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4.6 Application (Practical) 189
A typical filtration system comprises a self cleaning filter system, which will
pre-filter down to 50 µm. This is followed with cartridge filter units, which can
filter down to 2 µm.
A cross-linked gel is a water-based system with a specially designed polymer
loading composed of a refined guar gelling agent cross-linked by a borate solution.
For geothermal purposes it should include a fluid’s high-temperature stability and
atime-delayedcross-linked reaction.Using afluidwith aslow apolymer loading
as possible is essential to ensure that both the effective fracture half-length and the
polymer degradation are maximized.
A linear gel fluid may be used to carry proppant on slickwater treatments, as a
breakdown fluid on conventional proppant frac treatments, or as a flush fluid.
In order to fine tune fluid, the following additives may also be included in
this fluid, if required: temperature stabilizer, biocide, breaker to disintegrate the
cross-linked gel, corrosion inhibitor to protect the casing, potassium chloride or
acid to avoid scaling (iron etc.), and surfactant to reduce friction.
4.6.1.1 Induced Seismicity

Definition Generally speaking, induced seismic events can be described as ‘‘Earth-
quakes triggered by anthropogenic activity.’’ Induced seismicity has been observed
in association with the production of hydrocarbons from oil- and gas fields, at
large water reservoir dams, during deep mining activities and in geothermal de-
velopments. In connection with geothermal projects, induced seismic events may
occur either during the stimulation phase or in association with the production
and injection of fluids during the operation of a geothermal field (Figure 4.8).

Source Mechanisms There are several potential source mechanisms which, ac-
cording to Majer et al. (2007) may explain the occurrence of induced seismicity
linked to the development or operation of EGS.

• Reduction of effective stress: increasing pore pressure can lead to a reduction
of the effective stress on potential shearing planes like fractures and faults. In
the presence of a deviatoric stress field this reduction of static friction may cause
seismic slip.
• Stress redistribution: injection or production of fluid may lead to volumetric
changes within the reservoir. These volume changes can result in large-scale
stress redistributions, which may cause seismic slip at fractures and faults close
to failure (critically stressed) within or close to the reservoir.
• Thermoelastic strain: A temperature drop due to the injection of cold fluid can
cause a contraction of fracture surfaces which, like in the case of effective stress,
reduces static friction, and triggers slip along a fracture that is already near failure
in a regional stress field.
• Chemical alteration of fracture surfaces: Injection of fluids into the reservoir
may cause geochemical alterations of fracture surfaces which might change the
coefficient of friction on the fractures affected.