Frequesntly Asked Questions about Carbon Capture and Storage.

This list represents some of the most frequently asked questions about the science of carbon sequestration and storage that the STEP staff answer each day.

What is Geologic Carbon Dioxide Storage?

Geological storage of carbon dioxide (CO2) is one method of isolating CO2 from the earth's atmosphere. Reducing the amount of CO2 released into the atmosphere may slow the global warming trends that have been observed in the last decade or more. Geological storage can play a significant role in preventing continued buildup of CO2 in the atmosphere.

Carbon Capture, Utilization and Storage (CCUS) is the capture, transport, and storage of CO2 from a point source, such as an electrical generation or biofuel production facility. The CO2 is stored in the pore spaces of suitable geological formations in a porous rock, such as sandstone, and sealed in place with an impermeable caprock, such as shale. Utilization focuses on the use of CO2 to produce additional fossil fuel resources, primarily through enhanced oil recovery.

What are saline reservoirs?

Saline reservoirs are units of rock with pore spaces containing water with high amounts of total dissolved solids (TDS), especially sodium chloride (table salt), that make the water too salty for human consumption or for agricultural and industrial uses. Saline reservoirs are found deep underground in rock formations that are acceptable for long-term storage of large quantities of CO2.

How is the CO2 gas treated before injection at IBDP?

Carbon dioxide is being captured from the ADM ethanol production facility in Decatur, dehydrated, and transported via on-site pipeline to the storage site. The captured CO2 is compressed to a liquid like state so that it occupies less volume than CO2 gas. The liquid like CO2 is then injected more than a mile below ground into porous sandstone called the Mt. Simon Sandstone for long-term geological storage.

What kinds of problems do you anticipate with having CO2 in a liquid-like state?

We have not encountered any issues with the CO2 in liquid state. We have drawn on the experience in the oil and gas industry with transporting and injecting supercritical CO2. Using current technology, the pipelines and wells at IBDP have been engineered to handle the high pressures necessary to keep CO2 in this form.

Where is the Mount Simon Sandstone?

The Mt. Simon Sandstone lies more than one mile below Decatur, Illinois, and is 1,600 ft thick at this location. It is the deepest sandstone in the Illinois Basin, a bowl-shaped geological feature that holds thousands of feet of rock and underlies most of Illinois, southwestern Indiana, and western Kentucky. The Mt. Simon Sandstone is covered by the low-porosity Eau Claire Shale, which also has very low permeability (the ability of fluids to flow through a rock) and will therefore hold the CO2 in the Mt. Simon. The Eau Claire Shale acts as a seal or cap, holding CO2 deep in the Mt. Simon Sandstone. The Mt. Simon has a large storage capacity because of its thickness, porosity, and areal extent, and it can hold a million tons of CO2 within a radius of just about a quarter-mile from an injection well.

What happens to the CO2 once injected underground?

The CO2 is held in the small pore spaces present in rocks. The ideal saline reservoir for geological storage shares characteristics with the best oil producing rocks. This is sedimentary rock with good porosity (percentage of open pore spaces) and permeability (connectivity of open pore space).

Directly above the storage reservoir, at depths of at least 5,500 ft, is a relatively thick caprock unit (hundreds of feet thick) of very low porosity and permeability, such as shale. The caprock layer acts as a lid or a cover for the saline reservoir. The depth of the reservoir is also important. A suitable saline reservoir will be far below drinking water levels yet economically viable for drilling.

Also, the volume of the potential storage space must be great enough to justify the expense of drilling a well, and having more than one shale caprock which will help ensure containment of the CO2. The Illinois Basin - Decatur Site will store CO2 more than a mile below the surface, and have two backup shale seals in addition to the primary shale seal in the mile between the Mt. Simon Sandstone and the surface.

How much CO2 can the Mt. Simon Sandstone hold?

The Mt. Simon Sandstone is commonly used for natural gas storage in the Illinois Basin and has fair to good permeability and porosity. Its overlying strata contain impermeable limestone, dolomite, and shale intervals. The depth of the Mt. Simon ranges from approximately 2,000 to 14,000 ft below the surface. In the southern half of the Basin the reservoir is brine-filled, and no oil or natural gas resources have been discovered in this unit. At its greatest thickness in the Illinois Basin, the Mt. Simon is over 2,600 ft thick. The estimated storage capacity of the Mt. Simon Sandstone ranges from 27 to 109 billion metric tonnes of CO2. Given the current total emissions of 304 million metric tonnes per year from point sources in the Illinois Basin, the Mt. Simon Sandstone has several hundred years of storage potential.

Is carbon sequestration safe?

Yes, if the storage site is correctly selected, designed, and operated. Extensive monitoring takes place during and after injection to be sure the CO2 being stored stays in place. Monitoring techniques include using geophysical technology to confirm the position of the CO2 underground and wells to monitor groundwater and soils. For decades, the ISGS has collected background information on the subsurface rocks of the Illinois Basin and has been investigating the sequestration potential of the Mt. Simon Sandstone for more than five years. The Mt. Simon Sandstone has performed well as a natural gas storage reservoir in Illinois, and we expect the same performance in Decatur and other locations throughout Illinois, Kentucky, and Indiana.

Are there any risks to pumping so much carbon dioxide underground?

No significant health, safety, or environmental risks should arise from a properly designed and managed storage site. Appropriate risk mitigation and management plans are an integral part of the overall project planning. Extensive monitoring will take place during and after the injection to be sure the CO2 stays in place. The first line of monitoring begins deep below the ground, so we will know if any leakages occur well before the CO2 reaches the surface.

What happens to the CO2 in the event of an earthquake?

Earthquake energy traveling through bedrock is many times smaller than what we experience on the ground surface. In the event of an earthquake, the CO2 in the porous spaces of the rock may react slightly to the space being momentarily compressed.

In water wells, a very small, momentary change in water levels because of earthquake waves passing through the area of the well has occasionally been recorded. This is believed to be caused by the earthquake wave slightly compressing the rock as it passes through, and then letting the bedrock relax back to normal.

How many other projects like this exist around the world?

Geological sequestration of CO2 by injection into the Earth’s subsurface is a promising technology being studied around the world. There are currently projects underway in Canada, Norway, Algeria, Australia, and under development around the world. The Illinois Basin - Decatur project is one of the first projects similar in scope in the United States to begin injection of CO2 for geologic sequestration.

Link to MGSC Web site.
The Sequestration Training and Education Program
615 East Peabody Drive
Champaign, Illinois 61820

Office 217-244-4068
Fax 217-333-2830

STEP is a program of the Advanced Energy Technology Initiative, University of Illinois.
©2010 MGSC. All rights reserved. This material is based upon work supported by the U.S. Department of Energy under Award Number DE-FE0002462 and the Illinois Department of Commerce and Economic Opportunity #09-484002.
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