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In part 1, we looked at the likely impact of the recent EPA rules for greenhouse gas emissions from new power plants. We also looked at some of the technology options for carbon capture from new coal-fired plants, and how they might affect the cost of compliance. Now we need to consider the other side of CCS: sequestration. What options do we have for disposition of captured CO₂, and how do they affect the economics of the overall process?

There are three general options we'll examine here: (1) enhanced oil recovery (EOR); (2) other forms of geological sequestration; and (3) ocean sequestration. These are not the only sequestration options by any means, but they are the ones most widely considered.

Enhanced Oil Recovery

EOR, as it applies to sequestration of CO₂, involves a form of geological storage. It is considered separately here because of its unique position: it has a negative cost to utilities. It costs them less than nothing to dispose of captured CO₂ by supplying it to oil field operators for EOR; the operators will buy the CO₂ from them and deal with pumping it into their oil reservoir. CO₂ has positive value to the operators because it enables recovery of more oil from the reservoir. At the same time, the fact that it has held oil for millions of years virtually guarantees that the oil reservoir will be suited for semi-permanent sequestration of CO₂.

EOR is a complex subject, and my treatment of it here will be greatly simplified. One point to be aware of is that injection of CO₂ is by no means the only option for EOR. Nor is it presently the most widely used. Other options include water flooding, steam flooding, nitrogen injection, and injection of various types of chemical solutions. It's also common in fields that co-produce both oil and natural gas to re-inject the gas. In all cases, the objective is to maintain pressure in the formation and to "encourage" the oil to migrate through the porous rock of the reservoir toward the production wells tapping it.

Although there are other approaches for EOR, CO₂ injection is often the most effective -- when it is an option at all. It requires an adequate nearby source of CO₂, which may not exist. But where it is an option, CO₂ injection works well. Supercritical CO₂, at a pressure above 73 atmospheres, is an effective solvent for nearly all types of oil. The liquid oil and the supercritical gas are fully miscible, and the result is a thin fluid with little or no surface tension that seeps relatively easily through the porous source rock. When the oil and CO₂ mix is brought up to the surface and depressurized, the CO₂ "fizzes out", leaving the oil behind. The CO₂ is then mixed with makeup gas and returned to the injection well for another round of oil recovery.

For every barrel of oil recovered by CO₂ injection, the equivalent of one or more barrels of supercritical CO₂ will typically be left behind in the reservoir. The ratio isn't precise; it will depend on the size and state of the reservoir rock and on how the operators choose to employ CO₂ injection. If they're paying a high price for CO₂, they will use it sparingly, in a manner that maximizes the ratio of oil recovered to CO₂ left behind. If it's cheap, they will use it more freely, in a manner that leaves more CO₂ in the reservoir but maximizes the amount of oil recovered.

Given those complications, it's impossible to assign a precise value to CO₂ used for EOR. When the IPCC went through the exercise for its report on CCS in 2005, they cited a range of 10 -- 16 US$ per ton for what operators had actually paid¹. However, that was from data taken when oil had been selling in the range of $25 -- $30 per barrel. The report's authors asserted that an oil price of $50 per barrel "could justify a credit" of $30 per ton to offset the cost of carbon capture. With oil at $100 per barrel, the figure would presumably be $60 per ton of CO₂. I'm somewhat skeptical of that figure, but it amounts to a credit of roughly $60 per megawatt-hour of output from a coal-fired plant with CCS. If valid, that's ample to pay for the cost and operational overhead of the CCS system -- even using currently available technology. With the enhanced CCS technologies likely to be available within 5 -- 10 years, it would make CCS highly profitable.

Limitations of EOR

There is a significant "fly in the ointment" in relying on EOR to make CCS profitable without the political hurdle of a carbon tax. That's the limited size of the market. If CO₂ could be supplied to all of the oil fields in the US that could profitably use it for EOR, a generous estimate of the amount of CO₂ sequestered annually would be 250 megatons. In absolute terms that's a lot of CO₂. However, it's less than 10% of what is currently produced by our coal-fired power plants.

