Experimental study of pore size distribution effect on phase transitions of hydrocarbons in nanoporous media

Accepted Manuscript Experimental study of pore size distribution effect on phase transitions of hydrocarbons in nanoporous media Sheng Luo, Jodie L. Lutkenhaus, Hadi Nasrabadi PII:

S0378-3812(18)30485-0

DOI:

https://doi.org/10.1016/j.fluid.2018.11.026

Reference:

FLUID 12013

To appear in:

Fluid Phase Equilibria

Received Date: 12 July 2018 Revised Date:

21 November 2018

Accepted Date: 22 November 2018

Please cite this article as: S. Luo, J.L. Lutkenhaus, H. Nasrabadi, Experimental study of pore size distribution effect on phase transitions of hydrocarbons in nanoporous media, Fluid Phase Equilibria (2018), doi: https://doi.org/10.1016/j.fluid.2018.11.026. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Experimental Study of Pore Size Distribution Effect on Phase Transitions of Hydrocarbons in

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Nanoporous Media

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Sheng Luo,† Jodie L. Lutkenhaus,*, ‡,§ Hadi Nasrabadi*,†

Harold Vance Department of Petroleum Engineering, Texas A&M University, College Station,

Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station,

Texas 77843 USA §

Department of Materials Science and Engineering, Texas A&M University, College Station,

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Texas 77843 USA

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‡

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Texas 77843 USA

KEYWORDS. Confinement, Phase Equilibria, Hydrocarbons, Pore Size Distribution, Adsorption, Desorption, Thermal Analysis

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ABSTRACT

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The phase behavior of a fluid in a nanoporous system is altered from that in the bulk due to confinement effects. There have been many experimental studies on this important effect, yet the vast majority are limited to porous media with a narrow or singular pore size distribution. However, natural porous media exhibit a broad pore size distribution. For example, in shale rocks the pore size ranges from several

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nanometers to hundreds of nanometers. It is not clear if a broad pore size distribution has a significant effect on the phase transition of nanoconfined fluids, and, if effects are present, how and why the phase

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behavior is altered. The effects of a distributed pore size on the bubble point of confined fluids as measured by differential scanning calorimetry (DSC) are not known. Here, we present an experimental study on the effect of pore size distribution on the liquid-vapor phase transition of n-hexane, n-octane and n-decane confined in nonporous media. We report a method to discretize the pore size distribution of natural shale rocks so that it can be mimicked using mixtures of synthetic nanoporous media. Using DSC,

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we show that the presence of a broad pore size distribution significantly alters the vapor-liquid phase transition of confined hydrocarbons. In fully saturated (i.e. filled) nanoporous media, the largest pore size dictates the onset of liquid-vapor phase transition (.i.e. bubblepoint). However, when the porous media is

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partially saturated, smaller pore sizes influence the bubblepoint. These findings suggest that there is a dependency of the confined fluid phase behavior on the fluid saturation in nanoporous media with a broad

media.

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pore size distribution, which contributes to a broader understanding of phase transitions in natural porous

MAIN TEXT INTRODUCTION

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In nano-scale pores confinement effects alter the phase behavior of fluids from that of bulk [1, 2]. Fluid properties in nanoporous systems have attracted much attention because of applications in fuel cell

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development [3, 4], separation design [5, 6], and shale oil and gas evaluation [7, 8]. Our interest in this topic arises from the endeavor to understand the phase behavior of petroleum fluids in shale reservoirs. Confinement effects are influenced by several factors, such as pore size and fluid-surface interaction

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strength, etc. Theories and experimental studies are reported for the fluid phase behavior in pores of a singular size [1, 9-11]. However, the pore sizes in shale are in the range of several nanometers to

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hundreds of nanometers [12], with the average pore size varying from case to case [8, 13, 14]. This broad pore size distribution leads to challenges in studying the phase behavior of hydrocarbons in shale porous systems. Therefore, an overall picture of pressure-volume-temperature properties of hydrocarbons in complex porous systems has yet to be achieved. Our interest specifically lies in developing a method to mimic the pore size distribution of natural geological formations using synthetic nanoporous media in

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before.

