Idealized Shale Sorption Isotherm Measurements To DeterminePore Capacity, Pore Size Distribution, and Surface AreaR. Holmes,†,⊥ H. Aljamaan,‡,⊥ V. Vishal,§ J. Wilcox,∥,# and A. R. Kovscek*,‡

†Emmett Interdisciplinary Program in Environment and Resources and ‡Department of Energy Resources Engineering, StanfordUniversity, Stanford, California 94305-2220, United States§Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India∥Chemical and Biological Engineering Department, Colorado School of Mines, Golden, Colorado 80401, United States#Chemical Engineering Department, Worcester Polytechnic Institute, Worcester, Massachusetts 01609, United States

ABSTRACT: One potential method for mitigating the impacts of anthropogenic CO2-related climate change is thesequestration of CO2 in depleted geological gas and oil formations, including shale. The accurate characterization of theheterogeneous material properties of shale, including pore capacity, surface area, pore-size distributions, and composition isneeded to understand the potential storage capacities of shale formations. Powdered idealized shale samples were created toexplore reduction of the complications in characterization of pore capacity that arise from the heterogeneous rock compositionand pore sizes ranging over multiple orders of magnitude. The idealized shales were created by mechanically mixing incrementalamounts of four essential powdered components by weight and characterized with low pressure gas adsorption/desorptionisotherms. The first two components, organic carbon and phyllosilicates (such as clays, micas, and chlorite), have been shown tobe the most important components for CO2 uptake in shales. Organic carbon was represented by kerogen isolated from aSilurian shale, and phyllosilicate groups were represented by powdered illite from the Green River shale formation. Theremainder of the idealized shale was composed of equal parts by weight of SiO2 to represent quartz and CaCO3 to representcarbonate components. Three idealized sample groups were prepared to be approximately 10, 30, and 55% illite by weight. Eachof the sample groups consisted of four samples, incrementing the percent kerogen from 1.5 to 6%. Eagle Ford, Baltic, andBarnett shale sorption measurements were used to validate the idealized sample methodology. The sorption isotherms weremeasured volumetrically using low pressure N2 (77 K) and Ar (87 K) adsorbates on Quantachrome Autosorb IQ2. Bothidealized and validation samples were outgassed using a standardized procedure that produced repeatable results whileminimizing changes to the material properties of the shale. The idealized sample results indicated a positive linear correlationwith increasing total organic carbon and pore capacity. This work is essential toward the development of predictive modelsweighted and scaled by the corresponding mineral compositional description of the reservoir.

1. INTRODUCTION

With recent development of shale geological formations as asource of hydrocarbons, research and development regardingthese resources have accelerated at an unprecedented pace.The focus has been on determining estimates of the gas-in-place, easing gas extraction, and the overall integrity of thereservoir system during and after extraction. With the recentquest for finding shales as suitable CO2 storage sites, a newdomain of investigation has opened.1,2 The key reservoirproperties such as rock composition, porosity, pore capacity(i.e., the total mass of a chemical species retained by shale),permeability, and strength have been determined for many ofthe prospective shale systems. The results of measuring theseproperties have made it apparent that the compositions ofshales vary from basin to basin, strata to strata, within the samestrata, and even from opposite sides of the same core. Attemptsto correlate these properties are challenging given the hetero-geneous nature of shale on at least two fronts. First, thephysical composition of shales includes numerous componentsin the rock at various ratios and stages of geochemical and/orthermal maturity. Second, pore sizes in shales can span multi-ple orders of magnitude, from visible millimeter fractures allthe way down to the nanoscale width of individual gas

molecules. These issues can begin to be resolved by under-standing the fundamental physical properties and mechanics atvarious discrete scales across the strata or basin.To address the first issue, this study reduces the number of

essential components under consideration to just four: kerogen,illite, silica, and calcium carbonate. The highest purity powderedcomponents available were used, such as lab grade silica andcalcium carbonate, well-characterized illite, and a kerogenisolate. To address the second, this study focuses on a specificsize range that may later be incorporated into a separate pore-size study rather than span all of the length scales of the poresizes found in shale. The scale here is chosen because it repre-sents a pore-size range important for shale gas storage. Largerpore sizes are left for analysis using other methods and account-ing for processes associated with free gas and permeable pathways.It is important to recognize that gases stored in nanoscale

pores may be adsorbed or may be tightly packed dissolvedspecies. Hence, a greater volume of gas is stored than is sug-gested from a simple volumetric measurement that accounts

Received: August 8, 2018Revised: October 11, 2018Published: January 2, 2019

Article

pubs.acs.org/EFCite This: Energy Fuels 2019, 33, 665−676

© 2019 American Chemical Society 665 DOI: 10.1021/acs.energyfuels.8b02726Energy Fuels 2019, 33, 665−676

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for only free gas in pore space. As such, the term pore capacityis used here rather than the term pore volume to account forall gases within the porous medium.The analysis of pore capacity and pore-size distributions

(PSDs) for both individual components and natural shale sam-ples is also challenged by inconsistent sample preparation acrossthe literature. As a matter of new protocol proposed by Aljamaanet al. as well as Holmes et al., the idealized and validation shalesamples were outgassed at 250 °C and the sorption isothermsmeasured using low pressure N2 (77 K) and Ar (87 K) adsorbateson a Quantachrome Autosorb IQ2.

