Technical Overview of Carbon Dioxide Sequestration

TECHNICAL OVERVIEW OF CARBON DIOXIDE SEQUESTRATION
TECHNOLOGIES
R. Jason Gwaltney, MPR Associates, Inc.
320 King Street, Alexandria Virginia, 22314
703-519-0200/703-519-0224, [email protected]
INTRODUCTION
Purpose
This paper investigates the leading methods for carbon sequestration and evaluates them on the bases of cost,
capacity and environmental consequences. Other factors such as geographical constraints and the current
state of technology are also considered. This white paper covers the following post-capture aspects of carbon
sequestration:
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Compression
Transport
Storage
Monitoring, Mitigation and Verification (MM&V)
A complementary paper has been written that addresses Carbon Capture: Technical Overview of Carbon
Dioxide Capture Technologies for Coal-Fired Power Plants [Reference 4].
Background
Atmospheric levels of carbon dioxide (CO2) have increased from a pre-industrial level of 280 parts-per-million
(ppm) [Reference 15] to today’s level of 379 ppm [Reference 2]. The primary source of anthropogenic CO2 is
combustion of fossil fuels. Studies have shown that capture of anthropogenic CO2 and carbon sequestration
could help to stabilize the concentration of atmospheric CO2 [Reference 1].
There are several approaches to sequestering carbon. Some of them are naturally occurring and are already
sequester large amounts of carbon (denoted by asterisks):
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Deep Saline Aquifers
Oil and Gas Reservoirs
Unminable Coal Seams
Direct Injection into Oceans
Terrestrial Sequestration*
Natural Enhancement of Oceans*
It is estimated that terrestrial ecosystems sequester approximately 7 GtCO2/year1 and that the oceans sequester
an additional 7 GtCO2/year [Reference 15]. Over the next 1000 years, the ocean is expected to absorb
approximately 85% of today’s anthropogenic carbon [Reference 15]. Although the level of natural sequestration
1
Billion tons CO2 = 109 tons CO2 = Gigaton CO2 = GtCO2
is impressive, it is no match for the global production of CO2. The 2003 global emissions of CO2 were
approximately 24 GtCO2 of which 6.4 GtCO2 was emitted by the U.S. [References 2 and 14].
Several of these carbon sequestration methods are currently in use today. However, none of these methods are
sequestering a significant amount of carbon compared to the rate of carbon production. The rates of current
levels of sequestration are provided only as a means for comparing the “state of technology”. All of the current
sequestration methods should be considered pilot scale.
Cost and Capacity Estimates
This paper provides costs and capacity estimates for each method of carbon sequestration addressed. Capacity
numbers are in terms of billion tons of CO2 (GtCO2)2. Some sources describe storage capacity in units of
carbon rather than carbon dioxide3 or metric tons instead of U.S. tons4. Therefore caution must be taken
when comparing numbers between sources. Cost estimates are based on the cost for capture, compression,
transportation and storage of CO2. Storage costs are in terms of $ per ton of CO2 avoided. The “CO2 avoided”
rather than just “CO2” takes into account that the energy required for capture, compression, transportation and
storage actually increases the emissions of CO2. Additional details on these cost estimates are in the section
entitled Capture and Transport of Carbon Dioxide.
SEQUESTRATION IN DEEP SALINE AQUIFERS
Description
Deep saline aquifers are porous rock formations filled with saline or brackish water. Sites suitable for injection
are typically located at depths greater than 2,000 ft below the Earth’s surface. Locations for sequestration must
be separated from potable water by impermeable rock and they must be deep enough such that the injected CO2
will remain supercritical. CO2 is stored using three distinct mechanisms: hydrodynamic trapping, solubility
trapping and mineral trapping [Reference 6]. Each of these trapping mechanisms is described in Table 1
below. It is expected that initial containment of CO2 will be due to hydrodynamic trapping; however, over time,
solubility trapping and then mineral trapping are expected to dominate the CO2 containment.
Billion tons CO2 = 109 tons CO2 = Gigaton CO2 = GtCO2
One ton of CO2 equals approximately 3.67 tons of carbon
4 One metric ton equals approximately 1.1 U.S. ton
2
3
Table 1. CO2 Trapping Mechanisms in Deep Saline Aquifers
Experience to Date
Statoil Company is currently sequestering CO2 into a saline formation 1,000 m under the Norwegian North
Sea [Reference 15]. The Utsira formation is porous sandstone with a shale cap rock, capable of storing up to
600 GtCO2 [Reference 5]. Through March of 2005, over 6 million tons of CO25 have been injected. Time-lapse
seismic technology is used to monitor the formation and detect CO2 leakage. No leakage has been detected
through March of 2005.
