Geologic Sinks

• Geologic Sinks and CO₂ Sequestration
• Geologic Sink Requirements
• Regional Geology and Sequestration
• Regional Sedimentary Basins
• Sequestration in Saline Formations
• Sequestration in Coal
• Sequestration in Oil and Gas Fields
• Enhanced Oil Recovery

Geologic Sinks and CO₂ Sequestration
Underground geologic layers of rock that are fit for the storage of CO₂ are called geologic sinks. Geologic sequestration is the practice of putting anthropogenic CO₂ from large-scale stationary sources into long-term storage in geologic CO₂ sinks. This is also called carbon capture and storage or CSS. Geologic CO₂ sinks include unminable coal beds, oil and gas reservoirs, and very deep geologic layers that contain very salty water (brine).

In geologic sequestration, CO₂ is captured at stationary sources at the surface and is then injected into the underground geologic sink. Successful geologic sequestration requires that the CO₂ stay in place and not pose a danger to human health and the environment.

Geologic Sink Requirements
To be considered for sequestration, geologic sinks must be near a major source of CO₂, have the characteristics that can hold the CO₂ in place for a long period of time (for example, a seal above a permeable zone of rock similar to the situation that would trap and store oil or natural gas), be considerably deeper than the deepest water well, and be in a stable area (that is, an area that is not prone to earthquake activity).

Regional Geology and Sequestration
The geology in the western and northern portion of the PCOR Partnership region has many pluses for geologic sequestration. The area is stable.¹ The geologic basins in the area have many features that stored oil and gas for millions of years and are possible CO₂ sinks. In addition, these basins have major underground zones at great depth that contain very salty water that might be suitable CO₂ sinks. Because CO₂ physically "bonds" to coal, many of the unminable coal seams in these basins could be candidates for CO₂ sequestration.

Back to Top

Regional Sedimentary Basins
There are four relatively large and deep, intracratonic oil-producing basins intersecting the PCOR Partnership region, each with a sedimentary cover thousands of feet thick. The basins in the PCOR Partnership region have significant potential as geological sinks for sequestering CO₂. Geological sinks that may be suitable for long-term sequestration of CO₂ include both active and depleted petroleum reservoirs, deep saline formations, and coal seams, all of which are common in these basins.

While general information on the structural geology, lithostratigraphy, hydrostratigraphy, and petroleum geology of these basins is available, additional characterization data for specific geological sinks will be necessary. Rocks that have been explored or developed for hydrocarbon recovery have been geologically characterized to a great extent, while non-hydrocarbon-bearing zones (such as saline formations) will require much more geologic investigation prior to large-scale sequestration.

Back to Top

Sequestration in Saline Formations
Saline formations within the PCOR Partnership region have the potential to store vast quantities of anthropogenic carbon dioxide. Two saline aquifer systems, the Mississippian Madison and the Lower Cretaceous, have been evaluated for their regional continuity, hydrodynamic characteristics, fluid properties, and ultimate storage capacities using published data.

The lateral extent of these aquifers, the current understanding of their storage potential gained through injection well performance, and the geographic proximity to major CO₂ sources suggest they may be suitable sinks for future storage needs. For example, reconnaissance-level calculations on the Mississippian system in the Williston Basin and Powder River Basin suggest the potential to store upwards of 60 billion tons of CO₂ over the evaluated region, while the Cretaceous system has the potential to store over 160 billion tons.^2,3

Back to Top

Sequestration in Coal
Many coal seams throughout central North America are too deep or too thin to be mined economically. However, many of these coals have varying amounts of methane adsorbed onto pore surfaces, and wells can be drilled into the coal beds to recover this "coalbed methane" (CBM). In fact, CBM is the fastest growing source of natural gas in the United States and accounted for 7.2% of domestic production in 2003.⁴

As with oil reservoirs, the initial CBM recovery methods, dewatering and depressurization, can leave methane in the coal seam. Additional CBM recovery can be achieved by sweeping the coal bed with CO₂, which preferentially adsorbs onto the surface of the coal, releasing the methane. For the coals in the PCOR Partnership region, up to thirteen molecules of carbon dioxide can be adsorbed for each molecule of methane released, thereby providing an excellent storage sink for CO₂.⁵ Just as with depleting oil reservoirs, unminable coal beds are a good opportunity for CO₂ storage.

Three major coal horizons in the PCOR Partnership region have been identified for further study: the Wyodak-Anderson bed in the Powder River Basin, the Harmon-Hanson interval in the Williston Basin, and the Ardley coal zone in the Alberta Basin. The total maximum CO₂ sequestration potential for all three coal horizons is approximately 8 billion tons.^6,7

In northeastern Wyoming, the CO₂ sequestration potential for the areas where the coal overburden thickness is >1000 ft (305 m) is 6.8 billion tons (6.2 × 1012 kg). The coal resources that underlie these deep areas could sequester all of the current annual CO₂ emissions from nearby power plants for the next 156 years.⁸

Back to Top

Sequestration in Oil and Gas Fields
The geology of carbon dioxide sequestration is analogous to the geology of petroleum exploration; the search for oil is the search for sequestered hydrocarbons. Oil fields have many characteristics that make them excellent target locations for geologic storage of CO₂. Therefore, the geological conditions that are conducive to hydrocarbon sequestration are also the conditions that are conducive to CO₂ sequestration. The three requirements for sequestering hydrocarbons are a hydrocarbon source, a suitable reservoir, and an impermeable trap. These requirements are the same as for sequestering CO₂, except that the source is artificial and the reservoir is referred to as a sink.

