Jökull - 31.12.2001, Blaðsíða 49

Seismic monitoring during a wastewater injection experiment in Svartsengi
no seismometers were installed in Svartsengi until
1984 when an analog station was established at the
southern edge of the geothermal field.
Figure 2. A geological map of the Svartsengi geother-
mal field showing the location of borholes (black cir-
cles), volcanic fissures (thick black lines) and other
tectonic lineaments (thinner and dashed lines) vis-
ible on top of hyaloclastite hills (white areas) and
lavas from shield volcanoes extruding the younger
lavafield. The sea is also white. The geothermal field
has an elliptical shape and is elongated along the main
fault direction (NE-SW) as delineated by the 4 ohmm
isoresistivity line at 200 m depth, but seems to be
more NNW-SSE oriented at 400 m and 600 m depth.
The injection borehole (H-6) is denoted by a star. -
Jarðfrœðikort af jarðhitasvœðinu í Svartsengi og ná-
grenni þess. Svartur kassi táknar stöðvarhúsið og
svartir punktar borholurnar. Niðurdœlingarholan H-
6 er merkt með stjörnu. Efsti hluti jarðhitasvœðisins
hefur ellipsulögun eftir meginsprungustefnu svæðis-
ins, NA-SV, sbr. 4ohm-metra jafnviðnámslínuna á
200 m dýpi. Þar fyrir neðan virðist jarðhitinn frekar
fylgja NNV-SSA lœgari stefnu, efmiðað er við 4 ohm-
metra jafnviðnámslínuna á 400-600 m dýpi. Gíga-
raðir eru táknaðar með feitum svörtum strikum en
mjórri strik sýna aðrar tektónískar línur svæðisins,
sem greinilegastar eru á yfirborði eldri jarðmyndana,
á móbergshálsum (hvít svæði) og í dyngjuhraunumfrá
síðasta hlýskeiði (grá afmörkuð svœði). Hraun eru
gráskyggð en hafið hvítt.
The exploitation of geothermal fluids from the
Svartsengi field has generated more than 20 bar pres-
sure drawdown in the wellfield (Björnsson and Stein-
grímsson, 1990; Vatnaskil, 2002). The drawdown has
probably generated a slight compression of the rock
matrix. Levelling and gravity measurements show
that an area of lOOkm2 around Svartsengi has sub-
sided by 237 mm in a confined region around the well-
field (Eysteinsson, 2000). The subsided region has
an elliptical shape, being elongated WSW from the
Svartsengi wellfield towards the Eldvörp geothermal
field and the rate of subsidence seems to be linearly
related to well pressure (Eysteinsson, 1993).
The pressure drawdown in the geothermal reser-
voir has caused some production problems. Wellhead
pressures have had to be reduced in order to maintain
the flowrate and to diminish calcite scaling within the
boreholes. Mass inflow of colder fluids constitutes the
greatest potential danger to the geothermal exploita-
tion in Svartsengi. The drawdown induced a tempo-
rary inflow of colder fluids into the wellfield during
March-June 1983 causing a cooling of up to 13°C and
a pressure drop up to 5 bars within individual wells
(Björnsson and Steingrímsson, 1991). Fortunately,
subsequent influxes have all been smaller. The draw-
down has a diminishing effect on pore pressure which
again increases rock strength within the geothermal
reservoir, possibly preventing further inflow of colder
fluids.
Continued drawdown will lead to decreased pro-
ductivity of the wellfield. In order to avert this pro-
cess, an injection of wastefluids (70-80°C) has been
carried out intermittently since 1984. As a result,
the average rate of subsidence within the Svartsengi
wellfield decreased from 14mm/year during 1975-
1982 to 7-8 mm/year during 1982-1992 but increased
again to 14mm/year during 1992-1999 (Eysteins-
sson, 2000). Subsequently, the rate of pressure draw-
down decreased from 2.5 bars/year during 1975-1982
to 1.2 bars/year in 1982-1992 (Björnsson and Stein-
grímsson, 1991; Eysteinsson 1993).
SEISMIC ACTIVITY WITHIN THE
SVARTSENGI REGION
Persistent microearthquake activity, characterized
by both continuous activity and individual swarms, is
often found within geothermal regions (Ward et al.,
1969; Ward and Björnsson, 1971; Combs and Hadley,
1977; Gilpin and Lee, 1978; Majer and McEvilly,
1979; Foulger, 1988). The spatial distribution of seis-
micity within geothermal areas has been used to map
the region of natural cooling and thus infer the ex-
tent of active heat mining from a geothermal reservoir
(Lister, 1974; Foulger et al., 1989). Pressure vari-
ations and water level fluctuations are also possible
triggers of microearthquake activity within geother-
mal reservoirs.
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