Comparing the Effectiveness of SO2 with CO2 for Replacing

Apr 30, 2019 - Key Laboratory of Unconventional Oil & Gas Development, China University of Petroleum (East China), Ministry of Education, Qingdao 2665...
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Comparing the Effectiveness of SO2 with CO2 for Replacing Hydrocarbons from Nanopores Yueliang Liu,†,‡ Xiaomin Ma,§ and Jian Hou*,†,‡ †

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Key Laboratory of Unconventional Oil & Gas Development, China University of Petroleum (East China), Ministry of Education, Qingdao 266580, China ‡ School of Petroleum Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, China § College of Mining Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China ABSTRACT: Molecular dynamic simulation is employed to investigate the fluid distribution of pure hydrocarbons, i.e., C1, C2, nC3, nC4, and nC5, in a hydrocarbon-wetting nanopore. SO2 and CO2 are then introduced into this nanopore to explore how SO2 and CO2 affect the hydrocarbon adsorption in an organic pore. Adsorption selectivity and replacement efficiency of SO2 over hydrocarbons are subsequently calculated and compared with those of CO2. The performance of SO2 and CO2 in enhancing hydrocarbon recovery from nanopores is thus evaluated. After introducing SO2 or CO2 into the “hydrocarbonsaturated” pore, the density of hydrocarbons in the adsorption layer decreases, while the density in the pore center increases. It suggests that both SO2 and CO2 can replace the adsorbed hydrocarbons from the pore surface. In addition, higher adsorption capacity is observed for CO2 than that for C1 but smaller than those of the heavier hydrocarbons, i.e., C2, nC3, nC4, and nC5. Comparatively, SO2 exhibits a stronger adsorption capacity than C1, C2, nC3, and nC4, suggesting its potential for enhancing the recovery of heavier hydrocarbons from organic pores. We expect this strategy will inspire new perspectives for flue-gas treatment and recovery of shale resources using flue-gas injection.

1. INTRODUCTION Carbon dioxide (CO2) and sulfur dioxide (SO2) in flue gas are the main components causing air pollution. CO2, which causes global warming, is attracting extensive attention, while SO2, which may cause acid rain and ozone depletion, is strictly restricted for emission.1 Recently, many studies have been conducted to remove CO 2 and SO 2 from flue gas independently.1−10 However, the methods proposed in these studies have the disadvantages of either high-energy consumption or expensive equipment cost. The injection of flue gas into shale reservoirs may be a promising way of removing CO2 and SO2 by realizing CO2 and SO2 sequestration underground; on the other hand, shale hydrocarbons can be replaced due to competitive adsorption between hydrocarbons, CO2, and SO2 on the shale. Shale hydrocarbon is one kind of important unconventional resource; due to its considerable abundance, studies on the recovery of such resources from shale reservoirs have been extensively conducted.11−15 However, the unique characteristics of shale reservoirs, such as extremely low permeability and heterogeneity, make it difficult to recover shale resources from such reservoirs.16 Unlike conventional reservoirs, pore size in shale matrix is generally in the nanoscale; fluids confined in such pores are strongly attracted by the pore surface. Moreover, shale may also contain a large proportion of organic matter, such as kerogen, while hydrocarbons have an affinity to organic matter. Due to the existence of organic matter as well as nanopores, hydrocarbons tend to adsorb on the organic pore surface, showing a unique adsorption behavior from that in conventional reservoirs due to stronger fluid−pore surface interactions. A better understanding of the adsorption behavior of fluid in organic pores is beneficial for obtaining the © XXXX American Chemical Society

