CH4 Competitive Adsorption in Various Clay

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Characterization of CO/CH Competitive Adsorption in Various Clay Minerals in Relation to Shale Gas Recovery from Molecular Simulation Xiaofei Hu, Hucheng Deng, Chang Lu, Yuanyuan Tian, and Zhehui Jin Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01610 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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Characterization of CO2/CH4 Competitive Adsorption in Various Clay Minerals in Relation to Shale Gas Recovery from Molecular Simulation

Xiaofei Hu1,2,3, Hucheng Deng1,3*, Chang Lu2, Yuanyuan Tian1, Zhehui Jin1,2,3*

1College 2School

of Energy, Chengdu University of Technology, Chengdu 610059, China

of Mining and Petroleum Engineering, Faculty of Engineering, University of Alberta, Edmonton T6G 1H9, Canada

3State

Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology), Chengdu 610059, China

1

ACS Paragon Plus Environment

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KEYWORDS: CO2/CH4 mixture, Competitive adsorption, Clay minerals, Stratigraphic conditions, Enhanced gas recovery, Molecular simulation ABSTRACT CO2 sequestration and enhanced gas recovery (CS-EGR) is a viable option with enormous potentials to produce shale gas. However, the microscopic competitive sorption behaviors of CH4 and CO2 in various clay minerals which are an important constituent of shale at actual formation conditions are still less clear. In this work, we study CO2/CH4 binary mixture competitive sorption in various clay minerals (montmorillonite, illite, and kaolinite) by using grand canonical Monte Carlo (GCMC) simulations. The effects of the clay mineral types and possible stratigraphic conditions, considering temperature, pressure, CO2/CH4 molar fraction, and selectivity are discussed in detail. The results demonstrate that the CO2 sorption capacity in the clay mineral follows an order of montmorillonite > illite > kaolinite. CO2 molecules are prone to adsorb on the surfaces of montmorillonite and illite nanopores with cation exchange than in the kaolinite without cation exchange. Moreover, cation exchange could distinctly increase the CO2/CH4 adsorption ratio so that the first layer of CH4 molecules can be displaced by CO2 molecules. The replacement ratio of CH4 is related to the type of adsorbent, which is independent of the original formation pressure. In addition, a case study is designed to quantify the enhanced gas recovery (EGR) and CO2-CH4 displacement efficiency. With a higher reservoir initial pressure when injecting CO2, the EGR of adsorbed CH4 gas could increase up to 28.97%. Our findings provide insights into gas mixture sorption in shale reservoirs and provide important guidelines for CS-EGR projects.

2


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1. Introduction The continuous depletion of conventional oil/gas reservoirs has stimulated extensive shale gas exploitation activities in the past decade 1-3. Shale has the characteristics of extremely low permeability and complex rock compositions, resulting in a vastly different phase behavior from that of conventional formations 4-6. In addition, exploitation techniques and enhanced gas recovery (EGR) processes have greatly advanced. CO2 sequestration with enhanced gas recovery (CS-EGR) has received much attention from scientists and engineers and has been considered an economically feasible and technically viable approach for shale gas recovery 710.

Understanding the mechanisms of CO2/CH4 competitive sorption in shale nanopores plays

a crucial role in the large-scale CS-EGR technique and reduction in greenhouse gas emissions. At present, although the CS-EGR project in shale gas has not yet been commercialized, initial attempts in fields have been conducted geological reservoirs

12,

11.

The long-term CO2 sequestration in different

including gas-rich and oil-rich shale reservoirs, active or exhausted

conventional fields, deep saline aquifers, and unminable coal seams, have been given priority as the potential spots

13.

On the other hand, the CO2 injection technique can be utilized for

reservoir fracturing, which is an alternative to hydraulic fracturing

14.

Hydraulic fracturing

might be harmful to shale reservoir development due to clay swelling, which could seal pores and reduce the porosity

14.

In addition, hydraulic fracturing might cause underground water

pollution and potential earthquakes

15-17.

Compared to hydraulic fracturing, CS-EGR is an

environmental friendly technique that has numerous advantages 14, including (1) less damage to the shale reservoir, (2) lower initial fracturing pressure, (3) more complex and branched fracturing networks, (4) higher flowback rate of the fracturing fluid, and (5) potential methane 3


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displacement due to carbon dioxide injection. The CS-EGR technique has been investigated by field work, numerical simulations, experimental analysis methods, and molecular simulations. In 2018, field work was conducted in Ordos Basin of China utilizing three main techniques for CO2 injection 14, including CO2 foam fracturing, dry fracturing, and a combination with hydraulic fracturing. The CS-EGR technique has been used for enhanced lacustrine shale gas recovery in the Ordos Basin, which has resulted in a major breakthrough 14. Nevertheless, in field work, it is difficult to unlock the underlying mechanisms of EGR due to CO2 injection, in which CO2 can displace CH4 in shale nanopores. A number of numerical simulations and methods have been employed in reservoirs 18-20,

and the approaches can be used to investigate the performance of the CS-EGR project at

the actual production field. In particular, the numerical simulation approach has been used to study the effects of formation heterogeneity on performance of CS-EGR

21-24.

