Storage
40. This section focuses on geological storage of
CO2 and, more specifically, on the prospects for storage
of CO2 in depleted oil and gas fields and deep saline
aquifers. Saline aquifers are porous rock formations found deep
underground which contain salty water unsuitable for use as potable
water. Other approaches to storage, including storage in unmineable
coal deposits, are considered in paragraphs 48 to 50. Geological
storage of CO2 has been occurring naturally for millions
of years and accumulation of CO2 in underground reservoirs
for very long timescales is a common geological phenomenon.[55]
It is proposed that, by analogy, the CO2 captured from
energy production and industrial processes could be injected into
suitable geological formations in the earth's crust for extremely
long term (tens of thousands of years) storage.
41. The basic principle entails the trapping of CO2
in the poresspaces between the sand grainsin sedimentary
rocks.[56] At the pressures
encountered at depths below 800-1000 m, CO2 enters
a so-called 'supercritical' state in which it has a density similar
to that of a liquid;[57]
this enables efficient use of the storage space. There are four
mechanisms of CO2 trapping and all may contribute at
different times at any one site. Firstly, although the injected
CO2 is buoyant in the water in the rock formation,
the presence of an impermeable layer of rock above the storage
site, known as caprock, provides a seal to prevent the CO2
migrating upwards. Secondly, the CO2 can dissolve in
the water which normally fills the pore spaces in the sedimentary
rock, helping to trap it in the site. Thirdly, CO2
may move slowly through the rock pores, driven by buoyancy; small
bubbles of CO2 are left stranded during this movement
and become trapped in disconnected pores. Finally, the CO2
may become trapped through chemical reactions with minerals in
the surrounding rock and so, over the long term, may become immobilised
in the form of carbonate minerals, similar to those which form
natural limestone.
42. There are sufficient candidate geological storage
sites for CCS to make a significant contribution to reducing UK
CO2 emissions over many tens, or even hundreds, of
years. The BGS estimates that the theoretical storage potential
of the UK's offshore oil and gas fields is equivalent to at least
4.7 Gigatonnes (Gt) of CO2. This represents approximately
20 years' worth of all present day power generation emissions,
although not all of this may be suitable for commercial exploitation.[58]
According to the BGS, there is no significant potential for onshore
CO2 storage in oil or gas fields in the UK (with the
possible exception of the Wytch farm oil field in Dorset). The
UK's onshore aquifer storage potential has not yet been fully
investigated.
43. There is extensive potential for CO2
to be stored in deep saline aquifers in the North Sea. The BGS
memorandum stated:
"An estimate of 250Gt was calculated in
the BGS led Joule 2 (1995) Project for all the aquifers (open
and closed) in the UK N. Sea. With UK emissions at over 0.6Gt/year,
a third of which comes from power generation it is clear that
even if only 10% of this capacity was realized it would serve
the UK's needs to beyond the period of fossil fuel dependency".[59]
This would be equivalent to more than 1000 years
of storage, based on current UK power generation emissions. The
IPCC report estimates global geological storage capacity to be
sufficient for CCS to contribute up to 55% of the cumulative mitigation
effort needed worldwide until 2100.[60]
It should be noted that estimates of the storage potential offered
by saline aquifers can vary wildly, reflecting the lack of data
in this area.
44. The UK is especially fortunate in that many sources
of CO2 generation from onshore power plant and other
industries lie within tens or hundreds of kilometres of potential
storage sites in rocks beneath the offshore northern and southern
North Sea and Irish Sea. This is within easy reach of pipeline
transport, as proven by onshore CO2 developments in
the US and by offshore oil and gas transport around the UK. The
geographic co-location of storage sites and CO2 sources
may be less favourable in some other countries, such as Japan,
and has yet to be thoroughly investigated in many others. Professor
Haszeldine commented on the UK's fortuitous situation in written
evidence:
"From hydrocarbon exploration, we have unrivalled
knowledge of our offshore geology. These are some of the world's
best-known and most accessible sediment basins, and contain both
depleted oil and gas fields and deep aquifers of saline water".[61]
The UK is fortunate in being very well endowed
with potential CO2 storage sites, many of which
have been thoroughly characterised. This provides the UK with
a competitive advantage in terms of access to potential CO2
storage sites, both for its own use and to demonstrate UK geological
skills to the rest of the world.
