The Carbon Dioxide Dilemma Promising Technologies and Policies (2003) / Chapter Skim
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Sequestration in Geologic Formations
Sequestration in Geologic Formations
Pages 15-40
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Sequestration in Geologic Formations
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From page 17... ...
We do have some technologies already in place that are capable of injecting CO2 into geologic formations. The engineering issues associated with oil and gas recovery and sequestration are closely linked, and a considerable body of engineering expertise already exists.
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It is also true that options for recovering natural gas have not been as well explored as options for recovering oil. If CO2 were abundantly available, it would be much more widely used for oil recovery because it is a very efficient agent for recovering oil.
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About 5 percent of the natural gas produced in the United States is methane that comes from coal beds. The fact that methane is found in coal is well established and has been a problem for coal miners.
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Many are not oil and gas reservoirs but contain salt water, and these porous formations could be used to sequester CO2. The big difference between an oil and gas reservoir and an aquifer is that, in an oil and gas reservoir, we know there is a seal a geologic formation with some combination of impermeable rocks that traps the oil or gas.
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We already deal with related issues in large-scale engineering systems. For example, we transport natural gas around the country in large pipelines that present fire and explosion hazards that do not exist for CO2.
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Capture and storage are already used in the food processing industry, so we know it can work on that scale, but cost will remain a big issue. We are looking to find a cost-effective, verifiable, viable way to store at least a million tons of CO2 in reservoirs.
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The CO2 Capture Project is taking a distinctive integrated approach to the use of CO2 to recover natural gas and oil and, hence, to stretch our natural reserves. The project is exploring the possibilities in the context of real applications specific fields and operations where we would like to capture and store CO2.
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This technology is already in use in the food processing industry, which uses an amine absorption process to scrub CO2 from flue gas and compress it to make it available for geologic storage. The second method is called precombustion decarbonization, which takes fossil fuel, reforms it to make hydrogen and CO2, compressing and storing the CO2 and using the hydrogen to generate power and heat.
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Now we may be talking about periods of hundreds, perhaps thousands, of years. In the end, we are looking for geological storage sites and technologies that will be acceptable to the public and to regulatory agencies and that will pose the fewest safety and environmental risks.
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The CO2 Capture Project has adopted a whole-risk assessment approach, using safety assessment methodologies and establishing risk assessment frameworks important for public perceptions and public involvement. Monitoring is a major component of risk assessment, and we have monitors in place to tell us the effects of injection properties on reservoirs and cap rocks.
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28 THE CARBON DIOXIDE DILEMMA In 2003, we'll be planning how we will demonstrate the technology and determine if it will be appropriate at scale. Our objective is then to make that technology available and put it into general use throughout the industry.
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As the discussion broadened to include more interaction with industry, environmental organizations, nongovernmental organizations, and other stakeholders, some very pressing issues have emerged. The first of these is that we will need robust performance requirements to enable us to decide whether a site is suitable or not for CO2 storage.
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But a number of studies have shown otherwise, so we decided to study it more, in hopes of squeezing out the last bits of natural gas from a depleted reservoir. Natural gas reservoirs are also very attractive targets for CO2 sequestration because they offer known containment and proven isolation for gas in the subsurface geologic environment.
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Early simulations were highly conceptualized and idealized in many ways. In the next phase, simulations showed what would happen in a real reservoir engineering-type well pattern, the so-called five-spot pattern the typical way we inject CO2 into a reservoir from which we also want to produce natural gas (Plate 2~.
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We also tried electromagnetic methods, which complement seismic technologies by transmitting information about the saturation of water in the pore bases, a very important piece of information. We also looked at other monitoring technologies, such as high-resolution, single-well seismic monitoring, and we added a suite of tracers so we could begin looking at issues of solubility and mineral trapping (Figure 3~.
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The presence of multiple fluid phases, particularly the presence of free gas, makes it extremely difficult to track CO2. We are working on methodology for combining multiple geophysical techniques, such as high-resolution electromagnetic and seismic imaging technologies to tell us something about the saturation of CO2 in the formation (Plate 3~.
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Figure 4 shows how multiphase flow, buoyancy forces, and reservoir influence storage capacity heterogeneity. In Plate 4, the subsurface is assumed to be uniform, with no buoyancy forces.
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FICURE 4 Scbem~c doming sbowing bow mulUpb~se Cow, buoyancy, and reservoir be~rogenei~ influence Forage c~ci~ of ~ geologic ~rm~on. Source: Edited Mom Doupb~ et Hi., 2001.
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After 20 years, when we stop injecting, the CO2 concentrations actually start to decrease. Thus, after CO2 injections stop, factors such as buoyancy forces continue to move CO2 out of the system and upwards, until it either finds a stable trap or returns to the surface.
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The targets are permeable formations with sufficiently thick reservoirs and thick lowpermeability cap rocks. Detailed studies are being done of the Frio formation in Texas, a structure that underlies the Houston area and covers the entire Gulf Coast.
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One example of performancebased requirements is in the nuclear waste storage industry, which has very basic requirements for storing nuclear waste that limits the permissible radiation dosage the public might receive. To assess the safety of a nuclear waste storage site, engineers work backwards from these requirements through an elaborate set of models that tell them how to design the waste canister and other aspects of the site.
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From page 39... ...
2001. Process modeling of CO2 injection into natural gas reservoirs for carbon sequestration and enhanced gas recovery.
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