Numerical

simulations indicate that injecting CO2 into shale reservoirs for EGR is technically desirable 11.

Nevertheless, the main purpose of numerical simulation is to predict oil and gas production

under different production schemes, where the microscopic interactions of fluid-fluid and fluidrock surface cannot be revealed. The experimental analysis method is widely used to quantitatively study the CS-EGR technique. Some experimental results have led to the conclusion that CO2 injection is an effective method for CS-EGR

25-29.

While experiments

can measure the CH4 and CO2 competitive sorption in shale rocks from macroscopic perspectives, they cannot reveal the underlying mechanisms of sorption in nanosized pores. In addition, it is difficult to explicitly tailor the rock properties to study competitive sorption in various rocks. On the other hand, molecular simulation can capture the 4


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essence of the intermolecular interactions which play a central role in CH4 and CO2 competitive sorption in nanopores with well-defined pore property settings and reveal the underlying mechanisms from microscopic perspectives. Shale gas consists of free gas, adsorbed gas, and dissolved gas 30-32, in which the free gas and adsorbed gas account for a great proportion. The different gas storage mechanisms of gasin-place have been reported 32-34. Curtis 35 shows that adsorbed gas could contribute 20-85% of total gas-in-place in five shale reservoirs in the United States. Shale is composed of organic and inorganic materials. Both kerogen and clay minerals can contribute large amount of adsorbed gas

4,36-41.

Clay minerals can contain a significant amount of nanosized pores 42, in

which the fluid-surface interaction is strong and fluid distribution is inhomogeneous 43,44. Guo, et al.

45

contrastively studied the CH4 sorption capacity between kerogen and clays by

experiments and found that the maximum sorption in kerogen can several times of that in clay minerals. Xiong et al. 46 demonstrated that clay minerals could account for up to 70 wt% of a shale. Experimental measurements have shown that clay minerals could greatly offer the gasin-place volume in shale 40,47,48. Ji et al. 49 and Pang et al. 41 claimed that the specific surface area (SSA) is the major controlling parameter of CH4 sorption capacity in clay minerals. Some molecular simulation works have been implemented for studying CH4 sorption in dry 4,50,51 and moist

52,53

clay minerals. The results show that CH4 sorption in clay nanopores can be

comparable to that in kerogen 43. Thus, CH4 sorption in clay nanopores is an indispensable part of the gas-in-place in clay-rich shales. The performance of CO2/CH4 competitive sorption can reveal the potential of the CS-EGR project in various clay minerals. While there have been a number of works using molecular 5


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simulation approaches to study CO2/CH4 competitive sorption in organic nanopores 54-58, only limited number of works have been conducted on clay minerals. Yang et al.

13

used grand

canonical Monte Carlo (GCMC) simulations to investigate the sorption mechanism of CO2/CH4 mixtures on Na-montmorillonite with pressure up to 20 MPa. Lee et al.

59

used ab

initio molecular dynamics (AIMD) simulations to explore the mechanisms of intercalation and H2O/CH4/CO2 ternary mixture on Ca-montmorillonite at 323 K and 90 bar. Both studies show higher sorption capacity for CO2 over CH4 in Na- and Ca-montmorillonite nanopores. However, the pressure considered in these studies is much lower than the typical shale reservoir pressure. Chong and Myshakin 60 used Gibbs ensemble Monte Carlo (GEMC) simulations to study the CO2/CH4 mixture sorption behavior in illite nanopores at two temperatures (355 and 394 K) and pressures up to 60 MPa. While these works provided important insights into the competitive sorption between CO2 and CH4, comparative investigations on binary gas adsorption in various clay minerals under reservoir conditions have not yet been reported. In this work, three different clay mineral models are utilized, including montmorillonite, illite, and kaolinite, to comparatively investigate CO2/CH4 competitive sorption in nano-size pores. The CH4 sorption capacity, the sequestrated CO2, the selectivity of CO2/CH4 in clay mineral models, and possible stratigraphic conditions are discussed. Based on the simulation results, CO2 sequestration and enhanced CH4 gas recovery in shale reservoir is pragmatically quantified. This work could provide a quantitative analysis result and some implications for CS-EGR application and optimization. 2. Simulation 2.1 Molecular models 6


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In this study, we use montmorillonite, kaolinite, and illite with fixed surfaces as clay minerals. Montmorillonite is termed a 2:1 clay that contains two Si-O tetrahedral sheets and one AlO octahedral sheet. The neutral montmorillonite clay has the unit cell formula Si8Al4O20(OH)4 61.

