Capillary-condensation hysteresis in naturally-occurring nanoporous media

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Capillary-condensation hysteresis in naturally-occurring nanoporous media ⁎

Elizabeth Barsottia, , Sugata P. Tana, Mohammad Piria, Jin-Hong Chenb a b

Center of Innovation for Flow through Porous Media, Department of Petroleum Engineering, University of Wyoming, Laramie, WY 82071, USA Aramco Services Company: Aramco Research Center – Houston, Houston, TX 77084, USA

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Capillary condensation Hysteresis Adsorption Desorption Nanopore Shale

Persistent uncertainties in understanding fluid phase behavior in natural nanoporous media, including shale rock, remain a significant challenge to fully utilizing tight geological formations as both globally significant sources of hydrocarbon fuels and repositories for greenhouse gas sequestration. By measuring isotherms of nbutane and n-pentane in kerogen-rich shale cores at temperatures from 4.9 to 65.6 °C, we show that shale nanopores can induce a phase transition known as capillary condensation upon adsorption or capillary evaporation upon desorption. For both adsorbates, capillary condensation and capillary evaporation took different paths, thus forming hysteresis loops that increased in size with increasing temperature. While isotherms of nbutane were expectedly reproducible, surprisingly those for n-pentane were not. This was due to irreversible kerogen swelling induced by the n-pentane. To further investigate this phenomenon, we measured scanning isotherms of n-pentane at 4.9 and 65.6 °C. Similar to the primary hysteresis loops, successive scanning measurements during adsorption resulted in different isotherm shapes, while those for desorption remained consistent. This implies differences in the physics governing adsorption and desorption, which may rely on the pore structure and fluid elasticity, respectively. These results comprise the first observations of hysteresis loop broadening at high temperatures, irreproducible hysteresis, and scanning isotherms during capillary condensation measurements in a natural nanoporous medium. By viewing these results in the context of the current hypotheses on capillary condensation derived from previous studies using synthetic nanopores, we conclude that

⁎

Corresponding author. E-mail address: [email protected] (E. Barsotti).

https://doi.org/10.1016/j.fuel.2019.116441 Received 7 August 2019; Received in revised form 9 October 2019; Accepted 16 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Elizabeth Barsotti, et al., Fuel, https://doi.org/10.1016/j.fuel.2019.116441

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new core analysis and reservoir modeling procedures must be developed to account for the irreproducible hysteresis at reservoir temperature.

1. Introduction

condensation and evaporation phase changes within nanopores, i.e., pores with diameters between 2 and 100 nm [1]. Because the underlying causes of hysteresis and its relationship to thermodynamic equilibrium are still not fully understood, the presence of hysteresis may introduce significant uncertainties to any adsorbent or adsorbate properties determined from the isotherms [3].

Adsorption isotherms are commonly used to determine the properties of both nanoporous media and nanoconfined fluids [1,2]; however, challenges in interpreting isotherms arise when capillary condensation hysteresis occurs. Hysteresis is a difference in pressure between the

Fig. 1. (a) All possible scanning isotherm types. Adapted with permission from Tompsett et al. [18] Copyright 2005 American Chemical Society. (b) Scanning isotherm types typical of H1 hysteresis. Adapted with permission from Neimark. [28] Copyright 1991 Elsevier. (c) Scanning isotherm types typical of H2 hysteresis. Adapted with permission from Neimark. [28] Copyright 1991 Elsevier. 2

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adsorbed) and desorption (decreasing amount adsorbed) scans are shown in Fig. 1(a) to be converging, crossing, and returning. In simple adsorbents, scanning isotherms have been shown to provide more information about the intermolecular interactions during the phase change than the primary hysteresis loop, itself [21]. For example, scanning isotherms have been introduced as a means to distinguish the different classifications of hysteresis provided by the International Union of Pure and Applied Chemistry (IUPAC). To standardize discussion of hysteresis, IUPAC has identified five different shapes of hysteresis loops: H1, H2, H3, H4, and H5 [1]. However, these definitions arise more from the type of adsorbent than the origin of hysteresis. For example, the H1 hysteresis loop is attributed to welldefined, rigid, and unconnected pores, such as those in MCM-41 [1]. Likewise, H2, H3, H4, and H5 hysteresis are all attributed to adsorbents of varying pore size and connectivity [1]. H2 is for interconnected nanopores of different size, H3 is for nanopores and macropores, H4 is for nanopores and micropores, and H5 is for a combination of nanopores that are open at both ends and plugged on one end [1]. In keeping with the IUPAC standard definitions, we use the terms micropore, nanopore, and macropore to define pores with sizes smaller than 2 nm, between 2 and 100 nm, and larger than 100 nm, respectively [1]. Because the scope of each hysteresis definition is broad, it can sometimes be difficult to distinguish the different types of hysteresis by shape alone, so scanning isotherms are employed to aid in the differentiation. As shown in Fig. 1(b) and (c), H1 hysteresis is accompanied by crossing scans during both adsorption and desorption, while H2 hysteresis is accompanied by converging scans during both adsorption and desorption [21,29,20]. Although, this is the most widely accepted interpretation of scanning isotherms, there are some contradictory research results that indicate H1 hysteresis loops may be characterized by desorption returning and adsorption converging scans or by desorption and adsorption converging scans, while H2 hysteresis may be characterized by desorption crossing and adsorption crossing [25], desorption converging and adsorption crossing, or by desorption crossing and adsorption converging [18]. Returning scans during adsorption have not yet been experimentally observed [18]. Hence, scanning behavior is similar to other aspects of capillary condensation hysteresis in that the present scarcity of experimental data precludes scientific consensus. As an additional note, preliminary studies also indicate that H3 isotherms may be accompanied by crossing adsorption and desorption scans, however to the best of our knowledge [18], no scanning isotherms have yet been measured for H4 and H5 hysteresis loops. Therefore, although the interpretation is not yet straightforward, scanning isotherms may be useful both for understanding fluid phase behavior in complex nanopore networks and for gaining new insights into the morphologies of natural adsorbents. Similar to synthetic silicas and MOF’s, there are preliminary observations of hysteresis in complex natural nanoporous adsorbents, such as cell wall membranes [30–33] and shale rock [34–36], which comprise the characteristics able to induce all four types of hysteresis. Additionally, some hypotheses predict that natural adsorbents may also be prone to other causes of hysteresis, such as wettability alteration, that have not yet been experimentally proven [32,33]. Nevertheless, no scanning isotherms have been published for natural adsorbents, and therefore the effects of both fluid phase behavior and pore space morphology remain unknown. In the course of our work to examine nanoconfinement-induced phase behavior in shale oil and gas reservoirs, we present here a study on the capillary condensation of n-butane and n-pentane in kerogenrich shale reservoir cores. The objective of this work is to understand how and when hysteresis occurs in shale rock and the implications it has for the production of natural gas. To this end, we present isotherms over a wide range of temperatures to determine how temperature affects the shape of the isotherm. Subsequently, we measured scanning isotherms at selected temperatures to determine the origin of hysteresis. To the best of our knowledge, these are the first scanning isotherms measured in a natural nanoporous medium. We interpret the scans in

