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CO2 Geological Storage –Migration Processes and Pathways
Karsten Michael
CSIRO ENERGY
IEAGHG Summer School, Perth, Australia6-11 December 2015
Potential impacts of CO2 injection
The subsurface area, as projected to surface, beyond the physical presence of CO2, but in which reservoir pressures are above ambient conditions. Pressures decrease rapidly outward along with the potential to drive unwanted migration or impact other resources. This area would require targeted characterisation and monitoring of identified potential leakage conduits (i.e. faults, old wells).
SURFACE PROJECTION OF CO2 PLUMEThe subsurface area, as projected to the surface, in which CO2 is present as a physically distinct phase. Within this footprint, reservoir pressures are highest and may be sufficient to drive lateral or vertical migration of CO2 and brine. This area requires the highest standard regarding site characterisation, monitoring and consideration of remediation options.
SURFACE PROJECTION OF INCREASED PRESSURE
Potential migration pathways
Two-phase flow
• Interplay between 3 forces:• Viscous force (injection pressure, formation water flow: Darcy’s Law)
• Buoyancy (density difference between 2 fluids, vertical)
• Capillarity (pore throat radius, interfacial tension, wettability)
• Capillary number represents the relative effect of viscous forces versus surface tension:• Ca = dynamic viscosity x flow velocity / interfacial tension
• Flow is dominated by capillarity if Ca < 10-5
• Drainage & imbibition• Capillary entry pressure
• Relative permeability, residual saturation
• Hysteresis
CO2 injection
Bk
qtktr
rwc
wrc
CO
)(
2
k = permeability = viscosityq = injection volume = porosityB = thickness
Nordbotten et al., 2005
Post-injection
Szulczeski et al., 2012
buoyancyr
rrCO2
r
rFFF +
-+-
0
w
w0
w zHg
CO2
*
Pressure-related force component
buoyancyr
rrCO2
r F-
- Egw
w
w
00CO2 F- Hgr
Hydrodynamic impact on CO2 migration
q = F* k
r
r rCO2-
w
w
Tilt factor:
CO2-water tilt: 0H
r
r rCO2-
w
w
Effect of flow on CO2 trapping
Single fluid
Two fluids (say oil and water)
Effective permeabilities ko kw
Relative permeabilities kro krw
dx
dpkAq
w
w
-
.
,
dx
dpAkq
dx
dpAkq
w
w
w
o
o
o
-
-
,
,
kkk
kkk
rww
roo
Darcy’s Law
Residual trapping
Capillary pressure & relative permeability
Relative permeability depends on capillary pressure so hysteresis in Pc
results in different kr curves for imbibition and drainage
Sw
kr
Drainage
Imbibition
Irreducible water
saturationResidual CO2
saturation
Aquifer geometry
Aquifer fabric
Boundary conditions
Injectivity – Number of injectors
1.1 Mt/year
Total: 22 Mt
0.37 Mt/year per well
1.85 Mt/year
Total: 37 Mt
0.25 Mt/year per well
2.0 Mt/year
Total: 40 Mt
V = 72 km3
Porosity = 20%CO2 density = 570 kg/m3
E = 2 %Capacity = 160 MtCO2
Fracture pressure = 150 bar
0.8 Mt/year per well
6.4 Mt/year
Total: 128 Mt (open boundaries)
Regional context
Long-range migration – basin-scale simulations
Meckel et al., 2015
Inversion percolation
Long-range migration – basin-scale simulations
Gibson-Poole et al., 2008
Geoscience Victoria
Long-range migration – basin-scale simulations
Assessment scale
Pore-scale
Core-scale
Well-scale
Reservoir-scale
Blunt et al., 2013
2 mm
Pore-scale imaging & modelling
20 cmCore-flooding
Well tests & logging
Production tests & model calibration
~100 m
Drill stem testing
20 km
Top seal leakage
Goldie-Divko et al., 2010
Well leakage
Gasda et al., 2004
Fault leakage
Bouvier et al., 1989; Lindsay et al., 1993
A) Across-fault
B) Up-fault
Ciftci et al., 2012
Potential impacts
Summary
• CO2 storage system has many similarities to the petroleum system Reservoir-seal terminology; migration pathways
Main differences: CO2 is highly soluble in formation water; assessment of potential migration out of the reservoir
• CO2 migration occurs due to the interplay of viscous forces, buoyancy & capillarity Viscous forces dominate near the well and during injection; buoyancy and
capillarity take over after injection has ceased and in the far-field
Choice of modelling method: analytical, numerical (Darcy, inver. percol.)
• Need to consider lateral migration within injection interval versus vertical migration (leakage) permeable faults, poorly completed wells, fracture systems in seal
displacement of formation water
• (Relative) permeability is a key parameter governing migration How is permeability measured? What scale?
What is the assessment (model) scale?
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