Methane Adsorption Capacities of the Lower Paleozoic MarineShales in the Yangtze Platform, South ChinaYue Wu,*,† Tailiang Fan,† Shu Jiang,‡ Xiaoqun Yang,† Huaiyu Ding,§ Miaomiao Meng,† and Duan Wei†

†School of Energy Resources, China University of Geosciences, Beijing 100083, People’s Republic of China‡Energy and Geoscience Institute, University of Utah, Salt Lake City, Utah 84108, United States§Liaohe Oilfield Company, PetroChina, Panjin, Liaoning 124010, People’s Republic of China

ABSTRACT: The adsorption capacities of the Lower Silurian Longamxi and Lower Cambrian Niutitang marine shales in theYangtze Platform in China were investigated through methane adsorption experiments. The correlations between the adsorptioncapacities and major factors, e.g., total organic carbon (TOC) contents, thermal maturity, mineral composition, moisture content,pressure, and temperature, were discussed. The isosteric adsorption heat was calculated according to the temperaturedependency of the methane adsorption isotherms. The results show that, under the temperature of 30 °C and pressure range of0−12 MPa, the maximum adsorption capacity of the Longmaxi shales ranges between 0.47 and 3.08 m3/ton of rock and that ofthe Niutitang shales ranges between 1.59 and 7.43 m3/ton of rock. The Langmuir adsorption capacity varies from 0.54 to 3.84m3/ton of rock for the Longmaxi shales and from 1.98 to 9.73 m3/ton of rock for the Niutitang shales. The TOC content showsa significantly positive correlation with the adsorption capacity, indicating that organic matter is responsible for adsorbing gas inthe shales. For these high mature shales, the thermal maturity shows no effect on the adsorption capacity. The clay minerals showlittle contributions to the adsorption capacity in the shales because of the effect of the water content. For the studied shales, themoisture exhibits no distinct correlation with the adsorption capacity. The influence of the pressure on the adsorption capacityvaries from sample to sample, while the temperature shows a generally negative effect on the adsorption capacity. The isostericheat of adsorption ranges from 8.48 to 27.35 kJ/mol, with an average of 17.59 kJ/mol, indicating a dominant physical adsorptionbehavior of the methane molecule in the shales.

1. INTRODUCTION

It is widely known that natural gas can be stored in shalereservoirs as free gas, adsorbed gas, and dissolved gas.1−6 It wasreported that the adsorbed gas can account for 20−85% in totalgas amount in some shale gas plays.3 The methane adsorptioncapacity of shales is a complex function of geochemistry,mineral composition, pore structure, and reservoir condi-tions.1,2,5−8 Organic matter is generally thought to be theprinciple contributor to the adsorption capacity of shales.5−9

Type III organic matter has higher gas adsorption capacity thanthat of type I and type II organic matter because of the highercontent of aromatic compounds in type III organic matter.9 Asmaturity increases, more micropores (diameter, D < 2 nm) maybe developed in the organic matter during the process ofkerogen conversion and hydrocarbon generation and expulsion.Thus, the overmature and high total organic carbon (TOC)shale samples generally show larger adsorption capacity thanthe low mature and low TOC samples. Clay minerals with aporous structure also have a strong impact on gas adsorptioncapacities in shales.5−8 The adsorption capacities of montmor-illonite and illite/semectite are obviously higher than that ofkaolinite, chlorite, and illite in the dry state.10 However, theadsorption capacities of clay minerals for the methane moleculewould be reduced greatly in the presence of moisture. Theinfluences of the water content on the adsorption capacity inshales were investigated by comparing the adsorption capacitiesbetween moisture-equilibrated samples and dry samples. A 40%decrease of the adsorption capacity was found in the moisture-equilibrated samples.7,8 Reservoir conditions, e.g., temperature

and pressure, have also been recognized as important factors toinfluence adsorption capacities of shales.The Lower Silurian Longmaxi and Lower Cambrian

