This document discusses the potential for storing captured carbon dioxide in very deep ocean trenches. It begins by providing background on carbon capture and storage and past consideration of deep ocean storage. It then discusses some potential advantages of storing liquid CO2 deeper than 6km in trenches, where it would be denser than seawater and could remain permanently trapped. Specific very deep trenches are identified with enormous potential storage capacities. Compliance with international agreements and further research needs are noted. In conclusion, the document aims to restart discussion of deep ocean storage of CO2 and proposes further exploration of feasibility and environmental impacts.

1. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of GHGT-13.
doi:10.1016/j.egypro.2017.03.1686
Energy Procedia 114 (2017) 5417 – 5429
ScienceDirect
13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18
November 2016, Lausanne, Switzerland
Potential for Very Deep Ocean Storage of CO2
Without Ocean Acidification:
A Discussion Paper
Steve Goldthorpe *
Steve Goldthorpe Energy Analyst Ltd. Po Box 96, Waipu, 0545, New Zealand
Abstract
Carbon Capture and Storage (CCS) is an essential contributor to the mitigation of climate change. CCS will
require vast CO2 storage capacity. At present only geological storage is being considered. This paper revisits an
alternative CO2 storage possibility in enclosed basins on the deep and very deep ocean floor.
For example, the Indonesian Sunda trench, the Japanese Ryukyu trench and the Puerto Rico trench are more than
6 km deep. If liquid CO2 were to be placed in such a trench, it would be 7% more dense than seawater and could
remain permanently as a lake of liquid CO2 on the ocean floor, possibly becoming a solid hydrate over time which
could inhibit mixing between the stored CO2 and ocean currents.
At depths greater than about 4 to 5 km metres, seawater is under-saturated in calcium carbonate, so ocean
ecosystems are significantly different. Any impact on deep marine fauna would need to be investigated.
The London Dumping Convention has provisions for disposal of material into the ocean provided the absence of
adverse effects can be proven.
Deep ocean CO2 entrapment is more certain than geological CO2 storage in deep aquifers. A CO2 delivery
concept by ship and vertical pipe is suggested for exploratory trials, with subsea pipelines for permanent
installations, which might be much cheaper than geological CO2 storage.
There is vast capacity for storage of CO2 in the world’s very deep ocean trenches. The Sunda trench below 6 km
has the capacity to accommodate 19,000 gigatonnes of liquid CO2, which is greater than the CO2 yield from all
currently known global fossil fuel reserves. The Puerto Rico trench has capacity for 24,000 Gt of liquid CO2 deeper
* Corresponding author. Tel/fax.: +64 9 4320532;.
E-mail address: Steve.Goldthorpe@xtra.co.nz
Available online at www.sciencedirect.com

2. 5418 Steve Goldthorpe / Energy Procedia 114 (2017) 5417 – 5429
than 7 km. Enclosed basins of limited area could easily accommodate captured CO2.
China has the largest potential demand for CO2 storage from power generation and industrial sources, which
could be 3 Gt per year by 2050. The Ryukyu trench, which is 700 km from the Chinese coast and is in Japanese
water, has two sections deeper than 7 km. Those sections of the Ryukyu trench would have the capacity to
accommodate all the CO2 captured in China at 3 Gt per year for over 200 years.
In the event that very deep ocean storage of CO2 is found to be practicable and acceptable, the minimum
practical depth would need to be determined as a criterions for acceptable additional storage locations. For
consideration, there is an enclosed basin on the floor of the Mediterranean Sea 60 km off Southern Greece, with
capacity for 84 Gt of CO2 deeper than 4.5 km. Also, there is an enclosed basin in the Arabian Sea, 320 km south
west of Karachi, with capacity for 86 Gt of CO2 deeper than 3.5 km. The potential storage of CO2 in such locations
would be temperature dependent.
The global CCS community has previously considered ocean storage of CO2 on the basis of ultimate dissolution
and dispersion of CO2 in ocean water. Those studies have dismissed ocean storage as environmentally unacceptable
due to ocean acidification.
This paper postulates that very deep ocean trenches (>6 km) and deep ocean floor depressions (>4 km) are
environments for CO2 storage, where permanent storage without dissolution, acidification or adverse effects on
fauna may be possible.
The purpose of this paper is to pose the question “Why not?” to the CCS community and to suggest that active
research is timely.
© 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the organizing committee of GHGT-13.
Keywords: CO2 storage; deep ocean; CO2 hydrate
1. Introduction
If the global temperature rise due to Climate Change is to be limited to 2o
C then actions on many fronts to reduce
greenhouse gas emissions, particularly CO2 from fossil fuel combustion and industrial processes, will be required.
