Environmental Engineering Reference
In-Depth Information
446
within the drilling phase of a geothermal well. Due to its high cost, gas lifts based
on inert gases (such as nitrogen) do not constitute a practicable solution for con-
tinuous operation either.
This is why, for the production of geothermal fluid, exclusively pumps in-
stalled below water level within the production wells are exclusively used so far.
If low installation depths are possible, borehole shaft pumps are used. For such
units, the drive mechanism is located aboveground and linked to the pump via a
shaft. For that reason almost the entire borehole cross section is available for ex-
traction.
For high installation depths, borehole motor pumps are used. They are prefera-
bly powered by an engine driven by electrical energy. Also, turbine pumps are
available powered by diverted volumes of the geothermal stream, compressed
aboveground and pumped to the turbine/pumping unit below the surface. In this
case annulus between the well casing and the turbine feed pipe is used to pump
the geothermal fluid aboveground.
Such borehole motor pumps usually consist of an assembly of pump, protector,
and engine. The pump is installed within the production well below the geother-
mal fluid level. The required energy is provided to the pump by means of a cable
located within the annulus section, between the cemented casing and the produc-
tion pipe.
The pump is usually a multi-stage centrifugal pump, for which the number of
pumping stages is related to the height difference to be overcome, and the impel-
ler design influencing the volume flow. In most cases, asynchronous three-phase-
motors are used. The special grade oil ensures the isolation of the motor coils,
lubrication of the motor bearing and heat transfer to the motor housing. The inte-
rior oil pressure varies as a result of volume changes to the motor oil filling at
different temperatures (starting and stopping the motor). To prevent water from
entering the motor, the pump is provided with a pressure compensation system
allowing for compensation between the geothermal fluid inlet pressure and the
internal oil pressure. This protection device usually contains cascade systems pre-
venting water from entering the motor, taking advantage of the existing density
difference between geothermal fluid and oil.
The required pump installation depth depends on the expected maximum re-
duction of the water level during pumping operation, and the necessary minimum
immersion depth. During the operation of the pump (i.e. the production of geo-
thermal fluid), the water level falls below the idle level, and is dependant on the
delivered volume flow. The reason for that are the friction pressure losses be-
tween the borehole bottom and the pump inlet, and the pressure reduction between
the non-influenced reservoir and the interior of the downhole filter. Additionally,
water level changes occur due to temperature-related density changes of the liquid
column within the well.
The required production level between the geothermal fluid level and the well-
head determines pump dimensions. Additionally the pressure losses within the
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