CH4 Competitive Adsorption on Shale

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C: Energy Conversion and Storage; Energy and Charge Transport

Molecular Simulation of CO2/CH4 Competitive Adsorption on Shale Kerogen for CO2 Sequestration and Enhanced Gas Recovery Tianyu Wang, Shouceng Tian, Gensheng Li, Mao Sheng, Wenxi Ren, Qingling Liu, and Shikun Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02061 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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The Journal of Physical Chemistry

Molecular Simulation of CO2/CH4 Competitive Adsorption on Shale Kerogen for CO2 Sequestration and Enhanced Gas Recovery Tianyu Wang a, Shouceng Tian a,*, Gensheng Li a, Mao Sheng a, Wenxi Ren a, Qingling Liu a,b, and Shikun Zhang a a

State Key Laboratory of Petroleum Resources and Prospecting, China University of

Petroleum, Beijing 102249, China b

Department of Petroleum and Geosystems Engineering, University of Texas at

Austin, Austin, TX 78712, USA

ABSTRACT The adsorption behavior and underlying mechanism of CO2 and CH4 binary mixture in shale kerogen significantly affect the CO2 sequestration with enhanced gas recovery project (CS-EGR). In this study, we investigated the competitive adsorption behaviors of CO2 and CH4 in shale kerogen nanopores using grand canonical Monte Carlo (GCMC) method. Kerogen model takes into the effect of matrix and slit nanopores and moisture content based on Ungerer’s molecular model and Scanning Electron Microscope (SEM) analysis, and is successfully validated against experimental data. The effects of temperature, CO2 and CH4 distribution, moisture content, adsorption selectivity, and optimal formation for injection were discussed. The results show that adsorption amount of CH4 on the kerogen increases with increasing pressure and decreases with increasing temperature. The adsorption 1

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selectivity of CO2 over CH4 is 2.53–7.25, which indicates that CO2 is preferentially adsorbed over CH4 under different temperatures. H2O prefers to adsorb inside the kerogen matrix and decrease the volumes of matrix pores with increasing moisture contents, and even divide some of them into ineffective pores. Compared with kerogen matrix, H2O molecules have slightly effect on CO2 and CH4 adsorption capacity on the slit surface. Moist content have a negative effect on desorption amount of CH4. The optimal injection formation for CS-EGR project is in the shallow stratum. The study will reveal the micro mechanism of competitive adsorption of CO2 and CH4 on kerogen and provide some theoretical support for CS-EGR project.

1. Introduction Shale gas, due to the advantages of large reserves and high energy efficiency, is a promising alternative energy resource for global energy crisis

1-2

. Profitable

production in shale gas reservoirs through advanced horizontal drilling and hydraulic fracturing technologies improves the necessity for a deeper understanding of shale gas adsorption behavior

3-4

. Injecting CO2 into geological structures for storage has been

recognized as a remarkable technique for reducing carbon emissions and mitigating global warming 5-6. Therefore, Enhancing shale gas recovery by injecting CO2 brings the benefit for the combination of shale gas reservoir exploitation and CO2 geological storage. The percentage of adsorbed shale gas to the total gas-in-place (GIP) varies from 20% to 85% 7. Adsorbed shale gas is highly related to the organic matter, and clay minerals have less contribution to the shale gas adsorption 8-9. The hydrophilic nature 2


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of clay minerals considerably reduces the gas adsorption capacity

10-11

. However, due

to the complex chemically heterogeneous structure of kerogen and the complicated processes occurring in interfaces between the gas and kerogen

12

, a better

understanding of the adsorption behaviors associated with kerogen is still a great challenge. Besides, the microscopic mechanism displacement of CH4 by CO2 in authentic kerogen is still unclear, which is important for optimizing the formation of CO2 injection. Kerogen is sedimentary organic matter which insoluble in common polar solvents, and it shows a wide range of chemical compositions 13-15. Moreover, moisture of organic matter is one of the key parameter associated with competitive adsorption. The carbon skeleton in kerogen is hydrophobic, while the carboxylic and hydroxylic functional groups are hydrophilic which leads the kerogen mixed-wet

16-17

kerogen is originally moisture equilibrated under reservoir conditions

18

moisture content is even increased by hydraulic fracturing

. Shale

and the

19

. Pore throats and gas

sorption sites in kerogen can be occupied by moisture content, which will greatly reduce the gas adsorption capacity 20-21. A series of isothermal adsorption experiments were conducted to evaluate the adsorption properties of CH4 and CO2

8, 10, 22-24

.

