By Silke Schmidt
Releasing carbon dioxide (CO2) into the atmosphere by burning fossil fuels is primarily responsible for climate change. Therefore, a natural question to ask is: can we mitigate climate change by capturing and storing man-made CO2 , instead of releasing it into the air? A fair amount of research and development has been funded by governments around the world to shed light on the feasibility of so-called carbon capture and storage (CCS) systems.
One of the main challenges that CCS proponents have to address is the cost of capturing CO2 , relative to the current option of releasing it, free of charge to energy-producing companies, into the atmosphere. An example of an underground storage system that may not only pay for itself, but actually make a profit for the power plant that would build it has been proposed by Steven Bryant and colleagues; Bryant is a professor of petroleum and geosystems engineering at the University of Texas at Austin.
At the heart of the system is brine, a hot salty fluid that is found in deep underground aquifers along the U.S. Gulf Coast, and in many other oil-producing regions in the world. An aquifer is an underground layer of permeable rock, gravel, sand or silt from which a well can extract groundwater. Most of the deep (~3,000 m, ~9,000 feet) aquifers in the world contain hot and highly concentrated salt water, which is called brine.
Brine has two interesting properties: it contains a substantial amount of methane, the main component of natural gas, which is used in many U.S. households to heat water and air; and it easily dissolves CO2 in a pressurized environment.
In a nutshell, the closed-loop hot brine system proposed by Bryant and colleagues brings hot brine up to the surface, using an existing infrastructure of wells in the U.S. Gulf Coast. Next, it extracts the naturally occurring methane from the brine and ships it, through an existing system of methane-carrying pipelines, to various energy customers.
The system then sends the still-hot brine into a heat exchanger where it heats water, which is sent to a nearby town to provide warm air and water to homes; using geothermal energy to heat homes has been done in Europe for many years, but has not yet been implemented at a large scale in the U.S.
The remaining cold brine is sent to a mixer, which is hooked up with a nearby coal-burning power plant. The CO2 created inside the power plant is captured and sent to the mixer as well. Inside the mixer, CO2 is dissolved into cold brine under high pressure. This forces out more methane, which is sent to the same pipeline system that extracted methane from the brine when it was hot.
Since the pressurized brine that is now saturated with dissolved CO2 is denser than before, it has a natural tendency to sink deeper into the earth’s crust. This has two advantages: it reduces the energy demands for the final step of pumping the CO2 -enriched brine back down into the deep brine aquifer; and it enhances storage security, since the brine-captured CO2 is unlikely to rise up on its own. Back in the deep aquifer, the brine is re-heated by geothermal energy and eventually is brought up the well again, creating a closed-loop system.
Bryant’s proposed brine-sequestration system combines three processes: storing CO2 underground, tapping brine for methane fuel, and extracting geothermal heat from brine. On their own, these processes have thus far not held much interest to the energy industry: storing CO2 costs money, while releasing it into the atmosphere is free; natural gas in deep shale reservoirs has a much greater concentration than methane in brine, which is why hydraulic fracturing, or “fracking,” of deep shale has been heavily pursued by the U.S. during the past few years; and similarly, the energy obtained by heating water is some two orders of magnitude smaller than the energy obtained by burning the same volume of coal, oil or gas, which is why the U.S. has not yet exploited brine for the geothermal heating of homes.
But Bryant argues that the combination of the three processes, storing CO2 and selling both methane and geothermal heat for a profit that exceeds the storage technology cost, may be financially attractive to energy companies; and a financial incentive is what it takes to get a company interested in reducing carbon emissions, in order to mitigate climate change. At this time, several research laboratories are evaluating specific designs to efficiently inject CO2 into brine while extracting energy, and a few companies are considering the construction of pilot plants along the Gulf Coast.