As a result, supplying CO₂ for EOR can never be much of a factor in reducing CO₂ emissions to the degree needed to stabilize greenhouse gas levels in the atmosphere. It will, however, provide an economical means to fund prototype plants while CCS technology is being refined and tested.

Other Geological Sequestration

Formations of the type in which oil is found are well suited for sequestration of CO₂, but only a tiny fraction of formations that are suitable for the latter also happen to hold oil. Any extensive sandstone formation that lies buried below an intact capstone layer is a candidate for storing CO₂. Startup C12 Energy in Berkeley, CA, is conducting geological surveys to identify such locations [2]. When they've identified a good formation, they negotiate contracts with the local land owners for CO₂ storage rights. Storage rights can be secured cheaply, since there is as yet no price on carbon emissions and no certainty that the rights being secured will ever be exercised. But if and when a price is put on emissions, and the capacity for storage in EOR operations is exceeded, the rights held by the company will presumably rise in value.

It isn't actually necessary that a formation be covered by an intact capstone layer in order to sequester CO₂. As long as it is far enough down and saturated with water (a deep saline aquifer, for example), then CO₂ that has been compressed enough to inject into the formation will be denser than water. Part of it will dissolve into the water, but what doesn't dissolve will tend to sink deeper into the formation rather than rising. The pressure needed to achieve that is approximately 30 MPa. That corresponds to the pressure in the ocean at a depth of 3 km, or about 10,000 feet. That's also the minimum depth needed for a CO₂ injection well, if the formation into which the CO₂ is injected is not capped.

A third type of formation that's suited for CO₂ sequestration is offshore sandstone. If it's in water deep enough that the formation lies below the ocean thermo cline, the water temperature and the temperature in the upper layers of the formation will be only a few degrees above freezing. In that case the formation can safely hold CO₂ even if the injection point is much less than 10,000 feet down. Any CO₂ that diffuses to the upper boundary of the formation will form a solid CO₂ clathrate with the cold water, plugging the pores of the sandstone and allowing at most very slow diffusion of sequestered CO₂ into the water above.

The capacity of formations suitable for geological sequestration of CO₂ is large. Estimates vary and are all rough, but there's a consensus that availability of suitable formations won't be a limiting factor for at least the next century or so. However, the likely cost of drilling huge numbers of deep injection wells and compressing gigatons of CO₂ to pressures for geological sequestration makes for ongoing interest in other options.

Ocean Sequestration

The most widely considered of those other options is ocean sequestration. It's not one option, however, but rather a varied set whose common feature is that the CO₂ ends up being stored in the ocean.

As a carbon reservoir within the global carbon cycle, the oceans are immense. Especially the deep waters that comprise the bulk of the oceans' 1.37 billion cubic kilometers. Figure 1 is a diagram from a NASA web site that illustrates the major reservoirs and the annual fluxes of carbon that move between them. The atmosphere is estimated to hold 800 gigatons of carbon (GtC), plant biomass 550 GtC, soil carbon 2,300 GtC, and fossil carbon 10,000 GtC. Those reservoirs, however, are dwarfed by the oceans' 38,000 GtC. Of those 38000 GtC, 37000 reside in the deep ocean, while 1000 are in surface waters.

The division of ocean waters into "deep" and "surface" categories is not an arbitrary matter of labels. They are distinct subsystems with different characteristics. The surface waters are stirred by wind, waves, and shallow-water currents. They are "well mixed". But the effects of wind and waves fall off rapidly with depth. By 50 meters, only very major storm systems have much effect. Away from polar latitudes, the surface waters are also much warmer than the deep waters. With solar heat deposited at the top, they form a stratified system that floats on the cold and denser waters below. The transition between the two is the thermo cline, a narrow region in which temperature drops rapidly with depth.

