Molecular insight into inhibitory performance of CTAB surfactant for montmorillonite swelling; implications for drilling fluid design

Abstract

The swelling behavior of expansive clay minerals, specifically montmorillonite (MMT), can impose severe challenges (e.g., hole instability) during subsurface drilling when the inhibitory properties of drilling fluid are not appropriately optimized. In this research work, the molecular dynamics (MD) simulations were employed to quantitatively evaluate the inhibitory performance of cetyltrimethylammonium bromide (CTAB) in retarding the transition state between crystalline and osmotic swelling of sodium montmorillonite (Na-MMT). The combined impacts of CTAB and KCl were assessed, and inhibition mechanisms were also elucidated. Comprehensive trajectory-based analyses showed that an addition of 10 g/L CTAB into the Na-MMT interlayer reduces the d-spacing by 5.18% from 19.3 Å to 18.3 Å, through cation-exchange, wettability alteration, and surface charge compensation mechanisms. The parallel orientation of CTA+ cations to the clay surface, forming monolayer and bilayer structures, assists in bringing the clay layers closer together. The assessment of the combined impacts of inhibitors revealed that, despite the Na+ cations having a higher hydration enthalpy compared to K+ cations, the higher ratio of K+/Na+ diminishes the performance of the combined CTAB and KCl. Our results showed that the addition of KCl up to 8 wt% into the interlayer improves the inhibitory performance of 10 g/L CTAB by further decreasing the d-spacing to 17.6 Å.