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order to understand the effects of pore size distribution on the bubble point, as this has not been done

Kelvin’s equation is one of the first theoretical methods to describe the phase transition in nanopores. By

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considering a pressure difference between the vapor phase and confined adsorbate as capillary pressure, Kelvin’s equation describes the pore condensation pressure in relation to pore diameter and interfacial tension. However, Kelvin’s equation has been found to underestimate pore size at the nanoscale [15, 16], and the deviations can be attributed to not considering for fluid-surface interactions and to the questionable usage of the bulk phase interfacial tension. Modeling the phase behavior for the confined fluid by equation of state (EOS) and capillary pressure has been reported [13], along with several efforts to improve capillary pressure estimation by introducing correction parameters to the Young-Laplace equation [17, 18]. Recently, a pore-size-dependent equation of state based on the generalized van der 3

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Waals theory was developed by Travalloni et al. [9, 10, 19], which featured a fluid-surface interaction as a square-well potential and which utilized chemical potentials to relate the equilibrium among phases. Luo et al. applied this approach in modeling the phase boundaries between the liquid, vapor, and

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supercritical hydrocarbon phases at multiple pore sizes [11]. Besides, density functional theory (DFT) [20] and molecular simulations [21-23] have been widely used, and substantial insights of fluid density distributions and phase transition processes in nanopores were obtained. Both methods simulated the

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phase equilibrium of the confined fluid by describing the system with fluid-fluid and fluid-pore surface

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interactions.

Experimental studies of confined fluid phase behavior have largely been performed using isothermal sorption. Through the sorption of fluid in nanoporous media, the pore condensation pressure, which is lower than the bulk saturation pressure, can be identified. Adsorption hysteresis is commonly observed [20], and the effect of surface properties and heterogeneities have been examined [24, 25]. Other useful

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experimental techniques for studying fluid phase behavior under confinement include temperatureprogrammed desorption (TPD) [26, 27], neutron diffraction [28], X-ray diffraction [29], volumetric measurement [30], and microscopic observation [31]. Recently, we developed a method using differential

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scanning calorimetry (DSC) to investigate the phase transition of hydrocarbons in surface-modified silicate nanoporous media (2-50 nm, single pore size investigated on a case-by-case basis) [11, 32-34].

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We observed that the bubble point temperature of confined hydrocarbons shifted upwards with decreased pore size until the fluid entered a supercritical state that was induced by confinement effects [11]. We also applied this technique to study the phase transitions of hydrocarbon mixtures in nanoporous media [34]. Elsewhere, Pathak et al. applied this approach in studying the fluid thermodynamics properties in the nanoporous Kerogen and ordered nanoporous media with specific single pore sizes [23, 35].

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Despite numerous theoretical and experimental studies on this topic, most works are limited to the phase behavior of fluid in a single-sized nanoporous media. For the studies relevant to pore size distribution, most have focused on interpreting the pore size distribution (PSD) with adsorption isotherms for nitrogen,

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argon, methane or other gas species. Barrett-Joyner-Halenda (BJH) method has been one of the most commonly used methods to determine the pore size distribution of nanoporous media [36]; however, it suffers from the inaccuracy of using Kevin’s equation. Gelb et al. compared the determined PSD of

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controlled-pore glasses obtained from grand canonical Monte Carlo (GCMC) simulations and analysis of isotherms by BJH method and found that the BJH method gave overly sharp distributions with a

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systematical shift of ca. 1 nm to lower pore sizes [37]. Samios et al. investigated the determination of micropore size distribution from GCMC and found that the most probable pore size from simulation agreed with the one found by the conventional nitrogen porosimetry method, while overall deviations existed [38]. López-Ramón et al. examined the adsorption isotherms of CH4, CF4, and SF6 at various pore sizes by GCMC and used each of the adsorptives to give a partial description of the full PSD of the

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microporous carbon [39]. By comparing the integrated isotherms with experimental isotherms, the media’s overall PSD was estimated [39-41]. However, to the best of our knowledge, there are very limited experimental studies on the phase behavior of intermediate to heavy hydrocarbons in nanoporous

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media with a known PSD, especially in relation to shale rock.

