Page 193 - Geothermal Energy Systems Exploration, Development, and Utilization

3.11 Recent Developments, Perspectives in R&D 169
3.11.1.3 Multilateral Wells
Multilateral wells are quite common in special oil and gas applications. The idea
behind is to reduce overall drilling costs by using one top hole section for several
‘‘laterals’’ and to multiply production by these multiple laterals. Typical cases are
shown in Figure 3.32.
The technique of drilling multilateral wells may be advantageous also for
geothermal wells, but diameters of the laterals may be critical for the high flowrates
needed in geothermal applications.

3.11.2
Other R & D-Themes of high Interest

Future research is needed in large-diameter drilling specially through plastic,
creeping or swelling formations as salt or shale. Commonly, abnormal high fluid
pressures in such formations cause abnormal stresses that can differ considerably
from usual pressure gradients. To provide long-life completion systems in plas-
tic formations, new cementing technologies accommodating the geomechanical
behavior of plastic rock need to be defined, especially for deviated wells.
Another aim in accessing the reservoir is to minimize formation damage using
low mud pressures by means of near-balanced drilling (NBD) or even UBD,
particularly in reservoirs with depleted (underhydrostatic) formation pressure.
Whenever drilling mud is introduced into the borehole in an overbalanced pressure
situation, mud invasion, and formation damage occur. Hydrothermal wells are in
particular sensitive to this invasion because of the long completion zones, complex
chemistries, and high temperatures thus the aim is to keep the mud pressure in
a low condition that is near or below the fluid pressure of the formation. The
negative effect of NBD is, however, high stress concentrations on the borehole
wall probably causing borehole failure. NBD and borehole stability under changing
stress conditions must be well understood. In future, wellbore integrity needs to be
investigated by fracture mechanical experiments and simulations. These important
efforts require core testing and thus, an extensive core sampling campaign is
essential to describe and compile rock mechanical parameters of geothermal fields.
More importantly, the availability of drillcore sections are crucial for sophisticated
formation evaluation at borehole scale that characterizes the subsurface conditions
with less uncertainties than with logs, where disturbing factors, for example, from
mud or tool itself can basically mislead log interpretations (Norden et al., 2008).
The objective of new-generation geothermal drilling should be to promote
ways and means to reduce the cost of geothermal drilling through an integrated
effort which involves improved 3D well planning and geosteering, rock and
fracture mechanical modeling in context of borehole stability consequently reducing
borehole problems, and fostering an environment and mechanisms to share
methods, experiences, and means to advance the state of the art.