fundamental mechanisms of shale-hydrocarbon storage in shale reservoirs. The idea of CO2 injection into shale reservoirs has been proposed as a feasible technique to enhance shale-hydrocarbon recovery; extensive studies have been conducted on this subject, although this technique has not been widely commercialized. Recently, studies on enhanced shale C1 recovery using CO2 method mostly consider that C1 is generally the most commonly seen component in shale gas. Numerical simulations were performed, and it has been found that the introduction of CO2 into the depleted shale gas reservoirs is technically feasible for enhancing shale C1 recovery.17−22 To understand the fundamental mechanisms of recovering C1 using CO2 injection from organic shale, experimental studies were conducted on the adsorption behavior of C1/CO2 on typical shale samples.23−30 It has been found that CO2 exhibits a higher adsorption capacity than C1, suggesting that CO2 could be an effective agent to recover shale C1 from shale reservoirs.23−30 Compared with the experimental measurements, molecular simulation is a powerful theoretical approach to gain insights into the microphase behavior of gas mixtures from the molecular perspective. Using molecular simulation, many attempts have been made to investigate the competitive adsorption behavior of C1/CO2 mixtures in organic pores.31−42 Based on the simulation results, CO2 presents a stronger adsorption on the pore surface than does C1.43 Received: April 1, 2019 Revised: April 25, 2019 Published: April 30, 2019 A

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Figure 1. Schematic of the 3 nm nanopore in the MD simulation.

2. MOLECULAR DYNAMICS (MD) The Forcite module and Amorphous Cell Package is applied to conduct the MD simulations.48−51 In the Force module, we employ the condensed phase-optimized molecular potential for atomistic simulation studies (COMPASS) force field to describe the interatomic interactions; such a force field has been recognized as the first force field that enables an accurate simultaneous prediction for a wide range of molecules.51,52 In the COMPASS force field, the total potential energy (Etotal) is expressed as52

Besides C1, heavier hydrocarbons, such as C2, nC3, nC4, and nC5, generally coexist, which usually shows individual adsorption capacities on organic shale. Jin and Firoozabadi44 studied the influence of CO2 on the adsorption of different hydrocarbons (i.e., C1 and nC4) in nanopores. Compared to C1, adsorbed nC4 cannot be readily replaced by CO2 due to the stronger associations of nC4 to the organic pore surface. To date, studies on the recovery of heavier hydrocarbons from shale reservoirs using CO2 are still in the initial stages. Recently, experimental studies were conducted to compare the adsorption of SO2 and CO2 on some commercial adsorbents.1,45,46 It was found that the adsorption of SO2 on the surface of commercial adsorbents is significantly higher than that of CO2. It inspires us that SO2 may have a distinguished performance in replacing hydrocarbons from organic shale. To the best of our knowledge, only few studies have been conducted to investigate the efficiency of SO2 for enhanced shale-hydrocarbon recovery. Based on the previous study, SO2 has been experimentally found to be a more efficient agent than CO2 for recovering C1 and C2 from shale samples using the low-field nuclear magnetic resonance technique.47 In this work, CO2 and SO2 are introduced into a “hydrocarbon-saturated” (i.e., saturated with C1, C2, nC3, nC4, and nC5) 3 nm pore to investigate how CO2 and SO2 affect the fluid distribution of hydrocarbons in the nanopore. Based on the altered fluid distribution, the selectivity of CO2 and SO2 over different hydrocarbons is then calculated to analyze the competitive adsorption of CO2, SO2, and hydrocarbons in their binary mixtures in the organic pore. The replacement efficiency of CO2 and SO2 over these hydrocarbons is then calculated to compare the usage of CO2 and SO2 for enhanced shale-hydrocarbon recovery. The objective of this study is to propose SO2 injection as a new strategy for enhancing shale-hydrocarbon recovery and for assessing the efficiency of CO2 for the recovery of heavier hydrocarbons from organic shale. Carbon slit-pore has been widely used in the simulation works to represent kerogen walls in the shale, considering that carbon surface is hydrocarbonwet and can provide the underlying mechanisms on the adsorption behavior of CH4 in nanopores.43 As part of a comprehensive study on the adsorption behavior of gas on organic shale, we also use a carbon slit-pore model to describe nanopores for simplicity. This work is expected to provide the basic understanding of competitive adsorption behaviors of hydrocarbons, CO2, and SO2 in organic pores, wherein it helps to evaluate the efficiency of CO2 and SO2 in replacing hydrocarbons from organic pores and thereupon realizes CO2 and SO2 sequestration.

