Molecular Insights into the Enhanced Shale Gas Recovery by Carbon

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Molecular Insights into the Enhanced Shale Gas Recovery by Carbon Dioxide in Kerogen Slit-Nanopores Haoyang Sun, Hui Zhao, Na Qi, and Ying Li J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017

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The Journal of Physical Chemistry

Molecular Insights into the Enhanced Shale Gas Recovery by Carbon Dioxide in Kerogen Slit-nanopores Haoyang Sun, Hui Zhao, Na Qi and Ying Li* Key Laboratory of Colloid and Interface Chemistry of State Education Ministry, Shandong University, Jinan, Shandong 250100, P. R. China

Corresponding author: Ying Li Tel:(86) 0531-88362078 Fax: (86) 0531-88364464 Email: [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Fully exploitation and utilization of the unconventional reservoirs of shale gas has become a central issue following with the increasing worldwide energy demand. Enhancing shale gas recovery by injecting CO2 is a promising technique that combine shale gas extraction and CO2 capture and storage (CCS) perfectly. In this study, an kerogen based slit-shaped pore with a width of ~ 21 Å was constructed by two kerogen-matrix, the grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods were used to investigate the adsorption and diffusion properties of CH4 and CO2 in kerogen-matrix and slit-nanopores, and explore the displacement efficiency of the residual CH4 by CO2 in kerogen slit-nanopores. The adsorption energy of CH4 and CO2 on kerogen fragment surface, and the isosteric heat of CH4 and CO2 in kerogen slit-nanopores were examined to demonstrate the competitive adsorption of CO2 over CH4 in kerogen slit-nanopores, the different intensity of interactions between the CH4 and CO2 molecules with the pore surface play a key role. An effective displacement process of the residual adsorbed CH4 by CO2 in kerogen slit-nanopores was performed, the efficiency of displacement enhanced with the bulk pressure increasing, and the sequestration amount of CO2 in kerogen slit-nanopores increased at the same time. Moreover, it was found that part of CH4 adsorbed firmly inside the intrinsic pores of kerogen-matrix were very hard to be displaced by the CO2 injection. This work demonstrates the micro-behaviors of CH4 and CO2 in kerogen slit-nanopores, give out the microscopic mechanism of the


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displacement of CH4 by CO2, for the purposes to provide useful guidance for enhancing shale gas extraction by injecting CO2. 1. INTRODUCTION Shale gas was recognized as one of the promising unconventional resources with the global energy shortages in the last decade, which with the advantages of large reserves and high energy efficiency.1-5 The United States has achieved success in shale gas extraction and utilization,6,7 the shale gas had occupied almost 40% of the total natural gas production of U.S. as of 2012, and which is expected to reach 50% by 2035.8 China has abundant shale gas resources and has vigorously pursued shale gas extraction in the last decade.9-11 In 2013, the shale gas production of China had got to 0.2 billion cubic meters (BCM), and aims to reach an annual production rate of 60 ~ 100 BCM by 2020.12,13 Considerable efforts are still needed to make this resource successfully and economically exploited.14 Recently, with the exposure of the environmental threats caused by hydraulic fracturing in shale gas extraction,15-18 enhancing natural gas recovery by injecting CO2 (CO2-EGR) is recognized as one of the remarkable techniques, which not only achieving the CO2 capture and storage (CCS), but also conquering the energy resource exploitation problems.19-21 Comparing with other techniques in shale gas exploitation, to displace CH4 by CO2 is a feasible, economic, efficient and sustainable way to combine the environmental and industrial benefits perfectly.22,23 Shale gas is mainly reserved in the micro-pores (d < 2 nm) and meso-pores (d = 2～ 50 nm) in shale formation.24,25 The inorganic minerals and organic matter



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constitute the main body of gas shale. Quartz, clay, calcite and feldspar etc. constitute the inorganic portion, meanwhile the organic matter, which is mainly composed of kerogen, is considered to be the primary CH4 trapping.11, 26 According to literatures, kerogen is insoluble amorphous organic mixture with various chemical composition, which can be classified into four types based on the ratios of H/C and O/C.27-29 The type

kerogen, which is mainly composed of the various cyanobacteria and

dinoflagellates, the H/C ratio is larger than 1.25 and O/C ratio less than 0.15. The ratio of H/C of the Type

kerogen is less than 1.25 and O/C of 0.03 ~ 0.18, which is

largely obtained from the marine planktonic organisms. Type

kerogen has a ratio of

H/C less than 1 and the ratio of O/C is in 0.03 ~ 0.3, which is primarily derived from the higher plant remains in coals. And the type

