Article pubs.acs.org/EF
Adsorption Equilibrium of CO2 and CH4 and Their Mixture on Sichuan Basin Shale Shuo Duan, Min Gu,* Xidong Du, and Xuefu Xian State Key Laboratory of Coal Mine Disaster Dynamics and Control, College of Resources and Environmental Science, Chongqing University, Chongqing 400044, People’s Republic of China S Supporting Information *
ABSTRACT: Adsorption equilibrium isotherms of CO2, CH4, and mixtures of CO2/CH4 on shale sampled from Nanchuan, southeastern Sichuan Basin, were measured at 278, 298, and 318 K by an accurate gravimetric method. The adsorption equilibrium data of CO2 and CH4 were fitted using both the virial model and the Brunauer−Emmett−Teller (BET) model, and the isotherms of CO2/CH4 mixtures were fitted by an extended BET model. On the basis of adsorption data, the adsorption selectivity factors for CO2 over CH4 (αCO2/CH4) and thermodynamic parameters were estimated. Nanchuan shale was characterized with a high total organic carbon (TOC), having inorganic minerals and wide pore size distribution ranges. The adsorption heat, negative Gibbs free energy change, and negative surface potential of CO2 are larger than those of CH4, and the entropy loss of CO2 is larger than that of CH4, suggesting that adsorbed CO2 is in a more highly ordered arrangement than CH4 on shale. αCO2/CH4 values at different temperatures are all larger than 2.5. arrangement of CH4 molecules on shale.11−13 However, thermodynamic parameters of CO2 on shale receive less attention. Gas adsorption capacity of shale for gases relates not only to conditions of pressure and temperature but also to the pore structure and composition of shale. Many researchers have reported positive correlations of the adsorption amount of CO2 and CH4 with the total organic carbon (TOC) content.5,10,14 However, some results illustrate a complex relationship of the TOC with CH4 adsorption capacity.11,12 Inorganic components also influence the adsorption capacity of a shale for gases. It is regarded that clay minerals can contribute to the overall sorption capacity of shales for gases,12,19−22 because they have a high internal surface area6,12,19 and they also influence the pore size, total porosity, and sorption characteristics of shales.20 The pore structure also has great impact on the gas capacities of shale, which has been studied by many authors.10−14,23 Although adsorption of gases on shales has been extensively investigated, it is yet not understood well because of the complex composition and pore structure of shales from different geological formations. The purpose of this paper is to understand the competitive adsorption of CO2 and CH4 on shale based on the composition and pore structure of shale and the thermodynamic parameters. The shale studied was sampled from Nanchuan, the southeastern Sichuan Basin of China. The Sichuan Basin is a gas-rich basin, and the Lower Silurian Longmaxi formation shale is currently being explored as shale gas reservoirs. The equilibrium adsorption of CH4, CO2, and their mixtures (CO2 /CH4) are studied by gravimetric techniques. Considering the inhomogeneous properties of the shale surface, the Brunauer−Emmett−Teller (BET) model and
1. INTRODUCTION Shale gas whose main component is adsorbed methane (CH4) has been developed as a new unconventional gas. The emission of carbon dioxide (CO2) in the atmosphere increases fast with the increase of energy consumption. Injecting CO2 into shale reservoir could provide dual benefits of enhanced CH4 recovery and secure CO2 storage,1 whose principle is the competition adsorption between CO2 and CH4.2 At the initial stage of CO2 injection, the pressure of CO2 injected decreases quickly to a relatively lower value, because CO2 quickly migrated away from the well bores and into the fracture network.3,4 Therefore, the competition adsorption between CO2 and CH4 in shale during CO2 injection occurs at a wide range of pressures. The adsorption of pure CH4 on shales has been studied extensively to predict the gas shale reserve.5−19 However, the studies about equilibrium adsorption of CO2 in shales, which is related to geological sequestration of CO2,7 are very lacking. Various adsorption models have been used to describe equilibrium adsorptions of CH4 on shale, among which the Langmuir model and modified Langmuir model are the most often used models.5,6,8−15,17−19 Sometimes, the simplified localdensity (SLD) model7,16 and supercritical Dubinin−Radushkevich (SDR) model9 were also used. Thermodynamic parameters, including Gibbs free energy change (ΔG), entropy change (ΔS), and enthalpy change (ΔH) of adsorption heat (Qst), are important for understanding the adsorption behaviors. Adsorption heat (ΔH and −Qst) is the most widely investigated among the thermodynamic parameters.9−18 It has been found that the adsorption heat of CH4 on shales of different districts is reported in the range from about 12 to 20 kJ mol−1 at loading from 0 to 0.14 mmol/g, which indicates that the CH4 adsorption in shale belongs to physisorption. Some authors reported that entropy (ΔS) values of CH4 on shales range from −74 to −101.8 kJ mol−1 K−1, in which the loss of entropy indicates a highly ordered © 2016 American Chemical Society
Received: September 15, 2015 Revised: January 20, 2016 Published: January 25, 2016 2248
DOI: 10.1021/acs.energyfuels.5b02088 Energy Fuels 2016, 30, 2248−2256
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Energy & Fuels virial isotherm model are applied to fit the adsorption isotherm data at different temperatures. The competitive adsorption was analyzed on the basis of the composition and pore structure of Nanchuan shale, adsorption isotherms, and thermodynamic parameters of CH4 and CO2 on the shale.