3,4 Quantachrome’squenched solid density functional theory (QSDFT) datareduction software was used for the PSDs, pore capacities, andspecific surface areas. Thermogravimetric analyses were con-ducted separately to inform the relative mass loss at differenttemperatures of kerogen and illite. The thermogravimetricmethod was not used for measuring or analyzing pore capacities.The scale in the current study focuses on pore capacities that

result from pore sizes of approximately 1−27 nm. The 27 nmcutoff is approximately the midpoint of IUPAC mesopores thatare defined as pore widths or diameters of 2−50 nm, whereasmicropores are pore widths or diameters of <2.0 nm. Theresults of the current study also include a small (<0.1%)amount of the measured pore capacity attributed to largermicropores (1.0−2.0 nm); however, this should not be con-sidered a robust micropore measurement or analysis becauselow pressure adsorption in shales using the low temperaturesrequired for N2 (77 K) and Ar (87 K) has limitations arisingfrom kinetics and tortuosity of the shale. As this is an explor-atory study to determine the usefulness of idealized shale, theanalysis of micropores using a CO2 adsorbate was not con-ducted. Because surface area can increase by orders of magni-tude in the micropores, the analysis of surface area withoutmicropore measurements would be incomplete and less thanrobust. As such, this study focuses on pore capacities arisingfrom the 1−27 nm span. The surface area analysis was left tofuture work that includes a thorough micropore analysis.There is debate about the mechanisms underlying corre-

lations between pore capacities and variables such as totalorganic carbon (TOC), phyllosilicate content (specificallyclays), and thermal maturity and/or depth of reservoir.5 Theintuitive hypothesis is that in all cases, the pore capacity ofany particular shale sample should be a simple sum (e.g., aweighted superposition) of the pore capacity of its relativecomponents; however, this has proven to be problematic andnontrivial when attempting to use data in existing literaturethat alternately show positive correlations for TOC in the caseof Ross and Bustin, versus no correlation in high TOC shales,as shown in Gasparik et al.6,7 Kuila et al. further the inves-tigation on pore capacity correlations with variables such asTOC content, phyllosilicate content (specifically clays), andthermal maturity and/or depth of reservoir.8 The correlationalresults are not always consistent or statistically significant.Nevertheless, the kerogen and clay components of shale

have been shown to be the essential components for porecapacity in smaller mesopores and micropores,6,9−12 which areconsidered the main source of methane and possible eventualsinks for sequestered CO2. The composition of kerogen andtypes of clays also control the ultimate gas storage capacity inshales. For instance, organic matter such as kerogen, which ismeasured and reported here as TOC, is correlated with theporosity, gas content, PSDs, and even geomechanical param-eters.3,13−17 Fan et al. established that the methane adsorption

capacity of phyllosilicate minerals decreased in the order:montmorillonite > kaolinite > illite > illite/smectite mixedlayer > chlorite.18 This study uses the more general term ofphyllosilicates when referring to clays to ensure inclusion ofcomponents that are not 2:1 clays, but were still found to bepresent during the X-ray diffraction (XRD) analysis of thevalidation samples. The phyllosilicates will be abbreviated asCMC for clays, micas, and chlorite. Literature shows thatadsorption capacity studies have either been carried out onindividual components or on representative shale samples.The treatment of both individual shale components alongsideindividual heterogeneous shale samples in this paper unifies theseparate approaches.This study tests the hypothesis that, despite the hetero-

geneity of shales, the pore capacities of shales should be thesum of the pore capacities of the relative components.We develop a methodology to mix the purest available powderedcomponents of shale in the desired proportion to provide thebest representative composition for the chosen reservoir. These“synthetic” or “idealized” powdered shales have knownconcentrations of all four key components: kerogen, phyllo-silicates, silica, and calcium carbonates. Secondly, this studyattempts to confirm the credibility of using such idealizedshales by comparing their pore capacities to real shales. This isa proof of concept study to lay the groundwork for futurestudies that explore the possibilities of the predictive powerand scientific utility of such idealized shales if they are shownto not only confirm the hypothesis, but also represent the porecapacities of real shale at the 1−27 nm scale. Subsequentstudies could then follow in order to explore and explain thepossible phenomenon that underlie the variation in shale porecapacities when their pore capacities do not appear to reflectthe sum of the parts, such as the role of thermal maturity, com-paction, the micron-scale arrangement of minerals, and so on.

2. EXPERIMENTAL SECTIONThis section describes preparation of both the idealized and validationsamples followed by an explanation of the low pressure adsorptionexperiment.

2.1. Sample Meshing. A standard for the degree to which shalemust be crushed has not been established in the literature. As long asthe crushed particles are sufficiently small to eliminate the problemsof low permeability, it should suffice. Some groups report crushingshale and sieving at 40 mesh (<420 μm),19 others have opted for asize of 80 mesh.18 The idealized shales and validation shales werecrushed and sieved to 200 mesh (<75 μm), with the exception of thequartz component of the idealized shale, which was manufactured at325 mesh. There is growing concern over the size of mesh used forpowdered samples, as well as for the problems using shale core chips.While chips are problematic as they primarily measure the permeablepathways and consider only the accessible shale components, it is anongoing debate how much of the disconnected pores in a reservoirshould be considered as part of the total “space available” in areservoir. For the purpose of gas extraction, substantial amounts of gasare retrieved by induced permeability through hydraulic fracturing.However, for CO2 sequestration, the geologic time frame over whichthe CO2 must be stored needs to include pores that are considereddisconnected, as it is becoming apparent that given enough time,relatively impermeable shale formations allow for CO2 and other gasesto and reside in these small pore spaces.20 Furthermore, chips havebeen shown to be more problematic in that they are subject to changewith core handling, multiple testing, and successive dehydration andrehydration,8 as compared to powders, which have been shown tohave minimal variation in the low pressure gas adsorption (LPGA)isotherms with repeated tests on the same samples.4

