Environmental Engineering Reference
In-Depth Information
the casing or liner and the cement section located behind with a perforation
gun or by jet cutting.
Due to its more favourable hydraulic properties, within recent years, open hole
completion has been applied for production and injection wells of hydro-
geothermal heating plants more frequently. But in case of a low consolidation of
the reservoir rocks (for instance, sandstone reservoirs tends to crumble if the geo-
thermal fluid is removed from the reservoir) additional completion measures need
to be taken. One measure is the use of a so-called gravelpack. Here the borehole
has to be enlarged within the reservoir section. Then a wire-wrap filter is installed.
The remaining volume of the annulus is then filled with gravel adapted to the
sandstone grain size of the reservoir rocks. However, if good reservoir stability
exists, additional sandstone control measures may not be required.
A good cementation of the casing is mandatory for geothermal wells. Incom-
plete cementation can result in buckling or rupture of the casing due to thermal
expansion, to a collapse of the casing due to the thermal expansion of fluid
trapped in caves behind the casing, or to a failure of the casing due to corrosive
formation fluids being in contact with the outside of the casing. In most cases
temperature stabilised cementing compositions with low water content are used.
These compositions contain silica flour to prevent strength retrogression and to
achieve high temperature stability, perlite to reduce the density of the cement in
order to prevent hydro-fracturing of the formation and lost circulation during the
cementing procedure, dispersants to keep the viscosity of the cement at a moder-
ate level, and a retarder to keep the cement pump able over a sufficiently long
time at elevated temperatures. The cement is mixed on site and pumped inside the
casing often via the drill string into the annulus between the casing and the forma-
tion. The cement volume to fill the annulus is calculated by using the results of
caliper logs. In most cases the annulus is filled with cement up to the top of the
casing. Cementation at the high temperatures prevalent in geothermal wells is a
complex procedure and requires careful planning, testing and experienced special-
ised service companies.
Downhole part. The downhole part of the geothermal fluid circuit is needed to
extract warm or hot fluid from the reservoir rocks in order to make them available
for the use as an energy carrier /10-1/.
Well completion. The required end diameter of the well located within the reser-
voir rock is determined by the required geothermal fluid volume flow, and varies
between 200 and 300 mm.
Beginning at the drilling stage specific tests (with regard to the assessment of
hydrodynamic properties of geothermal reservoirs), measurements and analyses
should be performed in order to evaluate the reservoir capacities for energy provi-
sion. The most important factors are the possible production rate as well as the
temperature of the geothermal fluid.
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