Carbon-dioxide storage and phase transitions: towards an understanding of crack development in the cap-rock layer

Supercritical CO₂ can be injected into deep saline aquifers to reduce the amount of CO₂ in the atmosphere and thus, lessen the impact on the global warming. Qualified reservoirs should be in a sufficient depth to guarantee the thermodynamical environment for the supercritical state of CO₂ and should be confined by an impermeable cap-rock layer. It is crucial to guarantee the safety of the storage site. Therefore, deformation processes and crack development of the rock matrix and the cap-rock layer, which might be induced by the high pressure injection of CO₂, must be investigated. If cracks occur, CO₂ could migrate into shallower regions, where the temperature and pressure cannot support the supercritical condition of the CO₂ anymore. Thus, it is important to describe the phase transition process between supercritical, liquid and gaseous CO₂. The Theory of Porous Media (TPM), see e. g. [1], provides a useful continuum-mechanical basis to describe real natural systems in a thermodynamically consistent way. Hence, the TPM is applied to model multiphasic flow of CO₂ and water and to include elasto-plastic solid deformations of the porous matrix. The Peng-Robinson equation, e. g. [2], is implemented as a cubic equation of state to describe the phase behaviour of CO₂. However, the two-phase region cannot be represented by a continuously differentiable function such as the Peng-Robinson equation and thus, the Antoine equation provides additional information of the vapourisation curve. The Extended Finite-Element Method (XFEM) will be used to account for the discontinuities arising from crack development due to solid deformations [3]. Herein, special attention has to be paid to the matrix-fracture interaction of the fluid phases. Numerical examples are performed to investigate the injection of CO₂ into a saline aquifer. These are computed with the FE program PANDAS, which allows for solutions of strongly coupled multiphasic problems in deformable porous media. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)