Niutitang shales with large thickness, high TOC content, andhigh brittle mineral content are regarded as the most potentialshale gas plays in the Yangtze Platform, south China.11−18 Animproved understanding of the adsorption characteristics ofthese two shale intervals is important and meaningful. The aimsof this study are to access the adsorption capacities of theLower Silurian Longmaxi and Lower Cambrian Niutitang shalesin the Yangtze Platform and discuss the key factors influencingthe adsorption capacities. Here, some representative shalesamples in the Longmaxi and Niutitang Formations from wellsor outcrops in the Yangtze Platform were collected for thisstudy. Shale properties, e.g., TOC, vitrinite reflectance (Ro),mineral composition, porosity, and methane adsorptioncapacities, were measured on the basis of a series ofexperimental procedures.

2. MATERIALS AND METHODS2.1. Samples. A total of 14 shale samples with different TOC

contents and mineral compositions were collected. Specifically, threeLongmaxi shale samples coded CUGB1−CUGB3 are from Doucan1well in Anhui Province in the Lower Yangtze Platform (note that theLongmaxi Formation is called Gaojiabian Formation in the Lower

Received: February 5, 2015Revised: June 3, 2015

Article

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© XXXX American Chemical Society A DOI: 10.1021/acs.energyfuels.5b00286Energy Fuels XXXX, XXX, XXX−XXX

Yangtze Platform); another five Longmaxi shale samples codedCUGB4−CUGB8 are from Xiye1 well in Guizhou Province in theUpper Yangtze Platform; and six Niutitang shale samples codedCUGB9−CUGB14 are from Fenghuang and Yongshun outcrops inHunan Province in the Upper Yangtze Platform (Figure 1 and Table1).

2.2. Methods. 2.2.1. TOC and Ro. The TOC contents of 14samples were measured through a LECO CS230 carbon/sulfuranalyzer. Samples were first crushed to powder with a particle lessthan 100 mesh, and then 1−2 g samples were pyrolyzed up to 540 °C.The thermal maturity of samples was determined on the basis of thereflectance measurements on pyrobitumen particles. The pyrobitumenreflectance (Rb) was measured through a MVP-3 microscope in non-polarized light at a wavelength of 546 nm in oil immersion. On eachsample, 20 measurements were taken whenever possible. Because ofthe lack of vitrinite in these early Paleozoic marine shales, Ro wascalculated from the measured Rb on the basis of the followingarithmetic formula:19

= +R R0.3195 0.6790o b (1)

2.2.2. X-ray Diffraction (XRD). Bulk mineralogical composition ofshales was derived from the XRD patterns. Eight Longmaxi samples(CUGB1−CUGB8) were first ground into powder, and then XRDanalysis was performed on the randomly oriented powder through aRigaku D/max-2600 diffractometer with Cu Kα radiation, automaticdivergent and anti-scatter silts, and a secondary graphite mono-chromator with a scintillation counter. The generator settings were 40kV and 40 mA. The diffraction data were recorded from 2° to 76° 2θwith a step width of 0.02° and a counting time of 4 s per step. Themineral content was semi-quantitatively determined on the basis of theintensity of specific reflections, the density, and the mass adsorptioncoefficient (Cu Kα) of the identified mineral phases.

2.2.3. Porosity. The porosity for eight Longmaxi samples(CUGB1−CUGB8) was measured by mercury injection porosimetry.Samples were dried in an oven for 24 h at 50 °C. The measurementswere performed using a AutoPore IV 9520 series mercury porosimeter.The mercury pressure was increased continuously from 0.013 to 200MPa.

2.2.4. Methane Adsorption Experiments. Methane adsorptionmeasurements were performed on shale powders with a high-pressuregas adsorption and desorption instrument of PCT Pro E&E Sivertsmodel. Two sets of methane adsorption experiments were designed fordifferent objectives in this study. The first set is that the methaneadsorption experiments were conducted on three dry powered samples(CUGB1−CUGB3) under high pressures of up to 30 MPa anddifferent temperatures of 50 and 70 °C. The second set is that theadsorption experiments were measured on 11 moisture-equilibratedsamples (CUGB4−CUGB14) at a consistent temperature of 30 °Cand up to a pressure of 12 MPa. Moisture equilibration of samplesfollowed the ASTM procedure (ASTM D1412-04). Ground sampleswere placed in a sub-atmospheric desiccator over a saturated saltsolution of KCl with controlled relative humidity of 80% at 30 °C formore than 72 h. Equilibrium moisture occurs at the point when thesample weight remains constant. Moisture content was measured byoven-drying, weight-loss calculations.

