Influencing Factors and Selection of CH4 and CO2 Adsorption on

Feb 20, 2018 - The specific surface area was calculated using the Barrett–Joyner–Halenda (BJH) method and the density functional theory (DFT) meth...
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Influencing Factors and Selection of CH4 and CO2 Adsorption on Silurian Shale in Yibin, Sichuan Province of China Yue Niu, Changtao Yue,* Shuyuan Li, Yue Ma, and Xinyi Xu College of Science, China University of Petroleum, Beijing 102249, People’s Republic of China ABSTRACT: Shale gas is a very promising natural gas with substantial development potential. In this study, Longmaxi Formation shale samples from the Silurian system in Yibin, Sichuan province of China, were selected to characterize the geological parameters, such as total organic carbon, clay mineral content, and vitrinite reflectance. The pore structure of shale was analyzed by field-emission scanning electron microscopy and a low-temperature N2 adsorption−desorption method. Isothermal adsorption experiments for CH4 and CO2 single components and mixtures were performed using a volumetric method. The second virial coefficient was introduced to calculate for the compressibility factor of the gas mixture. Then, the influencing factors and selection of CH4 and CO2 adsorption capacity of shale were investigated. According to the experimental results, the selected shale samples are black mud shale with high maturity, and they possessed diversified surface morphologies, complicated structures, and various pore types. Calculated pore parameter results showed that microspores comprised the majority of developed pores in shale samples and play a major role in the adsorption process. Isothermal adsorption experimental results for single CH4 and CO2 show that adsorptions of CH4 and CO2 follow similar rules. The amount of CO2 adsorption was higher than that of CH4 adsorption, and a high pressure was required to reach CO2 adsorption saturation. The Langmuir−Freundlich equation can be used to fit the isothermal adsorption experimental results. Isothermal adsorption experimental results for the CH4 and CO2 gas mixture followed trends similar to those of a single-gas component. The adsorption isotherms were all I-type adsorption isotherms, indicating that micropores play a major role. The temperature, pressure, pore structure, organic carbon content, and clay mineral composition of shale are important factors that influence the gas adsorption capacity of shale. During the adsorption of the same gas, the adsorption capacity of shale samples is more sensitive to unit pressure changes under lower temperature and pressure. In competitive adsorption, shale prefers to adsorb CO2. Therefore, CO2 is easier to be adsorbed by shale and causes CH4 to be released from the adsorption site. This occurrence holds guiding significance to CO2 driving CH4.

1. INTRODUCTION Shale gas is a highly important unconventional energy source with abundant reserves.1 Moreover, it is formed from dense organic-rich rocks with gas-saturated formations, concealed accumulation mechanisms, multiple lithologic seals, and short migration distances. After the shale gas is generated, it is gathered near the source rock and has the characteristics of “in situ” accumulation, which also makes the shale itself a source rock, gas reservoir, and caprock. Given the special structure of the shale gas reservoir, shale gas can be stored in three ways: adsorbed, free, and dissolved.2 Different from conventional gas, the content of adsorbed shale gas can reach 20−85% of the total reservoir.3 The adsorption characteristics and the adsorption isotherm of different gases on shale are a basis for evaluating the reservoir and enhancing shale gas recovery.4,5 Gas adsorption of shale is influenced by a variety of factors, including the shale reservoir properties, shale mineral composition, pore structure of shale, and adsorption environment.6−10 Research11 reported that total organic carbon (TOC) can directly influence the pore distribution, and micropore volume, mesopore volume, and porosity of shale increase with TOC growth. Moreover, high-pressure injection of CO2 into shale reservoirs has been accepted not only as an important way to reduce the greenhouse effect but may also be beneficial for CH4 desorption.12−14 Shale rich in organic matter has significant carbon dioxide adsorption capacity, and preferential adsorption of carbon dioxide can increase methane recovery by competitive adsorption. This phenomenon involves © XXXX American Chemical Society

