Monitoring, Verification, and Accounting (MVA) Research Underway to Assure that CO₂ Injected Underground Stays There

	Perfluorocarbon tracers being added to the carbon dioxide as it is injected underground at a sequestration test site near Gaylord, Michigan.

The Department of Energy is conducting research and field trials aimed at getting the country technologically prepared for future regulations that may limit carbon dioxide (CO₂) emissions and thereby combat global warming. As discussed in the Feature Story previously posted in this spot, and at NETL’s carbon sequestration link, careful monitoring, verification, and accounting (MVA) is going to be a crucial component of underground storage of CO₂. After the effort and expense of capturing and placing the CO₂ deep underground, no one will want to see it escape to the atmosphere. Moreover, sequestration project developers will need MVA technology to obtain permits, ensure that human health and safety or the host ecosystem are not adversely affected by sequestration projects, and that they qualify for emission reduction credits, which will require that close to 100 percent of the injected CO₂ remains underground. Ensuring that less than 1 percent of the CO₂ stored underground at a site escapes will require a high level of confidence in MVA tools to detect the first signs of leakage. We need to be able to measure the movement of the injected CO₂ and its chemical and physical state deep underground; techniques to do this accurately are still being developed and refined. Since each site is different, the best MVA techniques for one site may not be the best at another. At NETL’s carbon sequestration link (provided above), you will be able to download a Best Practices manual on MVA that, if followed, can already ensure that less than 5% of the CO₂ stored underground escapes. Meanwhile, research is underway, at NETL and at field sites, to ensure by 2012 that less than 1% escapes. A small portion of the research being conducted on this topic by NETL scientists and engineers is described in these three LabNotes, but more than a dozen other MVA research projects being conducted by NETL are described in detail at NETL’s carbon sequestration link.

One approach that NETL has pioneered has been the use of perfluorocarbon (PFC) tracers, which can be added to the CO₂ during injection. These unique organic molecules are not found in nature, which means that any PFC found outside of the reservoir strata where the CO₂ was injected is associated with a leak. In addition, they are generally non-toxic and non-reactive, which means that they can move through the strata without being significantly impeded by chemistry, and most important, they can be detected at incredibly low concentrations (down to the femtogram level, which is equivalent to one billionth of parts per trillion, 10-16 mol PFC/L). Furthermore, they have a low solubility in water.

NETL has demonstrated that this novel technique works well at field sites. For example, using PFC tracers, NETL detected extremely low levels of CO₂ leakage (<0.01% per year loss) and, using ground-penetrating radar, determined that it was associated with subsurface thinning and faulting, at the West Pearl Queen, New Mexico sequestration test site. In contrast, again using PFC tracers, NETL found no evidence of near-surface CO₂ release at the Lower Michigan Basin test site pictured below and at two other test sites. NETL has received the prestigious R&D 100 Award for the development of a protocol for tracer detection and quantification in soil-gas and atmospheric plumes that can be used to detect leakage at sequestration sites.

Another approach that is used to see what is happening underground is seismology. We are all familiar with how seismic energy is monitored using seismographs, but these devices can be used to do more than measure earthquake intensity. Seismographs actually measure various types of acoustic energy that passes through strata deep underground, and is reflected and/or refracted by various factors, such as the type of rock, its porosity, its fluid content, etc. These variables are all important is assessing potential sequestration reservoir strata deep underground, so rather than wait for an earthquake, scientists set off small explosions and use sophisticated software to study how the resultant energy is reflected off of the rocks at depth. This approach is routinely used by geologists to find formations that may contain oil and/or natural gas. A three dimensional (3D) seismic image can then be constructed of the underground strata, and scientists can look at these images and interpret changes in the rock strata deep underground.

In an accompanying story (below), we discuss how laboratory research at NETL, using technology, such as computerized tomography (CT) (imaging the pores in the rock the same way that CT or CAT scans are used to image what is going on inside a person’s body), scanning electron microscopes, and high-pressure, high-temperature core flow equipment, are being used to refine the interpretation of seismic signals so that they can be used to track the movement of CO₂ deep underground. It is anticipated that this will be a vital component of future MVA efforts. Then, in an accompanying LabNote (below), we discuss an actual field test where the use of PFCs was combined with information from seismic imagery to provide information on strata permeability that will be useful in future sequestration and MVA at that location. An actual seismic image is provided to illustrate how useful such images can be.

Seismic Surveys are Being Used to Track Carbon Dioxide Movement Underground

	NETL researchers are measuring fluid flow at pressures and temperatures that are representative of conditions deep underground, and determining the effect that changing the amount of CO2 or other fluids has on ultrasonic seismic signal velocities in the rock.

NETL researchers and collaborators at the University of Pittsburgh have developed a correlation between acoustic wave velocities measured in the laboratory and relative carbon dioxide (CO₂) saturation, which allows researchers to calibrate and refine the interpretation of seismic reflection surveys. The researchers are able to simulate the flow of CO₂ in a core sample of reservoir rock at the temperatures and pressures that are encountered deep underground, and then measure how the amount of CO₂ present affects how seismic energy moves through the rock. Recent laboratory tests showed that there is a marked decrease in signal velocity when CO₂ is introduced into the pore space, and that this decrease is large enough to be detectable in a seismic survey. Although it was previously possible to observe apparent gas plumes underground, interpretation of the data was very complicated because of all of the unknowns, such as how different types of ultrasonic seismic signals are affected by the concentrations of various fluids at the temperatures and pressures encountered at various depths. So, it was assumed that the image changes were due to migration of the gas, but the margin of the migration was hard to define and the changes observed could possibly have reflected other changes caused by the injection, such as movement of other fluids, like brine. However, by conducting laboratory tests on actual samples of the rock that the seismic signals are passing through, the effects of CO₂ can be more easily and more precisely measured, which in turn makes it possible to much more easily track the movement of the CO₂ deep underground using surface instruments. In addition, subtle changes in the rock matrix, caused by the injection process and the CO₂, can be much better understood after modeling these experiments.

