Musings on potential CO2 migration along pre-existing (former) fluid flow pathways in the overburden - presentation by Andy Chadwick and Tom Bradwell of BGS at the UKCCSRC meeting Monitoring of the deep subsurface, 23 October 2014

1. Musings on potential CO2 migration along pre-existing
(former) fluid flow pathways in the overburden
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Andy Chadwick, Tom Bradwell
British Geological Survey
UKCCS RC Storage Meeting: Monitoring of the deep subsurface: Leakage Pathways –
understanding and monitoring the mechanics of CO2 storage

2. North Sea Basin
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3. Coalescing ice-sheets over central North Sea
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Ice-sheet loading and unloading

4. Effects of ice loading / unloading on store geology
• Geological properties of overburden
• Sediment characterisation
• Discontinuities & heterogeneities
• Geomechanical response to glacial loading-unloading cycles
• Stress fields & neotectonics
• Rock / sediment mechanics
• Porosity & permeability changes
• Hydromechanics
• Hydraulic pressurisation and depressurisation
• Natural gas expansion / expulsion
• Induced fluid flow pathways
• Freshwater ingress
• Diagenesis & lithification
• Porosity & permeability changes
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5. Southern North Sea
Northern North Sea
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Overburden pathways
Seismic data courtesy
of Schlumberger
WesternGeco

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Sleipner CO2 storage project
Sleipner ~18 years storage
> 15 million tonnes
3D time-lapse seismic monitoring
1994 (baseline)
1999
2001
2002
2004
2006
2010

7. Fluid escape features
at seabed
seismic 10 m
chimney?
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10 m
gas blanking

8. Pre-existing gas pathways above the CO2 plume
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Plume arrived ~2008
CO2
Plume arrived ~2001
CO2
CH4
CH4

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Time-lapse perspective
CH4
1994 2010
sbm

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So what is a gas-chimney (now)?
Sgas = 0 – 0.2 (= residual saturations)
Are chimneys dormant?
[Petersen et al 2010]
observed velocity model Sgas = 0 - 0.2 synthetic seismic

11. A dormant chimney model for CO2 migration into the
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overburden
High resolution TOUGH2 flow model (1D)

12. Overburden: k = 4 x 10-19 m2, Pcap = 0
Pressure: hydrostatic + buoyancy
Tgrad = 25 C km-1
Overburden: k = 1 x 10-18 m2, Pcap = 0
Pressure: 0.75 lithostatic
Tgrad = 37.5 C km-1
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TOUGH2 results
advection
advection
diffusion
diffusion
Advective transport 2 - 68 mm / century
Diffusive + advective transport 23 - 100 mm / century

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Monitoring constraints - reflectivity
Base of window ~ 55 m above reservoir
Minimum ‘no-change’ is ~ 20 m above reservoir
[closer than this impinges on the side-lobe energy of top CO2 reflection]

14. 2006 2010
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Monitoring constraints – time-shifts
Z = ΔT.(Vsw.Vsco2)
2(Vsw- Vsco2)
Timeshifts <1.0 ms
For SCO2 = 0.3 Z < 18 m
ΔT 2010 - 2006

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Concluding remarks
Following the last ice-sheet removal ‘event’, no tendency for
major fluid ‘dewatering’ at the present time
Assuming a ‘dormant’ regional hydraulic system then fluid
migration along fluid palaeo-pathways is likely to be very slow
and controlled by low intrinsic permeabilities in argillaceous
overburden
No evidence of CO2 out-of-reservoir migration at Sleipner along
presumed earlier fluid flow pathways
Current monitoring not of sufficient duration to ultimately
constrain migration rates to theoretical (very low) values