Metastability of the adsorbate [1], the adsorbate molecular chain length [4], pore blocking effects [1], and deformation of the adsorbent [5,6] have all been experimentally observed to cause hysteresis. Metastability is most often attributed to the formation of metastable states during adsorption [7–9] or prior to cavitation upon desorption [10–12]. The former has been experimentally observed using isotherms of argon, nitrogen, oxygen, and carbon dioxide in the well-ordered synthetic nanoporous silicas MCM-41 and SBA-15, which are both characterized by unconnected cylindrical pores of a singular size [7–9]. The latter has been observed using nitrogen, oxygen, and argon in MCM-41 and a similarly well-characterized synthetic nanoporous silica, COK-11, which also has unconnected cylindrical pores [10–12]. In essence, even for the same adsorbates in similar, simple adsorbents, identification of the equilibrium phase change remains unclear. Nevertheless, hysteresis caused by metastability has only been observed at relatively low temperatures and has been shown to decrease with increase in temperature, such that it disappears significantly below the pore critical temperature or above the critical pore size of the confined fluid [13]. This distinguishes it from hysteresis caused by adsorbate molecular chain length, which has been observed above the pore critical temperature and below the critical pore size for the chain molecules n-butane, n-pentane, and n-decane in MCM-41 and wettability-modified variants of MCM-41 [4,14,15]. The pore geometries of adsorbents have also been shown to cause hysteresis [1] through pore blocking effects during desorption [16,17]. Pore blocking can occur when larger pores are indirectly connected to bulk or unconfined fluid via smaller pores. Because pore size is directly related to condensation pressure, a liquid-like phase forms first (i.e., at the lowest pressure) in the smallest pores during adsorption, while increases in pressure result in capillary condensation in larger pores. However, during desorption, the liquid in the smallest pores evaporates last, blocking the fluid in the larger pores from evaporating into the bulk gas phase [16,17]. This causes delayed emptying of the larger pores, resulting in hysteresis. In studies using nitrogen and argon in well-characterized synthetic nanoporous silicas with interconnected pores of different size, such as KIT-5, SBA-16, and FDU-1, it has been shown that increase in temperature decreases the probability of pore blocking occurring relative to that of cavitation [17,16]. Therefore, a threshold temperature may exist for adsorbents with interconnected pores of different sizes above which cavitation, rather than pore blocking, occurs. However, because only small, simple adsorbates (i.e., nitrogen and argon) were employed in these studies, it is unknown whether pore blocking can occur at high temperatures for more complex fluids, such as those consisting of chain molecules. Some less rigid adsorbents – namely metal organic frameworks (MOF’s) – may exhibit deformation-induced hysteresis. MOF’s consist of well-characterized, cage-like nanopores synthesized by binding metal ions together with organic ligands. The ligands in some MOF’s can stretch or buckle during adsorption so that the pore sizes or geometries characteristic of adsorption and desorption are different, thereby inducing hysteresis. In studies using carbon dioxide in an indium-based MOF [5] and supercritical hydrogen in a cobalt-based MOF [6], the size of the hysteresis loop was found to increase with increasing temperature. In all previous studies, the adsorbents have been well-characterized and no more than two of the four causes of hysteresis have been considered concurrently. Nevertheless, longstanding uncertainties associated with each of the four types of hysteresis have necessitated the development of new methods with which to further investigate the underlying intermolecular interactions. Most commonly, scanning isotherms are used [18–27]. Scanning isotherms are isotherms in which desorption is induced on the adsorption branch of the hysteresis loop before the upper closure point of the hysteresis loop; or adsorption is induced on the desorption branch of the hysteresis loop before the lower closure point of the hysteresis loop. The three types of adsorption (increasing amount 3

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the instrument resolution, a more visually appealing image can be generated, but no features below the instrument resolution can be used reliably. Higher magnification images of the calcite showed it to comprise coccolithophore fossils and fossil fragments that contained no discernible nanoporosity except for the fine cracks between the fins of the coccolithophore shells. The presence of the fossils indicated a marine depositional environment, which is in agreement with the Type II classification of the kerogen. High-resolution images of the kerogen showed it to contain two classes of nanoporosity: large nanopores with effective diameters greater than 10 nm and small nanopores with diameters smaller than 10 nm, which are shown in Fig. 3(b). Finally, three-dimensional imaging was carried out. The imaging process was automated using Auto Slice and View software (version 1.5.0.46, Thermo Fisher Scientific). To produce a three-dimensional (3D) volume, slices of the rock were gradually removed using the gallium ion beam (30 kV, 0.78 nA). Between slices, the electron beam was used to take high-resolution images of the milling plane. Electron images were taken using a through-the-lens detector with backscatter electrons and a beam voltage of 2 kV, a beam current of 100 pA, a working distance of 3.38 mm, a horizontal field width of 21.35 μ m, and a dwell time of 300 ns. For each image, 16 consecutive scans of the milling plane were integrated. The combined ion and electron beam properties resulted in a voxel resolution of 5 nm depth (the approximate resolution of the ion beam), 5.21 nm width, and 5.21 nm height. Using Auto Slice and View, a series of 971 slices was completed. Segmentation of the 3D volume using Avizo software (version 9.4, Thermo Fisher Scientific) revealed inorganic minerals (i.e., calcite, quartz and pyrite) to comprise 83.42% of the total volume of the sample. Kerogen and pores accounted for 16.04% and 0.55% of the volume, respectively. 19.61% (volume percent) of the pores were found to have effective diameters below 100 nm. Due to the maximum image resolution of 5 nm, the 3D images underestimated the number of pores with diameters below 100 nm. However, they allowed for visualization of the interconnectivity of the pore space. The pores, which occurred mainly in the kerogen, expressed connectivity, forming large networks. As shown in Fig. 4, many macropores in these networks were observed to be connected by nano-sized throats. Here we follow the definitions for pore and throat presented by Dong and Blunt, who define pores as “larger void spaces” in the rock and throats as “narrow openings connecting the pores” [49]. A full description of the Avizo segmentation procedure along with additional 2D and 3D SEM images of the pores and extracted pore networks are included in the Supplementary material. The SEM images are useful for proving the presence of kerogen and nanopores in the shale and for giving examples of specific features in the rock, such as the large pores connected by nano-sized throats, that must affect the phase behavior of fluids through processes including pore blocking. However, we remind the reader that the representative elementary volume of shale is as-of-yet unknown. Therefore, to say that exact features observed in Figs. 3 and 4, which were obtained from single grains of the crushed rock, are representative of all of the thousands of grains employed in the isotherm measurements, which we discuss in the subsequent section, is unreasonable. This is exemplified, for example, in Fig. 3, where the types of nanoporosity, i.e., the ratio of large to small nanopores, is not uniform across the four grains selected

the context of the IUPAC-defined hysteresis loops and use two- and three-dimensional scanning electron microscopy to support our findings. 2. Materials and methods 2.1. Shale rock characterization Shale core samples were obtained from a gas reservoir in the Middle East. To increase the surface area available for fluid-rock interactions, the rock was crushed by hand using a mortar and pestle. Crushed shale is commonly employed throughout the literature both in adsorption studies [37–45] and for core analysis [46,47]. To prevent interparticle nanoporosity from forming among the shale grains, the shale was crushed in such a way as to maintain relatively large grain sizes, as shown in Table 1. Pieces of the crushed shale were randomly selected for scanning electron microscopy using a Helios 650 focused ion beam scanning electron microscope (FIB-SEM) from Thermo Fisher Scientific equipped with an Oxford X-MaxN 50 mm2 Silicon Drift Detector for energy dispersive X-ray spectroscopy (EDS). Before imaging, each piece of shale was mounted on a 45-degree pre-tilted aluminum sample stub (Ted Pella) using carbon tape (Ted Pella) and silver paint (Ted Pella) as adhesives. The samples were then sputter coated with carbon using an Q150R carbon coater (Electron Microscopy Sciences) to prevent charging during imaging. Initially, low resolution images were obtained to ensure the samples were not contaminated by any solids, such as halite, that could have precipitated from the reservoir fluids during the coring process and would necessitate cleaning of the crushed rock to reconnect the pore elements [48]. No contaminants were observed in any of the samples. Next, the gallium ion beam of the FIB-SEM was used to mill out and polish an area of the rock that was approximately 20 μ m wide by 60 μ m long. This effectively removed the carbon coating from only the polished cross section of the sample. Although the carbon coating improves the electrical conductivity of the specimen, it may obscure fine features in the rock, such as nanometer-sized pores, and introduce uncertainties into elemental analysis using EDS. However, because the carbon was only stripped from this small section of the rock, the carbon coating the rest of the sample still allowed for the dissipation of electrons, effectively preventing charging. Therefore, the polished cross section could be used for high resolution two- and three-dimensional imaging along with elemental analysis. High resolution two-dimensional (2D) imaging was carried out using a through-the-lens detector in backscatter electron, downhole vision, and secondary electron modes with voltages from 2 kV to 10 kV and currents from 13 to 100 pA. EDS was carried out under the same conditions. Based on the measured elemental concentrations from EDS, the mineralogy of the rock was found to consist primarily of kerogen and calcite. A high-resolution SEM image of the rock and its corresponding mineral map are shown in Fig. 2. Note that throughout this work, we use the term resolution to refer to the pixel and voxel sizes in the 2D and 3D images, respectively. The instrument resolution is the maximum resolution that can be physically achieved by the FIB-SEM. This resolution is factory-determined from the build and type of the microscope. In our case, the instrument resolution is 0.8 nm. The digital resolution is determined by the imaging magnification, as manifest in the horizontal field width, and the number of pixels in the image. In other words, for large horizontal field widths and low pixel counts, the digital resolution can be larger than the instrument resolution. Likewise, at high magnification and a high pixel count, the digital resolution can be better than the instrument resolution. When the digital resolution is worse than the instrument resolution, no features below the digital resolution can be used reliably, such as in segmentation procedures to determine pore volume or total organic carbon. Conversely, when the digital resolution is better than

Table 1 Grain sizes of the crushed shale rock sample determined using sieve analysis.

