Summary
Storage of carbon dioxide in deep formations is being actively considered
for the reduction of greenhouse gas emissions. Relevant experience in the
petroleum industry comes from natural gas storage and enhanced recovery using
carbon dioxide, but this experience is over a time scale less than the hundreds
or thousands of years required for carbon dioxide storage. On these long time
scales, different mechanisms need to be considered.
In the long term, the dominant mechanism for dissolution of carbon dioxide
in formation water is convective mixing rather than pure diffusion. This arises
because the density of formation water increases upon dissolution of carbon
dioxide, creating a density instability. Linear stability analysis has been
used to estimate the time required for this instability to occur in anisotropic
systems. For sufficiently thick formations with moderate vertical permeability,
this time ranges from less than a year up to a few hundred years. Further
approximate analysis shows that the time needed for the injected gas to
dissolve completely is typically much longer, on the order of hundreds of years
to tens of thousands of years, depending on the vertical permeability. This
theoretical analysis is compared with the results of numerical simulations.
Introduction
If the emissions of carbon dioxide from the use of fossil fuels continue on
the current scale, then it has been predicted that significant changes in the
global climate will occur in the next 100 years.1 Deep cuts in emissions will
be needed in the next few decades in order to stabilize atmospheric carbon
dioxide at reasonable levels so that the extent of the climatic changes can be
limited.2 Such cuts can ultimately only be achieved by a broad strategy that
encompasses both alternative energy sources and the “cleaner” use of existing
reserves of fossil fuels. In the latter category, one possible technical
solution is to store carbon dioxide emissions in a form where they will not
reach the atmosphere for decades to centuries.
Because current emissions from fossil-fuel usage are around 6 to 7 Gt of
carbon per year (equivalent to 22 to 5 Gt of carbon dioxide), any form of
storage must have considerable capacity if it is to make a significant
contribution to reducing emissions. Two leading options are storage deep in the
ocean or underground.3 In the absence of carbon credits or taxes, the most
economically attractive forms of underground storage in the short term are
enhanced recovery schemes, whereby carbon dioxide is injected both to increase
production and to provide storage. However, the potential storage capacity for
CO2 in enhanced recovery operations is not large compared to the scale of
emissions, nor are such opportunities always present close to sources of CO2.
An alternative form of underground storage is injection in deep saline
formations, because these are widely available and have a large total storage
capacity.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
1 March 2004
- Revised manuscript received:
14 October 2004
- Manuscript approved:
5 May 2005
- Version of record:
15 September 2005
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