2. Models and Methodology
2.1 Structure of illite and charged surface
chemistry
Kaolinite, illite, chlorite and
smectite are the most commonly occurring clay minerals43 with illite being the most common diagenetic
product in shales44. Generally, clay-hosted pores in
shales are slit-shaped or cylindrical, with few occurrences of oval- and
cone- shaped pores45,46. Because slit pores are the
most prevalent47, we focus on investigating fluid
transport in illite-hosted slit-shaped pores.
The chemical formula of illite is
K[Si7Al](Al4)O20(OH)4,
according to Zhang et al.48. Isomorphic substitutions
in each unit cell of our model are made by replacing one
Si4+ by one Al3+. Loewenstein’s rule
is used for ion substitution in clay minerals so that the locations of
two substitution sites are not adjacent to each other49. Interlayer cations (potassium cations,
K+) are placed randomly in the interlayer space of
illite to counterbalance the electrostatic charges induced by the
isomorphic substitutions. The K+ cations can move in
the interlayer space.
The simulation box contains 20 clay unit cells (forming a 10×2×1
supercell), with dimensions of 6.4042 nm×2.2308nm×1.0204 nm (x-, y- and
z-direction) as shown in Fig. 1. The slit pore is constructed with four
parallel illite layers confined in a three-dimensional simulation box. 2
illite layers form the top pore surface and the other two illite layers
form the bottom. Each illite pore model has three different basal
spacings (5nm, 10nm, and 15nm).
There are generally four illite
slit pore structures discussed in literature based on charged clay
surface chemistry: potassium-hydroxyl (P-H)50,
hydroxyl-hydroxyl (H-H)51, potassium-potassium
(P-P)48, and a structure52 between
the P-H and H-H configurations. The corresponding illustrations are
shown in the Fig.S-1 in Supporting Information. This work considers only
the potassium-hydroxyl (P-H) and hydroxyl-hydroxyl (H-H) structures and
a schematic of both are shown in Figs. 2a-b.