Page 351 - Fundamentals of Gas Shale Reservoirs

MS OF GAS ADSORPTION ON MINERALS 331

CH -O
4
CH 3 (ethane)_O
CH 3 (propane)_O
(a) CH (propane)_O (b)
2
2.0 Channel width: 10Å, pressure: 15 MPa
CH 3 CH 2 CH 3
1.5

RDF 1.0 CH 3 CH 2 CH 3

CH CH CH 4 r
3
3
0.5
0.0
2 3 4 5 6 7 8 9
r (Å)

FIGURE 15.4 MD simulation results: (a) RDF of CH , CH ‐ and CH ‐ with references to the O‐ atom in quartz showing that the adsorption
3
4
2
of CH , C H , and C H are all in single layer mode and (b) schematic illustration showing the attachment mode of CH , C H , and C H SiO .
3
8
3
6
2
8
2
2
6
4
4
r: RDF range.

FIGURE 15.5 Molecular structural model of the Wyoming‐type montmorillonite: Layered structure of the Wyoming‐type montmorillonite.

as potassium or sodium. Montmorillonite (Wyoming‐type) Clay surface with the dimensions 31.68 × 27.42 × 6.56 Å
3
was chosen as a typical clay mineral in gas shale as the was constructed with a layer spacing of 20 Å, which contains
molecular structure of montmorillonite has been well 36 clay unit cells details are in the literature (Pusch and
established and it is sensitive to moist. Madsen, 1995). The clay mineral studied is a Wyoming‐type
The composition of a typical nature gas was used in the montmorillonite, with a unit cell formula as M [Si Al ]
x
simulation with 92% CH , 6% C H , and 2% C H . The mod­ (Al Mg )O (OH) , a 8−a
20
4
3
8
4
2
6
b
4−b
eling temperature was set to 353.5 K or 80.5°C. Five pressure where M represents a monovalent cation; in the present
points were simulated including 5, 10, 15, 20, and 25 MPa. work Na is considered, x = 0.75, a = 7.74, and b = 3.5. The
+
The simulations were performed for both dry and moist sce­ Wyoming‐type montmorillonite is a member of the smectite
narios with a water content of 7.15 wt%. family and consists of two tetrahedral layers, sandwiching
MC simulations of natural gas adsorption in the inter­ an octahedral layer (2:1), shown in Figure 15.5. One out of
layers of 2:1 Na‐saturated clay minerals were performed. thirty‐two Si atoms in the tetrahedral sheet is replaced by an