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2.2. Idealized Shales. Powdered idealized shales are composed ofprecisely measured amounts of the four components commonly foundin shales: kerogen, phyllosilicates (here in the form of the clay, illite),quartz (here as SiO2), and calcium carbonate (here as CaCO3). Threesets of idealized shales were prepared and categorized by holdingconstant the percent illite in each set, labeled as low, medium, andhigh clay, adding incremental amounts of kerogen, and adding equalparts by weight quartz and carbonate to comprise the remaining massof the sample. All three idealized sample sets were analyzed using N2.The low clay set was also analyzed using Ar to compare to the N2 dataas well as compare possible outgassing effects to the validationssamples.2.2.1. KerogenThe TOC Component. The organic carbon

component of the idealized shale is a Silurian kerogen. The kerogenis composed of approximately 74.4% organic carbon. The percentkerogen by weight was converted to TOC by multiplying the percentkerogen by the TOC of the kerogen. The TOC was measured usingHCl acid digestion to remove the carbonates, with organic carboncontent detected using a Carlo-Erba 1500 NA Elemental Analyzer asdescribed in Psarras et al.22 The sample sets were created by mea-suring kerogen by weight so that each of the three sample sets wouldhave samples that incrementally increase. The TOC for each idealizedsample was calculated by multiplying the percent kerogen by 74.4%,which results in a set of idealized samples with sample TOC asapproximately 1.5, 3, 4.5, and 6%. Three samples of kerogen wereprocessed by outgassing at 60 and 250 °C with LPGA to show thevariation in the kerogen pore capacities between samples.2.2.2. IlliteThe Phyllosilicate or Clay Component. The idealized

shales were created using a Green River shale formation illite (Ward’sNatural Science Establishment, Rochester, NY). This is a non-expandable clay with each clay unit composed of two tetrahedrallayers each bonded to opposite sides of one inner octahedral layer,also called a 2:1 clay. The illite is a clay that has been compressed andheated at conditions associated with deeper geologic formations.Therefore, the idealized shales could be considered to have lowerbounds for pore capacity when compared to shales with expandableclays. Clays with greater pore capacities than the idealized samplesmay result from having been subjected to lower pressures, tem-peratures, and/or shallower depths. The three sets of idealized shaleswere prepared by mixing Green River shale formation illite to producesets of samples with low, medium, and high percentage by weightof illite. XRD on the Green River illite indicates a composition ofapproximately 96.0% illite, 3.0% quartz, and <1% mixture of othermaterials such as pyrite. The low clay set was designed to haveapproximately 10% illite, the second or medium clay set was designedto have approximately 30% illite by weight, and the third, or high clay,set was designed to have approximately 55% clay by weight. Raw illitesamples were prepared by crushing and grinding the illite to a powderusing a mortar and pestle and sieved to 200 mesh.4 Approximately 5 gof illite was measured for use in the idealized shales after homo-genizing the powder by shaking and stirring. Portions of the powderedillite were then used in the selected amounts when mixing theidealized shales. Additionally, at least three individual samples of250 mg or more of the illite were outgassed to create sets of samplesat temperatures of 60, 110, 200, and 250 °C. This was done to showpotential variations in pore capacities resulting from the differenttemperatures of outgassing. Comparisons between outgassing temper-ature sets were performed with the mean of the samples from eachtemperature set.2.2.3. Quartz and Carbonate Components. Quartz and carbon-

ates in geologic formations go through several transitions that result incementation of sedimentary layers. Quartz cements are the result ofthe dissolution of opal A to opal C/CT, which are known to fill porespaces, cementing the layer and inducing stronger rock with greaterbrittleness. Likewise, carbonates also provide cementation thatincreases rock strength.21 Møller-Pedersen & Koestler also indicatethat at depths of between 2 and 3 km there is a general transition zonefrom processes driven by mechanical compaction to kinetic andchemical interactions, especially for quartz and carbonates.21 Abovethe 2.5−3 km zone, biogenic quartz and carbonates are responsible

for cementation that can strengthen rock layers. Laboratory gradeSiO2 (Spectrum Chemical MFG, Crystalline SiO2, 325 mesh powder)and CaCO3 (Sigma-Aldrich 99.5% CaCO3 powder) were measuredby weight to contribute equally to the remaining sample weight afterthe illite and kerogen had been added, as shown in Table 1.

2.3. Validation Samples. A total of 29 samples from threedifferent shale plays were selected for comparison with the idealizedshales. The validation samples were chosen for their similarities incomposition, especially with respect to TOC, phyllosilicate content,including 2:1 CMC, and to a lesser extent quartz and carbonates.

2.3.1. Barnett. Six of the validation samples were from the Barnettformation. The Barnett samples range in TOC from 1.1 to 6.3%,CMC from 5.9 to 33%, quartz from 55.6 to 63.7%, calcite from 0 to35.6%, and pyrite from 0.8 to 2.8%. The Barnett samples are all in asimilar range of depth, from 2609 to 2632 m. Barnett 5 sample dataare reproduced with permission, as there was insufficient sample toconduct further analysis.

2.3.2. Eagle Ford. Eight of the validation samples were from theEagle Ford formation. The Eagle Ford samples range in TOC from2.1 to 5.3%, CMC from 5.8 to 40.1%, quartz from 11.4 to 28.7%,calcite from 30.1 to 74.7%, and pyrite from 0.3 to 3.7%. The EagleFord samples are all in similar range of depth, ranging from approx-imately 3863 to 3915 m. Eagle Ford 1 and Eagle Ford 2 sample datawere collected by the lead authors of this study with the same meth-odology,4,22 but the data were previously published and are repro-duced with permission as there was insufficient sample to conductfurther analysis on the shale for this study.

2.3.3. Baltic. Fifteen validation samples were analyzed from a Balticformation. The Baltic samples range in TOC from 0.2 to 5.37%, CMCfrom 44.3 to 71.1%, quartz from 18.6 to 43.8%, calcite from 0 to26.0%, and pyrite from 0.1 to 3.2%. The Baltic samples span a rangeof depths from 1416 to 4410 m (Table 2).

2.4. X-ray Diffraction. Semiquantitative XRD was used to obtainapproximate compositions of the raw illite, idealized series, Barnett,Eagle Ford, and Baltic samples. X-ray diffractograms were collectedusing a Rigaku Miniflex 600. The diffractograms were analyzed usingthe PDXL2 software.

