Cables from the electrodes to the surface are attached to the casing as it is lowered into the borehole.
A technique originally applied to monitor the flow of contaminants into shallow groundwater supplies has been repurposed by Lawrence Livermore National Laboratory researchers to monitor carbon dioxide pumped deep underground for storage.

Electric Resistance Tomography (ERT) has been installed to track where a plume of injected CO2 moves underground at Cranfield Oilfield, an oil field near Natchez, MS. The site is part of the Southeast Regional Carbon Sequestration Partnership (SECARB), a project that eventually will test more than one million tons of CO2 in underground formations.

At 10,000 feet, the ERT project at Cranfield is the deepest subsurface application of the method to date. ERT uses vertical electrode arrays, usually in a cross-well arrangement, to perform four-electrode measurements of changes in the spatial distribution of electrical resistance within a subsurface formation. Because the Cranfield site contains CO2, which is five times as resistive as its surroundings, ERT showed that significant resistance changes occurred during plume growth and movement.

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

A web of cables hangs over the drill-rig during the ERT electrode array installation process.
ERT, a technology developed for environmental and geologic applications at LLNL starting in the 1980s, is similar to a computed tomography scan. It images soil resistivity, and that gives scientists information on soil properties such as temperature, soil type, and saturation. In the case of the Cranfield project, it can provide Carrigan with critical information on what happens to the CO2 once it’s stored deep underground.

The ERT system was installed in two monitoring wells more than 10,000 feet deep and able to withstand more than 250 degrees Fahrenheit and 5,000 pounds per square inch (psi) of pressure.

Carrigan said monitoring plume characteristics requires sophisticated sensors, data acquisition devices, and imaging instruments involving different measurement techniques that are capable of operating in deep boreholes. The results of the monitoring are analyzed to ensure that the site is operating as expected. Even with the proper equipment, plume movement can be difficult to reconstruct due to uncertainties in reservoir structure and unknown multiphase fluid processes.

However, ERT can convert a large number of resistance measurements into an image of electrical resistivity distribution associated with the plume. “Because changes in CO2 concentration and saturation cause changes in resistivity, ERT is a useful monitoring tool,” Carrigan said. ERT monitoring is potentially capable of signaling leakage from a sequestration reservoir possibly years before it can reach an overlying aquifer causing damage to water supplies.

The ERT system currently takes 10,000 measurements per day that Carrigan can access remotely. So far the technology has shown that the CO2 plume produces a strong signal and ERT has captured the basic plume details.

The advantages to power companies looking to store and monitor CO2 movement and storage underground include a robust system with no moving parts, outside-the-casing installation leaving the well open for other uses, and relatively low cost to install and continuously operate.