Carbon capture and storage (CCS) is one contributor to that global objective for countries without sufficient
renewable energy resources to meet their energy needs. The IEA [1] estimates that in 2040 one third of the global
action required to stabilise the global atmosphere at 450 ppm CO2 would need to be provided by CCS. That would
require storage for 52 gigatonnes (Gt) of CO2 by 2040 and about 5 Gt per year of additional CO2 storage capacity
thereafter.
In the early days of assessment of CCS, three types of CO2 storage were considered:
Disused hydrocarbon well storage; with or without Enhanced Oil Recovery (EOR)
Deep saline aquifer storage; and
Deep ocean storage.
The original concept of deep ocean storage of CO2 is based on the differential compressibility of liquid CO2 and
seawater, which means that at depths greater than about 3km liquid CO2 is denser than seawater and would sink to
the ocean floor where it would form a lake of cold liquid CO2. [2]
The concept of ocean storage of CO2 was subsequently extended to include mid-ocean injection at depths greater
than about 500 metres; i.e. sufficient to ensure that liquid CO2 is delivered and dispersed without vaporizing. In that
mid-ocean disposal concept, although less dense than seawater, the liquid CO2 would be deliberately intended to
rapidly disperse and dissolve in seawater before it formed bubbles that rose to the surface. The thermocline was
expected to limit vertical mixing of CO2-rich mid-ocean water with surface water.
In 2002 there was a proposal by an international team to discharge of 50 tonnes of liquid CO2 at a depth of 800m
into the mid-ocean off Hawaii through an existing pipeline. This proposal raised fierce opposition from
environmentalists and funding was withdrawn. A subsequent plan to experiment with injection of 5 tonnes of liquid
CO2 at about 800m depth into the North Sea off Norway was also abandoned due to environmentalist pressure. The
© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of GHGT-13.

3. Steve Goldthorpe / Energy Procedia 114 (2017) 5417 – 5429 5419
environmentalists’ explicit concern is that direct dissolving of CO2 in seawater would result in local ocean
acidification with adverse impacts on marine life [2]. The prospect of CO2 disposal into the oceans has
subsequently been largely dismissed by the CCS community.
Deep Ocean Storage has not been seriously revisited in recent years due to concerns that:
The CO2 could have adverse effects on bottom-dwelling marine life, with consequences up the marine food
chain;
The stored CO2 could eventually dissolve in the ocean resulting in ocean acidification; and
The issues of public acceptability of the principle of ocean storage of CO2 and compliance with international
ocean dumping conventions remain unresolved.
CO2 storage has instead recently focussed on geological storage in depleted oil and gas wells and in deep saline
aquifers. However, underground geological CO2 storage is not without problems:-
Every potential geological storage location would be site specific and would require comprehensive geological
data acquisition prior to CO2 storage;
It would also require extensive costly indirect monitoring and verification after storage in an environment where
visual observation is impossible;
Monitoring of the areal spread of the CO2 plume is feasible, but volumetric accounting for all the injected CO2 is
very difficult;
Depleted oil and gas wells, whilst demonstrating seal integrity, are limited in capacity and timely availability;
Deep saline aquifers do not have demonstrated seal integrity, so entrapment of CO2 over centuries would be
uncertain;
CO2 injected deeper than 800 metres would be in the supercritical state (>7.4 MPa) due to the geothermal
gradient and its density would be much less than that of saline groundwater, making it buoyant.
Supercritical CO2 is a mobile low viscosity organic solvent without the surface tension effects of a liquid;
making it good for Enhanced Oil Recovery (EOR);
EOR from depleted hydrocarbon wells, whilst potentially generating revenue, would result in additional fossil
fuel production. It is estimated [3] that the potential global average demand for CO2 for EOR would correspond
to 1.7 tonnes of additional fossil carbon in oil being produced for each tonne of carbon in CO2 being sequestered
in a depleted oil field. This could defeat the principle of CCS.
Rapid rates of injection might result in minor earthquakes, whereas slow rates of injection would require more
injection wells to be drilled; and
The principled opposition by some environmentalist groups to any CCS remains.
As the required capacity of geological storage of CO2 increases, the cost would increase and the certainty of
permanent storage in sub-optimal locations would decrease. Furthermore, the availability of suitable geological
storage locations would eventually constrain the continued implementation of CCS for the life of CO2 sources. In
contrast, if deep ocean storage were to be demonstrated and large scale viability proven, access to effectively
unlimited replicable CO2 storage capacity at constant low cost would be established.
