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Exploring High Temperature reservoirs: new challenges for geothermal energy - Volterra, Italy, Workshop2
Exploring High Temperature reservoirs: new challenges for geothermal energy - Volterra, Italy, Workshop2
1-4 April 2007 Volterra, Tuscany, Italy
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Thermal and rheologic signatures of high-enthalpy resources
Surface heat flow measurements over high-enthalpy systems indicate strongly positive
thermal anomalies. Whereas in “normal“ geothermal settings the surface heat flow is
usually below 100-120 mW m-2, in active geothermal areas heat flow values as high as
several W m-2 can be found. Systematic interpretation of heat flow patterns sheds
light on heat transfer mechanisms at depth on different lateral, depth and time
scales. Borehole temperature profiles in active geothermal areas show various signs
of subsurface fluid movement, depending on position in the active system. The heat
transfer regime is dominated by heat advection (mainly free convection). The onset of
free convection depends on various factors, such as permeability, temperature
gradient, and fluid properties. The features of heat transfer are different for
single or two phase flow. 

Two main factors affect the rheology of the lithosphere in active geothermal areas:
steep temperature gradients and high pore fluid pressures. Combined with lithology
and structure, these factors result in a rheological zonation with important
consequences both for geodynamic processes and for harnessing geothermal energy. As a
consequence of high temperature, the mechanical lithosphere is thin and its total
strength can be reduced by almost one order of magnitude with respect to the average
strength of continental lithosphere of comparable age and thickness. The
brittle/ductile transition is located within the upper crust at depths less than 5-10
km, acts as the root zone of listric normal faults in extensional environments, and
at least in some cases is visible on seismic reflection lines. 

Characteristic heat flow and heat transfer signatures as well as structural and
rheological features in high-enthalpy systems will be demonstrated by examples from
Iceland, Italy, New Zealand and the USA. 

More in:
Ranalli, G., Rybach, L.: Heat flow, heat transfer and lithospheric rheology in
geothermal areas: Features and examples. J. Volc. Geotherm. Res. 148 (2005), 3-19
Id: 13
Place: Volterra, Tuscany, Italy
Campus SIAF, SP del Monte Volterrano
Localita' Il Cipresso
Volterra, Italy
Starting date:
02-Apr-2007   13:30
Duration: 30'
Primary Authors: Prof. RYBACH, Ladislaus "Ladsi" (GEOWATT AG)
Presenters: Prof. RYBACH, Ladislaus "Ladsi"
Material: slides Slides

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