The Department of Energy is funding a project in Frio, Texas to inject CO2 into a saline aquifer. In 2004,
approximately 1,900 tons of CO26 were injected approximately 1,500 meters underground [Reference 1]. The
goal of the project is to demonstrate the technology and prove that the underground storage of CO2 is safe for
both people and the environment.
Looking Ahead
Deep saline aquifers are located throughout the continental United States as shown in Figure 1. The storage
capacity is estimated to be up to 500 GtCO27; orders of magnitude greater than other forms of geologic storage
[Reference 9]. More research is needed to understand the hydrodynamic, solubility and mineral trapping.
Additional research is needed regarding the caprock strength, to evaluate it before injecting CO2 and to maintain
its strength during injection. There is also a question of whether there is induced seismicity associated with CO2
injection.
Courtesy of Oak Ridge Natl. Lab./
US Dept. Of Energy
Courtesy ORNL/DOE and USGS
6 million tons CO2 = 0.006 GtCO2
1,900 tons of CO2 = 0.0000019 GtCO2
7 500 GtCO is equivalent to approximately 80 years of the current U.S. anthropogenic carbon emissions
2
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Figure 1. Saline Aquifers in the United States [Reference 15]
There is a need for more studies on specific locations that are candidates for injection. Some aquifers are too
deep, while others are too shallow. Models need to be developed to understand how the CO2 will behave in the
candidate sites.
The cost of deep saline injection is higher than other forms of sequestration. It does not contain any of the
added economic benefits associated with other methods of sequestration (e.g. Oil and Gas Reservoirs). The
approximate cost for the capture, transportation and storage of CO2 into saline aquifers is $21 per ton CO2
avoided [Reference 8].
SEQUESTRATION IN OIL AND GAS RESERVOIRS
Description
There are two basic methods for sequestering CO2 in oil and gas reservoirs: injection into depleted reserves
and enhanced oil recovery. Injection into depleted reserves is just that; the injection of CO2 into oil and gas
reservoirs that have been exhausted. This method of sequestration is very similar to deep saline aquifers with
regard to risk and cost. The second method for sequestration in oil and gas reservoirs is enhanced oil recovery
(EOR). Standard extraction methods for oil and gas reservoirs only recover the first 20-40% of the oil and gas
[Reference 11]. Any additional extraction of oil requires the use of EOR. There are several methods to perform
EOR, only one of which uses CO2:
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Chemical Flooding
CO2 Injection
Hydrocarbon Injection
Thermal Recovery
CO2 injection is a recovery method in which CO2 is injected into the reservoir at a supercritical state. There are
actually two different mechanisms by which CO2 injection aids in the extraction of oil: miscible and immiscible
extraction. Miscible extraction is when the CO2 mixes thoroughly with the oil and lowers its viscosity.
Immiscible extraction is when the oil and CO2 remain separate; however the CO2 causes the oil to swell and
improves mobility. All large scale EOR projects in the U.S. use miscible extraction [Reference 11].
Experience to Date
By far, industry has the most experience with carbon sequestration through the use of EOR. This method
accounts for over 80% of the commercial CO2 use in the U.S. [Reference 15]. In 1998, there were 74 active
EOR sites in the U.S. using CO2 to aid in extraction. These sites accounted for approximately 3% of U.S.
production of oil. This effectively sequesters 3.3 million tons of CO28 per year [Reference 11]. However, most
of these facilities are using naturally occurring carbon dioxide from underground deposits, and therefore are not
helping to reduce anthropogenic carbon emissions. Even if anthropogenic carbon emissions were used for EOR
instead of naturally occurring CO2, the impact on overall carbon sequestration would be minimal.
There is a project in the Weyburn oil field in Saskatchewan, Canada to sequester anthropogenic carbon via
EOR. The Dakota Gasification Co. which operates the Great Plains Synfuels Plant in Beulah, North Dakota
constructed a 325 km pipeline between Beulah and Weyburn. CO2 is transported to Weyburn where it is used
to increase oil recovery. Over the life of the project an estimated 20 million tons of CO29 will be sequestered
[Reference 5].
