Page 162 - Geothermal Energy Systems Exploration, Development, and Utilization

138 3 Drilling into Geothermal Reservoirs
The fracture gradient can be determined by leakoff tests or mini-frac tests where a
drop during mud pressure increase indicates induced mud loss through a tensile
fracture. In particular, the fracture propagation pressure determined during leakoff
tests is similar to the minimum principal stress value which is a key parameter for
stimulation planning and treatments in enhanced geothermal systems (EGS).

3.5.1.3 Critical Formations/Fault Zones
Critical formations comprise high permeable layers with abnormal fluid pressures,
and highly ductile, unstable, or swelling successions like salt rock, shale, or clay. In
particular, the creeping process is often accelerated under high temperatures and
threatens both drill and casing string. To avoid complications in those creeping
formations, certain mud pressures and mud chemistry and thick-walled casing
strings need to be considered in well planning. Especially the long-term safety of
the large-diameter geothermal wells depends on the right thick-walled completion
design. Special effort should be focused on the cementation process of deviated
wells: in the bend sections of deviated wells the cement is not necessarily filling
the annulus completely – despite use of centralizers – and the resulting abnormal
hoop stresses might cause casing collapse.
Unstable formations like shale and also fault zones tend to result in break outs
and cavings (as evidenced in the San Andreas Fault Zone Observatory at Depth
(SAFOD) research well, drilled through the San Andreas fault, Prevedel, 2007);
Karstic structures like faults in the South German Molasse Basin are geologic
targets but bear high jeopardy of massive fluid loss while drilling. Drilling through
seismogenic zones threaten a complete well if the fault is activated by natural or
artificial influences (shearing of complete casing strings through faulting).

3.5.1.4 Hydrocarbon Bearing Formations
During the drilling operation and within the geothermal target hydro carbon (HC)
bearings can delay or stop a whole geothermal project. Generally, HC bearings
can be encountered in nearly all sedimentary environments. High gas contents
under high pressure bear the risk of a blow out, thus, a BOP needs to be
installed below the rig even in geothermal drilling projects. Large-scale crossing
faults can be pressurized by gas contents leading to unexpected gas kicks even in
presumed non-HC bearing or impermeable formations. High gas contents finally
can be mixed up with the geothermal fluid and need to be considered in the well
completion of a production well.

3.5.1.5 Permeabilities
Formations that transmit fluids readily, such as sandstones, are described as per-
meable and tend to have many large, well-connected pores. As such, sandstones
are typical geothermal targets in sedimentary basins. Special mud chemistries and
mud weights allow an optimization in reservoir assessment to mitigate formation
damage. If the mud pressure is kept slightly overbalanced, the risk of differen-
tial sticking is high in permeable sandstones. Impermeable formations such as
sandstones of mixed grain size, diagenetically cemented sandstone, shale, siltstone,