The simulation box contains two sheets with duplicated 32-unit cells (8 × 4 × 1) to form

montmorillonite clay nanopores, resulting in a clay patch of 4.224 nm × 3.656 nm in x and y direction, respectively, with a thickness of 0.656 nm 4,43. All the positions and charges in the unit cell of montmorillonite are given by Skipper et al. 62. The fixed distance of two clay sheets in the unit cell represents the pore size in the z direction 4. Similar montmorillonite interlayer structures with different cation exchanges, such as Na and Ca, have been studied to reveal the intercalation and hydration behavior

63-65

as well as CH4 or/and CO2 sorption by molecular

simulation 4,13,39,43,66. In this work, the montmorillonite model contains cation exchange so that the unit cell formula is Na0.75(Si7.75Al0.25)(Al3.5Mg0.5)O20(OH)4

67.

Accordingly, in the

octahedral sheet of each 32-unit cell, the divalent Mg atoms replace 16 isomorphous trivalent Al atoms; identically, the trivalent Al atoms substitute 8 isomorphous tetravalent Si atoms in the tetrahedral sheet; and 24 compensating monovalent sodium ions in the interlayer region that represent the cation exchange. The clay with cation exchange is discussed by Chávez-Páez, et al. 67 Illite is also 2:1 clay that consists of one Al-O octahedral layer and two Si-O tetrahedral layers 68. The unit cell formula of neutral illite clay is Si8Al4O20(OH)4, which contains 40 atoms. Similar to montmorillonite, illite consists of two sheets containing 32-unit cells (8 × 4 × 1), which form the clay slit-like nanopore with dimensions of 4.128 nm × 3.584 nm in the x and y 7


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directions, respectively 4,43. The coordinates of each atom are obtained from pyrophyllite-1Tc powder diffraction 68-70. Chong, et al. 60 used K-illite to study CH4 and CO2 sorption. Poinssot, et al.

71

investigated Cs, Sr, Ni and Eu sorption in Na-illite by experimental methods. In this

work, we use illite with potassium cation exchange, whose chemical formula is K(Si7Al)Al4O20(OH)4 72, as illite nanopore models in our simulation. In each unit cell, one Si atom in tetrahedral layers is replaced by one Al atom, where the clay sheet contains a negative charge 43, which is neutralized by monovalent potassium ions. Unlike montmorillonite and illite, kaolinite is a 1:1 clay that contains single Si-O tetrahedral layer and a single Al-O octahedral sheet 43. The chemical formula of the kaolinite unit cell is Al4Si4O10(OH)8 73. As in montmorillonite and illite, each kaolinite sheet consists of 32-unit cells (8 × 4 × 1) with a surface area of 4.1232 nm × 3.5768 nm. The pore structure of clay mineral models extends periodically in the x – y directions, while the length is finite in the z direction 4,39. For the slab geometry and the long-range electrostatic interactions, we employ a vacuum in the simulation cell along the z direction with the length much larger than those in in x and y directions 4. All of our models and simulations are robust and reliable, which have been calibrated in our previous works 4,39,43.

(a)

(b)

(c)

Figure 1. Schematic representations of (a) Na-montmorillonite; (b) K-illite; and (c) kaolinite 8


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nanopores. The red spheres are O atoms, yellow spheres are Si atoms, green spheres are Al atoms, white spheres are H atoms, purple spheres are Mg atoms, blue spheres are Na+ ions, and pink spheres are K+ ions. In this work, the clay atoms and interlayer ions are represented by a CLAYFF force field 74.

