Evaluation method of thermal stability of bentonite for water-based drilling fluids

The quality of bentonite directly affects the safety and economic benefits of oil well drilling engineering. Due to the fact that most of the qualified drilling-grade bentonite used in water-based drilling fluid under current API standards are modified by physico-chemical methods, its performance maybe extremely unstable in high temperature conditions, which implies that the viscosity of drilling fluid will decrease or increase rapidly and may subsequently lead to downhole accidents. Therefore, China National Petroleum Corporation (CNPC) has proposed a new evaluation standard of bentonite Q/SY 17009-2019, which will be used to test the quality of high temperature stability through monitoring the performance of bentonite after aging at high temperature. For scientific evaluation and better understanding of this new standard, this paper focuses on researching several natural bentonite and industrial modified drilling-grade bentonite samples. Exploring their differences in performance after different aging condition under ambient temperature and high temperatures (120 °C, 150 °C and 180 °C). In addition, it also investigates the rationality of drilling-grade bentonite under CNPCs' new standard. The results shown that the crystallization characteristics of natural bentonite causes it to form a network lattice structure under high temperature, leading to an increase in suspension viscosity with increasing temperature. On the contrast, the performance of industrial drilling-grade bentonite after aging at high temperature is quite different, the viscosity of some samples increases greatly, while others decrease rapidly. By analysis of experimental data, it can be concluded that the new standard Q/SY 17009-2019 is more scientific and reasonable than the old standards API SPEC-13A-2010 concerning “drilling fluids material specification”. Through supplementing different grades of the performances after aging at high temperature, and then grading the bentonite, it is more suitable for the actual needs of drilling engineering, which could be considered as a more reasonable scheme to evaluate drilling-grade bentonite.