Quantification of Adsorption of Water on Clay Surfaces and Electrical Double Layer Properties Using Molecular Simulations

Quantification of adsorption of water on the clay mineral surface at a molecular scale can provide fundamental insights on the properties of electrical double layer (EDL), cation exchange capacity (CEC), and production performance of clay-rich formations. However, there are limited fundamental studies on quantifying the impacts of reservoir temperature/pressure on water adsorption on clay surface, and on the factors controlling the properties of EDL. In this project, we use molecular simulations to (i)investigate water adsorption on clay minerals (ii)quantify the impacts of reservoir temperature onadsorption (iii) investigate the structure of the EDL onclay surface as a function of electrolyte concentrationand pore size and calculate diffusion coefficients. Grand Canonical Monte Carlo (GCMC) simulations are performed to calculate water adsorption. These simulations are performed at 330K at the pressure of 5MPa. Then, an electrolyte (including NaCl) is added to the system and Molecular Dynamics (MD) simulations are performed at temperature range of 330K to 380K. To investigate the impact of electrolyte concentration on the geochemistry of the solid-fluid interface, these simulations are performed at electrolyte concentrations ranging from of 0.7 mol/dm3, 1.4 mol/dm3 and 1.9 mol/dm3. To analyze the effects of confinement on water adsorption, MD simulations performed on 2 and 4 nm-wide illite slit pores. We applied the proposed methods on multiple types of clay minerals including illite and kaolinite. Our results show the formation of two hydration layers on the surface of illite and kaolinite. We found that the position of the adsorbed cations and anions inside the clay nanopore do not change significantly with ionic strength, and that clay geochemistry is the main factor determining the adsorption planes of ions. As temperature increases the mobility of water and ions increase, however when temperature is increased from 360 K to 380 K the increase in mobility is not significant. Results also showed that the diffusion coefficient of molecules across the surface of clay walls is smaller compared to that parallel to the surface. Besides that, we found that as confinement effect increases, spatial distribution of ions does not change, but the van der Waals interactions between clay surface and brine increases. Quantification of water adsorption and characterization of EDL in clay minerals at reservoir conditions cannot be easily assessed experimentally. The proposed method enabled quantifying water adsorption and EDL characterization in different types of clay minerals and elucidating the clay-water interface at such conditions. The outcomes of this work can potentially contribute to development of quantitative models for CEC and wettability assessment as a function of geochemistry of the rock.