How Well Does CO2 Adsorb onto Shale?

by Emil Morhardt

If one of the big advantages of using supercritical CO2 rather than water for fracturing shale is that it effectively disposes of the CO2 by absorption onto the shale (Middleton et al. 2013—see Jan 13 post), some experimental evidence would be useful. This is provided by Lafortune et al. (2014) who obtained a sample of shale from a Mesozoic marine basin in France, dried and crushed it, put it on an ultrasensitive balance, and flooded it with CO2 at various pressures and temperatures. The highest pressure was 9 MPa (90 bar, or 90 times atmospheric sea level pressure) and the highest temperature 328 K (131ºF), just at the combination of temperature and pressure at which CO2 becomes supercritical (see figure). This is only about a tenth of the pressure sometimes achieved in actual hydraulic fracking, and probably a somewhat lower temperature than normally used, but might be all that is necessary when using supercritical CO2. The higher the temperature the less CO2 adsorbed onto the shale so that the observation by Middleton et al. that temperatures of supercritical CO2 drop suddenly at the shale when the pressure is released augur well for increasing CO2 adsorption. There was a nearly linear increase in the amount of CO2 adsorption onto the shale with pressure, with no sign of leveling off at the pressures these experimenters used, so that too suggests an effective means of both sequestering CO2 and releasing methane, although the adsorption was not as high as it would have been on coal, someplace else it might be profitably sequestered. Continue reading

Using Supercritical CO2 Instead of Water for Fracking

by Emil Morhardt

The purpose of hydraulic fracturing is to use high pressure to open up pores in deep fuel-bearing shale deposits so that the oil or natural gas can escape through boreholes to the surface. To make this work, very high pressures (hence, much surface equipment) and a great deal of water are required. To keep the pores propped open when the pressure and water recede, something (usually sand) needs to be included. The inclusion of acid can increase pore efficiency, and because water is a good biological medium, antibacterial agents may be required to prevent fouling. Finally, most of the fracking fluid returns to the surface where it presents a treatment and disposal problem. But in theory, any liquid, or supercritical substance, would work, supercritical CO2, for example. According to a study underway at Los Alamos National Laboratory (Middleton et al. 2014) sCO2 has a number of potential advantages over water, and some potential disadvantages as well. Continue reading