Casing While Drilling (CwD)

CWD has been around for many years and it is one of proven technologies that can save both time and money. CwD is a process where a well is simultaneously drilled and cased; the casing is used for the drill string, and is rotated to the drill and cemented into the well at TD. One of the main benefits of this process is that it greatly cuts down on the tripping time needed to pull out the bottom hole assembly (BHA) and run the case- if not removing this need entirely. Therefore, the flat time is reduced, and the process is made more economically viable.

Types of Casing while Drilling Systems:
Three main types of CwD, which is determined by the configuration and operation of the drill, are as follows:

  • Non-Retrievable Casing While Drilling System
  • Retrievable BHA Casing While Drilling System
  • Drilling with Liner Systems

Non-Retrievable Casing While Drilling System:
The non-retrievable system is the simplest type of CwD. In this case, the system is made up of a drillable bit or drill shoe, a casing string, and a casing drive system. The drill shoe is fitted securely to the bottom of the casing string; the latter is rotated by a power swivel which is hooked up to the drive system. This system only offers a limited number of options- it can only drill in a straight hole, and to a pre-determined depth.

Retrievable BHA Casing While Drilling System:
The retrievable casing while drilling BHA system strikes a balance between conventional drilling tools and CwD. The main advantage of this system is that it can be steered, and used with both conventional measured while drilling (MWD) and logging while drilling (LWD) tools.

Advantages and disadvantages of all three hole enlargement methods are shown below;

Reaming Casing Shoe:
🔸Advantages:

  • Maintain good directional control and LWD reading
  • No risk of reamer not collapse when retrieving

🔹Disadvantages:

  • Rat hole equal to BHA length. This will leave the longest rat hole among three methods.
  • Limited reamer RPM equals to casing RPM

Near Casing Shoe Underreamer:
🔸Advantages:

  • Shorter rat hole
  • Enable to improve reamer performance with PDM

🔹Disadvantages:

  • Risk of underreamer stuck
  • Near Bit Underreamer

🔸Advantages:

  • Shortest rat hole
  • Enable to improve reamer performance with PDM

🔹Disadvantages:

  • Risk of underreamer stuck
  • Effect on LWD reading and directional control performance

Drilling with Liner Systems (DwL):
works in much the same way as the previous two systems, except it does not involve the use of a casing drive system. The liner hanger setting tool is connected to the drill pipe, and then attaches to the power swivel at surface.

There are three sub-types of this system: non-retrievable, wireline retrievable, drill pipe retrievable.