There are a number of systems that have been developed in an attempt to eliminate the need to use drill pipe by utilizing the casing string as the drillstring. These systems vary as to whether or not the casing is rotated or a downhole motor is used, and whether the bit is replaceable via wireline or "drillable."

With Casing Drilling™, a well is drilled with standard oilfield casing in place of drillpipe, allowing an operator to simultaneously, drill, case, and evaluate a well. Drill bits and other tools are lowered inside the casing to the bottom of the hole via wireline, where they are latched to the last joint of casing. Specially designed underreamers extend out from the bottom hole assembly (BHA) to create a hole sufficient for the casing and cement. For directional and horizontal wells, the BHA can be fitted with conventional mud motors and measurement-while-drilling (MWD) tools.

Illustration courtesy of Novatek, Inc.
Novatek's drilling system includes a casing generated in situ
(actual casing of the system with a composite pipe generated while drilling.)

A top drive system is used to rotate the casing, which remains in the hole at all times and is eventually cemented in place when the appropriate depth is reached. Special equipment is needed to handle the wireline-retrievable BHA; a split crown and split traveling blocks facilitate wireline access to the top of the casing through a wireline blowout preventer. A wireline unit sufficient to run and pull the BHA is also needed.

Casing Drilling™ was developed and patented by Tesco Corporation of Calgary, Alberta. The process is being commercialized through a joint venture with ConocoPhillips, where Tesco is conducting a two-year development drilling program in the operator's South Texas Lobo natural gas trend.

Weatherford International Ltd.'s Drilling with Casing (DwC™) utilizes a drillable bit that is made up onto the casing. When TD is reached, the casing can be cemented into place by pumping through the bit. If the hole must be deepened, the bit is designed to be drilled through to continue conventional drilling.

The advantages of avoiding the need for a drillstring include:

• Elimination of tripping and other pipe-handling reduces rig time and manpower requirements and related safety incidents and trip-related problems (e.g., kicks, swab and surge pressure problems, key seats and unintentional sidetracks),
• Elimination of rental expenses related to drill collars and drillpipe,
• Reduced required horsepower (and thus lower maintenance, lower fuel costs, and improved bit hydraulics),
• Reduced capital and logistic costs due to lighter weight substructure and derrick, and
• Reduced mud and cementing costs due to smaller wellbore diameter.

Another innovative approach to casing-while-drilling involves a drilling system from Novatek, Inc., illustrated above,  that generates a composite casing in situ while drilling. This new concept eliminates not only the need to trip out a drillstring and run a casing string into the hole, but the entire cementing operation as well.