The technique proposed would depend on the fact that carbon dioxide can be obtained as a solid by cooling to -78.50 C . The overall density is approximately one and a half times ~ 1.56 kg.dm-3 that of seawater. If the solid was shaped as a torpedo and then left to fall through the water column it would penetrate quite deeply into soft underlying sediments. This conclusion is based on in-situ investigations using penetrators that were studied as a disposal option for other solid wastes.

This concept should, therefore, provide permanent storage as the emplaced carbon dioxide will be chemically sequestered by the sediments (via the formation of an intermediate clathrate). Other than secure segregation of the emplaced CO2, the penetrator option has a further major advantage in that there should be no long-term effects to biological systems: penetrator disposal is deep within sedimentary formations which have zero or very low biological activity.

The use of the deep oceans had been proposed as early as 1977 [1], as a possible disposal medium for carbon dioxide. Subsequent investigations [2-3] underlined the need to dispose of the gas at great depths and to use sinking currents to avoid rapid outgassing to the atmosphere. More recent studies [4] have indicated that even shallow injection could be envisaged, by relying on the increase in water density that would result from carbon dioxide dissolution in seawater, to transport the dissolved gas to greater depths. However, although seawater itself contains large quantities of carbon dioxide as carbonate and bicarbonate ions and as dissolved carbon dioxide gas, the system is in dynamic exchange with atmospheric carbon dioxide and depending on temperature and salinity can act as a sink or a source for this gas. Thus ocean techniques proposed so far will only tend to displace the problem into the relatively near future [5] without ensuring permanent disposal.

A solution for permanently sequestering anthropogenic carbon dioxide may, however, be proposed on the basis of the observation of the occurrence of carbonate rich sediments which are ubiquitous in sedimentary formations of the ocean at depths above the carbonate compensation depth at around 4 km. They form a natural stable repository and are a result of environmental processes which have affected oceanic and thus atmospheric carbon dioxide concentrations over geological time. Alternatively use could be made of the soft alumino-silicate clay type sediments at the same or greater depths which would also act as an effective barrier to carbon release.

The technique proposed would depend on the fact that carbon dioxide can be obtained as a solid by cooling to -78.50 C. The overall specific gravity is approximately one and a half times ~ 1.56 that of seawater. Tests have shown [6] that if produced as a solid, carbon dioxide blocks would fall through the water column, although, such pieces would not enter into the underlying sediments and would slowly dissolve on the sea floor. However, if the solid was shaped as a torpedo and then left to fall through the water column it would penetrate soft sediments. This conclusion is based on studies using penetrator technology that were investigated as a disposal option [7]. A conceptual engineering design for an industrial disposal system has been described for this option [8].

Preliminary design characteristics of penetrators, free-fall velocity, embedment depth, and hole closure, have been described in some detail [9-10]. The results will be only briefly summarised here and the reader is referred to the above sources for fuller information. The attainable free-fall velocity or terminal velocity of a penetrator is determined by equilibrium of buoyant and hydrodynamic drag. A large range of parameters including overall weight, density, engineering form, sediment shear strength etc. need to be considered [11]. Figure 1 shows predicted free-fall velocities attainable by a series of penetrators having a length to diameter ratio of 10:1 and weights from 5-1000 tons, and for a range of densities for solid carbon dioxide when produced industrially, 1.1 - 1.5 kg.dm-3 [9]. Clearly tests would be necessary to determine the optimum quantities. However, experiments have been undertaken in the abyssal Atlantic Ocean using similarly designed streamlined projectiles [12] and have shown firstly, that sediment penetration depths greater than 30m can be obtained for torpedoes made out of steel, weighing as little as 2000 kg, and secondly that the in-situ results validated the simple hydrodynamic model used.

[ FIGURE 1 ] [ FIGURE 2 ]

A number of investigations have also been carried out to estimate the behaviour and attainable embedment depth of penetrators [13-15], as well the question of subsequent hole closure. Figure 2 shows the effects of penetrator weight and density on emplacement depth. It is clear from these results that an increase in density is a more efficient way to increase penetration than by augmenting overall weight. It also appears that there is a lower limiting weight to this penetrator technique to ensure complete tail embedment. It can be seen that pure CO2 penetrators (having a specific gravity of 1.56 km.dm-3) of around 100 tons can (tail) embed themselves to significant depths (~ 10m) in soft deep ocean sediment formations. Clearly the parameters "penetrator density and sediment shear strength" are critical factors for the final emplacement depth attained.

