Casing & Cementation

What is a Casing

Large-diameter pipe lowered into an openhole and cemented in place

As the drilling of a well progress, its necessary to line the walls of the hole with heavy steel pipe

The total length of casing (of uniform outside diameter) which is run in well during a single operation is called casing string

Its one of the most expensive parts of a well, around 20% of the cost of a completed well

Functions of CasingFollowing are the important functions of casing:

to prevent the caving of hole

to prevent contaminations of fresh waters in upper sand

to exclude water from the producing formations

to confine production to wellbore

to provide means of controlling pressures

to facilitate installation of subsurface equipment

Classification of Casing

The classification of casing is based on primary function of the casing string. There are four types of casing

Conductor casing

Surface casing

Intermediate casing

Production casing

Surface casing

Intermediate casing

Production casing

Conductor Casing Conductor casing, or drive pipe, is a short string of casing of large

diameter (16 to 48 in.) It is required for offshore operations locations and other conditions

in which extra wellhead support is necessary Its principal function is to keep the top of the wellbore open and to

provide a means of conveying the drilling fluid returns from the wellbore to the mud pit

The depth to which it is set varies but is usually 100 to 400 ft The part of the wellhead on the top of the conductor string is the

base plate, which is either an integral part of the casing head or a fabricated (welded-on) plate of steel connected to the casing head by gussets

Surface Casing

The surface casing is the first string of casing run on a land well and is generally thought of as the foundation of the wellhead

The size generally ranges from 8 5/8 to 20 in. outside diameter (OD) The length of this string varies greatly in different areas, from 200 to

several thousand feet It is set at a depth sufficient to protect all fresh water bearing sands

and to prevent an underground blowout The cement is circulated to the surface on this string The part of the wellhead at the top end of this string is the casing

head, which is either screwed or welded onto the last joint of casing

Intermediate Casing The Intermediate casing is the second string of casing run in a well The size generally ranges from 7 5/8 to 13 3/8 in. outside diameter

(OD) Setting depths vary to meet regulations and geological conditions in a

given well but generally ranges from 3000 to 8000ft Its purpose is to allow for deeper drilling by isolating weak formations

that could either cause the hole to cave in or lose circulation The cement is circulated to the surface on this string The part of the wellhead at the top end of an intermediate casing string

is a casing spool The intermediate casing may be cemented all the way back to the

surface, but generally it is cemented back to the end of the previous string or far back enough to isolate a particular formation

Production Casing

The final string of casing is called the production casing. It is usually set to or beyond the formation that is to be produced

Production casing typically ranges in size from 4 1/2 to 9 5/8 in. OD

The production casing isolates all the other formations from the producing zone or zones and is generally cemented, like the intermediate casing, back to the end of the previous string

The part of the wellhead that works over the top end of the production casing is the tubing head

Production Tubing The final string of tubular goods that goes into the well is the production

tubing Tubing size may range from 1 to 7 in. OD, and the bore size of the tree is

sized accordingly. Unlike casing, it is not cemented in the well. It is supported and sealed by

hanging it inside the top bowl of the tubing head, and it can be packed off down below with a packer

Tubing can be replaced when damaged and also the well can be deepened or plugged back and a new tubing string used to accommodate the new depth

The annular space between the OD of the tubing and ID of the production casing can also be used to lift the fluids artificially from the well or to inject chemicals when the well is being produced

The parts of the wellhead at the top end of the tubing are the tubing head adapter and the production tree.

Primary Oil Well Cementing Cement that is pumped down the casing and into the annulus between the casing and Wellbore is used as a sealant to help protect:

• Casing and Wellbore from external pressure that could collapse the pipe or cause a blowout

• Oil- and gas-producing strata from extraneous fluids • Casing from possible corrosion and electrolysis caused by

formation waters and physical contact with various strata • Drill string against loss in a key-seat or sticky hole • Down hole production and drilling equipment • Pipe from the stresses of formation movement

Classification of Cements• Most oil well-cementing operations

worldwide use slurries based on API Classes A, G, H, and C (high-early-strength cement).

• API Classes D, E, and F are intended for high-temperature applications, and each contains an organic set retarder added by the manufacturer. Classes D, E, and F are not widely distributed, and in some instances the set retarder is not compatible with materials added on-site.