Influence of the surfactants' nature on the structure and rheology of organo-montmorillonite in oil-based drilling fluids

Abstract Aiming to select appropriate organo-montmorillonites (OMt) for the use in oil-based drilling fluids, the influences of surfactants' nature on the structure of OMt, structure and rheology of OMt in oil-based drilling fluids were studied in this work. X-ray diffraction (XRD) results indicated a tendency that bigger modifiers resulted in higher basal spacing and more ordered internal arrangement. Scanning electron microscope (SEM) images testified a trend that modifiers with bigger size and low Hydrophile-Lipophile Balance (HLB) value resulted in more OMt lamellaes (thickness of ca. 50 nm) scattered. Contact angle tests demonstrated that lipophilicity of OMt was influenced by the HLB values of surfactants. XRD results of OMt in oil-based drilling fluids indicated a tendency that surfactants with bigger size and low HLB values led to easier exfoliation in oil. OMt modified with small size and high HLB values obviously shrank while OMt modified with bigger size and low HLB values kept exfoliation above 180 °C. The rheological flow model of OMt in oil-based drilling fluids was very different from the surfactants' size and polarity. Exfoliation of OMt in oil improved the rheological properties of oil-based drilling fluids, such as viscosity and thixotropy. Thixotropy of oil-based drilling fluids benefited from the exfoliation of OMt in oil while excessive exfoliation was harmful to increasing viscosity.