Environmental Engineering Reference
In-Depth Information
478
Enhancing the productivity.
Due to the relatively low energy content of steam
or hot water compared to hydrocarbons geothermal wells require a very high mass
flow in order to achieve a commercially viable power output. Commercial wells in
vapour dominated reservoirs produce several 10 t/h of dry steam. In liquid domi-
nated reservoirs a mass flow of one to several hundred t/h of hot water is required
for economic reasons. This can only be achieved in highly porous, fractured, or
karstic rock formations with a very high permeability. But the productivity of
geothermal wells can be enhanced to some extent by acid treatment and by hy-
draulic fracturing. In case of the Hot-Dry-Rock (HDR) concept no permeability of
the formation is required. The flow paths are artificially created by using the wa-
terfrac technique. These methods are described in the following sections.
Acid treatment.
Acid injection is a suitable means for productivity enhancement
when flow impedance close to the well has to be removed or minimised. In clastic
rocks, such as sandstone, the acid is dissolving the cement bridges between the
quartz particles and is thus increasing the pore space. Due to the relatively high
porosity (> 10 % of the rock volume) large volumes of the pore filling have to be
removed in order to achieve a measurable effect. In practice, the acid affects only
the first decimetres or metres around the borehole wall. Its effect on productivity
enhancement is therefore very limited. Better results are obtained in fractured
rocks. In this type of rock fluid flow is restricted to joints, natural flow pipes, or
channels. This secondary porosity forms only a small fraction (often < 0.01 %) of
the rock volume and can therefore be enhanced over much bigger distances
around the well. The best results are obtained in karstic carbonate rock forma-
tions. In this type of rock formation the effect of acid injection often consists in
establishing or improving a flow connection between the well and highly conduc-
tive flow pipes in the vicinity of the borehole.
Though quite often applied in oil or gas wells, acid treatment is still more an art
than a technique. The choice of acid (hydrochloric acid, fluoride acid, acetic acid,
formic acid and others), its concentration, the volumes and flow rates and of addi-
tives, e.g. corrosion inhibitors, is based more on experience and sense than on
formulas. Quantitative predictions are hardly possible. Common acid volumes for
an acid treatment are in the order of some to some tens of cubic meters. Since it
requires specialised equipment for on-site mixing and injection, acid treatment is a
relatively expensive technique /10-20/.
Hydraulic fracturing.
The hydraulic fracturing technique for oil wells was in-
vented in 1947 /10-21/ in order to improve their productivity. It is now a mature
technique and has been applied since then in more than a million oil and gas
wells. The basic process is to inject fluid into the borehole by using high-pressure
pumps. When the fluid pressure reaches a certain critical level the rock bursts.
Usually, a single axial fracture is initiated splitting the borehole in two halves over
a length of several metres or decametres. The fracture extends into the rock on
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