## ARTICLE IN PRESS

Energy 32 (2007) 1194–1201



# Natural emissions of CO 2 

# from the geosphere and their bearing on the

geological storage of carbon dioxide

S. Holloway a, 

### *



## , J.M. Pearce a



## , V.L. Hards a



## , T. Ohsumi b



## , J. Gale c

a British Geological Survey, Keyworth, Nottingham NG12 5GG, UK

b Civil Engineering Research Laboratory, Central Research Institute for Electric Power, 1646 Abiko, Abiko-shi, Chiba-ken 270-1194, Tokyo, Japan

c IEA Greenhouse Gas R&D Programme, Stoke Orchard, Cheltenham GL52 7RZ, UK

Received 18 January 2006 www.elsevier.com/locate/energy

Abstract

Carbon dioxide (CO 2 ) capture and storage has the potential to reduce CO 2 emissions from fossil fuel combustion. Although leakage

from monitored CO 2 injection sites has been minimal to non-existent, experience from the natural gas storage industry suggests that, if it

becomes a widely deployed technology, leaks may be expected from some storage sites. Natural occurrences of CO 2 in the geosphere,

some of which have been exploited, provide insights into the types of emissions that might be expected from anthropogenic CO 2 storage

sites. CO 2 emission sites are commonly found in clusters in CO 2 -prone geological provinces: the most common natural emissions sites in

sedimentary basins consist of carbonated springs and mofettes. These represent at worst only a local hazard. In volcanic and

hydrothermal provinces, more energetic emissions may occur due to active supply from degassing magma. These include rare, sudden

emissions from fissures and craters that have caused fatalities. It is unlikely that such provinces would be considered for CO 2 storage

Major lake overturn events such as occurred at Lake Nyos in 1986 are considered highly unlikely to occur as a result of CO 2 storage, not

least because CO 2 levels in lake waters can be monitored and remediated. Natural CO 2 fields indicate that under favourable conditions

CO 2 can be retained in the subsurface for millions of years. The main risk from man-made CO 2 storage sites that does not have any close

analogy in nature is considered to be a well blowout. A blowout that took place at a natural CO 2 field provides some indication of the

likely hazard.

r 2006 NERC. Published by Elsevier Ltd. All rights reserved.

Keywords: Carbon dioxide; Geological storage; Natural CO 2 emissions

1. Introduction

The single most significant contributor to carbon dioxide

(CO 2 ) emissions to the Earth’s atmosphere is the burning

of fossil fuels, which results in the emission of about

23 x 10 9 tonnes CO 2 /year [1]

. One way to reduce these CO 2

emissions to the atmosphere is to capture and store a

proportion of them in the geosphere, via a process known

as CO 2 capture and storage (CCS). This involves capturing

CO 2 from large industrial point sources such as power

plants, compressing it for transport by pipeline, or shipping

* Corresponding author. Tel: +44 115 936 3190: fax: +44 115 836 3437.

E-mail address: shol@bgs.ac.uk (S. Holloway). to a site where it can be injected into an underground

geological formation [2]

.

If CCS is to make a contribution to reducing future

atmospheric CO 2 levels, any CO 2 stored underground

would have to be retained until well past the end of the

fossil fuel era, which may last several hundred years [3]

. After the end of the fossil fuel era, atmospheric CO 2 levels

might begin a slow decline as land/ocean/atmosphere CO 2

levels re-equilibrate [4,5]

. Clearly it would not be desirable

for significant amounts of stored CO 2 to be released until

there had been a significant decline in atmospheric CO 2

levels. Thus, the most desirable time frame for storage

might be at least thousands of years. Nevertheless, shortterm storage of a few hundred years could be very valuable

in shaving the expected peak levels of CO 2 in the

0360-5442/$ - see front matter r 2006 NERC. Published by Elsevier Ltd. All rights reserved.

doi: 10.1016/j.energy.2006.09.001

## ARTICLE IN PRESS

1195 S. Holloway et al. / Energy 32 (2007) 1194–1201

atmosphere that might occur near the end of the fossil

fuel era.

