Illite Dissolution under Sodium Hydroxide Solution: Insights from Reactive Molecular Dynamics

Abstract

In alkali/surfactant/polymer (ASP) flooding systems, alkalis react with clay minerals such as Illite, montmorillonite, and kaolinite, leading to reservoir damage and impacting oil recovery rates. Therefore, studying the dissolution effects of strong alkalis on clay minerals is crucial for improving oil recovery. This study uses Illite as a representative clay mineral and employs the ReaxFF reactive force field and molecular dynamics simulations to model its dissolution in NaOH solution. We investigated the diffusion coefficients of metal cations in Illite and their interactions with hydroxide ions at various NaOH concentrations. The study also explores the evolution of dissolution products and protonation characteristics during the dissolution of Illite. By calculating the changes in ionic energy throughout the dissolution process, we analyzed variations in ionic reactivity within the system. Simulation results show that as the NaOH concentration increases, metal cations in Illite form stable chemical bonds with hydroxide ions, creating highly aggregated clusters with strong ionic interactions that hinder migration. Consequently, the diffusion coefficients of metal cations gradually decrease. During the reaction, water dissociates to produce hydrogen ions and hydroxide ions. Ion exchange occurs between the solution cations and Illite cations. Illite cations gradually precipitate and form metal hydroxides by combining with hydroxide ions under electrostatic forces. Protonation propagates from the surface to the internal structure during the reaction. Moreover, the degree of protonation increases with higher NaOH concentrations. Changes in the average ionic energy before and after the reaction indicate that K + exhibits the highest reactivity. Intermediate silicate products are unstable in NaOH solution, with some Si 4+ ions showing higher energy and stronger reactivity.