A single oil field can have multiple zones of accumulation which are commonly referred to as pools, although specific legal definitions of fields, pools, and reservoirs vary for each state or province. Once injected into an oil field, CO₂ may be sequestered in a pool through dissolution into the formation fluids (oil and/or water), as a buoyant supercritical-phase CO₂ plume at the top of the reservoir (depending on the location of the injection zone within the reservoir), and/or mineralized through geochemical reactions between the CO₂, formation waters, and formation rock matrix.

Oil is drawn from the many oil fields in the PCOR Partnership region from depths ranging from 2500 to 4000 feet for the shallower pools, to 12,000 to 16,000 feet for the deepest pools.

Although oil was discovered in this region in the late 1800s, significant development and exploration did not begin until the late 1940s and early 1950s. The body of knowledge gained in the past 60 years of exploration and production of hydrocarbons in this region is a significant step toward understanding the mechanisms for secure sequestration of significant amounts of CO₂.

Back to Top

Enhanced Oil Recovery (EOR)
Most oil is extracted from the ground in three distinct phases: primary, secondary, and tertiary (or enhanced) recovery. Natural pressures within the reservoir drive oil into the well during primary recovery, and pumps bring the oil to the surface. Primary recovery produces roughly 12%-15% of a reservoir's original oil. An additional 15%-20% of the original oil can be extracted through secondary recovery processes which involve injecting water to displace the oil.⁹

Conventional primary and secondary recovery operations often leave two-thirds of the oil in the reservoir. In the United States, EOR methods have the potential to recover much of that remaining oil, which is estimated to be 200 billion barrels.⁸ However, oil recovery is challenging because the remaining oil is often located in regions of the reservoir that are difficult to access, and the oil is held in the pores by capillary pressure.

Reconnaissance-level CO₂ sequestration capacities were estimated for selected oil fields in the Williston Basin, Powder River Basin, and Denver-Julesberg Basins. Two calculation methods were used, depending on the nature of the available reservoir characterization data for each field. The estimates were developed using reservoir characterization data that were obtained from the petroleum regulatory agencies and/or geological surveys from the oil-producing states and provinces of the PCOR Partnership region. Results of the estimates for the evaluated fields (using a volumetric method) in the three basins indicate a storage capacity of over 10 billion tons of CO₂.

Aside from non-market-based incentives, CO₂ sequestration in many geologic sinks is not generally economically viable under current market conditions. However, EOR miscible flooding is a proven, economically viable technology for CO₂ sequestration that can provide a bridge to future non-EOR-based geologic sequestration. For example, a portion of the revenue generated by CO₂ EOR activities can pay for the infrastructure necessary for future geologic sequestration in brine formations. It is expected that unitized oil fields subjected to this type of recovery process would retain a significant portion of the injected CO₂ (including the amount recycled during production) as a long-term storage solution.

Back to Top

References:

1. Fischer, D.W., LeFever, J.A., Sorensen, J.S., Smith, S.A., Helms, L.D., LeFever, R.D., Whittaker, S.G., Steadman, E.N., and Harju, J.A., 2005, The influence of tectonics on the potential leakage of CO₂ from deep geological sequestration units in the Williston Basin: Plains CO₂ Reduction (PCOR) Partnership topical report for U.S. Department of Energy and multiclients, Grand Forks, North Dakota, Energy & Environmental Research Center, October 2005.
2. Fischer, D.W., Smith, S.A., Peck, W.D., LeFever, J.A., LeFever, R.D., Helms, L.D., Sorensen, J.A., Steadman, E.N., and Harju, J.A., 2005, Sequestration potential of the Madison of the northern Great Plains aquifer system (Madison geological sequestration unit): Plains CO₂ Reduction (PCOR) Partnership topical report for U.S. Department of Energy and multiclients, Grand Forks, North Dakota, Energy & Environmental Research Center, June 2005.
3. Fischer, D.W., Sorensen, J.A., Smith, S.A., Steadman, E.N., and Harju, J.A., 2005, Potential CO₂ storage capacity of the Lower Cretaceous aquifer system in the PCOR Partnership area: Plains CO₂ Reduction (PCOR) Partnership topical report for U.S. Department of Energy and multiclients, Grand Forks, North Dakota, Energy & Environmental Research Center, September 2005.
4. www.fe.doe.gov/programs/oilgas/environment/Water_Resources_R&D.html (accessed September 2005).
5. Burruss, R., 2003, CO₂ adsorption in coals as a function of rank and composition - a task in USGS research on geologic sequestration of CO₂: The Second International Forum on Geologic Sequestration of CO₂ in Deep, Unmineable Coal Seams (Coal-Seq II), March 6-7, 2003, Washington, D.C., www.coal-seq.com/Proceedings2003/Burruss.pdf.
6. Nelson, C.R., Steadman, E.N., and Harju, J.A., 2005, Geologic CO₂ sequestration potential of lignite coal in the U.S. portion of the Williston Basin: Plains CO₂ Reduction (PCOR) Partnership topical report for U.S. Department of Energy and multiclients, Grand Forks, North Dakota, Energy & Environmental Research Center, June 2005.
7. Bachu, S.B., Steadman, E.N., and Harju, J.A., 2005, Carbon dioxide storage capacity in Upper Cretaceous-Tertiary Ardley coals in Alberta: Plains CO₂ Reduction (PCOR) Partnership topical report for U.S. Department of Energy and multiclients, Grand Forks, North Dakota, Energy & Environmental Research Center, June 2005.
8. Nelson, C.R., Steadman, E.N., and Harju, J.A., 2005, Geologic CO₂ sequestration potential of the Wyodak-Anderson coal zone in the Powder River Basin: Plains CO₂ Reduction (PCOR) Partnership topical report for U.S. Department of Energy and multiclients, Grand Forks, North Dakota, Energy & Environmental Research Center, June 2005.
9. www.netl.doe.gov/scngo/Petroleum/publications/eordrawings/Color/colop.pdf (accessed September 2005).