The effects of initial water saturation on retrograde condensation in natural porous media: An in situ experimental investigation of three-phase displacements

In this study, state-of-the-art imaging technology is integrated with a miniature core-flooding system to map pore-scale fluid occupancy during the retrograde condensation process in the presence of brine and in a natural porous medium. Two depletion experiments were performed using a three-component synthetic gas condensate mixture in a Fontainebleau sandstone core sample and after establishing different levels of initial water saturations. The results are analyzed to investigate the impact of water saturation on condensate formation, accumulation, and mobilization and to shed light on the relevant three-phase flow dynamics. The results provide the first direct pore-scale observation of gas, condensate, and brine residing in the pore space. The micro-scale fluid occupancy maps illustrated that the presence of water in the pores delays the pressure at which condensate nuclei form, slows down the initial growth of nuclei, and partially impedes the accumulation of condensate. The higher the water saturation is, the lower the amount of condensate will be accumulated. As the pore pressure decreases, the condensate clusters develop significantly in the pore space triggering chains of displacement events between various pairs of fluids. The key mechanisms include gas-to-brine/condensate and condensate-to-gas/brine. The displacement events eventually reduce the brine saturation and increase the gas and condensate saturations in the pore space. The condensate saturation increases monotonically as the pore pressure reduces and reaches a maximum value. Quantitative pore fluid occupancy data also reconfirm the visual observations and demonstrate that condensate accumulation is associated with displacing gas and brine from small- and medium-sized pore elements.