Futhermore, the TraPPE 75 and EPM2 76 models are utilized for methane and carbon dioxide

molecules, respectively. The interactions between gas molecules and clay atoms, and interlayer ions are represented by the pair wise-additive Lennard-Jones (LJ) 12-6 potentials and Coulomb interactions 4:

u (rij )  u

LJ

  ij  u  4ij    rij  C

12 6   ij   qi q j ,       r 4  r  0 ij   ij  

(2)

where rij , ij and ij are the separation, LJ well depth, and LJ size, respectively; and qi is the partial charge of the sites. The standard Lorentz-Berthelot combining rules 77 are used to calculate cross interactions between the different atoms and molecules, i and j: ij   ii   jj  / 2 ,

(3)

ij  ii  jj ,

(4)

The bond-bending potential ubending of CO2 molecule is given as:

1 2 ubending    k    0  , 2

(5)

where k =1236 kJ/mol/rad2 is the bond-bending force constant,  is the bond-bending angle between O-C-O atoms, and  0 = π rad is the equilibrium bond-bending angle 4. The short-range LJ interactions are truncated at a distance of 1.07 nm without a shift 43. In addition, we use the standard three-dimensional Ewald summation with a correction term 78,79. 9


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2.2 Computation details The CH4 and CO2 mixtures simulation on clay minerals are utilized in the grand canonical (μVT) ensemble with a rectangular simulation box periodically in the x and y directions, similar to our previous study 4,43. The pore size is the length in z direction 4. In this work, we use Multipurpose Simulation Code (MUSIC) 80 for the simulation. During the simulation processes, the trail random displacement is used to all methane molecules. Based on the chemical potential of the methane reservoir outside, methane molecules are randomly inserted into or removed out of the simulation cell with equal probability 4. Except the aforementioned MC moves, a trial random rotation is performed to simulate all carbon dioxide molecules in each MC cycle 4. Here, the configurational biased MC algorithm is utilized to insert and remove carbon dioxide molecules 61. We apply the Widom insertion method 81 in a canonical (NVT) ensemble without clay minerals to compute the chemical potentials of methane and carbon dioxide molecules in the exterior reservoir. While the bulk densities of CH4-CO2 mixtures are calculated from the Peng-Robinson equation of state

82

at given

temperature and pressure, the National Institute of Standards Technology (NIST) Chemistry Webbook 83 is used to get the bulk densities of pure CH4 and CO2. The Metropolis algorithm is used for MC moves

84.

The simulation processes contain 0.5 million MC cycles for

equilibrium and another 0.8 million MC cycles per adsorbate molecules for describing density profiles. The selectivity of the CO2/CH4 mixture is an important parameter for investigating exploitation, separation, and purification, which is defined as 85:

10


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SCO2  where xCO2 and xCH 4

x y

CO2

xCH 4

CO2

yCH 4

, 

(6)

are the molar fractions of the components CO2 and CH4 in the

nanopores, respectively; and yCO2 and yCH 4 are the molar fractions in the bulk phase. In addition, the average gas weight density ave in clay nanopores is given as 4:

ave 

1 H

H

  ( z )dz ,

(6)

0

where  ( z ) is the weight density at distance z from one of the clay surface sheets. 3. Results and Discussion 3.1 Adsorption mechanisms and influence factors of CO2/CH4 mixtures 3.1.1 Effect of the clay mineral models on binary mixtures adsorption

(a)

11


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(b)

(c) Figure 2. The total sorption isotherms of CH4 and CO2 in montmorillonite (a), illite (b), and kaolinite (c) at a temperature T = 333.15 K and a pore size H = 4 nm with different molar fractions.

12


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(a)

(b)

(c) Figure 3. Density profiles of the carbon dioxide (top) and methane (bottom) distributions along the z direction in montmorillonite (a), illite (b), and kaolinite (c) at T = 333.15 K and P = 30 MPa with different molar fractions of CO2 and CH4. Figure 2 presents the total sorption isotherms of the CO2/CH4 mixtures in clay 13


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nanopores of H  4 nm for various bulk molar fractions and pressures, which is represented by the average density in nanopores. All simulations are performed with a pore size H = 4 nm and a system temperature T = 333.15 K. Because the proportion of nanometer pores in the shale increased sharply from 4 nm and the ‘intra-tachoid’ porosity (pores with a diameter of 3-4 nm) did not change with compaction; we performed all the simulations in 4 nm clay pores 70,86. For both CO2 and CH4 molecules, gas sorption increases with increasing bulk pressure. For the CO2 molecules, sorption is stronger in the montmorillonite and illite nanopores than in the kaolinite nanopores. As shown in Figure 2, there are crossover points between the black dotted line (CO2) and the black solid line (CH4) in montmorillonite and illite, when the CO2 composition is 25%, while there is no crossover point in kaolinite. In general, cation exchange enhances CO2 sorption, while it plays a minor role for the decreases for CH4. The CH4 isotherms have almost the same shapes in the three different clay minerals. It is because CO2 has a strong quadrupole moment 87, which increases sorption significantly coupled with the partially charged atoms in montmorillonite and illite. On the other hand, CH4 molecules-clay and -ion interactions are from the LJ interactions; thus, the sorption of CH4 is much lower. Previous studies