4

Weight %

Particle Size [μ m]

74.52 24.50 1.89

> 250 90–250 < 90

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Fig. 2. SEM image and corresponding mineral map of the reservoir shale rock sample used in this study. The digital resolution of the images is 10 nm, and the instrument resolution is 0.8 nm.

Fig. 3. High-resolution SEM images of the kerogen pores (a–c) and calcite coccolithophore fossils (d). As shown in (a–c), two classes of porosity were observed in the kerogen: large nanopores with diameters larger than 10 nm and small nanopores with diameters smaller than 10 nm. In all images, the digital resolution is equal to or better than the instrument resolution of 0.8 nm.

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Table 2 Reservoir temperatures of globally prominent shale oil and gas reservoirs. Reservoir

Temperature [°C]

Bakken [52] Barnett [53] Fayetteville [54] Jafurah [55] Longmaxi [56] Marcellus [57] Montney [58] Utica [59]

115.56 65.6 47.78 129.44 30 60 95 65.6–93.3

pressure transducer (Emerson) or a Leybold TM101 vacuum gauge. Temperature was maintained constant across the entire experimental range from 4.9 to 65.6 °C with a uniformity of ± 0.1 °C using a Thermotron environmental chamber. The low temperature measurements were used for elucidation of the underlying physics of the confined phase behavior of the shale, as isotherms at the same temperatures had been measured previously in less complex adsorbents [4]. The higher temperatures were used to approximate reservoir conditions. For reference, the temperatures of many globally prominent shale oil and gas reservoirs are included in Table 2. Fig. 4. Example of large macropores connected by nano-sized throats visualized from the segmented 3D pore volume using Avizo software.

3. Results and discussion for imaging. In fact, this inhomogeneity of the rock properties observed using characterization methods such as FIB-SEM provides justification for alternative measurements, including isotherms. Isotherms contain information about fluid phase behavior, fluid-rock interactions, and pore space morphology. Patterns observed between different porous media with regard to these three attributes are the basis upon which IUPAC developed its classification scheme for hysteresis loops [1]. However, as we discussed in the previous section, no such classification system yet exists for natural nanoporous media, such as shale rock. This exemplifies the immediate importance of this work. So although FIBSEM allows for easy observation of potential contaminants like halite and provides useful examples of the features encountered in a given rock formation, larger scale measurements, including capillary condensation isotherms, that can average the rock and fluid properties at high-resolution across a more representative volume of rock, are more pertinent to reservoir characterization.

3.1. N-butane The isotherms for n-butane in shale measured from 4.9 to 65.6 °C are shown in Fig. 5. Isotherms are reported throughout this work in terms of grams of fluid adsorbed per gram of shale rock present in the core holder for easy comparison of the isotherms generated with each adsorbate. Starting from low pressure, the concave down slope of the

2.2. Isotherm measurements N-Butane (99% AirGas) and n-pentane (99.8% Alfa Aesar) were used as adsorbates. Capillary condensation and evaporation isotherms were measured using a novel gravimetric apparatus [50,51]. 67.30 grams and 62.90 grams of shale were packed into two titanium core holders for use in the n-butane and n-pentane measurements, respectively. Initially, the core holders were outgassed at 50 °C under vacuum for approximately 1 week or until the vacuum levels stabilized below 1 mbar. This was to remove any pre-adsorbed gases from the surfaces of the rock that could affect its sorption capacity. No additional cleaning procedures were employed because no evidence of contamination was observed during the SEM imaging. After outgassing, the isotherms were measured by injecting (during adsorption) or retracting (during desorption) fluid into the core holder using a 6000 Series dual cylinder Quizix pump (Chandler Engineering). Using the pump, the n-butane could be pressurized up to 2.23 bar, while the n-pentane could be pressurized to over 69 bar. A Welch Duo Seal Vacuum Pump (Seargant Welch, Co.) was used to aid in desorption, such that the system was momentarily subjected to vacuum in order to incrementally decrease pressure. As the amounts of fluid in the core holders were varied, their pressures were measured using a Rosemount

Fig. 5. Isotherms of n-butane measured over the entire temperature range from 4.9 °C to 65.6 °C. Solid and hollow symbols are used for adsorption and desorption, respectively. For ease of comparison, the isotherms at 4.9 °C, 9.8 °C, 16.1 °C, 20.3 °C, and 26.6 °C are shifted up by 0.011, 0.0089, 0.0067, 0.0045, and 0.0022 grams of n-butane/gram shale rock, respectively. The red arrows indicate the measured bulk saturation pressures. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 6

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information, where large macropores were observed to be connected by nano-sized throats. Furthermore, the width of the hysteresis loop for n-butane does not follow the general trend wherein it decreases monotonically with increases in temperature [4]. This is similar to our previous work on nbutane in 8 nm-diameter MCM-41, where the width of the hysteresis loop for n-butane fluctuated with temperature so that the widths of the hysteresis loops at some higher temperatures were greater than those at lower temperature [4]. This was attributed to the length of the n-butane molecule. Therefore, the hysteresis observed here may also arise from the molecular chain length of the adsorbate. Hysteresis and the absence of a steep condensation jump during adsorption for shale are both in agreement with other observations in the literature [34,35]. However, the shape of our hysteresis loop is different, which may be attributed to the differences in the chemistries and pore structures of shales obtained from different reservoirs. For example, it is known that even minor differences in pore size, shape, wettability, and interconnectivity can significantly impact the shapes of isotherms. As mentioned previously, this is the basis of the IUPAC classifications [1]. To determine the reproducibility of the measured features, the isotherms measured at 4.9 and 16.1 °C were each repeated up to three times. If we use the shape of the hysteresis loop to divide the isotherm into four different regions, as shown in Fig. 6, the isotherms at each temperature are observed to coincide except for over Region IV, where the ultimate amount adsorbed varies. The coincidence of the repeated isotherms over Regions I–III indicates that no permanent changes to the structure of the shale are occurring. The differences in amounts adsorbed over Region IV are an artifact of desorption crossing scanning behavior. For practical reasons, it is common both here and in the literature to terminate condensation of the bulk or unconfined gas before the whole core holder or sample cell is full of bulk liquid [4]. Practically this allows for conservation of the large amounts of fluid it would take to fill the bulk volume of the core holder, especially since the main feature of interest is the capillary condensation, not the bulk condensation. Therefore, the amount adsorbed at the termination of adsorption is not related to temperature or pore size. As such, even at the measured amount adsorbed at termination in Fig. 6, the core holder was not yet fully saturated with bulk liquid. By intentionally varying the amount adsorbed at termination, we induce scanning at different amounts adsorbed. Hence, no permanent changes to the structure of the shale were observed over Region IV either. The main differences in Region IV are due to our manual adjustment of the amount adsorbed at termination. Adsorption scanning measured at 4.9 °C, as shown in Fig. 6, was also observed to exhibit crossing behavior. We emphasize that care must be taken to define the nature of adsorption scanning in cases such as this, where the adsorption branch of the hysteresis loop almost coincides with the bulk saturation pressure, for if desorption is induced just as the adsorption scan attains the bulk saturation pressure, the scan might be mistakenly interpreted as converging. The crossing scans observed for both adsorption and desorption