2.5. Low-Pressure Gas Adsorption Analysis. LPGA isothermswere measured using an Autosorb iQ2 (Quantachrome Instruments)and interpreted from the adsorption branch using the QSDFT model,assuming a carbon adsorbent and cylindrical pores23 for the estimatesof pore capacities found in Table 3. The carbon adsorbent is chosenbecause of the propensity for gas adsorption to favor the kerogen ofshales, as well as to have consistent comparisons between validationsamples and the idealized shales. The carbon adsorbent model wasalso used for all of the individual raw components in order to have theability to compare the summation of the pore capacities of the indi-vidual components at their relative weights with the idealized and vali-dation shales. The samples were outgassed as stated in Section 2.5.1.The QSDFT calculations were based on a 55-point adsorption branch,

Table 1. Composition of Idealized Sample Sets UsingSemiquantitative XRD

quartz calcite pyrite illite % kerogen TOC

low 1 40.0 44.1 0.0 13.9 2.0 1.49low 2 42.8 43.2 0.0 10.3 4.0 2.98low 3 49.3 30.7 0.0 13.5 6.0 4.46low 4 35.6 41.4 0.0 14.7 8.0 5.95medium 1 40.6 32.4 0.0 25.4 2.0 1.49medium 2 37.4 24.2 0.0 34.3 4.0 2.98medium 3 29.8 22.6 0.0 41.9 6.0 4.46medium 4 35.9 26.7 0.0 29.4 8.0 5.95high 1 21.8 16.5 2.1 57.7 2.0 1.49high 2 22.7 16.9 0.0 56.4 4.0 2.98high 3 22.6 19.7 3.8 47.9 6.0 4.46high 4 21.1 15.4 0.0 55.2 8.0 5.95

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with measurements made on relative pressures (P/P0) spanning from10−6 to 0.995. Each point was required to maintain an equilibration timeof at least 2 min. The QSDFT model pore capacities were reportedthrough a pore diameter of approximately 27 nm, with IUPAC definedmesopores described as 2.0−50 nm and micropores <2.0 nm. Porecapacities for fine mesopores and micropores (1.0−27 nm) weremeasured with N2 (99.999%) as the adsorbate at 77 K for the Balticand idealized samples, with the temperature held constant by a liquidnitrogen bath. The low clay group of the idealized samples, Barnettand Eagle Ford validation samples were analyzed using Ar as theadsorbate at 87 K to the extent possible, with temperature held con-stant in a liquid argon bath.Micropore analysis using CO2 was not conducted in this study.

However, there is some overlap for both Ar and N2 into the micro-pore region. For instance, some samples indicated pore capacitymeasured in micropores in the range of 1.0−2.0 nm. The microporesthat were measured accounted for 0.07% or less of the cumulativepore capacity in the 1−27 nm range for both Ar and N2 as adsorbates.As such, the pore sizes contributing to the pore capacities in this study

are listed as ranging from 1.0 to 27 nm using Ar and N2, leaving themicropore features of the idealized shales using CO2 as an adsorbatefor another possible study. As detailed in Section 2.5.1, Ar wasselected as the adsorbate for analyzing how the idealized shalescompare to validation shales in response to outgassing at increasinglyhigh temperatures. The choice of Ar as the adsorbate is supported bythe IUPAC Technical Report by Thommes and Cychosz.24 While N2was used for the Baltic validation samples, medium, and high idealized

Table 3. Idealized Raw Components Pore Capacities Outgassed at 250 °C, Outgassed Using Ar or N2 as Labeleda

adsorbateillite(%)

TOC(%)

mean pore capacity(cm3/g) 1.0−15 nm

mean pore capacity(cm3/g) 1.0−27 nm

min pore capacity(cm3/g) 1.0−27 nm

max pore capacity max(cm3/g) 1.0−27 nm

DFT surface area(cm2/g) 1.0−27 nm

kerogenb Ar 0.0 74.4 0.140 0.222 0.178 0.249 121.1illiteb Ar 96.0 0 0.019 0.030 18.3silica N2 0.005 0.011 4.3carbonate N2 0.001 0.002 0.7pyrite N2 <0.001 0.001 0.5Barite N2 <0.001 0.002 0.4

aBarite and pyrite were outgassed at 50 °C. bAverage of three kerogen samples and four illite samples.

Table 2. Barnett, Eagle Ford, and Baltic XRDCompositional Data

depth (m) quartz calcite pyrite CMCb TOC

Barnett 1 2616 56.4 35.6 0.8 5.9 1.1Barnett 2 2609 63.7 0.0 0.7 30.4 4.9Barnett 3 2615 60.6 1.0 0.8 32.8 5.3Barnett 4 2613 55.6 3.1 2.8 33.0 5.4Barnett 5a 2625 64.3 0.0 5.4 24.1 6.1Barnett 6 2632 63.7 0.0 2.3 28.1 6.3Eagle Ford 1a 3864 11.9 86.0 0.0 <5 2.1Eagle Ford 2a 3863 12.1 85.5 0.0 <5 2.4Eagle Ford 3 3893 13.6 74.7 0.5 8.5 2.7Eagle Ford 4 3887 12.6 63.5 0.7 19.3 4.0Eagle Ford 5 3915 24.1 30.1 1.3 40.1 4.5Eagle Ford 6 3890 20.8 56.8 0.3 17.1 5.3Eagle Ford 7 3864 25.9 54.4 2.6 14.9 2.3Eagle Ford 8 no data 28.7 60.2 1.1 5.8 4.4Baltic 1 <1600 34.1 9.9 3.2 51.3 5.37Baltic 2 >1600 35.0 12.0 2.3 50.0 2.72Baltic 3 no data 22.3 9.0 2.6 65.3 3.66Baltic 4 no data 33.2 20.0 0.6 44.3 5.20Baltic 5 <2350 34.4 1.7 1.6 61.6 1.76Baltic 6 >2350 41.9 0.5 0.4 57.4 0.20Baltic 7 <2600 43.4 1.9 0.1 54.2 1.28Baltic 8 <2925 37.7 6.8 1.6 54.2 0.87Baltic 9 >2925 27.9 0.0 1.3 71.1 0.41Baltic 10 <3250 37.9 2.6 1.3 57.6 1.68Baltic 11 >3250 18.6 22.4 1.8 56.0 2.17Baltic 12 >4409 43.8 1.3 3.2 51.5 0.54Baltic 13 <4409 24.9 26.0 1.8 48.0 0.22Baltic 14 4409 34.1 1.6 0.91 62.8 1.70Baltic 15 >4375 37.4 0.62 2.2 58.3 1.63

aReproduced with permission.4,22 bClays, micas, chlorite (CMC).

