The concept of drilling and casing the hole in the same time is, of course, valuable by combining two time consuming processes. So the main purpose of Casing Drilling, or Casing while Drilling (CwD), is to eliminate classic casing runs and isolate formations while drilling. By using Standard casing string instead of conventional drill string, the drilling and casing are executed simultaneously, section by section. Casing while Drilling is also a hazard mitigation solution, having applicability in drilling soft shallow sections with high borehole instability and known losses.

How is it performed?

The introduction of the casing running tool (CRTi) has made way for Casing Drilling. When the casing is run with top drive and CRTi, you are able to operate the casing string much in the same manner as a drill string. The top drive casing running tool (TDCRTi) lets you push, pull, rotate and reciprocate, thus giving you extended control over the string.

On level two, CwD is used drilling vertical wells with non-retrievable bits, mostly for surface and soft sections. On level three, CwD is used drilling directional with casing, with retrievable BHA’s, mostly for deeper and challenging sections.

What are the benefits?

1- Maximizing efficiency
2- Two operations in one, each meter drilled will be cased.
3- Reduces time for tripping in and out, and the risk involved with it.
4- Improves drilling efficiency by reducing of the non-productive time.
5- Drilling time and cementing saving.
6- The smearing effect: Prevent and cure (or minimize) losses while drilling, i.e. good control of the annular pressure losses.
7- Applicability in depleted zones or underbalanced drilling applications.
8- High applicability in drilling soft shallow sections (high borehole instability with known losses).

What will decide if CwD is suitable for a specific project?

Geology:
With level 2 CwD (using drillable bits) most surface sections are suitable for casing drilling. These sections are characterized by various layers of clay (or marl), sand, conglomerates, gravels, sandstones, coals. However a detailed analyse is required in order to select the most suitable casing drilling bit (PDC type) for your section.

Rig type
The rig has to be equipped with a top drive.

Efficient pump system and mud system is required.