Drilling Engineering 2 Course (1st Ed.)

1. Laminar Flow in Pipes and Annuli

2. Turbulent Flow in Pipes and Annuli

3. Pressure Drop Across Surface Connections

4. Pressure Drop Across Bit

5. Optimization of Bit Hydraulics

6. Particle Slip Velocity

Casing functions

Casing costs compromise one of the largest cost items of the drilling project. Therefore proper planning of

casing setting depths and casing selection is vital to realize a cost effective and safe well.

The casings themselves fulfill multiple functions that can be summarized as:Isolate porous formations with different fluid-pressure

regimes from contaminating the pay zone,Prevent near surface fresh water zones

from contamination with drilling mud,Protect the hole from caving in,Provide a connection and support of the wellhead equipment,Provide exact dimensions for running

testing, completion and production subsurface equipment.

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Casing Types

According to the different functions, the total casing program consists of different casings strings.Conductor Casing

Surface Casing

Intermediate Casing

Production Casing

Liners

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Conductor Casing

The function of the conductor is to enable circulation of the drilling fluid to

the shale shakers without eroding the surface sediments directly below the rig foundation.

The conductor prevents the subsequent casings from corrosion and may partly support the wellhead weight.

Commonly a diverter is installed on top of the conductor casing to divert an unexpected inflow of formation fluids into the wellbore away from the rig-site and the personal.

Conductor setting depths are in the range of 150 to 600 [ft] (46-183 m), their seizes range from 36 to 20 [in] (0.91-0.5 m).

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Surface Casing

The function of the surface casing is to prevent cave in of unconsolidated, weak near-surface formations

as well as protect the shallow, freshwater sands from contamination with drilling mud.

As the conductors, surface casing protects the subsequent casings from corrosion.

Before the surface casing is set, no blow out preventers (BOP) are installed.

After setting the surface casing and installing the wellhead, a BOP is available to handle kicks when drilling the intermediate hole section.

Surface casing setting depths are in the range from 300 to 5,000 [ft] (91-1524 m), their diameters range from 24 to 17-1/2 [in]. the surface casing setting depth is often determined by government

or company policy and not selected due to technical reasoning.

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Intermediate Casing

The intermediate casing string is a purely technical casing.

One or more may be necessary to handle abnormal formation pressures, unstable shale formations, lost circulation or cave-in zones.

An intermediate casing may also be necessary to realize the planned mud weight profile. When for example an abnormally pressured formation is

encountered, it may have to be protected by an intermediate casing so when formation pressure of the formations below is normal, a lower mud weight can be applied.

Intermediate casing diameters range from 17-1/2 to 9- 5/8 [in].

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Production Casing

The production casing is set through the prospective production zone(s).

This casing string protects the environment in case of production tubing failure and permits the tubing string to be maintained or replaced during the production life.

Commonly production casing and production liners have gas-tight connections,

their diameters range from 9-5/8 to 5 [in]. A production casing diameter of 7 [in] is encountered often.

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Liners

To save cost, the casing installed sometimes doesn’t reach until the surface but finishes within the previous string. Such a casing configuration is called

liner.

A liner is mounted on a so called “liner hanger” to the previous casing string.

Commonly the liner head is several hundred feet into the previous casing to enable a good cement seal.

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Various typical casing programs

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develop a casing program

To develop a casing program, first the various casing setting depths have to be determined.

Since the primary reason to drill a well is to produce hydrocarbons out of a reservoir, the final casing inside diameters

have to be large enough to allow for the forecasted completion and production schemes. Factors like completion type

(open hole, cased hole, monobore production, etc.), expected amount of production (production tubing seize), expected production forecast (e.g. need of gas-lift, etc.) and seize of evaluation tools to be run have to be considered.

In general, for production purposes the well diameters shall be as large as possible.

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Advantages of small hole sizes

On the other hand, as small as possible hole seizes reduces the total cost of the well since:drilling times are faster,less mud has to be used (purchase and disposal ofmud),smaller mud equipment can be used (cleaning, pumps, etc.),smaller casings can be used (cheaper, higher strength at same

grade),smaller rig can be applied (lighter casings, smaller mud

volume),rig site can be smaller (especially important offshore and

platform types like TLP where weights are limited).

Out of this reason a technology called slim-hole drilling was developed.

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