The effect of oxygen partial pressure on oxidation of an oil base liquid

Oil base liquids are used as the continuous phase in the majority of oil-based drilling fluid systems in conventional drilling and as the liquid phase in gasified liquids (aerated muds) in some underbalanced drilling operations. These fluids can also be used as the friction reducing agents in situations where a stuck drillstring is to be freed. Understanding the oxidation behavior of these liquids is paramount in the drilling industry, as it may lead to explosion and cause changes in the rheological properties.An experimental investigation was conducted to establish the effect of oxygen partial pressure on the oxidation of an oil base liquid that is used as the continuous phase in oil-based drilling fluid systems. It was aged for 7 days at an initial total pressure of 14 MPa and temperatures of 120, 135, and 150 °C in the presence of oxygen/nitrogen mixtures with oxygen mole fractions of 0.1019, 0.2189, 0.3971, and 0.7968. Characterization of the post-test gases and liquid phases before and after oxidation and measurements of the amounts of solids formed and viscosity of the clear and oxidized liquid were carried out. The results show that the extent of oxidation is reflected by the cumulative concentration of CO plus CO2 as well as by the solids precipitated and viscosity. Pressure decline curves indicated two oxidation regions. The main consumption of oxygen occurs in the second region. The division between the two regions reflects an induction delay time. As the percentage of oxygen in the initial gas or oxygen partial pressure is increased, the induction delay time is decreased. Furthermore, the variations of the solid precipitated and viscosity ratio with oxygen mole fraction in the initial gas are in a direct relation to the amount of total CO and CO2 produced. These three parameters are in turn related to the extent of oxidation as measured by the cumulative oxygen uptake.