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DEEP HEAT MINING: Development of a cogeneration power plant from an enhanced
geothermal system in Basel, Switzerland.
Markus O. Haring, Geothermal Explorers Ltd
Key Words
enhanced geothermal system (EGS), heat mining, pilot plant, cogeneration, district heating,
Rhine Graben, seismic monitoring, induced seismicity
Abstract
In Basel, Switzerland a geothermal cogeneration pilot plant is being developed. The area at the
south-eastern margin is characterised by an increased heat flow and is known as a seismically
active area. A 2.7 km deep exploration well through the Rhine Graben sequence into the granitic
host rock has confirmed a temperature gradient of 4.0 °C/100m. The well has confirmed a stress
field that is favourable for creating an enhanced geothermal reservoir by hydraulic fraccing. It is
intended to drill one injection and two production wells to produce 3 MW electric power and 20
MW thermal power for the local district heating grid from an enhanced reservoir at five
kilometres depth. The pilot plant should demonstrate that geothermal power production from
enhanced geothermal reservoirs is a viable option of developing a sustainable and
environmentally benign resource outside known geothermal fields for the future global energy
demand.
Introduction
All global energy demand scenarios show that in the second half of this century additional energy
resource have to be available. By analysing all sustainable energy resources it becomes clear that
only geothermal energy is capable of producing baseload energy in a reliable manner without
additional storage. The conditions new energy resources have to fulfill are high: They must be
reliable, sustainable, environmentally benign and first of all economic. We are convinced that
geothermal energy is so abundant and ubiquituous, that a great effort is justified to make the
resource available to a larger market. Engineered or enhanced geothermal reservoirs are one
promising method to tap this resource outside classical geothermal fields.
Switzerland is not only a country with great sceneries, fabulous chocolate and secretive banks but
also a country devoid of fossil resources. Power is generated by hydropower and nuclear plants.
In the search for alternative resources it has been recognised that wind and solar are insufficient
to provide a reliable power supply beyond the age of fossil fuels. Switzerland is also a densely
populated country and land prices are high. New power plants should require little space and be
as inconspicuous as possible. The most striking part of geothermal plants are the cooling units.
Water is abundant in Switzerland and cooling of thermal power plants can be achieved in general
with running water from large rivers like the Rhine. Hence it is understandable that research into
geothermal technologies is widely supported by power producers, the public and the government.
The purpose of this paper is to demonstrate that it is possible to develop geothermal power plants
in areas outside known geothermal fields, when local geological characteristics, infrastructural
advantages and commercial requirements are adressed and dealt with in an integrated approach.
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Pilot Plant
In Basel, Switzerland a pilot plant is being developed to use energy won by EGS technology for
co-generation of electrical power and heat for local district heating. The core of the project called
DEEP HEAT MINING BASEL, is a well triplet into hot granitic basement at a depth of 5000
metres (Figure 1). Two additional monitoring wells into the top of the basement rock will be
equipped with multiple seismic receiver arrays. They will record the frac induced seismic signals
to map the seismic active domain of the stimulated reservoir volume. Reservoir temperature is
expected to be 200°C. Water circulation of 100 l/s through one injection well and two production
wells will result in gross 30 MW thermal power at well heads. It has not yet been decided what
conversion cycle will be used for electric power production. The plant is located in an industrial
area of Basel. The waste incineration of the municipal water purification plant provides an
additional heat source. In combination with this heat source and an additional gas turbine, a
combined co-generation plant can produce annually up to 108 GWh electric power and 39 GWh
of thermal power to the district heating grid.
Geology
Basel is situated at the south-eastern end of the Rhine Graben (Figure 2), a failed rift feature
cutting from north-east to south-west through central western Europe. This part of Europe is
characterised by strike-slip faulting dominated by compressive forces of the alpine collision
(Figure 3) (Reinecker et al., 2003).
Rifting of the Rhine Graben during the Oligocene has resulted in a thinned crust. Refraction
seismic data indicate buckling of the Moho with a minimum of 24 km below surface at the centre
of the Southern Rhine Graben (Bonjer, 1997). Strike slip faulting along the graben boundary fault
system led to vertical hydraulic circulation in various locations. Geothermal maps of the area are
not very detailed and are based on a restricted number of shallow observations. They show
however throughout the region an increased heat flow of at least 100 mW/m2 with up to 130
mW/m2 (Medici and Rybach, 1995).