However, it is the macroscopic adsorption behaviors and failed to deeply analyze the adsorption mechanism on microscopic theory of competitive adsorption of CH4 and CO2 on kerogen. Meanwhile, due to the structures of kerogen with nanometers and limit in experimental methods and conditions, many unclear or unknown problems still exist in the study of adsorption on shale. Molecular simulation is a natural choice 3



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to complement experimental approaches. It is an important tool for understanding the micro behavior of gases in complex porous media at microscopic level intuitively and accurately

25

. To reveal the microscopic mechanism of CS-EGR, it is necessary to

perform the research of CO2/CH4 competitive adsorption behaviors in nanopores of kerogen. Some of the simulations are performed on simplified shale organic model like graphene slit, carbon nanotubes and nanoporous carbons

26-29

. However, the

simplified shale organic model failed to accurately capture the chemically heterogeneous nature of authentic kerogen 30. Recently, investigations have been performed on the CO2/CH4 competitive adsorption on realistic kerogen models. Sui et al. 31 simulated the CO2/CH4 adsorption behaviors in kerogen nanopores, and discussed the volumetric strain of kerogen, which indicating that CO2 adsorption induced swelling on volumetric strain is bigger than CH4. Zhao et al. 32 studied the influence of maturity and moisture content on the adsorption characteristics of shale under the reservoir conditions, suggesting that the presence of the moisture content can greatly decrease the CH4 adsorption and has a greater effect on the kerogen with high maturity. Huang et al. 33 examined the effect of organic type and moisture on CO2/CH4 competitive adsorption in kerogen. Unfortunately, the effect of slit nanopores was not discussed in their work. In practice, kerogens usually have pore spaces that combine many length scales

34-37

. We

conducted Scanning Electron Microscope (SEM) analysis to investigate the pore size of shale, as shown in Fig. 1. It shows that in the case of shale kerogen, nanopores are not merely in the matrix, but also exist as slit pores. A series of research have revolved 4


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around adsorption behaviors of kerogen matrix

31-33, 38-39

. The effect of slit nanopores

is closely correlated with CO2/CH4 competitive adsorption. However, there are only few reported papers focusing on this topic 34, 40. Falk et al. 34 performed the adsorption in systems with a combination of nanopores and mesopores. Sun et al. 40 investigated the CO2/CH4 competitive adsorption behaviors of kerogen for enhancing shale gas extraction by injecting CO2. Although the effect matrix and slit nanopores was analyzed, the moisture content was not explored. The conventional model is inadequate to describe adsorption of CH4 on shale accurately. The microscopic mechanisms of moisture on the CO2/CH4 competitive adsorption characteristics in realistic kerogen matrix and slit nanopores remain undetermined. Therefore, a more realistic model capturing effect of slit nanopores and moisture content is urgently needed.

Fig. 1. Diversity of nanopores in shale kerogen. In this work, we first built a realistic kerogen molecular model. Next, we validated the model by experimental data from literatures. Then the grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) method were used to investigate the adsorption behavior of CO2/CH4 in shale kerogen. We also study the impacts of 5



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temperature, radial distribution function (RDF), and the distribution of water in kerogen on the competitive adsorption of CO2/CH4. Finally, we discussed the optimization of injection depth for CS-EGR project. The results can provide theoretical and instructional significance for CS-EGR project.

2. Models and methodology 2.1. Kerogen structure In this study, the molecular model of kerogen was developed by Ungerer et al.

