POWER SOURCES OF MWD

The MWD Tool works in two situations :
(a) When the circulation is ON
(b) When there is no circulation, i.e. while tripping when the pump is in OFF condition.

Also, once lowered, the MWD tool is not retrieved back to surface unless there is some kind of problem with tool.

Thus to continuously provide power to the tool, we require a power source.


The power source can be :
(a) Batteries
(b) Turbine Alternators

Batteries : (Lithium Battery, usually 24 V)
They are compact and reliable since they contain no moving parts.
They have a finite operational life and are temperature-dependent.
batteries.
Since it has no relation with drilling fluid motion, this enables the tool to operate while tripping and also enables operation independent of mud flow hydraulics.
They have been successfully used for applications in which only directional data are required. As they provide a limited power output, they are not preferred with multisensor tool.

Turbine Alternators :

With the trend of using multisensor tools for downhole survey, turbines are becoming more widely used to provide power to the MWD tool.
The flow of mud through the tool is harnessed by the turbine blades, which rotate a shaft connected to an alternator, hence generating electricity.

The electrical power generated must be controlled by a voltage regulator. Although this system provides more power and longer operating life than a battery pack, power failures can occur if the turbine is damaged.
To prevent this damage a screen can be installed upstream of the turbine to filter out any debris in the mud.
The screen may be positioned at the top of the drill string for ease of access if it requires to be emptied or removed to allow passage of wireline tools.