Basel is not only situated at the south eastern end of the Rhine Graben but also at the northern
front of the Jura mountains, the outermost expression and youngest part of the alpine fold belt.
The peculiar coincidence of north-northwest trending compression and west-northwest extension
creates a seismically active environment. Historically the worst earthquake occurred in 1356 with
an estimated magnitude of 6.5 to 7 (Weidmann, 2002). It destroyed the city of Basel and many
medieval castles in the wider area. Hypocentres are characteristically at a depth of around 15 to
20 km, a depth at which the main boundary faults are believed to sole out. Although depth
allocation of the regional seismic monitoring array is not very precise, it appears that substantial
seismic events may occur as shallow as 5 km (Figure 4).
The plant lies within this seismic active area (Deichmann, 2003). It is therefore indicated to
record and understand the natural seismic activity as accurately as possible, prior to stimulation
of a deep reservoir volume, characteristically accompanied with induced seismicity. The first
exploration well, Otterbach 2 drilled in 2001 into granitic basement at 2650 metres to a total
depth of 2755 metres. The well is planned to become a monitoring well equipped with a seismic
cable. The cable with a total length of 3‘000 metres consists of twelve mems sensors installed
into three tetrahedral (four component) sondes, twelve geophones likewise installed into three
tetrahedral sondes and a temperature sonde at the bottom. The sondes are placed at 10 metre
intervals from the bottom upwards. Deployment of the cable is planned in June 2004. The
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purpose of the monitoring is to detect and locate the induced seismic signals generated by
hydraulic fracs as well as natural regional seismic events.
A shallow seismic survey was shot in 2002 along the Rhine river with airgun and streamers. The
survey was part of the scientific EUCOR-URGENT programme with the focus on seismic
hazards and neotectonics in the Upper Rhine Graben. The seismic line through the city of Basel
cuts across the main boundary fault displaying a vertical throw of more than 2500 metres. The
Otterbach exploration well, drilled in 2001 is located on the downthrown side, about two
kilometres east of the main boundary fault. It is the first well in this area penetrating the entire
sedimentary sequence down to basement (Figure 5).
The temperature gradient is 4 °C/ 100 m (Figure 6). The sedimentary sequence consists of
Tertiary clastics, Mesozoic carbonates, shales and evaporites and Permian sandstones. Heat flow
and heat production measurements in the outcropping granites on the flanks indicate that the
same gradient is likely to persist down to the target depth.
Borehole deformation logging with acoustic and electric borehole televiewer tools shows induced
fractures pointing predominantely in a NNW direction and induced borehole breakouts in the
perpendicular direction. This trend is completely in line with the regional stress field (Plenefisch
and Bonjer, 1997). No pressure tests were performed. The well was drilled with a balanced mud
system. The fact that induced fractures are observed already in a balanced well, indicates that
fraccing in the granite will not require large hydraulic pressures.
The EGS project of the European Community at Soultz-sous-forêts, 150 kilometres north,
situated in the Rhine Graben too, experienced similar conditions. In an injection test over a period
of 126 days with flow rates around 25 l/s through a reservoir at 3.5 km depth, injection pressures
averaged 30 bars (Baumgärtner et al., 1998)
Project plan
The next well is planned to the targeted reservoir depth at 5000 metres. It will be drilled on a
industrial site in the city of Basel. It is intended to deviate the well at a depth of 3000 metres to
the east with an angle of 15° in order to improve chances to penetrate open fractures associated
with the main boundary fault system.
When the main targets of a minimal temperature of 190°C and a fractured reservoir rock is found
in a favourable stress field, the well will be suspended. A second monitoring well two kilometres
to the east will then be drilled and equipped with a similar seismic array like the Otterbach well.
The two extended seismic arrays provide a series of locally independent receiver points sufficient
to compute the location of a seismic source with the required accuracy. Subsequently injection
tests will be conducted in the deep well in order to develop an enhanced reservoir. The final two
wells will be drilled deviated from the same location. The conversion cycle for power production
will be selected upon proof of circulation.
The exploration phase (proof of circulation) should be completed within two years. Beside the
technical challenges to stimulate a fracture system along a fault system in a seismically active
area, other environmental challenges, like drilling noise mitigation in a city, have to be met.
