Jökull - 31.12.2001, Page 52

Bryndís Brandsdóttir et al.
Table I. Waste water injection volumes into well H-6 in 1993. Data from Geir Þórólfsson, Sudurnes Re-
gional Heating Company. - Yfirlit yfir niðurdœlingu í holu H-6. Gögn frá Geir Þórólfssyni, Hitaveitu
Suðurnesja.
Date 1993 Dags. 1993 Number of days Dagafjöldi Average injection rate Meðalrennsli (l/s) Total (tons) Heildarmagn (tonn)
19.07-20.07 1 30 2,592
20.07-22.07 2 22 3,802
23.07-27.07 4 15 5,184
28.07-04.08 8 10 6,912
04.08-10.08 6 11 5,702
11.08-28.09 48 18.5 76,723
15.10-15.11 30 15 38,880
01.12-31.12 30 30 77,760
Total/Sönzta/í 217,555
“earthquake threshold” lies in geothermal regions and
only by documenting various cases will our under-
standing of these processes be improved.
Results from various injection tests (Davis and
Frohlich, 1993) indicate that the wellhead pressure
needs to exceed 20-40 bars in order to induce seis-
micity. Applying the same criteria we conclude that
the injection pressure into the Svartsengi geothermal
field was probably far below the pressure needed. An
injection test within the Laugaland geothermal field in
N-Iceland also did not induce microearthquakes even
though wellhead pressures reached 30 bars (Axelsson
et al., 2000). These results are explained by outflow
of 70% of the injected water within a relatively low
stress regime in the uppermost 1000 m in Laugaland.
Fluid drawdown that preceded both the Svartsengi
and Laugarland tests is another factor which affects
the likelihood of induced seismicity. Drawdown de-
creases the pore pressure and increases rock strength,
which again reduces microseismic activity. The draw-
down of more than 20 bars in the Svartsengi since the
start of production in 1976 may have reduced the mi-
croseismic activity temporarily. Earthquakes are thus
not to be expected until the regional tectonic stress
reaches the new fracture limit.
Acknowledgements
This project was a part of the National Energy Au-
thority Environmental Assessment Program carried
out in cooperation with the Sudurnes Regional Heat-
ing Company. Benedikt Halldórsson, Brandur Karls-
son, Egill Axelsson, Jósef Hólmjárn, Karl Gunnars-
son and Olafur G. Flóvenz participated in the field-
work. We thank Albert Albertsson and his staff of the
Svartsengi geothermal power plant for providing fa-
cilities and general assistance. Guðni Axelsson and
William Menke provided constructive reviews.
ÁGRIP
Áhrif niðurdælingar á smáskjálftavirkni jarðhita-
svæða: Dæmisaga úr Svartsengi
Hérlendis hafa hita- og þrýstingsmælingar í borhol-
um fyrst og fremst verið notaðar til þess að fylgj-
ast með jarðhitasvæðum í vinnslu. Landhæðar- og
þyngdarmælingum hefur og verið beitt til eftirlits með
svæðisbundnum áhrifum vinnslunnar á jarðhitasvæð-
in. Jarðskjálftamælingar veita upplýsingar um innri
gerð jarðhitakerfa og þær má einnig nota til að fylgj-
ast með breytingum sem verða við vinnslu yfir langan
tíma. Smáskjálftavirkni og órói gefa upplýsingar um
ástand jarðhitakerfa, en notagildi þeirra við vinnslu og
eftirlit hefur enn lítt verið kannað hér á landi.
Vinnsla jarðhita hérlendis hefur í flestum tilvik-
um haft í för með sér vatnsborðslækkun (niðurdrátt)
innan viðkomandi svæða þar sem vökva hefur verið
tappað af þeim hraðar en nemur náttúrulegu aðrennsli.
Tilsvarandi þrýstingslækkun hefur einnig margvísleg
50 JÖKULL No. 51