Unique Drilling Challenges at Danville
Authors: Wisniewski J.W. , Tumlinson V.H. 
Published: December 2007

Summary
The 20,661 Danville USA A#1 well was the first successfully drilled deep Arkoma Basin Arbuckle test in central Arkansas. Extremely hard abrasive sands coupled with highly dipping beds, severe faulting, well bore instability, high temperature, hydrogen sulfide potential and environmental concerns in the Ouachita National Forest combined to make drilling Danville a unique challenge. Despite the failure of all previously attempted wells to reach the Arbuckle objective, current drilling techniques, technology and the appropriate application there of resulted in a successfully drilled well.

Introduction
At Danville, a number of engineering and operational techniques were utilized to achieve success. Several of the innovative solutions applied included bit selection, design and modification.
Large diameter flat bottom hammer bit use was extended beyond depths previously drilled. Record setting depth runs with 30" and 22" flat bottom hammer bits were achieved. Problems with the large bits were identified for future research and development efforts. Diamond enhanced carbide inserts were used to overcome accelerated wear in tricone bits. Nose and gauge cutters were retrofitted in 22" and 8-½" bits with a new diamond compound which overlaid carbide material. Conventional and domed PDC bits were successfully used in several hard, dense shale intervals. PDC bit use strategy was developed for future wells.
Multiple drilling fluid media were employed to achieve success at Danville. Air and foam operations in large hole sizes were performed with air volumes exceeding 8,000 scfin. Aerated water based mud, conventional water based mud and oil based mud were used. Also, a new environmentally safe, high temperature, chrome free water based mud system was used successfully for the first time.
A focused drill string quality program was implemented which reduced bottom hole assembly failures to zero with estimated savings of over one million dollars.
Extremely hard, faulted formations with dips in excess of 70 degrees contributed to directional drilling design challenges. Bottomhole assembly models which incorporated geologic and seismic interpretation aided in surface location placement. Theoretical directional profile information was used in casing wear and torque and drag modeling.
Due to the large number of expected rotating hours, casing wear prediction modeling, well design efforts and operating procedures were employed to reduce the effects of casing wear. Strict dogleg limits and an aggressive drillpipe protector program successfully reduced casing wear to manageable levels.

PLANNING
The Danville USA A#1 was drilled into the Arbuckle formation in section 33, township 5 north, range 22 west in Yell county just outside the community of Danville, Arkansas. The well was drilled to a depth of 20,661' with an 8-½" hole at total depth in 636 days.
The location was in the Mkoma Basin, located in southeast Oklahoma and west central Arkansas, and encompasses all or parts of Latimer, Leflore, Haskell and Pittsburgh counties in Oklahoma and Sebastian, Logan, Scott, Yell, Pope, Conway and Faulkner counties in Arkansas.
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