Article pubs.acs.org/EF
Adsorption Behavior of Hydrocarbon on Illite G. Chen,†,‡,§ J. Zhang,*,§ S. Lu,*,†,‡ M. Pervukhina,§ K. Liu,‡,§,∥ Q. Xue,⊥,# H. Tian,∥ S. Tian,†,‡ J. Li,†,‡ M. B. Clennell,§ and D. N. Dewhurst§ †
Research Institute of Unconventional Petroleum and Renewable Energy (RIUP&RE), China University of Petroleum (East China), Qingdao, Shandong 266580, China ‡ School of Geosciences, China University of Petroleum (East China), Qingdao, Shandong 266580, China § CSIRO Energy, 26 Dick Perry Avenue, Kensington, Western Australia 6151, Australia ∥ Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China ⊥ Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, Shandong, P. R. China # College of Science and Key Laboratory of New Energy Physics & Materials Science in Universities of Shandong, China University of Petroleum, Qingdao 266580, Shandong, P. R. China ABSTRACT: The adsorption of hydrocarbon (pure CH4 and C2H6) on illitic clay was investigated at temperatures of 333, 363, and 393 K (60, 90, and 120 °C) over a range of pressures up to 30 MPa using grand canonical Monte Carlo (GCMC) simulations. We first discussed the comparability of molecular simulation results with experimental measurements. Our results indicate that molecular simulation results of the excess adsorption are comparable with the experimental measurements if they are both expressed per unit surface area available for adsorption instead of per unit mass. The gas density profiles indicate that the adsorption of CH4 and C2H6 is mainly affected by the clay surface layers. In micropores smaller than 2 nm, the overlapping of the interaction of the simulated pore walls with the gas results in enhanced density peaks. For pore sizes of 2 nm or larger, the overlapping effect is significantly reduced, and the height of the gas density peak close to the surfaces is no longer affected by pore sizes. The maximum excess adsorption of illite for C2H6 is almost twice that for CH4 due to the stronger interaction between illite and C2H6 than between illite and CH4, but the saturation capacity (maximum loading) is the same for both. Our findings may provide some insights into gas adsorption behavior in illite-bearing shales and give some guidance for improving experimental prediction.
1. INTRODUCTION Growing global energy needs and continuous depletion of conventional oil and gas resources have spiked an increasing interest in unconventional energy resources.1−5 Shale gas has proven to be the most promising form of unconventional energy in the recent great success of exploration and development in North America.6−8 Shale gas exists both as free gas within intergranular pores or natural fractures and as adsorbed gas on or underneath the surface of insoluble organic matter and/or inorganic minerals.9 Adsorbed gas plays an important role in both exploration and development of shale gas. Therefore, the adsorption capacity of shale has been widely studied experimentally.10−16 In these experiments, the excess adsorption amount has been obtained over a limited range of pressures and temperatures. Molecular simulation methods have recently been introduced to investigate gas adsorption on both kerogen and clay over large pressure and temperature ranges.17−28 Both the GCMC and molecular dynamic (MD) methods have been used to investigate the shale/coal gas adsorption and diffusion behavior.24,26−29 Adsorption behaviors of CH4 and CO2 in slit-shaped carbon pores of varying sizes were studied.19,30 Recent molecular simulation studies on the shale gas adsorption behavior on clay have mainly focused on the Namontmorillonite structure although other clays, such as illite and kaolinite, play a more important and relevant role in shale gas adsorption behavior.31 Adsorption behaviors of CH4 and © XXXX American Chemical Society
CO2 in different pore sizes from 1 to 4 nm with/without water have been investigated using the Na-montmorillonite structure.17,18,25 Previous studies focused on the adsorption of CH4 and CO2 with little or no consideration of C2H6, which also is present in shale gas. The C2H6 content is not as high as CH4, but the adsorption capacity of C2H6 has been reported to be larger than that of CH4,32,33 and C2H6 adsorption is thus important, especially for wet gas. In previous molecular simulation studies of gas adsorption on shale or coal, the excess adsorption was usually calculated by subtracting the bulk gas density multiplied by pore volume from the total uptake, and the bulk gas density was calculated by the equation of state.19,20,24,26−28,30 The difference in the result of the bulk gas density obtained from the equation of state and from the molecular simulation may induce uncertainties in the excess adsorption density. In addition, the excess adsorption amount was usually expressed per unit mass of adsorbent24,26 to compare with experimental measurements. However, to the best of our knowledge, the comparability between these two has not been investigated and reported. In this study, the CH4 and C2H6 adsorption behaviors on Killite were investigated. The bulk gas density is obtained using Received: July 20, 2016 Revised: September 18, 2016
A
DOI: 10.1021/acs.energyfuels.6b01777 Energy Fuels XXXX, XXX, XXX−XXX
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Waals forces, which are described by pairwise-additive Lennard-Jones (LJ) 12-6 potentials:40
the GCMC simulation in an empty simulation box rather than from the equation of state. Comparability of our molecular simulations with experiments is discussed. Gas distribution in varying pore space and the effects of specific surface area, pore size, and temperature on excess adsorption are investigated.
