Phase Transitions and Condensation in Tight Porous Materials

In this work, we investigate the impact of pore sizes and their distribution on condensation and phase transitions in tight synthetic and natural porous materials. Nuclear magnetic resonance (NMR) spectroscopy and Thermogravimetric (TGA) experiments are used to detect phase transitions for ethane and CO₂ above their critical temperatures in these porous materials. Mesoporous silica materials with a narrow PSD's at XX nm and YY nm were designed to provide a basis for comparing and validating different experimental techniques, and to facilitate detailed analysis via molecular-dynamics (MD) simulations. Condensation phenomena in a natural sandstone sample with a strong bimodal PSD (inter-grain and clay porosity) were also investigated.

We demonstrate that ethane phase behavior can be detected through liquid state ¹H-NMR, whether in the bulk or in porous media. By sweeping trajectories of the ethane phase diagram around the critical point in the bulk, we can detect any of the fluid phases (gas, liquid or supercritical fluid) in known porous samples. Distinct NMR spectra responses clearly enable us to differentiate phases within an unknown sample.

For the sandstone sample, we observe a sharp increase in the sample weight (TGA) at CO₂ pressures above 800 psi (at 50 ^oC), and we demonstrate that the observed increase is consistent with condensation occurring within the clay structure of the sample.

The experimental observations are complemented with molecular-dynamics (MD) simulations to explain the observed behavior in porous materials with homogeneous as well as heterogeneous compositions. To this end, Monte-Carlo simulations are performed in canonical configurations by considering different chemical potentials. Finally, we apply the theory of Minkowski functionals to interpret/describe the observed phase behavior.