Theoretical investigations on hole closure [16] suggest that an open pathway to the sediment surface is unlikely, and that during the embedment process the penetrator creates large sediment deformations in its passage leading to an inflow of surface sediments due to the suction forces generated by the water that is drawn into the entry path. This causes the sediments to close behind the penetrator. No cavitation can occur due to the high ambient pressures at the depths being considered.

These studies are supported by the results of in-situ experiments which have been carried out in the Mediterranean Sea using small steel penetrators weighing between 2-3 tons. The data obtained from geotechnical testing and overcoring of a number of penetrator impact points provided evidence that the entry paths created by penetrators were indeed closed and filled with remoulded sediments with about the same density as the surrounding undisturbed sediments [17].

If one considers the phase diagram of carbon dioxide and water system (Fig 3), the physico-chemical behaviour of the solid carbon dioxide during its rapid descent through the water column and subsequent deep emplacement and sequestration in the underlying sedimentary formation can be assessed.

[ FIGURE 3 ]

Assuming that reaction times are fast, it can be postulated that equilibrium states are more or less reached instantaneously for the water column, but will depend on kinetic and thermodynamic properties in the sediments where chemical interactions with the surrounding environment will be the critical parameter in determining its fate. Two major processes can be identified that will depend upon the physico-chemical properties of carbon dioxide, these are thermal dissolution during penetrator free-fall through the water column, and subsequent clathrate formation after burial in the underlying geological formation.

Thermal dissolution in the water column. The penetrator is assumed to be industrially produced at a temperature between -800C to -100C and is travelling in relatively warmer water (~ 50C), the temperature difference will produce a heat flux from the water to the penetrator inducing a possible loss of penetrator mass [18]. The mechanism involved is forced convection across a turbulent boundary layer and conventional procedures can be used for an approximate prediction. From the diagram it can be seen that the behaviour of the solid carbon dioxide, after launch, will be as follows:
a) Sublimation regime:from the sea surface to a pressure of 5.2 bars (triple point) corresponding to a depth of 52m, the CO2 will sublimate and the boundary layer on the penetrator surface will be a two-phase flow (gas, liquid). Such conditions will only occur for a few seconds after penetrator launch.
b) Liquefaction-evaporation (mixed) regime: from 5.2 to approximately 40 bars (critical point), corresponding to depths ranging from 52-400m, CO2 at the surface of the penetrator will initially melt at a rate depending on the actual ocean temperature profile. The liquid carbon dioxide will then evaporate (if the local temperature near the penetrator surface is high enough), or later on while being mixed with seawater and progressively warmed up. Evaporation may occur during flow along the penetrator length or later in its wake; the boundary layer on the surface of the penetrator will progressively change from a two-phase flow to a single phase, two-species flow (liquid CO2 and dissolved aqueous CO2). At present any subsequent ion species formation within the water column have not been taken into account.
c) Pure melting regime: at pressures greater than the critical point (~ 40 bars, 400m) carbon dioxide will simply melt and cold liquid CO2 will progressively mix with seawater during its further descent through the water column and subsequently with pore water after embedment in the underlying sedimentary formation.
d) Chemical sedimentary interaction: the behaviour of the carbon dioxide once it has been successfully emplaced in a deep ocean geological formation will clearly depend on its interaction with the host formation under the prevailing in-situ conditions of temperature, pressure and geochemical environment.

Preliminary calculations have been carried out on thermal dissolution in the water column [18] taking into account the different regimes described above. For penetrators initially weighing 10, 100 and 1000 tons (vol. ~ 6.41, 64.1, 641 m3, dimensions ~ 9. 5m x 0.95m, 20.0m x 2.0m and 43m x 4.3m) figure 4 shows estimates of the loss of CO2 during free-fall, for various storage sites at depths down to 4000 m. It can immediately be seen that for small penetrators (10 t) the loss, as a percentage of the initial mass, even at disposal sites of only 1 km. depth are high, in the region of 40%. However, for a 100 t penetrator the loss is considerably less and estimated to be in the range of 17%. In the case of a 1000 t penetrator the loss is around 7%; for these large penetrators even for sites as deep as 3-4 km losses are only in the region of 25%.

The results show that mass loss is roughly proportional to velocity, and as time to depth is inversely proportional to velocity, the mass loss per unit of surface to a given depth is roughly independent of penetrator size. The increase of mass loss with penetrator size is thus only proportional to the increase of surface area, and independent of free-fall velocity.

