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This paper details the modifications performed and knowledge gained in 2007 and 2008 while drilling four underbalanced<br>(UB) vertical gas wells in the Travis Peak formation of the western east Texas deep Bossier play in the Amoroso field, using<br>jointed pipe. In spite of continuous improvements in drilling bit performance and better drilling practices, operators still face<br>the “80-20 rule.” Extremely hard and abrasive intervals represent about 20% of the total footage drilled, and they take about<br>80% of the time to drill out. Performance UB drilling (UBD) technology was used to increase the rate of penetration (ROP)<br>through the formation at depths of 9,800 ft (2,987 m) and greater. The surface and the intermediate casing intervals were<br>drilled conventionally. The next section (8 1/2-in. hole size, using hammer and tricone bits) was drilled using UBD<br>techniques that used straight air or membrane nitrogen (N2).