The rotational orientation
of a tool around the axis of the borehole is the
tool-facing measurement. Tools that require their facing direction
to be measured include cameras, wedges, directional drilling
devices, casing cutters, core orientation equipment, x-ray and
ultrasonic devices. Tool-facing surveys are procedure specific and
require accessories (running gear) that are not required in the
directional survey of the borehole heading. At Pajari Instruments we
emphasize the fact that directional surveying and tool-facing are
separate procedures, each with their specific requirements of
accuracy, efficiency, and costs.
illustrates the tool facing measurement system with an e-SYNC-t. The
standard instrument will give a 0° reading when the index mark on
the instrument is vertical upward in the borehole irrespective of
the inclination of the hole.
The instrument is attached
to the applicable tool and secured with the index marks of the
instrument and the tool being longitudinally aligned. Alignment can
be accomplished by variable thickness washers between two threaded
units or by using an alignment coupling.
The +/- error envelope for the tool-facing reading defines the maximum error expected from the
instrument. However, instruments which measure tool-facing by plumb
devices (gravity sensors/pendulums) cannot give a tool-facing in
vertical holes. The accuracy decreases rapidly in the last few
degrees approaching vertical and is not measurable at vertical.
Instruments which read tool-facing from gyro, gyro-compass or
gyro instruments will give the tool-facing in vertical holes to the azimuth accuracy of the
instrument. If the tool-facing in near vertical holes can be
surveyed at least 6 meters (20 ft) away from ferromagnetic metals,
tool facing can be done to ‘azimuth accuracy’ with magnetic compass