In comparison with geological storage of CO2, very deep ocean storage (>6 km) without dissolution and
dispersion in seawater offers the following potential advantages:-
Once the feasibility is established, and a delivery technology has been developed, it could be readily replicated
without further major data acquisition or cost, other than an assessment of deep ocean topography, geology,
biological presence and currents;
The capacity of the deep ocean trenches for liquid CO2 is effectively unlimited;
The density difference between CO2 and water at high pressure and low temperature would provide a physically
stable provable CO2 entrapment mechanism;

4. 5420 Steve Goldthorpe / Energy Procedia 114 (2017) 5417 – 5429
The formation of a solid hydrate at the CO2-water interface could inhibit mixing;
Rapid rates of CO2 placement would be free from risks of enhanced seismicity;
A single injection facility could be used at high capacity indefinitely; and
Deep water remote video imaging could be used to monitor the storage location
The Indonesian Sunda trench, the Japanese Ryukyu trench and the Puerto Rico Trench are three examples of
very deep trenches each with the capacity for storage of virtually unlimited volumes of CO2 deeper than 6 km. [4]
Other trenches and depressions deeper that 4 km, e.g. in the Arabian Sea and Mediterranean Sea may be suitable for
limited CO2 storage, but with a lesser density difference.
This paper is based on analysis of public domain data and is founded on the author’s desk research experience in
the 1990’s, [5] adapted in the light of recent knowledge and experience. Whilst every effort has been made to
ensure accuracy, no liability is taken for any errors. This paper is independent of any organisation or funding body.
This discussion paper is freely offered to the CCS community to stimulate discussion of the question “Why not?”
and to propose active research.
2. The London convention and public acceptability
The disposal of waste materials into the ocean is contrary to the principle of the London Dumping Convention
1972 [6]. However, in a recent meeting of the parties to the London Convention [7], a new article was added to
explicitly regulate geoengineering activities. Annex 4 to the revised convention is a list of such activities, but only
includes ocean fertilisation at present. Annex 5 to the revised convention provides draft guidance on a procedure for
considering the inclusion of new activities in Annex 4 to the London Protocol.
The CCS community has put research on deep ocean storage “on-hold” pending inclusion of that concept in
Annex 4 to the London Convention. The process described in Annex 5 to the Convention, together with the
necessary research, is likely to take many years. However, some countries, e.g. Indonesia, are not party to the
London Convention, so that legal barrier would not apply in the case of very deep ocean storage of CO2 in a location
such as the Sunda Trench and early exploratory trials could be possible. The other locations discussed in Section 7
all relate to areas under the ambit of the Convention, so progress towards in-situ trials would be slower.
In 2005 a special study of Ocean Storage was published by IPCC [2]. That wide ranging study identified several
issues to be addressed, including public acceptability.
In light of concerns about ocean acidification and consequent public acceptability, mid-ocean dispersal of liquid
CO2 has been dismissed by the CCS community as a potential storage option for captured CO2 [8]. A consequence
is that deep ocean storage without dispersion in the ocean water is also “off the agenda” of the CCS community.
3. The physics of CO2
Figure 1 shows the phase diagram for carbon dioxide [9]. The triple point for CO2 is at -56.6o
C temperature and
0.51 MPa pressure. The critical point is at 31.1o
C and 7.4 MPa. Figure 1 shows that the state of CO2 would be well
within the liquid phase at 6 km depth in cold deep ocean water, where the pressure would be about 61 MPa.
The maximum density of water at ambient pressure occurs at 4o
C, but at very high pressure the maximum water
density occurs at a slightly lower temperature of 1o
C to 2o
C. In bodies of water where polar meltwater circulates,
the water temperature of the deepest parts is typically at that temperature. At 2o
C seawater and liquid CO2 have the
same density at 2,715 meters depth. [10, 11]
However, in bodies of water that are not supplied by polar meltwater the temperature in the deep ocean may be
higher. For example, in the Mediterranean Sea west of Greece there is a depression in the sea floor that is over 5 km
deep at its deepest point. The temperature in that place is 14.3o
C and the salinity is 39.4 parts per thousand [12]. At
those conditions the water has the same density as liquid CO2 at 4,270 metres depth. Figure 2 shows the relationship
between the depth of equal density and the temperature and salinity of water.