Figure 1. Cartoons of kerogen pore conditions for low pressuresorption measurements resulting from the different outgassingtemperatures. The cylinders above the S curve represent pores andare situated above points on the S curve to show the hypotheticalstates of adsorbed or evolved gases as temperature increases. For eachshale, there is a hypothetical minimum between the S1 and S2 curvewhere the pore is completely free of adsorbed species and yet has notevolved any hydrocarbons from kerogen. The exact temperature foreach shale minimum is not known, but should be in the inter-S-curveminimum, which is proposed to be between 200 and 250 °C.

Table 4. Barnett and Eagle Ford Pore Capacities Using Ar(Except Where Noted) and Outgassed at 250 °C

CMC(%)

TOC(%)

porecapacity(cm3/g)

1.5−15 nm

porecapacity(cm3/g)

1.5−27 nm

DFTsurface area(cm2/g)

1.5−27 nm

Barnett 1 5.9 1.1 0.009 0.016 7.5Barnett 2 30.4 4.9 0.009 0.010 9.6Barnett 3 32.8 5.3 0.009 0.010 9.6Barnett 4 33.0 5.4 0.013 0.023 11.2Barnett 5a 31.9 6.1 0.019 0.025 13.3Barnett 6 28.1 6.3 0.016 0.026 13.7Eagle Ford 1a 14.5 2.1 0.013 0.020 9.4Eagle Ford 2a 17.2 2.4 0.013 0.021 9.6Eagle Ford 3 8.5 2.7 0.011 0.016 8.8Eagle Ford 4 19.3 4.0 0.013 0.023 14.5Eagle Ford 5 40.1 4.5 0.016 0.023 17.3Eagle Ford 6 17.1 5.3 0.015 0.025 16.3Eagle Ford 7 14.9 2.3 0.008 0.018 7.2Eagle Ford 8 5.8 4.4 0.015 0.026 15.8

aReproduced with permission.4,22 Nitrogen adsorbate used instead ofargon.

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samples, the Ar data matched closely to the N2 data on the lowidealized set, and therefore, running the N2 samples on Ar was notdetermined to be necessary.2.5.1. Outgassing Temperature. To show the maximum possible

pore capacities, all samples were outgassed at a pressure of 10−5 barfor at least 10 h at 523 K (250 °C) using methodology previouslyreported.4,22 The 523 K (250 °C) is expected to yield the largestpossible pore capacities while minimizing the structural alterations tothe shale. The 250 °C outgassing temperature is hypothesized to bevery close to the minimum between the S1 and S2 pyrolysis curve, asdepicted in the cartoon in Figure 1 as the inter-S-curve minimum,previously referred to by Holmes et al. as the “pregeneration trough.”4

Selected idealized and validation samples were also outgassed at multi-ple temperatures to compare the effects of outgassing temperature on

the idealized shales as compared to the validation samples. The addi-tional outgassing temperatures selected were at or near temperaturescommonly found throughout low-pressure adsorption experimentalliterature: 333 K (60 °C), 383 K (110 °C), and 473 K (200 °C). Theimpact of increasing the outgassing temperature is reinforced bythermogravimetric analysis (TGA). The low clay idealized shales wereoutgassed at all four temperatures. The Barnett and Eagle Ford sampleswere outgassed at 60 and 250 °C (Table 4).

2.6. Thermogravimetric Analysis. In order to analyze possibleimpacts of the outgassing temperature on illite and kerogencomponents, TGA was performed using TA Instrument Q500 TG.This TGA should not be confused with gravimetric measurementsusing high pressure adsorption instruments (such as a Rubotherm)that also create isotherms that can be used to analyze PSDs and porecapacities. No pore capacities were produced from this TGA. Sampleswere prepared as described in Section 2, with a sample mass of 20 mg.Each sample was run under an N2 atmosphere, with a temperatureincrease ramp of 10 °C/min from 30 to 1000 °C. Each run included ablank run on the sample crucible, followed by the sample being run inthe same crucible (Table 5).

3. RESULTS AND DISCUSSION

3.1. Interpretation of PSDs. Given the interest in thenanoporosity of shale, the ultimate goal of sorption isothermsis to determine pore capacities and PSDs. There are a numberof options for determining the PSD. On the basis of the limitsof the traditional methods such as BJH, which can under-estimate pores less than 10 nm by as much as 20−30%, densityfunctional theory (DFT)-based methods have been chosen forthe PSD interpretation.25 Pore characterization of shales usinglow-pressure adsorption must be limited to a region lower thanapproximately P/P0 = 0.9 as recommended in Holmes et al.4

P0 is held to approximately atmospheric pressure of 760 Torrby the Autosorb. Hysteresis between the adsorption and desorp-tion branches are consistent with complex pore networks, andthe closing of the hysteresis loops only at very low pressure,

Table 5. Baltic Pore Capacities and Surface Areas Using N2and Outgassed at 250 °C

CMC(%)

TOC(%)

porecapacity(cm3/g)

1.5−15 nm

porecapacity(cm3/g)

1.5−27 nm

DFT surfacearea (cm2/g)1.5−27 nm

Baltic 1 51.3 5.4 0.021 0.035 13.7Baltic 2 50.0 2.7 0.023 0.039 16.6Baltic 3 65.3 3.7 0.024 0.039 15.9Baltic 4 44.3 5.2 0.023 0.036 16.0Baltic 5 61.6 1.8 0.023 0.036 16.7Baltic 6 57.4 0.2 0.027 0.040 21.6Baltic 7 54.2 1.3 0.017 0.025 12.8Baltic 8 54.2 0.9 0.018 0.028 15.2Baltic 9 71.1 0.4 0.020 0.032 18.6Baltic 10 57.6 1.7 0.019 0.028 15.8Baltic 11 56.0 2.2 0.019 0.027 14.3Baltic 12 51.5 0.5 0.015 0.027 11.8Baltic 13 48.0 0.2 0.015 0.021 10.6Baltic 14 62.8 1.7 0.009 0.014 7.4Baltic 15 58.3 1.6 0.019 0.028 14.4

Figure 2. Low pressure adsorption measurements using Ar on illite and kerogen and using N2 on raw components. Kerogen shows the greatestadsorption capacity followed by illite. Adsorption: filled symbol. Desorption: unfilled symbol. Barite -blue circle-, pyrite -green triangle-,CaCO3 -red square-.

