CASING DESIGN THEORY AND PRACTICE

The way of how to select casing seats based on pore pressure requirements would really tough and very fuss to determine when casing seats would be placed, nevertheless, Casing design conpcepts relate to using the casing that meets requirements at the lowest possible cost.

Before considering economics, it’s vital to know the setting depths of the casing string,  plans must definetely be made in order to control corrosion, as well as taking account and considerations for casing wear. The setting depths of casing strings may be based on governmental regulations, hole requirements, or routines practices. Casing strings are commonly identified by their purposes in the hole. The types of casing string are described below.

Structural Casing or Drive Pipe:

This string of structural casing or drive pipe provides initial hole support close to the surface and usually varies in lenght from about 30 to 300 ft. The casing may be driven in or a hole drilled. If necessary to drill a hole, the casing might be cemented in place.

Conductor Casing:

Conductor Casing in some areas may be the first casing string set and cemented, the main purpose of this casing is to hold back the unconsolidated surface formations and prevent them from falling into the hole. In others, particularly offshore, this is the first casing string which provides proteccion against formation pore pressures.  And besides, where shallow water or gas flow is expected, the conductor pipe is fitted with a diverter system above the flowline outlet. This device permits the diversion of drilling fluid or gas flow away from the rig in the event of a surface blowout. Generally speaking, it is cemented around the string of casing back to the surface. The length of this pipe is from 40-50o ft onshore and up to 1000 ft offshore, is among 7 to 20 -in. in diameter. Generally, a 16-in. pipe is used in shallow wells and a 20-in deep wells.

Surface Casing:

Surface Casing was initially reffered to as the casing string set to isolate freshwater sands near the surface. however, depending on the location, surface casing also seals off shallow formations that may cause drilling issues as well as preventing contamination by fluids from deeper formations and to serve as a base on which to set the blowout preventers (BPO). Besides, it provides a conduit that permits the use of mud weights in the range of 12-14 Lb/gal in the pressure transicion zone of deep wells. Setting depths of the surface casing vary from 7 to 16-in. in diameter, with 10.75 and 13.375 in. being the most common sizes. On land, surface casing is usally cemented to the surface. For offshore wells, The cement column is frequently limited to the kickoff point.

Intermediate or Protective Casing:

This kind of casing string seals off open formations so the well can be drilled deeper. In some instances, such as when production liners are used, this casing may also be part of the production casing. That is to sat that it is set a depth between the surface and production casings. The main reason for setting intermediate casing is to case off the formations that prevent the well from being drilled to the total depth, more than one string of intermediate Casing may be required to get to the total depth. Troublesome zones encountered include those with abnormal formation pressures, lost circulation, unstable shales, and salt sections.

When abnormal formation pressures are present in a deep section of the well. intermediate casing is set to protect formations below the surface casing from the pressures created by the drilling fluid specific weight required to balance the abnormal pore pressure.

Intermediate casing varies in length from 7.000 ft to as nmch as 15.000 ft and
from 7 in. to 11.75 in. in outside diameter. It is commonly cemented up to 1,000 ft from the casing shoe and hung onto the surface casing. Longer cement columns are sometimes necessary to prevent casing buckling.

Production Casing:

This kind of casing string is set and cemented to isolate the productive formations. The Cement volume may be determined from Caliper log calculations or might be a given amount of excess based on experience. Keep in mind that cement  travels more readily up the large side of the hole. As a consequence, more excess than calculations indicate might have to be used to cover the casing adequately and suitably in the small part of the hole if the casing is not centralized in the hole.

Production casing varies from 4.5 in. to 9.625 in. in diameter, and is cemented
far enough above the producing formations to provide additional support for
subsurface equipment and to prevent casing buckling.

 Liners:

Liners are string of pipe which doesn’t extend back to the surface. A primary advantage of using liners is the savings in steel costs. Also liners, may be run more quickly than full casing strings, which improves the opportunities to get the liner to the desired depth. Both protective and production liners are set; the purpose is the same as that of a full string of casing. One problem with liners is getting a good cement seal around them. Liners are often neither moved nor centralized when cementing. As a result , getting cement on all sides of the liner is difficult. Liner hangers that permit liner movement during cementing are available, and to ensure a good cement job liners should be moved. Other problems associated with liners include normal clearances, which are sometimes small, especially with deep production liners.

Drilling liner:

Drilling liner is a section of casing that is suspended from the existing casing        ( Surface or intermediate Casing). In most cases, it extends downward into the open hole and overlaps the existing casing by 200 to 400 ft. It is used to isolate abnormal formation pressure. Lost circulation zones, heaving shales and salt sections, and to permit drilling below these zones without having well problems.

Production Liner:

production liners is run instead of full casign to provide isolation across the production or injectionn zones. In this cases, Intermediate casing or drilling liner becomes part of the completion string.

Tie-back liner:

Tie-back liner is a section of casing extending upwards from the top of the existing liner to the surface. This pipe is connected to the top of the liner with a specially designed connector. Production liner with tie-back liner assembly is most advantageous when exploratory drilling below the productive interval is planned. It also gives rise to low hanging-weighta in the upper part of the well.

Scab-Liner:

Scab-liner is a section of casing used for repairing existing damaged casing. It might be cemented or sealed with packers at the top and bottom.

Scab tie-back liner:

this section is extending upwards from the existing liner, but which does not reach the surface and is normally cemented in place. Scab tie-back liners are commonly used with cemented heavy-wall casing to isolate salt sections uin deeper portions of the well.

Furthermore, Casing string designs are based on tension, collapse pressure, and burst pressure. Compression will be considered when landing casing. Most casing design criteria use the minimum weight and grade of casing that meets the hole requirements.

Before designing the casing the following information is necessary:

  • Setting depths
  • Formation pore pressure and mud weights
  • Rotatinf time in the casing
  • Auxiliary operations that may be necessary, like squeeze cementing.
  • Formation behaviour, such as salt sections, that might result in collapsed , casing latere production operations.
  • Corrosive features of the fluids to be produced.

Casing depths rely on the following:

  • Governmental requirements.
  • Hole problems.
  • Routine practices.
  • pore pressures.

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