several issues, such as the adsorption characteristics of CH4 and CO2, the factors that affect adsorption, and the selection of CH4 and CO2 competitive adsorption. Further studying shale properties and evaluating factors that influence CH4 recovery growth by CO2 are also significant for evaluating the reservoir and enhancing shale gas recovery.15−17 In this study, Longmaxi Formation shale samples from the Lower Silurian system in Yibin, southern Sichuan province of China, were selected. The sedimentary environment is marine. There are several sets of source rocks developed in the Yibin area, dominated by black shale, with the features of large thickness, stable distribution, high degree of thermal evolution, and good gas availability. The selected samples taken from different depths of the same well, organic carbon content, organic matter maturity, and mineral composition are quite different, with good representation. Geological parameters of selected samples, including TOC, clay mineral contents, and vitrinite reflectance, were investigated. The pore structure and pore diameter distribution of shale were analyzed by fieldemission scanning electron microscopy (FE-SEM) and a lowtemperature N2 adsorption−desorption method. Isothermal adsorption experiments for single components (CH4 or CO2) and mixtures were performed with a volumetric method using a Received: December 5, 2017 Revised: February 9, 2018 Published: February 20, 2018 A

DOI: 10.1021/acs.energyfuels.7b03815 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Working principle of the isothermal adsorption instrument by the volumetric method.

Table 1. Analysis Results on the Geological Parameters of Shale Samples mineral species and content (%) sample

depth (m)

TOC (%)

organic matter type

vitrinite reflectace (%)

quartz

potassium feldspar

albite

calcite

dolomite

pyrite

total amount of clay minerals

1 2

2321 2333

1.85 3.83

II 1 II 1

1.88 1.97

32.4 28.5

0.5 0.2

4.7 3.1

8.1 17.1

3.0 15.1

3.1 2.0

48.2 34.0

2.4. Low-Temperature N2 Adsorption−Desorption. A SI analyzer (Kangta Company) was used in this experiment, and the pore structural features of shale samples were characterized. The specific surface area was calculated using the Barrett−Joyner−Halenda (BJH) method and the density functional theory (DFT) method. Before the experiment, samples were ground and 150 mg of samples was treated by 6 h of vacuumization under 105 °C. The experimental temperature was 77.35 K. 2.5. Isothermal Adsorption Experiment. Following the experimental method of GB/T19560-2004, a high-pressure isotherm adsorption instrument for the volumetric method was constructed independently (Figure 1). The large reactor kettle with 100 mL can load more shale samples, and a strong magnetic stirring device was installed below the kettle body to avoid stratification caused by the difference of gas molecular mass in the competitive adsorption, which provided better reliability and accuracy for the experiment. The main experimental steps were as follows: (1) Sample preprocessing: A total of 120 g of powder samples was collected and dried in vacuum conditions for 10 h under 105 °C. The purity of CH4 and CO2 experimental gases was 99.99%. (2) Gas tightness examination: Helium was injected into the system, and gas tightness was examined upon reaching the maximum experimental pressure. When the pressure was maintained constant for 3 h, gas tightness was ensured. (3) Pore volume test: The pore volume in the sample pool was calculated accurately on the basis of the pressure−volume− temperature (PVT) method. (4) Isothermal gas adsorption: Gases were initially injected into the reference kettle. When the pressure in the reference kettle reached a stable value, the valve was switched between the reference kettle and adsorption kettle. The adequate gas adsorption of shale was then protected by maintaining the adsorption equilibrium at each pressure point for at least 8 h. When the gas pressure became constant, adsorption reached the balance. The traditional gas adsorption device used in the multicomponent gas competition experiment has the problem that the sampling of gas inside the absorption kettle at each adsorption equilibrium pressure is difficult. The experimental device in this paper has been improved using the micro gas chamber. The micro gas chamber is connected to the absorption kettle and is able to sample the gas inside the adsorption kettle at each adsorption equilibrium pressure for chromatographic composition analysis. The sampling process will