41

based on the analysis data from Kelemen et al. 42. Kerogen molecule built to represent kerogen is indicated in Figure 2. The kerogen is a typical one which deposited in excellent conditions of preservation, such as anoxic lacustrine environments, and the composition is C251H385O13N7S3. As indicated in Table 1, the parameters of the model match analytical data of the X-ray and Solid-State 13C Nuclear Magnetic Resonance (NMR) with a reasonable accuracy 42. 10 basis molecules of kerogen were chosen to build a kerogen unit cell. Because of the irregularly combined fragments, kerogen is amorphous with the intrinsic nanopores inside and is regarded as rigid in the simulation

31

. We carried out high

temperature structural relaxation with NVT ensemble at 800 K for 300 ps with the time step of 1 fs for the configuration. After that, we use the MD method with NPT ensemble with a stepwise decreasing temperature from 800 to 300 K at 20 MPa

33, 43

Different moist kerogen models are built in Sorption module of Materials Studio (MS).

6


.

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Fig. 2. (a) Model of the kerogen slit nanopore with two kerogen matrices. (b) Kerogen molecules developed by Ungerer et al. 41 based on analysis data from Kelemen et al. 42. Color scheme: grey, white spheres, red, blue and yellow represent carbon, hydrogen, oxygen, nitrogen and sulfur atoms, respectively. Table 1 Composition and structural parameters of kerogen. Property

Quantity

Analytical Model unit

Composition

H/C

1.53

1.53

O/C

0.051

0.052

N/C

0.029

0.028

S/C

0.014

0.012

Aromatic C from XPS or NMR (%)

29

29

C atoms per aromatic cluster

16

14.6

Fraction of attached aromatic C

0.4

0.4

Protonated aromatic C (per 100 C)

7

6.3

O in C-O per 100 C

3.8

4.0

O in carboxylic groups (-COOH) per 100 C

0.8

0.8

O in carbonyl groups (>C=O) per 100 C

0.5

0.4

Pyrrolic (mol%)

57

72

Pyridinic (mol%)

20

14

C group

O group

N group

7



S group

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Quaternary (mol%)

13

14

Aromatic S (mol%)

35~45

33

Aliphatic S (mol%)

55~65

67

2.2. Simulation details The absolute adsorption isotherms were calculated by the GCMC method, which is carried out by the Sorption package in MS software. We adopted the COMPASS 44 force field for all simulations. There were four types of trial moves including exchange, conformation, rotation and translation. We used Metropolis algorithm to describe the acceptance or rejection of a trial move . The electrostatic potential and van der Waals (vdW) potential represented the nonbonding interactions. The electrostatic potential was described by the Ewald method and the Lennard-Jones 9-6 potential with a cutoff distance of 12.5 Å was used to perform the vdW interaction. Each simulation was performed with 1 × 107 steps. The former 5 × 106 steps were used to guarantee equilibrium, and the latter 5 × 106 steps were performed to calculate the thermodynamic parameters.

3. Results and discussion 3.1. Model validation We validated the model and simulation methods by comparing our simulation results with the experimental data from literatures. The laboratory experiments measure excess adsorption capacity while GCMC simulation gives absolute adsorption capacity. Several researchers32-33, 45 assume that the adsorbed phase volume is equal to the pore volume. However, the validity of this assumption is questionable in slit model. The shale gas adsorption in micropores involves the volume filling of 8


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the adsorption space. However, monolayers of adsorbed gas are mainly formed in mesopores

46

. In our slit model, the adsorbate molecules are not filling all of the

adsorption space, which indicates the adsorbed phase volume is lower than the pore volume. Thus, in our model, the adsorbed phase volume in shale matrix is limited by the “Connolly surface” probed by CH4, and for the slit pore, it is calculated based on monolayers of adsorbed gas. We converted the absolute adsorption capacity to excess adsorption capacity by the following equation47: nexc = nab s - ρVad s / M

(1)

where nexc is the excess adsorption capacity, mol/kg; nabs is the absolute adsorption capacity, mol/kg; Vads is the adsorption volume, m3/kg; ρ is the equilibrium density of CH4, kg/m3; M is molar mass of the gas, kg/mol.

Fig. 3. Comparison between the experimental data and simulated results at 338 K. Figure 3 plots the comparison between the experimental data and the simulated results at 338 K. The experimental data we used for comparison in this part are measured by Gasparik et al 8. As can been see in Figure 3, the simulated results agree satisfactorily with the experimental data, which validates our model and GCMC

9



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simulations. There are slight differences probably because the effects of thermal maturity and kerogen type play an important role in shale adsorption according to the experimental researches by Lu et al

29

. The comparisons indicate that the present

model, which considers both of the matrix and slit nanopores, can describe the real shale kerogen nanopores and be used to quantify CO2/CH4 competitive adsorption.