kerogen consists of abundant

polycyclic aromatic hydrocarbons, with the ratio of H/C less than 0.5.28 Researches about the micro-behaviors of gases in nanoporous materials have been widely done both in experimental studies and computer simulations.30-34 The simulation studies could provide more detailed micro-information accurately and easily, especially about the micro-behaviors of gases in the adsorbents which were related to the real nanoporous layers.35-41 Recently, studies about the micro-behaviors of CH4 and CO2 in nanopores with organic-matter mainly focused on the adsorbents of nanoporous carbons (NPCs) and carbon nanotubes (CNTs).42-45 Such as, Lu et al.,46 examined the effect of edge-functionalization on the competitive adsorption of a binary CO2–CH4 mixture in NPCs. Zhu et al.47 investigated the CH4 adsorption in CNTs, and reveal the adsorption structure of CH4 in CNTs with the pore size variation.


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Brochard et al.48 studied the competitive adsorption of binary CO2–CH4 mixture in NPCs-based model of coal seam, and described the desorption behavior of CH4 with the CO2 injection. Meanwhile, some researches about the displacement of CH4 by CO2 in nanopores with organic-matter have also been examined.49,50 Such as, Kowalczyk at al.49 performed the displacement of CH4 by coadsorbed CO2 in carbon nanopores, aiming to reveal the microscopic mechanism about enhanced coalbed methane recovery (ECBM). Yuan et al.50 examined the adsorption of CH4 and CO2 on graphene, and the micro-properties of confined CH4 displaced by CO2 in CNTs. The transport properties of CH4 in three type nanoporous media, mainly based on the CNTs, were also performed by Lee et al..51 All of these studies could give us some guidance about the micro-behaviors of CH4 and CO2 in the nanoporous organic matter, but is limited because of using the CNTs to represent the nanopores with organic matter, which is somewhere away from the real condition. So far the displacement of CH4 by CO2 in authentic kerogen nanopores is rare, and the microscopic mechanism is still unclear, and that is exactly what we desire to figure out. 2. MODELS AND METHODOLOGY 2.1 Atomistic Models



Figure 1. (a) Model of the kerogen slit-nanopore with two kerogen-matrix (the surface landscape is in pink), (b) fragments of kerogen. Atoms: C in grey, H in white, O in red, N in blue and S in purple. The model of kerogen slit-nanopore with a pore width of ~ 21 Å was constituted by two kerogen-matrix (fig 1a). Each kerogen-matrix was constructed by the fragments of mature kerogen (fig 1b) as described in the research work of Collell,29 with the H/C ratio of 0.5 ~ 0.8, O/C ratio of 0.05 ~ 0.1, and the density of the kerogen-matrix was 1.24 g/cm3. The kerogen-matrix was amorphous with the intrinsic nano-scaled pores inside due to the irregularly combined of the fragments. The whole sorbent was regarded as rigid during the simulation, and described by the COMPASS52 force field. The charges and bond parameters of CH4,53 CO254 and fragments of kerogen are shown in fig S1. 2.2 Simulation Details The COMPASS52 force field was used to perform the whole simulations in this study by using Materials Studio (MS) software package. The non-bonding interactions are represented by the electrostatic potential and van der Waals (vdW) potential, respectively. As described in our previous work,55 the adsorption of CH4 and CO2 in


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kerogen slit-nanopores were investigated by the GCMC method by using the Sorption program package of MS. Four types of trial moves were adopted, including creation, destruction, rotation and translation. The Metropolis algorithm was used to describe the acceptance or rejection of a trial move.56 The Lennard-Jones 9-6 potential was used to perform the vdW interaction, meanwhile the electrostatic interaction was examined by the Coulombic term. Each equilibration and calculation process includes 5×106 steps. The diffusion of CH4 and CO2, and the displacement of CH4 by CO2 in kerogen slit-nanopores were examined by the MD method, using the Forcite program package of MS. The NVT ensemble was used to perform the process with the temperature thermostat of Nosé-Hoover. Each MD simulation process was in a run time of 5.0 ns with a time step of 1 fs, and the last 1.0 ns of the MD process was used for analysis. Meantime, as a single CH4-CO2 molecule adsorbed on the basis unit of kerogen fragments, the minimum energy configurations were calculated by using the Dmol3 program package of MS.57 The exchange–correlation interaction was described by the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE)58 functional, which was widely used to describe the interactions between gases and surface of carbon material due to the high accuracy in the description of a long-range distance force, such as the vdW potential.59 The density functional semicore pseudopotential (DSPP)60 method, with the localized double-numerical basis with a polarization (DNP) functional was chosen for all the atoms, according to the previous work of Lu et al.,46 which could well perform the adsorption of CH4 and CO2



in nanoporous carbons. 3. RESULTS AND DISCUSSION 3.1 Adsorption and Diffusion of CH4 and CO2 as Single Component in Kerogen Slit-nanopores