Looking Ahead
In comparison to other methods, the technology for EOR and injection into depleted reservoirs is very
advanced. The long-term storage integrity of the reservoir is high as long as it is not over-pressurized. This
method of sequestration also has economic benefits; the estimated cost is $4 per ton of CO2 avoided when used
for EOR [Reference 8]. When injected into depleted oil and gas reservoirs, the cost is approximately $22-23
per ton of CO2 avoided, which is nearly the same as injection into saline aquifers [Reference 8]. Figure 2 shows
the locations of gas-producing areas in the U.S.
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9
3.3 million tons of CO2 per year = 0.0033 GtCO2 per year
20 million tons of CO2 = 0.02 GtCO2
Courtesy ORNL/DOE and USGS
Figure 2. Gas-Producing Areas in the United States [Reference 15]
Although EOR using CO2 is presently occurring in the U.S., it is not sequestering anthropogenic CO2. Oil
companies use the cheapest source of CO2 available: naturally occurring CO2 from underground deposits.
Without an economic benefit to carbon sequestration, EOR using anthropogenic carbon is unlikely to occur in
the short-term. The amount of CO2 capable of being sequestered through EOR is a small fraction of the total
estimated CO2 sequestration capacity of 40-50 GtCO2 in oil reservoirs and 80-100 GtCO2 in gas reservoirs
[Reference 9].
SEQUESTRATION IN UNMINEABLE COAL SEAMS
Description
Coal beds contain methane that has been adsorbed onto the pore surfaces of the coal. Unmineable coal seams
contain coal bed methane, but are either too deep or too narrow to be mined for the coal. By drilling down and
tapping into the beds, this coal bed methane can be extracted and used as a fuel. The extraction process can be
augmented through the use of CO2. The coal preferentially desorbs the methane and adsorbs the CO2. In this
manner, the CO2 is sequestering CO2 and facilitating the extraction of methane. For every molecule of methane
desorbed by the coal, 2-3 molecules of CO2 will be adsorbed [Reference 2]. Producing methane will result in
the burning of methane and an increase in CO2 emissions; however, for simplicity it is assumed that methane
production via coal bed methane will offset other existing forms of production. Furthermore, it is important that
the released methane be trapped as effectively as possible. Methane is over 20 times more effective than CO2 as
a greenhouse gas.
It should be noted that CO2 is not the only gas that can be used to extract the coal bed methane. In fact, nitrogen
is more effective at releasing the coal bed methane than CO2. There has been consideration for using a direct
flue gas mixture; however, this substantially reduces the sequestration capacity. Figure 3 shows a notional
graph of the effect of CO2:N2 ratio on the methane removal effectiveness and carbon sequestration effectiveness.
Mixtures of 100 % CO2 provide moderate levels of gas extraction while maximizing the sequestration potential.
Figure 3. Notional Effects of CO2:N2 Ratio on Methane Removal and
Carbon Sequestration Effectiveness [Based on Data from Reference 1]
Experience to Date
There are three test platforms in North America for the sequestration of CO2 into unmineable coal beds: the
San Juan Basin in New Mexico, Alberta Canada and the Central Appalachian Basin in Virginia. The current
plan for the San Juan Basin is to inject 280,000 tons of CO2 over a six-year period [Reference 2]. The Alberta
study began in July 1999 and is headed by the Alberta Research Council, Inc. (ARC). There are several phases
to the project, including preliminary modeling to determine feasibility, small-scale pilot testing and full-scale
pilot testing. The ARC is also evaluating the steps required to prepare the gas for injection, including flue gas
treatment and compression. Consol Energy plans to inject 26,000 tons of CO210 over 1 year into an unmineable
coal seam in the Central Appalachian Basin starting in 2005 [Reference 2].
There are countless other studies that attempt to model gas recovery from coal bed methane. For example,
the Dutch performed a program called the Feasibility of Carbon Dioxide Disposal and Coal Bed Methane
Production in the Netherlands [Reference 10]. The study indicates that theoretically, there is sufficient coal
10
26,000 tons of CO2 = 0.000026 GtCO2
bed methane in the Netherlands to meet the national energy demand for 20 years. Furthermore, the theorized
storage capacity for CO2 in unmineable coal beds is sufficient to store all of the Netherlands’ CO2 emissions for
the next 40 years.
