Experimental Study and Modeling of the Effect of Nanoconfinement on Hydrocarbon Phase Behavior in Unconventional Reservoirs

Abstract

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 nanochannels 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 microchannels 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.

Effect of Confinement on Pressure/Volume/Temperature Properties of Hydrocarbons in Shale Reservoirs

“…Ma et al (2013) and Jin et al (2013) used the same data set as Devegowda et al did with additional experimental data from Vishnyakov et al (2001) and Singh and Singh (2011) to develop models to approximate the shifts on critical properties of confined hydrocarbons. Similar to the model from Zarragoicoechea and Kuz (2004), the critical shifts of Ma et al (2013) and Jin et al (2013) are functions of pore width and type of contained fluid.…”

“…It is reported that the water mobility of confined water becomes significantly lower than that of bulk water (Hirunsit and Balbuena 2006). In addition, Zarragoicoechea and Kuz (2004) showed that the shifts on the critical temperature are proportional to the size of the mesopore. Normally, as the pore size decreases, the critical temperature and freezing/melting point tend to decrease (Kanda et al 2004;Moore et al 2010).…”

“…Zarragoicoechea and Kuz (2004) proposed a critical shift model derived from van der Waals interactions. According to their studies, the shifts of critical temperature and pressure behave similarly, and they are dependent on pore width and type of contained fluid.…”

“…All these studies imply that derivation of mixture phase diagram is not straightforward because the interaction between pore wall and fluid molecule is important to phase equilibrium in confined space. Teklu et al (2014) modified the conventional vapor/liquid equilibrium calculations to account for both the capillary pressure and the critical shifts in nanopores (Zarragoicoechea and Kuz 2004). In particular, they applied the critical properties obtained from the model of Zarragoicoechea and Kuz to an iterative algorithm including the capillary-condensation effect (Kelvin equation).…”

Effect of large gas-oil capillary pressure on production: A compositionally-extended black oil formulation

“…Results using this approach may not be valid, however, for mixtures. Recently, Wang et al (2014) conducted phase behavior experiments in micro-and nano-sized channels and concluded that bubble-point pressure for pure components is significantly lower in nano-sized channels as compared to the micro-sized channels. They also examined a three-component system and concluded that in addition to capillary pressure effects, liberation of lighter components from the oleic phase to the vapor phase further suppresses the bubble-point pressure in the nano-sized channels.…”