In the adsorption experiment, the amount of adsorbed gas iscalculated on the basis of the following mass balance:20

ρ= −m m Vadsorbed total gas void (2)

where madsorbed is the adsorbed gas content, mtotal is the total amount ofgas introduced into the system, the void volume (Vvoid) is determinedby helium expansion at the measured temperature prior to theadsorption measurement, and the gas density (ρgas) in thecorresponding pressure and temperature conditions is calculatedfrom the equation of state by Setzman and Wagner.21 The measuredresults are presented in volume unit normalized to the rock mass (CH4m3/ton of rock) or the TOC mass (CH4 m3/ton of TOC) understandard temperature (273.15 K) and standard pressure (105 Pa).

2.2.5. Parameterization of Adsorption Data. The measuredadsorption data can be parametrized using the Langmuir model,which is commonly applied to describe the relations between theadsorbed gas on a solid surface and measured pressure at a fixedtemperature22

=+

=+

V VP

P PV V

KPKP

or1L

LL

(3)

where V is the volume of adsorbed gas, VL is the Langmuir volume (onthe basis of the monolayer adsorption), which is the maximumadsorption capacity of the absorbent, P is the gas pressure, PL is theLangmuir pressure, at which the adsorbed gas content (V) is equal tohalf of the Langmuir volume (VL), and K is the Langmuir constant,which is the reciprocal of the Langmuir pressure (PL).

3. RESULTS3.1. Source Rock Characterization. The results of TOC

content and thermal maturity (on the basis of calculated Ro) for14 samples are listed in Table 2. On the basis of the measured

Figure 1. Locations of the sampled wells and outcrops in the YangtzePlatform, south China (modified with permission from ref 18): (1)Doucan1 well in Anhui Province, (2) Xiye1 well in Guizhou Province,(3) Fenghuang outcrop in Hunan Province, and (4) Yongshunoutcrop in Hunan Province.

Table 1. Provenance of the Studied Shale Samples and TheirLithostratigraphic Origina

sample ID well/outcrop formation depth (m)

CUGB1 Doucan1 LMX 123CUGB2 Doucan1 LMX 105CUGB3 Doucan1 LMX 95CUGB4 Xiye1 LMX 647.6CUGB5 Xiye1 LMX 636.5CUGB6 Xiye1 LMX 627CUGB7 Xiye1 LMX 569.3CUGB8 Xiye1 LMX 566.4CUGB9 Fenghuang NTT 3CUGB10 Fenghuang NTT 56.9CUGB11 Fenghuang NTT 132.1CUGB12 Yongshun NTT 12CUGB13 Yongshun NTT 100CUGB14 Yongshun NTT 106

aLMX = Longmaxi Formation, Lower Silurian. NTT = NiutitangFormation, Lower Cambrian. The locations of wells and outcrops areshown in Figure 1. The depth in outcrop is the distance to the ground.

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values, the TOC content of the Longmaxi shales rangesbetween 0.52 and 6.05% and that of the Niutitang shales rangesbetween 1.45 and 8.93%. Most of the samples are organic-rich(TOC > 2%), except for CUGB6, CUGB7, and CUGB8 withTOC contents of lower than 1%. The Niutitang samplesgenerally contain more organic matter than the Longmaxisamples. Both of the Longmaxi shales and Niutitang shales areovermature, with the calculated Ro values higher than 2% onaverage. Additionally, many previous publications reported asapropelic (type I) and humic−sapropelic (type II1) kerogenfor the Lower Paleozoic marine shales in the Upper YangtzePlatform.23−25

3.2. Mineralogical Composition and Porosity. TheXRD and porosity results for eight Longmaxi samples(CUGB1−CUGB8) are listed in Table 2. On the basis of theXRD data, quartz and clays are the major mineralogicalcomposition for the Longmaxi shales and little carbonate ispresent. The quartz contents of the samples from Doucan1 wellare more than 50% on average, and clay mineral contents aremore than 40% on average. Both of the quartz and clay mineralcontents of the samples from Xiye1 well are above 30% onaverage. The samples from Xiye1 well contain more carbonateminerals than the samples from Doucan1 well, which may becaused by their different depositional settings.