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with both N2 and Ar, is another indicator of complex porenetworks.23 The dropoff observed in the desorption brancharound the relative pressure of 0.4−0.5 is a result of metasta-bility issues with the adsorbates as discussed elsewhere,4,23 andas such, the trends illustrated in this paper make pore capacitycomparisons using the adsorption branch with a maximumpore diameter cutoff of approximately 27 nm for both Arand N2. This diameter is the maximum pore diameter cutoffavailable from the Quantachrome QSDFT using cylindricalpores on a carbon adsorbent. The Quantachrome model allowsN2 through 33 nm on carbon, but for consistency, the N2 oncarbon values were also reported only through 27 nm in thetables. Representative adsorption/desorption isotherms can bereferred to in Figure 2.3.2. Raw Components PSD and Adsorption Capacity.

The four main components have cumulative pore capacitiesthat increase as CaCO3/pyrite/barite < SiO2 < illite < Siluriankerogen. Additional trace amounts (i.e., less than 3%) of bariteand pyrite can also be found in some shales, but were found tohave pore capacities similar to CaCO3. As the relative amountof barite and pyrite are small compared to the other four com-ponents, little pore capacity should be attributed to barite andpyrite in these samples. Most of the pore capacity will be foundassociated with kerogen and illite. The isotherms and PSDs forthe four main components, as well as barite and pyrite, areshown in Figures 2 and 3, respectively. Notice the change ofy-axis scale for each subplot (Table 6).3.3. Impact of Varying TOC and Illite Content on

Idealized Samples. The primary factor for determining porecapacity in the idealized and validation shales is the TOC,followed by clay. When grouping the idealized shales togetherby constant TOC, each of the idealized shale sets showincreasing pore capacity with increased illite content, as shownin Figures 4 and 5. The 1.5% TOC group has some overlapbetween the high clay and medium clay groups. This could bethe result of the variations in PSD inherent to the kerogen andgives cause to conduct experiments at additional pore scales,especially the larger mesopores. For instance, 25−50 nm wouldbe appropriate drawing from these data. All four of the TOCgroups show the groups with small amounts of clay have theleast amount of pore capacity; however, the medium and high

clay groups have some overlaps. The 4.5% TOC group showsan increase in pore capacity from low, to medium, to high claycontent across the pore sizes shown. The 3.0 and 6.0% TOCgroups show the medium clay having slightly more pore capac-ity in smaller pore diameters (<10 nm), but the groups withhigher clay content have pore capacities that have surpassedthe medium clay pore capacities starting at approximately25 nm in diameter.

3.4. Validation Versus Idealized Pore CapacityMeasurements. The degree to which the idealized samplescan be said to approximate the validation samples can be judgedby (1) whether the pore capacities are similar given similar claycontent and TOC and (2) whether the physical processesappear to be similar to validation shales, such as strength andresponse to factors such as outgassing temperature. Figure 6shows the increasing nature of pore capacity of the idealizedsets with respect to both clay and TOC. Table 7 shows therepresentative pore capacities for each individual component,weighted by the mass percent each component has in theidealized shale. The mean, min, and max of the kerogen areused to show the possible spread of the idealized shales interms of what should be expected from the mixtures. Themixed idealized shales are shown. The medium 3 sample

Figure 3. Low pressure adsorption measurements using Ar on illite and kerogen are plotted through 27 nm and plotted through 33 nm foradditional raw components using N2. Kerogen shows the greatest adsorption capacity followed by illite. Cumulative pore capacity filled symbol.PSD unfilled symbol. Barite -blue circle-, pyrite -green triangle-, CaCO3 -red square-.

Table 6. Idealized Pore Capacities and Surface Areas UsingN2 and Outgassed at 250 °C

illite(%)

TOC(%)

N2 porecapacity(cm3/g)

1.0−15 nm

N2 porecapacity(cm3/g)

1.0−27 nm

N2 DFTsurface area(cm2/g)

1.0−27 nm

low 1 13.9 1.49 0.008 0.013 5.6low 2 10.3 2.98 0.013 0.019 8.9low 3 13.5 4.46 0.008 0.015 7.3low 4 14.7 5.95 0.019 0.026 13.9medium 1 25.4 1.49 0.013 0.019 9.7medium 2 34.3 2.98 0.017 0.023 11.7medium 3 41.9 4.46 0.014 0.021 11.7medium 4 29.4 5.95 0.023 0.031 17.8high 1 57.7 1.49 0.013 0.026 12.2high 2 56.4 2.98 0.016 0.026 14.1high 3 47.9 4.46 0.019 0.030 18.1high 4 55.2 5.95 0.022 0.034 20.7

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(TOC 4.5%) appears to be an outlier. As calculated in Table 7,it would be predicted by a summation of the relative weightedcomponents to be between 0.027 and 0.031 cm3/g. However,at 0.021 cm3/g, it falls outside natural variation calculated fromthe kerogen and clays in Table 7. It has been left in as anoutlier. The illite pore capacities had a standard deviation of0.0003 when outgassed at 250 °C, and as such, it is unlikely thatthe variation is resulting from illite, which statistically couldaccount for less than 0.001 cm3/g at 99% confidence interval.

The high clay idealized samples demarcate the lower end ofthe pore capacities of the Baltic series of validation samples.The low and medium clay idealized samples appear to straddlethe range of the pore capacities of the Barnett and Eagle Fordsamples. As has been explored by Bourg, the role of clay onporosity has been reported as less significant when the shale iscomposed of clay fractions less than approximately one-third,but more significant when the shale composition is above theapproximate one-third clay fraction.26 The limited 1−27 nm

Figure 4. Low pressure adsorption measurements varying TOC and illite idealized series. Adsorption: filled shapes. Desorption: unfilled shapes.Low -blue circle-, med -green triangle-, high -red square-.