where q is the adsorption amount per unit weight of adsorbent (mmol g−1), qm represents the amount of adsorption corresponding to a complete monolayer coverage (mmol g−1), p is the absolute pressure of adsorbate gas at equilibrium (MPa), p0 is saturated vapor pressure of adsorbate (MPa), and c is a parameter related to the difference in the heat of adsorption of first-layer adsorbate and that of the other layers. The thermodynamic virial isotherm model is very flexible to fit isotherms with different degrees of steepness in a wide range of pressure and temperature conditions.27−29 This model is obtained from the virial equation of state with Gibbs isotherm27 q p= exp(C1q + C 2q2 + C3q3) KH (2)
2. EXPERIMENTAL SECTION 2.1. Materials. Nanchuan shale belongs to the Lower Silurian Longmaxi formation, it was sampled from the southeastern Sichuan Basin, at a depth of 400 m from the ground surface. The thickness of the Silurian Longmaxi Formation shale is at depth ranging from 65 to 516 m.24 The sample was crushed and sieved to particles whose size range from 1.7 to 2.3 mm before characterization and measurement. CO2 and CH4 gases with a purity of 99.999% were provided by Chongqing Tianke Gas Company, Ltd. Hence, the uncertainty of the adsorption measurements as a result of gas purity was negligible. 2.2. Characterization of the Shale Sample. The TOC was measured using a Multi N/C 3100 TOC-TN analyzer (Analytik Jena, Germany). The measurement temperature was set as 1050 °C. The surface morphology the shale sample was observed by Hitachi TM-300 scanning electron microscopy (SEM) at an accelerating voltage of 20 kV. The structure of the crystalline phase of shale was determined by X-ray diffraction (XRD) of Rigaku D/Max2500PC using Cu Kα radiation. The elemental contents were analyzed by X-ray fluorescence (XRF) on a Rigaku ZSX Primus III+ spectrometer. The pore structure of the shale is characterized by mercury porosimetry and nitrogen (N2) adsorption. The mercury porosimetry was performed to measure the macropore size range from 0.1 to 160 μm using a Pascal 140/440 from Thermo Finnigan (Thermo Fisher Scientific, Inc., Waltham, MA). The pore size smaller than 0.1 μm was analyzed by a N2 adsorption isotherm of 77 K using the non-local density functional theory (DFT). The adsorption isotherm of N2 was measured using a Micromeritics ASAP2020M volumetric adsorption apparatus. On the basis of the isotherm data of N2, the BET surface area (SBET) was calculated using the BET equation. The total volume (Vt) was obtained at a relative pressure of 0.995. The microspore volume (Vmic) was obtained using the t-point method. The mesopore volume (Vmes) was calculated using the Barrett−Joyner−Halenda (BJH) model, and the macropore volume (Vmac) was received by the difference between Vt and Vmic and Vmes. The samples were outgassed under vacuum at 200 °C for 12 h prior to use. 2.3. Measurement of Adsorption Isotherms. Adsorption isotherms of CH4 and CO2 on the shale sample were measured using an Intelligent Gravimetric Analyzer (IGA-100B, Hiden Isochema, Ltd., U.K). Before the measurements, the sample of 170 mg was loaded in the vessel of IGA-100B and vacuumed up to 10−5 Pa at 378 K for 12 h. The pressure was performed at a range from 0 to 2 MPa. The temperature ranges were from 278 to 318 K. Single gas adsorption experiments were carried out in the static model, while mixed gas experiments were carried out in the flowing mode.25 The volume percent ratios of CO2/CH4 in the CO2/CH4 mixtures were 7:3, 5:5, and 3:7, respectively. 