CCS is beginning to be deployed as a greenhouse gas

mitigation technology. At the Sleipner West gas field in the

Norwegian sector of the North Sea, about 1 million tonnes

CO 2 per year are being stored underground [6,7] ; some 7.5

million tonnes has been stored to date. CO 2 is also being

injected underground in enhanced oil recovery (EOR)

operations worldwide. The greatest concentration of such

projects is in the Permian Basin of west TX, USA [8]

. EOR

projects at Weyburn, in Saskatchewan, Canada [9] and

Rangely, CO, USA [10–12] are being monitored for

leakage; the former as part of a major research project

studying the geological storage of CO 2

. More recently,

commercial-scale CO 2 storage projects have been started at

In Salah, Algeria [13,14] and reached the construction

phase at the Snohvit field in the Barents Sea, offshore

Norway [15]

. Smaller demonstration projects have been

undertaken at Nagaoka, Japan [16]

, Frio, Texas [17] and

the K12-B gas field offshore Netherlands [18] . As a

technology, CCS has the potential to create value in

depleted petroleum fields, especially if it is accepted as a

valid means of CO 2 emissions reduction in schemes such as

the EU Emissions Trading Scheme.

Long-term underground CO 2 storage by man, i.e. for

centuries or millennia, cannot be directly demonstrated but

there are analogies in nature, such as the many natural

underground CO 2 fields around the world [19]

, which are

identical to natural gas fields in every respect apart from

their gas composition. Furthermore, many natural gas

fields contain varying quantities of CO 2 mixed in with the

hydrocarbon gases [20]

. Some of these fields of both pure

CO 2 and CO 2 /hydrocarbon mixtures are thought to have

existed for millions of years, proving that under favourable

circumstances CO 2 can be retained underground for

geological timescales.

Nonetheless, if CCS were to become a large-scale, widely

deployed technology, leakage of CO 2 from man-made

underground storage sites could occur, and could have a

deleterious effect on the atmosphere, or local marine and

terrestrial ecology and/or be potentially directly hazardous

to man. In the last thirty years, three well-known natural

events involving the rapid emission of large masses of CO 2

from volcanoes have resulted in many deaths from

asphyxiation. These occurred as the result of a minor

volcanic eruption at the Dieng volcanic complex, Java,

Indonesia (night of February 20th 1979: 142 deaths), the

sudden degassing of the CO 2 -saturated waters of Lake

Nyos (August 21st 1986: 1746 deaths) and Lake Monoun

(August 15th 1984: 37 deaths), volcanic crater lakes in

Cameroon, West Africa. Also, stark evidence of tree kill at

natural CO 2 emission sites at Mammoth Mountain,

California [21–23]

, has led to concern that there may be

significant local damage to the natural environment and

hazards to human health if a CO 2 storage site were to leak.

More recently, the potential for local or regional damage to

marine ecosystems from increased levels of CO 2 in the atmosphere has been highlighted [24] and there is concern

that a persistent leak of CO 2 from an underground CO 2

storage site beneath the continental shelf could result in

local, more acute damage to marine ecosystems [25] . As a

result of these concerns, a review of some natural CO 2

emissions was undertaken to assess what might be learned

from them regarding emissions from CO 2 storage sites [26]

.

It is recognised that methane-dominated systems into

which CO 2 might be injected, such as oil, gas and coalbed

methane fields, may have the potential to leak methane as

well as CO 2

. However a discussion of natural analogues for

such leakage is beyond the scope of the current study.

The main risk drivers for CO 2 leakage are CO 2 flux and

concentration [27]

. The level of immediate risk to human

and animal life depends more on how effectively the

emitted CO 2 is dispersed than on the quantity released [28]

. Emissions that allow significant concentrations of CO 2 in

air to build up a few metres above or below ground are the

most dangerous in this respect [27]

. Such emissions may

involve only small masses of CO 2 if they are confined in a

small space, such as a building.

2. Origin of CO 2 in the geosphere and its migration through

sedimentary rocks

Much of the naturally occurring CO 2 emitted from the

geosphere ultimately originates from the degassing of

magma. When magma rises towards the Earth’s surface,

its pressure drops and dissolved CO 2 and other gases come

out of solution. Most of the CO 2 originating from magma

degassing migrates to the Earth’s surface and is emitted

through volcanic and associated hydrothermal areas.

