Page 195 - Low Temperature Energy Systems with Applications of Renewable Energy

182 Low-Temperature Energy Systems with Applications of Renewable Energy

another important physical property that is easily measured. The magnetic field in a
prospective area is of great interest and can be measured and interpreted to provide
a fuller picture of the subsurface structure.

5.2.1.4 Synthesis of findings
The most important lesson during exploration is that no one individual method is
dispositive about the value of a particular site; all of the information gathered from
exploration tools of various types must be used to piece together a full assessment
of the field’s potential. The data must be synthesized in a comprehensive manner,
lest one measurement be given undue excessive weight and influence the final
outcome. Ultimately, even a most promising area based on exploration studies must
be proven by drilling deep wells.

5.2.2 Drilling
The best use of the exploration studies is to determine targets for the first deep wells.
Since well-drilling is an expensive and risky operation, one needs to target the most
attractive site for the first well, along with sites for the next ones if the first well proves
successful. In the early days of geothermal exploration, obvious sites were targeted and
shallow wells were sufficient to tap into reservoirs that had existed for millennia
without exploitation. These wells were used to supply fluids for mineral recovery,
as at Larderello in Italy where boric acid was produced for several centuries until
the mid-1900s, or to generate electricity from geothermal steam, as was done also at
Larderello from about 1904 to the present day.
Nowadays, places far from tectonic plate boundaries in areas not known for conven-
tional geothermal resources, such as Western Europe, are being drilled for thermal wa-
ters to heat buildings, and other direct heat applications. Since the fluid temperature in

such places is controlled mainly by the natural geothermal gradient of about 30e33 C
per kilometer, it is necessary to drill quite deep, typically 3000e4000 m, to reach fluids
with usable temperatures. Drilling rigs have been developed for such deep operation,
and some even are automated to the extent that crews on the drilling platform are not
needed; all operations are controlled remotely from the rig cabin, including adding drill
pipe and screwing together the drill string.
Even though acceptable temperature may be found, a well still needs adequate perme-
ability to allow sufficient flow rate of geothermal fluid. A poor drilling operation can dam-
age the formation, plugging narrow fractures that serve as fluid conduits, and leave an
unproductive hole in the ground. Some damage can be repaired by chemical treatment
to clean out the plugged fractures or by thermal shock from injecting cold water and
thereby restore permeability and allow the well to be salvaged for commercial use.
In some basins in Europe, a deep sedimentary formation with good permeability
exists at drillable depths. In such places two wells can be drilled into the same layer
but with widely spaced points of penetration that permit fluid to be withdrawn from
one well, used for various heat applications, and then reinjected into the same forma-
tion. Done properly using directional drilling, the two wellheads may be close to each