13,52

indicate that CO2 molecules

preferentially assemble near the Na+ cations in montmorillonite and K+ cations in illite. The strong interactions among CO2 molecules and cations lead to a higher selectivity of CO2 over CH4 within clay nanopores. We present density distributions of CO2 and CH4 at a temperature T = 333.15 K and pressure P = 30 MPa in three different clay nanopores with varying bulk molar fractions in Figure 3. It shows that both CO2 and CH4 contain two peaks: CO2 and CH4 molecules first 14


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adsorb on the clay surfaces and then form a second adsorption layer. The two peaks in CO2 sorption are significantly enhanced in montmorillonite and illite with cation exchange, compared to those in kaolinite. As a result, the first adsorption peak of CH4 is lower than the second peak. Cation exchange could significantly increase the CO2/CH4 sorption ratio. This condition is the reason that the amounts of sorbed CO2 in montmorillonite and illite are higher than that in kaolinite, as shown in Figure 2. In contrast, the first CH4 sorption peak in kaolinite is higher than those in montmorillonite and illite because without cation exchange the weaker CO2 molecule adsorption near the clay surface could provide available space for CH4 adsorption. In addition, as the CO2/CH4 ratio increases, the first peak in the CH4 density profile is suppressed by CO2, and only minor first peaks remain in montmorillonite and illite. Furthermore, as the CO2 bulk molar fraction increases, the second peak of the CO2 density profile obviously increases. In montmorillonite and illite nanopores, the location of the first CO2 peak is at ~ 3.7 Å from the surfaces, due to strong CO2-surface interactions. Jaramillo, et al. 88 and Liu, et al. 89 reported the similar results for CO2 sorption in zeolite. On the other hand, The main peak (second peak) of CH4 is at a distance around ~ 6.5 Å from the surfaces in montmorillonite and illite nanopores, while the main peak (first peak) CH4 is at ~ 3.5 Å in kaolinite nanopores. 3.1.3 Effect of pressure

15


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(a)

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(b)

(c) Figure 4. The density distributions of methane and carbon dioxide at P = 10 MPa and 30 MPa, T = 333.15 K, and CO2:CH4 molar fraction = 50%:50%, in (a) montmorillonite, (b) illite, and (c) kaolinite nanopores with size H = 4 nm. The density distributions of CO2 and CH4 at P = 10 MPa and 30 MPa with size H = 4 nm are demonstrated in Figure 4. In this paper, the pressure and temperature are within the normal range of shale formation conditions 90. Both the density distributions of the CO2 molecules and CH4 molecules have two peaks at two different bulk pressures. The second group of adsorption peaks are more noticeable at higher pressures. With the higher pressure, the first layer of CO2 molecules adsorbed on the surfaces, and a second layer forms. With cation exchange, the values of the first sorption peaks of the CO2 and CH4 molecules are almost identical at 10 MPa and 16


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30 MPa in the montmorillonite and illite nanopores. Without cation exchange in the kaolinite interlayer, the first peak at 30 MPa is higher than that at 10 MPa. This phenomenon is probably because the first adsorption peak of CO2 reached a maximum at a relatively low pressure in the montmorillonite and illite nanopores. On the other hand, without cation exchange in kaolinite, the adsorption sites are not completely occupied by CO2 molecules at 10 MPa. There is residual space on the clay surfaces for the adsorption of CO2 and CH4 molecules at low pressures. 3.1.4 Effect of temperature

(a)

(b)

17


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(c) Figure 5. The sorption isotherms of CH4 and CO2 mixtures, in (a) montmorillonite, (b) illite, and (c) kaolinite with a CO2:CH4 molar fraction = 50%:50%, and a pore size H = 4 nm, at different temperatures. We show the effects of temperature on the sorption isotherms. Here we provide an example with a CO2:CH4 molar fraction = 50%:50% in Figure 5. The temperature has a negative impact on mixture sorption; as the temperature increases, the average density decreases for both the CO2 and CH4 molecules due to weaker fluid-surface interactions. The sorption process for both components is suppressed with the higher temperature, where the mixture of molecules possess more energy to surmount the layer barrier and separate from the adsorbed layers 91. Meanwhile, the CO2 can be more strongly absorbed than CH4 at the same temperature. Moreover, the adsorption molar density difference between CO2 and CH4 in montmorillonite and illite is larger than that in kaolinite under any temperature condition. Relevant studies about methane and carbon dioxide adsorption in clay models, excess and absolute adsorption characteristics, and phase behaviors, can be found in our previous studies 4,39,43,92,93. 3.2 Implications for CO2 injection and shale gas exploitation 18


