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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report 3 Advanced Subsurface Technologies To date, deep, long-term subsurface storage of CO2 has been demonstrated in conventional reservoir formation rocks (e.g., in depleted reservoirs). The workshop sessions on advanced subsurface technologies sought to broaden the menu of options beyond demonstrated techniques and identify novel ways to manage carbon utilizing the properties of the subsurface environment. The unique characteristics of deep environments can potentially be exploited for the following purposes: To store liquid CO2 under pressure with surety (in some cases as a liquid), To effect “permanent” chemical or biological subsurface sequestration of CO2, To convert CO2 to useful products, and To ensure environmental security. The discussion during the sessions accordingly focused on the following questions and challenges: Where do such environments exist? What engineering means could be employed to create or access such environments? What technologies would need to be developed to achieve exploitation? The subgroup discussed many potential areas of research and selected ideas in the following categories: Unconventional CO2 storage formations; Increased use of CO2 in enhanced oil recovery; Microbial hydrogen generation from fossil fuels at depth, with subsurface fuel cell coupling; and Deep-sea contained storage. UNCONVENTIONAL CO2 STORAGE FORMATIONS Deep, long-term underground storage of CO2 is being conducted today in various parts of the world in depleted reservoirs. The formations at these reservoirs essentially represent conventional reservoir formation rocks that have been well studied. To meet the potentially dramatic increase in demand for storage of CO2 in areas that may not have conventional storage formations available, unconventional formations must be explored as alternatives to the costly transport of CO2 over very long distances. Concepts of operation that may be used to stimulate thinking about this type of storage include the injection of CO2 into formations with particularly favorable containment properties. When injected into sandstone formations derived from basalt at depths of about 2 km, CO2 is expected to react with the chemical composition of the basalt and to form
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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report carbonate precipitates, fixing the CO2 at depth. Studies on the global availability and capacity of such formations overlain with competent, impermeable formations will be required. Modeling and experiments on CO2 interactions with basalt should point to the level of benefit that could be derived from exploiting these formations for storage. CO2 could potentially be injected into deep (greater than roughly 3 km) ocean sediments at depths where the pressure/temperature regime will result in a stabilized CO2 with a density greater than that of water. This would essentially isolate the CO2 from seawater to inhibit chemical interactions. Studies of the global availability of such environments and of their porosity, permeability, and capacity would be of specific interest. CO2 could also potentially be injected into subsea tectonic spreading regions, which are highly active geothermally, creating unique geohydrologic activity. Seawater flows downward into the sediment at distances of many meters to kilometers away from the spreading area, then turns toward the region at depth and is eventually heated to high temperatures and expelled back to the sea. The conditions surrounding these regions have many advantages for fixing CO2 in that the reaction kinetics are fast because of increasing heat and pressure in the direction of flow. It is theorized that CO2 could be injected into the geohydrologic flow field away from the spreading region and entrained in that flow. This would result in the development of several carbonate species (magnesite, magnesium carbonate, dolomite, and calcite) as the combined flow of seawater and CO2 is heated, pressurized, and released back to the sea. Another possibility is Arctic hydrate storage of CO2 below the permafrost layers in regions where methane and other gas hydrates form. The injected CO2 would form CO2 hydrates that would reside in the pore space of the host rock, with the permafrost layer above it serving, in effect, as the cap rock of a newly created CO2 reservoir. Research Areas Opportunities may exist in unconventional storage formations for utilizing the chemistry, temperature, and pressure (depth) to improve the long-term stability of sequestered CO2 through mineralization, precipitation, and other stabilizing reactions. Possible research concepts include the following: Characterization of promising, previously unstudied porous rock mass formations from a storage media perspective. This investigation would include examining porosity, permeability, capacity, and chemical composition. Identification of regional and global locations of favorable formations. Investigation, through modeling and experimental work, of the nature of rock/CO2 fluid interactions in various rock types over short and long periods. The goal would be to determine beneficial interactions that may occur in basalt or sandstones derived from basalt. Assessment of containment issues such as interactions that may occur in surrounding and overlying rock types and performance of typical rock mass characterization. Accumulation and analysis of existing data on CO2 storage. This activity should include examination of natural storage areas as well as engineered storage areas.