Most of the samples present a low porosity ranging between1 and 2%. Two organic-rich samples of CUGB1 and CUGB4exhibit a large porosity of more than 4%.

3.3. Methane Adsorption Isotherms. The experimentallymeasured and Langmuir-fitting methane adsorption capacitiesfor samples of CUGB4−CUGB14 are presented in Table 3 andFigure 2. Within the measured pressure range, the maximummethane adsorption capacity ranges between 0.47 and 3.08 m3/ton of rock for the Longmaxi shales and between 1.59 and 7.43m3/ton of rock for the Niutitang shales. The Langmuir volumesrange from 0.54 to 3.84 m3/ton of rock for the Longmaxi shalesand from 1.98 to 9.73 m3/ton of rock for the Niutitang shales.The Langmuir pressures are between 1.27 and 2.55 MPa for theLongmaxi shales and between 2.15 and 3.23 MPa for theNiutitang shales. The Niutitang shales show a generally largerLangmuir volume and Langmuir pressure than the Longmaxishales.A comparison of the adsorption capacity between the Lower

Paleozoic shales in the Yangtze Platform in China and some hotshales in North America was made (Table 4). Because theexperimental conditions and measurement methods for theadsorption capacities are different for all of those samples, thecomparison in this paper is simple and preliminary. Incomparison to those gas-producing shales in North America,the Lower Paleozoic marine shales in the Yangtze Platform

Table 2. Results of TOC, Calculated Ro, Porosity, and XRD Analysis

major minerals (wt %)

sample ID TOC (%) Ro (%) porosity (%) quartz clay carbonate

CUGB1 3.5 2.16 4.19 78.1 17.5 3.8CUGB2 3.17 1.52 1.47 53.3 41 1.9CUGB3 1.95 1.43 1.39 50.2 46.9 1.9CUGB4 6.05 3.12 4.40 63.5 18.8 11.8CUGB5 4.08 3.22 1.80 45 30.8 12.9CUGB6 0.95 3.23 0.60 38.7 32.4 17.3CUGB7 0.64 2.96 1.20 33.2 37.4 19.2CUGB8 0.52 2.94 0.90 38.9 29.9 18.5CUGB9 8.93 3.32 NAa NA NA NACUGB10 2.21 3.28 NA NA NA NACUGB11 1.68 3.41 NA NA NA NACUGB12 7.1 3.47 NA NA NA NACUGB13 2.91 3.43 NA NA NA NACUGB14 1.45 3.41 NA NA NA NA

aNA = not available.

Table 3. Maxima in the Adsorption Isotherms (30 °C) and Langmuir-Fitting Parameters for Samples of CUGB4−CUGB14a

Vmax VL

sample ID TOC (%) moisture (%) (m3/ton of rock) (m3/ton of TOC) Pmax (MPa) (m3/ton of rock) (m3/ton of TOC) PL (MPa)

CUGB4 6.05 1.18 3.08 50.91 10.86 3.84 63.47 2.55CUGB5 4.08 1.65 2.45 60.05 10.67 3.02 74.02 2.49CUGB6 0.95 1.68 1.01 106.32 10.65 1.01 106.32 1.27CUGB7 0.64 1.66 0.47 73.44 8.67 0.54 84.38 1.36CUGB8 0.52 1.36 0.5 96.15 8.67 0.6 115.38 2.3CUGB9 8.93 1.75 7.43 83.20 11 9.73 108.96 3.23CUGB10 2.21 2.14 2.36 106.79 11.01 2.93 132.58 2.67CUGB11 1.68 1.93 1.67 99.40 11.17 1.98 117.86 2.15CUGB12 7.1 1.67 6.37 89.72 11.63 8.19 115.35 3.02CUGB13 2.91 1.98 2.88 98.97 11.21 3.61 124.05 2.59CUGB14 1.45 1.97 1.59 109.66 11.13 1.98 136.55 2.29

aVmax means the maximum adsorption capacity within the measured pressure range, and Pmax is the corresponding pressure. VL is the Langmuirvolume, and PL is the Langmuir pressure.