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Herein, we present an experimental study on the effect of pore size distribution on the confined phase behavior of hydrocarbons in nanoporous media. We use the differential scanning calorimetry (DSC) technique to perform the experiments. The effect of two specific pore size distributions based on Eagle Ford shale and Bakken shale is studied in these experiments. We present a novel approach to mimic and discretize the pore size distribution of the shale rock samples using synthetic nanoporous media. nHexane, n-octane and n-decane are infiltrated into nanoporous media, and DSC is used to measure the phase transition temperature of the confined hydrocarbons. 5

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EXPERIMENTAL SECTION

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We choose two representative pore size distributions for shale rock: one for Eagle Ford shale [14] and the other for Bakken shale [13]. Eagle Ford shale has a majority pore size of ca. 16 nm, and Bakken shale has a majority pore size of ca. 40 nm. Experimental synthetic porous materials of specific pore diameters were physically mixed to mimic the shale pore size distribution. Figure 1 shows the pore size distributions

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of the two shale samples and the representative mixture samples used in this work.

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Figure 1. Pore size distribution are from two shale cores: (a) Eagle Ford shale [14] and (b) Bakken shale [13]. The laboratory pore size distribution of shale rocks are shown as blue lines. The shaded columns are

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the representative discretized pore volumes of synthetic nanoporous media with a specific pore diameter used in this work. The details are discussed in experimental section.

Surface modified porous materials CPG-35 (controlled pore glasses, dia. 37.9 nm), SLG-15 (silica gel, dia. 14.8 nm), MSU-H (Michigan State University silica-H, dia. 9.8 nm), SBA-15 (Santa Barbara amorphous silica-15, dia. 6.0 nm), CPG-4 (controlled pore glasses, dia. 4.1 nm), SBA-16 (Santa Barbara amorphous silica-16, dia. 3.3 nm), MCM-41 (Mobil Crystalline Material-41, dia. 2.2 nm) were used as 6

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the synthetic nanoporous media with specific nanopore diameters. The properties of the experimental media are shown in Table 1. The preparation of the materials are described in our recent work [11, 33]. The silicate porous media were cleaned in boiling nitric acid at 100 oC, rinsed with deionized water, and

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dried under vacuum. Then, the porous media were reacted with hexamethyldisilazane (HMDS) at 55 oC to make the surface oil-wetting. The prepared materials were stored in a desiccator. In this surface modification the silanol ≡SiOH groups are capped with trimethylsilyl groups by reacting with HMDS

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[42]. The modification, applied to all the silicate porous media in a mixture, changes the surface from hydrophilic (water-wet) to oleophilic (oil-wet) [43], analogous to the materials of organic matter that host

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the nanopores in shale rock [44, 45]. Figure 2 shows the DSC thermograms of empty nanoporous materials used in this study: prior to adding hydrocarbons, there is no fluid in the nanopores and the porous media are thermally stable.

Porous Materials Pore Diameter (nm)

MCM-41

SBA-16 CPG-4 SBA-15 MSU-H SLG-15 CPG-35

2.2

3.3

4.1

6.0

9.8

14.8

37.9

1000

700-900

170

600

750

300

64.7

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Surface Area (m2/g)

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Table 1. Properties of synthetic porous materials

0.98

0.98

0.22

0.68

0.91

1.15

1.17

Pore Size Distributiona (±%)

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11

10

12

12b

16

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Pore Volume (cm3/g)

a

pore size distribution is given as 90% of pores are within the ±% range of nominal pore diameter

b

typical value from the same porous media in literature [46].

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Figure 2. DSC thermograms empty nanoporous materials listed in Table 1. All of these materials were modified with HMDS. The lack of any peaks indicates that the materials did not uptake any water.

