Casing Drilling with Retrievable Drilling Assemblies
Authors: Warren Tommy , Tessari Robert , Houtchens Bruce 
Published: October 2008

Abstract
Retrievable Casing Drilling* tools have been used to drill more than 600,000 ft of hole in over 120 wells encompassing six casing sizes ranging from 4-1/2" to 13-3/8" and reaching inclinations of 90°. This drilling system is composed of downhole and surface components that provide the ability to use normal oil field casing as the drill string so that the well is simultaneously drilled and cased. The casing is rotated from the surface for all operations except slide drilling with a motor and bent housing assembly for oriented directional work.
A retrievable drilling assembly is attached to the casing inside a proprietary profile nipple located near the casing shoe. The drilling assembly extending below the casing usually includes an underreamer and a PDC or roller cone pilot bit. Other conventional drill-string components may be included so that almost any normal drilling activity can be conducted. These assemblies are retrieved with a wireline at the casing point or at any point in the drilling process when there is a need to change drilling tools.

Introduction
Tesco's Casing Drilling system combines the drilling and casing processes into a single operation to provide a more efficient well construction method. While other service providers offer casing while drilling (CwD) systems directed at drilling individual sections of wells, Tesco provides the first commercial system for drilling the entire well using the casing as the drill string.
Casing while drilling is gaining increasing acceptance as a practical method of reducing drilling costs and solving drilling problems.1,2,3,4,5,6 This activity includes both onshore applications where entire wells have been drilled with casing and offshore applications where only the first hole section or two have been drilled with casing.
Portions of vertical wells may be drilled with casing using a special bit attached to the casing that can be drilled out to run subsequent casing strings. In other instances, a conventional bit may be run on the casing and left in the hole. There are situations where both of these techniques provide the best solution, but when there is a need to drill with a motor without rotating the casing or the section cannot confidently be drilled with a single bit, then a retrievable drilling assembly that can be recovered and re-run is required.
The fully retrievable and re-runnable Tesco Casing Drilling system has been used in over 120 commercial wells to drill approximately 620,000 ft with casing since it was introduced in 1999. These applications range from drilling only shallow surface holes to drilling as deep as 9,400 ft. They include both vertical and directional wells and wells with up to three string of casing with sizes ranging from 4-1/2" to 13-3/8". About 250,000 ft has also been drilled with Tesco non-retrievable tools, to give a total of more than 270 strings of casing that have been drilled with both systems.
Most of this activity has been focused on drilling vertical intervals, but interest in using the Casing Drilling system in directional wells is increasing as the processes for drilling straight holes become proven, the benefits of the Casing Drilling system are demonstrated, and more versatile tools become available.