Page 37 - Geothermal Energy Systems Exploration, Development, and Utilization

1.2 Heat Flow and Deep Temperatures in Europe 13
highly permeable areas (fault zones) in which crustal fluids may freely circulate.
Depending on fluid velocity, permeability, or thickness of the porous layer, heat
from several kilometers depth may be entrained by fluid circulation and thus create
temperature anomalies.
Several studies have demonstrated the possibility to detect fluid motion by
temperature measurements within boreholes (Drury, Jessop, and Lewis, 1984;
Pribnow and Schellschmidt, 2000). Small-scale water flows through a fracture
crossing the borehole may disturb locally the measured geotherm by a few degrees
centigrade (Vasseur et al., 1991), and large-scale fluid circulation (convective flows)
may lead to cooling or warming effects exceeding tens of degrees centigrade (Lopez
and Smith, 1995; B¨ achler, Kohl, and Rybach, 2003; Wisian and Blackwell, 2004). In
some cases, the measured temperature anomalies cannot be explained by purely
conductive processes and one must account for free convection in highly permeable
fault zones (R¨ uhaak, 2009; Garibaldi et al., 2010).

1.1.7
Summary

Crustal temperatures are controlled by thermal boundary conditions and thermal
properties of rocks. In the pure conductive regime, knowledge of mantle heat flow
and crustal heat production enables to determine a probable averaged geotherm,
but the natural heterogeneity of crustal composition may lead to local variations
reaching several tens of degrees centigrade at a few kilometers depth. When
available thermal data are used to infer deep temperatures (as it is the case in the
next section), similar uncertainties can be assigned to extrapolated data.
Despite the fact that crustal temperatures are not easy to estimate, it is shown
in Section 1.1 that models of geothermal reservoirs depend on several other
parameters, which may be less constrained than thermal properties. In particular,
the presence of fluids, which is important in the development of geothermal
energy, may completely distort temperature field as soon as rock permeability
is high enough (Manning and Ingebritsen, 1999). Within sedimentary basins,
permeabilitycan varybyapproximatelyfour ordersofmagnitude,thusallowing
or preventing fluid circulation. The detailed knowledge of temperature field in an
area is probably not sufficient to characterize a geothermal reservoir.
Before defining the concept of geothermal reservoir, heat flow data from Europe
are reviewed and presented. The objective of this second section is to illustrate
how surface heat flow and deep temperatures are not necessarily correlated, and
how significant errors in deep temperature estimates can be made when shallow
measurements are extrapolated at depth.

1.2
Heat Flow and Deep Temperatures in Europe

Independent of numerical modeling of heat transfer within geological systems,
the best way to search for thermal anomalies in the shallow crust consists first