Page 180 - Geothermal Energy Systems Exploration, Development, and Utilization

156 3 Drilling into Geothermal Reservoirs

Drillhole
Overgauge hole
Breakout

Overgauge hole Brittle marly mudstone
Washout Sandstone Mudstone

weakly
lithified

Circulation lost
Limestone
Induced fractures

Sandstone

Hole closure
SH
Salt, plastic clay
Creap
Horizontal stresses
Sh
Figure 3.23 Possible geologically related drilling problems.

through the drillstring. Massive lost circulation can appear in highly fractured or
karstic rocks like the Malmkarst of the Molasse Basin in South Germany. Karstic
fault zones represent the geological target of high flow rates of geothermal fluids,
but when unexpectedly entered the fault zone, a total circulation loss may cause a
catastrophic loss of well control. If geothermal wells of large diameter are drilled
in formations that are prone to lost circulation or if underbalanced drilling (UBD)
is required foam drilling might the most suitable mud technique.
• High pressure zones (e.g., shales, particularly dangerous if gas bearing):
Gas-bearing high pressure zones like sealed shales threaten the well by a
sudden gas kick, especially with UBD.
• Keyseats: Keyseats often occur in doglegs and in alternating sequences of
harder and softer layers. This phenomenon is already explained in Section 3.6.4.
Eventually, larger tools of the bore hole assembly like tool joints, DCs, or
stabilizers will not pass the small channels (keyseats) and drillstring could get
stuck. Preventive measures especially in alternating strata include keeping any
turns in the wellbore gradual and smooth otherwise the keyseating necessitate
enlarging the worn channel by additional runs.
• Differential pressure sticking: This phenomenon belongs to the most critical
drilling problems worldwide in terms of time and financial costs. It crops up
especially in permeable sandstones. In general it occurs if drilling is done