3.2. Competitive adsorption of CH4 and CO2 on kerogen Fig. 4a depicts the adsorption isotherms of pure CH4 on dry kerogen at different temperature. The CH4 adsorption capacity on kerogen increases with increasing pressure and decreases with increasing temperature. The absolute adsorption isotherms of CO2 after injecting CO2 into shale kerogen (mole fraction is 0.5) are shown in Fig. 4b. Similar to the CH4 adsorption on kerogen, adsorption isotherms of CO2 are affected by temperature and pressure. The difference is that with increasing pressure, the adsorption isotherms of CO2 increases sharply compared with CH4. As can been see in Figure 4, both CH4 and CO2 absolute adsorption isotherms correspond to type I isotherm, and fit well with the Langmuir model 48. The driving force of gas concentration gradient causes the desorption of CH4 in shale formation. CO2 is used as the injected-gas to replace the residual CH4 in shale gas reservoirs. The CH4 replacement is calculated by the following equation ndes = nad - nre

(4)

where nad is the adsorption amount of gas in shale nanopores of the initial pressure, and nre is the adsorption amount of the residual gas after injecting CO2. Fig. 5 depicts CH4 desorption on dry kerogen at different temperature. Desorption of CH4 increases 10


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with increasing pressure and recedes with increasing temperature. The CH4 adsorption capacity decreases for the CO2 and CH4 binary mixture. This is because that, with the decreasing mole fraction of CH4 and reducing adsorption space, the CH4 adsorption capacity decreases gradually 49.

Fig. 4. Absolute adsorption isotherms of CH4 and CO2 on dry kerogen matrix at different temperature. (a) Absolute adsorption isotherms of pure CH4; (b) Absolute adsorption isotherms of CO2 with yCO2 = 0.5 after injecting CO2 into shale kerogen.

Fig. 5. Desorption of CH4 on dry kerogen matrix at different temperature. Adsorption is known to cause a differential swelling of kerogen, and the swelling can cause a closure of the cleat network which makes a reduce of the permeability of 11



shale50. Brochard et al.

28, 51

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developed an extended poromechanical model to study

the effect of adsorption on shale swelling:

ε=-

p C + K K

∫

p

0

nVbulk dp

(5)

where ε is volumetric strain, p is bulk pressure, Vbulk is the volume, K = 2.65 GPa is the bulk modulus, C = 6.05 for CH4 is the coupling coefficient52-53. Fig. 6 shows the volumetric strain of kerogen at different pressures at the temperature of 298 K, 328 K, 358 K and 388 K immersed in CH4. The general trend in all temperature is that the volumetric strain increases up to maximum as pressure increase, and then decrease gradually as pressure increase. This phenomenon can be explained as follows: swelling induced by adsorption and mechanical compression would have the opposite effect on the volumetric strain. At low pressure (p < 8 MPa), swelling induced by adsorption has a primary role which results in increasing of the volumetric strain of shale kerogen. At high pressure (p > 15 MPa), the influence of mechanical compression is getting more important which lead to decrease of the volumetric strain of shale kerogen.

Fig. 6. Volume strain of kerogen at different pressures and temperature 12


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Fig. 7 depicts the CH4 and CO2 distributions in kerogen nanopores under different pressures. For efficiency of observation and convenience to observers, we magnified the CH4 and CO2 molecules in kerogen model. It is found that the adsorption of CH4 and CO2 inside the kerogen matrix creates a slight variation, whereas it increases significantly in the slit nanopore with pressure increases. CH4 and CO2 adsorb in both the kerogen matrix and slit pores and the molecular density increases with increasing pressure. Specifically, at low pressures, the adsorption of CH4 and CO2 molecules almost the same between the kerogen matrix and slit nanopores, as shown in Fig. 6a and b. With pressure increases, the adsorption of CH4 and CO2 in kerogen matrix no longer increase, whereas there are more CH4 and CO2 molecules adsorbed in kerogen slit nanopore. It indicates that pore size in kerogen matrix restricts the adsorption of CH4 and CO2 and slit nanopores significantly affect the adsorption under high pressure, as shown in Fig. 6c and d.

