Page 201 - Geothermal Energy Systems Exploration, Development, and Utilization

4.3 Stimulation and Well path Design 177
of vertical wells like hexagonal or five spots patterns to enlarge the effective heat
transfer area (Armstead and Tester, 1987). The optimization of the number of wells
includes several constraints which depend crucially on the target depth and hence
the individual costs for drilling, the initial productivity of the reservoir rocks, and
the required costs for stimulation treatments to enhance this productivity.
The arrangement of two wells follows two conflicting goals and can be generalized
for a multiwell design. On the one hand, the wells should be located in such a
way, that the pressure in the reservoir will not drop significantly during production
resulting in a comparatively close distance of the wells. On the other hand, a short
circuit between the wells, implying a temperature drop in the production well,
should be avoided. In general, there are two options:

• Arrangement of stimulated fractures on the connection line of both wells,
corresponding to the classical HDR approach, that is, the orientation of the
doublet is in the direction of the maximum principle stress.
• Arrangement of stimulated fractures perpendicular to the connection line of
both wells, that is, the orientation of the doublet is in the direction of the least
principle stress and fluid flow propagation is through natural permeable rocks.
Both arrangements of the doublet (parallel and perpendicular to the minimum
principal stress) in a reservoir with some matrix permeability result in a pressure
decrease at the production well and a pressure increase at the injection well
(Huenges et al., 2006). The risk of a thermal short circuit of the system is most
probable in the parallel case. Therefore the perpendicular case is the appropriate
arrangement for a deep sedimentary reservoir with some matrix permeability and
is valid as long as no extended natural fracture systems are connected. (Legarth
et al., 2005; Zimmermann, et al., 2005).
A different concept includes the connection of an existing permeable fault zone
in a reservoir, which can be on one side connected to the wells via stimulation
treatments or on the other side through a well path intersecting the fault zone like
in the European HDR project in Soultz-sous-Forˆ ets (Baria et al., 1999; Hettkamp
et al., 2004; Zimmermann et al., 2009). In a natural fractured reservoir, a deviated
well can intersect multiple fractures and connect them to the well. This design
can be supported by multiple stimulation treatments to enhance the number of
connected fractures and hence the productivity of the well.
Considering vertical or deviated (nonhorizontal) wells, the use of multiple layers
for production and injection is another option to enhance the productivity of a
geothermal system. This design depends on the particular geological setting of the
reservoir. A potential option is a connection of two different layers via a hydraulic
fracturing treatment (Zimmermann et al., 2009). In this particular case, a vertical
fracture propagates from the bottom of the well (a fractured rock layer) upward
to the next layer (permeable sediments) where vertical fracture propagation may
stop, if the fracture reaches a superimposing impervious (and ‘‘soft’’) layer, and is
followed by a leak off into the sediments. If a doublet of wells is scheduled, the
flow between the wells is then performed through the sediments and the respective
connection to the wells is supported by the induced fractures.