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This image, created by computerized tomography (CT), shows connected pores in white and unconnected pores in red, within a sample of a rock type that may eventually be used to sequester carbon dioxide; the rectangular cube shown is 1.54 x 1.40 x 1.12 mm.

Associated laboratory tests indicate that it may also be possible to use changes in the seismic images, which symbolize reflection strength, variations in this strength between surveys, and other changes observed over time, to identify changes in fluid phases deep underground. Laboratory experiments conducted at NETL have examined how rock porosity can be changed by the dissolution or precipitation of minerals by the CO₂ that dissolves in the water that is present, but it is important to examine the large-scale effects of this over time to predict the long term fate of the injected CO₂, since changes in how rock pores are connected can dramatically affect the rate at which the CO₂ flows underground. The image below shows how some pores are connected, which allows fluids, such as CO₂, to flow through the rock.

Associated laboratory tests indicate that it may also be possible to use changes in the seismic images, which symbolize reflection strength, variations in this strength between surveys, and other changes observed over time, to identify changes in fluid phases deep underground. Laboratory experiments conducted at NETL have examined how rock porosity can be changed by the dissolution or precipitation of minerals by the CO₂ that dissolves in the water that is present, but it is important to examine the large-scale effects of this over time to predict the long term fate of the injected CO₂, since changes in how rock pores are connected can dramatically affect the rate at which the CO₂ flows underground. The image below shows how some pores are connected, which allows fluids, such as CO₂, to flow through the rock.

MVA Technology Proves Useful in Pilot-scale Field Test

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	Figure 1. A seismic image of a 9 square mile area of the lower Fruitland coal seam at a sequestration test site in New Mexico; the thickest areas of the coal seam are shown in red. The dots represent the injection well and the monitoring wells.

Accurate monitoring, verification, and accounting (MVA) requires knowing where the CO₂ will go once it is injected deep underground. After all, to determine if CO₂ is leaking, one must know where to look. Geological sequestration field tests are being conducted so that we can learn what works and what does not. However, we are still learning; this LabNote provides an example of how complex such predictions can be and the importance of conducting such research now, well before CO₂ sequestration becomes standard practice.
 
Models of how the CO₂ would flow at the San Juan Basin pilot test site (being conducted by the Southwest Regional Partnership (SWP) on Carbon Sequestration) were created using information from wells drilled down through the strata, geophysical logs, and rock samples collected during drilling of the wells. Movement of injected CO₂ into the Fruitland coal seam was expected to follow the prominent face cleat trend. Face cleats are fairly continuous fracture systems within coal, and it was expected that the injected CO₂ would migrate through these fracture systems. Because the face cleats there have a northeast-southwest orientation, it was anticipated that the injected CO₂ would first be detected at a well about 1500 feet to the southwest; however, the PFC tracer, and later, the CO₂, was first observed in the east offset well, opposite to what was expected.

The SWP obtained seismic data to help researchers better understand the subsurface geological environments deep underground and why the CO₂ moved the way it did. Seismic data is based on analysis of how energy associated with vibrations, in this case, locally imposed shock waves, passes through and reflects off of subsurface features. Seismic images can provide us with a more comprehensive picture of subsurface CO₂ storage environments than is possible from sparsely distributed well data, but can also be tricky to interpret because so many factors affect the path and speed of the reflected signals. The 3D seismic interpretations of the subsurface in the vicinity of the San Juan Basin pilot site revealed additional complexity in the coal depositional environments and their deformation. The seismic imagery indicated that the coal apparently thickened northeast of the injection well, and that this thickened zone has a northwest-southeast trend. The injection well happens to be located near the southwestern end of this thickened coal zone (see Figure 1).

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Figure 2. 3D seismic imagery reveals discontin-uities that cut through laterally coherent reflection events. This image represents a line about 3.4 miles in length and oriented in the NE-SW dip direction through the injection well (see Figure 1). Neighboring wells are located about 1500 feet from the injection well. Arrows point to the tips of linear features cross-cutting reflection events, which represent possible faults and fracture zones. MVA efforts should be concentrated near and above such zones.

The researchers working with these data speculate that the injected CO₂ that entered the thicker coal section migrated more rapidly, which would explain why the injected PFC tracers were first observed in the well east of the injection well, near the edge of the thickened coal zone. Researchers working with the 3D seismic data also observed that the area is more complexly deformed than expected (Figure 2). These subsurface structures may provide additional pathways along which injected CO₂ might escape from the reservoir interval (the Fruitland in Figure 2) and migrate to higher strata, and possibly to the surface.

Thus, MVA activities were critical to this pilot scale test. They revealed unexpected behavior in the subsurface CO₂ flood. This kind of information is critical in determining whether a given site would be appropriate for larger-scale sequestration and, if so, where monitoring wells should be concentrated.  Second generation flow simulations will be developed in the coming year to incorporate the results of ongoing seismic research.