Rheological properties of organo-palygorskite in oil-based drilling fluids aged at different temperatures

Abstract Aiming to develop a new excellent rheological additive for use in oil-based drilling fluids, palygorskite (Pal) was organically modified with three cationic surfactants using wet method. The surfactants were octadecyl trimethyl ammonium chloride (C18-A), benzyl dimethyl octadecyl ammonium chloride (C18-B) and dimethyl dioctadecyl ammonium chloride (DC18). The structure and properties of organo-palygorskite (OPal) and its use in oil-based drilling fluids were characterized by X-ray diffraction (XRD), scanning electronic microscope (SEM), thermal analysis, gel volume and rheological tests. Surfactant-modification did not change the crystal structure of Pal, but changed the aggregation of Pal fibers. Surfactants mainly coated on the surface of Pal. But the details were influenced by the molecular scale of surfactants and the coating level. The scale of polar heads of surfactants being smaller than the channels' scale is expected. Surface modification decreased the particle size of Pal and loosened the arrangement of Pal fibers. The thermal stability of OPal decreased as follows: C18-A-Pal > C18-B-Pal > DC18-Pal, as well as the thermal stability of corresponding oil-based drilling fluids. Particularly, C18-A-Pal fluids could bear heat of 200 °C. OPal could increase the viscosity of oil-based drilling fluids well. In addition, the oil-based drilling fluids containing OPal showed excellent thixotropy aged above 150 °C. Temperatures below 150 °C increased the rheological properties of oil-based drilling fluids while high temperature helped to disperse the OPal particles and fibers. The rheological properties of oil-based drilling fluids containing OPal were influenced by surfactants nature (polar head scale of surfactant cation, Hydrophilic-Lipophilic Balance (HLB) value, molecular conformation), coating level and temperature. The rheological properties of OPal fluids indicated that OPal was very suitable for oil-based drilling fluids.