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136 3 Drilling into Geothermal Reservoirs
3.4.6
Influence of Temperature on Casing and Cement

During the lifetime of a borehole, temperature changes will occur and affect
basically the open hole as well as the cased hole sections. Temperature effects start
even when drilling the well, because the circulating drill mud transports heat from
the lower hot wellsectionto the upper colder part. Hence, the temperature downhole
will be reduced while the upper part is heated up. After having stopped circulation,
for example, when tripping for a new drill bit, the temperature profile will tend to
go back to the undisturbed temperature curve. The magnitude and time span of
temperature changes are dependent on various factors like specific heat capacity of
rock and mud, circulation rate, duration of phases with and without circulation.
If a borehole is producing hot water (or steam) the whole length will be heated
up to nearly bottomhole temperature.
During well treatment, for example, acidizing, cold fluid (water) is injected,
which may cool down the hole significantly, depending on the amount and pump
rate of the treatment.
All temperature changes create stress and load changes on the casing strings and
the borehole wall; both are discussed in Sections 3.5 and 3.6.

3.5
Planning a Well

3.5.1
Geological Forecast

The whole complexity of planning a well is fundamentally based on the forecast of
a geological depth profile. Such a geological depth profile describes and evaluates
the thickness and lithology of rock mass which is crosscut by the presumed well
path. The geological pile should not only be provided as lithostratigraphy but
also as mechanical stratigraphy. A profound geologic-geomechanical knowledge is
important for several reasons: (i) the target can be defined by precise structural-
geologic description, (ii) wellbore safety can be evaluated by understanding the
geomechanical reaction to drilling process and (iii) design of the well path geometry
and completion by integrating detailed geological-rock mechanical knowledge. The
fundamental knowledge for geological forecasts comes from seismic data, offset
wells, and most importantly from regional geology.
Geological depth profiles along the drill path should be provided in MD
(measured depth, that is, the length of a wellbore), in TVD (true vertical depth, that
is, the vertical depth below drill rig from ground elevation to depth) and TVDMSL
(true vertical depth below mean sea level, that is, the vertical depth related to
mean sea level – these values are negative and are needed for geological modeling
because they are georeferenced to sea level). To complete a geological well section,
additional information might be integrated like the planned logging and coring