At a depth of 6.5 km and a temperature of 2o
C the density of seawater will be 1.06 gm/cm3
and the pressure will
be 66 MPa. At those conditions, CO2 has a density of 1.13 gm/cm3
[11]. Liquid CO2 is 7% more dense than
seawater at these conditions, thus providing a positive CO2 entrapment mechanism

5. Steve Goldthorpe / Energy Procedia 114 (2017) 5417 – 5429 5421
Figure 1 CO2 phase diagram
Figure 2 Isodensity lines
3.1 Comparison with CO2 properties in geological formations
Figure 1 shows that the state of CO2 would be a supercritical fluid in geological storage at a depth of 2000m
(based on a hydrostatic pressure of 20 MPa) and at 70o
C (343o
K) (based on a geothermal gradient is 25o
C per km
above 20o
C ambient). A supercritical fluid behaves as a gas or a liquid without surface tension properties and with a
low viscosity.
4500.0
4300.0
4100.0
3900.0
3700.0
3500.0
3300.0
3100.0
2900.0
2700.0
2500.0
0 5 10 15
Depthinmetres
Temperature in degrees Celcius
Equal density of Seawater and liquid CO2 vs temperature and salinity
35 ppt
40 ppt
Salinity
CO2 less
dense than
seawater
CO2 more
dense than
seawater
Deep
ocean
Mediteranean
Sea

6. 5422 Steve Goldthorpe / Energy Procedia 114 (2017) 5417 – 5429
At elevated temperature liquid CO2 is less dense. Figure 3 shows the pressure density relationships for CO2 at
various subcritical and supercritical temperatures that might be found in geological formations, compared with deep
ocean conditions.
The density of liquid CO2 on the vapour liquid equilibrium line is derived from data in the Dortmund Data Bank
[13] for conditions ranging from 1.4 MPa/-30o
C to 6.4 MPa/25o
C. The critical density is reported from
Thermophysical Properties at Critical and Super Critical conditions [14] and the data for the supercritical curves
are determined with a Critical Processes [15] calculator.
Figure 3 Density vs. pressure for CO2 at subcritical and supercritical conditions
These physical property data for CO2 show that at the warm condition found in depleted oil and gas wells, or
deep saline aquifers, the stored CO2 density would be substantially lower than the density of saline groundwater (~1
gm/cc). At 2km depth the density of supercritical CO2, would be about 60% of the density of saline ground water.
Therefore injected CO2 in geological storage reservoirs would be buoyant relative to ground water, would easily
flow through small cracks or faults, and would have to rely on the physical structure of the geological formation for
permanent entrapment.
3.2 Formation of CO2 hydrate
At high pressure conditions 6.5 km deep in the ocean, CO2 is very soluble in seawater to the extent that the two
liquids are effectively miscible. Therefore the interface between liquid CO2 will be a stratified layer where the
density transitions from water at 1.06 gm/litre at the top to CO2 at 1.13 gm/litre at the bottom. The thickness of that
density transition layer under steady state conditions is unknown.
Within that transition layer there will be a region where the liquid CO2 concentration in water is 30%. Under
those conditions and concentrations a solid CO2 hydrate may form. If CO2 hydrate forms within the water CO2
interface it could potentially form a solid barrier inhibiting mixing of the stored liquid CO2 with the overlying
seawater.
The physics of CO2 hydrate formation at these extreme pressure condition is largely unknown. Research would
be required to establish whether or not CO2 hydrate would form, whether CO2 hydrate would accumulate or
dissipate, whether the CO2 hydrate would float or sink and whether or not all the stored CO2 would eventually be
converted to CO2 hydrate. Such research could probably be carried out at modest cost in a high pressure laboratory
facility.
4. Ocean zones
Figure 4 shows the nomenclature of zones of the ocean. The Abyssal Zone ranging from 4 km to 6 km depth is
widespread, particularly in the Pacific Ocean. However, the deeper Hadal Zone is limited, mostly to trenches,
typically where oceanic geological plates are sub-ducted under continental plates. The deepest point in the global
ocean is the Marianas Trench at 11 km deep in the Western Pacific Ocean.
0
0.2
0.4
0.6
0.8
1
1.2
0 100 200 300 400
Densitygm/cc
Pressure bar
2 deg C (liquid) in
deep ocean
Vap liq equil. 30C to
25C
Critical point 31C and
73 bar
32 deg C supercritical
50 deg C supercritical
70 deg C supercritical
Subsurface
conditions

7. Steve Goldthorpe / Energy Procedia 114 (2017) 5417 – 5429 5423
Exploration of the concept of deep ocean storage of CO2 might pursue two lines of investigation. Firstly, the
hadal zone in very deep ocean trenches, where there is a substantial density difference between liquid CO2 and
seawater, and unlimited capacity. However suitable hadal zone locations may be distant from CO2 sources.
