High-Pressure Flammability of Drilling Mud/Condensate/Sour Gas Mixtures in Deoxygenated Air for Use in Underbalanced Drilling Operations
Authors: Mehta S.A. Raj , Moore R.G. , Pratt C.A. Kip , Gair S.D. , Hoyer C.W.J. 
Published: December 2007

Abstract
Underbalanced drilling (UBD) has proven to be a viable technique to reduce drilling damage caused in part by drilling fluid invasion. Companies are now starting to look at ways to optimize the UBD process. The most common method of reducing the hydrostatic pressure of the drilling fluid column is to inject a gas, typically nitrogen, into the drilling "mud" for circulation through the wellbore.
One of the major challenges for remote or long duration UBD operations is the liquid nitrogen supply. This has increased interest in the use of air or deoxygenated air (membrane) technology to reduce costs and logistical challenges associated with liquid nitrogen. However, while underbalanced drilling, there is potential for flammable, or explosive mixtures to be present in the wellbore and surface flowlines when air, or gas containing oxygen is injected with the drilling "mud".
The purpose of this investigation was to determine the flammability of several mixtures of gas, condensate and drilling mud in deoxygenated air (less than 21% oxygen by volume) in order to minimize the potential for ignition and/or explosion during underbalanced drilling or work-over operations. Two areas that received the main focus were membrane deoxygenated air (5% oxygen) and the role that hydrogen sulfide (H2S) played with respect to flammability.
Over three thousand individual flammability tests were carried out encompassing a spectrum of conditions which could be realistically encountered in the field. These tests were performed using different mixtures of gas, condensate and drilling mud at pressures and temperatures ranging from atmospheric to reservoir.
This paper outlines the test apparatus and procedures used. It will also detail the results of the tests and give recommended guidelines to stay below flammable/explosive limits found.

Introduction
There are significant reserves of hydrocarbons in the Foothills region of Alberta which could be more efficiently produced with the implementation of new technologies. One such technology involves the drilling of wells employing underbalanced or near-balanced drilling procedures in order to minimize the potential for formation damage due to drilling fluid invasion. Underbalanced drilling also has many other potential advantages, such as increase of rate of penetration, productivity testing while drilling, elimination of differential sticking while drilling, drilling fluid options, reduction in completion costs, etc.
In underbalanced drilling, gas is injected and/or circulated with the drilling mud to reduce the potential for formation damage due to whole mud, fluid filtrate and solids invasion into the hydrocarbon-bearing formation. Cases which are commonly used are nitrogen, natural gas, normal air or an oxygen-containing gas (usually vitiated air, which is air mixed with nitrogen, or de-oxygenated air, which is air with some of the oxygen removed). Of these choices, the oxygen-containing gases are the least expensive; however, there is a potential for flammable, or explosive, mixtures to be present in the wellbore and surface piping during this type of drilling operation.
When developing an underbalanced drilling strategy for a gas condensate well, the flammability of the condensate/drilling mud/gas mixture must be determined specifically for the reservoir and the conditions which will be present during the drilling operation. P. 267