The project is carried by a partnership of regional utility companies and subsidised by the local
government. All investors are aware of the pilot character of the project, but in view of the
chances to gain a leading know-how in harvesting an old but hitherto unreachable resource is
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worth the well known exploration risks.
References
Baumgärtner, J., A. Gérard, R. Baria, R. Jung, T. Tran-Viet, T. Gandy, L. Aquilina, and J.
Grarnish, 1998, Circulating The HDR Reservoir at Soultz: Maintaining Production and Injection
Flow in complete balance. -Initial results of the 1997 circulation experiment.: Twenty-third
Workshop on Geothermal Reservoir Engineering.
Bonjer, K.-.-P., 1997, Seismicity pattern and style of seismic faulting at the eastern borderfault of
the southern Rhine Graben: Tectonophysics, v. 275.
Deichmann, N., 2003, Seismicity Basel 1975 - 2002, in epimapBasel.cvx, ed.
Medici, F., and L. Rybach, 1995, Geothermal Map of Switzerland 1995 (Heat flow density):
Beiträge zur Geologie der Schweiz, v. Geophysik Nr. 30.
Plenefisch, T., and K.-.-P. Bonjer, 1997, The stress field in the Rhine Graben area inferred from
earthquake focal mechanisms and estimation of frictional parameters.: Tectonophysics, v. 275, p.
71 - 97.
Reinecker, J., O. Heidbach, and B. Mueller, 2003, The 2003 release of the World Stress Map
(available online at www.world-stress-map.org).
Weidmann, M., 2002, Erdbeben in der Schweiz, Verlag Desertina, Chur.
4
recharge
heat reservoir
exchanger cooling
unit
reservoir-
monitoring
power ORC or
Kalina
cycle
district
monitoring heating
well
injection
well
production monitoring
wells well
~ 4.5 - 5
.5 km
enhanced
reservoir
m
- 1. 5 k
~ 0. 5
Figure 1: Concept of EGS cogeneration pilot plant in Basel, Switzerland
cro s
s se
c ti o n
in Fi
g u re BLACK FORREST
4
N
VOSGES
BE
RA
EG
IN
Ml
RH
5.0
4.0 Basel
3.0
2.0
1.0
PLATEAU JURA
UR A
D J
DE
FOL
20 km MOLASSE-BASIN
Figure 2: Location of the EGS pilot plant with regional geological setting. Seismicity 1975 - 2002
(source Swiss Seismological Service, Deichmann 2004)
FRANCE GERMANY
discussed area
SWITZERLAND Display of maximal horizontal stress
Regime
semi-filled circles: strike-slip faulting
filled circles: thrust faulting
empty circles: normal faulting
Method
circle: focal mechanism
stars: hydro. fracture
open arrows: borehole slotter
closed arrows: breakouts
source: World Stress Map Rel. 2003
Heidelberg Academy of Sciences and Humanities
100 km ITALY Geophysical Institute University of Karlsruhe
Figure 3: Stress field Switzerland showing predominant strike-slip faulting regime in the Basel area.
WEST EAST
VOSGES RHINEGRABEN BLACK
FOREST
0 km Basel
100°C
200°C DEEP HEAT MINING
10 km 300°C no date Sierentzer-
hypocentre
400°C
upper crust
500°C (brittle)
20 km lower crust
Legend
30 km
Tertiary
Mesozoic
lithospheric mantle Paleozoic
crystalline basement
40 km Focal points 1971-1994
10 km
Figure 4: E-W section through the Southern Rhine Graben with focal points (1971 - 1994)
NW BS1 OT2 Rhinegraben SE
Huningue/F Main Boundary Fault Waldhus/Hard
1000 m 2000 3000 4000 5000 6000 7000 8000
250 m
0 Tertiary
Jurassic
- 1000 Triassic
Permian
- 2000
- 3000
Crystalline
Basement
- 4000
- 5000
Figure 5: SE - NW section across main Eastern Rhine Graben fault and situation of geothermal wells
Otterbach 2 (OT2) and Basel 1 (BS1, planned)
OT2 0°C 100°C 200°C
0 km 0 km
Tert.
Jur.
1 km 1 km
Tri. measured
2 km 2 km
Per.
124°C
3 km 3 km
Pre-Carb. basement
extrapolated
4 km 4 km
5 km Target temperature 200°C 5 km
6 km 6 km
Figure 6: Temperature gradient as measured in well Otterbach 2 (OT2) and extrapolated to target depth.
