabed pockmark in block A11multi-beam image & headspace gas analysis

1. Multi-beam image showsseabottom morphology:

depression = seabed pockmark

2. Geochemical analysis:

122.6 ppm CH4represents ageochemical anomaly



Area 2: Gas plumes in the water columnexample from block B13

Anomaly:up to 10,395 ppm methanein seabed sediment



Gas plumes in the water columnexample from block B13 (area # 2)

200 m

Core # 26510,395 ppm C1

Core # 26139 ppm C1

-43m

~12

m

W E

G as p lum esG as p lum es

High frequency sub-bottom profiler record (TNO, 2002):Active venting observed in block B13 over a Plio-Pleistocene shallow gas field

Associated : geochemical anomalies (up to 10,395 ppm methane)



Underlying Plio-Pleistocene shallow gas field (block B13)

Bright spot

Mioceneunconformity

plume



Area 3: block F3various subsurface indications for gas

Legend:

C1 concentrations in headspace (black)

δ13C of C1 in red

Key vibrocore numbers in blue



In line 695W E

G as pocke ts

B righ t spo t:U pper P liocenegas sands

2500 m

Mid Miocene unconf.

#260 #244#243 #250 #249

Zechsteinsalt dome Zechstein

salt dome

Gas leaking along faults and fracturesexpresses itself on seismic profiles

Example from Southern North Sea (Dutch sector)



Gas leaking along faults and fracturesexpresses itself on seismic profiles

Y=424782.0

21191440

21591440

21991440

22391440

22791440

23191440

23591440

23991440

24391440

24791440

25191440

25591440

25991440

26391440

26791440

27191440

27591440

27991440

XLIL

-2200

-2000

-1800

-1600

-1400

-1200

-1000

-800

-600

-400

-200

Example from offshore Nigeria, courtesy Addax Petroleum Ltd



Gas chimney seen on seismic data(block F3/F6)

Zechsteinsalt dome

Mid Miocene

Base Tertiary

Upper P liocenegas sands

2500 m

In line 190W E

#225 #224 #223

#222

#221 #220Gas Chimney



Gas chimney above a Plio-Pleistocene bright spot(gas accumulation)

W E

16 parallel seismic lines from a 3D surveyLine spacing 250 metersViewed from north to south

3750m

View direction

Map view



Velocity pull-down

Chimney

1000m

Gas chimney above Pliocene bright spot (Dutch offshore),indicating leakage to the seabed; expression on profiles

100 m

400 m

Pliocene shallow gas accumulation (~ 500 m)



Shallow Chimney

2500 m

Fault

Tim es lice a t 300 m sec

Gas chimney above Pliocene bright spot (Dutch offshore), indicating leakage to the seabed; expression in map view









• Conclusions



Conclusions

• Various geophysical and geochemical monitoring techniques

can be applied to reveal the presence of gas (CO2 or CH4) in the

subsurface

• Of these techniques 4D seismic monitoring is a very powerful method (covering large areas with high resolution)

• In case of leakage to the surface techniques exist to measure quantities and fluxes• Geophysical techniques can be used for ‘early warning’ • With geochemical techniques (at surface or in wells) more acurate quantifications

can be made

• Each case requires a site-specific monitoring strategy depending on an initial risk analysis and on subsurface modelling• How frequently and for how long a period should we monitor ?• Should we go for permanent monitoring systems ?

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How can injected CO 2 be monitored?