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show a larger adsorption capacity, indicating a great shale gaspotential.

4. DISCUSSION4.1. Effect of Organic Matter Abundance. It is widely

acknowledged that organic matter plays an important role inthe adsorption capacity of shales. The relationship betweenTOC contents and adsorption capacities in shales has beenstudied by many scholars.1,2,5−9,26 For example, Ross andBustin reported that there were positive correlations betweenthe TOC contents and adsorption capacities in shale samplesfrom the Western Canadian Sedimentary Basin.7,8 Theadsorption capacities increased linearly with TOC contents inthe Devonian shale samples from basins in northeastern

America.23 Tan et al. and Wang et al. reported a linearcorrelation between the adsorption capacity and TOC contentin the Lower Paleozoic shales in the Upper Yangtze Platform inChina.17,18 The correlations between the TOC contents andadsorption capacities for the studied samples are illustrated inFigure 3. The Langmuir volumes correlate positively with theTOC contents for both the Longmaxi and Niutitang shales,indicating that organic matter made a significant contributionto the adsorption capacity in these samples. Organic matter isusually porous in high-mature shales, with enough surface areaonto which natural gas can adsorb.7,8 Organic matter providesmost of the porosity in the Longmaxi samples indicated by thelinear correlation between the TOC content and porosity(Figure 4A). Intrapores were largely developed within theorganic matter revealed by scanning electron microscopy(SEM) (Figure 4B).

4.2. Effect of Thermal Maturity. Thermal maturity hasbeen reported to affect the adsorption capacity because oftexture changes in organic matter.5−9 Many micropores arecreated during thermal decomposition of organic matter, whichcan enhance the gas adsorption capacity of shales. Ross andBustin demonstrated that the adsorption capacity increasedwith thermal maturity in shale samples from the WesternCanadian Sedimentary Basin.7,8 Gasparik et al. reported thatadsorption capacity of overmature shales was generally higherthan that of low mature or immature shales.1,2 Tan et al. foundthat the Langmuir adsorption capacity generally increased fromimmature to overmature samples.18 The relationship betweenthe thermal maturity and adsorption capacity for the studiedsamples is shown in Figure 5. There is no correlation betweenthe TOC-normalized Langmuir volume and thermal maturity.The Niutitang shales with Ro values of 3.2−3.5% show agenerally larger adsorption capacity than the Longamxi shaleswith Ro values of 2.9−3.2%. For these samples within theovermature range, there is limited potential for porous structurecreation during the thermal maturation of organic matter.

4.3. Effect of Mineral Composition. In the present study,the correlations between the adsorption capacity and mineralcomposition were discussed on the Longmaxi samples, and theNiutitang samples may exhibit a similar relationship. Themineral composition of the Longmaxi shales in the YangtzePlatform is dominated by quartz and clay minerals. Quartz maybe irrelevant to the adsorption capacity because of its non-adsorptive nature. Clay minerals with high internal surface areaand adsorption energy are regarded as a significant factor toaffect the adsorption capacity of shales.5−8 The clay mineral

Figure 2. Methane adsorption isotherms measured at 30 °C forsamples of CUGB4−CUGB14. Plot A is for the Longmaxi shales, andplot B is for the Niutitang shales. Points are experimentally measureddata, and lines are Langmuir-fitting results.

