Casing DesignFunction and Classification of casing

Casing

Functions of Casing Isolate porous formations with different fluid-pressure regimes and also allow isolated communication with selectively perforated formation(s) of interest. Isolate troublesome zones (high pressured zones, weak and fractured formations, unconsolidated formations, and sloughing shale) and to allow drilling to the total depth. Prevent the hole from caving in

FunctionsServe as a high-strength flow conduit tosurface for both drilling and production fluids. Prevent near-surface fresh water zones fromcontamination with drilling mud.Provide a connection and support of thewellhead equipment and blowout preventers.Provide exact dimensions for running testing,completion, and production subsurfaceequipment.

Types of Casing StringsThere are different types of casing for differentfunctions and drilling conditions.They are run to different depths and one ortwo of them may be omitted depending on thedrilling conditions. They are: Cassion pipeConductor pipe Surface casingIntermediate casing Production casingLiners

Cassion pipe (26 to 42 in. OD)For offshore drilling only.Driven into the sea bed.It is tied back to the conductor or surface casing and usually does not carry any load.Prevents washouts of near-surface unconsolidated formations.Ensures the stability of the ground surface upon which the rig is seated.Serves as a flow conduit for the drilling mud to the surface

Conductor pipe (7 to 20 in. OD)The outermost casing string. It is 40 to 500 ft in length for onshore and up to 1,000 ft for offshore. Generally, for shallow wells OD is 16 in. and 20 in. for deep wells.Isolates very weak formations. Prevents erosion of ground below rig.Provides a mud return path. Supports the weight of subsequent casing strings.

Surface casing (17-1/2 to 20 in. OD)The setting depths vary from 300 to 5,000 ft10-3/4 in. and 13-3/8 in. being the most common sizes. Setting depth is often determined by government or company policy and not selected due to technical reasoning.Provides a means of nippling up BOP.Provides a casing seat strong enough to safely close in a well after a kick.Provides protection of fresh water sands.Provides wellbore stabilization.

Intermediate casing (17-1/2 to 9-5/8 in.OD)Also called a protective casing, it is purely a technical casing.The length varies from 7,000 to 15,000 ft.Provides isolation of potentially troublesome zones (abnormal pressure formations, unstable shale, lost circulation zones and salt sections). Provides integrity to withstand the high mud weights necessary to reach TD or next casing seat

Production casing (9-5/8 to 5 in. OD) It is set through the protective productive zone(s). It is designed to hold the maximal shut-in pressure of the producing formations. It is designed to withstand stimulating pressures during completion and workover operations. A 7-in. OD production casing is often used

Production CasingProvides zonal isolation (prevents migration of water to producing zones, isolates different production zones).Confines production to wellbore. Provides the environment to install subsurface completion equipment. Provides protection for the environment in the event of tubing failure during production operations and allows for the tubing to be repaired and replaced.

Casing diagram

Liners They are casings that do not reach the surface.They are mounted on liner hangers to the previous casing string. Usually, they are set to seal off troublesome sections of the well or through the producing zones for economic reasons (i.e. to save costs).Drilling liner Production liner Tie-back liner Scab liner Scab tie-back liner

LinerDrilling Liner Same as intermediate/protective casing. It overlaps the existing casing by 200 to 400 ft. It is used to isolate troublesome zones and to permit drilling below these zones without having well problems.Production Liner Same as production casing. It is run to provide isolation across the production or injection zones.Tie-back Liner it is connected to the top of the liner with a specially designed connector and extends to the surface, i.e. converts liner to full string of casing.Scab Liner A section of casing used to repair existing damaged casing. It may be cemented or sealed with packers at the top and bottom.Scab Tie-back LinerA section of casing extending upwards from the existing liner, but which does not reach the surface and normally cemented in place. They are commonly used with cemented heavy-wall casing to isolate salt sectons in deeper portions of the well.

Diagram