Rheological properties of bentonite-free water-based drilling fluids with novel polymer viscosifier

Water-based drilling fluids with no bentonite content was particularly advantageous in oil and natural gas exploration due to the excellent characteristics including faster rates of penetration, enhanced hole cleaning, less formation damage, etc. However, how to keep the excellent rheological regulation of bentonite-free drilling fluids was still one of the most serious issues for the mud designers. In this study, novel polymer viscosifier (SDKP) was prepared under optimal synthesis conditions through the copolymerization of N-vinylcaprolactam and sodium 2-acrylamido-2-methylpropane sulfonate. The effect of SDKP on rheological behavior of bentonite-free fluids was studied according to American Petroleum Institute standard. Results showed that SDKP could improve the rheological properties of bentonite-free fluids. In addition, the good thermal stability and salt stability in improving rheological behavior of fluids made SDKP more attractive. Rheological behavior of SDKP based bentonite-free fluids at different temperatures was also evaluated. Results showed that SDKP based bentonite-free fluids showed certain thermo-viscosifying behavior in the temperature range of 80-140 °C even in high salinity, which was benefit for carrying cuttings. Shear test showed that the magnitude of the thermo-thickening viscosity was very sensitive to shear rate, and SDKP containing fluids became better shear-thinning upon heating, which could be more easily pumped into the well. In addition, the mechanism of SDKP in improving rheological behavior of bentonite-free fluids was revealed based on the investigation of morphology of SDKP through Environmental Scanning Electron Microscopy experiment.