45. In general, more is known about oil and gas fields
than saline aquifers leading some people to suggest that the former
are better candidates for immediate development as storage sites.
The oil and gas fields in the North Sea have been particularly
well characterised and both Friends of the Earth and Green Alliance
were far more enthusiastic about CCS projects involving storage
in oil and gas fields than those involving aquifers. Germana Canzi,
Senior Climate Change Adviser for Friends of the Earth, told us:
"We feel that the oil industry has a lot of experience with
the geology of oil and gas fields and, therefore, we feel more
confident that the science about the permanence of the CO2
underground is more certain".[62]
The evaluation of offshore saline aquifers for storage purposes
is readily achievable, by application of the same techniques as
are normally used in offshore oil and gas exploration and production.
Oil and gas fields have, in general, been better characterised
than saline aquifers so may be more suitable for immediate development.
Nevertheless, the best way of furthering understanding of storage
of CO2 in aquifers, which provide very substantial
storage potential in the longer term, is through large scale demonstration
projects.
46. BP commented in its memorandum that the oil and
gas industry had "over one hundred years of experience identifying
and managing fluids in the deep sub-surface" and noted that
the "geological storage of CO2 is very similar
to the management of other liquids and gases routinely handled
by the industry throughout the world".[63]
In addition, BP asserted that "the industry has considerable
experience of oil and gas abandonment, and much of this will be
used to form the basis for secure [field] abandonment to ensure
long term safe, secure storage of CO2 in the rock".[64]
Furthermore, the British Geological Survey, which has developed
an enviable reputation over its 170 years of operation, has innovated
and maintained particular expertise in CCS since the early 1990s.
The UK's geological expertise through the hydrocarbon industry
and British Geological Survey is recognised to be amongst the
best in the world. This expertise should be leveraged to facilitate
and promote UK demonstrations of CCS and, ultimately, uptake of
CCS internationally.
47. Globally, most experience to date of relevance
to CO2 injection has been obtained as a result of enhanced
oil recovery (EOR) projects. There are more than 70 EOR projects
around the world. Extensive experience in CO2 injection
into oil fields, for the purposes of enhanced hydrocarbon production,
has been gained over a 30 year period, particularly in North America.
The BGS commented on the fact that although there have been many
commercial projects involving the injection of CO2
for EOR, "few have been accessible to researchers".[65]
At Weyburn and Rangely, there has been large scale CO2
injection for EOR purposes. Apart from EOR projects, there are
only two large-scale industrial projects worldwide, at In Salah
in Algeria and Sleipner in Norway, where CO2 is being
injected underground at a rate of around 1Mt/year. Several more
industrial size projects are planned to commence soon; Box 3 contains
a summary of current and forthcoming projects involving CO2
storage.
Box 3: CO2 storage: summary
of experience

OTHER FORMS OF STORAGE
48. Deep ocean storage has been proposed as an alternative
to geological storage. In this case, captured CO2 would
be directly injected into the deep ocean (below 3000 m) where,
being denser than water, it would be expected to form a 'lake',
delaying its dissolution into the surrounding water. This storage
mechanism is still at the research stage and there is unease about
its potential environmental implications. It is not considered
to be viable for the North Sea in any case due to the lack of
storage sites of appropriate depth. It would also be illegal under
current international law.
49. Unmineable coal seams offer another potential
CO2 storage opportunity. Enhanced Coal Bed Methane
Recovery (ECBM) involves injecting CO2 into coal seams,
where it displaces methane and thereby enhances recovery of methane
from the coal bed. This can give an economic return to help offset
the costs of storage. Professor Peter Hall from the University
of Strathclyde and Stephen Jewell from Composite Energy Ltd asserted
that "the bituminous coals that constitute most of the resource
of the UK have the ability to permanently store CO2".[66]
However, the majority of evidence published to date seems to suggest
that UK coal is too impermeable to make this a practical option.[67]
On the basis of current information, coal seems unlikely to
be a major storage option for the UK, at best being of small scale
and local significance. We do not, of course, exclude the
possibility that future research or developments in technology
will alter this situation.