Figure 5. PSD of illite samples grouped by TOC with varying illite content. Cumulative: filled shapes. PSD: unfilled shapes. The plots run through33 nm for QSDFT on carbon with cylindrical pores, but cumulative pore capacities are reported only through 27 nm in the tables. Low clay -bluecircle-, medium clay -green triangle-, high clay -red square-.

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span of pores considered in this study should not be con-sidered pore capacities that represent total porosity. However,the material properties reported by Bourg could be responsiblefor the results found even at this limited 1−27 nm fine meso-pore scale. This could be an explanation for why 12/15 of porecapacities of the Baltic samples were greater than the high clayidealized sample set, whereas the low and medium clay ide-alized shales have less consistency demarcating the various claycontent levels of the Barnett and Eagle Ford samples, 12/14 ofwhich are less than 1/3 clay fraction.The idealized shale samples show an increasing pore

capacity with increasing TOC for outgassing at both 60 and250 °C. The increase in pore capacity from 60 to 250 °C isexpected, with the validation shale having similar trends.4 The250 °C outgassing temperature results in a noticeable improve-ment in the positive correlation between pore capacity andTOC. While the idealized samples have excellent R2 corre-lations at both 60 and 250 °C (0.96−0.98), the validationshales at 60 °C are not well correlated with an R2 of 0.58,

perhaps signaling incomplete outgassing. After outgassing at250 °C, the R2 increases to 0.83. The idealized samples behavesimilar to the validation samples in that the pore capacityincreases with increasing outgassing temperature, as shown inFigure 7 (Table 8).

3.5. Effects of Temperature on Pore Capacity andStrength on Each of Four Components. There is a con-cern when working with natural samples that the high outgastemperatures may be fundamentally altering the shale andprevious experiments in the literature have used various out-gassing temperatures. As such, desorption of adsorbed species,such as water, methane, and so on, is required for LPGA,leading to the need for the highest possible temperature thatminimizes the structure alteration of the shale.4 Looking atthe key components associated with pore capacity in shale(kerogen and clay) at increasing outgassing temperatures canreveal indications of how structural changes may take place inshale (Figure 8).Outgassing kerogen at 250 °C could make the kerogen

transition from a glassy structure to a rubbery structure, whichmakes the kerogen relax. This could be the result of kerogencracking or possibly relaxation of the kerogen which could fillthe surrounding pore walls and spaces. When this happens,previously isolated gases that may have been trapped areallowed to escape from the kerogen. Additionally, the strengthof the kerogen is reduced as a result of the “rubberized” struc-ture.27 As another note, there may be a correlation between thematurity of the kerogen and changes in Young’s modulus.28

Because the idealized shales were made with kerogen that hasnot yet been characterized, there is little analysis to be done onhow this may have had an impact. Interesting progress is beingmade, especially in the realm of modeling kerogen that willhelp clarify the issues associated with kerogen and its role inshale porosity and strength.29 We can, therefore, infer fromthese results that outgassing temperatures ≥250 °C can impactthe porosity and strength of kerogen. The kerogen on the leftof Figure 9 has the most pore capacity found in the paper afteroutgassing at 250 °C, which is 0.070 cm3/g greater than thesample with the lowest pore capacity (right).In contrast to the trend with the kerogen, the illite used in

the idealized shale did not show significant changes in pore

Figure 6. Idealized series shows increasing pore capacity at27 nm with increasing clay content and increasing TOC. Low clayin blue on Ar, medium clay in forest green on N2, and high clay inmagenta on N2.

Table 7. Pore Capacities in (cm3/g) Are Calculated from the Summation of the Weighted Percentage of Each Component, orWeighted Superposition of the Partsa

estimated pore capacity asweighted sum of individual

components

quartz calcite pyrite illitemeankerogen

minkerogen

maxkerogen mean min max

pore capacity as idealizedmixture

low 1 0.004 0.001 0.000 0.004 0.004 0.004 0.005 0.014 0.013 0.014 0.013low 2 0.005 0.001 0.000 0.003 0.009 0.007 0.010 0.018 0.016 0.019 0.019low 3 0.005 0.001 0.000 0.004 0.013 0.011 0.015 0.023 0.021 0.025 0.015low 4 0.004 0.001 0.000 0.004 0.018 0.014 0.020 0.027 0.023 0.029 0.026medium 1 0.004 0.001 0.000 0.008 0.004 0.004 0.005 0.017 0.016 0.018 0.019medium 2 0.004 0.000 0.000 0.010 0.009 0.007 0.010 0.024 0.022 0.025 0.023medium 3 0.003 0.000 0.000 0.013 0.013 0.011 0.015 0.030 0.027 0.031 0.021medium 4 0.004 0.001 0.000 0.009 0.018 0.014 0.020 0.031 0.028 0.033 0.031high 1 0.002 0.000 0.000 0.017 0.004 0.004 0.005 0.024 0.024 0.025 0.026high 2 0.002 0.000 0.000 0.017 0.009 0.007 0.010 0.029 0.027 0.030 0.026high 3 0.002 0.000 0.000 0.014 0.013 0.011 0.015 0.031 0.028 0.032 0.030high 4 0.002 0.000 0.000 0.017 0.018 0.014 0.020 0.037 0.033 0.039 0.034

aThe individual component pore capacities are used and then added to the reported using the mean, min, and max of the kerogen samples andcompared to the pore capacities measured as idealized mixtures.