However, a proportion may travel up deep-seated faults

[20] to be released away from volcanic areas, or degas from

high-level intrusions [19] and may make its way into

sedimentary basins. CO 2 can also be produced in the

geosphere by the metamorphism of carbonate rocks (most

commonly by contact with magmatic intrusions or in

subduction zones), the thermal alteration of coal and other

organic material, the biodegradation of oil and gas and the

dissolution of carbonate rocks [29,30]

. Migrating CO 2

mixes readily with petroleum and is a common component

of natural gas and associated gas dissolved in oil, e.g. [20]

.

Carbon dioxide, in either the gaseous or supercritical

phase, is buoyant compared both to rock itself and water

(of varying salinity) which is the most common native fluid

found in the pore spaces of sedimentary rocks. Consequently, naturally occurring CO 2 originating at depth in

the geosphere tends to migrate upwards towards the

Earth’s surface. It can migrate through natural fracture

networks in the rock strata, and/or through the matrix of

porous and permeable sedimentary rocks. Consequently,

areas where CO 2 is being, or has been, generated in the

geosphere are commonly associated with distinct clusters of

natural CO 2 emission sites at the ground surface or sea bed,

and/or natural underground CO 2 fields. These result from

the migration and dispersion of the CO 2

. Well-known

## ARTICLE IN PRESS

1196 S. Holloway et al. / Energy 32 (2007) 1194–1201

examples are the French carbo-gaseous province [31]

, the

Paradox Basin [19,32] and the Yellowstone hydrothermal

area [33]

.

Most consolidated rocks at shallow depths of a few tens

of metres or less contain fractures such as joints and faults

that are open and highly permeable. Consequently, at most

natural CO 2 emission sites, CO 2 tends to emerge along

fractures [22,32,34–37] because these short-circuit the route

to the ground surface or sea bed at the outcrop of porous

and permeable reservoir rocks. However, these fractures

may not reach all the way to the ground surface or sea bed

and the emissions from them may be dispersed in overlying

soil or subsoil onshore and in unconsolidated or poorly

consolidated shallow sediments offshore.

Before reaching the ground surface the migrating CO 2

passes through two hydrogeological zones of the Earth’s

crust; the saturated zone and the unsaturated zone. The

saturated zone comprises the porous and permeable strata

below the water table, where any pore spaces and fractures

are filled with water or other pore fluids. The unsaturated

zone lies above the water table and its pore spaces and

fractures are commonly filled with a mixture of water and

soil gas (air modified by soil processes). Once through the

phreatic zone, natural CO 2 emissions rising up towards the

ground surface may be dispersed or naturally attenuated in

the vadose (unsaturated) zone [27,33,38]

. However, this is

not always the case; particularly where there is little soil or

subsoil above the bedrock, some will remain as concentrated release sites [39]

. Additionally, migrating CO 2 commonly dissolves in groundwater, which emerges at

the ground surface as springs. Thus the emission sites

within carbo-gaseous provinces commonly consist of

carbonated springs, mofettes (dry CO 2 gas emissions sites)

and, rarely, geysers.

When gas emerges through unconsolidated fine-grained

seabed sediments, it commonly produces pockmarks—

circular to elliptical depressions on the sea floor, metres to

hundreds of metres in diameter [40–43]

. Where gases

emerge along with mud or muddy water, a mud volcano

may develop. In areas where the sea bed sediments are

sandy, for example above the Tommeliten field, North Sea,

gas emerges as bubbles through small circular vents c.

10 mm in diameter in the sandy sea floor [40]

. The vents

commonly form a cone-shaped depression about 20 cm in

diameter. Gas bubbles at Tommeliten (and elsewhere) have

been observed using a remotely operated vehicle and have

been imaged on echo-sounder and shallow 3.5 kHz seismic

records. The gas may emerge through the sea bed

intermittently, in response to changes in hydrostatic

pressure or to self-sealing mechanisms [44,45]

. Carbon

dioxide bubbles are much more soluble than the commonly

detected natural gas bubbles (predominantly methane) and

may dissolve in the sea water column [46]

. Migrating gas

dispersed in shallow sediments can result in commonly

observed shallow submarine seismic responses such as

acoustic blanking, acoustic turbidity and reflector enhancement (bright spots) [45]

. 3. Natural emissions of CO 2 from the geosphere

Recorded natural emissions of CO 2 in sedimentary

basins generally have low fluxes ( Table 1 ). Many carbonated springs [31,47–50] and dry emission sites known as

mofettes [31,32,51–53] have been described. Additionally,

CO 2 geysers have been described at abandoned well sites

[32]