10

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(a)

8 7 6 5 4

298 K 323 K 343 K 373 K

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Adsorption Loading (mmol/g)

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298 K 323 K 343 K 373 K

5 4 3 2 1 0

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Figure 2. Adsorption isotherms of CH4 (a) and CO2 (b) as a function of adsorption pressure ranging from 10 kPa to 20000 kPa in kerogen slit-nanopores at various temperatures. The absolute adsorption isotherms of CH4 and CO2 in kerogen slit-nanopores at various temperatures, ranging from 298 K to 373 K, are shown in fig 2. It is found that the adsorption amount of gases increases gradually with the enhanced pressures, and decreases with the temperature increasing, which is consistent with the characteristic of CH4 and CO2 adsorption in nanoporous materials, including the inorganic mineral,40 CNTs44 and NPCs.46 In comparison with the CH4 adsorption in kerogen samples by experimental study,27 the simulation result is about five times of the experimental data, and considering the complexity and uncertainty of the composition and construction of the real shale, precise credible experimental results are hard to get, especially under harsh conditions, so the differences between the


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simulation and experimental results might be attributed to the complex influencing factors of real shale. Figure 2 also indicates that the adsorption amount of CO2 is larger than CH4 in kerogen slit-nanopores, which can be attributed to the different intensity of interactions between CH4 and CO2 molecules with the kerogen slit-nanopore, as discussed below.

Figure 3. Molecular density profiles of CH4 (a) and CO2 (b) in kerogen slit-nanopores at various pressures and 323 K, with the corresponding simulation snapshots of the adsorption density at 323 K and 10 MPa. The adsorption density distribution of CH4 and CO2 as single component in kerogen slit-nanopores versus the pressure variation at 323 K are shown in fig 3. It is found that in addition to adsorbed inside the slit-nanopore, a small number of CH4 and CO2 can also be adsorbed inside the intrinsic pores of kerogen-matrix. Therefore, gas molecules adsorbed in kerogen slit-nanopores are divided into three parts in this work: the part adsorbed close onto the pore surface (Ads-surface), the part adsorbed around the center of the pore (Ads-central), and the part adsorbed inside the intrinsic pores of the kerogen-matrix (Ads-matrix). It can be seen that, CH4 prefers to adsorb closely onto the pore surface, and the molecular density increases with the pressure increasing, of



which the Ads-surface occupies the main part at various pressures, which indicates that the CH4 adsorbed inside the slit-nanopore mainly distributes in locations close to the pore surface. Fig 3a also indicates that, the Ads-surface part of CH4 reaches a nearly saturated state when the pressure is larger than 10 MPa, after that the density of Ads-central part increases with the continued pressure increasing. The CO2 molecules also prefer to adsorb on to the pore surface as shown in fig 3b. And in contrast to CH4, the density of CO2 is much larger. Fig 3b also indicates that the density of CO2 increases acutely when the state changing from non-supercritical to supercritical state (the critical pressure of CO2 is 7.3 MPa) as pressure increasing. When the pressure is larger than 10 MPa, the density of the Ads-central part almost reaches up to that of the Ads-surface part, which demonstrates that in addition to adsorbing closely onto the pore surface, the CO2 can fill the whole slit-pore at the supercritical state. The peaks of the density of CH4 and CO2 in kerogen slit-nanopores are irregular, as shown in fig 3, which should be attributed to the very rough pore surface.

