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Introduction toCasing

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After each section of hole has been drilled astring of casing is run to line the inside of thewellbore. The main reasons for casing off the

open hole are:

to prevent caving of the wellbore, as if unsupported, thehole may be liable to fall in upon itself;to provide support for weak or fractured formations frommud weights which may cause these zones to break down;to isolate zones of abnormal pressure;to seal off zones of lost circulation (i.e. thief zones wheremud or cement slurry etc enters the formation);to provide a means of completing and producing the wellefficiently;to provide structural support for wellheads and blow-outprevention equipment

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D ifferent

CasingStrings

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Casing Programmes

Casing makes up a considerable part of the overall well cost(

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Casings have to

be cemented inplace to ensurea competentpressure control

system

Inner CasingString

Cement beinginjected

Cement Sheath

Outer CasingString

More about cementing later...

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T ypes of Casing

Well now have a look at:

Conductor CasingSurface CasingIntermediate Casing

Production CasingLiner

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Conductor Casing

Conductor casing (sometimes called pipe) is the firstcasing string to be run often driven into the formation

using large hydraulic jack-hammers. It ...consequently has the largest diameterprotects seabed formations - continuous fluid circulation willlikely create large washouts through erosionalso protects surface formations which may have lowfrictional strength which could easily be exceeded by thehydrostatic pressures exerted by the drilling fluid whendrilling a deeper section of the wellboreprovides a conduit for mud returns

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Surface Casing

T he main functions of surface casing are to sealoff any fresh water sands, and ...to provide structural support for the blow-outpreventer (BOP) and wellhead equipment

In an area where abnormal pressures areexpected the setting depth is important, as thecontrol equipment will need to be capable of withstanding that pressure. If the casing is settoo high there may not be sufficient formationstrength at the shoe to handle a kick whendrilling the next section

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Intermediate Casing

Intermediate, or protection casing is used to isolate anytroublesome formations which would/could cause drillingproblems in the wellbore - a few examples:

sloughing shale lost circulation high pressure zones

Several strings of intermediate casing may be requireddepending upon the number of problems encountered, if rock formations are stable, drilling can be undertaken forrelatively long periods of time prior to the setting of casingT

he setting depth depends on a knowledge of porepressures and fracture gradientsD uring drilling operations the mud weight controls porepressures, but must not exceed the fracture strength of shallower zones not cased (i.e. not protected) from thedrilling fluid

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Production Casing

T his is usually the last string of casing to be run in thewellbore and is either run through the pay zone(reservoir) or set just above the pay zone (for an

open hole completion). Its main purpose is to:

isolate the production interval from otherformations such as water bearing sands, and to protect the completion tubing (through which

the reservoir fluids will be transported to thesurface)

Production casing should be thoroughly pressuretested. Usually the casing will be pressure tested toaround 60% of its specified burst pressure, if thecasing withstands that pressure it is expected to becapable of withstanding higher pressures

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Liner

A liner is a short string of casing which does notextend back to the surface. It is run back inside the

previous casing string to provide some overlapLiners may be used either as an intermediate orproduction stringT he liner has the advantage of being much cheaperthan a full length casing string

If required, a tie back string can be run to extend theliner back to the wellhead

A liner is usually less than 5,000 feet long. Overlap issomewhere in region of 200500 feet and it isessential that the liner overlap is gas tight

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Y oull hear a lot about .. Pore Pressure (P f )

It is .... T he pressure of fluids within the pores of areservoir (usually hydrostatic pressure) or the pressureexerted by a column of water from the formation's depthto sea levelWhen impermeable rocks (such as shales) form assediments are compacted, their pore fluids cannot alwaysescape and must then support the total overlying rock

column, leading to anomalously high formation pressures

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Casing vs. Bit Size ( illustrative)

CasingSize ~20 ~13-3/8 ~9-5/8 ~7 ~ 5

Bit Size( Typical)

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