Table 4. Experimental Conditions and Adsorption Capacities for the Compared Shale Samplesa

experimental condition

sample temperature (°C) pressure (MPa) moisture (%) adsorption capacity (m3/ton) reference

Lower Silurian (UYP) 30 0−12 1.18−1.68 0.47−3.08 this paperLower Cambrian (UYP) 1.67−2.14 1.59−7.43Lower Silurian (UYP) 46 0−25 0.43−0.62 1.02−1.45 Tan et al.18

Lower Cambrian (UYP) 0.58−2.28 0.82−4.77Lower Silurian (UYP) 60 0−11 0 0.94−2.82 Wang et al.17

Lower Cambrian (UYP) 1.19−1.46D−M (western Canada) 30 0−6 1.6−5.2 0.1−1.6 Ross and Bustin8

Jurassic (western Canada) 0.6−8.5 0.1−2.0Lower Cretaceous (western Canada) 30 0−6 1.5−11 0.04−1.89 Chalmers and Bustin6

Barnett (U.S.) 65 0−25 0 0.16−3.47 Gasparik et al.2

aUYP = Upper Yangtze Platform. D−M = Devonian−Mississippian. Unit conversion factor: 1 mmol/g = 22.71 m3/ton.

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content correlates negatively with the rock-normalizedLangmuir volume in this study (Figure 6A), which may beascribed to the strong effect of the TOC content; e.g., there aresubstantially low TOC contents in the clay-rich samples. Thisphenomenon indicates that organic matter has a moresignificant impact on the adsorption capacity than clay mineralsin the studied samples.To more clearly present the contribution of clay minerals to

the adsorption capacity, the adsorption data were normalized tothe TOC content. The clay mineral content shows a weaklypositive correlation with the TOC-normalized Langmuirvolume (Figure 6B), indicating that the clay minerals havelittle impact on the adsorption capacity in the Longmaxi shales.This phenomenon may be caused by the effect of the watercontent. The surface of clay minerals has a high affinity ofwater, which would block the access of methane molecules tothe adsorption sites. The contribution of clay minerals to theadsorption capacity in shales may be reduced greatly in thepresence of water.

4.4. Effect of Moisture Content. The effect of themoisture content on the adsorption capacity was investigated inmany shale samples from around the world.1,2,5−8,27,28 Forexample, the adsorption capacity was negatively correlated withthe moisture content in shale samples from the WesternCanadian Sedimentary Basin.7,8 The presence of water mayswell the clay minerals, block the pore system, and occupy

Figure 3. Correlation plots between the Langmuir adsorption capacityand TOC content. Plot A contains all measured samples; plot B is forthe Longmaxi shales; and plot C is for the Niutitang shales.

Figure 4. (A) Linear correlation between the TOC content andporosity in the Longmaxi shales and (B) SEM image showing poreslargely developed within the organic matter.

Figure 5. Effect of thermal maturity on the adsorption capacity. Theadsorption capacity is represented by the TOC-normalized Langmuirvolume. The Niutitang shales show a generally larger adsorptioncapacity than the Longmaxi shales.

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potential adsorption sites. However, shale samples fromnortheastern British Columbia, Canada, showed a positivecorrelation between the adsorption capacity and moisturecontent, indicating that water and methane molecules mayoccupy different adsorption sites in shales.5,6 The LowerPaleozoic shale samples from the Upper Yangtze Platform inChina showed that the TOC-normalized adsorption capacitydecreased following polynomial-law relations with the moisturecontent increasing.18 In this study, the moisture content showsno correlation with the rock-normalized adsorption capacity(Figure 7A). This phenomenon may be caused by the strongereffects from other factors, e.g., TOC content. The samples withhigher TOC contents have remarkably larger adsorptioncapacities (Figure 7A). However, the moisture contents alsohave no obvious correlation with the TOC-normalizedadsorption capacity (Figure 7B). Higher moisture contents ofthe samples than their critical moisture contents may accountfor this phenomenon, which could be indicated from the weakeffect of clay minerals on the adsorption capacity and themoisture-equilibration condition with a relative humidity of80% for the samples. Gasparik et al. reported that the criticalmoisture content could be achieved at the relative humidity ofless than 75% for the Upper Cambrian−Lower OrdovicianAlum shale.2

