The Effect of Oxidation on Viscosity of Oil-Based Drilling Fluids
Authors: Shahbazi K. , Mehta S.A. , Moore R.G. , Ursenbach M.G. 
Published: March 2010

Abstract
True downhole rheological properties affect equivalent circulating density, hole cleaning, barite sag, surge/swab pressures during tripping, pump pressure, and bit hydraulics. Gelation and excessive viscosity are major concerns at high temperatures. When using oil-based drilling fluids lightened by an injection of de-oxygenated air (containing small amounts of oxygen, usually around five per cent) in underbalanced drilling (UBD) operations, there is a need to be able to predict the effect of oxidation on the viscosity of the oil-based muds as a function of temperature and pressure.
This paper presents the results of an experimental investigation that was aimed at establishing the effect of oxidation on the viscosity of an oil-based drilling fluid. The drilling fluid was aged for 2.5 days and for 7 days in the presence of air at temperatures ranging from 100 to 150 °C and at pressures ranging from 14 to 44 MPa. The viscosity of the drilling fluid samples after aging is compared with the corresponding fresh samples (before aging). The results show that oxidation causes an increase in viscosity. The amount of the increase depends on the amount of oxygen reacted, which is a function of temperature, pressure, and time. The higher temperatures of the reactors lead to the higher increases in viscosity. Furthermore, at higher temperatures, solid (mostly coke) formation was observed.

Introduction
Underbalanced drilling (UBD), in which drilling fluid pressure is lower than reservoir pore pressure, has been studied by many researchers since the late 1980s. This method can improve the financial returns on drilling a well by reducing formation damage (especially in horizontal wells and depleted reservoirs), minimizing lost circulation (especially in fractured reservoirs and depleted reservoirs), increasing the penetration rate of drilling, increasing bit life, minimizing differential sticking, reducing stimulation, and improving formation evaluation(1). The underbalanced condition is created by using different drilling fluids such as dry air, nitrogen, natural gas, mist, stable foam, stiff foam, gasified liquids, glass bubbles, and liquids.
In gasified liquids, a gas is injected into the liquid to reduce the density of the drilling fluid. In these mixtures, since the pressure difference between the formation and the wellbore is relatively low (1.72 - 3.45 MPa), these drilling fluids have fewer UBD related problems such as wellbore instability and formation fluid inflows(1). These specifications have made them the most appropriate drilling fluid in UBD.
The unique performance of oils makes them preferable to water as the liquid phase in gasified liquids. Oils prevent clay swelling and they are resistant to contaminants. Their rheological properties are stable at high pressures and temperatures. Also, they are highly lubricious and non-corrosive. On the other hand, to prevent downhole fires, de-oxygenated air from membrane units with maximum five per cent oxygen is usually used as the gas phase in gasified liquids(2). The presence of oxygen causes the oil in the gasified liquid to be oxidized. In this paper, the effect of oxidation on the viscosity of the gasified liquids is investigated.