The Gulf Coast region offers a wide variety of sequestration options.
Steve Blankinship, Associate Editor, Power Engineering
At an oil field near Dayton, Texas, in 2004 a group of researchers pumped 1,600 tons of carbon dioxide almost a mile down into a non-oil-bearing 20-ft-thick sandstone stratum in a geologic formation known as the Oligocene upper Frio Formation.
The research group, which included Sandia Technologies, Praxair, Air Liquide and Trimeric, were testing tools to monitor how carbon dioxide (CO2) moves and how it is contained belowground. They also tested their ability to predict CO2 movement using numerical modeling and CO2 flow prediction techniques. Follow-up tests done 16 months later showed the CO2 was still where it was placed and that it was difficult to get out, even using pumps. The tests provided evidence that monitoring tools are adequate to track the movement of CO2 underground and that conceptual and numerical models for injection and storage are fundamentally correct.
“The Frio Project represents a first-time injection of anthropogenic (man-made) CO2 into a saline formation in the United States to study sequestration,” says Scott Klara, manager of the Carbon Sequestration Program at the U.S. Department of Energy’s National Energy Technologies Laboratory, which supported the $6 million test. “The project is critical to the development of the scientific tools necessary to ensure that CO2 sequestration is safe and permanent, should widescale deployment of sequestration technologies become necessary.”
In coming years sequestration could play a major role in how fossil-fueled power plants manage CO2 emissions. Rather than simply pump the gas underground and forget about it, industry is looking at how to build markets to extract more of its economic value through sequestration.
Earlier this year, researchers at Battelle released a report on carbon capture technologies that documented their potential value in mitigating carbon related climate change.
James Dooley, senior scientist at the Joint Global Change Research Institute and the report’s chief author, said the principal merit of carbon capture and storage (CCS) technologies is their ability to significantly lower the long-term cost of addressing climate change, potentially on the order of hundreds of billions or even trillions of dollars. He said carbon dioxide capture and storage technologies can do this by allowing continued use of the earth’s fossil fuels, while still making progress on climate change by preventing the release of CO2 emissions into the atmosphere.
The Battelle study, “Carbon Dioxide Capture and Geologic Storage: A Core Element of a Global Energy Technology Strategy to Address Climate Change”, identifies a worldwide potential geological capacity to store more than 11,000 billion tons of CO2. That level is thought to be many times more than what would be required to respond to even the strictest climate mitigation policies that could be adopted over the next 100 years.
The Battelle report also identifies the electric power sector as the largest potential market for CCS technologies. It says CCS will be most economic when deployed with new advanced coal-fired baseload electric power plants. The report also projects that based on existing technologies, large-scale CCS technology deployment is likely to begin as CO2 prices rise above $25/ton, a level comparable to – and in some cases much less than – other large-scale CO2 emission reduction and abatement options.
The prevailing opinion among many geologists is that the most robust CO2 sinks are deep saline aquifers. Considerable research and development on saline aquifer storage has been conducted, notably involving aquifers in the North Sea off northern Europe.
A somewhat more limited resource is geological sequestration coupled with enhanced oil and natural gas recovery. Enhanced oil recovery (EOR) and enhanced gas recovery (EGR) consist of injecting CO2 into partially depleted deposits to force out more oil or natural gas. After being removed from fuel or combustion products, the CO2 is pumped into wells in a supercritical state pressurized to more than 70 atmospheres. This gives the CO2 properties of both a liquid and a gas.
Test wells drilled in Texas offered data that CO2 storage can be viable. Photo by Seay Nance, University of Texas.
At such pressures the CO2 acts as a solvent, squeezing out the oil. The CO2 then is moved to a subsurface saline brine aquifer for long-term storage. The process can increase oil field output by up to 60 percent and extend well life up to 20 years. EOR could accommodate around 10 percent of the total CO2 emitted from U.S. coal-fired plants. Other sequestration methods include placing CO2 into depleted or uneconomic coal seams or other unground cavities.
The Gulf Coast states of Texas, Louisiana and Mississippi account for about 16 percent of the CO2 produced each year in the United States. Texas alone emits 667 million metric tons. Paradoxically, the region could potentially be a part of the solution to CO2 storage. Some of the most ambitious geological sequestration research and development activities to date have taken place in Texas.
A Natural Vault for CO2
According to the Pew Center for Climate Change, the use of fossil fuels coupled with the loss of forests is causing the level of greenhouse gases (GHGs) – primarily CO2 – to increase in the atmosphere. Burning fossil fuels contributes an estimated 5.5 gigatons (billion metric tons) of CO2/yr., while land use changes account for another 1.1 gigatons. The world’s oceans absorb about 2 gigatons more CO2 from the atmosphere than they release, while the earth’s ecosystems appear to be accumulating another 1.2 gigatons/yr.
