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.