Page 217 - Geothermal Energy Systems Exploration, Development, and Utilization

4.6 Application (Practical) 193
at mitigating induced vibrations to an acceptable level. Although there are that
certain stimulation regimes (e.g., injecting at low pressures over longer periods,
or avoiding large pressure gradients by slowly increasing and decreasing the well-
head pressure at the beginning and the end of injections (Bromley and Mongillo,
2008)) might be beneficial, these topics are subjects of ongoing research, and will
still be so in the near future.
• Information and education: The benefits of an open information and education
policy, targeting the local population, regional authorities, and the stakeholders
cannot be overestimated. Such a proactive information policy should include
regular information (meetings, newsletters, articles in local newspapers, etc.)
about the general development objectives, recent project progress, and benefits
for the community. This will help to reduce public concern and avoid complaints
and unreasonable claims.
• Monitoring concept: It is generally desirable to have a kind of seismic monitoring
system available in order to observe and keep a record on any microseismic events
potentially induced during stimulation operations. This may either be done using
an existing regional or national seismic grid, in case there are seismic stations of
the grid in the vicinity of the EGS development site, or by installing a dedicated
seismic monitoring system which can be tailored to the site-specific conditions
and particular stimulation parameters.
• Implement emergency action plan: It is advisable, particularly if the upfront
assessment has shown the potential for induced seismic events to be felt by
the local population, to design and implement a clear action plan to monitor
and assess induced seismicity and vibrations. The aforementioned ‘‘traffic-light
approach’’ may serve as an example here, which can be modified to match local
conditions.

4.6.2
Thermal Stimulation

Up to now, thermal stimulation was actively applied almost exclusively in hightem-
perature systems associated to volcanic activity (e.g., Iceland, Axelsson et al., 2007,
or Bouillante, Tulinius, Correia, and Sigurdsson, 2000) or in high temperature
metamorphic environments like in Travale or Larderello (Italy). The stimulation
processes normally starts with water circulation through the drillstring followed
by pumping cold water into the well. Occasionally water is injected at increased
wellhead pressures. The injection can be interrupted by intervals of non-activity,
during which the well is allowed to heat up toward its natural temperature state.
In this way, a combination of thermally induced cracking forces and pressure im-
pulses can increase the permeability of existing fractures and possibly create new
◦
ones. Stimulations at low temperature fields (below 150 C reservoir temperature)
primarily involve pressure changes induced either directly at wellhead or down-
hole, where inflatable packers (seals) are placed to more effectively address deeper
well sections. Air-lift pumping is commonly used in low-temperature stimulation
operations. Thermal stimulation operations generally are applied for a period of a