Convective mixing of CO2 in saline aquifers is an important physical process that can significantly increase the amount of CO2 that is securely trapped in the aquifer. This process happens on a relatively small length scale, and as such, is difficult to accurately model in numerical simulations of geological storage on CO2. In this webinar, Christopher Green and Jonathan Ennis-King, from CSIRO Energy presented an overview of Convective mixing of CO2 in geological storage and discuss the state-of-the art research into this interesting phenomenon.

1. Convective mixing in geological storage of CO2
Webinar – Monday, 27 June 2016

2. Dr Chris Green is a research scientist with CSIRO
Energy. He has a BSc with honours, a BEng with
honours, and a PhD in applied mathematics from the
University of Melbourne (2006). He joined CSIRO as a
postdoctoral fellow working on upscaling methods for
subsurface flow. Since then, his research has focussed
on numerical and theoretical modelling of multiphase
flow in porous media, with particular emphasis on
buoyant migration of carbon dioxide in heterogeneous
saline aquifers. of carbon dioxide. He has been involved
in several field-scale simulations of CO2 storage
projects within Australia, and is currently focussed on
massively-parallel numerical simulations of subsurface
flow.
Research Scientist, CSIRO Energy
Dr Chris Green

3. Dr Jonathan Ennis-King is a senior research scientist with CSIRO
Energy. He received a BSc with first class honours in Mathematics
from the University of Melbourne in 1988, and a PhD in Applied
Mathematics from the Australian National University (1989-1993).
He subsequently held postdoctoral positions at the University of
Melbourne (1993-5), Lund University, Sweden (1996-7) and the
Australian National University (1998-9).
During this period he worked in the field of theoretical physical
chemistry, with research into surface forces, colloids, polymers,
polyelectrolytes and non-equilibrium statistical mechanics.
Senior Research Scientist, CSIRO Energy
Dr Jonathan Ennis-King
He joined CSIRO in 1999 to work on the geological storage of carbon dioxide. His general
research interests are in the modeling and simulation of multiphase flow in porous media,
and the coupling of that flow to thermal, mechanical and chemical processes. He has
specifically applied this to understanding the behaviour of carbon dioxide in the
subsurface, especially over long time-frames where density-driven convection can be
significant. He has had a key role in the reservoir engineering and simulation of the
CO2CRC’s Otway project, the first demonstration of underground storage of carbon
dioxide in Australia.

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5. ANLEC R&D is a not-for-profit agency, funded by the Australian Government Department Industry, Innovation and Science through the
National Low Emissions Coal Initiative, and by the ACA Low Emissions Technologies Ltd (ACALET) through the COAL21 Fund.
Enabling research to reduce greenhouse emissions from coal technologies
Australian National Low Emissions Coal Research & Development
ANLEC R&D is an Australian National Research
Initiative to support Carbon Capture and Storage
(CCS) deployment in Australia.
Founded in 2010, ANLEC R&D has deployed a
research effort of $100M+ in over 25 institutions
nationwide.
Our present focus supports CO2 storage across
3 Australian geological basins:
Surat Basin
Gippsland Basin
Perth Basin
This webinar addresses convective mixing in geological storage of CO2, commissioned by CTSCo
applied to the Surat Basin
For more information please visit www.anlecrd.com.au

6. Convective mixing in geological storage
of CO2
CSIRO ENERGY
Christopher Green & Jonathan Ennis-King
27 June 2016

7. Trapping mechanisms for
geological storage of CO2
Density-driven convective mixing
in porous media
Challenges
Conclusions
7 |

8. Structural trapping
CO2 immobilised by impermeable seal
8 |
Pros:
Can trap significant
amount
Cons:
Relies on suitable seal
Large amount of CO2
still in mobile state

9. Residual trapping
CO2 immobilised in the pore space
9 |
Pros:
Can trap significant
amount
Fast
Cons:
Total storage depends
on reservoir sweep
efficiency

10. Dissolution trapping
CO2 immobilised in solution
10 |
Pros:
Can trap significant
amount
Saturated brine sinks
Cons:
Can be slow acting

11. Mineral trapping
CO2 immobilised in mineral form
11 |
Pros:
Most secure trapping
Cons:
Takes significant time

12. Trapping contributions
12 |
After IPCC Special Report

13. Trapping mechanisms for
geological storage of CO2
Density-driven convective mixing
in porous media
Challenges
Conclusions
13 |

14. 14 |
Buoyancy leads to high CO2 concentration
in the upper reservoir
CO2 dissolves into the brine, slightly
increasing its density (by ~ 1%)
Leads to a gravitational instability – dense
fluid atop a less dense fluid
May lead to density-driven convection
Weir et al, 1996
Lindeberg & Wessel-Berg, 1997

15. Convection in porous media
The ratio of the importance of convection to diffusion is the Rayleigh number:
Density-driven convection can only occur for a critical value of
Ra > 32.5
Typically, Ra > 1,000
15 |
Slim & Ramakrishnan, 2010

16. 16 |

17. 17 |
Diffusive regime
• CO2 dissolution rate

18. 18 |
Onset of convection regime
• Downwelling fingers become
visible
• Rapid increase in dissolution rate

19. 19 |
Merging regime
• Downwelling fingers begin to
merge
• Dissolution rate decreases again