4.5. Effect of Pressure and Temperature. To explore theeffects of the pressure and temperature on the adsorptioncapacity, the methane adsorption experiments were performedon three Longmaxi samples (CUGB1−CUGB3) at the

temperatures of 50 and 70 °C and under a wide range ofpressures of up to 30 MPa. The experimental results are shownin Table 5 and Figure 8. The effects of the pressure on theadsorption capacity are quite complex. Figure 8A shows thatthe adsorbed gas content increases consistently with theincrease of the pressure. The adsorption isotherms in Figure 2also show a monotonous increase of the adsorption capacitywith a pressure increase, but the pressures applied in thosemeasurements are relatively low (<12 MPa). Plots B and C ofFigure 8 show that the adsorbed gas contents increase first andthen decrease over the measured pressure range.The effects of the temperature on adsorption capacity have

been summarized by Yee et al.29 A negative correlation betweenthe temperature and adsorption capacity was observed in thestudied samples. The adsorption capacities measured at 50 °Care generally larger than those measured at 70 °C (Figure 8).The methane adsorption process is exothermic, and highertemperatures are favorable for more gas in the free state than inthe adsorbed state. In addition, we can find that the impact ofthe temperature on the adsorption capacity was reduced by theTOC content. In comparison to the samples of CUGB2 andCUGB3, the sample of CUGB1 with a higher TOC contentshows less difference between the adsorption capacitiesmeasured at 50 and 70 °C (Figure 8).

4.6. Isosteric Heat of Adsorption. Isosteric heat ofadsorption is an important thermodynamic parameter and canbe used to characterize the methane adsorption behavior inshales. The concept of isosteric heat of adsorption has beendiscussed by many authors before.30−34 The isostericadsorption heat can be determined on the basis of adsorptionisotherms and the following equation:34

= −Δ

+PH

RTCln L

s(4)

where PL is the Langmuir pressure, ΔHs is the isostericadsorption heat, R is the universal gas constant, equal to 8.314 J

Figure 6. Correlation plots of clay mineral content with the (A) rock-normalized Langmuir adsorption capacity and (B) TOC-normalizedLangmuir adsorption capacity for the Longmaxi shales.

Figure 7. Correlation plots between the moisture content and the (A)rock-normalized Langmuir adsorption capacity and (B) TOC-normalized Langmuir adsorption capacity for the Longmaxi shales.No obvious relationship can be observed.

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mol−1 K−1 in this paper, T is the measured temperature, C is aconstant, and the negative symbol shows that the adsorptionprocess is exothermic.In the present study, the isosteric heats for samples of

CUGB1−CUGB3 are summarized in Table 6. The isostericadsorption heat varies from 8.48 to 27.35 kJ/mol, with anaverage of 17.59 kJ/mol. This indicates that the dominantmethane adsorption behavior in the shales is physicaladsorption because chemical adsorption usually shows anisosteric heat of 40−600 kJ/mol.35,36

5. CONCLUSIONIn this paper, methane adsorption experiments were conductedon some representative shale samples from the Lower Silurianand Lower Cambrian Formations in the Yangtze Platform inChina. Some conclusions from this study are summarized asfollows: (1) Under the measured temperature of 30 °C and thepressure range of 0−12 MPa, the maximum adsorption capacityof the Longmaxi shales ranges between 0.47 and 3.08 m3/ton ofrock and that of the Niutitang shales ranges between 1.59 and7.43 m3/ton of rock. The Langmuir adsorption capacity variesfrom 0.54 to 3.84 m3/ton of rock for the Longmaxi shales andfrom 1.98 to 9.73 m3/ton of rock for the Niutitang shales. TheLongmuir pressures are between 1.27 and 2.55 MPa for theLongmaxi shales and between 2.15 and 3.23 MPa for theNiutitang shales. In comparison to those hot shales in NorthAmerica, the studied shales in the Yangtze Platform in Chinashow a larger adsorption capacity. (2) For the Longmaxi andNiutitang shales in the Yangtze Platform, TOC is the primary

