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Safety considerations for underbalanced drilling of horizontal wells using air or oxygen-containing gas. S.A. Mehta et al. J. Can. Petrol. Technol., September 1998, 37(9), 30--35.

The use of gas injection during the underbalanced drilling of horizontal wells is discussed. Nitrogen prevents fire and explosion hazards but it is expensive. Air, de-oxygenated air, or nitrogen/air mixtures reduce costs but increase the potential for flammable or explosive mixtures. A test rig is described and illustrated which is used for determining the flammability of air, live heavy oil, and drilling mud as a function of pressure. Flammability data are used to determine the optimum composition of vitiated air needed to minimize ignition and explosion hazards. Effects of pressures from atmospheric to 2100 kPa at 21 C are determined. Guidelines are recommended for safe field operations during underbalanced drilling operations. 4 refs.

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