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capacity with increasing outgassing temperatures. In Figure 10,samples of illite were outgassed at increasing temperature.In order to help understand the possible impacts of out-

gassing the samples and raw components at 250 °C, we havetested the two components with the greatest pore capacity(kerogen and illite) in shales using TGA, as described inSection 2.5 with results shown in Figures 11 and 12. The250 °C outgassing temperature is an attempt to maximize removalof molecules dwelling in pores at the nanopore scale, whileminimizing structural changes.Results in Figure 13 for illite indicate a stable region up to

400 °C. An initial decrease in mass starts around 400 °C andends around 800 °C. The total mass loss in illite is only 3% and

could be attributed to water dehydration. These results arein alignment with Figure 10, where no change in illite porecapacity is observed at varying outgassing temperature rangingfrom 60 to 250 °C.Kerogen results illustrated in Figure 11, however, indicate a

total mass loss of approximately 17% within the same temperaturerange. The initial decrease in mass starts at around 200 °C with aslight decrease from 200 to 250 °C. The sharp decrease intemperature after 250 °C indicates possible kerogen cracking andevolution of hydrocarbons. The TGA results are consistent withthe LPGA analysis, indicating that temperatures up to 250 °Cminimize structural alteration for the outgassing temperatures,and reinforce 250 °C as the high temperature limit for outgassing.

Figure 7. PSD on varying TOC and illite idealized samples. Cumulative: filled shapes. PSD: unfilled shapes. The plots run through 33 nm forQSDFT on carbon with cylindrical pores, but cumulative pore capacities are reported only through 27 nm in the tables. TOC 1.5% -blue circle-,TOC 3.0% -green triangle-, TOC 4.5% -pink square-, TOC 6.0% -red diamond-.

Table 8. Idealized and Validation Pore Capacities with Increasing Outgassing Temperatures Using Argon as Adsorbate

TOC (%) CMC (%) 60 °C (cm3/g) 1.0−27 nm 110 °C (cm3/g) 1.0−27 nm 200 °C (cm3/g) 1.0−27 nm 250 °C (cm3/g) 1.0−27 nm

Barnett 1 1.1 5.9 0.008 0.016Low 1 1.5 13.9 0.013 0.013 0.014 0.015Eagle Ford 7 2.3 14.9 0.016 0.015 0.017 0.018Low 2 3.0 10.3 0.014 0.016 0.017 0.019Eagle Ford 4 4.0 19.3 0.017 0.018 0.024 0.020Eagle Ford 8 4.4 5.8 0.024 0.025 0.026 0.026Low 3 4.5 13.5 0.017 0.018 0.020 0.022Barnett 2 4.9 30.4 0.004 0.010Barnett 3 5.3 32.8 0.005 0.010Eagle Ford 6 5.3 17.1 0.020 0.022 0.024 0.023Low 4 6.0 14.7 0.021 0.021 0.024 0.026Barnett 6 6.3 28.1 0.016 0.023

Figure 8. Idealized series compared to validation samples outgassed at 250 °C. The low clay series is Ar (left), while the medium (left) and high(right) are N2 adsorbates.

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4. CONCLUSIONSIdealized powdered shale sorption isotherms were created bymechanically mixing incremental amounts of four essentialcomponents by weight. The first two components, organiccarbon and clay, have been shown to be the most importantcomponents for increased storage capacity in shales. Organic

carbon was represented by kerogen isolated from a Silurianshale, and clay groups were represented by illite from theGreen River shale formation. The rest of the idealized shalewas composed of equal parts by weight of SiO2 to representquartz and CaCO3 to represent carbonate components. EagleFord, Baltic, and Barnett shale sorption measurements wereused to validate the idealized samples. Shale sorption isothermswere measured using low pressure N2 (77 K) and Ar (87 K)adsorbates on a Quantachrome Autosorb IQ2. The idealizedsample results indicated a positive linear correlation withincreasing TOC and pore capacity. Samples from the Barnettand Eagle Ford formation showed reducing pore capacity withlower clay content.These results suggest strongly that idealized shales provide

insights into and reasonable estimates of natural shale char-acteristics for the pore-size range of 1−27 nm. Clearly, this isnot the only relevant pore scale for shales. Understanding thecharacteristics of additional pore sizes is of great importance;this study examines one part of the complexity of shales.A library of characterized idealized shales with varying com-positions and maturity can be examined to help benchmarkexisting shale formations. The experiments here reveal that it iscritical to find a type of organic matter that has reliable porecapacity properties and that can be produced and obtained byany interested party. When the basic composition of the shalelayer is known from wellbore logs, a synthetic core can becreated. This could potentially address the common problem ofobtaining similar rock cores for routine and special core analysis

Figure 9. Eagle Ford and Barnett validation vs small clay contentidealized pore capacity measurements. Four Eagle Ford and FourBarnett samples were outgassed at 60 °C. All validation samples wereoutgassed and plotted at 250 °C. Table 3 lists adsorbates.

Figure 10. Two kerogen samples showing the changes in pore capacity that results from increasing outgassing temperature. The plots illustrate thevariability in kerogen pore capacities, as well as the variations in responses to outgassing.

Figure 11. TGA for illite up to 1000 °C. Results indicate a total of 3% weight loss occurring after ∼400 °C.

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due to wellbore instability issues that limit sampling.30 Furtherreview of the underlying material properties of the idealizedshales should be undertaken. This includes further character-ization of the kerogen used to create the samples as well as thestructural changes caused by the different outgassing procedure.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: kovscek@stanford.edu.ORCIDR. Holmes: 0000-0002-5047-5528V. Vishal: 0000-0001-6342-9603J. Wilcox: 0000-0001-8241-727XA. R. Kovscek: 0000-0003-1336-384XAuthor Contributions⊥R.H. and H.A. contributed equally.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank Dr. Adam D. Jew and Dr. Erik Rupp for their helpand training on the X-ray diffraction and Quantachrome

Instrument, respectively. We thank Dr. Doug McCarty andChevron for contributing the Silurian kerogen samples, as wellas Prof. Mark Zoback and ConocoPhillips, for the natural shalesample contributions. H.A. acknowledges a graduate fellowshipfrom Saudi Aramco. Additional support was provided by theStanford Center for Carbon Storage.

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Figure 13. Average cumulative pore capacity for illite at varyingoutgassing temperatures. This uses silica as the DFT adsorbent, whichQuantachrome allows to extend through 27 nm. The range of claypore capacities at various temperatures overlaps in the plotted ±2σ bar.

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