Page 211 - Drilling Technology in Nontechnical Language

202 Drilling Technology in Nontechnical Language Second Edition

Designing the Casing String

When a drilling engineer has to design a set of casings for a well, there
are quite a few considerations to be made. First it is necessary to predict all
the physical forces that each casing may be subjected to throughout the life

of the well (from starting drilling until the well is finally abandoned).
The chemical environment also must be understood; sometimes,

corrosive fluids are produced from the reservoir. This will lead to special

steel alloys being used, which tend to be expensive and sometimes difficult

to handle when running into the hole. When corrosive pore fluids are
present in formations penetrated by the well, the outside of the casing can
be attacked and corroded. The cement has to form a protective barrier
around the casing.
The main casing design considerations are explained in the text that
follows.
Tension. Each piece of casing will be in tension from the weight of
the casing below it. The tension will therefore increase from the bottom to
the top.

Downhole tubulars (casing, tubing, and drillpipe) are specified by

weight per foot as well as other attributes. This might seem a strange way
to specify a casing, but casing design tables will give, for each casing OD,
a choice of weight per foot and also of grade (which is the type of steel
alloy used). A common production casing size is 9⅝" OD, and this comes
in weights per foot of 36.0, 40.0, 43.5, 47.0, 53.5, and 58.4. Neglecting
buoyancy forces for a moment, the weight of a 10,000 ft string of 9⅝",
53.5 lb/ft casing would be 535,000 lb, which is over 242 tonnes. A long
string of casing can be pretty heavy!
Additional tensile forces are imposed on the casing. In a deviated well
where the casing has to bend around, the tensile stress in the outside of the
bend is increased (while the tensile stress in the steel on the inside of the
bend is decreased). The maximum tension on any element of the casing
must be less than the tensile strength of the casing.
Casing is pressure tested after cementing, and this produces a force
trying to pull the casing apart—a tensile force. The inside diameter of 9⅝",
53.5 lb/ft casing is 8.535", which gives a cross-sectional area of 57.2 in .
2
A 3,000 psi pressure test will impose an additional 172,000 lb (78 tonnes)
of tensile force on every joint in the casing, in addition to the weight.

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