STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

902 Drilling and Well Completions
c. resume rotary drilling with the appropriate bottomhole assembly to build
angle, hold, or drop.
The kick-off procedure required numerous single-shot runs to start the
deviation in the correct direction. Since, during this phase, the drillpipe was
not rotating a steering tool was developed to be lowered on an electric wireline
instead of the single shot. The measurements were then made while drilling.
Measurements by electric cable are possible only when the drillstem is not
rotating, hence with a turbine or downhole motor. The logging tool is run in
the drillstring and is positioned by a mule shoe and key. The process is identical
to the one used in the single-shot measurements. The magnetic orientation
sensor is of the flux-gate type and measures the three components of the earth’s
magnetic field vector in the reference space of the logging tool. Three accelerometers
measure the three components of the gravity vector still in the same
reference space. These digitized values are multiplexed and transmitted by an
insulated electric conductor and the cable armor toward the surface. On the
surface a minicomputer calculates the azimuth and the drift of the borehole as
well as the angle of the tool face permanently during drilling. In the steeringtool
system, the computer can also determine the azimuth and slant of the
downhole motor underneath the bent sub and thus anticipate the direction
that the well is going to take. It can also determine the trajectory followed.
Figure 4-215 shows the steering tool system.
Figure 4-215. Typical steering tool unit with surface panel and driller readout.
(Courtesy of lnstitut Francois du Petrole.)