Article Cite This: Langmuir 2018, 34, 1967−1980 pubs.acs.org/Langmuir Capillary Condensation of Binary and Ternary Mixtures of n‑ − − Pentane Isopentane CO2 in Nanopores: An Experimental Study on the Effects of Composition and Equilibrium Elizabeth Barsotti,* Soheil Saraji, Sugata P. Tan, and Mohammad Piri Department of Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071, United States *S Supporting Information ABSTRACT: Confinement in nanopores can significantly impact the chemical and physical behavior of fluids. While some quantitative under- standing is available for how pure fluids behave in nanopores, there is little such insight for mixtures. This study aims to shed light on how nanoporosity impacts the phase behavior and composition of confined mixtures through comparison of the effects of static and dynamic equilibrium on experimentally measured isotherms and chromatographic analysis of the experimental fluids. To this end, a novel gravimetric apparatus is introduced and validated. Unlike apparatuses that have been previously used to study the confinement-induced phase behavior of fluids, this apparatus employs a gravimetric technique capable of discerning phase transitions in a wide variety of nanoporous media under both static and dynamic conditions. The apparatus was successfully validated against data in the literature for pure carbon dioxide and n-pentane. Then, isotherms were generated for binary mixtures of carbon dioxide and n- pentane using static and flow-through methods. Finally, two ternary mixtures of carbon dioxide, n-pentane, and isopentane were measured using the static method. While the equilibrium time was found important for determination of confined phase transitions, flow rate in the dynamic method was not found to affect the confined phase behavior. For all measurements, the results indicate qualitative transferability of the bulk phase behavior to the confined fluid. 1. INTRODUCTION lower pressures in an isothermal system or at higher fl temperatures in an isobaric systemin confinement than in Although the study of pure, single-component uids in fi nanopores has been broadly undertaken, there is very little the bulk. This con nement-induced phase change, called capillary condensation, has been reported in the literature, see knowledge as to how mixtures in nanopores behave. A 3 fi Barsotti et al. for a comprehensive review, yet most of the quantitative realization of nanocon nement-induced mixture fl behavior is prerequisite to breakthroughs in many fields from associated studies involve single-component uids in simple pore systems far removed from those encountered in the medicine and biology to materials science and electrochemistry. 3 An example of how significantly a comprehensive under- reservoir setting. Those studies that have been carried out on ff fl multicomponent fluids are scarce, providing little overall insight standing of the e ects of nanoporosity on uid mixtures can fi fl impact each of these scientific endeavors can be found in into the phase behavior of con ned uid mixtures. The majority petroleum engineering, where the ability to accurately predict of the experimental studies have been carried out under isobaric fi fi fl conditions to probe the confinement-induced bubble point. A con ned mixture behavior could signi cantly in uence the fi economic valuation of shale and tight gas reservoirs. limited number of studies have observed the con nement- induced dew point, while a few others have focused more on Within the next few decades, natural gas consumption is fi fl projected to increase more than that of any other energy the structure of the con ned uid during the phase transition 1 with emphasis on phase separation. To the best of our resource. Much of this growth in demand will be satiated by fi vastly increasing production from shale and tight gas knowledge, none have witnessed the con ned critical point of reservoirs.1 In spite of this, very little is known about the mixtures. fl fl Studies on the confined bubble point include the work of physics of uid ow, transport, and storage in these reservoirs. 4 5 6,7 8 In particular, there is virtually no understanding of fluid phase Cho et al., Luo et al., Jones and Fretwell, and Yun et al. fi While all the studies, except for that of Luo et al.,5 witnessed behavior in such systems. Shale gas reservoirs are typi ed by fi nanopores, which constitute a significant fraction of their total depression of the con ned bubble point with respect to that of porosity.2 The scale of these pores, alone, regardless of their chemistry or geometry, may alter the phase behavior of the Received: December 4, 2017 confined fluids from their bulk counterparts. Specifically, the Revised: January 9, 2018 vapor-to-liquid phase transition may occur earlierthat is, at Published: January 23, 2018 © 2018 American Chemical Society 1967 DOI: 10.1021/acs.langmuir.7b04134 Langmuir 2018, 34, 1967−1980 Langmuir Article the bulk, the results must be viewed in the context of the same pressure and temperature, so that the confined fluid did experimental path. The two paths available for studies