Experimental Study and Modeling of the Effect of Pore Size Distribution on Hydrocarbon Phase Behavior in Nanopores

Abstract Pore sizes of many shale and tight oil and gas reservoirs are in the range of nanometers. In these pores, the phase behavior of hydrocarbon mixture is affected by the capillary pressure and the surface forces and is different from that characterized in PVT cells. Many existing phase behavior models use a single pore size. This research investigates the effect of a pore size distribution on the phase behavior of hydrocarbon mixtures. Pure n-pentane and a ternary mixture of n-butane, i-butane, and n-octane were loaded into a nanofluidic device with microchannels and nano-channels to study phase transition due to evaporation. For n-pentane, evaporation in the nano-channels took place immediately after the liquid in the micro-channels completely evaporated. For the ternary mixture, however, evaporation in the micro-channels slowed down and did not progress into the nano-channels despite continuous heating, because evaporation in the micro-channels changed the composition of the remaining liquid. A vapor-liquid equilibrium calculation procedure that considers the effect of capillary pressure, the sequence of phase change due to pore size distribution and the associated compositional change was developed and used to simulate depressurization of light oil and retrograde gas inside nanoporous media. The pore size distributions were characteristic of tight reservoirs and the fluid compositions were representative of typical reservoir fluids. Predictions of the model show that phase transition in porous medium with pore size distribution is a process that cannot be described by a single phase boundary, because the initial phase change alters the composition of the remaining fluid, which in turn suppresses the next phase change. For the oil, capillary pressure due to nanoconfinement increased the level of supersaturation and the critical gas saturation had a strong influence on the properties of produced fluids; for the retrograde gas, the effect of capillary pressure was insignificant due to the low interfacial tension. Despite the choice of fluids, calculations indicate that the smallest pores are probably always occupied by hydrocarbon liquid during depressurization. Experiments and modeling presented in this research provide tools to investigate and understand the effect of nanoconfinement on phase behavior, which assist the development of shale oil and gas condensate reservoirs.