From: David A Gabel, ENN
Published June 16, 2010 11:28 AM

Monitoring the Sequestration of Carbon Dioxide in the Earth

Global Warming is caused by several factors such as the release of greenhouse gases like carbon dioxide into the atmosphere. One solution to the problem is to capture the carbon dioxide before it enters the atmosphere, and instead, deposit the CO2 into the ground. However, up to this point, scientists have been unable to effectively track how it might move underground. The desire is to get the CO2 in place and not have it move elsewhere and potentially cause problems. Now, with the advent of Electrical Resistivity Tomography (ERT), developed at the Lawrence Livermore National Laboratory (LLNL), tested by the Southeast Regional Carbon Sequestration Partnership (SECARB), and funded by the Department of Energy’s National Energy Technology Laboratory, sequestration of greenhouse gases may expand.

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ERT technology is similar to computer tomography scan. It takes images of soil resistivity which allows scientists to determine the soil's properties, such as temperature, saturation, and soil type. Tomography is imaging by sectioning through the use of energy waves. It can be applied to other fields such as radiology, archaeology, oceanography, and astrophysics.

SECARB conducted its ERT experiment at the Cranfield Oilfield near Natchez, Mississippi. The project will test over one million tons of CO2 in underground formations at a depth of 10,000 feet, the deepest application of ERT technology to date.

ERT uses vertical electrode arrays set up in a cross-well arrangement. Four-electrode measurements are taken to monitor changes in the distribution of electrical resistance within the underground formation. Since the CO2 at the Cranfield site has resistivity five times greater than its surroundings, the ERT system can determine where the CO2 is, and in what speed and trajectory it is moving.

"We can image the CO2 plume as the fluid is injected," said geophysicist Charles Carrigan, the LLNL lead on the ERT project. "What we've seen is a movement of the plume outward from the injection well into the geologic formation used for storage."

Scientists have constructed two monitoring wells of more than 10,000 feet in depth at the Cranfield site. The ERT system installed is capable of withstanding over 250 degrees Fahrenheit and 5,000 psi of pressure. It takes 10,000 measurements per day which Carrigan and his team can access remotely.

At this point, the ERT system seems to be performing its job well to capture images of the underground CO2 plume. However, human intelligence is still required to analyze the findings. This can be a difficult task, even with the proper equipment. Uncertainties in reservoir structure and unknown multiphase fluid processes are not easy factors to incorporate. The goal in the end is to ensure that the underground storage of CO2 is operating properly. What good is it to capture the carbon only to find it leaking out of the ground in two years?

According to Carrigan, the hope is to start applying ERT systems to commercial CO2 underground storage sites. This type of technology is vital for the future of carbon sequestration. Power companies will be employing these systems as they look to reduce their carbon footprint.

For more information: http://www.secarbon.org/

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