high-pressure isotherm adsorption instrument. The second virial coefficient was introduced to calculate the compressibility factor of the gas mixture. Then, the influencing factors and selection of CH4 and CO2 adsorption capacity of shale were investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. Shale samples, which were selected from the Longmaxi Formation of the Silurian system in Yibin, Sichuan province of China, were black mud shales. Prior to measurement, each sample was ground into powder of 60−80 mesh size. These powder samples were prepared for analysis of TOC, vitrinite reflectance test, X-ray diffraction (XRD), scanning electron microscopy (SEM), and lowtemperature N2 adsorption−desorption. 2.2. TOC, Organic Matter Type, Vitrinite Reflectance, and XRD. A WR-112 LECO carbon analyzer of LECO Corporation, St. Joseph, MI, U.S.A., was used to conduct the organic carbon analysis. The size of the sample particle is less than 0.2 mm. Iron and tungsten were added to the sample particle to aid combustion. An OGE Workstation oil and gas evaluation workstation was used with the standard of rock pyrolysis analysis (GB/T18602-2012) to classify the type of organic matter. A UMSP-50 micro spectrophotometer was used to test vitrinite reflectance. The test conditions were a temperature under 26 °C, a wavelength of 546 ±5 nm (green), and a 25× to 100× unstrained oil immersion objective. A total of 100 tungsten halogen lamps and electronic exchange regulator of 3 kVA were also used. The mineral composition test was performed using XRD under Cu Kα radiation. Emission and scattering slits are both 1°, and the receiving slit is 0.3 mm. The operating voltage is 30−45 kV; the electric current is 20−100 mA; the scanning speed is 2°/min; and the sampling step width is 0.02°. 2.3. FE-SEM. FE-SEM images of shale samples were collected using 200F SEM with energy-dispersive X-ray spectroscopy (EDS). A smooth sample surface was obtained by polishing using an argon ion, and this achieved an ideal mirror surface effect. SEM images of shale samples were collected under 35% humidity and 24 °C. For the convenience of understanding the pore structure, shale samples were observed under different magnifications. B

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Figure 2. SEM micrograph of pores in the shale sample. not affect the adsorption experiment, and the accuracy of the experiment has been increased. Isothermal adsorption experiments for single components (CH4 or CO2) and mixtures (CH4/CO2 = 2:8) in shale samples were carried out at 50, 70, and 90 °C. Pressure points were selected as 0, 0.5, 1, 2, 4, 6, 10, 12, and 15 MPa. Shale sample pretreatment was performed in a vacuum environment at 105 °C.

3. RESULTS AND DISCUSSION 3.1. Geological Parameters of Shale Samples. Analysis results on the geological parameters of shale samples are listed in Table 1. TOC test results demonstrated that the TOC contents in shale samples were 1.85 and 3.83%. The organic content was relatively high. Moreover, organic types were concentrated and belonged to II 1 type. XRD confirmed that quartz, feldspar, carbonatite (calcite and dolomite), pyrite, and clay minerals were the main mineral components of the shale. Brittle mineral contents of quartz, feldspar, and carbonatite were 30.45, 4.25, and 11.6%, respectively. Clay minerals, which are favorable for the adsorption capacity of shale, accounted for a high proportion at 41.1% on average. 3.2. Pore Structure of Shale. Micropore structures of shale sample 1 were observed by FE-SEM (Figure 2). Shale samples possessed diversified surface morphologies, complicated structures, and various pore types. The surface morphological structures include organic pores, clay mineral pores, intergranular pores of authigenic minerals, corrosion pores, and microcracks.18−21 With the influence of the squeezing action of clay minerals during the diagenetic process, numerous pores were formed (Figure 2a). Organic pores (Figure 2b) mainly refer to pores developed in organic matter, and these pores are mainly irregular. Some intergranular pores are developed between pyrite and other authigenic minerals (Figure 2c). Given the corrosion by some acid liquids, the pores of corrosion holes were typically triangular and square (panels d and e of Figure 2). Furthermore, microcracks played an important role in the seepage of shale gas (Figure 2f). Microcracks spread in samples to several micrometers long and several dozens of nanometers wide.22,23 3.3. Pore Distribution of Shale. Pore structure parameters and pore diameter distribution of shale were characterized using a low-temperature N2 adsorption−desorption experiment, and the results were shown in Figure 3. It is found that adsorption and desorption isotherms did not overlap at relatively high parts (P/P0 > 0.4). The adsorption isotherm

