Page 152 - Geothermal Energy Systems Exploration, Development, and Utilization

128 3 Drilling into Geothermal Reservoirs
−2
g (9.81 kg × ms ) = acceleration due to gravity,
L (m) = vertical depth.
Drilling problems occurring due to very high mud density (e.g., differential stuck
pipe, mud losses by hydraulic fracturing) can be accentuated by the ECD effect.
Estimates of formation pressures made before drilling are based primarily on
correlation of available data from nearby wells (leakoff tests, mud report of offset
wells) and seismic data. To estimate formation pressure from seismic data, the
average acoustic velocity as function of depth must be determined and the reciprocal
of the velocity – the porosity-dependent interval transit time – is generally displayed
(Bourgoyne et al., 1986). During the drilling operation, the formation pressure can
be determined by the pressure at the bottom of a well when it is shut in at the
wellhead. If abnormal formation pressures in critical formations (overpressure due
to underconsolidation or aquathermal effects in heated low permeability rocks) are
presumed and threaten the drilling process, the use of FPWD (formation pressure
while drilling logging tool) can resolve drilling safety (Charlez, 1997). If formation
pressure increases, mud density should also increase to balance pressure and to
keep the wellbore stable.
Assessing the reservoir top and during all further operations the initial reservoir
conditions are disturbed by the drilling process. Effectively, problems may arise
from formation damages when the well is putted on stream at a later stage (Perrin,
1999). In particular, the pay zone where the major geothermal water influx is
prospected may be damaged by the fluids, the mud solids, cement slurry, and so
on, causing a significant reduction in productivity. Thus, the careful and concerted
to the reservoir conditions selection of mud chemistry, mud solid type and size,
and mud weight belong, among others, to the critical influencing factors to
minimize formation damage. For example, near-balanced mud weight minimizes
the infiltration of mud solids within the pore space of siliciclastic formation.
However, a low mud weight might increase hoop stresses (that are the tangential
stresses) at the borehole wall initiating breakouts. Therefore, formation damage
and wellbore stability are two counteracting issues and essentially need to be
aligned to the well integrity.
In case of incurred or unavoidable formation damage, productivity can be restored
easily in carbonate formations than in sandstone formations by acidizing. In
metamorphic or granitic rock, chemical treatments can be successful, too (Tischner
et al., 2007). Commonly, hydraulic fracturing is finally applied to overcome the
damaged zone and to increase permeability in general. In any case, it requires
costly treatments in terms of rig time and treatment itself.

3.4
Casing and Cementation

Casing strings are needed to be run from time to time to secure drilled hole
sections (e.g., in unstable formation), to separate hole sections/formations with