The net effect is that the earth currently appears to be absorbing about 3.4 gigatons more CO2 than it is releasing. While the annual net increase in atmospheric carbon is small compared with the total 750 gigatons of carbon already present in the atmosphere, it adds up over time. If the current rate of carbon accumulation were to remain constant, a 25 percent net gain in atmospheric carbon is likely to occur over the next 50 years.
Beneath the Gulf Coast lies an unusually thick geologic formation made up of rock fragments interspersed with a variety of porous and permeable sand aquifers separated by shale. This sedimentary wedge provides an ideal CO2 storage repository whose capacity could total hundreds of gigatons. The blankets of shale separating the sand should seal CO2 stored there for thousands of years.
The region also offers a variety of sequestration options. They include lignite and coal formations that could be suitable to coal methane projects, depleted gas sands for enhanced natural gas recovery, depleted oil reservoirs for enhanced oil recovery and a large variety of saline brine formations with differing levels of porosity and permeability.
Several years ago, the Bureau of Economic Geology (BEG) at the University of Texas formed the Gulf Coast Carbon Center (GCCC) to carry out applied research in developing strategies and protocols for long-term geologic carbon storage along the Gulf Coast. The GCCC partnership includes BP, Chevron, Entergy, Kinder Morgan, Marathon, NRG Energy, Praxair and Schlumberger.
“If implemented on a massive scale, geologic storage of CO2 in Gulf Coast brine aquifers could help reduce the rate of increase of CO2 during a transition period of decades,” says Ian Duncan, associate director of environmental and earth systems for BEG.
The focus of GCCC’s work is based on gasifying coal, lignite, petcoke and even biomass using integrated gasification combined cycle (IGCC) technology, rather than extracting CO2 post-combustion from pulverized coal generation.
“Gasification has the advantage of producing CO2 in much higher concentrations than in pulverized coal power plants,” says Duncan. Gasifying coal produces syngas composed of hydrogen and CO2, the latter in sufficiently high concentrations to make traditional separation technologies cost effective.
“This gas mixture is also suitable for separation using membrane techniques, which is an active field of research,” he says. “Coal gasification provides an interesting nexus between the CO2 and hydrogen value chains. The Gulf Coast is already the largest market in the United States for hydrogen and has the potential to become the nation’s largest CO2 market.”
Before a CO2 sequestration industry based on EOR can be developed in the Gulf Coast region, however, a system of backbone pipelines and supporting trunk lines with distributed compression would have to be built. The GCCC is currently conducting economic modeling of a possible CO2 value chain for the Gulf Coast. Researchers are evaluating the idea that pipeline complexes linking many sources and storage sinks are more cost effective than linking source-sink pairs. The center is also testing whether the capital costs of adding capture to existing plants, and building compressors and pipeline complexes, are prohibitively expensive and require substantial carbon penalties, credits or other incentives. Perhaps most important of all, the GCCC is trying to determine a future fair market value for Gulf Coast CO2.
Although EOR is the largest existing CO2 market and the only one that results in long term CO2 storage, other markets exist.
CO2 is used to make carbonated beverages, to decaffeinate coffee and has niche applications in other food manufacturing processes. CO2 might also find use in the dry cleaning industry. A company called Cool Clean Technologies has developed a CO2 based alterative whereby clothes are put into a chamber pressurized with CO2 . The process separates oils and other stains from fabrics without using a toxic substance.
Perhaps most exciting of all, a new technology developed by Raytheon might make additional oil available from U.S. oil shale by heating the shale, then forcing out the oil with CO2. The mixture would be pumped to the surface, depressurized and the oil separated from the CO2. The CO2 would be re-pressurized and sent back down the well to repeat the process.
A variety of monitoring tools were used at the Frio Brine Pilot Project. Illustration courtesy BEG.
Texas has the largest CO2 market in the world and the largest quantity of CO2 being actively placed into long-term storage. The abundance of coal and lignite-fired power plants in East Texas adds to the potential sequester, capture and use possibilities.
Researchers at the GCCC envision the Gulf Coast region becoming a major, if not predominant, site for a new carbon sequestration industry. EOR in the Gulf Coast might provide an attractive market for CO2 produced by fossil fuel gasification.
From his office at the University of Texas, BEG associate director Ian Duncan believes there is a strong likelihood that CO2 markets will develop in step with the availability of a reliable source of CO2. Evidence increasingly suggests that electric power could be just such a source.