Figure 3. Nitrogen adsorption−desorption isotherms of shale samples.

curve was below the desorption isotherm curve, and a hysteresis loop was formed. On the basis of the division of the adsorption isotherm of the International Union of Pure and Applied Chemistry (IUPAC), the adsorption isotherm of shale samples presented characteristics of IV type, which indicated a wide and continuous distribution of pores.24 dV/dD−D curves can be calculated using the BJH and DFT methods, where V is the pore volume, D is the pore diameter, and dV/dD is the change rate of the pore volume with the pore diameter. The BJH and DFT methods can be used to observe the pore diameter distribution in the 1−100 and 1−10 nm ranges, respectively. Accoding to the calculated results shown in Figure 4a, in the 1−100 nm range, the pore volume of shale samples decreased gradually with an increased pore diameter. This finding reflected the great contributions of the micro- and mesopores to the pore volume, whereas the contribution of the macropores to the pore volume was small. Moreover, the results in Figure 4b show that, in the pore diameter distribution range of 1−10 nm, the pore volume peaked when the pore diameter was 1−2 nm, and the pore volume became the second largest when the pore diameter was 2−4 nm. The pore volume changed in small increments and stabilized when the pore diameter exceeded 4 nm. This observation indicated that the micropores in the shale samples contributed a higher pore volume than that of the mesopores. C

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Figure 4. Pore size distribution of shale nitrogen adsorption.

extent, the gas adsorption growth of shale samples tends to slow down, as manifested by a stable curve. Moreover, the amount of CO2 adsorption of shale samples was far higher than that of CH4 adsorption and required a higher pressure to reach CO2 adsorption saturation under equal conditions. The adsorption isotherm of the CH4 and CO2 adsorption of shale samples was a I-type adsorption isotherm, indicating that micropores play a major role.25−27 The Langmuir−Freundlich isothermal adsorption equation was chosen to describe the experimental results

The calculated pore parameters are also shown in Table 2. Results showed that the specific surface areas of the shale Table 2. Pore Parameters of Shale from Low-Pressure Nitrogen Adsorption sample

specific surface area (m2/g)

pore volume (cm3/g)

average pore size (nm)

1 2

15.97 21.35

0.0164 0.0159

4.112 3.950

V = VL

samples were 15.97 and 21.35 m2/g. The average pore volumes were 0.0164 and 0.0159 cm3/g, and the average pore diameters were 4.112 and 3.950 nm. These results revealed that sample 1 possessed a slightly larger average pore volume and average pore diameter than those of sample 2; however, the specific surface area of sample 1 was 25.2% smaller than that of sample 2. Hence, more micropores were present in sample 2 and account for a large proportion. 3.4. Isothermal Adsorption Characteristics of Gas on Shale. 3.4.1. CH 4 and CO 2 Single-Component Gas. Isothermal adsorption experimental results for single CH4 and CO2 are shown in Figure 5. It is obvious that adsorptions of CH4 and CO2 follow similar rules. At a low pressure, the gas adsorption of shale samples increased significantly with an increased pressure. When the pressure increased to some