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The porous materials in Table 1 were physically mixed to mimic the pore size distribution of shale rock (Figure 1). A specific portion of each type of material was used to represent the corresponding pore size

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portion of shale rock. An optimization method was used to determine the mass fractions of porous media. We note the pore diameters of n types of experimental porous media from smallest to the greatest as ,

,…,

and their pore volumes per mass (cm3/g) as

synthetic mixture are

,

,…,

,

,…,

and the pore volume fractions are

. Their mass fractions in the ,

,…,

. The mass fraction

relation and pore volumes can be written as: +

+⋯+

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= 1 (1)

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∙

=∑ ,

The discrete pore volumes

,…,

(2)

∙

represent the pore size distribution.

is the area of shaded

columns of the medium i in Figure 1. The height of the column is directly read from the PSD curve by the

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specific material’s pore diameter, as ℎ . The width

of the column is the range of PSD represented by

the pore diameter in the center.

(3)

Herein, we optimize the choices of column widths

,

,…,

by minimizing the gaps between the

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columns, where the target function g(w) can be written as:

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∙ℎ =

( )=

%

&

+ 2

!

"

−(

"

−

)$ (4)

The target function is minimized under the constraints of eq. 1 and the relations of eqs. 2 and 3 using the

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Matlab optimization toolbox. The results of mixing mass fractions for each case are listed in Table 2. The pore volume of the synthetic Eagle Ford sample is 1.09 cm3/g, and pore volume of the synthetic Bakken sample is 1.15 cm3/g.

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Table 2. Nanoporous media mixture representing the shale pore size distribution Porous Materials

MCM-41

SBA-16

CPG-4

SBA-15

MSU-H

SLG-15

CPG-35

0.4%

1.3%

1.2%

8.9%

1.6%

64.3%

22.3%

0.1%

0.2%

0.2%

2.1%

2.7%

11.6%

83.1%

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Eagle Ford

Mass

sample

Fraction

Bakken sample

a

weight measurement uncertainty is 0.01 mg by analytical balance. With a batch of Eagle Ford synthetic

sample as 66.62 mg and a batch of Bakken synthetic sample as 166.11 mg, the uncertainties for mass fractions are 0.015% and 0.006%, respectively. 9

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For sample preparation, the media mixture and hydrocarbons of interest were added to the DSC pan. The pan was sealed with a Tzero® hermetic lid bearing a laser-drilled pinhole. The hydrocarbons used in this studies are anhydrous n-hexane (purity ≥97%, GC), n-octane (purity ≥99%, GC) and n-decane (purity

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≥99%, GC). The hydrocarbons infiltrated into the nanopores because of capillary wetting. Based on our experience, the filling is nearly instantaneous. We allow our samples to interact with the fluid for 24 hr just to ensure filling. The filling rate is dz/dt = Rγcosθ/(4ηz) [47], where R is the radius of the pore, γ is

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the interfacial tension, θ is the contact angle, η is the viscosity. Given typical values for n-octane, the rate should be on the order of 0.01~0.1 m/s. The sample was subjected to thermal analysis using a TA

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Instruments Q200 differential scanning calorimeter. The heating rate was 10 K/min under nitrogen purge. The fluid in the nanoporous media evaporates and turns into vapor state as the temperature rises above bubble point. The pressure within the pan was considered as atmospheric pressure because of the connection to atmosphere through the pinhole in the lid [11, 48]. The liquid-vapor phase transition point was taken as the onset of the endothermic peak of fluid vaporization, which is determined as the

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intersection of the extrapolated baseline with the tangent line at the point of greatest slope of the peak [33]. Additionally, the temperature at the maximal point of the vaporization peak is also studied. Regarding the reproducibility, for each set of experiments, we examined three different samples that were

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prepared at separate times. We have observed a maximum of ±2% error in loading percentages, representing physical differences attributed to human error upon filling among simples. As our

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experiments were each produced in triplicate, we have observed good reproducibility among each data set. The standard error from the triplicated experiments was taken as reproducibility, shown as ± value in Table 3. The worst reproducibility was observed for loadings of 83-87% (octane in the Eagle Ford sample), where the bubble points were within 0.35% of each other; we attribute this variation in loading percentage to human error, where it is difficult to always get exactly the desired loading value. RESULTS AND DISCUSSION

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DSC thermograms of hydrocarbons confined in the synthetic Eagle Ford sample at different loadings are presented in Figure 3. The DSC thermograms of the bulk fluid are also included (green curves). The bubble point at atmospheric pressure (i.e. normal boiling point) is measured as the onset of the

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evaporation [33], which gives bubble point for n-hexane: 341.8 ± 0.1 K, n-octane: 398.72 ± 0.02 K, and n-decane: 447.4 ± 0.02 K. The DSC measured bubble points are in good agreement with literature values which are: n-hexane: 341.86 ± 0.01 K, n-octane: 398.74 ± 0.02 K, and n-decane: 447.21 ± 0.04 K [49].