Looking Ahead
The storage capacity for unmineable coal seams is estimated at 100 GtCO211; however, nearly 50% of this
capacity is located in Alaska [Reference 2]. One major hurdle to enhanced coal bed methane recovery is
the phenomena of coal swelling. When CO2 is injected into the coal it has a tendency to swell and reduce
permeability. This reduces both the flow of CO2 into the coal seam and the outlet flow of methane. Nitrogen
does not cause coal swelling, and given its superior ability to remove methane, its use is preferred over CO2.
There will probably need to be economic benefit to sequestering CO2 for CO2 injection to be selected over
nitrogen.
The cost of CO2 sequestration via unmineable coal seams is strongly dependent upon natural gas prices.
Sequestration costs decrease by approximately $1/ton for each $1/MMBTU increase in the natural gas price
[Reference 1]. At today’s price for natural gas (approximately $7/MMBTU), the cost of sequestration is
approximately $6-8 per ton CO2 avoided [References 1 and 8].
OCEAN SEQUESTRATION VIA DIRECT INJECTION
Description
At first glance, this process is as simple as the name implies: liquified CO2 is injected deep into the ocean.
At depths greater than 1000 m, the CO2 will be at approximately the same density as water and will remain
suspended. Upon further inspection, there are actually several different approaches to injecting CO2 as shown
in Table 2:
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100 GtCO2 is equivalent to approximately 15 years of the current U.S. anthropogenic carbon emissions
Table 2. Injection Methods for Ocean Sequestration [References 12 and 13]
The uncertainty surrounding CO2 injection is focused on three distinct questions:
1.
How does the CO2 behave once injected?
2.
How does the CO2 affect oceanic organisms?
3.
How much of the CO2 remains sequestered over time?
Injected CO2 may be immediately dissolved into the water. Once dissolved, the injected CO2 will be subjected
to underwater currents. Changes in temperature (and therefore density) will allow the water to rise or fall.
Fluid models of underwater currents have made attempts to estimate the spread of the CO2 once injected;
however, modeling underwater currents and diffusion of CO2 is very difficult. If ultra-cold CO2 is injected,
it will form a solid lake with an ice-like layer of CO2 hydrate as its exterior. CO2 hydrates are compounds
formed by CO2 and minerals in the water at low temperature and high pressure, which makes studying hydrate
formation very complex. Formation of carbon hydrates may add to the stability of injected CO2. Blocks of dry
ice could also be dropped into the ocean, but this is not practical due the large amount of energy required to
convert the CO2 from liquid to solid state.
The interaction of CO2 with oceanic organisms is of paramount interest to researchers. Higher concentrations
of CO2 in water can already be observed; however the effects of pH changes may be the most significant. The
pH level of the ocean is already 0.1 pH units lower than in the 19th century [Reference 12]. It is estimated that
changes of pH in excess of 0.2 pH units may have a detectable biological impact [Reference 13].
The sequestration of CO2 in the oceans is a temporary, albeit long-term, form of storage. Over a period of
centuries, the CO2 will gradually leak out into the atmosphere at rates between 0-0.5% per year [Reference 13].
Modeling suggests that >75% of carbon injected at 3000 m is sequestered for more than 500 years [Reference
13]. Regardless, the oceans have the potential to sequester enormous amounts of carbon and there are still
substantial benefits for the temporary storage of CO2.
Experience to Date
To date, there has been strong public resistance to large-scale injection of liquified CO2 into the ocean. In the
late 90’s the National Energy Laboratory of Hawaii Authority (NELHA) attempted to inject CO2 into the ocean
off the coast of the Big Island. From a scientific perspective, the location was suitable for the experiment.
However, local opposition derailed the project and to date, no large scale testing has been performed. The
technology to inject the CO2 is not complex; however, it is unclear how the CO2 will behave once injected.
Looking Ahead
By far, ocean sequestration has the largest environmental and sociological roadblocks of any method.
Temporary storage of CO2 means that over a period of centuries, the CO2 will eventually be released back into
the atmosphere. Additionally, this sequestration method must be paired with stationary CO2 sources near the
ocean. These sources only account for 15 to 20% of the anthropogenic CO2 emissions in the U.S. [Reference
15].