. The emerging CO 2 is usually dispersed by mixing with

the surrounding air, commonly at very short distances

from the emission site. However, at some sites there is an

area immediately surrounding the source where the

emerging CO 2 is less easily dispersed by the wind. This

commonly takes the form of a depression in the ground

around the emission point, on the scale of metres to tens of

metres diameter [39,48,54]

. Some such sites are fenced off

for safety reasons, for example at Ciampino, a satellite

community southwest of Rome, a natural seepage has been

fenced off to prevent people and livestock entering; a

similar approach could be taken if a CO 2 storage site were

to leak and produce emissions of this nature at the ground

surface. In some areas, e.g. the Paradox Basin of the USA

[32]

, the French carbogaseous area [31]

, and the Eifel

region of Germany [55]

, such emissions appear to pose, at

worst, only very localised hazards. Even in suburban areas,

low flux emissions have proved manageable [56]

.

Although the natural emissions of CO 2 from the

geosphere recorded in sedimentary basins do not appear

to pose major hazards, their remediation might be difficult.

If a CO 2 leak point along a fault was successfully blocked,

this might simply transfer the CO 2 migration path to

another permeable point further along the fault. The main

remediation strategies that have been proposed for such

leaks therefore involve lowering the pore fluid pressure in

the CO 2 storage reservoir either by removing CO 2 or water

from it, or increasing the pore fluid pressure in a (higher)

reservoir, if any, into which the CO 2 is leaking [57]

.

Table 1

Some estimated natural and anthropogenic CO 2 emissions and fluxes,

anthropogenic fluxes in italics

Emission Mass (tonnes)

Lake Nyos event, estimated total emission [68] a 1,240,000

Lake Monoun event, estimated total emission [68] a 47,000

Sheep Mountain CO 2 production well blowout, 13,136

calculated maximum flow potential per day [71]

Siena Basin, estimated flux per km 2 per day from 137

200 km 2 area of ground surface [35]

Tyrrhenian Basin, estimated minimum flux per km 2 per 68.5

day from 15 km 2 area of sea floor [46]

Rangely enhanced oil recovery project, estimated flux 10.4

per day of deep-sourced CO 2 from 78 km 2 site [10,12]

Snaefellsnes CO 2 springs, Iceland, per day [47] 0.03

a Witness indicated that the duration of the Lake Nyos gas emission was

approximately four and a half hours, in which case the flux through the

- 1 lake surface can be estimated as 42,000 m 3 s [66]

. The duration of the

Lake Monount event is not known precisely but was no more than a few

hours.

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Hazards recorded from natural CO 2 emissions in

volcanic regions are much greater than those from

recorded emissions in stable areas of sedimentary basins.

Volcanic regions, including areas of hydrothermal activity,

are tectonically unstable, prone to earthquakes and liable

to ground heave and fracturing. Moreover, because there is

natural CO 2 supply from the degassing of magma, and high

temperatures and steam are commonly present near the

ground surface, they can contain gas under great pressure

at shallow depths. Very large masses of CO 2 may be

emitted during volcanic eruptions, but in general the CO 2

itself poses no significant threat to life as the CO 2 is ejected

high into the atmosphere and disperses [28,58]

, although

Martini [59] suggests that CO 2 emission may have caused

many deaths during the AD 79 eruption of Vesuvius.

Occasionally, large, sudden emissions of CO 2 have

occurred through fissures or fumaroles on the flanks of

volcanoes [60,61]

, Table 1 ). Very large, relatively sudden,

emissions of CO 2 have resulted from the build-up of

dissolved CO 2 in Lakes Nyos and Monoun, two stratified

crater lakes in Cameroon, West Africa [62–70]

, Table 1 ).

Volcanically active areas are not likely to be considered as

sites for CO 2 storage. However, the possibility of leakage

of CO 2 into lake waters from man-made CO 2 storage sites

building up and causing a sudden large emission warrants

serious consideration and is discussed further below.

In terms of immediate danger to life, the main concern

may be that a large, relatively sudden, emission of CO 2 , similar in impact to the widely publicised fatal emissions

that occurred in 1979 at the Dieng volcanic complex

[60,61]

, and in 1986 and 1984 at Lakes Nyos and Monoun

respectively, could occur as a result of CO 2 storage. Either

the opening up of a natural fracture (putatively caused by

excess pore fluid pressure in the reservoir during injection)

or a well blowout might produce a large localised release of

CO 2 from a storage reservoir to the ground surface in a

relatively short time.

