Effect of Oxidation On Explosion Potential And Barite Sag Reduction In Oil- And Synthetic-Based Drilling Fluids
Authors: Shahbazi K. , Mehta S.A. , Moore R.G. , Ursenback M.G. 
Published: March 2010

Oil and synthetic-based drilling fluids are the drilling muds of choice in high pressure and high temperature wells. The hot drilling fluid is returned to the surface and is exposed to air in the mud tanks. Since the base liquids of these drilling fluid systems are expensive, they are used in many wells before disposal. The long exposure of the oil to air can result in a substantial oxidation of the oil; especially in hot reservoirs in summer temperatures.

To simulate the oxidation of these drilling fluids, an experimental study was conducted in which 20 oil-based and synthetic-based drilling fluid systems which are used in Western Canada, were examined. The drilling fluid systems were aged in stationary reactors in the presence of one charge of air for 52 hours at 150 ºC and 13.5 MPa initial pressure. While rapid heat releases or explosions in the synthetic-based drilling fluids were observed, oil-based systems were safe under the conditions tested. The rheological properties of the drilling fluid samples after aging were compared with the original (before aging) samples. The results at an ultra low shear rate range of 0.0 – 0.2 s−1 and a high shear rate range of 0 – 4,000 s−1 are presented. It is shown that the aged drilling fluid systems exhibited a tremendous increase in viscosity at ultra-low shear rates. Furthermore, relatively low increases in viscosity at medium and high shear rates were observed.

Introduction

Demand for energy sources is increasing throughout the world at a rapid rate due to industrial development and population growth. The search for new sources of oil and gas on land and offshore, and also developing current resources, has intensified due to this increasing demand. Oil production involves two main phases of drilling operations; namely, exploration and development. The exploration phase operations determine the potential hydrocarbon reserves and include drilling of exploratory wells, whereas, the development phase operations include drilling of production wells once a hydrocarbon reserve has already been discovered.

Drilling fluids are one of the essential components of the drilling operation. Inappropriate design of a drilling fluid system can lead to numerous problems such as stuck pipe, lost circulation, wellbore instability, gellation, insufficient hole cleaning, bit balling, barite sag, excessive torque and drag, corrosion, H2S contamination and plugging due to gas hydrates.

Due to some underbalanced drilling applications that employ gasified liquids, oxidation of oil and synthetic-based drilling fluids has recently received some attention. Historically, in situ combustion(1) and high pressure air injection (HPAI)(2, 3) are two major areas in thermal enhanced oil recovery techniques where the oxidation of oils are of special importance. In these processes, it is of vital importance to know what effects the oxidation has on rheological properties of the oil, because it affects the mobility of the oil and, therefore, the recovery factor. In drilling fluids, in addition to changes in rheological properties, safety considerations and explosion potential require investigation.

In the first phase of this research, the oxidation of different base oils was investigated.