50. Mineral carbonation has also been proposed as
an alternative means of storing CO2. Dr Sam Holloway,
Senior Geologist at the British Geological Survey, told us: "It
is a process that will work but the costs are likely to be extremely
high" and asserted that it "would require very considerable
advances in order for it to become anywhere near competitive with
other methods of sequestering carbon".[68]
Dr Riley, also from the BGS, put it in even stronger terms:
"I think mineral storage is a distraction.
The bulk of CO2 storage is going to have to be done
by injecting the carbon dioxide underground. The issue of can
we create dolomite from seawater will make the problem worse because
if you make carbonate from seawater you evolve carbon dioxide
back to the atmosphere. It sounds counterintuitive [but] it actually
makes the situation worse in the short term".[69]
It has also been argued that it may be possible in
future to use 'waste' CO2 as feedstock for other processes.
The Royal Society of Chemistry told us that "from a chemical
science perspective CO2 can also be seen as potential
feedstock for the manufacture of useful chemicals and as such
chemical conversion of significantly large amounts of carbon dioxide
to inert or commercially useful material is an option that cannot
be ignored".[70]
The IPCC considered this possibility in its Special Report and
concluded that "The potential for industrial uses of CO2
is small" and that "the CO2 is generally
retained over short periods".[71]
Furthermore, the IPCC cautioned that "Processes using captured
CO2 as feedstock instead of fossil hydrocarbons do
not always achieve net life-cycle emissions reductions".[72]
It is clear that storage in geological formations, providing
that it can be done safely and securely, is the most desirable
and competitive way of storing CO2 of the currently
available options.
Transport
51. Captured CO2 obviously needs to be
transported to the storage site and there is agreement that this
will generally be best done via pipeline. There are already more
than 2500 km of pipelines, mainly in the Western United States,
transporting 50 Mt CO2 per year from natural sources
to EOR sites.[73] The
recent IPCC report concluded that there was "no indication
that the problems for carbon dioxide pipelines are any more challenging
than those set by hydrocarbon pipelines in similar areas, or that
they cannot be resolved".[74]
The risks associated with transportation by pipeline are discussed
in paragraph 90.
52. Captured CO2 is usually compressed
prior to transportation in order to decrease the volume occupied
by the gas. Providing that the CO2 is dry and free
from hydrogen sulphide, the risk of pipeline corrosion is minimal.
The IPCC report suggests that it would be "desirable to establish
a minimum specification for 'pipeline quality' CO2".[75]
53. In the case of liquefied petroleum gas, liquefaction
is used to decrease the volume for transportation by ship. Although
similar technology could be applied to liquefaction of CO2,
Rodney Allam from Air Products pointed out that transportation
of CO2 in pressurised tankers would be expensive and
there was a consensus amongst witnesses that development of a
global market involving trading of emissions certificates between
countries was far more likely, and preferable to, one where countries
exported their CO2 for storage in sites in other countries.[76]
34 Q 70 Back
35
Q 124 Back
36
Q 125 Back
37
Ev 25 Back
38
Q 14 Back
39
Ev 85 Back
40
Ev 73 Back
41
Q 168 Back
42
Ev 23 Back
43
POSTnote 253 Back
44
Ev 21 Back
45
Q 122 Back
46
Personal communication. Back
47
Ev 74 Back
48
Ev 183 Back
49
Ev 183 Back
50
Ev 78 Back
51
Ev 182 Back
52
Ev 88 Back
53
As above. Back
54
Q 61 Back
55
IPCC, Special Report on Carbon Dioxide Capture and Storage,
Autumn 2005. Back
56
Sedimentary rocks are made up of sediments, laid down millions
of years ago, which may be second-hand fragments of other rocks,
or remnants of living organisms. Back
57
Supercritical CO2 occupies less than 1% of the space
that CO2 gas would fill at normal temperature and pressure. Back
58
Ev 72 Back
59
Ev 72 Back
60
IPCC, Special Report on Carbon Dioxide Capture and Storage,
Autumn 2005. Back
61
Ev 133 Back
62
Q 213 Back
63
Ev 138 Back
64
Ev 138 Back
65
Ev 71 Back
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Ev 85 Back
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Ev 60, 118, 142 Back
68
Q 79 Back
69
Qq 80-81 Back
70
Ev 145 Back
71
IPCC, Special Report on Carbon Dioxide Capture and Storage,
Autumn 2005. Back
72
As above. Back
73
As above. Back
74
As above. Back
75
As above. Back
76
Q 153 Back