The cost of carbon sequestration via a pipeline is $24 per ton CO2 avoided and for a tanker is $39 per ton CO2
avoided [Reference 8]. The depth of the injection also has an effect on price; the deeper the injection, the higher
the cost. The cost penalty for deep injections may be offset by the reduction in leakage back to the atmosphere
in comparison to shallow injection. This will require that there be some economic value to the temporary
sequestration of CO2.
TERRESTRIAL SEQUESTRATION
Description
Terrestrial sequestration is part of the natural CO2 cycle on Earth whereby trees and plants absorb CO2 via
photosynthesis. It is estimated that the net absorption of CO2 by the terrestrial biosphere is approximately
7 GtCO2 per year [Reference 15]. Storage of CO2 in soils, plants and trees is actually a temporary form of
storage. Events such as fire, insect infestations and changes in land-use allow the release of the stored carbon
dioxide. Basically, whenever the plants die and decompose, they release their carbon. The exception to this is
the creation of wood products that have a long-term use (e.g. construction materials).
Terrestrial sequestration is the development and maintenance of ecosystems such that carbon storage is
maximized. Preventing emissions of carbon is just as important as storing additional carbon; therefore some
terrestrial sequestration efforts focus on maintaining levels of sequestration in certain ecosystems rather than
developing additional growth. Methods for increasing terrestrial sequestration include:
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Increasing photosynthetic carbon fixation
Reducing decomposition of organic matter
Reversing land-use changes that contribute to global emissions
Creating energy offsets through biofuels or other products (e.g. wood products)
In addition to trees and plants, soils contain a great deal of carbon. Approximately 75% of the terrestrial carbon
is currently contained in soils [Reference 15]. It is important to understand what mechanisms increase the
carbon uptake of the soils and how carbon storage can be maximized. Soil erosion releases the carbon stored
within soils. Agricultural practices such as tilling greatly reduce the amount of carbon stored in the soil.
Experience to Date
For decades, the U.S. has been managing its forests with a focus on sustaining timber yields. Recently, more
attention has been given to forest management with regard to forest fires.
Countless projects have been demonstrated which improve carbon storage, including: no-till farming, erosion
prevention, planting forests, slowing deforestation and wetland preservation.
Looking Ahead
It is estimated that the net storage of CO2 by terrestrial ecosystems is 7 Gt/CO2 per year (globally), but could
be over 20 Gt/CO212 per year [Reference 15]. There is some upper limit on carbon storage in the terrestrial
ecosystem, but it is unclear what that limit is. Further research is needed to:
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Develop methods for measuring carbon uptake in a given ecosystem
Estimate the potential for total carbon storage in a given ecosystem
Estimate the length of time over which the increased uptake could be sustained
Develop methods for monitoring the uptake in an ecosystem
OCEAN SEQUESTRATION VIA NATURAL ENHANCEMENT
Description
CO2 concentrations in the atmosphere and the ocean attempt to maintain equilibrium. Currently, there is an
excess of CO2 in the atmosphere and the ocean is absorbing CO2 faster than it is releasing it to the atmosphere.
Once dissolved into the ocean, a portion of the CO2 is consumed by ocean organisms, such as phytoplankton.
Eventually the phytoplankton are either eaten by larger organisms, or die and sink to deeper waters. Through
this process of CO2 absorption, consumption and sinking, the ocean acts as a biological pump for CO2. It is
estimated that this natural pathway will eventually remove 85% of today’s anthropogenic carbon [Reference
15]. Ocean sequestration via natural enhancement increases the rate at which this natural pump operates. This
is accomplished through the injection of micronutrients into the ocean. For example, phytoplankton require
ammonia, phosphorous and iron to grow. Some areas of the ocean contain sufficient levels of ammonia and
phosphorous, but are lacking in iron. When iron is added to these areas, the phytoplankton flourish and there is
a temporary, but substantial uptake in CO2. There is a distinct advantage to adding iron over the other nutrients
(e. g. phosphorous). Provided below are the ratios of nutrients consumed by the phytoplankton, which include
carbon (C), nitrogen (N), and phosphorous (P) [Reference 7]. The ratio of carbon to iron consumption is quite
large, such that one ton of iron will sequester at least 23,000 tons of carbon (or 84,000 tons of CO2).