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3.2.1 Selectivity

(a)

(b)

(c) Figure 6. The selectivity for different molar fractions at T = 333.15 K in (a) montmorillonite, (b) illite, and (c) kaolinite nanopores. In this section, we focus on the effect of surface chemical heterogeneity on binary mixture selectivity. The CO2/CH4 adsorption selectivity in various clay nanopores is presented in Figure 6. SCO2  1 indicates preferential adsorption of CO2 over CH4. A consistent trend for montmorillonite and illite can be perceived where SCO2 decreases as the pressure increases at low pressures, which is the same as that of SCO2 in kerogen nanopores 1. In kaolinite, SCO2 is generally constant at low pressures, and shows a small drop when P > 20 MPa. With cation 19


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exchange, the CO2 molecules prefer to occupy adsorption sites with more energy

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11

at low

pressures in montmorillonite and illite. With a higher pressure, CO2 and CH4 mixtures begin to occupy the middle of a pore when the adsorption layer becomes saturated. Overall, when a shale reservoir possesses a higher selectivity for CO2, it is more conducive for CO2 sequestration and more efficient for the displacement of CH4. Moreover, SCO2 decreases with increasing CO2 molar fraction for all clay nanopores. Generally, the order of SCO2 selectivity for clay nanopores is montmorillonite > illite > kaolinite. 3.2.2 CO2 sequestration with enhanced shale recovery

Figure 7. The main three periods of CO2 injection and CH4 production in a single-well (modified from Kalantari-Dahaghi 94, Fathi and Akkutlu 95). Figure 7 depicts the CO2 injection and CH4 production operation for shale gas reservoirs in a single well with a multiple-fracture setting. There are three main steps involved in the configuration of CS-EGR, including CO2 injection, soaking, and CH4 production. Based on the 20


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different subsurface environment conditions, the soaking stage could last for several months to a year

94,95.

After the new equilibrium states reached, the shale gas production period could

ideally be maintained for over 30 years. Figure 7 also illustrates the desorption of CH4 molecules, competitive sorption processes, diffusion in the matrix, and flow through fractured systems in shale reservoirs. The (a), (b), (c), and (d) show the four states and the main flow mechanism of CH4 molecules and CO2 molecules.

(a)

(b)

(c) Figure 8. The displaced average molar density of CH4 for different CO2/CH4 molar fractions, a pore size H = 4 nm, and a temperature T = 333.15 K in montmorillonite (a), illite (b), and kaolinite (c). 21


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Figure 9. The displacement ratios of CH4 for CO2:CH4 molar fractions of 75%:25% (solid lines), 50%:50% (dashed lines), and 25%:75% (dotted lines), at a temperature T = 333.15 K in montmorillonite (red), illite (blue), and kaolinite (black) of pore size H = 4 nm. The injection and sorption of CO2 can potentially extract CH4 from the pores into the fracture network 94. Shale gas is produced from wells, and CO2 is preserved in the formation. In this work, we further study the effects of clay minerals for subsurface CH4 and injected CO2 that is closely related to the CS-EGR efficiency. The displaced average molar density of CH4

 aveR

( CH 4 )

is defined as,

 aveR

( CH 4 )

where aveT( CH

4)

  aveT( CH )   aveS( CH 4

4)

(7)

is the average molar density of pure CH4 in nanopores and aveS( CH

4)

is the

adsorption average molar density of CH4 in the binary mixtures in nanopores. All these molar densities are calculated with the same pressure and temperature. To further compare the displacement capacities in the three different clay minerals, the displacement ratio r(CH given as, r(CH 4 ) 

aveR

( CH 4 )

 aveT

100% .

( CH 4 )

22


(8)

4)

is

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Figure 8 shows the molar density of the displaced CH4 by CO2 for various molar fractions in the three clay nanopores. It can be observed that the more CO2 is injected, the more CH4 is displaced. Furthermore, we investigate the displacement ratio of CH4 in the three clay minerals, as shown in Figure 9. For the same bulk CO2/CH4 molar fraction, the displacement ratios are almost equal in these three clay nanopores and are independent of the pressure. Therefore, we choose montmorillonite as an example to study the CS-EGR project.

Figure 10. The methane production amount for different CO2 injection ratios in montmorillonite nanopores of pore size H = 4 nm at T = 333.15K.