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The fluid loading is defined as the volume of added fluid to the cumulative volume of the pores (Table 1 and 2). A loading of 100% represents that all pores are completely filled, above 100% the pores are over-

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filled, and below 100% the pores are under-filled. Uncertainty in the loading percentage arises from the uncertainties in the fluid volume and the pore space volume. The fluid density is close to bulk fluid density in nanopores larger than 5 nm in diameter, but in small nanopores (2-4 nm) the density is ca. 9498% of bulk fluid density [50]. Here, the bulk fluid density was applied for all loading calculations, so the uncertainty in fluid volume within the nanopores was estimated as about 2-6%. In determining the pore

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volume, adsorptive characterization typically gives a confidence range ±5% [51]. Under experimental DSC conditions at atmospheric pressure and temperatures of 300-500 K, the pore volume is expected to be insensitive to pressure [52] and temperature [53]. Overall, the uncertainty in calculating the loading

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percentage is estimated to be about 3-11%.

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For the case of n-octane, two major peaks were observed along with minor features below and above these peak temperatures at a loading of 107%. At a loading of 93%, four peaks are clearly observed. As found in our previous observations, the bulk fluid bubble point temperature is lower than that of the confined fluid [11]. The peak at the lowest temperature relates to the vaporization of bulk fluid, which exists outside the pores. It needs to be clarified that bulk fluid may be observed at a loading slightly less than 100% because fluid may fill the porous media heterogeneously and some minor amount of fluid may stay outside the nanopores. Consistent with our previous observations [33], the bulk peak disappeared 11

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upon lowering the loading to 83%. At 83%, the onsets of the two major peaks were 396.7 K and 401.8 K. Further decrease of the pore loading to 68% resulted in a smaller peak at 396.7 K. At a pore loading of 46%, there was only one major peak onset at 401.8 K and a minor one at the higher temperature. By

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comparing with the scans of octane confined in porous media of a single pore size [11], the bubble points (onset) were associated with vaporization from the corresponding pore sizes: 398.9 K for 37.9 nm, 401.8 K for 14.8 nm and 410.3-416.6 K for 9.8-4.1 nm. By analyzing the phase transitions using peak maxima

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of hydrocarbon vaporization, the same conclusions are obtained (Table 3). Thus, for the two major peaks at 83%, the one with a lower temperature is from fluid in the 37.9 nm pore, and the higher one is from the

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14.8 nm pore. Additionally, the loading tests indicate that when the pores are under-filled, the fluid will preferably fill the pores of smaller sizes. n-Hexane and n-decane showed similar discrete vaporization behavior among different pore sizes for the synthetic Eagle Ford PSD (Figure 3b and 3c): the vaporization of bulk, confined fluid in 37.9 nm, 14.8 nm and smaller pores could be found (peaks from low to high

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temperature, respectively) and fluid fills small prior to large nanopores.

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Figure 3. DSC thermograms for (a) octane (b) hexane (c) decane infiltrated in the sample with the synthetic Eagle Ford PSD at atmosphere pressure (101.325 kPa). The percentages on the right are the pore

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loadings (Loading percentage = Vfluid/Vtotal pore volume × 100%). Bulk is the scan of bulk fluid (green curves).

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DSC thermograms of octane confined in the sample with the synthetic Bakken PSD at different loadings are presented in Figure 4a. At a loading of 108%, two major overlapping peaks were observed. At a loading of 89%, the left peak associated with bulk fluid vaporization nearly disappeared. The onset of the main peak was measured as 398.6 K, in agreement with the bubble point (onset) of octane in 37.9 nm pores (398.9 K) [33]. For loadings of 89% to 48%, the main vaporization onset at 398.9 K remains, and several minor peaks at higher temperatures were found. The minor peaks are rationalized to be associated with the vaporization of fluid in small pores (14.8 nm and below). Hexane and decane showed similar

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vaporization behavior for the synthetic Bakken PSD, where a clear separation of bulk fluid and a major confined peak from 37.9 nm can be recognized (Figure 4b and 4c). Analysis of phase transitions using

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peak maximum of hydrocarbon vaporization yields the same results (Table 3).

