Air Drilling in the Presence of Hydrocarbons: A Time for Pause
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 108357, "Air Drilling in the Presence of Hydrocarbons: A Time for Pause," by K.P. Malloy, SPE, Stress Engineering Services, and G.H. Medley, SPE, and C.R. Stone, SPE, Signa Engineering, prepared for the 2007 IADC/SPE Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition, Galveston, Texas, 28–29 March.

Drilling with compressed air continues to enjoy great popularity and is often the application of choice in dry, hard rock because the drilling-fluid medium is inexpensive and respectable rates of penetration can be achieved. In terms of economic considerations with respect to drilling-fluid expenditures, air drilling has virtually no equal. However, air drilling is not without its drawbacks, not the least of which is its reactivity with hydrocarbons.

Introduction

Historical Perspective. The first recorded use of compressed air in drilling was in the early 1860s when a piston-type compressed-air mechanical drill bit bored the 8-mile-long Mount Cenis Tunnel in the Alps. In the late 1940s and early 1950s, air drilling became a popular departure from standard rotary-drilling operations.

Safety Considerations. Drilling with compressed air provides many benefits, but misapplication of this technology has led to losses that include both downhole and surface equipment. More seriously, the most devastating of these losses are injuries to rig personnel.

For too long, some operators, drilling contractors, and rig and service personnel have been under the mistaken impression that air drilling is somehow an inherently safer operation than conventional drilling over a broad range of applications. While that can be true, the basic chemical concept of the classical combustion triangle (Fig. 1) often has been ignored or forgotten. Fire is an oxidative process, and combustion can occur only when the three legs of the combustion triangle are present.

Classical studies performed jointly in the 1920s by the US Department of Interior, Bureau of Mines, and the Safety in Mines Research Board of Great Britain describe the hazards of working in an oxygen-rich environment in the presence of hydrocarbons. As long as hydrocarbons are not present, air drilling can be just as safe as drilling with nonreactive fluids common to drilling operations, such as water and water-based, oil-based, or synthetic-based mud.

Chemistry

Air is a mixture of many gases with approximately 21 vol% oxygen. The minimum oxygen concentration needed for combustion of methane (CH4) is only 12 vol% at sea level (Fig. 2).

The primary constituent of natural gas is CH4. In the presence of an ignition source and sufficient oxygen, complete combustion of hydrocarbons occurs to create carbon dioxide (CO2), water, and heat. When there is too much fuel (CH4) or insufficient oxygen, the chemical reaction cannot oxidize the hydrocarbons completely to CO2 and water. The reaction yields carbon black, commonly known as soot, as a solid carbon product along with water and heat.