Adsorption-induced swelling of porous media

This paper studies deformation and swelling of porous media induced by sorption of fluids, and in particular carbon dioxide. The phenomena are significant, not only to the capacity of porous formations for storing CO2, but also to the effect that they have on the mechanical properties of the formations and microseismic events that they might possibly trigger. To study the phenomena we formulate the problem by energy consideration in which the Hamiltonian (total energy) of a porous medium and its fluid content is represented as the sum of the elastic energy of the system, and the energies associated with the interactions between the adsorbates and between them and the medium's solid matrix. The gas phase is described by a density functional theory, while the solid matrix can be either linearly or nonlinearly elastic. The model provides predictions for the sorption isotherms, the dependence of the strain on the bulk pressure, and the change in the porosity. The change in the strain is anisotropic due to the deformation of the solid, as well as the difference between the stiffness of the matrix and the fluid phase. During desorption the strain is also released and, similar to the sorption isotherms, exhibits hysteresis. This opens up the possibility of using mechanical measurements during sorption experiments to gain insight into the structure of a porous medium. When the model was applied to sorption of CO2 in clay particles, all the reported experimental features of the phenomena were reproduced.