23


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Figure 11. The percentage of enhanced gas recovery (EGR) with CO2 injection at different pressures in montmorillonite nanopores of pore size H = 4 nm at T = 333.15K. Based on the simulation data, we further investigated the EGR efficiency. Figure 10 shows an example of a CH4 gas production curve versus different CO2 injection ratios in montmorillonite nanopores. With actual shale gas production, the average reservoir pressure or abandonment pressure cannot reach a very low value. Therefore, CH4 gas needs to be exploited as much as possible, before the bottom hole flowing pressure reaches the abandonment pressure. Here we propose that the original pressure of a shale gas reservoir is 35 MPa. The pink line represents the production amount of CH4 gas in natural depressurization without CO2 injection. The blue, red, and black lines show the CH4 production curves with CO2 injected with bulk CO2/CH4 molar fractions of 25%, 50%, and 75%, respectively. These curves show that CO2 has a significant displacement effect on CH4, which could greatly enhance shale gas recovery. The rhombus, triangle, square, and circular crossover points between the gray dotted line and the pink solid line represent CO2 injection at different pressures, while the other crossover points represent the CH4 production amount with different CO2 injection molar fractions at the same pressure. For example, during the early stage of shale gas exploitation, natural drawdown is adopted for production. After a certain period of production, when the reservoir pressure drops to 6 MPa, we assume that CO2 gas is injected and shale reservoir depressurization occurs. The crossover rhombus point of the gray dotted line with the blue line indicates that CO2 gas injection reached a bulk CO2/CH4 molar fraction of 25%:75%. Shale reservoirs go through three stages of CO2 injection, soaking, and depressurization to produce CH4 again. If more CO2 is injected, then the crossover points of 24


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the gray dotted line with the red and black lines represent the CO2/CH4 molar fraction reaching 50% and 75%, respectively. Notably, the final CO2 injection pressure is too high to inject CO2 gas to reach the 75% CO2/CH4 molar fraction when the initial injection pressure is 15 MPa. Therefore, the crossover point between the gray dotted line and black line is not presented. EGR refers to the ratio of the incremental CH4 gas amount exploited by CO2 injection to the original total CH4 content in the reservoir. The ratio can reflect the increase rate of the CH4 yield achieved by injecting CO2, which is defined as follows,

 CO

2

EGR 

aveP( CH ) 4

  aveP( CH ) 4

 aveT

100% ,

(9)

( CH 4 )

CO where  aveP2

( CH 4 )

is the produced CH4 gas amount by CO2 injection when the pressure drops to

the same pressure before the CO2 injection, mol·L-1, which can be recognized as the gray points on the black, red, or blue lines in Figure 10;  aveP( CH ) is the produced CH4 gas amount before 4

injecting CO2, mol·L-1, which can be found from the gray points on the pink line in Figure 10; and  aveT( CH

4)

is the total CH4 gas amount stored in the shale reservoir, mol·L-1. Figure 11

presents the EGR (%) with CO2 injection at different initial pressures in montmorillonite. The histogram indicated that with more CO2 being injected, the EGR of CH4 shows an increasing trend. Moreover, with a higher reservoir initial pressure when injecting CO2, the EGR of CH4 also increases. The maximum EGR could reach 28.97%, which is a considerable value. It is worth noting that this EGR value is only applicable to 4-nm MMT pores. In fact, clay contains wide range pore size distributions from a few nanometers to several micrometers 70,86. In large pores, surface adsorption plays a negligible role and gas behaves as a free gas. Thus, the EGR in these pores would be much less than the values presented in this work. The overall EGR 25


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should to be quantified according to the actual shale rock properties and pore size distributions. On the other hand, as mentioned before, obtaining economic benefits is the ultimate purpose of industrial shale gas production. Implementing a CS-EGR project is a capital-intensive undertaking; nonetheless, the single largest project expense is generally the production (or purchase) of CO2. The costs for CO2 production can vary widely based on CO2 capture and purification, storage and transportation, and the amount of CO2 required. Therefore, operators have historically strived to optimize and reduce the cost of CO2 production (or purchase) and injection wherever possible. Furthermore, we investigated the efficiency for enhanced CH4 gas recovery with different amounts or densities of injected CO2 gas, where the CO2-CH4 displacement efficiency ( ED ) can primarily evaluate the EGR per molar density unit of CO2 gas at different pressures. The CO2-CH4 displacement efficiency varies as a function of EGR and amount of CO2 injected, which is defined as follows,

ED =

EGR

 aveI (CO )

,

(10)