Secondly, the abyssal zone, where the density difference would be less and the CO2 capacity would be limited, but
storage locations may be closer to sources of captured CO2.
Figure 4 Oceanic provinces and zones [16]
4.1 Deep ocean biota
Light does not penetrate into the ocean water below about 1 km. Deep ocean biology studies have identified a
large number of species of creatures living in the absolute darkness of the Abyssal Zone. These creatures are
believed to be supplied by detritus falling from above to sustain bottom feeders in the Abyssal Zone (4-6 km deep).
In turn the creatures of the Abyssal Zone provide a food source for creatures living in the Bathypelagic and
Mesopelagic zones further up the ocean. Accordingly, there is concern that widespread deposition of liquid CO2 on
the floor of the Abyssal Zone would have the potential to disrupt the ocean ecosystem.
Manned and un-manned research expeditions have also identified life forms existing in the deeper Hadal Zone
trenches. Detritus in the ocean tends to accumulate in the deep trenches, providing a source of nutrients. Endemic
ecosystems may also exist around hydrothermal vents, providing nutrients and energy. Whether or not creatures
living in the Hadal Zone contribute significantly to the ocean food chain in overlying water would need to be
determined by research.
Observed fauna in hadal trenches is predominantly in the form of holothurians (sea cucumbers). However some
exoskeletal species such as amphipods, isopods and bivalves have been observed. Research at these extreme depths
is very limited. If storage of CO2 in a specific contained basin in the Hadal Zone is considered then an investigation
would be required to determine the presence, species, mobility and uniqueness of any species.
It is conceivable that a volcanic seep in the target area could be home to a uniquely evolved endemic species. A
photographic investigation of the area to be affected, via a remotely controlled submarine, would be necessary prior
to the start of CO2 storage, to check for any signs of endemism in the area.
5. Carbonate compensation depth
The carbonate compensation depth is the depth in the oceans below which the rate of supply of calcite lags
behind the rate of solvation, such that no calcite is preserved. That means that sea creatures with an external
skeleton, such as shell fish, coral etc. cannot theoretically exist below the carbonate compensation depth, because

8. 5424 Steve Goldthorpe / Energy Procedia 114 (2017) 5417 – 5429
their shells would dissolve in the seawater.
The carbonate compensation depth varies from 3.5 km to 5 km deep in the oceans. The Hadal Zone is generally
below the Carbonate Compensation Depth. However, some exoskeletal creatures have been observed at greater
depths. Research would be required to determine whether such creatures are adapted to accommodate the ocean
chemistry or whether they only exist where there is an anomaly in the carbonate compensation depth.
Since the ocean chemistry below the carbonate compensation depth is significantly different from the overlying
water it could be inhospitable to mid-ocean dwelling creatures, such that they do not feed on the biota from the
deeper water. If so, then the carbonate compensation threshold might act as a natural interface between two separate
independent ocean ecosystems.
6. Storage capacity of enclosed basins in the Sunda Trench
The following capacity assessment illustrates a technique used in this research to harvest ocean floor topography
data from the GoogleEarth software system for the purpose of estimating deep ocean volumes. This analysis has
been carried out using a public access version of GoogleEarth in which the bathymetric data is referenced to the US
navy. The absolute values of the depth data may be questionable.
This capacity assessment is based on the premise that permanently stored CO2 would be in the liquid phase. In
the event that the deposited CO2 becomes completely converted to CO2 hydrate over time, then the storage capacity
for CO2 would be about 30% of the volumes calculated below.
Figure 5 shows an image of part of the Sunda ocean trench south of West Java and South Sumatera. This trench
results from the subduction of the Indo-Australian Plate underneath the Eurasian Plate. Using GoogleEarth
bathymetric data, three locations have been identified where there are enclosed basins in the trench with a depth of
6.7 to 6.8 km. These three locations are labelled A, B and C in Figures 5 and 6.
Figure 5 Image of part of the Sunda Trench south of Java and Sumatra
The analysis presented in Figure 6 shows the seafloor areas deeper than 6.5 km with colour-coded contours at 50
metre intervals. The grid squares are sixths of a degree of latitude and longitude; i.e. 18km x 18km or 335 km2
per
grid square. The colour-coded areas are surrounded by areas less than 6.5 km deep. This analysis identifies a small
isolated basin (A) and a larger trench including enclosed basins B and C.
