Fracture networks are used for underground heat exchangers, i.e. reservoirs, in the
Enhanced Geothermal
Systems (EGS). Each fracture composed of the network is spread over a relatively
large area of several
hundreds square meters, but its aperture is limited to several millimeters at
maximum. There is no way to
detect directly such a thin structure nor the fluid flow in it from ground surface
through a huge rock mass
with few thousands meters in thickness, while those factors are important for
construction of the EGS. On the
other hand, a number of microseismic events are observed during hydraulic
stimulation. It is believed that the
occurrence of those events is associated with the fluid flow through fracture
networks caused by the
stimulation. Therefore it may be possible to estimate the fluid flow from the
observed micoseismic events.
To this end, we have considered the sequence in which hydraulic stimulation leads to
the occurrence of
microseismic events. Then, based on those consideration, we have come up with an
idea to integrate the data
of microseismic events for estimating pore pressure propagation along the fracture
network associated with
hydraulic stimulation. Furthermore, we have shown that the location of flow pathways
and the hydraulic
conductivity along them could be estimated from the pore pressure distribution
estimated from microseismic
events. To do this, we assume an appropriate model of flow pathway structure and
adjust it as the pore
pressure distribution computed by the model agrees well with that estimated from
micro-seismic events. We
have applied these methods to the Soultz and Cooper Basin HDR fields. As a result,
we have succeeded to
reach plausible estimations of pressure propagation during hydraulic stimulation and
flow pathway structures
for each field.
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