4. Well failures

A well blowout that occurred during the drilling of a

production well in the Sheep Mountain natural CO 2 field

[71] resulted in a large flux of CO 2 through fissures in the

ground surface to the atmosphere. CO 2 from the reservoir

moved up an open-hole section of the well, into the

surrounding rock at the top of the open hole section

and emerged at the ground surface through three

0.6–0.9 m-diameter fissures, topographically above the well

site, that opened up as a result of the blowout. After the

blowout was controlled, the flow capacity from the

reservoir was calculated to be between 5.6 x 10 6 and

7.1 x 10 6 m 3 CO 2 /day (approximately 10,400–13,200

tonnes CO 2 /day). This flow was sufficient to throw chunks

of dry ice and rock projectiles a few centimetres in diameter

hundreds of feet into the air above the natural fractures

through which the CO 2 emerged at the ground surface.

Significant build-up of CO 2 at ground level occurred during this blowout, in particular during periods of calm, windless

and cool conditions. For example, on one of the first

mornings of the blowout period a car engine stopped when

being driven through a low spot on the way to the drillsite.

CO 2 levels were closely monitored using hand-held and fixed

devices, and full self-contained breathing apparatus was

used when working on the drillsite when concentrations

exceeded safe levels. At times, concentrations at ground level

were high enough to cause asphyxiation. Nonetheless, the

blowout was safely and successfully controlled. Such a

blowout clearly represents a serious hazard to life.

Carbon dioxide injection well blowouts are therefore a

potential hazard, which may be exacerbated where injection occurs into unconsolidated sediments, because, in such

cases, it might be difficult to control the emerging gas.

A blowout of a mineral water exploration well that

occurred near Florina in northern Greece exemplifies this

[Hatziyannis in 54]. In the summer of 1990, the Department

of Hydrogeology of the Greek Institute of Geology and

Mineral Exploration drilled an exploration well for the

location of mineral water in the Florina basin, where the

existence of CO 2 -rich water in the wider basin area had

been known since the early 1960s. The well was drilled to a

depth of 559 m and cased throughout with a steel tube with

an external diameter of 70 mm. The well completion also

included the installation of a wellhead with a valve. Carbon

dioxide was found from a depth of 97 m to the final depth

of 559 m. After the well completion and while the valve of

the wellhead was closed, CO 2 leakage was observed at a

distance of 100 m from the well. Later, the CO 2 leakage

advanced towards the well and created a hole around it

having an area of more than 25 m 2 (5 x 5 m) and a depth of

50 m. The cement base used for the drill rig collapsed and a

small lake was created. Access to the lake, for people and

animals, was restricted by the installation of a fence. Some

years later, the local authorities created a circular pool with

a diameter of 4–5 m with a cement lining, around the

leaking well. The pool was used by local people as a health

cure, by immersion of their feet only in the pool whilst

keeping their face about 1.5 m above the water surface.

This continued for some years until a fatal accident in

which a man was asphyxiated when swimming in the pool

due to the concentrated CO 2 layer lying just above the

water surface. At this point, the local authority prohibited

the use of this pool. In 2000, water and CO 2 were still

flowing from the pool, with the flowing water causing a

red-brown iron-rich deposit on the banks of small streams

created by the flowing water. During field work in 2003, the

pool and the well were found to be dry. No water or gas

was flowing, suggesting that after 12 years of continuous

CO 2 flow, the pressure was lowered and/or the borehole

collapsed and closed. From a CO 2 storage perspective, this

illustrates that in relatively poorly consolidated geological

successions CO 2 leakage can be induced by drilling wells.

The events described above support the observation

from nature that, outside the built environment, a high flux

emission that takes place in a naturally confined area where

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1198 S. Holloway et al. / Energy 32 (2007) 1194–1201

the CO 2 does not mix well with the surrounding air

potentially can allow a layer of concentrated CO 2 to build

up to dangerous levels over a large area relatively quickly.