isotherm below 0.1 bar indicates that the n-butane wets the shale pores [1]. At relatively high pressures, the abrupt increases in amount adsorbed indicate most of the capillary condensation and eventually the gas-to-liquid phase change of the bulk fluid. Note that this jump is absent from temperatures above 20.3 °C due to the maximum experimentally achievable pressure of 2.23 bar for n-butane. In other words, the isothermal technique that we employ here allows us to measure both the nanoconfined and bulk phase behavior altogether within the range of achieveable experimental pressure. The bulk measurements are indicated in Fig. 5 by red arrows. The ability to measure both the nanoconfined and bulk phase behavior in a single isotherm provides a practical way to gauge the accuracy of each measurement. Here, we deteremine the accuracies of the measurements by comparing our measured numerical values for the bulk condensation to values reported by the National Institute of Standards and Technology (NIST). The accuracies of our measurements at each temperature are given in Table 3. As shown in Table 3, the bulk condensation pressures measured here all fall within 5% of those reported by NIST [60]. The accuracies of the measurements fell within 1%, 5%, 4%, and 1% at 4.9 °C, 9.8 °C, 16.1 °C, and 20.3 °C, respectively. Although it is important to note that the bulk saturation measurements were not entirely vertical as one would expect. Rather there are positive slopes to the measurements. This is consistent with measurements throughout the literature [61,62,4,13]. Due to the slight slope of the bulk saturation pressure, we have selected data points near the midpoint of the jump to represent the bulk saturation pressure. No scientific justification has yet been given for this slope in the literature. Because all of our measurements are within 5% of the values reported by NIST, such a justification is beyond the scope of this work. However, it will be the topic of our future work. Capillary condensation in simple porous media, such as MCM-41, can be easily identified as an abrupt step in the isotherm at a pressure lower than the bulk staturation pressure [2,1]. We observed this step in our previous work with n-butane in MCM-41 [4]. However, here, for the n-butane, we do not observe any distinct capillary condensation steps in the isotherms, despite the fact that the SEM images presented in the Materials and Methods section and Supplementary information show at least 20% of the effective diameters of the kerogen pores to fall below 100 nm, which is the IUPAC-defined pore size cutoff below which capillary condensation is commonly observed to occur [1]. Rather, we use the hystersis loop to identify capillary condensation [36]. This unexpected presentation of capillary condensation in the isotherm, is simply due to the wide pore size distribution of the shale [36]. The wide range of pore sizes from less than 2 nm to 1 μ m causes condensation to occur gradually, starting in the smallest pores at low pressures and ending in the larger macropores at the bulk saturation pressure [36]. Upon desorption, the pore size distribution may also be the cause of the large hysteresis loop, in which case, hysteresis arises as fluid in the larger pores is blocked until capillary evaporation occurs in the smaller throats and pores. The interconnectivity of the pores is further supported in the three-dimensional scanning electron microscope images presented in the Materials and Methods section and Supplementary

Table 3 Comparison of the bulk saturation pressures that we measure in the n-butane isotherms compared to those published by NIST [60]. Temperature [°C]

Measured Bulk Saturation Pressure [bar]

NIST Saturation Pressure [bar] [60]

% Error

Figure in the Text

4.9 °C 4.9 °C 9.8 °C 16.1 °C 16.1 °C 16.1 °C 20.3 °C 26.6 °C 65.6 °C

1.2787 1.2921 1.4698 1.8974 1.8926 1.8998 2.1074 N/A N/A

1.2380 1.2380 1.4742 1.8274 1.8274 1.8274 2.0967 2.5563 7.3021

3.23% 4.28% 0.30% 3.76% 3.51% 3.88% 0.51% N/A N/A

Figs. 5 and 6 (1st Measurement) Fig. 6 Fig. 5 Figs. 5 and 6 (2nd Measurement) Fig. 6 Fig. 6 Fig. 5 Fig. 5 Fig. 5

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Fig. 6. Repeated isotherm measurements of n-butane at 16.1 °C and 4.9 °C. Solid and hollow symbols are used for adsorption and desorption, respectively.

to n-butane, displaying all four regions reported in the previous section. The highest reported temperature of 65.6 °C is representative of shale reservoirs, such as Marcellus. Hence, the persistence and broadening of the hysteresis loop up to 65.6 °C indicates that at reservoir conditions gas may be stored in a liquid-like state. This contradicts the current gas storage model for shale reservoirs, which hypothesizes that gas is stored either as an adsorbed fluid on the pore walls or as free gas in the pore bodies [2,63–69]. In other words, this work justifies a need to improve the current models used for estimating total gas in place and ultimate production, as they do not account for the physics responsible for capillary condensation [2]. Furthermore, hysteresis indicates that injection and production pressures may differ. Therefore, a phase diagram used to describe one may be inappropriate to describe the other. For example, if a phase diagram outlining the capillary condensation phenomenon is used to guide decision making during primary production,

indicate H1 hysteresis, which is not a realistic characterization of the rock-fluid system in the context of the current IUPAC definition [1]. H1 hysteresis is primarily attributed to very simple adsorbents, such as MCM-41, which are characterized by a single pore size, unconnected pores, and uniform surface chemistries [1]. None of these characteristics is representative of the shale rock, which has a wide pore size distribution, interconnected pores, and non-uniform surface chemistry. Finally, for ease of interpreting the more complex scanning behavior in the following section on n-pentane, we consider any data points collected as a continuation of any scanning measurements to be the chronologically same measurement as the primary hysteresis loop. For example, in Fig. 6, the isotherm denoted as “Continuation of 2nd Measurement after Adsorption Scan” will henceforth be denoted as part of the 2nd measurement. Only if the fluid amount is reduced to below the lower closure point of the hysteresis loop will we consider new data points as part of a subsequent measurement. 3.2. N-pentane The isotherms for n-pentane in shale measured from 4.9 to 65.6 °C are shown in Fig. 7. The measured bulk saturation pressures all fell within 1% of those reported by NIST except for that measured at 9.8 °C, which fell within 5%. The higher accuracy of the n-pentane isotherms is due to the higher purity of the fluid, i.e., the n-butane was 99% pure while the n-pentane was 99.8% pure. Numerical data for the measured bulk saturation pressures and the accuracies of the measurements are provided in Table 4. In comparison to the n-butane isotherms presented in Fig. 5, less npentane is adsorbed per gram of shale rock. In other words, the density of the nanoconfined n-pentane is less than that of the nanoconfined nbutane. This may indicate either heterogeneity of the two shale packs, in which case, we emphasize the important role that capillary condensation measurements, such as these, may play in providing information regarding the representative elementary volume of the rock. Likewise, size exclusion effects may also play a role in the adsorption capacity of the rock, such that not as many n-pentane molecules can fill the pores due to steric obstruction induced by the small sizes of the pores. Hysteresis was observed at all temperatures and occurred similarly

Fig. 7. Isotherms of n-pentane measured over the entire temperature range from 4.9 °C to 65.6 °C. Solid and hollow symbols are used for adsorption and desorption, respectively. For ease of comparison, the isotherms at 4.9 °C, 9.8 °C, and 16.1 °C are shifted up by 0.0089, 0.0067, and 0.0045 grams of n-butane/ gram shale rock. The red arrows indicate the measured bulk saturation pressures. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 8

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Table 4 Comparison of the bulk saturation pressures that we measure in the n-pentane isotherms compared to those published by NIST [60]. Temperature [°C]

Measured Bulk Saturation Pressure [bar]

NIST Saturation Pressure [bar] [60]

% Error

Figure in the Text

4.9 °C 4.9 °C 4.9 °C 9.8 °C 16.1 °C 20.3 °C 65.6 °C

0.3022 0.3022 0.3022 0.3827 0.5023 0.5684 2.4987

0.30421 0.30421 0.30421 0.37519 0.48537 0.57217 2.5133

0.66% 0.66% 0.66% 4.56% 3.4% 0.66% 0.58%

Figs. 7 and 8 (2nd measurement) Fig. 8 Fig. 8 Fig. 7 Fig. 7 Fig. 7 Figs. 7 and 11

