Page 36 - Geothermal Energy Systems Exploration, Development, and Utilization

12 1 Reservoir Definition
formations are present in a basin, the role of compaction has to be accounted
for since porosity decreases with compaction pressure and particles’ orientation
becomes horizontal with increasing pressure (Vasseur, Brigaud, and Demongodin,
1995). Both effects together with temperature-dependence effect induce important
changes in thermal conductivity. First, the decreasing porosity (and thus amount
of water) with depth tends to increase thermal conductivity, while temperature
dependence tends to decrease it (see Harcou¨ et et al., 2007 for details). Second,
the horizontal orientation of individual clay particles develops anisotropy, favoring
lateral heat transfer and hindering vertical heat flow.
An example of the effect of thermal conductivity anisotropy on thermal field is
illustrated in Figure 1.7, where the Paris basin is modeled according to Demongodin
et al. study (1991)). Anisotropy ratio is increased with depth and thermal boundary
conditions enable to reproduce measured surface heat flow values. Figure 1.8b
shows horizontal temperature profiles at 1500 m depth, with and without anisotropy
effect. When anisotropy is accounted for, heat accumulates more efficiently within
◦
the basin and a 20 C difference with the isotropic case is reached at basin
boundaries. Obviously, the importance of the anomaly critically depends on thermal
conductivity values and anisotropy ratios. Measurements on representative core
samples, and scaling with in situ conditions are thus of major importance when
thermal modeling of a sedimentary basin is performed (Gallagher et al., 1997).

3.1– 2.9
1500 m 2.6 –1.8
2.3 –1.5
3.1 – 2.0
3.0
Vertical exageraion ×20
6 km
(a) 400 km
80 No anisotropy
With anisotropy
T (°C) 60 Depth = 1500 m
70
50
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
5
(b) Horizontal distance (×10 m)
Figure 1.7 (a) Chosen model for the Paris accounted for, the first value is considered
basin (after Demongodin et al., 1991), as homogeneous. (b) Horizontal temperature
with thermal conductivity values indicated profiles at 1500 m depth across the basin
as follows: ‘‘horizontal component–vertical (see text).
component.’’ When anisotropy is not

1.1.6
Fluid Circulation and Associated Thermal Anomalies

In previous sections, heat transfer was described by pure conduction, where no
heat transfer by fluid motion could occur. However, shallow geological systems
are sometimes characterized by sufficiently porous layers (sedimentary units) or