Retrograde condensation in natural porous media: An in situ experimental investigation

Abstract

Complex in situ behavior of fluids during a retrograde condensation process is experimentally investigated in a miniature sandstone core sample. Two depletion experiments were conducted with various pressure decline rates using a three-component synthetic gas mixture with a dew point of 3610 psi. A state-of-the-art miniature core-flooding system integrated with a high-resolution micro-computed tomography scanner was employed to acquire pore-scale evidence of condensate nucleation, growth, accumulation, and mobilization in a natural porous medium under different depletion conditions. Analysis of pore-scale fluid occupancy maps demonstrates the formation of discrete nuclei of the condensate in pore throats and crevices as the pressure drops slightly below the dew point. The in situ fluid configurations show that a greater pressure drawdown rate significantly increases the condensate growth and accumulation. The results also illustrate the occurrence of condensate-to-gas imbibition displacements, i.e., snap-off and piston-like events, and the consequent trapping of the gas phase in the pore space. As the pore pressure is reduced, the condensate droplets are found to connect to each other through wetting layers, whereas the large gas clusters are continuously fragmented into smaller globules with reduced hydraulic connectivities. This effect was more pronounced in the case of the high depletion rate experiment. Furthermore, the condensate banking was not completely eliminated (through evaporation) by re-injecting the gas phase. This implies that in the development of a gas condensate reservoir, condensate dropout and banking should be minimized in the first place by, for instance, producing at lower pressure drawdown rates.