This paper details the modifications performed and knowledge gained in late 2007 and early 2008 while drilling three underbalanced (UB) vertical gas wells in the Travis Peak formation of the western east Texas deep Bossier play in the Amoroso field, using jointed pipe. The deep Bossier represents one of the most active onshore plays in the United States. The Travis Peak is also known as ‘tragic peak’ during conventional drilling operations because the sandstone formation is hard and abrasive in nature, leading to an extreme low rate of penetration (ROP). Performance UB drilling (UBD) technology was used to increase the ROP through the formation at depths of 9,800 ft (2,987 m) and greater. The surface and the intermediate casing intervals were drilled conventionally. The next section (8 1/2-in. hole size, using hammer and tricone bits) was drilled using UBD techniques that used straight air or membrane nitrogen (N2).
The injection rates for compressed air or membrane generated N2 ranged from 2,800 to 3,400 scfm, and the liquid rates during mist-foam drilling were 14 to 16 gpm (53 to 61 lpm). The section in the first well was drilled with three hammer bits and one tricone bit with an average ROP (including connection times) of 42 fph (13 m/h). Instantaneous ROP in the range of 350 fph (107 m/h) was observed with the hammer bits. A total of 2,378 ft (725 m) in 55.5 hr was drilled UB. During the entire UB phase, penetration rates were significantly greater than the offset records. The operator carried forward the lessons learned from the first well into the second well. A total of 2,750 ft (838 m) in 58.75 hr (using three hammer bits and one tricone bit) was achieved. Based on the second well's success, planning was initiated for the third well with additional improvements in bit design proposed. Enhanced hammer bit design, however, did not yield promising results; the decision was made to drill the section with tricone bits. A total of 3,010 ft (917 m) in 61.7 hr was drilled UB using four hammer bits and three tricone bits. Improved ROP and overall cost savings were the primary objectives for the client and were achieved.
For the rig crew, drilling with compressed air or N2 was a new experience. Rig training was conducted before the drilling phase by the service company personnel.