The significance of this layering of ocean waters as it applies to the carbon cycle is that the CO₂ content of surface waters is in rough equilibrium with CO₂ in the atmosphere, while that of the deep waters is not. It takes on average from one to two thousand years for water that has been conveyed from the surface to the deep ocean to make its way back to the surface. That means that the bulk of deep ocean waters carry a dissolved CO₂ content that was in equilibrium with the polar atmosphere of one to two thousand years ago -- before the industrial revolution when atmospheric CO₂ levels were much lower.

Gases do not have fixed solubility limits in water. The amount of gas that will dissolve in water (or any liquid) is proportional to the partial pressure of that gas at the gas-liquid interface -- a relationship known as Henry's Law. As the partial pressure of CO₂ in the atmosphere goes up, the amount of CO₂ dissolved in the surface waters goes up with it. As a result, for every ton of CO₂ released into the atmosphere in burning fossil fuels, only about half of it remains there for any length of time. A substantial fraction of it is quickly absorbed into the surface waters of the ocean.

There is a direct relationship between dissolved CO₂ and the pH of water. The anthropogenic CO₂ that has been added to the atmosphere and to the surface waters of the ocean has lowered the water's pH (increased its acidity), but it has barely touched the vast body of deep ocean waters.

If the entire ocean were mixed over a period of just decades to the degree that the surface waters are, then we would have no problem with CO₂ levels or ocean acidification. All the fossil carbon that has been burned since the start of the industrial revolution would have raised atmospheric and ocean CO₂ levels slightly, but not to anything like the extent we have seen. From the pre-industrial 280 PPMv, we might have risen to around 295 PPMv. However we could continue burning coal, oil, and gas at the current rates for another two centuries before hitting the 350 PPMv regarded as the maximum safe level over the long term for avoiding a disruptive degree of climate change. (We're already beyond that now, at 395 PPMv and rising.)

Unfortunately for us, the time scale for mixing of the deep ocean waters is millennia, not decades. Every year, the ocean's thermohaline circulation carries a sizable volume of frigid surface water from polar regions down to the deep ocean basins. A matching volume of deep ocean water is pushed up and begins to mix with the surface waters around the globe. The volumes of water involved are huge -- many times larger than the combined flows of all the world's rivers -- but they amount to less than 0.1% of the total volume of the world's oceans. Thus it takes more than 1000 years to complete one turnover of the ocean waters.

Because the polar surface waters that sink each year have been exposed to higher atmospheric CO₂ levels than the water pushed up elsewhere to replace them, they carry more CO₂. In this manner, the natural thermohaline circulation carries about two GtC to the deep ocean annually -- about a quarter of what burning of fossil fuels and production of cement are currently dumping into the atmosphere. If we were somehow able to stop all such activity tomorrow, CO₂ levels would stop rising and would begin a slow relaxation back toward 295 PPMv. It would take a few thousand years to get there, however.

Direct Injection

Most of the proposed forms of ocean sequestration are methods of bypassing the thermohaline circulation to get CO₂ quickly into the deep ocean waters. They shortcut the process by using direct injection.

Figure 2 illustrates options that have been considered. They differ in how the injection is accomplished, but are otherwise similar. The preferred method depends on local geographic factors. For instance, use of pipelines from the shore is generally more economical than use of ships, but not always an option. It requires the continental shelf to be close to shore, with a significant coastal current at the injection depth to disperse the CO₂ plume. Similarly, the shallower injection options require less energy for compression of CO₂ and would be more economical than deeper injection. However, they are only viable in areas where there is a sinking current around the injection point to carry dissolved CO₂ down to deep ocean basins.

Boosting Ocean Alkalinity

An alternative to direct injection involves boosting the capacity of the ocean surface waters to pull CO₂ from the atmosphere. That avoids the need for carbon capture at point sources, and can be done by increasing the water's alkalinity.