Etotal = Einternal + E cross ‐ coupling + Evander Waals + E electrostatic (1)

E

internal

=

∑E

E cross ‐ coupling =

(b)

+

∑E

(θ )

+

∑E

(φ)

+

∑E

(γ )

∑ E(bθ) + ∑ E(bφ) + ∑ E(b ′ φ) + ∑ E(θθ ′) + ∑ E(θφ) + ∑ E(θθ ′ φ)

(2)

(3)

where b and b′ are the lengths of two adjacent bonds, θ and θ′ are the angles between two adjacent bonds, ϕ is the angle resulting from dihedral torsion, and γ is the out-of-plane angle.51 Einternal is the energy derived from each of the internal valence coordinates; Ecross‑coupling is the cross-coupling terms between internal coordinates. Evander Waals is the sum of repulsive and attractive Lennard-Jones terms.53 Evander Waals and Eelectrostatic are obtained according to the atom-based method, with a cutoff distance of 13.0 Å.54 In addition, we use the Andersen thermostat55 for the temperature conversion.

3. SIMULATION MODEL Carbon materials are generally applied to simulate organic matter in the shale considering that carbon surface is hydrocarbon-wet and can provide the underlying mechanisms of adsorption of hydrocarbons in organic pores.43,44,56 In this simulation, the full atomistic structure of graphite layers, formed by carbon atoms, is used to simulate the organic nanopore. As shown in Figure 1, one carbon-slit pore is connected by a fictitious reservoir with a given volume and temperature. CO2 or SO2 is first introduced into the fictitious reservoir, which can spontaneously enter the organic nanopore due to the affinity to the pore surface. The periodic boundary condition is applied for the created pore in all three directions. Moreover, two graphic layers are used to form one carbon sheet. The separation distance between two carbon-atom centers in two opposite graphite layers is 0.335 nm. In the same graphite layer, the distance between two adjacent carbonatom centers is 0.142 nm. During the MD simulations, the position of each carbon sheet is fixed. The size of the created pore is (Lp + LF) nm × WP nm × (HP + 2Wc) nm in the OC, B

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Energy & Fuels OB, and OA directions, respectively (Lp is the length of the nanopore, 6.619 nm; LF is the length of the fictitious reservoir; Wp is the width of the nanopore, 2.952 nm; and Wc is the separation distance between the two carbon-atom centers in the two graphite layers, 0.335 nm). In our work, MD simulations are performed in a canonical NVT ensemble, which has fixed number of particles (N), volume (V), and temperature (T). A binary mixture composed of given number of molecules is initially loaded in the NVT ensemble. The dynamic density distribution of each species is obtained after achieving a competitive adsorption/desorption equilibrium of the binary mixture on the pore surface. The basic inputs in the MD simulations are summarized in Table 1. Table 1. Basic Inputs in the MD Simulationsa mixture

molar fraction

simulation time (ns)

simulation temperature (K)

hydrocarbon/CO2 hydrocarbon/SO2

0.5:0.5 0.5:0.5

35 35

363.15 363.15

a

It is noted that Hydrocarbon represents pure C1, C2, nC3, nC4, or nC5.

When placed in this nanopore, C1, C2, nC3, nC4, nC5, CO2, and SO2 molecules in their binary mixtures will exhibit competitive adsorption on the organic pore surface. In this work, we use adsorption selectivity to characterize the competitive adsorption of C1, C2, nC3, nC4, nC5, CO2, and SO2 molecules on the pore surface. As for binary mixtures, the adsorption selectivity (SA/B) is calculated as33 SA/B

(x / x ) = A B (yA /yB )

Figure 2. Density distributions of pure C1, (a) CO2/C1, and (b) SO2/ C1 mixtures in the 3 nm pore at 363.15 K.

