Investigation on the inhibition mechanism of alkyl diammonium as montmorillonite swelling inhibitor: Experimental and molecular dynamics simulations

Abstract

Effective inhibition of mud shale hydration and swelling is the major challenge for water-based drilling fluids (WBDFs). In this paper, a series of novel low molecular weight alkyl diammoniums with different alkyl chain lengths was synthesized and evaluated as montmorillonite swelling inhibitor. The inhibition behavior of alkyl diammonium was studied by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), cation exchange capacity (CEC), and molecular dynamics simulation. The XRD results illustrated that the inhibitors (Gs-9, Gs-10, Gs-11, Gs-12, Gs-13 and Gs-14) rapidly lowered the basal spacing (d001 value) of hydrated sodium montmorillonite (Na-Mt) from 1.92 nm to 1.41 nm, which indicates that a flat monolayer of inhibitors was inserted into the interlamination of Na-Mt. The XPS and CEC results showed that the alkyl diammonium displaced exchangeable cations in the interlamination of Na-Mt. The SEM results showed that the alkyl diammonium effectively improved the surface structure and curled edges of hydrated Na-Mt. The simulated results showed that the quaternary ammonium functional groups were fixed firmly on the six- oxygen rings of Na-Mt. The adsorption energy of alkyl diammonium increased with the increase in alkyl chain length. Therefore, alkyl diammonium with long alkyl chain can decrease the amount of inhibitor adsorbed on Na-Mt, thus reducing the dosage of inhibitor used in the WBDFs. This is conducive to reducing drilling costs. Therefore, this research on the inhibition behavior of alkyl diammonium with different hydrophobic chain lengths has important reference value for the development of WBDFs and suitable inhibitors for them.