The Geo-Science Division of the
National Energy Authority of Iceland (NEA) provides services to the geothermal
industry in Iceland. The services consist mainly of geothermal exploration,
consultancy and services in geothermal drilling, reservoir modelling and
consultancy on exploitation. NEA also
consults on selecting sites for
surface installations (power
houses) for geothermal power plants, as regards earthquake, faulting and
volcanic hazards.
The role of NEA, as and end user in
the RETINA project, is therefore somewhat different form the other end users,
which are mainly concerned with natural hazards and public safety. For NEA the
main emphasis is on adapting techniques and understanding of dynamical
processes developed in the TETINA project to geothermal exploration. NEA and
the geothermal industry will also benefit from and make use of hazard and risk
estimation procedures developed in the project.
High-temperature geothermal
resources in Iceland are related to magmatic heat sources (intrusions) in the
crust and hence closely related to crustal movements and volcanism.
Low-temperature resources are in most cases related to deep ground-water
circulation in faults and fissures and hence strongly related to tectonics and
seismicity.
Main issues in geothermal
exploration are therefore to locate relatively shallow heat sources and faults
and fractures that maintain permeability for water to percolate to depth and
convect heat towards the surface. Improved knowledge and techniques in
volcanology, crustal movements and earthquake monitoring are therefore of great
importance in geothermal exploration.
Geothermal exploration and exploitation
is a multi disciplinary process, with input from many disciplines, such as
geology, geochemistry, geophysics and reservoir engineering. Results and data
from different disciplines are used to construct conceptual models of the
geothermal systems, where temperature distribution (likely heat sources),
permeability distribution (flow paths) and flow pattern are the crucial issues.
The models are progressively refined as new experience and data is acquired.
Experience has clearly shown that the success rate in geothermal drilling and
utilization is ultimately dependent on how good the conceptual models are.
Electrical methods that measure the
resistivity of the subsurface rocks are among the most important surface
exploration tools, because the resistivity structure can, to a certain extent,
be interpreted directly in terms of temperature distribution. It lies, however,
in the very nature of electromagnetic fields in conducting media, that the
resolution decreases strongly with depth and resistivity structure becomes
increasingly fuzzy. Data with higher spatial resolution are therefore generally
needed to define drilling targets at depth.
Surface geology (e.g. mapping of
faults and estimation of volcanic centers) and geochemistry can to some extent
be used to sharpen the picture. Temporal resolution, inferred from geological
features visible on surface is, however, often too limited to distinguish which
structures are relevant at present. Monitoring of earthquake activity and
accurate location of events and active fault planes (relative location and
focal mechanism) direct measurements of crustal movements revile presently
active faults.
High-temperature geothermal wells
are normally only drilled to the depth of about 2000 m (the deepest well is
about 2.400 m) and the physical and geological conditions in the reservoirs are
knot well know below that depth. Intrusive rocks (dikes and sills) are
increasingly common below about 1000 m depth, showing that transient heat
sources can be found at relatively shallow depth. It is therefore of importance
to be able to determine where intrusions are formed in crustal movement and
magma transport episodes, like in Krafla-fires and the resent episode in the
Grændalur-Hengill-Hellsiheidi area.
Measurements and monitoring of
crustal movements (both lateral and vertical) give important information on
crustal dynamics, tectonics and magma transport in the crust. This information
is of fundamental importance in understanding the geothermal activity and
construction of conceptual models of geothermal systems (heat sources, active
faults etcet.). Exploitation of geothermal resources (mass extraction) can, and
frequently does, lead to measurable crustal moments, mainly subsidence.
Precision monitoring of crustal movements, with GPS and INSAR techniques can
give quantitative information on the lateral extent of influences of
exploitation (size of the reservoir), and, when combined to gravity monitoring,
also on mass balance in the reservoir (how much of the extracted mass in compensated
by re-charge).
Seismicity, tectonics and magma
movements do not only give valuable information about the geothermal resources.
They also pose a threat of damaging both the underground resource (the
geothermal reservoir) and installations (wells) as well as surface
installations (power plants and pipelines). Since high-temperature geothermal
resources are ultimately linked to volcanism, volcanic eruptions are a
potential danger and gases from shallow magma intrusions can contaminate the
geothermal waters. This happened during the Krafla-fires in North Iceland. Earthquakes, faulting and rifting can damage
wells, pipelines and buildings. This has to be taken into account when
designing and installing such structures. Evaluation of these hazards and
development of precursor recognition is therefore of importance for geothermal
utilization, as well as for general public safety.
The requirements fall into two main
categories :
a) Development of new and improved
techniques for geothermal exploration,
reservoir assessment and exploitation monitoring.
b) Development of techniques and
procedures for estimation of natural hazards and risks to operations.
The requirement in the above two
categories can be further specified in terms of seismic and crustal deformation
monitoring:
As regards seismicity, the
requirements are specifically:
a. Techniques for precise mapping
(to accuracy of the order of 10 m) of active faults at depth in the geothermal
systems, determined by relative locations of micro-earthquakes, for the purpose
of identifying possible flow-channels into and within the geothermal reservoirs
and defining of drilling targets.
b1 Mapping of previously and
presently active faults, including amount and direction of slip, for the
purpose of constructing a hazard maps, defining seismic risk to plant
structures and operations.
b2. Monitoring of seismicity over time in and around the geothermal
systems in order to determine seismicity patterns in time and space, for the
purpose of alerting of changes, which can be expected to be precursors to a
damaging earthquakes or a volcanic events.
As regards monitoring of crustal
deformation, the requirements are:
a1. Cost-efficient methods for
monitoring crustal movements (GPS, CGPS and INSAR) for understanding tectonics
and crustal dynamics in space and time and identification of magmatic transport
events in the crust.
a2. Monitoring of production
induced subsidence.
b. Mapping and monitoring of active
faults and magma transport for hazard and risk assessment.