Page 176 - Geothermal Energy Systems Exploration, Development, and Utilization

152 3 Drilling into Geothermal Reservoirs
between the tubing and the annulus or are used to make it easier to use
measurement and maintenance tools.
• An extra safety valve, called the subsurface safety valve for high risk wells under
high pressure.
3.7.4
Well Completion with Pumps

There are two different pump types in use; it is the line shaft pump and the electric
submersible pump.
• Line shaft pump: This type of pump is driven mechanically by a long hollow
shaft which is rotated from surface while the pump is mounted at a certain depth
of up to several 100 m inside the inner casing at this depth.
• Electrical submersible pump (ESP): This pump type is driven by an electric motor
which is installed downhole below the pump; both hangs under a pump string
(casing string) of sufficient diameter to handle the high flow rates. A cable for
the electric power supply is clamped outside onto the pump string.
• Production/pump string: Pump strings normally consist of normal casing mate-
rial; however, it is recommended to use a flush or near-flush connection instead
of a standard coupled connection in order to reduce the risk of cable damage by
the rectangular shoulder of the standard couplings.
To reduce the pressure loss inside the pump string an internal plastic coating
(IPC) can be used if the temperature of the hot water is not exceeding the
temperature limits of the coating material.
When using a pump string together with a high torque electrical submersible
pump (ESP) a casing thread should be selected which is capable to with-
stand high torque without being unscrewed, particularly when starting the
ESP.
• Injection string: For injection strings also standard casing material is used.
Because normally the pump (if needed) is installed at surface no torque will be
transmitted to the injection string during use standard coupled threads can be
used. In order to reduce the pressure loss inside the injection string an IPC can
be used.
• Protection string (retrievable): Sometimes it may be advised to install a retriev-
able protection string which protects the cemented casing against wear and/or
corrosion (e.g., if highly corrosive fluids will be produced). However, such a
string has to be taken into consideration from early planning stage because it
reduces the available space for pumps and pump string.

3.8
Risks

Occurrence of risks leads always to delay due to additional measures and need
for extra tools/services (fishing, additional casing/liner strings, additional cemen-
tations, and other services); always additional time and costs; often to additional