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.
The authors used only two shale samples with dissimilar compositions, and unsurprisingly found differences in the amount of adsorption, so this research cannot be generalized broadly, but the results are encouraging and suggest that more studies would be useful.
Lafortune, S., Adelise, F., Garrido, D.R.R., Pokryszka, Z., 2014. Assessing CO2 Adsorption Capacities onto Shales Through Gravimetric Experiments: A First Step in the Feasibility Study of Coupling “Fracking” with Carbon Storage. Energy Procedia 63, 5933-5937. http://www.sciencedirect.com/science/article/pii/S1876610214024448