As we observe here, and as can be seen in the literature, it is common for different solvents to produce different swelling behaviors [74,73,76]. For example, using crushed shale from the same cores that we use here, we have found that the capillary condensation of propane can cause the kerogen to swell so aggressively that it induces fractures [36]. Therefore, based on investigations in our lab, propane can fracture the rock, n-butane may reversibly swell the rock, and n-pentane can irreversibly swell the rock. The main significance of these findings lies in the changes to the shapes of the isotherms. The shape of an isotherm is directly related to the pore size distribution [1]. In fact, isotherms can easily be converted to pore size distributions using techniques including non-local density functional theory [1]; however, the extension of such techniques to shale requires parameterization of the energy functionals to account for the surface chemistry and roughness of kerogen. Because the surface chemistry and roughness of kerogen are open areas of research in and of themselves [77], the application of such techniques are unjustified here. Rather, we take the qualitative equivalency of the change in isotherm shape to change in pore size as evidence that the pore size distribution changes from one measurement to another. Because the different fluids affect the changes to the pore size distribution differently, in practice, it is inadvisable to measure porosity or perform core analysis with fluids that cause a strong swelling interaction if only non-swelling fluids exist in the reservoir or vice versa. In other words, care must be taken to ensure that any measurements encompass rock-fluid interactions that are representative of the interactions in situ; otherwise, cores could be

which is a capillary evaporation process, the absence of hysteresis data from the phase diagram may impart significant errors to production estimates. In essence, different phase diagrams may be necessary to accurately describe both. Similar to n-butane, the width of the hysteresis loop for n-pentane was also observed to increase with increases in temperature. Note that hysteresis loops are commonly known to either increase or decrease monotonically with temperature depending on the molecular size of the adsorbate [4]. In our previous work we found that alkanes smaller than n-butane were characterized by a hysteresis loop that decreased with temperature. Conversley, molecules larger than n-butane were characterized by hysteresis loops that increased with temperature. For example, n-pentane in MCM-41 remained hysteretic at temperatures as high as 72.1 °C [4]. Because our observation of increasing hysteresis loop width here is in agreement with the findings in our previous work, molecular size may also contribute to the hysteresis that we observe here. However, neither pore blocking effects nor adsorbent deformation can be ruled out as potential causes of hysteresis for n-pentane in this study. Unlike n-butane, repeated isotherms of n-pentane at 4.9 °C were observed to change significantly, especially at pressures marking the transition between Regions III and IV, as shown in Fig. 8. Despite large differences in the amounts adsorbed prior to the bulk saturation pressure, all measured bulk saturation pressures for the three isotherms presented in Fig. 8 fell within 1% of the pressures reported by NIST [60]. This provides evidence that contaminants or leaks are not responsible for the different shapes of the isotherms. Therefore, differences in the shapes of the isotherms may indicate irreversible swelling of the kerogen. In essence, the changes in the shapes of the isotherms are only revealed by the excessive strain [70] brought on by capillary condensation. Although reversible swelling has previously been reported for shale during both isothermal [34,36] and isobaric [71,36] capillary condensation measurements, this is, to the best of our knowledge, the first set of isotherms displaying irreversible swelling. In other studies focused specifically on kerogen swelling with no considerations for capillary condensation, n-pentane has been shown to swell shale irreversibly, where the irreversibility may be attributed to physical rearrangement of the kerogen structure or the extraction of chemical components from the kerogen [72–75]. Because our measured bulk saturation pressures were similar to those reported by NIST, we give greater credence to the first hypothesis, which states that the kerogen in its as-received form is not characterized by the global free energy minimum but is frozen in a “glassy” state at a local free energy minimum. Introduction of certain solvents to the kerogen softens it, facilitating rearrangement of the kerogen structure to achieve the global free energy minimum [72,76]. However, in kerogens that rearrange slowly, the global free energy minimum might not be achieved during the first contact with solvent, so that the structure of the kerogen can continue to change with multiple contacts [72,76]. Conversely, during reversible swelling, the kerogen maintains the same structure throughout multiple contacts with solvent [76]. Because calcite does not swell, the irreversible swelling observed in the isotherms here indicates that the n-pentane is condensed predominately in the kerogen pores.

Fig. 8. Three isotherms of n-pentane measured consecutively at 4.9 °C. Note that the isotherms do not coincide over the transition between Regions III and IV. 9

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chronology of the measurements (i.e., scanning isotherm A was measured before isotherm B), crossing scans of widely varying shapes were observed. To be consistent with our discussion of scanning in the section on n-butane, we do not identify any of the scans as converging because, even though bulk condensation was nearly achieved, the core holder was not yet saturated with the bulk liquid. Conversely, all of the desorption scans were crossing with similar shapes regardless of chronology. Similar to the behavior observed with n-butane, crossing during both adsorption and desorption indicates H1 hysteresis despite the evident complexity of the shale rock. Further investigations of this behavior were made by also measuring scanning isotherms for the second and third n-pentane measurements at 4.9 °C, which are shown in Fig. 10. In all cases, desorption and adsorption scanning isotherms were crossing. However, whereas the

permanently damaged through interactions like irreversible swelling to the point that the pore space no longer geometrically represents the pores that are in the reservoir. This finding provides additional support for criticisms in the literature of current core analysis techniques, which employ fluids like mercury, helium, and nitrogen – fluids that must interact with cores differently than hydrocarbons due to their different chemical and physical properties, such as molecular size [78]. To determine the relationships of all four regions of the n-pentane isotherms to irreversible kerogen swelling, scanning isotherms were measured for the first isotherm at 4.9 °C. Regardless of chronology, the adsorption scanning isotherms in Regions II and III retained consistent shapes. However, the shapes of the adsorption scanning measurements in Region IV changed significantly with chronology. As shown in the inset in Fig. 9, where the alphabetic ordering corresponds to the

Fig. 9. Adsorption (a) and desorption (b) scanning isotherms for the first measurement made with n-pentane at 4.9 °C. 10

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Fig. 10. Adsorption (a, c) and desorption (b, d) scanning isotherms for the second and third n-pentane measurements made at 4.9 °C.

which are analogous to the reservoir conditions, scanning curves at 65.6 °C were measured and are shown in Fig. 11. At 65.6 °C, the adsorption scans were measured first followed by the desorption scans. In decreasing and increasing the pressure to achieve these measurements, swelling of the kerogen was also observed. At this elevated temperature, the isotherms were observed to change more. For example, in repeating portions of the adsorption branch of the primary hysteresis loop to elevate pressure enough to perform the desorption scanning measurements, large differences were observed between the primary adsorption measured at different times. This is shown in Fig. 11(b), where the repeated adsorption measurements of the primary hysteresis loop do not coincide with the initial measurements. In comparing the high and low temperature scanning isotherms, it is evident that the low temperature isotherms change most in shape after outgassing between measurements – i.e., when the shale sample is pressurized, it maintains its configuration better. However, at high

desorption scans for all three primary isotherms retained similar shapes, those for adsorption varied greatly. In Fig. 9, adsorption scans generally cross parallel to the X axis, while a tendency to ascend during crossing and adopt the form of the primary adsorption measurement was observed in Fig. 10(a) and (c). This has two implications. First, these are the first observations of changes in scanning shape without changes in scanning type. This may indicate insufficiencies in the current scanning classifications for adequately defining scanning behavior in complex natural adsorbents. Second, the consistency of the desorption isotherms and scans and the simultaneous variability of the adsorption isotherms and scans support the hypothesis that the adsorption branch of the primary hysteresis loop primarily depends on the morphology of the pore space, while the desorption branch, where capillary evaporation essentially occurs via cavitation, primarily depends on the elasticity of the fluid [79]. To determine if this relationship held true at high temperatures, 11

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conditions that are relevant to globally prominent shale reservoirs. Note that the capillary condensation and evaporation of n-pentane occurs at pressures much lower than those encountered in petroleum reservoirs. Therefore, at the higher pressures in petroleum reservoirs, fluids like npentane must exist as a liquid-like, capillary condensed phase inside the pores. However, capillary condensation is not yet accounted for in reservoir modeling procedures. Through systematic studies such as the one we present here, new reliable tools [80] can be built with which to characterize and model shale reservoirs to the point that ultimate production can be increased. Along these lines, future work will include the addition of additional fluid compositions and phases, e.g., brine and gas, which may further improve our understanding of the capillary condensation of nanoconfined fluids. 4. Conclusions In comparing isotherms measured for n-butane and n-pentane in kerogen-rich shale cores, we observed differences in the reproducibilities of the isotherms and the corresponding hysteresis measured with both adsorbates. While the n-butane isotherms were reproducible, the n-pentane isotherms were not. Rather, the shapes of both the isotherms and the associated scanning curves measured for n-pentane changed significantly throughout consecutive measurements at the same temperature. The unpredictability of the isotherm shapes indicates irreversible kerogen swelling, thus irreproducible hysteresis. During the n-pentane measurements, the observed changes in the shapes of the isotherms occurred only during adsorption, supporting the hypothesis that the adsorption branch of the isotherm depends primarily on the rock properties, while desorption depends more on the fluid properties. Therefore, the determinations of pore size distributions for shale should be made using the adsorption data. However, the unpredictable adsorption indicates that it may be challenging to predict the exact pore size of the rock a priori. Finally, the hysteresis observed for n-pentane at temperatures up to 65.6 °C indicates that hysteresis can persist at reservoir temperatures. Therefore, condensation and evaporation are characterized by different pressures, which necessitate the use of multiple phase diagrams in reservoir simulators to accurately predict scenarios associated with pressure increase and decrease. Because the relationship of hysteresis to thermodynamic equilibrium has not been fully understood, especially at reservoir temperatures, additional experimental studies are needed to accurately define the equilibrium. This will be the topic of our future work. Declaration of Competing Interest There are no conflicts to declare. Acknowledgements We gratefully acknowledge the financial support of Saudi Aramco, Hess Corporation, and the School of Energy Resources at the University of Wyoming. We thank Dr. Wendi Kuang of the Piri Research Group at the Center of Innovation for Flow Through Porous Media of the University of Wyoming for his assistance in segmenting the 3D FIB-SEM images using Avizo software, and we extend our gratitude to Evan Lowry for his technical support.