Fig. 7. The snapshots of the CO2/CH4 in kerogen slit at different pressures. (a) 1 MPa; (b) 6 MPa; (c) 16 MPa; (d) 30 MPa. In general, radial distribution function (RDF), defined as the ratio of the number of atoms at a distance from a given atom compared with the number of atoms at the same distance in an ideal gas with the same density 54, acquires structural information 13



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of gas adsorbed on shale kerogen. We compare the RDFs between CH4/CO2 and atoms in the kerogen model to study the affinity between the two particles, as shown in Figure 8. The close contact peaks between CH4 and S atoms and CH4 and N in kerogen model are higher than these between CH4 and other atoms. This indicates that nitrogen- and sulfur-containing functional groups are the higher energy sites for CH4 in kerogen. However, the RDFs between CO2 and kerogen atoms show that peaks between CO2 and S atoms and CO2 and O atoms are higher than these between CO2 and other atoms, which is consistent with previous studies

33, 54

. It indicates that

compared with CH4, oxygen-containing functional groups have a stronger affinity for CO2. This was also found in previous study by Dang et al.

55

. Moreover, the RDFs

between CO2 and kerogen atoms and between CH4 and kerogen atoms show that the first peak of CO2 is sharper than that of CH4, which means the stronger interaction between the CO2 and kerogen atoms than CH4.

Fig. 8. RDFs between gas and atoms in the kerogen model (atom C, H, O, N, and S). (a) RDFs between CH4 and kerogen atoms; (b) RDFs between CO2 and kerogen atoms. 14


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3.3. Adsorption selectivity of CO2 over CH4 Adsorption selectivity, which represents the ratio of the mole fractions of the two species in adsorbed phase relative to the ratio of the mole fractions in bulk phase54, is a criterion to assess the relative adsorption priority between CH4 and CO2 in binary mixture47. The greater the adsorption selectivity of CO2 over CH4, the more preferential adsorption of CO2 over CH4. The adsorption selectivity of CO2 over CH4 on shale kerogen is defined as the follow equation: SCO2 / CH 4 =

xCO2 / xCH4

(6)

yCO2 / yCH 4

where xCO2 and xCH4 denote the average mole fraction of component CO2 and CH4 quantities of adsorbed phase, and yCO2 and yCH4 denote the average mole fraction of component CO2 and CH4 quantities of bulk phase, dimensionless. Fig. 9 shows the adsorption selectivity of CO2 over CH4 on shale kerogen in function of the pressures at different temperature. All the adsorption selectivity values are greater than one. This indicates that CO2 is preferentially adsorbed on kerogen over CH4 in function of the pressures at different temperature. The simulated results of selectivity (2.53–7.25) are fairly close to the reported value (1.87–8.9)

33, 54

. The

adsorption selectivity initially gets enhanced with increasing pressure, then recedes quickly and finally tends to stays at a constant. This regular is consist with previous studies 33, 54-55. Wang et al. 47reported that the adsorption selectivity stays at a constant as the pressure reaches the critical pressure (7.38 MPa for carbon dioxide). The effect of temperature on adsorption selectivity of CO2 over CH4 is further investigated. Fig. 9 also shows that the adsorption selectivity values decrease with increasing 15



temperature. This can be ascribed to the fact that the CO2 adsorption decreases faster than that of CH4 on shale kerogen in function of the pressures.