⎧ ⎡⎛ ⎞12 ⎪ ⎢⎜ σij ⎟ ⎪ 4εij⎢⎜ ⎟ − E(rij) = ⎨ ⎣⎝ rij ⎠ ⎪ ⎪ 0 ⎩
2. SIMULATION METHODS 2.1. Models. Our adsorption system consists of both adsorbent (illite) and adsorbate molecules (CH4 or C2H6). The illite is represented by pyrophyllite-1Tc34,35 with the unit cell formula of Kx[Si(8−x)Alx](Al4)O20 (OH)4 (x = 1) and an overall clay charge of −1.0e per unit cell.36 In each unit cell, one silicon (Si) is substituted by one aluminum (Al) in the tetrahedral sheet. The negative charge caused by the substitution is balanced by the interlayer potassium cations (K+).37 The interlayer cations are mobile. To investigate the effect of specific surface area on the adsorption, we built two types of clay layers (types I and II) shown in Figure 1. In type I, illite is
⎛ σij ⎞6 ⎤ ⎜⎜ ⎟⎟ ⎥ for (rij ≤ rcutoff ) ⎥ ⎝ rij ⎠ ⎦ for (rij ≤ rcutoff )
(1)
Here, rij, εij, and σij are the separation, LJ well depth, and LJ radius, respectively, for the pair of atoms i and j. The cutoff distance rcutoff in the LJ potentials is set as 1.25 nm in this simulation. Interactions longer than this distance are omitted from the energy and force computations since they are negligible. Cross interactions between unlike atoms i and j are calculated by the Lorentz−Berthelot combining rules41,42 given below: σij = εij =
1 (σii + σjj) 2
(2)
εiiεjj
(3) 43,44
simulations are 2.2. Simulation Details. In this study, GCMC performed to investigate the adsorption behavior of CH4 and C2H6 on illite with varying pore sizes, temperatures, and pressures using the open source package RASPA 1.0 developed by Dubbeldam et al.45 In the GCMC simulations, the chemical potentials of the gas (μ), the volume (V), and the temperature (T) of the system are fixed. The chemical potential (μ) relates to reservoir gas or bulk gas pressure (P)
( ), where p and μ are the standard pressure
by μ = μ0 + RT ln
φP
0
0
p0
and chemical potential, respectively, and φ is the fugacity coefficient. The temperature is fixed to be 363 K for both the simulations of type I and type II. In order to study temperature effects on adsorption, we run simulations of type I at varying temperatures of 333, 363, and 393 K for the pore size of 2 nm. To obtain the bulk density of CH4 and C2H6 at each temperature and pressure, the GCMC simulations are performed in an empty box of 3 × 3 × 3 nm3 in x-, y-, and zdimensions. For each simulation run, the GCMC simulation consists of 1 × 105 steps for equilibration and 1 × 106 steps for the ensemble averages. We run MPI parallel programming in the Raijin supercomputer in the National Computational Infrastructure (NCI) Australia.
Figure 1. Snapshot of the simulation cell (clay and ethane) with the following color scheme: O, red; H, white; Si, yellow; Al, pink; K, purple; C, gray. Clay layers are represented by bond structures; ethane molecules are represented by spheres. (a) Structure I (15 clay unit cells) with two half layers of the clay sheet; (b) structure II (30 clay unit cells) with two complete layers of the clay sheet. Both types have the same layer surface area and pore size, defined as the distance between the planes of the centers of oxygen atoms in the inner surface layers, but different masses in order to investigate the effect of the specific surface area on the adsorption behavior.
