According to the different functions, the total casing program consists of different casings strings.1.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], their seizes range from 36 to 20 [in].
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], their diameters range from 24 to 17-
1/2 [in]. Note that the surface casing setting depth is often determined by government or company policy and not selected due to technical reasoning.3.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 17.5 to 9(5/8)[in].
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.5.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, see sketch 8.2. Commonly the liner head is several hundred feet into the previous casing to enable a good cement seal. Various typical casing programs are shown in sketch 8.5. 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. On the other hand, as small as possible hole seizes reduces the total cost of the well since:
1. drilling times are faster,
2. less mud has to be used (purchase and disposal of
3. smaller mud equipment can be used (cleaning, pumps, etc.),
4. smaller casings can be used (cheaper, higher strength at same grade),
5. smaller rig can be applied (lighter casings, smaller mud volume),
6. 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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