Figure 4. DSC thermograms for (a) octane (b) hexane (c) decane infiltrated in synthetic Bakken PSD at

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atmosphere pressure (101.325 kPa). The percentages on the right are the pore loadings (Loading percentage = Vfluid/Vtotal pore volume × 100%). Bulk is the scan of bulk fluid (green curves)

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vaporization in nanopores at 101.3 kPa 2.2 nm 345.5 ± 0.3 K

3.3 nm 355.5 ± 0.3 K

4.1 nm 360.4 ± 0.7 K

6.0 nm 359.5 ± 0.4 K

9.8 nm 357.8 ± 0.06 K

Eagle Ford PSD

-

-

-

-

-

Bakken PSD

-

-

-

-

Single-Sized Porea

403.0 ± 0.6 K

412.6 ± 0.2 K

419.2 ± 1.7 K

417.4 ± 0.7 K

Eagle Ford PSD

-

-

-

-

-

Bakken PSD

-

-

-

-

-

-

Single-Sized Porea

448.7 ± 1.3 K

462.1 ± 0.3 K

466.8 ± 0.2 K

467.0 ± 0.4 K

463.4 ± 1.5 K

Eagle Ford PSD

-

-

-

-

-

459.7 ± 0.4 K 457.5 ± 0.2 K

Bakken PSD

-

-

-

-

-

a

Single-Sized Pore n-Hexane

n-Octane

n-Decane

b

-

-

415.7 ± 0.4 K

410.0 ± 0.4 K 409.3 ± 0.4 K

-

37.9 nm 350.5 ± 0.6 K 345.7 ± 0.3 K 348.9 ± 0.5 K 403.4 ± 0.2 K 402.0 ± 0.9 K 404.9 ± 0.7 K 452.2 ± 0.9 K 450.3 ± 0.4 K 452.9 ± 0.2 K

Bulk 343.6 ± 0.08 K 342.3 ± 1.1 K 343.0 ± 1.5 K 400.9 ± 0.05 K 398.1 ± 1.3 K 398.9 ± 2.1 K 449.2 ± 0.2 K 447.3 ± 0.08 K 447.0 ± 1.3 K

Data of single-sized pore are from previous work [11].

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a

14.8 nm 353.7 ± 0.2 K 351.5 ± 0.7 K

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Porous Media

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Hydrocarbon

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Table 3. Phase transition temperature (peak maximum) and its reproducibility (±) for hydrocarbon

Fluid vaporization peaks of 2.2~9.8 nm in Eagle Ford PSD and 2.2~14.8 nm in Bakken PSD are

c

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observed as minor peaks and the maximums are not determined. DSC scans are shown in Figure 2 and 3. The uncertainty of pressure is 0.1 kPa. The uncertainty of temperature is 0.01 K.

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From the DSC analysis of hydrocarbons in the heterogeneous nanoporous media, it is concluded that the phase behavior of hydrocarbons in nanoporous media of a varying PSD varies with fluid saturation or loading. It is generally observed that from high loadings (>100%) to low loadings (<100%), the bulk fluid disappears first, then the large pores are unfilled, and at last the small pores are left empty. These observations indicate that, in the presence of different pore sizes, fluid infiltrates into the smaller pores before the larger pores, both prior to existing in bulk space. This preferred infiltration to the smaller pores happens since the pore surface in the smaller pores have stronger overall interaction with the fluid

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compared to the larger pores [54]. On the other hand, under a given pore volume, smaller nanopores have the larger surface area, and fluid would preferably fill the smaller pores to lower the total system energy. Russo et al. experimentally studied the adsorption enthalpies of organic compounds (e.g., toluene, n-

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pentane, neopentane) in different silica nanopore sizes and found that the isosteric enthalpies with capillaries are higher in narrower mesopores, indicating a stronger interaction or lower energy in narrower

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nanopores [25].

