English Edition (6 MB pdf) - Saudi Aramco

Fig. 6. Motor performance of the ‘Uthmaniyah 12” curve section.
the-bit. Figure 7 shows a point-the-bit RSS tool used during
this bit optimization exercise, which operates by offsetting the
bit and creating a bend that changes the course of the well in
the desired direction. The bit and tool body rotate at the same
speed, and the magnitude of the bit tilting effect is controlled
electronically from the surface. By eliminating the need to
slide to follow the well plan, the system creates high quality
wellbores that improve almost every aspect of drilling by
lowering vibration, extending bit life, minimizing downhole
tool failures, and increasing ROP.
These systems operate by placing a relative bit offset bend in
the system, much like a standard motor assembly. This bend is
held geostationary (nonrotating) with respect to the formation.
To understand the point-the-bit principle, one can make
comparisons to conventional drilling systems that use motors
or turbines. In a conventional system, a bent housing and
stabilizer on the bearing section enables the motor to drill in
either an oriented (sliding) mode or a rotary mode. In the
rotary mode, the bit and the drillstring both rotate. The
rotation of the drillstring negates the effect of the bent housing,
and the bit drills an over gauge straight path parallel to the
axis of the drillstring above the bent housing. In the sliding
mode, only the bit rotates. The motor changes the well course
in the direction of the bent housing, and the drillstring slides
down the hole behind the bit. In the point-the-bit system, the
bent housing is contained within the collar of the tool. This
bent housing is controlled by means of an electric motor that
rotates counter to the direction of and at the same velocity as
the drillstring. This control enables the bent housing to remain
geostationary while the collar is rotating.
POWERED ROTARY STEERABLE SYSTEM (PRSS)
Fig. 7. Point-the-bit RSS tool.
ROTARY STEERABLE SYSTEM (RSS)
RSSs have gained ground rapidly because they can
revolutionize the way that directional wells are drilled. RSSs
can be categorized by the mode of operation. There are two
steering concepts for these systems: point-the-bit and push-
A PRSS is a high-performance RSS with a fully integrated high
torque power section that converts mud hydraulic power to
mechanical energy, Fig. 8. This energy, combined with the
rotation provided by the rig’s top drive, significantly increases
downhole power at the bit. The additional torque capacity
enables the use of aggressive PDC bits for directional
application and greater weight on the bit, resulting in
increased ROP and more cost-effective drilling. This system
can drill faster and further.
The integrated power section rotates the bit and enables the
drillstring rotation to be slowed. Stick/slip and other
damaging vibration modes common to conventional rotary
drilling are reduced. All available energy is used to drill the
hole optimally. Casing wear and drillstring fatigue is reduced
as a result of slower drillstring rotation, which minimizes the
possibility of drillstring or casing failure 2 .
All external parts rotate at drillstring speed, which reduces
drag. The rotation also helps to clean and condition the hole,
reducing the risk of differential or mechanical sticking.
For deep gas operations in Saudi Arabia, point-the-bit
systems were used in conjunction with downhole power
sections, i.e., PRSS.
48 SUMMER 2010 SAUDI ARAMCO JOURNAL OF TECHNOLOGY