Fig. 11. Adsorption (a) and desorption (b) scanning isotherms for the n-pentane measurement made at 65.6 °C. In (b), significant differences between the repeated adsorption measurements and the initial adsorption measured along the primary hysteresis loop are attributed to kerogen swelling.

temperature, we observe significant changes in the shape of the isotherm even at relatively high pressures. In this way, data points measured at high temperature as continuations of the hysteresis loop between scanning measurements could no longer be considered as representative of the original measurement of the primary hysteresis loop. We associate this with our observation that the system pressure and adsorbed mass fluctuate at high temperature without reaching true equilibrium, even over timespans longer than 2 weeks. Further investigations of this phenomenon are the topic of our future work. We employ high temperature to provide measurements at

Appendix A. Supplementary data 2D and 3D focused ion beam scanning electron microscopy (FIBSEM) characterization of the shale and isotherm data corresponding to Figs. 5–11 in the text in tabular format. Supplementary data associated with this article can be found, in the online version, athttps://doi.org/ 10.1016/j.fuel.2019.116441. 12

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References [1] Thommes M, Kaneko K, Neimark AV, Olivier JP, Rodriguez-Reinoso F, Rouquerol J, et al. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (iupac technical report).https://doi.org/10.1515/pac2014-1117. [2] Barsotti E, Tan SP, Saraji S, Piri M, Chen J-H. A review on capillary condensation in nanoporous media: implications for hydrocarbon recovery from tight reservoirs. Fuel 2016;184:344–61. https://doi.org/10.1016/j.fuel.2016.06.123. URL:http:// www.sciencedirect.com/science/article/pii/S0016236116305865. [3] Groen JC, Peffer LA, Pérez-Ramírez J. Pore size determination in modified microand mesoporous materials. pitfalls and limitations in gas adsorption data analysis, Microporous and Mesoporous. Materials 2003;60(1):1–17. https://doi.org/10. 1016/S1387-1811(03)00339-1. [4] Barsotti E, Tan SP, Piri M, Chen J-H. Phenomenological study of confined criticality: insights from the capillary condensation of propane, n-butane, and n-pentane in nanopores. Langmuir 2018;34(15):4473–83. https://doi.org/10.1021/acs. langmuir.8b00125. pMID: 29611709. [5] Yang S, Lin X, Lewis W, Suyetin M, Bichoutskaia E, Parker JE, et al. A partially interpenetrated metal-organic framework for selective hysteretic sorption of carbon dioxide. Nat Mater 2012;11:710–6. [6] Choi HJ, Dinca M, Long JR. Broadly hysteretic h2 adsorption in the microporous metal- organic framework co (1, 4-benzenedipyrazolate). J Am Chem Soc 2008;130(25):7848–50. [7] Morishige K, Nakamura Y. Nature of adsorption and desorption branches in cylindrical pores. Langmuir 2004;20(11):4503–6. https://doi.org/10.1021/ la030414g. pMID: 15969158. [8] Morishige K, Ito M. Capillary condensation of nitrogen in mcm-41 and sba-15. J Chem Phys 2002;117(17):8036–41. https://doi.org/10.1063/1.1510440. [9] Grosman A, Ortega C. Nature of capillary condensation and evaporation processes in ordered porous materials. Langmuir 2005;21(23):10515–21. https://doi.org/10. 1021/la051030o. pMID: 16262315. [10] Morishige K. Nature of adsorption hysteresis in cylindrical pores: effect of pore corrugation. J Phys Chem C 2016;120(39):22508–14. https://doi.org/10.1021/acs. jpcc.6b07764. [11] Ravikovitch PI, Domhnaill SCO, Neimark AV, Schueth F, Unger KK. Capillary hysteresis in nanopores: theoretical and experimental studies of nitrogen adsorption on mcm-41. Langmuir 1995;11(12):4765–72. https://doi.org/10.1021/ la00012a030. [12] Hiratsuka T, Tanaka H, Miyahara MT. Mechanism of kinetically controlled capillary condensation in nanopores: a combined experimental and monte carlo approach. ACS Nano 2017;11(1):269–76. https://doi.org/10.1021/acsnano.6b05550. pMID: 28001354. [13] Horikawa T, Do D, Nicholson D. Capillary condensation of adsorbates in porous materials. Adv Colloid Interface Sci 2011;169(1):40–58. https://doi.org/10.1016/j. cis.2011.08.003. URL:http://www.sciencedirect.com/science/article/pii/ S0001868611001497. [14] Barsotti E, Piri M, Althaus S, Chen J-H. Solution gas drive in tight oil reservoirs: New insights from capillary condensation and evaporation experiments. Unconventional Resources Technology Conference. [15] Lowry E, Piri M. Effect of surface chemistry on confined phase behavior in nanoporous media: an experimental and molecular modeling study. Langmuir 2018;34(32):9349–58. https://doi.org/10.1021/acs.langmuir.8b00986. pMID: 30008204. [16] Morishige K, Tateishi M, Hirose F, Aramaki K. Change in desorption mechanism from pore blocking to cavitation with temperature for nitrogen in ordered silica with cagelike pores. Langmuir 2006;22(22):9220–4. https://doi.org/10.1021/ la061360o. pMID: 17042533. [17] Ravikovitch PI, Neimark AV. Experimental confirmation of different mechanisms of evaporation from ink-bottle type pores: equilibrium, pore blocking, and cavitation. Langmuir 2002;18(25):9830–7. https://doi.org/10.1021/la026140z. [18] Tompsett GA, Krogh L, Griffin DW, Conner WC. Hysteresis and scanning behavior of mesoporous molecular sieves. Langmuir 2005;21(18):8214–25. https://doi.org/10. 1021/la050068y. pMID: 16114924. [19] Morishige K. Dependent domain model of cylindrical pores. J Phys Chem C 2017;121(9):5099–107. https://doi.org/10.1021/acs.jpcc.6b12566. [20] Cychosz KA, Guo X, Fan W, Cimino R, Gor GY, Tsapatsis M, et al. Characterization of the pore structure of three-dimensionally ordered mesoporous carbons using high resolution gas sorption. Langmuir 2012;28(34):12647–54. https://doi.org/10. 1021/la302362h. pMID: 22853806. [21] Cimino R, Cychosz KA, Thommes M, Neimark AV. Experimental and theoretical studies of scanning adsorption-desorption isotherms. Colloids Surf A 2013;437:76–89. https://doi.org/10.1016/j.colsurfa.2013.03.025. characterization of Porous Materials: From Angstroms to Millimeters A Collection of Selected Papers Presented at the 6th International Workshop, CPM-6 April 30 – May 2nd, 2012, Delray Beach, FL, USA Co-sponsored by Quantachrome Instruments. URL:http:// www.sciencedirect.com/science/article/pii/S0927775713002148. [22] Payer KR, Hammond KD, Tompsett GA, Krogh L, Pratt MN, Conner WC. The effects of mechanical and thermal perturbations on states within the hysteresis of sorption isotherms of mesoporous materials. J Porous Mater 2009;16(1):91–9. https://doi. org/10.1007/s10934-007-9172-9. [23] Morishige K, Tarui N. Capillary condensation of nitrogen in ordered mesoporous silica with bicontinuous gyroid structure. J Phys Chem C 2007;111(1):280–5. https://doi.org/10.1021/jp064946s. [24] Hitchcock I, Lunel M, Bakalis S, Fletcher RS, Holt EM, Rigby SP. Improving