This is essentially what happened in the Lake Nyos and

Lake Monoun emission events [62,66,70]

. Although the gas

outbursts (limnic eruptions) from the lakes may have been

quite violent at their peaks [72]

, the exhalation of CO 2 at

Lake Nyos may have continued for approximately four

and a half hours. A fatal concentration of CO 2 was able to

accumulate above the lake surface to a height of 120 m,

confined by the crater walls, before it moved down the lake

spillway into nearby valleys. A dissolved CO 2 build-up and

sudden emission of this sort could only take place as a

result of a leak from a CO 2 storage site into the bottom of a

lake that had the potential to maintain long-term density

stratification. Lakes found in temperate regions commonly

overturn seasonally, so CO 2 build-up in lakes in temperate

regions is less likely than in the tropics CO 2 build-ups in

lake waters can be detected by regular water sampling and

remediated, and CO 2 levels in the atmosphere near lakes

can be metered and alarmed ( http://www.biology.lsa.

umich.edu/

� gwk/research/nyos.html ). Nevertheless, the

risk of such an occurrence should be considered on a

case-by-case basis. The most likely scenarios in which a

high flux emission from a man-made CO 2 storage site could

occur are thought to be an injection well blowout or the

opening of a fracture between the reservoir and the ground

surface or sea bed.

5. Emissions in offshore areas

Emissions from the geosphere in offshore areas pose

little hazard to man but could adversely affect the marine

environment. At a natural (hydrothermal) CO 2 /CH 4

emission in the Tyrrhenian Sea, offshore from Panarea,

Aeolian islands, Italy [46]

, emerging gases generally issue

from open fractures in the rocky sea floor as bubbles,

although several areas of gas emergence through sand have

also been observed. The gas bubbles consist dominantly of

CO 2 with a small component of methane and their

temperature ranges from 40 to 95 1 C. By the time the

bubbles reach the sea surface the majority of the CO 2 has

dissolved and the bubble gas composition has changed

from predominantly CO 2 to predominantly (less soluble)

methane. Bubble sizes decrease from approximately 3–4 cm

at emergence to about 0.8 cm over a vertical distance of

about 20 m. Thus the seawater column absorbs most of the

CO 2 flux. A minimum CO 2 output estimated for an area of

15 km 2 of sea bed in the Tyrrhenian Basin is 25,000 tonnes

per year, most of which dissolves in the overlying sea water.

The extent to which this flux affects marine biota has not

been studied.

6. CO 2 emissions to date from man-made CO 2 storage sites

The Sleipner project started storing CO 2 in 1996 and

stores approximately 1 million tonnes of CO 2 per year. It has been monitored by means of time-lapse seismic surveys

and no leakage from the storage reservoir has been

detected to date [73]

. The In Salah gas fields project in

Algeria began injecting approximately 1.2 million tonnes of

CO 2 per year in late 2004. It is most unlikely to leak

because the CO 2 is injected just below a natural gas field,

which by its very presence demonstrates low-permeability,

high quality cap rocks. The K12-B project, in the Dutch

sector of the Southern North Sea, is injecting up to

60 tonnes per day of CO 2 into a compartment of a largely

depleted gas field. As this compartment of the field

formerly held natural gas it is considered that it is very

unlikely to leak.

Two EOR operations, at Weyburn in Saskatchewan and

Rangely, in Colorado, that use CO 2 as an injectant and

serendipitously store this CO 2 underground, have also been

closely monitored. CO 2 injection into the Weyburn field

began in 2000 [9]

. A wide range of monitoring techniques

has been deployed, including the monitoring of production

data, reservoir pressure and the geochemistry of produced

fluids and gases, soil gas monitoring, time-lapse seismic

imaging and passive seismic monitoring. No leakage has

been detected to date [9]

. Moreover, risk assessment

suggests that no CO 2 will leak out of the field via natural

pathways over a 5000 year period, and the estimated

maximum cumulative leakage from an estimated 1,000

wells in the field over this time frame is 0.03 Mt CO 2 , i.e. an

average release rate of 6 tonnes per year over 5000 years [9]

. At Rangely, where CO 2 injection started in 1986, a deepsourced flux of up to 3,800 tonnes/yr CO 2 to the surface has

been detected [10,12]

. However it is likely that at least part,

if not all, of this is CO 2 formed by the microbial oxidation

of methane migrating upwards from the reservoir or

overlying strata [11]

, rather than being CO 2 that was

injected into the reservoir for EOR.

