This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 92195, “Casing Directional Drilling Using a Rotary Steerable System,” by Robert Strickler, SPE, and Todd Mushovic, SPE, ConocoPhillips; Tommy Warren, SPE, Tesco; and Bill Lesso, SPE, Schlumberger, prepared for the 2005 SPE/IADC Drilling Conference, Amsterdam, 23–25 February.
The hardware used in casing-while-drilling operations has proved to be robust and reliable. Several directional wells have been drilled successfully with casing using positive-displacement motors (PDMs), but the drilling efficiency was low. A rotary steerable system (RSS) improved drilling efficiency when drilling with casing.
Introduction
ConocoPhillips began a field-development program in 1997 in the Lobo trend of south Texas. More than 900 wells have been drilled through the Wilcox (Lobo) section ranging in depth from 7,500 to 13,000 ft. In 2001, after drilling approximately 600 wells, a program was undertaken to find ways to reduce drilling costs sufficiently to extend the development potential for several years. Any major reduction in drilling time had to address the flat time more than the making-hole time. A casing-while-drilling system was chosen for the Phase 1 five-well pilot project. Sufficient progress was made in drilling these first wells to justify moving to a second phase of drilling.
The second phase proved that casing while drilling could eliminate the formation-related trouble time experienced with conventional drilling. Lost circulation was almost completely eliminated, allowing the drilling of additional wells formerly considered uneconomical. The wells were not drilled trouble-free, but the trouble was associated with the mechanical-equipment limitations, which were overcome on the Phase 2 wells.
Retrievable Casing-While-Drilling Process
Casing can be used as all or part of the drillstring in several ways. Systems are either retrievable or nonretrievable. Nonretrievable systems include both liner-drilling applications and applications with full strings of casing with which a fixed bit is used for drilling. The bit may be drillable (i.e., drilled out to drill the next hole section), or it may be a conventional bit that is left in the hole at total depth.
Retrievable systems allow the bit and bottomhole assembly (BHA) to be changed without tripping the casing. Use of a retrievable system is the practical choice for directional wells because of the need to recover the directional-drilling and guidance tools, the need to replace failed equipment before reaching casing point, and the need for quick and cost-effective access to formations beyond the casing shoe.
A retrievable casing-while-drilling system has downhole and surface components that enable standard oilfield casing to be used as the drillstring so that the well is simultaneously drilled and cased with the wireline-retrievable drilling BHA suspended in a profile nipple near the bottom of the casing. The top component of the BHA that facilitates attachment to the profile nipple is the drill lock assembly (DLA). The casing is rotated from the surface, and the drilling fluid is circulated down inside the casing and back up the annulus.