106 C : 16 N : 1 P : (0.001 - 0.005) Fe
12
Global emissions of CO2 are approximately 24 GtCO2 per year [Reference 2]
Experience to Date
A series of iron fertilization experiments have been performed over the last few years [Reference 3]:
Table 3. Ocean Fertilization Experiments
In these experiments iron was added to areas deficient in iron, but containing excess nitrogen and phosphorous.
Within hours of the addition of iron, the phytoplankton growth increased dramatically and the partial pressure of
CO2 above the water surface decreased [Reference 15]. Several patents have already been registered for ocean
fertilization, including details such as a spiral pattern of fertilization.
Looking Ahead
This method of carbon sequestration has environmental concerns similar to those for direct injection. Questions
regarding the effects of increased carbon uptake on the overall ecosystem are still unanswered. Natural
enhancement should be thought of as a temporary method for CO2 storage. Eventually the captured carbon will
be released into the atmosphere.
Given that this is an indirect method of sequestration, there are no geographical limitations with regard to
anthropogenic sources of CO2. However, to be considered a candidate for natural enhancement, the site will
have to contain high levels of nutrients, but low chlorophyll. There are three areas that have been identified as
candidate sites for natural enhancement [Reference 7]:
4. Eastern Equatorial Pacific
5. NE Subarctic Pacific
6. Southern Ocean
The Southern Ocean represents the largest potential source for carbon sequestration. It is estimated that it could
sequester 180 to 550 GtCO213 over 100 years of fertilization [Reference 7]. However, through global ocean
currents, waters in Southern Ocean reemerge in the tropics. Therefore increased growth of phytoplankton in the
Southern Ocean may reduce growth in the tropics by 30 to 70% [Reference 7]. There are other drawbacks to
ocean fertilization. Dimethyl sulfide production by phytoplankton increases during fertilization, and N20 may
be produced as a result of high nitrogen content in low oxygen areas. N20 is approximately 250 times more
powerful as a greenhouse gas than CO2. There are many issues that need to be resolved before large-scale ocean
fertilization is utilized.
13 At
the current carbon emission rates, the world will produce 2,400 GtCO2 over the next 100 years [Reference 2]
CAPTURE, COMPRESSION AND TRANSPORT OF CARBON DIOXIDE
As discussed in the introduction, the scope of this white paper is limited to the sequestration, compression,
transport and storage of CO2. A complementary paper has been written that addresses Carbon Capture:
Technical Overview of Carbon Dioxide Capture Technologies for Coal-Fired Power Plants [Reference 4]. Note
that all of the direct methods for sequestering carbon require the transport of CO2 from the power plant to the
sequestration site (e.g. wellhead). CO2 is most economically shipped when liquified. In general, it is expected
that the CO2 will be pressurized to approximately 2000 psi and have a 500 psi pressure drop due to piping losses
[Reference 1 and 8]. All of the sequestration costs provided in the previous sections have accounted for the
costs of capture, compression, transportation and storage. It is important to consider all of these costs when
comparing two diverse methods such as Ocean Sequestration through Direct Injection and Ocean Sequestration
through Natural Enhancement. Both methods sequester carbon in the ocean, but direct injection requires the
capture, compression and transport of liquified CO2. The following assumptions were used to calculate the
capture, compression and transportation costs:
Table 4. Parameters for Carbon Sequestration Cost-Estimates [Reference 8]
POST-INJECTION MONITORING, MITIGATION AND VERIFICATION
Without exception, each method of carbon sequestration requires careful monitoring, both to ensure safety
and to verify the storage effectiveness. The developing industry of carbon sequestration has coined the term
MM&V to describe this step in carbon sequestration. MM&V is the Post-Injection Monitoring, Mitigation and
Verification of sequestered carbon. The objectives of MM&V are to measure the amount of sequestered carbon,
monitor the area for leaks or problems that may lead to leaks, mitigate leaks or damage to the host environment
and verify that the sequestered carbon is interacting with its host environment in a manner that was expected.
The following list highlights several methods that can be used to monitor sequestered carbon:
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Subsurface - Monitors the sequestered CO2 to detect leakage
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Ultrasonic testing
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Observation wells
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3D seismic
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Cross-well seismic
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Soils - Monitors the carbon uptake of the soil above the sequestration area
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Soil monitors
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Leakage pathways into the atmosphere
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Above-ground - Monitors the carbon uptake of vegetation
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Vegetation monitoring
Modeling of sequestered carbon is a key element to MM&V. When measuring the actual conditions of stored
carbon, it is important to be able to compare the findings to predicted values.