(4)

after the introduction of CO2. CO2 has a higher adsorption capacity on the organic surface than that of C1; thereby, the adsorbed C1 can be replaced by CO2 on the pore surface, which thereby decreases the density in the adsorption layer of C1. As shown in Figure 2a, in addition to the first adsorption layer, C1 also exhibits a second weak adsorption layer when CO2 is introduced. Recently, Tian et al. (2017)58 also observed this second weak adsorption layer but named it “transition zone”, whose density is significantly smaller than that of the adsorption layer. However, the possible mechanisms forming such a zone were not elucidated in their study. With more adsorbed C1 being replaced by CO2, the desorbed C1 can still be attracted by the pore surface, as well as the adsorbed molecules in the first adsorption layer, which enhances the formation of the second weak adsorption layer. In other words, the second weak adsorption layer formed by C1 results from the competitive adsorption between CO2 and C1 on the pore surface and the attractions from the pore surface and the first adsorption layer. Comparatively, we introduce equal molar amount of SO2 into the 3 nm pore instead of CO2, which is initially saturated with C1. Figure 2b presents the density profiles of pure C1 and C1 and SO2 in the equal molar mixture of C1/SO2 in the 3 nm pore at 363.15 K. We observe that the density of the adsorption layer of SO2 is significantly higher than that of C1, indicating the higher adsorption capacity of SO2 on the organic pore surface. Similarly, the adsorption-layer density of C1 is reduced heavily after introducing SO2, suggesting the feasibility of SO2 in recovering C1 from organic pores. Compared to CO2, the adsorption-layer density of SO2 is much higher. In

where A and B represent the species in the binary mixture, xA and xB represent the molar fractions of adsorbates A and B in the adsorbed phase, respectively, and yA and yB represent the molar fractions of adsorbates A and B in the reservoir, respectively. If the calculated SA/B is less than 1, it indicates that the adsorption capacity of A is lower than that of B.57

4. RESULTS AND DISCUSSION In this subsection, the influence of SO2 on the fluid distribution of C1, C2, nC3, nC4, and nC5 in the 3 nm pore is analyzed and then compared with that of CO2. The adsorption selectivity of CO2 and SO2 over different hydrocarbon species is then calculated to investigate the competitive adsorption between hydrocarbons and CO2 and SO2 molecules in organic nanopores. Finally, the replacement efficiency of CO2 and SO2 on different hydrocarbon species is computed to assess the efficiency of CO2 and SO2 in replacing hydrocarbons in organic pores. 4.1. Comparison of the Influence of CO2 and SO2 on the Fluid Distribution of Hydrocarbons in Nanopores. Figure 2a shows the density profile of pure C1 in a 3 nm pore at 363.15 K. Pure C1 can only form a single-adsorption layer on the pore surface, which is in line with the previous findings.44,58 When an equal molar amount of CO2 is introduced into this 3 nm pore, which is initially saturated with C1, the density of the adsorption layer of CO2 at equilibrium is much higher than that of C1, indicating the higher affinity of CO2 on the organic pore surface. Moreover, the density of the adsorption layer of C1 decreases significantly C