2

where

ED is the CO2-CH4 displacement efficiency, %/mol·L-1, which represents the

efficiency or capability of enhanced CH4 gas recovery (%) with different molar densities of injected CO2 gas, and  aveI (CO2 ) is the molar density of the injected CO2 at the reservoir pressure, mol·L-1. Figure 12 presents an example of the CO2-CH4 displacement efficiency in montmorillonite considering the cost of CO2. From the CO2-CH4 displacement efficiency, it is possible to acquire information on the CS-EGR per unit of injected CO2. The CO2-CH4 displacement efficiency increases with increasing initial reservoir pressure when injecting CO2; meanwhile, the CO2-CH4 displacement efficiency also increases with increasing CO2 injection 26


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ratio. Based on the information from the CO2-CH4 displacement efficiency data, it is profitable that the more CO2 gas injected and CO2 injection at the early stage of well production (at a relatively high reservoir pressure). The CO2-CH4 displacement efficiency of illite and kaolinite model are given in Supporting Information. The results show the same tendency as the montmorillonite in the illite model, while the CO2-CH4 displacement efficiency decreases with higher initial pressure under the same CO2 injection ratio in the kaolinite model. All the results provide the idea that practitioners can design a good cost-effective solution for improving shale gas recovery as much as possible while controlling the cost of CO2 injection.

Figure 12. The CO2-CH4 displacement efficiency at different CO2 injection pressures in montmorillonite nanopores of pore size H = 4 nm at T = 333.15 K. 4. Conclusions In this work, we studied the competitive adsorption behaviors of CO2/CH4 mixtures in montmorillonite, illite, and kaolinite nanopores using GCMC molecular simulation. Full atomistic models are employed to represent the clay nanopores, and intermolecular interactions are explicitly considered. 27


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Compared to that for CH4 sorption, the partial charge of clay is the main factor contributing to the sorption of CO2 molecules. Due to cation exchange in montmorillonite and illite, the sorption peaks of CO2 molecules are significantly enhanced and the selectivity for CO2 over CH4 is much higher than that in kaolinite. As a result, CH4 molecules can be easily displaced and are mainly adsorbed on the second layer. Without cation exchange, the weaker CO2 molecule sorption could provide available space for CH4 molecule sorption near the clay surface. The influence on the sorption capacity of CO2 molecules is more sensitive to cation exchange than pressure. The CO2/CH4 selectivity is consistently higher than 1 for all circumstances, showing that CO2 has a stronger adsorption capacity than CH4. With cation exchange, CO2 molecules prefer to occupy adsorption sites with more energy at low pressures. With higher pressure, CO2 and CH4 begin to occupy the middle space of the nanopores. In general, the interactions between Na+ cations and CO2 molecules in montmorillonite are stronger than the interaction between K+ cations and CO2 molecules in illite. The CS-EGR project case study in montmorillonite reveals that the amount of methane extracted from a shale reservoir by injecting CO2 is related to the CO2 injection amount but is independent of the original formation pressure. Accordingly, the CO2-CH4 displacement efficiency increases with higher reservoir initial pressure when injecting CO2, and the CO2CH4 displacement efficiency increases with increasing of CO2 injection ratio. Finally, the injection of more CO2 gas at the early stage of well production or when the reservoir is at a relatively high pressure is also suggested.

28


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Author information Corresponding Author *Corresponding Author 1: Dr. Hucheng Deng Professor, Petroleum Engineering College of Energy Chengdu University of Technology Phone: 0086-13402851216 Email: [email protected]

*Corresponding Author 2: Dr. Zhehui Jin Assistant Professor, Petroleum Engineering School of Mining and Petroleum Engineering Department of Civil and Environmental Engineering University of Alberta Phone: 1-780-492-6633 Email: [email protected] Author Contributions Xiaofei Hu performed the GCMC simulations, analyzed data, and drafted the main manuscript. Hucheng Deng supervised the project and contributed to the conception of study. Zhehui Jin provided the main idea, defined the statement of problem and helped draft the manuscript. Chang Lu helped acquire data. 29


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Yuanyuan Tian performed the data analyses. All authors reviewed the manuscript and have given approval to the final version of the manuscript. Notes The authors declare the approve of publication and no competing interests. Acknowledgements The first and second authors acknowledge the financial support provided by the National Science and Technology Major Project (2017ZX05036003-007) and the National Science and Technology Major Project (2017ZX05036004-006). The first, third, and fifth authors acknowledge an Open Fund (PLC201704) provided by State Key Laboratory for Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology). This research was enabled in part by support provided by Westgrid (www.westgrid.ca) and Compute Canada (www.computecanada.ca). Z.J. also greatly acknowledges a Discovery Grant from Natural Sciences and Engineering Research Council of Canada (NSERC RGPIN2017-05080).

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CH4 Competitive Adsorption in Various Clay

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