Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 476

both sides of the borehole as the injection continues. It is assumed that the fracture
plane is orienting vertically to the direction of the minimum principal compressive
stress component in the rock. Since for most tectonic settings the direction of the
minimum principal stress is horizontal, most fractures are vertical. For hydraulic
fracturing operations in oil or gas wells, usually several hundreds of cubic metres
of fluid are injected at flow rates between 10 and 100 l/s and at well head injec-
tion pressures of several tens of MPa. Fracture operations in oil and gas wells are
designed such that the height of the fracture is limited to the thickness of the oil or
gas-bearing layer. Typical fracture lengths in the horizontal direction are in the
order of hundred metres. In order to keep the fractures open after pressure relief,
sand or other fine grain substances (proppants) are mixed to the fracture fluid and
pumped into the fractures. For this reason gel-type fluids are used, that are suit-
able to transfer the propping material. At the same time the high viscosity of these
fluids minimises fluid losses into the surrounding rocks during the fracturing op-
eration. The viscosity breaks down after the placement of the propping material
thus enabling the transfer of oil or gas in the fracture.
Though hydraulic fracturing is a mature technique in oil and gas wells, there is
only little experience with this technique in geothermal applications. It has to be
stated however that the requirements in geothermal applications are considerably
higher. The very high mass flows in geothermal applications (except for steam
wells), for instance, require higher hydraulic fracture conductivity, higher tem-
perature fracturing fluids, and for the more aggressive fluids a higher chemical
resistance of the proppants. For these reasons the costs of fracturing operations are
in the range of several hundred thousands to some Mio. €. Therefore, it is hard to
predict, which role hydraulic fracturing will play in future geothermal applica-
tions.
Waterfrac technique. The Hot-Dry-Rock (HDR) and related concepts are based on
creating artificial fractures or opening natural fractures with surface areas of sev-
eral square kilometres. This is a great technical challenge and can never be
achieved by the conventional hydraulic fracturing technique. The only promising
technique to date is the so-called waterfrac technique. This technique is similar
but simpler than the conventional frac technique. Instead of injecting small to
moderate quantities of viscous fluids containing proppants, very large volumes of
water are injected to fracture the formation.
The waterfrac technique has been widely applied for HDR projects /10-22/, and
it was demonstrated that in crystalline rock the desired large fracture surfaces can
be created by injecting water volumes of several ten thousands of cubic metres. It
was also proved that these fractures are kept open after pressure relief due to a
self-propping effect. This effect is not totally understood yet. The most likely
explanation is that the two opposite fracture surfaces are laterally displaced
against each other by irreversible shearing during the fracturing process and that
due to the roughness and unevenness of the fracture surfaces they no longer
match, and open spaces remain after releasing the fluid pressure. It has been dem-