(bP)m 1 + (bP)m

(1)

where V is the equilibrium gas adsorption quantity of shale (mL/g), P is the adsorption pressure (MPa), VL is the Langmuir volume, that is, the theoretical saturation adsorption capacity of shale (mL/g), PL is the Langmuir pressure (MPa; the corresponding adsorption capacity was half of the saturated adsorption capacity), b = 1/PL, and m is the heterogeneity coefficient (m ≤ 1). The characteristic parameters calculated on the basis of eq 1 are listed in Table 3. It can be seen that, with the increase of the temperature, the PL and VL values of adsorbing CH4 and CO2 decrease gradually. However, the VL and PL values of adsorbing CO2 are relatively larger than those of adsorbing CH4, which also show the high adsorption capacity of CO2.

Figure 5. Isothermal adsorption curve of CH4 and CO2 of shales. D

DOI: 10.1021/acs.energyfuels.7b03815 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 3. Characteristic Parameters of Langmuir−Freundlich Model Fitting CH4

CO2

sample

temperature (°C)

VL

PL

m

VL

PL

m

1

50 70 90 50 70 90

1.40 1.24 1.18 2.09 1.61 1.39

4.23 3.20 3.03 9.15 5.867 3.52

0.79 0.81 0.93 0.57 0.69 0.91

9.45 5.35 3.72 11.28 6.78 5.35

11.75 10.68 8.79 14.00 8.75 8.69

0.55 0.481 0.46 0.51 0.55 0.54

2

⎛ ωj + ωi ⎞ ωij = ⎜ ⎟ ⎝ 2 ⎠

3.4.2. Calculation of the CH4 and CO2 Mixed Gas Compression Factor. For real gas, the partial pressure theorem and the partial volume theorem cannot describe the gas mixtures accurately as a result of the non-ideality of the pure components of the gas and the interaction between the mixed gases. Therefore, the compressibility factor was introduced to describe the real gas state. In this paper, we use the generalized virial equation with the second virial coefficient that reflects the intermolecular forces to calculate the CH4 and CO2 mixed gas compression factor. The virial equation is a semi-empirical equation that has been used to modify the ideal gas state equation and is proven by the method of statistical thermodynamics. The virial equation is suitable for mediumand low-pressure gas, and the experiment is carried out under the conditions of 0−15 MPa, belonging to the range of low and medium pressure; thus, the virial equation was chosen. Statistical mechanics can derive the second virial coefficient of the gas mixture as BM =

∑ ∑ yyi j Bij i

where kij is the binary exchange interaction parameter and is crucial for calculating the compressibility factor. kij is related with mixture properties, generally ranging between 0 and 0.2. For the binary system of CH4 and CO2, kij can be approximately 0. The steps for calculating the compressibility factor of the CH4 and CO2 gas mixture using the generalized second virial coefficient are as follows. The generalized second virial coefficients B11 and B22 of CH4 and CO2 were calculated, and each interaction of critical parameters was computed on the basis of equations from eqs 5 to 9 using the critical parameters of pure CO2 and CH4 from Table 4. Table 4. Critical Parameters of CO2 (1)−CH4 (2)

(2)

j

where i and y are the molar fractions of components in the mixture and Bij is the interaction between components i and j. For the mixture of CH4 and CO2, we have 2

2

BM = y1 B11 + 2y1y2 B12 + y2 B22

RTcij Pij

(B(0) + wijB(1))

(3)

Tcij (K)

Pcij (MPa)

Vcij (m3 kmol−1)

Zcij

ωij

11 22 12

304.2 190.6 240.8

7.382 4.599 5.839

0.0940 0.0986 0.096

0.274 0.286 0.28

0.228 0.011 0.1195

Table 5. Second Virial Coefficient of CO2 (1)−CH4 (2) (4)

