Casing Drilling (including liner drilling) is a technology / well construction method that allows simultaneous drilling and casing of a well. While in the conventional drilling transferring torque, fluids and drill pressure is done through a drill pipe, during casing-drilling, the casing itself replaces the functions of the drill pipe. Once drilling is completed, the casing remains in the well. This technology allows drilling and running casing as one operation.
Casing-drilling technology is a fairly new technology, less than 20 years old, and has not been widely used. Yet, casing-drilling technology was able to demonstrate its advantages of overall faster drilling, elimination of hole-drilling related problems, such as mud losses and wellbore stability by the casing and isolating the unstable formations while drilling. This isolation while drilling removes the risk of the well collapse and fluids losses. The plastering effect is another benefit of casing drilling, whereby drill cuttings stick to the wellbore as a result of the rotation of the casing and reduced gap between casing and wellbore. This provides not only a seal of the well bore/pores, but fewer drill cuttings are returned to the surface, hence less handling.
There are two major systems used in casing-drilling- 1) simple drill-through casing bit (non-retrievable) system and 2) Advanced wire-line retrievable bottom hole assembly (BHA) system used for directional casing-drilling. While in more than 70% of the cases, the simple system is used, the advanced directional casing-drilling system is best suited for the offshore mature oilfields, where fluids losses and hole stability issues are evident.
A specially developed drilling rig or modified rig for casing drilling is required to accommodate the new equipment setup. A casing drive system (CDS) must be installed as well.
Today, casing drilling is best used when drilling soft formations with casing sizes above 7”. While penetration rate during casing-drilling is lower, fewer trips and handling makes it economical.
Major disadvantages of casing-drilling are:
- Reduced penetration rate
- High cost
- Size of the casing, whereby in smaller casing sizes ( 7” and less) down-hole motors cannot be powerful enough to drill efficiently, due to its small size
- Redesign of surface rig equipment and down-hole systems
In addition, in high torque, high tension and high compression loads applications, casing / OCTG that are designed to accommodate it, will be required and may significantly increase costs.