Development of a new approach for modelling the tribocorrosion of cemented tungsten carbide in oil and gas drilling environments

• Main Author: Herd, Stephen
• Other Authors: Wood, Robert
• Published: University of Southampton 2017
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.741690

One of the primary activities involved in oil & gas recovery is the drilling operation that allows access to the reservoir rock. Cemented tungsten carbide (WC-Co) is used to protect downhole components and is subject to both abrasive wear and corrosive degradation, called a tribocorrosion system. The combined result of these degradation mechanisms produces high and sometimes unpredictable wear rates that lead to challenges in the prediction of component service life. The objective of this study is to lay the foundations for a new way of understanding of WC-Co when exposed to tribocorrosion through use of numerical modelling and experimental techniques. A new numerical modelling technique, the combined finite-discrete element method (FDEM), is proposed as a suitable method for the study of WC-Co. FDEM software Y-Geo, originally used for geomechanical applications, is used to simulate fracture explicitly in a WC-Co microstructure. For the first time a model was able to capture the discrete steps of fracture behaviour described from experimental observations. Furthermore, the simulated final fracture path matched closely with that obtained experimentally. Through the process of creating a fracture model a number of knowledge gaps were highlighted in the current literature. In order to move towards a greater collaboration between the modelling and experimental sides of the field a new experimental focus on the degradation of microstructure and material properties at the mesoscale is proposed. Established mechanical and electrochemical techniques are coupled in new ways in order to bring a greater understanding to the degradation of WC-Co when exposed to corrosive environments. Potentiodynamic polarisation tests in acidic solutions (sulphuric acid) indicate ‘pseudo passive’ behaviour, whereas alkaline solutions (sodium hydroxide) show no signs of passivation although is more prone to pitting. Nanoindentation and Palmqvist tests (used for measuring fracture toughness) are performed on the corroded surfaces and compared against uncorroded samples. There is a clear decline in the fracture properties with acidic conditions having the largest impact. This project looks to set a new course for the study of tribocorrosion by explicitly modelling tribocorrosion through understanding the effect of corrosion on both microstructure and intrinsic material properties.