3. SIMULATION RESULTS AND DISCUSSION 3.1. Specific Surface Area and Its Effect on Adsorption. In the clay adsorption simulation, the total uptake refers to the total amount of adsorbate present in the clay slit nanopores. As shown in Figure 1, illite is built up of a certain number of unit layers. By changing the number of unit layers, the mass is changed, and therefore, the specific surface area (generally expressed in the unit of m2/kg or m2/g) is altered. Specific surface area is a property of solids defined as the total available surface area for adsorption per unit mass of the adsorbent. We perform adsorption simulations in the illite slit pores with structures of type I (Figure 1a) and type II (Figure 1b). They have the same pore size and layer surface area, but different mass in order to investigate the effect of the specific surface area on the adsorption behavior. Comparison of the total number of gas molecules loaded in types I and II is displayed in Figure 2 for two pore sizes of 0.74 nm (Figure 2a) and 2 nm (Figure 2b). It shows that, for a given pore volume, the effect of the specific surface area has a negligible effect on the illite adsorption for the gases specified. In other words, for a given pore volume, altering the thickness of solid (mass) hardly affects the adsorption amount in the nanopores. Therefore, it does not make sense to compare our amount of gas adsorbed on the clay slit pore in the unit of mol/kg or kg/kg with
represented by a patch with two half layers of the clay sheet; in type II, illite is a patch with two complete layers of the clay sheet (shown in Figure 1). Type I and type II are composed of 15 (5 × 3 × 1) and 30 (5 × 3 × 2) illite unit cells. They have the same layer surface area of 2.58 nm × 2.69 nm, but different masses. A three-dimensional periodic boundary condition is applied to both. In order to investigate the influence of slit pore size (defined as the distance between the planes of the centers of oxygen atoms in the inner surface layers) and the number of clay sheet layers, we select 4 pore sizes of 0.74, 1, 2, and 3 nm for type I simulations and 1 pore size of 0.74 nm for type II. In our simulations, the cutoff distance is 1.25 nm, so we repeat the simulation box in z-dimension if its size is less than 2.5 nm to make sure it is not smaller than 2 times the cutoff distance. In our simulations, the illite layers are considered to be rigid. The two layers form a slit-like nanopore structure. Counter cations and gas molecules are present in the slit-shaped pore. The illite is modeled using the CLAYFF force field.38 The adsorbate molecules, CH4 or C2H6, are simulated using optimized potentials for liquid simulations (OPLS model).39 In the model, the carbon and the hydrogen that is bonded to it are treated as a united atom (pseudoatom). In the C2H6 model, the pseudo atoms are connected by bond with a fixed length of 0.153 nm. For CH4 and C2H6, the gas−clay and gas−gas interactions are dominated by van der B
DOI: 10.1021/acs.energyfuels.6b01777 Energy Fuels XXXX, XXX, XXX−XXX
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Subtraction of the latter from the former can thus yield the additional gas amount in the slit pore as excess adsorption.30 The pore volume is the volume free to be occupied by gases in adsorption processes. It is essential to derive the excess adsorption from the absolute adsorption. In the experiment, it is measured using helium, because helium hardly adsorbs. The point is that helium is a reference gas for measuring excess adsorption of all other gases, and the requirement is that the procedure for measuring pore volume should be identical for simulation prediction and experimental determination. In this study, pore volume is obtained by probing the structure with helium at a room temperature of 25 °C. It was obtained from a separate simulation using the Widom particle insertion method, as the pore volume fraction corresponds to the new Rosenbluth weight.47 We obtained an average helium pore fraction of 31.6%, 42.2%, 64.0%, and 73.9% for the pore sizes of 0.74, 1, 2, and 3 nm, respectively. The best way to examine the simulation results is to compare them with available experimental measurements. To make this comparison possible, the simulation and experimental results should be expressed in the same unit. The adsorption amount is commonly expressed per unit mass of adsorbent in experiments.12,48,49 As discussed in section 3.1, it does not make sense to compare our simulation results for the adsorption in the clay slit pore with the experimental measurements in units of mol/kg or kg/kg. One possible option is to express both simulation and experimental excess adsorption amount per unit pore volume (excess adsorption amount divided by pore volume) or per unit clay surface area given by
Figure 2. Comparison of loading number of CH4 and C2H6 at a temperature of 363 K (90 °C) in types I and II pores of (a) 0.74 nm and (b) 2 nm. This indicates that, for a given channel width or pore volume, the specific surface area has negligible effect on the illite total uptake for the gases specified.
