Page 224 - Geothermal Energy Systems Exploration, Development, and Utilization

200 4 Enhancing Geothermal Reservoirs
(SPME) has reduced the limit of detection by a factor of about 30 (Mella et al.,
2006a).

4.7.1.4 Monitoring Techniques
In order to assess the success of the thermal stimulation measures, and to
obtain information of the relevant reservoir properties, it is recommended to
perform a monitoring program together with the well treatment. The most obvious
and basic monitoring parameters will be the injection flow rate, the wellhead
pressure during stimulation and the temperature of the injected fluid. Very useful
information is provided by monitoring the injection pressure downhole, while
profiles of temperature and pressure can serve in identifying relevant feed in
zones.
Although not a standard measure up to now, it might be useful to perform a
seismic monitoring program as well. Microseismic imaging of induced fractures is
a technique originating from earthquake seismology. It significantly evolved over
time, and has often been successfully applied in recent EGS development projects
(e.g., Soma et al., 2004; Charl´ ety et al., 2007, or Kwiatek et al., 2008) Axelsson,
Th´ orhallson, and Bj¨ ornsson (2006) report about cases, in which the analysis of
seismic monitoring data showed that the thermal resource might have a much
wider extent, and therefore a much higher generation potential than previously
assumed. In order to record compressional and shear waves emitted during
fracture stimulation, three-component geophones (3C) are placed in a monitoring
well, or, if possible, at the surface location, to determine the location of the seismic
event.
Tiltmeter surveys of the borehole wall and/or the surface, measure the tilt of the
earth from deformation caused by a displacement of a hydraulic fracture, with very
high resolution (up to 1 nano radian). Depending on the registration geometry,
they can be used to resolve for geometric parameters of the induced fracture like:
azimuth, dip, height, and length.
Microseismic monitoring and tiltmeter surveys work most reliably when sensors
are deployed at the target depth in an offset well. In case no offset monitoring
well exists, the methodologies can be applied form surface locations, but in
these instances, a successful application is limited through local parameters
like treatment depth, volume of the fracture, overburden material, and strength
of the source signal. Although techniques are available for application in the
treatment well as well, these surveys are subject to technical restrictions in terms
of applicability and of force of expression. Barree, Fisher, and Woodproof (2002)
or Cipolla and Wright (2000) have given comprehensive and general outlines of
fracture diagnostic technologies, frequently applied in the hydrocarbon industry.
A chemical monitoring program, which can be performed alongside conventional
well tests, will provide useful information on any changes in the chemistry of
the produced fluid or steam, and help to asses any changes in the hydraulic
state of the reservoir (see e.g., Sanjuan, 2000), which may be caused by the
stimulation program. Still the best evaluation of the treatment effect is achieved
by a combination of pressure, flow, and temperature monitoring at the wellhead