[25]

[26]

[27]

[28]

[29]

[30]

[31] [32]

[33] [34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

13

sensitivity and accuracy of pore structural characterisation using scanning curves in integrated gas sorption and mercury porosimetry experiments. J Colloid Interface Sci 2014;417:88–99. https://doi.org/10.1016/j.jcis.2013.11.025. URL:http:// www.sciencedirect.com/science/article/pii/S0021979713010096. Mitropoulos AC, Stefanopoulos KL, Favvas EP, Vansant E, Hankins NP. On the formation of nnaobubbles in vycor porous glass during the desosrption of holgenated hydrocarbons. Scientific Rep 5.https://doi.org/10.1038/srep10943. Kruk M, Jaroniec M, Sayari A. Nitrogen adsorption study of mcm-41 molecular sieves synthesized using hydrothermal restructuring. Adsorption 2000;6(1):47–51. https://doi.org/10.1023/A:1008995015347. Esparza J, Ojeda M, Campero A, Domínguez A, Kornhauser I, Rojas F, et al. N2 sorption scanning behavior of sba-15 porous substrates. Colloids Surf A 2004;241(1):35–45. Proceedings of the Third International TRI/Princeton Workshop. Neimark A. Rodriguez-Reinoso F, Rouquerol J, Sing K, Unger K, editors. Characterization of porous solids II, Vol. 62 of studies in surface science and catalysis Elsevier; 1991. p. 67–74. https://doi.org/10.1016/S0167-2991(08)61310-5. Cychosz KA, Guillet-Nicolas R, García-Martínez J, Thommes M. Recent advances in the textural characterization of hierarchically structured nanoporous materials. Chem Soc Rev 2017;46:389–414. https://doi.org/10.1039/C6CS00391E. Chang S-S, Clair B, Ruelle J, Beauchêne J, Di Renzo F, Quignard F, et al. Mesoporosity as a new parameter for understanding tension stress generation in trees. J Exp Bot 2009;60(11):3023–30. https://doi.org/10.1093/jxb/erp133. Hill CAS, Norton A, Newman G. The water vapor sorption behavior of natural fibers. J Appl Polym Sci 2009;112(3). https://doi.org/10.1002/app.29725. Al-Muhtaseb A, McMinn W, Magee T. Moisture sorption isotherm characteristics of food products: a review. Food Bioprod Process 2002;80(2):118–28. https://doi.org/ 10.1205/09603080252938753. URL:http://www.sciencedirect.com/science/ article/pii/S0960308502703052. Kapsalis JG. Influences of hysteresis and temperature on moisture sorption isotherms. Water activity. Routledge; 2017. p. 173–213. Zhao H, Lai Z, Firoozabadi A. Sorption hysteresis of light hydrocarbons and carbon dioxide in shale and kerogen. Scientific Rep 7: 16209.https://doi.org/10.1038/ s41598-017-13123-7. Zandavi SH, Ward CA. Characterization of the pore structure and surface properties of shale using the zeta adsorption isotherm approach. Energy Fuels 2015;29(5):3004–10. https://doi.org/10.1021/acs.energyfuels.5b00244. Barsotti E, Lowry E, Piri M, Chen J-H. Using capillary condensation and evaporation isotherms to investigate confined fluid phase behavior in shales. In The Proceedings of the 33rd International Symposium of Core Analysts – 26–30 August, Pau, France. Thern H, Horch C, Stallmach F, Li B, Mezzatesta A, Zhang H. Low-field nmr laboratory measurements of hydrocarbons confined in organic nanoporous media at various pressures. Micropor Mesopor Mater.https://doi.org/10.1016/j.micromeso. 2017.11.047. URL:http://www.sciencedirect.com/science/article/pii/ S1387181117307680. Zhang T, Ellis GS, Ruppel SC, Milliken K, Yang R. Effect of organic-matter type and thermal maturity on methane adsorption in shale-gas systems. Organ Geochem 2012;47:120–31. https://doi.org/10.1016/j.orggeochem.2012.03.012. URL:http:// www.sciencedirect.com/science/article/pii/S0146638012000629. Heller R, Zoback M. Adsorption of methane and carbon dioxide on gas shale and pure mineral samples. J Unconventional Oil Gas Resour 2014;8:14–24. https://doi. org/10.1016/j.juogr.2014.06.001. URL:http://www.sciencedirect.com/science/ article/pii/S2213397614000329. Ross DJ, Bustin RM. Impact of mass balance calculations on adsorption capacities in microporous shale gas reservoirs. Fuel 2007;86(17):2696–706. https://doi.org/10. 1016/j.fuel.2007.02.036. URL:http://www.sciencedirect.com/science/article/pii/ S0016236107001202. Chen M, Kang Y, Zhang T, You L, Li X, Chen Z, et al. Methane diffusion in shales with multiple pore sizes at supercritical conditions. Chem Eng J 2018;334:1455–65. https://doi.org/10.1016/j.cej.2017.11.082. URL:http://www.sciencedirect.com/ science/article/pii/S1385894717320053. Rani S, Prusty BK, Pal SK. Adsorption kinetics and diffusion modeling of ch4 and co2 in indian shales. Fuel 2018;216:61–70. https://doi.org/10.1016/j.fuel.2017.11. 124. URL:http://www.sciencedirect.com/science/article/pii/ S0016236117315387. Xing J, Hu S, Jiang Z, Wang X, Wang J, Sun L, et al. Classification of controlling factors and determination of a prediction model for shale gas adsorption capacity: a case study of chang 7 shale in the ordos basin. J Natural Gas Sci Eng 2018;49:260–74. https://doi.org/10.1016/j.jngse.2017.11.015. URL:http://www. sciencedirect.com/science/article/pii/S1875510017304365. Hu H, Hao F, Guo X, Dai F, Lu Y, Ma Y. Investigation of methane sorption of overmature wufeng-longmaxi shale in the jiaoshiba area, eastern sichuan basin, china. Mar Petrol Geol 2018;91:251–61. https://doi.org/10.1016/j.marpetgeo. 2018.01.008. URL:http://www.sciencedirect.com/science/article/pii/ S0264817218300084. Liu Y, Li HA, Tian Y, Jin Z, Deng H. Determination of the absolute adsorption/ desorption isotherms of ch4 and n-c4h10 on shale from a nano-scale perspective. Fuel 2018;218:67–77. https://doi.org/10.1016/j.fuel.2018.01.012. URL:http:// www.sciencedirect.com/science/article/pii/S0016236118300127. Guidry K, Luffel D, Curtis J. Development of laboratory and petrophysical techniques for evaluating shale reservoirs final technical report: gas research institute, chicago, illinois. Report GRI-95/0496; 1995: 286. Fisher Q, Lorinczi P, Grattoni C, Rybalcenko K, Crook AJ, Allshorn S, et al. Laboratory characterization of the porosity and permeability of gas shales using the crushed shale method: insights from experiments and numerical modelling. Mar Petrol Geol 2017;86:95–110. https://doi.org/10.1016/j.marpetgeo.2017.05.027.