There are two existing projects that highlight unique methods for monitoring carbon sequestration: Ultrasonic
testing and M3DADI. Ultrasonic testing has been used in the Sleipner field in the Norwegian North Sea.
Results to date indicate that no leakage has occurred. It is estimated that using 3D seismic methods, CO2
deposits as small as 2,750 tons can be detected [Reference 5]. M3DADI is a Multi-Spectral 3-Dimensional
Aerial Digital Imagery camera being developed by the DOE. It uses two cameras and a laser attached to a plane
to map out forest vegetation and estimate carbon storage [Reference 2]. The technology is currently being
validated against conventional methods for measuring carbon storage in forests.
Accurate verification of carbon storage will also be an essential aspect to the economy of carbon storage. Both
enhanced oil recovery and coal bed methane recovery have the potential to sequester carbon, but are lacking
the economic motivation. Without a value associated with the sequestration of carbon, easy opportunities
for sequestration will be missed. It is also important to assign an economic value to the temporary storage of
carbon. Some methods are guaranteed to leak some percentage of the sequestered carbon over the next few
centuries. Accurate verification through MM&V will enable trading of carbon storage credits.
CONCLUSIONS
The long-term feasibility of any carbon sequestration method will be governed by its cost, capacity, technical
feasibility, and environmental consequences. Each of the sequestration methods in this paper has its own
unique opportunities and challenges. Table 5 provides a comparison of the six different sequestration methods
discussed in this paper.
Terrestrial sequestration and oil and gas reservoir sequestration are the most likely to be the forerunners because
industry has the most experience with these methods. On the other hand, ocean sequestration, both by direct
injection and natural enhancement, presents some of the largest risks. There is a higher rate of CO2 leakage
back into the atmosphere and greater uncertainty with regard to environmental impact. Carbon sequestration in
oceans is a technology that will have to be proven more thoroughly before implementation.
Carbon sequestration is in its infancy. The test programs underway have the capacity to sequester less than
0.1% of the annual global production of CO2. The infrastructure required to sequester approximately 10% of
the annual U. S. production of CO2 is estimated to be of the same order as the existing natural gas production
and distribution infrastructure. A strong commitment from the public and significant economic incentives, that
are currently absent, will be required to make carbon sequestration a reality.
Acceptance of carbon sequestration will require the development of reliable Monitoring, Mitigation and
Verification (MM&V) of sequestered carbon. Reliable MM&V goes hand-in-hand with public acceptance and
placing an economic value on carbon sequestration. There must be reliable data that ensure the CO2 is being
effectively sequestered.
Table 5. Comparison of Sequestration Options
1 There is approximately 55 GtCO2 of capacity in the continental U.S. with an additional 45 GtCO2 of capacity
in Alaska.
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14. Caldeira, K., Herzog, H.J., and Wickett, M.E. Predicting and Evaluating the Effectiveness of Ocean Carbon Sequestration by Direct Injection.
First National Conference on Carbon Sequestration, Washington, D.C. May 14-17, 2001.
15. Annual Energy Review 2003, Table 12: Environmental Indicators. Energy Information Administration, 2003.
16. Department of Energy. Carbon Sequestration Research and Development. Office of Science and Office of Fossil Energy, December 1999.
(Footnotes)
1
Billion tons CO2 = 109 tons CO2 = Gigaton CO2 = GtCO2
2
Billion tons CO2 = 109 tons CO2 = Gigaton CO2 = GtCO2
3
One ton of CO2 equals approximately 3.67 tons of carbon
4
One metric ton equals approximately 1.1 U.S. ton
5
6 million tons CO2 = 0.006 GtCO2
6
1,900 tons of CO2 = 0.0000019 GtCO2
7
500 GtCO2 is equivalent to approximately 80 years of the current U.S. anthropogenic carbon emissions
8
3.3 million tons of CO2 per year = 0.0033 GtCO2 per year
9
20 million tons of CO2 = 0.02 GtCO2
10
26,000 tons of CO2 = 0.000026 GtCO2
11
100 GtCO2 is equivalent to approximately 15 years of the current U.S. anthropogenic carbon emissions
12
Global emissions of CO2 are approximately 24 GtCO2 per year [Reference 2]
13
At the current carbon emission rates, the world will produce 2,400 GtCO2 over the next 100 years [Reference 2]