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. We refer the readers to
our previous work and those of others42,48 for a more
detailed description of the illite unit cell with chemical formula,
K[Si7Al](Al4)O20(OH)4.
In Fig.1, we show one illite layer comprising 20-unit cells. 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 (PH)49,
hydroxyl-hydroxyl (HH)50, potassium-potassium
(PP)51, and a structure52 between
the PH and HH pore systems. The corresponding illustrations are shown in
the Fig.S-1 in Supporting Information. This work considers only the PH
and HH pore systems and a schematic of both are shown in Figs. 2a-b.