Deep underground, reservoirs of salty water could be the answer to the carbon dioxide (CO2) floating far overhead. Geologic carbon storage is the process by which CO2 is captured at large sources, such as coal-fired power plants, and pumped into these reservoirs to be held permanently beneath the earth. One of the safest processes that traps CO2 in these reservoirs is when the CO2 dissolves into the brine. Unfortunately, this can be difficult to predict because of the high pressure and temperature of deep reservoirs. Recently, at the Center for Frontiers of Subsurface Energy Security (CFSES), a team of scientists, led by Philip Bennett, a geochemist at the University of Texas at Austin, discovered a way to predict CO2 solubility during geologic storage.
One of the difficulties in understanding how CO2 behaves in these deep reservoirs is figuring out how to conduct safe experiments at high temperature and pressure. In other words, before the CFSES scientists could figure out how to predict CO2 solubility, they first had to figure out how to measure it. Kimberly Gilbert, a recent Ph.D. graduate working with Bennett, devised a clever system to experimentally measure CO2 solubility under high temperature and pressure.
First, Gilbert took a stainless-steel reactor full of a high-pressure mixture of CO2 and brine and incubated it at a constant temperature. Once the CO2 was dissolved into the brine, a small amount of brine was removed and weighed. Next, the brine with the dissolved CO2 was connected to a large empty chamber. No longer under high pressure, the CO2 left the brine and filled the chamber. Once the CO2 was at room temperature and low pressure, the amount of CO2 in the chamber could be calculated using the ideal gas law, which relates gas temperature, pressure, and volume. Measuring CO2 in the large chamber told the researchers how much CO2 was dissolved in the high-pressure reactor.
Once the researchers knew how much CO2 dissolved into these high-pressure brines, they began looking for a way to correlate their results. Most efforts at predicting solubility rely on the concentration of ions, or the ionic strength, of the brines. However, the researchers found that this did not work well for their high temperature and pressure experiments. Instead, they examined the relationship between CO2 solubility and the amount of water tightly held by dissolved ions.
Gilbert explains, “An ion in water has several shells of water molecules surrounding it. In the innermost shell, the water molecules are held so tightly that the water is incompressible. This water, known as electrostricted water, does not easily move away from the ions to surround CO2 molecules, which is required for CO2 dissolution.”
The amount of water molecules held by an ion is complicated and relies upon the size, shape, and charge of the ions as well as what other ions are found in the brine. Using data gathered from other scientists, Gilbert closely correlated her experimental results with the amount of electrostricted water in her brines. In the future, this discovery will allow scientists to quickly predict how much CO2 will dissolve during geologic CO2 storage just by knowing the composition of the brine.
After the success of this work, Bennett’s team is continuing to develop novel experiments. They are now exploring how clay minerals interact with CO2 under high pressures and temperatures.