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Energy & Fuels addition, the density drops in the adsorption layer of C1 caused by SO2 is more significant than that caused from CO2. It suggests that SO2 can replace the adsorbed C1 more efficiently than CO2 from the organic surface. Interestingly, as shown in Figure 2b, the density of the second adsorption layer of C1 is observed to be higher than that of the first adsorption layer after introducing SO2. It is not proper to recognize the second adsorption layer as the socalled transition zone58 only according to the relatively density values. Due to the stronger adsorption capacity of SO2, SO2 replaces the adsorbed C1 from the pore surface, releasing more adsorption sites; such released adsorption sites are then immediately occupied by SO2 molecules. The desorbed C1 molecules are still highly attracted by the pore surface as well as the first adsorption layer, resulting in the formation of the second stronger adsorption layer adjacent to the first adsorption layer. As shown in Figure 2, after introducing SO2, the density of C1 in the free-phase zone is higher than that after introducing CO2 as well as that of the pure C1, indicating that more adsorbed C1 can be replaced by SO2 and becomes the free-state C1 in the pore center. Thereby, SO2 is possibly more efficient than CO2 for enhancing shale C1 recovery from organic pores. Besides C1, heavier components, such as C2, nC3, nC4, and nC5, may also be important components in shale gas or shale condensate. In Figures 3−6, we show the density profiles of

Figure 5. Density distributions of pure nC4, CO2/nC4, and SO2/nC4 mixtures in the 3 nm pore at 363.15 K.

Figure 6. Density distributions of pure nC5, CO2/nC5, and SO2/nC5 mixtures in the 3 nm pore at 363.15 K.

pure C2, nC3, nC4, and nC5 and the equal molar mixtures of CO2/C2, SO2/C2, CO2/nC3, SO2/nC3, CO2/nC4, SO2/nC4, CO2/nC5, and SO2/nC5 in the 3 nm pore at 363.15 K. Similarly, the density of hydrocarbons in the adsorption layer in their binary mixtures is lower than that of pure hydrocarbons, i.e., C2, nC3, nC4, and nC5. It indicates that CO2 and SO2 can somewhat replace the heavier hydrocarbons from the organic pore surface. However, as hydrocarbon becomes heavier, the density of hydrocarbons in the adsorption layer approaches that of the pure hydrocarbons, suggesting that the heavier hydrocarbons may not be readily replaced. It is because the adsorption capacity of CO2 or SO2 relative to that of hydrocarbons decreases when hydrocarbon species in shale fluids become heavier. In addition, the density of hydrocarbons in the adsorption layers drops more significantly after introducing SO2 than that after CO2 introduction, indicating the superiority of SO2 in enhancing the heavier hydrocarbons from organic pores. This observation is in line with our previous finding using the lowfield nuclear magnetic resonance technique that SO2 is more efficient than CO2 for recovering C1 and C2 from shale samples.47 Interestingly, the introduction of SO2 and CO2 increases the density of hydrocarbons in pore center (i.e., the free-phase zone); compared to that for CO2, the density in pore center is observed to increase more significantly for SO2. The desorbed hydrocarbons due to the introduction of SO2 and CO2 become free-state gas residing in the pore center; further developing method should be proposed to recover the

Figure 3. Density distributions of pure C2, CO2/C2, and SO2/C2 mixtures in the 3 nm pore at 363.15 K.

Figure 4. Density distributions of pure nC3, CO2/nC3, and SO2/nC3 mixtures in the 3 nm pore at 363.15 K.

