PDC bits have afforded drillers a means to improve penetration rates in many applications. However, previous attempts to apply PDC technology in air/foam drilling environments have proven unsuccessful primarily due to excessively rapid cutter wear.
A successful test of modern PDC bit designs has been conducted in a foam drilling application of the Appalachian Basin, a widely drilled basin with well-known geological properties. The initiative included identifying applications and refining operating practices while measuring the economic viability of PDC bits in this environment. Encompassing a period of 24 months, this initiative has accumulated performance data from dozens of bit runs, with drilling cost savings and reduction/elimination of borehole deviation as its objectives.
The principle area of focus involves Ordovician period formations in eastern Ohio. Foam is the primary drilling medium with an occasional need to finish the section using light mud for well control. PDC performance has delivered an ROP increase of 300% and a 30% reduction of days-on-well. This proves the ability to reduce drilling costs with PDC bits in certain air and foam applications.
This paper details methods, results, and lessons learned from this study, and translates findings into practical techniques. Operating aspects are discussed, as well as formation characteristics and down-hole conditions. A case study is provided, including performance data and dull bit photographs.
Oil and gas wells in the Appalachian Basin have long been drilled using air-based fluids as circulating media. This ability has been afforded through the combination of stable, competent formations, and relatively low formation pressures. The use of these low density drilling fluids creates a significantly lower pressure down-hole than is seen with conventional liquid drilling fluids. The resultant lower confining stress allows the formations to fail more readily, which produces higher penetration rates, as discussed in W. C. Lyons et al (1984) 1, J. McLennan et al (1997)2, and L. L. Payne et al (1952)3. The combination of faster drilling rates and lower-cost circulating media makes air drilling very desirable to value-conscious operators.
However, there are complications from air drilling, including heat- and vibration-related cutting structure damage, as well as increased abrasive wear due to reduced fluid lubricity. Concerns over increased heat due to poor cooling conditions in air drilling have inspired the use of specialized lubricants and heat-tolerant seals. Dull bit analysis has revealed that mud-drilling bits used in air environments develop flat-crested tooth wear, leading to ROP decline over the run. To combat inherent weaknesses in traditional tungsten carbide insert (TCI) roller cone bits when used in an air drilling application, design modifications have been made to tooth shape, count, and composition, to maximize bit life, as covered in K. C. Brannon et al (1994)4.