Page 352 - Fundamentals of Gas Shale Reservoirs

332 MOLECULAR SIMULATION OF GAS ADSORPTION IN MINERALS AND COAL

TAbLE 15.4 Different types of atoms and their LJ force‐field

−1
Atom type Label C (kJ nm mol ) C (kJ nm mol ) Q(e)
(12)
12
(6)
6
−1
Oxygen (surface) O 0.25400 × 10 −2 0.24781 × 10 −5 −0.80
Oxygen (apical) O 0.25400 × 10 −2 0.24781 × 10 −5 −1.00
Oxygen (OH) O 0.25400 × 10 −2 1.24781 × 10 −5 −1.52
Hydrogen H +0.52
Silica Si 0.182 × 10 −2 0.626 × 10 −7 +1.20
Sodium Na 0.12500 × 10 −2 0.58453 × 10 −7 +1.00
Aluminum Al +3.00
Magnesium Mg +2.00
Methane CH 4 1.32400 × 10 −2 0.35651 × 10 −4
CH in propane CH (P) 1.03800 × 10 −2 0.36789 × 10 −4
3
3
CH group CH 2 0.70000 × 10 −2 0.24806 × 10 −4
2
CH in ethane CH (E) 1.00200 × 10 −2 0.28987 × 10 −4
3 3
A1 atom, and one out of eight A1 atoms in the octahedral coefficient is 209E‐5 cm /s along the channel direction for
2
network are substituted by one Mg atom. These substitutions a channel width of 2 nm with an external acceleration of
are typical for the Wyoming‐type montmorillonite and give 10 N/s at 353.5 K; and (iii) monolayer adsorption behavior
2
rise to an overall negative charge on the clay framework. of methane along montmorillonite channel was observed.
This negative charge is balanced by the presence of monova­
lent interlayer counter ions M (Na ). The LJ potential param­ 15.2.3 MD Simulation of Gas Adsorption on Zeolite
+
eters and partial charges are taken from Jorgensen et al.
(1984) (Table 15.4). 15.2.3.1 Zeolite Structure, Compositions, and Model
Construction MS has been played an important role in
15.2.2.2 Results and Discussion The simulations of understanding the fundamentals of gas adsorption in zeolite
natural gas adsorption on the Wyoming‐type montmoril­ at the molecular level (Beerdsen et al., 2003; Demontis et al.,
lonite results are summarized in Figure 15.6. The simulated 1992; Dubbeldam and Smit, 2003; Dubbeldam et al., 2004a, b;
isotherm for CH fits to the classic Langmuir curve extremely García‐Pérez et al., 2006; Granato et al., 2007; Wender et al.,
4
well with an R of 0.999 (Fig. 15.6a). The fitting parameters 2007; Zhang et al., 2012). Calero et al. (2004) developed a
2
for the maximum adsorption capacity and adsorption rate are force field to accurately describe the adsorption properties
5.744 mol/kg and 0.112 MPa , respectively. The adsorption of n‐alkanes in the sodium form of FAU‐type zeolites.
−1
of CH increases with pressure from less than 2 mol/kg to García‐Pérez et al. (2006) extended the force field of Calero
4
over 4.0 mol/kg from 5 to 25 MPa, while that of C H appears et al. (2004) by including calcium‐type ions and the force
6
2
to be unaffected by pressure (Fig. 15.6b). The adsorption of field was applied to study the adsorption properties of
C H decreases from 0.3 to 0.2 mol/kg with pressure from 5 n‐alkanes in LTA 5A over a range of temperatures and pres­
8
3
to 25 MPa (Fig. 15.6b). The adsorption of CH on dry the sures. In our simulation, we primarily focused on the effect
4
Wyoming‐type montmorillonite is consistently higher than of the number density of the nonframework cations, the ratio
that for the moist one with 7.15 wt% of water by up to 40% of Si/Al and Na /Ca on gas adsorption.
2+
+
over the pressure range simulated (Fig. 15.6c). The partial FAU unit cell composition with Si/Al = 1.18 corresponding
density of the CH in the z‐direction of the simulation box to 88 aluminum atoms per unit cell is used in our simulation
4
attains a maximum of 570 kg/m at a distance of 0.15 nm (Fig. 15.7). The negative charges introduced by Al replacing
3
from Na and 0.38 nm from the clay surface (Fig. 15.6d). Si are compensated by Na (NaX). Atomic charges are
+
+
RDF indicates that CH has a close contact of 3.2 Å with Na chosen as q = +1, q = +2.05, q = +1.75 (Zhang et al.,
+
Na
Al
Si
4
in the Wyoming‐type montmorillonite followed by CH –O 2012, 2014b). The crystal structures of Na Al Si O is
88
88
104
4
384
(3.9 Å) and CH –Si (4.6 Å). The mole fraction of methane in used for FAU zeolite X, with a lattice parameter of 25.099 Å.
4
the adsorbed phase is lower than that in the bulk phase. Cations present in the zeolite framework are Na . The zeo­
+
However, both mole fractions of ethane and propane in the lite is assumed to be rigid. We studied the influence of
adsorbed phase are higher than those in the bulk phase. temperature on methane and carbon dioxide adsorption in
It is noticed that the presence of water in the Wyoming‐ FAU zeolite.
type montmorillonite reduces the adsorption capacity of The interactions of adsorbate molecules with the zeolite
methane. During the nonequilibrium MS on methane diffu­ host framework are dominated by the forces between the
sion in montmorillonite, we have found that (i) the diffusion adsorbate and the oxygen atoms of zeolite. The contribution
coefficient increases with external force; (ii) the diffusion of Si and Al are taken into account through an effective