Page 172 - Geothermal Energy Systems Exploration, Development, and Utilization

148 3 Drilling into Geothermal Reservoirs
24 and 72 hours, a directional drilling assembly (with DHM and MWD, as
described in Section 3.2) is used to sidetrack from the hard cement into the
borehole wall, favorably into the proper direction from the early beginning.
When the new hole is drilled out of the old one, the desired new well path can
be followed up.
– Sidetrack from a whipstock (Figures 3.20 and 3.21): Inside the casing a
‘‘whipstock’’ (= a wedge) is set oriented, either on a cement plug or is fixed to
apacker.
Then a milling assembly is used to mill out the casing wall into the direction
of the wedge which pushes the mill into the casing wall.
Depending on the system another milling run may be necessary to cut the
window to full size prior to run a directional drilling assembly as described
above to drill the new well path.

3.7
Well Completion Techniques

The well completion starts with the casings which are planned to be used directly
for injection or production and includes all tubulars and safety equipment need to
handle the water flow in the wells.

3.7.1
Casing (Please Refer Also to ‘‘Casing String Design’’)

In a very early phase (planning the well) the decision has to be made whether
the casing strings which will be used either for injection or for production will be
allowed to move under temperature changes or not (Section 3.5).

3.7.1.1 Allowance of Vertical Movement of Casing
If a traveling of the upper part of a casing string due to thermal expansion or
contraction should be allowed only the lower part is to be cemented. With this
technique several meters of up- and downward movement of the casing head may
occur and has to be taken into consideration! Critical with this technique may be that
the uncemented casing may buckle under its own weight – particularly in caved hole
sections – and that a sliding seal may be necessary to seal the annulus of the casing.

3.7.1.2 Pretensioning
If no casing movement should be allowed either the casing has to be cemented to
surface or the noncemented upper section has to be pulled into sufficient tension to
prevent later movement during production. To determine the ‘‘sufficient’’ pulling
force for pretensioning the following work steps have to be done:

• Estimate the average expected temperature change over the noncemented string
length.
• Calculate the elongation which will be generated by this temperature change.
• Calculate which load is necessary to generate the same elongation.