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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report Specifically, experiences with injection and fate of CO2 in enhanced oil recovery should be studied. Examination of the characteristics of geologic formations that now hold and have held CO2 over geologically significant time scales. INCREASED USE OF CO2 IN ENHANCED OIL RECOVERY Some tens of millions of tons of CO2 are currently used for enhanced oil recovery (EOR), but CO2 is not available in many oil fields where it could be used. A relatively concentrated CO2 stream can be generated in integrated gasification combined cycle (IGCC) coal-burning power plants. An economic synergy might be achieved if new IGCC plants could be located near oil fields that could use the concentrated CO2 output. Additionally, regulations could be established to create incentives for the use of CO2 in enhanced oil recovery and to discourage the use of naturally stored CO2 in ways that ultimately vent it to the surface. Research Areas The current practice of using enhanced oil recovery indicates that CO2 disposal could be applied almost immediately. This process would result in immediate incremental increases in CO2 sequestration, provided that CO2 from energy-producing plants can be efficiently delivered to the oil fields. Oil reservoirs could be simultaneously filled with CO2 in a long-term storage scenario as oil is removed from the reservoir. This would avoid the near-certain loss of valuable storage space when the oil reservoir is completely depleted before it is ever reengaged to serve as a storage medium. Possible research concepts include the following: Identifying economical means of transporting CO2 from energy-producing plants to the oil fields and quantifying costs in rough orders of magnitude; Identifying additional infrastructure requirements and costs to employ EOR on many more reservoirs than those that current practice uses; Identifying the potential for CO2 consumption through EOR if EOR were used in various percentages of all oil production worldwide; and Considering credit and penalty schemes to create incentives for using CO2 for this purpose. MICROBIAL HYDROGEN GENERATION FROM FOSSIL FUELS AT DEPTH, WITH SUBSURFACE FUEL CELL COUPLING The objective of microbial hydrogen generation from fossil fuels at depth, with subsurface fuel cell coupling, is to use microorganisms to degrade fossil fuels and couple the generation of hydrogen from this process with a fuel cell or other device to capture energy and prevent the formation of CO2. It has been known for several decades that certain microorganisms under anaerobic conditions degrade hydrocarbons. For one microbial degradation scenario, the process is inhibited by low concentrations of H2, which is a product of the microbial hydrocarbon degradation. The premise of this technology concept is to
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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report harvest the low concentrations of aqueous H2 using a novel fuel cell or to apply other novel technologies to obtain energy from this microbial degradation without generation of CO2. Microorganisms of the family Geobacteriaceae have been shown to oxidize a variety of aromatic hydrocarbons with the reduction of Fe(III). When Fe(III) was replaced with a graphite electrode as the electron acceptor, current production with organic oxidation was achieved.1 It may be possible to harvest energy from the hydrocarbon oxidation using an electrode configuration. In situ application of this technology would be highly desirable from an environmental standpoint, because energy generation would occur in the subsurface, minimizing the environmental impacts of mining the fossil fuel, bringing the material to the surface, and processing it at existing power plants. Also, no CO2 would be produced during the energy generation process using the proposed microbial technology. If such a process could be effectively developed, it could play an important role in limiting CO2 levels in the atmosphere. Research Areas Because of the novelty of this process, considerable basic research and substantial process development are required as well as new energy-capturing technologies. Engineering design and pilot testing of the proposed microbial technology are likely only with major breakthroughs in microbiology as well as in energy-capturing technology. Major research needs include the following: Identification and culture of microorganisms capable of rapid fossil fuel degradation; Optimization of conditions for rapid fossil fuel biodegradation to enhance the kinetics of the reactions; Optimization of technology to rubbilize coal in the subsurface, maximizing surface areas for reactions; and Development of a fuel cell or an electron-accepting device capable of effectively harvesting extremely low concentrations of H2 or other forms of energy from aqueous solutions. DEEP-SEA CONTAINED STORAGE The idea behind deep-sea contained storage (DSCS) is that CO2 is piped under pressure to a depth greater than about 3000 m in the ocean, a depth at which the CO2 is a liquid that is denser than seawater. The CO2 is then stored in very large containers on the ocean floor. The first concept for a storage container is a 100-m diameter bladder, perhaps 1 km long. The container could be compartmentalized to minimize CO2 release in the event of container failure. Other containment options could be considered. 1 Bond D.R., Holmes D.E., Tender L.M., and Lovley D.R.. Electrode-Reducing Microorganisms that Harvest Energy from Marine Sediments. Science 295:483, 2002.
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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report Deep-sea contained storage of CO2 may also make H2 production from methane clathrates more attractive economically and environmentally. One of several issues surrounding the harvesting of methane clathrates for H2 has been the question of what to do with the CO2. DSCS may offer a solution. A seafloor manufacturing facility could be constructed for methane clathrate processing. The CO2 from this processing facility could be pumped directly to nearby CO2 deep-sea storage containers. Following are some key arguments that point to the importance of DSCS: Capacity could be expanded indefinitely. Some 125 containers, as described above, would store 1 km3 or 1 Gt of CO2. CO2 could be recovered, if desirable, or transferred to another container, if there was danger of leakage. Ordinary leakages would be comparatively benign. First estimates of infrastructure development, costs, and time appear to be far less than estimates for sequestering CO2 in traditional subsurface storage formations, given that the volume of CO2 that needs to be sequestered is equal to or greater than the total production capacity of the global petroleum industry. Thus, if used exclusively to meet storage requirements, conventional subsurface storage would require essentially duplicating the existing worldwide petroleum production infrastructure. There is precedence for this type of storage, as fuel is already stored in large containers in the sea. A considerable body of knowledge and technology already exists for this type of ocean storage. Research Areas At this early stage in concept development, operational concepts are needed that consider types of bladders or other containers, their manufacture, deployment, filling, CO2 diffusivity, and longevity. Concepts must include rough order-of-magnitude engineering cost estimates. Ocean floor pipeline considerations must also be included. Major research areas include the following: The cost of compressing CO2 and pumping it to the deep-sea bed for disposal in the containers; Environmental issues; Security issues—the total seabed area that would be required to sequester approximately 1 Gt of CO2 for this type of disposal is not large (estimated at 5 km ×5 km×100 m deep) in comparison with deep-sea disposal without containment; limiting the disposal area would enable better surveillance of the area; and Container protection—the container could be engineered so that it is encapsulated with stronger materials (e.g., cement) to afford it greater protection.
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