Potential environmental effects of CO2 leakage in the marine and terrestrial environments

1. Marine Monitoring
Ian Wright
National Oceanography Centre,
Southampton, UK
CCS Meeting – University of Nottingham
21st February 2012

2. Presentation
Outline
• Storage site characteristics and monitoring strategy;
• Leakage scenarios and temporal sequence of
emission;
• Current monitoring state of art;
• Marine monitoring for CO2 volume loss - challenges;
• Conclusions

3. CCS Storage Sites
EOR Sites Saline Aquifers Sites
• ~ 250 km2 reservoir / • ~22500 km2 reservoir /
seafloor area; seafloor area;
• ~25-30 km3 ocean; • ~2500-3000 km3 ocean;
• point > dispersed seep • point = dispersed seep
sources. sources.

4. CCS Leakage Scenarios
A continuum from:
• High discharge (e.g., >200 tonnes d-1) point source
leakage (due to acute well-casing leakage or
hydro-fracturing of a seal cap) in a relatively small
depleted reservoir site, 
• Low discharge (e.g., <20 tonnes d-1), dispersed
source discharges from an extensive saline aquifer
system.
• Necessitates diverse, and responsively staged
monitoring.
+ Transport + Storage +
Leakage Assurance CCS
Capture + Monitoring = Implementation
Impact

5. Monitoring
strategies
How do we monitor sites 250 - 22500 km2 in area, with
ocean volume of 25 - 2500 km3, with potentially known and
unknown point and dispersed seep sources?
• Baseline monitoring;
• Seafloor / ocean leakage detection;
• Quantification of CO2 leakage.
Point source, high discharge
 dispersed, low discharge
leakage

6. Deep geophysical detection
Current regulatory monitoring practice
places significant emphasis on “deep”
geophysical monitoring of reservoir
formation, integrity of the capping
seal, and migration of CO2 within the
reservoir.

7. 4-D Geophysical monitoring of Sleipner from
Geophysical monitoring of CO2 storage
4-D MCS has been very successfulet al., 2003
Chadwick
Very clear imaging and
modelling of CO2
accumulation and
expansion of “gas
plume” within the
reservoir.
Kaarstad, 2004 1996 1999 2001

8. Deep geophysical detection
Requires known quantitative
relationship between any geophysical
parameter and supercritical CO2, but
probably not sensitive enough for
regulation and carbon emission
trading.

9. Seafloor detection -1
Two additional significant opportunities
for CCS monitoring:
1. Probable that pre-cursory fluids will
be emitted at the seafloor before
CO2 due to buoyancy pressure of
CO2 displacing stratigraphically
higher fluids.
2. Seafloor, and lesser extent the
overlying ocean, provide a site for
more direct and quantitatively
explicit measurement of CO2 flux
(both as free gas and dissolved
phases) that is potentially more
sensitive for measurement and
verification of CO2 leakage.

10. Leak Detection
Signature of seeping fluids on to seafloor and overlying water-column
Reduced unconsolidated sediment pore fluids;
• Increase Fe, [Fe (II)], Mn
• Increase H2S
• Decrease Eh
Reservoir formation fluids / brines;
• Increase salinity
• Increase temperature
• Increase noble / inert gases (e.g., radon, neon, argon)
CO2 fluids, free gas, dissolved phase
• Increase CO2 (dissolved phase ± free gas)
• Decrease pH
• ?Increase trace / heavy metals

11. Seafloor detection - 2
Physical and chemical signatures of
CO2 loss from the seafloor, either as
direct CO2 measurement, a decrease
in pH, or emission of gas bubbles, are
arguably more tractable both in the
sense of making the observation and
understanding its relationship to CO2
volume loss.

12. Seafloor physical detection
Physical techniques developing
around passive and active acoustic
bubble detection that would determine
free gas leakage. Hydrophones
acoustically detect bubble oscillation
and expansion, while active sonar
record the acoustic back-scatter
response of ascending gas bubbles.
Multi-frequency acoustic data can be
inverted to determine bubble size
populations.

13. Seafloor chemical detection
Chemical techniques could include
elevated salinity, Mn, ferrous Fe,
acidity, H2S, and lower dissolved
oxygen.
Typical LOD’s for dissolved Fe and
Mn are nM, methane 0.2 nM, salinity
0.00001, temperature 0.005°C, and
for pH is currently 0.005-0.003 pH
unit, but could be improved to 0.0005
pH unit in the near future. Similarly, a
CO2 sensor with a detection limit of ~3
ppm is possible using microfluidic
techniques.

15. Monitoring platforms
Temperature
Salinity
Turbidity
Dissolved oxygen
MBARI

16. Conclusions (1)
• CCS sites with both large spatial seafloor extent and overlying ocean
volumes (with potentially dispersed and localised emission sources)
provide a monitoring challenge;
• Essential rationale for monitoring will be baseline studies, leakage
detection, and flux emission quantification;
• Potential CO2 leakage will have precursor fluid release of reducing
sediment pore fluids ± aquifer brines (each of which has a unique
chemical signature);
• New marine sensor and underwater platform technology is developing to
deploy long-term point observing and remotely surveyed monitoring of
the critical fluid parameters at the necessary sensitivity and spatial
scales for CCS sites (and at relative low cost);

17. Conclusions (2)
• Monitoring can comprise “deep” remote passive / active geophysical
imaging and direct measurement of dissolved & free gas emissions at
seafloor / ocean.
• The former is deployable now but will require inversion of applicable
geophysical parameters to CO2 flux loss;
• The latter is not yet deployable but will directly measure emission fluid
fluxes (including precursory fluids) but requires more baseline
knowledge;
• The regulatory framework emphasises monitoring of the reservoir, but if
leakage occurs, then quantification of CO2 loss is probably easier and
more accurate at the seafloor.