While many are focusing on atmospheric solutions to reduce greenhouse gases, some researchers are setting their sights on the ground -- deep underground. Li Li, an assistant professor of energy and mineral engineering at Penn State, is investigating geologic carbon sequestration (storing carbon dioxide deep beneath the surface of the Earth) as a way to reduce the amount of carbon dioxide in the atmosphere.
The process, which at this point is in early stages of development with a few pilot tests, would involve the collection of carbon dioxide from stationary sources such as power plants, the compression of carbon dioxide into a supercritical fluid form with a density similar to a liquid, and the injection of the supercritical liquid into various formations deep below the Earth's surface (at about one kilometer or deeper). Because of the high pressure underground, the injected fluid will maintain its supercritical form and could be stored underground for long periods of time in deep saline aquifers (which cannot be used as a source for drinking water), coal seams and depleted oil and gas reservoirs.
Li's research, with partial funding from the Department of Energy's National Energy and Technology Lab (NETL), is focused on the possibility for and potential impact of the leakage of carbon dioxide from underground sequestration sites. For example, her group is studying the effect of carbon dioxide and brine on wellbore cement integrity, and also is studying the potential impact of carbon dioxide leakage on the quality of drinking water.
The primary leakage pathways for CO2 are fractures in rock formations, and abandoned oil and gas wells. If CO2 does leak from fracture and reaches sediments and freshwater, dissolved CO2 can lead to an increase in acidity (decrease in pH), which can in turn result in other reactions, including heavy metal mobilization from sediments, increase in alkalinity, and accelerated mineral dissolution and precipitation. All these reactions can potentially change aqueous geochemistry and water quality. Figure courtesy of Li Li (from “Evaluation of Potential Changes in Groundwater Quality in Response to CO2 Leakage from Deep Geological Storage,” by J.A. Apps et al, Transp Porous Med (2010) 82:215–246).
In paper recently published in the journal Environmental Science & Technology, her graduate student Evan Frye and co-authors have shown that the leakage of carbon dioxide into aquifers can cause heavy metals to mobilize. The extent of the mobilization depends on several factors, including the velocity and amount of carbon dioxide leaking from underground storage areas, and the properties of the aquifer into which the gas is leaking.
NETL is working with researchers to pioneer the Carbon Sequestration Program, whose goal is the development of "a technology portfolio of safe, cost-effective, commercial-scale carbon dioxide capture, storage and mitigation technologies that will be available for commercial deployment beginning in 2020." The goal of Li's research is to quantify the risks associated with carbon sequestration, understand the coupled processes in the complex subsurface, and determine the optimal injection conditions and sequestration sites.
Li said a primary challenge for researchers is that the scale of the work is so large, both in terms of time and space.
"Typically it is much easier for researchers to fully understand the scope of their work when the scale is small, in a laboratory where scales range from microns to meters and within time frames from seconds to years," she said. "However, geologic carbon sequestration involves areas of tens or hundreds of kilometers and time scales of hundreds or thousands of years. The issue of scaling is a big challenge as we work to quantify and predict the ultimate fate of carbon dioxide stored deep beneath the Earth's surface."
Also, it is prohibitively expensive to drill wells and take samples from the depths that would be involved in carbon sequestration, so research often relies heavily on mathematical models. Li uses a combination of numerical simulations and experimental work that aims to mimic relevant underground conditions. She and her team, composed of a few graduate students and post doctoral scholars, split their time between experimental work and numerical simulation to better understand the subsurface complexities associated with geologic carbon sequestration.
Li, who holds a doctoral degree in environmental engineering from Princeton University, is an assistant professor of energy and mineral engineering in the John and Willie Leone Family Department of Energy and Mineral Engineering in Penn State's College of Earth and Mineral Sciences. She can be reached by email at firstname.lastname@example.org.