Fig. 9. CO2/CH4 adsorption selectivity values on kerogen at different pressure 3.4. Effect of moisture on mixed gases adsorption Moist kerogen is modeled in Sorption module of MS software. To be consistent with the kerogen moisture of previous studies

56-57

, we generated moist kerogen

models with different moisture contents (0.6, 1.2, 1.8 and 2.4 wt%) in this work. Fig. 10 depicts the H2O distributions in kerogen slit model under different moisture contents. For efficiency of observation and convenience to observers, we magnified the H2O molecules in kerogen slit model. In addition to adsorption inside the kerogen matrix, a small number of H2O can be adsorbed in the slit nanopore. H2O prefers to adsorb inside the kerogen matrix and the molecular density increases with increasing moisture contents. Specifically, at low moisture contents, most of H2O molecules adsorb inside the kerogen matrix, as shown in Fig. 10a-c. When the moisture contents reach at 2.4 wt%, there are some H2O molecules in kerogen slit nanopore, and H2O mainly distributes close to the pore surface. To understand the effect of moisture on 16


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CO2 and CH4 competitive adsorption behaviors on kerogen more thoroughly, it is necessity to study the distribution of H2O in the kerogen matrix. Fig. 10 shows the pore volumes distributions of kerogen matrix under different moisture contents. With increasing moisture contents, the pore volumes gradually decrease. As shown in Fig. 11a and b, H2O has little influence on the pore volumes at low moisture content. However, when the moisture content is higher, the pore volumes are greatly affected, as shown in Fig. 11c and d. This indicates that H2O molecules adsorb in the kerogen matrix pores and decrease the volumes of these pores with increasing moisture content. Moreover, H2O molecules even divide some effective pores into ineffective pores. The similar result was also presented by Huang et al.

33

with the molecular

simulation.

Fig. 10. The snapshots of the H2O in kerogen slit at different moisture contents. (a) 0.6 wt% moist kerogen slit model; (b) 1.2 wt% moist kerogen slit model; (c) 1.8 wt% moist kerogen slit model; (d) 2.4 wt% moist kerogen slit model.

17



Fig. 11. Pore volumes distributions of kerogen at different moisture contents. (a) 0.6 wt% moist kerogen slit model; (b) 1.2 wt% moist kerogen slit model; (c) 1.8 wt% moist kerogen slit model; (d) 2.4 wt% moist kerogen slit model. Pore volumes are colored in blue. Fig. 12 shows the RDFs between H2O and atoms (C, H, O, N and S) in kerogen model at the moisture content of 2.4 wt%. The contact peak between H2O and C atoms in kerogen and between H2O and H atoms are much smaller than the peaks between H2O and S atoms and H2O and O atoms, showing weak interaction between H2O and C atoms or H atoms in kerogen. This indicates that the H2O molecules are strongly adsorbed around sulfur- and oxygen-containing groups in kerogen model.

18


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Fig. 12. RDFs between H2O and atoms in the kerogen model (C, H, O, N, and S). The effect of moisture contents on CO2 and CH4 adsorption at 30 MPa is shown in Fig. 13. With moisture contents increasing, the adsorption capacity of CO2 and CH4 gradually decrease. Specifically, the adsorption capacity of CO2 and CH4 decrease 9.91% and 20.55%, respectively. This is because that the H2O molecules clusters occupied in the kerogen nanopores and even divide them into ineffective pores. Furthermore, H2O molecules are preferentially adsorbed on the hydrophilic groups, converting the interaction between adsorbate and adsorbent to the interaction between adsorbate and liquid through the steric effect

45

. Previous studies

32-33, 45

have

researched the effects of moisture contents on CO2 and CH4 adsorption capacity by GCMC simulation, and the decrease of CO2 and CH4 adsorption capacity is slightly higher than our results. This phenomenon can be explained as follows: (1) compared with their kerogen models, our model is immature kerogen; (2) H2O molecules prefer to adsorb inside the kerogen matrix and moisture has a small influence on the slit of kerogen, which results in that H2O molecules have slightly effect on CO2 and CH4 adsorption capacity on the slit surface, as shown in Fig. 9. In addition, we compute the 19



CO2/CH4 adsorption selectivity under different moisture contents. Fig. 13 suggests that the adsorption selectivity of CO2 over CH4 has no obvious correlation with moisture content. Weniger et al. 58 and Huang et al. 33 reported a slight variation in the CO2/CH4 adsorption selectivity. However, the results change little under different moisture contents. For shale kerogen slit nanopores, the effect of moisture contents on adsorption selectivity is not obvious.