[ FIGURE 4 ] [ FIGURE 5 ]

It is believed, however, that these losses could be reduced close to zero by using frozen seawater as an insulating material. The option proposed is to coat the penetrator by a shell of solid ice. Such a coating would effectively shield the penetrator from the surrounding water, since water-ice has a surface temperature around 0 C and thus would markedly reduce heat flux from the seawater. Figure 5 shows the predicted minimum ice coating required to shield the carbon dioxide from the water for disposal sites from 1-4 km. Ice thickness vary from 4-12 cm for this range of depths. Any ice remaining at the end of the sediment penetration phase could also provide a further source of water for the subsequent transformation of carbon dioxide into clathrate.

Clearly other material (plastic foam etc.) could also be investigated for insolation if this were to be required for the industrial production of large penetrators.

Carbon dioxide sequestration within deep ocean sediments. Two generic types of pelagic environments could be particularly suitable for permanent storage, calcareous rich oozes and soft alumino-silicate clays. In both types of environment the initial sequence of events will be probably similar. After emplacement (say at 4000m), a strong thermal gradient will be establish between the solid CO2 penetrator (-80 to -100°C) and the surrounding environment (2°C). Initially the penetrator will freeze the pore water in the surrounding sediments, and its temperature will increase because of latent heat of ice freezing. As the temperature of the penetrator reaches ~ -50oC (phase change temperature at the in-situ pressure of 400 atm) the solid CO2 will go to a liquid state as:

CO2 (s) = CO2(l)

After completion of this phase change the temperature will start to increase again. During this thermal transient, diffusion transport will mobilise aqueous components towards the cold zone. This will counteract the movement of reactive solutes away from the carbon dioxide along the chemical gradient. Reaction with surrounding pore water will start to take place with the formation of a carbon dioxide clathrate. This compound is a solid under conditions of temperature and pressure at such a site (fig. 3):

CO2 (l) + 5.75 H2O (l) = CO2-Clathrate (s)

a) Calcareous rich oozes: after the formation of a clathrate, in the case of soft carbonate sediments, there will be an ensuing formation of carbonate ions (however over very long time periods) due to the interaction between the clathrate and the surrounding calcium carbonate:

CO2-Clathrate (s) + CaCO3 (s) = Ca2+(aq) + 2CO32- + 2H+ + 4.75 H2O(l)

The formation of dissolved carbonate ions will subsequently (again over very long time periods) be neutralised by excess anions in the sedimentary environment and re-precipitation of calcium carbonate will follow. These reactions will occur in carbonate rich sedimentary formations that have been set down above the carbonate compensation layer at around 4000 m.

In the sea of Japan geothermally heated fluids containing very high concentrations of carbon dioxide at depths greater than 1500 m [19] have been discovered. These have been observed to precipitate on cooling once the temperature is less than 10°C, thus confirming solid clathrate as the equilibrium state of carbon dioxide in the NaCl-CO2-H2O ternary system.

b) Alumino-silicate soft clays: an alternative storage environment is that of a sediment rich in calcium and magnesium silicates [20], such sediments exist at both lesser and greater depths than the carbonate compensation layer. In this case sequestration will occur through precipitation as solid carbonate following a reaction of the type:

3MgCa(SiO3)2 + 3CO2 +H2O = H4Mg3SiO9 + 3CaCO3 + 4SiO2

In this case there may be competition for the carbon dioxide between the silicates and water for the formation of calcium carbonate or clathrates. Although the limited data available tend to indicate a favourably stable environment for both types of sediment, much more work clearly needs to be undertaken on the physico-chemical and kinetic interaction and evolution between hydrates and the different geochemical environments.

Preliminary theoretical studies appear to show that permanent storage of CO2 is possible using the natural geochemical properties of selected deep ocean sediments. Such a storage option would circumvent the problem of the eventual return of this gas to the atmosphere. Further evidence for the stable nature of clathrates can be drawn from existence of a natural analogue of CO2 hydrate, methane hydrate[21]. This hydrate is believed to be responsible for the bottom-following (or bottom simulating) reflector observed in many ocean areas from marine seismic reflection investigations. This reflector is thought to represent the lower face of sediment stiffened by interstitial methane clathrates [22]. By analogy, it appears reasonable to postulate that a zone of sediment indurated and stifferened by CO2-hydrate would form above the final resting point of the penetrator. This would supress both advection and diffusion, leading to the effective isolation of the CO2 for periods of millions of years.

A final point to consider is that such a disposal option should have negligible biological impact on pelagic fauna as there is no acidic dissolution of CO2 at the sediment-water interface, thus inducing no alteration to local levels of seawater pH which could cause damage to local populations. Microbial activity at the foreseen burial depths (>10-15m) is expected to be very low and will probably only be linked to residual organic carbon deposition [23].

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