6.1 Basin A
The deepest point in Basin A is at 6.725 km. The area of the shallow basin below 6.7 km metres is about 7 km2
and
the estimated volume below 6.7 km is 63 million cubic metres; i.e. capacity for 71 million tonnes of CO2. That

9. Steve Goldthorpe / Energy Procedia 114 (2017) 5417 – 5429 5425
capacity at the bottom of Basin A, below the 6.7 km contour, would be sufficient to accommodate the CO2 captured
from a 2 GW coal fired power station for 6 years. Such a location may be suitable for a limited and contained field
trial of the very deep ocean storage concept.
Figure 6 Sea floor contours in the Sunda trench (via GoogleEarth)
Table 1 shows the additional enclosed CO2 storage capacity that Basin A would have if it were progressively filled
up to the 6.55 km depth level with CO2 captured from 2 GW coal fired power stations. On that basis, the small
enclosed Basin A would have the capacity to store CO2 captured from 27 2GW coal power stations operating for 25
years before overflowing into adjacent areas.
Table 1 CO2 storage capacity of Basin A
Depth (m) of CO2-
water interface
Affected Area (km2
)
Storage capacity
Gt of CO2
Number of 2GWeplants
(CCS for 25 years)
>6700 7.3 0.07 0.25
>6650 30.5 1.18 4
>6600 37.8 3.33 15
>6550 63.4 6.07 27
6.2 Basins B and C
A similar analysis of the storage potential of the trench shown on Figure 6, including basins B and C is shown in
Table 2. This analysis suggests that the enclosed trench areas identified on Figure 6 would have the capacity to
store CO2 captured from nearly a thousand 2 GW coal fired power stations each operating for 25 years
Table 2 CO2 storage capacity in 170 km long trench with Basins B and C
Depth (m) of CO2-
water interface
Capacity Gt CO2
Basin B
Capacity Gt CO2
Basin C
Number of 2GWe plants
(CCS for 25 years)
>6750 0.7 0 2.5
>6700 4.1 1.48 20
>6650 9.6 21.40 113
>6600 19.7 97.9 430
>6550 263 963

10. 5426 Steve Goldthorpe / Energy Procedia 114 (2017) 5417 – 5429
6.3 Whole of Sunda Trench
In a separate analysis, Figure 7 shows plots of the 6 km contours to the north and south of the whole of the Sunda
trench that is south of Java and Sumatera.
Figure 7 Boundaries of the hadal zone of the Sunda trench
These contours have been derived by examination of depth data reported by GoogleEarth at quarter degree
increments of longitude. The deepest point on each north-south cross section was also identified. From this data the
cross section area of the Hadal Zone deeper than 6 km at each increment is estimated and from that data the volume
of the whole Hadal Zone is estimated. Figure 7 shows the main trench, labelled D, which includes the areas
analysed in more detail in Figure 6. Four smaller enclosed areas of the Hadal Zone to the South East are also
identified.
Table 3 Estimates of dimensions of Hadal Zone of Sunda trench below 6 km depth
Area Length – km Average width km Maximum depth
(below 6000 m)
CO2 storage Gt of CO2
D 1155 48 831 14,600
E 83 16 440 200
F 165 17 679 600
G 248 24 671 1,600
H 330 15 1,071 1,600
All 2200 31 1,071 18,700
6.4 Natuna gas field situation
The Natuna gas field in the South China Sea has a CO2 content of 71%, which has to be separated to produce a
saleable natural gas. That gas processing operation will produce about 7 volumes of CO2 for each 3 volumes of
natural gas product. The planned peak rate of gas production from the Natuna field is about 4 billion standard cubic
feet per day after processing, which is expected to commence in 2024[17].
That production rate would yield 190 million tonnes per year of CO2, which is about the same as would be
produced by CCS from seventeen 2 GW coal-fired power stations. The plan is for the Natuna gas field to be
exploited at that rate for 20 years, before production declines. If the total recoverable reserves are exploited, the
Natuna field would release about 6 Gt of CO2, which is equivalent to the storage capacity of Basin A.
The scope for ultimate geological storage of CO2 in depleted oil and gas fields in the region is limited and may
13
12
11
10
9
8
7
6
5
4
100 105 110 115 120
DegreesofLatitude
(Southfromtheequartor)
Degrees of Longitude
Northern 6000m
depth contour
Southern 6000m
depth contour
JAVA
SUMATERA
D
E
G H
F

11. Steve Goldthorpe / Energy Procedia 114 (2017) 5417 – 5429 5427
be inadequate to store all the CO2 from the Natuna gas field. As noted above, permanent retention of CO2 stored in
deep saline aquifers is less certain. In contrast, Table 2 shows that the enclosed trench (B and C) south of
Java/Sumatera has the capacity to store all the CO2 from the Natuna field forty times over. This CO2 storage
location could therefore provide scope to store CO2 from other countries.