Fig. 13. CH4 and CO2 adsorption capacity and adsorption selectivity of CO2 over CH4 in the mixtures under different moisture contents at 30MPa with yCO2 = 0.5. 3.5. Implication for CS-EGR For CS-EGR project, CO2 is pumped into the shale reservoir firstly, and then it diffuses in shale cracks and pores so that it is delivered to the adsorption sites. Because of the preferential adsorption behavior of CO2, it replaces CH4 on the adsorption sites of kerogen surface, and CH4 are desorbed from the kerogen. Finally, CH4 will be extracted from the developed mixture species, and CO2 will be recycled into the injection well. Herein, the pressure and temperature conditions as a function of depth can be estimated by the pressure coefficient (10 MPa/km) and the geothermal 20


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gradient (25 °C/km)59. The effect of pressure and temperature on the CS-EGR efficiency can provide some fundamental guidance for the designing of injection of CO2. The adsorption capacity of CH4 and CO2 for dry and moist kreogen in various buried depth are plotted in Fig. 14a. Pure CH4 adsorption and competitive CH4 adsorption (mole fraction of CH4 is 0.5) increase with buried depth increasing. However, competitive CO2 adsorption capacity increase first until it reaches maximum, and then CO2 adsorption is substantially retained with increasing formation depth. This is because that, both temperature and pressure have effect on adsorption capacity of CO2. The adsorption capacity increases with increasing pressure, while decreases with increasing temperature. At shallow formation，the effect of temperature on adsorption is negligible whereas it becomes obvious and dominates over the effect of pressure once the site gets deeper. However, it cancels out the effects of temperature and pressure on CO2 adsorption at deep formation. Desorption amount of CH4 for dry and moist kerogen in various buried depth are shown in Fig. 14b. Similar with the adsorption capacity of CO2, the desorption amount of CH4 first increases quickly, and then shows a slight changes with increasing formation depth. Moist content have a negative effect on desorption amount of CH4. Specifically, the desorption of CH4 decrease 11.67%, 8.92% and 8.25% for the site of 1000 m, 2000 m and 3000 m, respectively. Based on the analysis of adsorption capacity of CO2 and desorption of CH4, the optimal injection formation for CS-EGR project is in the shallow stratum, for example 1000~2500 m, which is consist 21



with our previous study in shale matrix

47

. The more CH4 can be desorbed and

extracted, the more profit will be obtained, and the drilling cost increase with increasing well depth. Thus, our study can provide some theoretical support for CS-EGR project.

Fig. 14. (a) Adsorption capacity of CH4 in various buried depth. (b) Desorption of CH4 for dry kerogen and 1.8 wt% in various buried depth.

4. Conclusions A realistic shale kerogen molecule model with various moisture contents is used to investigate the competitive adsorption behaviors of CO2 and CH4 in shale kerogen for CS-EGR project. The model is successfully validated againt experimental data from literatures. The study offers a theoretical base for the demonstrated CO2 sequestration and enhanced gas recovery, and can be applied to reliably predict adsorption of CO2 and desorption of CH4 in shale kerogen. Major conclusions are summarized as follows. The CH4 and CO2 adsorption isotherms correspond to type I isotherm, and fit well with the Langmuir model. Compared with CH4, oxygen-containing functional groups have a stronger affinity for CO2. The adsorption selectivity of CO2 over CH4 is 2.53– 22


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7.25 which indicates that CO2 is preferentially adsorbed over CH4 under different temperatures. H2O prefers to adsorb inside the kerogen matrix and decrease the volumes of these pores with increasing moisture contents, and even divide some of them into ineffective pores. Compared with kerogen matrix, H2O molecules have slightly effect on CO2 and CH4 adsorption capacity on the slit surface. Moist content have a negative effect on desorption amount of CH4. The optimal injection formation for CS-EGR project is in the shallow stratum, for example 1000~2500 m. In our future research, we intend to focus on the effect of organic type of kerogen on CO2/CH4 competitive adsorption. Several researchers have applied the simulations in shale matrix, and we will refer to both the matrix and slit nanopores in shale.

Acknowledgements This work was financially supported by the General Projects of the Natural Science Foundation of China (No. 51674275) and by the State Major Program of the National Science Foundation of China (No. 51490652).

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CH4 Competitive Adsorption on Shale

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