The pore sizes of shale and other unconventional plays are of the order of tens of nanometers.Based on the fundamental theory of thermodynamics,several studies have indicated that,in such small pores,phase behavior is affected by the capillary pressure and surface forces and is different from that characterized in PVT cells.No experimental evidence of this phenomenon,however,has been presented in the literature.In this study,we apply nanofluidic devices to visualize phase changes of pure alkane and an alkane mixture under nanoconfinement as a means to approach oil/gas phase behaviors in nanoporous rocks.Pure alkane starts vaporizing in the micro-channels first,and then the meniscus flashes into the nano- channels immediately after the complete vaporization of the liquid in the micro-channels.The vaporization of the ternary hydrocarbon mixture,however,is very different from pure alkane.Although the liquid starts to vaporize in the micro- channels first,as expected,the meniscus cannot propagate into the nano-channels in a comparable time scale as the pure alkane.The reason is that the liberation of lighter components from the liquid phase to the gas phase in the micro-channels increases the apparent molecular weight of the liquid in the nano-channels,suppressing the bubble point of the remaining fluid.A modified flash calculation procedure that uses the sizes of micro-channels and nano-channels as the characteristic lengths and assumed contact angle can reproduce the vaporization propagation sequence in the experimental observations. Experiments and modeling presented in this paper provide the proof of the concept and promote the understanding of phase behavior in nanoporous unconventional reservoirs.
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