35000

7000

CO2 of Ads-surface CO2 of Ads-central

MSD (Å )

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0

(b)

CO2 of Ads-matrix 6000

CH4 of Ads-central

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CH4 of Ads-matrix CH4 of Ads-surface

30000

MSD (Å )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 0

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t (ps)

400

600

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1000

t (ps)

Figure 4. MSDs of CH4 (a) and CO2 (b) adsorbed at different locations in kerogen slit-nanopores at 10 MPa and 323 K. In order to investigate the microscopic diffusion properties of CH4 and CO2 in


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kerogen slit-nanopores, the self-diffusion of the molecules were examined by using the mean squared displacement (MSD) with the follow equation:

MSD(t ) =

1 N 2 〈 ri (t ) −ri (0) 〉 ∑ N i=1

(1)

where N is the number of same type particles, ri(t) is the particle position when the time is t and ri(0) is the initial position. Gases molecules adsorbed at different positions of kerogen slit-nanopores were marked to calculate the MSDs respectively, as shown in fig 4. The MSDs of the gas molecules can also be divided into three part: MSD-surface, MSD-central and MSD-matrix. It is found that the self-diffusion for CH4 and CO2 in kerogen slit-nanopores can both be demonstrated as: MSD-matrix < MSD-surface < MSD-central. So, gas molecules of Ads-matrix part are adsorbed much more firmly comparing with the part of Ads-surface and Ads-central, which is primarily because of the confinement of the tiny pores of kerogen-matrix, generating the gas molecules being trapped tightly. Moreover, gas molecules of Ads-surface part are adsorbed more tightly than the Ads-central part. Because the Ads-surface part is close to the pore surface, therefore the adsorption interaction of gas molecules of Ads-surface part to the surface is stronger. And beyond that, the density of gases close to the pore surface is large, and the surface roughness also play an important role. Figure 4 also indicates that the MSDs of CO2 are much weaker than CH4, which is mainly attributed to the different intensity of interactions between the CH4 and CO2 molecules with the pore surface. 3.2 Adsorption Properties of CH4 and CO2 in Kerogen-matrix



2.5

3.5

(b)

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298 K 323 K 343 K 373 K

0.5

0.0


(a) Adsorption Loading (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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fugacity (kPa)

Figure 5. Adsorption isotherms of CH4 (a) and CO2 (b) as a function of adsorption pressure ranging from 10 kPa to 20000 kPa in kerogen-matrix at various temperatures. The adsorption isotherms of CH4 and CO2 in each kerogen-matrix are shown in fig 5, it is found that, similar to the gases adsorption in kerogen slit-nanopores, the adsorption amount of CH4 and CO2 in kerogen-matrix gets enhanced with the pressure increasing, and recedes with the increasing temperature. Meantime, the adsorption amount of CO2 is slightly larger than CH4. The snapshots of the adsorption equilibrium states of gases inside the intrinsic pores of kerogen-matrix also demonstrate that the CH4 and CO2 can be adsorbed inside the inner space of the amorphous kerogen-matrix (fig S2).


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Figure 6. Stable adsorption configurations (side view (up) and top view (down)) of CH4 (a–d) and CO2 (e–h) on the kerogen fragments. To further investigate the microscopic adsorption of CH4 and CO2 in kerogen nanopores, the adsorption energy (Eads) between each CH4 and CO2 molecule on the fragments of kerogen surface is examined with the follow equation, Eads = Ea+s – Es – Ea

(2)

in which Ea is the energy of the gas species, Es is the energy of the kerogen fragment surface, and Ea+s is the total energy of the gas molecule adsorbed on the surface of kerogen fragment. As shown in fig 6, the Eads of CO2 is weaker than CH4 on each kerogen fragment, which indicates the CO2 molecule has stronger adsorption interaction with the kerogen fragment comparing with CH4, and which can be a candidate in displacing CH4. For the adsorption sites, the CH4 molecule prefers to adsorb onto the hollow site of a carbon ring (fig 6a-c), while the CO2 mostly chooses the bridge site of C-C bond as the favorable adsorption site (fig 6e and f), which in agreement with the adsorption of CH4 and CO2 on the graphene surface studied by Yuan et al..50 Figure 6g and h also indicate that the functional group of hydroxyl has significant influences on the adsorption of CO2 comparing with CH4, it is because of



the O atom of CO2 molecule can form hydrogen bonds with the hydroxyl group,36 and which also consistent with the research work by Lu et al..46 3.3 Competitive Adsorption of CO2 over CH4 in Kerogen Slit-nanopores

Figure 7. Selectivity of CO2 over CH4 at various temperatures, with the corresponding simulation snapshot of the adsorption density of the binary mixed CH4-CO2 in kerogen slit-nanopore at 323 K and 10 MPa. The competitive adsorption of CO2 over CH4 in kerogen slit-nanopores is found to be broadly occurred, due to the different interaction intensity of CH4 and CO2 with the pore surface. Therefore, the selectivity parameter (S) is employed to perform the competitive adsorption of the binary mixed CH4 and CO2 in kerogen slit-nanopores that defined as

