Casing and Intervals of Casing
A casing is a type of large diameter pipe designed to protect the wellbore from geological formations that can potentially fill the borehole. A casing is placed into newly drilled areas of a borehole then cemented in place inside the drilled well. Once cemented, casing facilitates the drilling process in a variety of ways.
First, casing prevents potential cave-ins and blowouts. It also prevents contamination from water sands and fluid loss in the wellbore. Furthermore, casing provides a sturdy upper foundation, isolates various zones and delivers a smooth internal bore, which can be used for the installation of production equipment. Additionally, casing allows for the installation of artificial lift equipment, blowout preventers, wellhead equipment, zonal production treatments, production packers and production tubing.
INTERVALS OF CASING
A single casing string can be applied to shallow, non-corrosive wells. However, in many cases, a single string of casing is insufficient and multiple strings are used to provide additional strength. The basic types of casing strings include conductor casing, surface casing, intermediate casing, production casing, and liners, or tieback casing.
The conductor casing is the initial string set. It is used near the top of the well to prevent the collapse of loose soil. The conductor casing provides added support and serves as a conduit to return drilling fluids to mud pits during drilling operations. Conductor casings vary in size and are available in common OD sizes being NPS 16” to NPS 36” and is set in depths of 50-250 feet.
The next casing after the conductor pipe is surface casing. Surface casing is typically cemented for its full length and is often required by law to protect groundwater from contamination during drilling and completion. Surface casing is the only string engineered to carry compression loads. It is strictly regulated. Surface casing sizes are typically NPS 13 ⅜” through NPS 20” and are set in depths of 200-5000 feet.
The intermediate casing lies between the surface casing and the production string. It does not have to be cemented full length. Intermediate casing is used to seal off unstable hole sections, weaker zones, production zones and lost-circulation zones. It prevents blowouts and provides support for the liner casing. Generally, the intermediate casing is sized NPS 4 ½” through NPS 13 ⅜” and is set in depths of 2000-20,000 feet. Some wells need more than one intermediate string.
The production casing is the main pressure containment string in the event of a tubing leak. It extends throughout the good depth and is not typically cemented full length. Production casing protects production equipment and prevents the equipment from fluid contamination. It is set in depths of 2000-25,000 feet. Common sizes include NPS 4 ½” to NPS 13 ⅜”.
Liners / Tieback Casing
Liners, or tiebacks, are short casing strings used to enhance pressure integrity, spanning from the liner top to the wellhead. Typically cemented full length, liners extend above the bottom of the previous casing string. Sometimes liners are used in place of full casing strings to enhance hydraulic performance during deeper drilling operations and to lower costs. They also enable larger tubing to be used above the liner top. Liners are typically sized from NPS 4 ½” to NPS 9 ⅝” and is set in depths of 5000-20,000 feet.
Costs associated with casing make up a large portion of the overall well cost. Production tubing is frequently applied without cement in the smallest casing of a good completion. This helps contain production fluids and transport them from an underground reservoir onto the surface.