6.5 Summary of storage capacity considerations
This analysis of the CO2 storage capacity of the Sunda Trench can be summarised as:
The Hadal Zone of the Sunda trench south of Indonesia is about 2200 km long and on average 31 km wide. It
occupies about 68,000 km2
of ocean floor deeper than 6 km, which is 0.019% of the global ocean floor. It would
have the capacity to store about 19 trillion tonnes of liquid CO2.
The entire CO2 content of the Natuna natural gas resource (6 Gt CO2) could be stored in basin A below 6.55 km
depth, affecting an area less than 40 km2
, which is 0.06% of the area of the Hadal Zone of the Sunda trench.
The entire CO2 from 90% CCS of all existing coal fired power plants in Indonesia for 50 years (9.6 Gt CO2)
could be stored in Basin B below 6.65 km metres affecting an area less than 60 km2
, which is 0.09% of the
Sunda trench area.
The entire CO2 from 90% CCS of all coal fired power stations in the world (~12 Gt CO2 per year) for 20 years
could be stored in Basins B and C below 6.55 km, affecting an area of 2,900 km2
, which is 6% of the Hadal Zone
of the Sunda trench or 0.00084% of the global ocean floor.
The foregoing assessment of CO2 storage capacity is based on the volume being filled with liquid CO2. The
discussion in Section 2 identifies scenarios under which the stored CO2 could be converted to CO2 hydrate with 5.75
to 6 molecules of water per molecule of CO2. Thus a cubic meter of CO2 might be converted to 3.35 to 3.45 cubic
metres of hydrate. In the case of that outcome the estimated CO2 storage capacity could be reduced from 18.7
trillion tonnes to 5.5 trillion tonnes, which is still more that the total potential global fossil fuel CO2 emissions.
7. Capacity assessments in some other locations
7.1 China
The East China Sea is shallow. Ocean of adequate depth for deep ocean storage does not occur until the Pacific
Ocean trench is reached beyond the ridge of the Southern Japanese Islands. The Ryukyu Trench south east of the
island of Okinawa includes areas deeper than 7 km. The Ryukyu trench is 700 km from the Chinese coast and is in
Japanese water. The greatest depth is 7.5km. A survey with GoogleEarth indicates that the areas deeper than 7 km
in two parts of the Ryukyu trench have the capacity to accommodate 760 Gt of liquid CO2, with much more capacity
in less deep areas.
China has the largest potential storage demand for CO2 captured from power generation and industrial sources,
which could be 3 Gt per year by 2050. The Ryukyu trench below 7 km would have the capacity to accommodate all
the CO2 captured in China at 3 Gt per year for over 200 years.
7.2 Mediterranean Sea
There is an enclosed basin, with a maximum depth of 5 km on the floor of the Mediterranean Sea, 60 km off
Southern Greece. Figure 2 shows that the depth of equal density is substantially greater than in the open ocean.
Therefore it is likely that secure CO2 entrapment by density difference could only be achieved in the area deeper
than 4.5 km. That area is 274 km2
. That potential CO2 storage location is large enough to accommodate 84 Gt of
CO2, which would correspond to the CO2 captured for 25 years from 370 2GW coal fired power plants. Hence that
CO2 storage location has the potential to service the CCS requirements of Europe.
7.3 Pakistan
The ocean adjacent to Pakistan is the Arabian Sea, which lies between Northern India and the Arabian Peninsula.
Away from the coasts, the Arabian Sea is generally 2 -3 kilometres deep, which is inadequate for deep ocean CO2
storage. However, a survey with Google Earth reveals that there is a small depression in the floor of the Arabian
Sea, about 320 km WSW of Karachi in the centre of the Pakistani Exclusive Economic Zone, where the depth
exceeds 4 km. The volume of that depression below 4 km deep would be sufficient to accommodate 1 Gt of liquid

12. 5428 Steve Goldthorpe / Energy Procedia 114 (2017) 5417 – 5429
CO2, which would be sufficient to accommodate the CO2 that could potentially be captured from coal fired power
generation in Pakistan until 2040. Capacity for over 60 Gt of CO2 exists below 3.5 km. Deep ocean storage is
theoretically feasible at depths greater than 3 km, provided the abyssal zone at that location has a low temperature,
with a capacity for over 500 Gt CO2 at this location. So there is scope for that location to accommodate greater
volumes of CO2, including that which might be captured in adjacent Middle East Countries.
