Casing-drilling-system design considerations

The casing-drilling system (CDS) provides an alternative to conventional drilling systems by use of ordinary casing for the drillstring. This enables the well to be cased as it is drilled, which reduces well costs. While potential savings from reduced drillstring-tripping and -handling times are important, savings from reduced hole problems may be more significant. Because the CDS process provides the ability to circulate the well continuously, it is safer than leaving the well static, without a means of circulating, while a conventional drillstring is tripped. Reduced pipe tripping with the CDS also should reduce surge and swab pressure fluctuations. The CDS has been under development for 3 years and has been used to drill two directional test wells. Recently, it was used to drill portions of 10 wells as part of the first field trials.