Application of carbon coated bentonite composite as an ultra-high temperature filtration reducer in water-based drilling fluid

During high temperature and high pressure well drilling, effective filtration loss control is a great challenge for a water-based drilling fluid. In this study, environmentally benign carbon coated bentonite composite (CCBC) was designed and fabricated with a hydrothermal carbonization using glucose, sodium bentonite and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) as raw materials. CCBC was characterized by Fourier transform infrared (FT-IR) spectrometry, Raman spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The effectiveness of CCBC to control the rheological and filtration properties of bentonite suspension and water-based drilling fluid before and after thermal aging at 220 °C and 240 °C was investigated. The results indicated that CCBC imparted minor influence on the rheological characteristic of bentonite suspensions before and after thermal aging, but decreased the filtration loss of bentonite suspension and water-based drilling fluid remarkably after thermal aging, superior to some conventional commercial high temperature resistant filtration reducers. CCBC functionalized with abundant oxygenated groups and sulfonic acid groups enhanced the colloidal stability of bentonite particles at high temperatures. Meanwhile, part of free carbon nanospheres in CCBC particles could fill the micro-pores of filter cake contributing to thin and impermeable filter cakes.