Geological Sequestration Research at NETL: an Overview

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	In geologic sequestration, CO2 is captured from large point sources, such as fossil-fueled power plants, and injected into geologic formations, such as unmineable coal beds, saline formations, and depleted oil and gas wells, for long-term storage.

Within the past 200 years, carbon dioxide (CO₂) emissions from human activity have increased from an insignificant level to more than 33 billion tons a year. The United States is one of the largest emitters of CO₂ in the world, second only to China, which has a huge impact on global warming. At NETL, researchers are developing technology to capture CO₂ generated at fossil fuel power plants before it can escape into the atmosphere and then safely store it deep underground. The technical term for this capture-and-store process is geologic sequestration. Based on research currently underway, NETL should have a technology portfolio of safe, cost-effective, proven, commercial-scale greenhouse gas capture, storage, and mitigation technologies available for widespread use by 2020.

A major goal of NETL’s sequestration research is to thoroughly understand how CO₂ behaves when it’s stored deep underground so that it will stay there and not adversely affect our environment. Our research focuses on the specific physical and chemical changes that occur to a geologic formation when CO₂ is injected, and the extent to which the CO₂ moves within that formation.

Even though private companies have been safely injecting CO₂ deep underground for decades at sites around the world, their motivation has not been to dispose of carbon dioxide. Rather, the CO₂ is injected to reduce oil viscosity so that more oil can be extracted. The United States is the world leader in this technology, called enhanced oil recovery, using about 32 million tons of CO₂ per year for this purpose alone.

Sequestration can also take advantage of unmineable U.S. coal resources, estimated at 6 trillion tons—90 percent of which can’t be mined due to seam thickness, depth, or structural integrity. These coal beds typically contain large amounts of methane-rich gas that is adsorbed on the coal’s surface. Today, the usual way to recover this methane (commonly called natural gas) is to depressurize the coal bed by pumping water out of the strata, but an alternative approach is to inject CO₂ into the deep coal seams. Because the adsorption rate for CO₂ is approximately twice that of methane, the CO₂ displaces the methane, taking its place in the coal bed, and releases the methane, which is pumped to the surface. This method of coal bed methane extraction has been demonstrated in limited field tests, but more work must be done to fully understand and optimize the process. For example, NETL researchers have documented how the coal swells as it adsorbs CO₂, which affects the permeability of the coal, and could cause problems in CO₂ injection.

	Injection wellhead at a sequestration test site near Natchez, Miss.

The sequestration option with the greatest potential is to pump CO₂ into saline formations very deep underground. It has been estimated that deep saline formations in the United States could potentially store up to 550 billion tons of injected CO₂. Locations suitable for sequestration must have impermeable rock strata above the aquifer that will prevent the CO₂ from escaping, but this is not unusual. In fact, the Norwegian oil company, Statoil, is already injecting over a million tons a year of recovered CO₂ into a saline formation under the North Sea. The amount being sequestered is equivalent to the output of a 150-megawatt coal-fired power plant.

Storage Retention Goals for Carbon Sequestration

The effectiveness of carbon sequestration depends greatly on storage permanence, so it’s not surprising that a key goal of NETL’s carbon sequestration research program is to retain 99 percent of carbon dioxide (CO₂) in underground reservoirs over a century. The trouble is that variability in field conditions greatly complicates quantitative predictions of leakage risk. To improve these predictions, NETL is collaborating with four other U.S. Department of Energy national laboratories in a new effort, the National Risk Assessment Program (NRAP). The program’s objectives are to integrate scientific insights from across the sequestration research community and to ensure development of the science base needed for appropriate risk assessment to support large-scale underground carbon storage projects. The NETL-led effort includes researchers from Lawrence Berkeley, Lawrence Livermore, Los Alamos, and Pacific Northwest National Laboratories.

Experience since the early 1970s with CO₂-enhanced oil recovery in the Permian Basin of West Texas—along with more recent large-scale, international sequestration operations underway—indicates that a well-characterized geologic site can be engineered to store CO₂ indefinitely. But ensuring the efficacy of large-scale CO₂ storage requires predicting accurate movement and reactivity of the CO₂ in a reservoir, as well as scientific monitoring of actual performance.

NRAP scientists are evaluating gaps in current scientific knowledge and targeting five primary areas for collaborative research. Although several laboratories are contributing in more than one area, each is taking primary responsibility for steering efforts in one area:

• NETL is responsible for research on wellbore risk assessment.
• Lawrence Berkeley is overseeing research related to monitoring for risk assessment.
• Lawrence Livermore is responsible for systems modeling for risk assessment.
• Los Alamos is overseeing research on natural seal integrity.
• Pacific Northwest  is coordinating research on risks to groundwater systems.

