Safety Considerations For Underbalanced Drilling of Horizontal Wells Using Air Or Oxygen-containing Gas
Authors: Mehta S.A. , Moore R.G. , Laureshen C.J. , Samuel P. , Teichrob R.R. , Bennion D.B. 
Published: March 2010

Abstract
The underbalanced drilling of horizontal wells involves the injection of gas either with the drilling mud or via a separate string to that through which the drilling mud is injected. Nitrogen is generally used to prevent fire and explosion hazards during drilling; however, this adds considerably to the cost. Costs can be reduced by the replacement of nitrogen injection with normal air, de-oxygenated air or vitiated air (air/nitrogen mixture), but this increases the potential for flammable, or explosive, mixtures to be present in the wellbore and surface piping during underbalanced drilling. Safe operational ranges of oxygen-containing drilling mud mixtures can be determined if the flammability of the mixtures is known. Several studies were carried out to determine the flammability of these mixtures as a function of pressure to minimize the potential for downhole and surface explosion, and to set out the guidelines for safe underbalanced drilling operations of horizontal wells.
A specially-designed apparatus was calibrated utilizing methane/air mixtures, then the ignition and the flammability characteristics of various mixtures of air, drilling mud and live oil were established over a range of pressures from atmospheric to 2,100 kPa at 21 ° C. Additional tests were carried out utilizing drilling mud, live oil, and either a nitrogen/air mixture (vitiated air) or de-oxygenated air. Flammability limits were determined for each series of experiments, and the results used to design the safety features for the underbalanced drilling of several horizontal wells. The field trials proved successful, and significant cost savings were realized.

Introduction
A growing demand for maximizing heavy oil production in a cost-effective manner has led to the development of a variety of novel technologies. One such technology involves the drilling of wells employing underbalanced or near-balanced drilling procedures in order to minimize the possibility of formation damage due to drilling fluid invasion. There is a potential for flammable, or explosive, mixtures to be present in the wellbore and surface piping during this type of drilling operation. In underbalanced drilling, 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) 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. The study presented in this paper was performed to determine the safe conditions for underbalanced drilling of Husky Wainwright Well 15B-31-44-4 W4 using compressed air and liquid nitrogen. This well, located in the Camrose reservoir, which is a fractured vugular carbonate at 690 m TVD, was proposed to be drilled 550 m laterally(1). The main objectives of this investigation were:To establish the flammability of mixtures of air, live heavy oil and drilling mud as a function of pressure, andTo determine the optimum composition of vitiated air (nitrogen- air mixture), based on the flammability data, in order to minimize the potential for ignition or explosion during the underbalanced drilling operation.