Absorption of CO₂ in a water solution involves a complex balance of equilibrium reactions:

CO₂ (g) + H₂O (l) -- H₂CO₃ -- H⁺ + HCO₃^- -- 2H⁺ + CO₃^-2

When the alkalinity of the solution is increased, as for example by adding caustic soda (NaOH), it reduces the concentration of H⁺ ions and shifts the above equilibrium in the direction that produces more H⁺ (to the right). That reduces the concentration of H₂CO₃, which also shifts the first reaction to the right, pulling in more gaseous CO₂ from the atmosphere. It works out that for every mole of hydroxyl ions added, an additional 0.89 moles of CO₂ are dissolved.

The most efficient means for boosting ocean alkalinity involves dissolving calcium carbonate (CaCO₃). Carbonate-rich minerals, in the form of vast chalk or limestone beds, are the "final resting place" for most of the CO₂ that volcanoes over the eons have spewed into the atmosphere. It is counter-intuitive that dissolving large quantities of them -- we're talking potentially gigatons per year -- could be a way to counter the rise in atmospheric CO₂ levels. Yet it is -- or theoretically could be.

A bare carbonate ion in solution is not a happy camper. With its double negative charge, it really wants to latch onto a free hydrogen ion to become a bicarbonate ion. Consequently, adding carbonate ions to a solution has a similar effect to adding hydroxyl ions. It soaks up H⁺ and allows more CO₂ to be dissolved. The net reaction can be written as:

CO₃^-2 (aq) + H₂O (l) + CO₂ (g) -- 2HCO₃^-

It seems like there ought to be a catch, and indeed there is. In fact there are two. The first is that the reaction is reversible, and if calcium carbonate should precipitate out of the solution, it will end up releasing as much CO₂ back to the atmosphere as dissolving the limestone would absorb. The net reaction is:

Ca⁺²(aq) + 2HCO₃^-(aq) -- CaCO₃(s) + CO₂(g)

The second catch is that added calcium carbonate cannot be dissolved into the the oceans' surface waters to begin with. They are already super-saturated with dissolved CaCO₃ . However, under the high pressures and low temperatures of the deep ocean waters -- below what's known as the carbonate compensation depth -- added CaCO₃ will dissolve. So one way to boost alkalinity is simply to dump massive amounts of crushed chalk or limestone into the deep ocean and allow it to dissolve naturally. When the carbonate-enriched waters begin to reach the surface they will start pulling CO₂ out of the atmosphere³. That would be around 1000 years later, in the normal course of events.

If the natural dissolution of CaCO₃ in deep waters is to be useful within this century, it has to be coupled with a mechanism that brings the carbonate-enriched waters directly to the surface. One can envision a large OTEC system (Ocean Thermal Energy Conversion) with the inlet to its deep riser pipe buried in a heap of crushed carbonate minerals on the sea floor. The system would do triple service: (1) power generation from surface water as a heat source and cold deep water as a heat sink; (2) absorption of CO₂ from the atmosphere when the high alkalinity water brought up is discharged at the surface; and (3) fertilization of surrounding surface waters for ocean farming or fishery enhancement.

Does that sound like a plan? Perhaps. But things are never simple. OTEC, and even the very idea of large scale sequestration of CO₂, face opposition from multiple quarters. We'll step back and look at some of the larger issues around sequestration next week, in part 3.

References

1. For Chapter 8 (Costs and Economic Potential) from IPCC Special Report on CCS -- http://www.ipcc-wg3.de/publications/special-reports/.files-images/SRCCS-Chapter8.pdf

2. For home page of C12 Energy -- http://c12energy.com/

3. One may wonder, if the surface water are already super-saturated and we're bringing up water with yet higher levels of dissolved CaCO₃, won't eh excess CaCO₃ simply precipitate? That would neutralize the alkalinity added when the carbonate was dissolved, and defeat the purpose of the whole operation. That doesn't happen however. At moderate super-saturation levels even higher than found in warm surface waters, CaCO₃ will not spontaneously precipitate. The potential energy barrier that prevents it is similar, conceptually, to the barrier that prevents free oxygen in the atmosphere from spontaneously oxidizing the carbon compounds in plant material.