English Edition (6 MB pdf) - Saudi Aramco

needed a fit-for-purpose bit design to match the system and to
increase the steerability and stability, which were the weak
points indentified in the first run with the PRSS. The first step
included an in-depth analysis of Bit A to identify its strengths
and weaknesses, and to establish an action plan for the next
iteration. Among the strong points, it was clear that the use of
eight blades and 19 mm cutters as the primary cutting
structure provided a good balance between bit life and ROP;
consequently, it was decided to continue with this geometry.
Bit A recorded several vibration incidents, with slip-stick
recorded at the surface and measured while drilling (MWD)
shocks registered by the downhole tools. The cutters exhibited
medium wear, and several chipped cutters indicated a need for
improvements in the cutting structure. The force imbalance
values, which were one of the few analytical components
initially available when Bit B was designed, showed relatively
high values. The ideal bit force imbalance scenario occurs
when the values are near zero; values of less than 4% are
accepted as the minimum standard.
The cutting structure of the new Bit B included few
changes, as compared with Bit A. To gain stability and
improve the force imbalance values, the angle of the back
rakes was adjusted by 15% to decrease the aggressiveness,
and the blades were moved to obtain better figures. The 13
mm cutters were eliminated in the bit face to increase the
ROP, and the gauge length was reduced to 2½” to improve
the steerability, particularly because planned future wells
would require a greater BUR than the first two runs in the
Haradh field. Rather than a straight gauge pad, Bit B used a
spiral gauge, which considerably increased the borehole
contact, Fig. 12.
The cutter type in Bit B was updated to the Z3 ® cutter,
which provided additional impact resistance. All force
imbalance values were improved by up to 50%.
Bit B was used in the next four wells, including two wells in
the Haradh zone and two in the ‘Uthmaniyah zone. The 12”
section in the ‘Uthmaniyah wells required building the angle
from the vertical up to 74°, which tested the system’s
steerability. In the first three wells of this batch, the 12”
directional section was completed with one bit, but because of
a long directional section in the last ‘Uthmaniyah well, the
ROP became too low before the interval was completed,
requiring the bit to be pulled out of the hole and the use of
one more bit to complete the interval. Bit B showed better
stability than Bit A; the steerability was good but not
optimum because the PRSS had to be set at the maximum
deflection in some intervals to achieve the required doglegs.
The bit came out of the hole with a great deal of cutter wear,
which explained the reduction in ROP that required the bit to
be pulled out of the hole, Fig. 13.
The Z3 cutter could not drill sections of more than 2,500 ft
long because of the abrasiveness of the Khuff formation in the
‘Uthmaniyah zone. This was the first well with such a long
section, and it set a new challenge to be overcome.
Fig. 12. Straight gauge pad vs. spiral gauge pad.
Fig. 13. Bit B condition after drilling in the ‘Uthmaniyah zone.
SAUDI ARAMCO JOURNAL OF TECHNOLOGY SUMMER 2010 51