Bij (m3 kmol−1)

where B(0) = (0.083 − 0.422)/Tr1.6 and B(1) = (0.139 − 0.172)/ Tr4.2. Critical parameters were calculated by the following mixing rule: Tcij = (1 − kij) TciTcj

ij

Equation 4 was used to calculate the crossing second virial coefficient B12, and the results were shown in Table 5. Then, BM of the CH4 and CO2 gas mixture was calculated using eq 3. Finally, the compressibility factor of the mixture was calculated using the following equation: B P Z=1+ M (10) RT

where B11 and B22 are the second virial coefficients of pure CO2 and pure CH4. B12 is called the crossing second virial coefficient and refers to the properties of CH4 and CO2 mixed gas. B12 can be calculated by the following equation: Bij =

(9)

ij

50 °C

70 °C

90 °C

11 22 12

−0.102 −0.033 −0.058

−0.088 −0.028 −0.050

−0.076 −0.023 −0.042

(5)

⎛ Zci + Zcj ⎞ ⎟⎟ Zcij = ⎜⎜ 2 ⎠ ⎝

(6)

⎛ Zci + Zcj ⎞ Zcij = ⎜⎜ ⎟⎟ 2 ⎠ ⎝

(7)

⎛ ZcijRTcij ⎞ ⎟⎟ Pcij = ⎜⎜ ⎝ 2 ⎠

(8)

3.4.3. CH4 and CO2 Mixed Gas. Isothermal adsorption experimental results of CH4 and CO2 mixed gas calculated in accordance with section 3.4.2 are shown in Figure 6. Maximum adsorption capacities of sample 1 at 50, 70, and 90 °C were 4.648, 2.743, and 2.089 mL/g, while those of sample 2 were 5.548, 3.391, and 2.567 mL/g, respectively. It can be seen that the isothermal adsorption process of the gas mixture followed trends similar to those of a single-gas component. The isothermal adsorption curve of the CH4 and CO2 mixture also showed characteristics of the I-type isothermal adsorption curve. E

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Figure 6. Isothermal adsorption curve of mixed gases of shales.

Figure 7. Change in the gas adsorption capacity for unit change in the pressure.

adsorption capacity of gas also increase. The TOC of sample 1 was lower than that of the sample 2 and is positively correlated with the gas adsorption amount. Mineral composition also influences the adsorption capacity of shale. According to the results shown in Table 1, terrigenous detritals and clay minerals were the major components of shale samples. The terrigenous detritals are mainly quartz, feldspar, and carbonatite, whereas the clay minerals mainly consisted of kaolinite, montmorillonite, illite−smectite mixed strata, illite, and chlorite. Their particle sizes, crystal structures, and intracrystalline pore diameters can influence the specific surface area significantly and, thus, affect the adsorption capacity of shale.29−31 3.5.2. Pore Structure. The results in Table 2 show that the average pore volume and average pore diameter of sample 2 are slightly smaller than those of sample 1. The specific surface area of sample 1 is 25.2% smaller than that of sample 2. According to the experimental results of isothermal adsorption, the maximum CH4 adsorption capacity of sample 1 was 18.5% less than that of sample 2 on average in the range of 0−15 MPa and the maximum CO2 adsorption capacity of sample 1 was 19.9% less. As shown by the fitting results of the Langmuir−

Figure 6 also shows that the equilibrium adsorption capacity of CH4 was significantly smaller than that of CO2, especially during the low-pressure stage. With the increasing pressure, the adsorption capacity tends to be saturated and that of CH4 reached saturation early. It is obvious that the adsorption capacities of the gas mixture and single component all declined with the increasing temperature. With the increase of the adsorption temperature, potential energy should increase when gas molecules move from the bulk gas phase to the surface of shale. In theory, the increase in the temperature would benefit the adsorption. However, the thermal motion of gas molecules, which enables gas to leave more easily from the surface of shale, also increases with the increase of the temperature and will play a greater influence on the decrease of the adsorption capacity. 3.5. Factors Influencing the Adsorption Capacity of Shale. 3.5.1. Geological Parameters. The organic matter content in shales has an important influence on the micropore volume and specific surface area. Kang et al.28 pointed out that the average pore diameter in organic matter is far smaller than that in inorganic matter. With the increase of the organic matter content, the number of pores and the corresponding saturated F

