Capillary Condensation-Induced Imbibition in Nanoporous Media

Nanoporous rocks have considerable importance for subsurface energy applications, in particular as source rocks for unconventional oil and gas production and as low-permeability seals for geologic carbon sequestration or nuclear waste disposal. Improved knowledge of the fundamental interactions of fluids with nanoporous rocks would have significant energy, water and environmental benefits. This study presents a theoretical investigation of the processes controlling adsorption, capillary condensation and imbibition in such nanoporous media, using water uptake experiment in a shale core. The theoretical model, which is based on the density functional approach, explicitly includes the relevant interaction forces among fluids and solids while allowing for a continuum representation of the porous medium. The experimental data include gravimetrically measured mass changes in an initially dry core sample exposed to varying levels of relative humidity. Our model suggests that under a low rh condition (0.31) flow within the shale sample is controlled by adsorption and diffusion type processes. After increasing the rh to 0.81, the uptake of water vapor becomes more significant, and according to our model this can be explained by capillary condensation followed by immiscible displacement in the core sample. It appears that strong fluid-pore wall attractive forces cause condensation near the inlet which then induces water imbibition further into sample.