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C H A P T E R

8Casing and Casing String Design

O U T L I N E

8.1 Types of Casing 385

8.2 Casing Data 393

8.3 Combination Casing Strings 425

8.4 Running and Pulling Casing 434

8.1 TYPES OF CASING

Based on the primary function of the casing string, there are five typesof casing to be distinguished.

Stove or Surface Casing The stovepipe is usually driven to sufficientdepth (1560 ft) to protect loose surface formation and to enable circulationof the drilling fluid. This pipe is sometimes cemented in predrilled holes.

Conductor String This string acts as a guide for the remaining casingstrings into the hole. The purpose of the conductor string is also to coverunconsolidated formations and to seal off overpressured formations. Theconductor string is the first string that is always cemented to the top and

equipped with casing head and blowout prevention (BOP) equipment.

Surface Casing This is set deeply enough to protect the borehole fromcaving-in in loose formations frequently encountered at shallow depths,

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386 8. CASING AND CASING STRING DESIGN

and protects the freshwater sands from contamination while subsequentlydrilling a deeper hole. In case the conductor string has not been set, thesurface casing is fitted with casing head and BOP.

Intermediate Casing Also called protection string, this is usually set inthe transition zone before abnormally high formation pressure is encoun-tered, to protect weak formations or to case off loss-of-circulation zones.Depending upon geological conditions, the well may contain two oreven three intermediate strings. Production string (oil string) is the stringthrough which the well is produced.

Intermediate or production string can be set a liner string. The linerstring extends from the bottom of the hole upward to a point about150250 ft above the lower end of the upper string.

Casing Program Design Casing program design is accomplished by twosteps. In the first step, the casing sizes and corresponding bit sizes shouldbe determined. In the second step, the setting depth of the individual casingstrings ought to be evaluated. Before starting the casing program design,the designer ought to know the following basic information:

The purpose of the well (exploratory or development drilling) Geological cross-sections that should consist of type of formations,

expected hole problems, pore and formations fracture pressure, numberand depth of water, oil, gas horizons Available rock bits, reamer shoes and casing sizes Load capacity of a derrick and mast if the type of rig has already been

selected

Before starting the design, it must be assumed that the production casingsize and depth of the well has been established by the petroleum engineer incooperation with a geologist, so that the hole size (rock bit diameter) for thecasing may be selected. Considering the diameter of the hole, a sufficient

clearance beyond the coupling outside diameter must be provided to allowfor mud cake and also for a good cementing job. Field experience showsthat the casing clearance should range from about 1.0 in. to 3.5 in. Largercasing sizes require greater value of casing clearance. Once the hole sizefor production string has been selected, the smallest casing through whicha given bit will pass is next determined. The bit diameter should be a littleless (0.05 in.) than casing drift diameter. After choosing the casing withappropriate drift diameter, the outside coupling diameter of this casingmay be found. Next, the appropriate size of the bit should be determined

and the procedure repeated.Expandable casing technology, expandable drill bits, under-reamers and

other tools for optimizations of borehole and/or string designs, are notcovered in this section.



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8.1 TYPES OF CASING 387

Example 8.1The production casing string for a certain well is to consist of 5-in. casing.Determine casing and corresponding bit sizes for the intermediate, surfaceand conductor string. Take casing data and bit sizes from Table 8.1.

SolutionFor production hole, select a 6 34 -in. rock bit. Therefore, the casing

clearance= 6.75 5.563= 1.187 in.For intermediate string, select a 7 5/8-in. casing, assuming that wall

thickness that corresponds to drift diameters of 6.640 or smaller will notbe used. For the 7 5/8-in. intermediate string, use a 9 7/8-in. bit. Thecasing clearance= 9.875 8.5= 1.375 in.

For surface string, select a 1034 -in. casing. Note that only unit

weights corresponding to drift diameters of 10.036 and 9.894 in. canbe used. For the 10 34 -in. casing, use a 13

34 in. bit, so the casing clear-

ance= 13.75 11.75= 2.0 in.For conductor string, select 16 in. casing; the bit size will then be 20 in.

and the casing clearance= 20 17= 3in.Having defined bit and casing string sizes, the setting depths of the

individual strings should be determined.The operation of setting is governed by the principle according

to which casing should be placed as deep as possible. However, the

designer must remember to ensure the safety of the drilling crew frompossible blowout, and to maintain the hole stability, well completionaspects (formation damage) and state regulations.

In general, casing should be set

Where drilling fluid could contaminate freshwater that might be usedfor drinking or other household purposes

Where unstable formations are likely to cave or slough into the bore-hole

Where loss of circulation may result in blowout Where drilling fluid may severely damage production horizon

Currently, a graphical method of casing setting depth determinationis used. The method is based on the principle according to which theborehole pressure should always be greater than pore pressure and lessthan fracture pressure. (Drilling with borehole pressures lower than porepressure requires the use of under-balanced drilling technologies notcovered in this chapter.)

For practical purposes, a safety margin for reasonable kick conditions

should be imposed (Figure 8.1). Even when borehole pressure is adjustedcorrectly, problems may arise from the contact between the drilling fluidand the formation. It depends upon the type of drilling fluid and for-mation, but in general, the more time spent drilling in an open hole, the



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