S=

xCO 2 / xCH 4 yCO 2 / yCH 4

(3)

where x is the fraction of gas component in the adsorbed phase, y is the fraction of gas component in the bulk phase. As shown in fig 7, the SCO2/CH4 decreases with the pressure increasing from the initial to P ~ 5 MPa at various temperatures, that is because of at low pressures (equal to with low adsorption loading), the CO2 is more


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preferentially adsorbed than CH4 in kerogen slit-nanopores. While the active sites decrease rapidly with the increasing loading, and further adsorbed gases molecules have to locate at less favored sites, so the SCO2/CH4 starts to decrease. Moreover, when the pressure is larger than 5 MPa, a significant change of SCO2/CH4 occurs as the temperature changing. At the higher temperature of 373 K, the SCO2/CH4 is kept nearly a constant value with the further enlarged pressure, it is because the active sites cannot strongly capture gas molecules with a larger kinetic energy at such a higher temperature.61 As the temperature decreasing (T < 373 K), the SCO2/CH4 increases gradually with the pressure increasing, especially at T = 298 K, which demonstrates that the temperature has a significant influence on the competitive adsorption of CO2 over CH4 in kerogen slit-nanopores. And the snapshot of the adsorption density distribution of the binary mixed CH4-CO2 in kerogen slit-nanopores also clearly shows that the CO2 has stronger adsorption capacity than CH4 in kerogen slit-nanopore. 12

CH4 adsorbed in kerogen slit-nanopore 10

isosteric heat (kcal / mol)

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CO2 adsorbed in kerogen slit-nanopore

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Figure 8. Isosteric heats of CH4 (black line) and CO2 (red line) with the variation of adsorption amount (percentage) in kerogen slit-nanopores at 323 K. To further investigate the differences of adsorption capacities between CH4 and



CO2 molecules in kerogen slit-nanopores, the isosteric heat ( ) is used to examine the adsorption properties expressed as = −

〈 〉〈 〉〈 〉 〉〈 〉 〈

(4)

where is the gas constant, T is the temperature, is the number of adsorbate and is the adsorption energy. As shown in fig 8, the values of both CO2 and CH4 decrease with the enlarged adsorption amount in nanopores, this is because of less favorable sorption sites would exist at high adsorption loading.62 The value of CO2 is larger than CH4, which further demonstrates that the CO2 molecule has stronger adsorption capacity in kerogen slit-nanopores comparing with CH4 as mentioned above. 3.4 Displacement of CH4 by CO2 in Kerogen Slit-nanopores 1.0

(a) Desorption of CH4 (ratio)

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fugacity (kPa)

Figure 9. Desorption of CH4 in kerogen slit-nanopores with the variation of equilibrium pressures, with the corresponding snapshots of the residual CH4 in kerogen slit-nanopores at 5 (b) and 10 (c) MPa. The desorption of shale gas in shale formation is mainly caused by the driving force of gas concentration gradient,63 the CH4 desorption in kerogen slit-nanopores is determined by the ratio ( ) of the gas molecules diffusing out from the nanopores


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by the following equation, = ( − )/

(5)

where Nad is the amount of gas in nanopores of the initial adsorption equilibrium conformation (P = 20 MPa, T = 323 K), and Nre is the amount of the residual gas left in nanopores after desorption. Four different systems with equilibrium pressures of ~ 5, 8, 10 and 15 MPa were investigated respectively, as shown in fig 9a. It is found that, the gets enhanced with the decreasing equilibrium pressures, and about 68% of CH4 could diffuse out from the kerogen slit-nanopores freely when the pressure is decreasing to 5 MPa. Figure 9b and c show the microscopic state of the residual CH4 in kerogen slit-nanopores after desorption at pressures of 5 and 10 MPa, respectively, which clearly indicate that majority of the residual CH4 adsorbed close onto the surface of kerogen slit-nanopores. The CO2 is used as the “bulk” phase to displace the residual CH4 in kerogen slit-nanopores at different bulk pressures, the model of displacement is shown in fig S3. The initial state of the residual CH4 in nanopores is derived from the equilibrium state of desorption at P = 5.0 MPa and T = 323 K. The displacement bulk pressure is controlled by adjusting the number of CO2 molecules, and is calculated by the Peng-Robinson equation of state.35