experimental data for gas adsorption in shale powder or particles with a wide range of pore distribution from microscopic to macroscopic. It is generally believed that the adsorption behavior is mainly caused by the gas−mineral surface interaction. The adsorption capacity of the clay at a given pore size depends mainly on the total surface area available for adsorption. The specific surface area of natural illite-rich rocks between individual publications varies from 2.26 m2/g analyzed by nitrogen31 to 64 m2/g analyzed by atomic force microscopy.46 Different measurement techniques measure distinct features of clay minerals (e.g., external surface area only as compared to clay interlayers and external surfaces), hence the variable results. Here we need to emphasize that illitic clays are built up of a certain number of unit layers. By changing the number of unit layers, the mass can be changed, and thus the specific surface area (generally expressed in the unit of m2/g) is altered. Obviously, the specific surface areas from the simulations are significantly larger than that in the natural illite-rich rocks.31,46 That is due to the fact that in our simulation model the solid clay layers are not thick enough (clay mass is not high enough) to compare with the natural illite. As a result, in the case that adsorption is mainly controlled by the available surface area, the adsorption expressed per unit mass of clay from the simulations is expected to be markedly larger than that obtained from experiments. So, there would be no comparability between the simulation results and experimental measurements if they are expressed per unit mass. We need to find a sensible way to compare simulation results with experimental measurements. As simulation results from types I and II are hardly distinguishable, all the simulations mentioned in the rest of the paper are therefore for type I only. 3.2. Excess Adsorption. The output of the GCMC simulations is the total uptake of adsorbate gas molecules. Excess adsorption is defined as the difference between the total uptake and the amount of bulk gas in the same pore volume.
Q ex =
Zst·nex ·R ·Tst PstS
(4)
where Zst is the compression factor of adsorbate molecules under a standard state, R is the ideal gas constant of 8.314 J/ (mol K), and Qex (in the unit of cm3/m2) is the volume of excess adsorption per unit clay surface area (S, in the unit of m2) under the standard state. The temperature (Tst) and pressure (Pst) of the standard state are 273.15 K and 0.101 MPa, respectively. The number of moles of excess adsorption, nex, is defined as nex =
ρb ·Vp Nt − NA Ma
(5)
where Vp is the slit pore volume, Nt is the total number of adsorbate molecules in the pore, NA is the Avogadro constant of 6.02 × 1023 mol−1, and Ma is the molar mass of the adsorbed gas molecules in g/mol. The bulk gas density ρb (g/cm3) is calculated using the GCMC simulations in the 3 × 3 × 3 nm3 empty box. Effect of Pore Size. To examine the effect of the pore size on the excess sorption, we calculate the excess sorption isotherms at temperature of 90 °C for various pore sizes from 0.74 to 3 nm and compare them in Figure 3. The results show that the excess adsorption of different pore sizes increases with increasing pressure to a maximum and then decreases at higher pressures. As the pore size increases, the positions of the maxima shift to higher pressure. In other words, the pressure corresponding to the maximum of the excess sorption increases with the increase in pore size. This observation was also reported by Tan and Gubbins for methane sorption in carbon micropores50 and Liu et al. for methane sorption in shale.51 We also observe that the initial slope of the excess adsorption C
DOI: 10.1021/acs.energyfuels.6b01777 Energy Fuels XXXX, XXX, XXX−XXX
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rocks at 30, 60, and 90 °C reported by Fan et al.,52 Tian et al.,53 and Heller and Zoback54 in Figure 4a. We have converted their experimental adsorption amount expressed per unit mass to per unit surface area (cm3/m2) by normalizing their original data by the specific surface area. We used the illite specific surface area of 41 m2/g obtained by Macht et al.55 to convert the experimental data from Fan et al.52 and Heller and Zoback.54 The value of 41 m2/g was derived by combining the external specific surface area obtained by N2 adsorption (BET) and the specific edge surface area from atomic force microscopy (AFM).55 For the experimental data of Tian et al.,53 we used the value of 57 m2/g to convert as they recently obtained this value for their illite samples using the low-temperature nitrogen adsorption method. The simulation results and the experimental data are of the same order of magnitude, although they do not match perfectly. The simulation results are reasonable and acceptable for the following reasons: (1) The illite specific surface area of 7.2 m2/g obtained by Ji el al.31 was measured by the nitrogen method. This value is much smaller than the value of ∼30 m2/g obtained by Ross and Bustin,9 23.25 m2/g by Jeon et al.,56 41 m2/g by Macht et al.,55 and 62 m2/g by Dewhurst et al.57 If the specific surface area is underestimated, the excess adsorption expressed per unit surface area will be much larger than the real value. Dewhurst et al.57 have reported a surface area for illite-bearing London clay of around 62 m2/g. The surface area of the clay-rich sample resides almost entirely in pores smaller than the 20 nm pore throat radius. The total surface area in those pores (>4 nm) penetrated by mercury is 22 m2/g compared with 62 m2/g measured by the nitrogen method. Between 57% and 65% of the total surface area resides within pores of radius
