Title:

Reservoir Conditions at 3-6 km Depth in the Hellisheidi Geothermal Field, SW-Iceland, Estimated by Deep Drilling, Cold Water Injection and Seismic Monitoring

Authors:

Grimur Bjornsson

Key Words:

reservoir conditions, Hellisheidi Field, deep drilling, water injection

Geo Location:

Hellisheidi, Iceland

Conference:

Stanford Geothermal Workshop

Year:

2004

Session:

FIELD STUDIES

Language:

English

File Size:

1223KB

View File:

Abstract:

Reykjavik Energy recently drilled a 2808 m deep
exploration well HE-8 in the Hellisheidi high-temperature
field. The well is located at the western
boundary of a NNE trending fissure swarm. Two
main feedzones were encountered at 1350 og 2200 m
depth. The bottomhole temperature appears not to
exceed 300 ?C, suggesting considerable fluid
convection at this depth. When stimulating the well
by cold water injection at the end of drilling and
again after 3 months of warm-up, the wellís
injectivity increased from 1-2 up to 6-7 kg/s/bar. An
intrinsic reservoir permeability of 3-6 milli-Darcys
was estimated by modeling the transient pressure data
collected. A total of 22 small quakes accompanied
the cold water injection into well HE-8, mostly at 4-6
km depth. Fluid pressure changes inside the reservoir
fracture network, during injection, are strongly
suspected as a trigger for these quakes. It also implies
that there is a pressure communication and good
permeability between the feedzones at 1350 and 2000
m depth and the 4-6 km depth of the quake centers.
Large normal faults, dipping to the east, are
suspected as likely surfaces of quake generation.
These downhole and seismic data suggest
considerably deeper fluid convection cells than
previously assumed and may result in an increased
generating potential estimate for the Hellisheidi
resource. The field therefore appears feasible as a
target for drilling a very deep exploration well. The
study shows that the regional stress field and the
permeability field are related. Seismic monitoring
during injection may become a valuable tool for
locating permeable fault surfaces at great depths.


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