control on the adsorption capacity. Thermal maturity, clayminerals, and moisture show slight or no effects on theadsorption capacity. Therefore, the TOC content can be usedas a proxy to determine the intervals with potentially largeadsorbed gas content in the Yangtze Platform. Specifically, (a)the TOC content shows a significantly positive correlation withthe adsorption capacity, indicating that organic matter isresponsible for adsorbing gas in the shales; (b) the thermalmaturity has no obvious effect on the adsorption capacity forthese high mature shales; (c) the contribution of clay mineralsto the adsorption capacity may be irrelevant because of theeffect of the water content; and (d) the moisture exhibits nocorrelation with the adsorption capacity in the shales. (3) Thepressure can help to increase the adsorption capacity to someextent, while the temperature may decrease the adsorptioncapacity. The effects of the pressure and temperature on theadsorption capacity in the shales can be used as guidance for

Table 5. Methane Adsorption Capacities for Samples ofCUGB1−CUGB3 at Different Temperatures

50 °C 70 °C

P (MPa) CH4 (m3/ton of rock) P (MPa) CH4 (m

3/ton of rock)

Sample of CUGB10.42 0.28 0.43 0.220.82 0.50 0.81 0.361.53 0.80 1.55 0.583.01 1.16 3.11 0.915.96 1.63 6.01 1.3310.54 1.98 10.26 1.7216.11 2.39 15.02 2.1021.32 2.82 20.35 2.70

Sample of CUGB20.42 0.22 0.42 0.180.81 0.37 0.80 0.291.50 0.59 1.52 0.432.94 0.82 3.03 0.645.80 1.04 5.85 0.8410.26 1.02 9.99 0.8816.17 0.99 14.67 0.9121.33 0.92 19.96 0.84

Sample of CUGB30.45 0.12 0.46 0.090.88 0.20 0.84 0.151.58 0.31 1.59 0.243.06 0.43 3.13 0.355.95 0.78 6.04 0.4610.57 0.77 10.29 0.4616.14 0.73 15.05 0.4421.34 0.72 20.43 0.4328.04 0.72 27.62 0.42

Figure 8. Methane adsorption isotherms at temperatures of 50 and 70°C for samples of CUGB1−CUGB3.

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gas desorption in the shale gas production stage. (4) Theadsorption behavior of methane molecules in the shales belongsto physical adsorption, with an average isosteric adsorption heatof 17.59 kJ/mol, ranging from 8.48 to 27.35 kJ/mol.The research results from this study can provide some useful

information to characterize the gas adsorption capacity of theLower Paleozoic marine shales in the Yangtze Platform inChina. However, some questions still exist, and further researchis necessary. For example, the experimental temperature andpressure for the adsorption measurements are much lower thanthose in reservoir conditions. The effects of the pressure andtemperature on the adsorption capacity of shales should betaken into account together. Some experimental uncertaintiesexist in the measured adsorption data, such as thereproducibility of measurements and experimental methods.More samples and more systematic measurements are needed.

■ AUTHOR INFORMATION

Corresponding Author*Telephone: 86-17888835218. E-mail: wuyue0906@gmail.com.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work is supported by the National Oil and Gas StrategicInvestigation Program (Grant 2009GYXQ-15), the NationalNatural Science Foundation Research (Grant 40672087), andthe Shale Gas Resource Investigation and Evaluation Program,Guizhou Province (Grant 2012GYYQ-01). The authors alsosincerely appreciate the support from the Energy & GeoscienceInstitute (EGI) of the University of Utah.

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Table 6. Calculated Isosteric Heat of Adsorption for Samples of CUGB1−CUGB3

50 °C 70 °C

sample ID VL (m3/ton) PL (MPa) VL (m

3/ton) PL (MPa) isosteric heat (kJ/mol)

CUGB1 3.41 1.20 3.29 2.32 27.35CUGB2 1.25 1.41 0.94 1.73 8.48CUGB3 0.84 5.30 0.47 7.97 16.94

Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.5b00286Energy Fuels XXXX, XXX, XXX−XXX

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