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Abstract This paper details the modifications performed and knowledge gained in 2007 and 2008 while drilling four 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. In spite of continuous improvements in drilling bit performance and better drilling practices, operators still face the "80-20 rule." Extremely hard and abrasive intervals represent about 20% of the total footage drilled, and they take about 80% of the time to drill out. Performance UB drilling (UBD) technology was used to increase the rate of penetration (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 remaining 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. The fourth well was drilled using only tricone bits under UB conditions to compare the ROP between hammer bits and tricone bits. A total of 2,985 ft (910 m) in 76.9 hr was drilled UB using five 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.
Introduction Conventional drilling techniques in hard-rock areas tend to be slow and are more expensive with increasing daily rig rates. The Travis Peak formation is located on the western flank of Robertson County, Texas, USA. The deep Bossier represents one of the most active onshore gas plays and has been assessed as one of the United States' largest gas fields, with estimated reserves of 4 Tcf of gas. UBD technology was used to increase the ROP at depths of 9,800 ft (2,987 m) and greater.
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