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Figure 8. Adsorption capability of single-component CH4 and CO2 of shale samples.

defined as M, and the values of M were shown in Figure 8. It is shown that M was always higher than 1.8. During the lowpressure stage, M was relatively high, declined gradually, and became stable with the increasing pressure. This behavior indicated that CO2 can replace CH4 at the adsorption sites and CH4 is easily replaced at a low pressure, which offers a guiding significance to CO2 gas flooding of CH4. It can also be seen from Figure 8 that the M value is negatively correlated with the temperature. This is mainly due to the intense movement of gas molecules when the temperature rises, while the molecular weight of CH4 is lower than CO2 and irregular movement is more severe, which makes it easier for the adsorbed CH4 molecules to desorb from the shale surface. Under the same conditions, sample 2 has a higher value of M as a result of its higher organic content, larger specific surface area, and more adsorption sites. Because the CO2 molecule has a higher quadrupole moment and dipole moment than those of the CH4 molecule, it makes the adsorption site with high energy of shale have a higher binding force with CO2 and makes it more difficult for desorption, which then resulted in a high M value. 3.7. Adsorption Capability of the CH 4 and CO2 Mixture. Results in Figure 6 demonstrated that, given the same temperature and pressure, CO2 adsorption of shale samples was higher than CH4 adsorption. The adsorption amount ratio of the CO2 and CH4 mixture on the same conditions was defined as the selectivity coefficient S of CO2. Because CO2/CH4 in the original gas was 8:2, the value of S was equal to the ratio between 1/4 of the CO2 adsorption amount and the CH4 adsorption amount. The calculated value of S was presented in Figures 9 and 10. It can be seen from Figures 9 and 10 that the value of S is always greater than 1, indicating that CO2 is more easily adsorbed by shale and can displace adsorbed CH4. Mixed gases compete against each other, and as a result of the van der Waals intermolecular force, the critical temperature and diffusion rate forces of CO2 are all greater than those of CH4; thus, CO2 adsorption is preferred. Moreover, the value of S is relatively high at the low-pressure stage. With the increase of the pressure, the value of S gradually becomes gentle, indicating that CH4 can be replaced more easily under a low pressure, which is instructive for CO2 driving CH4.

Freundlich model, the CH4 and CO2 saturated adsorption capacities of sample 1 were 23.7 and 22.6% smaller than those of sample 2 on average. These results indicated that the adsorption capacity of shale is positively correlated with the specific surface area of shale. The specific surface area of shale is closely related with the porosity of shale. The pore size distribution of the two shale samples is mainly dominated by micro- and mesopores. The former can make shale have a larger specific surface to provide more adsorption sites for gas adsorption, while the latter can make shale have a larger pore volume to provide additional adsorption and storage space for gas adsorption.32,33 3.5.3. Unit Pressure Change. The effect of pressure changes on the adsorption capacity of shale was investigated by calculating the partial derivative of pressure in eq 1 to obtain eq 11. The numerical result of eq 11 shown in Figure 7 can reflect the influences of pressure changes on CH4 or CO2 adsorption capacity of two shale samples at different temperatures. Obviously, pressure changes affected the adsorption capacity of CH4 and CO2 significantly during the low-pressure stage of 0−2 MPa, and the influence declined dramatically with the pressure increase. When the pressure was higher than 2 MPa, the effect weakened gradually with the rising pressure. ∂V bmP m − 1 = VL ∂P [1 + (bP)m ]2

(11)