D

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adsorption capacity of SO2 on the organic pore surface is stronger than that of C1, C2, nC3, and nC4. As for CO2, the adsorption selectivity is higher than 1 only for C1, suggesting that the adsorption capacity of CO2 is higher than that of C1 but lower than that of C2, nC3, nC4, and nC5. One may infer that SO2 is efficient for enhancing C1, C2, nC3, and nC4 recovery but is not proper for nC5, while CO2 is only suitable for the recovery of C1 from organic pores. However, as has been observed in Figure 6, after introducing SO2 into the “nC5saturated” pore, the adsorption-layer density of nC5 also decreases slightly, while the density of nC5 in the free-phase zone increases. Moreover, as shown in Figures 3−6, after introducing CO2, the adsorption-layer density of C2, nC3, nC4, and nC5 decreases slightly, while the density in the free-phase zone increases. That is, the presence of CO2 and SO2 somewhat increases the recovery of C1, C2, nC3, nC4, and nC5 from the organic pore due to the effect of competitive adsorption. 4.3. Replacement Efficiency of CO2 and SO2 on Different Hydrocarbon Species. Hydrocarbons residing in organic pores can be possibly replaced by the introduction of SO2 or CO2, resulting in the enhancement of the recovery of hydrocarbons. In Figure 8, we show the replacement efficiency of CO2 or SO2 on different hydrocarbon species. It is noted that the replacement efficiency is defined as the molar percentage of hydrocarbons replaced by CO2 or SO2 from the nanopores. We observe that the recovery of C1, C2, nC3, nC4, and nC5 is enhanced to some extent after introducing CO2 or SO2 into the organic pore. Specifically, CO2 can recover C1 by more than 25 mol % from the 3 nm pore, while the recovery degree is generally less than 20 mol % for the other hydrocarbon species. On the contrary, SO2 can achieve the recovery degree by more than 25 mol % for both C1 and C2. Moreover, recovery degree for different hydrocarbon species achieved by SO2 is always higher than that of CO2. It confirms our viewpoint that SO2 can be a more efficient agent than CO2 for enhancing hydrocarbon recovery from organic pores. In addition, as hydrocarbon becomes heavier, the recovery achieved by CO2 and SO2 decreases; it is mainly caused by the decreasing adsorption capacity of CO2 or SO2 relative to hydrocarbon species.

free-state gas confined in nanopores to enhance the recovery of shale hydrocarbons. 4.2. Adsorption Selectivity of CO2 and SO2 over Different Hydrocarbon Species. In nanopores, different components in mixtures generally exhibit individual adsorption capacities on organic pore surface, which is defined as the socalled “competitive adsorption”. In this work, the adsorption selectivity is calculated to illustrate the relative adsorption capacity of CO2 or SO2 over different hydrocarbon species in their binary mixtures. Figure 7 presents the adsorption

Figure 7. Adsorption selectivity of CO2 and SO2 over C1, C2, nC3, nC4, and nC5 in their binary mixtures in the 3 nm pore at 363.15 K.

selectivity of SO2 and CO2 over C1, C2, nC3, nC4, and nC5 in their binary mixtures in the 3 nm pore. As shown in Figure 7, the adsorption selectivity of SO2 over C1, C2, nC3, nC4, and nC5 is always higher than that of CO2 in the same condition; meanwhile, the difference enlarges as the hydrocarbon becomes lighter. It proves our aforementioned statement that SO2 has more affinity to the organic pore surface than CO2. In addition, as the carbon number increases, the adsorption selectivity of SO2 and CO2 over hydrocarbons decreases, suggesting that the adsorption capacity of SO2 and CO2 relative to hydrocarbons decreases with increasing carbon number in the hydrocarbons. In Figure 7, the adsorption selectivity of SO2 over C1, C2, nC3, and nC4 is always higher than 1; it means that the

Figure 8. Replacement efficiency of CO2 and SO2 on C1, C2, nC3, nC4, and nC5 from the 3 nm pore. E

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single pores. Thereby, molecular dynamic simulations are recommended to study the adsorption behavior in porous media with pore size distribution, which is more practical for shale reservoir modeling. Additionally, based on scanning electron microscopy imaging, it is found that the pore structures in the real shale sample are not only slit-shaped but also include ink-bottleshaped and cylindrical pores.60,61 The adsorption behavior of the hydrocarbon/SO2 (or CO2) mixtures is expected to be different from those in the slit-shaped pores. For instance, depending on the pore-diameter ratio of “ink” and “bottle”, N2 adsorption/desorption isotherms may behave differently as a result of pore-blocking and cavitation effects within the inkbottle model.62,63 Future work is suggested to explore the effect of pore geometry on the adsorption of nanoconfined hydrocarbon/SO2 (or CO2) mixtures. Flue gas is a mixture composed of multiple components. SO2 may behave differently in flue gas or as a single component. In the future works, it is necessary to investigate the role of flue gas in enhancing shale reserve recovery. In addition, besides kerogen, some other minerals, such as illite, kaolinite, and montrmorillonite, may also exist. Hydrocarbons generally behave differently on different core minerals. Thereby, it is necessary to investigate the efficiency of CO2 or SO2 for recovering hydrocarbons due to difference in their mineral composition.