2.4. Adsorption Models. 2.4.1. Adsorption Models for Single Gas. Adsorption isotherms reflect the interaction between the adsorbate and adsorbent, and hence, the adsorption mechanism can be identified by different adsorption isotherm curves. The correlation of experimental equilibrium data using either a theoretical or empirical equation is essential for mechanism interpretation and adsorption data prediction. After applying several models often used for shale in the literature, we found that the BET and virial isotherm models fit the adsorption equilibrium data better than the Langmuir, Toth, and Sips models. The BET model assumes that an arbitrary number of adsorbate molecules may be accommodated for each adsorption site, and the rate of adsorption of the first layer is different from those of other layers. The BET equation is described as eq 126
q cp = qm (p0 − p)[1 + (c − 1)(p /p0 )]
where C1, C2, and C3 are virial coefficients and KH is the Henry constant (mmol g−1 MPa−1). The Henry constant is related to the temperature (T) through the van’t Hoff equation ⎛ −ΔH0 ⎞ ⎟ KH = K∞ exp⎜ ⎝ RT ⎠
(3)
where K∞ is the adsorption constant at infinite temperature (mmol g−1 MPa−1), −ΔH0 is the limiting heat of adsorption at zero coverage (kJ/ mol), and R is the universal gas constant (J mol−1 K−1). ln KH = ln K∞ −
ΔH0 RT
(4)
−ΔH0 is calculated from the slops of the plots of ln KH versus 1/T according to eq 4. 2.4.2. Adsorption Models for Mixture Gases. Many methods for predicting multicomponent gas adsorption have been proposed.30 In this study, adsorption isotherms of the CO2/CH4 mixtures were fitted by an extended ideal multicomponent BET model31,32
qCH =
qmCH t ̅XCH4[1 + (τCH4 − 1)(1 − Y )] 4
(1 − Y )[1 + ( t ̅ − 1)Y ]
4
qCO =
qmCO t ̅XCO2[1 + (τCO2 − 1)(1 − Y )] 2 (1 − Y )[1 + ( t ̅ − 1)Y ]
2
XCH4 =
pCH
4
p0CH
XCO2 =
,
4
(6)
pCO
2
p0CO
2
Y = XCH4 + XCO2 t̅ =
(5)
(7) (8)
cCH4XCH4 + cCO2XCO2
τCH4 =
(9)
Y cCH4 t̅
,
τCO2 =
cCO2 t̅
(10)
where qCH4 and qCO2 are adsorption amounts (mmol/g) of CH4 and CO2, respectively, pCH4 and pCO2 are the partial pressures (MPa) in a CO2/CH4 mixture, respectively, and qmCH4 and cCH4 and qCO2 and cCO2 are the parameters of pure component CH4 and CO2 calculated by eq 1, respectively. p0CH4 and p0CO2 are vapor pressures (MPa) of CH4 and CO2, respectively. The total adsorbed amount (Δm) of a mixture of CH4/CO2 measured by IGA is in weight. Δm correlates qCH4 and qCO2 as
Δm = qCH MCH4 + qCO MCO2 4
2
(11)
where MCH4 and MCO2 are the molecular weights of CH4 and CO2 (16 and 44 g/mol), respectively. 2.4.3. Adsorption Selectivity. The adsorption selectivity is an important parameter to evaluate the competitive adsorption between two gases. The Langmuir model6 fit well the experimental adsorption isotherms of CH4 at all temperatures and CO2 at 318 K. However,
(1) 2249
DOI: 10.1021/acs.energyfuels.5b02088 Energy Fuels 2016, 30, 2248−2256
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Energy & Fuels there is a small error for isotherms of CO2 at 278 and 298 K. The selectivity factor of CO2 over CH4 (αCO2/CH4) was estimated by eq 12 related to Langmuir parameters33 xCO2 yCH4 VLCO2/PLCO2 αCO2 /CH4 = = xCH4 yCO VLCH4 /PLCH4 (12) 2
