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3.4 Casing and Cementation 129
different pressure gradients and to seal formations against influx of fluids (gas, oil,
and water) from the previous or next hole section. Normally, the annulus between
the casing and the borehole is filled with cement to achieve a sealing ‘‘connection’’
between both (Figure 3.11).

3.4.1
Casing and Liner Concepts

In order tobeabletorun thecasingintoaborehole thecasing size (diameter)must
fit into the hole which means it must be smaller than the hole diameter. Because
generally more than one casing string is necessary to complete a borehole, the bit
size for the next drill section has to pass through the previous casing. So, every
well starts with a large diameter at the top and ends in a smaller diameter at final
depth; for more details see Section 3.5.
Some casing strings are designed as ‘‘surface strings’’ which means that the pipe
string leads from bottom to surface. Other casing strings are run as a ‘‘liner,’’ only
reaching 10–100 m into the previous casing string. Liners are run on drillpipe to
the desired depth and are equipped with a special ‘‘liner hanger’’ which is a tool
that allows hanging the weight of the complete liner into the lower part of the
previous casing (Figure 3.12). Liners offer the advantage to have available a larger
casing inner diameter above the liner hanger.
In geothermal wells in particular, large diameters are necessary to allow the
circulation of high flow rates through the casings because otherwise the circulation
pressure drop will be very high and will increase the energy costs for the production
and reinjection pumps; for details see Section 3.9.

3.4.2
Casing Materials

There are different steel materials (steel grades) in use for casing, depending on
strength, corrosion resistance, and so on. Generally, casing sizes, wall thicknesses,
and material grades are ‘‘standardized.’’ The most common standard used world-
wide is the ‘‘API Specification 5CT’’ from the American Petroleum Institute; here,
all material grades and properties are specified, including chemical composition of
the different grades.
Common grades for carbon steel material are: H40, J55, K55, L80, N80, C95,
P110, Q125; in all cases the numbers give the ‘‘minimum yield strength’’ of
the material in ‘‘pound/square inch’’ ×1000(= psi × 1000). Other materials are
available, for example, corrosion resistant materials for highly corrosive fluids.
Common sizes (outer pipe diameters) are 24 1/2, 20, 18 5/8, 13 3/8, 11 3/4, 10 3/4,
9 5/8, 8 5/8, 7 5/8, 7, 6 5/8, 5 1/2, 5, 4 1/2, 3 1/2, 2 7/8, 2 3/8 in. From these sizes
a casing program for the wells can be gathered, together with the desired drill bit
diameters for the corresponding hole section (Section 3.5).