8. Delivery of CO2 to a very deep ocean basin
Pipelines are the most effective way to transport large volumes of CO2 over moderate distances. The natural gas
industry provides the technology for pipe-laying on the sea floor, even at great depths. A 24-inch diameter pipeline
would be able to transport 1500 tonnes of CO2 per hour from a 2 GW power station with CCS for over 150 km
without the need for recompression. If a subsea pipeline cost is US$75,000 per km-inch, such a pipeline might cost
US$270 million. Over a 25 year lifetime the pipeline could deliver about 300 million tonnes of CO2. Hence the
cost per tonne of CO2 transported and stored would be about US$1 per tonne. That cost compares with estimates in
the region US$5 - US$10 per tonne for geological storage of CO2.
For longer distances liquid CO2 might also be transported at low temperature and elevated pressure (-20o
C, 2
MPa) in refrigerated and pressurised insulated containers on 3,000 tonne ocean-going vessels. These parameters are
within the range of technologies currently used for the ocean transportation of liquefied petroleum gas.
A long vertical CO2 delivery pipe might be suspended from a geostationary floating platform above a very deep
ocean storage location. A preliminary evaluation results in the following outline design concept in Box 1.
Box 1 Outline concept for CO2 placement in a deep basin from a geostationary vessel
A 16-inch diameter, thick walled high density plastic gas pipe (as used for gas transmission) might be used,
which would be supported by its buoyancy. If liquid CO2 is delivered to the top of the pipe at 2 MPa and -20o
C, its
differential head over seawater would overcome the pipeline pressure drop of about 100 kPa per kilometre at a
flowrate of 1,500 tonnes per hour, i.e. the same as the rate of CO2 production at a 2 GW power station with CCS.
Each ship would be unloaded in two hours. Lights and cameras mounted on the bottom end of the delivery pipe
could be used to monitor the CO2 delivery into the deep ocean trench. The refrigerated CO2 would flow down the
vertical delivery pipe with no additional pressurisation requirement.
Delivery of CO2 300-350 km from a 2,000 MW power station to the geostationary injection platform would
require a fleet of fifteen 3,000 tonne vessels operating on a 30-hour round trip schedule, with each ship taking two
hours to load CO2 as it is captured from the power plant, 12 hour voyages each way (12 hours slow steaming at 14
knots = 30 kph) and two hours to unload the liquid CO2. If the steaming fuel consumption of the ship is 1.5 tonnes
per hour, then the CO2 emissions from the ship would be about 100 tonnes per 3,000 tonne CO2 load delivered; i.e.
3% loss of CO2. Optimisation of ship fuel use from a CO2 emission perspective would be necessary.
9. Suggested further investigation
The confirmation of physical property data for liquid CO2 at the extreme pressure and low temperature
conditions corresponding to very deep ocean, which could be via laboratory experiments;
The potential for crystalline CO2 hydrate to form at an interface between liquid CO2 and seawater at very deep
ocean conditions would be investigated, which could also be researched in the laboratory;
The potential for CO2 dissolution in an ocean current from the hydrate or liquid CO2 surface would need to be
researched, which could possibly be in the laboratory;
The defined procedure could be followed for considering the inclusion of very deep ocean storage of CO2 in
Annex 4 of the London Convention;
The legal status of the storage of large amounts of liquid CO2 in an enclosed basin in the hadal zone within the
Exclusive Economic Zone of Indonesia would need to be explored within the context of the London Dumping
Convention;
Initial listing of biota recorded in the OBIS database for specific deep ocean locations of potential interest;
Further research would be required to characterise any biota in the area of interest, perhaps via remotely
controlled submarines with cameras;

13. Steve Goldthorpe / Energy Procedia 114 (2017) 5417 – 5429 5429
Biological research might be needed to consider the impacts of CO2 storage on any biota identified in the area of
interest;
The interaction, if any, between ocean biota communities on either side of the carbonate compensation threshold;
Geological research might be needed to consider the potential impact of deep ocean CO2 placement on the plate
subduction process;
An engineering contractor evaluation of outline CO2 transport and delivery schemes would be required;
Engagement with principled environmentalist groups to address their concerns would be advisable; and
If no barriers are found to progressing with the concept of very deep ocean storage, then a well monitored and
observed in situ trial would need to be carried out.
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