Page 161 - Geothermal Energy Systems Exploration, Development, and Utilization

3.5 Planning a Well 137
program, the casing shoe depths, bit and casing diameters, mud program, and
specification of presumed geologically risk zones. Additionally, a 3D geological
model helps to define well planning and finally serves as communication platform
between the various disciplines from geoscientist, engineers, and drillers at
the drill site. The geological forecast should be distributed before drilling to all
participating parties and drilling services.

3.5.1.1 Target Definition
The target within a geothermal field is defined by preceding exploration efforts.
If the starting point of a well (its elevation) and the target horizon is known, the
well path can be defined by the starting point at ground surface, entry point in the
reservoir, landing points in the pay zone and end of well. Directional drilling is
often used and necessarily includes kick-off points for deviation and/or azimuthal
changes. During directional drilling, the MWD tools allow to ‘‘steer’’ the well into
the geologic target.
The decision-making process in geologic target definition is governed by 2D/3D
geological/geophysical models that visualize the geometrical constrains of geolog-
ical layers and their properties. The latter are generally interpreted by borehole
logs from offset wells or estimated in connection with regional/local geological
knowledge.
Important formation properties for the drilling process are described in the
following paragraphs.

3.5.1.2 Pore Pressures/Fracture Pressure/Temperature
The pore pressure is the pressure exerted by the fluid that is in the pore space or
within fractures. The often used term for the pore pressure is formation pressure.
Usually the pore pressure is calculated by the depth of formation and the density of
fluid. When the formation pressure is approximately equal to theoretical hydrostatic
pressure (i.e., the compaction of water with depth is negligible due to existence of
relatively permeable flow path to the surface) for a given depth, formation pressure
is called normal, expressed by the hydrostatic gradient. Abnormal fluid pressures
are pressures above the hydrostatic gradient, for example, caused by high fluid
temperatures (e.g., in volcanic areas of Iceland), whereas subnormal formation
pressures are fluid pressures below the hydrostatic gradient (e.g., in the Malmkarst
of the South German Molasse Basin).
Generally, abnormal formation pressures are found in most of the sedimentary
basins worldwide, caused by mechanisms like compaction effects, diagenetic
effects, differential density effects, and fluid migration effects. Specially, compacted
shales and sealed permeable formations can bear fluids that cannot escape, leading
to anomalously high formation pressures. Thus, abnormal pressures that effect
wellbore safety in drilling and completion need to be considered in assessing
geothermal reservoirs in sedimentary basins and high temperature fields.
The fracture pressure is the pressure where a certain rock type cracks, that is,
the pressure where tensile fractures occur. This pressure is an important value for
drilling operation to avoid mud or cementation losses by inducing tensile fractures.