In Figure 8, we depict the snapshots of molecular distributions of C1, C2, nC3, nC4, nC5, CO2, and SO2 in their binary mixtures in the 3 nm pore at 363.15 K. We observe that SO2 molecules are mainly adsorbed on the pore surface, forming adsorption layers. The adsorbed hydrocarbons, i.e., C1, C2, nC3, nC4, and nC5, are replaced by SO2 molecules residing in the pore center or in the reservoir. On the contrary, among the five hydrocarbon species, obvious adsorption layers formed by CO2 are observed only when CO2 is introduced into the “C1-saturated” pore. It seems that CO2 cannot replace hydrocarbons heavier that C1; we observe that the introduced CO2 mainly appears in the pore center and outside the organic pore. However, the fact that CO2 enhances the recovery of C2, nC3, nC4, and nC5 is probably due to the accidental adsorption of CO2 on the pore surface when it is introduced to coexist with heavier hydrocarbons, wherein the adsorption sites of hydrocarbons is partly occupied by the adsorbed CO2.

5. CONCLUSIONS In this study, we compare SO2 with CO2 for enhanced shalehydrocarbon recovery from an organic nanopore using MD simulations. Based on the simulation results, SO2 is observed to be more efficient than CO2 in replacing the adsorbed hydrocarbons from the organic pore surface, indicating its higher feasibility for enhanced shale-hydrocarbon recovery. The specific conclusions can be drawn as follows: • Density of SO2 in the adsorption layer is much higher than that of CO2, suggesting its higher affinity to the organic pore surface. Compared to CO2, density of hydrocarbons in the adsorption layer is reduced more substantially after introducing SO2 into the nanopore, indicating its higher feasibility to recover hydrocarbons from the organic pores. • CO2 has a higher adsorption capacity than C1 but is less than that of C2, nC3, nC4, and nC5; it indicates that CO2 is properly used for enhanced C1 recovery. On the contrary, SO2 has a stronger adsorption capacity than C1, C2, nC3, and nC4, suggesting that SO2 may be a potential agent in flue gas for enhanced recovery of heavier hydrocarbon from shale reservoirs. • CO2 and SO2 can recover hydrocarbons from organic pores to some extent. Compared to CO2, SO2 has a higher replacement efficiency for hydrocarbons from the organic pore, especially for C1 and C2. CO2 and SO2 in the flue gas can be used as a replacement agent for enhanced shale-hydrocarbon recovery; meanwhile, it realizes the sequestration of greenhouse gases in shale reservoirs, which thereby reduces the impact of the emission of flue gas into air. This work may provide some fundamental understanding of the mechanisms of enhancing shale-hydrocarbon recovery using flue-gas injection method. It is also significant for understanding the basic mechanisms of CO 2 and SO 2 sequestration in shale reservoirs. In future works, SO2 may also be applied for enhanced coal-bed methane recovery besides CO2. Moreover, the adsorption behavior is suggested to be studied at varied pressure and temperature conditions, which is practical for designing schemes for shale reservoirs. In shale matrix, pore size varies, showing pore size distribution; adsorption behavior modeling for porous media with pore size distribution is different from that in single nanopore. However, recent works59 mainly investigated the adsorption behavior in

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 86-15192665837. ORCID

Yueliang Liu: 0000-0002-1041-8377 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors greatly acknowledge the Grant from the National Science Foundation for Distinguished Young Scholars of China (Grant No. 51625403) to J.H.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.9b00995 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.9b00995 Energy Fuels XXXX, XXX, XXX−XXX