of not comprise a homogenous mixture.10 Thus, confinement confined phase phenomena are adsorption and desorption. In could not only affect the phase transitions of fluids, such as adsorption, the initial phase of the bulk fluid is gaseous. natural gas, but also their compositions, including the pore fluid Adsorption experiments are exemplified in the literature by the occupancy in the event of confinement-induced phase works of Jones and Fretwell7 and Yun et al.,8 who used positron separation. This could prove important to the ultimate recovery annihilation spectroscopy and a volumetric flow-through of shale gas, because the pore fluid occupancy dictates the approach, respectively. In desorption, the initial phase of the mechanisms by which various phases will be produced. bulk fluid is liquid. Desorption experiments are represented in In an effort to better understand the effect of confinement on the literature by studies employing density scanning calorimetry both the phase transitions and compositions of fluids in measurements, such as that of Luo et al.5 nanopores, a novel gravimetric apparatus11 is introduced for the Although the adsorption and desorption experiments both study of both pure fluids and mixtures in a variety of porous result in confinement-induced shifts of the fluid phase media using both static and dynamic processes. Unlike the transitions, the results are quantitatively different. Alam et al. apparatuses used in previous studies of confined fluid mixtures, explained this difference in their positron annihilation spec- such as the isobaric differential scanning calorimetry5 and troscopy study of the confined dew points of binary mixtures of positron annihilation spectroscopy measurements9 or the nitrogen and argon.9 They found inequalities between the isothermal volumetric measurements of Yun et al.,8 our confined dew points measured using adsorption and desorption apparatus uses changes in mass to directly measure the amount to result from enrichment of the confined fluid by the bulk fluid of fluid adsorbed. This allows for isothermal measurements that during desorption.9 Thus, although adsorption and desorption are more relevant to shale gas recovery than isobaric both qualitatively indicate confinement-induced shifts of the measurements, that is, temperature can generally be considered phase transition, the degree to which those shifts occur is highly constant in gas reservoirs, and more accurate12 than volumetric dependent on whether desorption or adsorption is taking place. measurements, which cannot measure the adsorbed amount Furthermore, the studies can be made either statically or directly but rather depend on equations of state to calculate it. using a flow-through method, such as that used by Yun et al.8 Similarly, this apparatus has the ability to facilitate large Whereas the other studies involving gas mixtures used a static quantities of adsorbents, including core plugs housed in high- approach in which the fluid within the pores was stationary at pressure core holders. Such a high capacity gravimetric equilibrium, the study of Yun et al. involved fluids that were apparatus has not been previously reported in the literature. always flowing and therefore experienced dynamic equilibrium.8 Although the evaluation of the confined phase behavior of Putting this into the context of natural gas production, the reservoir fluids in core plugs was beyond the scope of this static and dynamic experiments approximately represent study, the ability of the apparatus to support a core holder and different yet complimentary situations throughout the life of a its associated plumbing was tested and validated throughout reservoir. For example, the static experiments best approximate this study by utilizing a titanium core holder packed with unproduced reservoirs in which fluids are stationary, the MCM-41 for all experiments herein. situation of which is relevant to the original gas in place In this work, the apparatus was first validated using calculations. Conversely, dynamic experiments best approx- isothermal capillary condensation data in the literature for imate reservoir processes in which fluids are flowing, such as pure carbon dioxide and pure n-pentane and with bulk production and injection, but with a constant flow rate. condensation data for both compounds from the National 13 Experimentally, the two methods differ in that during static Institute of Standards and Technology (NIST). Next, experiments, the overall (confined plus bulk) composition of building upon the data for the pure component isotherms, the fluid is constant while during the flow-through experiments, binary isotherms of carbon dioxide and n-pentane were only the bulk composition of the fluid is maintained constant measured for the first time using a static method and then a by the flow. Except for this methodological difference, there is dynamic, flow-through method. Finally, two ternary mixture no evidence for any difference in the underlying concept as isotherms for CO2, n-pentane, and isopentane were measured. both can provide the desired capillary condensation.