The toolface angle is the angle made by the tool face on a horizontal plane with respect to the magnetic North. The angle is measured in a plane perpendicular to the drill string axis between a fixed reference and a reference direction on the drill string.
For vertical wells, North is the fixed reference and the angle is called the magnetic toolface or gyro toolface depending on the instrument used to measure it.
For deviated wells, the fixed reference is considered to be the top of the borehole, and the angle is called the gravity toolface or high side toolface.
A steerable tool face system can be adjusted in the desired direction to continue drilling with the required deflection within the wellbore. Tool face orientation is expressed as either magnetic/gyro tool face or gravity/high side tool face.
The magnetic or gyro toolface is the orientation of the tool face measured on a perpendicular plane. When measured using a magnetic tool, it is called a magnetic toolface and when measured by a gyroscope, it is called a gyro toolface.
The gravity or high side tool face is measured from the high side of the borehole in a plane perpendicular to the hole axis. It is used in well sections which have an inclination greater than 5 degrees.
The formula for converting a magnetic or gyro tool face to high side toolface reading is:
High side tool face angle = Magnetic / Gyro tool face - azimuth of hole
The tool face angles indicate the following about the drilling progress such as:
At 0° - No change in azimuth and mud motor is lined up to build angle
At 45° - The mud motor is lined up to turn right and build angle
At 90° - No change in inclination and mud motor is lined up to turn right
At 135° - The mud motor is lined up to turn right and drop angle
At 180° - No change in azimuth and mud motor is lined up to drop angle
At 225° - The mud motor is lined up to turn left and drop angle
At 270° - No change in inclination and mud motor is lined up to turn left
At 315° - The mud motor is lined up to turn left and build angle