Page 169 - Geothermal Energy Systems Exploration, Development, and Utilization

3.6 Drilling a Well 145
• pick up and run in hole drill bit of fitting size for the casing;
• drill ahead (and so on until final well depth is reached).

3.6.4
Problems and Trouble Shooting

Many problems affect the success of a geothermal drilling operation. Since the
main function of a drilling rig is to penetrate and to seal off formations, any single
technical failure may halt this process thereby causing additional expenditures.
Therefore, the success of a geothermal drilling operation is strongly dependent on
avoiding problems causing downtime. Problems may occur mainly during drilling,
but can also occur during other work steps, for example, during running casing
strings. In all cases, problems will lead to delay and higher costs.
Bradely (1979) identified the human element as a key factor in avoiding borehole
problems like stuck pipe. Therefore, in addition to sound engineering practices
the operation culture may also strongly affect the outcome of a potential borehole
problem. The mud density selection in context with formation geomechanics is
essential for the success of a drilling operation, but good planning of related
elements are important as well. Examples are torque and drag considerations in
well path planning as discussed by Sheppard, Wick, and Burgess (1987).
Of course, proper hole cleaning is of importance for fast and safe drilling.
Particularly in deviated well sections, care has to be taken for sufficient carrying
capacity of the drill mud. Occasionally, some junk may be left in the hole (e.g.,
inserts of TCI bit) and has to be removed specifically when PDC bits are to be used
Following problems often occur during drilling operation:

• Mud losses: Trouble shooting by decrease of mud density and/or adding sealing
additives to the mud.
• Influx: Trouble shooting by increase of mud density.
• Borehole stability problems: Borehole problems such as fracturing, collapse, lost
circulation with severe drop of hydrostatic head of the mud column, and others
may affect borehole stability and need to be considered in a rock mechanical con-
text. It is shown in many wells that by maintaining the mud pressure close to the
level of the in situ stresses, most borehole problems will be minimized (Aadony,
1999). On the other hand, a low mud density, near to formations pressure,
will minimize formation damage through mud infiltration. Borehole stability
and mitigation of target formation damage must be carefully weighted. In any
case, a key issue in evaluating and avoiding borehole problems is understanding
rock failure within the in situ stress field. Hence, stress distributions along the
borehole wall and geomechanical parameters belong to the essential knowledge
of determining the optimal mud density.
A case study of the deep geothermal well Gt GrSk 4/05 in Groß Schoenebeck,
the key site in the Northeastern German Basin, exemplifies a combined approach