Oxidation Studies of Light Hydrocarbons for Underbalanced Drilling Safety
Authors: Ferguson H.A. , Metha S.A. , Moore R.G. , Okazawa N.E. , Ursenbach M.G. 
Published: March 2010

Economic development of many of the world's oil and gas reserves will require the application of improved drilling methods such as underbalanced drilling (UBD), a technique used to minimize formation damage in hydrocarbon reservoirs. In UBD operations, oxygen-reduced air is often injected along with an oil-based drilling fluid. This oxygen-reduced air may contain up to 5% oxygen which can react with oil and/or an oil-based drilling fluid during the drilling process to form potentially hazardous oxidized hydrocarbons. This could lead to the presence of potentially flammable or explosive mixtures in the surface piping and/or drill string, particularly the return annulus. Discussion of the oxidation behaviour of light oils and oil-based drilling fluids in the literature is limited, particularly regarding the effects of long-term exposure to oxygen.

This paper characterizes the oxidation behaviour of two light oils and an oil-based drilling fluid using thermogravimetry (TG) and differential scanning calorimetry at 89 and 9,811 kPa (1,410 psig). The effects of core, pressure, and pre-exposure to oxygen on subsequent oxidation behaviour were studied.

The results show that the addition of crushed core enhanced the oxidation reactions of the oil samples and delayed exothermic activity of the drilling fluid. Elevated pressure accelerated the oxidation reactions. Pre-exposure to oxygen was shown to increase the reactivity of the oils and drilling fluid during subsequent oxidation. The results indicate that under certain conditions, oxidation processes could begin at significantly reduced temperatures, possibly leading to ignition of the hydrocarbon mixtures.

Introduction

Underbalanced drilling is a technique used to minimize formation damage caused by drilling fluid invasion in hydrocarbon reservoirs. In order to achieve this technique, the hydrostatic head of liquid in the hole must be decreased, which can be accomplished by co-injecting nitrogen, natural gas, flue gas, vitiated air (air mixed with nitrogen), or oxygen-reduced air (air with some of the oxygen removed). The oxygen containing gases are the least expensive, but the possibility of flammable or explosive mixtures is an important safety consideration. Oxygen-reduced air is normally obtained using portable membrane separation systems but often still contains up to 5% oxygen by volume. During the drilling process, this oxygen can react with produced hydrocarbons or the oil-based drilling fluid to form peroxides and other oxidized hydrocarbons. These oxidized hydrocarbons could pose a safety hazard. While the oxygen concentrations under consideration are relatively small, the pressures and temperatures in the reservoir and along the drill string are significant. Added to this is the effect of prolonged exposure to oxygen. Drilling fluid is re-circulated throughout the drilling operation and may be used in multiple operations. It may also be exposed to oxygen while in the drilling mud tanks at the surface (during the drilling operation) and in storage tanks. Over time, the fluid may be gradually oxidized, posing an increased risk of combustion or even explosion.

Due to the serious ramifications of undesired oxidation processes, it is necessary to establish fundamental information on the oxidation behaviour of the hydrocarbons involved in these processes.