where xCO2 and xCH4 are the molar fractions of CO2 and CH4 in the adsorbed phase, respectively, yCO2 and yCH4 are the molar fractions of CO2 and CH4 in the gas phase, respectively, and VLCO2 and VLCH4 are Langmuir volumes of CO2 and CH4, respectively. They are the adsorption volume at infinite pressure. PLCO2 and PLCH4 are Langmuir pressures, at which half of the Langmuir volume (1/2VL) is adsorbed. 2.4.4. Adsorption Thermodynamics. Adsorption thermodynamic parameters, including the isosteric heat of adsorption (Qst), surface potential (Ω), free energy change (ΔG), entropy change (ΔS), and enthalpy change (ΔH) were numerically analyzed by the following equations, respectively:34−38
⎛ ∂ ln p ⎞ ⎟ Q st = RT 2⎜ ⎝ ∂T ⎠q
ln p = −
Q st RT
Ω = −RT
∫0
(13)
+C
(14)
p
q d(ln p)
(15)
p
ΔG =
RT ∫ q d(ln p) Ω 0 =− q q
(16)
ΔH = − Q
ΔS =
(17)
ΔH − ΔG T
Figure 1. (a) XRD pattern of Nanchuan shale: (1) quartz, (2) calcite, (3) smectite, (4) dolomite, (5) illite, (6) gypsum, (7) orthoclase, (8) chlorite, (9) kaolinite, (10) siderite, and (11) pyrite. (b) Contents of the components of Nanchuan shale.
(18)
where q, R, T, and p are the same as in eq 2.
3. RESULTS AND DISCUSSION 3.1. Characterizations of the Shale. 3.1.1. Composition Analysis. The TOC of a shale is an important parameter for assessment of the methane reservoir. The measured TOC of the Nanchuan shale is 2.58 wt %, which is in agreement with the reported TOC content of Lower Silurian Longmaxi shale (ranging from 0.51 to 4.55 wt %).21 Table 1 shows the contents of the elements of Nanchuan shale measured by XRF, which are calculated in the form of
minerals include illite [KAl2[(Al,Si)Si3O10](OH)10·nH2O], kaolinite [Al4(Si4O10)(OH)8], smectite [(Na,Ca) 0.33 (Al,Mg) 2 [Si4 O 10 ](OH)2 ·nH 2O], and chlorite [X5−6Y4O10(OH)8, where X = Li, Al, Fe3+, Fe2+, Mg, Mn, and Cr and Y = Al and Si]. In addition, there are some carbonates and sulfates, including dolomite [CaMg(CO3)2], calcite (CaCO3), siderite (FeCO3), gypsum (CaSO4·2H2O), and pyrite (FeS2). The reason for the high content of carbon is that the shale contains not only inorganic carbon, such as carbonate minerals, but also organic carbon. The content of element Al is relatively higher than other metallic elements, because Al is the composition of orthoclase and also of the clay minerals. It has been reported that shales from the same districts contain brittle minerals (quartz, orthoclase, gypsum, etc.) of 60−80 wt % and clay minerals of 20−40 wt %.21,22 Nanchuan shale has a lower amount of clay minerals (23.69 wt %) but a higher amount of brittle minerals (74.76 wt %). It has been regarded that the commercial exploitation conditions for gas shales are that its brittle mineral content should be higher than 40 wt % and the TOC should be larger than 2.0 wt %.39 Hence, Nanchuan shale is suitable to be developed considering its composition. 3.1.2. Pore Structure. According to the International Union of Pure and Applied Chemistry (IUPAC) definition, micropores are of a diameter of 50 nm, and mesopores are of a diameter range from 2 to 50
Table 1. Element Contents and Pore Structure Parameters of the Nanchuan Shale content of the element (wt %) SiO2 CO2 Al2O3 CaO SO3
52.69 22.41 12.11 1.51 1.45
Fe2O3 K2O MgO
2.94 2.7 2.3
pore structure parameter SBET (m2 g−1) Vt (cm3 g−1) Vmic (cm3 g−1) Vmes (cm3 g−1) Vmac (cm3 g−1)
35.41 0.0349 0.0009 0.0302 0.0038
their corresponding oxides. The main elements found in the shale are Si, Ca, C, Al, Fe, K, and Mg. There are some minor components, such as Ti, Na, Ba, etc., whose contents are less than 1 wt %. The powder XRD pattern of the shale is shown in Figure 1a. The contents of the components calculated by the XRD peak area are shown in Figure 1b, which shows that the most abundant phases are quartz and clay minerals. Clay 2250