From the other aspect, in the porous media with a nanoscale pore size distribution, the fluid in the bulk

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space vaporizes first, then the fluid in larger pores vaporizes next at a higher temperature, and the remaining fluid in smaller pores vaporizes last at the highest temperature. The evaporation temperaturepore radius relation can be qualitatively analyzed with a modified Kelvin’s equation [54, 55]:

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+ 2. ln * , - = − (5) + /0123 − 45 6 where the pressure-temperature (p-T) relation of liquid-vapor phase transition in nanopores is related by surface tension (γ), molar liquid volume (Vm), pore radius (rp), and thickness of the adsorbed film prior to

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condensation (tc). Under a given pressure and parameters of surface tension, molar liquid volume and film thickness, temperature is negatively correlated to pore radius; i.e., smaller pores leads to a higher phase

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transition temperature. Experimentally, the vaporization order from large to small pores agrees with our previous findings from single pore size media [33]. A similar pore evaporation process has also been seen in desorption, whereby lowering the pressure, the fluid in large mesopores vaporizes first, followed with vaporization in small mesopores, and at last in micropores [56, 57]. It should be noted that fluid in each pore was observed to vaporize independently into the bulk vapor in the presence of multiple pore scales. It can be concluded that, in media of broad pore size distribution, the bubble point temperature (the first peak) changes with pore saturation (loading).

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At last, we include the study of binary mixture octane:decane = 50mol%:50mol% confined in the synthetic Eagle Ford PSD. Different from single-component bulk fluid evaporation, the bulk mixture

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thermogram is featured with a main peak followed by the sharp shoulder on the right (Figure 5). This is associated with the changing fluid compositions in the evaporation process and the evaporation of heavy component residue results in the shoulder peak [34]. For the mixture confined in Eagle Ford PSD (loading

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97%), several evaporation events are observed. The minor shoulder peak at the lowest temperature is associated with the minor population of bulk fluid outside the nanopores. The sharp peak at ca. 430 K is

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due to the fluid evaporation from 37.9 nm pores, which is very close to the bulk fluid evaporation. These observations are consistent with our previous studies on hydrocarbon mixtures in single-sized pores [34]. At the higher temperature of 97% mixture in Eagle Ford PSD, there is a broad peak (435-465 K) and a minor peak at 460 K. These peaks are affected by both the compositional change during the evaporation process and the pore size distribution, complicating exact interpretation. Further studies are needed to

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evaporation process.

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understand the joint influence of the compositional change and pore size distribution during the

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Figure 5. DSC thermograms for mixture octane:decane = 50mol%:50mol% infiltrated in synthetic Eagle Ford PSD at atmosphere pressure (101.325 kPa). The percentages on the right are the pore loadings

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(Loading percentage = Vfluid/Vtotal pore volume × 100%). Bulk is the scan of bulk fluid (green curves).

CONCLUSIONS

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In this paper, we presented a study on the effect of pore size distribution on the phase transition of hydrocarbons in nanoporous media using differential scanning calorimetry. The experimental nanoporous

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media carried the pore size distributions similar to two types of shale rock samples: Eagle Ford shale and Bakken shale. The phase behavior of n-hexane, n-octane and n-decane confined in the synthetic shale rock systems was studied. It was concluded that, in the presence of a broad pore size distribution, hydrocarbons fill the smaller pores prior to filling the larger pores, and that the phase transition occurs in larger pores at a lower temperature than the phase transition in smaller pores. The fluid phase behavior in

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variation with pore loading indicated that the fluid heterogeneously filled the nanoporous system with a broad pore size distribution, and the phase behavior was dependent on fluid saturation. This approach, in general, shows a means to access the phase behavior of fluids confined in natural heterogeneous

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nanoporous media.

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AUTHOR INFORMATION Corresponding Author

*(H.N.) E-mail: [email protected]. *(J.L.L.) E-mail: [email protected].

ACKNOWLEDGEMENTS 18

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We thank the Crisman Institute at the Petroleum Engineering Department of Texas A&M

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University for financial support.

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