|
Theory and practice in CCS technology There’s a huge worldwide drive towards the use of carbon capture and sequestration, but broad implementation faces several hurdles Alan Bailey Petroleum News
Widely blamed for an acceleration in global warming, man-made carbon dioxide has become something of a symbol for human-induced environmental degradation. And in the debate about how to minimize future volumes of this gas in the Earth’s atmosphere and oceans, given our likely continued dependence on carbon-based fossil fuels, many people are pinning their hopes on somehow storing the gas in places where it can do no harm. In fact, this type of long-term storage, a technique referred to as carbon capture and sequestration, or CCS, appears to underpin most schemes for minimizing future levels of atmospheric carbon dioxide.
But, how close is the possibility of the widespread use of CCS? And what are the issues involved in the most commonly considered CCS approach, that of taking the carbon dioxide out of fuel or exhaust gas streams, and then pumping it deep underground?
Several countries, including Canada, European Union countries and Australia, have initiated major CCS research and development projects while, in the United States, the Department of Energy is sponsoring a major CCS research program involving partnerships between DOE, state agencies, universities and private companies. The DOE program is transitioning from a phase involving small-scale field tests of CCS technologies into the start of some large-scale tests of prototype storage facilities in various parts of the country: The idea is to test different techniques for carbon dioxide storage; to determine infrastructure requirements; and to work out what type of regulations might be required for a commercial storage operation.
Three commercial applications But although worldwide there is a long list of planned CCS projects, there are in practice only three CCS applications in commercial operation, Diane Shellenbaum, a petroleum geophysicist with Alaska’s Division of Oil and Gas and a member of a state team investigating options for reducing carbon dioxide emissions from Alaska, told Petroleum News May 29. Those applications are at Weyburn in Saskatchewan, Canada; the Sleipner gas field in the North Sea; and the In Salah natural gas project in Algeria.
The Weyburn system, operating since 2000, involves piping carbon dioxide about 200 miles from a coal gasification plant in North Dakota for enhanced oil recovery in the Weyburn oil field (there are many projects that sequester produced carbon dioxide as part of enhanced oil recovery programs, but these projects do not typically capture carbon dioxide from industrial processes). At Sleipner and In Salah, carbon dioxide, removed from produced natural gas as part of purification prior to gas export, is injected back into an underground reservoir.
In general, a CCS implementation involves four distinct components: capturing the carbon dioxide; dehydrating and transporting the carbon dioxide to the sequestration site; compressing and injecting the carbon dioxide into storage; and monitoring what happens to the sequestered carbon dioxide, Shellenbaum said.
Because produced gas can naturally contain some carbon dioxide, an initial step in carbon dioxide capture, the first of the CCS components, may be fuel gas treatment, as at Sleipner and In Salah. Then, in a situation where carbon dioxide is captured from a power plant, one option is to process the fossil fuel into hydrogen, to use as fuel for the plant, and carbon dioxide for sequestration. Alternatively, carbon dioxide can be scrubbed from the flue gas after combustion using a chemical fluid such as amine. Another possibility in a power station is to use oxygen rather than air in the fuel combustion process, thus producing an exhaust containing carbon dioxide and water, from which the water can be condensed to leave a carbon dioxide-rich gas stream.
Stored as liquid If the carbon capture site is distant from the carbon sequestration site, the carbon dioxide must be shipped by pipeline or road tanker between the two sites. Then, for sequestration, the carbon dioxide would be compressed for injection down a well into a suitable rock formation to a depth in excess of perhaps 2,500 to 3,000 feet, where pressures and temperatures would cause the carbon dioxide to remain stable as a liquid, Shellenbaum said.
Storing the carbon dioxide as a liquid rather than as a gas greatly reduces the volume of rock required for storage while also reducing the likelihood of the sequestered material escaping from the reservoir, Shellenbaum explained. And, like oil, liquid carbon dioxide tends to float above water in a reservoir, she said.
Then, over time, the carbon dioxide may dissolve in the water, to form a relatively heavy liquid that sinks.
“Ideally that’s what you’d like for long-term storage,” Shellenbaum said. Or, better still, the carbon may react chemically with material in the reservoir to become a solid mineral, she said.
However, a key to successful underground sequestration is the location of a suitable impervious rock that will seal the carbon dioxide into an underlying reservoir rock — for successful carbon sequestration, carbon dioxide needs to remain trapped underground for perhaps thousands of years, thus rendering a storage reservoir with even quite slow leakage somewhat worthless.
Depleted fields Given the importance of having an effective seal rock in a situation that can form a fluid trap, the usual first choice for carbon dioxide storage is a depleted oil or gas field, thanks to the fact that the properties of the field reservoir and seal rocks will already be well established. Trying to develop a storage facility at some new site would require an exploration project, involving the determination of underground structures and rock properties, Shellenbaum said.
However, where a power plant is located far from existing oil and gas fields, as is the situation for some coal-fired power plants in the Lower 48, it will be necessary to explore for a suitable carbon dioxide reservoir or build a carbon dioxide pipeline. In this type of situation deep, well-sealed reservoirs containing brine are likely candidates, in part because this type of reservoir will avoid contamination of potable water that might be tapped by water wells. Other possibilities being investigated include fractured volcanic rocks, where the carbon dioxide may react with the rock material to form solid minerals.
Once a CCS system starts operating it will probably be necessary to use techniques such as seismic or gravity surveys to monitor what happens to the carbon dioxide in the underground reservoir, both to verify that the reservoir is not leaking and to track the migration of the carbon dioxide within the reservoir. Tracking the migration of the carbon dioxide will be important because of the possibility of the material migrating from the originally intended reservoir location into a new reservoir site, perhaps subject to some different subsurface land ownership rights, Shellenbaum explained.
And the various complications that will be inherent in any CCS arrangement will require government regulations, regulations that do not currently exist in the United States but which the Environmental Protection Agency is currently developing, Shellenbaum said. Lack of regulations would impede the development of a commercial CCS project, in part because of the risk of environmental lawsuits.
“There are still a lot of unknowns,” Shellenbaum said. “I wouldn’t think anyone would take the risk.”
Very expensive However, the biggest impediment to a commercial CCS development is the high cost of building and operating the various components of a CCS system. At Weyburn, the value gained from carbon dioxide enhanced oil recovery pays for the CCS costs; at Sleipner, where carbon dioxide has to be removed from produced gas regardless of whether the gas is sequestered or vented to the atmosphere, the cost of sequestration is presumably offset by savings in Norwegian carbon emissions taxes.
An operational CCS system at a power plant, for example, will significantly increase the cost of power — according to a 2005 report by the Intergovernmental Panel on Climate Change, CCS may increase the cost of electricity by anywhere from 21 to 91 percent, depending on the type of power generation technology involved. Consequently, there’s no real way of making CCS financially viable without some form of carbon tax, carbon cap-and-trade system or enhanced oil recovery application to offset the costs.
But with Congress debating carbon cap-and-trade legislation as part of a general trend towards government-driven initiatives to reduce carbon emissions, CCS research programs being conducted by DOE and by individual states are preparing the road for a carbon-constrained future.
“We’re seeing it coming down the line, and states like Virginia that are heavily coal-dependent are trying to get ahead of the curve,” Shellenbaum said.
|
