MWD SENSORS

An MWD tool is equipped with the combination of following sensors depending upon the requirement :

Directional Sensor
Gamma ray Sensor
Temperature Sensor
Downhole WOB/ Torque Sensor
Turbine RPM Sensor

Directional Sensor :

The directional sensors currently being used in MWD tools uses triaxial magnetometers and accelerometers.

These sensors measure the required angles of inclination, azimuth and toolface.

Since the magnetometers measure azimuth relative to Magnetic North, the correct magnetic declination must be applied to the results.

The C axis is aligned with the axis of the tool, and the B axis defines the reference for measuring toolface angle.

The angular offset between the B axis and the scribe line of the bent sub must be measured before running in the hole.

Both magnetometers and accelerometers give voltage outputs that have to be corrected by applying calibration coefficients. The corrected voltages can then be used to calculate the required directional angles.

Some kind of signal (like when drill string rotation stops or when the pumps are shut off) are sent from surface to the MWD control system.

The control system after receiving such signals power up the sensors.

A transducer or motion sensor within the downhole tool recognizes this signal and initiates the survey.

During the time when the sensors are actually taking the measurements the drill string must remain stationary for accurate results to be obtained. This period is generally less than 2 min., after which normal drilling can resume.

The driller resumes the normal drilling process once the MWD rig display unit displays the updated survey.

The measurements of inclination azimuth and toolface are sent in a predetermined order. It generally takes 2-4 min. for transmission of a complete directional survey.

Accuracy of the survey :
± 0.25° for inclination, ± 2.0° for Azimuth and ± 3.0° for Toolface, which may vary from one tool manufacturer to other.