7. Discussion

Experience from natural gas storage operations [74]

suggests that fluxes to the biosphere might be expected

from a proportion of man-made CO 2 storage sites. Such

fluxes are likely to be highly site-specific, because the

subsurface is an extremely variable natural system and each

site will be geologically unique. Consequently, it is

considered that engineered CO 2 storage sites should be

subject to a regulatory process that, prior to injection of

any CO 2 , should include: detailed geological and hydrogeological characterisation of the storage site and surrounding area, the construction of geological and reservoir

models, simulation of CO 2 injection into the storage

reservoir, simulation of the long-term fate of the injected

CO 2 , risk assessment based on the site characterisation,

models and simulation [75] and the acquisition of baseline

monitoring surveys. During CO 2 injection, monitoring of

the subsurface and ground surface or sea bed should be

combined with history matching of the monitoring results

with the models, to provide feedback to improve the

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1199 S. Holloway et al. / Energy 32 (2007) 1194–1201

understanding of the site. Agreement between the modelled

and observed results, to support the predictive modelling of

future CO 2 behaviour in the reservoir, would be desirable

before the site is closed [76]

. Leakage at monitored man-made CO 2 storage sites and

EOR projects to date is either non-existent or very low

( Table 1 ), although it should be borne in mind that even

the oldest monitored site (Rangely) has been injecting CO 2

for only 20 years and injected CO 2 may need to be stored

underground for thousands of years. However, once

injection has ended, the stability of storage of CO 2 in

some sites may increase rather than decrease through time.

This is because reservoir pore fluid pressure is likely to be

greatest during the injection period and will gradually fall

off after injection has ended. Enhanced pore fluid pressure

is an important factor driving the migration of both free

and dissolved CO 2

. Geomechanical stress will also decrease

as pore fluid pressure decreases. Moreover, modelling of

the long-term fate of CO 2 at the Sleipner storage site

suggests that as time progresses, more and more CO 2 will

dissolve into the surrounding pore waters [77]

. The

resulting CO 2 -saturated pore water is slightly denser than

the native pore water and will tend to migrate downwards

through the reservoir rather than upwards, greatly

decreasing the chances of it reaching the biosphere.

8. Conclusions

Natural CO 2 emission sites commonly occur within

large, ‘CO 2 emission-prone’ provinces, but at a local level

they consist of relatively small, concentrated emission

sources. In sedimentary basins, these consist principally of

carbonated springs and mofettes. In general, there is only a

local hazard to man if the CO 2 from these emissions is able

to build up in confined areas. The really major emissions

that have resulted in significant fatalities are confined to

volcanic areas.

Leaks from existing monitored CO 2 injection sites are

low to non-existent, although they have only a short track

record of injection (up to 20 years).

It is envisaged that the majority of leaks from underground CO 2 storage sites would be similar to those that

occur naturally in sedimentary basins. However, larger,

potentially more dangerous, leaks could occur from the

storage reservoir through unidentified natural pathways or

well blowouts. Leaks through natural pathways may be

difficult to remediate. However properly designed wells,

soundly executed drilling programmes and prudent storage

operations should mitigate such risks

, as evidenced by very

similar hydrocarbon storage and production facilities

around the world.

The major CO 2 emission events that resulted from limnic

eruptions in Lake Nyos and Lake Monoun are considered

extremely unlikely to occur as a result of properly executed

underground CO 2 storage, as they require a leak of stored

CO 2 into the bottom of a deep lake that is capable of

becoming stratified. Such events could be avoided by monitoring CO 2 levels in any lake waters near a storage site

and, if necessary, remediating the lake by gas-lift processes,

as is occurring at Lake Nyos today.

Finally, geological studies of some of the many natural

CO 2 fields around the world give assurance that in suitable

locations very large quantities of CO 2 can be trapped in the

subsurface for millions of years.

Acknowledgements

The authors thank the IEA Greenhouse Gas R&D

Programme for supporting the study. S. Holloway, J.M.

Pearce and V.L. Hards publish with the permission of the

Executive Director, British Geological Survey (NERC).

We particularly thank Richard Lynch and George Hatziyannis for the contributions they have made to this paper

and we thank Dr R.A. Chadwick and Dr G.A. Kirby for

critically reviewing the typescript.

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