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3.5 Planning a Well 139
and limestone, and metamorphic or magmatic rocks can be affected by high frac-
turization especially in tectonically stressed zones. Fractures themselves typically
do not have much volume, but by joining preexisting pores or interconnected
faults, they may enhance permeability and hence productivity, significantly.

3.5.2
Well Design

Borehole design means to plan the trajectory of the well, select casing setting
depths, select sizes of casing and corresponding borehole (drill bit) diameters, and
calculate the necessary wall thicknesses and material grades for the casing strings.

3.5.2.1 Trajectory
The borehole trajectory is the ‘‘connection’’ from the spud point at surface through
the target(s) to final well depth. Designing the well path the casing diameters have
to be taken into account particularly for the sections where changes are planned in
inclination and azimuth in order to avoid sharp bends with high impact on casing
collapse resistance.

3.5.2.2 Casing Setting Depths
Setting depths have to be chosen according to the geological profile forecast.
Generally, casings have to be set to separate formations with strongly different
pressure gradients from each other in order to (i) to separate formations which need
different drill mud types (e.g., fresh water mud against saturated salt mud), (ii) to
seal formations against influx of fluids (gas, oil, and water) from the previous or the
next section, and (iii) to secure drilled hole sections (e.g., in unstable formation).

3.5.2.3 Casing Sizes
As shown above the needed number of casing/liner strings depends mainly on the
geology. To find the fitting casing sizes, the whole well planning process should
always start at final depth with the desired diameter there and move upward
stepwise (casing size by casing size, Figure 3.17). Casing diameters depend on bit
diameters, and bit diameters on casing diameters. Casing (pipe) diameters and
corresponding bit diameters are – more or less – ‘‘standardized’’ as already pointed
out in Section 3.4. Typical diameters of hole and casing are shown below. Starting
from the smallest casing/liner size at final well depth the corresponding hole size
can be chosen by either following a full line (conventional scheme) or a dotted line
(feasiblebut uncommon) to theholesize, from theretothe next size of casing,and
so on, until the planned well scheme has reached the starting diameter.
A ‘‘golden rule’’ for designing a casing scheme is ‘‘drill as small as possible, but
as large as necessary!’’
The reason for this is the simple fact that drilling a large-diameter hole is
normally more expensive than drilling a smaller one. However, this should not lead
to a well with insufficient diameters of casing strings, because this may adversely
affect the cost of the operational phase of the power plant for the whole lifetime!