Page 182 - Geothermal Energy Systems Exploration, Development, and Utilization

158 3 Drilling into Geothermal Reservoirs
Table 3.2 Potential geological risks of different lithologies.

Lithology Risk
Washout Sand Caving/ Collapse Swelling Creeping Lost Gas
production breakouts circulation kick

Sandstone xx xxx x xxx
Siltstone X xx xx x
Mudstone xx xxx
Claystone x xx xx x
Unconsolidated xxx xxx
sand
Clay/salt xx xxx xxx
Marl xx x
Limestone xxx x xxx
Shale xx x xx xxx
Schist/phyllite xx x x
Gneiss xx x
Granite xxx xx
Basalt xx xx

in geologic units during the drilling process. Generally, a sampling rate of 5 m
steps will be sufficient unless critical strata like casing shoe formations, or entry
points to the reservoir are expected. In critical strata and in the reservoir rock
a sampling rate of 1–2 m is advisable in order to react prompt to changes in
lithology or to recognize expected geologic boundaries. A careful and continuous
analysis and description of the cuttings is an essential requirement to this special
rock identification. Additionally, conventional LWD tools can provide the necessary
information and insight for steering boreholes geologically and for petrophysical
evaluation. However these costly tools require a detailed cost-benefit calculation for
the geothermal project.
A sound geological well planning can considerably minimize geological risks and
involves delineating the targets on the basis of (i) seismic maps, (ii) top structure
maps, (iii) offset well logs, (iv) geographic target selection, and (v) 3D modeling
and visualization. If one or more of these points are missing, following major
complications may emerge:
• drilled geological profile differs from expected (poor seismic interpretation,
deviated well path is insufficiently considered);
• fault/fracture zones found which had not been detected in seismic profiles
(subseismic features);
• borehole stability/caving problems/sand production in weakly consolidated for-
mations;
• borehole stability depending of trajectory in the current stress field (high stress
concentration along well path);