20. 20 |
Constant flux regime
• Large fingers form
• Dissolution rate oscillates about a
constant value

21. 21 |
Constant flux regime
• Large fingers dominate mixing
• Dissolution rate oscillates about a
constant value

22. Total dissolved CO2
22 |
Convective mixing can significantly increase
the amount of dissolved CO2

23. Critical time
Convective mixing begins in a homogeneous system after a critical onset time
Initially, the wavelength of the instability is given by
23 |
Ennis-King & Paterson, 2003

24. Long-term flux
The average long-term flux in a homogeneous system is
This can be characterised by the Sherwood number
Expect n = 1 for large H, but various values have been reported
24 |
Hesse et al, 2006; Pau et al, 2010

25. Typical fluid properties
Unsaturated brine density 1,000 kg m-3
Saturate brine density 1,010 kg m-3
Viscosity 0.6 x 10-3 kg m-1 s-1
CO2 solubility 0.049 kg kg-1
Diffusion coefficient 2 x 10-9 m2 s-1
Typical reservoir properties
Porosity 0.2
Permeability 10-12 m2 (1 d)
Convective mixing parameters
Rayleigh number 2,000
Critical onset time 2 x 105 s ( < 3 days)
Critical wavelength 0.2 m
Constant long-term flux 1.4 x 10-7 kg m-2 s-1
25 |

26. Trapping mechanisms for
geological storage of CO2
Density-driven convective mixing
in porous media
Challenges
Conclusions
26 |

27. Modelling
Analytical solutions are limited to simple
cases and estimates
Numerical models require significant
computational resources
(particularly in 3D)
Experiments can only be performed on
simplistic representations (often using bead
packs and analogue fluids)
27 |

28. Capillary transition zone
28 |
Most studies use a single-
phase approximation
In reality, the flow is two-
phase and a transition zone is
present
This can reduce the critical
onset time and increase the
long-term flux
Elenius et al, 2014
Emami-Meybodi & Hassanzadeh, 2012

29. Heterogeneity
29 |
Real reservoirs are highly
heterogeneous at multiple
length scales
Heterogeneity can strongly
influence dissolution by
convection
Role of heterogeneity on
theoretical estimates is still
not well understood

30. Heterogeneity
Broadly, heterogeneity can be categorised as
1. Weak – little influence on convection
2. Moderate – convection via preferential flow paths
3. Strong – dispersive flux only
30 |
Farazadeh et al, 2011
Ranganathan et al, 2012
Chen et al, 2013

31. Heterogeneity
31 |
Green & Ennis-King, 2014
CO2 flux in heterogeneous
model can be approximated
by anisotropic model with
effective permeability

32. Geochemistry
32 |
Geochemical reactions can
act to enhance or inhibit
convection
Reactions can increase
porosity and permeability
(dissolution), or decrease
porosity and permeability
(precipitation)
Geochemistry is site-specific,
so is hard to generalise

33. Geochemistry
33 |
The importance of geochemical reactions can be expressed by the Damköhler
number
If Da << 1 – convection dominates and the effect of geochemistry is only late
mineralisation
If Da >> 1 – reactions are rapid, and significant coupling can occur
A critical value exists, below which convection is
suppressed by geochemical reactions (assuming indefinite reactants)
Ennis-King & Paterson, 2007
Andres & Cardoso, 2011

34. Field-scale modelling
34 |
Field-scale models feature grids with a metre (or greater) length scale
Convective mixing may occur on a mm or cm length scale (far below the
field-scale resolution)
Makes it difficult to accurately include enhanced dissolution due to
convective mixing

35. Field-scale modelling
35 |
Some attempts at upscaling convective mixing have been made
These assume that convection occurs instantly, and that the flux is
constant over all time
Macminn et al, 2011
Gasda et al, 2011, 2012

36. Trapping mechanisms for
geological storage of CO2
Density-driven convective mixing
in porous media
Challenges
Conclusions
36 |

37. Conclusions
Dissolution of CO2 into brine in a saline aquifer slightly increases the density of
the brine, leading to a gravitational instability
The subsequent density-driven convective mixing can significantly increase the
amount of CO2 that is dissolved
Dissolved CO2 sinks to the base of the aquifer, reducing the risk of leakage
We have a good understanding of an idealised model (isotropic, homogeneous,
single-phase, 2D)
Including the effects of reservoir heterogeneity and geochemistry, and
incorporating these results in field-scale simulations is still challenging
37 |

38. Acknowledgements
Green & Ennis-King, 2013. Convective mixing in geological storage of CO2. EP13096
Green & Ennis-King, 2014. Steady dissolution rate due to convective mixing in anisotropic porous
media. Adv. Water Res. 73, 65–73
Ennis-King & Paterson, 2005. Role of convective mixing in the long-term storage of carbon dioxide in
deep saline formations. Soc. Pet. Eng. J. 10 (3), 349–356
Ennis-King & Paterson, 2007. Coupling of geochemical reactions and convective mixing in the long-
term geological storage of carbon dioxide. Int. J. Greenh. Gas Control 1 (1), 86–93
Ennis-King, Preston & Paterson, 2005. Onset of convection in anisotropic porous media subject to a
rapid change in boundary conditions. Phys. Fluids 17 (8), 84107
38 |

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