In comparison of the degree of influence of unit pressure changes on CH4 and CO2 adsorption, it is found that CO2 adsorption is more sensitive than CH4 adsorption. For the adsorption of the same gas, a low temperature leads to an increased sensitivity for adsorption capacity to pressure changes, which deduced that only a low pressure can desorb large amounts of gas in the gas desorption process. Moreover, sample 2 was more sensitive to unit pressure changes than sample 1. This difference is related to the larger specific surface area and more adsorption sites of sample 2 than those of sample 1. 3.6. Adsorption Capability of Single-Component CH4 and CO2. Isothermal adsorption experimental results on single CH4 and CO2 adsorption revealed that CO2 adsorption of shale was higher than CH4 adsorption at the same temperature and pressure. The adsorption amount ratio of CO2 and CH4 was G

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Langmuir−Freundlich equation can be used to fit the isothermal adsorption experimental results. The isothermal adsorption experimental results for the CH4 and CO2 gas mixture followed trends similar to those of a single-gas component. The adsorption isotherms were all I-type adsorption isotherms, indicating that micropores play a major role. (4) The temperature, pressure, pore structure, organic carbon content, and clay mineral composition of shale are important factors that influence the gas adsorption capacity of shale. A low temperature and high pressure favor the gas adsorption of shale. The total pore volume and specific surface area of shale are significantly and positively correlated with the saturated adsorption capacity, whereas the pore structure is related with the TOC and clay mineral content of shale. (5) During the adsorption of the same gas, the adsorption capacity of shale samples is more sensitive to unit pressure changes under a lower temperature and pressure than at a higher temperature and pressure. The CO2 adsorption of shale samples is highly sensitive to unit pressure changes. During gas desorption, shale can desorb abundant gases in the lowpressure period. (6) Given the same temperature and pressure, the CO2 adsorption of shale samples is higher than the CH4 adsorption. In competitive adsorption, shale prefers to adsorb CO2. Therefore, CO2 is easier to be adsorbed by shale, and this causes CH4 to be released from the adsorption site. This occurrence holds a guiding significance to CO2 driving CH4. Moreover, the pressure and temperature can affect the selective adsorption of CO2.

Figure 9. Adsorption capability of the CH4 and CO2 mixture of shale sample 1.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-89735669. E-mail: [email protected].

Figure 10. Adsorption capability of the CH4 and CO2 mixture of shale sample 2.

ORCID

Changtao Yue: 0000-0001-8438-3878 Shuyuan Li: 0000-0001-7428-3988

It can also be seen that the temperature has an effect on the value of S. The value of S under 70 °C was higher than that under 50 °C; this effect was attributed to the intensified gas molecular motion and, especially, CH4 motion under a high temperature. Thus, CH4 at adsorption sites was more easily released than CO2. The S value under 90 °C changed slightly and even decreased; this behavior was related to the intensified molecular motion of CO2 under a high temperature. When more CO2 molecules were adsorbed, the probability to release CO2 would also increase.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (41372152) and the National Basic Research Program of China (973 Program, 2014CB744302).

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4. CONCLUSION (1) The selected shale samples are black mud shale with high maturity, and they possessed diversified surface morphologies, complicated structures, and various pore types. The surface morphological structures include organic pores, clay mineral pores, intergranular pores of authigenic minerals, corrosion pores, and microcracks. (2) Calculated pore parameter results showed that the specific surface areas of the shale samples were 15.97 and 21.35 m2/g. The average pore volumes were 0.0164 and 0.0159 cm3/g, and the average pore diameters were 4.112 and 3.950 nm. Microspores comprised the majority of developed pores in shale samples and play a major role in the adsorption process. (3) Isothermal adsorption experimental results for single CH4 and CO2 show that adsorptions of CH4 and CO2 follow similar rules. The amount of CO2 adsorption was higher than that of CH4 adsorption, and a high pressure was required to reach CO2 adsorption saturation. The

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DOI: 10.1021/acs.energyfuels.7b03815 Energy Fuels XXXX, XXX, XXX−XXX