	Carbonate precipitate within cement pores leads to formation of a barrier that significantly slows the reaction between CO2 and cement.

Wellbores are obvious potential leakage pathways for CO₂ injected into geologic formations for storage. This is not just from the wells that have been drilled into the strata to inject and monitor the CO₂, but also from old wells that may have been drilled for oil and natural gas exploration and/or production. NETL’s seal integrity research aims to predict leakage rates based on an increased understanding of the chemical and physical processes affecting seals.

Early NETL research focused on the integrity of wellbore cement after CO₂ injection. Deep wells are typically lined with cement to prevent leakage of gases and fluids, such as saline or oil, to the surface or into underground drinking water resources. However, because CO₂ dissolved in water is acidic, and cement is alkaline, there is the potential for chemical reactions that could adversely affect seal integrity. Recent NETL research shows that the CO₂ reaction with typical wellbore cement is too slow to cause leakage in a properly constructed well that is in good condition. But the cement in old or abandoned wells could still be a problem; these would all need to be located and sealed before sequestration at any given site is initiated.

The flow path of an acidic fluid through a fracture in cement is discolored by chemical reactions. In every case studied, the chemical reaction “softened” the surface of the fracture, allowing the confining pressure to partially close it.

Carbon sequestration calls for CO₂ to be injected under pressure in a supercritical state, which means that it is similar to a liquid but is more compressible and less viscous than water. Ideally, this allows it to be injected at higher pressures without fracturing the reservoir. However, the geomechanical responses to increased fluid pressures in a fluid-rock system could cause faults and fractures to either open or close, affecting both natural seal integrity and wellbore integrity. Because of this, fluid flow and subsequent chemical reactions in the reservoir are being studied to determine whether a pre-existing flow path will open or close over time as a result of changing stress on the fractured rocks and/or chemical dissolution or precipitation.

Experiments were conducted in which an acidic fluid was allowed to flow through a fracture in cement while under a confining pressure that simulates deep subsurface conditions. This research suggests that the flow of CO₂-saturated brine along an open pathway in wellbore cement, which one might think would cause the cement to dissolve, can actually have a positive impact over time by sealing a pathway as minerals that first dissolve then re-precipitate. Additional research is now being conducted to verify this phenomenon and determine whether there are other conditions under which flow pathways may open instead.

After the NRAP develops findings and recommendations, field tests being conducted by DOE’s Regional Carbon Sequestration Partnerships, will provide ideal opportunities to apply and validate the new risk-assessment tools.

NatCarb Offers Public Access to Sequestration Data

Successfully developing commercial-scale carbon capture and storage projects requires an integrated knowledge of carbon dioxide (CO₂) sources, CO₂ transportation pipelines, and candidate geologic storage formations. NETL has compiled this information in the National Carbon Sequestration Database and Geographic Information System (NatCarb) to help researchers assess carbon capture and storage potential across the United States and Canada. Ultimately, NatCarb will provide public access to a wide range of CO₂ sequestration data and analytical tools drawn from a wide variety of scientific resources.

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	NatCarb-generated map of evaluated oil and gas reservoirs (red), saline formations (blue),and coal seams formations (yellow), which can potentially be used to store CO2.

Currently, NETL’s NatCarb webpage shows interactive Google Maps™ with locations of CO₂ sources and geologic storage formations, geographic information system (GIS) data downloads, and links to an external interactive data viewer that queries data directly from Regional Carbon Sequestration Partnership GIS servers. Visitors can access additional CO₂ source and storage formation data via links to the Carbon Sequestration Atlas of the United States and Canada.

NatCarb is also being used to collect laboratory- and field-based research results that can be directly translated into user-accessible GIS tools. For example, NETL’s Office of Research and Development is revising the methods used to calculate CO₂ storage resource estimates, which are high-level assessments of the amount of CO₂ that can be stored in a geologic formation. These revised calculation methods are being developed into a GIS-based tool that can be used to calculate storage resource estimates for different geologic basins.

Future development of NatCarb resources includes incorporating data from American Recovery and Reinvestment Act–funded CO₂ sequestration site characterization projects, links to other carbon capture and storage projects, and the development of additional GIS tools for high-level planning of CO₂ sequestration projects. NatCarb is managed through NETL’s Office of Research and Development and NETL’s Strategic Center for Coal, and includes key team members from NETL’s GIS personnel, the Regional Carbon Sequestration Partnerships, West Virginia University, and the Kansas Geological Survey.