Fuel xxx (xxxx) xxxx

E. Barsotti, et al.

2018. URL:https://books.google.com/books?id=5sZmDwAAQBAJ. [65] Huang H, Sun W, Xiong F, Chen L, Li X, Gao T, Jiang Z, Ji W, Wu Y, Han J. A novel method to estimate subsurface shale gas capacities. Fuel 2018;232:341–50. https:// doi.org/10.1016/j.fuel.2018.05.172. URL:http://www.sciencedirect.com/science/ article/pii/S0016236118310184. [66] Wang P, Chen Z, Jin Z, Jiang C, Sun M, Guo Y, Chen X, Jia Z. Shale oil and gas resources in organic pores of the devonian duvernay shale, western canada sedimentary basin based on petroleum system modeling. J Natural Gas Sci Eng 2018;50:33–42. https://doi.org/10.1016/j.jngse.2017.10.027. URL:http://www. sciencedirect.com/science/article/pii/S1875510017304584. [67] Richardson J, Yu W. Calculation of estimated ultimate recovery and recovery factors of shale-gas wells using a probabilistic model of original gas in place. SPE Reservoir Eval Eng 2018;21(3). https://doi.org/10.2118/189461-PA. [68] Tinni A, Sondergeld C, Rai C. New perspectives on the effects of gas adsorption on storage and production of natural gas from shale formations. Petrophysics 2018;59(1). [69] Yan B, Mi L, Wang Y, Tang H, An C, Killough JE. Multi-porosity multi-physics compositional simulation for gas storage and transport in highly heterogeneous shales. J Petrol Sci Eng 2018;160:498–509. https://doi.org/10.1016/j.petrol.2017. 10.081. URL:http://www.sciencedirect.com/science/article/pii/ S0920410517308665. [70] Gor GY, Paris O, Prass J, Russo PA, Ribeiro Carrott MML, Neimark AV. Adsorption of n-pentane on mesoporous silica and adsorbent deformation. Langmuir 2013;29(27):8601–8. https://doi.org/10.1021/la401513n. pMID: 23758155. [71] Pathak M, Kweon H, Deo M, Huang H. Kerogen swelling and confinement: its implication on fluid thermodynamic properties in shales. Scientific Rep 2017;7. https://doi.org/10.1038/s41598-017-12982-4. [72] Larsen JW, Flores CI. Kerogen chemistry 5. ydride formation in, solvent swelling of, and loss of organics on demineralization of kimmeridge shales. Fuel Process Technol 2008;89(4):314–21. https://doi.org/10.1016/j.fuproc.2007.11.019. [73] Larsen JW, Li S. An initial comparison of the interactions of type i and iii kerogens with organic liquids. Organ Geochem 1997;26(5):305–9. https://doi.org/10.1016/ S0146-6380(97)00016-8. URL:http://www.sciencedirect.com/science/article/pii/ S0146638097000168. [74] Ballice L. Solvent swelling studies of göynk (kerogen type-i) and beypazar oil shales (kerogen type-ii). Fuel 2003;82(11):1317–21. https://doi.org/10.1016/S00162361(03)00026-7. URL:http://www.sciencedirect.com/science/article/pii/ S0016236103000267. [75] Larsen JW, Li S. Solvent swelling studies of green river kerogen. Energy Fuels 1994;8(4):932–6. https://doi.org/10.1021/ef00046a017. [76] Larsen JW, Parikh H, Michels R. Changes in the cross-link density of paris basin toarcian kerogen during maturation. Organ Geochem 2002;33(10):1143–52. https://doi.org/10.1016/S0146-6380(02)00102-X. URL:http://www.sciencedirect. com/science/article/pii/S014663800200102X. [77] Chiang W-S, Georgi D, Yildirim T, Chen J-H, Liu Y. A non-invasive method to directly quantify surface heterogeneity of porous materials. Nat Commun 2018;9(1):784. [78] Bertier P, Schweinar K, Stanjek H, Ghanizadeh A, Clarkson CR, Busch A, et al. On the use and abuse of n2 physisorption for the characterization of the pore structure of shales. The clay minerals society workshop lectures series, vol. 21. 2016. p. 151–61. [79] Thommes M, Smarsly B, Groenewolt M, Ravikovitch PI, Neimark AV. Adsorption hysteresis of nitrogen and argon in pore networks and characterization of novel micro- and mesoporous silicas. Langmuir 2006;22(2):756–64. https://doi.org/10. 1021/la051686h. pMID: 16401128. [80] Tan SP, Barsotti E, Piri M. Application of material balance for the phase transition of fluid mixtures confined in nanopores. Fluid Phase Equilibria 2019;496:31–41. https://doi.org/10.1016/j.fluid.2019.05.011. URL:http://www.sciencedirect.com/ science/article/pii/S0378381219302213.

URL:http://www.sciencedirect.com/science/article/pii/S0264817217301861. [48] Alizadeh AH, Akbarabadi M, Barsotti E, Piri M, Fishman N, Nagarajan N. Salt precipitation in ultratight porous media and its impact on pore connectivity and hydraulic conductivity. Water Resour Res 2018;54(4):2768–80. https://doi.org/10. 1002/2017WR021194. [49] Dong H, Blunt MJ. Pore-network extraction from micro-computerized-tomography images. Phys Rev E 2009;80(3):036307. [50] Barsotti E, Saraji S, Tan SP, Piri M. Capillary condensation of binary and ternary mixtures of n-pentane-isopentane-co2 in nanopores: an experimental study on the effects of composition and equilibrium. Langmuir 2018;34(5):1967–80. https://doi. org/10.1021/acs.langmuir.7b04134. pMID: 29360363. [51] Barsotti E, Saraji S, Piri M. Nanocondensation apparatus; 2017. [52] Kurtoglu B, Kazemi H, Rosen R, Mickelson W, Kosanke T. A rock and fluid study of middle bakken formation: Key to enhanced oil recovery, SPE/CSUR Unconventional Resources Conference – Canada, 30 September-2 October, Calgary, Alberta, Canada. https://doi.org/10.2118/171668-MS. [53] Yu W, Sepehrnoori K. Simulation of gas desorption and geomechanics effects for unconventional gas reservoirs. Fuel 2014;116:455–64. https://doi.org/10.1016/j. fuel.2013.08.032. URL:http://www.sciencedirect.com/science/article/pii/ S0016236113007606. [54] Wattenbarger RA, Alkouh AB. New advances in shale reservoir analysis using flowback data. In SPE Eastern Regional Meeting, 20–22 August, Pittsburgh, Pennsylvania, USA.https://doi.org/10.2118/165721-MS. [55] Lubis W, Mulhim N, Al-Sultan A, Asiri K, Buraiki M, Bartko K, et al. Does the horizontal fracture limit the fracture height growth in jubaila source rock?, SPE/ IATMI Asia Pacific Oil & Gas Conference and Exhibition, 17–19 October, Jakarta, Indonesia.https://doi.org/10.2118/187022-MS. [56] Chen S, Zhu Y, Wang H, Liu H, Wei W, Fang J. Shale gas reservoir characterisation: a typical case in the southern sichuan basin of china. Energy 2011;36(11):6609–16. https://doi.org/10.1016/j.energy.2011.09.001. URL:http://www.sciencedirect. com/science/article/pii/S0360544211005986. [57] Williams RH, Khatri DK, Keese RF, Roy-Delage SL, Roye JM, Leach DLR, et al. Flexible, expanding cement system (fecs) successfully provides zonal isolation across marcellus shale gas trends. In Canadian Unconventional Resources Conference, 15–17 November, Calgary, Alberta, Canada.https://doi.org/10.2118/ 149440-MS. [58] Kuppe FC, Nevokshonoff G, Haysom S. Liquids rich unconventional montney: the geology and the forecast. In SPE Canadian Unconventional Resources Conference. https://doi.org/10.2118/162824-MS. [59] Orangi A, Nagarajan N. Unconventional shale gas-condensate reservoir performance: Impact of rock, fluid, and rock-fluid properties and their variations. In Unconventional Resources Technology Conference (URTEC).https://doi.org/10. 15530/urtec-2015-2170061. [60] Lemmon E, McLinden M, Friend D. Thermophysical properties of fluid systems. In Linstrom P, Mallard W (Eds.), NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg MD, 20899, 2018.https://doi.org/10.18434/T4D303. [61] Dantas S, Struckhoff KC, Thommes M, Neimark AV. Phase behavior and capillary condensation hysteresis of carbon dioxide in mesopores. Langmuir 2019;35(35):11291–8. https://doi.org/10.1021/acs.langmuir.9b01748. pMID: 31380648. [62] Neimark AV, Ravikovitch PI. Capillary condensation in mms and pore structure characterization. Micropor Mesopor Mater 2001;44–45:697–707. https://doi.org/ 10.1016/S1387-1811(01)00251-7. http://www.sciencedirect.com/science/article/ pii/S1387181101002517. [63] Inwood J, Lovell M, Fishwick S, Morgan N, Pritchard T, Davies S, et al. Assumptions and uncertainties in petrophysical models for shale gas formations and their effect on resource calculations. In SPWLA 59th Annual Logging Symposium, 2–6 June, London, UK. [64] Yu W, Sepehrnoori K. Shale Gas and tight oil reservoir simulation. Elsevier Science;

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