14

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Loading amount of CH4 (mmol/g)

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0 20000

Bulk pressure (kPa)

Figure 10. (a) Loading amount of CH4 (black line) and sequestration amount of CO2 (read line) with the variation of bulk pressures in kerogen slit-nanopores at 323 K, with the corresponding snapshots of the residual gases in kerogen slit-nanopores at the bulk pressure of 6 (b) and 20 (c) MPa. The microscopic property of the residual CH4 displaced by CO2 in kerogen slit-nanopores at different CO2 injection pressures is shown in fig 10a, along with the snapshots of the equilibrium state after displacement at bulk pressures of 6 and 20 MPa at T = 323 K (fig 10b and c). The efficiency of displacement of CH4 by CO2 is performed by calculating the ratio (Rdis) of the displaced CH4 molecules from the initial residual adsorbed CH4 molecules in kerogen slit-nanopores that described as,

=

(6)

in which Ndis is the amount of CH4 molecules displaced by CO2 from the nanopore, and N’re is the initial amount of the residual CH4 adsorbed in nanopore after desorption (P = 5.0 MPa, T = 323 K). As is shown in fig 10a, it is found that the residual CH4 can be displaced gradually by CO2 with the bulk increasing pressure, and at the bulk pressure of 20 MPa, the Rdis could reach about 84%. Meantime, according to the research work of Yuan et al.,50 almost all the CH4 could be displaced by CO2 in


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the CNTs. As the research study by Zhang et al.,41 about 78% of CH4 could be displaced by CO2 in montmorillonite-methylnaphthalene based shale nanopores. And in our previous work about the displacement of CH4 by CO2 in calcite slit-nanopores, the Rdis could reach over 90%.55 All these detailed evidences that given by the research have demonstrated that the CH4 can be displaced by injecting CO2 in nanopores, and the differences of the efficiency could be attributed to the different nanoporous matter, pore structure and the uncertain displacement conditions. Besides, fig 10a demonstrates that CO2 can be captured and reserved in kerogen slit-nanopores during the displacement process, and the sequestration amount of CO2 gets enhanced with the enlarged bulk pressure. Figure 10b and c also clearly indicate that, the residual CH4 can be displaced by the CO2 flow, while a small number of CH4 molecules (Ads-matrix) is still adsorbed and fixed in the original adsorption position, which is hard to be displaced by injecting CO2. 4. CONCLUSION In this study, the microscopic adsorption and diffusion properties of CH4 and CO2 in kerogen slit-nanopores are investigated, and the displacement of CH4 by CO2 is performed by molecular dynamics simulations. It is found that gas molecules adsorbed in kerogen slit-nanopores can be divided into three part: the Ads-surface, Ads-central and Ads-matrix part. Meantime, the self-diffusion of gas molecules in kerogen slit-nanopores can be demonstrated as: MSD-matrix < MSD-surface < MSD-central. A competitive adsorption of CO2 over CH4 in kerogen slit-nanopores broadly occur, due to the stronger adsorption capacity of CO2 than CH4 in nanopores, and the difference



of adsorption energy of CH4 and CO2 on kerogen fragments. The isosteric heat of CH4 and CO2 in kerogen slit-nanopores is compared to give the evidence. The micro-behaviors of the residual CH4 displaced by CO2 in kerogen slit-nanopores are investigated, which is found that the displacement efficiency gets enhanced with the enlarged bulk pressures, accompanied by the CCS in kerogen slit-nanopores during the displacement process. Moreover, it is found that a small part of CH4 molecules still be adsorbed firmly inside the intrinsic pores of the kerogen-matrix after the displacement by CO2, and are hard to be displaced by the injected CO2. This study provide detailed micro-informations about the adsorption and diffusion behaviors of CH4 and CO2 molecules in kerogen slit-nanopores, accompanied by the efficiency of the displacement of CH4 by CO2, which should be helpful for better understanding about the microscopic states of gas molecules in shale, and might give out useful guidance for shale gas extraction by injecting CO2. ASSOCIATED CONTENT Supporting Information Detailed parameters of gases and fragments of kerogen, equilibrated configuration snapshots of gases in kerogen-matrix, and the model of the displacement of CH4 by CO2. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes


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The authors declare no competing financial interests. ACKNOWLEDGEMENTS The funding of National Science Fund of China (No. 21473103 and 61575109) are gratefully acknowledged.



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Molecular Insights into the Enhanced Shale Gas Recovery by Carbon

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