Hydromechanical Aspects of CO₂ Breakthrough into Clay-rich Caprock

Caprock formations are intended to prevent upwards carbon dioxide (CO₂) migration to the surface during CO₂ geological storage. Caprock interaction with CO₂, as well as its potential consequences, requires to be predicted, and thus, need to be studied experimentally. Laboratory investigations of caprock behavior are complex due to its low permeability, and the scarcity of experimental studies involving high-pressure CO₂ injection into caprock representatives puts this difficulty into manifest. In this study, we perform laboratory experiments in an oedometric cell on intact and remolded Opalinus clay (Jurassic shale), evaluating the breakthrough pressure and permeability for liquid and supercritical CO₂. Intact and remolded shale specimens present intrinsic permeabilities of 10^-21 m² to 10^-20 m², respectively. Applied axial stress ranges from 27 MPa to 42 MPa and the pressure and temperature conditions are representative of a caprock at a depth of 800 m. We found that the microstructure of the caprock has a great effect on the material properties. The intrinsic permeability of a more tight material (intact Opalinus clay) is around two times lower than that of remolded shale, which has a more open microstructure. Additionally, the intact rock becomes 30 times less permeable to CO₂ than the remolded shale, which implies that the CO₂ relative permeability is 15 times smaller for intact rock than for remolded shale. On the other hand, CO₂ breakthrough pressure for the tighter material is almost three times lower than for the more permeable remolded shale. Breakthrough pressure of the remolded shale ranges from 3.9 MPa to 5.0 MPa for liquid CO₂ and from 2.8 MPa to 4.6 MPa for supercritical CO₂. For the intact shale, breakthrough pressure is 0.9 MPa for liquid CO₂ and 1.6 MPa for supercritical CO₂. Thus, the breakthrough pressure cannot be correlated with the intrinsic permeability of the caprock.