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intraparticle porosity of 0.36 and interparticle porosity of 0.14 are obtained by the mercury porosimetry measurement. Figure 4a shows the adsorption and desorption isotherms of nitrogen at 77 K. According to the classification of IUPAC, the
nm. The pore size distribution (PSD) of Nanchuan shale that ranged from 0.1 to 160 μm is illustrated in Figure 2. It is shown
Figure 2. Macropore size distribution of Nanchuan shale determined by mercury intrusion.
that there are mainly two kind of pores, distributed around 1.1 μm (0.3−3 μm) and around 10 μm (7−12 μm), respectively. This result is confirmed by the SEM measurement results shown by Figure 3, in which a large amount of macropores with a size of 0.5 μm (B2) and 10 μm (A1) is found. The
Figure 4. (a) Adsorption isotherm of N2 at 77 K and (b) PSDs of Nanchuan shale.
N2 adsorption isotherm belongs to type II and the hysteresis loop between the desorption and adsorption isotherms belongs to type H2. Hysteresis loop H2 suggests that the pore of Nanchuan shale is slit shape. The PSD of shale distributed from 1.0 to 100 nm is shown in Figure 4b. The main micropores are distributed 1.2 and 1.6 nm. It is noted that the meso- and macropore are continuously distributed. SEM images of the shale also show that there are many small pores with different sizes. Table 1 summarizes SBET and pore volumes of the shale. In combination of Figure 4b and Table 1, it is concluded that shale has a complex pore structure with a wide pore size distribution, ranging from nanometers to micrometers. 3.2. Equilibrium Adsorption. 3.2.1. Adsorption Isotherms of Pure CO2 and CH4. The adsorption equilibrium isotherms of CO2 and CH4 on the shale sample at 278, 298, and 318 K are shown in Figure 5. It is clear that the adsorption capacity of CO2 is always larger than those of CH4 (about 2.41−11.54 times at 273 K, 2.63−12.05 times at 298 K, and 1.97−5.30 times at 318 K) at the experimental pressure range.
Figure 3. SEM images of Nanchuan shale. 2251
DOI: 10.1021/acs.energyfuels.5b02088 Energy Fuels 2016, 30, 2248−2256
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Table 2. Average Relative Errors of the Fitting Results for CO2 and CH4 Isotherms by BET and Virial Modelsa adsorbate
T (K)
error BET (%)
error virial (%)
CO2
278 298 318 278 298 318
0.316 0.457 0.374 0.163 0.109 0.128
0.909 0.644 0.976 1.392 1.766 2.388
CH4
a Error is the average relative deviation. Error = 1/N∑|(qcal − qexp)/ qexp| × 100%, where N is the number of experimental data points and qexp and qcal are the experimental adsorbed amount and the adsorbed amount calculated by equation, respectively.
The KH values of the virial model of CH4 and CO2 decrease with the increase of the temperature. The −ΔH0 of CH4 calculated by KH is 21.58 kJ/mol, which is close to values reported for other Longmaxi shale of 23.91 and 27.76 kJ/ mol.17,18 It is noted that −ΔH0 of CH4 is close that of CO2. The pore structure and surface properties are the main factors influencing the adsorption behavior of gas. The pore structure and surface properties of shale actually depend greatly upon its composition, of which detail mechanisms are not well understood. Large adsorption energies of CO2 and CH4 are only found in micropores (
