Page 200 - Geothermal Energy Systems Exploration, Development, and Utilization

176 4 Enhancing Geothermal Reservoirs
First, stimulation measures may be necessary because the target formation have
been clogged or damaged within the course of the drilling process. ‘‘Clogging’’ may
even be caused deliberately by adding weighting material in order to protect the
target formation throughout the drilling process. In these cases, acid stimulations
may be applied to clean up the well. Natural fractures can become mechanically
blocked by drill cuttings as well during the drilling process. This will especially
happen, depending on the mechanical rock properties and the drill bit applied, if
drill cuttings are very small. Thermally stimulating these wells can be an effective
way of restoring their productivity (Axelsson, Th´ orhallson, and Bj¨ ornsson, 2006;
Tulinius et al., 1996).
Secondly, and most importantly, geothermal wells are being stimulated in
order to significantly increase their productivity such that they flow at economic
levels. This can be achieved by generating new fractures or fracture systems,
by connecting the well to an existing natural fracture network, or by expanding
natural joints and fractures. In high temperature systems like in volcanic and
in some metamorphic settings, the most cost-effective way of stimulation is a
thermal treatment, provided reservoir temperatures are sufficiently high. It is
difficult to give a lower temperature limit above which thermal stimulation will be
possible, but successful thermal stimulations have been reported for reservoirs with
◦
temperatures of T > 210 C (see e.g., Tulinius, Correia, and Sigurdsson, 2000). For
the vast majority of other cases, namely deep sedimentary and granitic settings, a
hydraulic stimulation will be the method of choice, which can be followed by an
acid treatment. On the contrary, in the particular case of the fractured or karstic
carbonates, an acid stimulation or acid frac will be the preferred method, in order
to sustainably increase hydraulic conductivity.
Thirdly, in case a well is damaged or clogged by mineral deposits or scaling during
the production phase of a geothermal reservoir, an acid treatment may be indicated.
Although main stimulation methods will consecutively be dealt with separately,
hybrid treatments are not uncommon. Thermal stimulations are being conducted
applying a certain wellhead pressure, and in acid fracs, acid is injected into the
reservoir above fracture pressure, thus creating hydraulic fractures. The faces of
these fractures are being etched in a nonuniform way, which creates hydraulic
conductivity.

4.3
Stimulation and Well path Design

There are several options to exploit a geothermal reservoir by obtaining the
optimum design to extract the maximum heat with minimum costs. The simplest
design consists of a single production well. Sustainability is only given if ground
water recharge compensates production; otherwise one takes the risks of depletion
of the geothermal reservoir. The basic types of a sustainable design are doublets or
triplets consisting of one or two production wells and one injection well to complete
the water cycle. These can be enhanced to a multiwell design with several patterns