18890 | Phys. Chem. Chem. Phys., 2017, 19, 18890--18901 This journal is © the Owner Societies 2017 Cite this: Phys. Chem. Chem. Phys., 2017, 19, 18890 Retrograde behavior revisited: implications for confined fluid phase equilibria in nanopores Sugata P. Tan * and Mohammad Piri Many fluid mixtures exhibit retrograde behavior, including those that define natural gases. While the behavior is well understood for mixtures in bulk, it is not so in nanosize porous space that dominates shale formations in unconventional reservoirs. The lack of experimental data creates the need for modeling works to make estimates as good as possible due to immediate needs in gas recovery. However, such efforts have been straying without firm guidance from systematic studies over what we have known so far. This article is intended to present the results of such a study that would incite further investigations in this area of research. Revisiting the retrograde behavior in the bulk is appropriate to start with, followed by a short review of what we know about fluids confined in nanosize pores. Based on this information, implications for the behavior of confined mixtures in the retrograde region can be inferred. The implied features that have been supported by experimental evidence are the locations of the confined dew point and bubble point at low temperatures, which are both at pressures lower than their bulk counterparts. Another feature found in this study is completely new, and therefore still open for further investigation. We reveal that the dew-point and bubble-point curves of confined mixtures end at moderate pressures on a multiphase curve, beyond which equilibrium occurs among the bulk and confined phases. The well-known points in the bulk retrograde region, i.e. the critical point and cricondenbar, are consequently absent in confined mixtures. 1. Introduction Retrograde condensation in vapor–liquid phase equilibria was first noticed by Kuenen in 1892. 1 It occurs when a fluid mixture condenses upon isothermal decompression or isobaric heating, where the opposite, i.e. , evaporation, is the more frequently observed process. Nevertheless, the retrograde behavior has been discovered to be very common in fluid mixtures, including oil and natural gases. In the recovery of natural gases, which starts with fluid at high pressure and high temperature in the reservoir, the gas-like fluid undergoes pressure reduction on its way to the production well. On many occasions, the conditions of this process allow the fluid to partly condense into liquid. This retrograde condensation is the physics responsible for the obstruction of the gas flow to the well, consequently reducing the production. There have been experimental and theoretical studies as well as careful reviews on retrograde condensation. 2–5 However, comprehensive thermodynamic studies are still required that can establish ways to understand more advanced situations such as what will happen if the retrograde fluid is entrapped in nanosize porous mediums, e.g., unconventional natural gas to be recovered from shale formations. It is well known that fluids confined in nanopores have significantly different behavior than in bulk. 6–9 The ultra-tight confinement enhances the interactions between the fluids and the surrounding solid walls, which effectively introduces unusual behaviors. For confined pure gases, the vapor phase condenses at the so-called capillary-condensation pressure that is experimentally lower than the bulk saturation pressure, or equivalently condensa- tion temperature higher than the bulk saturation temperature. Moreover, the critical point is lowered both in pressure and temperature in the nanosize confinement. Interestingly, despite the long-studied behavior of pure gases in nanopores, comparable experimental studies on con- fined fluid mixtures are rare, and studies on their retrograde behavior are virtually nonexistent. Even for dew points and bubble points, which are important in phase transitions, the experimental data are scarce for confined fluids. The data available in the literature were measured in the nonretrograde environment using various methods, but none of them was verified against each other. A differential scanning calorimeter (DSC) and volumetric method were used to measure the bubble points of binary mixtures of octane–decane in CPG 10 for the former and methane–octane and methane–decane in SBA 11 for the latter. They both applied the evaporation path, i.e., isobaric heating for the former and isothermal depressurizing for the latter, but provided opposite conclusions. DSC found that Department of Petroleum Engineering, University of Wyoming, Laramie, WY 82071, USA. E-mail: sptan@uwyo.edu Received 15th April 2017, Accepted 26th June 2017 DOI: 10.1039/c7cp02446k rsc.li/pccp PCCP PAPER Published on 26 June 2017. Downloaded by University of Wyoming Libraries on 07/09/2017 03:22:18. View Article Online View Journal | View Issue